AN ABSTRACT OF THE THESIS OF
Frank M. Sprtel for the degree of Master of Science in Oceanography presented on June 23, 1997. Title: Two Styles of Oceanic Near-RidgVo1canism for the South Indian Ocean and the NE Pacific Oean Redacted for privacy Abstract approved: David M. Christie
Near-ridge volcanism is an integral component of a mid-ocean ridge system that defines the off-axis extent of the magmatic region sunounding the ridge. Perhaps the two most common expressions of near-ridge volcanism in the ocean basins are near-ridge seamounts and intra-transform volcanic centers.
Seafloor near-ridge seamount volcanism was examined for the Southeast Indian
Ridge (SEIR), a part of the global spreading system that lies between the Australian and
Antarctic continents. During the 1995 WESTWARD 9 & 10 cruises, 104 seamounts were discovered adjacent to a undersurveyed and undersampled 2500km section of the SEW from 88°E to 120°E. SEW seamounts are evenly distributed on the Australian and Antarctic plates. Australian plate seamounts are generally larger in size than Antarctic plate seamounts. Moreover, all seamounts aligned in chains were found on the Australian plate.
Such an asymmetric distribution of seainount chains also occurs at the Juan de Fuca ridge in the NE Pacific and may be related to mid-ocean ridge migration. Four of the seven SEW seamount chains are aligned in a direction oblique to absolute plate motion. The presence of oblique trending sealnount chains may be related to along-axis asthenospheric flow.
Twelve of the 104 seamounts surveyed were sampled by WESTWARD 10. Seamount basalt glasses were analyzed for major, minor, and trace element content. These data indicate that the seamount lavas are similar in composition to axial lavas and have undergone crystal fractionation. Intra-transform volcanism was examined for the Blanco Fracture Zone, a transform fault that connects the Juan de Fuca and Gorda Ridges in the NE Pacific. A 1994 NOAA cruise utilizing the U.S. Navy's Advanced Tethered Vehicle (ATV) discovered fresh- looking lava flows in the East Blanco Depression (EBD), a small basin within the Blanco
Fracture Zone. Glasses from pillow basalts recovered by the ATV were analyzed for major, minor, and trace element content. The glass data span a large range in MgO (-5-9.2 wt.%). Minor and trace elements were modeled by batch and open system fractional melting with 6 % retained melt. These data suggest that both batch melting and open system fractional melting may have been involved in the initial stages of the Blanco lava development while a minor crystal fractionation signature was superimposed on these lavas prior to eruption. Two Styles of Oceanic Near-Ridge Volcanism for the Southeast Indian Ocean and the NE Pacific Ocean
by
Frank M. Sprtel
A THESIS
submitted to
Oregon State University
in partial fulfillment of the requirements for the degree of
Master of Science
Presented June 23, 1997 Commencement June 1998 Master of Science thesis of Frank M. Snrtel presented on June 23. 1997
Redacted for privacy
Major Pro1ssor, representing
Redacted for privacy
Dei of College of Oceanic & Sciences
Redacted for privacy
Dean of
I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. Redacted for privacy
Frank M. SrteVauthor Acknowledgments
I would like to whole-heartedly thank my two advisers, Dave Christie and Bob
Duncan, for their advice and support of my interests during my tenure at Oregon State
University. Special credit goes to Dave Christie, who has certainly improved my scientific writing! Andy Ungerer, Roger Nielsen, and Peter Michael were great to have as mentors in the lab because of their broad experience in different analytical techniques.
A very special thanks to my family for all of their love, support, and reassurance during my thesis work. I owe as much gratitude to my partner, Kate Metzger, for her constant love, understanding, and encouragement. As a fellow Wisconsinite, Brendan
Sylvander, was a good friend and extremely helpful in easing the burden of day-to-day graduate level research. And last but not least...I am forever grateful to my fellow distance running partners, Dave, Jeremy, and Dylan for offering me a daily escape from the rigors of my graduate program.
This research was supported by a grant from the National Science Foundation. Contribution of Authors
Trace element analyses were performed in the laboratory of Dr. Dave Christie who also provided guidance and advise for the two projects contained in this thesis. Dr. Roger
Nielsen helped interpret the data from the East Blanco Depression and ran the electron microprobe lab that produced the major element analyses for both projects. Table of Contents
Page
Chapter I: Introduction I
Chapter II: A Morphologic, Kinematic, and Petrologic Study of Near-Axis Southeast Indian Ridge Seamounts 4
Abstract 5
Introduction 5
Methods 9
Observations 11
Distribution of seamounts along the SEIR 11 Seamount distribution with individual segments 13 SEW seamount chain orientation 21 SEW seamount chemistry 23
Discussion/Interpretation 31
Distribution of seamounts along the SEW 31 Seamount distribution within individual segments 35 SEW searnount chain orientation 39 SEW seamount chemisny 42
Conclusion 45
References 46
Chapter ifi: The Petrogenesis of Intrabasin Volcanics from the East Blanco Depression 49
Abstract 50
Introduction 50
Geologic History 51
Methods 54
Results 54
Petrography and mineral chernistiy 54 Chemistry of East Blanco Depression lavas 56 Table of Contents, Continued
Page
Discussion 59
Polybaric fractional crystallization model 59 Batch and fractional melting models 63 Open system fractional melting model 67 Crystal fractionation correction for the open system fractional melting model 69 Two scenarios for the petrogenesis of the Blanco lava suite 69
Conclusion 71
References 73
Chapter IV: Summary 75
Bibliography 78
Appendices 83 List of Figures
Figure Page
11.1 a Map of the Southeast Indian Ridge from 88°-120°E 6
II.lb-e Boxed sections of the SEW 7
11.2 Map of the East Pacific Rise from 5°-15°N 10
11.3a-b Histograms of seamounts on the Australian Plate 12
11.4a-b Histograms of seamounts on the Antarctic Plate 12
11.5 The ll1°ESeamountField,Box4 14
11.6 SEW seamount height versus longitude 15
11.7 SEW seamount basal diameter versus longitude 16
11.8a-b Histograms of SEW and EPR seamount heights 17
11.9 a-b Histograms of SEW and EPR seamount basal diameters 18
11. lOa-b SEW seamount chains and individual seamounts position in segment 19
11.1 la-b SEW and EPR seamount chain position in segment 20
11.12 Bathymetry map of 89°E seamount chain 22
IL 13 Bathymetry map of 99°E seamount chain 24
11.14 Bathymetiy map of 90°E seamount chain 25
11.15 Bathymetry map of 101°E seamount chain 26
11.16 Bathymetry map of 101 °50'E seamount chain 27
IL 17 Bathymetry map of 107°E seamount chain 28
11.18 SEW seamount major and minor element variation diagrams 29
11.19 SEW seamount trace element spider diagram 30
11.20a-b Davis and Karsten (1986) seamount chain evolution model 33
11.21 Along-axis mantle flow model of West and Christie (1996) 36
11.22 SEW seamount chain apparent flow velocity versus longitude 40 List of Figures, Continued
Figure Page
11.23a-f Oblique seamount chain formation model 41
11.24 SEIR seamount La/Sm versus MgO variation diagram 43
111.1 East Blanco Depression location map 52
111.2 Bathymetry map of the East Blanco Depression 53
111.3 Anorthite composition of plagioclase phenocrysts in glass versus Na20/CaO of host glass 55
111.4 Fosterite composition of olivine phenocrysts versus Mg# compared with calculated olivine composition for host glass versus Mg# 55 ffl.5a-b K20 versus MgO and Ti02 versus MgO variation diagrams for Juan de Fuca, Gorda, and Blanco lava suites 57 ffl.6a-b CaO versus MgO and K .OITiO2 versus MgO variation diagrams for Juan de Fuca, Gorda, and Blanco lava suites 58
111.7 Blanco lava suite spider diagram 60 ffl.8a-b CaO versus MgO and Ti02 versus MgO variation diagrams for polybaric crystal fractionation 61
HJ.9a-b K20 versus MgO and (La/Sm)n versus MgO variation diagrams for polybaric crystal fractionation 62 IIJ.lOa-b Ti versus Zr and Ti/Zr versus (La/Sm)n variation diagrams for batch and open system fractional melting 64
111.1 la-b (La/Sm)n versus Ba and (La/Sm)n versus K20 variation diagrams for batch and open system fractional melting 65
111.12 (CeIYb)n versus Sm variation diagram for batch and open system fractional melting 66 List of Appendices
Page
Appendix I: Electron Microprobe and ICP-MS Analytical Techniques 84
Electron Microprobe Analytical Technique 84
ICP-MS Analytical Technique 84
Sample preparation 84 Sample digestion and dilution 85
Appendix II: Basalt Glass Analyses for SEIR Seamount Lavas 86
Table All.1 87 Appendix III: Basalt Glass Analyses for East Blanco Depression Lavas 106
Table All. 1 107
Appendix IV: Batch and Open System Fractional Melting Model Parameters 110 Two Styles of Oceanic Near-Ridge Volcanism for the Southeast Indian Ocean and the NE Pacific Ocean Chapter I
Introduction
The global mid-ocean ridge system, traced for 80,000 km throughout the world's oceans (Mammerickx, 1989), is the largest volcanic construct on earth. Mid-ocean ridges mark divergent plate boundaries where two oceanic plates spread apart from each other as new oceanic crust forms. Although mid-ocean ridges are loci of most oceanic volcanism, their volcanic activity is supplemented by eruptions at other locations on the seafloor such as near-ridge seaxnounts, intra-transform extensional basins, and hotspots. Seamount volcanism is much more widespread than intra-transform volcanism. Seamounts are very common throughout the world's oceans and can potentially tell an important story about the magnitude and composition of volcanism during different periods of earth's past. In particular, near-ridge seamounts can be used to examine the extent and magnitude of volcanism on the flanks of a mid-ocean ridge.
Near-ridge seaxnount volcanism is covered in Chapter II of this thesis which discusses the distribution, orientation, and petrology of seamounts from a previously unstudied and unsurveyed 2500 km section of the Southeast Indian Ridge (SEIR). During the austral summer of 1995, WESTWARD 9 & 10 cruises aboard theR/VMelvillewere the first research expeditions to survey and sample the SEW from 88°E to 120°E. The small size and numbers of seamounts discovered on the flanks of the SEW indicate that the
SEIR's flanks are not as magmatically active as the flanks of other intermediate spreading centers such as the Juan de Fuca Ridge. Although spreading at nearly the same rate as the
Juan de Fuca Ridge, the SEW changes in ridge morphology from an axial high in the west to an axial valley in the east. This suggests that mantle temperature and flux change in magnitude and result in the movement of mantle toward the Australian Antarctic
Discordance (AAD).
The study of SEIR near-axis seamounts is important for contstraining the extent and
magnitude of the flow of mantle off-axis toward the AAD. For example, four seamount
chains oblique to the Australian absolute plate motion vector may have originated from a
source in the upper mantle that became entrained in the large scale regional flow field.
Since these chains form 10-20 km off-axis, their source indicates that the movement of
mantle away from the ridge is different from that beneath the ridge.
Although not as commonplace as near-ridge seamounts, intra-transform extensional basins contain volcanic centers such as intra-transform spreading ridges and single vents or cones. Intra-transform volcanism is relatively rare with volcanic activity documented in three major transforms; Garrett, Siqueiros, and Blanco Transform Faults. This style of oceanic volcanism occurs when extensional forces from plate boundary realignment occur along a transform boundary, causing the seafloor to rift within the transform fault zone. As a passive response to this rifling event, the mantle upwells and produces magma.
The chemical characteristics of this type of magma are addressed in Chapter ifi which focuses on lava erupted from the East Blanco Depression of the Blanco Transform
Fault Zone. This vent was discovered during a 1994 NOAA cruise that utilized the U.S.
Navy's Advanced Tethered Vehicle (ATV) to conduct surveillance and sampling in the East
Blanco Depression. The fresh-looking pillow basalts recovered by the ATV were analyzed for major, minor, and trace element concentrations and then modeled using Ariskin et al.'s
(1993) polybaric fractionation model, simple batch and fractional melting models, and
Johnson and Dick's (1992) open system fractional melting model to deduce the petrogenesis of the Blanco lavas. Model results indicate that the petrogenesis of the East
Blanco Depression was complex and was probably dominated by fractional melting with a minor fractional crystallization imprint superimposed on the lavas late in their evolution. 3
These results aie significant for two reasons. First, they demonstrate that aspects of the melting processes of MORB can be determined if the liquids did not pool and aggregate in a magma chamber prior to eruption. Second, the results show that batch melting can play a role in the formation of MORB. 4
Chapter II
A Morphologic, Kinematic, and Petrologic Study of Near-Axis Southeast Indian Ridge Seaznounts
Frank M. Sprtel David M. Christie 5
Abstract:
The 1995 WESTWARD 9 and 10 cruises confirmed thepresence of 104 near-axis seamounts adjacent to the intermediate spreading Southeast Indian Ridge (SEJR). SEIR seamounts are evenly distributed between the Australian and Antarctic plates. Seamounts located on the Australian plate are generally larger in size than those locatedon the Antarctic plate. Twelve SEIR seamounts were dredged by WESTWARD 10. Basalt glasses recovered from these dredges are N-MORBs and similar in composition to glasses sampled from the ridge axis. Incompatible element ratios (K2OIFiOZ arid La/Sm) indicate that the seamount lavas underwent ciystal fractionation. SEIR seamounts, especially seamount chains, cluster near the western end of individual ridge segments that display either axial high or transitional axial-high to axial-valley ridge axis mohology. Seven seamount chains were discovered on the Australian plate whileno chains were found on the Antarctic plate. SEW chains contain edifices thatare smaller in size than seamount chains adjacent to the East Pacific Rise (EPR). Unlike EPR chains, SEW chains azimuthsare largely oblique to absolute plate motion. The azimuths of these oblique chains and the relative motion ''ere used to calculate a difference vector. The oblique trends for SEW chains suggest that the seamount chain source is moving in a direction parallel to the east-west along-axis mantle regional flow field.
Introduction:
During the austral summer of 1995, cruises WESTWARD 9 and 10 of RN Melville surveyed and sampled 2500 km of a previously uncharted section of the Southeast Indian
Ridge from 88°E-120°E (Figure IL la) including off-axis bathymetric coverage out to 0.7
Ma along several sections of the ridge. The WESTWARD cruises also discovered numerous individual off-axis seamounts including those seamounts belonging to seven chains (Figures ll.la, II. lb-e & Table 11.1). The survey confirmed that the Southeast Southeast Indian Ridge
4OS
1 40°S 42S I
44S 60°S
2 46S 3 80°E 120°E 16OE ox4 48S
50 S
52 S
9OE 95°E 100°E 1O5E 11OE 115E 12OE
Figure II.la. An axial ridge crest trace of the Southeast Indian Ridge surveyed by WESTWARDcruises 9 & 10. Inset map shows the geographic position of the Southeast Indian Ridge. Circleson the large map mark seamount locations. The boxed-in sections of the ridge representareas under investigation in this study of SEIR near-ridge seamount volcanism. 7
Box 1 Box 2 31E BTE 95E 96E 9TE 98E 99E 110E b) - a. I.
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200-300m. $900-bOOm. 300-400 m. 1000-l100 m. 400-SOOm. J...... b- SMT Azimuth 50O-00m.1100-1200m. -- AIxohiteVectc 600-700 m. I eIath Vt 700-800 m. 1200-1300 m. 800-900m.13001400m.
Figure ILlb-e. Inset box sections from Figure 11.1a depicting the location of seamounts and individual seamount chains. The alignment of SEIR seamount chains is depicted with light gray arrows and is adjacent to the vectors of relative (medium gray) and absolute (darkgray)motion of the Australian Plate. Table 11.1. Small seamount chains near the Southeast Indian Ridge 0990 lot 11)671 ,,_ --41,550 -- 09'i)'E .-...._=....,..,,....AunIrIllo 350 lSu0,IOiamnlrr OIuIoceFroni 17.1 SnO Chain Alsunluit 30.24 AI.onIulr 7.30 l'CV RelatiOn 37.52 iinIoilr'n 7.21 SmI Chain 20.80 Azimuth GROut AoIn.uIh 0111.1 Along Aol. Slur, 9.52 M80fl 100MAU%nlul1 0.24 0.6! 900ItlnflS00mEflI 2.5 IS 09'O00809'S 4(7.3,,)2nd ll'43'S441303 I°40'S _..__. 50'13'E09'24'E09'I4t *0011110,4uulrallaAuritallr 10357)03500 11.020.7III6,41 ,.._., 29.625.) 27 38.2430.24 7.807.0151.80 37.3237.8237.52 zflrr 7.217,2!7,2) fltfl 2040)288029.00 9.529.32 (0.240.34 0,610.6!00! 22512.5 2.5 09'E 41'41'S 09'19'E 1250 3.28 AVGorg lilt (0652 Oifl 4i'70'3 080'55'800'108 AmlialigAuslIalla i 040150 4124 leA2.3 27.2239.!10,2 22 35.7438,2438.24 7.037.007.0) 31.1237.3207.55 7.213.28 20.0024.0020,00 9329.52 80,2410.24 0,24 04300.60.01 82.5825 2.3 SO'S00'EOS'S9&E ails.Ini21o4In 42'Ot'S4214242905 R)I'34'S09'59'E AusirnIlaAuui,a)iaAusiralia 280130 0.021.002.772.20 70,9228 51.12315257,71 7.043.841.04 37.0)37.0!33,0) 7.137.237.25 20.80(0.0010.80 27,0113.0127,08 27,3227,7227.72 1.092.85 .85 34.934.0 AVG 42'OJ'S 00°0 AtulInnilo 3Q.,,,,,,,,950 2IL.,_ 21.4048.7 28.5 37,1237.72 7341.84 37.01 7.237,23 . 10.00 21.0)27.00 27.7221.12 8,05 34.834.9 99°C09°090'9 2uid(Wc540581(093)ItIOWoS)) 47'59'S41°51'S43'Oi'S 99357099'22'E995630 AustialliAustralIaAuOnrIia 390205 00 5.3!(4321.10 (8.3II.! 0 ' 38.0931.403)40 0,226.228.22 503230.72 '5.457.407,49 2326)23.0023.135 '7.127,727.12 0,408.400.49 0.3)0,510.51II! 0059)1.590.5 96'S9)Pfi 5161004) 111194th 47'45'S47'14'S 99°l9'S95'09'2 AustraliaAosl,alia 65038)) 0.196,49 45,327,0 31,4038.40 0,270.22 30,1250,17 7,401,49 2539125.00 7.111,72 0.408,40 0.3!0,52 90.590,5 SQ'SOS'S90'E 0(18 (095) 'Silt 47'30'242'53'S47'40'S 00°22'E00°10'R99'la'E *o.traliaAustraliaAunIritilO 5809001047 7.250,273,02 8,2.650.948.1 31,4951,4931.43 0.220,22 30,7130,72 1.407.40 23.0323.0023,80 7.121.121.32 0,100,445.49 (8.520,52052 90.590090.3 lOt'Slot'SAVG 1)1(097) intl 479(5'S47'II'S ltO'06'E130'S8'E *08011154011r0111 0301169(511 1,89597$31 34,0198.023.8 01,9 208029,003144 8.220,24124 29,2)29,2138.72 _j,4, 1,401.40 23,002.002.03 27,7!27,1!7.12 23.0821.038.40 .._3,5L,,, 1,931,93 NA19,5 9.0 AVGI0i'E 3rd 46'So'S 100'54t Au00116 _,,,,,457,,__ 4010 4.839,22 31,0339.2 20.00 0.240,24 ...J!,1L,, 20,2) 1.491.09 ,,,,,,,,,j,05,,,,,,,,, 2.03 27.2827,2! 21.00 1.93 (0,9 l0i'SO'ElO('705 2su)IntlIs) 43'll'S4399'S 1010500100495IOl'48'E AusleSlInAuslnsiiuAtiaIrItil (70(30440 2.955,02 .73 41.645.330,1 jIL,, ,_P,9!_ 29,3329.3929,30 0,260.208.26 20,7228,72 7.307.50 14.8))14,8))44,00 (4,7214,32 ._.JZA!_. 25.5915,39 0,990.098.05 51.431.489.0 I9l'50'E(OS'SAVG 1,1 44'01'S47'ti'S 104'SS'S104'578 AustralIa 450235 3.902.25 31,3044*8 8.2 26,7829,30 0.320.26 .. 20.8725.7228.72 9.9*5403,50 20.115584.00 - 44.1210.124.1'? 78.5925.394.15 0.200,990,09 31.4$1.4 47 AvelOS'S 0390 .0(i)2nd 47°52'O4799'S l05'05'E AustraliaA,09Iialiu 347310350 7.625,720,16 32.7771,0049,05 34.20 20,7026,3910.20 0.320,31 26,8728,1126.11 1337.533.53 221825.0022.00 1.174.114,17 4.704.754,18 0,230,200.25 4707 lOS'S1117'S107°8 rI (01199 dull2ndlid 40'Il'S49'i5'S40'lI'S l07'13'S107'92'SI01'lO'E AnutrullaAustrolIlAtraaIla 000330220350 4.073,))0.20 27,029,016,10.03 24,7424.742434 0,360.568.34, 24)024.1014.19 - 7.55155 350.05550.80258.00 76,1026.1026,10 26.1426,7426.74 1.001.061,06 20,836,876.0 AVGtIll'S03'S Sin 40°06'S48'IO'S l01'29'El07'34'S AtsirnillAurInaltu 380250 4.8402,905.04 20.444 20.4 1237 24.7420,162474 0,300,16836 24,4024,1024.10 7.051,557.53 300,0355000350,03 ,_,,,,,,j4,j!,,,,,,,, 26.1816.18 26.7426.1426,14 1.061,061.06 24.026,0 Indian Ridge (SEIR) changes its along-axis morphology froman axial ridge in western ridge segments to an axial valley in eastern ridge segments. These morphologic changes occur not only along the entire length of the ridge, but also along certain individual ridge sections.
The role that near-axis seamounts can play in defining the melting regime of mid- ocean ridges is beginning to be understood (Batiza and Vanko, 1983; Batiza and Vanko,
1984; Batiza et al., 1989a; Batiza et al., 1990; Niu and Batiza, 1991; Edwards et al., 1991;
Cordery and Phipps Morgan, 1993; Shen et al., 1993; Schierer and Macdonald 1995; Shen et al., 1995). Batiza et al. (1990) argued that since off-axis seamounts are near zero-age volcanic centers like the ridge axis itself, they suggest a broad (-1001cm) neo-volcaniczone near the spreading center. The presence of active near-axis seamounts and their distribution about the SEIR provide important evidence for the extent of the melting regime for this ridge system.
This paper first examines the abundance and distribution of seamounts from the
88°E- 120°E section of the SEIR and the 5- 15°N area of the EPR (Figure 11.2) to ascertain the factors involved in off-axis seamount production for a fast (EPR) and an intermediate spreading (SEW) mid-ocean ridge. The paper then focuses on the orientation and chemical composition of SEIR searnount chains and the potential importance of oblique chains in examining the dynamics and extent of along-axis mantle flow beneath the SEW.
Methods:
SEIR and EPR near-axis volcanoes were measured for basal diameter, height, distance from the ridge axis, and distance from first order segment boundaries. For determining seamount abundance along 2500 km of mid-ocean ridge a 100 km by 100 km. grid was placed over bathymetric maps of the SEIR. Within each cell, all seamounts circular or ellipsoidal with a basal diameter greater than 2km anda height greater than 200m were counted. Smaller edifices that belonged to linear chains were also counted if they 10
;.5 14°
'l.°. '4C-4 \
'2° \ '.
c- (1 /cocos
RC.2 9.O' 9.
Figure 11.2. The 5°-15°N area of the East Pacific Rise from Batiza et al., 1990. Filled circles represent seamounts sampled by the 1988 RA1T 02 cruise. Circles with diagonal lines represent seamounts previously sampled and open circles represent seamounts unsampled. The sizes of the circles are roughly proportional to seamount sizes. Labels "RC" and "RP" denote seamount chains parallel to the relative motion of the Cocos and Pacific plates, respectively. Labels "AC" and "AP" denote chains parallel to the absolute motion of the Cocos and Pacific plates, respectively. 11
attained a height of 150 m. Two ormore closely spaced (within 10 km) seamounts that
formed a linear array were considered to forma chain and azimuths of such chains were
recorded. Chain azimuths were then compared with relative plate motionvectors to
calculate a difference vector, whose magnitude is interpretedas the velocity of along-axis mantle flow. Seamount positions were recorded and the distance to ridge axis from the mid-point of the seamount was measured.
Seamount basalt glasses were analyzed for major and minor elements using Oregon
State University's Cameca SX-50 electron microprobe (Appendices I & II). A 5mm beam
was used to perform the analyses as recommended by the techniques for microprobe
analysis of Nielsen and Sigurdsson (1981). Trace elements from SEWseamount glasses were analyzed with Oregon State University's Fision Quadrapole ICP-MS instrument
(Appendix II). A comprehensive description of sampleprep and procedures used for the analysis are found in Appendix I.
Observations:
-Distribution of seamounts along the SEIR
One hundred and four seamounts greater than 200m in height were counted on crust less than 1 Ma (Figure ll.la). Fifty-four seamounts are on the Australian plate and fifty seamounts are on the Antarctic plate. West of 1 10°E searnountsare predominantly on the Australian plate while east of 1 l0°E seainountsare predominantly on the Antarctic plate
(Figure ll.la). Although individual seamountsare almost evenly distributed between the
Australian and Antarctic plates, all seamount chainsare located on the Australian plate.
The Australian plate seaniount population differs from the Antarctic plate population. Nine of the ten largest seamounts (>800m in height; >8 km in basal diameteT) adjacent to the SEW formed on the Australian plate. Heights and basal diameters of
Australian plate seamounts (Figures ll.3a & Il.3b)cover a broader range and are larger than the heights and basal diameters of most Antarctic plateseamounts (Figures 11.4a & Figure 3a Figure 4a SEIR Seamounts on Australian Plate 16.. c,.,,.,i.,.,.p,,.,,,..,. SEIR Seamounts on Antarctic Plate 14 -..--.------+ 14 . g 12 12- ...... 10 10-
IIiiIiI'IiIiiiiIIi.1IiiIiIiiIiiiiIIIiiiIiitIiIIIIiIIiiiiIiIi1 I! i
0 500 1000 15 0 200400600800 1000 1200 1400 Height (m) Height (m)
Figure 4b SEIR Seamounts on Antarctic Plate 16 14- ......
12- ......
I I:
0 ,i,,1L.iL., 0 2 4 6 8 10 12 0 2 4 6 8 10 12 14 Basa' Diameter (km) Basal Diameter (km)
Figures II.3a-b & II.4a-b. Histograms depicting the heights (m) and basal diameters (km) of seamounts located on the Australian and Antarctic plates. 13
11.4b). On the Antarctic plate, seamountsare smaller (<800 m in height; <8 km in basal diameter) than those formed on the Australian plate. For example, Antarctic plate seamounts in Box 4 (Figure 11. le) make up the largest cluster of seamounts found inany
200 km2 area adjacent to the SEIR. This largegroup of seamounts comprises the 111 °E seamount field and consists of small, nanow-based cones of 200-400 m in height that lie within the wake of the 11 l°E propagating ridge (Figures ILid & 11.5).
Seamounts 200-250 mill height are most common for both the Australian and
Antarctic plates (Figures 11.3 & 11.4). Furthermore, maximum height and maximum basal diameter of seamount on both plates decreases from west to east along the entire length of the SEIR (Figures 11.6 & 11.7).
When compared to the 5°-15°N area of the EPR (Figure 11.2), the 88°-120°E section of the SEIIR (Figure 11. la) contains fewer large seamounts (>1000m in height (Figures
11.8a & 11.8b); basal diameter >5 km (Figures 11.9a & IL9b)). Seamount chainsare also more abundant adjacent to the EPR than adjacent to the SEIR. Seamounts 200-300 m in height are most common for both the SEW and the EPR (Figures 11.8a & 11.8b).
-Sea,nount distribution within individual segments
The asymmetric distribution of SEIR seamount chains is not observed for individual SEW seamounts (Figures 11. lOa, II. lOb, & 1111 .a). Seamount chainsappear to form in certain locations within individual segments of the SEW. Seamount chainsare initiated within 10-20 kin of the axis andare absent from seafloor within 15 km of segment ends, positions similar to those found at other spreading ridges (Batiza, 1982; Jordanet aL, 1983; Fomari et al. 1987; Smith and Jordan, 1987; Smith and Jordan, 1988; Edwardset al., 1991). Chain distñbution is asymmetric about mid-segment (Figures 11.lOa & 11.1 la), with most seamount chains within 50% of first order segment length (distance of chain from the western end of the segment divided by total segment length). 14
The 111°E Seamount Field, Box 4
Sr .
4c
0. o.
Figure 11.5. A wire-mesh surface plot of the 11 1°E Seamount Field superimposed ona contour plot of the same area. The seamounts lie in the wake of the 111 °E propagating ridge and are depicted by the surface plot as small, conical mounds behind the outer pseudofault of the propagator. C17 1400 C16 C15I
£ 1200 4
£LA E 800 - LA : 600 LA 400 A 200 ALA -, 4
0 s__ . it - 88 93 98 103 108 113 118 Longitude A Austr&ian Plate Antarctic Plate
Figure11.6.Heights of SEIR seamounts vary from west to east. Ridge segment labels begin with a'IC"followed by the segment number as observed by Christie et al., 1995. Segments are shaded basedon their topography with light grey colors indicating axial rise morphology and dark grey colors indicating axial valley topography. Segments that have gradient shading show a transition in morphology from axial rise to axial valley (left to right in this figure).
01 14.0 C17 C16 E 10.012.0 £ E 406.080 £ A £ 0.020 88 93 98 Longitude 103 108 113 118 Figurea °C" 11.7. followed Basal by diameters the segment of SEIR number seamounts as observed vary by from Christie west toet al.,east. 1995. Ridge Segments segment arelabels shaded begin based with on their £ Australian Plate Antarctic Plate (lefttopography.topography to right withinSegments this light figure). grey that havecolors gradient indicating shading axial showrise morphology a transition andin morphology dark grey colors from indicatingaxial rise to axial axial valley valley C' 17
a) SEIR (88°E-120°E) 35
30
25 'N.
0 20 E 15 ri 10
5
0 0 500 1000 1500 Height (m)
b) EPR (5°N-15°N) 35
30
25
15
10
5
0 0 500 1000 1500 Height (m)
Figures ll.8a & 8b. These histograms depict the distribution of heights (m) for seamounts adjacent to the 88°E-120°E section of the SEIR and the 5°-15°N section of the EPR. a) SEIR (88°E-120°E) 4 p.
I
0 5 10 15 20 25 Basal Diameter (km)
b) EPR (5°N-15°N) 12
10
8 0 E 6
4 4r
2
0 0 5 10 15 20 25 Basal Diameter (kin)
Figures II.9a & II.9b. These histograms depict the distribution of basal diameters (1cm) for seamounts adjacent to the 88°E- 1 20°E section of the SEIR and the 5°-i 5°N section of the EPR. 19
a) SEIR Seamount Chain Distribution 1 5 1111111! hull 11111
I Austrahan PlateI
10 J5
0 0 20 40 60 80 100 Position in Segment
b) SEIR Individual Seamount Distribution 15 Australian Plate Q Antarctic Plate
10
nv k1!11. 0 20 40 60 80 100 Position in Segment Figures II.lOa & II.lOb. These diagrams displayseamount chain and individual seamount locations as a percentage of total ridge segment length. The distance ofsearnount chains and individual seamounts from the west end of their adjacent ridgesegment was measured and then "normalized" to the total length of that segment. Each bar in Figure ll.lQa represents one seamount chain while the height of the barrepresents the number of edifices per chain. The black bars in both figures representseamounts on the Australian Plate while the white bars represent seamountson the Antarctic Plate. 20
a) SEIR Seamount Chains 15 I, SEIR chains removed from OSCs SEIR chains adjacent to OSCs
10
0 B 5
0 0 20 40 60 80 p JI Position in Segment
b) EPR Seamount Chains 15 EPR chains removed from OSCs EPR chains adjacent to OSCs
.e 10
0 B 5
I 20 40 60 80 100 Position in Segment
Figures ILlia & ILlib. These diagrams display SEIR and EPRseamount chain locations as a percentage of total ridge segment length. The distance ofseamount chains from the west end of their adjacent ridge segmentwas measured and then "normalized" to the total length of that segment. Each bar in Figures 11.11a & 11.11 b represents one seamount chain while the height of the bar represents the number of edificesper chain. 21
By contrast, the distribution of seamount chains adjacent to the EPR is symmetrical about mid-segment (Figure 11.1 ib). Figure 11.1 lb shows a symmetrical pattern of seamounts per chain as the number of seamounts decreases from either segment end toward mid-segment. Keeley et al. (1995) noticed a similar pattern about mid-segment for the
Southern EPR (SEPR) just north of the Wilkes Transform. However, Keeley et al. (1995) stated that seamounts adjacent to the SEPR do not preferentially form adjacent to inflated sections of ridge, but do increase in size near these inflated sections (Keeley et al., 1995).
-SEIRseamount chain orientation
Relative and absolute plate motions for the Australian plate are essentially parallel
(within 1° each other) in a 024-038° direction with a magnitude of 7.2-8.4 cm/yr
(calculations based equations from Fowler (1993) using NUV.EL-1 global plate motion models of DeMetts et al. (1990) and Gripp and Gordon (1990). Three of the seven SEIR chains trend nearly parallel (within 10°) to relative and absolute plate motions. Two of these chains, the 89°E and 99°E, are comparable to EPR chains from 5°-15°N area in terms of the number and size of seamounts. The remaining four SEIR chains are oblique to plate motions (14-27°) (Table 11.1) and contain fewer numbers of edifices (3-4 edifices) and edifices smaller in size (150-950m in height and -'1-9km in basal diameter) than those belonging to SEIR and EPR chains parallel to either relative or absolute plate motions.
Two SEIR chains within 10° of absolute parallel motion are the 89°E and 99°E seamount chains. The 89°E seamount chain is a unique example of SEIR chain that appears to have been produced by a stationary, persistent source (Figure 11.12). The 89°E chain consists of at least six edifices which extend out from the axis in a direction nearly parallel
(within 9.5°) to both relative and absolute plate motion. Furthermore, the 89°E chain is morphologically distinct since it consists of a group of closely spaced, partially overlapping edifices (Figure 11.12). 22
89E Seamount Chain
Boxi
89° 12'E 89° 20E
41° 44S 41° 443
41° 52'S 41° 52'S
42° DO'S 42° 00'S
89° 12'E 89 20'E
-3600 -3500 -3400 -3300 -3200 -3100 -3000 -2900 -2800 -2700 -2600 Depth in Meters
Figure 11.12. Contour plot of the 89°E seamount chain, Box 1. The long, darkgray arrow is the azimuth of the 89°E chain. The light gray arrow is the absolute vector for the Australian Plate while the black arrow is the relative vector (representing full spreading rate) for the Australian Plate. 23
The 99°E seamount chain contains at least eight edifices align nearly parallel (within
8°) to plate motion directions (Figure 11.13). The height and basal diameters of the first
three edifices within this chain increase with distance from the axis, but the fourth through
eight edifices are roughly the same size (Figure 11.13).
Oblique-trending seamount chains include the 90°E, 1O1°E, 101°50'E, and 107°E
chains (Figures 11.14-11.17). These chains are much smaller than either SEW or EPR
absolute parallel chains. For example, they consist of only 3-4 seamounts that are 150-950
m in height and - 1-9 km in basal diameter compared to the 6-8 edifices of the 89°E and
99°E chains that are 200-1250 m in height and 1.6-11.6 km in basal diameter.
Orientations of SEW chains contrast with orientations of chains of the EPR. Over
half of SEW chains are oblique to relative and absolute plate motions while EPR chains
between 5°N and 15°N are parallel to either relative or absolute plate motions (Batiza et al.
1990; Schierer and Macdonald, 1995). Oblique chains are not found at other intermediate
spreading ridges such as the Juan de Fuca and Pacific-Antarctic ridges.
-SEIR seamount chemistry
Seamount lavas fall into a enriched and depleted chemical groups based on variation
diagrams on concentrations of incompatible elements versus MgO (Figure 11.18; Appendix
11). The enriched group (E-MORB) and the depleted group (N-MORB) display separate
fractionation trends when plotted on Harker diagrams. The enriched group has elevated 1(20 (.5-.8 wt.%), P205 (-. 15-.27 wt.%), Na20 (-2.9-3.7 wt.%), and K20/Ti02 (.3-
.45 wt.%) when compared with the depleted group (Figure 11.18). Searnount lava data display positive correlations between A1203 and MgO and negative correlations between
CaO, Na20, K20, and P205 and MgO. Trace element data for SEW lavas also show two
distinct chemical groups--an E-MORB and N-MORB group (Figure 11.19). These two
groups for compositional end members of the SEW axial lava suite. A1though most SEW seamounts are N-MORB in composition, two seamounts display E-MORB compositions 24
99E Seamount Chain Box2
04'E 99° 12'E 20'E 99° 28'E
28'S 47° 28'S
47° 36'S 36'S
47° 44'S 0 47° 44'S
.5 52'S 52'S
°
48° 00'S ,-. 480 00'S
99° 04'E 12'E 99° 20'E 99° 28'E
-3600 -3500 -3400 -3300 -3200 -3100 -3000 -2900 -2800 -2700 -2600 Depth in Meters
Figure 11.13. Contour plot of the 99°E seamount chain, Box 2. The long, darkgrey arrow is the azimuth of the 99°E chain. The light grey arrow is the absolute vector for the Australian Plate while the black arrow is the relative vector (representing full spreading rate) for the Australian Plate. 25
90E Seamount Chain
Boxi
89° 52'E 90° 00'E 900 08'E
42° 08'S 420 08'S
O 0 0
42° 16'S 42°16'S
0 t
'0. 42°24'S 42°24'S
#00I,,
89° 52'E 90° 00'E 90° 08'E
-3400 -3300 -3200 -3100 -3000 -2900 -2800 -2700 -2600 -2500 -2400 Depth in Meters
Figure 11.14. Contour plot of the 90°E seamount chain, Box 1. The long, darkgrey arrow is the azimuth of the 90°E chain. The light grey arrow is the absolute vector for the Australian Plate while the black arrow is the relative vector (representing full spreading rate) for the Australian Plate. 101 E Seamount Chain
10040'E 10048'E lOOBboOl.O4,E1010 12'E
46 56'S 46 56'S
I,
II 4T 04'S 47' 04'S
0
- 47' 12'S 47° 12'S
47' 20'S 47' 20'S
47' 28'S 47' 28'S
100' 40'E 1000 48'E 100' 56'E 101° 04'E 101' 12E
-3500 -3400 -3300 -3200 -3100 -3000 -2900 -2800 -2700 -2600 -2500 Depth in Meters Figure ILlS. Contour plot of the 10l°E seamount chain, Box 3. The long, light gray arrow is the azimuth of the 101 °E chain. The medium gray arrow is the absolute vector for the Australian Plate while the black arrow is the relative vector (representing full spreading rate) for the Australian Plate. 27
101 :50E Seamount Chain Box3
101°36'E 1O1°44E 101°52'E
4T 12'S 47° 12'S
4T 20'S 47° 20'S
47° 28'S 47° 28'S
0
47° 36'S 47° 36'S
., _c.
101° 36'E 101° 44'E 101° 52'E
-3700 -3600 -3500 -3400 -3300 -3200 -3100 -3000 -2900 -2800 -2700 Depth in Meters Figure 11.16. Contour plot of the IOl°50'E seamount chain, Box 3. The long, dark grey arrow is the azimuth of the 1Ol°50'E chain. The light grey arrow is the absolute vector for the Australian Plate while the black arrow is the relative vector (representing full spreading rate) for the Australian Plate. 107E Seamount Chain Box4
107 04'E 107° 12E lOT 20'E 107° 28E 107° 36'E II
-'9 48° 08'S
48° 16'S 48° 16'S
48° 24'S 48° 24'S -
-
107° 04E 107° 12'E 107° 20'E 107° 28'E 107° 36'E
-3600 -3500 -3400 -3300 -3200 -3100 -3000 -2900 -2800 -2700 -2600 Depth in Meters Figure 11.17. Contour plot of the 107°E seamount chain, Box 4. The long, darkgrey arrow is the azimuth of the 107°E chain. The light grey arrow is the absolute vector for the Australian Plate while the black arrow is the relative vector (representing full spreading rate) for the Australian Plate. 13,000 CaO 0,000 1(20 13.000 . :V.,afl . 0,000
MgO 0.100 Mg() 00000 0000 5.00 6.0Ol 7.00 O.O01 9.000 - l0.00l 0,000 6.00I i.01 O.00OS.OJ IQ.01O ñWE Sit Onk. Lag.. soot Sit 0th lava, bSMsl Sit lava [awE SI On). lag., Last sit Gal). Lava, sht.htS Sit lava,1
10.000 .A1203 0.300 17305 0.250 07.000 00300 0.200 ______
16.000 a 0.100 00.509 5,100 ,.. 05000 ,0.5 S.... .0. MgO 0.050 14,005 . 0.050 5.00 )..OL3 ISiO 1.001 0.05k 10,0013 5.0 6. 7.0l6 0,5,13 a.o4 so.010 [iii 07* thaI. Lava, 5090 Sit On). lava, a OMAn) SI Lain 0th Lava, SW On). Lava. ld, STO Iav'1 1
Na20 :4.0
3,200
3,000 0,290 2.400 0.10* 2.200 0100 0g A 2000 a nsa 5, 3). b 0,000 1.05) 0.0013 9.001 l030) a,007 7.0(10 s.o(10 o.oJ
aOSt SI Oslo IMa, 499,0 Si-I OS. lava, ht.t.l OTt(15)1LaYS, S5E Sit OS). lava, Mvjdit SI 1
Figure 11.18. Major and minor element data for SEW seamounts. The darkgrey fields represent an enriched chemical group (K20/Ti02>1 .0) while the light grey fields represent a depletedgroup (K2OITiO2<1 .0). The enriched group (E-MORBs) have higher concentrations of incompatible minor elements (K, P, and Ti) than the depleted group (N-MORBs). 100.000 SEIR Seamounts
10.000 c)
A---89°E Smt Chain Lavas 1.000 A----99°E Suit Chain Lavas A--Individual Smt Lavas I a- E-Morb N-Morb
0.100
I- E 0
Figure 11.19. Trace element data of samples recovered from twelve off-axis SEIR seamounts. Searnount data form end member enriched and depleted chemical groups. E-Morb and N-Morb values are from Sun and McDonough (1989) and are based on a literature survey and internal consistency of elemental ratios. 31
(Figure 11.19). These E-MORB seamount lavas also show an enrichment in Nb and Ta when compared to Cl chondrite (Sun and McDonough, 1989).
Discussion/Interpretation:
-Distribution of sealnounts along the SEJR
The change in ridge morphology from an axial high in the west (88°E-96°E) to an
axial valley in the east (108°E-120°E), despite a constant spreading rate, suggests a decrease
in melt flux from west to east along the SEIR (Sempere et al. 1996; West et al. 1997). The
decrease in melt flux is also manifested in a decrease in abundance and size of searnounts from west to east (Figures 11.6 & 11.7). On a regional scale, the greater abundance of large
seamounts (>800 m in height) in the far west section of the SEIR (88°E-93°E) (Figures 11.6
& 11.7) may also reflect the larger flux of melt to this region. Moreover, the large seamounts and seamount chains are found predominantly on the Australian plate which may suggest a greater flux of melt beneath the Australian plate.
The greater abundance of large seamounts and seamount chains on the Australian plate may be related to migration of the SEIR. The SEIR is inferred to move northward in the 'hotspot reference frame' based on the sum of absolute vectors of the Australian (-7.5 cm/yr) and the Antarctic (-1.0 cmlyr) plates. This situation is similar to that for the Juan de
Fuca ridge where Davis and Karsten (1986) developed a model to explain why there are large numbers of seamounts on the west flank of the Juan de Fuca ridge but very few seamounts on the east. In order to understand their model, one must first examine its four main assumptions.
First, Davis and Karsten (1986) assumed that mantle upwelling is vertical and related only to ridge-normal velocity. Second, they ignore spreading asymmetry. Third, they assumed that axis-parallel horizontal flow within the asthenosphere is negligible. 32
Fourth, they assume that the sources for seamount chains adjacent to the Juan de Fuca
ridge are heterogeneities that reside within the upper mantle.
After outlining their assumptions, Davis and Karsten (1986) cite two factors that
help to explain why ridge migration over a heterogeneous asthenosphere allows the
production of off-axis seamounts on the leading plate. First, ridge migration causes the
asthenosphere upwell beneath the Pacific plate in order to resupply material lost to accretion
(Figure IL2Oa). The ridge migrates in an absolute motion framework based on a large difference in absolute plate velocities of the Pacific (5.5 cm/yr) and Juan de Fuca plates (2.5 cm/yr) (Figure ll.20a).
Second, as the ridge migrates westward, "the zone of rapid upwelling beneath the ridge axis sweeps across previously unaffected upper mantle material" (Davis and Karsten
(1986)). This causes the mantle containing heterogeneities to ascend in advance of the migrating spreading center (Davis and Karsten (1986)) (Figure ll.20b). As the mantle continues to rise, it's embedded heterogeneities melt before the surrounding mantle until they axe reduced to a composition whose solidus is similar to that of their host peridotite or until the ridge passes overhead (Figure ll.20b), As the ridge passes overhead, the mantle is flushed of its heterogeneities as its ascent velocity abruptly diminishes (Davis and
Karsten (1986)) (Figure ll.20b). In this manner, the ridge strips the mantle of most of its heterogeneities such that few of them remain in shallow enough depths beneath the trailing plate to produce seamounts.
The model is relevant for explaining the distribution of seamount chains adjacent to the SEW for two reasons. First, the Australian plate is moving faster than the essentially stationary Antarctic plate. Second, seamount chains are only found on the faster moving
Australian plate.
SEW seamount chains are less abundant and contain edifices that are smaller in number and size than Juan de Fuca chains. This suggests that the flanks of the SEW experience a smaller melt flux than the Juan de Fuca ridge. A smaller flux may reflect 33
a) b)
Figure ll.20a. Diagram displays the response of the asthenosphere to plate spreading for a fixed ridge and a ridge migrating with respect to a hotspot' frame of reference. Arrow lengths indicate the ascent velocities of the upwelling mantle. Horizontal velocities in the asthenosphere are assumed to be small. Diagram is a modification of Figure 2 from Davis and Karsten, 1986. Figure ll.20b. Diagram shows time sequenced events involving ridgemigration that causes the early melting of an enriched heterogeneity in the mantle. Volcanism begins at time 1, 20 km away from the ridge axis on crust -.700,000 years old. As the ridge migrates, the heterogeneity continues to melt as it ascends in the lithosphere, producing a chain of seainounts. After a period of time (Time 5 in diagram), the heterogeneity no longer produces melt because either the lithosphere incorporates the heterogeneity or the ridge passes overhead which flushes the heterogeneity of its early melt fraction. Figure II.20b isa modification of Figure 5 from Davis and Karsten, 1986. 34 cooler lithosphere beneath the SEIR' s flanks and small amounts of melting that a heterogeneous mantle undergoes as it ascends vertically in response to ridge migration.
Along-axis mantle flow occurs beneath the SEW. Phipps Morgan and Sandwell
(1994) suggested that the gravity and bathymetric gradient along the SEW is responsible for ridge propagation down-gradient toward the Australian-Antarctic Discordance. This may affect the ascent trajectories of heterogeneities at shallow depths in the mantle. For example, as a parcel of heterogeneous mantle rises vertically, it moves through mantle of lower viscosity (West et aL, 1997). The higher the parcel rises, the more it becomes influenced by the horizontal component of along-axis flow because along-axis flow may cause the advection of warmer, less viscous mantle on top of cooler, more viscous deep mantle, leading to a decrease in sub-axial mantle viscosity (West et aL, 1997). Eventually, a heterogeneity may stop rising vertically altogether and move parallel to the ridge axis.
Heterogeneities that reside deeper in the mantle may not be influenced by along-axis flow. These may have acted as the source for the large 89°E and 99°E seamount chains
(Figures 11.12 & 11.13). The 89°E and 99°E chains are parallel to absolute motion of
Australian plate and are adjacent to broad, robust sections of ridge. This ridge morphology suggests a larger magma flux than nearby sections of ridge. Fornari et al. (1988) observed a similar robust ridge morphology for a section of EPR adjacent to the Lamont Seamounts which they interpreted to indicate the current position of the Lamont chain source.
Following Fornari et aL's interpretation, the robust ridge morphology near 89°E and 99°E may indicate the current location of the source that produced the 89°E and 99°E chains.
Third, SEW seamount abundance shifts from the Australian plate west of I lO°E to the Antarctic plate east of I lO°E (Figure lila). This shift may result from 1) the effects of asymmetric spreading associated with ridge propagation and 2) upwelling associated with along-axis mantle flow. The asymmetric wake of the 111 °E propagator suggests that crustal accretion is greater on the Antarctic plate (Phipps Morgan and Sandwell, 1994).
Higher crustal accretion to the Antarctic plate coupled with a dense concentration of small 35 seamounts in the wake of the outer pseudofault of the 111 °E propagator (Figure 11.5) suggests that more melt may exist beneath the Antarctic plate than the Australian plate in the area near 1 lO°E.
Further east along the SEW, a small group of seamounts located on the Antarctic plate near the 1 14°E transform (Figure 11. la) may be explained by an along-axis flow model proposed by West and Christie (1996) (Figure 11.21). West and Christie (1996) suggest that the focusing of magmatism (inferred from robust, axial high/rif ted high ridge morphologies in these sections) in western sections of SEW segments may result from a disruption of along-axis asthenospheric flow by large transform offsets. As mantle flows along-axis and encounters a transform boundaiy, it 'overshoots' the western or upstream portion of the next ridge segment, thus isolating it from the regional flow (West and
Christie, 1996) (Figure 11.21). West and Christie (1996) postulate that this generates more vigorous mantle upwelling in the western portions of segments adjacent to major transform offsets. More vigorous upwelling in the western portion of the ridge segment near the
1 14°E transform may generate a higher, more focused melt flux in this location than elsewhere along the ridge. Consequently, this higher relative melt flux could generate excess melt that then produced the cluster of small seamounts on the Antarctic plate near the inside corner of the I 14°E transform.
-Seamount distribution within individual segments
Based on their distribution within individual segments, SEW seainount chains and individual seamounts can be infeffed to have formed by different processes. SEW seamount chains grow from the west end of segment 10-35% of the segment length and begin to form 10-20 km from the ridge axis. Percent segment length was determined by dividing the distance of the first seamount within a chain from a segment's west end by the total length of the first order ridge segment which lies adjacent to the chain. These Oblique Seamount Transform Fault Plane Plate Motion 0chain 2D: -1 Pattern1Propagating \ L Ridge
I
I
0 I Along-Axis Asthenospheric Flow
4<: -z
Figure 11.21. Along-axis flow model proposed by West and Christie (1996). Their modeldepicts the pathway that mantle follows as it flows beneath the SEIR from near the Kuergulen Platformto the Australian Antarctic Discordance (AAD). As the mantle encounters a transform boundary, its flow is disturbed andfollows a path of least resistance around the transform. This results in the flow 'bending' around the transformand overshooting the upstream portion of the ridge on the opposite site of the transform. The 'overshot'causes a 3-D style of upwelling beneath the upstream portion of the segment which gradually diminshes tomore 2-D style of upwelling further downstream.
C' 37
observations suggest that chain production requires a specific balance between the supply
of melt, presence of axial magma chambers, and the thickness of overlying lithosphere.
Lithospheric thickness is an important factor to consider when determining where
seamounts and seamount chains are produced. The presence of the youngest SEW
seamounts in chains on crust 10-20 km from the ridge axis may suggest that the thickness
of the lithosphere brackets the area off-axis where new edifices of seamount chains are
formed. For example, at 10-20 km from the ridge axis, melt that ascends through the
upper mantle may escape subaxial convective currents and penetrate the lithosphere off-axis
,which Parker and Oldenburg (1973) estimate to increase in thickness at a rate equal to the
square root of crust.al age. However, beyond this optimal distance range of 10-20 km from
the axis, the lithosphere may become too thick (17-20 kin) for melt to penetrate. Thus, the
lithosphere may act as a filter for excess melt from the upper mantle by becoming more
impenetrable as distance from the axis increases.
Another potentially important factor controlling seamount chain distribution is the degree of heterogeneity of the upper mantle. Davis and Karsten (1986) and Allan et al.
(1989) suggest that a single, large melting heterogeneity may act as the source for an absolute-motion parallel chain such as the Lamont Seamounts.
SEW chains such as the 89°E and 99°E were, perhaps, produced by a single heterogeneity as envisioned by Davis and Karsten (1986) and Allan et al. (1989). Given that the Davis and Karsten (1986) model may explain the predominance of SEll seamount chains on the Australian plate, the skewed upwefling flow fields described by the model would decrease the likelihood that a series of diapirs could follow the same melting pathway to produce a seamount chain. However, the single source that generated the 89°F chain is probably different from the single source that generated the 99°E based on the morphologies of the seamounts within each chain. For example, the 89°E chain consists of at least six overlapping, cratered cones (Figure 11.12) which suggests two possibilities for the physical nature of its source. First, the source could be elongated by the upwelling flow field and lie beneath several edifices within the chain. The increase in size of the first
three edifices suggests that an elongated source may lie beneath multiple seaniounts (Figure
11.12). Such a source could contribute melt to several seamounts simultaneously. Second,
the 89°E source may lie beneath the seamount closest to the ridge, but still supply several
seamounts with melt through an network of multiple conduits.
Based on overlapping nature of 89°E seamounts and the smaller size of the first three seainounts when compared with the latter three, the 89°E chain was probably generated by an elongate source. Such a source could simultaneously feed melt to the first group of three seamounts in the 89°E chain.
By contrast, the 99°E chain consists of at least eight discrete, non-cratered cones whose heights and basal diameters are smaller than those of the 89°E chain and which no longer increase after the third edifice in the chain (Figure 11.13). These observations suggest that the source feeding the 99°E chain produces smaller amounts of melt than the source feeding the 89°E chain. They also suggest a possible scenario for the development of the 99°E chain. The decrease in seamount size toward the axis may indicate that the source is elongate and may lie beneath the first three edifices within the chain. While this scenario appears similar to the scenario proposed for the development of the 89°E chain, it differs from the 89°E scenario since the source is not as robust and may be feeding each of the first three searnounts in the chain with different volumes of melt. For example, the third edifice is the same size as the rest of the seamounts in the chain which suggests that either it no longer receives any melt, or receives small amounts of melt which do not significantly contribute to the seamount's growth.
Individual seamounts are more common and their distribution is much more random than the distribution of seaniount chains adjacent to the SEIR. They may arise from smaller blobs that bypassed axial convective upwelling by forming at distance off-axis that is beyond the influence of axial upwelling. 39
-SEIR seamowzt chain orientation
The orientation of a seamount chain is related to the location of its source in the
upper mantle and to plate motion. Seamount chains parallel to absolute plate motions,
such as the 89°E and 99°E chains, necessarily originate from a source below any along-axis mantle flow field.
Seamount chains oblique to absolute motions must originate from a moving source
within an along-axis flow field in the upper mantle (Figures 11.14-11.17). The off-axis
mantle that contains the source would be cooler and more viscous than the mantle directly
beneath the axis, resulting in an along-axis flow velocity that is less than the flow velocity
at the axis (West et al., 1997). This hypothesis is supported by apparent searnount chain
velocities (calculated from the angle between the chain azimuth and relative plate motion)
which are lower than along-axis velocities calculated from propagating ridges, which may approximate the movement of mantle directly beneath the ridge axis (West et aL, 1997)
(Figure 11.22). The obliquity for all seven SEW chains is consistent with the mantle moving from west to east.
Since the sources for oblique SEW chains are infened to migrate, they are fundamentally different from fixed chain sources upon which conventional models, such as the Davis and Karsten (1986) model, are based. Thus, a new model is required in order to explain the kinematics behind the formation of a seamount chain from a migrating source. The model first assumes that along-axis mantle flow causes seamount sources to migrate from west to east. Second, the model assumes that oblique chain sources are much smaller in volume than the absolute parallel chain sources and produce seamounts over a short period of time followed by a period of quiescence when no seamounts are produced.
Oblique chains begin when a blob within the mantle begins to melt in response to convective upwelling that is possibly driven by ridge migration (Figures 11.23a & 11.23b).
As the blob rises, it begins to produce melt which is quickly separated from the blob
(Figure 11.23c). This melt rises vertically through the asthenosphere and lithosphere until it SEIR Seamount Chains
C17 C16Cis:?
'- -' -
, 4
II
J. £89
88 93 98 103 108 113 118 Longitude
£ Australlan Plate Antarctic Plate Propagating Ridge
Figure 1122. Apparent along-axis velocities for SEIR seamount chains using difference vectors calculated from relative-motion vectors and seamount chain azimuths. 41
a) d) .4Ridge Migraiic E 4-Ridge Migra1i E
w
Vertical Upwelling & HorTzontal Flow
b) .4Ridge Migrolizz -4--Ridge Migratici
w
f) E .4-Ridge Migrathm .4_...RidgeMigTwion
Figure 11.23. Time sequenced model depicting the formation of oblique SEW seamount chains. Heterogeneous mantle in a fixed location within the asthenosphere begins to upwell in response to ridge migration as outlined by Davis and Karsten, 1986 (a). Mantle continues to vertically ascend until a heterogeneity in the mantle begins to melt (b-c). This melt eventually reaches the surface and produces a seamount (c). The mantle continues its vertical ascent until it becomes entrained in the along-axis flow and moves horizontally in a direction parallel to the ridge (d). As the mantle moves horizontally 'downstream,' its heterogeneity continues to melt and produce seamounts until it is exhausted of its early-melt fraction (e-f). When melting stops, a chain of seamounts is left on the seafloor with an azimuth that reflects the migratory history of its source (f). 42 erupts at the surface (Figure ll.23c). As the blob continues to rise and melt, it passes through different along-axis velocity fields (West et aL, 1997). It continues to ascend vertically, until the horizontal velocity within the along-axis flow field causes the blob to begin to move in a direction parallel to the ridge axis at a distance from the axis based on the off-axis extent of axial upwelling currents (Figure I1.23d). While the blob moves horizontally, the plates continue to spread in a ridge-normal direction, causing the first seamount the blob produced to move away from its point of origin. Meanwhile, the blob begins to produce melt again, which rises vertically through the asthenosphere, lithosphere, and crust to produce a new seamount (Figure 11.23d). This second edifice is further "down range" than the first since its source has moved down range from the site where it produced the first seamount. This process of seamount production continues until the blob becomes exhausted of melt (Figure 11.230.
-SEIR seamount chemistry
Major, minor, and trace element data from SEIR seamount lavas suggest that they are similar in. composition to SEIR axial lavas. Both seamount and axial ridge lavas display a wide fange in MgO values (-6--9.7 wt.%). Moreover, little variation in incompatible element ratios of (La/Sm)n (Figure 11.24) and K2OIFiO2 (Figure 11.19) with decreases in
MgO content may indicate that the seamount lavas underwent crystal fractionation. If crystal fractionation played an important role in the petrogenesis of SEW seamount lavas, then these lavas probably mixed while pooled in a small, subcrustal magma reservoir prior to eruption since such a chamber would allow the liquid melt to cool and crystallize. The presence of small (<1km in diameter) craters on several SEW seamounts supports the hypothesis that some seamounts may have had small reservoirs that collapsed from magma drained-back after an eruptive event.
Although SEW seamount and axial lavas are similar in composition, the seamount lavas are generally higher in MgO (-8.5-9.7 wt.%) (Figure 11.18). This may indicate that 43
2.50
2.00 1 U
1.50 E
1.00 U
++ 0.50 +$9°SrtCbi +Xx
X99 Sni Chjn
IfldiVidWJSmI
0.00 I I I I II I I I I I I I Il-I I I I I I I I 10
Figure 11.24. La/Sm values were normalized to the Cl Chondrite values of Sun and McDonough, 1989. Samples Dl 14 and Dl 19 from individual seamounts form the enriched suite (LaJSm> 1.0) while samples from the seamount chains form the depleted suite (La/Sm < 1.0). some seamounts (e.g. seamounts from the 99°E chain) are less evolved than the ridge axis, an observation made for lavas from seamounts adjacent to other mid-ocean ridges (Batiza and Vanko, 1984; Zindler et al. 1984; Graham et al. 1988; Allan et al. 1987; Fornari et al.
1987; Allan et al. 1989; Batiza et al. 1989a; Batiza et al. 1989b; Niu and Batiza, 1991;
Cordery and Phipps Morgan, 1993; Shen et al. 1995). SEIR seainount lava compositions also indicate that the seamounts may tap a wide range of melts from the melting column as predicted by the searnount production models developed by Niu and Batiza (1991) and
Cordery and Phipps Morgan (1993). 45
Conclusion:
Off-axis volcanism, manifested in seamount production, is a vital component to understanding the dynamics of a mid-ocean ridge system. Seamount abundance, distribution, orientation, and chemistry offer a means of examining the distribution of melt about a spreariing ridge segment. These characteristics were examined for the88°E-120°E section of the SEW and the 5°-15°N section of the EPR in order to determine the effect of different spreading rates on seamount volcanism. SEW seamount chains are more abundant in the western ends of ridge segments while EPR seamount chains do not vary in abundance across the length of a spreading segment. Furthermore,SEWseamounts vary in terms of their heights and basal diameters, showing a decrease in both toward the west end of the88°E- 1 20°Eridge section. The heights and basal diameters of EPR seamounts, on the other hand, do not vaiy along the length of the axis, suggesting a much more even distribution of melt supply in the Eastern Pacific. SEW seamount chains are also often oblique to absolute and relative plate motions while EPR seamounts are mostly parallel.
The oblique nature of SEW seatnount chains provides strong evidence for along-axis mantle flow. Flowing mantle from west to east along the SEW may also affect the distribution of seamounts as well the chemical composition of seamount lavas. This phenomenon provides an ideal environment for studying the relationship between convecting mantle and the distribution of melt off-axis. 46
References:
Allan, J.F., R. Batiza, M.R. Perfit, DJ. Fornari, and R.O. Sack, Petrology of lavas from the Lamont Seamount Chain and adjacent East Pacific Rise, 10°N, J. Petrology, 30, 1245-1298, 1989.
Batiza, R., Abundances, distribution and sizes of volcanoes in the Pacific Ocean and implications for the origin of non-hotspot volcanoes, Earth Planet. Sci. Lett., 60, 195- 206, 1982.
Batiza, R. arid D. Vanko, Volcanic development of small oceanic central volcanoes on the flanks of the East Pacific Rise inferred from narrow-beam echo-sounder surveys, Mar. Geol., 18, 53-90, 1983.
Batiza, R. and D. Vanko, Petrology of young Pacific seamounts, J. Geophys. Res.,18, 11,235-11,260, 1984.
Batiza, R., J. Brodholt, J. Karsten, D. Vanko, and R.O. Sack, Petrogenesis of young seamounts near transforms and OSCs of the EPR 5-15°N, Eos, Trans.AGU, 69, 1475, 1988.
Batiza, R., T.L. Smith, and Y. Niu, Geological and petrologic evolution of seamounts near the EPR based on submersible and camera study, Mar. Geophys. Res., 11, 169-236, 1989a. Batiza, R., P.J. Fox, P.R. Vogt, S.C. Cande, N.R. Grindlay, W.G. Melson, and T. O'Hearn, Morphology, abundance, and chemistry of near-ridge seamounts in the vicinity of the Mid-Atlantic Ridge-. 26°S, J. Geology, 97,209-220, 1989b. Batiza, R., Y. Nm, and W.C. Zayac, Chemistry of seamounts near the East Pacific Rise: implications for the geometry of subaxial mantle flow, Geology, 18, 1122-1125, 1990. Corderey, M.J. and J. Phipps Morgan, Convection and melting at mid-ocean ridges, J. Geophys. Res., 98, 19,477-19,503, 1993.
Davis, E.E. and J.L. Karsten, On the cause of the asymmetric distribution of seamounts along the Juan de Fuca ride: ridge-crest migration over a heterogeneous asthenosphere, Earth Planet. Sci. Lett., 79, 385-396, 1986. DeMetts, C., R.G. Gordon, D.F. Argus, S. Stein, Current plate motion, Geophys. J. mt., 101, 425-478, 1990.
Edwards, M.H., D.J. Fornari, A. Malinverno, W.B.F. Ryan, and J. Madsen, The regional tectonic fabric of the East Pacific Rise from 1 2°50'N to 150 10'N, J. Geophys. Res., 96, 7995-8017, 1991. Fornari, D.J., R. Batiza, and J.F. Allan, Irregularly shaped seamounts near the East Pacific Rise: Implications for seamount origin and rise axis properties, in Seamounts, Islands and Atolls, Geophys. Monogr. Ser., vol. 43, edited by B .H. Keating, P. Fryer, R. Batiza, and G.W. Boehlert, pp. 35-47, AGU, Washington, D.C., 1987. Fornari, D.J., M.R. Perfit, J.F. Allan, R. Batiza, R. Haymon, A. Barone, W.B.F. Ryan, T. Smith, T. Simkin, and M.A. Luckman, Earth Planet. Sci. Lett., 89, 63-83, 1988. 47
Fowler, C.M.R., The Solid Earth: An Introduction to Global Geophysics, 412p., Cambridge University Press, New York, 1990. Graham, D.W., A. Zindler, M.D. Kurz, W.J. Jenkins, R. Batiza, and H. Staudigel, He, Pb, Sr and Nd isotope constraints on magma genesis and mantle heterogeneity beneath young Pacific seaniounts, Contrib. Mineral. Petrol., 99,446463, 1988.
Gripp, A.E. and R.G. Gordon, Current plate velocities relative to the hotspots incorporating the N1JVEL-1 global plate motion model, Geophys. Res. Lett., 17, 1109- 1112, 1990.
Jordan, T.H., H.W. Menard, and D.K. Smith, Density and size distribution of seamounts in the eastern Pacific inferred from wide-beam sounding data, J. Geophys. Res., 88, 10,508-10,518, 1983.
Keeley, C., A. Macario, and W.B.F. Ryan, Observations and simulations of seamount populations, Ridge Events, 6, 11-12 & 27, 1995.
Nielsen, R.L. and H. Sigurdsson, Quantitative methods of electron microprobe analysis of sodium in natural and synthetic glasses, Amer. Mineral., 66, 547-552, 1981.
Niu, Y. and R. Batiza, An empirical method for calculating melt compositions produced beneath mid-ocean ridges: application for axis and off-axis (seamounts) melting, J. Geophys. Res., 96, 21,753-21-777, 1991. Parker, R.L. and D.W. Oldenburg, Thermal model of ocean ridge, Nature Phys. Sd., 242, 137-139, 1973.
Phipps Morgan, J. and D.T. Sandwell, Systematics of ridge propagation south of 30°S, Earth Planet. Sci. Leit., 121, 245-258, 1994.
Scheirer, D.S. and K.C. Macdonald, Near-axis seamounts on the flanks of the East Pacific Rise, 8° to 17°N, J. Geophys. Res., 100, 2239-2259, 1995. Sempere, J.-C., B.P. West, and L. Geli, The Southeast Indian Ridge between 127° and 132°40'E: contrasts in segmentation characteristics and implications for crustal accretion, in Tectonic, Magmatic, Hydrothermal and Biological Segmentation of Mid- Ocean Ridges., Geol. Soci. Spec. Pubi., vol. 118, edited by MacLeod, C.J., P.A. Tyler, and C.L. Walker,pp. 1-15, 1996. Shen, Y., D.W. Forsyth, D.S. Scheirer, and K.C. Macdonald, Two forms of volcanism: implications for mantle flow and off-axis crustal production on the west flank of the Southem East Pacific Rise, .1. Geophys. Res., 98, 17,875-17, 889, 1993. Shen, Y., D.S. Scheirer, D.W. Forsyth, and K.C. Macdonald, Trade-off in production between adjacent seamount chains near the East Pacific Rise, Nature, 373, 140-143, 1995.
Smith, D.K. and T.H. Jordan, The size distribution of Pacific seamounts, Geophys. Res. Lett., 14, 1119-1122, 1987. Smith, D.K. and T.H. Jordan, Seamount statistics in the Pacific Ocean, J. Geophys. Res., 93, 2899-29 18, 1988. Sun, S.-s., and W. F. McDonough, Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes, in Magmatism in the Ocean Basins., Geol. Soc. Spec. Pubi., vol. 42, edited by A.D. Saunders and M.J. Norry, pp. 313-346, Blackwell Scientific, Boston, Mass., 1989.
West, B.P.W, and D.M. Christie, Evidence for asthenospheric flow toward the Australian- Antarctic Discordance from gravity anomalies along the Southeast Indian Ridge, Eos, Trans.AGU, 77, 685, 1996.
West, B.P.W., W.S.D. Wilcock, J.C. Sempere, and L. Geli, Three-dimensional structure of asthenospheric flow beneath the Southeast Indian Ridge, J. Geophys. Res., 102, 7783-7802, 1996.
Zindler, A., H. Staudigel, and R. Batiza, Isotope and trace element geochemistry of young Pacific seamounts: implications for the scale of upper mantle heterogeneity, Earth Planet. Sci. Lett.,70, 175-195, 1984. 49
Chapter III
The Petrogenesis of Intrabasin Volcanics from the East Blanco Depression
Frank M. Sprtel Roger L. Nielsen David M. Christie 50
Abstract:
The Blanco Transform includes several small basins that may have formed from
extensional forces applied across the transform boundary diring episodes of plate
readjustment (Embley and Wilson, 1992). Fresh basaltic glass was recovered from several
sites in one of these basins, the East Blanco Depression, by the U.S. Navy's Advanced
Tethered Vehicle (ATV). Major element data span a wide range of MgO values (-5-9.2
wt.%) which encompasses almost the entire range for mid-ocean ridge basalts (MORB).
The major element trends follow fractional crystallization trends at 4-8 Kbar, but minor elements (Ti02, K20, and P205) and trace element ratios (e.g. (La/Sm)n) are over-
enriched relative to crystal fractionation trends. These over-enrichment trends can be
reproduced using either simple batch melting calculations or the open system fractional
melting calculations of Johnson and Dick (1992). The closeness of the Blanco data to the
batch melting and open system fractional melting curves may suggest that these lavas
originated from a single source. It also may suggest that both types of partial melting were involved in the petrogenesis of the Blanco suite. Major, minor, and trace element data indicate an early release of enriched melt, which underwent extensive fractionation during transport through the upper mantle and crust. Later more depleted melts experienced less melting due to heating of the conduit system by the earlier packets of magma.
Introduction:
Intra-transform volcanism has been well studied at three oceanic transform boundaries, the Garrett Transform (East Pacific Rise) (Hekinian et al., 1992; Hekinian et al., 1995), the Siqueiros Transform (East Pacific Rise) (Natland, 1989; Perfit et aL, 1996), and the Blanco Transform (Juan de Fuca Ridge) (Gaetani et al., 1995). This style of volcanism may result from diffuse extension within a transform caused by reorganizations in plate geometry (Embley and Wilson, 1992). Intra-transform volcanism that occurs in extensional basins within transform fault zones, such as the East Blanco Depression (East 51
Blanco Depression) of the Blanco Transform, produces lavas that are often enriched in incompatible elements when compared to N-MORB (Perfit et al., 1996). These lavas may enable petrologists to examine petrologic processes that are often masked by the production of N-MORB from the mixing and crystal fractionation of magmas in chambers beneath mid-ocean ridges.
One of the most commonly observed characteristics of MORB suites is the correlation of incompatible element contents and ratios with decreasing MgO (O'Hara,
1968; Stakes et aL, 1984; Frey et aL, 1993). In some lava suites, this correlation produces a trend that is divergent and displays higher incompatible element concentrations for the most evolved lavas than can be attributed by crystal fractionation alone. Several different processes have been cited to explain this enrichment phenomenon, including assimilation of altered crust (Michael and Schilling, 1989), paired fractionation and recharge (O'Hara and
Mathews, 1981), and boundary layer fractionation (Langmuir, 1989; Hekinian and Walker,
1987).
In this paper we describe a suite of lavas from the East Blanco Depression. We then use the diversity exhibited by this suite to address the role that cold, crustal environments, such as extensional basins within fracture zones, play in influencing the chemistry of lavas, and the relationship between this suite and suites erupted at other axial magma systems.
Geologic History:
The Blanco Transform Fault is a right-lateral oceanic transform fault linking the
Juan de Fuca and Gorda Ridges in the Northeast Pacific Ocean off the coast of Oregon
(Figure ifi. 1). It consists of five right-stepping strike-slip sections separated by deep, rhombohedral-shaped extensional basins (Dziak et al. 1991). One of these basins, the East
Blanco Depression, contains an area of recent volcanism discovered during a 1994 survey and sampling cruise (Figure ffl.2). Complex structural features within the East Blanco 52
130°W 125GW 12OW
5ON 5O N
45 N 45N
N 4O N
130°W 125GW 12OW
Figure 111.1. The sma), square box marks the location of the East Blanco Depression within the Blanco Transform Fault Zone. 53
East Blanco Depression
12944W 12940W 12936W II
S . S I .
C, 12'N
:
44'W 40W 36W
-4000 -3900 -3800 -3700 -3600 -3500 -3400 -3300 -3200 -3100 -3000 Depth in Meters
Figure 111.2. Bathymetry map of the East Blanco Depression with 50m contour intervals. The small, white dots represent sample locations. 54
Depression led Embley and Wilson (1992) to suggest that this basin is accommodating the
full spreading rate of the Juan de Fuca ridge with a single transform fault connecting the northern end of the East Blanco Depression to the Juan de Fuca Ridge. The presence of
active volcanism suggests that the tectonic stresses associated with this spreading rate
accommodation fracture the brittle crust within the East Blanco Depression, providing conduits for mantle derived melts.
Methods:
Major and minor element analyses of fresh glasses were performed using the
Cameca SX-50 electron microprobe of Oregon State University (Appendices I & Ill and
Table Affi. 1). Trace element analyses of the glasses were performed using Oregon State
University's Fision VG PlasmaQuad ICP-MS (Appendices I & ifi and Table Aill.1).
Sample dilution for analysis contained 100 ml of sample stock solution, 100 ml of 500 ppb internal standard spike (comprised of In, Be, Re, and Bi), and 5 ml of 1% HNO3. The
ICP-MS was tuned to a sensitivity of 5.5 x iø for 10 ppb 1 151n prior to the sample runs.
Results:
-Peirography and mineral chemistry
The East Blanco Depression lavas range from plagioclase ultraphyric basalts (with plagioclase crystals up to 2 cm. in length and 15% of the total mineral assemblage) to aphyric basalt. Olivine phenociysts and spinel and pyroxene microphenocrysts are relatively rare, never constituting more than 3-5% of the phenocryst assemblage. Pyroxene is present only in lavas with less than 6.5% MgO. The matrix is unaltered, fresh and ranges from glassy to microcrystalline.
Within each lava, plagioclase phenocrysts exhibit a wide range in anorthite composition (Figure 111.3). All phenocryst-rich basalt glasses contain high An plagioclase 55
90
85 - 80 75 70 65 60
55 - Plagioclase 50 micro pharncryst 0.15 0.25 035 Na2OICaO in host glass Figure 111.3. Double arrows represent the range of anorthite content of plagioclase phenocrysts> 1OOm correlated to Na20/CaO composition of the host glass for each sample. Diamonds represent anorthite compositions of plagioclase phenocrysts < lOOj.im correlated to Na20/CaO composition of the host glass.
o Calculated olivine crystal composition 74 Measuredolivine 72 crystal composition 70 40 50 60 70 Mg# Figure 111.4. Open circles represent the olivine phenocryst composition calculated to be in equilibrium with the host glass using the model of Ariskin et al. (1993). Diamonds represent the measured olivine phenocryst composition correlated to the Mg# of the host glass. 56
(>An80), but only the more evolved basalt glasses (<7% MgO) contain plagioclase below
An70. In contrast to the plagioclase phyric lavas from the Endeavour (Sours Page et al. in
prep) and Gorda Ridges (Nielsen et aJ., 1995), there is no apparent correlation of An
content with phenocryst size. In spite of their diversity, the plagioclase phenocrysts exhibit
relatively little zoning, generally less than 5% anorthite within any single crystal. In effect,
the phenocryst population is diverse and exhibits internal homogeneity.
The chemical composition of olivine and plagioclase microphenocrysts displays a
coherent correlation with the lava composition. In both cases, the mineral compositions are
within analytical error of the calculated fosterite and anorthite compositions for the host lavas (Figure ffl.4).
Again in contrast to the Endeavour and Gorda Ridge lavas, melt inclusions are
common in plagioclase phenocrysts regardless of An content. These inclusions range up to
300 microns in diameter and contain quench crystals and glass. The inclusions and host
glass containing quench crystals in these lavas were the topic of an independent
investigation of the effect of quench crystals on Na analysis (Nielsen et al., 1996).
-Chemistry of East Blanco Depression lavas Blanco glasses cover a wide range of MgO values (5.2-9.2 wt.%). Ti02, K20,
P205, and Na20 increase with decreasing MgO and trends increase for CaO and Al203.
Minor element ratios such as KZOIFiO2 increase with decreasing MgO.
East Blanco Depression data span a broader range in MgO than Juan de Fuca, Gorda Ridge, and Blanco Trough data (Figures ffl.5a & b and ffl.6a & b). East Blanco Depression glasses were not as enriched in K20 as some glasses from the Juan de Fuca
(Figure ffl.5a), but are more enriched than Juan de Fuca, Gorda, and Blanco Trough lava suites in K2OfriO2 at 5.2-6 wt.% MgO. Furthermore, East 57
a) 0.6
0.5
0.4
03
02
0.1
0 5 6 7 S 9 10 MgO b)
3.0 . I 2.5 Blanco Trough
2.0 C rda
1.5
1.0 juaft'FucaMlW
0.5
0 5 6 7 8 9 10 MgO
Figure 111.5. Variation diagrams of K20 versus MgO (a) and Ti02 versus MgO b) for the Blarico lava suite. Juan de Fuca data from Karsten et al. (1990). Gorda data from Davis and Clague (1987). Gorda '96 data is unpublished. Blanco Trough data from Gaetani et al. (1995). a) 13.5 Gorda '% 13.0 Juan de Fuca
12.5 .. fIII
12.0
11.5
11.0
10.5
10.0 Blanco Trough
9.5
5 6 7 8 9 10
b) MgO
0.5
0.4 0.4 I 0.3: 0
0.2 Gorda / Blanco Trough 0.1 .1"
0.1
0.0 -
0 5 6 7 8 9 10 MgO
Figure 111.6. Variation diagrams of CaO versus MgO (a) and I(20/TiC)2 versus MgO (b) for the Blanco lava suite. Juan de Fuca data from Karsten et al. (1990). Gorda data from Davis and Clague (1987). Gorda 96 data is unpub1ished. Blanco Trough data from Gaetani et al. (1995). 59
Blanco Depression K20/Ti02 data increase by a factor of four from 9 wt.% MgO to 5.2
wt.% MgO while Juan de Fuca and Gorda Ridge K2OITiO2 data for the show little change
with decreasing MgO. East Blanco Depression lavas are also more enriched in K20 and
Ti02 than Blanco Trough lavas which were recovered from a single dredge haul from the
north-facing wall of the Blanco Transform adjacent to Parks Seamount (Figures ffl.5a &b and ffl.6a & b) (Gaetani et aL, 1995).
Cl/K ratios for Blanco glasses (<0.08) are less than Cl/K for basalt glasses from the
Southern East Pacific Rise (SEPR) (Michael and Schilling, 1989). East Blanco Depression
Cl/K ratios are comparable to CIIK ratios for the North Mid-Atlantic Ridge (NMAR),
Southwest Indian Ridge (SWIR), and the Explorer Ridge (Michael and Schilling, 1989).
However, East Blanco Depression CI data span a much broader range in MgO and peak at lower values for the most MgO depleted sample.
Blanco lavas below 6 wt.% MgO are E-MORBs (La/Sm)n >1.0) and those above 6 wt.% MgO are N-MORBs (La/Sm)n >1.0). The lavas also exhibit a relatively flat REE pattern (Figure ffl.7). Incompatible trace elements such as La are overenriched at low wt.% MgO. Incompatible ratios such as TiiZr increase with decreasing MgO while Ti/Zr increases with decreasing (La/Sm)n.
Discussion:
-Polybaric fractional crystallization model
Major, minor, and trace element ratio trends of the Blanco suite were first compared with trends produced by fractional crystallization. The Ariskin et al. (1993) fractional crystallization model produced trends that diverge from the data trends for major elements
Ca and Al (Figures ffl.8a) and minor elements Ti, K, P and trace element ratio La/Sm (Figure ffl.8b and Figures ffi.9a & b). Ti, K, P. and La/Sm are enriched with respect to the calculated fractionation trends, a phenomenon referred to as over-enrichment. Similar 100.0
10.0
1.0 A Blanco FZ lavas E-MORB
N-MORB 0.1 z-c- Figure 111.7. Spider diagram of the Blanco lava suite. Samples are normalized to the Cl Chondrite values of Sun and McDonough (1989). 61
a) 13
12
U 11 / I 10 'a ..Jatm
_4 kbar 9
8 kbar
8 5 6 7 8 9 10 MgO b) 3.0 a pBiancoFZ I Lavas 2.5
2.0
1.5
1.0
0.5
0 5 6 7 8 9 10 MgO
Figure 111.8. Variation diagrams of CaO versus MgO (a) and Ti02 versus MgO (b) for the Blanco lava suite. Liquid line of descent curves for these diagrams at 1 atmosphere, 4 Kbar, and 8 Kbar pressures were calculated using the crystal fractionation model of Ariskin et al. (1993). The parent melt used for the model runs was the Blanco sample (A9 1-1 R2) with the highest MgO content of the suite. Each symbol on the calculated curves represents 1% crystal fractionation. 62
a) 0.50 BIallcoFZ 0.45 Lavas
0.40
0.35
0.30 :::: 0.25
0.20
0.15
0.10
0.05
0 6 7 8 9 1U b) MgO 1.30 Banw FZ - Lavas 1.20 I
1.10 S 1.00 _4kbr
FL90 U
0.80
0.70
0.60
0.50
0.40 5 6 7 8 9 IU MgO
Figure 111.9. Variation diagrams of 1<20 versus MgO (a) and (La/Sm)n versus MgO (b) for the Blanco lava suite. Liquid line of descent curves for these diagrams at I atmosphere, 4 Kbar, and 8 Kbar pressures were calculated using the crystal fractionation model of Ariskin et al. (1993). The parent melt used for the model runs was the Blanco sample (A91-1R2) with the highest MgO content of the suite. Each symbol on the calculated curves represents 1% crystal fractionation. 63 over-enrichment is well documented for other MORB suites (Stakes et. al., 1984; Frey et. al., 1992; Nielsen et aL. 1995). MORB glasses from the Juan de Fuca and Gorda ridges do not exhibit these overenrichment trends and span a narrower range in MgO than the
Blanco glasses (Figures Ill.5a & b and ffi.6a & b).
The degree of over-enrichment varies from one element to the next. For example, K20 is enriched by a factor of nine at 5 wt % MgO while Ti02 is enriched by a factor of two. La/Sm versus MgO trends also display an enrichment which cannot be produced by fractional crystallization but may be produced by partial melting (Johnson and Dick, 1992).
-Batch and fractional melting models
In order to ascertain the petrogenesis of the Blanco lava suite, simple fractional and batch melting models were calculated at 1%, 2%, 5%, 10%, 15%, and 20% partial melting for Ti, Zr, La, Sm, Ce, Nb, Yb, K, and Ba (Appendix IV). The Blanco data are near the batch melting trend for plots of Ti versus Zr (Figurern_iOa), Ti/Zr versus (LaISm)n (Figure ifi. lOb), (La/Sm)n versus Ba (Figure ifi. 11 a), (La/Sm)n versus K20 (Figure ffl.i ib), and (Ce/Yb)n versus Sm (Figure ifi. 12). However, the observed amount of batch melting that the lavas underwent changes for different trace element variation diagrams. For example, batch melting curves in plots of Ti versus Zr (Figure ifi. lOa),
Ti/Zr versus (La/Sm)n (Figure ffl.lOb), and (Ce/Yb)n versus Sm (Figure ffl.12) showed that the Blanco lavas underwent 2.5-12.5% partial melting whereas batch melting curves in plots of (LaISm)n versus Ba (Figure ifi. 1 la) and (La/Sm)n versus K20 (Figure ffl.i ib) show that the lavas underwent 2.5-20% partial melting.
In order to assert that batch melting was primarily responsible for the petrogenesis of the Blanco suite, the batch melting curves in all the plots should be in agreement about the amount of partial melting that produced the lavas. Since the calculated amount of batch melting was inconsistent among the variation diagrams, a different petrologic process must be primarily responsible for the Blanco suite's initial development. The Johnson and Dick a) 10000
9000 2% 8000
7000
6000
1; 5000 ObsdB1.icoFZ p4000 La'as -3-SoU1CWOSitiO 3000 0- BachMdting
2000 -- FnnMdtirg -II- pomsity=2% 1000 -4- pomsity=6% 0 0 50 100 150 b) Zr (ppm)
Obsved B1aico 140 Lav -- SouCan,osiuoi 20% -0-sthMng 120 -é.. FionMdting
-44- pomity=2% 10% 100 .-4- pomicy.6%
80 : 2% 60 1%
40 0.5 0.7 0.9 1.1 1.3 1.5 (LafSm)n
Figure 111.10. Variation diagrams of the Blanco lava suite for Ti versus Zr (a) and TiJZr versus (La/Sm)n (b). Partial melting curves were generated for a parent source with 0.6% retained melt (porosity) by using the open system fractional melting model of Johnson and Dick (1992). Fractional and batch melting curves were also calculated using the equations of Johnson et al. (1990). The open system model calculates partial melt compositions of a clinopyroxene parent source in the spinel stability field. The source composition was determined by adjustingC0and the porosity of the open system melting model unti the model trends closely matched the data trends. Crystal fractionation affects were removed from the data using the phase equilibrium calculation of Ariskin et al. (1993). a) 2.0 BIico F2lavas 1.8 FionJionUconti Obs& RlaFZIav 1.6 -s--SorntConosition _cBatdiMting 1.4 6 IctionMdting -41.. Porosity2% 1.2 Foro.tyr6% 110
5% 120% 0.6
0.4
0.2
0.0 b) 0 10 20 30 40 Ba (ppm)
1.4
1.2
1.9 5% S 8ac FZLav 10.8 10% g Fionioo Uncd 5% Obvd BIa,coFZLavas -- SouieCirçosition Q BthMdthg 0.4 -- *ionMdting
0.2 -41-- msity2% .4 Fiosity6% 0.Oi I 0.0 0.1 0.2 0.3 0.4 0.5 0.6 K20 Figure 111.11. Variation diagrams of the Blanco lava suite for (La/Sm)n versus Ba (a) and (La/Sm)n versus K20 (b). Partial melting curves were generated for a parent source with 2% and 6% retained melt (porosity) by using the open system fractional melting model of Johnson and Dick (1992). Batch melting curves were also calculated using the equations of Johnson et al. (1990). The open system model calculates partial melt compositions of a clinopyroxeneparent source in the spinel stability field. The source compos- ition was determined by adjustingC0and the porosity of the open system melting model until the model trends closely matched the data trends. Crystal fractionation affects were removed from the data using the phase equilibrium calculation of Ariskin et al. (1993). 3.0
2.5 1%
1% 2.0 2%
1.5 UBloPZ Iav p FiooionUucv 2% ObsbiBIjFZ1w1as 1.0 10% - SoceCcnoition 15% -0-BadMeJthg
0.5 b- onM1ting
-a'-. lioSfly2% 5% 0.0 0 F,m6 0 10 20 30 40 50 Sm(ppm)
Figure 111.12. Variation diagrams of the Blanco lava suite for (CeIYb)n versus Sm. Partial melting curves were generated for a parent source with 2% and 6% retained melt (porosity) by using the open system fractional meltingmodel of Johnson and Dick (1992). Fractional and batch melting curves were also calculated using the equations of Johnson et aT. (1990). The open system model calculates partial melt compositions of a clinopyroxene parent source in the spinel stability field. The source composition was determined by adjustingC0and the porosity of the open system melting model until the model trends closely matched the data trends. Crystal frac- tionation affects were removed fromthe data using the phase equilibrium calculation of Ariskin et al. (1993).
L 67
(1992) open system fractional melting model was next used to model the East Blanco Depression glasses.
-Open system fractional melting model
Johnson and Dick (1992) developed their model around three assumptions. (1)
There is retained melt after the partial melting of the host peridotite, (2) partial melting
occurs in only one stability region (e.g. the spinel stability field), and (3) the host undergoing melting is a clinopyroxene peridotite.
To provide a set of values that have a consistent degree of differentiation, the East
Blanco samples were corrected for fractionation prior to running the model. The fractionation corrections were calculated using the Ariskin et al. (1993) phase equilibria calculation: F=.03(70Mg#) (1) where F is the fraction of the solid assuming 3% crystallization per Mg#. Each sample composition is "back-fractionated" to Mg# 70 assumed to approximate the Mg# of the mantle prior to melting. An initial concentration was then calculated using the Rayleigh fractionation equation: C0=C(1F) (2) assuming a partition coefficient of zero for the incompatible elements of interest.
The open system fractional melting model was run at 0.1%, 2%, 6%, and 15% retained melt for trace elements Ti, Zr, La, Sm, Ce, Nb, Yb, K, and Ba (Appendix IV).
The model produced an array of liquid compositions at 0.5% increments from 0% to 24% partial melting of the source composition. The model trends for 6% retained melt most closely matched the Blanco suite trends (Figures ifi. lOa & b, ifi. 11 a & b, and ifi. 12).
The close fit of the 6% trend to the Blanco data trend for plots of Ti versus Zr (Figure
UI. 1 Oa), Ti/Zr versus (La/Sm)n, (Figure UI. lob), (La/Sm)n versus Ba (Figure 111.11 a),
(La/Sm)n versus K (Figure ifi. 1 ib), (CefYb)n versus Sm (Figure ifi. 12) indicates that the Ler.
samples were likely generated by 2-10% partial melting of the parent source, with a high
degree of retained melt. Unlike the batch melting model, the open system model produced
arrays on all variation diagrams consistent with 2-10% partial melting that produced the
lavas (Figures ffi.lOa & b, ffl.11a & b, and 111.12). This consistency in Figures Ill.lOa
& b, ifi. 1 la & b, and 111.12 also suggests that the Blanco lava suite may have evolved
from a single, homogeneous source that underwent differential partial melting.
However, the 6% retained melt calculated for the Blanco suite is higher than the 2%
retained melt calculated for melt inclusion data for lavas from the Gorda Ridge (Nielsen et
al., 1995) and the 0.1% and 0.5% retained melt calculated for residual abyssal peridotite
diopsides from the Atlantis 11 and Bouvet Fracture Zones (Johnson and Dick, 1992). The model results indicate that open system fractional melting does not adequately explain the
generation of high degrees of retained melt (-6%) since this model was developed to account for the <0.2% resident melt fraction for a dunite residue that experimental evidence
suggests is in the mantle under mid-ocean ridges (Riley and Kohlstedt, 1991).
The open system fractional model results also suggest that batch melting may play a role in the development of melt in the cool mantle environment near oceanic transforms. In this environment, batch melting may account for the initial high retention of melt in the mantle which may increase the buoyancy of the mantle. This could create a narrowly focused zone of upwelling where melt is transported vertically through the upper mantle, a concept referred to as buoyancy-driven, dynamic flow by Forsyth and Chave (1994). As mantle upwells and continues to melt, it may then undergo open system fractional melting at shallower depths than melt that forms beneath a mid-ocean ridge. However, to better constrain this depth of partial melting, modeling of the Blanco data in the plagioclase and garnet stability fields is required. -Crystal fractionation correction for the open system fractional melting model
The Blanco lavas were not significantly altered by crystal fractionation during their initial development. This assertion is based on a fractionation correction that caused the data to plot along a partial melting trend. Fractionation-corrected trace element data points depicted in Figures ifi. 11 a & b and Figure ifi. 12 are connected to fractionation-uncorrected points by lines to show the effect of crystal fractionation on the petrogenesis of MORB.
The results of this correction suggest that a minor crystal fractionation signature was superimposed on the Blanco suite prior to eruption.
-Two scenarios for the petrogenesis of the Blanco lava suite
Scenario #1: Figures ffl.8a and ffl.9a & b show an increase in incompatible element concentrations with decreasing MgO content. One possible way to replicate this trend is to produce melts sequentially. Perhaps the first melts (e.g. samples A91-1OR and
A91-1 lR) produced were emiched in incompatible elements because such elements are higher in concentration at the onset of melting and become depleted as melting progresses.
Their low MgO values suggest low eruption temperatures, possibly the result of the melts quickly cooling when they came into contact with the cold lithosphere of the transform.
The plagioclase porphyritic texture of these lavas indicate that the lavas crystallized plagioclase over a period of time beneath the cold lithosphere of the transform. Such affects are observed to occur at other oceanic transforms. For example, Hekinian et al.
(1995) proposed that the fast cooling environment of the Garrett Transform should increase the viscosity of the first magmas produced and, therefore prevent a large degree of crystal settling. This leads to the extrusion of highly porphyritic lavas (Hekinian et aL, 1995).
As these first, enriched magmas were erupted, they may have warmed the conduits that channeled them to the surface. Subsequent melts produced by continued partial melting of the source would experience less ambient heat loss as they travel through the insulated conduits. This might explain the higher eruption temperatures (higher MgO 70
content) of these subsequent melts. These later forming melts would also have lower
concentrations of incompatible elements since progressive partial melting depletes the parent source of these elements.
Scenario #2: The wide range of % anorthite for individual plagioclase phenocrysts
within a single Blanco glass sample (Figure ffl.3) and the increase of incompatible
elements as MgO decreases (Figures ffl.Sa and ffl.9a & b) may indicate that the Blanco
magmas did not form at a uniform depth and ascend sequentially, but formed and mixed
over a range of depths and ascended simultaneously. Three lines of evidence support this hypothesis.
First, the relatively neutral buoyancy of plagioclase crystals may suggest that newly formed crystals remain with their parent melt as it ascended through the asthenosphere.
Thus, plagioclase crystals from one melt could mix with crystals derived from a different parent melt as the plagioclase phyric magmas ascend together.
Second, the wide range of % anorthite of plagioclase phenocrysts within the Blanco glasses may suggest that the crystals formed at different depths. For example, crystals with high % anorthite (>An80) axe found in all Blanco glasses, but low % anorthite crystals
( Blanco glasses are also enriched in incompatible elements relative to the less evolved glasses, they probably formed from deep, incompatible element enriched mantle. Third, trace element variation diagrams (Figures ffl.lOa &b, ffi.1 Ia & b, and ffl.12) show that lavas enriched in highly incompatible elements result from lower degrees of partial melting than lavas more depleted in these elements. This may provide further evidence that the enriched lavas formed deeper than the depleted lavas. These magmas may have then simultaneously ascended through the asthenosphere. Scenario #1 appears to better explain the major, minor, and trace element trends for the Blanco suite than scenario #2. Scenario #1 provides a plausible explanation for the increase in incompatible elements as MgO content decreases. As proposed in scenario #1, 71 a sequenced eruption of different lava types might reduce the loss of heat to the cold surrounding crust, thus allowing the magma from subsequent injections to retain more heat. This style of eruption also seems to explain the petrogenesis of enriched and depleted melts through progressive partial melting of the parent source.Scenario #1 may also better explain the apparent crystal fractionation imprint on the Blanco glasses (Figures ffl.1 la & b and Figure ifi. 12). As the first melts reach the surface, they may have more time to cool than subsequent melts which enables the primary melts to perhaps undergo a small degree of crystal fractionation. Conclusion: 1) Fresh-looking pillow basalts were recovered from the East Blanco Depression of the Blanco Transform Fault Zone. These basalts span a wide range in MgO content (-5 to 9.2 wt.%). 2) The minor, and trace element data do not appear to follow crystal fractionation trends. They display trends in variation diagrams that diverge from crystal fractionation trends as MgO content decreases. These trends indicate that some additional petrologic process could control the evolution of the Blanco lava suite. 3) A combination of batch melting and open system fractional melting (Johnson and Dick, 1992) at 6% retained melt may explain the origin of the Blanco suite. These model trends suggest that the Blanco melts formed from low degrees of partial melting (-2-10 %). The ability to 'back fractionate' the Blanco lavas to Mg#70 and to produce a data trend that can be modeled by open system fractional melting at 6% retained melt may suggest that the Blanco lava suite was overprinted prior to eruption with a minor crystal fractionation signature. 4) The petrogenesis of the Blanco lava suite may be explained by two scenarios. Scenario #1: The Blanco suite was possibly produced by sequential eruptions of lavas with varying enrichment in incompatible elements. Perhaps the first lavas erupted were enriched 72 in incompatible elements followed by subsequent lavas that were depleted in those elements. While the first lavas may have chilled quickly upon reaching the cold lithosphere beneath the transform, they warmed the walls of their feeder conduits. This allowed subsequent melts to erupt at higher temperatures. Scenario #2: The varied % anorthite of plagioclase phenocrysts within single Blanco glass samples may suggest that the Blanco suite was produced by lavas that mixed as they simultaneously ascended through the asthenosphere. Enriched lavas may have formed at deeper depths than the depleted melts since batch and open system modeling of the Blanco suite suggests that the enriched lavas were formed by lower degrees of partial melting than depleted melts. 73 References Ariskin, A.A., U. Mikhail, G.S. Barmina, and R.L. Nielsen, COMAGMAT: A FORTRAN program to model magma differentiation processes, Computers & Geosciences, 19, 1155-1170, 1993. Dziak, R.P., C.G. Fox, and R.W. Embley, Relationship between the seisrnicity and geologic structure of the Blanco Transform Fault Zone, Mar. Geophys. Res., 13, 203- 208, 1991. Enibley, R.W. and D.S. Wilson, Morphology of the Blanco Transform Fault Zone--NE Pacific: Implications for its tectonic evolution, Mar. Geophys. Res., 14, 25-45, 1992. Forsyth, D.W. and A.D. Chave, Experiment investigates magma in the mantle beneath mid-ocean ridges, EUS Trans. AGU, 75 (46), 537-540, 1994. Frey, F.A., N. Walker, D. Stakes, S.R. Hart, and R. Nielsen, Geochemical characteristics of basaltic glasses from the AMAR and FAMOUS axial valleys, Mid-Atlantic Ridge (36°-37°N): Petrologic implications, Earth Planet. Sci. Lett., 115, 117-136, 1993. Gaetani, G.A., S.E. DeLong, and D.A. Wark, Petrogenesis of basalts from the Blanco Trough, northeast Pacific: Inferences for off-axis melt generation, J. Geophys. Res., lOU, 4197-4214, 1995. Hekinian, R. and D. Walker, Diversity of spatial zonation of volcanic rocks from the East Pacific Rise near 21 °N, Contrib. Mineral. Petrol., 96, 265-280, 1987. Hekinian, R., D. Bideau, M. Cannat, J. Francheteau, and R. Hthert, Volcanic activity and crust-mantle exposure in the ultrafast Garrett transform fault near 13°28'S in the Pacific, Earth Planet. Sci. Lett., 108, 259-275, 1992. Hekinian, R., D. Bideau, R. Hébert, and N. Yaoling, Magmatism in the Garrett transform fault (East Pacific Rise near 13°27'S), J. Geophys. Res., 100, 10,163-10,185, 1995. Johnson, K.T. and H.J.B. Dick, Open system melting and temporal and spatial variation of peridotite and basalt at the Atlantis II Fracture Zone, J. Geophys. Res., 97, 9219- 9241, 1992. Karsten, J.L., Delaney, J.R., Rhodes, J.M., Liias, R.A., Spatial and temporal evolution of magmatic systems beneath the Endeavour Segment, Juan de Fuca Ridge: Tectonic and petrologic constraints, J. Geophys. Res., 95, 19235-19,256. Langmuir, C.H., Geochemical consequences of in situ crystallization, Nature, 340, 199- 205, 1989. Michael, P.J. and J.-G. Schilling, Chlorine in mid-ocean ridge magmas: Evidence for assimilation of seawater-influenced components, Geochim. Cosmochim. Acta, 53, 3131-3143, 1989. Natland, J.H., Partial melting of a lithologically heterogeneous mantle: inferences from crystallization histories of magnesian abyssal tholeiites from the Siqueiros Fracture Zone, in Magmatism in the Ocean Basins., Geol. Soc. Spec.PubL,vol. 42, edited by A.D. Saunders and M.J. Norry,pp. 41-70, Blackwell Scientific, Boston, Mass., 1989. Nielsen, R.L., J. Crum, R. Bourgeois, K. Hascall, L.M. Forsythe, M.R. Fisk, and D.M. Christie, Melt inclusions in high-An plagioclase from the Gorda Ridge: An example of the local diversity of MORB parent magmas, Contrib. Mineral. Petrol., 122, 34-50, 1995. Nielsen, R.L., D.M. Christie, and F.M. Sprtel, Anomalously low sodium MORB magmas: Evidence for depleted MORB or analytical artifact?,Geochim. Cosmochim. Acta, 59, 5023-5026, 1996. O'Hara, M.J., Are any ocean floor basalts primary magma?, Nature, 220, 683-686, 1968. O'Hara, M.J. and R.E. Mathews, Geochemical evolution in an advancing, periodically replenished, periodically tapped, continuously fractionated magma chamber, J. Geol. Soc. London, 138, 237-277, 1981. Pefit, M.R., Di. Fornari, W.I. Ridley, P.D. Kirk, J. Casey, K.A. Kastens, J.R. Reynolds, M. Edwards, D. Desonie, R. Shuster, S. Paradis, Recent volcanism in the Siqueiros transform fault; pictritic basalts and implications for MORB magma genesis, Earth Planet. Sci. Leit., 141, 91-108, 1996. Riley, G.N., Jr., and D.L. Kohlstedt, Kinetics of melt migration in upper mantle-type rocks, Earth Planet. Sci. Lett., 105, 500-521, 1991. Stakes, D.S., J.W. Shervais and C.H. Hopson, The volcanic-tectonic cycle of the FAMOUS and AMAR valleys, Mid-Atlantic Ridge (36°47'N): Evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley mid-sections, AJvIAR 3, J. Geophys. Res., 89, 6995-7028, 1984. 75 Chapter IV Summary Two styles in which near-ridge volcanism in the ocean basins is manifested are at near-ridge seamounts and in intra-transform spreading centers. Southeast Indian Ridge seamounts and the East Blanco Depression of the Blanco Transform, examples of these two styles, were reviewed. Chapter II addressed near-ridge seaniount volcanism adjacent to the SEIR. Previous studies of near-ridge seamounts (Batiza and Vanko, 1983; Batiza and Vanko, 1984; Batiza et al., 1989a; Batiza et al., 1990; Niu and Batiza, 1991; Edwards et aL, 1991; Cordery and Phipps Morgan, 1993; Shen et al., 1993; Schierer and Macdonald 1995; Shen et aL, 1995) indicate that seaxnounts in the eastern Pacific Ocean are products of excess melt from a single sources in the upper mantle. The WESTWARD 9 & 10 cruises provided an opportunity to study seamount volcanism at a previously unsurveyed section of the SEIR whose axis represents a wide range of ridge axis morphology despite a constant spreading rate. SEIR seamount fields are important to examine since they can be used as tools to ascertain the magnitude and lateral extent of the melting region of the SEW. The WESTWARD surveys showed that SEIR seamounts are fewer in number and smaller in size compared to seamounts from the EPR or Juan de Fuca ridge. This might indicate that the melting anomalies responsible for producing SEW seamounts are less voluminous than melting anomalies in the Pacific Ocean. It also might suggest that Indian Ocean mantle is colder and/or less fertile than Pacific Ocean mantle which supports a large population of near-axis seamounts. SEW seamounts decrease in number from west to east and are not evenly distributed along ridge segments as EPR and Juan de Fuca seamounts. This may reflect a change in off-axis melt flux along the SEW. The WESTWARD surveys also uncovered very small seamount chains trending in a direction oblique to the absolute motion of the Australian plate. These chains are possible 76 indicators of along-axis mantle flow (West et al., 1997). Their small size (in terms of number and size of edifices in each chain) compared with chains parallel to absolute plate motion from the SEIR and other mid-ocean ridges suggests that their source is 1) small and 2) located at shallow off-axis depths where it moves parallel to the ridge axis toward the AAD. Chapter ifi addressed intra-transform volcanics from the East Blanco Depression of the Blanco Transform Fault Zone. Such intra-transform volcanic eruptions are documented only for two other transform boundaries--the Garrett and Siqueiros (Hekinian et al., 1992; Hekinian et al., 1995; Natland, 1989; Pefit et al., 1996). Intra-transform volcanism differs from seamount volcanism because it occurs as a passive response to a rifting event initiated by extensional forces on a transform boundary. This style of volcanism differs from mid-ocean ridge volcanism because it occurs in a cold environment. This type of environment prevents a long-lived magma chamber from forming. Consequently, melt is erupted quickly and undergoes smaller degrees of ciystal fractionation than melt that pools and aggregates in a magma chamber prior to eruption. Since the Blanco lavas most likely did not pool in a reservoir prior to eruption, their petrologic evolution is preserved. This allows one to determine the melting processes responsible for the generation of oceanic basalt whose origin is otherwise masked by the affects of crystal fractionation within a magma chamber prior to eruption. Fresh-looking pillow basalts were discovered in the East Blanco Depression and samples by a 1994 NOAA survey cruise. Major, minor, and trace element chemical data from the pillow glasses suggest that the Blanco lavas are highly enriched in incompatible elements when compared with lavas erupted along the Juan de Fuca and Gorda Ridges. The minor and trace element enrichments shown by the Blanco lava suite may be explained by a combination of batch melting and open system fractional melting. Batch melting model curves for different elements were inconsistent in depicting the amount of partial melting that produced the lavas whereas the open system fractional melting curves 77 consistently indicated between 2 and 10% partial melting of the source for all elements plotted. This suggests that both batch melting and open system fractional melting may have been involved in different initial stages of the lava development. This combination of melting processes also covers the entire compositional range of the Blanco lavas suggesting that both the depleted and enriched end member lavas can be explained by a partial melting process. This may suggest that the Blanco lavas were erupted in a short period of time and probably originated from a single parent source. 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Keller, R.A., The Petrology, Geochemistry, and Geochronology of Hotspot Seamounts in the North Pacfic andArc/Backarc Volcanism on the Northern Anta rctic Peninsula, Ph.D. Thesis, Oregon State University, 1996. Klein, E.M. and C.H. Langmuir, Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness, .1. Geophys. Res., 92, 8089-8 115, 1987. Langmuir, C.H., Geochemical consequences of in situ crystallization, Nature, 340, 199- 205, 1989. Marnmerickx, J., Large-scale undersea features of the northeast Pacific, in The Eastern Pacific and Hawaii, The Geology of North America, vol. N, edited by E.L. Winterer, D.M. Hussong, and R.W. Decker, pp.5-14, The Geologic Society of America, Boulder, Colorado, 1989. Michael, P.J. and J.-G. Schilling, Chlorine in mid-ocean ridge magmas: Evidence for assimilation of seawater-influenced components, Geochim. Cosmochim. Acta, 53, 3131-3143, 1989. 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Macdonald, Trade-off in production between adjacent seamount chains near the East Pacific Rise, Nature, 373, 140-143, 1995. Smith, D.K. and T.H. Jordan, The size distribution of Pacific seamounts, Geophys. Res. Lett.,14, 1119-1122, 1987. Smith, D.K. and T.H. Jordan, Seamount statistics in the Pacific Ocean, .1. Geophys. Res., 93, 2899-29 18, 1988. Stakes, D.S., J.W. Shervais and C.H. Hopson, The volcanic-tectonic cycle of the FAJvIOUS and AMAR valleys, Mid-Atlantic Ridge (36°47'N): Evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley mid-sections, AMAR 3, 1. Geophys. Res., 89, 6995-7028, 1984. Sun, S.-s., and W. F. McDonough, Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes, in Magmatisin in the Ocean Basins., Geol. Soc. Spec. Pubi., vol. 42, edited by A.D. Saunders and M.J. Norry, pp. 313-346, Blackwell Scientific, Boston, Mass., 1989. West, B.P.W, and D.M. Christie, Evidence for asthenospheric flow toward the Australian- Antarctic Discordance from gravity anomalies along the Southeast Indian Ridge, Eos, Trans.AGU, 77, 685, 1996. West, B.P.W., W.S.D. Wilcock, J.C. Sempere, and L. Geli, Three-dimensional structure of asthenospheric flow beneath the Southeast Indian Ridge, J.Geophys. Res., 102, 7783-7802, 1997. Zindler, A., H. Staudigel, and R. Batiza, Isotope and trace element geochemistry of young Pacific seamounts: implications for the scale of upper mantle heterogeneity,Earth Planet. Sci. Lert., 70,175-195, 1984. r.] Appendices Appendix I Electron Microprobe and ICP-MS Analytical Techniques Electron Microprobe Analytical Technique: Basalt glass samples were crushed into 1-2 mm chips and mounted in 1 inch diameter epoxy disks prior to analysis using Oregon State University's Cameca SX-50 electron microprobe. Electron beam conditions and data reduction are all identical to those used by Keller (1996) except for CI and S which were analyzed with a beam width of 40 ElementsCounting TimesCounting Times Analyzed(secs.) for Blanco(secs.) for SEIR Sample Run Sample Run Na 10 10 Mg 10 10 Al 20 20 Si 10 10 P 60 30 K 30 30 Ca 10 10 Ti 20 20 Cr 10 10 Mn 10 10 Fe 20 10 S 100 * Cl 1000 * *S and Cl were not analyzed for SEIR glass samples. ICP-MS Analytical Technique: -Sample preparation Basalt glass samples were crushed using a mortar and pestle and to a 0.5-1.0 mm grain size. They were then washed in an ultrasonic bath to remove excess particulate matter and dried in a oven for 12 hours at 80°C. Once dried, the samples were hand-picked to [1 remove crystals and glass chips containing large phenocrysts. 60-80 mg of this hand- picked material was then placed in SaviIlexTM beakers for acid digestion. -Sample digestion and dilution Samples were first dissolved with a concentration ratio of 333 ml HF and 666 ml 16N HNO3 in the Savillex' beakers. The beakers were capped and placed in an oven for 24 hours at 80°C. They were then removed from the oven, uncapped, and placed on a hotplate at 60°C in a fume hood where the samples were allowed to thy-down to a small bead. Subsequent digestions steps are listed below in the order that they occurred. 1) Add 500 ml of 6N HCI &lded to the sample residue and dry-down to a small bead on hotplate at 60°C. 2) Add 400 ml of 8N HNO3 to the sample residue and thy-down to a small bead on hotplate at 60°C. 3) Same procedure as step #2. 4) Add 10 ml of 2N HNO3 to small, dry bead in the SavillexTM beakers. 5) Pipette 100 ml of the sample and standard (BCR-3, BIR-1, BHVO-1, rn-la, and W-2 were used for the analysis) solutions made in step #4, 100 ml of an internal standard spike (consisting of 500 ppb Be, In, Re, and Bi), and 5 ml of 1% HNO3 into test tubes. 6) Stir the run solution made in step #5 for 6-8 seconds using a Vortex. 7) Sample run procedures and data reduction are identical to those employed by Keller (1996). rTA Appendix II Basalt Glass Analyses for SEIR Seamount Lavas Table All.! represents major, minor, and trace element analyses of SEIR seamount basalt glasses. Major and minor elements were analyzed by electron microprobe and the trace elements were analyzed by ICP-MS. Major and minor element data was normalized to the BASL standard. La/Sm ratios were normalized to the CI Chondrite values of Sun and McDoriough (1989). Samples were grouped based on the clustering of data points in variation diagrams of Na20 versus MgO. This variation diagram was selected because it reflects data trends that can be used to approximate the extent of melting that produced the seamount lavas (Klein and Langmuir, 1987). Table AI1.1 Group D66.1 D66.1 D66.1 066.1066.1 066.1 066.1 D66.1 D66,1 Sample No. 66-01 66-02 66-93 66-04 66-0566-6a.2 66-(ib.1 66-6b..266-6ci Avg. No. of Analyses n=4 n=4n=14 n8 n=13n=2 n=2 n=2 n=2 Si02 50.02049.93849.85749.82549.84649.854 49.769 49.633 49.757 Ti02 1.485 1.467 1.484 1.469 1.468 1.475 1.473 1.508 1.500 A1203 15.263 15.18015.144 15.13915.098 15.173 15.229 15.095 15.045 FeO 10.325 10.287 10.237 10.19810.13910.125 10.136 10.202 10.191 MaO 0.133 0.126 0.153 0.140 0.134 0.153 0.134 0.142 0.134 MgO 7.737 7.709 7.727 7.666 7.770 7.655 7.689 7.661 7.570 CaO 12.496 12.43312.345 12.37412.225 12.253 12.374 12.364 12.273 Na20 3.044 3.020 3.039 3.019 3.050 3.078 2.973 2.987 2.961 1(20 0.109 0.101 0.107 0.106 0.103 0.107 0.112 0.102 0.097 P205 0.108 0.103 0.091 0.090 0.090 0.095 0.079 0.052 0.086 Cr203 0.043 0.043 0.041 0.044 0.040 0.034 0.041 0.061 0.069 Total 100.762100.407100.223100.07099.964100.002100.009 99.807 99.683 Sc Cr Co Ni Cu Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm En Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th Ti La/Sm(n) Table AII.1 (Continued) Group D66.1 D66.1 D66.1 D66.1 D66.1 D66.1 D66.1 D66.1 D66.1 Sample No. 66-6c.266-6d.166-6d.2 66.07 66-7a.1 66-7a.266-7a366.08 66-09 Avg. No. of Analyses n=2 n=2 n=2 n8 n=2 n=2 n=2 n=4n=4 Si02 49.479 49.577 50.185 49.891 49.465 49.732 49.67849.80249.662 T102 1.471 1.49% 1.415 L486 1.427 1.500 1.461 1.489 1.471 A1203 15.034 15.071 15.253 15.133 14.985 15.171 15.140 15.05215.030 FeO 10.058 10.148 10.238 10.030 10.012 10.010 10.134 10.06510.219 MnO 0.149 0.182 0.166 0.124 0.159 0.169 0.149 0.147 0.140 MgO 7.559 7.697 7.606 7.751 7.614 7.610 7.663 7.707 7.629 CaO 12.074 12.281 12.186 12.550 12.161 12.199 12.248 12.37412.240 Na20 2.965 2.991 2.951 2.956 2.980 2.973 3.024 2.985 2.997 1(20 0.108 0.098 0.111 0.102 0.105 0.102 0.111 0.094 0.101 P205 0.102 0.071 0.110 0.102 0.088 0.115 0.133 0.084 0,067 Cr203 0.055 0.049 0.042 0.047 0.053 0.044 0.041 0.045 0.047 Total 99.053 99.655 100.324100.17299.049 99.625 99.78399.84399.602 Sc 40.321 Cr 282.064 Co 44.199 Ni 53.835 Cu 87.726 Zn 71.005 Rb 0.477 Sr 120.341 Y 29.183 Zr 83.024 Nb 1.732 Cs 0.034 Ba 6.35 1 La 2.630 Ce 8.604 Pr 1.569 Nd 8.698 Sm 2.985 Eu 1.124 Gd 3.782 Th 0.727 Dy 4.761 ho 1.030 Er 2.979 Tm 0.462 Yb 2.906 Lu 0.440 111 2.286 Ta 0.129 Tb 0.053 U 0.041 La/Sm(n) 0.561 Table AII.1 (Continued) Group P66.1 D66.1 D66.1 D66.1 D66.1 P66.2 D66.1 D66.1D66.2 D66.1 Sample No. 66-09 66-10 6640a 66-11 66-12 66-13 66.14 66-15 66-16 66-17 Avg. No. of Analyses n2n=6 n=2 n3 n=3 n=3 n=3 n=3n=3 n3 Si02 49.92349.86450.25850.16350.08050.28549.86349.72850.06349.942 Ti02 1.5271.473 1.490 1.494 1.519 1.488 1.464 1.465 1.473 1.471 A1203 15.22415.07415.201 15.27615.147 15.591 15.20215.08815.13615.198 FeO 10.18510.10010.196 10.371 10.216 10.079 10.22610.05110.10410.310 MnO 0.143 0.123 0.130 0.125 0.126 0.127 0.136 0.1460.112 0.125 MgO 7.678 7.682 7.765 7.748 7.730 6.940 7.754 7.632 7.588 7.657 CaO 12.36312.18012.291 12.351 12.371 12.641 12.28712.24812.20812.355 Na20 3.010 2.985 3.010 3.018 2.967 3.106 3.013 2.9942.932 2.973 K20 0.104 0.105 0.106 0.101 0.104 0.107 0.106 0.102 0.093 0.098 P205 0.101 0.098 0.072 0.089 0.086 0.094 0.105 0.090 0.081 0.082 Cr203 0.0530.042 0.041 0.041 0.045 0.047 0.049 0.041 0.048 0.039 Total 100.309 99.728 100.560 100.777 100.392 100.506 100.205 99.585 99.838 100.248 Sc Cr Co Ni Cu Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Er Tm Yb Lu Hf Ta Th U La/Sm(n) Table AIL1 (Continued) Group D66.1 D66.1 D66.1 D66.1 D66.1D66.1D66.1 D66.1 D66.1D66.1 Sample No. 66-18 66-19 66.20 66-21 66.2266-2366-24 66-25 66-2666-27 Avg. No. of Analyses n3 n=3 11=3 n=3n=3n=3n=3 n=3 n3 n3 Si02 50.38750.22050.028 50.52949.81749.45649.40949.58449.21249.340 Tf02 1.512 1.481 1.479 1.487 1.487 1.460 1.476 1.482 1.465 1.465 A1203 15.357 15.252 15.147 15.32615.07115.02215.01715.02215.00114.994 FeO 10.210 10.211 10.164 10.24910.09810.01310.18310.30110.19410.186 MnO 0.119 0.122 0.132 0.172 0.1600.1360.148 0.121 0.1780.168 MgO 7.742 7.629 7.612 7.712 7.6447.691 7.747 7.842 7.708 7.650 CaO 12.29812.290 12.296 12.42912.20212.16112.30812.38812.20112.214 Na20 3.022 2.998 2.955 2.994 3.015 3.031 3.030 3.029 3.048 2.988 1(20 0.109 0.111 0.109 0.105 0.106 0.1040.098 0.100 0.093 0.113 P205 0.071 0.095 0.086 0.099 0.0860.088 0.091 0.095 0.1050.073 Cr203 0.018 0.056 0.039 0.058 0.0450.048 0.041 0.052 0.041 0.033 Total 100.844100.465100.049101.16099.73199.21299.547100.01799.24599.225 Sc 38.923 Cr 273.372 Co 44.427 Ni 53.598 Cu 90.040 Zn 70.475 Rb 0.486 Sr 117.635 Y 28.018 Zr 79.481 Nb 1.636 Cs 0.016 Ba 7.121 La 2.528 Ce 8.246 Pr 1.505 Nd 7.954 Sin 2.792 Eu 1.100 Gd 3.769 Tb 0.691 Dy 4.462 Ho 0.992 Er 2.802 Tm 0.440 Yb 2.728 Lu 0.413 Hf 2.139 Ta 0.105 Tb 0.090 U 0.046 La/Sni(n) 0,590 91 Table AII.1 (Continued) Group D66.lD66.lD66.1 D66.3 D66.1 D66.1 D66.1D66.1 D67.1D67.2 Sample No. 66-28 66.29 66-36 66-56 66-57 66-58 66-59 66-61 67-01 67-10 Avg. No. of Analyses n=3n=3n=3 n=3 n=3 n=3 n3 n3 n3 n3 Si02 49.087 49.352 49.20750.285 50.01049.89949.883 49.50550.87950.306 Ti02 1.447 1.482 1.453 1.471 1.448 1.472 1.447 1.450 1.097 1.192 A1203 15.03115.01315.08715.193 15.116 15.06515.04214.90216.05615.097 FeO 10.04210.14810.14910.273 10.294 10.36010.22110.102 8.617 8.989 MnO 0.1480.1650.154 0.144 0.118 0.146 0.1240.148 0.124 0.134 MgO 7.6037.6727.622 7.396 7.647 7.767 7.6677.587 7.876 8.204 CaO 12.17112.30012.18212.547 12.43412.47612.41012.33413.095 12.839 Na20 3.008 2.981 2.950 2.889 2.930 2.939 2.943 3.000 2.506 2.436 1(20 0.0940.0990.097 0.105 0.106 0.113 0.102 0.111 0.079 0.079 P205 0.1060.081 0.105 0.083 0.106 0.095 0.098 0.107 0.060 0.082 Cr203 0.0570.0570.034 0.039 0.045 0.031 0.0570.043 0.071 0.064 Total 98.79499.34999.041100.426100.252100.36399.99499.289100.46199.423 Sc Cr Co Ni Cu Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Th Dy 10 Er Tin Yb Lu Hf Ta Tb U LaISm(n) 92 Table AII.1 (Continued) Group D67.2D67.3 D67.3D67.3D67.3D67.3D67.4 D67.4 D67.4D67.4 D72.1 Sample No. 67.13 67-1467-1567-1867-1967-24 67-0267-0467-06 67-08 72-01 Avg. No. of Analyses n=3 n=3n=3n=3n=3n=3n=3 n=3n=3n=3 n=6 Si02 50.43650.15349.99450.28249.77749.78350.51250.37550.22850.46651.391 Ti02 1.151 1.163 1.140 1.164 1.110 1.124 0.989 0.973 1.013 0.997 1.250 A1203 15.34015.20015.18615.07615.88815.13115.46315.38115.36715.472 15.5Th FeO 8.895 8.830 8.811 8.9148.425 8.699 8.845 8.747 8.743 8.795 9.505 MnO 0.150 0.124 0.1570.1380.1360.139 0.115 0.115 0.123 0.125 0.126 MgO 8.350 8.398 8.3958.3998.0258.405 8.745 8.680 8.675 8.800 8.472 CaO 13.01512.64412.62512.55112.69012.58513.17813.14113.07713.128 12.715 Na20 2.525 2.5002.4502.408 2.4562.413 2.158 2.2092.163 2.231 2.527 1(20 0.082 0.0740.0800.0960.0820.072 0.069 0.0690.074 0.072 0.060 P205 0.068 0.0820.085 0.085 0.0900.086 0.055 0.0620,056 0.050 0.079 Cr203 0.071 0.0620.059 0.0670.045 0.061 0.059 0.058 0.066 0.055 0.051 Total 100.08299.22998.98199.17898.72398.497100.18799.81099.587100.191101.553 Sc 36.131 Cr 359.297 Co 45.334 Ni 99.223 Cu 79.752 Zn 62.482 Rb 0.390 Sr 106.235 Y 23.131 Zr 59.458 Nb 1.266 Cs 0.031 Ba 5.801 La 1.937 Ce 6.346 Pr 1.165 Nd 6.291 Sm 2.216 Eu 0.880 Gd 3.027 Tb 0.552 Dy 3.580 Ho 0.810 Er 2.252 Tm 0.357 Yb 2.213 Lu 0.331 hf 1.637 Ta 0.079 Th 0.065 U 0.031 LaISni(n) 0.572 93 Table AII.1 (Continued) Group D72.1 D72.1 D72.1 P723 D72.4 D72.1 D72.1 D72.1 P72.2 D72.1 Sample No. 72-02 72-03 72-04 72-05 72-06 72-07 72-09 72-11 72-12 72-14 Avg. No. of Analyses n=6 n=6 n=6 nr6 n=6 n=6 n=6 n=6 n=6 n=6 Si02 51.304 51.18551.373 51.111 51.70251.215 50.72350.772 51.13750.941 Ti02 1.219 1.214 1.216 1.209 1.252 1.224 1.203 1.213 1.322 1.238 M203 15.375 15.302 15.413 15.311 15.111 15.384 15.310 15.309 14.974 15.234 FeO 9.401 9.372 9.361 9.454 9.532 9.368 9.252 9.386 9.732 9.462 MnO 0.103 0.124 0.118 0.130 0.118 0.134 0.117 0.137 0.129 0.125 MgO 8.462 8.450 8.318 8.603 8.107 8.312 8.350 8.368 7.873 8.317 CaO 12.726 12.711 12.731 12.458 12.735 12.709 12.586 12.648 12.643 12.654 Na20 2.481 2.513 2.541 2.604 2.627 2.495 2.446 2.479 2.594 2.519 K20 0.062 0.061 0.068 0.062 0.057 0.069 0.065 0.063 0.071 0.057 P205 0.061 0.084 0.075 0.068 0.071 0.059 0.075 0.091 0.096 0.076 Cr203 0.047 0.062 0.048 0.058 0.043 0.055 0.067 0.058 0.038 0.040 Total 101.242101.077101.261101.069101.356101.024100.194100.523100.609100.661 Sc 37.781 Cr 296.375 Co 45.523 Ni 62.655 Cu 74.628 Zn 66.721 Rb 0.398 Sr 92.447 Y 26.180 Zr 59.440 Nb 1.090 Cs 0.035 Ba 6.401 La 1.856 Ce 6.153 Pr 1.152 Nd 6.513 Sm 2.406 Eu 0.955 Gd 3.361 Tb 0.634 Dy 4.019 110 0.899 Er 2.506 Tm 0.399 Yb 2.463 Lu 0.367 Hf 1.657 Ta 0.064 Th 0.066 U 0.030 LaISm(n) 0.496 94 Table AIL1 (Continued) Group D72.5 D72.2 D72.1 D72.1 D72.1 D72.1D72.1D72.1D72.1D72.1 Sample No. 72-15 72-16 72-17 72-19 72-20 72-2172-2272-2372-2472-24 Avg. No. of Analyses n=6 n=6 n6 n=6 n=6n=6n=6 n=6n=6n=5 Si02 51.17550.98650.861 50.68050.78750.02350.02150.07250.10250.926 Ti02 1.156 1.296 1.222 1.220 1.228 1.183 1.158 1.166 1.143 1.135 M203 15.755 14.961 15.30615.249 15.31414.99114.99415.01014.81015.065 FeO 9.172 9.488 9.276 9.256 9.324 9.111 9.058 8.9708.820 8.919 MnO 0.117 0.133 0.113 0.128 0.132 0.1180.337 0.1290.1230.119 MgO 8.110 7.863 8.330 8.327 8.227 8.229 8.195 8.1698.171 8.248 CaO 12.971 12.526 12.649 12.630 12.62412.48912.43312.36612.45512.452 Na20 2.552 2.667 2.482 2.506 2.463 2.4652.4572.4882.5462.553 K20 0.054 0.092 0.065 0.064 0.064 0.0740.0640.0650.0440.049 P205 0.072 0.076 0.066 0.077 0.079 0.0830.0970.0750.0670.083 Cr203 0.040 0.057 0.055 0.062 0.052 0.0520.0470.0390.046 0.051 Total 101.174100.146100.425100.198100.29498.81898.64098.55198.32799.601 Sc Cr Co Ni Cu Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sin Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U La/Sm(n) 95 Table AIL1 (Continued) Group D72.1D72.1D72.1D72.1 D93.1 D93.2 D93.2D93.2 D93.3D93.3D93.3 Sample No. 72-2572-2572-2672-27 93-01 93-19 93-21 93-0293-05 93-07 93-08 Avg. No. of Analyses n=6 n=6n=6n=6n=6 n=3 n=3 n6 n=6 n3 n=3 Si02 49.79950.28249.76350.54549.731 50.95551.12649.43948.94949.63249.537 Ti02 1.153 1.169 1.186 1.198 1.040 1.095 1.103 0.8450.809 0.856 0.854 AJ203 14.90515.06714.91115.10615.363 15.941 16.09616.99116.93516.94116.962 FeO 8.9068.9708.9829.236 8.403 9.155 9.062 7.7887.477 7.744 7.744 MnO 0.1320.155 0.1420.148 0.112 0.127 0.150 0.091 0.092 0.130 0.094 MgO 8.221 8.2288.2448.281 8.700 8.877 8.335 9.7689.836 9.863 9.690 CaO 12.38412.42612.54712.52912.178 12.941 12.98012.70712.411 12.66712.668 Na20 2.477 2.4102.494 2.518 2.412 2.492 2.447 2.1132.147 2.120 2.216 K20 0.0630.0590.0830.056 0.071 0.069 0.060 0.0570.054 0.052 0.055 P205 0.073 0.071 0.096 0.08 1 0.090 0.063 0.088 0.0480.050 0.036 0.060 Cr203 0.0580.045 0.0450.057 0.054 0.048 0.057 0.0620.070 0.056 0.049 Total 98.17298.88298.49499.75498.154101.764101.50699.91098.830100.09899.929 Sc 35.501 Cr 319.844 Co 47.786 Ni 85.890 Cu 69.311 Zn 67.495 Rb 0.466 Sr 96.584 Y 24.409 Zr 55.569 Nb 1.223 Cs 0.032 Ba 7.206 La 1.831 Ce 6.027 Pr 1.123 Nd 6.139 Sm 2.233 Eu 0.910 Gd 3.089 Th 0.579 Dy 3.716 Ho 0.856 Er 2.408 Tm 0.384 Yb 2.360 Lu 0.348 Hf 1.599 Ta 0.077 Th 0.061 U 0.026 LalSm(n) 0.52 1 Table AIL1 (Continued) Group D93.3 D93.3 D93.3 D93.3 D93.3 D93.3 D93.4 D93.3 D93.3 P94 Sample No. 93-09 93-10 93-11 93-12 93-13 93-14 93.15 93-16 93-23 94-05 Avg. No. of Analyses n=3 n=3 n=3 n=3 n=3 n=3 n=3 n=3 n=6 n=3 Si02 49.585 49.88249.67 1 49.90049.86250.00049.90049.86450.023 50.472 Ti02 0.849 0.848 0.842 0.828 0.855 0.910 0.892 0.870 0.835 1.097 A1203 17.145 16.935 17.142 17.106 17.171 16.723 16.629 16.950 17.350 16.328 FeO 7.693 7.856 7.640 7.899 7.882 8.123 8.230 7.899 7.968 8.851 MnO 0.071 0.101 0.106 0.113 0.092 0.090 0.102 0.096 0.088 0.101 MgO 9.811 9.664 9.873 9.594 9.711 9.359 9.421 9.579 9.843 9.318 CaO 12.805 12.822 12.837 12.910 12.812 12.926 12.942 12.895 12.970 12.763 Na20 2.222 2.139 2.126 2.138 2.182 2.165 2.192 2.130 2.082 2.451 K20 0.040 0.043 0.041 0.047 0.063 0.063 0.063 0.059 0.040 0.074 P205 0.032 0.061 0.071 0.073 0.062 0.079 0.075 0.060 0.063 0.106 Cr203 0.066 0.059 0.038 0.060 0.054 0.064 0.050 0.072 0.064 0.068 Total 100.316100.409100.385100.669100.745100.503100.497100.475101.325101.629 Sc Cr Co Ni Cu Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U La/Sm(n) 97 Table AIL1 (Continued) Group D94 D94 D94 D94 D94 D94 D94 D95 D95 D95 Sample No. 94-06 94-07 94-08 94-09 94-10 94.11 94-12 95-01 95.02 95-03 Avg. No. of Analyses n3 n=3n3 n=3 n=3 n=3 n=3 n=6n=6 n=6 Sj02 50.42749.61050.565 50.00549.871 50.043 50.34450.26351.227 51.082 Ti02 1.031 1.015 1.029 1.107 0.987 1.016 1.035 1.076 1.117 1.106 A1203 11.01116.63416.932 16.59816.923 16.972 17.07716.65216.322 16.648 FeO 8.392 8.37 1 8.546 8.533 8.281 8.463 8.503 8.035 8.692 8.299 MnO 0.131 0.116 0.094 0.075 0.102 0.101 0.101 0.091 0.127 0.112 MgO 9.069 8.905 9.186 8.542 8.879 8.959 9.084 8.585 8.638 9.061 CaO 12.71112.52612.739 12.562 12.576 12.549 12.75712.25912.608 12.458 NaZO 2.408 2.314 2.424 2.465 2.329 2.345 2.401 2.578 2.583 2.658 K20 0.074 0.078 0.077 0.079 0.076 0.072 0.074 0.089 0.093 0.097 P205 0.088 0.063 0.064 0.098 0.093 0.062 0.080 0.095 0.103 0.092 Cr203 0.036 0.059 0.074 0.060 0.066 0.063 0.059 0.067 0.060 0.062 Total 101.37999.692101.730100.125100.181100.646101.51499.789101.568101.676 Sc 19.141 Cr 228.556 Co 29.748 Ni 85.962 Cu 43.118 Zn 38.608 gb 0.286 Sr 72.274 Y 12.900 Zr 33.103 Nb 0.819 Cs 0.028 Ba 4.056 La 1.126 Ce 3.600 Fr 0.677 Nd 3.739 Sm 1.344 Eu 0.544 Gd 1.810 Tb 0.346 Dy 2.197 Øo 0.503 Er 1.381 Tm 0.225 Yb 1.385 Lu 0.192 HI 0.949 Ta 0.059 Tb 0.005 U 0.014 LaISm(n) 0.538 Table ALLI. (Continued) Group D95 D95 D95 D95 1)95 1)95 D95 D95 D95 D95 Sample No. 95-04 95-05 95-Sal95-8b195-Sd 95-8e195-811 95-9b1 95-9c195-9e1 Avg. No. of Analyses n=3 n=3 n=3 n=3 n=3 n=3 n=3 n=3 n=3 n=3 Si02 51.285 51.73951.276 51.76051.45951.276 51.61550.334 51.287 51.460 Ti02 1.108 1.138 1.099 1.128 1.122 1.071 1.118 1.089 1.104 1.159 A1203 16.505 16.574 16.740 16.594 16.419 16.287 16.384 16.130 16.395 16.464 FeO 8.481 8.567 8.232 8.533 8.480 8.276 8.427 8.282 8.281 8.462 MnO 0.119 0.105 0.119 0.113 0.106 0.114 0.114 0.141 0.124 0.081 MgO 8.801 8.839 8.478 8.793 9.332 8.592 8.764 8.829 8.921 8.859 CaO 12.362 12.578 12.467 12.635 12.300 12.210 12.309 12.302 12.422 12.623 Na20 2.569 2.544 2.628 2.591 2.525 2.563 2.628 2.620 2.632 2.661 K20 0.091 0.091 0.091 0.091 0.092 0.095 0.099 0.092 0.095 0.093 P205 0.091 0.085 0.062 0.112 0.087 0.095 0.082 0.099 0.077 0.072 Cr203 0.055 0.061 0.057 0.052 0.067 0.054 0.056 0.043 0.054 0.053 Total 101.467102.321101.250102.403101.990100.632101.59499.961101.393101.987 Sc 32.161 Cr 336.222 Co 42.038 Ni 82.412 Cu 64.547 Zn 58.640 Rb 0.576 Sr 114.217. Y 21.921 Zr 58.557 Nh 1.597 Cs 0.021 Ba 8.020 La 2.150 Ce 6.677 Pr 1.216 Nd 6.552 Sm 2.288 Eu 0.885 Gd 2.962 Tb 0.554 Dy 3.593 Ho 0.796 Er 2.240 Tm 0.354 Yb 2.140 Lu 0.321 Hf 1.648 Ta 0.105 Th 0.060 U 0.039 La/Sm(n) 0.590 Table AII.1 (Continued) Group D97 D97 D97 D97 D97 D97 D97 D97 D97 D97 Sample No. 97-06 97-07 97-OS 97-09 97-10 97-1197-1297-1397-14 97-15 Avg. No. of Analyses n=3 n=3 n3 n=3 ri=3 n=3n=3n=3n=3 n=3 Si02 4933649.051 49.51249.34949.12448.76948.60049.012 49.06349.359 Ti02 1.282 1.260 1.271 1.253 1.276 1.277 1.276 1.307 1.245 1.275 A1203 17.996 18.167 17.955 17.944 17.90117.79017.74917.74017.91318.021 FeO 8.113 8.134 8.083 8.042 8.028 7.8667.943 8.2477.920 8.020 MnO 0.146 0.115 0.122 0.140 0.116 0.1300.1530.122 0.098 0.120 MgO 8.731 8.853 8.842 8.761 8.699 8.6448.693 8.5828.774 8.809 CaO 11.555 11.558 11.498 11.495 11.43111.43911.43911.42111.41711.569 Na20 3.254 3.204 3.238 3.240 3.222 3.2393.2893.236 3.195 3.250 K20 0.121 0.108 0.125 0.126 0.131 0,12! 0.1300.126 0.113 0.111 P205 0.106 0.105 0.098 0.088 0.071 0.111 0.1460.095 0.095 0.100 Cr203 0.042 0.044 0.025 0.054 0.050 0.0470.0190.042 0.035 0.043 Total 100.682100.599100.770100.493100.05099.43499.43799.93099.866100.677 Sc 27.688 Cr 243.964 Co 42.596 Ni 140.031 Cu 59.517 Zn 58.018 Rb 0.841 Sr 195.085 Y 23.257 Zr 86.131 Nb 2.428 Cs 0.033 Ba 11.282 La 3.518 Ce 10.656 Pr 1.815 Nd 8.915 Sm 2.754 Eu 1.103 Gd 3.439 Th 0.602 Dy 3.723 Ho 0.796 Er 2.237 Tm 0.344 Yb 2.167 Lu 0.329 Hf 2.099 Ta 0.157 Th 0.133 U 0.059 La/Sm(n) 0.822 100 Table AlI.1 (Continued) Group D104 D104 D104 D104 D104 D104 D114.1D114.2D114.1 Sample No. 104-1 104-11 104-12104-13 104-14 104-15114-01 114-02114-03 Avg. No. of Analyses n=3 n=3 n=3 n=6 n=6 n=6 n=3 n3 n=3 Si02 50.79050.348 50.614 50.981 50.815 50.553 49.395 49.93649.165 Ti02 1.035 1.056 1.059 1.083 1.078 1.060 1.613 1.779 1.581 A1203 16.323 16.268 16.097 16.118 16.116 16.242 16.899 16.284 16.912 FeO 8.405 8102 8.328 8.516 8.458 8.225 7.858 8.230 7.729 MnO 0.097 0.146 0.095 0.128 0.130 0.132 0.087 0.086 0.071 MgO 8.845 8.775 8.601 8.569 8.539 8.6.44 7.674 7.073 7.582 CaO 12.792 12.547 12.667 12.662 12.624 12.597 11.187 11.225 11.103 Na20 2.599 2.709 2.692 2317 2.722 2.727 3.144 3.268 3.102 K20 0.059 0.062 0.068 0.060 0.066 0.065 0.738 0.805 0.713 P205 0.066 0.054 0.08 1 0.077 0.077 0.087 0.247 0.262 0.233 Cr203 0.064 0.057 0.064 0.049 0.064 0.058 0.049 0.080 0.055 Total 100.30900.123100.368100.960100.690100.39098.891 99.021 98.246 Sv 31.969 Cr 346.183 Co 42.774 Ni 116.094 Cu 68.435 Zn 57.834 Rb 0.592 Si- 107.051 Y 22.185 Zr 55.412 Nb 1.098 Cs 0.033 Ba 8.278 La 1.897 Ce 6.211 Pr 1.121 Nd 6.142 Sm 2.167 Eu 0.850 Gd 2.883 Tb 0.523 Dy 3.433 Ho 0.759 Er 2.146 Tni 0.341 Yb 2.121 Lu 0.314 Hf 1.531 Ta 0.069 Th 0.088 U 0.034 LaISm(n) 0.566 101 Table AII.1 (Continued) Group D114.1D114.iD114.2D114.1D114.1D114.1D114.1D114.1D114.1 Sample No. 114-04 114-OS 114-06114-07114-08114-09114-10 114-11 114-12 Avg. No, of Analyses n3 =3 n=3 n=3 n=3 n=3 n=3 n=3 n3 Si02 49.79649.196 49.97849.549 49.883 49.974 49.635 49.791 49.802 T102 1.641 1.603 1.778 1.589 1.639 1.622 1.585 1.644 1.639 A1203 16.931 16.882 16.350 16.921 17.101 16.928 16.893 16.906 16.966 FeO 7.839 7.826 8.281 7.737 7.882 7.911 7.696 7.903 7.825 MnO 0.099 0.085 0.070 0.076 0.102 0.061 0.110 0.104 0.095 MgO 7.632 7.672 7.079 7.657 7.711 7,651 7.679 7.645 7.659 CaO 11.208 11.163 11.048 1L182 11.361 11.274 11.275 11.264 11.224 Na20 3.222 3.113 3.147 3.167 3.184 3.094 3.121 3.181 3.170 K20 0.749 0.743 0.826 0.731 0.750 0.743 0.740 0.759 0.750 P205 0.251 0.255 0.267 0.266 0.247 0.239 0.235 0.232 0.241 Cr203 0.041 0.039 0.054 0.060 0.03 1 0.035 0.030 0.037 0.055 Total 99,40998.578 98.877 98.93499.890 99.533 99.00099.466 99.427 Sc 30.249 Cr 244.219 Co 37.526 Ni 114.031 Cu 61.090 Zn 61.223 Rb 11.493 Sr 301.001 Y 26.051 Zr 114.476 Nh 21.063 Cs 0.148 Ba 141 .554 La 11.607 Ce 24.837 Pr 3.288 Nd 14.356 Sm 3.724 Eu 1.343 Gd 4.039 Tb 0.693 Dy 4.219 0.900 Er 2.443 Tm 0.380 Yb 2.332 Lu 0.352 Hf 2.671 Ta 1.220 Th 1.402 U 0.388 La/Sm(n) 2.005 102 Table AII.1 (Continued) Group D114.1D114.1D114.1D114.iD114.1D114.1 D114.1D114.3D114.4 Sample No. 114.13114-14114-15 114-16114-17 114-18 114-19114-20114-21 Avg. No. of Analyses n=3 n=3 n=3 n=3 n=3 n=3 flr=6 n=6 n=6 Si02 49.290 49.523 50.05649.946 49.941 50.206 50.079 50.1 18 50.069 Ti02 1.606 1.611 1.607 1.641 1.596 1.654 1.647 1.540 1.544 A1203 16.882 16.916 17.137 17.021 16.906 17.030 16.978 17.226 17.310 FeO 7.747 7.839 7.806 7.877 7.751 7.921 7.896 7.988 7.851 MnO 0.101 0.080 0.104 0.102 0.069 0.103 0.098 0.085 0.096 MgO 7.632 7.648 7,680 7.665 7.711 7.641 7.726 8.418 8.113 CaO 11.143 11.196 11.248 11.267 11.216 11.246 11.222 11.169 11.187 Na20 3.142 3.075 3.195 3.169 3.222 3.180 3.280 3.270 3.268 K20 0.724 0.739 0.736 0.718 0.719 0.749 0.752 0.692 0.687 P205 0.269 0.234 0.246 0.261 0.248 0.234 0.276 0.248 0.218 Cr203 0.047 0.041 0.049 0.022 0.027 0.041 0.044 0.044 0.047 Total 98.583 98.903 99.86499.688 99,405100.00799.998100.799100.389 Sc 30.246 Cr 269.938 Co 39.385 Ni 142.158 Cu 60.968 Zn 63.127 Rb 10.790 Sr 300.652 Y 24.985 Zr 107.856 Nb 19.469 Cs 0.155 Ba 133.217 La 10.787 Ce 23.111 Pr 3.094 Nd 13.396 Sm 3.546 Eu 1.262 Gd 3.891 Th 0.656 Dy 4.124 Ho 0.851 Er 2.390 Tm 0.365 Yb 2.254 Lu 0.346 Hf 2.478 Ta 1.139 Tb 1.279 U 0.362 La/Sm(n) 1.956 103 Table AII.1 (Continued) Group D114.1D114.lD114.1 D114.1D119JD119.1D119.2D119.2D119.2 Sample No. 114-22114-23114-24114.25 119-01 119-01 119-05 119-06 119-07 Avg. No. of Analyses n=3 n=3 n=3 n=3 n=6 n=6 n=6 n=6 n=6 Si02 50.186 50.205 50.431 49.27950.82950.829 50.569 50.423 50.762 T102 1.616 1.642 1.645 1.593 1.902 1.902 1.803 1.799 1.821 A1203 17.043 17.218 17.106 16.838 15.562 15.562 15.481 15.405 15.552 FeO 7.966 7.957 7.935 7.778 9.708 9.708 9.256 9.267 9.381 MnO 0.104 0.084 0.087 0.088 0.129 0.129 0.154 0.136 0.125 MgO 7.776 7.770 7.705 7.667 6.192 6.192 6.560 6.601 6.562 CaO 11.305 11.204 11.224 11.157 10.906 10.906 11.005 11.087 11,148 Na20 3.356 3.380 3.273 3.342 3.703 3.703 3.568 3.546 3.507 1(20 0.748 0.783 0.751 0.741 0.582 0.582 0.543 0.540 0.556 P205 0.246 0.237 0.250 0.236 0.214 0.214 0.232 0.202 0.211 Cr203 0.026 0.028 0.035 0.024 0.020 0.020 0.033 0.032 0.035 Total 100.373100.509100.44298.743 99.74699.74699.202 99.039 99.659 Sc Cr Co Ni Cu Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Th Dy 110 Er Tm Yb Lu Hf Ta Th U LaISm(n) 104 Table AII.1 (Continued) Group D119.2D119.2D119.2D119.2D119.2D119.2D119.2D119.2D119.2 Sample No. 119-08 119-09 119-10119-11 119-12119-13 119-14119-15119-16 Avg. No. of Analyses n=6 n=6 n=6 n=6 n=6 n=6 n=6 n=6 n=6 Si02 50.49450.705 50.994 50.771 51.097 51.030 51.16250.888 51.417 Ti02 1.800 1.827 1.826 1.811 1.849 1.846 1.841 1.823 1.858 Al2.03 15.461 15.416 15.591 15.542 15.650 15.638 15.693 15.621 15.745 FeO 9.305 9.327 9.436 9.319 9.518 9.446 9.363 9.428 9.537 MnO 0.144 0.153 0.138 0.164 0.110 0.135 0.141 0.146 0.134 MgO 6.619 6.591 6.591 6,586 6.726 6.696 6.706 6.650 6.701 CaO 11.091 11.106 11.191 11.105 11.215 11.193 11.181 11.299 11.286 Na20 3.554 3.531 3.540 3.511 3.592 3.546 3.486 3.517 3.569 1C20 0.546 0.542 0.547 0.551 0.545 0.556 0.541 0.559 0.552 P205 0.213 0.204 0.202 0.204 0.210 0.197 0.214 0.232 0.222 Cr203 0.027 0.036 0.039 0.03 1 0.037 0.027 0.029 0.038 0.040 Total 99.253 99.439100.09599.594100.551100.310100.358100.200101.062 Sc 40.143 Cr 243.432 Co 39.084 Ni 59.269 Cu 75.815 Zn 81.727 Rb 9.805 Sr 289.661 Y 31.576 Zr 147.074 Nb 18.057 0.143 Ba 120.953 La 12.411 Ce 28.522 Pr 3.841 Nd 16.867 Sm 4.391 Eu 1.511 Gd 4.819 Tb 0.825 Dy 5.040 Ho 1.055 Er 2.910 Tm 0.460 Yb 2.721 Lu 0.394 Hf 3.370 Ta 1.108 Th 1.278 U 0.375 La/Sm(n) 1.824 105 Table AII.1 (Continued) Group D119.2D119.2D119.2D119.2 SainpleNo. 119-17 119-18119-19119-20 Avg. No. of Analyses n=6 n=6 n=6 n6 Si02 51.264 50.94250.758 51.231 Ti02 1.842 1.824 1.829 1.815 A1203 15.746 15.641 15.646 15.698 FeO 9.502 9.459 9.410 9.497 MnO 0.156 0.118 0.140 0.142 MgO 6.695 6.653 6.705 6.679 CaO 11.246 11.222 11.244 11.199 Na20 3.553 3.528 3.513 3.534 K20 0.558 0.550 0.554 0.55 1 P205 0.217 0.203 0.220 0.215 Cr203 0.024 0.036 0.046 0.033 Total 100.802100.176100.063100.595 Sc Cr Co Ni Cu Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu HI Ta Tb U LaISm(n) 106 Appendix III Basalt Glass Analyses for East Blanco Depression Lavas Table AIH.1 represents major, minor, and trace element analyses of East Blanco Depression basalt glasses. Major and minor elements were analyzed by electron microprobe and the trace elements were analyzed by ICP-MS. S and Cl data was also collected by electron microprobe andrepresents an average of two analyses per sample. Major and minor element data was normalized to the BASL standard. La/Sm ratios were normalized to the Cl Chonthite values of Sun and McDonough (1989). 107 Table AJIL1 Sample No. A91-1kA91-1R2A91-2RA91-3RA91-SRA91-SRIA91-8R2A91-9R Avg n=10 n10 n=10 n=10 n=10 n=10 n=1O n=10 Si02 48.126 48.462 48.850 47.860 48.422 50.266 49.876 50.433 1102 0.987 0.999 0.987 1.020 0.996 2.013 1.852 2.134 A1203 17.116 17.114 17.126 17.218 17.187 14.592 14.621 14.351 FeO 9.528 9.660 9.642 9.588 9.654 11.056 11.071 11.392 MnO 0.121 0.133 0.126 0.122 0.117 0.149 0.147 0.156 MgO 9.119 9.140 9.268 9.018 9.237 6.768 8.019 6.520 CaO 12.339 12.286 12.245 12.328 12.311 11.363 11.047 11.261 Na02 2.281 2.286 2.266 2.301 2.280 2.926 2.980 3.043 K20 0.093 0.097 0.085 0.087 0.090 0.235 0.217 0.258 P205 0.077 0.074 0.072 0.068 0.080 0.187 0.169 0.212 Cr203 0.045 0.052 0.058 0.064 0.033 0.013 0.033 0.035 Total 99.327 99.797 100.260 99.168 99.906 98.996 99.489 99.261 S 0.21485 0.21445 0.2171 0.2281 0.2064 0.2887 0.29375 0.3051 Cl 0.00325 0.0037 0.0032 0.0047 0.00405 0.01085 0.0085 0.0132 Sc 36.4526 37.1017 37.23935 36.434 40.902 41.756 Cr 306,8422313.0115 300.9852301.235 112.722 103.654 Co 49.2175 49.4119 48.65198 48.817 42.947 43.598 Ni 166.7986 164.1787 163.0712 164310 58.226 54.897 Cu 91.0040 85.5947 85.89891 87.394 68.992 71.722 Zn 64.8924 66.1070 65.9667 61.914 90.105 96.531 Rb 1.1011 1.1000 1.165379 1.2335 3.3203 3.7581 Sr 135.3296 138.7443 138.0224 141.688 175.639 175.622 Y 21.9752 22.4856 22.88522 23.099 38.135 43.967 Zr 55.4051 56.5852 57.02697 57.032 132.932 155.358 Nb 2.4586 2.4502 2,454552 2.869 6.689 7.737 Cs 0.0139 0.0162 0.011052 0.0285 0.0574 0.0620 Ba 13.2810 13.3570 13.63155 11.811 31.920 34.633 La 2.0577 2.2060 2.198465 2.310 5.907 6.988 Ce 6.5470 6.5859 6.885671 6.763 16.602 18.667 Pr 1.0792 1.1098 1.151366 1.238 2.743 3.132 Nd 6.0429 6.1671 6.344503 6.013 13.474 15.229 Sm 2.0631 2.1385 2.192231 2.042 4.247 4.765 Eu 0.7798 0.7783 0.811194 0.802 1.468 1.560 Gd 2.4757 2.5460 2.610882 2.560 4.812 5.238 Th 0.5107 0.5156 0.546871 0.502 0.878 0.924 Dy 3.3028 3.3402 3.37424 3.221 5.588 5.932 Ho 0.7269 0.7279 0.749993 0.742 1.227 1.314 Er 2.1079 2.1843 2.250646 2.100 3.553 3.828 Tm 0.3417 0.3535 0.365932 0.350 0.525 0.612 Yb 2.0951 2.1246 2.21404 2.203 3.429 3.721 Lu 0.3130 0.3399 0.347754 0.346 0.528 0.586 Hf 1.4125 1.4486 1.529616 1.512 3.266 3.574 Ta 0.1580 0.1474 0.157209 0.142 0.431 0.510 Th 0.1951 0.1822 0.176442 0.1679 0.4355 0.5416 U 0.0559 0.0469 0.065541 -0.0002 0.0912 0.1312 (LaISm)n 0.644 0.666 0,647 0.645 0.898 0.947 Table AIII.1 (Continued) Sample No. A91-IOR A91-11R A91-11R2 A91-11R3 A91-12R A91-13R1 A91-16R1 Avg n=10 n10 n=10 n=10 n=10 n=l0 n=10 Si02 50.520 49.837 49.881 50.179 51.395 50.771 50.597 Ti02 2.872 2.570 2.571 2.542 1.567 2.102 2.728 A1203 13.539 13.951 13.946 13.993 15.494 14.720 14.513 FeO 13.460 12.499 12.498 12.532 10.326 11.377 12.690 MnO 0.173 0.157 0.165 0.153 0.123 0.153 0.149 MgO 5.260 5.686 5.730 5.734 7.679 6.822 6.056 CaO 9.936 10.204 10.200 10.216 10.893 11.156 10.192 Na02 3.162 3.188 3.235 3.202 2.730 3.077 3.190 K20 0.453 0.416 0.418 0.418 0.133 0.260 0.397 1>205 0.315 0.284 0.289 0.279 0.141 0.225 0.328 Cr203 0.016 0.042 0.025 0.027 0.050 0.025 0.013 Total 99.177 98.318 98.453 98.781 99.977 100.132 100.299 S 0.38345 0.3339 0.3339 0.33315 0.2393 0.28925 0.3023 Cl 0.0198 0.01515 0.0169 0.0172 0.0035 0.00975 0.01355 Sc 40.223 40.062 41.930 41.890 41.7130 Cr 73.361 82.331 71.210 81.447 93.8275 Co 42.622 42.326 43.950 43.757 41.4708 Ni 40.441 38.704 44.781 49.708 51.6695 Cu 61.752 61.734 61.707 68.378 54.4998 Zn 119.907 117.697 117.832 114.469 115.9078 Rb 6.6712 6.4960 6.5924 4.1890 6.2809 Sr 197.302 196.916 197.179 179.697 165.8156 Y 50.906 49.342 49.770 47.608 55.6330 Zr 201.117 197.482 197.773 169.778 224.6705 Nb 13.299 13.241 13.304 8.705 14.1641 Cs 0.1154 0.1221 0.1030 0.0629 0.0989 Ba 62.170 61.451 61.450 38.001 58.0336 La 10.890 10.740 10.785 7.561 11.1338 Ce 27.252 26.935 26.964 20.537 30.0296 Pr 4.251 4.199 4.181 3.396 4.6554 Nd 19.839 19.805 19.955 16.686 22.6714 Sm 5.761 5.772 5.713 5.105 6.7548 Eu 1.782 1.873 1.883 1.674 1.9227 Gd 6.077 6.326 6.379 5.607 7.1953 Tb 1.056 1.084 1.079 0.978 1.3453 Dy 6.818 6.847 6.878 6.225 9.0198 Ho 1.461 1.460 1.485 1.342 1.8232 Er 4.364 4.399 4.3 16 3.998 5.4099 Tm 0.665 0.663 0.662 0.604 0.8505 Yb 4.191 4.153 4.128 3.746 5.0175 Lu 0.642 0.627 0.662 0.575 0.7405 Hf 4.540 4.323 4.347 3.693 5.4461 Ta 0.931 0.880 0.871 0.548 0.9701 Tb 0.9702 0.8646 0.8552 0.5333 0.9147 U 0.2856 0.2761 0.2542 0.1377 0.3376 (La/Sm)n 1.220 1.201 1.219 0.956 1.064 Table AIII.1 (Continued) Sample No.A91-16R2 Avg n=10 Si02 50.823 Ti02 2.670 A1203 14.506 FeO 12.696 MnO 0.161 MgO 6.098 CaO 10.086 Na02 3.116 K20 0.387 P205 0.354 Cr203 0.025 Total 100.388 S 0.306 Cl 0.01255 Sc 42.0551 Cr 93.3744 Co 41.2353 Ni 50.7244 Cu 55.1964 Zn 115.8688 Rb 6.3037 Sr 167.9510 Y 56.0556 Zr 224.5712 Nb 14.1090 Cs 0.0948 Ba 57.6464 La 11.2180 Ce 30.1559 Pr 4.6084 Nd 22.2634 Sm 6.7897 Eu 1.9461 Gd 7.0193 Tb 1.3592 Dy 8.9451 Ho 1.8252 Er 5.4162 Tm 0.8229 Yb 5.1108 Lu 0.7875 Hf 5.4556 Ta 1.0075 Th 0.9398 U 0.3378 (LaISm)n 1.067 110 Appendix IV Batch and Open System Fractional Melting Model Parameters Starting mineral phase proportions. Mineral StartingMelt Mode (Vol %) Phase Mode (vol%) 0! 0.56 -0.1 Opx 0.24 0.38 Cpx 0.16 0.67 Sp 0.04 0.05 Crystal/liquid partition coefficients used in open system fractional melting model Element D Cpx D 01 P OpxP Sp Ti 0.3 0.015 0.14 0.07 Zr 0.15 0.0005 0.014 0.04 La 0.05 0.000001 0.0009 0.0006 Ce 0.1 0.000001 0.0009 0.0006 Nd 0.15 0.000007 0.009 0.0006 Sm 0.2 0.0007 0.02 0.0006 Yb 0.4 0.023 0.1 0.005 Ba 0.00500 0.00000 0.000000.00000 K 0.000001 0.0000000.0000000.000000 Model starting parameters Element Do* Co Ti 0.09199 0.255 890 Zr 0.02907 0.10736 6.1 La 0.00812 0.03364 0.88608 Ce 0.016 0.06691 1.06209 Nd 0.02583 0.10323 1.2848 Sm 0.03666 0.14059 1.37255 Yb 0.0998 0.3018 1.8235 Ba 0.00079 0.00333 0.80000 K 0.000000 0.000001 55.00000 Do initial bulk solid partition coefficient of element. P=weighted partition coefficient of iiquitL Co=starting composition of the source. *The equation used to calculated this parameter is foundin Johnson at al. (1990). F (degree of meling) =O%-24% Melting increment= 0.5%