GEOLOGIC MAPPING AND PETROLOGY OF THE
SUBMARINE RADIAL VENTS ON MAUNA LOA
VOLCANO HAWAI'I
A THESIS SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI'I IN PARTIAL FULFILLMENT OF THE REQUIREMENTS OF THE DEGREE OF
MASTER OF SCIENCE
IN
GEOLOGY AND GEOPHYSICS
DECEMBER 2004
By V. Dorsey Wanless
Thesis Committee:
Michael Garcia, Chairperson Scott Rowland Frank Trusdell ACKNOWLEDGEMENTS
This study was supported by the National Science Foundation grant OCE-97-29894. I thank the Halloween 2002 Mauna Loa expedition science team, the intrepid JASON2 team, and the R/V T. Thompson captain and crew for their help in collecting data and samples for this study and for subsequent discussions. This manuscript would not have been possible without the reviews, discussions and data provided by Mike Rhodes
(Mauna Loa XRF data), Marc Norman (ICPMS data), and Dominique Weis (Isotope data). Mahalo to Mike Vollinger for instruction in the black art of XRF analysis, Angie
Miller for petrography, Bruce Applegate, Nathan Becker, and Akel Sterling for help with bathymetric data processing, Steve Schilling and Steve Sahetepy-Engle for teaching and problem solving in GIS, and Dan Fornari and Jack Lockwood for comments and advice.
Thanks to my committee members for their inspiration, discussions, comments and reviews. To Scott Rowland for his thought provoking conversations, insights, and understanding of the Hawaiian Island volcanoes. And to Frank Trusdell for sharing his
GIS expertise and his wealth of knowledge and passion for Mauna Loa Volcano. And finally, a special thank you to my advisor, Mike Garcia, for his guidance, support, and patients over the last two and a half years.
iii TABLE OF CONTENTS Acknowledgements .iii
List of Tables vii
L1st· 0 fF'19ures Vll1...
Preface x
Chapter 1: Geologic Mapping and Petrology of the Submarine Radial Vents on Mauna
Loa Volcano Hawai'i
1. Abstract l
1.1 Introduction 3
1.2 Mauna Loa's Radial Vents: Definitions, Post-1832 Eruptions and Models.....7
1.2.1 Radial Vent Definition 7
1.2.2 Post-1832 Eruptions 7
1.2.3 Radial Vent Formation Models 9
1.3 Seafloor Bathymetry and Geologic Map 11
1.3.1 Location and Collection 11
1.3.2 Bathymetry Collection and Processing 13
1.3.3 Geologic map 14
1.3.4 Eruption Volumes 21
1.3.5 Ages 22
1.4 Petrology 22
1.4.1 Outcrops and Samples 22
1.4.2 Tholeiite Petrography 23
1.4.3 Glass Analyses 25
IV 1.4.4 XRF 29
1.4.5 ICPMS 34
1.4.6 Isotopes 39
1.5 Discussion 39
1.5.1 Radial Vent Ages 39
1.5.2 Surface Area and Basic Geology .45
1.5.3 Implications of Submarine Lava Volume Estimates .46
1.5.4 Degassing History and Lava Flow Length .47
1.5.5 Radial Vent Cone Shape .49
1.5.6 Correlations between Radial Vent Activity and Eruption Rates on
Mauna Loa 53
1.5.7 Compositional Heterogeneity and Temporal Trends for Submarine
Mauna Loa Lavas 55
1.5.8 Evidence of Low-pressure Crystallization 56
1.6 Summary 57
Chapter 2: Shield Stage Alkalic Volcanism on Mauna Loa Volcano, Hawai'i
2. Abstract 59
2.1 Introduction 60
2.2 Regional Geology 62
2.3 Location and Sample Collection 63
2.4 Petrography and Geochemistry Results 63
2.4.1 Petrography 63
v 2.4.2 Glass Analysis 64
2.4.3 XRF 68
2.4.4 ICPMS 71
2.4.5 Isotopes 71
2.5 Discussion 76
2.5.1 Volcanic Sources of Radial Vent Alkalic Magmas 76
2.5.2 Origin of Mauna Loa Radial Vent Alkalic Lavas 77
2.5.3 Implications of Alkalic Volcanism for Mauna Loa's Evolution 82
2.5.4 Structural Implications of Mauna Loa's Alkalic Volcanism 84
2.6 Conclusions 86
References 88
VI LIST OF TABLES
1.1 Physical characteristics of Mauna Loa's radial vent deposits and flows 17
1.2 Petrography of Mauna Loa's submarine radial vent and other submarine lavas 24
1.3 Microprobe glass analyses of Mauna Loa's submarine radial vent and other submarine lavas 26
1.4 XRF whole rock compositional analyses for Mauna Loa's submarine radial vent and other submarine lavas 30
1.5 ICPMS analyses of Mauna Loa's submarine radial vent and other submarine lavas , 36
1.6 Pb, Sr and Nd isotope analyses of Mauna Loa's submarine radial vent and other submarine lavas 40
2.1 Petrography, microprobe and XRF analyses of the alkalic radial vents 65
2.2 ICPMS and isotope analyses of the alkalic radial vents 72
vii LIST OF FIGURES
Figure
1.1 Location Map of Mauna Loa's Submarine Radial Vents 4
1.2 Geologic Map of Mauna Loa's Western Submarine Flank 6
1.3 Bathymetric Map of Mauna Loa's Western Submarine Flank 12
1.4 Slope Map of Mauna Loa's Western Submarine Flank 15
1.5 Hillshade Map of Mauna Loa's Western Submarine Flank 16
1.6 Sidescan Sonar Map of Mauna Loa's Western Submarine Flank 18
1.7 Major Element Diagram: CaO, Ah03, KzO and CaO/Ah03 vs. MgO 27
1.8 Sulfur Diagram of All Submarine Lavas 28
1.9 Trace Element Diagrams: Sr, Cr, Nb vs. K 35
1.10 REE Plot of All Submarine Lavas 37
1.11 Spider Plot of All Submarine Lavas , 38
1.12 Pb Isotope Diagrams 41
1.13 Sr, Nd Isotope Diagram .42
1.14 Cone Shapes 50
1.15 Trace Elements and Ages: Sr/Y vs. K/Y 54
2.1 Location Map of Mauna Loa's Submarine Alkalic Lavas 61
2.2 Total Alkalies vs. SiOz 66
2.3 Sulfur Diagram: Alkalic glasses and glass inclusions 67
2.4 Cpx Fractionation Diagrams: Sr/Zr and CaO/Ah03 vs. MgO 69
2.5 Source Diagrams: K vs. Nb and Ba 70
Vlll 2.6 REE Diagram for Mauna Loa's Alkalic Lavas 73
2.7 LalYb vs. Zr/Nb and the Alkalinity Index 74
2.8 Pb Isotopes and Zr/Nb 75
2.9 Ol-Cpx-Qtz Ternary Diagram 80
2.10 Schematic Diagram 85
IX PREFACE
This Master's thesis consists of two separate chapters on Mauna Loa's submarine radial vents. Chapter One focuses on the physical characteristics of all of the submarine radial vent eruptions. In this chapter, the first detailed geologic map of the area is presented and the ages, volumes, cone shapes, and lava flow lengths are examined. In addition, the major and trace element geochemistries of the eight tholeiitic radial vent lavas are examined to determine crystallization sequences, the number of parental magmas required to produce the geochemical variations, and to compare the lavas to possible Mauna Loa source endmembers.
Chapter Two examines the petrology of the two alkalic submarine radial vent lavas. This chapter focuses on the source, formation, and interpretations of the first sampled alkalic lavas from Mauna Loa volcano. Isotopes, major and trace elements and glass analyses are used determine the origin of the alkalic lavas and major and trace elements are used to explore the mechanisms involved in their formation.
This thesis combines a variety of different types of geochemical data, including petrography, microprobe, XRF, ICPMS, and isotope analyses. The bulk of the petrographic data was determined at the University of Hawai'i (UR) by Angie Miller (an undergraduate student at UR). ICPMS data was collected by Mark Norman at Australia
National University and the Pb, Sr, and Nd isotope data was done by Dominique Weis at the University of British Columbia. The XRF analyses were undertaken at the University of Massachusetts (under the supervision of Mike Rhodes). The XRF samples were washed and powdered by Michael Vollinger (a University of Massachusetts graduate student) and I prepared and ran the major element discs and trace element pellets with the
x University of Massachusetts XRF. I also prepared and conducted microprobe analyses for major elements and S on all glass samples at UR.
Thoughtful reviews and discussions were also provided by all of the geochemical contributors, including Mike Rhodes, Mark Norman, and Dominique Weis. They are co authors on a paper submitted to a special issue of JVGR (2004) based on chapter two of this thesis.
xi CHAPfER 1: GEOLOGIC MAPPING AND PETROLOGY OF THE SUBMARINE RADIAL VENTS ON MAUNA LOA VOLCANO HAWAI'I
1. ABSTRACT Radial vents play an important role in the eruptive history of Mauna Loa volcano.
Among the 44 previously identified radial vents on Mauna Loa, only one, which erupted in 1877, was submarine. A detailed bathymetric survey of the western flank of the volcano in 2002, revealed nine new submarine radial vent eruptions and three submarine lava flows. This bathymetry, along with sidescan sonar, shaded relief images, slope maps, photography, and lava geochemistry, was used to produce a geologic map of
Mauna Loa's western submarine flank. The ages of these vents, constrained by eye- witness accounts, geologic relationships and Mn-coatings, range from 127 years to -47 ka. Eight of these radial vents produced degassed lavas despite eruption in water depths sufficient to inhibit sulfur degassing. The amount of volatiles present in the magma appears to be a determining factor in cone shape. Degassed magmas formed truncated cones and short lava flows whereas, the undegassed magmas created "irregular" cones and longer lava flows. Similar variations in bulk composition changed the mineral crystallization order. Compositionally, most of the submarine radial vent lavas lie within the previously defined field of Mauna Loa for major and trace elements except for lavas erupted from two alkalic cones. Isotopically, all of the submarine lavas (including the alkalic lavas) are similar to previously established fields for Mauna Loa. Lavas from most of the submarine eruptions lie along different liquid lines of descent indicating distinct parental magmas. The submarine radial vents and flows cover a total of 29 km2
1 of seafloor and comprise a total of -2 x 109 m3 of lava, suggesting that submarine eruptions are important in the growth of Mauna Loa.
2 1.1. Introduction
Nine radial vent eruptions and three lava flow fields have been discovered on the western flank of Mauna Loa in an area previously thought to consist primarily of fragmental debris. Only one radial vent eruption (A.D. 1877) was previously known to have occurred in this area. These new eruptions are radial vents, of which 44 subaerial examples younger than 4 ka have been identified (Trusdell, pers. comm.; Lockwood and
Lipman, 1987). Mauna Loa has erupted from radial vents, two rift zones and its summit during its shield-building stage of volcanism.
The formation of radial vents and rift zones on ocean islands can be attributed to stresses from regional processes, the injection of magma into the volcano, and gravitational controls. There are minimal regional or structural controls in the central part of the Pacific plate and those that exist (i.e. Molokai fracture zone) are unlikely to playa role in Mauna Loa's stress field (Fiske and Jackson, 1972; Walker, 1993). The injection of magma into the center of an axisymetric volcano will result in the formation of radial fractures if no external stresses are acting on the volcano (Pollard, 1987; Walker,
1993). These fractures are common at many isolated volcanoes (e.g., Spanish Peaks;
Ode, 1957; Galapagos; Chadwick and Howard, 1991; Naumann and Geist, 2000).
However, the stress field of a volcano can also be affected by growth on a pre-existing structure and/or buttressing by neighboring structures (Fiske and Jackson, 1972; Swanson et aI., 1976). The stress field in a young volcano can also be influenced by its construction on an older volcano (e.g. KIlauea on Mauna Loa; Fig. 1.1) leading to the preferential development of rift zones parallel to the flank of the older volcano (Fiske and
Jackson, 1972; Swanson et aI., 1976).
3 Glass
0.20 ~ .&.
.&. 0.15 • Mo'ikeha Glass .&. I!I Mo'ikeha Glass ...... - Inclusions undegassed ...... Akihimoana Glass '*3= 0.10 ... .&. Akihimoana Glass r.1 --en Inclusions partially degassed
0.05 11111,. I.' 1 degassed 0.00 9 10 11 12 13 14 15 16 17 18 19 FeD (wt%)
Figure 2.3: Total iron as FeD vs. S plot for glasses and melt inclusions from radial vent alkalic lavas. Glasses from Mo'ikeha are distinct with low S «0.04 wt %), similar to subaerially erupted Mauna Loa lavas (Davis et aI., 2003), despite being collected in water depths sufficient enought to inhibit S degassing (Moore and Fabbi, 1971). In contrast, Akihimoana lavas are undegassed. Both Mo'ikeha and Akihimoana lavas have undegassed (>0.09 wt %) glass inclusions in plagioclase and olivine phenocrysts, indicating that degassing occurred after phenocryst formation. Solid lines represent boundary between degassed, partially degassed, and undegassed magmas (Moore and Clague, 1987; Davis et aI., 2003). Analytical error for sulfur is < 2%.
4 3 Mauna Loa, the largest Hawaiian volcano (80,000 kIn ; Lipman, 1995), grew on the flanks of two older volcanoes (Mauna Kea and Hualalai) and has a younger volcano
(KIlauea) growing on its southeast flank (Fig. 1.1). This creates a complicated stress field within the volcano, which apparently allows for the formation of both rift zones and radial vents. No study has been devoted to the mechanics of radial vent formation on
Mauna Loa, however, a few previous studies have noted their presence and attempted to rationalize their development (e.g. Stems and Macdonald, 1946; Lockwood and Lipman,
1987; Walker, 1990; Lipman, 1995; Trusdell, 1995). The only previously known submarine radial vent was active in 1877 and has been the focus of several shallow submarine studies (Normark et al., 1979; Fomari et al., 1980; Moore et al., 1985). Three submarine radial vents were alluded to by Davis et al. (2003), but they were not surveyed, so their existence was in question.
Our study builds on those earlier works by exploring the ten submarine radial vents (including the 1877 eruption), which greatly expand the area of known radial vent eruptions (Fig. 1.1 and 1.2). A new geologic map of the submarine Kealakekua Bay area is presented here based on high-resolution multibeam echo sounding (EM300), photography, and sampling by the Jason2 ROV (Fig. 1.2). Volume estimates for the cones and flows are compared to other known Mauna Loa values. Cone shapes and flow lengths are examined and the affect of volatiles in cone formation is discussed. Multiple parental magmas are required to explain the geochemical variations of the submarine radial vent lavas. Trace element ratios, which are compared to subaerial Mauna Loa lavas, indicate that the submarine radial vent lavas may have erupted during periods of high eruption rates on the volcano.
5 N \ (~~",J~ .. A ~. Kealake~ Bay z ~ <0 N °Ol Big Island
.z N :;;,;.,;;..._~~ ..r~
156°6Q'Q"W 155°56'Q''W °....Ol Figure 1.2: Geologic map of Mauna Loa's western submarine flank near Kealakekua Bay. Stippled patterns are used to distinguish between two adjacent flows but do not represent a change in lithology. See legend for rock types. The Island of Hawai'i is shown in pale green. White areas represent places where no bathymetric data was collected. See text for details on how map was produced. 6 1.2. Mauna Loa's Radial Vents: Defmitions, Post-1832 Eruptions, and Models
1.2.1 Radial Vent Definition
In this study, Mauna Loa's radial vent population includes all vents located outside the volcano's summit and rift zone regions that are oriented radially to the summit caldera. Ifmultiple fissures were produced during a single event then they are considered part of one radial vent eruption. This definition differs from that of
Lockwood and Lipman (1987) in that they counted all eruptions that occurred outside the summit and two rift zone regions, regardless of fissure orientation. Twelve of the 66 radial vents they identified are related to northeast rift zone eruptions and are oriented orthogonal to the rift zone, not radial to the summit. Therefore, these vents are not included in our radial vent count. Lockwood and Lipman (1987) also counted all fissures produced during a single eruption as separate radial vents. For example, there were six different active fissures during the 1859 eruption. They formed two parallel tracks «15 km apart) down the northwest side of the volcano. Lockwood and Lipman (1987) counted these as six separate vents, whereas we consider them part of one episode and therefore, a single radial vent eruption. Using our definition, Mauna Loa has had 44 subaerial radial vent eruptions.
1.2.2 Post-1832 Eruptions
Three radial vent eruptions have occurred since 1832 (1852, 1859, and 1877), which is the first year that a published account of a Mauna Loa eruption appears in the literature (Barnard, 1995). These eruptions occurred during a period of relatively high eruption rates on the volcano (Lipman, 1995). The 1852 eruption had a short phase of
7 radial vent activity that was followed by a larger northeastern rift zone eruption. The
1852 radial fissure broke out at 3,920 m and produced a lava flow that traveled -5 km down the northwestern flank of the volcano. The entire eruptive episode, including the effusion along the northeastern rift zone following the radial vent activity, lasted 21 days, producing an estimated volume of -182 x 106 m3 of lava (Lockwood and Lipman, 1987).
One XRF analysis has been reported for lava from this eruption (Rhodes and Hart, 1995), and it is from the later rift zone flow and not the radial portion.
The 1859 radial eruption began after a short summit eruption that lasted for <1 day (Barnard, 1995). The activity shifted to the northwest flank where eruptions occurred from multiple vents, ranging from 3,375 m to 2,630 m above sea level (Stearns and Macdonald, 1946). The eruption lasted for -300 days (Barnard, 1995) producing a lava flow 51 km in length (the longest on Hawai'i; Rowland and Walker, 1990), which reached the coastline eight days after the eruption began (Dana, 1891). The total volume
6 3 2 of lava produced during the 1859 eruption was 383 x 10 m , covering an area of 91 km and burying the village of Wainanali 'i (Barnard, 1995). There have been limited geochemical studies of this eruption, although a single analysis indicates that the eruption fits Mauna Loa's temporal sequence (Rhodes and Hart, 1995).
The 1877 submarine radial vent eruption in Kealakekua Bay began ten days after a short-lived summit phase «1 day). It was witnessed from land and by passengers aboard the steamer KIlauea, who reported bubbling water, blocks of lava floating to the surface, and the smell of sulfur in the air (Whitney, 1877). Since then, several submersible programs have been conducted in the area to determine the extent of the eruption. These began in 1975, with 15 Seacliff submersible dives, which discovered
8 vent structures and associated extremely glassy pillow lavas (Fornari et al., 1980), and led to the first bathymetric study of the area (Normark et al., 1979). The vents for this eruption were determined to be located at depths of 690 to 1,050 m below sea level
(Fornari et al., 1980). Several additional dives were also conducted in 1979 and 1983
(Moore et al., 1985). The full extent of the flow field from this eruption was not determined from these studies. Lavas from this eruption have been petrographically described and geochemically characterized (Fornari et al., 1980; Moore et al., 1985).
1.2.3 Radial Vent Fonnation Models
Several models have been proposed to explain the origin of Mauna Loa's radial vents. Stearns and Macdonald (1946) originally identified the subaerial radial vents and proposed that they were part of a diffuse northern rift zone. However, a recent gravity survey of the volcano does not support this proposal (Kauahikaua et al., 2000) and subsequent mapping (Lockwood and Lipman, 1987) has shown that the radial vents are non-parallel and too widely dispersed to define a rift zone.
Movement along the Kealakekua Bay Fault was invoked to explain the 1877 submarine eruption (Moore and Clague, 1987). This east-west trending fault is located
-2 km north of the 1877 eruption and makes a sharp bend south -4 km inland. It was thought that an earthquake felt during the night of the eruption may have signaled activity along this fault and caused the eruption in Kealakekua Bay. However, examination of a coral reef that crosses this fault indicates that there has been no offset since reef emplacement (Moore and Clague, 1987) at 13,250 ± 380 years ago (Moore and Fornari,
1984). This suggests that the earthquake was not created by the movement along the
9 fault and instead, may have been related intrusion or emplacement of a dike within the volcano.
A third idea is based on the observation that -20% of the subaerial radial vent eruptions lack near-vent structures, indicating that the erupted lava was degassed
(Lockwood and Lipman, 1987). This observation led to the hypothesis that these eruptions resulted from leaky summit lava lakes (Lockwood, 1995), which is supported by the fact that most of the degassed vents are thought to have erupted during a period of high lava lake activity (1.5-0.75 ka, Lockwood and Lipman, 1987). This model, however, provides no explicit structural explanation for the formation of radial vent fractures.
Walker (1990) offered a model for radial vent formation while attempting to rationalize the curvature of many Hawaiian rift zones. He examined how unequal distributions of magma injection in the proximal versus distal portion of the rifts could create a bend in the once linear rift zones. This theory predicts the migration of Mauna
Loa's upper rift and summit northwestward toward the two buttressing volcanoes (Mauna
Kea and Hualalai) while the lower portions of the rift zones remain immobile. Although it has been suggested that the lower portion of Mauna Loa's southwest rift zone has migrated westward (Lipman, 1980), there is no evidence that the summit has shifted over time. However, the presence of a bend in the rift might create an area of extension on the northwestern flank of the volcano allowing for the formation of a third rift zone or radial vents (Walker, 1990). Any injection of magma into the rift zones will cause compression perpendicular to dike emplacement causing the area inside the acute angle of the bend
(southeastern flank of Mauna Loa) to be in compression and the area outside of the bend
10 (northwest flank) to undergo extension, possibly allowing for radial intrusions (Walker,
1990). This provides an explanation for the presence of the radial vents on the western and northern flanks and their absence on the southeastern flank of Mauna Loa.
Our discovery of nine radial vents on the submarine flanks of Mauna Loa greatly expands the known area affected by this type of volcanism and highlights the importance ofthese vents in the growth of this volcano. Below we document the geology of these vents and the petrology of their lavas.
1.3. Seafloor Bathymetry and Geologic Map
1.3.1 Location and Collection
An area off the western coast of the Island of Hawai'i (Fig. 1.1) was selected for this study after a few conical features were noted in previous bathymetry of the region, although they were not specifically identified in the larger scale study (e.g., Moore and
Chadwick, 1995). A 1999 expedition also noticed and dredged the same three possible cones in the Kealakekua Bay area (Davis et aI., 2003) but was unable to properly survey the region. Eight samples from these cones that were analyzed for whole-rock and glass compositions (Davis et aI., 2003) are included in this study. A new high-resolution survey of the Kealakekua Bay region was undertaken in the fall of 2002 to gain a better understanding of the area's geology (Fig. 1.3). This survey was augmented by photography and sampling using the Jason2 ROY.
A total of ten submarine vent eruptions were mapped during the 2002 cruise and nine of them were given Hawaiian names (excluding the 1877 eruption; Fig. 1.3; Table
1.1), in accordance with Hawaiian traditions. These Hawaiian names tell the story of the
11 156°60'O'W 155°56'0"W
Legend . ealakekua Bay (} Radial vent --100 m contours Bathymetry ? High: -80 m N ? Co N N Co O'l N ° o .....O'l Big Island
z N ~ °.....O'l
z N b N °O'l ... 155°56'0"W Figure 1.3: Bathymetric map underlain by a shaded relief image (Fig. 1.5) of Mauna Loa's western submarine flank near Kealakekua Bay. The approximate locations of the 10 submarine radial vent coness are indicated by a yellow asterisk. The contour interval is 100 m. Illumination is from the northwest. 12 ninth migration of sharks. Nine of the ten vents mapped in the area were sampled. Six of these eruptions were sampled by only the Jason2 ROV, two others were only dredged, and Pa'ao vent was sampled using both methods.
1.3.2 Bathymetry Collection and Processing
The bathymetry and sidescan data were collected using a Simrad EM300 multibeam echo sounder aboard the University of Washington's R/V Thomas Thompson in October and November of 2002. The EM300 operates ata frequency of 30 kHz
(Hughes-Clark et aI., 1998) and is capable of producing ocean floor images in water depths ranging from 10 to 5,000 m (Simrad, 1997). The 30 kHz frequency allows for high-spatial resolution imagery in both shallow and relatively deep (5,000 m) water. The vertical resolution of the EM300 is 0.2% of the water depth or < 5 m for the Kealakekua
Bay area (Simrad, 1997; Hughes Clark et aI., 2004). The Kealakekua Bay region was mapped at a ship speed of 8-10 knots. Processing of the data was done using MB
Systems (Guth et aI., 1987; Guth, 2001) and GMT (Wessel and Smith, 1995). Maps are projected in North American 1983 UTM zone 5 datum and have a spatial resolution of 25 m. ArcGIS software was used to produce bathymetry, sidescan sonar, 3D, slope, and shaded relief images of the study area. These and other maps of the submarine southwest rift zone of Mauna Loa, which were also surveyed during the 2002 cruise, will be made available through the U.S. Geological Survey's open-file report series (Trusdell et aI., in prep.).
13 1.3.3 Geologic Map
Our mapping revealed variety of geologic features on the western submarine flank of Mauna Loa, including volcanic cones and associated lava flows, lavas fields (with no identifiable connection to volcanic cones), volcaniclastic and/or highly sedimented terrain, a subaerially erupted lava flow, slump terraces, and a landslide scar.
Classification of a feature as a cone was based on several criteria. It must have at least one 20m closed contours, relief >50 m, and slopes opposing the western dip of Mauna
Loa's flank (Fig. 1.3 and1.4). Summit depressions were identified on four of the cones
(>10m; Fig. 1.5). The 1877 eruption flow field has a depression, with drainback features surrounded by spatter ramparts seen in Jason2 photography. Similar features were seen during submersible cruises in 1975 to the shallow parts of the 1877 flow field and were described as "primary vents and eruptive centers" (Fornari et aI., 1980). Based on these criteria, there are ten submarine cones in the Kealakekua Bay area. These can be divided into three categories based on classifications of Clague et ai. (2000): seven truncated cones, two irregular cones, and one flat-topped cone.
The extents of Mauna Loa's submarine radial vent lava flows were determined using several parameters including sidescan imagery, bathymetry, shaded relief images, photography, and geochemistry (Table 1.1). Sidescan was particularly useful in determining the full extent of the youthful 1877 eruption and in mapping the contact between the 1877 flow and the neighboring flow (Fig. 1.6). When sampled, rock geochemistry was important in relating the distal portion of the lava flows to their back to their respective cones. These geochemical interpretations are supported by raised topography seen in the bathymetry (Fig. 1.3), and surface roughness in shaded relief
14 156°60'O"W 155°56'O"W
Legend Slope (degrees) High: 75 z ~ OJ N ....°C) Big Island
z ~
N ....°"""C)
z ~ z o ~ N °....C) illiil;E~~~;I~~;~....:.._-iL~~~ J~ 156°60'O''W 155°56'O"W Figure 1.4: Slope map of Mauna Loa's western submarine flank near Kealakekua Bay. Brownish-red represents areas with slopes >20°. Blue represents the shallowest slopes (>5°). Cone locations are indicated by yellow asterisk. Steep slopes surround most of the radial vent cones.
15 156°60'O'W
Legend Sample location ,- Alkalic Tholeiitic Geologic Un" Boundaries '? z N llld'elltlc Conest Co •N N F~ (0 eno N .edkalic Cones} oen 1=lows .,.-...fiIlOW Lava Fields ~baerlally o~inated flow
z ~
N o"*' en......
z ~ N eno ...... 156°60'O"W 1550 6'O'W Figure 1.5: Shaded relief image of Mauna Loa's western submarine flank near Kealakekua Bay with illumination from the northwest. The locations of both alkalic (red stars) and tholeiitic (blue circles) samples collected using the Jason2 are shown. Flow boundaries from the geologic map are overlain.
16 Table 1. Physical characteristics of Mauna Loa's radial vent deposits and flows Ka-wohi- Kahole Kua-o- C B A Feature 1877 Lau'ehu Pa'ao Akihimoana Mo'ikeha Au'aulana Hinamolioli kui-ka- a-kane wakea Flow Flow Flow moana
aElevation (mbsl) -990 -960 -1560 -1375 -1400 -1080 -800 -1540 -1160 -1640 -111 0 -1145 -1120 Cone ht (m) 110 65 375 230 260 150 230 190 100 200 Base diameter (m) 620 440 1600 800 1050 960 1000 1100 700 2100 Top diameter (m) 310 190 400 380 425 500 400 500 580 1200 Aspect ratio (h/B.D) 0.18 0.15 0.23 0.29 0.25 0.16 0.23 0.17 0.14 0.10 bFlattness ratio 0.50 0.43 0.25 0.48 0.40 0.52 0.40 0.45 0.83 0.57 Flow thickness (m) 10 10 20 25 20 20 80 25 -- -- 10 10 15 Flow length (km) 3.6 0.8 1.5 1.1 0.6 4.2 2.8 1.7 0 1.5 2 Area (km ) 7.0 0.2 0.3 1.2 1.1 3.8 1.6 1.6 0.7 3.8 1.5 0.7 5.1 6 3 GIS volume (x 10 m ) 434 10 331 211* 400 163 100 29 344 60 5 206 6 3 d d eVol comparison (x 10 m ) 112 5 330 183* 113 94 99 31 330 Sed. cover (1- 5=highest) 1 na 3 3.5 3 4 5 na 5 na 4 na na I--' ...... :J Relative age from sidescan Black Black Gray Gray Gray Gray White Gray White White White White Black Mn thickness (mm) 0 na 0.029 0.029 na 0.039 0.049 0.078 0.078 0.118 0.049 na na Age from Mn-coating (ka) 0 na 12 12 na 16 20 32 32 47 20 na na Composition Thol ? Thol Thol Thol Alk Alk Thol Thol Thol Thol ?? *volume estimates were combine due to geochemical, physical, and spatial evidence indicated that they were produced during the same eruption aElevation (mbsl) equals the shallowest point bFlatness ratio = summit diamterl basal diamter 3 eCone Vol. (m ) were calculated using the equation for a truncated cone except where indicated. Flows not included in volumes. 3 dCone Vol. (m ) were calculated for entire flow and cone using the equation surface area x flow height 156°60'O"W 155°56'Q"W Legend Backscatter High
low z N Geologic Unit Boundari z C:o ~_.....I Tholeiitic flow •N N ..., field Co °0) N .... ~"~:::::: Alkalic flow °0) H field r!----' lava fields ~i;;;::==: Subaerially. ~'I~---'" originated flow Sediment! volcaniclastics
Big Island
Figure 1.6: Sidescan sonar image of Mauna Loa's western submarine flank near Kealakekua Bay overlain by outlines from the geologic map. Darkest colors represent areas of higher reflectivity (Le. thinner sediment cover).
18 (Fig. 1.5) and sidescan images (Fig. 1.6). These criteria can therefore be used to determine the extent of unsampled lava flows. Thus, any high-standing lava flow (>10 m) that can be traced back to a cone in the bathymetry and shaded relief maps is considered to be related to that cone if the sidescan images confirm this interpretation.
Seven of the vents produced flows>1 km in length, with the longest (4.2 km) originating from Akihimoana vent (Table 1.1).
Surface irregularity is commonly used to distinguish areas of submarine lava, when no other data are available (Moore and Chadwick, 1995; Mitchell et aI., 2002).
Both sidescan images and bathymetric textures can be used to differentiate submarine lava fields from the surrounding sea floor (Moore and Chadwick, 1995). Lava fields are areas with "lobate and hummocky morphology" or "wrinkled and rubbly lava surface"
(Moore and Chadwick, 1995). Following this definition, three submarine pillow lava
2 fields were identified. In the central part of the map area, Flow C (-1.5 km ) was photographed and sampled. The two other flows to the south were identified only from bathymetry, sidescan, and shaded relief images. The larger of these two flows (Flow A)
2 2 covers -5.1 km . The other flow (Flow B) is much smaller (-0.7 km ) but is highly reflective in sidescan images, suggesting it is young. We were unable to link these flows to upslope cones, so they are not considered radial vent eruptions here. Alternatively, these vents could be related to subaerial lavas whose shallower portions have been buried by sediment.
Mauna Loa's western submarine flank, from nearshore to -10 km offshore is characterized by smooth textures in shaded relief imagery (Fig. 1.3 and 1.5). This area is thought to be covered by sediment or volcaniclastics produced by explosions from
19 subaerial lava entering the ocean (Moore and Fiske, 1969; Moore and Chadwick, 1995).
Similar smooth textures were seen elsewhere in the nearshore regions around the island and have been characterized as fragmentally quenched lava (Moore and Chadwick,
1995). Several samples consisting of a mixture of fine-grained carbonate sand and basaltic glass chips (up to 2 cm in size) were collected in our study site by Jason2 using sediment scoops. A thin layer of similar material was seen on top of pillow lavas from the radial vents. Sidescan imagery indicates that the nearshore slope is streaked with channels of sediment debris.
The submarine extension of one subaerial lava flow was identified in the study area. The location of the flow indicates that it is related to the Waiea subaerial eruption
(Trusdell, 2004 pers. comm.). It overlies the Alika 2 landslide scar (Moore and
Chadwick, 1995) at the southern end of the Kealakekua Bay study site (Fig. 1.2 and 1.5).
Bathymetry data indicates that the flow stands 10 -50 m above the surrounding seafloor.
This flow was not sampled during this study.
The Kealakekua Bay area consists of three sediment-covered terraces, which may have been related to the North Kona slump (Moore and Chadwick, 1995). The last major movement of this slump is thought to have occurred prior to 130 ka (Moore and Clague,
1992). Nine of the radial vents sit on top of the terraces. The southern portion ofregion is cut by the Alika 2 landslide (Moore and Chadwick, 1995). The age of Alika 2 landslide has been estimated at 112 ± 15 to 127 ± 5 ka (McMurtry et aI., 1999). The
Pa'ao vent erupted on the Alika 2 landslide scar.
20 1.3.4 Eruption Volumes
Volumes of the volcanic products produced by each submarine radial vent eruption were estimated in ArcGIS using gridded elevation data from the bathymetric map. This was done by creating a plane through the average base height of each cone and estimating the volume of material above the plane for each eruption. This approach worked well for the cones and lava flows erupted on a relatively low angled slopes (i.e.,
Kahole-a-kane or Pa'ao; Fig. 1.4). However, several of the radial vents erupted lava flows that travel over steep slopes, which causes overestimates flow volumes. To minimize this problem, volumes for flows that traveled over a slope of>10° (Mo'ikeha,
Akihimoana, and 1877) were calculated in multiple steps. For example, Mo'ikeha eruption was broken down into three segments for volume calculations, the cone, the portion of the lava flow on flat terrain, and the portion of the lava flow draping the cliff.
The volume of lava produced by the submarine radial vent eruptions varies from 10 x 106
6 3 to 430 X 10 m (± 10 %; Table 1.1) To check the accuracy of the GIS volume estimates, a second set of volume estimates were made using two different methods depending on cone shape. For the flat-topped cone and the truncated cones, the GIS estimates were checked using the equation for a truncated cone (v = (h/3)*At+A2+ -V (At*A2); At=II
2 2 (Dt /4), A2=II (D2 /4), Dl = basal diameter, D2 = top diameter). The volumes determined using this equation are smaller than the GIS estimates because they do not include the associated lava flows. The fact that this method for estimating cone volumes is similar to the GIS estimates for the truncated cones supports the GIS estimated volumes. This equation, however, does not provide a reasonable check for the GIS estimated volumes for the two irregular cones due to their long lava flows. Instead, their
21 GIS volumes are compared to volumes estimated using surface area and average flow thickness (Table 1.1).
1.3.5. Ages
Attempts were made to date seven of the submarine radial vent samples (13-06,
14-04,15-05,17-04,13-13,18-06 and 26-10) using unspiked K-Ar methods, which have proven useful for dating young Hawaiian basalts (e.g. Guillou et aI., 1997; Quane et aI.,
2000). These samples are fresh, holocrystalline, with low vesicularity and relatively high
K20 content for Mauna Loa basalts (0.33 to 0.46 wt %), making them potentially viable candidates for dating. Unfortunately, all of the samples returned zero ages suggesting that they are relatively young. Other attempts at relative dating of these eruptions are discussed in section 1.5.1.
1.4. Petrology
This section describes the petrography and geochemistry of the tholeiitic radial vent lavas, including outcrop descriptions. A more detailed examination of the petrology of 27 alkalic samples is presented in Chapter 2.
1.4.1 Outcrops and Samples
The submarine radial vent eruptions and lava fields are composed primarily of striated bulbous and elongate pillow lavas. They range in color from black to red and have glassy rims of variable thickness. A sheet flow was encountered while sampling the
1877 flow field, and spatter-like ramparts were found at its vent. A total of 71 lava
22 samples were collected using Jason2. The analyses of eight rocks dredged from three vents in 1999 are also included in this study. Sixty-seven samples were collected from the radial vent cones and their associated lava flows, seven samples come from the C flow, and five samples of other flows from an unknown origin on the submarine flanks of
Mauna Loa. Four of these other samples were collected between cones and have distinct compositions. One sample (13-01) is highly vesicular (-29%), has high modal olivine
(-17 %), and was not collected in-situ. It is most likely a subaerially erupted lava that has rolled down slope. This sample is not displayed in geochemistry plots.
1.4.2 Tholeiite Petrography
Modes on 48 Mauna Loa radial vent and eight C flow lavas reveal they are mostly aphyric «1 vol.%), weakly vesicular (average - 1.7 vol.%) basalts (Table 1.2). Among phenocrysts (width 2: 0.5 mm), olivine is the most common (-0.4 vol.%), followed by plagioclase (-0.2 vol.%), with rare clinopyroxene (cpx; -0.1 vol.%). Microphenocrysts
(width 0.1-0.5 mm) are more abundant (olivine - 1.9 vol.%, plagioclase - 3.3 vol.%, and cpx - 2.0 vol.%). Orthopyroxene (opx) was seen in lavas from only one vent (1877) where it occurs as rare microphenocrysts (-0.6 vol. %) and is mantled by a thin cpx rim.
This distinctive texture has been used previously to identify lavas from the 1877 eruption
(Moore et al., 1985). Several of the 1877 lavas show slight petrographic differences.
The three samples from the south arm of the flow contain fewer phenocrysts and microphenocrysts, and are more vesicular than samples from the north arm (Table 1.2).
Ka-wohi-kui-ka-moana lavas have much higher olivine abundances (23-24 vol.%) than any other submarine radial vent lavas. Cpx microphenocrysts are most abundant in Pa'ao
23 Table 2 Petrography of Mauna Loa's submarine radial vents and other submarine lavas in vol % Olivine Plag Cpx Opx Opaq Vent Sample Ves Matrix ph mph ph mph ph mph ph mph mph J2-14-11 <0.1 1.8 0 0.2 <0.1 1.2 0 <0.1 0.2 8 88.4 South 1877 J2-14-14 <0.1 0.8 0.6 0.4 0 0.4 0 <0.1 <0.1 8.8 88.6 J2-14-12 <0.1 0.2 0 0.2 <0.1 0.6 0 <0.1 <0.1 7.8 91 J2-18-12 0.6 0.2 0 0.4 <0.1 3.8 0 <0.1 <0.1 4.6 90.2 J2-18-01 1.2 3 <0.1 1.4 0.2 1.6 0 0.2 <0.1 0.2 91.6 J2-18-05 0.6 0.6 <0.1 2.4 <0.1 2.4 0 0.4 <0.1 0 93.2 J2-18-10 0.8 0.8 0 1.8 0 2 0 1.2 <0.1 1.6 91.8 North 1877 J2-18-11 0.2 1.4 <0.1 1.6 <0.1 0.8 0 1.2 <0.1 3 91.2 J2-18-13 0.2 0.8 <0.1 0.6 <0.1 2.2 <0.1 0.8 <0.1 6 88.8 J2-18-04 1 1.2 0.4 2.6 <0.1 1.2 0 0.4 <0.1 0 92.4 J2-18-03 0.4 0.8 <0.1 1 <0.1 1.2 0 0.6 <0.1 1 94.4 J2-18-02 0.2 1.4 0.2 3 0 2.2 0 0.4 <0.1 0.2 92.4 J2-15-06 0.4 0.6 1 2.2 0.4 3.4 0 0 0 0.2 91.8 J2-15-03 0.2 1.2 0.2 4.2 1.2 2.8 0 0 <0.1 0.8 89.4 J2-15-05 0.2 0.6 0.2 3.8 0 5.2 0 0 <0.1 0.4 89.6 J2-15-04 0 1.2 0.6 7 0.4 3.8 0 0 <0.1 0.6 86.2 Pa'ao J2-15-01 0.2 1 0.6 3 0.6 2.6 0 0 0 0.2 91.8 J2-15-02 0 0.8 1 3 0.8 4.4 0 0 0 0.1 90 *M27-2 0 0.8 0.6 3.4 0.4 3.2 0 0 0 0 91.2 *M27-11 0.2 2 0.6 4.8 1.4 5.8 0 0 0 0 85 J2-13-02 <0.1 1.2 <0.1 5.6 0.2 1.8 0 0 <0.1 1.2 89.8 J2-13-058 0.6 0.8 <0.1 4.8 0 1 0 0 <0.1 0 92.6 J2-13-048 <0.1 1.2 <0.1 5.6 0 1 0 0 0 0 91.8 Kahole-a- J2-13-06 0.4 5.6 0.2 4.4 0 2 0 0 0 0.2 86.8 Kane J2-13-04A 0.2 1 <0.1 10.4 0.4 1.8 0 0 0 0 86 J2-13-05A 0.8 0.8 0.4 7 0 0.8 0 0 0 1.2 88.8 J2-13-03 <0.1 1 0.2 5.6 0 1.6 0 0 0 0 91 Kua-o- J2-13-07 <0.1 5.6 <0.1 6.6 0 1.6 0 0 <0.1 0 85.8 wakea J2-13-08 <0.1 2.9 0.2 2.6 0.4 2 0 0 <0.1 1 90.2 J2-18-07 0 1.7 0 1.4 0 2.8 0 0 0 0.2 94 Hinamolioli J2-18-08 0 0.6 0 2 0 1.6 0 0 0 0.2 95.6 J2-18-06 0 1 0 2.8 0 3 0 0 0 0.4 92.8 Ka-whohi- *M22-11 24.2 2.2 0 0.4 0.8 0.2 0 0 0 71.6 kui-ka- *M22-4 23 3.8 0 0.6 0 0.2 0 0 0 72.4 J2-13-12 0.8 1.8 <0.1 3.2 0 1.6 0 0 <0.1 <0.1 92.4 J2-13-14 1.6 5.6 <0.1 2.2 0 0.8 0 0 <0.1 1 88.2 J2-13-13 1.6 1.8 <0.1 0.8 0 0 0 0 <0.1 0 95.8 J2-13-15 <0.1 4.6 <0.1 10.4 0 2.4 0 0 <0.1 0.8 81.8 CFlow J2-13-11 1.4 5.6 <0.1 1.4 0 0.6 0 0 <0.1 0.2 90.2 J2-13-16 0.2 4.6 <0.1 13.2 0 7 0 0 0.4 0.2 74.2 J2-13-09 0.6 4.6 0.2 3.6 0 1.8 0 0 0 0.6 88.6 J2-14-01 1.2 2.2 0 2.6 0 1.2 0 0 0.2 1.4 91 J2-14-03 <0.1 0.2 0 0.4 0 0.2 <0.1 0.4 0.4 <0.1 98.2 J2-14-02 0.8 0.2 0 0.4 0 0.4 0 0 <0.1 2.4 96 Other Flows J2-14-10 0.2 2.6 0.4 4.6 0 0 0 0 <0.1 0.6 91.2 J2-15-07 2 0.4 1.8 3.2 0.6 3.6 0 0 <0.1 <0.1 88.4 J2-15-08 3 0.8 0.2 4 0.2 2.4 0 0 <0.1 <0.1 88 TJ2-13-01 17.4 0.2 0.4 4.4 0.8 0.2 0 0 0.2 29.4 45.6 based on bUU pt count mo :les; samplies listed In order 0 whole-roCK N glU conetnt *dredge samples (data from Davis 2001); '+sample not taken in place; all other modes by A. Miller 24 lavas, which also have more plagioclase than the other submarine radial vent lavas.
Although most of the submarine lavas have undergone little alteration, 13 samples contained microscopic amounts of manganese-iron (Mn-Fe) coating, ranging in thickness from 0.028 to 0.118 mm (± 0.001 mm). Ka-wohi-kui-ka-moana has the thickest Mn-Fe coating, whereas there is no visible coating on the 1877 lavas. A thin palagonite layer
C'S0.390 ± 0.001 mm thick) occurs under most of the Mn-Fe coatings.
1.4.3 Glass Analyses
Microprobe analyses were conducted for major elements and S on 44 submarine radial vent glasses (Table 1.3) at the University of Hawai 'i, following methods described in Garcia et aI. (1995). All of the glasses are tholeiitic with silica values ranging from
50.8 to 52.9 wt% (Table 1.3), which are typical for subaerial Mauna Loa lavas (e.g.,
Garcia,1996). MgO contents are relatively low (6.5 to 5.1 wt %) in the submarine radial vent glasses. These glass compositions are indicative of eruption temperatures of 1,118 to 1,165 °C based on the MgO geothermometer of Montierth et aI. (1995). Several of the glasses have lower Ah03 (13.2-13.9 wt %) and CaD (9.7-11.0 wt %) than their respective whole rock compositions (Fig. 1.7). These values are consistent with the petrography for these lavas and indicate that the lavas have fractionated beyond olivine control by crystallizing plagioclase and cpx. Their glass vs. whole rock CaDIAh03 ratios show little or no variation for all lavas except those from the 1877 eruption (Fig. 1.7) indicating the proportions of cpx:plag fractionation are similar. Most of the glasses have low sulfur values (0.01 to 0.03 wt %; Fig. 1.8), which is typical of subaerial Mauna Loa lavas (e.g.,
Davis et aI., 2003). The 1877 eruption, however, produced lavas with both undegassed
25 Tabl e 3 M'Icroprobe glass ana yses 0 fMauna L'oa s submanne rad'laI ven and 0 ther submanne avas Vent Label Si02 Ti02 AI20 3 FeO MnO MgO CaO Na20 K20 P20S S Total J2-14-11 51.53 2.85 13.72 12.93 0.16 5.08 9.66 2.52 0.54 0.26 0.135 99.39 South 1877 J2-14-14 51.85 2.47 13.84 11.81 0.16 6.24 10.33 2.41 0.45 0.23 0.117 99.89 J2-14-12 51.51 2.65 13.52 12.63 0.18 5.86 9.88 2.48 0.49 0.23 0.124 99.56 J2-18-12 52.19 2.49 13.24 12.90 0.18 5.87 9.95 2.53 0.46 0.23 0.127 100.16 J2-18-01 52.92 2.59 13.54 12.07 0.19 5.58 9.88 2.59 0.50 0.25 0.075 100.18 J2-18-05 52.10 2.51 13.54 12.02 0.18 5.80 10.03 2.38 0.44 0.26 0.083 99.35 J2-18-10 51.97 2.51 13.10 12.88 0.20 5.92 9.85 2.53 0.47 0.25 0.139 99.83 North 1877 J2-18-11 52.06 2.52 13.14 12.75 0.19 5.98 9.91 2.58 0.46 0.25 0.135 99.99 J2-18-09 52.11 2.43 13.60 11.85 0.16 5.96 10.24 2.40 0.44 0.28 0.134 99.59 J2-18-13 52.34 2.40 13.26 12.80 0.21 5.90 9.92 2.56 0.47 0.23 0.132 100.22 J2-18-04 52.54 2.55 13.24 12.58 0.18 5.90 9.96 2.47 0.49 0.24 0.075 100.24 J2-18-02 52.18 2.59 13.47 12.13 0.19 5.64 9.93 2.42 0.46 0.28 0.069 99.38 J2-15-06 51.04 2.58 13.94 11.86 0.18 6.52 10.90 2.47 0.44 0.22 0.023 100.17 J2-15-03 50.97 2.47 13.94 11.98 0.17 6.62 11.00 2.44 0.43 0.23 0.016 100.27 J2-15-05 50.84 2.52 13.83 12.19 0.17 6.49 10.78 2.47 0.46 0.24 0.022 100.00 J2-15-01 50.96 2.54 13.81 12.22 0.19 6.50 10.86 2.47 0.45 0.23 0.013 100.23 J2-15-02 51.01 2.49 13.95 11.99 0.17 6.53 10.88 2.46 0.44 0.26 0.030 100.20 Pa'ao *M27-01 51.59 2.32 13.75 10.83 0.20 6.37 10.61 2.38 0.46 0.29 0.009 98.81 *M27-02 51.57 2.48 13.48 11.51 0.21 6.29 10.85 2.37 0.43 0.26 0.014 99.46 *M27-11 51.60 2.45 13.40 11.540.17 6.25 10.75 2.41 0.44 0.28 0.013 99.30 *M27-12 51.50 2.49 13.49 11.91 0.16 6.29 10.85 2.43 0.43 0.30 0.014 99.86 *M27-28 52.38 2.47 13.55 10.26 0.14 6.31 10.82 2.40 0.40 0.30 0.009 99.04 *M27-37 52.76 2.21 13.76 10.39 0.19 6.49 10.61 2.46 0.44 0.23 0.006 99.55 J2-13-02 52.09 2.40 13.55 12.03 0.19 6.27 10.65 2.34 0.43 0.23 0.016 100.18 Kahole-a- J2-13-058 51.60 2.39 13.46 12.04 0.19 6.26 10.62 2.37 0.43 0.22 0.013 99.59 kane J2-13-06 51.71 2.41 13.42 12.13 0.18 6.21 10.52 2.36 0.43 0.23 0.012 99.60 J2-13-03 52.10 2.39 13.55 11.99 0.18 6.29 10.66 2.35 0.43 0.24 0.014 100.19 Kua-o- J2-13-07 51.72 2.52 13.32 12.32 0.19 6.06 10.41 2.42 0.44 0.23 0.016 99.64 wakea J2-13-08 52.21 2.50 13.51 12.16 0.18 6.20 10.49 2.27 0.40 0.21 0.010 100.13 *M28-01 52.15 2.65 13.02 12.83 0.19 5.50 9.97 2.50 0.50 0.30 0.014 99.62 *M28-10 51.66 2.70 12.77 12.90 0.21 5.29 9.84 2.57 0.53 0.32 0.013 98.80 Au'aulana *M28-11 52.29 2.67 12.97 12.87 0.22 5.54 9.91 2.47 0.53 0.30 0.013 99.78 *M28-14 52.19 2.66 13.00 12.91 0.19 5.54 9.94 2.46 0.54 0.30 0.014 99.74 *M28-21 52.16 2.36 13.63 10.17 0.22 6.62 10.99 2.38 0.40 0.28 0.016 99.23 Hinamolioli J2-18-06 51.67 2.72 13.11 13.01 0.20 5.55 9.97 2.49 0.52 0.32 0.011 99.56 Ka-wohi- *M22-04 52.69 2.37 14.06 9.94 0.18 6.79 10.77 2.37 0.39 0.23 0.011 99.80 kui-ka- moano *M22-11 52.11 2.49 13.74 10.51 0.18 6.33 10.62 2.48 0.40 0.27 0.011 99.14 J2-13-14 51.02 2.38 13.59 11.91 0.17 6.48 11.02 2.39 0.42 0.22 0.019 99.62 J2-13-15 50.89 2.49 13.57 11.95 0.20 6.43 10.95 2.41 0.44 0.23 0.026 99.58 CFlow J2-13-11 50.80 2.64 13.54 12.17 0.18 6.29 10.83 2.41 0.42 0.22 0.023 99.52 J2-13-09 51.96 2.56 13.41 12.10 0.18 6.15 10.46 2.33 0.39 0.20 0.014 99.75 J2-15-07 52.12 3.00 13.45 12.38 0.17 5.78 9.87 2.60 0.54 0.29 0.018 100.22 Other Flows J2-15-08 51.94 3.06 13.38 12.79 0.19 5.49 9.75 2.65 0.57 0.30 0.018 100.13 +J2-13-01 52.45 2.46 13.65 11.35 0.17 6.48 10.75 2.34 0.40 0.20 0.008 100.24 +sample not collected In place; *dredge samples (data from DaVIS, 2001)
26 0.85 I I 14.51 AI 0 (wt%) CaO/AI203 x Subaerial Radial Vents 2 3
0.80 • o 1877 1\ Pa'ao 0.75 o Kahole-a-kane o Kua-o-wakea + Au'launa ;( Hinamolioli x Ka-wohi-kui-ka-moano 0.7 ,.< i '" I I I I I I 11.bl I I i I I , I x D C Flow CaD (wt%) x other flows 0.6 • Pa'ao Glass • Hinamolioli Glass 0.5 • Kahole-a-Kane Glass • Kua-o-wakea Glass IV 10.5 -...J 1843 • 1877 Glass 0.4 188 0.3 • -w •• • • Mauna Loa K20 (wt%) • 0.2 I, I I f I I , I 9.5 1 K I I I I I I I 1 5 6 7 895 6 7 8 9 MgO (wt%) MgO (wt%)
Figure 1.7: MgO variation diagrams for CaO, A120 3, K20 and CaO/AI20 3 for Mauna Loa's submarine tholeiitic lavas. Fields for Mauna Loa subaerial lavas are black solid lines and for Mauna Loa's subaerial radial vent lavas by the blue field. Whole-rock data are represented by open symbols, whereas corresponding glass data for each flow are shown as shaded symbols. The 1 atmosphere liquid lines of descent (calculated using MELTS; Ghiorso and Sack, 1995) for two suggested Mauna Loa compositional endmembers (1843 and 1880 eruptions; Rhodes and Hart, 1995) are shown. See text for modeling conditions. Several radial vent samples plot along different liquid lines of descent and therefore, these endmembers cannot account for the compositional variability in Mauna Loa's tholeiitic submarine radial vent lavas. Several samples have compositions that lie outside of the suggested 1880 and 1843 endmembers. Data for Mauna Loa fields from Rhodes and Hart (1995), Rhodes, (1995), and Rhodes and Vollinger (2004), and Rhodes (unpub. data). 0.16 ,..------, S (wt%)
o f 0 1877 048 0.12 o ,r Pa'ao 0 Kahole-a-kane undegassed Kua-o-wakea I <> ------0------1+ Au'aulana 0.08 o 0 I D Mo'ikeha o 0 Akihimoana )( Hinamolioli 0.04 )~ ~a;:;i-kui-ka-moanal degassed- -- -ll ------c I•
+ a ~ C 0.00 +----r------.-----,...----.------.----,~ 10 12 14 16 FeO (wt%)
Figure 1.8: Total iron as FeD vs. S in glasses from Mauna Loa's submarine radial vents and other lavas. Dashed lines separate glasses that are partially and strongly degassed as defined by Moore and Clague (1987) and Davis et al. (2003). Most of the radial vent lavas have degassed signatures «0.04 wt%), similar to subaerially erupted Mauna Loa lavas (Davis et aI., 2003) despite being erupted at water depths great enough to inhibit degassing (1000 m; Moore and Fabbi, 1971). However, the 1877 glasses show partially degassed to undegassed signatures (0.04 to >0.09 wt %). Akihimoana alkalic glasses are also degassed. Analytical error for sulfur is < 2%.
28 (0.12 to 0.14 wt %) and partially degassed S values (0.07 to 0.08 wt %). The partially degassed samples were collected further from the vent.
1.4.4 XRF
XRF major and trace element analyses were made on 53 submarine radial vent lava samples (Table 1.4) at the University of Massachusetts following methods described in Rhodes and Vollinger (2004). The weakly phyric nature of most of these rocks (Table
1.2) suggests that the whole-rock compositions are indicative of magmatic compositions.
All of the tholeiitic submarine lavas have major element compositions comparable to subaerial Mauna Loa lavas (Fig. 1.7). Some of the submarine radial vent lavas lie within or near the field for subaerial radial vent lavas in whole-rock major element compositions
(Fig. 1.7). Multiple samples from individual vents and the C flow field cluster together and in many cases are distinct in composition when compared to lavas from other vents.
Kahole-a-kane and Kua-o-wakea (two adjacent cones) are an exception, with very similar geochemical compositions (Fig. 1.3 and 1.7). Most of the submarine radial vent lavas have relatively low MgO and a lower K20 for a given MgO compared to typical summit and rift zone Mauna Loa lavas (Fig. 1.7). Lavas from Ka-wohi-kui-ka-moana have much higher MgO values (15-18 wt %) but are within the subaerial Mauna Loa field (e.g.,
Rhodes, 1995; Rhodes and Hart, 1995). These higher MgO values are consistent with their high modal olivine (23-24 vol. %). Whole rock CaO/Ah03 ratios for the submarine lavas range from 0.71 to 0.84. The most fractionated lavas have the lowest ratios. Rocks with MgO ~7.0 have CaO/Ah03 ratios ranging from 0.74 to 0.80, possibly indicating a range of parental magma compositions (Fig. 1.7).
29 Table 4. XRF whole rock compositional analyses for Mauna Loa's submarine radial vent and other submarine lavas
Vent North 1871 South 1877
Sample J2-14-11 J2-14-14 J2-14-12 J2-18-12 J2-18-01 J2-18-05 J2-18-10 J2-18-11 J2-18-09 J2-18-13 J2-18-04 J2-18-03 J2-18-02 Si02 51.95 52.06 51.89 51.79 51.90 51.88 51.81 51.98 51.81 52.03 51.69 51.76 51.89 Ti02 2.01 2.00 2.00 2.02 2.02 2.01 2.02 2.02 2.00 2.01 2.03 2.01 2.03 AI20 s 13.29 13.44 13.32 13.29 13.34 13.26 13.37 13.31 13.33 13.36 13.36 13.32 13.36 Fe20S* 11.84 11.84 11.88 11.82 11.83 11.96 11.86 11.85 11.81 11.93 11.91 11.81 11.97 MnO 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 MgO 7.60 7.66 7.71 7.55 7.60 7.61 7.62 7.65 7.68 7.68 7.69 7.70 7.71 CaO 10.19 10.26 10.19 10.21 10.22 10.20 10.21 10.20 10.16 10.23 10.21 10.14 10.23 Na20 2.24 2.16 2.18 2.10 1.97 2.02 2.30 2.12 2.17 1.83 2.31 1.94 2.35 K20 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 P20S 0.22 0.22 0.22 0.22 0.22 0.22 0.23 0.22 0.22 0.22 0.22 0.22 0.23 Total 99.87 100.18 99.92 99.52 99.64 99.69 99.95 99.88 99.71 99.83 99.96 99.43 100.32 w Nb 8.2 8.1 8.2 8.1 8.2 8.3 8.3 8.3 8.3 8.2 8.3 8.3 8.2 o Zr 125 124 124 124 125 125 124 125 124 124 126 125 125 Y 23.1 23.1 23.0 23.0 23.3 23.1 23.0 23.4 23.0 23.2 23.3 23.1 22.9 Sr 289 290 287 289 290 290 289 290 289 288 292 289 290 Rb 5.6 5.7 5.6 5.4 5.7 5.9 5.5 5.4 5.7 5.6 5.7 5.6 5.7 Ga 18 19 19 18 18 18 19 18 19 18 19 18 18 Zn 111 110 110 110 111 111 110 110 111 111 110 111 110 Ni 103 100 106 102 108 106 104 109 110 106 104 113 105 Cr 394 396 394 397 402 404 402 404 393 401 393 399 412 V 248 245 246 246 247 250 244 246 252 250 245 249 242 Ce 23 21 23 23 23 24 21 22 24 22 21 23 22 Sa 77 80 77 78 76 77 78 78 76 77 82 81 75 *total iron values in weight percent for oxides, ppm for trace elements Table 4. (Continued) XRF whole rock compositional analyses for Mauna Loa's submarine radial vent and other submarine lavas
Vent Pa'ao Kahole-a-kane
Sample J2-15-06 J2-15-03 J2-15-05 J2-15-04 J2-15-01 J2-15-02 dM27-2 dM27-11 J2-13-02 J2-13-058 J2-13-04B J2-13-06 J2-13-04A Si02 51.01 50.88 50.81 50.88 50.94 50.96 51.0 50.8 51.46 51.51 51.62 51.61 51.45 Ti02 2.15 2.13 2.13 2.11 2.12 2.13 2.13 2.1 2.10 2.09 2.09 2.10 2.09 AI20 3 14.09 14.12 14.05 14.16 14.10 14.15 14.05 14.06 13.79 13.84 13.83 13.88 13.76 Fe203* 11.86 11.85 11.78 11.73 11.78 11.83 11.95 11.82 12.20 12.15 12.13 12.15 12.16 MnO 0.18 0.17 0.18 0.18 0.17 0.18 0.17 0.17 0.18 0.18 0.18 0.18 0.19 MgO 6.69 6.70 6.71 6.73 6.75 6.76 6.97 6.93 6.55 6.55 6.56 6.56 6.58 CaO 11.25 11.22 11.24 11.28 11.26 11.28 11.28 11.28 10.69 10.66 10.71 10.68 10.69 Na20 2.19 2.30 2.10 2.12 2.31 2.27 2.28 2.24 2.22 2.26 2.32 2.35 2.25 K20 0.38 0.37 0.37 0.37 0.37 0.38 0.38 0.38 0.38 0.38 0.38 0.38 0.38 P20S 0.24 0.24 0.24 0.24 0.24 0.24 0.24 0.23 0.24 0.24 0.23 0.23 0.23 Total 100.03 99.97 99.61 99.80 100.03 100.18 100.41 100.01 99.80 99.87 100.05 100.11 99.79 w Nb 8.9 8.9 8.9 8.6 8.9 8.7 8.7 8.4 9.0 9.2 8.9 9.1 9.0 ...... Zr 136 136 136 133 135 134 132 131 135 136 135 135 135 Y 25.3 25.1 25.1 24.8 25.0 24.8 24.6 24.3 24.6 24.7 24.3 24.4 24.3 Sr 306 305 304 306 305 308 301 302 302 302 303 303 304 Rb 5.5 5.9 5.5 5.6 5.7 5.8 5.5 5.3 5.6 5.6 5.7 5.7 5.7 Ga 20 20 19 20 20 20 19 19 19 19 19 20 19 Zn 109 110 110 107 110 106 107 105 113 114 111 112 112 Ni 96 98 100 99 100 97 98 98 79 78 79 78 79 Cr 206 209 223 209 217 201 206 213 158 158 179 156 154 V 268 268 269 265 269 269 252 252 263 269 263 261 264 Ce 24 25 25 21 25 23 22 22 23 24 24 27 24 Ba 81 79 78 79 74 74 71 69 81 82 81 84 81 *total iron; ddredge samples values in weight percent for oxides, ppm for trace elements Table 4. (Continued) XRF whole rock compositional analyses for Mauna Loa's submarine radial vent and other submarine lavas
Vent Kahole-a-kane Kua-o-wakea Au'aulana Hinamolioli Ka-wohi-kui-ka-moano
Sample J2-13-05A J2-13-03 J2-13-07 J2-13-08 dM28-2 J2-18-07 J2-18-08 J2-18-06 dM22-21 dM22-2O dM22-11 dM22-17 dM22-4 Si02 51.71 50.88 51.61 51.61 51.53 51.58 51.64 51.57 49.02 48.75 48.67 48.83 48.47 Ti02 2.10 2.08 2.08 2.09 2.28 2.31 2.30 2.31 1.83 1.80 1.72 1.78 1.65 AI20 3 13.84 13.70 13.83 13.93 13.57 13.69 13.66 13.67 10.6 10.32 10.2 10.15 9.85 Fe20 3* 12.20 12.12 12.08 12.12 12.91 12.89 12.84 12.92 11.87 12.02 12.11 12.04 12.08 MnO 0.19 0.19 0.18 0.18 0.19 0.19 0.19 0.19 0.16 0.17 0.17 0.17 0.17 MgO 6.60 6.61 6.64 6.57 6.21 6.04 6.04 6.13 15.7 16.2 16.9 16.9 18.0 CaO 10.76 10.67 10.68 10.73 10.33 10.31 10.31 10.31 8.21 8.02 7.95 7.82 7.69 Na20 2.42 2.25 2.23 2.35 2.48 2.22 2.32 2.56 1.85 1.78 1.69 1.8 1.61 K20 0.38 0.38 0.38 0.38 0.47 0.45 0.46 0.46 0.31 0.301 0.29 0.296 0.28 P20S 0.23 0.23 0.23 0.23 0.27 0.28 0.28 0.28 0.20 0.19 0.18 0.19 0.18 Total 100.42 99.10 99.94 100.19 100.24 99.95 100.03 100.39 99.75 99.57 99.89 99.99 99.96
W Nb 8.9 8.8 9.1 8.9 10.1 10.4 10.2 10.3 7.6 7.6 7.3 7.4 7.1 tv Zr 135 135 135 134 146 150 149 148 116 114 108 114 104 Y 24.3 24.4 24.2 24.1 25.9 26.8 26.5 26.6 20.7 20.4 19.7 20.2 18.6 Sr 302 304 301 301 322 325 323 324 222 217 216 214 204 Rb 5.6 5.7 5.7 5.8 7 6.9 7.1 7 4.8 4.6 4.2 4.7 4.1 Ga 19 19 20 20 20 20 20 20 15 16 14 15 14 Zn 111 111 111 110 116 122 120 121 107 108 103 112 103 Ni 79 78 80 79 62 65 65 63 713 707 744 787 886 Cr 158 150 172 167 78 85 88 79 949 992 899 988 1023 V 259 264 259 259 273 294 292 288 217 215 203 210 190 Ce 24 23 24 23 24 31 28 29 19 20 18 19 18 Sa 75 86 78 81 89 101 100 94 53 53 53 54 46 *total iron; ddredge samples values in weight percent for oxides, ppm for trace elements Table 4. (Continued) XRF whole rock compositional analyses for Mauna Loa's submarine radial vent and other submarine lavas
Vent CFlow Other Flows
Sample J2-13-12 J2-013-14 J2-13-13 J2-13-15 J2-13-11 J2-13-16 J2-13-09 J2-14-01 J2-14-03 J2-14-02 J2-14-10 J2-15-07 J2-15-08 +J2-13-01 Si02 51.31 51.33 51.16 51.40 51.36 51.26 51.49 51.17 51.46 51.53 49.77 51.57 51.70 48.80 Ti02 2.15 2.16 2.14 2.16 2.15 2.13 2.15 2.15 2.00 1.98 2.79 2.18 2.20 1.72 AI20 3 13.31 13.34 13.26 13.32 13.31 13.25 13.38 13.37 13.43 13.38 13.59 13.93 13.88 10.61 Fe203* 12.25 12.26 12.22 12.24 12.25 12.64 12.28 12.29 12.06 12.05 12.01 11.50 11.64 12.64 MnO 0.18 0.19 0.20 0.18 0.18 0.19 0.19 0.21 0.18 0.19 0.18 0.18 0.19 0.18 MgO 7.32 7.32 7.34 7.34 7.39 7.39 7.43 7.47 7.52 7.73 6.46 7.13 7.13 15.24 CaO 10.55 10.52 10.53 10.52 10.54 10.53 10.54 10.54 10.29 10.27 11.36 10.30 10.31 8.46 Na20 2.16 2.09 2.23 2.16 2.16 2.22 2.19 2.08 2.11 2.35 2.58 2.14 2.16 1.17 K20 0.34 0.33 0.33 0.33 0.33 0.33 0.33 0.33 0.35 0.33 0.53 0.41 0.42 0.27 P20S 0.21 0.22 0.22 0.22 0.22 0.22 0.21 0.22 0.22 0.23 0.29 0.25 0.25 0.18 Total 99.78 99.75 99.64 99.89 99.90 100.15 100.18 99.82 99.62 100.04 99.56 99.60 99.87 99.71 w Nb 8.9 9.2 9.0 9.1 9.1 9.1 9.1 9.0 7.8 7.8 13.4 9.8 9.8 7.0 w Zr 128 128 127 129 128 126 128 126 123 123 191 143 143 104 Y 23.7 24.0 23.5 23.7 23.7 23.5 23.4 23.7 22.9 23.2 30.6 26.0 26.1 19.1 Sr 286 284 286 286 284 287 284 288 292 294 458 304 304 229 Rb 5.2 5.1 4.9 5.1 5 5.1 5 4.7 5.2 4.8 8.3 6.5 6.4 4.3 Ga 19 19 19 19 19 19 19 19 18 19 24 20 20 15 Zn 112 114 113 114 114 110 112 111 110 108 133 109 110 111 Ni 90 87 89 90 91 89 88 90 105 117 108 137 142 590 Cr 393 388 381 391 394 374 394 395 360 381 88 302 304 985 V 269 271 267 266 269 267 268 265 260 264 290 269 269 213 Ce 23 26 24 22 24 23 25 25 23 25 34 26 25 18 Sa 73 71 73 74 73 68 75 74 73 70 122 85 91 63 *total iron; ddredge samples; '+sample not taken in place values in weight percent for oxides, ppm for trace elements The submarine tholeiitic lavas have compatible and incompatible trace element compositions similar to other Mauna Loa lavas, including subaerial radial vent lavas (Fig.
1.9). The submarine lavas plot in the lower part of the Mauna Loa field for K-Cr except for the Ka-wohi-kui-ka-moana (Fig. 1.9). The Hinamolioli and Au'aulana lavas plot towards the lower part of the Mauna Loa field in the Sr plot (Fig. 1.9), which is consistent with petrographic and major element evidence for plagioclase fractionation in these lavas
(Fig. 1.9). The C flow has relatively high Nb and Sr contents.
1.4.5 ICPMS
ICPMS analyses were done on 15 tholeiitic submarine lavas (Table 1.5) at
Australian National University following methods described in Norman et al. (1998).
These submarine lavas, like other Mauna Loa lavas, have light REE enriched patterns
(Fig. 1.10). Although samples with lower MgO tend to have higher REE concentrations
(Table 1.5), the mild fanning nature of the REE patterns cannot be explained by low pressure fractionation of the observed minerals in these rocks (Table 1.2; Fig. 1.10). The sample with the highest light REE content and steepest pattern (14-10) is the most alkaline (alkalinity --1.6 to 3.7 ± 0.2 for the other samples). The fanning patterns and variable alkalinity of the radial vent lavas is a result of variable degrees of partial melting of a gamet-bearing source (e.g., Lanphere and Frey, 1987). Overall, the tholeiitic radial vent lavas show remarkably similar trace element patterns (Fig. 1.11) except for Pb, which ranges from 0.7 to 2.9 ppm (Table 1.5). The high Pb values may be the result of low temperature sea floor alteration (Jochum and Verma, 1996).
34 12 -.------~------__._____, Nb (ppm) 11
10
9
8
7
Sr (ppm) Mauna Loa 350
300
250 01877 L;Pa'ao o Kahole-a-kane 200 <> Kua-o-wakea +Au'aulana ~ Hinamolioli Ka-woh-ku i-ka-moana a C Flow
800
600
Mauna Loa 400 Subaerial Radial Vents 200
Cr (ppm) ~+ O+------,r------r-----.------l 2300 2800 3300 3800 4300 K (ppm) Figure 1.9: K variation diagrams for Cr, Sr, and Nb in Mauna Loa's submarine radial vent and other tholeiitic lavas. Fields for Mauna Loa's subaerial radial vent lavas are shown in blue. The larger field (black solid line) is for all Mauna Loa lavas. Data for Mauna Loa fields from Rhodes and Hart (1995), Rhodes, (1995), and Rhodes and Vollinger (2004), and Rhodes (unpub. data). 35 Table 5. ICPMS analyses, of Mauna Loa's submarine radial vent and other submarine lavas Kahole- Vent 1877 Pa'ao iAu'aulam Hinamolioli Ka-wohi-kui-ka-moana CFlow Other Flow a-k"'nA -Sample J2-18-12 Jl-14-12 *M27-11 J2-15-05 J2-13-06 *M28-02R J2-18-07 *M22-21 *M22-20 *M22-11 R *M22-17 *M22-04F U2-13-1 U1-14-1 CJ2-15-08 Li 4.88 4.76 5.35 5.19 5.11 6.04 6.01 4.95 4.94 4.72 4.98 4.51 4.83 5.71 5.53 Sc 30.6 29.6 32.8 33.0 31.3 34.4 32.4 26.4 26.3 26.1 25.8 24.6 30.9 25.6 31.2 V 250 238 304 274 256 340 298 259 258 257 255 240 261 288 276 Cr 374 378 -- 215 160 n 77 n -- n n -- 362 93 293 Co 65.2 56.6 46.1 62.2 55.6 55.7 65.6 70.4 72.0 83.1 74.0 91.6 60.8 57.3 58.5 Ni 114 114 113 109 85 79 77 773 791 826 844 922 95 127 163 Cu 122 114 125 125 122 130 127 109 108 108 107 103 122 66 112 Zn 107 99 104 105 104 119 118 103 104 104 106 102 103 126 107 Ga 19.3 18.1 20.1 20.3 19.4 21.2 21.0 16.2 16.1 15.9 15.6 15.1 18.8 22.6 20.6 Rb 5.69 5.60 5.88 6.04 5.95 7.54 7.35 4.88 4.81 4.68 4.75 4.43 5.09 8.72 6.75 Sr 297 290 311 322 302 339 337 231 227 232 223 220 288 459 322 V 23.4 22.9 28.6 26.2 24.4 31.0 27.6 24.4 23.9 24.0 23.6 22.4 23.6 30.8 27.3 Zr 124 122 138 138 133 158 150 120 119 119 118 112 125 188 146 Nb 8.4 8.3 9.2 9.4 9.1 11.3 10.7 8.4 8.2 8.3 8.2 7.9 9.0 13.5 10.3 Mo 0.72 0.77 0.66 0.79 1.08 0.77 0.87 0.55 0.51 0.60 0.62 0.70 0.82 0.93 0.80 Cd 0.072 0.066 -- 0.072 0.066 n 0.074 n ------0.065 0.074 0.069 Sn 1.57 1.56 1.55 1.51 1.60 1.77 1.77 1.32 1.32 1.32 1.27 1.26 1.48 2.04 1.57 UJ Sb 0.026 0.026 0.025 0.030 0.031 0.03 0.047 0.025 0.025 0.025 0.023 0.021 0.045 0.046 0.047 0\ Cs 0.062 0.057 0.063 0.066 0.068 0.08 0.084 0.051 0.049 0.051 0.049 0.046 0.058 0.091 0.083 Sa 83.0 76.5 76.8 80.3 83.2 101.9 105.5 60.6 59.8 60.8 59.2 56.7 73.1 117.5 95.0 La 8.6 8.0 9.1 9.2 9.3 11.1 11.3 7.6 7.6 7.5 7.5 7.1 8.6 13.2 10.3 Ce 22.3 20.6 23.5 24.0 23.8 28.2 29.0 19.8 19.9 19.6 19.8 18.5 22.6 33.8 26.6 Pr 3.26 3.06 3.44 3.49 3.59 3.98 4.09 2.84 2.89 2.83 2.86 2.68 3.28 4.89 3.82 Nd 16.7 15.4 17.3 17.7 17.3 19.8 20.4 14.6 14.5 14.6 14.2 13.7 16.2 23.8 19.2 Sm 4.88 4.45 4.88 5.11 4.96 5.61 5.69 4.18 4.22 4.27 4.18 3.94 4.69 6.54 5.45 Eu 1.70 1.59 1.75 1.78 1.73 1.96 1.96 1.43 1.43 1.45 1.43 1.36 1.65 2.26 1.86 Gd 5.36 5.01 5.58 5.66 5.47 6.15 6.23 4.68 4.65 4.85 4.62 4.47 5.18 7.03 5.89 Tb 0.866 0.808 -- 0.912 0.869 n 1.001 -- n n n -- 0.824 1.096 0.946 Dy 4.93 4.62 5.04 5.26 4.98 5.55 5.73 4.43 4.30 4.43 4.22 4.22 4.78 6.20 5.50 Ho 0.96 0.92 1.00 1.04 0.98 1.08 1.12 0.86 0.85 0.86 0.83 0.80 0.94 1.21 1.09 Er 2.37 2.31 2.66 2.55 2.59 2.86 2.77 2.25 2.23 2.24 2.19 2.10 2.41 3.06 2.69 Vb 2.02 1.93 2.07 2.18 2.09 2.26 2.38 1.81 1.79 1.82 1.79 1.71 1.99 2.59 2.33 Lu 0.29 0.27 0.30 0.31 0.29 0.33 0.34 0.26 0.25 0.26 0.25 0.24 0.28 0.36 0.33 Hf 3.23 3.05 3.31 3.42 3.32 3.73 3.81 2.95 2.92 2.90 2.87 2.76 3.10 4.55 3.60 Pb 0.69 0.73 0.86 1.10 1.45 1.09 2.34 0.71 0.71 0.80 0.70 0.75 2.86 1.36 2.36 Th 0.55 0.52 0.59 0.61 0.60 0.70 0.71 0.53 0.53 0.52 0.52 0.48 0.63 0.85 0.69 U 0.194 0.178 0.189 0.206 0.200 0.226 0.244 0.164 0.164 0.165 0.164 0.155 0.209 0.286 0.229 all vaTues In oom: analVSI - M. I'liorman: '"areaae samDies mala Trom uavls ZUU II 50
Q) ;:lc co ~ ~ :.;::; "E 10 "Ca.. Q) -"is. Vent Sample Alkalinity E -e- Ka hole-a-kane 13-06 -1.93 enco C Flow 13-13 -1.94 ---tr- Pa'ao 15-05 -1.90 W -.....l --e- 1877 18-12 -2.28 ~ Hinamolioli 18-07 -1.99 -+- Au'aulana M28-02R -1.68 Ka-wohi-kui-ka- ---*-- M22-17 -3.64 moana Other 15-08 -2.13 - 14-10 -0.87 1 -!-I------i La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Yb Lu Figure 1.10: Primitive mantle normalized REE plot for several Mauna Loa submarine radial vent lavas. The mild fanning of the LREE patterns and crossing HREE patterns cannot be explained by low pressure fractionation alone. These features probably represent small variations in partial melting in the garnet field. The sample with the highest LREE concentrations also has the lowest alkalinity (total alkalies ((Si02 x 0.37)-14.43)). Primative mantle normalizing values from McDonough and Sun (1995). Accuracy and precision were <1 to 2% for all elements based on repeat analyses of several samples. 50 r-.------:
-E 0.. 0.. -c ..o ~ .oJ C Bc 810 "0 ,§ m Eo c .....1877 Q) -il-Pa'ao w ;; 00 C -s-Kahole-a-kane tV E __Au'aulana Q) ...... Hinamolioli > ___ Ka-wohi-kui-ka-moana ..'E 'C -Cflow a.. ___Other
1 Cs Rb Ba U Th Nb La Ce Pb Sr Nd Zr Hf Sm Eu Gd Dy Ho Y Er Yb Lu SC
Figure 1.11: Spidergram of fifteen submarine radial vent and other tholeiitic lavas. Elements are plotted in order of increasing compatibility to the left. Most samples show similar trace element patterns. Primative mantle normalized values from McDonough and Sun (1995), Accuracy and precision were <1 to 2% for all elements based on repeat analyses of several samples. 1.4.6 Isotopes
Pb, Sr, and Nd isotope analyses were completed on 13 tholeiitic and 5 alkalic submarine radial vent samples (Table 1.6 and 2.2) at the University of British Columbia, following methods described in Weis and Frey (2002). The radial vent and other submarine lavas from the western flank of Mauna Loa plot in a restricted portion of the isotope fields for Mauna Loa lavas (Fig. 1.12). On a plot of 206PbPo4Pb vs. 208PbPo4 Pb, the submarine lavas form a small elongate field, whereas for 207PbP04Pb there is one large group and two smaller groups (Fig. 1.12). The high 207PbPo4Pb group consists of two flows of unknown origin, whereas the two samples with low 207PbPo4Pb are from the oldest tholeiitic radial vent lava and an alkalic flow. Nd isotopes ratios also show a somewhat restricted range compared to Sr isotopes, which form two groups (Fig. 1.13).
The smaller group with lower Sr isotope ratios contains the C-flow sample (13-13) and the oldest radial vent lava (M22-4). This group plots with the older Mauna Loa lavas, whereas most of the radial vent lavas plot along a Nd-Sr isotope trend (Fig. 1.13) similar to the 14C dated samples of Kurz et al. (1995).
1.5. Discussion
1.5.1 Radial Vent Ages
The subaerial radial vents cover an age range from 145 years (1859) to -4 ka
(Trusdell, 2004 pers. comm.). Older radial vent eruptions almost certainly occurred but evidence of them has been buried by later eruptions. The more remote location of Mauna
Loa's submarine vents on the western flank of the volcano (-35 km from the summit caldera and> 20 km from either rift zone) may have allowed for the preservation of older
39 Table 6. Pb. Sr and Nd isotope analyses of Mauna Loa's submarine radial vent and other submarine lavas o4 04 Vent Sample # 206Pbfo4Pb 2 sigma 207 pbf Pb 2 sigma 208Pbf Pb 2 sigma 87Sr;86Sr 2 sigma 143Ndl44Nd 2 sigma 1877 J2-18-12 18.15019 0.00159 15.46206 0.00133 37.89987 0.00358 0.703851 0.000007 0.512907 0.000006 J2-14-12 18.15019 0.00234 15.46069 0.00176 37.89552 0.00448 0.703839 0.000007 0.512896 0.000008 Pa'ao J2-15-5 18.13812 0.00164 15.46261 0.00148 37.88411 0.00364 0.703761 0.000008 0.512926 0.000006 *M27-2-3 18.1377 0.0013 15.4615 0.0011 37.8839 0.00280 0.703773 0.000009 0.512936 0.000004 *M27-11-3 18.1362 0.0014 15.4606 0.0013 37.8744 0.00340 0.703773 0.000005 0.512941 0.000008 Kahole-a- J2-13-06 18.14067 0.00194 15.45867 0.00170 37.88714 0.00404 0.703800 0.000007 0.512915 0.000006 kane Kua-o- J2-13-08 18.13963 0.00182 15.45814 0.00166 37.88617 0.00414 0.703821 0.000008 0.512913 0.000006 wakea Au'aulana *M28-2 18.09562 0.00191 15.46318 0.00162 37.88083 0.00408 0.703890 0.000008 0.512887 0.000005
+;:. Hinamolioli J2-18-7 18.09346 0.00170 15.46127 0.00138 37.87608 0.00392 0.703885 0.000006 0.512891 0.000007 o Ka-wohi-kui *M22-4 18.17692 0.00296 15.44970 0.00276 37.90896 0.00634 0.703670 0.000006 0.512959 0.000007 ka-moano
CFlow J2-13-13 18.17895 0.00087 15.47368 0.00085 37.93497 0.00240 0.703684 0.000007 0.512945 0.000010 Other J2-14-10 18.11724 0.00109 15.45774 0.00088 37.86873 0.00240 0.703799 0.000006 0.512919 0.000007 Flows J2-15-8 18.18845 0.00125 15.47180 0.00130 37.93246 0.00326 0.703799 0.000008 0.512935 0.000006 *dredge samples; analyst - D. Wels 15.49...------.
15.48
15.47 Mauna Loa 15.46
15.45
15.44
38.00
37.90
01877 • Mo'ikeha 1::0. Pa'ao + Au'aulana 37.80 o Kahole-ka-moana ::tC Hinamolioli o Kua-o-wakea 0 C Flow • Akihimoana x Other ". Ka-wohi-kui-ka-moana 37.70 +---,------.--..-----~===::;::====:;:::::::===;=====-1 18.00 18.10 18.20 18,30 18.40
206pbP04Pb
Figure 1.12: Pb isotope ratios for Mauna Loa's submarine radial vent and other tholeiitic lavas. These lavas have similar Pb isotope signatures to other Mauna Loa lavas (fields from WeiSt umubl. data). The submarine data form a single elongate field for 208pbP 4pb. In contrast the 207PbP04pb data reveal three separate groups. The higher 207Pb/204pb samples are not from radial vent lavas but from other lavas in the area. The two low 207pbP04pb ratios are from the oldest (-47 ka) submarine radial vent (based on Mn-coating thickness) and from an alkalic vent. Analytical errors are smaller than the symbols for all Pb analyses.
41 0.51300 ,------r==~======il 01877 6 Pa'ao o Kahole-ka-moan o Kua-o-wakea 0.51296 • Akihimoana • Mo'ikeha o ::t( Hinamolioli DC Flow + Au'aulana Ka-wohi-kui 0.51292 ka-moana o x Other o Mauna Loa 0.51288
0.51284
0.51280 -I------r-----.,..------,------1 0.70360 0.70370 0.70380 0.70390 0.70400 87Sr~6Sr
Figure 1.13: Nd and Sr isotopes for Mauna Loa submarine radial vents and other lavas. Nd isotopes show a restricted range and one elongate group. However, the Sr isotope data from two groups. The smaller group with lower Sr isotope ratios contains the C-f1ow sample (13-13) and the oldest radial vent lava (M22-4). This group plots with the older Mauna Loa lavas (Weis, unpubl. data; Kurz, 1995), whereas most of the radial vent lavas plot along a Nd-Sr isotope trend similar to the 14C dated samples of Kurz et al. (1995).
42 radial lavas. In the absence of radiometric ages, we have inferred vent ages from several indirect methods to establish both a relative eruption sequence based on geologic constraints, sidescan sonar, and sediment thickness, and approximate ages from Mn-Fe coatings (using procedures from Moore and Clague, 2004).
The upper and lower age limits for the submarine radial vent eruptions can be constrained by known geologic events. The most recent submarine eruption was witnessed in 1877 and reported in the local newspaper (Whitney, 1877). An upper bound on the age for one of the eruptions, Pa'ao, can be assigned based on its location within a landslide scar that is thought to have formed between 115-127 ka (McMurty et aI., 1999).
Photographs taken with the Jason2 provide a rough estimate of sediment thickness, which can be used to infer the relative ages of the vents (assuming sediment accumulation increases with age). Each eruption was assigned a number (1-5) that describes the amount of sediment draping the lava outcrops (Table 1.1). The vent with the thinnest amount of sediment (A.D. 1877) was appointed value of one; the eruption with the most sediment was allocated a five (Hinamolioli). All other vents were given intermediate values. A second method of relative dating utilized on sidescan data. In these images, lava flows normally appear as zones of high reflectivity (dark; Fig. 1.5), whereas sedimented areas should be less-reflective and have lower backscatter (Appelgate, 1990).
This approach can be complicated by bathymetric variations (cliffs), which can produce a false impression of age due to irregular surfaces (Appelgate, 1990). Also, youthful sheet flows can have low backscatter because of their extremely flat surfaces (Embley et aI.,
1990). Despite these problems, sidescan data are useful indicators of sediment thickness and therefore can be used to determine relative ages (Appelgate, 1990; Hagen et aI.,
43 1990). The submarine radial vents were divided into three categories based on reflectivity: 1. black (low to no sediment-young) 2. gray (intermediate sediment and ages) and 3. white (thick sediment-old). In general, the relative ages assigned from sediment cover matched the order assigned by sidescan reflectivity (Table 1.1).
Mn-Fe coatings have been used previously to estimate ages for Hawaiian basalts
(e.g., Moore et aI., 1982) based on an average rate of growth of -2.5 mm/m.y. around the
Hawaiian Islands (Craig et aI., 1982). These crusts begin to form upon exposure to seawater. Microscopic thicknesses of Mn-Fe coatings (down to 0.015 mm) have proven useful in dating young Hawaiian basalts (6,000 to 380,000 ka; Moore and Clague, 2004).
Thin patches of Mn-Fe coatings were found on lavas from most of the radial vents and flow fields sampled, except those from the A.D. 1877 eruption (Table 1.1). Although errors associated with Mn-Fe thickness dating are not discussed in the previous study, errors based on uncertainties for thickness measurements are reported for the radial vent lavas. The thickest coating was found on a Ka-wohi-kui-ka-moana vent lava (max. 0.118
± 0.001 mm), yielding an age estimate of -47 ka, whereas a sample from Hinamolioli vent had a maximum Mn thickness of 0.078 mm (± 0.001 mm) and an approximate age of 32 ka. The other submarine lavas have thinner coatings (0.029 to 0.049 ± 0.001 mm), corresponding to ages that range from -12 to 20 ka (Table 1.1). The age sequence based on Mn-Fe coatings is consistent with the sequence inferred from sidescan and sediment thickness estimates (Table 1.1). These results indicate that the submarine radial vents may span an age range of 127 years (1877 A.D.) to -47 ka. These results indicate that the submarine radial vent eruptions occurred fairly recently in Mauna Loa's past, and may span a wider age range than the subaerial radial vent eruptions.
44 1.5.2 Surface Area and Basic Geology
The discovery of numerous radial vents on the submarine flanks of Mauna Loa impacts models for the construction of the submarine flanks of Hawaiian volcanoes. For example, a debate exists on the relative proportions of lava flows and fragmental debris that comprise the submarine flank of Hawaiian volcanoes (Moore and Chadwick, 1995;
Garcia and Davis, 2001). Based on bathymetric data collected around the island of
Hawai'i using a combination of single beam hydrographic data, multibeam sonar, and single beam soundings, as well as observations from two recent eruptions from KIlauea volcano, Moore and Chadwick (1995) suggested that the flanks ofHawaiian volcanoes are composed primarily of fragmental debris. During submersible dives on the flanks of
Mauna Loa (at depths of 1-2 km), Garcia and Davis (2001) observed that pillow lavas are predominate, especially in steep sections exposed by landslides. These sections however, are often draped with many centimeters of mud, which would mask the lavas from being detected by remote sensing. Glasses from these pillows are degassed, which led Garcia and Davis (2001) to propose that some Mauna Loa flows are able to cross the shoreline without fragmenting and travel up to 10 km offshore. These lavas are thought to have avoided fragmentation by forming lava tubes, which were observed on the submarine flanks of Mauna Loa (Garcia and Davis, 2001). These tubes would isolate the lava from the surrounding seawater and prevent fragmentation. Lengthy lava flows are also noted by Moore and Clague (1987) off the western coast of the island. Previous studies overlooked the contributions of submarine eruptions, which our bathymetry, sidescan sonar, ROV Jason2 photography and sampling suggest are important in the construction of the submarine flanks of the volcano.
45 The Kealakekua Bay area contains ten volcanic cones with associated lava flows, and three subaqueous pillow lava fields (Fig. 1.2). These volcanic features cover 29 km2 of the Mauna Loa's flank in the study area (Table 1.1). Most of the glasses from these lavas (7 of 9) are degassed. Along the -28 km of the coastline adjoining this area there is evidence of only one coherent lava flow that crossed the shoreline. There are three submarine lava fields of uncertain origin (they have no obvious submarine source vents and any possible landward connection is buried by sediment) that cover 4.5% of the study area. Submarine radial vent lavas cover at least 12.5% of the area suggesting that subaqueously erupted lavas contribute to the submarine growth of Mauna Loa's western flank. Capping these and perhaps other flows is a layer of sediment composed of fine grained calcareous and siliceous sand, with bits of angular, blocky basalt, glass chips, olivine, and limu-o-Pele.
1.5.3 Implications ofSubmarine Lava Volume Estimates
Although only three of Mauna Loa's 39 eruptions since 1832 have been from radial vents, our new volume estimates for the 1877 eruption demonstrate that these eruptions produced a significant portion of the lava erupted during this period. The previous volume estimate for the 1877 eruption concluded that it was only 8 x 106 m3
(Lockwood and Lipman, 1987). Our new bathymetric, sidescan sonar, ROV video and sampling show that this eruption was much larger (-430 x 106 m3 ± 10 %), making it the second largest post-1832 eruption on Mauna Loa, surpassed by only the 1872 summit
6 3 eruption (630 x 10 m ; Lockwood and Lipman, 1987). The 1877 eruption was thought to have lasted for less than one day. However, the extensive flow field related to this
46 eruption (Fig. 1.2 and 1.6) is unlikely to have occurred in a single day given Mauna Loa's typical eruption rate (e.g., 110 m3/sec for the 1984 eruption; Lockwood et aI., 1987) and the range of 4 m3/sec to -1,000 m3/sec for other known eruptions (Rowland and Walker,
1990). Even with the highest inferred eruption rates, the 1877 eruption is likely to have taken at least 5 days and probably much longer.
Our new volume for the 1877 eruption increases the estimate for Mauna Loa's post-1832 eruption rate by -10%, from 29 x 106 m3/yr (Lockwood and Lipman, 1987), to
32 x 106 m3/yr. More importantly, it indicates that the three recent radial vent eruptions
(1852, 1859, and 1877) have played a significant role in the growth of the volcano, accounting for 22% of the volume of lava produced since 1832. Submarine radial vents eruptions tend to be large but highly variable in volume (10 to 430 x 106 m3 ± 10 %;
Table 1). The average lava volume produced by a submarine radial vent is larger than the
6 3 6 3 average post-1832 eruption on Mauna Loa (226 x 10 m ± 10 % vs. 129 X 10 m , excluding the 1877 eruption; Barnard, 1995). Furthermore, few (5/33; 15%) post-1832, non-radial vent eruptions produced volumes greater than 200 x 106 m3 (Lockwood and
Lipman, 1987), compared to most (five of nine; 56%) submarine radial vent eruptions.
Thus, radial vent eruptions are important contributors to the submarine growth of Mauna
Loa.
1.5.4 Degassing History and Lava Flow Length
The sulfur content of basaltic glass has been shown to be a useful indicator of depth of a submarine eruption (e.g., Moore and Fabbi, 1971). Sulfur will remain dissolved in basaltic magmas until degassing, which typically begins at crustal depths of
47 -300 m or water depths of -1000 m for Hawaiian basalts (Garcia, 1996). However, glasses from some submarine rift zone and flank lavas on Kilauea and Mauna Loa have low volatile contents despite their considerable eruption depth (Dixon et aI., 1991; Garcia and Davis, 2001; Davis et aI., 2003). Many of these lavas have glass inclusions in olivine with higher sulfur contents (Davis et aI., 2003) indicating they originally had higher volatile contents. The low S glasses are interpreted as having degassed under lower crustal/water pressures before transportation and eruption on the submarine portion of the rift zone (Dixon et aI., 1991; Davis et aI., 2003).
Most of the glasses from the Mauna Loa submarine radial vent lavas are degassed
(exceptions: 1877 and Akihimoana lavas; Fig. 1.8) despite having been erupted at depths of 900 to 2,200 mbsl, which should have inhibited degassing. It is unlikely that these lavas have subsided significantly (>150) since their formation given the slow rate of island subsidence (-2.5 mmlyr) and the relatively young ages of the lavas «50,000 years; Table 1.1). Many of the subaerial radial vents lack near vent structures, which was interpreted as being indicative of the eruption of degassed magmas (Lockwood and
Lipman, 1987). These subaerial lavas are thought to have been originally erupted and degassed at the summit, perhaps during a period of extensive lava lake activity, and then drained out the side of the volcano (Lockwood and Lipman, 1987). Many of the submarine vents may have a similar history.
The amount of volatiles dissolved in a lava has been positively correlated with the distance a submarine flow can travel (Gregg and Fornari, 1998). The low viscosities of submarine lavas that travel large distances are related in part to the presence of dissolved gasses (Gregg and Fornari, 1998; Clague, 2000). Seven of the Mauna Loa submarine
48 radial vents have associated lava flows, however, only two (1877 and Akihimoana) traveled farther than three km (Fig. 1.2). These two cones produced the only lavas with undegassed glasses. This agrees with the previous studies showing that the presence of dissolved volatiles in the submarine radial vent lavas seems to enhance flow lengths (e.g.,
Gregg and Fornari, 1998).
1.5.5 Radial Vent Cone Shape
Thousands of seamounts litter the ocean floor (e.g. Batiza, 1982) but the vast majority of these volcanic constructs have not, and may never be, sampled. However, many have been surveyed along mid-ocean ridges and oceanic island chains (e.g., Searle,
1983; Batiza and Vanko, 1983; Fornari et aI., 1987; Bridges, 1997; Rappaport et aI.,
1997; Clague et aI., 2000). Various studies have attempted to utilize sonar data (Bridges,
1997; Rappaport et aI., 1997) or sonar combined with limited field work (Fornari et aI.,
1988; Batiza et aI., 1989; Clague et aI., 2000) to infer the origin of seamounts in various tectonic settings. Several different types of volcanic constructs have been identified, including pancakes, starfish shaped cones, ridge mounds, flat-topped cones, upturned soup bowl shapes, and truncated cones.
Multibeam data (Clague et aI., 2000) and Gloria sidescan images (Bridges, 1997) have been utilized to describe a combined total of 524 submarine cones from near the
Hawaiian Islands. Most of these cones (328 or 63%) were categorized as flat-topped
(Fig. 1.14). These pancake or nickel-shaped cones, are characterized by low aspect ratios (height/basal diameter < 0.14), nearly horizontal tops (only 10-20 m of offset from
49 h=265 m ~ Basal Diameter = 1450 m
h = 200 m
/ Truncated ~ Basal Diamter = 1060 m
h = 150 m / Flat-topped ~ Basal Diameter = 2000 m
Cone Sha es Flat-to ed Truncated Pointed aspect ratios <0.14 0.14-0.28 0.11-0.25 f1atnesses 0.01-0.57 0.25-0.57 <0.03 slopes >25 >20 5-15 summit depression sometimes sometimes no de assed es no no
Figure 1.14: Schematic diagram depicting the differences in shape between Hawaiian submarine flat-topped, pointed, and truncated cones. The flat-topped and pointed cones are drawn using the average cone dimensions from Clague et al. (2000). The truncated cone dimensions are an average from the Mauna Loa submarine radial vent, truncated cones.
50 one edge to the other), steep flanks (>25°; Clague et aI., 2000) and a range of flatness ratios (minimum summit diameter to minimum basal diameter) from -0.01 to
-0.57 (Rappaport et aI., 1997). The second most common cones around the islands are
"pointed cones" (aspect ratios of 0.11 to 0.25; Clague et aI., 2000). These cones have pointed tops, low flatness ratios (-0.03), low slopes (5-15°), lack summit craters and are thought to be associated with post-shield alkalic volcanism (Rappaport et aI., 1997;
Clague et aI., 2000). Other types of submarine features that have been associated with the Hawaiian Islands include heaps, truncated cones, shields, and star-shaped seamounts
(Bridges, 1997).
Among Mauna Loa's ten submarine radial vents, which are all related to shield stage volcanism (Chapter 2), only one (Ka-wohi-kui-ka-moana) has the low aspect ratio
(0.1; Table 1.1) and steep flanks of a flat-topped cone. The other nine radial vents have higher aspect ratios from 0.14 to 0.28, flatnesses of 0.25 to 0.57 (excluding Hinamolioli, whose base is covered by the 1877 flows), and steep slopes (Table 1.1). The summits of seven of Mauna Loa's submarine radial vent cones are truncated and four of these contain well-developed summit craters (Fig. 1.5). Therefore, these seven are neither flat-topped nor pointed cones. Their shape may be best described as a truncated cone (Fig. 1.14).
The two irregular cones (1877 and Akihimoana) do not fit in any previously described cone shape categories and may define a different type of volcanic cone on the submarine flanks of the Hawaiian Islands.
Submarine cone shape is thought to be controlled by several parameters, including viscosity of the erupting magma, effusion rate, initial magmatic volatile content, separation of the gas phase, eruption duration, lava composition and water depths (Clague
51 et aI., 2000). The formation of a flat-topped cone is thought to require an effusive, long duration eruption on flat pre-existing slopes with low to moderate effusion rates, low viscosities, and low volatile contents (Clague et aI., 2000). The flat-topped cones can be composed of either alkalic or tholeiitic lava compositions (Clague et aI., 2000). The
Mauna Loa submarine radial vent truncated cones formed under many of these same conditions. They were found at similar water depths, on low angled underlying slopes
(Fig. 1.3 and 1.4). Glass S analyses reveal that both the flat-topped and truncated cones erupted lavas that are degassed (Fig. 1.8). Although this characteristic may be important in the formation of both cone shapes, the distinction between these two cone types is unlikely to be related to volatile content. The viscosity of the truncated cone and flat topped cone lavas may be similar (20 to 170 Pa s; Clague et aI., 2000), given their similar major element compositions and volatiles, and their low crystallinity. Therefore, the development of a truncated cone vs. a flat-topped cone cannot be due to any of the parameters discussed above. Perhaps, the distinction between the formation of these two cone shapes is in their effusion rates and/or eruption duration, two parameters which are unknown for the Mauna Loa submarine radial vent eruptions.
In contrast to the degassed flat-topped and truncated cones, pointed and irregular cones are thought to be products of undegassed magmas (Clague et aI., 2000). Pointed cones around the Hawaiian Islands have circular bases and are though to be composed of alkalic lavas (Clague et aI., 2000). These undegassed magmas undergo gas exsolution upon eruption, which may cause a more vigorous eruption and produce fragmental ejecta around the vent (Clague et aI., 2000). There are no pointed cones in the study site however the undegassed lavas of Mauna Loa's western submarine flank instead formed
52 irregular cones with extensive lava flows (Fig. 1.2). Unlike pointed cones (Clague et aI.,
2000), the low vesicularity of the irregular cone lavas (0.2 to 8 vol. %; Table 1.2) suggests that the volatiles in the irregular cone lavas remained dissolved in the lava during eruption. The presence of these dissolved volatiles decreases the viscosity of the lavas, allowing longer submarine flow lengths (Gregg and Fornari, 1998), suggesting that the presence of volatiles hinders the formation of symmetrical cones.
1.5.6 Correlations between Radial Vent Activity and Eruption Rates on Mauna Loa
Lockwood and Lipman (1987) suggested that during the last 4,000 years subaerial radial vent eruptions are more common during periods of decreased volcanic activity.
This is based on the mapping of almost all the surficial Holocene lava flows on Mauna
Loa. Thus, the submarine radial vent eruptions might also be expected to have occurred during these intervals. Because our age estimates for most of the submarine radial vent eruptions are pre-Holocene, (Table 1) too long ago for detailed knowledge of eruptive activity to be known, they cannot be related to a specific type of activity on the volcano.
However, fluctuations in Mauna Loa's eruption rate are thought to be correlated with variations in lava trace element compositions during the last 150 years (Rhodes and Hart,
1995). The more enriched (higher K/Y, Sr/Y, LalYb) of Mauna Loa's two source components (1880 lavas) dominates during periods of low eruptive activity and a more depleted component (1843 lavas) dominates during periods of higher activity (Rhodes and Hart, 1995). Similar trace elements vs. eruption rate variations were noted for
KIlauea (Pietruszka and Garcia, 1999). Trace element ratios for the submarine radial vent lavas are relatively low (Fig. 1.15), indicating domination by a more depleted source
53 17 SrlY
16
1843 15
14
13 01877 6. Pa"ao cgIJ o Kahole-a-kana 12 0 o Kua-o-wakea +Au"aulana 188 )l( Hinamolioli 11 Ka-woh-kui-ka-moana a C Flow 10 100 110 120 130 140 150 160 170 180 KJY Figure 1.15: SrlY vs. KN for Mauna Loa submarine lavas. The field for the subaerial radial vents is shown in blue. The thick solid black line represents the field for Mauna Loa lavas. Two endmember compositions for post-1832 Mauna Loa lavas are shown as blue circles (1880 and 1843; Rhodes and Hart, 1995). The 1880 endmember, which is thought to represent periods of high eruption rates, has low SrlY and KN ratios. The red box represents compositional ranges that are thought to be dominated by lavas formed during higher eruption rates (Rhodes and Hart, 1995). Most of the submarine lavas fall within this box. However, lavas from Hinamolioli and Au'aulana vents have more intermediate KIY ratios. The picritic lavas from Ka-wohi-kui-ka-moana have SrlY compositions lower than any other submarine radial vent lava.
54 (1880) and suggesting eruptions occurred during periods of higher eruptive activity.
6 3 Furthermore, the more voluminous submarine radial vent eruptions (> 300 x 10 m ) have low K/Y ratios «132; Table 1.1). These results support the Rhodes and Hart (1995) hypothesis that a more depleted source is tapped during periods of high eruptive activity.
1.5.7 Compositional Heterogeneity and Temporal Trendsfor Submarine Mauna Loa
Lavas
Mauna Loa's submarine radial vent lavas span a relatively wide age (117 years to
-47 ka) and major element range (alkalic to tholeiitic; Table 1.1; Figure 1.8, 1.12, and
1.13). In addition to the two compositionally distinct alkalic lavas (Chapter 2), the two most diverse tholeiitic, major element compositions, the 1877 and Pa'ao lavas (Fig. 1.7), require distinct parental magmas. To evaluate the fractionation trends for Mauna Loa magmas and determine how many distinct parental magmas may be represented by the submarine radial vents, we modeled two lavas that are thought to span the post-1832 compositional range for Mauna Loa (1843 and 1880; Rhodes and Hart, 1995), using the
MELTS program (Ghiorso and Sack, 1995). Modeling parameters included low amounts of water (0.16 wt % to accommodate the degassed nature of most of the radial vent lavas fractional crystallization, low pressure (1 to 10 bars), and Quartz Fayalite Magnatite
(QFM) buffer oxygen fugacity, with 1DC fractionation steps. Although the two post-1832 parental lavas span a wide range of K 20 (Fig. 1.7), trace elements (Fig. 1.13), and isotope ratios compared to the Mauna Loa radial vent lavas, their predicted fractionation trends have relatively similar and low CaO/Ah03 compared to the more MgO-rich radial vent and other submarine lavas (Fig. 1.7). MELTS modeling with slightly higher water
55 contents (0.3%) produced better fits for Ah03 trends but do not explain the large
CaO/Ah03 difference between the predicted crystallization trends and many radial vent lava compositions (Fig.1.7). Thus, the submarine radial vent lavas with distinct
CaO/Ah03 ratios were derived from parental magmas other than the 1880 and 1843 endmembers (Rhodes and Hart, 1995). Several submarine radial vent lavas that have similar CaO/Ah03 also have distinct Sr isotope and trace element ratios (Fig. 1.7, 1.9, and 1.13), which requires different parental magmas. This indicates that the submarine radial vent lavas probably were derived from numerous independent magmatic pulses, which may be a consequence of their possible formation over the last -47,000 years.
1.5.8 Evidence ofLow-pressure Crystallization
The glassy margins of the nearly aphyric Mauna Loa radial vent lavas allow us to examine their late-stage crystallization trends. Olivine is expected to be the first mineral to form in most Hawaiian tholeiitic magmas (e.g., Wright, 1971) but the next phase to form at low-pressures is dependant upon the magma's bulk composition. For Mauna Loa compositions, one atmosphere experiments found that after olivine, the second phase to crystallize is plagioclase (at -1160°C), followed by cpx (- 1150°C) and pigeonite (
1140°C; Montierth et aI., 1995). The petrography of most of the radial vent lavas suggests a similar sequence, although no pigeonite was identified (Table 1.2). This order of crystallization was also supported by MELTS (Ghiorso and Sack, 1995) modeling calculations using a Mauna Loa parental endmember magma composition (1843; Rhodes and Hart, 1995). The 1877 lavas, however, follow a different crystallization sequence with opx crystallizing as the second phase.
56 The early appearance of opx in the crystallization sequence for the 1877 lavas is probably related to their somewhat different bulk composition compared to the other radial vent lavas. The 1877 lavas have relatively low CaO and somewhat lower
CaOlAh03 based on whole-rock analyses (Fig. 1.7), which we consider to be indicative of magmatic compositions based on the scarcity of phenocrysts in these lavas (Table 1.2).
Attempts at modeling the magmatic history of the 1877 lavas were made using both the
1880 and 1843 parental compositions. The 1880 parent, which has major element compositions similar to the 1877 lavas, followed a similar crystallization sequence as the
1877 compositions. This modeling predicted opx as the second phase to crystallize. In contrast, no variations in the MELTS modeling parameters (oxygen fugacity, pressure, and water content) predicted opx as the second phase to crystallize using the 1843 parental composition. The 1843 parent has slightly higher CaO and Ah03 than the 1880 lavas but lower Si02• Thus, the small difference in bulk composition is sufficient to shift the opx cotectic to allow it to crystallize second in the 1877 lavas.
1.6. Summary
The discovery of nine new vents on Mauna Loa's submarine western flanks has increased the known radial vent population on the volcano by 17%. These vents possibly span a wider age range (127 years to -47 ka) than exposed subaerial radial vent lavas «
4 ka). Our detailed geologic map indicates that Mauna Loa's western submarine flank has undergone extensive volcanism and produced -2 x 109 m3of lava. This map also shows that the 1877 lava flow field is much more extensive than previously thought.
57 The collection of detailed bathymetric data has allowed for in-depth studies of the physical characteristics and geology of the Kealakekua Bay radial vent cones. This area contains three different types of submarine cones including flat-topped, truncated, and irregular cones. While flat-topped cones are common around the Hawaiian Islands, only one was found on the west flank of Mauna Loa (Ka-wohi-kui-ka-moana). Instead, truncated cones are the preferred cone shape. These cones have larger aspect ratios than flat-topped cones (0.14-0.28 vs. <0.14), larger flatness ratios than pointed cones (0.25
0.57 vs. <0.3), steep slopes (>20°), degassed lavas, and often summit depressions. The presence of volatiles in the radial vent lavas appears to increase lava flow lengths and inhibit cone growth, resulting in the irregular cone shape. Therefore, it appears that volatile content directly influenced the shape of the submarine radial vent cones.
Mauna Loa's submarine lavas underwent low-pressure crystallization. All of the submarine radial vents followed a similar low-pressure crystallization sequence (olivine followed by plagioclase and then cpx), except for the 1877 lavas. The second phase to crystallize in the 1877 lavas was opx, which is a similar to MELTS modeling predictions for the 1880 lavas. This difference in the crystallization sequence thought to be related to slight differences in bulk composition. Multiple different parental magmas are required to account for the compositional variations in the submarine radial vent lavas. The high
K/Y, Sr/Y and LalYb ratios of the submarine radial vent lavas may indicate that these lavas were erupted during periods of high eruptive activity on the volcano.
58 CHAPfER 2: SIDELD STAGE ALKALIC VOLCANISM ON MAUNA WA VOLCANO, HAWAI'I
2. ABSTRACT Alkalic lavas have been discovered at two young (-15-20 ka) submarine radial vents on the western flank of Mauna Loa, a volcano that was thought to have produced only tholeiitic lavas for at least the last -240,000 years. Major and trace element data show that the alkalic lavas were formed by variable degrees of partial melting and moderate pressure (-1.0 GPa) fractionation of clinopyroxene. Clinopyroxene fractionation is also indicated by MELTS modeling and phase diagram projections.
Although lavas with similar compositions have been erupted from neighboring Mauna
Kea and Hualalai volcanoes, Pb isotope and trace element data indicate that the submarine alkalic lavas were produced from the same source generating Mauna Loa tholeiites. Thus, these are the first alkalic lavas to be identified from this volcano. The major element composition of the vast majority of Mauna Loa lavas has remained tholeiitic throughout the Holocene and recent eruption rates are relatively high. Thus, the volcano has probably not entered the postshield stage of Hawaiian volcanism, which is characterized by alkaline lavas and less frequent eruptions. The occurrence of the alkalic lavas only at radial vents may indicate that these magmas bypassed the primary conduit that supplies the summit reservoir with predominantly tholeiitic magma.
59 2.1. Introduction
3 Mauna Loa, the largest volcano on earth (-80,000 km , Lipman, 1995), has been producing tholeiitic lavas for hundreds of thousands of years during its shield building stage of volcanism. Thus, the discovery of two young alkalic cones and associated lava flows on the volcano's western flanks was completely unexpected (Fig. 2.1). These alkalic cones, Akihimoana and Mo'ikeha, lie in a field of ten radial vents, the other eight of which are tholeiitic in composition (Chapter 1). Both of these alkalic vents are relatively young «20 ka), based on the thickness of Mn-coatings and sparse sediment cover. Attempts to date these flows using the K-Ar unspiked method, which has been successful for some young Hawaiian lavas (Guillou et aI., 1987), have been unsuccessful
(H. Guillou, 2004, pers. comm.).
Hawaiian volcanoes typically experience four phases of volcanism during their lifecycle; the preshield, shield, postshield, and the rejuvenation stages (Clague and
Dalrymple, 1987), although evidence for the pre-shield stage is limited to only L5'ihi lavas. The appearance of alkalic lavas on other Hawaiian shield volcanoes is thought to signal the end of the shield building stage of volcanism and the beginning of the postshield stage, when eruptions are less frequent and mostly alkalic in composition
(Macdonald et aI., 1983). The switch from tholeiitic to alkalic lavas in Hawai'i is interpreted to reflect a gradual decrease in the amount of partial melting as the volcano drifts away from the center of the plume (e.g., Frey et aI., 1990).
On some other oceanic islands, tholeiitic and alkalic volcanism are coeval.
Examples include the Galapagos Islands (Naumann and Geist, 1999; Geist et aI., 1998),
60 156"W 155"W
20' N
Akihimoanlo
00& o~ 0*/ Mo'ikeha
19' N
o 10 20 30 . IIIIIII Kilometers
Figure 2.1: Map of the island of Hawai'i showing its five volcanoes, the locations of their rift zones (thick lines) and summit calderas (Peterson and Moore, 1987). The locations of Mauna Loa's 44 subaerial radial vents are drawn as short dashes on its northern and western flanks. Submarine tholeiitic radial vents on the western flank of Mauna Loa, including the location of the 1877 eruption are shown as open circles. The two vents that produced alkalic lavas are noted by asterisks.
61 Kerguelen (Damasceno et aI., 2002) and Reunion Island (Albarede et aI., 1997). At these islands, the occurrence of alkalic lavas is explained by moderate (0.5 to 1.5) to high pressure (>1.5 GPa) fractionation of clinopyroxene. This mechanism was also advocated by Murata (1960) and Macdonald (1968) for the origin of some Hawaiian alkalic lavas.
Here, major, trace element, and isotopic data are used to evaluate the volcanic source for the alkalic radial vent lavas on the submarine western flank of Mauna Loa, and to examine their petrogenesis. We show that the alkalic lavas have geochemical signatures characteristic of Mauna Loa, and present evidence that both moderate pressure cpx fractionation and lower degrees of partial melting played a role in their petrogenesis.
2.2. Regional Geology
Mauna Loa is one of five subaerial volcanoes that make up the island of Hawai'i.
It has erupted 39 times since 1832 (when the first written accounts appear, Barnard,
1995) and produced only tholeiitic lavas (Rhodes, 1995). Like most Hawaiian shield volcanoes, Mauna Loa has two primary rift zones (Fig. 2.1). Additionally, 44 vents
(Trusde1l2004, pers. comm.) are radially distributed on the western and northern subaerial flanks of the volcano, including three post-1832 eruptions (1852,1859 and an
1877 submarine eruption; Fornari et aI., 1980; Macdonald et aI., 1983). Mauna Loa is the only Hawaiian volcano known to have radial vent eruptions during the shield building stage of volcanism (Lockwood and Lipman, 1987). The 1877 eruption was the only known submarine radial vent prior to a 1999 dredging cruise on the western flank of
Mauna Loa, when three of the ten cones in the area were discovered and sampled (Davis et aI., 2003). The other six cones were unknown until our 2002 survey.
62 The ten submarine radial vents are almost equidistant from the summit of both
Mauna Loa and Hualalai volcanoes (Fig. 2.1). Hualalai volcano is in its postshield stage of volcanism. The entire subaerial portion of the volcano has been resurfaced with alkalic lavas (Clague et aI., 1980; Moore et aI., 1987) with ages between 203 years (1801
A.D.) and 105 ka (Clague, 1980). The newly discovered submarine alkalic vents lie 17
20 km west of Hualalai's southeast rift zone (Fig.2.1). Thus, one of the goals of this study is to determine which shield volcano was their source.
2.3. Location and Sample Collection
A detailed multibeam survey of the region was conducted off the west coast of the island of Hawai 'i in the fall of 2002 (Chapter 1). This survey revealed a total of ten submarine radial vents on the flanks of the volcano, two of which are alkalic. Twenty seven samples were collected from the two alkalic radial vents using the ROV Jason2.
2 Akihimoana vent, with a summit at -1,080 mbsl, has a surface area of 3.8 km , erupted
400 x 106 m3 (± 10%) of lava, and stands 160 m above the surrounding seafloor.
2 Mo'ikeha vent, with a summit at 780 mbsl, has a total surface area of 1.6 km , erupted
163 x 106 m3 (±1O%) of lava and stands 280 m above the surrounding seafloor. It produced a lava flow -60 m thick.
2.4. Petrography and Geochemistry Results
2.4.1. Petrography
The submarine alkalic lavas are petrographically simple, with sparse olivine and plagioclase phenocrysts. The Akihimoana vent lavas contain 1.2 vol. % olivine
63 phenocrysts (> 0.5 mm) and 0.9 vol.% plagioclase phenocrysts, based on 500 point count modes of six samples (Table 2.1). Lavas from Mo'ikeha (19 samples examined) have somewhat lower amounts of olivine (0.4 vol. %) but similar plagioclase abundances as
Akihimoana lavas (Table 2.1). Most plagioclase and olivine phenocrysts are euhedral to subhedral, although some show moderate signs of resorption. No pyroxene phenocrysts or microphenocrysts were observed in lavas from either vent. Vesicularity is low « 0.1
4.0 vol.%) in these predominantly glassy lavas (65-87 vol.%; table 1).
2.4.2. Glass Analysis
Fifteen alkalic glasses were analyzed for major elements and sulfur (Table 2.1) using the University of Hawai'i Cameca SX-50 electron microprobe following methods described in Garcia et al. (1995b). The Mo'ikeha glasses have lower SiOz and higher alkalis than the Akihimoana glasses (Fig. 2.2; Table 2.1) but both groups of glasses are alkalic (Fig. 2.2). The Mo'ikeha glasses are hawaiites with high total alkalies - 5.5 wt%
(Fig. 2.2). Compared to other Mauna Loa lavas, the alkalic glasses have low MgO « 4.9 wt %) and CaOlAlz03 ratios « 0.65) but high FeD (14.8-15.5 wt %) and TiOz (4.0-5.4 wt
%). Mo'ikeha vent glasses also have low sulfur values « 0.04%; Fig. 2.3), similar to subaerially erupted Hawaiian basalts (e.g., Swanson and Fabbi, 1973; Davis et aI., 2003).
Akihimoana glasses and those from the nearby (tholeiitic) 1877 eruption (Moore et aI.,
1985) have higher sulfur contents (> 0.09 wt %; Fig. 2.3), which are typical for lavas erupted at >1,000 m water depths (Moore and Fabbi, 1971). Glass inclusions in olivine and plagioclase phenocrysts were analyzed to determine if the Mo'ikeha magmas degassed prior to or during eruption. The phenocrysts contain melt
64 Table 2.1. Petro ra h ,micro robe, and XRF anal ses of the alkalic radial vents Vent Mo'ikeha Akihimoana Sample J2-17-05 J2-17-10 J2-17-14 J2-13-10 J2-14-04 J2-14-07 J2-14-08 Petrography Oliv ph 0.2% 0.0% 0.4% 0.4% 2.4% 0.8% 1.4% 1.6% Oliv mph 3.0% 4.0% 4.0% 5.4% 1.8% 5.8% 3.4% 5.0% Plag ph 1.0% 0.2% 0.8% 1.6% 4.4% 1.4% 1.2% 0.2% Plag mph 15.4% 8.2% 10.2% 12.6% 4.8% 14.4% 11.6% 20.2% Opaques 0.2% <0.1 0.4% <0.1 <0.1 <0.1 <0.1 <0.1 Ves 0.2% <0.1 <0.1 <0.1 <0.1 3.2% 4.0% 2.0% Matrix 80.0% 87.4% 84.2% 79.8% 85.0% 73.0% 77.8% 69.6% Glass Si02 45.97 46.11 45.93 45.79 47.69 47.38 47.42 Ti02 4.94 4.89 4.90 5.42 4.01 4.14 4.27 AI203 13.96 14.24 14.07 13.37 13.47 13.23 13.08 FeO 15.15 14.83 14.87 15.46 15.01 15.18 15.30 MnO 0.17 0.17 0.19 0.20 0.21 0.21 0.20 MgO 4.59 4.68 4.74 4.33 4.95 4.87 4.80 CaO 9.15 9.12 8.91 9.00 9.72 9.75 9.80 Na20 4.14 4.24 4.19 4.19 3.35 3.27 3.19 K20 1.24 1.16 1.18 1.44 0.77 0.81 0.79 P205 0.64 0.59 0.59 0.64 0.49 0.49 0.49 S 0.03 0.02 0.02 0.02 0.13 0.14 0.10 Total 99.98 100.05 99.59 99.87 99.79 99.47 99.44 XRF Malors Si02 46.695 46.54 46.56 46.91 48.32 47.493 47.28 47.08 Ti02 3.512 3.432 3.507 3.495 3.12 2.898 2.878 2.844 AI203 15.74 15.66 15.61 15.73 14.82 15.51 15.45 15.42 Fe203 14.07 13.89 14.19 13.99 13.93 13.81 13.84 13.73 MnO 0.16 0.164 0.17 0.17 0.20 0.18 0.183 0.183 MgO 5.92 6.22 6.01 6.06 5.40 6.54 6.83 6.78 CaO 8.80 8.58 8.85 8.59 9.77 9.62 9.35 9.55 Na20 3.68 3.86 3.63 3.71 3.20 2.98 3.14 3.23 K20 0.87 0.847 0.87 0.89 0.63 0.63 0.594 0.587 P205 0.54 0.528 0.57 0.54 0.40 0.38 0.377 0.366 Total 100.00 99.72 99.96 100.08 99.77 100.05 99.92 99.77 XRF Traces Rb 12.3 12.1 12.8 12.5 9.3 8.8 8.4 8.3 Sa 186 182 185 178 137 126 113 119 Nb 20.1 19.5 20.1 19.8 14.5 13.5 13.3 13.2 La 18 17 19 18 13 11 13 11 Ce 50 50 54 49 39 37 35 34 Sr 663 648 659 665 414 459 452 459 Zr 263 259 267 264 208 192 189 189 Y 28.0 27.9 29.7 28.2 35.5 30.9 30.2 30.0 Zn 162 162 163 160 140 134 136 133 V 263 265 273 262 326 293 289 284 Ni 93 103 99 95 61 108 113 108 Cr 71 88 84 72 87 86 89 94
65 6.0 i • Mo'ikheha whole rock Mo'ikeha glass Akihimoana whole rock ~ 5.0 - Akihimoana glass o ~ + Kilauea alkalic glass -o '::t:,N4.0 + o N # ro Z 3.0
Mauna Loa
2.0 45 46 47 48 49 50 51 52 Si02 (wt%) Figure 2.2: The total alkalis versus silica classification diagram (alkalic-tholeiitic line from Macdonald and Katsura, 1964) for Mauna Loa tholeiites, lavas and glasses from the two alkalic radial vents, and glasses from Kilauea alkalic lavas. The alkalic lavas (> 3.5 wt % total alkalies) are distinct from the other Mauna Loa samples. Kilauea glass data from Johnson et al. (2002). Data for Mauna Loa fields from Rhodes and Hart (1995), Rhodes (1995), and Rhodes and Vollinger (2004), and Rhodes (unpub. data).
66 156" N 155' N N A
2(1 N
o 10 20 30 . 1go N IIIIII, kilometers
Figure 1.1: Map ofthe island of Hawar i showing its five subaerial volcanoes, the location oftheir rift zones (thick black lines) and summit calderas (after Peterson and Moore, 1987). Mauna Loa's 44 subaerial radial vents are shown as short dashes on its northern and western flanks. The locations ofthe 10 submarine radial vents are shown as circles. The rectangle indicates the study site adjacent to Kealakekua Bay and the area of bathymetric data and geologic mapping shown in figures 1.2-1.6. The gray field shows the boundary ofsubaerial Mauna Loa lavas.
67 inclusions with higher sulfur values (0.05 to 0.20 wt %). This suggests that degassing of the Mo'ikeha lavas occurred after crystallization of the phenocrysts phases but prior to the eruption. All of the Akihimoana glass inclusions were undegassed.
2.4.3 XRF
XRF major and trace element analyses were made on 27 samples (Table 2.1) at the University of Massachusetts using methods described by Rhodes and Vollinger
(2004). These analyses confirm the alkalic nature of the lavas from the two cones. The lavas have higher total alkalis (Na20+K20 =3.1-4.8 wt %) at a given Si02 than previously observed for Mauna Loa lavas (Fig. 2.2), placing them well above the alkalic/tholeiitic line of Macdonald and Katsura (1964). The alkalic lavas are also distinct from Mauna Loa tholeiites in their lower CaO/Ah03 (Fig. 2.4) and higher Ti02
(Table 2.1).
Incompatible element abundances are higher in the alkalic versus the tholeiitic submarine radial vent samples despite their similar MgO contents (Table 2.1). The alkalic lavas have similar abundances of Ni but lower abundances of Cr than radial vent tholeiites. Zr, Sr, Nb, Rb, and Ce abundances are significantly higher, whereas Zn, Y,
Ba, La, and K are slightly higher in the alkalic lavas compared to other Mauna Loa tholeiite lavas (Fig. 2.5). The more alkalic Mo'ikeha lavas have higher Rb, Ba, Nb, La,
Ce, Sr, and Zr than the Akihimoana lavas. Mauna Kea alkalic lavas have higher Nb and
Ba contents for a given K content compared to Mauna Loa lavas (Fig. 2.5). Hualalai alkalic lavas are also distinct with a higher Nb and Ba for a given K content (Fig. 2.5).
68 0.25 Sr/Zr olivine control 4------..... Mauna Loa 0.2
0.15
Mo'ikeha 0.1 Akihimoana
0.80 CaO/AI203 olivine control'Ao------~
0.75 Mauna Loa
0.70
0.65
0.60 A
0.55
0.50 5 6 7 8 MgO (wt 0/0) Figure 2.4: MgO variation diagrams for Sc/Zr and CaO/AI 0 in 2 alkalic radial vent lavas. The alkalic radial vents have lower:Sc/Zr and CaOI AI 0 ratios than Mauna Loa tholeiites and lie well below 2 3 their olivine control trend. The location of the alkalic samples is consistent with cpx fractionation. Data for Mauna Loa fields from Rhodes and Hart (1995), Rhodes (1995), and Rhodes and Vollinger (2004). and Rhodes (unpub. data).
69 8000II} .....?i K (ppm) K (ppm)
6000
4000
...;j o 2000 MO'ikeha~ Akihimoana l 0 Mauna Loa
o i""" I I I ,I'" Ii' I I I o 5 10 15 20 0 50 100 150 200 250 300 Nb (ppm) Ba (ppm) Figure 2.5: K vs. Nb and Sa variations in the radial vent alkalic lavas, Mauna Loa tholeiites, and alkalic lavas from Mauna Kea and Hualalai. These highly incompatible elements define a source trend (black line) for Mauna Loa that is distinct from those for Mauna Kea and Hualalai lavas. The alkalic radial vent lavas lie along the extension of the Mauna Loa line, indicating a similar source. Data for Mauna Kea fields from Frey (1991). Hualalai data from Moore et al. (1987) and Moore (1996). Fields for Mauna Loa data from Rhodes and Hart (1995), Rhodes (1995), Rhodes and Vollinger (2004) and Rhodes (unpub. data). 2.4.4 ICPMS
Five alkalic lavas were analyzed by solution ICPMS at the Australian National
University (Table 2.2) using methods described by Norman et al. (1998). Compared to
Mauna Loa tholeiites, the more alkalic Mo'ikeha lavas have steeper REE patterns with similar abundances of HREE but higher abundances of LREE, yielding higher LalYb ratios (Figs. 2.6, 2.7). In contrast, the Akihimoana lavas have REE patterns that are parallel to that of the tholeiites, but with higher abundances of all elements and lower
LalYb ratios (Fig. 2.6, 2.7). Thus, there is no simple correlation of LalYb with alkalinity for these lavas compared to Mauna Loa tholeiites, although the more alkaline Mo'ikeha lavas have the highest ratio (Fig. 2.7). Sc/Zr ratios are significantly lower in both suites of alkalic lavas compared to Mauna Loa tholeiites with similar MgO values (Fig. 2.4).
2.4.5 Isotopes
Pb isotope compositions of five alkalic samples (Table 2.2) were measured at the
University of British Columbia following methods described in Weis and Frey (1991) and Weis and Frey (2002) to evaluate whether the alkalic lavas could have been erupted from HuaHilai rather than Mauna Loa. Pb isotopes ratios for lavas from these adjacent volcanoes are distinct (Fig. 2.8). The 206PbP04Pb and 208pbP04Pb ratios of the Mo'ikeha and Akihimoana alkalic lavas as well as all the tholeiitic lavas from other radial vents, fall well within the field for Mauna Loa tholeiites (Fig. 2.8). In contrast, the Pb isotope ratios for alkalic lavas from HuaHilai volcano are distinctly lower (Fig. 2.8).
71 Table 2.2. ICPMS and isotope analyses of the alkalic radia:.;.,l...:..ve;;.;.n,;.;..t.;.;;,;Ia;.;-va~s~ _ Vent I Mo'ikeha I Akihimoana Sample J2-15-13 J2-17-10 J2-17-14 J2-13-10 J2-14-04 ICPMS Li 8.5 8.3 8.6 6.5 5.6 Sc 19.8 19.9 19.4 29.8 25.7 V 274 274 269 322 289 Cr 76 92 83 95 91 Co 61 67 63 60 59 Ni 118 130 125 73 126 Cu 59 61 60 81 65 Zn 161 161 160 133 127 Ga 27.7 27.4 27.5 23.2 22.6 Rb 13.2 13.6 13.1 9.4 9.1 Sr 689 669 662 408 457 Y 29.8 30.1 29.0 36.0 30.9 Zr 264 263 264 208 189 Nb 2004 20.1 20.0 14.5 13.5 Mo 1.2 1.3 1.3 1.2 1.0 Cd 0.089 0.088 0.085 0.083 0.078 Sn 2.81 2.8 2.7 2.33 2.04 Sb 0.058 0.119 0.075 0.049 0.056 Cs 0.154 0.148 0.145 0.121 0.144 Sa 198 194 183 136 119 La 21.0 21.8 20.8 14.8 13.4 Ce 53.5 54.9 53.1 37.4 34.4 Pr 7.39 7.60 7.32 5.59 4.96 Nd 36.0 36.9 35.7 26.5 24.1 Sm 9.30 9.53 9.20 7.34 6.65 Eu ·3.06 3.11 3.03 2.49 2.28 Gd 9.01 9.23 8.99 8.00 7.09 Tb 1.31 1.35 1.32 1.26 1.11 Oy 6.77 7.04 6.80 7.16 6.27 Ho 1.21 1.27 1.21 1.41 1.22 Er 2.75 2.93 2.77 3.73 3.09 Yb 2.09 2.27 2.10 3.03 2.58 Lu 0.29 0.31 0.29 0.43 0.37 Hf 6.33 6.39 6.30 4.98 4.55 Pb 1.69 3.39 2.58 2.48 1.82 Th 1.37 1.41 1.34 0.98 0.86 U 0.47 0.57 0.50 0.33 0.30 Isotopes 4 206Pblo Pb 18.1199 18.1158 18.1769 18.1185 18.1148 2S0* 0.0015 0.0016 0.0025 0.0017 0.0010 o4 207Pbl Pb 15.4648 15.4601 15.4482 15.4586 15.4606 2S0* 0.0014 0.0014 0.0022 0.0014 0.0010 208 Pblo4Pb 37.8871 37.8762 37.8909 37.8718 37.8746 2S0* 0.0035 0.0037 0.0053 0.0036 0.0029 *standard deviation
72 "0 50~------' Q) .~ r 15 • _. • Avg Mo'ikeha E'-cen -- .- - Avg Akihimoana 00 z:.o:; ~ Avg Mauna Loa Q)~ -+oJ+oJc ffi~10 :::2:c Q)O >(,) :.0:; CO E ;t 3L...------I La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Yb Lu
Figure 2.6: Primitive mantle normalized REE diagram for Mauna Loa showing average values for lavas from the two alkalic radial vents and an average Mauna Loa tholeiite lava (Rhodes, unpubl. data). The Mo'ikeha lavas have a steeper REE pattern than typical Mauna Loa tholeiite lavas, but they have identical HREE abundances, which might be explained by lower degrees of melting of a common garnet-bearing source. In contrast, the Akihimoana lavas have a similar pattern but elevated REE compared to the average Mauna Loa tholeiite, suggesting similar degrees of melting but greater extent of fractionation. Primitive mantle normalizing values from McDonough and Sun (1995). Accuracy and precision were <1 to 2% for all elements based on repeat analyses of several samples.
73 Decreasing Mo'ikeha 10 LalYb I• 10- -!I• Melt Fraction .. Akihimoana I I Tholeiitic vents 8 81
6 6 i ¢¢ £::.. A /} '¢ 4 Mauna Loa 4 1
Kilauea Component lII( • Mauna Loa Component -....J 2 I I i I I I I .$>. 8 9 10 11 12 13 14 15 16 -2 -1 0 1 2 Zr/Nb Alkalinity Index
Figure 2.7: LalYb VS. Zr/Nb and alkalinity index (Rhodes, 1996; total alkalies - (Si02 * 0.37-14.43)) for Mauna Loa radial vent lavas. The alkalinity index reflects the departure of the sample composition from the alkali/tholeiite line (Rhodes, 1996). Mo'ikeha lavas are more alkaline than the Akihimoana lavas. The higher LalYb values seen in the Mo'ikeha lavas are probably related to lower degrees of partial melting based on their high LalYb ratios (e.g., Norman and Garcia, 1999). Zr/Nb ratios of the alkalic radial vent lavas are similar to Mauna Loa tholeiites, suggesting a similar source. Kilauea and Mauna Loa data from Rhodes et al. (1989). Primative mantle normalizing values from McDonough and Sun (1995). Accuracy and precision were <1 to 2% for La, Yb, Zr, and Nb based on repeat analyses of several samples. 38.0 .,..------:------.
37.9 - Mauna Loa Tholeiites 37.8 Hualalai Alkalic Lavas \ . ··..t ·. . I• Mo'ikeha 37.7 ...... A Akihimoana ...... C..... '.' '.:"'>
37.6 ..---r-----,,--~--_,..-----y---_.._--__.__-----l Zr/Nb 16 A A II 12 Mauna Loa Tholeiites
Hualalai Alkalic Lavas 8 c=. .. .J.... ;;:,
4+---r-----,r--~--____r--_.__--____._--__.__-~ 17.9 18.0 18.1 18.2 206pb/204Pb Figure 2.8: Pb isotope and Zr/Nb ratio plots for radial vent alkalic lavas compared to Mauna Loa tholeiites and alkalic lavas from nearby Hualalai volcano. Hualalai alkalic lavas are distinct in these plots from Mauna Loa tholeiites. The alkalic radial vent lavas plot within the Mauna Loa field for these ratios indicating they have a similar source. Data for Mauna Loa fields from Rhodes and Hart (1995) and Weis (unpub. data). Hualalai data from Park (1991) and Weis (unpub. data). Analytical errors are smaller than the symbols for all Pb analyses.
75 2.5. Discussion
2.5.1. Volcanic Sources ofRadial Vent Alkalic Magmas
Mauna Loa was thought to have erupted only tholeiitic lavas for at least the last
240,000 years (Rhodes et aI., in review; Garcia, 1995). The appearance of alkalic lavas on other Hawaiian shield volcanoes has marked the onset of postshield activity and therefore, the slow demise of the volcano over -250,000 years (Frey et aI., 1990). Before interpreting the implications of the radial vent alkalic lavas for Mauna Loa's future, we need to demonstrate the parentage of these lavas. Another possible source for the alkalic lava is a neighboring volcano HuaHilai, which has been in the alkalic postshield stage of volcanism for at least 100,000 years (Moore et aI., 1987). HuaHilai is a possible source due to its recent (Kauahikaua et aI., 2002) eruptions of alkalic basalts and it's proximity to the radial vents.
With only one recent eruptive sequence ending in 1801 (Kauahikaua et aI., 2002;
Moore et aI, 1987), HuaUilai has received limited geochemical characterization compared with the many detailed studies of active Kilauea and Mauna Loa volcanoes. However, comparisons with existing isotopic data (Park, 1990; D. Weis, 2004 unpub. data) reveal that Mo'ikeha and Akihimoana alkalic submarine radial vent lavas are distinct from
HuaHilai alkalic lavas. For example, the alkalic radial vent lavas have a higher K for a given Ba than HuaUilai and Mauna Kea volcanoes, defining distinct source trends (Fig.
2.5). However, the radial vent alkalic lavas lie along the same trend as Mauna Loa lavas.
Likewise, Pb isotope and Zr/Nb ratios for the radial vent lavas are similar to Mauna Loa tholeiitic lavas but are distinct from HuaHilai alkalic lavas (Fig. 2. 8). Thus, incompatible
76 element and isotope data indicate that Mauna Loa is the source for the radial vent alkalic lavas, making these lavas the first alkalic magmas reported from this volcano.
2.5.2. Origin ofMauna Loa Radial Vent Alkalic Lavas
Various mechanisms have been invoked to explain consanguineous tholeiitic and alkalic lavas on many ocean island volcanoes. Early ideas included the formation of tholeiitic magma from alkalic magma by crystal fractionation (Bowen, 1928) and assimilation of siliceous crust (Bailey et aI., 1924). Alternatively, alkalic magmas were thought to be produced from tholeiitic magmas by assimilation of carbonates (Daly,
1933). These hypotheses have subsequently been discarded (e.g., McBirney, 1993).
Experimental studies in the 1960s explored high pressure origins for tholeiitic and alkalic suites, which included eclogite fractionation (O'Hara, 1968), variable depths of partial melting (O'Hara, 1968; Green and Ringwood, 1967), and variable extents of partial melting (Green and Ringwood, 1967). A higher pressure origin (>0.4 GPa) allows hypersthene normative tholeiitic magmas to fractionate clinopyroxene and form nepheline normative alkalic magmas, whereas at low pressures a thermal divide separates alkalic and tholeiitic magmas (Yoder and Tilley, 1962).
Today, the most widely accepted mechanism for the production of both alkalic and tholeiitic magmas at a single Hawaiian volcano is variable degrees of partial melting of a peridotite source, with tholeiites produced by larger degrees of melting compared to alkalic compositions (Feigenson et aI., 1983). For Hawaiian volcanoes, an increase in the extent of partial melting has been advocated for the transition from alkalic to tholeiitic volcanism at the end of the preshield stage (Moore et aI., 1982; Garcia et aI., 1995a) and
77 a decrease in partial melting for the transition from tholeiitic to alkalic lavas at the start of the postshield stage (e.g., Feigenson et al., 1983; Chen and Frey, 1983).
If the alkalic and tholeiitic lavas formed by varying degrees of melting of a common source, as suggested by their similar Pb isotopic compositions (Fig. 2.8), then ratios of highly over moderately incompatible trace elements (e.g. LalYb) in these magmas should be different (e.g., Garcia et aI, 1995a). The range in LalYb ratios for the two alkalic radial vent lavas requires different partial melting histories.
The more alkalic Mo'ikeha lava has higher LalYb ratios than the Mauna Loa tholeiites, yet the HREE abundances are similar (Fig. 2.6, 2.7). This is consistent with an origin of the Mo'ikeha magma either by a lower degree of melting of a garnet-bearing source similar to that which produced the tholeiites, or by a greater amount of residual garnet in the source. Zr/Nb ratios appear to be relatively insensitive to moderate variations in partial melting in Hawaiian shield lavas, and therefore, are more indicative of mantle source composition (Rhodes, 1989). In contrast, the less alkaline Akihimoana lavas have lower LalYb ratios and REE patterns parallel to those of the tholeiites, but elevated absolute abundances (Fig. 2.6, 2.7). This is suggestive of similar degrees of melting for the Akihimoana and tholeiitic magmas to establish the slope of the REB. An alternative mechanism for producing alkalic magmas is higher pressure fractionation of cpx (e.g., Bohrson and Reid, 1995; Albarecte et al., 1997; Geist et al., 1998; Naumann and Geist, 1999; Damasceno et al., 2002).
In addition to variations in the extent of partial melting, moderate pressure (0.5
1.5 GPa) clinopyroxene fractionation may have also played a role in the production of the
Mauna Loa radial vent alkalic lavas. Several recent studies have shown that moderate
78 pressure fractionation of clinopyroxene can be important in the production of alkalic magmas (e.g., Bohrson and Reid, 1995; Albarede et aI., 1997; Geist et aI., 1998;
Naumann and Geist, 1999; Damasceno et aI., 2002). Crystallization of clinopyroxene in basalts is enhanced when magmas fractionate at higher pressures because its liquidus surface expands (Yoder and Tilley, 1962; O'Hara, 1968). Pressures >0.4 GPa are needed for it to be the dominate crystallizing phase. The fingerprints of cpx fractionation are seen in the major and trace element data for the Mo'ikeha and Akihimoana lavas.
CaOIAh03 and Sc/Zr ratios are sensitive to clinopyroxene fractionation because CaO and
Sc are highly compatible in cpx (Henderson, 1992). These ratios are distinctly lower in the alkalic lavas compared to Mauna Loa tholeiitic lavas with similar MgO contents (Fig.
2.4) suggesting that these lavas have undergone cpx fractionation.
To further evaluate the role of cpx fractionation in the Mauna Loa alkalic lavas, we utilized the Grove et ai. (1992) phase diagram projections. These projections draw on experimental results to show how the location of the liquidus surfaces of basaltic minerals change as a result of variable pressures. The composition of the magma, especially the TiOz wt%, total alkalis, and MglFe ratio, affect these projections. We therefore, recalculated invariant points for Mauna Loa compositions following steps outlined in the appendices of Grove et ai. (1992) for pressures of 0.0001,0.5,0.7, 1.0
GPa. Alkalic radial vent lava compositions were normalized and projected from plagioclase onto the Ol-Cpx-Qz triangle (Fig. 2.9). Fields for evolved alkalic lavas « 7 wt% MgO) for neighboring Mauna Kea and Hualalai volcanoes are also shown, along with Mauna Loa tholeiitic lava compositions.
79 Hualalai Alkalic
Kilauea Alkalic
Mauna Loa Radial Vent Tholeiites o00
Olivine Silica Mauna Loa Tholeiites Figure 2.9: Ol-Cpx-Si phase diagram projected from plagioclase. The invariant points were recalculated following procedures in Grove et al. (1992) for 0.5,0.7, 1.0 GPa of pressure. Fields for alkalic lavas from Hualalai, Kilauea, and Mauna Kea (Laupahoehoe only) and tholeiite lavas from Mauna Loa volcanoes are shown. Mauna Loa's alkalic radial vent lavas straddle and extend into the nepheline-normative field from the silica saturated field. They plot along a trend that indicates cpx fractionation at - 1.0 GPa of pressure, slightly lower pressures than for Mauna Kea alkalic lavas (Frey et aI., 1991). Fractionated Mauna Loa tholeiitic lavas (6-7% MgO) cluster near the 1 atm cotectic (Fig. 2.9). In contrast, the Mauna Loa alkalic radial vent samples plot along a trend that extends out of the silica-saturated field and into the nepheline field. They follow a trend similar to that of Mauna Kea alkalic lavas and plot between the evolved alkalic lavas « 7 wt% MgO) from Hualalai and Mauna Kea, which are thought to have fractionated at moderate pressures from an alkalic parent (Frey et al., 1990; Fay et al.,
2002). The Mo'ikeha and Akihimoana alkalic radial vent lavas appear to have fractionated at pressures -1.0 GPa or -33 km depths based on Grove et al. (1992) projections.
MELTS modeling (Ghiorso and Sack, 1995) was undertaken to test this interpretation. Multiple MELTS runs were run on compositions from the both the submarine radial vents and other tholeiitic Mauna Loa lavas, with MgO values ranging from 6 to 13 wt%. All Mauna Loa magmas contain some water (e.g. Davis et al., 2003), which affects their crystallization sequence. Therefore, water was added to the compositions using the equation H20 = K20 *1.2, which is based on the parental magma estimate of Wallace and Anderson (1998) and Davis et al. (2003). The magma compositions were normalized assuming a QFM oxygen fugacity (appropriate for Mauna
Loa magmas; Davis et al., 2003), and run at pressures (0.0001 to 1.0 GPa) assuming fractional crystallization in a closed system. The MELTS results show that at pressures
>0.4 GPa, pyroxene is the liquidus phase, whereas at lower pressures, olivine is the liquidus phase.
It is evident from the three approaches discussed here that moderate pressure
(-1.0 GPa) clinopyroxene fractionation probably played a significant role in the
81 production of Mauna Loa's alkalic lavas. This is supported by the low CaOIAh03 and
Sc/Zr ratios of these lavas (Fig. 2.4), the location of the alkalic lava compositions on the
Ol-Qz-Cpx ternary diagram (Fig. 2.9), and the MELTS modeling. However, clinopyroxene has not been found as a phenocryst or microphenocryst in any of the alkalic Mauna Loa radial vent lavas (Table 2.1). No pyroxene was found in the evolved alkalic Laupahoehoe lavas from Mauna Kea, which are also thought to have formed by moderate pressure cpx fractionation (Frey et al., 1990). Thus, cpx must have been removed from the magma prior to eruption, perhaps in a subcrustal magma chamber or by reaction with the magma as it becomes metastable during ascent (Frey et al., 1990).
2.5.3. Implications ofAlkalic Volcanism for Mauna Loa's Evolution
The transition of a Hawaiian volcano from shield stage to postshield stage is typically signified by a decrease in eruption rates (e.g. Frey et al., 1991) and the gradual shift of lava compositions from tholeiitic to transitional basalts (which have compositions lower in silica and somewhat higher in total alkalis than a tholeiitic lava) and then to alkalic basalts (e.g. Feigenson and Spera, 1981). Below, we examine these indicators to determine if the discovery of the radial vent alkalic lavas on Mauna Loa is consistent with the volcano having entered the postshield building stage of volcanism.
Estimates of lava accumulation rates and drowned shorelines have been used to suggest that the growth rate of Mauna Loa is declining (Moore et al., 1990; Lipman,
1995), and that the volcano may be entering the postshield stage. However, eruption rates on Mauna Loa have varied markedly during the Holocene with periods of relative quiescence (e.g., 6-8 ka) alternating with periods of frequent eruptions (e.g., 1.3-1.8 ka,
82 Lockwood, 1995). Kilauea, which is undoubtedly in the shield building stage, has also
6 undergone large variations in eruption rate during the last 200 years « 10 to > 100 X 10 m3fyr; Dvorak and Durizin, 1993; Pietruszka and Garcia, 1999), but shows no signs of postshield activity. In comparison, Mauna Kea's eruption rate during the basaltic
6 3 substage of postshield volcanism decreased from 10 to 2 X 10 m fyr (Frey et aI., 1990) and continued to decrease during its hawaiite substage (-0.4 x 106 m3fyr; West, 1988).
HuaHilai is in the basaltic substage of postshield volcanism with an eruption rate of 2 x
106 m3/yr (Moore et aI., 1987). Thus, although there are fluctuations in Mauna Loa's
Holocene eruptive activity, its post-1832 relatively high rate of eruptive activity (average of 32 x 106 m3fyr from 1832 to 1984; Wanless et aI., in press), would seem to indicate that it has not entered the postshield stage.
Typically, Hawaiian volcanoes erupt transitional basalts and alkalic lavas as they begin the postshield stage (e.g., Chen et aI., 1991). These basalts are intercalated with tholeiitic lavas on Mauna Kea and Haleakala (e.g., Frey et aI., 1990; Chen et. aI, 1991).
This change in major element chemistry is thought to reflect a gradual decrease in partial melting as these volcanoes drift off the center of the hotspot and enter the postshield stage (Feigenson et aI., 1983). In contrast, there have been numerous eruptions on Mauna
Loa, including 39 historical eruptions (Barnard, 1995), since the emplacement of the alkalic lavas and no transitional basalt compositions have been found. In fact, with the exception of these alkalic lavas, the overall major element composition of Mauna Loa lavas has not changed significantly in 240,000 years (Garcia et aI., 1995b; Rhodes et aI., in review). Thus, despite the appearance of alkalic lavas on the flanks of Mauna Loa, we suggest that the volcano has not moved into the postshield stage of volcanism.
83 The eruption of alkalic magma on the flanks of a Hawaiian volcano during the shield stage, although rare, is not unique to Mauna Loa. On KIlauea's northeast rift zone, the Puna Ridge, sidescan sonar images (Clague et aI., 1995) revealed a relatively young, large lava flow (-70 kmz), which was subsequently sampled by submersible and shown to be alkalic in composition (Johnson et aI., 2002). Unlike the Mauna Loa alkalic lavas, these KIlauea alkalic lavas plot near the 1 atm cotectic on the OI-Cpx-SiOz ternary diagram (Fig. 2.9), an indication that cpx fractionation probably did not playa significant role in their petrogenesis. However, the discovery of KIlauea alkalic lavas also indicates that shield stage volcanoes can produce and erupt alkalic magmas.
2.5.4. Structural Implications ofMauna Loa's Alkalic Volcanism
It is commonly assumed that during shield building volcanism magma ascends through a primary conduit from the mantle into a crustal magma reservoir, from where it may erupt in the summit region or along rift zones (e.g. Tilling and Dvorak, 1993). With the normally high eruption rates (-30 x 106 m/yr during the shield stage; Frey et aI., 1990), magma is thought to be nearly continuously flowing through Hawaiian shield volcanoes and mixing in their summit reservoirs (e.g. Pietruszka and Garcia, 1999; Garcia et aI.,
2003). Thus, if a relatively small volume of alkalic magma entered Mauna Loa's plumbing system, it would be overwhelmed by the more voluminous tholeiites. To avoid mixing with tholeiitic magma, the alkalic magmas must either bypass the highly active main conduit and the summit reservoir or be formed during periods of much lower magma production. We believe that the voluminous activity in main conduit of a shield stage volcano would not allow for the preservation of the alkalic magmas and therefore,
84 Mauna Loa Mauna Kea Kilauea
radial vents
t t - I II\I MOHO - • '" ; t t t t t Okm t t t t t t 00 t t t VI . It pnmary magma t ~t Okm alkalic magmas conduits ~~ t
Figure 2.10: Cartoon illustrating a model for the eruption of Mauna Loa's alkalic radial vent lavas. These lavas are produced from Mauna Loa's source by lower degrees of partial melting and moderate pressure fractionation of clinopyroxene (at -33 km) prior to intruding into the crust. These magmas branch off or bypass the main conduit system of the volcano and avoid mixing with the voluminous tholeiitic magmas. They then rise Into the edifice of the volcano. The Mo'ikeha vent lavas are degassed. so they must have risen to near the surface where they could degassed before eruption. The magmas then followed radial fractures to the submarine flanks of Mauna Loa, erupting at a depths >800 m below sea level. we favor the separate conduit model which would allow the magma to undergo storage and cpx fractionation at - 30 km depth prior to eruption (Fig. 2.10).
A schematic of Mauna Loa illustrates a scenario for the occurrence of alkalic lavas on Mauna Loa's flank during the main shield stage (Fig. 2.10). Alkalic magmas ascend outside of the main mantle conduit and are intruded within the volcano's edifice.
The emplacement of these shield stage alkalic lavas within the volcano is rare compared to tholeiitic lavas. S data indicate that the magmas that produced the Mo'ikeha lavas degassed prior to eruption (Fig. 2.3) and the water pressures where it erupted are too great to allow for this degassing. Therefore, the magma must have been near the surface of the volcano «300 m depths) prior to eruption. After degassing, the magma was then transferred to the submarine flank of the volcano.
Alkalic magmas may have been generated routinely during fluctuations in melt production. This idea can be tested by examining melt inclusions in early formed olivine crystals for signs of alkalic parental magmas. However, hundred's of such inclusions in olivine phenocrysts have been analyzed and no alkalic signatures have been found
(Norman et al., 2002).
2.6. Conclusions
Field and geochemical data point to Mauna Loa, not neighboring Hualalai volcano, as the source of the two newly discovered alkalic lavas on the submarine western flanks of the island of Hawai'i. Thus, these are the first alkalic lavas reported from Mauna Loa. Lavas from both vents, which are oriented radially to the summit,
86 acquired their alkalic signature from moderate pressure fractionation of clinopyroxene.
The high LalYb and incompatible element abundances of the lavas from Mo'ikeha vent indicate that lower degrees of partial melting from a common mantle plume source was important in their formation. This interpretation is supported by CaO/Ah03, Sc/Zr, and
REE, isotope data, phase diagram projections, and MELTS modeling. Despite the appearance of alkalic lavas on Mauna Loa, it has not entered the postshield stage of volcanism. Whereas the onset of alkalic eruptions typically accompanies the beginning of the postshield building stage, there has been no long-term change in the major element chemistry of Mauna Loa lavas in the last 240,000 years and no other alkalic or transitional basalts are known despite numerous Holocene eruptions and hundreds of chemical analyses (Rhodes and Hart, 1995; Rhodes, 1995, Rhodes and Vollinger, 2004;
Rhodes, unpub. data). These shield stage alkalic magmas were able to erupt without being assimilated by avoiding the highly productive mantle conduit, which is dominated by voluminous tholeiitic magma.
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