9. Low-Pressure Melting Studies of Basalt and Basaltic Andesite from the Southeast Greenland Continental Margin and the Origin of Dacites at Site 9171

Home , Basaltic andesite, Partial melting, Restite

Larsen, H.C., Duncan, R.A., Allan, J.F., Brooks, K. (Eds.), 1999 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 163

9. LOW-PRESSURE MELTING STUDIES OF BASALT AND BASALTIC ANDESITE FROM THE SOUTHEAST GREENLAND CONTINENTAL MARGIN AND THE ORIGIN OF DACITES AT SITE 9171

P. Thy, 2 C.E. Lesher,2 and J.D. Mayfield2

ABSTRACT

A series of 1-atm. melting experiments on basaltic flows collected from Holes 918D and 989B in the oceanic succession of the East Greenland continental margin can be used to define possible phase equilibria and liquid lines of descent. A sample from Site 918 (Section 152-918D-108R-2) shows the melting order low-Ca pyroxene (1153°C), augite (1182°C), olivine (1192°C), and plagioclase (1192°C). A sample from Site 989 (Section 163-989B-10R-7) melts in the order of low-Ca pyroxene (1113°C), augite (1167°C), plagioclase (1177°C), and olivine (1184°C). In particular, the relatively early appearance of low-Ca pyroxene distinguishes the melting relations for the oceanic succession from those observed for the basaltic continental succession at Site 917. A basaltic andesite flow from the Middle Series at Site 917 (Section 152-917A-27R-4) shows the melting order of low-Ca pyroxene (1142°C), plagioclase (1173°C), and olivine (1173°C). This melting order is difficult to

reconcile with the observed large compositional variations in SiO2 and FeO for the Middle Series, which imply early magnetite fractionation. Major element considerations and rare-earth element modeling of the dacites of the Middle Series suggest that they formed by low extent of melting (<20%) of continental hydrated gabbroic or mafic amphibolite at pressures <8 kbar. These crustally derived melts represent possible contaminants of basaltic magmas of the Lower and Middle Series at Site 917.

INTRODUCTION The transition into the oceanic succession is represented by the Up- per Series at Site 917 and is composed of basaltic and picritic flows The East Greenland Early Tertiary rifted margin is composed of compositionally and morphologically resembling the Lower Series extensive seaward-dipping reflector sequences (SDRS) and conti- flows (Larsen, Saunders, Clift, et al., 1994; Larsen and Saunders, nental flood basalts. The magmatic development during the conti- 1998). The Upper Series is less contaminated than the Lower and Mid- nental rifting and initiation of oceanic spreading were investigated dle Series (Fitton et al., 1998a, 1998b; Saunders et al., 1998). The by researchers on both Leg 152 (Holes 915–919) and Leg 163 slight evidence of contamination and the mid–ocean-ridge basalt (Holes 988–990) of the Ocean Drilling Program. Hole 917A pene- (MORB)-like source characteristics of most lavas of the Upper Series trated the early part of a subaerially erupted continental succession lead Fitton et al. (1998b) to include the Upper Series in the synrift suc- divided into a Lower Series of basalt and subordinate picrite, domi- cession. Fram et al. (1998) showed that trace-element compositions of nantly pahoehoe flows (444 m), and a Middle Series of basaltic and the succession of Upper Series lavas recorded a rapid removal of the dacitic aa flows and minor tuffs (193 m) (Larsen, Saunders, Clift, et thick continental lithosphere that accompanied plate separation and al., 1994). Werner et al. (1998) showed that the volcaniclastic depos- initiation of subaerial seafloor spreading in the region. This was re- its of the Middle Series represented primary or reworked tephra that flected in an increase in the extent of partial melting and a decrease in originated from rare explosive hydroclastic and pyroclastic erup- pressure of melt separation from the mantle. The oceanic succession tions during the terminal phases of the effusive continental flood ba- was drilled at Site 918 (and 915) in the main portion of seaward- salt succession. The continental succession was shown to have been dipping reflectors. In contrast to the interpretation of Fitton et al. derived from a relatively depleted mantle and was modified by crust- (1998b) and Fram et al. (1998), the boundary between the continental al contamination (Fitton et al., 1998a, 1998b; Saunders et al., 1998). and oceanic successions was defined by Larsen and Saunders (1998) Thy et al. (1998) suggested that the assimilation of a hydrous silicic by the unconformity and thin sediment horizon between the Middle component suppressed plagioclase crystallization and permitted ex- and Upper Series of Hole 917A. tended olivine fractionation and formation of accumulative picrites. The principal scientific drilling objectives of Leg 163 were to sam- Fitton et al. (1998a, 1998b) postulated that granulitic gneiss was the ple the basal portion of the continental succession (Site 989), to drill principal contaminant for the lower part of the Lower Series, while into the breakup unconformity, and to document more completely the above this level, including the Middle Series, contamination by am- continent/ocean transition (Site 990). Thereby, Leg 163 would bridge phibolitic gneiss became increasingly important. The most heavily the gaps in the record believed to exist between the Upper Series (Site contaminated lavas were recovered from the Middle Series (Larsen, 917) and lavas recovered at Sites 915 and 918 (Duncan, Larsen, Allan, Saunders, Clift, et al., 1994). Fitton et al. (1998b) conjectured that et al., 1996). Only two units of aphyric basalt were recovered at Site the compositional variations up through the Lower and Middle Se- 989. Based on flow composition and morphology, the Shipboard Sci- ries at Site 917 reflected a temporal decline in the supply of primitive entific Party (Duncan, Larsen, Allan, et al., 1996) concluded that the melts to the crust and a progressive shoaling of the site by magma lavas belonged to the oceanic succession and not to the Lower Series. storage and fractionation. Lesher et al. (Chap. 12, this volume) concluded that this unusual com- pound flow most likely represents a lava fan delta formed by multiple breakouts from a lava tube at a considerable distance from the erup- tion site at the axial rift. At Site 990, 13 units of aa and pahoehoe flows 1 Larsen, H.C., Duncan, R.A., Allan, J.F., Brooks, K. (Eds.), 1999. Proc. ODP, Sci. Results, 163: College Station, TX (Ocean Drilling Program). were recovered that also resembled the aphyric to phyric basalts of the 2 Department of Geology, University of California, Davis, CA 95616, U.S.A. oceanic succession recovered from Sites 915 and 918 (Duncan, Lar- [email protected] sen, Allan, et al., 1996). At Site 988 we drilled north of the main

95 P. THY, C.E. LESHER, J.D. MAYFIELD transect into the featheredge of the seaward-dipping reflectors and ume). The phase compositions for these experiments are reported in sampled two flows. this study. We also relate the new data to our previous results on the Most of the lava succession of the Southeast Greenland margin magnesium-rich flows from Site 917, which included Unit 13 from displays systematic compositional variations that have previously the Upper Series (Section 152-917A-11R-4) and Unit 84 from the been related qualitatively to crystal fractionation and assimilation of Lower Series (Section 152-917A-86R-7) (Thy et al., 1998). primitive, mantle-derived melts (e.g., Fitton et al., 1998b; Larsen et The selected sample from the Middle Series (Section 152-917A- al., 1998). Quantitative information on the low-pressure phase rela- 27R-2) is compositionally a basaltic andesite with 5.6 wt% MgO, tions and liquid lines of descent of these primitive melts was provided while the two other samples (Sections 152-918D-108R-1 and 163- in the experimental study of Thy et al. (1998). Here we present new 989B-10R-7) are basalts with MgO contents of 8–8.2 wt% (Table 1). low-pressure experimental data for a number of more evolved com- The basaltic andesite sample (Section 152-917A-27R-2) contains positions belonging to the oceanic succession (Sites 918 and 989) and small amounts (~2%) of plagioclase (An53–65) and trace amounts of from the silicic flows of the Middle Series of the continental succes- augite phenocrysts in a groundmass of fine-grained pyroxene, plagio- sion (Site 917). These results constrain the locations of low-pressure clase, and mesostasis (Larsen, Saunders, Clift, et al., 1994; Demant, cotectics that permit assessments of fractionation assemblages in- 1998). The sample from the oceanic succession (Section 152-918D- volved in crustal differentiation and of the generation of the silicic la- 108R-1) is aphyric with <1% plagioclase phenocrysts (An72–86) and a vas of the Middle Series. fine-grained groundmass of plagioclase, augite, Fe-Ti oxide minerals, and mesostasis (Larsen, Saunders, Clift, et al., 1994). The near vent lava from Leg 163 (Section 163-989B-10R-7) is aphyric with a EXPERIMENTAL AND ANALYTICAL TECHNIQUES groundmass assemblage of plagioclase, augite, Fe-Ti oxide minerals, Selected Samples and mesostasis (Duncan, Larsen, Allan, et al., 1996; Lesher et al., Chap. 12, this volume). All samples are altered by very low-grade Three lava samples were selected for experimental study. They metamorphism and contain secondary assemblage of zeolites and include Unit 41 from the Middle Series at Site 917 (Sample 152- clay minerals (Duncan, Larsen, Allan, et al., 1996; Demant et al., 917A-27R-2 [Piece 4, 38–43 cm]), Unit 12B from oceanic succession 1998). Despite the secondary replacements, the samples are relatively at Site 918 (Sample 152-918D-108R-1, [Piece 2B, 54–58 cm]), and fresh and, in view of their aphyric to sparsely phyric nature, likely Unit 1 from Site 989 (Sample 163-989B-10R7 [Piece 5A, 55–59 represent magmatic liquid compositions. cm]). All samples in the following are referred to by their respective section identifications. The samples are all from relatively evolved, Experimental Techniques hypersthene normative basaltic flows with Mg/(Mg+Fetotal) atomic ratios of <0.54 (Table 1). The phase relations for the Site 989 sample The experimental techniques used in the present study are identi- were determined as part of a companion experimental study of nucle- cal to those used by Thy et al. (1998), and only the main points need ation and crystallization kinetics (Lesher et al., Chap. 12, this vol- to be given. The rock samples were ground in a tungsten-carbide

Table 1. Composition of starting materials.

152-917A-27R-2 152-918D-108R-1 163-989B-10R-7 (Piece 4, 38–43 cm) (Piece 2B, 54–58 cm) (Piece 5A, 55–59 cm)

XRF* XRF† Probe‡ XRF* XRF† Probe‡ XRF* XRF† Probe‡ SiO2 51.98 54.28 54.58 50.29 51.29 50.50 49.99 50.93 48.48 TiO2 1.21 1.26 1.25 0.94 0.95 0.99 1.03 1.05 1.11 Al2O3 14.82 15.48 15.82 13.56 13.77 13.94 13.41 13.66 13.95 Fe2O3 10.27 3.61 6.11 FeO 1.54 8.81 7.01 FeO** 10.78 11.26 12.53 12.06 12.24 13.22 12.51 12.74 13.57 MnO 0.12 0.13 0.15 0.23 0.23 0.25 0.20 0.20 0.21 MgO 5.36 5.60 5.56 7.84 7.96 7.54 8.03 8.18 0.04 CaO 7.27 7.59 7.60 11.39 11.56 11.43 10.67 10.87 11.06 Na2O 3.28 3.43 3.34 2.01 2.04 1.84 2.11 2.15 0.24 K2O 0.80 0.84 0.77 0.08 0.08 0.14 0.12 0.12 0.13 P2O 50.12 0.13 0.08 0.09 0.09 0.06 0.10 0.10 0.03 LOI2.841.311.33 Total 99.61 100.00 101.68 100.15 100.00 99.91 100.11 100.00 98.82 Mg# 0.470 0.442 0.537 0.504 0.534 0.514

V 239 323 355 Cr ND 114 58 Ni 52 101 81 Cu 78 221 186 Zn 143 81 102 Sr 501 72 78

CIPW weight norm†† Or 4.96 4.47 0.48 0.83 0.72 0.78 Ab 29.02 27.46 17.27 15.58 18.19 19.15 An 24.35 25.46 28.16 29.38 27.25 27.90 Hy 27.57 29.81 25.38 26.85 25.34 12.41 Di 10.41 9.32 23.74 22.48 21.57 22.02 Ol 0.00 0.00 2.95 2.86 4.70 15.19 Il 2.39 2.33 1.81 1.88 1.99 2.13 Ap 0.31 0.19 0.22 0.14 0.24 0.38 Cm 0.04

Notes: * = from Larsen et al. (1998) and Larsen et al. (this volume) as determined by XRF. † = XRF analysis calculated anhydrous with all iron as FeO. ‡ = Electron microprobe analysis of superliquidus glass. FeO** = total iron as FeO. Mg# = Mg/(Mg+Fe) ratio with all iron as Fe2+. LOI = loss on ignition. ND = no data. †† = CIPW weight norm calculated with all iron as FeO.

96 LOW-PRESSURE MELTING STUDIES AND DACITE ORIGIN shatterbox and subsequently by hand in an agate mortar under ace- ju, 1989) was analyzed concurrently and used to evaluate analytical tone to an estimated average grain-size below ~10 µm. The powder precision and accuracy (Thy et al., 1998, table 1). The analyses were was mixed with polyvinyl alcohol and pressed into pellets and dried. screened by compositional and stoichiometric criteria and are consid- These pellets were broken into ~50-mg pieces and fused to an 0.004- ered representative for the experiments. The average glass and min- in Fe-Pt alloy suspension wire prepared by iron electroplating eral compositions for the individual experiments are reported in Ta- (Grove, 1981). The powder from the sample taken from Section 163- bles 3, 4, 5, and 6. The iron in liquids has been redistributed between 989B-10R-7 was pressed into 50-mg pellets without using an organic Fe2O3 and FeO appropriate for the fayalite-magnetite-quartz oxygen binder. The experimental charges were suspended in a 1-atm. vertical buffer using the equations of Kilinc et al. (1983). quench furnace at the run temperatures. Temperature was monitored The standard deviations of the replicate analyses are reported in by a Pt/90Pt10Rh thermocouple (S-type) calibrated against the melting Tables 3, 4, 5, and 6 and reflect the homogeneity of the phases. In par- point of gold. The furnace atmosphere was controlled to the fayalite- ticular, the results for plagioclase and augite for the low-temperature magnetite-quartz oxygen buffer using a CO-CO2 gas mixture and experiments reflect a high degree of heterogeneity and incomplete re- monitored by a solid ZrO2-ceramic oxygen probe calibrated against action of the starting material. Glass and olivine, on the other hand, the Ni-NiO reaction. Run duration varied from 8–239 hr, generally are generally homogeneous. increasing with decreasing melting temperature. The experimental The modal proportions of the experimental phases were estimated products were quenched in air. A summary of the experimental con- by weighted mass balance using the compositions of the phases de- ditions and results are given in Table 2. termined by electron microprobe (Bryan et al., 1969). All of the oxides were assigned a weighting factor of 1.0, except SiO2 and Al2O3, Analytical Techniques which were weighted by 0.4 and 0.5, respectively. Losses of sodium to the furnace gas and under the electron microbeam during analysis The phase compositions of the experimental products were deter- were estimated by including Na2O as a phase in the mass balance cal- mined using a Cameca SX50 electron microprobe operated with an culation. Table 2 presents the analyzed modes and shows that <20% accelerating voltage of 15 kV and a beam current of 10 nA. Details by weight of Na2O is unaccounted for in a given experimental charge. of the analytical procedures can be found in Thy et al. (1998). A focused beam was used for minerals, while glasses were normally analyzed with a 10-µm broad beam to minimize volatilization of sodium. EXPERIMENTAL RESULTS The small residual volumes of glass in some low-temperature exper- Phase Appearances and Proportions iments prohibited the use of a broad diameter beam, and these, therefore, were analyzed with a narrow focused beam. An internal glass The results of the melting experiments are given in Table 2. Sec- standard prepared from international rock standard W-1 (Govindara- tion 152-917A-27R-2 shows the melting order of low-Ca pyroxene at

Table 2. Experimental conditions and results.

Phase assemblage Phase proportions

Temp fO2 Duration Na2O loss Run # (°C) log (hr) Gl Ol Pl Lpx Aug Σr2 (wt%)

152-917A-27R-2 (Piece 4, 38–43 cm) 27R2-6 1177 –9.13 23 Gl 1.000 1.178 0.20 27R2-7 1168 –8.64 21 Gl Ol Pl 0.862 0.032 0.106 0.369 0.30 27R2-9 1158 –8.81 69 Gl Ol Pl 0.832 0.047 0.121 0.115 0.40 27R2-10 1148 –9.25 68 Gl Ol Pl 0.752 0.065 0.183 0.170 0.30 27R2-11 1136 –9.26 92 Gl Ol Pl Lpx 0.679 0.014 0.209 0.098 0.034 0.50 27R2-12 1130 –9.39 71 Gl Ol Pl Lpx 0.574 <0.010 0.276 0.150 0.042 0.50 27R2-13 1120 –9.55 91 Gl Ol Pl Lpx 0.504 <0.010 0.311 0.199 0.087 0.50 27R2-8 1111 –9.58 68 Gl Ol Pl Lpx 0.470 <0.010 0.331 0.198 0.036 0.60 27R2-19 1101 –9.65 239 Gl Ol Pl Lpx 0.425 0.066 0.370 0.139 0.408 0.50

152-918D-108R-1 (Piece 2B, 54–58 cm) 108R1-4 1197 –8.66 20 Gl 1.000 1.187 0.20 108R1-5 1186 –8.81 27 Gl Ol Pl 0.851 0.050 0.099 0.686 108R1-6 1177 –9.13 23 Gl Ol Pl Aug 0.816 0.041 0.085 0.058 0.011 0.20 108R1-7 1168 –8.64 21 Gl Ol Pl Aug 0.694 0.041 0.148 0.117 0.019 108R1-9 1158 –8.81 69 Gl Ol Pl Aug 0.597 0.055 0.198 0.150 0.283 108R1-10 1148 –9.25 68 Gl Ol Pl Aug Lpx 0.496 0.036 0.226 <0.010 0.253 0.020 108R1-11 1136 –9.26 92 Gl Ol Pl Aug — 0.390 <0.010 0.289 0.226 0.153 0.017 0.20 108R1-12 1130 –9.39 71 Gl Ol Pl Aug Lpx 0.315 <0.010 0.315 0.218 0.176 0.005 0.20 108R1-8 1111 –9.58 68 Gl Ol Pl Aug Lpx 0.117 0.020 0.410 0.154 0.299 0.129 0.10 108R1-21 1103 –9.56 70 Gl Ol Pl Aug Lpx 0.221 <0.010 0.366 0.214 0.219 0.015 0.20 163-989B-10R-7 (Piece 5A, 55–59 cm) 989B-8 1186 –8.83 8 Gl 1.000 989B-7 1182 –8.81 22 Gl Ol 0.981 0.019 0.020 989B-4 1172 –8.91 25 Gl Ol Pl 0.918 0.045 0.037 0.012 989B-10 1612 –9.00 13 Gl Ol Pl Aug 0.825 0.069 0.097 0.009 0.018 989B-9 1158 –9.07 20 Gl Ol Pl Aug 0.737 0.078 0.137 0.048 0.005 989B-1 1150 –9.55 22 Gl Ol Pl Aug 0.697 0.078 0.143 0.082 0.218 0.10 989B-5 1125 –9.48 27 Gl Ol Pl Aug 0.412 0.119 0.290 0.179 0.009 989B-3 1101 –9.59 21 Gl Ol Pl Aug Lpx 0.263 0.121 0.356 0.036 0.224 0.002 0.10

Notes: Abbreviations of experimental phase assemblage are gl = glass; ol = olivine; pl = plagioclase; lpx = low-Ca pyroxene; aug = augite; — = phase not observed. Temp = the experimental temperature in °C. The phase proportions (wt%) are calculated by least squares mixing calculations, as explained in the text. Σr2 = the sum of squares of residuals. The loss of Na2O has been estimated by including Na2O as a phase in the mixing calculations. Since the mineral phases were not analyzed for Runs # 10R7-1 and 108R1-8, appropriate compositions were used in the calculating of the mode.

97 P. THY, C.E. LESHER, J.D. MAYFIELD 5 O 2 OP 2 O K 2 SD FeO MnO MgO CaO Na 3 O 2 Al 2 TiO = the number of analyses used for calculating each average. FeO* = total iron as FeO; = total FeO* average. each calculating for used analyses number of = the N 2 C. C. ° SiO mperature in O 2 (wt% corr.) Mg# Na 5 O 2 P (wt%) O values are based on the estimated losses from Table 2. from Table on the estimated losses basedare O values 2 O 2 K (wt%) O 2 Na (wt%) Table 3. Compositionexperimental of glass. Table CaO (wt%) MgO (wt%) MnO (wt%) FeO* (wt%) 3 O 2 Al (wt%) 2 TiO (wt%) 2 SiO (wt%) N C) ° 59 cm) 58 cm) ( 1111 3 48.73 3.11 10.63 21.84 0.31 3.21 9.18 2.22 0.33 0.37 2.32 0.208 0.21 0.13 0.33 0.33 0.02 0.70 0.10 0.16 0.01 0.02 – Temp – 43 cm) – is below detection is limit;= Mg/(Mg+Fe) Mg# = corrected.calculated Na with allFeO. ironas Corr. 3 O 2 Run # Cr 27R2-627R2-7 117727R2-9 116827R2-10 6 115827R2-11 1148 727R2-12 54.58 1136 5 627R2-13 55.05 1130 827R2-8 55.94 1120 1.25 55.63 627R2-19 1.34 56.19 15.82 6 1111 1101 1.45 55.12 15.57 1.52 12.53 55.37 6 14.92 6 1.67 14.56 12.53 2.19 14.20 0.15 56.35 11.90 58.38 2.17 12.44 13.45 0.20 13.15 5.56 13.15 0.16 0.21 2.27 14.49 2.37 5.06 0.20 14.90 4.66 7.60 13.25 13.10 4.30 0.16 7.34 4.19 0.17 3.34 12.04 7.15 14.77 3.92 7.09 3.13 3.57 0.77 7.12 0.16 3.05 3.04 0.18 0.86 7.22 2.99 0.08 2.58 0.95 7.12 0.97 3.32 2.78 0.08 1.04 2.79 3.54 0.10 5.95 0.13 1.09 3.43 7.06 0.442 0.13 1.20 2.46 3.45 3.34 0.14 0.419 2.60 0.27 3.49 0.17 0.411 1.75 0.381 0.18 3.28 1.36 0.06 0.362 0.08 3.29 0.22 0.74 0.06 0.325 0.09 0.28 0.20 0.07 0.299 2.96 0.10 0.15 0.26 0.18 0.13 0.07 0.26 3.20 0.276 0.22 0.19 0.09 0.05 0.17 0.36 0.05 0.286 0.83 0.65 0.06 0.11 0.26 0.12 0.01 0.11 0.81 0.13 0.04 0.26 0.08 0.04 0.37 0.06 0.12 0.22 0.06 0.08 0.04 0.14 0.07 0.03 0.82 0.13 0.11 0.10 0.08 0.16 0.07 0.10 0.10 0.05 0.42 0.13 0.15 0.05 0.16 0.11 0.13 0.06 0.03 0.11 0.06 0.08 0.07 0.02 0.21 0.06 0.10 0.04 0.04 0.08 0.03 0.16 0.07 0.08 0.03 0.04 0.05 0.12 0.10 0.05 0.06 0.09 0.05 989B-8989B-7 1186989B-4 1182989B-10 10 1172989B-9 10 1162 48.48989B-1 7 48.73 1158 8989B-5 1150 1.11989B-3 49.08 5 49.01 1125 1.12 13.95 6 1101 49.27 14.14 7 1.21 1.33 13.57 51.04 5 14.07 13.39 50.54 13.42 1.51 0.21 50.15 1.61 13.77 13.05 0.17 14.25 2.33 8.04 13.21 0.17 3.58 14.61 7.38 12.33 0.21 11.06 14.89 11.21 11.12 6.76 0.16 17.22 6.41 2.24 0.24 19.27 2.28 11.44 11.54 5.99 0.30 0.13 5.58 0.31 2.41 3.63 0.12 2.27 11.14 4.58 0.16 10.65 0.14 2.43 0.16 0.11 8.69 9.50 2.46 2.24 2.36 0.18 0.15 2.28 0.18 2.69 0.514 0.29 0.496 0.42 2.41 0.21 2.27 0.21 0.83 0.20 0.38 0.467 0.45 0.445 2.43 0.06 0.30 2.53 0.08 0.49 0.40 0.422 2.46 0.16 2.69 0.400 0.17 0.03 0.04 0.78 0.251 1.02 0.322 0.26 0.09 0.39 0.10 0.08 0.25 0.04 0.52 0.08 0.27 0.06 0.44 0.07 0.23 0.34 0.15 0.15 0.02 0.12 0.50 0.07 0.26 0.24 0.22 0.27 0.10 0.39 0.05 0.07 0.18 0.59 0.06 0.06 0.12 0.17 0.16 0.20 0.06 0.10 0.09 0.91 0.23 0.14 0.03 0.12 0.09 0.22 0.03 0.25 0.21 0.10 0.02 0.03 0.02 0.06 0.02 0.11 0.10 0.02 0.04 0.04 0.02 0.03 0.03 0.05 0.05 0.05 0.03 108R1-4108R1-5 1197108R1-6 1186 11108R1-7 1177 7108R1-9 50.50 1168 7108R1-10 1158 51.39 1148 6108R1-11 0.99 51.54 1136 6108R1-12 6 51.71 1130 13.94 1.05108R1-8 6 50.69 1.13108R1-21 50.68 13.99 4 13.22 1103 1.32 49.61 13.54 1.38 13.69 49.38 13.40 0.25 4 7.54 1.82 13.23 12.85 1.85 12.40 0.27 14.19 11.43 48.50 2.36 11.85 0.25 15.04 15.56 1.84 6.93 11.53 0.24 3.77 18.68 6.53 0.28 11.75 0.28 19.19 0.14 6.28 10.21 11.26 0.30 5.82 2.07 5.07 10.80 0.06 0.30 20.82 1.97 5.17 10.44 0.13 2.20 4.64 2.04 9.67 0.34 0.12 2.34 9.72 0.09 0.16 0.504 2.31 3.53 9.54 0.07 0.18 2.27 2.07 0.16 0.09 0.19 2.21 9.44 2.17 0.13 0.24 0.474 0.05 2.20 0.16 0.23 1.77 0.468 2.34 0.14 0.14 0.11 0.441 2.31 0.18 0.26 0.29 0.408 2.47 0.08 0.33 0.25 0.367 2.41 0.07 0.30 0.24 0.330 0.14 0.05 0.09 0.29 0.301 0.10 1.97 0.09 0.28 0.43 0.08 0.11 0.12 0.44 0.27 0.232 0.15 0.07 0.01 0.23 0.31 0.19 0.04 0.03 0.44 0.32 0.07 0.05 0.04 0.16 0.23 0.10 0.17 0.04 0.05 0.28 0.14 0.10 0.05 0.54 0.25 0.06 0.14 0.04 0.02 0.13 0.22 0.03 0.99 0.06 0.08 0.14 0.08 0.20 0.07 0.03 0.03 0.11 0.01 0.26 0.06 0.25 0.02 0.13 0.09 0.03 0.08 0.03 0.02 0.18 0.02 0.05 0.03 0.02 0.03 0.15 0.04 0.01 0.04 0.04 0.04 0.05 0.04 152-917A-27R-2 38 4, (Piece 152-918D-108R-1 54 2B, (Piece 163-989B-10R-7 55 5A, (Piece Notes: Average of individual analyses and associated one standard deviations (SD) of oxides and Mg#. Temp = the experimental te individual of analyses deviations one standard and associated oxides and Mg#. of (SD) Temp Notes: Average

98 LOW-PRESSURE MELTING STUDIES AND DACITE ORIGIN

Table 4. Composition of experimental olivine.

SD Temp SiO2 FeO MnO MgO CaO NiO Fo ° Run # ( C) N (wt%) (wt%) (wt%) (wt%) (wt%) (wt%) (mol%) SD KD SiO2 FeO MnO MgO CaO NiO 152-917A-27R-2 (Piece 4, 38–43 cm) 27R2-7 1168 5 38.22 25.00 0.20 38.02 0.28 0.13 73.1 0.6 0.303 0.24 0.80 0.05 0.56 0.04 0.03 27R2-9 1158 4 37.14 26.74 0.24 36.28 0.31 0.13 70.7 0.4 0.329 0.30 0.46 0.02 0.56 0.03 0.04 27R2-10 1148 5 37.62 29.18 0.27 34.87 0.31 0.10 68.1 0.5 0.330 0.41 0.37 0.03 0.70 0.03 0.06 27R2-11 1136 5 37.76 30.88 0.29 32.51 0.32 0.17 65.2 0.6 0.347 0.33 0.43 0.05 0.98 0.05 0.05 27R2-12 1130 5 36.18 33.64 0.33 30.89 0.40 0.11 62.1 0.5 0.338 0.41 0.71 0.06 0.35 0.03 0.05 27R2-13 1120 5 35.88 34.87 0.39 28.94 0.41 0.13 59.7 0.7 0.332 0.29 0.75 0.03 1.08 0.03 0.09 27R2-8 1111 5 36.03 37.06 0.35 29.15 0.42 0.15 58.4 0.5 0.245 0.22 0.42 0.04 0.31 0.05 0.06 27R2-19 1101 4 34.25 37.97 0.41 27.08 0.42 0.11 56.0 0.5 0.343 0.22 0.45 0.05 0.27 0.03 0.02 152-918B-108R-1 (Piece 2B, 54–58 cm) 108R1-5 1186 8 39.13 20.97 0.34 41.38 0.37 0.19 77.9 1.4 0.293 0.33 1.29 0.03 1.02 0.05 0.06 108R1-6 1177 4 38.59 21.51 0.36 40.44 0.39 0.14 77.0 0.3 0.300 0.14 0.37 0.03 0.09 0.08 0.06 108R1-7 1168 8 37.34 25.10 0.37 37.15 0.37 0.12 72.5 0.5 0.341 0.40 0.54 0.11 0.54 0.08 0.06 108R1-9 1158 4 37.60 27.46 0.45 35.64 0.44 0.16 69.8 0.6 0.342 0.58 0.30 0.04 0.66 0.03 0.03 108R1-10 1148 5 36.32 29.71 0.42 32.40 0.46 0.17 66.0 0.5 0.342 0.19 0.85 0.02 0.25 0.07 0.05 108R1-11 1136 4 37.11 29.78 0.48 32.22 0.42 0.21 65.9 0.4 0.295 0.13 0.73 0.04 0.59 0.05 0.11 108R1-12 1130 4 35.42 34.52 0.55 29.81 0.48 0.16 60.6 0.8 0.323 0.24 0.81 0.07 0.54 0.06 0.10 108R1-9 1111 4 35.33 40.46 0.57 25.19 0.54 0.12 52.6 0.2 0.274 0.25 0.45 0.07 0.07 0.06 0.03 108R1-21 1103 4 35.02 43.74 0.58 23.17 0.60 0.15 51.4 0.9 0.370 0.59 0.74 0.07 0.22 0.04 0.04 163-989B-10R-7 (Piece 5B, 55–59 cm) 989B-7 1186 6 37.26 21.66 0.33 40.06 0.37 0.17 76.7 0.9 0.341 0.84 0.76 0.05 0.61 0.04 0.08 989B-4 1172 7 37.43 21.00 0.25 41.28 0.34 0.09 77.8 1.6 0.286 0.28 1.41 0.06 1.06 0.07 0.05 989B-10 1162 6 37.05 23.53 0.34 37.27 0.93 0.04 73.8 1.8 0.326 0.54 2.44 0.11 3.40 1.19 0.04 989B-9 1158 3 36.87 26.36 0.37 35.64 0.47 0.10 70.7 1.1 0.349 1.01 0.88 0.02 1.45 0.05 0.02 989B-5 1125 7 35.55 37.05 0.50 28.54 0.50 0.02 57.8 1.1 0.397 0.45 1.11 0.08 0.55 0.07 0.04 989B-3 1101 6 35.00 39.96 0.48 24.87 0.65 0.08 52.6 1.0 0.349 0.37 1.08 0.06 0.49 0.09 0.09

° Notes: Averages of individual analyses and associated one standard deviations (SD) of oxides and Mg#. Temp = the experimental temperature in C. N = number of analyses. KD is calculated as discussed in text.

1142° ± 6°C and plagioclase and olivine at 1173° ± 5°C. Section 152- liquid. The relationship between plagioclase and liquid composition ° Na/Ca 918D-108R-1 melts in the order of low-Ca pyroxene at 1153° ± 5 C, can be expressed in terms of the exchange of Na and Ca (KD [pl/ ° ° augite at 1182° ± 5 C, and olivine and plagioclase at 1192° ± 6 C. liq] = [Na/Ca]pl/[Na/Ca]liq) (Fig. 4A). The calculated KD values range Section 163-989B-10R-7 melts in the order of low-Ca pyroxene at between 0.6 and 1.6 and show a general increase with decreasing 1113° ± 12°C, augite at 1167° ± 5°C, plagioclase at 1177° ± 5°C, and temperature (and liquid fraction) (Fig. 4B). The results indicate that olivine at 1184° ± 2°C. Fe-Ti oxide minerals were not detected in any the KD for the basaltic andesite (Section 152-917A-27R-2) is lower of the melting experiments. than for any of the other samples. These values are largely in accord There is a remarkably good correlation between melting temper- with previous experimental results (Grove et al., 1982; Mahood and ature, phase proportions, and compositional variables of the experi- Baker, 1986; Tormey et al., 1987; Ussler and Glazner, 1989). mental liquids. The variation of melting temperature and liquid frac- The covariation between the An content (mol%) of plagioclase tion (Fig. 1) allows a minimum estimate of the solidus temperature and Fo content (mol%) of olivine shows systematic differences be- by extrapolation to zero melt fraction and gives solidus temperatures tween the low-Na2O (Sections 152-918D-108R-1 and 163-989B- of ~1110°C for the most magnesian-rich lavas (Sections 152-917A- 10R-7) and the high-Na2O basaltic samples (Sections 152-917A- 11R-4 and 152-917A-86R-7) and ~1050°C for the most magnesian- 11R-4 and 152-917A-86R-7) as a steepening in the slopes of the Fo- poor lava (Section 152-917A-27R-2). An covariation for the former group (Fig. 5), likely to be related to the depletion rates of Na2O of the coexisting liquids. The errors asso- Phase Compositions and Liquid Lines of Descent ciated with plagioclase compositions are especially large for the low- temperature runs because of incomplete equilibration between pla- The compositional liquid lines of descent for the investigated gioclase and liquid. samples (Table 3) (Thy et al., 1998) are represented in Figure 2 as a The pyroxenes form two compositional groups, a high-Ca group function of MgO content (wt%). The main variation is caused by co- and a low-Ca group. The high-Ca group are salites to augites, while saturation of olivine, plagioclase, and augite as reflected in the de- the low-Ca group are magnesian to intermediate pigeonites (Polder- creasing Al2O3 and CaO with decreasing MgO. vaart and Hess, 1951; Deer et al., 1978). Both augite and pigeonite Olivine is on the liquidus for all samples and present throughout show increasing ferrosalite content with decreasing melting temper- the melting intervals. The composition of olivine correlates with ature (Table 6). The exchange of Fe and Mg between pyroxene and melting temperature and liquid Mg/(Mg + Fe2+) ratio (Table 4). The liquid can be calculated as for olivine, except that all iron in the liquid 2+ FeO/MgO distribution coefficient for the moles of MgO and FeO between oliv- is calculated as Fe . The calculated KD (aug/liq) vary between FeO/MgO ine and liquid (KD [ol/liq] = [FeO/MgO]ol/[FeO/MgO]liq) is 0.22 ± 0.02 (Sections 152-917A-11R-4 and 152-917A-86R-7; n = 15) 0.32 ± 0.03 for 49 determinations (Fig. 3A) and is consistent with and 0.26 ± 0.02 (Sections 152-918D-108R-1 and 163-989B-10R-7; n previous determinations (Roeder and Emslie, 1970; Ulmer, 1989). = 8) (Fig. 3B) and are consistent with other experimental results as These results indicate that olivine and liquid are well equilibrated for well as natural pyroxene-liquid pairs (Grove and Bryan, 1983; Perfit most experiments. and Fornari, 1983; Toplis and Carroll, 1995). The pigeonites indicate FeO/MgO Plagioclase composition (Table 5) shows wide and irregular vari- a KD (pig/liq) of 0.23 ± 0.01 (Section 152-917A-27R-2; n = 5), ations with melting variables and the composition of the coexisting 0.25 (Section 152-918D-108R-1; n = 2), and 0.23 (Section 163-

99 P. THY, C.E. LESHER, J.D. MAYFIELD O 2 O K 2 SD FeO MgO CaO Na 3 O 2 Al 2 = number of analyses. N TiO C. C. ° 2 mperature in in mperature An (mol%) O 2 K (wt%) SiO SD O 2 Na (wt%) CaO (wt%) Table 5. Composition 5. of experimentalplagioclase. Table MgO (wt%) FeO (wt%) 3 0 2 Al (wt%) 2 TiO (wt%) 2 SiO (wt%) N C) ° 58 cm) 58 59 cm) 59 ( 1111 5 53.69 0.06 29.11 1.21 0.11 12.64 3.94 0.08 63.6 7.9 0.92 0.04 0.72 0.16 0.03 0.44 0.41 0.01 Temp – – 43 cm) – Run # 27R2-727R2-9 116827R2-10 115827R2-11 1148 827R2-12 1136 627R2-13 1130 4 53.8127R2-8 1120 5 54.2927R2-19 54.06 8 1111 0.06 54.15 1101 5 0.08 55.61 0.13 10 16 55.17 29.46 0.11 28.79 0.09 55.81 28.06 55.42 0.08 1.10 28.72 1.20 27.74 1.27 0.11 0.11 27.92 0.17 1.29 0.26 1.42 27.35 28.37 0.35 1.43 12.26 0.17 11.57 0.26 1.27 11.48 1.41 0.16 4.26 11.59 4.49 10.97 4.40 0.18 0.17 10.94 0.17 4.50 0.16 4.85 10.47 0.21 10.96 4.86 0.21 60.8 0.22 58.2 5.07 58.3 4.95 0.27 58.0 5.6 0.32 54.8 4.9 5.6 0.31 54.6 2.7 1.53 52.3 1.07 2.2 1.50 54.0 3.3 0.69 0.03 0.03 2.4 0.64 0.07 0.70 2.4 0.95 0.05 0.98 0.71 0.04 0.81 0.04 0.89 0.72 0.11 0.10 0.43 0.04 0.08 0.51 0.07 0.05 0.04 0.05 0.19 0.77 0.08 0.15 1.03 0.03 0.68 1.05 0.17 0.06 1.24 0.03 0.64 0.56 0.15 0.50 0.49 0.07 0.54 0.63 0.28 0.05 0.03 0.03 0.23 0.39 0.04 0.35 0.05 0.60 0.35 0.02 0.06 0.24 0.04 0.03 108R1-5108R1-6 1186108R1-7 1177108R1-9 1168 6 11108R1-10 1158 1148108R1-11 50.43 9 49.36 1136108R1-12 5 1130 8108R1-8 50.77 0.08 5108R1-21 0.07 50.70 50.85 1103 6 0.06 31.94 50.64 31.24 0.07 51.51 6 0.06 28.77 1.18 0.06 1.11 31.29 51.61 0.08 29.89 1.35 30.67 0.32 1.13 0.08 0.24 30.79 1.04 0.33 1.23 15.26 29.91 15.10 0.21 1.14 0.21 13.97 2.58 0.18 1.00 2.74 14.95 0.14 13.60 3.30 14.16 0.03 3.04 0.17 0.06 13.88 3.41 0.04 3.25 76.4 13.91 0.04 3.57 75.0 0.04 0.06 69.9 3.75 1.2 0.07 72.9 1.6 68.6 70.4 3.8 0.08 0.15 68.0 2.6 0.85 3.9 6.4 1.27 66.9 0.06 4.8 0.73 0.04 1.43 1.68 0.03 6.3 1.35 1.39 0.05 0.49 0.01 0.85 0.03 0.15 2.08 0.75 0.08 0.03 1.15 0.23 1.05 0.12 0.29 0.05 1.13 0.03 0.16 0.23 0.10 0.23 1.10 0.06 0.06 0.76 0.02 0.73 0.07 0.12 0.25 0.53 0.13 0.04 0.79 0.42 1.28 0.01 0.11 0.29 0.99 0.02 0.42 0.73 0.01 1.46 0.55 0.01 0.01 0.02 0.71 0.03 0.04 989B-4989B-10 1172989B-9 1162989B-5 1158 6989B-3 7 1125 49.48 1101 7 49.92 6 50.45 9 0.05 0.07 52.05 53.01 0.05 30.98 30.68 0.05 0.07 30.59 1.14 1.33 29.88 30.19 1.42 0.25 0.26 1.51 1.44 0.38 14.90 14.37 0.21 0.18 14.45 3.26 3.44 13.68 12.94 3.32 0.03 0.03 3.82 4.20 0.04 71.5 0.04 69.7 0.06 70.5 4.8 66.3 3.5 62.8 2.6 1.49 3.5 1.03 3.9 0.69 0.03 1.01 0.03 1.27 0.02 0.84 0.47 0.03 0.04 0.52 0.24 0.19 0.66 0.78 0.19 0.03 0.20 0.02 0.15 0.19 0.97 0.70 0.05 0.05 0.67 0.56 0.42 0.79 0.70 0.26 0.02 0.38 0.02 0.49 0.02 0.02 0.04 152-917A-27R-2 38 4, (Piece 152-918D-108R-1 54 2B,(Piece 163-989B-10R-7 55 5B,(Piece Notes: Averages of individual analyses and associated one standard deviations (SD) of oxides and An. Temp = the experimentalte = the Temp oxides and An. of one standard deviationsassociated and (SD) individual of analyses Notes: Averages

100 LOW-PRESSURE MELTING STUDIES AND DACITE ORIGIN 3 O 2 OCr 2 SD FeO MnO MgO CaO Na 3 O 2 Al 2 TiO 2 SiO D = number of analyses. Mg# = Mg/(Mg+Fe) calculated on an atomic basis. En = enstatite, En = basis. on atomic an calculated Mg# = Mg/(Mg+Fe) of analyses. = number N C. C. o emperature inemperature Wo (mol%) Mg# SD K Fs (mol%) En (mol%) 3 O 2 Cr (wt%) O 2 Na (wt%) Table 6. Composition 6. of experimental pyroxenes. Table CaO (wt%) MgO (wt%) is calculated as discussedcalculated as in text. is D MnO (wt%) FeO (wt%) 3 O 2 Al (wt%) 2 TiO (wt%) 2 SiO (wt%) N C) ° 58 cm) 59 cm) ( 11111111 7 4 51.60 52.88 0.63 0.32 1.76 0.89 16.92 23.25 0.33 0.54 14.01 18.91 15.57 5.32 0.21 0.06 0.16 40.4 27.4 52.9 32.3 36.5 59.6 10.7 4.7 59.2 0.178 1.7 0.77 0.181 0.21 0.4 0.54 50.04 2.14 0.07 0.06 1.29 1.10 0.04 1.23 0.46 0.06 0.79 0.09 0.01 Temp – – 43 cm) – Fs = = ferrosilite,wollastonite; Wo all on a molecular basis. K Run # 27R2-1127R2-12 113627R2-13 1130 527R2-8 1120 327R2-19 52.91 3 1111 1101 52.71 0.28 3 52.44 12 0.35 1.19 0.28 51.83 53.41 1.21 0.42 18.45 0.94 0.33 19.12 0.32 0.89 21.12 0.29 0.91 23.70 0.45 22.01 23.55 21.81 0.36 3.53 22.23 0.39 3.68 19.88 0.06 3.72 21.61 0.05 4.61 0.07 3.89 0.09 0.09 64.8 63.8 60.5 28.3 29.1 55.9 32.2 6.9 59.0 7.2 34.8 69.6 7.3 68.7 33.4 9.3 1.6 65.2 0.9 0.248 7.6 60.8 1.5 0.220 0.44 2.9 63.8 0.227 0.33 0.06 0.238 0.19 0.6 0.05 0.08 0.33 0.66 0.227 0.03 0.09 1.56 0.20 0.27 0.91 0.17 0.14 0.11 0.95 0.02 0.61 0.53 0.19 0.31 0.29 1.20 0.52 0.04 0.75 0.25 0.03 0.93 0.47 0.01 0.04 0.45 0.04 0.35 0.04 0.25 0.04 108R1-6108R1-7 1177108R1-9 1168 16108R1-10 1158 6 1148108R1-10 51.82 6 1148108R1-11 9 0.30 50.71 1136108R1-12 3 52.30 1130108R1-12 0.35 50.95 10 1.88 1130108R1-8 0.35 52.12 4 0.60 50.66 2.47108R1-8 5 0.31 8.92 1.68108R1-21 0.45 51.10 2.72 10.64 1103 0.21108R1-21 52.83 1.15 0.50 11.50 1103 1.85 0.28 12.65 5 0.30 16.58 0.30 18.19 3 1.70 0.33 13.24 17.50 50.67 18.97 1.02 0.46 16.57 0.30 14.88 52.47 17.47 17.35 0.59 19.41 0.20 17.67 21.55 0.32 0.31 14.05 15.15 0.16 2.17 0.52 0.15 5.47 17.40 0.17 14.54 0.91 0.17 16.40 0.19 21.13 47.1 16.85 21.49 0.08 0.21 0.13 0.34 48.6 0.19 5.88 0.42 46.4 0.20 14.2 50.2 0.09 0.10 14.74 16.6 0.06 20.90 38.7 60.4 43.2 15.77 18.1 0.09 20.6 34.9 41.5 4.76 0.05 76.8 35.6 0.16 28.6 21.2 29.2 74.6 58.3 2.4 0.06 23.9 72.0 35.6 11.0 0.14 71.0 3.6 30.1 0.266 34.6 2.6 41.8 67.9 67.1 1.0 0.269 11.7 0.67 0.269 63.5 0.6 4.3 26.1 0.237 0.92 0.05 66.0 57.5 0.39 4.3 0.275 0.242 32.1 0.08 0.82 0.04 0.8 0.248 0.15 33.2 0.24 0.62 0.30 61.6 0.70 0.222 0.04 0.15 1.21 2.52 0.09 9.4 7.4 0.62 1.53 0.12 0.46 0.03 1.14 0.189 0.15 0.30 63.4 0.96 0.04 0.03 0.56 0.12 0.22 0.06 0.38 1.92 0.7 1.88 0.03 0.44 0.23 0.51 0.62 2.18 0.174 0.96 0.01 0.05 2.13 0.04 0.80 0.83 0.16 0.82 0.07 1.79 0.08 3.68 0.02 0.05 0.03 0.77 0.05 0.11 1.11 0.02 0.06 0.04 0.10 0.07 0.57 0.91 0.03 0.01 0.04 0.08 1.40 1.09 0.01 0.05 0.06 2.16 0.60 0.08 0.03 0.06 0.03 0.08 0.10 0.34 0.09 0.03 989B-10989B-9 1162989B-5 7 1158989B-3 1125989B-3 8 50.60 1101 5 1101 0.42 50.53 3 51.05 3 0.51 2.24 51.32 0.48 51.82 3.04 0.53 10.96 2.59 0.65 0.27 10.93 2.73 11.20 1.63 0.27 17.54 11.74 0.23 21.55 17.24 17.31 0.28 16.33 0.44 17.35 0.19 15.77 18.26 17.50 0.21 18.34 0.12 0.21 7.23 0.21 48.6 0.01 0.06 0.17 48.2 0.05 17.0 45.6 44.4 17.1 34.4 17.8 34.7 18.5 74.1 36.6 50.3 73.8 37.1 5.1 72.2 34.8 3.5 0.281 70.2 4.7 14.9 0.259 3.6 0.91 0.183 0.14 59.1 1.07 0.140 0.86 0.15 2.4 1.59 0.30 0.10 0.21 0.232 1.02 2.02 0.74 0.44 1.22 0.42 0.04 0.39 2.03 1.65 0.05 3.56 0.06 0.97 1.34 1.52 0.08 1.20 1.18 0.06 1.06 1.77 1.28 0.07 0.04 1.35 0.08 0.05 0.03 1.20 0.04 0.04 0.03 1.25 0.22 152-918D-108R-1 54 2B, (Piece 152-917A-27R-2 38 4, (Piece 163-989B-10R-7 55 5B, (Piece Notes: Averages of individual analyses and associated one standard deviations (SD) of oxides and Mg#. Temp = the experimentalof t individual and associated analyses deviations one standard (SD) oxides of and Mg#. Temp Notes: Averages

101 P. THY, C.E. LESHER, J.D. MAYFIELD

989B-10R-7, n = 1). However, the very low KD for one of the pigeonite and liquid pairs (Section 152-918D-108R-1) suggests disequilib- rium (Table 6; Fig. 3B).

Phase Equilibria

The experimentally determined phase relations are portrayed graphically in terms of the normative components quartz (q), plagioclase (pl), olivine (ol), and diopside (di) in Figure 6. Figure 6A shows the multiply-saturated liquids and coexisting pyroxenes for projections from pl to the plane of ol-di-q. Figure 6B shows the same data set as projected from di to the plane ol-pl-q. In contrast to olivine and plagioclase that plot near their ideal normative compositions (not shown), the experimental augites and pigeonites plot significantly away from their ideal normative compositions and form an array between normative hypersthene (hy) and a point off normative diopside (Fig. 6B). Augite becomes increasingly subcalcic in the sequence Sections 152-917A-11R-4, 152-917A-86R-7, 163-989B-10R-7, and 152-918D-108R-1. The experimental liquids for Section 152-918D-108R-1 and 163- Figure 1. Melting temperature as a function of liquid proportion. The first 989B-10R-7 show a systematic increase in normative quartz with de- appearance of pyroxene is indicated (aug = augite; lpx = low-Ca pyroxene). creasing temperature and move away from a fairly constant volume The lines are best linear fits to the data. defined by coexisting olivine, plagioclase, and augite. The displace-

Figure 2. Compositions of the experimental liquids as a function of MgO content (wt%). All analyses have been calculated to 100% with iron distributed between Fe2O3 and FeO using the equation of Kilinc et al. (1983). The curves are best polynomial fits to the data.

102 LOW-PRESSURE MELTING STUDIES AND DACITE ORIGIN

A B

Figure 3. The molecular ratios FeO/MgO of mafic minerals as a function of the FeO/MgO ratio of the liquid. A. Olivine and coexisting liquid with iron of the liquid distributed according to Kilinc et al. (1983). B. Pyroxenes and coexisting liquid with iron calculated as total FeO (FeO*).

A B

Na/Ca Figure 4. Plagioclase liquid relations. A. The albite (Ab)/anorthite (An) ratio plotted against the Na/Ca ratio of the coexisting liquid. B. The calculated KD (pl/liq) as a function of melting temperature. See text for calculations.

ment of the multiply-saturated cotectics for Sections 163-989B-10R7 clase, and 41% augite. Of these three sections, only Section 163- and 152-918D-108R-1 toward lower normative plagioclase (Fig. 6) 989B-10R-7 reaches low-Ca pyroxene saturation. are principally caused by the variation in composition of the experi- The liquid line of descent for the basaltic andesite sample (Section mental augite not fully accounted for in projecting from normative di- 152-917A-27R-4) is controlled by olivine, plagioclase, and pigeon- opside. However, the differences are small and the near linear varia- ite. This multiply-saturated relation is a reaction boundary along tion in the liquid trends in these projections indicate that the cotectic which olivine reacts with the liquid to form pigeonite (Grove et al., proportions of olivine, plagioclase, and augite are nearly constant. 1982, 1983; Grove and Juster, 1989; Juster et al., 1989). The result is Sections 152-917A-11R-4, 152-917A-86R-7, and 163-989B-10R-7 that the liquid compositions lie outside the triangle defined by equi- show early crystallization of olivine and/or plagioclase (cotectic pro- librium plagioclase, pigeonite, and olivine. The results for Section portion: 29% ol and 71% pl), followed by augite. The olivine, plagio- 152-917A-27R-4 define a point on the pigeonite-saturated reaction clase, and augite cotectic corresponds to 7% olivine, 52% plagio- curve. By extrapolating pigeonite-olivine-plagioclase relations to in- 103 P. THY, C.E. LESHER, J.D. MAYFIELD

tersect with the olivine-plagioclase-augite cotectic as defined by Sec- Section 152-918D-108R-1 shows an extended interval of olivine, tions 152-917A-11R-4 and 152-917A-86R-7, we can estimate an ap- plagioclase, augite, and pigeonite saturation, but defines the low- proximate location of the pseudoinvariant point in Figure 6B, where variance pigeonite-saturated conditions at a relatively higher temper- olivine + liquid → plagioclase + augite + pigeonite. ature (1153°C) and lower silica saturation compared to typical MORB compositions (Fig. 6B) (Juster et al., 1989; Grove and Juster, 1989). Section 163-989B-10R-7 is saturated in pigeonite at the lowest melting temperature (1113°C) and slightly higher normative quartz composition than 152-918D-108R-1. Sections 152-917A- 86R-7 and 152-917A-11R-4 were not saturated in low-Ca pyroxenes, and low-Ca saturation likely will be below the lowest temperature melted (1111°–1120°C) and the highest normative quartz of any of the investigated sections. The present results extend the stability of low-Ca pyroxenes to lower normative quartz than obtained by Juster et al. (1989) and Grove and Juster (1989) for basalts and basaltic andesite lavas (Fig. 6).

MAGMATIC EVOLUTION OF THE EAST GREENLAND RIFTED MARGIN Phase Equilibria Constraints

The analyzed flows sampled during Legs 152 and 163 (Larsen, Saunders, Clift, et al., 1994; Duncan, Larsen, Allan, et al., 1996; Larsen et al., 1998; Larsen et al., this volume) have been recast into the same normative components using the same calculation proce- dure as for illustrating the experimental phase equilibria. The results are shown in Figure 7 and highlight several important features. As was observed by Thy et al. (1998), the primitive flows from Hole 917A in the Lower and Upper Series plot away from the experimentally determined multiply-saturated cotectic. This is consistent with Figure 5. Forsterite content (Fo mol%) of olivine as a function of anorthite the phenocryst assemblages found in these lavas (Larsen, Saunders, content (An mol%) of coexisting plagioclase. The lines are best linear fits to Clift, et al., 1994; Demant, 1998). Thy et al. (1998) used inferred the data. shifts in the locations of the olivine-plagioclase cotectic to suggest

Figure 6. Normative projections of the olivine (ol), plagioclase (pl), diopside (di), quartz (q) basalt tetrahedron on the triangular diagrams (A) ol-pl-q and (B) ol- di-q. Shown are liquids multiply-saturated in olivine, plagioclase, and pyroxene together with coexisting pyroxenes. The normative components have been calculated using a CIPW molecular norm with iron distributed as for Figure 2. The pyroxenes have been plotted assuming that all iron occurs as FeO. The vertical lines marked 108R1 and 10R7 (stippled) and 27R2 (full drawn) are schematic illustration of pigeonite saturation (marked by ‘+’) for the three melted sections, respectively 152-918D-108R-1, 163-989B-10R-7, and 152-917A-27R-4. Section 152-917A-86R-7 is not saturated in pigeonite at any temperature melted. The low-Ca pyroxene saturated liquids marked by ‘|’ are from Juster et al. (1989) and Grove and Juster (1989) and range in composition from basaltic to andesitic. Other abbreviations used are hy = hypersthene, pig = pigeonite, and aug = augite.

104 LOW-PRESSURE MELTING STUDIES AND DACITE ORIGIN

that some differentiated Lower Series lavas were modified by crustal Low-Pressure Fractional Crystallization Modeling contamination and concluded that excessive olivine fractionation caused by suppressed plagioclase crystallization at elevated partial Quantitative modeling of fractional crystallization is presented in water pressures could explain some of the compositional features of Figure 8. The modeling is accomplished by incrementally removing the Lower Series lavas. In general, however, both the Lower and Up- the calculated solid fractionate in equilibrium with the residual liq- per Series were controlled by near-anhydrous crystallization condi- uid. The calculation of the solid fractionate is based on the experi- tions. In contrast, lavas of the Middle Series show large variations in mentally determined mineral compositions and their proportions the normative projections and plot systematically below the projected from this study, Thy et al. (1996, 1998), and Toplis and Carroll olivine-plagioclase-augite cotectic in Figure 7B. This position im- (1995). The first model illustrates the effect of fractionation on the es- plies fractionating phase assemblages of olivine or low-Ca pyroxene timated primary liquid in equilibrium with mantle olivine (Fo91) for and plagioclase. Such fractionating assemblages would not lead to Section 152-917A-86R-7 and uses the crystallization order experi- the large variation in SiO2 content shown by the Middle Series lavas. mentally determined (Thy et al., 1998). The second model shows the Recovery at Site 988 consisted of differentiated olivine, plagio- results for the section from Hole 989B using the crystallization order clase, and augite phyric lava from only one fresh flow. The analyzed from Table 2. samples from this flow cluster near the multiply-saturated experi- We can draw several conclusions from this modeling. Most of the mental cotectic, consistent with the evolved nature of the flow and its compositional variation within groups of flows is accounted for by phenocryst assemblage. In these respects, the Site 988 lava is similar <15% fractionation. The exceptions are a few TiO2-rich flows from to the onshore plateau lavas in the Scoresby Sund area (Larsen et al., Site 917 that suggest extensive olivine, plagiocase, and augite re- 1989; Fram and Lesher, 1997). The flows analyzed from Sites 989 moval approximating 50%–60% fractionation. The flows from Site and 990 are compositionally very similar to each other and narrowly 988, as well as a small subset from Site 918, also suggest derivation cluster along the multiply-saturated cotectic, consistent with their by extensive fractionation and may require a more Ti-rich parental aphyric to highly olivine, plagioclase, and augite phyric nature. composition than used in the calculations (Fig. 8). The multiply-saturated liquid line of descent for the Site 918 sample is slightly offset from the trend determined for the basaltic Site Origin and Evolution of the Middle Series Magmas 917 samples by lower normative diopside and plagioclase (Fig. 6). The flows recovered from the oceanic succession at Site 918 have The Middle Series at Site 917 deviates from the experimental liq- compositions that deviate from their experimentally determined liquid lines of descent (Fig. 7), as well as the modeling of compositional uid line of descent (compare Figs. 6 and 7). The majority of the Site effects of fractional crystallization (Fig. 8). This is especially true for 918 lavas are aphyric and contain microphenocrysts of olivine and FeO and TiO2 that do not show increasing concentrations with de- plagioclase and, occasionally, augite phenocrysts (Larsen, Saunders, creasing MgO as predicted for typical tholeiitic evolution trends. An Clift, et al., 1994). Therefore, these are likely to have been controlled alternative fractionation model involving early low-Ti magnetite sat- by olivine and plagioclase crystallization and thus plot away from the uration (~1180°–1200°C) could account for an early decrease in FeO olivine, plagioclase, augite cotectic toward lower normative diopside and only modest variation in TiO2. However, early saturation of Fe- (Fig. 7). Lesher et al. (Chap. 12, this volume) discuss important ki- Ti oxides is neither supported by the present study nor predicted netic controls on the nucleation of augite that further help to explain based on Fe-Ti oxide solubility in basaltic systems (e.g., Toplis and the discrepancies between the petrographic observations and the ex- Carroll, 1995). Furthermore, magnetite and ilmenite are conspicu- pectations based on the equilibrium phase relations for the basalts of ously absent in the lavas of the Middle Series (Larsen, Saunders, the oceanic series. Clift, et al., 1994).

Figure 7. Normative projections of the olivine (ol), plagioclase (pl), diopside (di), quartz (q) basalt tetrahedron on the triangular diagrams (A) ol-pl-q and (B) ol- di-q. Shown are the analyzed flows sampled during Leg 152 (Sites 917 and 918) and Leg 163 (Sites 988, 989, and 990), excluding analyses with >5% loss on ignition. Calculation, projection, and abbreviations are as for Figure 6. The solid lines are the pyroxene, olivine, and plagioclase cotectics taken from Figure 6.

105 P. THY, C.E. LESHER, J.D. MAYFIELD

Figure 8. Oxide variation diagrams with MgO as abscissa for all available analyses of flows from Leg 152 and Leg 163. All in wt% oxides calculated anhydrous and iron distributed between FeO and Fe2O3 (not shown) following Kilinc et al. (1983). Analyses with >5% loss on ignition has been excluded. Model calculation of fractional crystallization is shown and calculated as described in the text for Sections 163-989B-10R-7 and 152-917A-86R-7 (Table 2). The latter is extended to the inferred “Primary Melt” to show the early olivine fractionation (Thy et al., 1998). The model calculation for 152-917A-86R-7 is ticked by intervals of 5% fractionation. The appearances of Fe-Ti oxides in the crystallization models are at 75% fractionation for 152-917A-86R7 and 65% for 163-989B-10R- 7. The curve marked olivine illustrates the effect of suppression of plagioclase (and augite). The marked bends of the model curves are for the appearances of plagioclase, augite, and Fe-Ti oxide minerals, except for the latter, as experimentally determined.

A consequence is that the major element variation observed for duce the stability of plagioclase relative to olivine and augite. This the Middle Series can neither be accounted for by the phenocryst as- could explain the occurrence of pyroxene phenocrysts and sieve tex- semblages nor by the experimental phase equilibria. The flows of the tured plagioclase phenocrysts. Middle Series are aphyric to plagioclase phyric basalts with embayed The Leg 152 Shipboard Scientific Party concluded that flows and plagioclase and rare augite phenocrysts. The dacites and silicic tuffs tuffs of the Middle Series were contaminated by significant amounts are generally restricted to the lower and uppermost parts of the Mid- of crustal material and suggested a process of magma mixing be- dle Series and contain abundant embayed and sieved textured plagio- tween basaltic and silicic components (Larsen, Saunders, Clift, et al., clase phenocrysts and minor amounts of augite phenocrysts. Olivine 1994). Fitton et al. (1998a, 1998b) argued that the flows of the Lower and quartz are only observed as rare xenocrysts. The occurrence of Series at Site 917 were contaminated by granulite facies leucogneiss, augite phenocrysts, although not common, is inconsistent with the while the Middle Series was contaminated by amphibolite facies low-pressure phase relations shown in Figure 7, and it is possible that leucogneiss. This temporal change in crustal contaminant is inter- these augites are also xenocrystic. Alternatively, Thy et al. (1998) preted as caused by a shallowing of magma chambers during devel- proposed that differentiation of Lower Series basaltic magmas was opment of the Lower and Middle Series succession. The modeling of influenced by the addition of water, possibly during the early stages Fitton et al. (1998a) implies relatively high extents of fractionation of crustal contamination. The effect of elevated water content is to re- (50%–70%) and/or assimilation to account for the dacitic flows of the

106 LOW-PRESSURE MELTING STUDIES AND DACITE ORIGIN

Middle Series. The contaminants were suggested by Fitton et al. 1989; Springer and Seck, 1997). It was illustrated by Beard and Lof- (1998a) to have been relatively similar to average Lewisian leuco- gren (1989), Thy et al. (1990), and Wolf and Wyllie (1994) that the cratic gneisses analyzed by Hamilton et al. (1978) and Dickin (1981). Al2O3 concentrations of partial melts were indicative of the stability Fitton et al. (1998a) proposed a contaminant for the Middle Series of plagioclase relative to pyroxene and amphibole in the source and 87 86 143 144 flows with high Sr/ Sr (0.7166) and low Nd/ Nd (0.5104) and, consequently of the partial pressure of H2O (pH2O). High pH2O for modeling purposes, used a late Archean granitic gneiss from the would tend to stabilize amphibole and destabilize plagioclase result- Skjoldungen area of Southeast Greenland (Blichert-Toft et al., 1995). ing in melts with high Al2O3. In contrast, low pH2O destabilizes am- Another possible contaminant is granophyric material described by phibole and thus reduces the Al2O3 content of a partial melt (Beard Blichert-Toft et al. (1992) from the Tertiary Miki Fjord macrodike of and Lofgren, 1989). The relatively low Al2O3 contents of the ande- central East Greenland. This granophyre represents anatectic melt of sites and dacites recovered from the Middle Series at Site 917 are Precambrian basement with a 87Sr/86Sr ratio of 0.7544 and a 143Nd/ similar to partial melts experimentally produced by dehydration 144Nd ratio of 0.5109. The major element compositions of these two melting of amphibolitic protoliths (Table 7). potential contaminants are given in Table 7, and their chondrite-nor- Drilling of the SDRS at both Site 642 on the Vøring Plateau and malized rare-earth element (REE) concentrations are given in Figure in the Rockall Trough recovered cordierite-bearing dacite flows (Ed- 9A. Figures 9B to 9D illustrate the results of combined fractional holm, Thiede, Taylor, et al., 1987; Morton et al., 1988) with average crystallization and assimilation processes (AFC) (DePaolo, 1981; Al- 87Sr/86Sr of 0.7120 and 143Nd/144Nd of 0.5122 (Taylor et al., 1989). barède, 1995) using these two possible contaminant types and Unit These dacite flows were interpreted to represent partial melts of up- 89 of the Lower Series as the differentiating magma. The fraction- per crustal metasedimentary rocks (Viereck et al., 1988; Morton et ation assemblage is 30% augite and 70% plagioclase, as experimen- al., 1988; Parson et al., 1989; Taylor and Morton, 1989). Melting of tally determined by Beard and Lofgren (1991) and Thy et al. (1998). mica-bearing metasediments and gneisses produces large melt frac- The results of the calculations are illustrated in Figure 9 for ranges in tions at relatively low temperatures (Patiño Douce and Johnston, liquid remaining (F = 1 to 0.5) and constant ratios of assimilation and 1991). For identical melting temperature and pressure (e.g., 950°C, crystallization masses (R = 0 to 0.5). Figures 9B and 9C show the ef- 10 kbar), metapelites and tonalite gneisses produce 30%–60% melt, fects of using the granophyre and Figure 9D of using the gneiss as while amphibolite protoliths may produce around 10% melt (Gardien contaminants. The AFC modeling of the REEs illustrates that it is et al., 1995). Therefore, the compositions of partial melts of metased- possible to account for the observed variation in the heavy rare-earth imentary rocks are strongly controlled by the protolith and often elements (HREEs) for small amounts of fractionation and assimila- highly silicic, aluminous, and alkalic (Johnston and Wyllie, 1988; Pa- tion. However, none of the models will account for the relatively tiño Douce and Johnston, 1991; Skjerlie and Johnston, 1993; Patiño steep slope in the light rare-earth elements (LREEs). In addition, the Douce and Beard, 1995; Scaillet et al., 1995; Singh and Johannes, model compositions show large negative Eu anomalies that are in- 1996; Stevens et al., 1997). For example, a biotite gneiss melted by consistent with the REE abundance data for the Site 917 dacites. Patiño Douce and Beard (1995) produced 42% melt at 925°C, which It has been experimentally demonstrated that dehydration melting is granitic in composition, containing >6 wt% K2O (Table 7). These of amphibolitic protoliths will produce partial melts ranging from melts differ from the tonalite, trondhjemite, or granodiorite melts rhyolitic near the solidus to dacitic at slightly higher extent of melting produced from melting quartz amphibolite at similar experimental (~10%–15%) (Beard and Lofgren, 1989, 1991; Wolf and Wyllie, conditions (Patiño Douce and Beard, 1995). The silicic volcanic 1991, 1994; Rushmer, 1991; Rapp et al., 1991; Sen and Dunn, 1994; rocks of Site 917 are tonalitic or granodioritic using the Ab-Or-An Patiño Douce and Beard, 1995; Springer and Seck, 1997). The com- classification (Baker, 1979), and, therefore, more akin to melts pro- position of such melts is strongly dependent on the extent of melting duced from amphibolite than from leucogneiss protoliths. The dacitic and the restitic phase assemblage. Sen and Dunn (1994) showed that flows recovered from the Vøring and Rockall Plateaus (Table 7) have melts produced at up to 15% melting in 10-kbar melting experiments relatively high Al2O3 contents and are corundum normative (3–8%). are quartz saturated and broadly dacitic in composition, while melts Elevated Al2O3 concentrations can reflect the presence of a vapor produced between 20% and 30% melting are andesitic. The SiO2 con- phase during partial melting of an amphibolitic source (Beard and tents of melts produced in experiments conducted at lower pressures Lofgren, 1989) and may not uniquely identify a metasedimentary (<3 kbar) are relatively similar to those produced at higher pressures source as suggested by Viereck et al. (1988) and Morton et al. (1988). for identical melt fractions (5–15 kbar) (e.g., Beard and Lofgren, In terms of SiO2 and K2O contents, both the Vøring and Rockall dac-

Table 7. Leg 152 dacites and comparisons.

Granophyre Gneiss Exp. melt Exp. melt Exp. melt 1000°C 1000°C 925°C Unit 47* Unit 55* Rockall† Vøring‡ CL87-52E** 324721†† 0.1 GPa‡‡ 0.3 GPa*** 0.3 GPa†††

SiO2 60.50 64.89 64.52 63.78 74.85 63.26 64.21 60.77 75.90 TiO2 0.99 0.74 0.90 1.38 0.34 0.78 0.28 1.53 0.32 Al2O3 14.86 13.32 16.08 16.85 12.30 17.25 16.46 14.07 13.04 FeO 7.07 5.19 6.03 5.21 1.96 2.11 5.56 9.58 1.27 MnO 0.14 0.14 0.14 0.06 0.03 0.06 0.22 0.37 0.03 MgO 2.67 3.04 1.53 0.62 0.59 1.60 2.13 3.08 0.37 CaO 4.78 4.42 2.22 4.64 1.07 3.30 3.99 6.35 0.64 Na2O 4.21 3.62 2.12 2.35 3.53 3.99 4.88 2.41 2.07 K2O 1.57 2.99 2.57 1.38 4.34 3.94 1.27 0.88 6.36 P2O5 0.27 0.07 0.16 0.61 0.07 0.30 0.96 Total 97.06 98.44 96.27 96.88 99.08 96.59 100.00 100.00 100.00

Notes: * = Site 917 dacite from Fitton et al. (1998b). Unit 47 = Sample 152-917A-32R-4, 76–60 cm. Unit 55 = Sample 152-917A-51R-1, 42–45 cm. † = Dacite from the Rockall trough, well 163/6-1A (Morton et al.,1988, table 4, analysis C7.1). ‡ = Dacite from the lower series at Site 642 on the Vøring Plateau (Parson et al., 1989, table 1, analysis of flow F112). ** = Granophyre component in the hybridized zone of the saddle part of the Miki Fjord macrodike (Blichert-Toft et al., 1992). †† = Agmatitic gneiss from the late Archean Skjoldun- gen alkaline igneous province, SE Greenland (Blichert-Toft et al., 1995). ‡‡ = Experimental melt in dehydration melting of basalt at 0.1 GPa and 1000°C (Beard and Lofgren, 1991). *** = Experimental melt in dehydration melting of basalt at 0.3 GPa and 1000°C (Beard and Lofgren, 1991). ††† = Experimental melt in dehydration melting of biotite gneiss at 0.3 GPa and 925 °C (Patiño Douce and Beard, 1995). Iron = total FeO. The experimental melts are calculated anhydrous to 100%.

107 P. THY, C.E. LESHER, J.D. MAYFIELD

C D

Figure 9. Modeling of rare-earth element (REE) variation for combined assimilation and fractional crystallization (AFC). The partitioning coefficients are from Martin (1987). Fractionation assumes 70% plagioclase and 30% augite. REE normalization values are from Sun and McDonough (1989). A. The samples used in the modeling: the andesitic to dacitic flows from the Middle Series (Fitton et al., 1998b) (Units 47 and 55), granophyre CL87-52E from Blichert-Toft et al. (1992, table 2), and Archean felsic gneiss 324721 from Blichert-Toft et al. (1995, table 2). The composition of Unit 89 is from Fitton et al. (1998b). B. AFC modeling using the granophyre as contaminant and Unit 89 as the differentiating magma. The model results are shown for 50% crystallization and constant assimilation/ fractionation ratios (R = 0 to 0.5 in step of 0.1). C. The same modeling as in (B) shown for constant R = 0.5 and variable liquid remaining (F = 1.0 to 0.5 in incre- ments of 0.1). D. AFC modeling using the gneiss as contaminant. The modeling variables are as for (B).

ites are relatively similar to the Site 917 dacites and to the melts pro- mafic amphibolite (either amphibolite with 27% plagioclase, 70% duced from amphibolitic protoliths (Beard and Lofgren, 1989). amphibole, and 3% quartz or metagabbro with 35% plagioclase, 48% The lower crust along the prerifted continental margin may act as amphibole, 14% augite, and 3% quartz). Further, we assume that a barrier to magma migration and may consequently trap large vol- likely protoliths are represented by a range of rare-earth compositions umes of mantle-derived magma (Fram and Lesher, 1997; Larsen and from average depleted to relatively enriched lower basaltic crust Saunders, 1998). Repeated injection and underplating may provide (Rudnick and Fountain, 1995). The melting reactions used in the cal- the heat for partial melting of lower to middle crustal granulites and culations are from Sen and Dunn (1994) and the partitioning coeffi- amphibolites (Hildreth and Moorbath, 1988; Kay and Kay, 1991). cients from Martin (1987). The REE analyses available of Middle Se- Wolf and Wyllie (1995) suggest that the amphibolite dehydration ries silicic flows are also shown in Figure 10A (Units 47 and 55) (Fit- melting solidus is equivalent to the H2O-saturated basalt solidus and ton et al., 1998b). The first set of modeling of partial melting of mafic show that liquid interconnectivity can be achieved at temperatures amphibolite assumes the melting reaction quartz + plagioclase + am- <900°C and with <5 vol% liquid. The melting is related to the (sim- phibole = clinopyroxene + melt (Fig. 10B), while the second set uses plified) breakdown of amphibole at low pressure to pyroxene + liquid the reaction quartz + plagioclase + amphibole = clinopyroxene + gar- and at pressures >10 kbar to pyroxene + garnet + liquid (Sen and net + melt (Fig. 10C). The source modes only have minor effects on Dunn, 1994; Wolf and Wyllie, 1995). the model results (Figs. 10D) (e.g., Martin, 1987; Rapp et al., 1991; The possibility that the andesites and dacites of the Middle Series Sen and Dunn, 1994; Johnson et al., 1997; Springer and Seck, 1997). at Site 917 are produced by partial melting of mafic continental Of most importance in governing the rare-earth pattern is the propor- sources can be tested by examining the rare-earth systematic of the tion of garnet in the residue. To account for the relatively flat slopes Middle Series dacites (Fitton et al., 1998b). The model calculations of the HREEs and the steepening of the LREEs, a garnet-absent resi- shown in Figure 10 assume nonmodal batch melting of continental due is required (Fig. 10B). This restricts the melting regime to middle

108 LOW-PRESSURE MELTING STUDIES AND DACITE ORIGIN

and upper crustal conditions (<8 kbar; Wolf and Wyllie, 1995). It can Archean to Proterozoic granulite and amphibolite gneiss complexes be seen that the results based on both amphibolitic and gabbroic that are dominated by tonalitic to granodioritic ortho- and paragneiss- crustal xenolith compositions reproduce relatively well the general es, metapelitic supercrustral rocks, metadolerite dikes, and granitic, pattern observed in the REEs of the dacites (Figs. 10B and 10C). syenitic, and basic intrusives (Taylor et al., 1992; Kalsbeek et al., However, only for very low extents of melting (<5%) or by assuming 1993; Blichert-Toft et al., 1995). The isotopic signature of many of an enriched protolith composition can the dacitic flow Unit 55 be re- these orthogneisses and metagneisses fulfill the basic requirement for produced. In order to reproduce Unit 47, a source with slightly higher Fitton et al. (1998a, 1998b) contamination model (Fig. 11). However, LREE/HREE ratios is required (Fig. 10B) and suggests some vari- the validity of the alternative proposal argued in this paper can equally ability in the source LREE/HREE ratio. be substantiated by the available isotope data. Metadolerite dikes and The model suggested by Fitton et al. (1998a) requires contami- basic intrusions present in the basement suggest late Archean and Pro- nants with 143Nd/144Nd ratios <0.5109 and 87Sr/86Sr ratios >0.7140 to terozoic underplating and intrusion of basaltic magma into the conti- account for the isotopic relations seen in the Middle Series dacites nental crust. Many of these basic dikes and intrusions have Nd-Sr iso- (Fig. 11). The Southeast Greenland coastal areas are composed of tope relations relatively similar to the dacites from Site 917 and the

C D

Figure 10. Partial melting modeling of the rare-earth elements (REE) of the silicic flows from the Middle Series at Site 917. The models assume nonmodal batch melting and uses partitioning coefficients from Martin (1987). The source is assumed to be hydrated and metamorphosed basalt or gabbro (amphibolitic: 26% plagioclase, 3% quartz, 71% amphibole; metagabbroic/basaltic: 35% plagioclase, 3% quartz, 48% amphibole, 14% augite). The model calculations, all shown at 20% melting, are given for a range of trace-element compositions based on the average concentration of depleted mafic xenoliths of lower-continental origin (in table 6 of Rudnick and Fountain, 1995). The modeling is shown for average mafic xenoliths without enrichment, average mafic xenoliths enriched by 25% REE, and average mafic xenoliths enriched by 100% REE. The REE normalization values are from Sun and McDonough (1989). A. The two Middle Series flow units referred to in the calculations (Fitton et al., 1998a; Units 47 and 55). “Crustal mafic xenolith” is the average of continental mafic xenoliths compiled by Rudnick and Fountain (1995). B. Modeling assuming the melting reaction quartz + plagioclase + amphibole = clinopyroxene + melt. The melting proportions were set at 0.85 amph, 0.41 pl, 0.04 q, and –0.30 cpx. Melting modes for garnet-free melting are based on Rushmer (1991) and Sen and Dunn (1994). The restite produced at 20% melting is 20% pl, 13% cpx, 3% q, and 64% amph. C. Modeling assuming the melting reaction quartz + plagioclase + amphibole = clinopyroxene + garnet + melt with melting proportions taken from Sen and Dunn (1994, equation 5). The restite at 20% melting is 24% grt, 15% pl, 34% cpx, and 27% amph. D. Modeling assuming a metagabbroic source mode and the same melting reactions as Figures 9B and 9C (garnet-free and garnet-bearing restite). The restite at 20% melting for the garnet-free reaction is 34% pl, 25% cpx, 2% q, and 39% amph and for the garnet-bearing restite 27% pl, 49% cpx, 23% grt, and 1% amph.

109 P. THY, C.E. LESHER, J.D. MAYFIELD

Vøring Plateau (Fig. 11). In particular, a Proterozoic basic dike constraints for low-pressure crystal fractionation to be evaluated. The swarm in the Ammassalik area reveals Nd-Sr isotope ratios (Kalsbeek experimental liquid lines of descent as well as modeling of crystal et al., 1993) very similar to the Vøring dacites and to one of the Site fractionation indicate near-multiple saturation and modest extent of 917 dacites (Fig. 11). Further, the Proterozoic Ammassalik igneous fractionation for the majority of recovered flows. The melting rela- complex and the late Archean Skjoldungen complex, at about the tions for samples from the oceanic succession (Sites 918 and 989) same geographic latitude as Site 917, contain basic and intermediate differ from those obtained from the continental succession (Thy et al., intrusive components (Kalsbeek et al., 1993; Blichert-Toft et al., 1998) by the early appearance of low-Ca pyroxene. Therefore, the 1995) that isotopically compare well to other Site 917 dacites. Hence, continental and oceanic successions are not comagmatic and related there is evidence that old mafic crustal material was available for re- to identical low-pressure liquid lines of descent. The results of melt- melting during an early Tertiary continental breakup and that this can ing basaltic andesite from the Middle Series are difficult to reconcile account for the high-Sr and low-Nd isotope ratios observed in the dac- with the compositional variation in the extruded lavas. The principal ites. reason is that the variation in SiO2 and FeO suggests early magnetite The experimental and compositional modeling of the Site 917 crystallization and that this phase was not saturated in the melting ex- dacites provide support for their formation as partial melts from basic periments. The REE modeling shows that the dacites can be produced protoliths. The relatively steep REE patterns and the lack of Eu by low degrees of melting (<20%) of hydrated gabbroic or basaltic anomalies can be closely modeled by partial melting using reason- material at middle to upper crustal levels. Such crustal melts may able amphibolitic source compositions and melting reactions. In con- have been important contaminants of the Lower and Middle Series trast, modeling of combined fractionation and assimilation with local basaltic magmas. The trace-element modeling does not support gneissic contaminants produces relatively flat REE patterns and leucogneiss contaminants as suggested by Fitton et al. (1998a, strong Eu anomalies that are not observed for the dacites. The origin, 1998b). as partial melts from amphibolitic protoliths, is further attractive because it does not require extensive fractional crystallization of Fe-Ti ACKNOWLEDGMENTS oxide-containing assemblages. This latter point is important since Fe- Ti oxides do not occur as phenocrysts and are not saturated in the low-pressure melting experiments. Critical reviews of an early version of the manuscript by J. Allan, J. Beard, and R. Batiza were very helpful. Dr. L.M. Larsen (Geolog- ical Survey of Greenland and Denmark) provided XRF analyses of SUMMARY starting materials and the whole-rock analyses of the Leg 152 and Leg 163 samples before publication. Support for this research was The results of melting lavas from the continental and oceanic suc- provided by NSF-94-19382 and JOI/USSAC (CEL). cessions of the seaward-dipping reflectors allow the phase equilibria REFERENCES

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