HR: 0800h
AN: V51B-0523    [Abstracts]
TI: The Role of Garnet Pyroxenite in High-Fe Mantle Melt Generation: High Pressure Melting Experiments
AU: * Tuff, J
EM: jtuf02@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Takahashi, E
EM: etakahas@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo, 152-8551 Japan
AU: Takahashi, E
EM: etakahas@geo.titech.ac.jp
AF: Institute for Research on Earth Evolution, Jamstec, 2-15 Natsushima-Cho, Yokosuka, 237-0061 Japan
AU: Gibson, S
EM: sally@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AB: Evidence for the existence of heterogeneous or 'marble cake' convecting mantle$^{1}$ is provided recently by rare, high MgO ($\sim$ 15 wt.%) primitive magmas with anomalously high abundances of FeO* ($\sim$ 13.5 to 16 wt. %$^{2,3}$; where FeO* = total Fe as FeO). These high-Fe mantle melts show a limited occurrence in the initial stage of magmatism in large igneous provinces (e.g. Deccan, Ethiopia and Paran\'{a}-Etendeka) and some have incompatible trace-element and radiogenic-isotopic ratios (Sr, Nd and Pb) that resemble those of ocean-island basalts. This suggests that they are predominantly derived from the convecting mantle$^{2}$. The ferropicrites are mildly- to sub-alkaline and have low contents of Al$_{2}$O$_{3}$ ($<$ 10 wt.%) and heavy rare-earth elements (e.g. Lu $<$ 0.18ppm) that are consistent with the increased stability of garnet, due to the high FeO* content in the ferropicrite mantle source. It has been proposed that the source of the high FeO* may be garnet-pyroxenite streaks derived from subducted mafic oceanic crust$^{2}$. We have undertaken melting experiments between 1 atmosphere and 7 GPa in order to determine the anhydrous phase relations of an uncontaminated ferropicrite lava from the base of the Early-Cretaceous Paran\'{a}-Etendeka continental flood-basalt province. The sample has high contents of MgO ($\sim$ 14.9 wt.%), FeO* (14.9 wt.%) and NiO (0.07 wt.%). Olivine phenocrysts have maximum Fo contents of 85 and are in equilibrium with the host rock, assuming a K$_{d}$ of 0.32 and we believe that the sample is representative of a primary Fe-rich mantle plume derived melt. In total, 75 experimental runs were carried out. Melting phase relations as well as compositions and modal proportions of all coexisting phases were successfully determined in 60 run products. Phase relations indicate that the ferropicrite melt was generated either at $\sim$ 2.2 GPa from an olivine-pyroxene residue or $\sim$ 5 GPa from a garnet-pyroxene residue. A low bulk-rock Al$_{2}$O$_{3}$ content (9 wt.%) and high [Gd/Yb]$_{n}$ ratio (3.1) are consistent with residual garnet in the ferropicrite melt source and favour high-pressure melting of garnet-pyroxenite. The garnet pyroxenite may represent subducted oceanic lithosphere entrained by the upwelling Tristan mantle plume starting-head. During adiabatic decompression, intersection of the garnet pyroxenite solidus at $\sim$ 5 GPa would occur at mantle potential temperatures of $\sim$ 1550$\deg$C. Subsequent melting of peridotite at $\sim$ 4.5 GPa may be restricted by the thick overlying sub-continental lithosphere such that dilution of the garnet-pyroxenite component would be significantly less than in intra-plate oceanic settings. This model accounts for the limited occurrence of ferropicrite magma in the initial stage of continental large igneous provinces and its absence in ocean-island basalt successions. $^{1}$ All\`{e}gre {\it et al}., Philosophical Transactions of the Royal Society of London A297, 447-477 (1980). $^{2}$ Gibson {\it et al}., Earth and Planetary Science Letters 174, 355-374 (2000). $^{3}$ Gibson, Earth and Planetary Science Letters 195, 59-74 (2002).
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting