The Role of Garnet Pyroxenite in High-Fe Mantle Melt Generation: High Pressure Melting Experiments

Evidence for the existence of heterogeneous or 'marble cake' convecting mantle¹ is provided recently by rare, high MgO ( ∼ 15 wt.%) primitive magmas with anomalously high abundances of FeO* ( ∼ 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 Paraná-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 mantle². The ferropicrites are mildly- to sub-alkaline and have low contents of Al₂O₃ (< 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 crust². 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 Paraná-Etendeka continental flood-basalt province. The sample has high contents of MgO ( ∼ 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 ∼ 2.2 GPa from an olivine-pyroxene residue or ∼ 5 GPa from a garnet-pyroxene residue. A low bulk-rock Al₂O₃ 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 ∼ 5 GPa would occur at mantle potential temperatures of ∼ 1550° C. Subsequent melting of peridotite at ∼ 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 Allègre et al., Philosophical Transactions of the Royal Society of London A297, 447-477 (1980). ² Gibson et al., Earth and Planetary Science Letters 174, 355-374 (2000). ³ Gibson, Earth and Planetary Science Letters 195, 59-74 (2002).