Generation of mantle heterogeneity via reaction between MORB-pyroxenite derived partial melts and peridotite

Characterization of mineralogy and lithology of mantle heterogeneities is important as they affect melting processes and physical properties of the mantle in turn influencing mantle dynamics. Direct evidence of such heterogeneities are found as lithospheric xenoliths and in tectonically exposed mantle sections. However, possible major element compositions of heterogeneities that can be produced in the convective mantle remain unclear. Reaction between silica-rich melt from subducted oceanic crust and peridotite has been deemed responsible for generation of secondary pyroxenite and olivine-free mantle beneath Hawaii [1]. However, no studies till date have systematically characterized the evolution of mineralogy and lithology owing to such a melt-rock reaction. We performed reaction experiments between two MORB-pyroxenite derived basaltic andesites and a volatile-free lherzolite using piston-cylinder apparatus and Pt/Gr capsules, at 2.5-3 GPa, 1375°C and 1440°C. The fraction of melt was varied from ~8 to 50%. Melt was introduced either in a layered geometry or mixed homogeneously with peridotite to simulate channelized and porous flow, respectively. Layered experiments produced a zone of (± garnet-) websterite separating a pool of reacted melt from four-phase lherzolite. They show that olivine-exhaustion is possible only locally along melt channels and the peridotite domains away from melt bands remain effectively unaltered in terms of modal mineralogy although slight diffusive enrichment in TiO₂, Al₂O₃ and FeO is observed. In the mixed experiments, olivine mode diminishes at the expense of orthopyroxene producing a continuous spectrum from lherzolite to olivine websterite and finally websterite with increasing melt-rock ratio. With increasing orthopyroxene modal enrichment, orthopyroxene composition displays increase in TiO₂ (~0.2 to ~0.9 wt%), Al₂O₃ (~4.5 to ~6.7 wt%), FeO (~6 to ~8 wt%), Na₂O (~0.2 to 0.8 wt%) and decrease in Cr₂O₃ (~0.35 to 0.15 wt%) compared to that in the starting lherzolite. Similarly, clinopyroxene shows an increase in TiO₂ (~0.4 to 1.8 wt%), Al₂O₃ (~6 to ~9 wt%), FeO (~4.2 to ~6.8 wt%), Na₂O (~0.86 to ~2.06 wt%) and decrease in Cr₂O₃ (~0.5 to 0.11 wt%) while garnet displays an increase in TiO₂ (~0.36 to 1.28 wt%), FeO (~6.7 to ~9.8 wt%) and decrease in Cr₂O₃ (~1.2 to ~0.2 wt%) with respect to those in the starting peridotite. Mg# of the bulk lithology decreased from 90 (starting lherzolite) to 85 (websterite) with increasing orthopyroxene enrichment. The formation of olivine-poor lherzolite, olivine websterite, and websterite with increasing melt-rock ratio adds to the pre-existing heterogeneities in the mantle. While these lithologies resemble some high Mg# natural pyroxenites, their compositional trends in major element space are distinct compared to high-MgO pyroxenites sampled from lithosphere. Future experimental studies will need to constrain the melting behavior of the lithologies produced in our melt-rock reaction experiments. [1] Sobolev A.V. et al. 2005, Nature 434, 590-597.