Fe2+-Mg partitioning between olivine and basaltic melts: Applications to genesis of olivine-phyric shergottites and conditions of melting in the Martian interior

摘要

Fe2+-Mg partitioning between olivine and basaltic melt, expressed by the exchange coefficient, KDol-meltFe-Mg [=(XmeltMg/XolivineMg)/(X meltFe2+/XolivineFe2+)] is widely used to check if a rock composition may represent a mantle-derived magma, to demonstrate equilibrium between coexisting olivine and groundmass in mafic-ultramafic systems, both in experiments and in natural assemblages, and to constrain liquid lines of descent where olivine is the dominant fractionating phase. However, KDol-meltFe-Mg of 0.30, which is appropriate for understanding most terrestrial basalts petrogenesis may not apply for Martian basalts as KDol-meltFe-Mg is known to depend strongly on the melt compositions and Martian systems produce basalts that are distinctly richer in iron than terrestrial basalts. Here we compiled experimental data on olivine-melt equilibria of Martian and terrestrial basalt compositions to parameterize the effect of magma composition on KDol-meltFe-Mg and derive the KDol-meltFe-Mg applicable for Martian magmatic systems. We find that the equilibrium relationship between olivine and basaltic melt in Martian systems is described by KDol-meltFe-Mg of 0.35±0.01.Applying the newly parameterized values of KDol-meltFe-Mg to olivine-phyric shergottites suggest that the only known Martian meteorites where the olivine cores and the bulk composition are in equilibrium and therefore could represent magma compositions are: Yamato 980459, NWA 5789, and NWA 2990. LAR 06319, which has been suggested to represent a near magma composition, actually contains ~11wt.% excess olivine. All other ol-phyric shergottites contain significant excess olivine (20-52 wt.%). Further, assuming that the basalts analyzed by the Mars Exploration Rovers at Gusev crater and the Bounce Rock in Meridiani Planum lie on olivine control lines, we have used our newly parameterized KDol-meltFe-Mg to estimate primary magmas in equilibrium with the model Martian mantle. Application of geothermobarometers to new primitive magma compositions suggest that basalt generation in the Martian mantle occurs at greater depths and higher temperatures than previously thought.