Experimental study of polybaric REE partitioning between olivine, pyroxene and melt of the Yamato 980459 composition: Insights into the petrogenesis of depleted shergottites

Abstract

A synthetic composition representing the Yamato 980459 martian basalt (shergottite) has been used to carry out phase relation, and rare earth element (REE) olivine and pyroxene partitioning experiments. Yamato 980459 is a sample of primitive basalt derived from a reduced end-member among martian mantle sources. Experiments carried out between 1–2 GPa and 1350–1650 °C simulate the estimated pressure–temperature conditions of basaltic melt generation in the martian mantle. Olivine-melt and orthopyroxene-melt partition coefficients for La, Nd, Sm, Eu, Gd and Yb ( D REE values) were determined by LA-ICPMS, and are similar to the published values for terrestrial basaltic systems. We have not detected significant variation in D-values with pressure over the range investigated, and by comparison with previous studies carried out at lower pressure. We apply the experimentally obtained olivine-melt and orthopyroxene-melt D REE values to fractional crystallization and partial melting models to develop a three-stage geochemical model for the evolution of martian meteorites. In our model we propose two ancient (∼4.535 Ga) sources: the Nakhlite Source, located in the shallow mantle, and the Deep Mantle Source, located close to the martian core-mantle boundary. These two sources evolved distinctly on the ε 143Nd evolution curve due to their different Sm/Nd ratios. By partially melting the Nakhlite Source at ∼1.3 Ga, we are able to produce a slightly depleted residue (Nakhlite Residue). The Nakhlite Residue is left undisturbed until ∼500 Ma, at which point the depleted Deep Mantle Source is brought up by a plume mechanism carrying with it high heat flow, melts and isotopic signatures of the deep mantle (e.g., ε 182W, ε 142Nd, etc.). The plume-derived Deep Mantle Source combines with the Nakhlite Residue producing a mixture that becomes a mantle source (herein referred to as “the Y98 source”) for Yamato 980459 and the other depleted shergottites with the characteristic range of Sm/Nd ratios of these meteorites. The same hot plume provides a heat source for the formation of enriched and intermediate shergottites. Our model reproduces the REE patterns of nakhlites and depleted shergottites and can explain high ε 143Nd in depleted shergottites. Furthermore, the model results can be used to interpret whole rock Rb–Sr and Sm–Nd ages of shergottites.