La Réunion Island dunites as analogs of the Martian chassignites: Tracking trapped melts with incompatible trace elements

Abstract

Fractional crystallization of basalts produces a variety of ultramafic cumulates poor in incompatible elements. In order to discuss the effect of trapped liquids on the concentrations of incompatible elements in these cumulates, we studied a series of dunites and pyroxenites from the Piton de la Fournaise (La Réunion Island, Indian Ocean). These rocks have a high variability in the abundance of these elements, which is only partially explained by their proportions of olivines and pyroxenes. Mass balance calculations indicate that ≈0.25 to 1 wt% of trapped melts are required to account for the budget of the highly incompatible element abundances such as U, Rb, Nb or light Rare Earth Elements (REE). Using our results, we revisited the REE geochemistry of chassignites, a grouplet of Martian meteorites. Chassignites are mineralogically, texturally and chemically very similar to the dunites from La Réunion Island, which can be seen as very close terrestrial analogs. The variations of REE abundances and ratios in Chassigny, one of the three known chassignites, can be explained by inhomogeneous distribution of trapped melts in this meteorite at the scale of the size of the fragments (typically <1 g) used for the chemical analyses. Using these variations, we deduced the shape of the REE pattern and abundances of the Chassigny parent melt.