REDOX CONDITIONS FROM XAS SPECTROSCOPY ON BRACHINITES AND BRACHINITE-LIKE UNGROUPED ACHONDRITES

Brachinites are olivine-rich primitive achondrites which record planetary differentiation onset on asteroidal bodies, displaying equilibrated textures and homogenous mineral phases with oxidised mineral chemistry (Fa26-36, Fig. 1) [1]. Ungrouped brachinite-like achondrite meteorites (UBAs) share many similarities with brachinites [2], whose asteroidal counterpart, as well as for brachinites, has not been identified to date. Their close compositional and textural affinities, pose questions about the possible genetic link among these two meteorite groups, i.e. whether they derive from a single heterogeneous or more compositionally affine parent body(ies) [3]. Constraining the oxidation state of starting material could help to identify the chondritic parental body, but this information has been obtained only indirectly, through thermodynamic calculation, starting from mineral composition assemblage and equilibrium temperature between mineral phases [4]. Few works tried to directly constrain oxygen fugacities of brachinites source material, by measuring Cr valence in olivines of some brachinites and brachinite-likes [5], while V valences in chromites from different meteorite groups, including brachinites, was investigated in [6]. Our purpose is to obtain a large and complete cation valence dataset on olivines, low/high-Ca pyroxenes and chromites from brachinites and brachinite-like achondrites by means of X-ray absorption spectroscopy (XAS), to directly constrain oxygen fugacity of source material and possibly better define both the genetic relationship between them and which kind of material formed the(ir) parental body(ies).