Fe and V oxidation state in chromites of brachinites and brachinite-like ungrouped achondrites.

Introduction: 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 (Fa_26-36, 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]. Studying the oxidation state of these meteorites helps to understand the nature and evolution of their chondrite parent body(ies). This information was previously only available indirectly through thermodynamic calculations [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, were investigated in [6].

Our purpose is to obtain a large and complete cation valence dataset on chromites, low/high-Ca pyroxenes and olivines 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).

Fig. 1. Fayalite mol% versus FeO-MnO ratio for brachinite and UBA studied olivines.

Samples and Method: A whole set of XAS measurements at V, Fe and Ti K-edges has been performed on 3 brachinites, NWA 4969, NWA 12733, NWA 13489, and 4 UBAs, AlH 010, MIL 090206, NWA 5400, NWA 6112 (Fig. 1).

Micro-XAS measurements were carried out at ID21 beamline, ESRF, using Si(311) monochromator crystals. Beam spot size on the sample surface was approximately 1000*500 nm². Spectra were measured in fluorescence mode using a silicon drift detector. For each meteorite, we collected multiple spectra for each metal edge on several mineral grains after XRF elemental maps inspection (for minerals identification) of several meteorite areas. 

Preliminary Results: Preliminary results from XANES on chromite crystals show that Fe is mainly present as Fe²⁺ in tetrahedral coordination while V is present as V²⁺. Quantitative EXAFS fit performed at V K-edge shows that divalent V is hosted in octahedra with a V-O distance of 2.00(1) Å and II and III shell distances (V-Cr=2.99(2) Å; V-Fe=3.49(2) Å in excellent agreement with V positioned at the 16d position.

V divalent state has been rarely described in natural samples [7], thus indicating extremely reduced oxygen fugacity values for brachinites and UBAs source material. We will expand XAS analysis to pyroxenes and olivines of all meteorite dataset, comparing valence states of Fe, V and Ti.

 

References: [1] Keil K. (2014) Chemie der Erde 74, 311–329. [2] Day J.M.D. et al., (2012) Nature Geosc. 5, 614-617. [3] Cuppone et al., in publication [4] Gardner-Vandy K.G. et al., (2013) Geochimica et Cosmochimica Acta 122, 36-57. [5] Goodrich C.A. et al., (2014) Geochimica et Cosmochimica Acta 135, 126-169. [6] Righter K. et al. (2016) American Mineralogist 101, 1928-1942. [7] Camara et al. (2019) Minerals 9, 4.

Acknowledgements: This work is supported by the ASI-INAF agreement n.2018-16-HH.0, Ol-BODIES project. We thank the Natural History Museum Bern and the National Institute for Polar Research for the meteorite samples (AlH010 and MIL 090206, respectively). We acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities (Exp. ES-1400) and we thank ID21 staff for the support during the measurements. This study was carried out within the Space It Up project funded by the Italian Space Agency, ASI, and the Ministry of University and Research, MUR, under contract n. 2024-5-E.0 - CUP n. I53D24000060005.