Oxygen Isotopic Composition of Ca-fe-rich Silicates in and around an Allende

Introduction: Most components of CV chondrites show variable degrees of late-stage alteration resulting in the modification of preexisting mineral assemblages and/or in the formation of new minerals. The conditions and environment under which these processes occurred are highly debated, with arguments in favor of solar nebular as well as asteroidal settings [1]. In the particular case of calcium-aluminum-rich inclusions (CAIs) these processes are typically evidenced by an Fe-alkali-halogen metasomatism, resulting in the formation of anorthite, nepheline, sodalite, grossular, wollastonite, diopside-hedenbergite-series pyroxene, andradite, and kirschsteinite. Similar mineral assemblages are also found in the matrices of these chondrites and in and around Allende dark inclusions. Detailed analysis and comparison of these late stage alteration phases in two different petrographic settings might offer better constraints on the formation conditions and environment. Therefore, we studied and compared the texture, mineralogy and chemical and oxygen isotopic composition of CaFe rich assemblages both in CAIs and the matrix of Allende, a member of the oxidized CV group in which these assemblages are very abundant. Petrography: Allende TS24F1 is a large fluffy type A CAI [2] surrounded by an accretionary rim (AR) [3,4]. High-resolution X-Ray-area mapping highlights the distribution of CaFe rich silicates throughout the thin section (Fig. 1). The phases occur lining the exterior of Wark-Lovering rims (WLR), mostly filling internal cavities within the irregularly shaped CAI and in areas where the CAI is fractured, but they are also concentrated along the outer boundary of the accre-tionary rim (AR) where it contacts the matrix. The CaFe rich minerals also occur uniformly throughout the matrix [5]. Petrographically, the assemblages in contact with the CAI consist of relatively large (~20 µm) euhedral grains of andra-dite (And), hedenbergite (Hed), and massive to acicular wol-lastonite (Wo) and intermediate Ca-rich pyroxene (Ca-px). Directly contacting the WLR and the CAI itself, there are assemblages of And and Hed (Fs >45Wo~50) intergrown with Ca-px (Fs 0-20Wo~50; Al2O3<15%), either as inclusions or as rims around them (Fig. 2). The FeO and Al 2O3 contents of the Ca-px from this setting are highly variable over a micron-scale. Acicular fine-grained Ca-px (Fs 10-35Wo~50; Al2O3<5%) occurs as highly porous masses around the most FeO-rich minerals, in many cases with interstitial nepheline. Wo (En <6Fs<3Wo>95) can occur as needles intergrown with Hed (Fig. 2b) or Ca-px as well as massive Wo inside euhedral Hed grains. Relict nodules of forsteritic olivine with sulfide cores occur within some of the …