Fine‐grained, spinel‐rich inclusions from the reduced CV chondrites Efremovka and Leoville: I. Mineralogy, petrology, and bulk chemistry

Abstract

Abstract-Fine-grained, spinel-rich inclusions in the reduced CV chondrites Efremovka and Leoville consist of spinel, melilite, anorthite, Al-diopside, and minor hibonite and perovskite; forsterite is very rare. Several CAIs are surrounded by forsterite-rich accretionary rims. In contrast to heavily altered fine-grained CAIs in the oxidized CV chondrite Allende, those in the reduced CVs experienced very little alteration (secondary nepheline and sodalite are rare). The Efremovka and Leoville fine-grained CAIs are 16 O-enriched and, like their Allende counterparts, generally have volatility fractionated group II rare earth element patterns. Three out of 13 fine-grained CAIs we studied are structurally uniform and consist of small concentrically zoned nodules having spinel ± hibonite ± perovskite cores surrounded by layers of melilite and Al-diopside. Other fine-grained CAIs show an overall structural zonation defined by modal mineralogy differences between the inclusion cores and mantles. The cores are melilite-free and consist of tiny spinel ± hibonite ± perovskite grains surrounded by layers of anorthite and Al-diopside. The mantles are calcium-enriched, magnesium-depleted and coarsergrained relative to the cores; they generally contain abundant melilite but have less spinel and anorthite than the cores. The bulk compositions of fine-grained CAIs generally show significant fractionation of Al from Ca and Ti, with Ca and Ti being depleted relative to Al; they are similar to those of coarsegrained, type C igneous CAIs, and thus are reasonable candidate precursors for the latter. The finegrained CAIs originally formed as aggregates of spinel-perovskite-melilite ± hibonite gas-solid condensates from a reservoir that was 16 O-enriched but depleted in the most refractory REEs. These aggregates later experienced low-temperature gas-solid nebular reactions with gaseous SiO and Mg to form Al-diopside and ±anorthite. The zoned structures of many of the fine-grained inclusions may be the result of subsequent reheating that resulted in the evaporative loss of SiO and Mg and the formation of melilite. The inferred multi-stage formation history of fine-grained inclusions in Efremovka and Leoville is consistent with a complex formation history of coarse-grained CAIs in CV chondrites.

Refractory inclusions in the CH/CB‐like carbonaceous chondrite Isheyevo: I. Mineralogy and petrography

“…5a-c, 6-8, 9a, 10) is texturally and mineralogically similar to CAIs in other chondrite groups (MacPherson 2003 and references therein; Lin et al 2006), which commonly show evidence for gas-solid condensation reactions, resulting in replacement of high-temperature minerals by the lowtemperature mineral assemblages; e.g., replacement of melilite and spinel by anorthite and Al-diopside (e.g., Krot et al 2004d), and replacement of forsterite by low-Ca pyroxene (Krot et al 2004a(Krot et al , 2004b.…”

Metasomatism in the Early Solar System: The Record from Chondritic Meteorites

“…Towards the CAI core, melilite and, to a smaller degree, anorthite are replaced by grossular, monticellite, wollastonite, and wadalite. Al-px Al-and Fe-bearing high-Ca pyroxene, an anorthite, andr andradite, fa ferroan olivine, grl grossular mel melilite; mnl monticellite, nph nepheline px Al,Ti-diopside; sod sodalite, sp spinel, wdl wadalite, wol wollastonite in the distribution of secondary minerals in Allende fine-grained CAIs is partly due to the primary mineralogical zoning observed in many fine-grained CAIs from the CV R chondrites Efremovka and Leoville (Krot et al 2004a). The CAI cores are enriched in anorthite, whereas the CAI mantles contain a high abundance of melilite.…”

Petrographic comparison of refractory inclusions from different chemical groups of chondrites

“…In contrast, hibonite-rich inclusions are the intensely studied objects in CM2 chondrites (e.g., Macdougall 1979;MacPherson et al 1983;Ireland 1990;Simon et al 1998), whereas grossitebearing inclusions appear more abundant in CR clan chondrites (e.g., Kimura et al 1993;Weber and Bischoff 1997;Aléon et al 2002;Krot et al 2002). Fine-grained anorthite-spinel-rich inclusions (ASI) were reported in the Ningqiang carbonaceous chondrite (Lin and Kimura 1998), with their counterparts in CR chondrites (Weber and Bischoff 1997;Aléon et al 2002), reduced CV chondrites (Krot et al 2004a), Yamato-81020 (CO3) (Itoh et al 2004) and the unique Acfer 094 carbonaceous chondrite (Krot et al 2004b). In addition, fine-grained spinel-pyroxene and melilite-spinelrich (fluffy type A) inclusions have been widely reported from CV chondrites.…”