A. Bischoff

Trace element abundances and magnesium, calcium, and titanium isotopic compositions of grossite-containing inclusions from the carbonaceous chondrite Acfer 182

The carbonaceous chondrite Acfer 182 contains Ca,Al-rich inclusions (CAIs) that differ from most CAIs in other meteorites. Many of them contain grossite (CaAl4O7), whose modal abundance exceeds 30 vol % in most of these CAIs (“grossite-rich inclusions”). Similar inclusions have been found only in the CH chondrite ALH85085 and the CR chondrite Acfer 059-El Djouf 001. Ion microprobe analyses of trace elements were made on nineteen CAIs from Acfer 182 and two CAIs from Acfer 059-El Djouf 001, of AlMg isotopes on eighteen Acfer 182 and the two Acfer 059-El Djouf 001 inclusions, and of calcium isotopes on ten Acfer 182 inclusions (on six of them also of titanium isotopes) and one Acfer 059-El Djouf 001 inclusion. Trace element and isotopic signatures of grossite-containing inclusions from Acfer 182 resemble those of some inclusions from ALH85085. Volatility-fractionated trace element abundance patterns, ranging from ultrarefractory-depleted (similar to Group II pattern) to volatile-enhanced, are predominant (17 CAIs, including both inclusions from Acfer 059-El Djouf 001). Two inclusions have ultrarefractory patterns, one has a Group III pattern, and one inclusion has a Group III-related pattern similar to patterns found in HAL-type inclusions. Only one CAI from Acfer 182, but both inclusions from Acfer 059-El Djouf 001, have nonlinear excesses of 26Mg corresponding to the initial 26Al27Al ratio of ∼ 5 × 10−5. None of the twenty inclusions has a significant intrinsic isotopic mass fractionation of Mg. Titanium is normal in all analyzed CAIs and only small 48Ca excesses (4.3 and 3.8‰) are present in two inclusions. Isotopes of both Ca and Ti show no significant intrinsic mass fractionations. Grossite-containing inclusions from Acfer 182 occur in different petrographic contexts. In many cases petrographic characteristics are correlated with trace element abundances. Almost all inclusions studied in this work have a condensation history but no evidence for extensive evaporation processes is present. Many of the CAIs must have formed by direct gas-solid condensation, while some inclusions with clearly identifiable igneous textures formed from a refractory melt. Grossite, whose formation was the subject of controversy in the past, probably condensed from a reservoir with a higher-than-chondritic Ca/Al-ratio and depleted in the highly refractory trace elements, or crystallized from a Ca,Al-rich melt. The two ultrarefractory inclusions from Acfer 182 have extremely high refractory trace element abundances (sometimes exceeding 105 × CI). The lack of isotopic fractionation indicates a formation at very high temperatures by condensation rather than evaporation. The lack of 26Mg excesses in almost all inclusions from Acfer 182 is best explained by heterogeneous distribution of 26Al in the primitive solar nebula. Grossite-rich inclusions having volatility-fractionated ultrarefractory-depleted trace element abundance patterns and lacking 26Mg excesses could be the complement of platy hibonites (PLACs) from Murchison with Group III or ultrarefractory patterns and likewise no 26Mg excesses.

The occurrence of grossite (CaAl4O7) in chondrites

The mineral grossite (CaAl4O7; Ca-dialuminate, also termed CA2) is a rare phase in terrestrial and extraterrestrial rocks. In meteorites, grossite was found in Ca,Al-rich inclusions (CAIs) from Allende (CV3), Vigarano (CV3), Leoville (CV3), Efremovka (CV3), Colony (CO3), Murchison (CM2), ALH 85085 (CH), Acfer 182 (CH), Acfer 059-E1 Djouf 001 (CR2), and the anomalous C-chondrite Adelaide. In Ca,Al-rich inclusions from most of these meteorites, grossite is an accessory phase; however, in many CAIs from ALH 85085, Acfer 182, and Acfer 059-E1 Djouf 001 it is a major phase. In this paper, we summarize the results of literature reports on grossite-bearing inclusions and focus, in detail, on the occurrence, mineralogy, and chemistry of CaAl4O7-containing Ca,Al-rich inclusions in Acfer 182 and Acfer 059-E1 Djouf 001. In these carbonaceous chondrites, abundant CAIs with high modal abundances of grossite (> 30 vol%, often >60 vol%) were discovered. To classify Ca,Al-rich inclusions with such high modal abundances of CaAl4O7 we suggest to introduce a new type of Ca,Al-rich inclusion: grossite-rich inclusions. In Acfer 059-E1 Djouf 001 and Acfer 182, grossite-containing inclusions occur with different petrographic characteristics. The most common type consists of a core of grossite rimmed by melilite, Ca-pyroxene ± spinel ± hibonite. In an other type, anhedral, polycrystalline grossite complexes of irregular shape are enclosed in melilite. In some spherical CA2-inclusions abundant euhedral to subhedral grossite and/or hibonite crystals are embedded in melilite. In two inclusions, a core of hibonite is mantled by grossite. Additionally, some grossite-bearing inclusions with unique texture and/or mineralogy were found. One of these inclusions is rich in Zr,Y-rich perovskite and contains phases rich in the ultrarefractory elements Sc and/or Zr. Almost all of the grossite-containing inclusions are rich in Al2O3 (>50 wt%), even if grossite is only a minor phase. Based on textural appearance and/or on REE contents in some inclusions (core minerals with Group II patterns) a condensation history for most inclusions is suggested. A single inclusion rich in perovskite and other phases containing high concentrations of ultrarefractory elements appears to have formed as an early condensate from the solar nebula.