We surveyed 270 ordinary chondrites (115 H, 116 L and 39 L/LL and LL) for chrome-spinel (Cr-Sp) chondrules and inclusions and Cr- Sp-rich mafic silicate chondrules. Here we discuss Cr-Sp inclusions. These inclusions are most common among H (52) chondrites and 3-4X less common in L (15) and L/LL+LL (5) chondrites. We divide the inclusions into two types chiefly on the basis of Cr/(Cr+Al) in the Cr-Sp: high (>0.84) in type I, low (<0.84) and, in many cases, variable in type II. Type I inclusions are irregularly shaped aggregates of Cr-Sp grains embedded in or surrounded by plagioclase mesostasis (Pl), and having merrillite (Mrl) and/or chlorapatite (Apt) rims. Some contain Mrl/Apt in the cores. Most rims are framed by low-Al and low-Ti clinopyroxene (Px). A few inclusions show core-to-rim changes in modal composition: cores consisting of Pl, Px and/or olivine (Ol) are surrounded by a Cr-Sp-rich zone and a Pl rim. Cr-Sp is uniform in composition with a Cr/(Cr+Al) ratio of 0.84- 0.86. Plagioclase occurs as coexisting Na- and K-rich varieties, the K-rich Pl generally occurring in cores. Type II inclusions are also irregularly shaped; they consist of a Pl core or fine- grained Pl+Cr-Sp with accessory ilmenite (Ilm), a surrounding Cr-Sp-rich zone and a rim of Pl and Px enclosed by Mrl. Some type-II inclusions consist of compact Cr-Sp cores surrounded by Pl and Mrl rims. One inclusion contains a Mg-Ca-Na-Si phase in the core. Cr-Sp has a uniform or slightly variable composition within the individual inclusions but has significant grain-to- grain variability in Cr/(Cr+Al): ~0.29-0.80, increasing from core to rim. Pl is igneously zoned: cores have higher Ca than rims. Cr-Sp inclusions have >10 wt% bulk Al2O3 and differ from Al-rich objects in ordinary chondrites by having high contents of Cr2O3 and FeO, and low contents of SiO2 and MgO. Models for the formation of Cr-Sp inclusions include the following: (1) The similarity in mineralogy and chemistry of Cr- Sp inclusions and chondrules and the occurrence of both within the same meteorite suggest a genetic relationship. The formation of Cr-Sp chondrules was explained by Krot et al. (1992) as having involved melting and incomplete evaporation of presolar lumps, perhaps during infall into the nebula; this resulted in enrichment of the residue in Cr and Al. Subsequent melting and fractional crystallization produced the high Cr/Al ratios. Differences between Cr-Sp chondrules and inclusions indicate that either (a) inclusions formed by lower degrees of melting than chondrules, (b) inclusions formed by chondrule disruption, or (c) inclusions are relicts of nebular precursors. (2) The presence of phosphates within some Cr-Sp inclusions and the presence of inclusions within metal suggest formation of Cr-Sp inclusions from oxidized precursor materials that originally consisted of Cr-P-Si-rich metal. Melting of oxidized metal grains together with an alkali-rich nebular component resulted in the formation of immiscible Cr-P-rich silicate and metal melts. (3) Based on the similarity of textures and mineralogy of Cr-Sp and Sp inclusions in ordinary chondrites, A. Bischoff (pers. comm.) proposed that Cr-Sp objects were formed by alteration of Sp inclusions during parent-body metamorphism; Sp was replaced by Cr-Sp, perovskite by Ilm, and anorthite by Na-Pl. The occurrence of Cr-Sp of variable composition in equilibrated ordinary chondrites implies low diffusion rates in Cr-Sp, inconsistent with this model; nebular reaction between Sp- and Cr-rich phases is still less likely. (4) The presence of shock veins and melt pockets having mineralogy similar to those of Cr-Sp inclusions suggests a shock origin. However, this seems unlikely because (a) Cr-Sp inclusions and melt pockets have different textures, (b) Cr-Sp within melt pockets has high Cr/(Cr+Al) ~0.86, similar to matrix chromite, and (c) most of the chondrites containing Cr- Sp inclusions are of low shock stage (S1-S3). References: Krot A., Ivanova M.A., and Wasson J.T. (1992) Earth Planet. Sci. Lett., submitted.
- Pub Date:
- July 1992