Amoeboid olivine aggregates from CH carbonaceous chondrites

Abstract

Amoeboid olivine aggregates (AOAs) in CH carbonaceous chondrites are texturally and mineralogically similar to those in other carbonaceous chondrite groups. They show no evidence for alteration and thermal metamorphism in an asteroidal setting and consist of nearly pure forsterite (Fa<3; in wt%, CaO=0.1–0.8, Cr2O3=0.04–0.48; MnO<0.5), anorthite, Al-diopside (in wt%, Al2O3=0.7–8.1; TiO2<1), Fe,Ni-metal, spinel, and, occasionally, low-Ca pyroxene (Fs1Wo2–3), and calcium–aluminum-rich inclusions (CAIs). The CAIs inside AOAs are composed of hibonite, grossite, melilite (Åk13–44), spinel, perovskite, Al,Ti-diopside (in wt%, Al2O3 up to 19.6; TiO2 up to 13.9), and anorthite. The CH AOAs, including CAIs within AOAs, have isotopically uniform 16O-rich compositions (average Δ17O=−23.4±2.3‰, 2SD) and on a three-isotope oxygen diagram plot along ∼slope-1 line. The only exception is a low-Ca pyroxene-bearing AOA 1-103 that shows a range of Δ17O values, from −24‰ to −13‰. Melilite, grossite, and hibonite in four CAIs within AOAs show no evidence for radiogenic 26Mg excess (δ26Mg). In contrast, anorthite in five out of six AOAs measured has δ26Mg corresponding to the inferred initial 26Al/27Al ratio of (4.3±0.7)×10−5, (4.2±0.6)×10−5, (4.0±0.3)×10−5, (1.7±0.2)×10−5, and (3.0±2.6)×10−6. Anorthite in another AOA shows no resolvable δ26Mg excess; an upper limit on the initial 26Al/27Al ratio is 5×10−6. We infer that CH AOAs formed by gas–solid condensation and aggregation of the solar nebula condensates (forsterite and Fe,Ni-metal) mixed with the previously formed CAIs. Subsequently they experienced thermal annealing and possibly melting to a small degree in a 16O-rich gaseous reservoir during a brief epoch of CAI formation. The low-Ca pyroxene-bearing AOA 1-103 may have experienced incomplete melting and isotope exchange in an 16O-poor gaseous reservoir. The lack of resolvable δ26Mg excess in melilite, grossite, and hibonite in CAIs within AOAs reflects heterogeneous distribution of 26Al in the solar nebula during this epoch. The observed variations of the inferred initial 26Al/27Al ratios in anorthite of the mineralogically pristine and uniformly 16O-rich CH AOAs could have recorded (i) admixing of 26Al in the protoplanetary disk during the earliest stages of its evolution and/or (ii) closed-system Mg-isotope exchange between anorthite and Mg-rich minerals (spinel, forsterite, and Al-diopside) during subsequent prolonged (days-to-weeks) thermal annealing at high temperature (∼1100°C) and slow cooling rates (∼0.01Kh−1) that has not affected their O-isotope systematics. The proposed thermal annealing may have occurred in an impact-generated plume invoked for the origin of non-porphyritic magnesian chondrules and Fe,Ni-metal grains in CH and CB carbonaceous chondrites about 5Myr after formation of CV CAIs.