Amoeboid olivine aggregates from CH carbonaceous chondrites

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 (Fs(1)Wo(2-3)), and calcium-aluminum-rich inclusions (CAIs). The CAIs inside AOAs are composed of hibonite, grossite, melilite (angstrom k(13-44)), 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 O-16-rich compositions (average Delta O-17 = -23.4 +/- 2.3 parts per thousand, 2SD) and on a three-isotope oxygen diagram plot along similar to slope-1 line. The only exception is a low-Ca pyroxene-bearing AOA 1-103 that shows a range of Delta O-17 values, from +/- 24 parts per thousand to +/- 13 parts per thousand. Melilite, grossite, and hibonite in four CAIs within AOAs show no evidence for radiogenic Mg-26 excess (Delta Mg-26). In contrast, anorthite in five out of six AOAs measured has Delta Mg-26 corresponding to the inferred initial Al-26/Al-27 ratio of (4.3 +/- 0.7) x 10(-5), (4.2 +/- 0.6) x 10(-5), (4.0 +/- 0.3) x 10(-5), (1.7 +/- 0.2) x 10(-5), and (3.0 +/- 2.6) x 10(-6). Anorthite in another AOA shows no resolvable Delta Mg-26 excess; an upper limit on the initial Al-26/Al-27 ratio is 5 x 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 O-16-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 O-16-poor gaseous reservoir. The lack of resolvable Delta Mg-26 excess in melilite, grossite, and hibonite in CAIs within AOAs reflects heterogeneous distribution of Al-26 in the solar nebula during this epoch. The observed variations of the inferred initial Al-26/Al-27 ratios in anorthite of the mineralogically pristine and uniformly O-16-rich CH AOAs could have recorded (i) admixing of Al-26 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 (similar to 1100 degrees C) and slow cooling rates (similar to 0.01 K h(-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 5 Myr after formation of CV CAIs. (C) 2014 Elsevier Ltd. All rights reserved.