Evidence for oxygen-isotope exchange in refractory inclusions from Kaba (CV3.1) carbonaceous chondrite during fluid-rock interaction on the CV parent asteroid

We report on the mineralogy, petrology, and in situ measured oxygen-isotope compositions of three Fluffy Type A Ca,Al-rich inclusions (FTA CAIs) and two amoeboid olivine aggregates (AOAs) from the CV3.1 carbonaceous chondrite Kaba. The FTA CAIs are aggregates of several inclusions composed of spinel, Al,Ti-diopside, and gehlenitic melilite replaced to various degrees by anorthite; they are surrounded by the Wark-Lovering rim layers made of spinel, anorthite, Al-diopside, and forsterite. One of FTA CAIs contains a relict ultrarefractory inclusion composed of Sc-rich Al,Ti-pyroxene, spinel, and Zr-rich oxides. The AOAs are aggregates of Ca- and/or Al-rich minerals (spinel, anorthite, and Al, Ti-diopside) surrounded by forsterite +/- Fe,Ni-metal condensates; Fe, Ni-metal is almost entirely replaced by magnetite and Fe,Ni-sulfides. Neither the FTA CAIs nor the AOAs show evidence for being melted after aggregation, and contain very minor secondary alteration minerals resulted from fluid-rock interaction on the CV parent asteroid. These include magnetite, fayalite, hedenbergite, phyllosilicates, and Fe-bearing Ti-free Al-diopside; secondary anorthite of asteroidal origin is absent in Kaba CAIs and AOAs. There are large variations in Delta O-17 (deviation from the terrestrial fractionation line = delta O-17 - 0.52 x delta O-18) within the individual FTA CAIs and AOAs: anorthite and melilite are systematically O-16-depleted (Delta O-17 range from similar to-14 to similar to-2 parts per thousand) relative to the uniformly O-16-rich forsterite and Al, Ti-diopside (Delta O-17 similar to -25 to -20 +/- 2 parts per thousand, 2 sigma). Scandium-rich Al, Ti-pyroxene has O-16-poor composition (Delta O-17 similar to -4 parts per thousand). Many anorthite and melilite analyses plot close to or along mass-dependent fractionation line with Delta O-17 of -1.5 +/- 1 parts per thousand(average +/- 2SD) defined by the aqueously-formed magnetite and fayalite from Kaba, and, therefore, corresponding to Delta O-17 of an aqueous fluid that operated on the CV parent asteroid. We conclude that anorthite and probably melilite in the Kaba FTA CAIs and AOAs experienced postcrystallization oxygen-isotope exchange with this fluid. The similar process must have affected plagioclase/plagioclase mesostasis and probably melilite in refractory inclusions and chondrules from CV3 chondrites of higher petrologic types [e.g., Allende (CV(oxA)3.6) and Efremovka (CV(red)3.1-3.4)], which appear to have experienced higher temperature metasomatic alteration than Kaba and were subsequently metamorphosed. We conclude that the carbonaceous chondrite anhydrous mineral (CCAM) line defined by oxygen-isotope compositions of whole-rock and mineral separates of Allende CAIs and having a slope of 0.94 is not the primary nebular line. Instead this line results from superposition of the nebular slope-1 line recorded by the primitive chondrule mineral (PCM) line, the mass-dependent fractionation line with slope of similar to 0.5 defined by the secondary minerals, and the minerals which experienced oxygen-isotope exchange with an aqueous fluid. (C) 2018 The Author(s). Published by Elsevier Ltd.