Oxygen and aluminum-magnesium isotope systematics of the metasomatically altered coarse-grained igneous calcium-aluminum-rich from CK3.7-3.8 carbonaceous chondrites

We report on oxygen and aluminum-magnesium isotope systematics of Compact Type A (CTA), Type B (B), and Forsterite-bearing Type B (FoB) Ca,Al-rich inclusions (CAIs) from the Northwest Africa (NWA) 5343 (CK3.7) and NWA 4964 (CK3.8) chondrites that experienced metasomatic alteration in the presence of aqueous solution that resulted in replacement of primary melilite, AlTi-diopside, grossmanite, anorthite, and perovskite by secondary minerals. The primary minerals have excesses of radiogenic Mg-26 (Mg-26*) that correlate with Al-27/Mg-24 ratio; the only exception is melilite in the CTA CAI. The calculated internal Al-Mg isochrons in the CTA (excluding melilite), Type B, and FoB CAIs correspond to the initial Al-26/Al-27 ratios [(Al-26/Al-27)(0)] of (5.09 +/- 0.58) x 10(-5), (2.58 +/- 3.2) x 10(-5), and (5.05 +/- 0.66) x 10(-5), respectively. The gehlenitic melilite (& Aring;k(<1)) in the CTA CAI has resolvable Mg-26* but very high Al-27/Mg-24 (up to similar to 660) and does not belong to the internal isochron defined by hibonite, spinel, and grossmanite. The high Al-27/Mg-24 in melilite containing submicron inclusions of grossular is due to redistribution of Mg between these minerals during thermal metamorphism. Hibonite, spinel, forsterite, rh & ouml;nite/louisfuchsite, and a grossmanite inclusion in spinel have O-16-rich compositions (Delta O-17 similar to - 23 +/- 2 parts per thousand), whereas melilite, anorthite, and perovskite are O-16-poor (Delta O-17 similar to - 3 +/- 2 parts per thousand). Grossmanite and AlTi-diopside are O-16-depleted to various degrees: Delta O-17 ranges from similar to - 24 to similar to - 3 parts per thousand; the degree of O-16-depletion correlates with titanium content in pyroxene. On a three-isotope oxygen diagram secondary grossular, FeAl-diopside, FeMg-olivine, and plagioclase plot along mass-dependent fractionation line with Delta O-17 of similar to - 3.7 +/- 1.9 parts per thousand that corresponds to Delta O-17 of metasomatic fluid in the host meteorites. This value is indistinguishable from Delta O-17 of metasomatic fluid that resulted in alteration of Allende (CV > 3.6) CAIs. Coarse-grained igneous CAIs in CKs and CVs have similar size distribution, textures and primary mineralogy, formed in a gas of approximately solar O-isotope composition (Delta O-17 similar to - 24 +/- 2 parts per thousand) and had the canonical (Al-26/Al-27)(0), suggesting they belong to the same generation of refractory inclusions, further supporting genetic relationship between CVs and CKs. Oxygen-isotope heterogeneity in CV > 3.6 and CK3.7 - 3.8 CAIs resulted from postcrystallization O-isotope exchange with O-16-depleted metasomatic fluid (Delta O-17 similar to - 3.7 +/- 1.9 parts per thousand) on their parent asteroid(s). This exchange preferentially affected melilite, anorthite, perovskite, and AlTi-pyroxenes, whereas hibonite, spinel, rh & ouml;nite/louisfuchsite, and forsterite retained their original O-16-rich compositions established during igneous crystallization in a gas of approximately solar composition. Metasomatic alteration and thermal metamorphism of CAIs from CK3.7 - 3.8 and CV > 3.6 chondrites disturbed their Al-Mg isotope systematics to various degrees.