Oxygen isotopic alteration in Ca-Al-rich inclusions from Efremovka: Nebular or parent body setting?

In situ SIMS oxygen isotope data were collected from a coarse-grained type B1 Ca-Al-rich inclusion (CAI) and an adjacent fine-grained CAI in the reduced CV3 Efremovka to evaluate the timing of isotopic alteration of these two objects. The coarse-grained CAI (CGI-10) is a sub-spherical object composed of elongate, euhedral, normally-zoned melilite crystals ranging up to several hundreds of μm in length, coarse-grained anorthite and Al, Ti-diopside (fassaite), all with fine-grained (∼10 μm across) inclusions of spinel. Similar to many previously examined coarse-grained CAIs from CV chondrites, spinel and fassaite are ^16O-rich and melilite is ^16O-poor, but in contrast to many previous results, anorthite is ^16O-rich. Isotopic composition does not vary with textural setting in the CAI: analyses of melilite from the core and mantle and analyses from a variety of major element compositions yield consistent ^16O-poor compositions. CGI-10 originated in an ^16O-rich environment, and subsequent alteration resulted in complete isotopic exchange in melilite. The fine-grained CAI (FGI-12) also preserves evidence of a 1st-generation origin in an ^16O-rich setting but underwent less severe isotopic alteration. FGI-12 is composed of spinel ± melilite nodules linked by a mass of Aldiopside and minor forsterite along the CAI rim. All minerals are very fine-grained (<5 μm) with no apparent igneous textures or zoning. Spinel, Al-diopside, and forsterite are ^16O-rich, while melilite is variably depleted in ^16O (δ^{17, 18O} from ∼-40% to -5%). The contrast in isojtopic distributions in CGI-10 and FGI-12 is opposite to the pattern that would result from simultaneous alteration: the object with finer-grained melilite and a greater surface area/volume has undergone less isotopic exchange than the coarser-grained object. Thus, the two CAIs were altered in different settings. As the CAIs are adjacent to each other in the meteorite, isotopic exchange in CGI-10 must have preceded incorporation of this CAI in the Efremovka parent body. This supports a nebular setting for isotopic alteration of the commonly observed ^16O-poor melilite in coarse-grained CAIs from CV chondrites.