Other
Scientific paper
Aug 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004m%26ps...39.1257f&link_type=abstract
Meteoritics & Planetary Science, Vol. 39, No. 8, p.1257-1272
Other
8
Scientific paper
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 mm in length, coarse-grained anorthite and Al, Ti-diopside (fassaite), all with finegrained (~10 mm 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 mm) with no apparent igneous textures or zoning. Spinel, Al-diopside, and forsterite are 16O-rich, while melilite is variably depleted in 16O (d17, 18O from ~-40? to -5?). The contrast in isotopic 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.
Fagan Timothy J.
Keil Klaus
Krot Alexander N.
Yurimoto Hisayoshi
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