Oxygen- and magnesium-isotope compositions of calcium-aluminum-rich inclusions from CR2 ca…

• Geochimica et Cosmochimica Acta

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Makide et al - 2009 - Oxygen- and magnesium-isotope compositions of calc.pdf

We report both oxygen- and magnesium-isotope compositions measured in situ using a Cameca ims-1280 ion microprobe in 20 of 166 CAIs identified in 47 polished sections of 15 CR2 (Renazzo-type) carbonaceous chondrites. Two additional CAIs were measured for oxygen isotopes only. Most CR2 CAIs are mineralogically pristine; only few contain secondary phyllosilicates, sodalite, and carbonates - most likely products of aqueous alteration on the CR2 chondrite parent asteroid. Spinel, hibonite, grossite, anorthite, and melilite in 18 CAIs have O-16-rich (Delta O-17 = -23.3 +/- 1.9 parts per thousand, 2 sigma error) compositions and show no evidence for postcrystallization isotopic exchange commonly observed in CAIs from metamorphosed CV carbonaceous chondrites. The inferred initial Al-26/Al-27 ratios, (Al-26/Al-27)(0), in 15 of 16 O-16-rich CAIs measured are consistent with the canonical value of (4.5-5) x 10(-5) and a short duration (<0.5 My) of CAI formation. These data do not support the "supra-canonical" values of (Al-26/Al-27)(0) [(5.85-7) x 10(-5)] inferred from whole-rock and mineral isochrons of the CV CAIs. A hibonite-grossite-rich CAI El Djouf 001 MK #5 has uniformly O-16-rich (Delta O-17 = -23.0 +/- 1.7 parts per thousand) composition, but shows a deficit of Mg-26 and no evidence for Al-26. Because this inclusion is O-16-rich, like CAIs with the canonical (Al-26/Al-27)(0), we infer that it probably formed early, like typical CAIs, but from precursors with slightly nonsolar magnesium and lower-than-canonical Al-26 abundance. Another O-16-enriched (Delta O-17 = -20.3 +/- 1.2 parts per thousand) inclusion, a spinel-melilite CAI fragment Gao-Guenie (b) #3, has highly-fractionated oxygen- and magnesium-isotope compositions (similar to 11 and 23 parts per thousand/amu, respectively), a deficit of Mg-26, and a relatively low (Al-26/Al-27)(0) = (2.0 +/- 1.7) x 10(-5). This could be the first FUN (Fractionation and Unidentified Nuclear effects) CAI found in CR2 chondrites. Because this inclusion is slightly O-16-depleted compared to most CR2 CAIs and has lower than the canonical (Al-26/Al-27)(0), it may have experienced multistage formation from precursors with nonsolar magnesium-isotope composition and recorded evolution of oxygen-isotope composition in the early solar nebula over 0.9(+2.2)(-0.7) My. Eight of the 166 CR2 CAIs identified are associated with chondrule materials, indicating that they experienced late-stage, incomplete melting during chondrule formation. Three of these CAIs show large variations in oxygen-isotope compositions (Delta O-17 ranges from -23.5 parts per thousand to -1.7 parts per thousand), suggesting dilution by O-16-depleted chondrule material and possibly exchange with an O-16-poor (Delta O-17 > -5 parts per thousand) nebular gas. The low inferred (Al-26/Al-27)(0) ratios of these CAIs (<0.7 x 10(-5)) indicate melting >2 My after crystallization of CAIs with the canonical (Al-26/Al-27)(0) and suggest evolution of the oxygen-isotope composition of the inner solar nebula on a similar or a shorter timescale.

Because CAIs in CR2 and CV chondrites appear to have originated in a similarly O-16-rch reservoir and only a small number of CR2 and CV CAIs were affected by chondrule melting events in an O-16-poor gaseous reservoir, the commonly observed oxygen-isotope heterogeneity in CAIs from metamorphosed CV chondrites is most likely due to fluid-solid isotope exchange on the CV asteroidal body rather than gas-melt exchange. This conclusion does not preclude that some CV CAIs experienced oxygen-isotope exchange during remelting, instead it implies that such remelting is unlikely to be the dominant process responsible for oxygen-isotope heterogeneity in CV CAIs. The mineralogy, oxygen and magnesium-isotope compositions of CAIs in CR2 chondrites are different from those ill the metal-rich, CH and CB carbonaceous chondrites, providing no justification for grouping CR, CH and CB chondrites into the CR clan.