Chronology of formation of early solar system solids from bulk Mg isotope analyses of CV3 chondrules

1Hsin-Wei Chen, 2Jennifer L. Claydon, 1Tim Elliott, 1Christopher D. Coath, 1,2,3Yi-Jen Lai, 2Sara S. Russell
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://www.sciencedirect.com/science/article/pii/S0016703718300814]
1Bristol Isotope Group, School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK
2Natural History Museum, Meteoritic and Cosmic Mineralogy, Cromwell Road, SW7 5BD, London, UK
3Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland
Copyright Elsevier

We have analysed the petrography, major element abundances and bulk Al-Mg isotope systematics of 19 ferromagnesian chondrules from the CV3 chondrites Allende, Mokoia, and Vigarano, together with an Al-rich chondrule and refractory olivine from Mokoia. Co-variations of Al/Mg with Na/Mg and Ti/Mg in our bulk chondrules suggest their compositions are dominantly controlled by reworking of different proportions of chondrule components (e.g. mafic minerals and mesostatis); their precursors are thus fragments from prior generations of chondrules. Our samples show a range in fractionation corrected 26Mg/24Mg (Δ’26Mg) ∼60ppm, relative to precisions <±5ppm (2se) and these values broadly covary with 27Al/24Mg. The data can be used to calculate model initial 26Al/27Al, or (26Al/27Al)0, of the chondrule precursors. Our resolvably radiogenic chondrules yield model (26Al/27Al)0 ∼1-2×10-5, equivalent to model “ages” of precursor formation ≦1Ma post CAI. However, many of our chondrules show near solar Δ’26Mg and no variability despite a range in 27Al/24Mg. This suggests their derivation either from younger precursor chondrules or open system behaviour once 26Al was effectively extinct ((26Al/27Al)0<0.8×10-5, given the resolution here). Evidence for the latter explanation is provided by marked rims of orthopyroxene replacing olivine, indicating reaction of chondrules with a surrounding silicate vapour. Concurrent isotopic exchange of Mg with a near chondritic vapour during late reworking could explain their isotopic systematics. One ferromagnesian object is dominated by a high Mg# olivine with elevated Ti and Ca abundances. This refractory olivine has a markedly negative Δ’26Mg = -16±3 ppm (2se), reflecting its early removal (model age of <0.5Ma post CAI), from a reservoir with evolving Δ’26Mg. If representative of the chondrule forming region, this grain defines a minimum interval of radiogenic ingrowth for CV chondrites commensurate with (26Al/27Al)0>3.4±0.6×10-5. Overall, our samples record a sequence of events from the formation of ferromagnesian objects within 0.5Ma of CAI to re-equilibration of chondrules and silicate vapour >2Ma post CAI, assuming an initially homogeneous 26Al/27Al. Metamorphism on the asteroid parent body may have played a subsequent role in affecting Mg isotope composition, but we argue this had a minor influence on the observations here.

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