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

Chronology of formation of early solar system solids from bulk Mg isotope analyses of CV3 chondrules 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) ∼ 60 ppm, relative to precisions <±5 ppm (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 ≦1 Ma 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 <0.5 Ma 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 0.5 Ma of CAI to re-equilibration of chondrules and silicate vapour >2 Ma 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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geochimica et Cosmochimica Acta Elsevier

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

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Publisher
Elsevier
Copyright
Copyright © 2018 Elsevier Ltd
ISSN
0016-7037
eISSN
1872-9533
D.O.I.
10.1016/j.gca.2018.02.011
Publisher site
See Article on Publisher Site

Abstract

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) ∼ 60 ppm, relative to precisions <±5 ppm (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 ≦1 Ma 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 <0.5 Ma 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 0.5 Ma of CAI to re-equilibration of chondrules and silicate vapour >2 Ma 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.

Journal

Geochimica et Cosmochimica ActaElsevier

Published: Apr 15, 2018

References

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