Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
8910803 | Geochimica et Cosmochimica Acta | 2018 | 41 Pages |
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 (Îâ²26Mg)â¯â¼â¯60â¯ppm, relative to precisions <±5â¯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)0â¯â¼â¯1-2â¯Ãâ¯10â5, equivalent to model “ages” of precursor formation â¦1â¯Ma post CAI. However, many of our chondrules show near solar Îâ²26Mg 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)0â¯<â¯0.8â¯Ãâ¯10â5, 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 Îâ²26Mgâ¯=â¯â16â¯Â±â¯3â¯ppm (2se), reflecting its early removal (model age of <0.5â¯Ma post CAI), from a reservoir with evolving Îâ²26Mg. If representative of the chondrule forming region, this grain defines a minimum interval of radiogenic ingrowth for CV chondrites commensurate with (26Al/27Al)0â¯>â¯3.4â¯Â±â¯0.6â¯Ãâ¯10â5. Overall, our samples record a sequence of events from the formation of ferromagnesian objects within 0.5â¯Ma of CAI to re-equilibration of chondrules and silicate vapour >2â¯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.
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geochemistry and Petrology
Authors
Hsin-Wei Chen, Jennifer L. Claydon, Tim Elliott, Christopher D. Coath, Yi-Jen Lai, Sara S. Russell,