The origin of volatile element depletion in early solar system material: Clues from Zn isotopes in chondrulesOPEN ACCESS

14/04/2017 09:51 · by karmaka · in carbonaceous chondrite, CB, CH, chondrule, chondrule formation, CI, CK, CM, CO, early chronology, solar system

Emily A. Pringle, Frédéric Moynier, Pierre Beck, Randal Paniello, Dominik C. Hezel

Earth and Planetary Science Letters,
Volume 468, 15 June 2017, Pages 62-71
Open Access funded by European Research Council

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Update (28 April 2017): LINK

“Highlights
• Allende chondrules are enriched in the light isotopes of zinc relative to the bulk.
• Chondrules did not experience open-system evaporation during formation.
• Sulfides in UOC are enriched in heavy zinc isotopes relative to silicates.
• Removal of sulfides can explain the zinc isotope compositions of chondrules.
• Zinc systematics of bulk CC is consistent with a nebular origin of volatiles.”

“Volatile lithophile elements are depleted in the different planetary materials to various degrees, but the origin of these depletions is still debated. Stable isotopes of moderately volatile elements such as Zn can be used to understand the origin of volatile element depletions. Samples with significant volatile element depletions, including the Moon and terrestrial tektites, display heavy Zn isotope compositions (i.e. enrichment of 66Zn vs. 64Zn), consistent with kinetic Zn isotope fractionation during evaporation. However, Luck et al. (2005) found a negative correlation between δ66Zn and 1/[Zn] between CI, CM, CO, and CV chondrites, opposite to what would be expected if evaporation caused the Zn abundance variations among chondrite groups.

We have analyzed the Zn isotope composition of multiple samples of the major carbonaceous chondrite classes: CI (1), CM (4), CV (2), CO (4), CB (2), CH (2), CK (4), and CK/CR (1). The bulk chondrites define a negative correlation in a plot of δ66Zn vs 1/[Zn], confirming earlier results that Zn abundance variations among carbonaceous chondrites cannot be explained by evaporation. Exceptions are CB and CH chondrites, which display Zn systematics consistent with a collisional formation mechanism that created enrichment in heavy Zn isotopes relative to the trend defined by CI–CK.

We further report Zn isotope analyses of chondrite components, including chondrules from Allende (CV3) and Mokoia (CV3), as well as an aliquot of Allende matrix. All chondrules are enriched in light Zn isotopes (∼500 ppm on 66Zn/64Zn) relative to the bulk, contrary to what would be expected if Zn were depleted during evaporation, on the other hand the matrix has a complementary heavy isotope composition. We report sequential leaching experiments in un-equilibrated ordinary chondrites, which show sulfides are isotopically heavy compared to silicates and the bulk meteorite by ca. +0.65 per mil on 66Zn/64Zn. We suggest isotopically heavy sulfides were removed from either chondrules or their precursors, thereby producing the light Zn isotope enrichments in chondrules.”