Origin of the volatile element depletion in carbonaceous chondrites: Insights from Zn and Cu isotope compositions of chondrules

Introduction:

Carbonaceous chondrites (CCs) are among the most primitive rock samples in our solar system. All CC groups are depleted in volatile elements (i.e., elements having 50% condensation temperature lower than ~1300 K) relative to CI chondrites [1]. This unique characteristic reveals mass-dependent enrichments of light isotopes by decreasing the mass fraction of moderately volatile element ([e.g., 2]; e.g., Zn, Rb, K, Cu, Te). The exact origin of this depletion is not fully understood with primary explanations including incomplete condensation from nebular gas [e.g., 2] or the varying mixing of volatile-rich and volatile-poor components [e.g., 3]. The recognition of light Zn and Te isotope enrichments in CV chondrules seems to suggest that the contribution of these depleted components is the main cause of the light isotope enrichments in the bulk rock CCs [4-6]. However, neither the impact on the chemical budget of bulk CCs nor the origin of the variation among them has been fully understood.

In this study, we obtained Zn and Cu isotope data for 24 bulk rock CCs, along with isotope data of matrix and single chondrules in the CR2 chondrite MIL 15328, with the aim to better understand the origin of the volatile depletion in carbonaceous chondrites. 

Results/Discussion:

Bulk rock CCs exhibit variable isotopic range from 0.05 to 0.43±0.02‰ and from -2.01±0.03‰ to -0.14±0.07‰ with an associated mass fraction range from 53 to 271μg/g and 55 to 170μg/g for Zn and Cu, respectively. In both isotopic systems, bulk rock CCs show mass-dependent light isotope enrichments with decreasing element mass fractions. The matrix-rich aliquot from the CR2 chondrite MIL 15328 has a lighter δ^66/64Zn (0.20±0.04‰) but heavier δ^65/63Cu (-1.51±0.04‰) compared to the respective bulk rock, which has value of 0.38±0.06‰ and -1.68±0.06‰ for Zn and Cu, respectively. The three analyzed single chondrules from MIL 15328 consistently display lighter compositions in both isotopic systems. Their δ^66/64Zn and δ^65/63Cu span a range from -3.17±0.17‰ to 0.28±0.06‰ and from -2.36 to -1.89±0.02‰, respectively, and correlate linearly with chondrule size. The data suggest that isotopically heavy Zn-Cu sulfide(±metal) was variably expelled during chondrule formation [e.g., 4]. Smaller chondrules, which contain fewer sulfides, are more strongly influenced by interaction with isotopically light gas under kinetic isotope fractionation during cooling of the local nebular domain. On the other hand, in larger chondrules, the expulsion of sulfide droplets from rotating silicate melt is less efficient, resulting in a composition that includes a mixture of isotopically heavy Zn and Cu sulfides in chondrule rims, while their cores remain sulfide-poor and isotopically lighter [5].

Taken together, the observed mass-dependent light isotope enrichments in bulk CCs with decreasing element mass fractions result from nebular fractionation and mixing between volatile-rich and isotopically heavy CI-like matrix with volatile-poor, isotopically light chondrules. These findings suggest that chondrule size affects sulfide distribution and isotopic composition, shedding light on the processes contributing to volatile element depletion in carbonaceous chondrites.

[1] Palme et al. (2014) Treat. on Geochem. 2 ed., 15-36.

[2] Nie et al. (2021) Sci. Adv. 7(49).

[3] Braukmüller et al. (2018) GCA 239, 17-48.

[4] Pringle et al. (2017) EPSL 468, 62–71.

[5] Van Kooten & Moynier (2019) GCA 261, 248-268.

[6] Hellmann et al. (2020) EPSL 549, 116508.