Volatile depletion and evolution of Vesta from coupled Cu-Zn isotope systematics

The moderately volatile elements, Cu and Zn, are not strongly affected by magmatic differentiation [1, 2] and are important tracers of volatile depletion in planetary bodies, particularly low-mass, airless bodies [3]. New isotopic ratio and abundance measurements for both Cu and Zn are presented for eucrites to more fully understand volatile depletion processes that affected the parent-body of the howardite-eucrite-diogenite (HED) meteorites, the asteroid 4-Vesta. Zinc isotope ratios are reported for twenty-eight eucrite samples, which along with prior data [4] yield a range of δ⁶⁶Zn from -1.8 to +6.3 ‰, excluding one outlier, PCA 82502 (δ⁶⁶Zn = -7.8 ‰) and a Zn concentration range from 0.3 to 3.8 p.p.m. Heavy Zn isotopic ratios (positive δ⁶⁶Zn compositions) in eucrites form a negative trend with Zn concentration, reflecting volatile depletion processes on Vesta that are similar to the Moon [5, 6]. Within the combined sample set, eleven eucrites have light Zn isotopic compositions from δ⁶⁶Zn of -0.02 to -7.8 ‰, with the majority having more negative compositions than likely chondritic precursors (maximum δ⁶⁶Zn of ~ -0.2 ‰ [7]). These samples are interpreted to reflect condensates formed subsequent to surface volatilization and outgassing, such as during impact bombardment. Measurements of Cu compositions are also reported for nineteen of the samples, yielding a range of δ⁶⁵Cu from -1.6 to +0.9 ‰, and range of Cu concentrations from 0.2 to 2.8 p.p.m., with the exception of Stannern (Cu > 10 ppm). As with Zn, negative Cu isotopic ratios that are lighter than chondritic compositions (δ⁶⁵Cu ~ -0.5 ‰ [8]) are attributed to recondensation that occurred following impact-induced vaporization (cf. [9]). Within the wide ranges of Zn and Cu isotopic compositions measured in eucrites, most samples cluster within ~ 0 ‰ < δ⁶⁶Zn < +3 ‰ and ~ 0.2 ‰ < δ⁶⁵Cu < +0.9 ‰. This range is interpreted to reflect volatile depletion processes similar to those that affected the Moon (BSM: δ⁶⁶Zn +1.4 ± 0.5‰ [5, 6, 10, 11] and δ⁶⁵Cu = +0.92 ± 0.16‰ [9-11]). The greater heterogeneity in eucrite Zn and Cu isotopic compositions compared to lunar samples can be attributed to the smaller size of the HED parent asteroid, which may have experienced more limited homogenization of these signatures following volatile depletion and for eucrites which have experienced complex impact addition and metamorphic processes.  

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