Zinc isotopes in HEDs: Clues to the formation of 4-Vesta, and the unique composition of Pecora Escarpment 82502

Abstract

The d 66 Zn (permil deviation of the 66 Zn/ 64 Zn ratio from a terrestrial standard) values for a suite of 20 non-Antarctic HED (howardite-eucrite-diogenite) meteorites and one mesosiderite, and for eight Antarctic eucrites and diogenites, were measured in order to determine the role of volatization in the formation of their presumed parent body, the asteroid 4-Vesta. The 20 non-Antarctic HEDs had d 66 Zn values that ranged from À2.0& to +1.67&, with a mean value of À0.01 ± 0.39& (2 se); this range likely represents a small-scale heterogeneity due to brecciation induced by multiple impacts. The non-Antarctic eucrites (d 66 Zn = +0.00 ± 0.58& (2 se), n = 12) were isotopically the same as the diogenites (d 66 Zn = À0.31 ± 0.80& (2 se), n = 4), and the howardites (d 66 Zn = +0.26 ± 0.37& (2 se), n = 4). On average, non-Antarctic eucrite falls were isotopically heavier (+0.50&) than non-Antarctic finds (À1.00&). The Antarctic finds studied were all unbrecciated samples, and they were significantly heavier than the non-Antarctic samples with a d 66 Zn range of +1.63& to +6.22& for four eucrites (mean, +4.32&) and +0.94& to +1.60& for three diogenites (mean + 1.23&), excluding one anomalous sample, while their Zn concentration is significantly lower than the brecciated samples. These data suggest that the unbrecciated eucrites probably represent the eucritic crust shortly after differentiation and cooling of the parent asteroid, at which time volatization of lighter zinc isotopes led to an isotopically heavy crust. Early impact events caused the ejection of these unbrecciated meteorites, which were subsequently spared from brecciation caused by multiple additional impacts on the much larger Vesta. The range of d 66 Zn values and Zn concentration for the brecciated HEDs in this study supports a major contribution to the Vestan surface by chondritic impactors (À1.30 < d 66 Zn < +0.76& for ordinary and carbonaceous chondrites). The anomalous eucrite PCA 82502 (d 66 Zn = À7.75&) is significantly isotopically lighter than the other HEDs and is the natural sample with the lightest Zn isotopic composition reported in the solar system to date. This meteorite most likely originated from a distinct parent body.

Lithium isotope constraints on crust–mantle interactions and surface processes on Mars

“…Notably, Li systematics appear to follow different isotope grouping for possible sources of large inner Solar System planets (carbonaceous + ordinary chondrites versus enstatite chondrites) than that discussed by Warren (2011; ordinary + enstatite chondrites versus carbonaceous chondrites). The available data also suggest slight difference in mean d 7 Li values between Earth and Moon and resolved differences were also observed for Zn (Paniello et al, 2012) and Cl (Sharp et al, 2010), for example, while identical compositions were measured for other elements including O (Wiechert et al, 2001), K (Humayun and Clayton, 1995), Mo (Burkhardt et al, 2014), or W (Touboul et al, 2007). Despite the uncertainties associated with estimating the mean d 7 Li values for Earth, Moon, Mars and the HED parent body(ies), their mean d 7 Li values are, on average, isotopically slightly heavier than those of most chondrites ( Fig.…”

Evaporative fractionation of volatile stable isotopes and their bearing on the origin of the Moon

“…The high δ 66 Zn of the regolith and soil samples reflect both sputtering effects and impact gardening of the lunar regolith [70,71] and do not represent primary magmatic compositions. There is a class of mare basalts with isotopically light δ 66 Zn (figure 3d, inset) that result in a similar range in lunar δ 66 Zn compared with eucrite meteorites (−7.8 to +6.2 ) [36] and have been interpreted as the consequence of condensation of isotopically light zinc after mare basalt crystallization [34,75].…”

“…lunar magma ocean prior to crust formation, relative to more prolonged crustal growth above a magma ocean on smaller planetesimals, can partly explain the greater enrichment of δ 66 Zn in some unbrecciated eucrites [36]. Second, for loss of volatiles, atoms would be required to exceed the escape velocity, or would need to be lost during a process such as atmospheric blow-off, and this process may inhibit ideal Rayleigh distillation of moderately volatile stable isotopes.…”

“…Panel (b) is modified from the data compilation of Davis [19]. [34][35][36][37]. planetesimals), large-scale global (e.g.…”

Zinc isotopic composition of iron meteorites: Absence of isotopic anomalies and origin of the volatile element depletion

“…; Paniello et al. , 2012b). Comparing the Zn isotope compositions in iron meteorites may help further our understanding of the physical and chemical conditions involved in the variation in the volatile element abundance among different groups.…”