Nature of volatile depletion and genetic relationships in enstatite chondrites and aubrites inferred from Zn isotopes

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Abstract

Enstatite meteorites include the undifferentiated enstatite chondrites and the differentiated enstatite achondrites (aubrites). They are the most reduced group of all meteorites. The oxygen isotope compositions of both enstatite chondrites and aubrites plot along the terrestrial mass fractionation line, which suggests some genetic links between these meteorites and the Earth as well.

For this study, we measured the Zn isotopic composition of 25 samples from the following groups: aubrites (main group and Shallowater), EL chondrites, EH chondrites and Happy Canyon (impact-melt breccia). We also analyzed the Zn isotopic composition and elemental abundance in separated phases (metal, silicates, and sulfides) of the EH4, EL3, and EL6 chondrites. The different groups of meteorites are isotopically distinct and give the following values (‰): aubrite main group (−7.08 < δ66Zn < −0.37); EH3 chondrites (0.15 < δ66Zn < 0.31); EH4 chondrites (0.15 < δ66Zn < 0.27); EH5 chondrites (δ66Zn = 0.27 ± 0.09; n = 1); EL3 chondrites (0.01 < δ66Zn < 0.63); the Shallowater aubrite (1.48 < δ66Zn < 2.36); EL6 chondrites (2.26 < δ66Zn < 7.35); and the impact-melt enstatite chondrite Happy Canyon (δ66Zn = 0.37).

The aubrite Peña Blanca Spring (δ66Zn = −7.04‰) and the EL6 North West Forrest (δ66Zn = 7.35‰) are the isotopically lightest and heaviest samples, respectively, known so far in the Solar System. In comparison, the range of Zn isotopic composition of chondrites and terrestrial samples (−1.5 < δ66Zn < 1‰) is much smaller (Luck et al., 2005, Herzog et al., 2009).

EH and EL3 chondrites have the same Zn isotopic composition as the Earth, which is another example of the isotopic similarity between Earth and enstatite chondrites. The Zn isotopic composition and abundance strongly support that the origin of the volatile element depletion between EL3 and EL6 chondrites is due to volatilization, probably during thermal metamorphism. Aubrites show strong elemental depletion in Zn compared to both EH and EL chondrites and they are enriched in light isotopes (δ66Zn down to −7.04‰). This is the opposite of what would be expected if Zn elemental depletion was due to evaporation, assuming the aubrites started with an enstatite chondrite-like Zn isotopic composition. Evaporation is therefore not responsible for volatile loss from aubrites. On Earth, Zn isotopes fractionate very little during igneous processes, while differentiated meteorites show only minimal Zn isotopic variability. It is therefore very unlikely that igneous processes can account for the large isotopic fractionation of Zn in aubrites. Condensation of an isotopically light vapor best explains Zn depletion and isotopically light Zn in these puzzling rocks. Mass balance suggests that this isotopically light vapor carries Zn lost by the EL6 parent body during thermal metamorphism and that aubrites evolved from an EL6-like parent body. Finally, Zn isotopes suggest that Shallowater and aubrites originate from distinct parent bodies.

Introduction

Enstatite meteorites include the undifferentiated enstatite chondrites and the differentiated enstatite achondrites known as aubrites. The enstatite meteorites are the most reduced group of meteorites (Larimer, 1968, Keil, 1989, Keil, in press): their metal contains dissolved Si and a fraction of the lithophile elements such as Ca, Mn and Mg are found in sulfides for lack of oxygen. In the δ17O vs δ18O plot, both enstatite chondrites and aubrites plot along the terrestrial mass fractionation line and close to the Earth mantle (Clayton and Mayeda, 1996).

Enstatite chondrites are divided into two groups: the Fe-rich EH chondrites and the Fe-poor EL chondrites. Each of these groups comprises several petrographic types according to their degree of thermal equilibration: EH3,4,5 chondrites and EL3,5,6 chondrites. The genetic relationship between EL and EH is still unresolved (Keil, 1989, Keil, in press, Kong et al., 1997). Based on the inverse variation of moderately volatile element abundances with petrogaphic type between EH chondrites and EL chondrites, Kong et al. (1997) proposed that both meteorite families originated from a single parent body. In contrast, Keil (1989) considered that they were derived from two separate parent bodies based on the absence in each group of clasts from the other group and, from a detailed petrographic study, Lin and El Goresy (2002) considered that EL3 and EH3 could not have evolved of the one from the other. Both Mn–Cr and I–Xe systematics indicate older ages for EH4 chondrites (∼4565 Ma) than EL6 chondrites (∼4560 Ma) (Kennedy et al., 1988, Shukolyukov and Lugmair, 2004). 53Mn has been detected in aubrites and one external isochron gives an age of ∼4561–4563 Ma for the end of the planetary differentiation of the aubrite parent body (Shukolyukov and Lugmair, 2004).

Aubrites are FeO-poor orthopyroxenites, which again formed under very reducing conditions (Keil, 1989). They contain an unusual assemblage of highly reduced minerals such as Si-bearing Fe–Ni metal, Ti-bearing troilite, niningerite: (Mg,Mn,Fe)S, caswellsilverite: NaCrS2, schreibersite: (Fe,Ni)3P, alabandite: (Mn,Fe,Mg)S, oldhamite: CaS, and daubreilite: FeCr2S4 which is the main carrier of Zn in aubrites (Keil, in press and references therein). Elements that are lithophile under moderate ƒO2 conditions are hosted in chalcophile or siderophile minerals, notably oldhamite (CaS) (Graham and Henderson, 1985, Keil, 1989, Keil, in press). All the aubrites but Shallowater are brecciated and seem to have formed within the same parent body. Shallowater is the only unbrecciated aubrite and has been suggested to have originated from a parent body distinct from all the others (Keil et al., 1989).

The large variability in the abundances of moderately volatile elements such as Zn observed in enstatite meteorites is conspicuous (Table 1). Zinc abundances decrease in the sequence (average concentrations in parenthesis): EH chondrites (∼290 ppm; Lodders and Fegley, 1998), Shallowater (∼32 ppm; Biswas et al., 1980), EL chondrites (19 ppm; Lodders and Fegley, 1998), aubrites main group (0.5–2 ppm; Wolf et al., 1983, Lodders et al., 1993). In addition, Zn abundances show also some variations within each group with a trend of decreasing concentrations with stronger degrees of metamorphism: for example the average Zn concentration of un-metamorphosed EL3 chondrites (213 ppm; Kong et al., 1997) are much higher than for the highly metamorphosed EL5,6 chondrites (6 ppm; Kong et al., 1997). Likewise, the Zn concentration for the EH5 chondrite St. Mark’s (48–100 ppm; Kong et al., 1997) is less than the mean for EH chondrites (290 ppm; Lodders and Fegley, 1998).

Nebular processes or volatilization during thermal metamorphism have both been proposed as possible causes of the loss of moderately volatile elements (Biswas et al., 1980, Kallemeyn and Wasson, 1986, Kong et al., 1997). Volatilization is expected to fractionate isotopes (Richter, 2004). Comparing the isotope compositions of volatile elements in meteorites may therefore help understand the physical and chemical conditions of the evaporation process. Zinc is a moderately volatile element with a 50% condensation temperature T50(Zn) of ∼730 K (Lodders, 2003) and may be even more volatile under conditions relevant to the metamorphism of ordinary chondrites (Schaefer and Fegley, 2010).

The Zn isotope variability in terrestrial rocks is relatively narrow (∼0.3‰ per amu, for review, see Albarède, 2004). Luck et al. (2005) showed that the different ordinary (OC) and carbonaceous chondrites (CC) groups have distinct Zn isotopic compositions with a total range of ∼0.35‰ per amu but that the relative depletion of volatiles elements in CC correlates with isotopically lighter, not heavier, Zn. In contrast, the very large isotopic variability of Zn in lunar soils (up to 3‰ per amu) (Moynier et al., 2006, Herzog et al., 2009) and their depletion in light Zn isotopes are consistent with vaporization upon impact by micrometeorites or sputtering at the lunar surface. Albarède et al. (2007) extended the study of isotopic fractionation of Zn to shocked rocks from a terrestrial impact site, Meteor Crater, Arizona, USA. They observed a negative correlation between isotope compositions and shock grade in seven Coconino sandstone samples and concluded that, on Earth, vaporization at high temperature by impacts fractionates the isotopic compositions of rather heavy elements to a measurable extent. Likewise, tektites (hypervelocity impact glasses) were enriched in heavy isotopes of Zn, most likely by evaporation upon melting during impact (Moynier et al., 2009a). Zn isotopic composition of meteorites has been shown to follow a mass-dependent fractionation line in meteorites, with no evidence of nucleosynthetic anomalies (Moynier et al., 2009b).

Preliminary data by Mullane et al., 2005a, Mullane et al., 2005b indicate a strong isotope fractionation of Fe and Zn in enstatite chondrites and aubrites. Here, we return to this issue and investigate Zn isotope fractionation in aubrites (aubrite main group and Shallowater), and in EL, and EH chondrites by multiple-collector inductively coupled plasma mass-spectrometry (MC-ICP-MS). In this study, we assess to what extent igneous processes such as partial melting and fractional crystallization, and impact-related processes such as brecciation, collisional disruption, impact-melting and the incorporation of foreign xenoliths, have affected the Zn isotope compositions of enstatite meteorites.

Section snippets

Sample description

Twenty-five samples representing the four main enstatite–meteorite groups (EH, EL, aubrites and Shallowater) were studied. They include eight samples from the aubrite main group (Aubres, Bishopville, Bustee, Cumberland Falls, Khor Temiki, Mayo Belwa, Norton County, Peña Blanca Spring), and Shallowater, which may be from a different parent body (Keil, 1989, Keil et al., 1989). We also analyzed 16 enstatite chondrites EL3 (2), EL6 (8), EH3 (3), EH4 (1), EH5 (1) plus Happy Canyon. The latter

Isotopic composition of the whole-rock meteorites

Isotope ratios are given in Table 1, Table 2, Table 3 and plotted in Fig. 1. The overall range of values for each group is consistent with the preliminary findings of Mullane et al., 2005a, Mullane et al., 2005b. As expected from mass-dependent isotopic fractionation, all the samples plot onto a straight line of slope 1.94 in a δ68Zn vs δ66Zn diagram (Fig. 1). The isotopically heaviest sample is the EL6 chondrite North West Forrest (δ66Zn = 7.35‰) and the lightest sample is the aubrite Peña

Enstatite meteorites are the rocks with the most extreme Zn isotopic compositions in the Solar System

The aubrite Peña Blanca Spring (δ66Zn = −7.04‰) and the EL6 chondrite North West Forrest (δ66Zn = +7.35‰) represent the planetary objects with the isotopically lightest and heaviest Zn known so far, respectively.

The Zn isotope of terrestrial rocks is very homogeneous, with δ66Zn typically measured between 0.20‰ and 0.40‰ in igneous rocks (Maréchal, 1998, Ben Othman et al., 2006, Cloquet et al., 2008) and up to 0.70‰ in some sedimentary rocks and ores (Albarède, 2004). Some tektites are enriched in

Conclusions

We measured the Zn isotopic composition of five EH(3,4,5) chondrites, 10 EL(3,6) chondrites, eight aubrites, Shallowater and Happy Canyon and discovered the most extreme range of mass-dependent fractionation yet measured in natural samples. EH chondrites and EL3 chondrites have normal terrestrial Zn isotopic compositions whereas EL6 chondrites are highly enriched in heavy isotopes. The Zn isotopic composition and abundance show that volatile element depletion in EL3 chondrites and EL6

Acknowledgments

We thank Philippe Telouk for maintaining the MC-ICP-MS in Lyon in perfect condition and Joyce Brannon for taking care of the clean lab in St. Louis. We thank Joel Baker, Ahmed El Goresy and an anonymous reviewer for thorough and constructive reviews.

We acknowledged the support of the Programme National de Planétologie (INSU-CEA) and the Agence Nationale de la Recherche (F.A.) and NASA LASER #NNX09AM64G (F.M.). We also are indebted to Joseph Boesenberg and Denton Ebel (American Museum of

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