Petrology and geochemistry of the unbrecciated achondrite Northwest Africa 1240 (NWA 1240): an HED parent body impact melt

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.

Section: Sample Descriptionmentioning

confidence: 99%

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

Paniello¹,

Moynier²,

Beck

et al. 2012

“…In addition, three modern island arc basalts (IABs) from New Britain (NMNH 116852-1, 116852-3 and 116852-11) were also analyzed here to test the accuracy of the measurements (see Dauphas et al (2009a) for a detailed discussion about IABs). (Jarosewich, 1990); NWA1950 (Gillet et al, 2005); Los Angeles (Warren et al, 2000); EETA79001, ALHA77005, Nakhla, ALH84001 (Lodders, 1998); Mil03346 (Day et al, 2006); GRO95555, MET00436, MET00424 ; NWA1461 (Warren et al, 2009);Shalka (McCarthy et al, 1972); ALHA77256 (Sack et al, 1991); NWA1240, NWA049 (Barrat et al, 2003); Cachari (Barrat et al, 2000); Agoult, Dag945 (Yamaguchi et al, 2009); Nuevo Laredo, NWA4523 (Barrat et al, 2007b); D'Orbigny, Sahara99555 (Mittlefehldt et al, 2002);LEW86010 (McKay et al, 1988); NWA1670 (Jambon et al, 2008); NWA1296 (Jambon et al, 2005); NWA2999 (Gellissen et al, 2007). i Weathering conditions are from MetBase (version 7.1).…”

Section: Sample Descriptionsmentioning

confidence: 99%

“…Pyroxenes are highly unequilibrated and their compositions range from En 76 to En 0.6 . Although its texture indicates a rock formed from a rapidly-cooled melt, its composition is more akin to cumulate eucrites and led Barrat et al (2003) to propose an impact-melt origin. Nevertheless, these ungrouped eucrites are probably not from asteroid 4-Vesta, but from Vesta-like asteroid parent-bodies (Scott et al, 2009).…”

Section: Sample Descriptionsmentioning

confidence: 99%

Iron isotope fractionation in planetary crusts

Wang

Moynier

Dauphas

et al. 2012

We present new high precision iron isotope data (d 56 Fe vs. IRMM-014 in per mil) for four groups of achondrites: one lunar meteorite, 11 martian meteorites, 32 howardite-eucrite-diogenite meteorites (HEDs), and eight angrites. Angrite meteorites are the only planetary materials, other than Earth/Moon system, significantly enriched in the heavy isotopes of Fe compared to chondrites (by an average of +0.12& in d 56 Fe). While the reason for such fractionation is not completely understood, it might be related to isotopic fractionation by volatilization during accretion or more likely magmatic differentiation in the angrite parent-body. We also report precise data on martian and HED meteorites, yielding an average d 56 Fe of 0.00 ± 0.01&. Stannern-trend eucrites are isotopically heavier by +0.05& in d 56 Fe than other eucrites. We show that this difference can be ascribed to the enrichment of heavy iron isotopes in ilmenite during igneous differentiation. Preferential dissolution of isotopically heavy ilmenite during remelting of eucritic crust could have generated the heavy iron isotope composition of Stannern-trend eucrites. This supports the view that Stannern-trend eucrites are derived from main-group eucrite source magma by assimilation of previously formed asteroidal crust.These new results show that iron isotopes are not only fractionated in terrestrial and lunar basalts, but also in two other differentiated planetary crusts. We suggest that igneous processes might be responsible for the iron isotope variations documented in planetary crusts.

“…This unbrecciated vesiculated eucrite was interpreted by Barrat et al (2003) to be an impact melted cumulate eucrite as it has a positive Eu anomaly and low concentrations of incompatible elements like cumulate eucrites, but it has a porphyritic texture with very fine-grained matrix (Fig. 6d) and unequilibrated pyroxenes.…”

Section: Nwa 1240mentioning

confidence: 99%

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

Scott

Greenwood

Franchi

et al. 2009

147

177

A few eucrites have anomalous oxygen isotopic compositions. To help understand their origin and identify additional samples, we have analyzed the oxygen isotopic compositions of 18 eucrites and four diogenites. Except for five eucrites, these meteorites have ∆ 17 O values that lie within 2σ of their mean value viz., -0.242±0.016‰, consistent with igneous isotopic homogenization of Vesta. The five exceptional eucrites-NWA 1240, Pasamonte (both clast and matrix samples), PCA 91007, A-881394, and Ibitira-have ∆ 17 O values that lie respectively 4σ, 5σ, 5σ, 15σ, and 21σ away from this mean value. NWA 1240 has a δ 18 O value that is 5σ below the mean eucrite value. Four of the five outliers are unbrecciated and unshocked basaltic eucrites, like NWA 011, the first eucrite found to have an anomalous oxygen isotopic composition. The fifth outlier, Pasamonte, is composed almost entirely of unequilibrated basaltic clasts. Published chemical data for the six eucrites with anomalous oxygen isotopic compositions (including NWA 011) exclude contamination by chondritic projectiles as a source of the oxygen anomalies. Only NWA 011 has an anomalous Fe/Mn ratio, but several anomalous eucrites have exceptional Na, Ti, or Cr concentrations. We infer that the six anomalous eucrites are probably derived from five distinct Vesta-like parent bodies (Pasamonte and PCA 91007 could come from one body). These anomalous eucrites, like many unbrecciated eucrites from Vesta, are probably deficient in brecciation and shock effects because they were sequestered in small asteroids (~10 km diameter) during the Late Heavy Bombardment following ejection from Vesta-like bodies. The preservation of Vesta's crust and the lack of deeply buried samples from the hypothesized Vestalike bodies are consistent with the removal of these bodies from the asteroid belt by gravitational perturbations from planets and protoplanets, rather than by collisonal grinding.