Petrology and chemistry of the basaltic shergottite North West Africa 480

Abstract

— North West Africa (NWA) 480 is a new martian meteorite of 28 g found in the Moroccan Sahara in November 2000. It consists mainly of large gray pyroxene crystals (the largest grains are up to 5 mm in length) and plagioclase converted to maskelynite. Excluding the melt pocket areas, modal analyses indicate the following mineral proportions: 72 vol% pyroxenes extensively zoned, 25% maskelynite, 1% phosphates (merrillite and chlorapatite), 1% opaque oxides (ilmenite, ulvöspinel and chromite) and sulfides, and 1% others such as silica and fayalite. The compositional trend of NWA 480 pyroxenes is similar to that of Queen Alexandra Range (QUE) 94201 but in NWA 480 the pyroxene cores are more Mg‐rich (En77‐En65). Maskelynites display a limited zoning (An42–50Ab54‐48Or2–4). Our observations suggest that NWA 480 formed from a melt with a low nuclei density at a slow cooling rate. The texture was achieved via a single‐stage cooling where pyroxenes grew continuously. A similar model was previously proposed for QUE 94201 by McSween et al. (1996). NWA 480 is an Al‐poor ferroan basaltic rock and resembles Zagami or Shergotty for major elements and compatible trace element abundances. The bulk rock analysis for oxygen isotopes yields Δ17O = +0.42%, a value in agreement at the high margin, with those measured on other shergottites (Clayton and Mayeda, 1996; Romanek et al., 1998; Franchi et al., 1999). Its CI‐normalized rare earth element pattern is similar to those of peridotitic shergottites such as Allan Hills (ALH)A77005, suggesting that these shergottites shared a similar parent liquid, or at least the same mantle source.

Concordant Rb–Sr, Sm–Nd, and Ar–Ar ages for Northwest Africa 1460: A 346Ma old basaltic shergottite related to “lherzolitic” shergottites

“…Both specimens consist mainly of pyroxene (72 vol.%) and maskelynite (25 vol.%) with accessory merrillite, apatite, titanomagnetite, ilmenite, chromite, pyrrhotite, silica, baddeleyite, and mesostasis K-Al-Si-rich glass. The pyroxene grains in both NWA 480 and NWA 1460 have identical patterns of progressive compositional zonation from magnesian orthopyroxene cores through augite mantles to more ferroan pigeonite rims (Barrat et al, 2002;Irving and Kuehner, 2003), which is the pattern expected for closed system crystallization of mafic igneous magmas of this bulk composition.…”

“…Like NWA 480 (described by Barrat et al, 2002), NWA 1460 is a medium grained mafic igneous rock with an ophitic texture (see Fig. 1).…”

“…There is no evidence at scales down to microns for any infiltration by extraneous fluids or alteration of any mineral grains (including phosphates), either on Mars or in the terrestrial desert environment. Barrat et al (2002) concluded from pyroxene zoning patterns that NWA 480 and QUE 94201 (an unrelated depleted basaltic shergottite) have very similar igneous cooling histories, and they proposed that the pyroxenes in each rock had grown continuously by single-stage cooling at a slow rate within a closed igneous system. The cosmic-ray exposure (CRE) ages of NWA 480 and NWA 1460 are indistinguishable at 2.6 ± 0.2 Ma (Mathew et al, 2003;Connolly et al, 2006), and also similar to those for QUE 94201, Zagami, Shergotty, and Los Angeles (Eugster et al, 1997(Eugster et al, , 2002Nyquist et al, 2001a;Nishiizumi and Caffee, 2006).…”

“…doi:10. /j.gca.2009 year earlier near the same location by the same nomad (Barrat et al, 2002;Irving and Kuehner, 2003). Both specimens are composed mainly of pale yellowish-green prismatic grains of low-Ca pyroxenes (up to 5 mm long) with subordinate gray, glassy maskelynite laths (converted from plagioclase by shock) and accessory phases.…”

“…Although NWA 480 and NWA 1460 are completely lacking in olivine, the bulk chemical composition (especially the rare earth element pattern) of NWA 480 led Barrat et al (2002) to suggest that it is petrogenetically related to olivine-bearing ultramafic shergottites like ALH 77005. Rocks of the ALH 77005 clan have commonly been called ''lherzolitic" shergottites in the literature on Martian meteorites, and in this paper we adhere to this convention, even though these rocks do not contain sufficient orthopyroxene nor the more than 60% olivine necessary to be so named according to established requirements for terrestrial lherzolitic ultramafic rocks (Le Maitre et al, 2002).…”