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A Partial Melt Residue of Enstatite Chondrite Parentage

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A Partial Melt Residue of Enstatite Chondrite Parentage Meteoritics & Planetary Science 43, Nr 7, 1233–1240 (2008) Abstract available online at http://meteoritics.org Northwest Africa 2526: A partial melt residue of enstatite chondrite parentage Klaus KEIL1* and Addi BISCHOFF2 1Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, Hawai’i 96822, USA 2Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany *Corresponding author. E-mail: [email protected] (Received 04 October 2007; revision accepted 05 February 2008) Abstract–NWA 2526 is a coarse-grained, achondritic rock dominated by equigranular grains of polysynthetically twinned enstatite (~85 vol%) with frequent 120° triple junctions and ~10–15 vol% of kamacite + terrestrial weathering products. All other phases including troilite, daubreelite, schreibersite, and silica-normative melt areas make up <~1 vol% of the rock. Oxygen isotopic analyses are well within the range of those for enstatite chondrites and aubrites. We show that the “enstatite achondrite” (Russell et al. 2005) Northwest Africa (NWA) 2526 is actually a partial melt residue of an enstatite chondrite-like lithology that experienced ~20 vol% partial melting. We suggest that the heat source was internal to the parent body. The FeS-Fe,Ni and plagioclase-enstatite partial melts were removed from the parent lithology, leaving NWA 2526 as a residue highly depleted in troilite and lacking plagioclase. Sub-solidus slow cooling and annealing is responsible for the coarse- grained, recrystallized texture of the rock. We also suggest that the parent lithology of NWA 2526, prior to partial melting, experienced a shock event which formed the curvilinear trails of blebs of minor troilite and rare metal that are enclosed in enstatite crystals; thus, these represent relicts. After partial melting and annealing, NWA 2526 experienced a second, relatively mild impact event (<~20 GPa) that caused formation of the polysynthetic twinning in the enstatite. We suggest that the meteorite Zak≥odzie, which has been referred to as a “primitive enstatite achondrite” (Przylibski et al. 2005), did not form from a magma of internal origin, but that it is an impact-melt breccia of enstatite chondrite-like parentage, as previously discussed by Burbine et al. (2000) and Keil (2007). Finally, the “metal-rich enstatite meteorite with achondritic texture” Itqiy (Patzer et al. 2001) formed by processes very similar to those responsible for formation of NWA 2526 and is also the residue of ~20 vol% partial melting of an enstatite chondrite-like parent lithology, with the FeS-Fe,Ni and plagioclase-enstatite partial melts having been removed from the residue. It also experienced an impact event after partial melting that was responsible for the formation of the mixed Mg-Mn-Fe- sulfides and the shock stage S3 features of the enstatite. These similarities indicate that NWA 2526 and Itqiy may have formed on the same parent body. This body was different from the EH, EL, Shallowater and aubrite parent bodies, and NWA 2526 and Itqiy may represent samples from yet another, fifth enstatite meteorite parent body. INTRODUCTION While the parent lithologies were heated by internal heating to temperatures exceeding their solidus temperatures, thus causing In the world’s meteorite collections, there are rocks that are partial melting and recrystallization, these rocks did not interpreted as the residues of partial melting of ordinary crystallize from melts, as did the differentiated achondrites (e.g., chondrite-like precursor lithologies. These are the acapulcoites- Weisberg et al. 2006) and impact-melt rocks (e.g., McCoy et al. lodranites and winonaites-silicate inclusions in IAB-IIICD irons 1995; Keil 2007). These rocks have recrystallized textures but (McCoy et al. 1996, 1997a, 1997b; Benedix et al. 1998, 2000). petrologic characteristics (including occurrence of occasional 1233 © The Meteoritical Society, 2008. Printed in USA. 1234 K. Keil and A. Bischoff relict chondrules), and whole-rock chemical and/or isotopic RESULTS compositions that indicate close affinities to the primitive ordinary chondritic parent rocks from which they were derived Texture and Mineralogy (e.g., Prinz et al. 1983). The partial melts may have been mobilized but not removed from the parent lithology (e.g., the Hand specimen inspection of the small meteorite acapulcoites; McCoy et al. 1996), or they may have been indicates that it is relatively coarse-grained, lacks chondrules, removed to varying degrees (e.g., the lodranites; McCoy et al. and contains ~10 vol% of unweathered metallic Fe,Ni. Thin 1997a, 1997b). The parent lithologies of these rocks were section microscopy confirms the coarse-grained, achondritic similar in composition, but not identical, to ordinary chondrites texture of the rock. It is dominated by equigranular grains of (e.g., Palme et al. 1981), but no meteorites of the unadulterated polysynthetically twinned enstatite (~85 vol%) which range chondritic precursor rocks seem to exist in the world’s meteorite in size from ~0.4–1.0 mm, although some as large as 2 mm collections (McCoy et al. 1996, 1997a, 1997b; Benedix et al. were also observed (Fig. 1), and ~10–15 vol% of kamacite + 1998, 2000). terrestrial weathering products of kamacite parentage; all The Northwest Africa (NWA) 2526 meteorite, a other phases make up <~1 vol% of the rock. The texture is single stone originally of 42.9 grams, was purchased on distinctly recrystallized (metamorphic), with frequent 120° June 26, 2003, from a dealer at St. Marie-aux-Mines, France, triple junctions between enstatite crystals, suggesting that the and had been recovered at an unknown locality in northwest rock was extensively annealed. Kamacite is fresh in places Africa. It was originally classified as an enstatite achondrite and partially altered to terrestrial hydrated iron oxide with ~10% metal (Russell et al. 2005). However, our weathering products in others. An XRF bulk rock analysis mineralogic, petrologic and chemical studies indicate that this courtesy of Herbert Palme and Tony Schulz indicates a total highly recrystallized metal-rich rock, while achondritic in of 18.5 wt% Fe and 1.07 wt% Ni. The Ni content of kamacite texture and consisting of major enstatite, is not an enstatite in NWA 2526 is ~5.7 wt% (Table 2), which suggests that the achondrite (aubrite) sensu stricto, i.e., a rock that crystallized bulk kamacite content of the meteorite must have been from a melt. Rather, we suggest that it is the residue of a ~18.8 wt%, roughly in agreement with the bulk Fe content of relatively low degree (>10–20 vol%) of partial melting and the rock determined by XRF of 18.5 wt%. Assuming a subsequent annealing not of an ordinary chondrite-like, but of density of the metallic Fe,Ni of 7.5 g/cm3 and a bulk density an enstatite chondrite-like precursor lithology, although a of the rock of 3.7 g/cm3 (Keil 1962), the 18.5 wt% kamacite direct link to known EH or EL enstatite chondrites cannot be convert to ~9.2 vol%. In spite of the many uncertainties established with certainty. We base this conclusion on the introduced, for example, by our assumption of closed-system relatively high content of metallic Fe,Ni and very low content weathering and by possible unrepresentative sampling of the of troilite, the absence of plagioclase, and the recrystallized coarse-grained rock for XRF analysis, this result is texture of the rock. We suggest that the parent lithology was nevertheless roughly in agreement with the estimated modal heated to sufficiently high temperatures to cause partial kamacite content of 10–15 vol%. melting and formation of FeS-Fe,Ni and plagioclase-enstatite Minor phases include troilite, daubreelite, schreibersite, partial melts, which were removed from the parent lithology, and Si-rich melt areas (Figs. 2a–d). Feldspar, a common thus explaining the very low FeS and the relatively high Fe,Ni phase in enstatite chondrites and achondrites (aubrites) (e.g., contents and the lack of plagioclase in the rock. Keil 1968; Watters and Prinz 1979), was not observed. Within the enstatite crystals occur tiny grains and blebs of opaque SAMPLES AND ANALYTICAL TECHNIQUES minerals (mostly troilite and some kamacite) that are arranged in curvilinear trails (Figs. 2c and 2d) which do not follow the Two polished thin sections (PL03170, PL06056) as well cleavage planes of enstatite. We speculate that these as a 6 gram specimen are located at the Institute für inclusions formed by impact-generated “shock-darkening” Planetologie, University of Münster, Germany. We studied (Rubin 1992) of the parent lithology of NWA 2526 prior to the polished thin sections by optical microscopy in partial melting and annealing (see discussion below). transmitted and reflected light. A JEOL 840A scanning Considering the bulk rock, most of the minor sulfide and electron microscope (SEM) was used to resolve the fine- schreibersite grains occur closely associated with metallic grained interstitial material of the rock and to identify phases Fe,Ni and are usually embedded in terrestrial hydrated iron with a Pentafet detector (Oxford Instruments) for energy oxide weathering products which resulted from the dispersive (EDS) semiquantitative elemental analysis using weathering of the associated metallic Fe,Ni. Troilite grains are the INCA analytical program provided by Oxford partly converted to terrestrial weathering products (Figs. 2a and Instruments. Quantitative mineral analyses
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