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Thermal History of the Enstatite Chondrites from Silica Polymorphs

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Thermal History of the Enstatite Chondrites from Silica Polymorphs Meteoritics & Planetary Science 40, Nr 6, 855–868 (2005) Abstract available online at http://meteoritics.org Thermal history of the enstatite chondrites from silica polymorphs M. KIMURA1*, M. K. WEISBERG2, Y. LIN3, A. SUZUKI4, E. OHTANI4, and R. OKAZAKI5 1Faculty of Science, Ibaraki University, Mito 310-8512, Japan 2Department of Physical Sciences, Kingsborough College of the City University of New York, Brooklyn, New York 11235, and Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York 10024, USA 3State Key Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 4Institute of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan 5Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan *Corresponding author. E-mail: [email protected] (Received 18 January 2005; revision accepted 14 April 2005) Abstract–Here we report the results of our petrologic and mineralogical study of enstatite (E) chondrites in order to explore their thermal histories. We studied silica phases in 20 E chondrites by laser micro Raman spectroscopy to determine the silica polymorphs they contain. Silica phases are commonly present in E chondrites and their polymorphs reflect the physical conditions of formation. The samples studied here include EH3–5, EL3–6, E chondrite melt rocks, and an anomalous E chondrite. We identified quartz, tridymite, cristobalite, and silica glass in the samples studied. EH4– 5 and EH melt rocks are divided into high and low temperature classes based on niningerite- alabandite solid solutions. EH3, EL3, and some EH melt rocks of the high temperature class contain tridymite and cristobalite. We suggest that tridymite and cristobalite crystallized in chondrules and E chondrite melts, followed by rapid cooling, leading to the survival of these silica polymorphs. EH4 and EL4 chondrites also contain tridymite and cristobalite in their chondrules, indicating that these silica polymorphs survived low temperature metamorphism (as estimated from opaque mineral geothermometers) because of the sluggishness of the transition to a more stable polymorph. Tridymite and cristobalite in EL6 chondrites reflect the high temperature processes experienced by these meteorites. On the other hand, some EH5 chondrites and EH melt rocks of the low temperature class contain quartz, which may be a product of the transition from tridymite or cristobalite during a long period of low temperature metamorphism. Although the thermal history of E chondrites have been previously estimated from opaque minerals, such compositions mainly reflect low temperature processes. However, we can reconstruct the primordial thermal processes and subsequent cooling histories of E chondrites from their silica polymorphs. The E chondrites have complicated thermal histories, which produced the observed variations among them. INTRODUCTION solution) phase is a characteristic mineral in E chondrites and is not present in other chondrite groups. 5) Fe-Ni metal The enstatite (E) chondrites are unusual meteorites, contains Si. These observations indicate that the E chondrites characterized by following features (e.g., Keil 1968; Sears formed in an environment with remarkably low oxygen et al. 1982): 1) Low Mg/Si relative to other chondrites, fugacity. resulting in enstatite, instead of olivine, as the dominant The E chondrites are classified into EH and EL groups, silicate phase. 2) Silica minerals are commonly present. 3) with petrologic types ranging from 3 to 6 (or 7). EH Most silicate minerals have extremely high Mg/(Mg + Fe) chondrites are somewhat more reduced than the EL ratios. 4) Elements such as Mg, Mn, K, and Ca, which are chondrites and are distinguished from EL chondrites by their typically lithophile in ordinary and carbonaceous chondrites, higher contents of siderophile elements, higher Si content in often occur as sulfides (chalcophile) in the E chondrites. For their Fe-Ni metal and presence of niningerite (Mg-rich example, (Mg,Mn,Fe)S (niningerite-alabandite solid sulfide) instead of alabandite (Mn-rich sulfide). 855 © The Meteoritical Society, 2005. Printed in USA. 856 M. Kimura et. al. In addition, melt rocks and melt breccias of EH and EL silica grains in E chondrites are small in size. Therefore, we groups have been abundantly reported (McCoy et al. 1995; systematically identified silica polymorphs using laser micro Rubin and Scott 1997; Weisberg et al. 1997; Leroux et al. Raman spectroscopy. We use the silica polymorph as well as 1997; Lin and Kimura 1998; Kimura and Lin 1999). the other mineralogical data to shed light on the complicated Anomalous and unclassified E chondrites have also been thermal history of the E chondrites from chondrule formation, discovered (Grossman et al. 1993; Weisberg et al. 1997; Lin through thermal metamorphism, and to brecciation of the and Kimura 1998; Kimura and Lin 1999). QUE 94204 and Y- parent body. 793225 show intermediate mineral chemistry between EH and EL chondrites (Weisberg et al. 1997; Lin and Kimura SAMPLES AND EXPERIMENTAL METHODS 1998). In addition to texture and mineralogy, many We investigated polished thin sections of 20 E chondrites geothermometers have been studied in order to explore the selected from the meteorite collections of the American complicated thermal history of the E chondrites (e.g., Skinner Museum of Natural History and the National Institute of Polar and Luce 1971; Larimer and Buseck 1974; Fogel et al. 1989; Research, and DaG 734 from Kyushu University. Zhang and Sears 1996). Most of them are based on opaque Backscattered electron (BSE) images and mineral minerals because geothermometry by silicate and oxide analyses were obtained using the JEOL 733 electron probe phases is usually not useful for E chondrites. Opaque mineral microanalyzer (EPMA) at Ibaraki University. The analytical geothermometers reveal that highly metamorphosed EL procedure was the same as that outlined by Lin and Kimura chondrites cooled slowly to low temperatures (Skinner and (1998). After the identification of silica phases by EPMA, we Luce 1971; Fogel et al. 1989; Zhang et al. 1995). El Goresy measured Raman spectra of these silica phases by the JASCO and Ehlers (1989) estimated the size of the EH chondrite NRS-2000 spectrometer with a nitrogen-cooled CCD detector parent body from sphalerite composition. Zhang et al. (1995) at Tohoku University. A microscope was used to focus the proposed that mineralogy reflects reequilibration during excitation laser beam (514.5 nm lines of a Princeton regolith or other postmetamorphic processes. Lin and El Instruments Ar+ laser) to a 2 μm spot. The laser power was Goresy (2002) suggested that opaque nodules in EH3 and 40 mW. Raman spectra were collected from 5 to 26 grains in EL3 chondrites experienced variable thermal histories before each sample. accretion to the parent bodies. However, these geothermometers are not always a PETROGRAPHY AND MINERALOGY reflection of the thermal processing that the meteorite experienced. They do not indicate peak metamorphic Outline and Classification of Samples temperatures, but instead give the last thermal conditions (Zhang et al. 1995, 1996). It is therefore difficult to estimate Table 1 shows the list of 20 samples studied here. We the primordial formation conditions of E chondrites from selected samples to cover the range of E chondrite materials: opaque geothermometers. In order to clarify the primordial two EH3, three EH4, three EH5, three EH melt rocks (or melt conditions, other minerals need to be considered. breccia), two EL3, two EL4, three EL6, an EL melt rock, and The common occurrence of silica phases in E chondrites an anomalous E chondrite. The composition of the is one of the characteristics that distinguish them from other (Mg,Mn,Fe)S phase clearly distinguishes these EH and EL chondrite groups. The high abundance of silica minerals is a chondrites (Table 2 and Fig. 1). This phase in Y-793225 has characteristic of primitive EH3 chondrites in particular (Hicks an intermediate composition between the two end members et al. 2000). Silica polymorphs are useful indicators of (Lin and Kimura 1998). We refer to it as “EI” in this paper pressure-temperature conditions. The study of silica because its mineralogical features are intermediate between polymorphs in the E chondrites could therefore help provide EH and EL chondrites. The Y-793225 chondrite shows a constraints on the thermal history of the E chondrites. highly recrystallized texture like an EL6. In this study, we focus on silica phases in the E Y-793246 and DaG 734 have not yet been documented in chondrites. The various silica polymorphs should preserve the detail. We describe these E chondrites and summarize their primary temperature and pressure conditions under which petrographic and mineralogical features in the appendix. they formed because of the transition rate between silica They are both EL4 chondrites, although DaG 734 was heavily polymorphs is relatively sluggish (Putnis 1992). Thus, we weathered and lost most of its opaque minerals, including the expect that silica polymorphs will help constrain the thermal (Mg,Mn,Fe)S phase. histories of the E chondrites. The petrographic and mineralogical features of most of Mason (1966) and Binns (1967) identified quartz, these E chondrites have already been reported in detail tridymite, and
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