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Bulk Mineralogical Changes of Hydrous Micrometeorites During Heating in the Upper Atmosphere at Temperatures Below 1000 °C

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Bulk Mineralogical Changes of Hydrous Micrometeorites During Heating in the Upper Atmosphere at Temperatures Below 1000 °C Bulk mineralogical changes of hydrous micrometeorites during heating in the upper atmosphere at temperatures below 1000 °C Item Type Article; text Authors Nozaki, Wataru; Nakamura, Tomoki; Noguchi, Takaaki Citation Nozaki, W., Nakamura, T., & Noguchi, T. (2006). Bulk mineralogical changes of hydrous micrometeorites during heating in the upper atmosphere at temperatures below 1000 °C. Meteoritics & Planetary Science, 41(7), 1095-1114. DOI 10.1111/j.1945-5100.2006.tb00507.x Publisher The Meteoritical Society Journal Meteoritics & Planetary Science Rights Copyright © The Meteoritical Society Download date 06/10/2021 08:34:17 Item License http://rightsstatements.org/vocab/InC/1.0/ Version Final published version Link to Item http://hdl.handle.net/10150/656158 Meteoritics & Planetary Science 41, Nr 7, 1095–1114 (2006) Abstract available online at http://meteoritics.org Bulk mineralogical changes of hydrous micrometeorites during heating in the upper atmosphere at temperatures below 1000 °C Wataru NOZAKI1, Tomoki NAKAMURA1*, and Takaaki NOGUCHI2 1Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University 33, Hakozaki, Fukuoka 812-8581, Japan 2Materials and Biological Science, Faculty of Science, Ibaraki University, Bunkyo, Mito 310-8512, Japan *Corresponding author. E-mail: [email protected] (Received 04 August 2005; revision accepted 25 April 2006) Abstract–Small particles 200 μm in diameter from the hydrous carbonaceous chondrites Orgueil CI, Murchison CM2, and Tagish Lake were experimentally heated for short durations at subsolidus temperatures under controlled ambient pressures in order to examine the bulk mineralogical changes of hydrous micrometeorites during atmospheric entry. The three primitive meteorites consist mainly of various phyllosilicates and carbonates that are subject to decomposition at low temperatures, and thus the brief heating up to 1000 °C drastically changed the mineralogy. Changes included shrinkage of interlayer spacing of saponite due to loss of molecular water at 400–600 °C, serpentine and saponite decomposition to amorphous phases at 600 and 700 °C, respectively, decomposition of Mg- Fe carbonate at 600 °C, recrystallization of secondary olivine and Fe oxide or metal at 700–800 °C, and recrystallization of secondary low-Ca pyroxene at 800 °C. The ambient atmospheric pressures controlled species of secondary Fe phase: taenite at pressures lower than 10–2 torr, magnesiowüstite from 10−3 to 10−1 torr, and magnetite from 10−2 to 1 torr. The abundance of secondary low-Ca pyroxene increases in the order of Murchison, Orgueil, and Tagish Lake, and the order corresponds to saponite abundance in samples prior to heating. Mineralogy of the three unmelted micrometeorites F96CI024, kw740052, and kw740054 were investigated in detail in order to estimate heating conditions. The results showed that they might have come from different parental objects, carbonate- rich Tagish Lake type, carbonate-poor Tagish Lake or CI type, and CM type, respectively, and experienced different peak temperatures, 600, 700, and 800~900 °C, respectively, at 60–80 km altitude upon atmospheric entry. INTRODUCTION Maurette et al. 1991). Based on the mineralogy of such weakly heated micrometeorites, we can elucidate the types of Micrometeorites are extraterrestrial material accreting to parental objects and the formation history of these objects. the Earth with sizes typically ranging from 50 to 100 μm, but Major dust suppliers in the solar system are asteroids and occasionally up to several hundred micrometers. During comets, although the ratio of dust from the two sources is accretion and with few exceptions, they are heated by under debate (i.e., Kortenkamp and Dermott 1998; Ishimoto deceleration in the atmosphere. Considering the difference in 2000). Some unmelted micrometeorites and interplanetary micrometeorite accretion rates between at upper atmosphere dust particles (IDPs) consist mainly of phyllosilicates and the (Love and Brownlee 1993) and at Antarctic ice fields (e.g., phyllosilicate mineralogy is basically similar to that of Taylor et al. 1998; Yada et al. 2004), most micrometeorites are hydrous carbonaceous chondrites such as CI, CM, and Tagish evaporated during heating in the atmosphere; only ~10% Lake type (e.g., for IDPs, see Bradley and Brownlee 1991 and survive to reach the Earth’s surface. Among the accreted Rietmeijer 1996; for micrometeorites, see Klöck and micrometeorites, approximately half are totally melted Stadermann 1994; Kurat et al. 1994; Nakamura et al. 2001; spherules that have completely lost their preheating Noguchi et al. 2002; Nakamura and Noguchi 2004). This mineralogical signatures. The remaining samples are also suggests that hydrous asteroids are one of parental objects that heated to some extent, but escaped high-temperature heating supply micrometeorites. However, there is no evidence to and form irregularly shaped unmelted or partially melted suggest that hydration in active comets is impossible. In fact, micrometeorites (e.g., Love and Brownlee 1991, 1994; there are observations that a sizeable fraction of CI-like 1095 © The Meteoritical Society, 2006. Printed in USA. 1096 W. Nozaki et al. Fig. 1. An experimental set-up for heating hydrous carbonaceous chondrites. A small particle approximately 200 μm in diameter was placed on a small sample container made of Si glass, which can slide in and out from an electric furnace using a hand magnet. A Pt-Rh thermocouple was held 2 mm above the sample container. Both the sample and the thermocouple were put inside of silica tube. The silica tube was evacuated using a rotary pump for pressures of 10−2 to 1 torr and using both a turbo molecular pump and the rotary pump for 10−4 to 10−3 torr. The atmospheric pressure was monitored using a pirani vacuum gauge and controlled using a leak valve to keep pressures constant throughout the heating. meteors enter the Earth’s atmosphere on cometary orbits and reproduced textures very similar to partially or totally melted with cometary velocities (Ceplecha et al. 1998; Rietmeijer micrometeorites. Most of the heating runs in Toppani et al. 2000). Therefore, the dust-supply contribution from hydrous (2001) were conducted at temperatures higher than 1000 °C, asteroids to the total micrometeorite accretion is far from while in this study, heating was performed at temperatures known. In addition, phyllosilicates in hydrous particles are lower than 1000 °C. Therefore, the temperature regime in this decomposed and dehydrated even by heating at low work is complementary to that in Toppani et al. (2001). We temperature during atmospheric entry. In the present study, a have succeeded in reproducing the bulk mineralogy of small particle of a hydrous meteorite was heated step-by-step unmelted micrometeorites by heating hydrous-chondrite up to 900 or 1000 °C in 100 °C intervals at variable particles at subsolidus temperatures. Mineralogical atmospheric pressures. The heating duration at each comparison between experimental products and natural temperature was 120 s. At each temperature step, the heated micrometeorites allows us to estimate temperatures and particle was investigated by synchrotron X-ray diffraction ambient pressures during atmospheric heating of the method in order to demonstrate the changes of bulk micrometeorites. mineralogy of hydrous particles during heating at subsolidus temperature. In addition to stepped heating, one-time 10 s SAMPLES AND EXPERIMENTS heating was also performed to see effects of short-duration heating. Stepped heating was performed on particles with Bulk mineralogy is an effective measure for diameters of ~200 µm that were separated from Orgueil CI characterization of micrometeorites because they are and matrix of Tagish Lake and Murchison CM chondrites. composed of numerous fine mineral particles, mostly The reason we chose these three meteorites as starting submicron in size (e.g., Klöck and Stadermann 1994; Kurat materials is that the bulk mineralogical characteristics of et al. 1994; Engrand and Maurette 1998). Any process such as phyllosilicate-rich micrometeorites are basically similar to hydration and heating, and any condition of the process such that of these chondrites (Nakamura and Noguchi 2004). as oxygen fugacity and temperature, are reflected in bulk Tagish Lake is composed of two representative lithologies: mineralogy. Therefore, bulk mineralogy can be used to carbonate-rich and carbonate-poor (Zolensky et al. 2002). establish a link between meteorites and micrometeorites The former contains small grains of Mg-Fe carbonate in (Nakamura and Noguchi 2004) and also to clarify the heating matrix in high density, while the latter does not. Therefore, we conditions of micrometeorites upon atmospheric entry. Some performed heating experiments on particles from both experimental simulations of the atmospheric entry of lithologies in the case of Tagish Lake. The experimental micrometeorites were already reported to investigate changes configuration is shown in Fig. 1. The experiment was carried of properties other than bulk mineralogy: microtextures and out using the following procedure: a particle of carbonaceous abundances of mobile trace elements (Fraundorf et al. 1982; chondrite was placed in a sample container and the container Klöck and Stadermann 1994; Greshake et al. 1998), IR was set inside a silicon glass tube that was evacuated down to spectroscopic signatures (Sandford and Bradley 1989), and a designated ambient pressure. A leak valve allowed
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