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

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

Download date 06/10/2021 08:34:17

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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 small textures and mineral compositions of spinels in particular amounts of atmosphere to flow into the glass tube to maintain (Toppani et al. 2001; Toppani and Libourel 2003). By flash- constant pressure. Prior to heating the sample particle, the heating a particle of Murchison CM and Orgueil CI chondrite glass tube was inserted into a furnace to heat it to a designated at controlled ambient pressures, Toppani et al. (2001) temperature. Then the container with the sample particle was Bulk mineralogical changes of hydrous micrometeorites during upper-atmosphere heating 1097 inserted into the furnace. The sample particle was heated by radiation from a hot spot of the furnace. After 120 s, the container with the sample particle was pulled out of the furnace in order to cool the particle rapidly. An example of temperature profiles is shown in Fig. 2. The heating experiments were carried out at variable temperatures and ambient atmospheric pressures, as listed in Table 1. An important aspect of this experiment is that a single particle was heated step by step at intervals of 100 or 300 °C (Table 1), so as to follow continuous mineralogical changes of the single particle with increasing temperature. For instance, a particle from carbonate-rich lithology of Tagish Lake meteorite was heated at 400 °C and for 120 s (hereafter 400 °C/120 s), 500 °C/120 s, 600 °C/120 s, 700 °C/120 s, 800 °C/ 120 s, and 900 °C/120 s. After heating at each temperature, the sample particle was analyzed by synchrotron X-ray diffraction to characterize bulk mineralogy and then it was heated again to a higher temperature. The stepped heating and the 120 s duration at each temperature step adopted in our experiments is not relevant to the actual heating of micrometeorites in the atmosphere, because the real heating is one time and much shorter Fig. 2. Temperature profiles of experimental heating measured by a duration. Calculations simulating the atmospheric-entry Pt-Rh thermocouple fixed close to the sample particle. The black heating indicated that micrometeorites were heated one time bold line is a profile adopted for stepped heating (in this case, 900 °C) for less than 10 s at peak temperature (e.g., Love and with duration of 120 s at each temperature. The thin gray line is a Brownlee 1991; Flynn 2001). In order to understand the profile for one-time, short-duration heating of approximately 10 s, adopted for a particle of carbonate-poor lithology of Tagish Lake differences between mineralogical changes induced by long- meteorite. duration stepped heating and those by short-duration one-time heating, we conducted a brief heating experiment in which a of Tagish Lake, each of which was heated at 500 °C and 10−1 particle from carbonate-poor lithology of Tagish Lake torr, 700 °C and 10−2 torr, and 900 °C and 10−2 torr, and a meteorite was heated once at 900 °C for 10 s. The temperature particle of Murchison heated at 1000 °C and 10−2 torr were profile of this experiment is given in Fig. 2. The mineralogy observed by a transmission electron microscope (TEM). The of the particle was compared with that of the particle having former two Tagish Lake samples and the Murchison sample been heated step-by-step at 400 °C/120 s, 500 °C/120 s, 600 were prepared only for TEM observation, while the latter °C/120 s, 700 °C/120 s, 800 °C/120 s, and 900 °C/120 s. Tagish Lake sample was analyzed by X-ray diffraction prior For characterization of the bulk mineralogy of to TEM observation (Table 1). The particle samples were experimentally heated particles and some natural embedded in epoxy resin EMbed-812, and 60 to 100 nm micrometeorites, we utilized synchrotron radiation X-rays at sections were microtomed by Leitz-Reichert Super Nova beam line 3A of the Photon Factory Institute of Material ultramicrotome for TEM observation. The microstructures, Science, High Energy Accelerator Research Organization, mineralogy, and chemical compositions of minerals in Tsukuba, Japan. A particle sample was mounted on a thin samples were obtained by JEOL JEM-2000FX II equipped glass fiber 5 µm in diameter and exposed to synchrotron with EDAX DX4 energy dispersive spectrometer (EDS). A X-rays in a Gandolfi camera that gives a powder diffraction 100 nm beam was used for the analysis of anhydrous silicates pattern of the particle (Nakamura et al. 2001). The X-rays and Fe oxides. The beam diameter was changed from 300 to were monochromated to 0.2161 ± 0.0001 nm and 500 nm for phyllosilicates to minimize compositional change concentrated by mirrors to a position of the Gandolfi camera. due to heating during analysis and contamination from the The X-ray diffraction pattern was recorded on a high- surrounding phases. Semiquantitative analysis was based on resolution imaging plate (IP) and the pattern was transformed the Cliff-Lorimer thin film approximation. Experimental to an 8-bit TIF image by using an IP reader (Fuji film BAS k-factors were obtained from many mineral standards. The 2500). The ultra-high intensity and well-monochromated k-factors were determined by using ultramicrotomed mineral synchrotron X-rays allows us to obtain a clear powder X-ray standards. Because the remainders after ultramicrotomy of diffraction pattern of each small particle with short exposure each embedded sample have quite flat surfaces, the surfaces of 15–20 min. of each remainder were coated by carbon and observed by Three particles from the carbonate-rich lithology JEOL JSM-6700F FE-SEM. 1098 W. Nozaki et al.

Table 1. Temperatures and ambient atmospheric pressure during heating of a particle of hydrous carbonaceous chondrite. Tagish Lake Method of carbonate-rich 200 °C 300 °C 400 °C 500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C characterization 2.5 × 10−4 torr x x SR-XRDa 2.0 × 10−3 torr x x SR-XRD 1.5 × 10−2 torr x x x x x x SR-XRD and TEM 1.5 × 10−2 torr x TEM 1.0 × 10−1 torr x x SR-XRD 1.0 × 10−1 torr x TEM 0.8 torr x x SR-XRD Tagish Lake carbonate-poor 200 °C 300 °C 400 °C 500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C 1.5 × 10−2 torr xb SR-XRD 1.5 × 10−2 torr x x x x x x SR-XRD Orgueil CI 200 °C 300 °C 400 °C 500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C 2.5 × 10−4 torr x x SR-XRD 2.0 × 10−3 torr x x SR-XRD 1.5 × 10−2 torr x x x x x x x x x SR-XRD 1.0 × 10−1 torr x x SR-XRD 0.8 torr x x SR-XRD Murchison CM 200 °C 300 °C 400 °C 500 °C 600 °C 700 °C 800 °C 900 °C 1000 °C 2.5 × 10−4 torr x x SR-XRD 2.0 × 10−3 torr x x SR-XRD 1.5 × 10−2 torr x x x x x x x x x SR-XRD 1.5 × 10−2 torr x TEM 1.0 × 10−1 torr x x SR-XRD aSR-XRD = synchrotron X-ray diffraction. bShort-duration (10 s) heating.

RESULTS AND DISCUSSION molecular water between t-o layers and thus no loss of the water molecules takes place from serpentine, as shown in Bulk Mineralogical Changes of a Tagish Lake Particle later in the results of Murchison heating. from Carbonate-Rich Lithology The abundance of Mg-Fe carbonate continuously decreases from 400 to 600 °C (Fig. 3). The decomposition Matrix of natural (unheated) Tagish Lake with carbonate- temperature of Fe-rich carbonate is lower than that of Mg-rich rich lithology consists mainly of saponite and Mg-Fe carbonate, and thus the gradual depletion of the Mg-Fe carbonate with minor amounts of pyrrhotite (Fig. 3) (detailed carbonate can be interpreted to mean that Fe-rich carbonate description of XRD is given in Nakamura et al. 2003). starts to decompose at 400 °C and Mg-rich carbonate Average Mg/(Mg + Fe) atomic ratio of coarse saponite is 0.79 decomposes almost completely at 600 °C. Magnesiowüstite ± 0.01 and that of Mg-Fe carbonate is 0.39 ± 0.11 (Nakamura forms at the expense of Mg-Fe carbonate and its X-ray et al. 2003). Heating at 10−2 torr and temperatures from 400 to reflections become narrower with increasing temperature, 900 °C with intervals of 100 °C changed the mineralogy which is suggestive of coarsening of the crystals. The drastically (results are shown in Fig. 3). The first change formation of magnesiowüstite was confirmed by TEM observed is shrinkage of (001) interlayer spacing of saponite observation of two samples heated at 500 and 10−1 torr and at 400 °C and higher temperatures. The spacing reduced from 700 °C and 10−2 torr, respectively: the crystal sizes of 1.32 nm to 1.06 nm at 600 °C, which resulted from a loss of magnesiowüstite are larger in the 700 °C sample (50–150 nm water molecules having been present between t-o-t layers of across) (Fig. 5b) than in the 500 °C sample (5–30 nm across) saponite (Fig. 4). This water loss from saponite only causes (Fig. 5c). Some Mg-Fe carbonate grains are decarbonated to shrinkage along the c-axis and does not bring about other magnetite (Fig. 5d), which depends on the FeCO3 content of crystallographic changes. Indeed, TEM observation of a the carbonate grains (Noguchi et al. 2002). sample heated at 500 °C and 10−1 torr showed that saponite Saponite reflections including 001 at 10°, prism at 28– exhibits 1.21 nm (001) lattice fringes that indicate slight 30°, and 300 at 90°, are weakened at 700 °C (Fig. 3), shrinkage due to a slight depletion of interlayer water indicating that saponite decomposes to become amorphous at molecules (Fig. 5a). On the other hand, serpentine has no this temperature. Consequently, magnesiowüstite and Bulk mineralogical changes of hydrous micrometeorites during upper-atmosphere heating 1099

Fig. 3. Synchrotron X-ray diffraction patterns showing changes of bulk mineralogy during a stepped heating at 10−2 torr of a particle from matrix of Tagish Lake with carbonate-rich lithology. pyrrhotite are major crystalline phases and the hydrous Stadermann (1994). Pyrrhotite abundance decreases at 700 silicates are mostly amorphous at 700 °C (Fig. 3). TEM °C, but it continues the presence until 900 °C (Fig. 3). observation of a sample heated at 700 °C showed that some Heating at lower (10−4 and 10−3 torr) or higher (10−1 and parts of saponite survive as poorly crystalline with 0.991 nm 1 torr) ambient pressures resulted in mineralogical changes (001) lattice fringes (the most dehydrated saponite), but most similar to heating at 10−2 torr (Table 1; Fig. 6). Mg-Fe parts become amorphous (Fig. 5e). At 800 °C, secondary carbonate decomposes at 600 °C, which is consistent with the olivine and low-Ca pyroxene crystallizes from amorphous temperature, as in the case of 10−2 torr heating. But a silicates (Fig. 3). At 900 °C, olivine and pyroxene reflections decomposition product of Mg-Fe carbonate being formed at become larger and thinner, suggesting that recrystallization 600 °C differs according to ambient pressures: proceeds with an increase of temperature. Crystal sizes of the magnesiowüstite from 10−4 to 10−2 torr, magnetite and secondary olivine and pyroxene range from 10 to 50 nm magnesiowüstite at 10−1 torr, and magnetite at 1 torr. At (Fig. 5f). The grain size of the recrystallized secondary olivine 900 °C (Fig. 6), both saponite and carbonate decompose and is consistent with that of the secondary olivine in natural the resulting secondary Fe phases are taenite, micrometeorites reported by Flynn et al. (1993) and Klöck and magnesiowüstite, and magnetite. The difference in the 1100 W. Nozaki et al. decomposed products reflects differences in oxygen fugacity during heating. Taenite formed at both 10−3 and 10−4 torr (Fig. 6) probably through the following two pathways: 1) from magnesiowüstite by further heating to 900 °C, and 2) from saponite together with formation of secondary olivine and low-Ca pyroxene. This suggests that the final decomposition product of Mg-Fe carbonate in the heating at 10−3 and 10−4 torr is taenite, but during decomposition, magnesiowüstite formed as a metastable phase because magnesiowüstite can be formed from the carbonate merely by 2+ loss of CO2 without any reduction of Fe . Small amounts of taenite are also detected in a product heated at 900 °C and 10−2 torr. It is worth pointing out that Fe metal forms as taenite, not as kamacite (Fig. 6). Taenite gives only three, albeit strong, reflections in the diffraction angle we detected: 0.208 nm at 63°, 0.180 nm at 74°, and 0.127 nm at 116°. All three reflections are clearly observed in the samples heated at low ambient pressures. Since the metal is a decomposition product of Mg-Fe saponite and Mg-Fe carbonate, both of which contain little Ni (e.g., Nakamura et al. 2003), the secondary Fe metal is expected to occur as kamacite. However, the phase diagram of Fe-Ni alloy (Goldstein and Fig. 4. A schematic drawing of the crystal structure of saponite along Ogilvie 1965) indicates that taenite is the stable phase at the c-axis. Multi-layers of water molecules are present between SiO4 temperature higher than 900 °C irrespective of Ni content. tetrahedron (t)-(Mg, Fe)(OH)2 octahedron (o)- t layers. X-ray Currently it is uncertain why taenite forms instead of diffraction analysis indicates that (001) interlayer spacing is reduced kamacite. We suggest that the presence of abundant from 1.32 nm (nonheated sample) to 1.06 nm (heated at 600 °C). taenite in the products heated at 900 °C and pressures lower Heating at 700 °C changes most parts of saponite to amorphous phases, but TEM observation suggests that in places saponite −3 than 10 torr can be interpreted in the following way: 1) survives and shows 0.99 nm spacing. during heating at 900 °C, taenite forms through the decomposition of saponite and carbonate, and 2) the taenite and thus the transformation from olivine to laihunite occurs at retains its crystallographic structure down to room relatively low temperatures ~800 °C (Tamada et al. 1983; temperature because of a very fast cooling rate after heating Kondoh et al. 1985). In contrast to previous studies on (Fig. 2). Therefore, the presence of taenite suggests that the laihunite that formed by oxidation of olivine, in this study heating occurs at temperature at 900 °C followed by rapid laihunite forms in the course of decomposition of hydrous cooling. In fact, heating at 900 °C and lower pressures also silicate saponite (Fig. 6). The laihunite in the particle of results in formation of abundant taenite in samples of both Tagish Lake heated at 1 torr (Fig. 6) seems to crystallize from Orgueil and Murchison, as shown later in this paper. amorphous silicates at 800 °C, because at 700 °C no Heating at higher ambient pressures also produces anhydrous silicates are detected and at 800 °C laihunite starts variable silicates. Secondary silicates that formed by the to form. The laihunite contains appreciable amounts of Mg2+ decomposition of saponite are olivine and low-Ca pyroxene as substitution for Fe2+ because X-ray reflections from the in products heated at pressures from 10−4 to 10−1 torr and laihunite (Fig. 6) appear at slightly higher diffraction angles, 900 °C, while those formed in a product heated at 1 torr are and thus with slightly smaller interlayer spacings, than quartz and probably laihunite (Fig. 6). Laihunite is known to laihunite with no Mg2+ (Tamada et al. 1983). form during subsolidus oxidation of olivine (Tamada et al. 1983; Kondoh et al. 1985) and has a distorted olivine Bulk Mineralogical Changes of a Tagish Lake Particle structure with Fe3+ charge balanced by vacancies with a from Carbonate-Poor Lithology and an Orgueil Particle ()3+ 2+ 2+ composition Fe Fe1 – xMgx 1.6SiO4 (Banfield et al. 1990; Banfield et al. 1992). It commonly occurs together with Bulk mineralogy of Tagish Lake matrix from carbonate- magnetite and quartz or amorphous silica in both terrestrial poor lithology is similar to that of Orgueil: both consist of rocks (e.g., Banfield et al. 1990) and experimentally heated saponite, serpentine, and magnetite with minor amounts of olivine at high oxygen fugacity (e.g., Kondoh et al. 1985). Mg-Ca carbonate and pyrrhotite (Figs. 7 and 8). Detailed Olivine and laihunite are similar in crystallographic structure, mineralogy of both meteorites is given in Tomeoka and sharing the framework of hexagonal close-packed oxygens, Buseck (1988) and Zolensky et al. (2002). Integrated Bulk mineralogical changes of hydrous micrometeorites during upper-atmosphere heating 1101

Fig. 5. TEM photomicrographs of ultramicrotomed sections of the matrix of the experimentally heated Tagish Lake meteorite. From (a) to (f), samples were heated, 500, 700, and 900 °C at 10−2 torr. a) A high-resolution image of phyllosilicate in the matrix heated at 500 °C. The spacing of (001) of saponite is 1.208 nm, slightly shrunk from unaltered spacing. b) An aggregate of fine-grained (50–150 nm in diameter) magnetite Fe3O4 and magnesiowüstite (Mg, Fe)O that was formed by decarbonation of Mg- and Fe-rich carbonate, heated at 700 °C. c) An aggregate of very fine-grained (5–30 nm in diameter) magnesiowüstite formed by decarbonation of Mg- and Fe-rich carbonate, heated at 500 °C. d) An aggregate of very fine-grained (5–30 nm in diameter) magnetite and magnesiowüstite in the same sample shown in (c). e) High-resolution image of phyllosilicate in the matrix heated at 700 °C. The majority of phyllosilicate has lost their sheet-like structure. Where (001) lattice fringes observed, they are shrunk to 0.991 nm, which indicates that the phyllosilicate completely lost their interlayer H2O molecules. f) When heated to 900 °C, the phyllosilicate predominant matrix recrystallized to the fine-grained (10–50 nm across) aggregates of olivine, low-Ca pyroxene, and magnetite. 1102 W. Nozaki et al.

Fig. 6. Synchrotron X-ray diffraction patterns showing the bulk mineralogy of a particle from Tagish Lake matrix with carbonate-rich lithology heated at 900 °C and atmospheric pressures of 10−4, 10−3, 10−2, 10−1, and 1 torr. intensities of saponite (001) reflection are roughly similar to the unheated sample (1.38 nm) to the one heated to 600 °C those of serpentine 001 reflection (Figs. 7 and 8), suggesting (1.23 nm), while (001) spacing of serpentine remains constant that the abundance of the two phyllosilicates is similar in at 0.73–0.74 nm (Figs. 7 and 8). This indicates that interlayer Orgueil and carbonate-poor lithology of Tagish Lake. A molecular waters are derived from saponite by heating, while minor difference is that Tagish Lake carbonate-poor lithology they are not derived from serpentine because no molecular tends to have slightly higher abundance of saponite than water is present in serpentine. At 600 °C, serpentine is serpentine. Magnetite reflections are very sharp and large in decomposed. At 700 °C, saponite is almost decomposed and both meteorites; they probably come from framboidal amorphous silicate seems to be dominant (Figs. 7 and 8). Mg- magnetites that occur in abundance in both meteorites Ca carbonate dolomite is decomposed at 700 °C. Its (Tomeoka and Buseck 1988; Zolensky et al. 2002). The decomposed products appear at higher temperatures as results of heating particles from the two meteorites are magnesiowüstite, but CaO lime cannot be detected by X-ray described together in the following section, since they show diffraction. Secondary olivine forms at 800 °C and its similar preheating mineralogy and similar heat-induced reflections become well-defined from 800 to 1000 °C due to mineralogical changes. an increase of crystallinity (Figs. 7 and 8). Transformation of As in the case of a Tagish Lake particle from carbonate- hydrous phases to anhydrous phases such as olivine is rich lithology, 100 °C interval step-by-step heating from 400 therefore almost complete at 800 °C. The temperature is to 1000 °C at 10−2 torr was performed for particles of Tagish consistent with that reported by Greshake et al. (1998). Lake with carbonate-poor lithology and Orgueil (Figs. 7 and Heating at variable ambient pressures from 10−4 to 1 torr 8). Bulk mineralogy of particles from both meteorites was performed only for Orgueil (Table 1; Fig. 9), because, changes greatly. The 001 spacing of saponite is reduced from again, the bulk mineralogy of Orgueil is similar to that of Bulk mineralogical changes of hydrous micrometeorites during upper-atmosphere heating 1103

Fig. 7. Synchrotron X-ray diffraction patterns showing changes of bulk mineralogy during a stepped heating at 10−2 torr of a particle from matrix of Tagish Lake with carbonate-poor lithology.

Tagish Lake with carbonate-poor lithology. Heating at low produces magnetite as a secondary Fe phase and olivine, pressures (10−4 and 10−3 torr) results in the formation of quartz, and probably laihunite as secondary silicates (Fig. 9). taenite and magnesiowüstite as secondary Fe phases, while heating at high pressures (10−2 and 10−1 torr) results in the Comparison Between Long-Duration Stepped Heating formation of magnesiowüstite and probably magnetite and Short-Duration One-Time Heating (Fig. 9). The formation of secondary magnetite is difficult to detect due to the presence of abundant primary magnetite. But The duration of heating in our experiment, 120 s heating secondary magnetite is small and low crystalline, which pulses, is much longer than the actual heating pulse results in broad reflections as is observed at a experienced by micrometeorites on atmospheric entry. In this diffraction angle of 83° for products heated at pressures section, we will explain why the pulse-heating for more than higher than 10−2 torr. Heating at the highest pressure of 1 torr an order-of-magnitude longer than the actual time produces 1104 W. Nozaki et al.

Fig. 8. Synchrotron X-ray diffraction patterns showing changes of bulk mineralogy during a stepped heating at 10−2 torr of a particle from Orgueil CI carbonaceous chondrite. results that are relevant to the actual case of micrometeorite 120 s, 500 °C/120 s, 600 °C/120 s, 700 °C/120 s, 800 °C/ heating. For comparison between long-duration stepped 120 s, and 900 °C/120 s: the results are shown in Fig. 7). The heating and short-duration one-time heating, we conducted a 10 s heating at peak temperature is similar to the heating heating experiment on a particle from carbonate-poor duration obtained by computer simulation of atmospheric lithology of Tagish Lake. The particle was heated to 900 °C at entry (Love and Brownlee 1991; Flynn 2001). 1.5 × 10−2 torr and for only 10 s (the results are shown in The comparison indicates that the mineralogies of the Fig. 10) and compared its mineralogy with the particle two particles heated in different thermal regimes are very heated step-by-step at temperatures from 400 to 900 °C at similar: saponite and serpentine are completely decomposed, 1.5 × 10−2 torr and for 120 s at each temperature step (400 °C/ dolomite is decomposed, secondary olivine and pyroxene Bulk mineralogical changes of hydrous micrometeorites during upper-atmosphere heating 1105

Fig. 9. Synchrotron X-ray diffraction patterns showing bulk mineralogy of a particle from matrix of Orgueil heated at 900 °C and atmospheric pressures of 10−4, 10−3, 10−2, 10−1, and 1 torr.

Fig. 10. Synchrotron X-ray diffraction patterns showing changes of bulk mineralogy during one-time 10 s heating at 10−2 torr of a particle from Tagish Lake carbonate-poor lithology. formed, Fe phases are mixtures of taenite, magnetite, and thus it is expected that a longer heating duration promotes the magnesiowüstite, and pyrrhotite survived (Figs. 7 and 10). growth. But the particle heated at 900 °C/10 s gave reflections This demonstrates that bulk mineralogical changes during of secondary olivine and pyroxene much sharper and stronger heating depend largely on heating temperature rather than than the particle heated stepwisely to 800 °C/120 s, but is heating duration. In addition, the growth of secondary more similar to the particle heated to 900 °C/120 s (Figs. 7 minerals during heating is a kinetically controlled process and and 10). This indicates that 100 °C difference in heating 1106 W. Nozaki et al. temperature is more critical to the growth of secondary (1973). Serpentine starts to decompose at 400 °C and minerals than one-order-magnitude difference in heating completes the decomposition at 600 °C. At 600 °C, duration. All these experimental facts verify the application amorphous materials, which were formed by the of our stepwise heating experiments to the estimation of peak decomposition of serpentine, are dominant. At 700 °C, temperatures experienced by micrometeorites during actual secondary olivine and magnetite start to form, but they are atmospheric entry. still low crystalline. The reflections of secondary olivine and The mineralogical changes observed in our long-duration magnetite become narrower and larger with increasing stepped heating experiments are similar to those observed in temperature, indicating promoting of recrystallization. At short duration (10–60 s) heating experiments of Orgueil CI 900 °C, magnesiowüstite forms, probably from magnetite by chondrite done by Greshake et al. (1998). They observed reduction. onset of phyllosilicate decomposition at 700 °C/20 s and TEM observation of a Murchison sample heated at complete of the decomposition at 800 °C/20 s. The 1000 °C and 10−2 torr shows that silicates are mostly small, temperatures are exactly the same as those observed in our equigranular crystals of olivine less than 200 nm in diameter long-duration stepped heating experiments. This coincidence (Fig. 12a). A low-magnification TEM image of the sample further supports the validity of our experiments to apply shows that there are abundant small vesicles (less than actual micrometeorite heating in the atmosphere. Sandford 200 nm in diameter) that were probably formed during and Bradley (1989) carried out short-duration (12–15 s) dehydration of hydrous phases (Fig. 12b). There are Fe- and heating experiments of Murchison CM chondrite. They used S-bearing object and Fe-, Ni-, and S-bearing object. In the fourier transform infrared spectroscopy to detect hydration former case, only magnetite is identified by selected area bands at 3 and 6 microns. The results showed that dehydration electron diffraction (SAED) (Fig. 12c), although S is detected starts at 395 °C; at 825 °C, small amounts of H2O and OH are by EDS analysis as well as abundant Fe. In the latter case, left in the Murchison sample, which can be interpreted to only Ni-rich magnetite is identified by SAED, although S is mean that at 825 °C, phyllosilicates was decomposed but detected by EDS (Fig. 12d). These data suggest that S-bearing some H2O and OH still remained in the particle. The results material exists as amorphous material that fills the interstices are also essentially consistent with those of our experiments. of magnetite. Magnetite with S-bearing amorphous materials is probably a decomposition product of tochilinite. There are Bulk Mineralogical Changes of a Murchison Particle olivine crystals that contain lamellar or breb-like magnetite (Fig. 12e). The SAED pattern of the olivine shows that Matrix of the unheated Murchison CM chondrite (100)olivine is parallel to (111)magnetite, which indicates that contains serpentine as a dominant phyllosilicate, which is oxygen packing planes are common to both of the minerals. verified by the presence of strong basal reflection of (001) at The olivine with abundant lamellar or breb-like magnetite 0.71 nm (2θ = 18° in Fig. 11) and (002) at 0.36 nm (35°) and was originally coarse Fe-rich serpentine that topotactically prism reflections rising up from 27°. Prism reflections are decomposed into these minerals during heating. peculiar to phyllosilicates with heavy stacking disorder along Variations of secondary Fe phases and silicates among the c-axis. 001 spacing of serpentine (0.71 nm) (Fig. 11) is Murchison samples heated at different ambient pressures are smaller than that of normal serpentine (0.72 nm), which similar to those of other meteorites. Heating at lower ambient suggests that some amounts of Si4+ in tetrahedral sites are pressures of 10−4 and 10−3 torr produces Fe metal taenite and substituted by Fe3+ to make cronstedtite (Wicks and magnesiowüstite as secondary Fe phases, while heating at O’Hanley 1991; Nakamura and Nakamuta 1996). The other elevated ambient pressures produces magnetite and hydrous phase present is tochilinite (Fig. 11), which consists magnesiowüstite for 10−2 torr and magnetite for 10−1 torr of 1:1 stacking of brucite (Mg, Fe)(OH)2 and mackinawite (Fig. 13). FeS layers (Mackinon and Zolensky 1984; Zolensky 1984). Overall results of bulk mineralogical changes in Large reflection of (002) at 0.54 nm relative to (001) at Murchison during heating are consistent with those of Akai 1.08 nm indicates dominant occupancy of Fe2+ in the brucite (1992), who heated a powder of Murchison CM chondrite at layer (Nakamura and Nakamuta 1996). In addition to the variable temperatures of 250–1100 °C for long durations of hydrous phases, magnetite and very small amounts of olivine 1–350 h under vacuum condition of 10−4 to 10−5 torr, with an and low-Ca pyroxene are present in the unheated Murchison aim to construct the T-T-T diagram of CM serpentine. particle (Fig.11). Experimentally heated Murchison samples were investigated Heating of a particle of Murchison matrix was performed by TEM and the results showed that below 300 °C no phase at temperatures from 200 to 1000 °C at intervals of 100 °C change was observed, at 300–500 °C serpentine starts and ambient pressure of 10−2 torr (Fig. 11). Tochilinite is the decomposition and transitional structure between serpentine first mineral decomposed: its abundance reduces at 300 °C and olivine appears, and at 500–750 °C olivine starts to and completely disappears at 400 °C. The decomposition crystallize and becomes more crystalline with increasing temperature is consistent with that reported in Fuchs et al. temperature. Bulk mineralogical changes of hydrous micrometeorites during upper-atmosphere heating 1107

Fig. 11. Synchrotron X-ray diffraction patterns showing the changes in bulk mineralogy during a stepped heating at 10−2 torr of a particle from Murchison CM carbonaceous chondrite matrix.

Application to Micrometeorite Bulk Mineralogy saponite, while that of carbonate takes place at temperatures from 500 to 600 °C for Mg-Fe carbonate and 700 °C for Mg- Conditions of heating, temperature, and ambient pressure Ca carbonate. Subsequent to decomposition of the primary of hydrous micrometeorites can be estimated based on the phases, many secondary phases such as olivine, pyroxene, results of our experiments. Since our experiments were and Fe metal or oxides start to form. But it is important to note carried out at relatively low temperatures, the results can that at an intermediate temperature between primary-phase apply only to thermal regime of unmelted, nonvesicular decomposition and secondary-phase recrystallization, low- micrometeorites that experienced heating at temperatures crystalline amorphous materials dominate hydrous lower than the melting point. Our results suggest that many meteorites. The temperatures are 600 °C for Murchison and primary phases in hydrous micrometeorites such as 700 °C for both Orgueil and Tagish Lake. The secondary phyllosilicates and carbonates decompose at relatively low phases then form at the expense of the amorphous materials. temperatures. Decomposition of hydrous phases occurs at The secondary silicates are mostly olivine and their 400 °C for tochilinite, 600 °C for serpentine, and 700 °C for crystallinity increases with increasing temperatures. 1108 W. Nozaki et al.

Fig. 12. TEM photomicrographs of ultramicrotomed sections of fine-grained matrix of Murchison meteorite experimentally heated at 1000 °C and 10−2 Torr. a) A secondary olivine formed by decomposition of serpentine. b) A low-magnification image of the sample. There are many vesicles among the recrystallized matrix. The matrix is composed of coarse (100–300 nm across) olivine and magnetite embedded in glass. Wavy cracks running from lower left to upper right are artifacts during ultramicrotomy. Pore spaces (indicated by arrows) are present prior to the ultramicrotomy, because they are filled with epoxy resin. c) Magnetite probably formed by the decomposition of troilite. Troilite was not found around this crystal, although a small amount of S was detected by EDS. d) Fe-, Ni-, and S-bearing objects where only Ni-rich magnetite is identified by selected area electron diffraction, although S is detected by EDS (inset). e) An olivine single crystal containing elongated magnetite almost parallel to the b-axis. SAED pattern of the olivine and magnetite inclusions shows that (111) planes of all the magnetite inclusions are parallel to the (100) of olivine. It was probably formed by decomposition of tochilinite or Fe-rich serpentine. Bulk mineralogical changes of hydrous micrometeorites during upper-atmosphere heating 1109

Fig. 13. Synchrotron X-ray diffraction patterns showing the bulk mineralogy of a particle from Murchison matrix heated to 900 °C and atmospheric pressures of 10−4, 10−3, 10−2, and 10−1 torr.

Secondary low-Ca pyroxene is detected in abundance in the phyllosilicates together with secondary Fe phases. The heated samples of Tagish Lake with carbonate-rich lithology occurrence of secondary Fe phases and silicates is consistent (Fig. 3) that were rich in saponite prior to heating. It is known with the results of our heating experiments at atmospheric that the decomposition product of saponite is low-Ca pressures from 10−2 to 10−1 torr. This pressure range pyroxene (e.g., Akai 1992), while that of serpentine is olivine corresponds to 60–80 km altitude and at this altitude most (e.g., Hiroi and Zolensky 1999). Therefore, the formation of micrometeorites have experienced the highest temperature. abundant secondary low-Ca pyroxene in Tagish Lake with We have characterized in detail three unmelted, carbonate-rich lithology is ascribed to the high abundance of nonvesicular micrometeorites recovered from Antarctica saponite prior to heating. (Nakamura et al. 1999; Yada and Kojima 2000), by SEM/ Our experiments produced a variety of secondary Fe EDS and synchrotron X-ray diffraction. Nonvesicular phases such as metal, magnesiowüstite, and magnetite, micrometeorites are those that are free of gas bubbles depending on the ambient atmospheric pressures. Taenite because of low-temperature heating upon atmospheric entry is abundantly produced in experimental products heating at (Genge et al. 1997). Therefore, bulk mineralogical and 10−3 and 10−4 torr (Figs. 6, 9, and 13). On the other hand, textural features of the three micrometeorites are well magnetite and magnesiowüstite are common Fe phases and explained by comparison to the results of our experiments. metal is minor in micrometeorites (Nakamura et al. 2001), F96CI024 (Fig. 14a) is a fine-grained micrometeorite with indicating that heating of micrometeorites took place at skeletal crystals of magnetite. It consists of saponite, atmospheric pressures of 10−2 torr or higher. Heating at 1 torr magnetite, magnesiowüstite, and pyrrhotite (Fig. 14b). in our experiments produces laihunite, which has never been Detailed mineralogy of this micrometeorite was already detected in micrometeorites. This suggests that atmospheric reported in Noguchi et al. (2002). The diffraction pressures are lower than 1 torr. Laihunite is therefore a good pattern (Fig. 14b) is very similar to that of Tagish Lake measure of oxygen fugacity during decomposition of with carbonate-rich lithology that was heated to 600 °C and 1110 W. Nozaki et al.

Fig. 14. Antarctic micrometeorite F96CI024. a) A backscattered electron (BSE) image indicating the presence of abundant magnetite in dark gray fine-grained phyllosilicates. b) A synchrotron X-ray diffraction pattern showing that the micrometeorite consists of saponite, pyrrhotite, magnetite, and magnesiowüstite. at 10−2 or 10−1 torr (Fig. 3). At 600 °C, saponite survives but micrometeorite that contains framboidal magnetite. The its 001 basal spacing is shrunk, while the large amounts of occurrence of bundles of fibrous materials in submicron size magnesiowüstite are formed by decomposition of suggests the presence of abundant phyllosilicates (Fig. 15a). carbonates. Therefore, this micrometeorite is consistent with However, the X-ray diffraction pattern (Fig. 15b) indicates an origin from a Tagish Lake–type asteroid and heating to that crystalline phases of the micrometeorites are only 600 °C at 60–80 km altitude upon atmospheric entry. The magnetite and neither phyllosilicates nor secondary result confirms the interpretation given in Noguchi et al. anhydrous silicates are detected: amorphous material (2002). dominates the micrometeorite. Therefore, the bundles of Kw740052 is also a fine-grained nonvesicular fibrous materials are apparently decomposed phyllosilicates. Bulk mineralogical changes of hydrous micrometeorites during upper-atmosphere heating 1111

Fig. 15. Antarctic micrometeorite kw740052. a) A BSE image of a portion of the micrometeorite showing the presence of abundant fibrous phyllosilicates-like materials. b) A synchrotron X-ray diffraction pattern showing that in the crystalline phase in the micrometeorite only magnetite and that the fibrous materials are amorphous.

The X-ray diffraction pattern of the micrometeorite (Fig. 15b) primary magnetite suggests that the micrometeorite is a piece is very similar to that of Orgueil or Tagish Lake with of neither Orgueil nor Tagish Lake with carbonate-poor carbonate-poor lithology heated to 700 °C at 10−2 or 10−1 torr lithology. The lack of abundant magnesiowüstite suggests that (Figs. 7 and 8). the micrometeorite is not a decomposed product of Tagish The last micrometeorite is kw740054, which is also Lake with carbonate-rich lithology. The presence of abundant classified to nonvesicular type (Fig. 16a). Compact fibrous secondary olivine and minor amounts of secondary low-Ca materials occur in an entire region of this micrometeorite, pyroxene suggests that pre-heated mineralogy of this which again indicates the presence of abundant micrometeorite would have been dominated by serpentine. phyllosilicates. X-ray diffraction analysis indicates that Therefore, the best candidate for the micrometeorite prior to olivine and magnetite are major phases and no phyllosilicates heating is a hydrous particle whose mineralogy is similar to are detected (Fig. 16b). The olivine and magnetite are low CM carbonaceous chondrites. The X-ray diffraction pattern of crystalline because their reflections are broad. This suggests this micrometeorite (Fig. 16b) is similar to that of a that they are secondary phases recrystallized from Murchison particle heated to temperatures of 800 and 900 °C decomposed hydrous phases. The lack of sharp reflections of at 10−2 or 10−1 torr (Figs. 11 and 13). 1112 W. Nozaki et al.

Fig. 16. Antarctic micrometeorite kw740054. a) A whole view of the micrometeorite showing that compact, fibrous phyllosilicates-like materials are dominant. BSE image. b) A synchrotron X-ray diffraction pattern showing that the micrometeorite consists of olivine, magnetite, and low-Ca pyroxene. No phyllosilicates are detected.

In summary, on the basis of bulk mineralogical changes materials, or low-crystalline anhydrous phases, respectively, observed during experimental brief heating of various are identified as heated hydrous micrometeorites. They might hydrous carbonaceous chondrites at variable atmospheric have come from different parental objects, carbonate-rich pressures, heating conditions of unmelted, nonvesicular Tagish Lake type, carbonate-poor Tagish Lake or CI type, and micrometeorites are successively evaluated. Three CM type, respectively, and experienced different peak micrometeorites, F96CI024, kw740052, and kw740054, temperatures, 600, 700, and 800~900 °C, respectively, at 60– which now consist of shrunk phyllosilicates, amorphous 80 km altitude upon atmospheric entry. Bulk mineralogical changes of hydrous micrometeorites during upper-atmosphere heating 1113

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