Bulk mineralogy of individual micrometeorites determined by X-ray diffraction analysis and transmission electron microscopy

Geochimica et Cosmochimica Acta, Vol. 65, No. 23, pp. 4385– 4397, 2001 Copyright © 2001 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/01 $20.00 ⫹ .00

Pergamon

PII S0016-7037(01)00722-0

Bulk mineralogy of individual micrometeorites determined by X-ray diffraction analysis and transmission electron microscopy TOMOKI NAKAMURA,1,* TAKAAKI NOGUCHI,2 TORU YADA,1 YOSHIHIRO NAKAMUTA,3 and NOBUO TAKAOKA1 1

Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan 2 Department of Materials and Biological Science, Ibaraki University, Bunkyo 2-1-1, Mito, Ibaraki 310-8512, Japan 3 Kyushu University Museum, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan (Received May 11, 2000; accepted in revised form June 7, 2001)

Abstract—Bulk mineralogy of individual fine-grained micrometeorites from 50 to 200 ␮m in diameter was determined on the basis of the powder X-ray diffraction patterns and the observation of internal textures by a transmission electron microscope (TEM). X-ray diffraction analysis of 56 micrometeorites indicated that 42, 11, and 3 samples are olivine-rich, pyroxene-rich, and phyllosilicate-rich micrometeorites, respectively. Among the phyllosilicate-rich micrometeorites, one contains saponite and other two contain serpentine. No samples contain both saponite and serpentine. We found that saponite-rich micrometeorite was weakly heated, which results in shrinkage of 001 basal spacing of saponite down to 9.7 Å, and that cronstedtite, which is commonly contained in CM chondrites, occurs in serpentine-rich micrometeorites. Micrometeorites that consist entirely of anhydrous minerals and amorphous phases are predominant in the samples studied. The major phases of such micrometeorites are olivine, low-Ca pyroxene, magnetite, and Fe-sulfide and the average abundances are 65, 17, 11, and 7 wt%, respectively, when the total abundance of the four minerals are normalized to 100 wt%. The relative mineral abundance varies greatly between samples: low-Ca pyroxene/olivine ratios range from 0 to 3.5, with a mean of 0.3. TEM observations of inner portions of some micrometeorites revealed that they are aggregates of very small equigranular grains (⬃100 nm) of olivine ⫹ magnetite, or low-Ca pyroxene ⫹ olivine ⫹ magnesiowu¨stite. The textures are very similar to those of hydrous carbonaceous chondrite that was experimentally heated to temperature below melting point, thus suggesting that the micrometeorites had been hydrous particles but were decomposed by the brief heating upon atmospheric entry. It is newly found that magnesiowu¨stite was formed in micrometeorites instead of magnetite as a product of phyllosilicate decomposition under low oxygen fugacity. The decomposed hydrous micrometeorites gave two types of characteristic X-ray diffraction patterns: (1) broad olivine and magnetite reflections or (2) variable intensities of magnesiowu¨stite reflections together with magnetite, low-Ca pyroxene, and olivine reflections. Twenty-nine olivine- or pyroxene-rich micrometeorites showed such diffraction patterns, thus suggesting that more than half of micrometeorites investigated must be decomposed hydrous particles. The results confirmed that hydrous dust particles are much more abundant in the interplanetary space than in the micrometeorites recovered on the Earth. Copyright © 2001 Elsevier Science Ltd their alteration history, such as decomposition of hydrous minerals at subsolidus temperature (e.g., Kurat et al., 1994; Greshake et al., 1996, 1998). The bulk mineralogy is therefore a key indicator of the formation and alteration processes of individual micrometeorites. In this article, we have characterized the bulk mineralogy of 56 unmelted to partially melted micrometeorites. The species and relative abundances of minerals, average Mg/Fe ratios of olivine and pyroxene, bulk elemental concentrations, and microtextures of individual micrometeorites were investigated by X-ray diffraction technique and electron microscopy.

1. INTRODUCTION

Micrometeorites up to several hundred micrometers in diameter are supposed to represent the major fraction of the extraterrestrial material accreting to the Earth (Gru¨n et al., 1985; Love and Brownlee, 1993). Small particles in the solar system in the size range of micrometeorites are moving in spiral orbits toward the sun as a result of the Poynting-Robertson effect (Wyatt and Whipple, 1950), and parts of the particles are captured by the Earth. Therefore, micrometeorites are samples of a relatively unbiased population of small particles that traverse the solar system. Unmelted micrometeorites, which escaped melting and vaporization during atmospheric entry (e.g., Flynn and McKay, 1990; Love and Brownlee, 1991, 1994; Maurette et al., 1991), are composed of numerous fine mineral particles of mostly submicron size (Klo¨ck and Stadermann, 1994; Yano and Noguchi, 1998; Noguchi and Nakamura, 2000). Thus, the least altered micrometeorites are expected to retain in their bulk mineralogy the primary dust characteristics from which they were formed. On the other hand, the altered micrometeorites must have recorded in the bulk mineralogy

2. SAMPLES AND ANALYTICAL TECHNIQUES

We have studied 56 micrometeorites larger than 50 ␮m in diameter (Table 1), which are irregular in shape with rough surfaces, suggesting that they have never experienced total melting. All samples were identified as extraterrestrial particles because they show “chondritic” X-ray energy spectra except for the depletion of sulfur when measured by a scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS) with a broad electron beam 100 ␮m in diameter. The validity of the identification was verified by the

* Author to whom correspondence should be addressed. 4385

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Table 1. Diameter, classification, relative mineral abundance, average, mineral, composition, bulk Si, Fe, and Mg atomic ratio, and opx and cpx abundance of individual micrometeorites.

Sample

Relative mineral abundancea Average Low-Ca dimension Phyllosili- Magnesiopxb wustited (␮m) Classification Olivine Magnetite Sulfide catec

Micrometeorites F96AK012 F96AK021 F96CI005 F96CI009 F96CI020 F96CI024 F96CI025 F96DI010 F96DI015 F96DI021 F96DI030 F96DI042 F96DI045 F96DK007 F96DK011 F96DK012 F96DK014 F96DK015 F96DK019 F96DK020 F96DK022 F96DK025 F96DK028 F96DK030 F96DK032 F96DK035 F96DK042 F96DK046 F96DK056 F96DK057 F96DK060 F96DK062 F96DK063 Micrometeorites EUR0001 EUR0002 EUR0003 EUR0004 EUR0005 EUR0006 EUR0007 EUR0008 EUR0009 EUR0010 EUR0011 EUR0012 EUR0013 EUR0014 EUR0015 EUR0016 EUR0017 EUR0018 EUR0019 EUR0020 EUR0021 EUR0022 EUR0023 Mean a

from Dome Fuji Station 200 Olivine rich 120 Pyroxene rich 190 Pyroxene rich 120 Olivine rich 80 Olivine rich 120 Phyllosilicate 80 Olivine rich 70 Olivine rich 70 Olivine rich 70 Olivine rich 90 Olivine rich 60 Olivine rich 150 Pyroxene rich 140 Olivine rich 100 Olivine rich 80 Olivine rich 200 Olivine rich 200 Olivine rich 120 Olivine rich 120 Olivine rich 90 Olivine rich 90 Olivine rich 270 Olivine rich 130 Olivine rich 120 Pyroxene rich 160 Pyroxene rich 130 Pyroxene rich 150 Pyroxene rich 100 Olivine rich 100 Olivine rich 160 Olivine rich 100 Olivine rich 80 Olivine rich from EUROMET collection 130 Olivine rich 90 Olivine rich 90 Phyllosilicate 70 Olivine rich 100 Pyroxene rich 130 Olivine rich 110 Olivine rich 90 Pyroxene rich 120 Olivine rich 70 Olivine rich 90 Olivine rich 80 Olivine rich 60 Olivine rich 120 Olivine rich 90 Pyroxene rich 130 Olivine rich 170 Olivine rich 80 Olivine rich 100 Olivine rich 80 Phyllosilicate 60 Olivine rich 80 Olivine rich 140 Pyroxene rich 113

0 37 49 1 1 0 23 2 34 21 21 0 36 33 0 10 11 1 0 27 22 9 0 1 35 42 30 42 0 5 3 23 11

98 17 14 85 95 0 38 84 43 32 44 95 17 41 91 88 75 91 87 44 60 89 90 80 19 18 22 14 89 85 81 66 58

2 20 31 10 4 7 22 11 8 41 16 0 21 8 9 2 10 8 10 4 9 3 9 18 16 15 13 21 10 9 11 3 8

0 26 6 4 1 20 17 3 16 5 19 5 27 18 0 0 3 0 3 26 8 0 1 1 30 25 35 22 1 1 4 9 23

0 0 0 0 0 73 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

9 0 0 0 46 0 0 72 22 0 36 0 20 0 69 0 4 0 38 0 0 0 59 16

82 94 73 68 45 91 94 20 67 88 49 99 71 90 0 92 89 100 48 0 97 90 33 64

9 5 13 32 9 9 6 7 10 12 15 1 8 9 31 8 8 0 12 15 1 10 8 11

0 2 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 2 0 0 7

0 0 11 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 85 0 0 0 2

⫹⫹

⫹⫹⫹

⫹⫹ ⫹

⫹⫹ ⫹

⫹⫹

⫹ ⫹

⫹⫹⫹ ⫹⫹⫹ ⫹ ⫹⫹

⫹

⫹⫹ ⫹

⫹ ⫹⫹ ⫹⫹

⫹⫹

⫹

⫹

Bulk Si, Fe, and Mg atomic ratio

Average silicate composition

Low-Ca px polymorphe

Fe/ Mg/ (Fe⫹Mg⫹Si) (Fe⫹Mg⫹Si)

En#

Fo#

— 87 89 100 94 — 98 92 87 100 91 — 88 86 — 86 91 87 — 97 66 79 — 77 74 87 86 90 — 62 91 95 65

55 91 100 34 60 — 59 38 59 73 72 62 92 64 46 61 53 54 80 70 55 88 59 59 93 75 90 79 57 28 48 60 43

0.40 NA NA NA 0.41 0.24 0.24 NA NA 0.18 NA NA 0.28 NA 0.34 0.14 NA 0.22 NA 0.31 0.26 0.27 0.22 0.34 NA NA 0.44 0.29 0.18 NA NA 0.26 0.29

66 — — — 63 — — 89 89 — 93 — 96 — 91 — 76 — 81 — — — 73 85

21 62 64 71 43 37 51 84 69 49 56 48 58 60 — 47 33 52 63 — 77 37 69 60

0.34 NA NA NA 0.36 NA NA NA NA 0.49 0.45 0.36 0.22 NA 0.47 0.36 0.33 0.26 0.32 0.58 0.27 0.36 NA 0.32

opx

cpx

0.26 NA NA NA 0.31 0.34 0.44 NA NA 0.38 NA NA 0.38 NA 0.33 0.37 NA 0.39 NA 0.31 0.36 0.37 0.45 0.31 NA NA 0.25 0.33 0.44 NA NA 0.38 0.33

— 11 100 63 47 — 24 5 26 0 26 — 27 35 — 24 28 80 — 14 22 75 — 4 37 21 26 18 — 28 5 8 26

— 89 0 37 53 — 76 95 74 100 74 — 73 65 — 76 72 20 — 86 78 25 — 96 63 79 74 82 — 72 95 92 74

0.31 NA NA NA 0.23 NA NA NA NA 0.20 0.26 0.32 0.30 NA 0.23 0.33 0.33 0.35 0.29 0.13 0.40 0.32 NA 0.32

73 — — — 39 — — 49 18 — 65 — 22 — 43 — 42 — 13 — — — 42 33

27 — — — 61 — — 51 82 — 35 — 78 — 57 — 58 — 87 — — — 58 67

Bulk mineral abundance is expressed as relative abundances of olivine, pyroxene, magnetite, sulfide, and phyllosilicate and total is normarized to 100 wt%. Opx and cpx abundance is shown in weight percent. c Phyllosilicate abundance is obtained based on the intensity calcularion as shown in the Appendix. d Magnesiowu¨ stite abundance is expressed as ⫹ (⫺5 wt%), ⫹⫹(⬃10 wt%), and ⫹⫹⫹ (⬃20 wt%). e Elemental abundance is not analyzed. b

Bulk mineralogy of micrometeorites

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Fig. 1. Histogram showing a distribution of low-Ca pyroxene/olivine weight ratios in micrometeorites, which indicates a wide range of the ratio from 0 to 3.5.

presence of solar noble gases in all five samples that were randomly selected from the samples used in this study (Osawa et al., 2000), and noble gas signatures of individual samples are similar to those reported previously (Olinger et al., 1990). Thirty-three of 56 samples were collected at the Dome Fuji Station located on the top of ice sheets in Queen Maud Land in Antarctica (Nakamura et al., 1999); 23 samples were from the EUROMET collection, which was collected at blue ice fields of Cap-Prudhomme in 1991 (Maurette et al., 1991, 1992, 1994; Yano and Noguchi, 1998). For the X-ray diffraction analysis, individual micrometeorites were mounted on a thin glass fiber 5 ␮m in diameter and exposed to Cr K␣ X-ray in a Gandolfi camera. A clear X-ray powder photograph can be taken from a single micrometeorite with an exposure duration from 12 to 60 h by the Gandolfi method (Gandolfi, 1967). The X-ray photograph was scanned by a microdensitometer, and the read data were transferred to a computer to determine precise peak positions and integrated intensity of X-ray reflections by means of a profile fitting technique (Nakamuta and Motomura, 1999). The relative abundance of major constituent minerals was determined on the basis of the integrated intensities of X-ray reflections (see Appendix for detailed procedure). After X-ray diffraction measurements, 33 micrometeorites were embedded in epoxy resin, polished with microdiamond paste, and analyzed by SEM-EDS (JEOL JSM-5800LV and -5300LV) and electron microprobe analyzers (JEOL JXA-733 and -733 superprobe) equipped with a wavelength-dispersive X-ray spectrometer. Quantitative analyses with wavelengthdispersive X-ray spectrometer were performed at 15 kV accelerating voltage and 9 or 10 nA beam current with a defocused beam 10 ␮m in diameter to determine bulk elemental composition of individual micrometeorites. Natural minerals such as diopside and olivine were used for standard material. Quantitative chemical compositions were obtained via the ZAF correction method. The detection limit is 0.05 wt% for elements analyzed, and the reproducibility is less than ⫾5% of concentrations of each element on the basis of the repeated analysis of the standard minerals. Six micrometeorites were microtomed by Leitz-Reichert Super Nova ultramicrotome for transmission electron microscopy (TEM) (JEOL JEM-2000FX II) observation. Semiquantitative elemental analyses were performed by

Philips DX4 EDS system. Although the analyses were based on Cliff-Lorimer approximation, the effects by absorption were considered by regarding k factors as the function of total counts of X-rays. These functions were determined by some mineral standards. 3. RESULTS

3.1. Bulk Mineralogy and Chemistry The results of X-ray diffraction and electron microprobe analyses of individual micrometeorites are shown in Table 1 for the relative mineral abundance, the bulk major-element concentration, the average Fo# and En#, and the abundance of orthopyroxene (opx) and clinopyroxene (cpx) in low-Ca pyroxene, and in Figure 1 for the ratio of low-Ca pyroxene to olivine. SEM and TEM observation showed that most micrometeorite samples investigated are aggregates consisting of very small minerals of mainly less than 1 ␮m in diameter, although some samples contain relatively large (⬃20 ␮m) silicate crystals. They are classified as fine-grained unmelted to scoriaceous partial-melted micrometeorites on the basis of an existing textural classification scheme for micrometeorites (e.g., Kurat et al., 1994; Engrand and Maurette, 1998). The mineralogy and mineral chemistry of individual micrometeorites (Table 1 and Fig. 1) are therefore integrated properties of numerous fine mineral particles. Our X-ray diffraction analysis showed that the major minerals in micrometeorites are olivine, low-Ca pyroxene, magnetite, and Fe sulfide (troilite and pyrrhotite), findings consistent with the results of previous TEM analyses (Klo¨ ck and Stadermann, 1994; Greshake et al., 1996; Yano and Noguchi, 1998). But the relative abundance of the four minerals in weight percent was found to vary greatly between samples (Table 1). The most common mineral in micrometeorites is olivine, which occurs all but three samples in a wide range of relative abundance from 14 to 100 wt%. Magnetite is also common but less abundant: it occurs all but three samples in a range from 1 to 32 wt%. The occurrence of low-Ca pyroxene and Fe sulfide is relatively similar: the former is contained in 36 of 56 samples in a range from 1 to 72 wt%, whereas the latter is present in 32 samples in a range from 1 to 30 wt%. Our study revealed some correlations in the relative abundance between the four major

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Fig. 3. Relationship between the average Fo# and En# of individual micrometeorites. No obvious tendency is observed in the diagram, but the ranges of Fo# and En# determined from the interlayer spacing in this study are overlapped with those of micrometeorites reported by Michel-Levy and Bourot-Denise (1992), Flynn et al. (1993), and Beckerling and Bischoff (1995). A line corresponding to Fo#/En# ⫽ 1 is shown in the diagram, and most data fall below the line, indicating that Mg/Fe ratios of olivine are lower than those of low-Ca pyroxene.

Fig. 2. Diagrams showing relationships between (a) olivine and low-Ca pyroxene abundance and (b) olivine and magnetite abundance in micrometeorites. Low-Ca pyroxene and magnetite abundances increase with a decrease of olivine abundance, although the correlation in (b) is weak.

minerals in micrometeorites. Olivine abundance decreases with an increase of low-Ca pyroxene abundance (Fig. 2a), and the anticorrelation is observed in a quite wide range of abundances. Magnetite abundance increases with decreasing olivine abundance, although the correlation is not so evident (Fig. 2b). Sulfide shows no clear correlation with other three minerals, but its abundance is lower than magnetite abundance in most cases. Next to the four major minerals, magnesiowu¨ stite (Fe, Mg)O commonly occurs in micrometeorites: it is contained in 22

samples, especially in those having low-Ca pyroxene abundance greater than 20 wt% (Table 1). Phyllosilicates occur in only three micrometeorite samples (Table 1) and therefore they are not major constituents in micrometeorites recovered from Antarctica. The average Fo# (Mg/(Mg ⫹ Fe) ⫻ 100 in olivine) and En# (Mg/(Mg ⫹ Fe) ⫻ 100 in low-Ca pyroxene) of individual micrometeorites were determined on the basis of the interlayer spacing of particular X-ray reflections (Appendix). Micrometeorite samples showed variable Fo# ranging from Fo20 to Fo100 and En# from En60 to En100 (Table 1 and Fig. 3). The ranges of Fo# and En# obtained in this study cover those determined by electron probe analyses (Fig. 3) (Michel-Levy and Bourot-Denise, 1992; Flynn et al., 1993; Beckerling and Bischoff, 1995). The Fo# and En# showed no clear correlation among samples studied (Fig. 3). This is probably due to a limited degree of Mg/Fe interdiffusion between olivine and pyroxene in micrometeorites, being consistent with the fact that fine-grained micrometeorites escape severe heating. In spite of a wide diversity in mineralogy and mineral

Bulk mineralogy of micrometeorites

Fig. 4. Si-Mg-Fe ternary diagram showing bulk elemental abundance of individual micrometeorites analyzed in this study. Most data are plotted around the center of the diagram. The compositional ranges of CM (Zolensky et al., 1993) and CI (Tomeoka and Buseck, 1988) cover most of data points of micrometeorites. The range is also consistent with that of micrometeorites reported previously (Kurat et al., 1994; Genge et al., 1997; Engrand et al., 1999).

chemistry, micrometeorites have relatively similar bulk elemental abundance. Mg, Si, and Fe abundances (Fig. 4) are within the range of those for micrometeorites reported previously (Kurat et al., 1994; Genge et al., 1997; Engrand et al., 1999) and are similar to those of CI (Tomeoka and Buseck, 1988) and CM (Zolensky et al., 1993) chondrites. Silicon is accommodated in both olivine and low-Ca pyroxene, but the latter contains Si with Si/(Mg ⫹ Fe) ratio twice as high as the former. Therefore, it is expected that micrometeorites rich in low-Ca pyroxene tend to show high Si/(Mg ⫹ Fe) ratios. But it is not the case. Such pyroxene-rich micrometeorites do not always show high Si/(Mg ⫹ Fe) ratios (Fig. 5) because they commonly contain minerals such as magnesiowu¨ stite that reduces the bulk Si/(Mg ⫹ Fe) ratios. The mean mineralogical and compositional characteristics of micrometeorite samples are obtained by averaging all values in Table 1. The “average” micrometeorite contains Si, Mg, and Fe in an atomic ratio of 1.0:0.9:0.9, which is very close to that of solar system abundance—1.0:1.0:0.9 (Anders and Grevesse, 1989). It consists of 64 wt% olivine, 16 wt% low-Ca pyroxene, 11 wt% magnetite, 7 wt% Fe sulfide, and 2 wt% phyllosilicate, when the total abundance of the five minerals are normalized to 100 wt% (Table 1). Opx:cpx ratio in low-Ca pyroxene is approximately 1:2 (Table 1). The bulk mineralogy of the “average” micrometeorite is not similar to that of any chondritic matrix material found so far. Like the classification for chondritic interplanetary dust particles (IDPs; Sandford and Walker, 1985), we have classified 56 micrometeorite samples into olivine-, pyroxene-, and phyllosilicate-rich classes on the basis of the relative mineral abundance. Compared with the infrared spectra (Sandford and

4389

Fig. 5. Relationship between low-Ca pyroxene abundance and Si content in individual micrometeorites. There are no clear correlations. Si/(Mg ⫹ Fe ⫹ Si) ratios of low-Ca pyroxene, serpentine, and olivine are shown for comparison.

Walker, 1985), X-ray diffraction patterns are better for mineral identification and estimation of relative mineral abundances because overlapping of the X-ray reflections is not so complex compared with that of absorption bands in the infrared spectra. Micrometeorites show a wide range of a weight ratio of low-Ca pyroxene to olivine from 0 to 3.5 (Fig. 1). The pyroxene-rich class has the weight ratio higher than 1. Micrometeorites having the ratio 1 or less are classified as olivine-rich class, and those containing phyllosilicates were classified as such, even if they contain olivine and pyroxene. Bulk mineralogical and compositional characteristics of the three classes of micrometeorites are summarized below. 3.2. Characteristics of Micrometeorites Classified by Mineralogy 3.2.1. Olivine-Rich Class Forty-two of 56 samples belong to this class. Texturally and mineralogically, three distinct types of population were observed in this class. The first type is rich in amorphous material. Olivine is the only silicate present, and low-Ca pyroxene is very minor if present. Magnetite and Fe sulfide are present in variable amounts, but magnesiowu¨ stite is absent. This type of micrometeorites shows weak and broad reflections of olivine and magnetite in the X-ray diffraction patterns (Fig. 6a). SEM observation of polished surfaces indicates that micrometeorites of this type show compact or fibrous texture (Fig. 6a), and their constituent minerals are too small to be identified by SEM. TEM observation of F96DK056 and F96CI020 revealed that they are nonporous aggregates consisting of small, equigranular grains mainly of olivine and magnetite with average diameter

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Fig. 6. X-ray diffraction patterns in a range of diffraction angle from 12 to 52° (2␪) and back-scattered electron (BSE) images of (a) olivine-rich, (b) pyroxene-rich, (c) smectite-rich, and (d) serpentine-rich micrometeorites. The bulk elemental composition of these samples is shown in Table 1. (a) EURO010 shows fine-grained and fibrous texture. Broad reflections of olivine and magnetite are observed in the X-ray diffraction pattern. (b) F96DK046 is a fine-grained and porous micrometeorite. Reflections of low-Ca pyroxene, olivine, magnetite, and Fe sulfide are observed. Olivine reflections are sharper than those in (a). (c) F96CI024 is a compact particle containing relatively coarse Fe-sulfide and magnetite (white grains) within very fine grained material. A large and broad smectite reflection at 13.5° and a prism reflection at 30 to 40° are observed. (d) EURO020 shows a very fine grained and compact texture; 001 basal reflection of serpentine at 18.4° and a prism reflection at 30 to 40° are observed. Abbreviations: Ol ⫽ olivine; Px ⫽ low-Ca pyroxene; Mag ⫽ magnetite; Sul ⫽ Fe sulfide; Smec ⫽ smectite; Serp ⫽ serpentine.

⬃40 nm (Fig. 7a). Si- and Al-rich amorphous material fills interspaces between the grains. In F96DK056, there is a portion showing fibrous texture where a train of magnetite and a small amount of olivine form each fiber, and Si-rich amorphous material fills interstices (Fig. 7b). The bulk chemical compositions of such areas determined by TEM-EDS are 7 to 14 wt% MgO, ⬃2 wt% Al2O3, 21 to 24 wt% SiO2, 2 to 4 wt% Na2O, and 58 to 66 wt% FeO. Crystallographic orientation of the trains of magnetite is strongly preferred. On the basis of the results of X-ray diffraction analysis and electron microscopy, 9 of 42 olivine-rich class of micrometeorites belong to this type. The second type of olivine-rich micrometeorites is also fine grained but more porous than the first type. It contains low-Ca pyroxene, but its abundance is lower than olivine. X-ray reflections of olivine are sharper than those of olivine in the first type. Magnetite and sulfides also occur in many samples. Twenty-five samples belong to this type. A noteworthy feature is that magnesiowu¨ stite is contained in many samples (approximate abundance is shown in Table 1, and also see the last paragraph of the Appendix for identification of magnesiowu¨ s-

tite by X-ray diffraction). Magnesiowu¨ stite is one of the major phases in the samples of this type because its X-ray reflection intensities are comparable to those of low-Ca pyroxene in many cases. The interlayer spacing of the strongest reflection of magnesiowu¨ stite varies slightly among samples from 2.12 to 2.14 Å, which suggests that the composition varies between Mg-rich (periclase, whose spacing is 2.11 Å; datum from ASTM 4-829) and Fe-rich (wu¨ stite, whose spacing is 2.15 Å; datum from ASTM 6-615) components. TEM-EDS analysis confirms this interpretation: magnesiowu¨ stite of this type contains 35 to 40 wt% MgO. On the other hand, some samples of this type do not contain magnesiowu¨ stite. They are samples having 10-␮m-sized low-Ca pyroxene grains within fine-grained material like EURO013 and samples consisting almost entirely of finegrained material such as F96CI025. F96CI025 is a very porous particle that contains discrete grains of kamacite and forsterite (Fig. 7c). X-ray diffraction analysis indicates that it contains 23 wt% of low-Ca pyroxene, 38 wt% of olivine (Table 1), and both pyrrhotite and troilite. TEM observation uncovers the

Bulk mineralogy of micrometeorites

Fig. 7. (a, b) TEM images of olivine-rich micrometeorite F96DK056. (a) Dark-field (DF) image of an inner portion. Small olivine and magnetite with average diameter of ⬃40 nm form equigranular texture. The triple junctions are observed (indicated by arrows) at some boundaries, suggesting crystal growth at subsolidus temperature. (b) Trains of magnetite and a small amount of olivine with interstices of Si-rich material exhibit fibrous texture. Selected area electron diffraction (SAED) of the fibrous area shows preferred orientation of magnetite. Bright-field (BF) image. (c) A BSE image of olivine-rich micrometeorite F96CI025. It is a very porous aggregate containing discrete kamacite and forsterite grains (indicated). (d) BF TEM image of F96CI025. Rectangular to lath-shaped Mg-rich olivine crystals (indicated by arrows) approximately 1 ␮m in size are set in very small grains of olivine with various composition and Mg-rich low-Ca pyroxene. (e) High-resolution TEM image of pyroxene-rich micrometeorite F96DI045, where small, subhedral grains of magnesiowu¨ stite (MW) and minor amounts of low-Ca pyroxene (Px) form equigranular texture. Insets are a SAED photograph and a TEM-EDS spectrum of a magnesiowu¨ stite grain. Diffraction rings from magnesiowu¨ stite are indicated by arrowheads. The interlayer spacing derived from the electron diffraction photograph matches those determined by X-ray diffraction. The TEM-EDS spectrum shows only strong Mg and Fe lines. Cu peaks are from Cu grids. (f) BF TEM image of smectite-rich micrometeorite F96C1024. An aggregate consisting of small grains of Fe-sulfide, magnetite, and magnesiowu¨ stite occurs in fibrous saponite-rich material. Insets are a SAED photograph and an TEM-EDS spectrum of a magnesiowu¨ stite-enriched portion. Diffraction rings from magnesiowu¨ stite are indicated. The TEM-EDS spectrum indicates that magnesiowu¨ stite in this particle is richer in Mg than that in (e).

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unique mineralogy of this sample: (1) it consists mostly of fine-grained minerals, but the size of the minerals varies greatly from less than 10 nm to 2 ␮m (Fig. 7d); (2) rectangular to lathlike-shaped Mg-rich olivine crystals (⬎Fo90) ⬃1 ␮m in longest dimension occur throughout the sample (Fig. 7d); and (3) very fine grained portions are composed of olivine with various compositions (Fo40-98); Mg-rich, low-Ca pyroxene (⬎En90); and Fe sulfide. The third type of olivine-rich class is partially melted micrometeorites, the outer portions of which are vesiculated by melting. Olivine and magnetite are the major constituent minerals. Most samples of this type show sharp X-ray reflections of olivine due to high degree of crystallinity, although some samples show olivine reflections in an asymmetrical shape, most likely due to Mg/Fe chemical zoning. Eight samples are classified in this type.

Table 2. Major element composition of cronstedtite in micrometeorite EUR0020 and CM chondrite matrix. Composition

EUR0020a

CM chondriteb

SiO2 TiO2 Al2O3 FeO MnO MgO CaO K2O Na2O Cr2O3 NiO S Total (wt%)

23.4 0.2 1.6 53.4 0.1 7.9 0.2 ⬍0.05 0.5 0.2 ⬍0.05 0.6 88.0

21.2 — 2.2 54.2 — 7.6 0.0 — — 0.0 0.2 2.0 87.4

a b

Average composition of 11 EPMA analyses. Data from Tomeoka and Buseck (1985).

3.2.2. Pyroxene-Rich Class Eleven samples belong to this class. The pyroxene-rich class of micrometeorites are porous aggregates of very fine grained mineral particles (Fig. 6b). X-ray diffraction analysis indicates that low-Ca pyroxene in general shows broad reflections, suggesting an extremely small size of the mineral (Fig. 6b). Intensity ratios of reflections at 2.88 and 3.16 Å greatly differ between samples, indicating that low-Ca pyroxene has a wide range of opx/(opx ⫹ cpx) ratio from 0.2 to 1 (Table 1). Variable amounts of olivine and Fe sulfide and abundant magnetite are also contained. Magnesiowu¨ stite occurs in all micrometeorites of this class except for one micrometeorite F96AK021. This and the fact that magnesiowu¨ stite is present in the pyroxene-bearing olivine-rich class suggest that magnesiowu¨ stite coexists mainly with low-Ca pyroxene in micrometeorites. TEM observations confirm this interpretation: magnesiowu¨ stite occurs mainly with low-Ca pyroxene in the interior portions of micrometeorite F96DI045, where many small mineral grains with average diameters of 50 nm exhibit equigranular texture, and triple junctions are also observed at some intersections of grain boundaries (Fig. 7e). 3.2.3. Phyllosilicate-Rich Class Three of 56 samples belong to this class. Smectite occurs in one sample; serpentine occurs in other two samples. There are no samples containing both smectite and serpentine. This class of micrometeorites comprises less porous aggregates than other classes. Micrometeorite F96CI024 consists of smectite, Fe sulfide, magnetite, and magnesiowu¨ stite. It shows large and broad reflection at 9.7 Å, which is assigned to 001 reflection of smectite (Fig. 6c). The 001 basal plane spacing is shorter than that of normal smectite (14 Å), indicating that the smectite is partly dehydrated by loosing water having been present between tetrahedral layers. TEM observation of F96CI024 showed that it consists of submicron-sized fibrous smectite with finely dispersed magnetite and sulfide. The results of selected-area electron diffraction and EDS analyses indicate the smectite is saponite. The average composition of saponite is 55 wt% SiO2, 5 wt% Al2O3, 15 wt% MgO, and 21 wt% FeO. The mean Mg/(Mg ⫹ Fe) ratio is ⬃0.7. Small grains of magnesio-

wu¨ stite, magnetite, and Fe sulfide form aggregates with diameter of ⬃1 ␮m in many places in this micrometeorite, and the aggregates are enclosed by coarse-grained phyllosilicate (Fig. 7f). Magnesiowu¨ stite in this sample show different modes of occurrence from that in pyroxene-rich class of micrometeorites (Fig. 7e), suggestive of different origin and formation. The magnesiowu¨ stite contains 40 to 52 wt% MgO, which is more magnesian than that in pyroxene-rich class micrometeorite F96DI045 (Fig. 7e). Two micrometeorites EURO003 and EURO020 contain serpentine-type phyllosilicate whose 001 basal spacing measures 7.15 Å. In EURO003, serpentine is a minor component and olivine with low crystallinity is a major component, whereas in EURO020, serpentine is a major component with a minor component magnetite (Fig. 6d). Major-element composition of finegrained portions of EURO020 is almost identical to that of cronstedtite in the matrix of CM chondrites (Table 2), suggesting that the serpentine is cronstedtite. This is confirmed by the fact that the serpentine has a 001 basal spacing (7.15 Å) shorter than that of normal serpentine (7.3 Å) due to exchange of parts of Si4⫹ by Fe3⫹ in the tetrahedral layers, as is observed in cronstedtite in CM chondrites (Nakamura and Nakamuta, 1996). 4. DISCUSSION

4.1. Heating Effects on Hydrous Phases in Micrometeorites at Subsolidus Temperature Our X-ray diffraction analyses of 56 micrometeorites larger than 50 ␮m revealed that only three samples contain hydrous minerals. The low abundance of hydrous micrometeorites is consistent with the results of previous attempts to search the hydrous phases in micrometeorites by TEM (Flynn et al., 1993; Klo¨ ck and Stadermann, 1994) and by infrared transmission spectroscopy (Alexander et al., 1992). On the other hand, hydrous phases occur as high as 37% of IDPs whose typical diameter is in the range of 10 to 25 ␮m (Schramm et al., 1989). The very low abundance of hydrous phases in micrometeorites relative to IDPs suggests that decomposition and dehydration reactions took place during deceleration in the atmosphere, in which large particles (micrometeorites)

Bulk mineralogy of micrometeorites

are heated to higher temperatures than small particles (IDPs) due to large hydrodynamic drags (e.g., Love and Brownlee, 1991, 1994). Entry heating modeling by Flynn and McKay (1990) indicates that ⬃99% of the 100-␮m-sized micrometeorites are heated above 600°C. In fact, much evidence of the phase transformation from hydrous to anhydrous minerals was found in many micrometeorites analyzed in this study. Therefore, direct comparison of anhydrous silicate abundance between IDPs and micrometeorites makes no sense. Major fraction of anhydrous silicates in IDPs is primary phase, as is evident by the presence of solar flare tracks (Bradley et al., 1984) and rod-shaped, low-Ca pyroxene crystals suggestive of direct growth from the nebular gas (Bradley et al., 1983). On the other hand, fine-grained anhydrous silicates in many micrometeorites appear to be secondary produced by thermal modification during atmospheric entry, as discussed in next several paragraphs. There are at least nine olivine-rich micrometeorites (the first type of olivine-rich class; see Results section), the internal textures of which suggest the presence of hydrous phases (Fig. 6a). These micrometeorites are less porous, and thus the low totals of the electron microprobe analyses can be ascribed to the presence of water. The results of X-ray diffraction analysis, however, indicate that no hydrous minerals remained in the samples. These samples may still contain limited amounts of structural water, but no more have crystal structures of phyllosilicates. This confirmed the interpretation that even micrometeorites that belong to the “phyllosilicate type” are thermally altered (Klo¨ ck and Stadermann, 1994; Kurat et al., 1994; Genge et al., 1997). Such micrometeorites seem to have experienced heating at relatively low temperatures around 600°C, resulting in the formation of very small equigranular olivine, magnetite, and Si-rich glass (Fig. 7a) from phyllosilicates (e.g., Brindley and Zussman, 1957; Brindley and Hayami, 1965; Zolensky et al., 1991; Akai, 1992; Klo¨ ck and Stadermann, 1994; Greshake et al., 1998). The fibrous texture that consists of magnetite and olivine with preferred orientation (Fig. 7b) is also strong evidence of pseudomorphic replacement of Fe-rich phyllosilicate such as cronstedtite by magnetite and olivine. Formation of low-Ca pyroxene from phyllosilicate needs higher temperature than formation of olivine (Brindley and Hayami, 1965; Akai, 1992). Klo¨ ck and Stadermann (1994) and Greshake et al. (1996, 1998) performed pulse-heating experiments of the Orgueil CI chondrite at temperatures and heating duration applicable to the atmospheric entry heating. They found that phyllosilicates in Orgueil samples were completely transformed into aggregates of very small, equigranular grains of olivine and pyroxene by the heating at temperature higher than 800°C. The microtexture of the decomposed phyllosilicates in Orgueil (Greshake et al., 1998) is similar to that of micrometeorites F96DI045 and F96DI021, both of which consist of olivine, low-Ca pyroxene, Fe-sulfide, and magnetite (Table 1). Thus, the two micrometeorites have experienced phase transformation of phyllosilicates at temperature higher than 800°C. TEM observations of the inner parts of the two micrometeorites also revealed that magnesiowu¨ stite coexists with low-Ca pyroxene and olivine to form an equigranular texture (Fig. 7e), which suggests simultaneous formation of the magnesiowu¨ stite with pyroxene and olivine at subsolidus

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temperature. The texture can be interpreted that magnesiowu¨ stite has formed there instead of magnetite under low oxygen fugacity during decomposition of phyllosilicates. During atmospheric entry heating, the most stable form of Fe oxide is magnetite (e.g., Yada et al., 1996), but after completion of dehydration of hydrous phases, the oxygen fugacity at the inner portions of micrometeorites would be lowered to allow formation of magnesiowu¨ stite, most likely due to the presence of abundant carbonaceous material (Engrand and Maurette, 1998). Therefore, the presence of magnesiowu¨ stite in anhydrous micrometeorites is likely a key indicator to identify the completely dehydrated hydrous micrometeorites on the basis of the X-ray diffraction pattern. Twenty anhydrous micrometeorites (9 of the second type of olivinerich class and 11 of the pyroxene-rich class) contain magnesiowu¨ stite, and thus they must have been hydrous particles before atmospheric entry. In summary, the results of X-ray diffraction and TEM analyses suggest that, in total, 29 anhydrous micrometeorites (9 of the first type and 9 of the second type of olivine-rich class, and 11 of the pyroxenerich class) are decomposed hydrous micrometeorites. Heating effects were observed even in phyllosilicate-rich micrometeorite F96CI024, the saponite basal spacing of which was reduced to 9.7 Å by partial dehydration. This suggests that the micrometeorite has been heated weakly at least up to several hundreds degree centigrade. Magnesiowu¨ stite is also contained in this hydrous micrometeorite (Fig. 7f), although the mode of occurrence differs from that in anhydrous micrometeorites (Fig. 7e). It is evident that this magnesiowu¨ stite is not a decomposed product of phyllosilicates because saponite still remains in this micrometeorite; rather, it would be a decomposed product of some hydrous phases such as ferrihydrite (e.g., Tomeoka and Buseck, 1988) or ferromagnesian carbonate, whose decomposition temperatures are lower than that of saponite (Noguchi and Nakamura, 2001). 4.2. Original Mineralogical Characteristics of Micrometeorites before Atmospheric Entry Mineralogically, many micrometeorites have been largely modified during high-temperature periods of the atmospheric entry, but a few samples are found to retain the original mineralogical characteristics. These pristine samples include three types of mineral assemblages: saponite-rich, serpentinerich, and olivine–pyroxene types. The saponite-rich type of micrometeorite F96CI024 (Fig. 6c) consists mainly of saponite. No serpentine and no anhydrous silicates such as olivine and pyroxene were found. Saponite occurs in meteorites, but coexists with serpentine as a coherent, unit-cell scale intergrowth in CI chondrites (Tomeoka and Buseck, 1988) and coexists with anhydrous silicates in CV and unequilibrated ordinary chondrites (e.g., Alexander et al., 1989). Therefore, saponite in F96CI024 seems to have formed by a process different from meteoritic saponite. In IDPs, saponite was found in three cases: (1) it occurs as a dominant constituent with no serpentine and no anhydrous silicates (e.g., Blake et al., 1988; Germani et al., 1990; Rietmeijer, 1991); (2) it coexists with anhydrous phases, especially pyroxene (e.g., Bradley, 1988; Germani et al., 1990); and (3) it makes intimate intergrowth with serpentine (Keller et al., 1992). Textures and

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mineralogy of F96CI024 are similar to those of case 1. F96CI024 is also similar to a micrometeorite consisting entirely of saponite and magnetite reported in Klo¨ ck and Stadermann (1994). The presence of saponite-dominated particles in both IDPs and micrometeorites suggests a common parental object made of primitive chondritic but aqueously altered material such as CI chondrites. Complete lack of serpentine in such particles, however, may indicates that the parental object is not identical to the normal CI chondrite parent body. Two micrometeorite samples are found to be serpentine-rich type, and crystallographic and compositional features indicate that the serpentine is cronstedtite (Table 2). The presence of abundant cronstedtite in EURO020 (Table 1 and Fig. 6d) indicates that it is a pristine serpentine-rich micrometeorite. In CM chondrites, cronstedtite is present as an intimate mixture with tochilinite (Zolensky et al., 1993) and in many cases interstratified with tochilinite producing a mix-layered mineral (Nakamura and Nakamuta, 1996). But tochilinite is absent in EURO020. This may deny a direct relation between EURO020 and CM chondrites, although it cannot be ruled out that tochilinite was preferentially decomposed in EURO020 as a result of slightly lower dehydration temperature (440°C determined by Zolensky et al., 1991) than that of cronstedtite (470°C determined by Caille`re and He´ nin, 1957). In case of IDPs, serpentine-rich IDPs are present (e.g., Brownlee, 1978; Rietmeijer, 1996), but much rarer than saponite-rich IDPs (Bradley, 1988). Serpentine-rich IDPs that consist almost exclusively of wellcrystallized serpentine with no olivine and pyroxene (Bradley, 1988) are very similar in textures and mineral assemblage to the serpentine-rich micrometeorite EURO020. Pristine olivine–pyroxene micrometeorites identified in this study are only three samples (F96AK021, F96CI025, and F96DI030) of 53 olivine- or pyroxene-rich class samples. They are samples that were originally formed as anhydrous particles and escaped strong heating upon atmospheric entry. Textural and mineralogical characteristics set them apart from the micrometeorites that were originally hydrous but decomposed to anhydrous particles by heating. F96CI025 is a very porous particle containing discrete grains of relict kamacite and forsterite (Fig. 7c). It does not contain magnesiowu¨ stite. Its finegrained parts are made of anhydrous minerals with quite a wide range of sizes and contain rectangular to lathlike Mg-rich silicates of mostly olivine (Fig. 7d), which is in clear contrast to microtextures of decomposed hydrous micrometeorites where equigranular grains of olivine ⫹ magnetite or low-Ca pyroxene ⫹ olivine ⫹ magnesiowu¨ stite fill inner portions (Figs. 7a,e). X-ray diffraction analysis showed that the three pristine olivine–pyroxene micrometeorites contain roughly equal amounts of olivine and low-Ca pyroxene with a cpx/opx ratio of ⬃3. Troilite and pyrrhotite coexist in a single micrometeorite. In chondritic porous anhydrous IDPs, equal amounts of olivine and pyroxene (Thomas et al., 1993) and coexistence of troilite and pyrrhotite (Zolensky and Thomas, 1995) are observed in many samples. Therefore, it is suggested that the pristine olivine–pyroxene micrometeorites have some textural and mineralogical features in common with the chondritic anhydrous IDPs. For the confirmation of the relation between pristine anhydrous micrometeorites and anhydrous IDPs, hydrogen isotopic measurement is needed to see if the microme-

teorites carry large D anomalies, although the high D anomaly is not found so far in micrometeorites (Engrand et al., 1999). In summary, before the atmospheric entry, micrometeorites were in the form of at least three different mineralogical types: saponite-rich, serpentine-rich, and olivine–pyroxene particles. Textures and mineralogy of the three types of particles are basically similar to those of saponite-rich, serpentine-rich, and anhydrous IDPs, respectively. 4.3. Relative Mineral Abundances of Micrometeorites According to the above discussion, only six particles (one saponite-rich, two serpentine-rich, and three olivine–pyroxene samples) retain mineralogical characteristics before the atmospheric entry, which in turn means that a major fraction of micrometeorites appears to have modified their original preatmospheric mineralogical features during brief heating in the atmosphere. Except for volatile elements such as sulfur, selenium, gallium, germanium, and zinc, bulk elemental abundances of micrometeorites do not change by evaporation at temperatures below melting point of micrometeorites (Greshake et al., 1998). Indeed, all micrometeorites investigated show roughly chondritic bulk elemental compositions (Fig. 4). On the other hand, the bulk mineralogy of micrometeorites varies widely (Figs. 2a,b) but retains chondritic elemental abundances, and thus certain paths of mineralogical changes are expected. Olivine abundance in micrometeorites decreases with an increase of low-Ca pyroxene (Fig. 2a) and magnetite (Fig. 2b) abundances. The Mg:Si:Fe ratio of micrometeorites is approximately 1:1:1 (Fig. 4); thus, in principle, the variation of the relative mineral abundances (Figs. 2a,b) can be explained as a mixing of two components, olivine MgFeSiO4 and a mixture of low-Ca pyroxene MgSiO3 and magnetite FeO4/3. In this interpretation, the average composition of olivine must be richer in Fe than that of low-Ca pyroxene, which is verified by the results of our X-ray analysis (Fig. 3). 5. SUMMARY

A series of mineralogical investigation revealed that a few micrometeorite samples retained the primitive nature of dust, whereas most samples seem to have modified their bulk mineralogy during atmospheric-entry heating and have many mineralogical features in common. Pristine micrometeorites found in this study are in the form of saponite-rich, serpentine-rich, and olivine–pyroxene particles. Textures and mineralogy of the three types of micrometeorites are basically similar to those of saponite-rich, serpentine-rich, and anhydrous IDPs, respectively. More than half of micrometeorites investigated had been hydrous dust particles but were decomposed and dehydrated by heating. Such decomposed hydrous micrometeorite samples showed characteristic X-ray diffraction patterns and are aggregates of very small equigranular grains of olivine ⫹ magnetite or low-Ca pyroxene ⫹ olivine ⫹ magnesiowu¨ stite ⫹ magnetite; thus, they can be distinguished from the pristine anhydrous micrometeorites. Acknowledgments—We thank Drs. Maurette and Yano for giving us the opportunity to study the micrometeorites of the EUROMET collection; the National Institute of Polar Research for providing micrometeorite samples; Dr. Imae for providing compositional data of one

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cles: Proof of an extraterrestrial origin and comparison with EDX and morphological analyses. Earth Planet. Sci. Lett. 100, 77–93. Osawa T., Nagao K., Nakamura T., and Takaoka N. (2000) Noble gas measurement in individual micrometeorites using laser gas-extraction system. Antarctic Meteorite Res. 13, 322–342. Rietmeijer F. J. M. (1991) Aqueous alteration in five chondritic porous interplanetary dust particles. Earth Planet. Sci. Lett. 102, 148 –157. Rietmeijer F. J. M. (1996) CM-like interplanetary dust particles in lower stratosphere during 1989 October and 1991 June/July. Meteor. Planet. Sci. 31, 278 –288. Sandford S. A. and Walker R. M. (1985) Laboratory infrared transmission spectra of individual interplanetary dust particles form 2.5 to 25 microns. Astrophys. J. 291, 838 – 851. Schramm L. S., Brownlee D. E., and Wheelock M. M. (1989) Major element composition of stratospheric micrometeorites. Meteoritics 24, 99 –112. Shinno I. (1980) Relation between (130) spacing, chemical composition, and cation site preference of olivine. J. Jpn. Assoc. Min. Petr. Econ. Geol. 75, 343–352. Thomas K. L., Blanford G. E., Keller L. P., Klo¨ ck W., and McKay D. S. (1993) Carbon abundance and silicate mineralogy of anhydrous interplanetary dust particles. Geochim. Cosmochim. Acta 57, 1551–1566. Tomeoka K. and Buseck P. R. (1985) Indicators of aqueous alteration in CM carbonaceous chondrites: Microtextures of a layered mineral containing Fe, S, O and Ni. Geochim. Cosmochim. Acta 49, 2149 –2163. Tomeoka K. and Buseck P. R. (1988) Matrix mineralogy of the Orgueil carbonaceous chondrite. Geochim. Cosmochim. Acta 52, 1627–1640. Virgo D. and Hafner S. S. (1969) Fe2⫹, Mg order-disorder in natural orthopyroxene. Am. Mineral. 55, 201–223. Wyatt S. P. and Whipple F. L. (1950) The Pointing Robertson effect on meteor orbits. Astrophys. J. 111, 134 –141. Yada T., Nakamura T., Sekiya M., and Takaoka N. (1996) Formation processes of magnetic spherules collected from deep-sea sediments—Observations and numerical simulations of the orbital evolution. Proc. NIPR Symp. Antarct. Meteorites 9, 218 –236. Yano H. and Noguchi T. (1998) Sample processing and initial analysis techniques for Antarctic micrometeorites. Antarct. Meteorite Res. 11, 136 –154. Yoder H. S. and Sahama T. G. (1957) Olivine X-ray determinative curve. Am. Mineral. 42, 475– 491. Zolensky M. E., Prinz M., and Lipschutz M. E. (1991) Mineralogy and thermal history of Y-82162, Y-86720, and B-7904 [abstract]. 16th Symp. Antarctic Meteorites, 195–196. Zolensky M. E., Barrett R., and Browning L. (1993) Mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites. Geochim. Cosmochim. Acta 57, 3123–3148. Zolensky M. E. and Thomas K. L. (1995) Iron and iron–nickel sulfides in chondritic interplanetary dust particles. Geochim. Cosmochim. Acta 59, 4707– 4712. APPENDIX Determination of Average Mg/Fe Ratios of Olivine and Low-Ca Pyroxene, opx/cpx Ratio, and Relative Mineral Abundances from X-ray Diffraction Patterns The average Mg/Fe ratios of olivine and low-Ca pyroxene in individual micrometeorites were estimated on the basis of the interlayer spacing, because the cell dimension of the minerals increases with increasing site occupancy of large cations (e.g., Yoder and Sahama, 1957). For olivine, the (130) spacing at ⬃2.77 Å was converted to Fo# by means of an equation given in Shinno (1980). For low-Ca pyroxene, a relationship between En# and the (610) spacing of low-Ca opx was calculated by means of the cell parameter given in Brown (1967). In the calculations, two assumptions were made: (1) low-Ca opx contains little amounts of Al, and (2) the interlayer spacing of low-Ca opx (610) is identical with that of low-Ca cpx (310). The low Al content of low-Ca pyroxene was verified by TEM-EDS analysis. Through the processes shown above, the average Fo# and En# in each micrometeorite are estimated from the interlayer spacing of the measured X-ray reflections. The interlayer spacing was precisely determined from the peak center positions of X-ray reflections optimized by profile fitting technique; therefore, any irregularity in the reflection profiles did not affect the obtained interlayer spacing.

Table A1. Calculated relative intensity of reflections from low-Ca pyroxene polymorph.a Interlayer spacing (Å) En100 En90 En80 En70 En60

⬃2.88

2.97

⬃3.16

opx (610)

opx (321)

opx (420) ⫹ (221)

52.3 54.1 52.6 47.9 45.9

186.3 205.3 204.1 190.4 181.4

100 100 100 100 100

cpx (310) ⫹ (⫺310) cpx (⫺221) En100 En90 En80 En70 En60

151.9 157.5 163.3 169.7 173.9

58.8 71.4 83.1 95.3 105.1

cpx (220) 64.9 57.8 53.8 50.3 48.7

a Opx and cpx are the same weight, and all intensities are relative values when opx (610) is defined as 100.

Our X-ray diffraction analysis showed that micrometeorites consist mainly of four kinds of minerals: olivine, low-Ca pyroxene, magnetite, and Fe sulfide. Relative abundance of the four minerals in individual micrometeorites was determined on the basis of the comparison of the integrated intensities of X-ray reflections between micrometeorites and standard minerals. For that purpose, the integrated intensities of particular X-ray reflections need to be known: magnetite (220) reflection at 2.97 Å, olivine (130) reflection at ⬃2.77 Å, a reflection at ⬃2.88 Å for low-Ca pyroxene, and a reflection at 2.65 Å for Fe sulfide. Phyllosilicate abundance was obtained separately from the four minerals, and the procedure for the estimation of phyllosilicate abundance is described in the last two paragraphs of this section. X-ray reflections are lowered and widened if minerals were in low crystallinity, were of very small size, or were heterogeneous in chemical composition. Thus, we used the integrated intensity, not height, of the reflections for determination of relative mineral abundances. Our study showed that troilite and pyrrhotite are major Fe sulfide phases in micrometeorites. The reflection at 2.65 Å is the sum of troilite (112) and pyrrhotite (20l) reflections. The integrated intensity of 2.65 Å reflection from micrometeorites is thus defined as that of Fe sulfide ((Isul)MM). The olivine (130) reflection at ⬃2.77 Å is a single reflection with no overlaps from other three minerals; thus, we define the integrated intensity of the reflection at 2.77 Å as that from olivine ((Iol)MM). Low-Ca pyroxene in micrometeorites occurs as opx and cpx. Both opx and cpx give strong reflections at ⬃2.88 and ⬃3.16 Å, but they differ in an intensity ratio between the two reflections. Thus, an opx/cpx ratio in individual micrometeorites can be estimated from the intensity ratio I2.88 Å/I3.16 Å. The reflection at 2.88 Å includes opx (610), cpx (310), and cpx (⫺310), whereas the reflection at 3.16 Å includes opx (420), opx (221), and cpx (220). Relative intensities of these reflections from the same weight of low-Ca opx and cpx are calculated (Table A1), using the structural parameters of opx (Morimoto and Koto, 1969) and cpx (Morimoto et al., 1960), where Fe in both pyroxenes is distributed between M1 and M2 sites with the site preference K ⫽ 0.1 (K ⫽ (Fe/Mg)M1/(Fe/Mg)M2) (Virgo and Hafner, 1969; Hafner and Warburton, 1971). The calculation shows that (I2.88 Å/I3.16 Å) of opx is much lower than (I2.88 Å/I3.16 Å) of cpx in the compositional range from En60 to En100. On the other hand, the intensity ratio of pyroxenes in micrometeorites is intermediate between (I2.88 Å/I3.16 Å) of opx and (I2.88 Å/I3.16 Å) of cpx, indicating the coexistence of cpx and opx (Table 1). Integrated intensities of low-Ca opx ((Iopx)MM) and cpx ((Icpx)MM) in individual micrometeorites are obtained by partitioning I2.88 Å using the relative abundance of opx (Aopx) and cpx (Acpx) (Table 1), (Iopx)MM ⫽ Aopx I2.88Å (Icpx)MM ⫽ Acpx I2.88Å.

Bulk mineralogy of micrometeorites The magnetite (220) reflection at 2.97 Å is overlaps with reflections from troilite, pyrrhotite, low-Ca opx and low-Ca cpx. Therefore, the true intensity of the magnetite (220) reflection is obtained by subtracting intensities of the Fe sulfides and the low-Ca pyroxenes from the measured intensity at 2.97 Å ((Imeas)2.97 Å). For the correction of Fe sulfides, it is known that the intensity of troilite and pyrrhotite reflections at 2.97 Å is close to that at 2.65 Å (e.g., ASTM 37-477). Thus, the integrated intensity at 2.65 Å, having been defined as ((Isul)MM), is considered as a contribution from Fe sulfides to that at 2.97 Å. From low-Ca pyroxenes, two reflections appear at 2.97 Å: opx (321) and cpx (⫺221). The intensities of the two reflections relative to those at 2.88 Å are calculated and shown in Table A1. The relative intensity of the 2.97 Å reflection for low-Ca opx ((Iopx)2.97 Å/(Iopx)2.88 Å) and low-Ca cpx ((Icpx)2.97 Å/(Icpx)2.88 Å) is variable depending on the average En#. (Iopx)2.97 Å/(Iopx)2.88 Å and (Icpx)2.97 Å/(Icpx)2.88 Å of each micrometeorite are obtained applying the average En# for both opx and cpx. Then, the true intensity of the magnetite (220) reflection (Imag) is calculated as follows: Imag ⫽ (Imeas)2.97Å ⫺ (Isul)MM ⫺ (Iopx)MM {(Iopx)2.97Å/(Iopx)2.88Å} ⫺ (Icpx)MM {(Icpx)2.97Å/(Icpx)2.88Å}. Through the process stated above, the integrated intensity of the particular X-ray reflections from olivine, low-Ca pyroxene, magnetite, and Fe-sufide (troilite ⫹ pyrrhotite) are determined for individual micrometeorites. To convert the integrated X-ray intensity into relative mineral abundance, the relative intensity of the four minerals at a constant weight (ISTD) needs to be known. For this purpose, three particles approximately 100 ␮m in diameter, which are mixtures of equal weights of fine-grained olivine (Fo100), low-Ca opx (En100), troilite, and magnetite, are prepared as standard samples. The X-ray–integrated intensities were determined in the same way as applied for micrometeorites, and the relative X-ray intensity of the four minerals at a constant weight is obtained by averaging the intensities derived from the three standard particles. In principle, a weight fraction of each mineral in a single micrometeorite was determined such as (Iol)MM/(Iol)STD. But between the standard particles and micrometeorites, there are three mineralogical and compositional differences that need to be corrected: (1) troilite is the only sulfide in the standard particles, whereas both troilite and pyrrhotite occur in micrometeorites; (2) the standard particles contain only low-Ca opx, whereas micrometeorites contain both opx and cpx; and (3) the average Fo# and En# are variable in micrometeorites, whereas those of the standard particles are Fo100 and En100, respectively. For difference 1, we assume that same weight of troilite and pyrrhotite gave the same intensity of X-ray diffraction because of very similar chemical compositions and crystal structure. For difference 2, (Icpx)MM was converted to (Iopx)MM with the same weight and En# by use of an intensity ratio of opx to cpx at 2.88 Å (Table A1), and then the total intensity of low-Ca pyroxene in micrometeorites is expressed as ˚ /(Icpx)2.88A ˚ }. (Iopx-total)MM ⫽ (Iopx)MM ⫹ (Icpx)MM {(Iopx)2.88A

For difference 3, in the case of olivine and low-Ca pyroxene, the X-ray intensity at a constant weight changes with Mg/Fe ratios. Therefore, to compare X-ray intensities between micrometeorites and the standard particles, the X-ray intensities of olivine and low-Ca pyroxene in micrometeorites, having various Fo# and En#, need to be converted to the intensities of Fo100 and En100, respectively, with the same

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weights. For this reason, the relationship between X-ray intensity and Fe/Mg ratio at a constant weight was calculated for olivine and low-Ca opx by using the structural parameters given in Brown (1982) for olivine and those in Morimoto and Koto (1969) for low-Ca opx. In the calculation, Fe in olivine prefers M2 site and that in low-Ca opx is distributed between M1 and M2 sites with the site preference K ⫽ 0.1 (e.g., Virgo and Hafner, 1969). From the estimated Fo# and En# and the relationship between X-ray intensity and Fe/Mg ratio, the intensities of olivine and low-Ca pyroxene in individual micrometeorites were converted to those of Fo100 and En100, respectively: (IMM-converted)Fo100 ⫽ (Iol)MM/(IFo#/IFo100) (IMM-converted)En100 ⫽ (Iopx-total)MM/(IEn#/IEn100). Then the relative mineral abundance in weight percent (W), when the total abundance of the four minerals is normalized to 100 wt%, was determined by comparing IMM-converted with ISTD. For olivine, for instance, Wolivine ⫽ 100 (IMM-converted/ISTD)Fo100/{(IMM/ISTD)mag ⫹ (IMM/ISTD)sul ⫹ (IMM-converted/ISTD)Fo100 ⫹ (IMM-converted/ISTD)En100}. The precision of the mineral abundance is estimated to be less than 15% of each mineral abundance—that is, if mineral abundance is 40%, then the error is less than 6%, and the error is mainly due to simplified assumptions made on low-Ca pyroxenes. The error, however, does not affect any of the conclusions we make in this article. Phyllosilicates were identified in some micrometeorites on the basis of the interlayer spacing of basal reflections and the presence of prism reflections, and their abundances are approximated (Table 1) from the X-ray intensity calculation alone without the use of any standard minerals. Two micrometeorites, EURO003 and EURO020, contain cronstedtite where, based on the 001 basal spacing (7.15 Å) and chemical composition (Table 2), tetrahedral sites are occupied by 25% Fe3⫹ and 75% Si4⫹ and octahedral sites are occupied by 75% Fe2⫹ and 25% Mg2⫹. X-ray intensity of 001 reflection of 1-mole cronstedtite (I001) was calculated by the equations I001 ⫽ Lp 兩F001兩2␤ Lp ⫽ (1 ⫹ cos22␪)/sin2␪cos␪, where Lp is the Lorentz polarization factor, F001 is the structural factor of 001 reflection of cronstedtite that we calculated by means of atomic scattering factor and cation site occupancy stated above, ␤ is the temperature factor that we assumed unity, and ␪ is the diffraction angle. Calculated I001 is divided by the molar weight (MW) of cronstedtite to compare I001/MW with calculated Ihkl/MW of other minerals. Then, by means of I001/MW, the integrated intensity of the measured 001 reflection of cronstedtite was converted to the relative mineral abundance (Table 1). Micrometeorite F96CI024 contains saponite, and the abundance of saponite in this sample was also approximated (Table 1) in the same way as cronstedtite. X-ray reflections of magnesiowu¨ stite in micrometeorites appear at 2.44, 2.12 (strongest), 1.50, 1.28, and 1.22 Å, which are almost identical to those of magnesiowu¨ stite reported in ASTM 35-1393. The precise relative abundance of magnesiowu¨ stite has not been obtained in this study. The approximate abundance is roughly estimated (Table 1) from the integrated intensity of reflection at 2.12 Å, assuming that structural factor of magnesiowu¨ stite is the same as that of magnetite.