A new variant of saponite-rich micrometeorites recovered from recent Antarctic snowfall

Authors


Corresponding author. E-mail: kanako@geo.kyushu-u.ac.jp

Abstract

Abstract– Eight saponite-rich micrometeorites with very similar mineralogy were found from the recent surface snow in Antarctica. They might have come to Earth as a larger meteoroid and broke up into pieces on Earth, because they were recovered from the same layer and the same location of the snow. Synchrotron X-ray diffraction (XRD) analysis indicates that saponite, Mg-Fe carbonate, and pyrrhotite are major phases and serpentine, magnetite, and pentlandite are minor phases. Anhydrous silicates are entirely absent from all micrometeorites, suggesting that their parental object has undergone heavy aqueous alteration. Saponite/serpentine ratios are higher than in the Orgueil CI chondrite and are similar to the Tagish Lake carbonaceous chondrite. Transmission electron microscope (TEM) observation indicates that serpentine occupies core regions of fine-grained saponite, pyrrhotite has a low-Ni concentration, and Mg-Fe carbonate shows unique concentric ring structures and has a mean molar Mg/(Mg + Fe) ratio of 0.7. Comparison of the mineralogy to hydrated chondrites and interplanetary dust particles (IDPs) suggests that the micrometeorites are most similar to the carbonate-poor lithology of the Tagish Lake carbonaceous chondrite and some hydrous IDPs, but they show a carbonate mineralogy dissimilar to any primitive chondritic materials. Therefore, they are a new variant of saponite-rich micrometeorite extracted from a primitive hydrous asteroid and recently accreted to Antarctica.

Introduction

Micrometeorites are extraterrestrial particles <1 mm in dimension (Maurette et al. 1991) and are the predominant extraterrestrial material accreting to the Earth (Grün et al. 1985; Love and Brownlee 1993; Taylor et al. 1998). They come from asteroids and comets, but one of the major sources is a primitive class of hydrated asteroids with mineralogy similar to carbonaceous chondrites (e.g., Kurat et al. 1994; Genge et al. 1997; Nakamura et al. 2001b). This interpretation is supported by the high concentration of carbon in micrometeorites (Engrand and Maurette 1998). The large contribution of hydrated asteroids to the production of dust in interplanetary space is ascribed to the volatile-rich, fragile nature of these asteroids, resulting in explosive dispersion of dust particles during impacts, as demonstrated by shock-loading experiments on hydrous carbonaceous chondrites (Tomeoka et al. 2003; Flynn et al. 2009; Levison et al. 2009). Dust particles are extracted from a variety of hydrated asteroids and therefore micrometeorites are expected to represent a much wider variation of primitive hydrous materials than meteorites (Beckerling and Bischoff 1995).

Phyllosilicates are major hydrous phases in micrometeorites, which decompose and dehydrate even at low temperatures during atmospheric entry. Saponite and serpentine, major phyllosilicates found in micrometeorites (Nakamura et al. 2001a, 2001b; Noguchi et al. 2002), are decomposed at 600 and 500 °C, respectively, by brief heating (Nozaki et al. 2006). As a result, the abundance of phyllosilicate-rich micrometeorites is very low and usually does not exceed 1% (Noguchi et al. 2002). Although the mineralogical data on phyllosilicates in micrometeorites are limited, saponite-rich micrometeorites are much more abundant than serpentine-rich or saponite-serpentine micrometeorites (Nakamura and Noguchi 2004). This tendency is consistent with that observed in interplanetary dust particles (IDPs; Bradley et al. 1989), extraterrestrial dust particles smaller than micrometeorites.

Many studies describe the mineralogy of micrometeorites collected from blue-ice fields of Antarctica (e.g., Maurette et al. 1991; Kurat et al. 1994; Genge et al. 1997; Engrand and Maurette 1998; Engrand et al. 1999; Nakamura et al. 2001a, 2001b; Noguchi et al. 2002; Genge 2008; Genge et al. 2008). The blue ice is old snow, formed by the compression of successive snowfalls, and thus micrometeorites in the blue ice came to the earth, for instance, several tens of thousands of years ago (e.g., Yada et al. 2004). During this long residence in the blue ice, micrometeorites are slowly altered, resulting in the dissolution or replacement of particular minerals and depletion or enrichment of certain elements (Kurat et al. 1994). In the case of phyllosilicate-rich micrometeorites, the alteration effects are severe on carbonates and sulfides: carbonates are dissolved into water and pyrrhotite is easily oxidized because it is a non-stoichiometric FeS mineral.

Recently, Duprat et al. (2005a, 2005b, 2007) reported that micrometeorites from Antarctic snow, collected from layers deposited in the 1970s and 1980s, preserve intact mineralogical and chemical characteristics due to very low degrees of alteration compared to blue-ice micrometeorites. The snow micrometeorites are characterized by high contents of Fe sulfides and an undepleted CI elemental abundance pattern of their fine-grained matrix. Noguchi et al. (2006) also succeeded in collecting micrometeorites from recent snow around the Dome Fuji Station. In this study, the mineralogy of eight micrometeorites with abundant phyllosilicates is described. The micrometeorite samples were collected with the same procedure described in Noguchi et al. (2006), but a year later. The samples have escaped severe heating in the atmosphere and have preserved mineralogical signatures of a hydrous parent asteroid. This is the first report of hydrous micrometeorites collected from recent snowfall in Antarctica.

Samples and Experimental Procedures

The micrometeorite samples from this study accreted to Earth between 2003 and 2005, because they were separated from snow that fell during the two years before 2005. The surface snow at DF80 site, 7 km south from the Dome Fuji Station, was collected by the 46th and 47th Japan Antarctic Research Expedition Team in 2005. It was cryogenically preserved and transferred to the National Institute of Polar Research and then transferred to a class 1000 clean room at Ibaraki University. During transfer, the snow sample was kept well below 0 °C. In the clean room, the snow was melted and the resultant water, approximately 30 kg, was filtered using the Millipore filters to collect fine-grained particles. The opening of Millipore filters is 2 μm. A total of 317 micrometeorite candidates with dull black color were picked from the filters and under a stereoscope. They were placed on a Cu plate without carbon coating and examined by a scanning electron microscope equipped with an energy dispersive spectrometer (SEM/EDS); JEOL JSM-5800LV at Kyushu University). Thirty particles with chondritic elemental abundances were identified as true micrometeorites after SEM/EDS observation and analysis.

We investigated the bulk mineralogy of individual micrometeorites by synchrotron radiation X-ray diffraction (XRD) using a Gandolfi (1967) camera at beam line 3A of the Photon Factory Institute of Materials Structure Science, High Energy Accelerator Research Organization. Each micrometeorite was mounted on a thin glass fiber of 5 μm thickness using a small amount of acetone-soluble bond and placed in the Gandolfi camera. The sample was exposed to a monochromated synchrotron X-ray with a wavelength of 2.161 ± 0.001 Å and a diameter of 0.5 mm. The exposure duration is 30 min for each micrometeorite. The details of the XRD technique are described by Nakamura et al. (2008).

Scanning electron microscopic observation and XRD analysis indicate that 11 micrometeorites show very similar mineralogy, surface morphology, and elemental abundances. Hereafter, these samples are collectively referred as “SK series micrometeorites”; the identification numbers of the individual samples are SK05P2H1, SK05P2D10, SK05P2A2, SK05P2E6, SK05P2C9, SK05P2H6 (Figs. 1a–f), SK05P2I4, SK05P2I6, SK05OTP6A3, SK05OTP6A6, and SK05OTP6H5.

Figure 1.

 X-ray diffraction patterns and backscattered electron (BSE) images of a) SK05P2H1, b) SK05P2D10, c) SK05P2A2, d) SK05P2E6, e) SK05P2C9, and f) SK05P2H6. Major phases are saponite, Mg-Fe carbonate, and pyrrhotite. Minor phases are serpentine and magnetite. SK05P2A2 in (c) and SK05P2H6 in (f) contain large pyrrhotite (Py) grains on their surfaces. SK05P2E6 in (d) contains carbonaceous material (C) that does not show any diffraction and is, thus, amorphous.

Following XRD analysis, three micrometeorites (SK05P2H1, SK05P2D10, and SK05P2A2; Figs. 1a–c) were embedded in epoxy resin. SK05P2H1 and SK05P2D10 were microtomed according to the procedure in Noguchi et al. (2002). In this study, we used the Reichert-Nissei Ultracut N ultramicrotome at Ibaraki University to make thin foils (70 nm) for transmission electron microscope (TEM) observation. SK05P2A2 was polished to make a flat surface using diamond-polishing paste. After polishing or microtome cutoff, approximately half of each micrometeorite was left in potted butts with flat surfaces. The polished surfaces of the three micrometeorites were carbon coated and observed with a field emission-scanning electron microscope equipped with an energy dispersive spectrometer (FE-SEM/EDS; JEOL JSM-7000F at University of Tokyo and JEOL JSM-7001F at Osaka University) to characterize microtextures. Then they were analyzed with an electron-probe microanalyzer equipped with a wavelength-dispersive X-ray spectrometer (EPMA/WDS; JEOL JCXA 733 at Kyushu University) to obtain quantitative major element concentrations. The WDS quantitative analyses were performed at 15 kV accelerating voltage and 10 nA beam current with a 3 μm focused beam. The compositions are tabulated in Appendix 1. SK05P2H1 and SK05P2D10 were further observed by TEM (JEOL JEM-2000FX) and quantitative chemical analyses were made by EDS (EDAX DX4EDS) at Ibaraki University. Detailed TEM procedures are described elsewhere (Noguchi et al. 2002).

In addition to the micrometeorites, small pieces (<1mm in diameter) of chondrites were analyzed to compare SK series micrometeorites. Small pieces (40–200 μm) of the Orgueil CI chondrites were analyzed by SEM/EDS prior to XRD analysis to compare C/O ratios between CI chondrite and SK series micrometeorites. Polished sections of the Tagish Lake carbonate-poor lithology and carbonate-rich lithology were also analyzed by EPMA to compare the major element abundances between Tagish Lake matrix and SK series micrometeorites. The compositions are tabulated in Appendix 2. A variety of chondrites (CI and matrices of CR, CM, CH, Tagish Lake carbonaceous chondrites) were analyzed by synchrotron XRD to compare the bulk mineralogy of these fine-grained materials to SK series micrometeorites. Each chondrite was exposed to a monochromated synchrotron X-ray with the same analytical condition as SK series micrometeorites. The exposure duration is 20 min for each chondrite.

Results

SK series micrometeorites have many morphological and mineralogical features in common. They range in size from 30 to 80 μm (Figs. 1a–f) and show similar fine-grained morphology except for SK05P2H6 which contains a large pyrrhotite crystal (Fig. 1f). SEM/EDS spectra from the surfaces of all eight samples show similar chondritic elemental patterns except for enrichments of S and Fe in SK05P2H6. In addition, unlike typical micrometeorites, they lack the fusion crusts or magnetite rims (Figs. 2a and 2b) that are formed by frictional heating upon atmospheric entry of small particles (Jessberger et al. 1992). This suggests that they might be fragments of a larger dust clump that broke up on the Earth.

Figure 2.

 BSE images of the SK series micrometeorites observed by FE-SEM. a) SK05P2H1. (Inset) An enlarged view of framboidal magnetite (white box). b) SK05P2A2. c) Close-up view of SK05P2A2. The area is shown by the white box in (b). Carbonates occur as hollow aggregates and are indicated by arrows. d) Platy pyrrhotite in SK05P2A2.

Bulk Mineralogy, Bulk Composition, and Overall Texture of SK Series

The results of the synchrotron XRD analysis show that the eight SK series micrometeorites exhibit similar XRD patterns and have the following mineralogical features in common (Figs. 1a–f): (1) the major minerals are phyllosilicates, sulfides, and Mg-Fe carbonates, (2) the phyllosilicates are saponite ([001] spacing of approximately 1.4 nm) and serpentine ([001] spacing of 0.7 nm) and the former tends to be much higher in abundance than the latter, (3) pyrrhotite is the dominant sulfide, (4) Mg-Fe carbonate has high Mg/Fe ratios, (5) magnetite is absent or a very minor component, and (6) anhydrous silicates are entirely absent.

The SK series micrometeorites show high carbon abundances. They were placed on a Cu plate without carbon coating and analyzed by SEM/EDS. A large carbon X-ray peak was detected (Fig. 3a) and C/O peak height ratios are comparable to those of Orgueil CI chondrite (Fig. 3b). The high carbon concentrations suggest that SK series micrometeorites are related to carbonaceous chondrites. Concentrations of major and minor elements (Al, Ti, Ca, Mg, Si, Cr, Mn, Fe, Ni, Na, K, P, and S) in three of the micrometeorites (SK05P2H1, SK05P2D10, and SK05P2A2) were determined by spot-beam electron-probe analysis of polished surfaces: the composition is the average of approximately 50 analyses per one micrometeorite and the number of analyses depends on the size of the micrometeorite. The three samples have similar elemental abundances, which confirms their mineralogical similarities. The abundances are roughly similar to CI (Fig. 4), except for large depletions in the elements Ca and Ni and minor depletions in some other elements. The Fe and Ni abundances are apparently correlated (Fig. 4), suggestive of a common cause of depletion: aqueous alteration of FeNi metal is probably responsible for leaching out of Fe and Ni into solution (cf. Hanowski and Brearley 2000).

Figure 3.

 EDS spectra of a) SK05P2D10, b) Orgueil CI chondrite. The Cu peaks come from the Cu plate substrate.

Figure 4.

 CI-normalized bulk chemical compositions of SK series micrometeorites and saponite-rich type chondrites. Elements are arranged in order of increasing cosmochemical volatility from left to right.

Large enrichments of K are observed in two micrometeorites SK05P2D10 and SK05P2A2 (Fig. 4). TEM/EDS and EPMA/WDS analyses of SK05P2D10 indicate that K occurs in some portions of the phyllosilicates with concentrations up to 1 wt% (Fig. 5a). K-rich phyllosilicates show a (001) spacing similar to saponite but tend to have higher SiO2 content (Fig. 5a), suggesting that some amounts of K-bearing mica are contained in saponite-rich regions.

Figure 5.

 SiO2 oxides variation diagrams of SK series micrometeorites and matrix of Semarkona LL3 chondrite, Tagish Lake carbonate-poor lithology, and Bells CM chondrite. Compositions were obtained by focused-beam EPMA analysis. a) SiO2 versus K2O. b) SiO2 versus Al2O3. c) SiO2 versus Na2O.

The Al/Si ratios are close to solar, irrespective of SiO2 contents (Fig. 5b). The SiO2 concentrations are proportional to the phyllosilicate contents in the areas analyzed by electron microprobe, because only phyllosilicates contain SiO2 among the major minerals in the SK series micrometeorites. Therefore, the Al/Si ratios of phyllosilicates in the SK series micrometeorites are consistent with the solar ratio (Fig. 5b). On the other hand, Na2O and K2O distribute heterogeneously within individual samples and also between samples (Figs. 5a and 5c). The Si–Mg-Fe ratios (Fig. 6) cluster around solar, but extend downward in two different directions, toward the Fe apex and toward a position with high Mg/Fe, which corresponds to the compositions of pyrrhotite and carbonate, respectively.

Figure 6.

 Bulk chemical compositions of SK series micrometeorites (SK05P2H1, SK05P2D10, and SK05P2A2) determined by EPMA analysis. Compositional fields of Semarkona, Tagish Lake carbonate-poor, and Bells are also shown for comparison. Solar ratio is approximately Si/Mg/Fe = 1/1/1 and is plotted at the center of diagram.

FE-SEM observation of polished surfaces shows that the SK series micrometeorites are fine-grained porous aggregates (Figs. 2a and 2b), with many grains of pyrrhotite and Mg-Fe carbonate dispersed within a fine-grained (<100 nm in size) matrix of phyllosilicates. Fibrous phyllosilicates (∼100 nm long) can be observed in some places in the fine-grained matrix, but most phyllosilicates cannot be resolved in the FE-SEM images. Carbonate often occurs as hollow aggregates (Fig. 2c) with diameters from 500 nm to 1 μm. Most pyrrhotite grains show round or subround morphology, ranging in size from several tens to hundreds nm, but occasionally up to 1 μm in size. Platy morphology rarely occurs (Fig. 2d). Magnetite is rare, being detected as small peaks in XRD patterns (Figs. 1a–f), and occurs as framboidal aggregates (Fig. 2a).

TEM Observation of SK Series

Phyllosilicates

TEM observations revealed that the phyllosilicates have variable crystallinities. Fine-grained (∼30 nm wide, ∼100 nm long) crystalline phyllosilicates are composed mainly of saponite (1.2–1.3 nm interlayer spacing) with a small amount of interstratified serpentine (0.7 nm interlayer spacing). Some fine-grained phyllosilicates are structured with serpentine cores and saponite envelopes (Fig. 7a). This implies changes in the physico-chemical conditions of solution at late stages of aqueous alteration. Poorly crystalline phyllosilicates coexist with the fine-grained ones (Fig. 7b) and are composed of one to five unit layers with a curved morphology. The interlayer spacing suggests that they are exclusively saponite.

Figure 7.

 Phyllosilicates in SK series micrometeorites. a) A high-resolution image of fine-grained crystalline phyllosilicates in SK05P2H1. The saponite is often accompanied by small amounts of serpentine. b) A high-resolution image of poorly crystalline phyllosilicates in SK05P2D10. c) Chemical composition of phyllosilicates determined by TEM/EDS analysis.

Chemical compositions of phyllosilicates obtained by TEM/EDS analyses show that they plot in a compositional field slightly below and subparallel to the saponite solid solution field (Fig. 7c). The phyllosilicates with a core–rim structure are slightly more magnesian and less siliceous than the fine-grained ones. Saponite to serpentine molar ratios vary from 4 to 6 within a given sample and also between samples on the basis of TEM/EDS chemical analysis. The ratios are positively correlated with the abundance ratios of poorly crystalline to fine-grained phyllosilicates, because poorly crystalline phyllosilicates have higher saponite/serpentine ratios than the fine-grained ones.

Sulfides

Pyrrhotite is the most abundant sulfide (Figs. 8a and 8b). It shows hexagonal to subhedral morphology and contains small amounts of Ni (<5 atomic%). Most pyrrhotites have Fe/S ratios close to unity, but some are largely depleted in S, suggesting partial decomposition (Fig. 8e). Pentlandite is much less abundant than pyrrhotite and only one pentlandite grain was identified, in SK05P2D10 (Fig. 8e).

Figure 8.

 Sulfides in SK series micrometeorites. a) A bright-field image of pyrrhotite in SK05P2H1. b) A bright-field image of pyrrhotite in SK05P2D10. c) A low-magnification bright-field image of SK05P2D10. Fine-grained (typically <50 nm across) pyrrhotite-rich aggregates exist in the phyllosilicate-rich matrix. In this image, an aggregate is framed by a white box. Carb, carbonate; Py, pyrrhotite; Mt, magnetite. d) An enlarged view of the pyrrhotite-rich aggregate in SK05P2D10. There are many pore spaces around small pyrrhotites in the aggregate. e) Chemical composition of sulfides determined by TEM/EDS analysis.

Pyrrhotites show two types of occurrences. Large pyrrhotites (100–500 nm in diameter) occur independently within phyllosilicates (Figs. 8a and 8b), while small pyrrhotites (typically <50 nm in diameter) occur as inclusions within unique fine-grained aggregates (Figs. 8c and 8d). The aggregates are ubiquitously distributed throughout the SK series micrometeorites and consist mainly of nm-sized pyrrhotites and poorly crystalline phyllosilicates. Most of the small pyrrhotites have thin rinds that are probably organic materials, because the rinds are almost transparent in a bright-field TEM image (Fig. 8d). In addition, many voids occur within and around the aggregates (Fig. 8d). Such aggregates have never been found in Antarctic micrometeorites recovered from blue-ice field.

The sizes (typically 100 nm in size) and the textures of the aggregates bear a resemblance to those of glass embedded with metal and sulfide (GEMS) that are commonly found in anhydrous IDPs (e.g., Keller and Messenger 2004), although the constituent minerals of the aggregates are different from those of GEMS. There is a possibility that the aggregates may be remnant GEMS that have been pseudomorphically replaced during aqueous alteration. Glass and metallic particles in the GEMS would have been replaced by phyllosilicates and pyrrhotites, respectively, during the course of alteration. Hydrothermal experiments of anhydrous IDPs revealed that GEMS are resistant to moderate degrees of aqueous alteration (Nakamura et al. 2005), but the intense alteration experienced by the SK series micrometeorites would have replaced all anhydrous minerals and GEMS. The aggregates in the SK series micrometeorites and GEMS from IDPs (Keller and Messenger 2004) have similar compositions (Figs. 9a–c), although the former show a depletion of Ca (Fig. 9c). The Ca-depletion is probably caused by aqueous alteration in a parent object, because Ca is leached out to solutions during the alteration. Such Ca-depletion is common in some hydrous chondrites (Fig. 4).

Figure 9.

 Bulk chemical compositions of GEMS-like pyrrhotite-rich aggregates in SK05P2H1. a) Si–S–Fe ternary atomic ratio diagram. b) Si normalized Mg versus Fe. c) S versus Ca diagrams. Almost all the data are within the compositional fields of GEMS from IDPs. IDP data are from Keller and Messenger (2004).

Carbonates

Carbonates are exclusively Mg-Fe carbonate. Ca- and Ca–Mg carbonates are not found. Many carbonate crystals occur as rhombohedral grains from 200 to 1000 nm in size. Some have corroded shapes and others have subhedral to euhedral shapes (Figs. 10a and 10b). The Mg/(Mg + Fe) ratios of the carbonates range from 0.35 to 0.70 (Fig. 10c). Most carbonates lie in a narrow compositional field from 0.6 to 0.7, but smaller crystals tend to have lower Mg/(Mg + Fe) ratios. MnO and CaO in the Mg-Fe carbonates do not exceed 3.3 and 3.1 wt%, respectively. Almost all Ca in the SK series micrometeorites is present in carbonate based on TEM/EDS analysis. The carbonates often show mottled textures (Fig. 10a), which may indicate incipient decarbonation during pulse heating upon atmospheric entry. In many places, the carbonates occur as hollow aggregates and show concentric ring structures on the polished or microtomed sections (Figs. 2c and 10d). The hollows are smaller than 1 μm in diameter and the rings are approximately 100 nm thick (Fig. 10d).

Figure 10.

 Carbonates in SK series micrometeorites. a) High-magnification image of a carbonate in SK05P2H1, showing mottled textures. b) High-magnification image of a carbonate in SK05P2D10. c) Chemical compositions of carbonates determined by AEM analysis, compared to those in the Tagish Lake carbonaceous chondrite. d) Low-magnification bright-field image of a carbonate hollow in SK05P2H1.

Organic Nanoglobules

We also found amorphous organic nanoglobules (carbonaceous nanoparticles) in SK05P2D10 (Figs. 11a and 11b), that are similar in size to the nanoglobules in heavily altered carbonaceous chondrites such as Tagish Lake and CI chondrites (Nakamura et al. 2001a; Noguchi et al. 2005; Nakamura-Messenger et al. 2006). However, unlike typical nanoglobules found in carbonaceous chondrites, those in the SK series micrometeorites have incomplete multi-ring structures and contain layers of voids where small pore spaces are entrained to form void-rich ring structures (Figs. 11a and 11b). These nanoglobules are similar to the ones in Bells CM chondrite reported in Messenger et al. (2008).

Figure 11.

 Bright-field images of hollow organic nanoglobules in SK05P2D10. Nanoglobules showing vesiculation a) at the outer layer and b) at the center layer.

Discussion

Atmospheric-Entry Heating and Terrestrial Weathering Effects

There are two possibilities to explain why SK series micrometeorites dominate the collection: (1) they are fragments of a larger object and (2) they represent a recent micrometeoroid shower that dominated the dust flux. The latter can be discounted on the basis that even a micrometeoroid stream with a single geocentric velocity will produce more heated particles and yet these are absent. The almost identical mineralogies and major-element abundances, the absence of igneous rim (Genge 2006), and their concentration in the same layer and location of the snow indicate that the SK series micrometeorites came to Earth as a single meteoroid and broke up into pieces on Earth. It is highly probable that other SK series materials are still dispersed in the same layer of snow in Antarctica.

The presence of abundant phyllosilicates and carbonates suggests that the SK series micrometeorites experienced weak heating at temperatures lower than 500 °C during atmospheric entry, because the decomposition temperatures by short-duration heating are 500, 600, and 700 °C for Mg-Fe carbonate, serpentine, and saponite, respectively (Nozaki et al. 2006). Only the carbonates show incipient decomposition (Fig. 10a), suggesting that temperatures were approaching 500 °C. The survival of carbonaceous nanoglobules also indicates that the SK series micrometeorites were heated at temperatures lower than 500 °C, because oxidation of amorphous carbon occurs at approximately 500 °C during combustion (i.e., high oxygen fugacity: Rai et al. 2003). The nanoglobules are not decomposed at 500 °C when heated in vacuum (i.e., low oxygen fugacity: Aoki and Akai 2008). The partial vesiculation of the globules (Figs. 11a and 11b) also suggests that incipient decomposition took place during heating in the atmosphere and that the temperature of heating was close to 500 °C. Vesiculation took place at the interior of the globule (Fig. 11b), which may indicate that volatile components are highly enriched at the interior rather than the surface. Some pyrrhotite grains show depletion of S relative to Fe (Fig. 8c), but do not show terrestrial weathering such as replacement by Fe oxy-hydroxides or ferrihydrite. Therefore, the depletion of sulfur in the grains may be caused by the pulse heating.

The survival of pyrrhotite and carbonates (which are soluble in water) confirm that the SK series micrometeorites experienced a very low degree of weathering in Antarctica. K enrichments similar to those observed in the SK series micrometeorites, up to one order of magnitude higher than CI, were reported from CI-like micrometeorites by Kurat et al. (1992) and were ascribed to terrestrial contamination. However, the K-enrichment of SK series micrometeorites is most likely pre-terrestrial because (1) the SK series micrometeorites have the short residence time in Antarctica and (2) other micrometeorites which were collected from the same snow as the SK series micrometeorites do not show K-enrichment. In addition, the Semarkona LL3 chondrite, which consists largely of hydrous phases, is also enriched in K (Fig. 4 and Matsunami 1984) and therefore the K enrichment may be a feature common to certain hydrated chondritic materials. Taken together, the data indicate that the SK series micrometeorites experienced little atmospheric-entry heating and terrestrial weathering, and thus largely preserve the mineralogical and chemical characteristics of their parental source object.

Parent Body Characteristics Derived from Mineralogy

The presence of abundant phyllosilicates and carbonates (Figs. 7 and 10), the absence of anhydrous silicates (Fig. 1), and the high concentration of carbon (Fig. 3) clearly indicate that SK series micrometeorites came from a carbonaceous-chondrite-like parental object where aqueous alteration prevailed. The large depletion of Ca suggests that Ca dissolved into solution during alteration and was redistributed to other regions in the parent body. The presence of serpentine cores with saponite sheaths implies an increase of SiO2 content in solution during progressive aqueous alteration, because saponite formation requires a higher Si/(Mg + Fe) ratio than serpentine. During the course of alteration, the solution temperature might have increased and the pH of aqueous solution from which phyllosilicates crystallized might have decreased, resulting in precipitation of abundant Mg-Fe carbonates, because CO2 becomes less soluble with increasing temperature. Subtraction of MgO and FeO due to precipitation of carbonates can account for the enrichment of SiO2 in solution. The temperature elevation of the solution is consistent with large extent of aqueous alteration in the parent body of SK series micrometeorites.

Comparison with Chondrites that Experienced Aqueous Alteration

Bulk mineralogy is important to identify the parental objects of the micrometeorites. To compare the bulk mineralogy of the SK series micrometeorites, a variety of XRD patterns, taken from CI and matrices of CR, CM, CH, and the Tagish Lake carbonaceous chondrites, are shown in Figs. 12a–f. In the following discussion, we concentrate on the mineralogical comparison between the SK series micrometeorites (Figs. 1a–f) and the fine-grained matrices of some representative carbonaceous chondrites (Figs. 12a–f).

Figure 12.

 X-ray diffraction patterns of various types of chondrites. a) Matrix of Tagish Lake carbonate-rich lithology. b) Matrix of Tagish Lake carbonate-poor lithology. c) Orgueil CI chondrite. d) Matrix of Murchison CM chondrite. e) Matrix of Renazzo CR chondrite. f) Matrix of Northwest Africa (NWA) 470 CH chondrite.

Phyllosilicates

There are many carbonaceous and some ordinary chondrites whose matrices consist largely of phyllosilicates. They are classified into the following four groups based on phyllosilicate mineralogy (Table 1).

Table 1.   Comparison of SK series with chondrites.
 SK series micrometeoritesCI chondrite Orgueil1Tagish Lake carbonate-poor2Tagish Lake carbonate-rich3CR chondrite4Semarkona and Bishumpur5CM chondrite Bells6
  1. S, saponite type; SR, saponite-rich type; SS, saponite–serpentine type; Py, pyrrhotite; Pe, pentlandite; Tr, troilite; Px, pyroxene.

  2. – No data available from literature.

  3. aPhyllosilicate in CR chondrite: varying degree of aqueous alteration.

  4. Weisberg et al. (1993) and Endreß et al. (1994): serpentine is abundant.

  5. Zolensky et al. (1993) and Krot et al. (2002): similar to CI and IDPs.

  6. References: 1. Tomeoka and Buseck (1988), 2. Zolensky et al. (2002), 3. Nakamura et al. (2003), 4. Bischoff et al. (1993), 5. Alexander et al. (1989), 6. Brearley (1995), 7. Krot et al. (2002), 8. Hutchison et al. (1987), 9. Garvie and Buseck (2004), 10. Zolensky (1991).

PhyllosilicateSaponiteSaponiteSaponiteSaponiteSaponiteSmectiteSmectite
SerpentineSerpentineSerpentine Serpentine Serpentine
TypeSRSSSRSSSaSRSR
SulfidePy
Pe (only one)
Py (rare)Pe, Py (abundant)Pe, Py (lesser amounts)Tr, Py (abundant)
Pe (rare)
Tr, PePe, Py (common)
CarbonateFe–Mg carbonate
Abundant
Mg–Ca–Fe carbonateMg–Ca–Fe carbonate
Minor
Fe–Mg carbonate
Abundant
Ca carbonate7
Rare
Pure Ca carbonate8
Minor
Ca carbonate (minor)
Mg-carbonate9
MagnetiteMinor (framboids, platelets)Abundant (framboids, platelets)Abundant (framboids, placquettes)RareAbundant (framboids, platelets)Common
Anhydrous mineral and metalAbsentAbsentOlivine
Px (rare)
Olivine (rare)Olivine10
Metal
Olivine, low-Ca PxOlivine, low-Ca Px
Bulk composition
 Ca relative to CIDepletedCIDepletedDepletedDepleted
 K relative to CIEnrichedCIClose to CIEnrichedDepleted
1. Saponite-Serpentine Type

The Orgueil CI chondrite and the matrices of some CR chondrites contain both saponite and serpentine (Figs. 12c and 12e and SS in Table 1).

2. Saponite-Rich Type

The matrices of Semarkona ordinary chondrite, Bells CM chondrite, and Tagish Lake carbonate-poor lithology contain both saponite and serpentine, but saponite is predominant (Fig. 12b and SR in Table 1).

3. Saponite Type

The matrix of Tagish Lake carbonate-rich lithology contains only saponite and no serpentine (Fig. 12a and S in Table 1). The matrices of some CV chondrites also contains saponite, but in many cases, saponite is a minor phase and anhydrous silicates such as olivine are the major phases. Thus, CV chondrites are excluded from this classification.

4. Serpentine Type

The matrices of many CM chondrites, except for Bells, and CH chondrites are dominated by serpentine (Figs. 12d and 12f).

The SK series micrometeorites contain both saponite and serpentine, and saponite is much more abundant. Thus, they are classified into (2) saponite-rich type. Therefore, we compare the mineralogy of the SK series micrometeorites with that of saponite-rich type chondrites, Semarkona, Bells, and the Tagish Lake carbonate-poor lithology. The matrix of Semarkona contains anhydrous silicates as a minor but ubiquitous component (Alexander et al. 1989), suggesting that the degree of aqueous alteration in Semarkona is much lower than in the other two chondrites and SK series micrometeorites. Major element abundances obtained by focused-beam (∼3 μm in diameter) electron-probe analysis indicate that phyllosilicates in SK series are most similar to those in Tagish Lake carbonate-poor lithology. Mg, Fe, and Si abundance ratios of SK series overlap considerably with those of Tagish Lake carbonate-poor matrix (Fig. 6). This is confirmed by TEM/EDS analysis of the phyllosilicates (Fig. 7c). The matrix of Semarkona is characterized by high SiO2, Na2O, K2O, and Al2O3 contents (Matsunami 1984), and that of Bells is characterized by high Al2O3 contents (Brearley 1995). Therefore, their compositions of phyllosilicates differ greatly from those of the SK series micrometeorites (Figs. 5a–c). In contrast, Tagish Lake carbonate-poor matrix shows major element abundances similar to the SK series micrometeorites, but the former is lower in Na2O and K2O abundances than the latter (Figs. 5a–c).

Carbonates

Mg-Fe carbonates are abundant in SK series micrometeorites and most of them have similar Mg-rich and Ca-poor compositions (Fig. 10c). In contrast, Tagish Lake carbonate-poor matrix contains calcite, dolomite, and Mg-Fe carbonates with high Fe/Mg ratios (Fig. 10c and Zolensky et al. 2002). Semarkona matrices contain minor amounts of Ca carbonate and no Mg-Fe carbonates (Table 1). Bells matrices contain minor amounts of Ca carbonate and Mg carbonate (Table 1). The Tagish Lake carbonate-rich matrix, although its phyllosilicate mineralogy differs from the SK series micrometeorites, contains abundant Mg-Fe carbonate (Nakamura et al. 2003). However, their Fe/Mg ratios are much higher than those of carbonates in the SK series micrometeorites (Fig. 10c). In addition, carbonates in the SK series micrometeorites show unique concentric hollow structures (Fig. 10d), which implies that they crystallized on the surfaces of volatile materials such as organic, H2O, or CO ice. In summary, carbonates in SK series micrometeorites are different from any other carbonates found so far in hydrated carbonaceous and ordinary chondrites.

Sulfides

Sulfides in the SK series micrometeorites are, in most cases, low-Ni pyrrhotite (Fig. 8e). In contrast, the major sulfide in the Tagish Lake carbonate-poor matrix is pentlandite. Thus, compositions of sulfides differ between SK series micrometeorites and Tagish Lake carbonate-poor matrix. On the other hand, Bells and Semarkona contain both pentlandite and pyrrhotite/troilite (Alexander et al. 1989; Brearley 1995), but relative abundances between the two minerals are unknown due to lack of XRD data in the literature.

In summary, the mineralogical characteristics of the SK series micrometeorites are most similar to the Tagish Lake carbonate-poor lithology, but differ in potassium abundance and carbonate and sulfide mineralogy. Hence, the SK series micrometeorites are different from any known chondrites in terms of both mineralogy and chemistry. The results suggest that the SK series micrometeorites came from a hydrous asteroid with a mineralogy and a chemistry slightly different from the Tagish Lake carbonate-poor lithology. However, there are chemical and mineralogical variations between Tagish Lake carbonate-poor and -rich lithologies (Fig. 12 for mineralogy and Fig. 4 for chemistry). This suggests that, even in a single asteroid, there are large elemental variations. Abundances of some elements (Ni and Ca) in SK series micrometeorites are intermediate between those of Tagish Lake carbonate-poor lithology and carbonate-rich lithology (Fig. 4), while potassium abundance is higher than both lithologies. Therefore, SK series micrometeorites are a new variant of saponite-rich micrometeorites, but it cannot be ruled out that SK series micrometeorites were derived from a hydrated Tagish lake–like asteroid.

Comparison to Hydrous IDPs

The majority of hydrous IDPs contain variable amounts of saponite. Only a few are serpentine-rich (Tomeoka 1991). The saponite-bearing IDPs are similar to the SK series micrometeorites in the following respects: anhydrous silicates are rare, the main carbonate is Mg-Fe carbonate, pyrrhotite is a major sulfide, and Ca is depleted in bulk composition (summarized in Table 2). But there are also some conspicuous differences: most saponite-bearing IDPs contain only saponite and no serpentine, and thus are classified as (3) saponite type if the above-mentioned classification is applied (Tomeoka and Buseck 1984, 1986; Zolensky et al. 1988; Tomeoka 1991; Ritmeijer 1991; Zolensky and Lindstrom 1992) and some IDPs also contain glassy material (Zolensky et al. 1988). In addition, there are also a few IDPs that contain both saponite and serpentine (Table 2), such as W7013F5, which contains Mg-Fe serpentine, Fe-bearing saponite, Mg-Fe carbonate, pyrrhotite and pentlandite (Keller et al. 1992). The Mg/(Mg + Fe) ratios of the Mg-Fe carbonate range from 0.3 to 1, similar to the carbonates in the SK series micrometeorites. However, the saponite/serpentine ratio of W7013F5 seems to be close to 1, because the main phyllosilicates in this IDP are an intergrowth of saponite and serpentine and, thus, are similar to CI chondrite phyllosilicates (Keller et al. 1992). Therefore, W7013F5 is different from SK series micrometeorites.

Table 2.   Comparison with hydrous IDPs.
 Hydrous IDPs
Calrissian1aLOW-CA IDP2bW7013F53W7029A23&A244cW7029E54W7017A35W7027A175L2001-16, U230A39, W7017A875
  1. S, saponite type; SR, saponite-rich type; SS, saponite–serpentine type; Py, pyrrhotite; Pe, pentlandite; Tr, troilite; Px, pyroxene.

  2. – No data available from literature.

  3. aCalrissian: r21-M1-9A.

  4. bLOW-CA IDP: U2-21-2-1.

  5. cDispersed sample allocations W7029A23 and -24 of parent CP IDP W7029C1.

  6. dFMA-silicate is a fluffy silicate that contains Fe, Mg, and Al.

  7. References: 1. Tomeoka and Buseck (1986), 2. Tomeoka and Buseck (1984), 3. Keller et al. (1992), 4. Rietmeijer (1991); Rietmeijer and Mackinnon (1985), 5. Thomas et al. (1990a,b), 6. Zolensky and Lindstrom (1992).

PhyllosilicateSmectite, micaFMA-silicatedFe-bearing saponite
Mg-Fe serpentine
Smectite
Kaolinite
Saponite
Illite/nontronite
Saponite
Serpentine
SaponiteSaponite
TypeSSSSSSSRSS
SulfidePy, PePy, Pe, low-Ni PePy,Pe (abundant)
Zn-bearing Fe sulfide
Fe–Ni sulfide (minor)
(∼0.5–30 wt%)
Fe–Ni sulfide (minor)
(∼0.5–30 wt%)
Fe–Ni sulfide (minor)
(∼0.5–30 wt%)
CarbonateMg-Fe carboanateMg-Fe carbonate
Abundant
MagnetiteAbsentMinor
Anhydrous mineral and metalRareOlivine (rare)KamaciteOlivine, low-Ca PxOlivine, Px (minor)
Glass
LIME olivine
Glass
Olivine, Px (minor)
Glass
Bulk composition
 Ca relative to CIDepletedDepletedDepleted
 K relative to CI
 L2005 P176L2005 K26L2005 K76L2005 Q66L2005 Q26L2005 K96L2007 156L2005 L26
PhyllosilicateSaponite
Serpentine
SaponiteSaponiteSaponiteSaponiteSaponiteSaponite
Serpentine (rare)
Saponite
TypeSRSSSSSSRS
SulfidePyPyPyPyPyPy, PePy. PePy, Pe
CarbonateMagnesite, siderite––
Magnetite
Chromite, Spinel
PresentPresentPresent
Spinel

Chromite
Anhydrous mineral and metal  En83–99 Glass (abundant)DiopsideEn61, diopsideFo100
  Fo77–99 Diopside Augite, Fo99–100En100
Bulk composition
 Ca relative to CI
 K relative to CI

Micrometeorites may better represent the mineralogy of their parent bodies than IDPs because of their much larger mass. For example, the IDPs LOW-CA (U2-21-2-1) (Tomeoka and Buseck 1984), Calrissian (Tomeoka and Buseck 1986), and W7017A3 (Thomas et al. 1990a, 1990b) may be similar to the SK series micrometeorites (Table 2), but detailed descriptions of carbonates and sulfides are missing and, therefore, comparisons to the SK series micrometeorites cannot be made. On the other hand, Zolensky and Lindstrom (1992) reported 12 chondritic IDPs that were all large enough to permit detailed mineralogy and trace element analysis. In their IDPs, there are two saponite-rich IDPs; phyllosilicates in L2005 P17 and L2007 15 have high saponite/serpentine ratios. In addition, the major sulfides in L2005 P17 and L2007 15 are pyrrhotite, and pyrrhotite and pentlandite, respectively, which is similar to the SK series sulfides. However, carbonates are not found in the saponite-rich IDPs and thus the two saponite-rich IDPs are different from SK series micrometeorites. In summary, the SK series micrometeorites are also different from known hydrous IDPs and represent a new variant of saponite-rich micrometeorite.

Conclusions

A group of hydrated micrometeorites with high saponite/serpentine ratios was found from recent snowfall in Antarctica. In addition to phyllosilicates, they contain Mg-rich carbonates with hollow structures, Ni-bearing pyrrhotite, and carbonaceous nanoglobules. No anhydrous silicates were found, suggestive of pervasive aqueous alteration of the parent object. Mineralogical comparison of the SK series micrometeorites to hydrated chondrites and IDPs suggests that the micrometeorites do not match the mineralogy of any known primitive materials. They are a new variant saponite-rich micrometeorite extracted from a primitive hydrous asteroid that came to Earth in the last few years.

Acknowledgments

Acknowledgments— We thank Drs. C. Floss, M. Genge, C. Engrand, and K. Nakamura for constructive and insightful reviews, Mr. K. Shimada for technical assistance during EPMA analysis, Messrs. N. Ohashi and S. Nishida for valuable supports during clean room work at Ibaraki University, Messrs. K. Ando and Y. Yamauchi for supports during SEM analysis, and Dr. K. Murata and Mr. H. Yoshida for the help during FE-SEM analysis at Osaka University and University of Tokyo, respectively. This work was supported by Grant-in-Aid (No. 20654054 and 21340165 to Nakamura) of the Japan Ministry of Education, Culture, Sports, Science and Technology.

Editorial Handling—Dr. Christine Floss

Appendices

Appendix 1

The average composition and the range of variation for each element in SK series micrometeorites determined by microprobe analyses (wt%).

 SKP2A2SKP2D10SKP2H1
AverageMin.Max.AverageMin.Max.AverageMin.Max.
  1. n.d., not detected. The limit of detection is 0.1 wt%.

Number of analysis63  43  58  
SiO231.121.141.227.415.940.725.40.336.5
TiO20.1n.d.0.10.1n.d.0.10.1n.d.0.1
Al2O31.10.12.41.60.72.31.5n.d.2.2
FeO20.311.229.022.29.727.726.815.579.9
MnO0.2n.d.0.60.20.10.50.2n.d.0.5
MgO16.310.023.016.67.224.914.80.423.9
CaO0.20.11.00.30.10.60.2n.d.0.4
Na2O0.40.10.80.60.21.10.30.10.6
K2O0.50.10.90.50.10.90.1n.d.0.2
Cr2O30.30.20.80.40.10.60.4n.d.0.6
NiO0.30.11.20.50.10.70.6n.d.1.2
P2O50.30.10.70.30.10.60.3n.d.0.5
S3.80.76.73.90.65.83.9n.d.6.3
Total75.1  74.7  74.5  

Appendix 2

The average composition and the range of variation for each element in the chondrites determined by microprobe analyses (wt%).

 Tagish Lake carbonate-poorTagish Lake carbonate-richSemarkona ordinary chondriteBells CM chondrite1
Average2Min.Max.AverageMin.Max.Average3Min.4Max.4AverageMin.Max.
  1. n.d., not detected.

  2. – No data available from literatures.

  3. aLow total is due to high porosity.

  4. References: 1. Brearley (1995), 2. Zolensky et al. (2002), 3. Hutchison et al. (1987), 4. Matsunami (1984).

SiO239.3514.338.419.33.434.437.532.042.225.621.327.0
TiO2 0.09n.d. 0.2 0.1n.d. 0.2 0.1n.d. 0.2 0.1 0.1 0.1
Al2O3 3.56 1.0 2.5 1.40.3 2.6 3.3 1.5 4.3 2.2 2.0 2.6
FeO14.3411.536.219.38.336.025.916.825.632.327.039.0
MnO 0.2n.d. 0.3 0.2n.d. 0.5 0.2 0.1 0.4 0.1 0.1 0.2
MgO28.26 8.018.711.73.918.211.5 8.316.713.912.316.6
CaO 0.14 0.2 2.2 0.90.2 2.7 0.70.50.9 0.50.1 1.1
Na2O 0.13n.d. 0.3 0.2n.d. 0.6 2.52.02.9 0.50.3 0.7
K2O 0.05n.d. 0.1n.d.n.d. 0.1 0.60.20.7 0.1n.d. 0.1
Cr2O3 0.56 0.2 0.8 0.3n.d. 1.7 0.30.10.6 0.40.3 0.5
NiO 0.15 0.1 3.2 0.60.2 1.9 1.40.20.7 2.61.2 4.1
P2O5 0.04n.d. 0.3 0.1n.d. 0.3n.d.0.2 0.1n.d. 0.1
S 0.09 0.312.2 2.80.5 7.7 0.20.10.2 4.82.1111.3
Total86.96  56.8a  84.2  83.2  

Ancillary