1žĢ?'>>ABBC:\WORD4\STANDARD.DFVIBMQUIETC@¨KÅt?>Aĩ Micrometeorites *) Gero Kurat, Franz Brandst„tter and Thomas Presper, Naturhistorisches Museum, Burgring 7, A-1010 Vienna, Austria Christian Koeberl, Institut fr Geochemie, Universit„t Wien, Dr.Karl-Lueger-Ring 1, A-1010 Vienna, Austria Michel Maurette, C.S.N.S.M., Batiments 104-108, F-91405 Campus Orsay, France Submitted to Geologia i Geofizika January 1993 --------------------------------------------------------- *) A birthday present to the late Vladimir S. Sobolev Abstract Dust of the size between 50 and 500 æm is the dominant extraterrestrial matter accreted by the Earth today. A surprisingly large proportion of this dust reaches the Earth's surface in an (almost) unaltered state. Such unmelted micrometeorites (UMMs) can be collected from the Antarctic ice where they are accompanied by partially melted scoriaceous micrometeorites (SMMs), totally melted cosmic spheres (CSs), and variabel amounts of terrestrial dust. The UMMs comprise phyllosilicate-rich particles (about 60%) and coarse-grained crystalline particles consisting of the anhydrous minerals olivine and low-Ca pyroxene. Such mineralogical composition is comparable to the rare classes of CI and CM chodndites. The match between UMMs and CM chondrites is, however, not perfect. As compared to CM chondrites, UMMs, * have a much lower olivine/low Ca pyroxene ratio, * lack Ca-, Al-, and Ti-rich forsterites, * are depleted in Ca, S, Ni, and CO and some also in Na, Mg, and Mn, and * appear to be enriched in K, As, and Au. The mineralogical differences between UMMs and CM chondrites appear to be indigeneous and could indicate a new type of solar system matter. Comparison of UMMs with SMMs and CSs reveals that * the precursor of SMMs and CSs must have been similar to UMMs in mineralogy and chemical composition, * the volatile elements Na, K, Zn, Br, and Se are increasingly depleted in SMMs and CSs with increasing degree of melting, * the siderophile elements Ni and Co are much less depleted in SMMs but are much more depleted in some CSs * most CSs are not depleted in Ca, none in Mg, and on average they are not depleted in Mn and * enrichments in K, As, and Au of SMMs are common and similar to that in UMMs but less pronounced. The fact that SMMs and CSs do on average not show the elemental depletions exhibited by the UMMs is taken as to indicate terrestrial causes for these depletions. The principal mechanism is probably leaching of carbonates (loss of Ca, Mg, and Mn), sulfates (loss of S, Ni, CO, and Na), and possibly also halite (loss of Na). Partially and totally melted dust (SMMs and CSs) shows loss of volatile elements due to thermal volatilization (Na, K, Zn, Br, and Se) and occasional loss of siderophile elements due to seperation of an immiscible metal melt. The enrichment of UMMs and some SMMs in K, As, and Au is very likely of terrestrial origin. The source of these elements needs to be identified. The whole micrometeorite family ranging from UMMs over SMMs to CSs appears ot originate from the same primitive matter which bears some similarities to CM chondrites, but which is sufficiently different from the latter to be considered a hitherto unknown solar system matter. Considering the fact that this matter comprises 99% of today's infall on Earth, we are surprisingly ignorant of its pre-terrestrial history. Introduction: The most common matter falling onto the Earth today is dust in the size-range from about 20 to 500æm (e.g. Hughes, 1978). Although this matter by far outweighs all other meteoroid classes very little is known about its nature and its origin. The major cause of our ignorance is the curious fact that the most common extraterretrial matter was not available for study until very recently. Meteoroids entering the Earth's atmosphere at high velocity experience frictional heating, the degree of which depends on the meteoroid mass and its velocity. Theory predicts (e.g., Kornblum, 1969; Hughes, 1978; Love and Brownlee, 1991) that most meteoroids experience destruction by melting and partial evaporation and that only meteoroids of sizes well below 100æm diameter will survive atmospheric entry grossly unchanged. Such dust can be (and has been) collected from the lower stratosphere utilizing airplanes and ballons (Brownlee, 1985; Stephens et al., 1991). Since 1981 the collection of such interplanetary dust particles (IDPs) has grown to >1000 pieces. However, their typical size is between 5 and 10æm and therefore IDPs are not representative of the most common interplanetary dust. Completely melted cosmic matter has been known as cosmic spherules (CSs) for a long time to occur in a variety of environments (e.g., Brownlee, 1985). Such spherules haven been collected from deep sea sediments (recent and fossile ones), recent (and ancient) soils, and from snow and ice (e.g., Chevallier et al., 1987; Ivanov and Florenskiy, 1968; ElGoresy, 1968; Murell et al., 1980; Taylor and Brownlee, 1991; Koeberl and Hagen, 1989; Crozier, 1966; Thiel and Schmidt, 1961). They are virtually omnipresent albeit in different concentrations due to different degrees of dilution by terrestrial sediments and different rates of destructions. Cosmic spherules have been well studied. However, they are mostly referred to as "ablation spherules" produced by the partial melting of large meteoroides. This is certainly an incorrect assumption since micrometeoroides of the proper size ( 1 mm) are about 104-times more abundant (by mass, not number) than meteorite-producing meteoroids (e.g., Hughes, 1978; Halliday et al., 1989).The presence of cosmogenic iosotopes and solar energetic particle gases in a considerable fraction of CSs strongly supports this view (e.g., Maurette et al., 1989d, 1991; Nier et al., 1990; Nishiizumi et al., 1991, 1992). However, cosmic spherules, even though they were melted (and were somewhat chemically fractionated during the melting event), are a valuable sample of micrometeoroids of a size which is not available for study in any other form. Like IDPs, CSs do probably also not represent the most abundant interplanetary dust particles but are possibly representative of the dust in the size-fraction >1mm. Dust of the proper size (50-500æm) has recently been collected from Arctic and Antarctic ice (Maurette et al., 1986, 1989a, b, 1991, 1992a) and has been dubbed micrometeorites (MMs). Apparently a considerable fraction of particles in the MM size range is decelerated in the Earth's atmosphere without extensive heating - in contrast to theoretical predicitions. Preliminary studies showed that MMs are dominated by a carbonaceous chondrite matter resembling CM chondrites (Maurette et al., 1991; Alexander et al., 1992; Brownlee et al., 1991; Christophe Michel-Levy and Bourat-Denise, 1992; Kl”ck et al, 1992; Koeberl et al., 1992; Kurat et al., 1992a; Steele, 1992). Micrometeorites seem to be mineralogically and chemically similar to phyllosilicate-bearing IDPs (Brownlee, 1981, 1985, 1987; Brownlee et al., 1989; Bradley, 1988; Bradley and Brownlee, 1991; Bradley et al., 1988; Rietmeijer, 1992; Stephens et al., 1991). Samples of the very fine-grained anhydrous olivine aggregates - common among IDPs (e.g., Bradley et al., 1988; Thomas et al., 1992; Zolensky and Lindstrom, 1991) - have not yet been encountered among MMs. Micrometeorites provide a pristine sample of the most common interplanetary matter. It is the purpose of this report to present the first results of our combined investigation of a suit of MMs from Antarctica by instrumental neutron activation analysis and microanalytical techniques. For this report we make the following distinctions: cosmic spherules (CSs) are (almost) totally melted micrometeoroides, the scoriaceous micrometeorites (SMMs) are partly melted, foamy MMs, and the unmelted (although occasionally thermally metamorphosed) micrometeorites (UMMs) represent the coarse (50-400æm) particle size fraction (Fig.1). Unmelted cosmic dust collected in the stratosphere we will call interplanetary dust particles (IDPs) in accordance with today's common usage. Particles of sizes typical of IDPs ( 10æm) are not yet available from the icy Arctic and Antarctic micrometeorite ores. This situation, however, could change soon because the collection of < 25æm particles appears feasible (Maurette et al., 1992a). Samples and methods. During the Antarctic summers of 1987 and 1991 French teams collected about 60g of dust (particle-size > 25æm) from melt ice water produced from about 360 tonnes of ice (e.g., Maurette et al., 1991, 1992a). The dust was collected in the size-fractions 20-50 æm, 50-100 æm, 100-400 æm, and >400 æm which comprised 70, 20, 7 and 2% of the total sample, respectively. Contamination by terrestrial debris is largest in the coarsest size-fraction and decreases with particle size with about 20% of the grains <100 æm being extraterrestrial. This extraterrestrial sample consists of unmelted micrometeorites (UMMs - 50%), partially melted scoriaceous micrometeorites (SMMs - 30%), and totally melted cosmic spherules (CSs - 20%). The ratio of non-spherules (UMMs + SMMs) to spherules (CSs) is usually >5 in the 50-100 æm size fraction but lower among coarser-grained particles. The proportion between phyllosilicate UMMs and coarse-grained crystalline UMMs is about 2:1. Two collections of particles were prepared by hand-picking under the binocular from the 50-100 æm and 100-400 æm size fractions. Both spherules and black non-spherical particles were mounted in epoxy and polished and utilized for optical microscopy, scanning electron microscopy, and electron microprobe analysis. The sample numbers give the year of the mount preparation plus an identification letter (e.g., 89/A) or number (e.g., 91/2). One polished mount of grains from the 50-150æm size-fraction of the collection was prepared for statistical purposes and labeled Ku3. These mounts were first used to collect bulk and mineral analyses and texturally characterize the particles. A second set of particles was hand-picked from the 100-400 æm size fraction for instrumental neutron activation analysis (INAA). Analysis was performed as outlined by Koeberl et al.(1992). After cooling for several months the analyzed particles were mounted on a glass slide, carbon coated and investigated in a scanning electron microscope. Subsequently the samples were mounted in epoxy and polished for optical microscopy and electron microprobe analysis. Analyses were made with an ARL-SEMQ microprobe operated at 15kV acceleration potential and 15 nA beam current. Measurements were made against mineral standards and the usual corrections for matrix effects were applied. Mineralogical composition and mineral chemistry Unmelted micrometeorites (UMMs) mostly consist of phyllosilicates with variable amounts of anhydrous silicates. Extreme cases consist either solely of phyllosilicates or of anhydrous silicates. These two extremes will be referred to as phyllosilicate UMMs and (coarse-grained) crystalline UMMs. This distinction is necessary because the partially to totally melted particles are also "anhydrous". Thus the mineralogy of UMMs resembles carbonaceous chondrites of the CM type which are an approximate 1:1 mixture of hydrous and anhydrous silicates. There are no sufficient data on crystalline UMMs available so far. We will, therefore, not discuss them here. Some phyllosilicate dominated UMMs contain framboidal and plaquette magnetites (Kurat et al., 1992b) which resemble those commonly (but not exclusively) found in CI chondrites (Jedwab, 1967) (Fig.1). Phyllosilicates have grain-sizes ranging from 1 æm to >5 æm and usually are fairly densely intergrown. They consist mainly of serpentine and saponite with minor contributions by other phases and are commonly intergrown with Fe-oxide phases. The most common one is tochilinite, a complex oxide containing Si,P,Mg, S, and other elements, the typical oxide phase of CM carbonaceous chondrites (Zolensky, 1987). Less common is magnetite which displays a variety of morphologies like framboids and plaquettes. An usual morphological type of magnetite, dubbed "Swiss-Cheese-magnetite" by Kurat et al.(1992b), has not yet been identified in meteorites. It is characterized by abundant round voids filled with phyllosilicates and is closely associated with ferroan brucite. The third type of magnetite envelops the micrometeorites. It is best developed at the surface of partly melted micrometeorites but is also present at the surface of UMMs (Fig.2). This magnetite is unique to micrometeorites. Anhydrous silicates are commonly present as isolated grains in phyllosilicate-dominated UMMs and as relics in partially melted SMMs. Olivine and low-Ca pyroxene are the most common phases. Plagioclase and Ca-rich pyroxenes are rare. The olivine/ orthopyroxene ratio, however, appears to be much smaller than usually found in carbonaceous chondrites (Presper et al., 1992; Christophe Michel-Levy and Bourot-Denis, 1992). Very rare are phases commonly present in refractory Ca-Al-rich inclusions such as spinel, perovskite, melilite, and Al-rich pyroxenes. Sulfides are present mainly as fine-grained pyrrhotite and pentlandite intergrown with the phyllosilicates. Occasionally larger grains ( 10-20 æm) of pyrrhotite are present. Metals are rare and are confined to inclusions in anhydrous silicates or to phyllosilicate-free particles. Carbonates and sulfates - which are common in carbonaceous chondrites (e.g., Bostroem and Fredriksson, 1966; Fredriksson et al., 1980; Dodd, 1981; Brandst„tter et al., 1987, 1992) - are usually not present in UMMs. Dolomite and calcite have been observed only in a few IDPs but so far they have not been found in micrometeorites (Tomeoka and Buseck, 1985, 1986; Hartmetz et al., 1990; Rietmeijer, 1990; Thomas et al., 1991;). In general, phyllosilicate-rich UMMs display large voids, sometimes in the shape of crystals which were possibly the sites where carbonates resided and from which they were leached in the terrestrial environment (see discussion below). The mineralogical compositions of UMMs suggest that the majority of them is similar to CM carbonaceous chondrites, a fairly rare class of chondrites. However, a clear difference between UMMs and CM chondrites is the olivine/low-Ca pyroxene ratio which is much lower in UMMs than in CM chondrites. A small proportion of UMMs resembles CI carbonaceous chondrites and some anhydrous UMMs could be fragments of CV3 chondrites or unequilibrated ordinary chondrites. No clear indication for the presence of particles of an achondritic heritage has been found so far but that could be a result of our selection criteria. Mineral chemistry Phyllosilicate compositions are characterized by nearly chondritic abundances of several minor elements such as Al, Ti, and Cr. However, they are depleted in alkalies, Ca, and Ni (Table 1). Furthermore, their Fe/(Fe+Mg) ratios can vary considerably, indicating partitioning of Mg into another phase (probably dolomite). The Fe/Mn ratio shows a similar tendency with a range from chondritic to super-chondritic - again apparently due to preferred partitioning of Mn into carbonates, especially dolomite. Thus, the composition of phyllosilicates recorded the degree of carbonatization of a given UMM. Although the carbonates have mostly been lost by terrestrial leaching, their influence on the chemical composition of phyllosilicates is preserved. Chemical compositions of anhydrous silicates are typically Fe-poor and rich in minor elements (Table 1). Olivine compositions range from Fo0.5 to Fo50 but are most commonly between Fo1-7 (Fig.2). Most FeO-poor olivines contain ~0.2wt % CaO and ~0.5 % Cr2O3, but are poor in Al2O3 and TiO2. Olivines with intermediate FeO contents (8-26 wt.%) contain in addition some NiO, as do the FeO-rich olivines. A few grains with high FeO and NiO ( ~0.4 wt.%) contents could indicate the presence of CK chondrite matter (e.g., Kallemeyn et al., 1991; Kurat et al., 1991). The FeO/MnO ratios are usually low (< 50) for most low-FeO olivines, indicating an overabundance of MnO (Fig.3), similar to what has been reported for isolated olivines from carbonaceous chondrites and from IDPs (Christophe Michel-Levy and Bourot-Denis, 1992; Thomas et al., 1990, 1991, 1992; Kurat and Kracher, 1975; Kurat et al., 1989; Steele, 1991, 1992). Low-Ca pyroxenes have, on average, lower Fe/Mg ratios as compared to olivines (Table 1, Fig.3). They are typically rich in minor elements (Ca, Al, Cr), similar to pyroxenes from carbonaceous chondrites. However, Fe/Mn ratios - although mostly within the range known from carbonaceous chondrites - sometimes approach unity. A large proportion of low-Ca pyroxenes has a pigeonitic composition with higher minor element contents. High-Ca pyroxenes are much less abundant than low-Ca pyroxenes. Their composition can be either diopsidic (with some Al, Ti, Cr) or Al-augitic (high Al, Ti) (e.g., Brandst„tter et al., 1991; Presper et al., 1992; Maurette et al., 1991). The latter composition is clearly related to refractory mineral associations commonly present in carbonaceous chondrites comprising Al-augite, Mg-spinel, and perosvkite. Chromium-rich spinels are common in partially or severly melted MMs and crystalline UMMs but are rare in phyllosilicate-rich UMMs. The composition of the most common spinel, magnetite, ranges from pure, stoichiometric, Fe3O4 to iron oxide with high contents of Mg, Al, Si, Cr, and Mn and unknown Fe/O ratio. Some correlation exists between the morphological type of magnetite and its composition: Plaquettes and framboids from CI-like MMs are relatively poor in minor elements, the Swiss Cheese-type magnetite is rich in Mg (+Si, Mn) and grades in composition into ferrobrucite (Kurat et al., 1992b). Magnetites replacing metal commonly contain some Ni and tend to compositionally approach tochilinite ( containing Mg, Al, Si, S, P, Cr, Mn) (compare Zolensky, 1987). The rim-forming magnetite (the magnetite enveloping MMs) is highly variable in composition but usually contains appreciable amounts of Mg, Al, Si, and other elements (Tab.2). The most common sulfide is Ni-free pyrrhotite, less common are Ni-bearing pyrrhotite and pentlandite. Metals commonly have low-Ni contents (Ni ~5 wt.%) and Fe/Ni and Ni/Co ratios close to the primitive (CI) values. In summary, the chemical composition of minerals in UMMs are close to those known from CM and CI chondrites. However, the compositional ranges of olivines and pyroxenes appear to exceed those of CM chondrites. Furthermore, there apparently is a lack of very high-Mg olivines rich in refractory minor elements (Al, Ti, Ca) in UMMs (see also Steele, 1992). This together with the low olivine/low-Ca pyroxene ratio of UMMs clearly distinguishes them from CM chondrites with which they otherwise share many characteristics. The best match is apparently provided by the Kaidun chondrite, a unique CM-CI-EC chondritic breccia (e.g., Ivanov, 1989; Brandst„tter et al., 1992). Bulk chemical composition A variety of bulk chemical data exists for cosmic spherules, micrometeorites, and IDPs. Techniques include analytical transmission electron microscopy, electron microprobe analysis (EMPA), synchrotron X-ray fluorescence analysis, proton-induced X-ray emission analysis, secondary ion mass spectrometry, and INAA (e.g., Bradley, 1988; Brandst„tter et al., 1991; Chevallier et al., 1987; Flynn and Sutton, 1987, 1990, 1992a,b,c; Ganapathy and Brownlee, 1979; Jessberger et al., 1992; Kl”ck et al., 1992; Koeberl and Hagen, 1989; Koeberl et al., 1992; Kurat et al., 1992a,b; Maurette et al., 1992b; Maurette and Passoja, 1989; Papanastassiou et al., 1983; Robin et al., 1988; Schramm et al., 1989; Sutton and Flynn, 1988, 1989; Thomas et al., 1992; Wallenwein et al., 1989). The general result of the studies is that micrometeorites, IDPs, and cosmic spherules keep a memory of the solar nebula in their primitive (chondritic) chemical composition. However, some differences between the compositions of chondrites and that of micrometeorites do exist. The questions is, why they do exist and how they came into being. Table 2 gives some representative chemical bulk and rim compositions of several UMMs from sample collection 89A as obtained by EMPA. The elemental abundances are at first approximation chondritic. The totals are low, indicating the presence of H- and C-bearing phases. In Fig.4 the compositions given in Tab.2 and some additional compositions of UMMs from the same sample are compared to compositions of carbonaceous chondrites (from Palme et al., 1981, and Wasson and Kallemeyn, 1988). Although there is quite a spread in the data, the best fit appears to be for CM chondrites - in accordance with mineral chemical data as outlined above. However, UMMs are depleted in Ca, Mn, Na, and Ni as compared to CM chondrites. The existence of most of these depletions was known before, except for Mn and some depletions in Mg (Kurat et al., 1992). The questions arising are whether these depletions are primary (extraterrestrial) or of a secondary (terrestrial) origin. A comparison of the compositions of UMMs with those of cosmic spherules ( Table 3, Fig.5) could provide some answer because the spherules were produced from micrometeoriods during their entry into the Earth's atmosphere. Deviations of their chemical compositions from that of chondrites must be either related to the melting event or could be primary. From the data shown in Fig.5 it is evident that most refractory lithophile elements are enriched in CSs over CI levels to an extent comparable to enrichments displayed by CO chondrites. Calcium follows that trend in some but not in all cases as it is depleted in several CSs. Since Ca is not a volatile element, the depletion of Ca in some spherules cannot be the result of high temperature melting. Consequently, the Ca abundance pattern observed among CSs must reflect primary compositional differences. From investigations of matrices of carbonaceous chondrites (McSween and Richardson, 1977; Brandst„tter et al., 1987, 1992) it is evident that Ca strongly partitions into carbonates (calcite and dolomite) which commonly form large poikiloblasts. Sampling of such a matrix in small pieces will produce a pronounced variability in Ca contents and also some depletions. This apparently is an appropriate explanation for the Ca variability in CSs. However, such a process cannot explain why all UMMs are depleted in Ca. An additional terrestrial depletion mechanism must have been active. A possible clue for answering this question is provided by the general lack of carbonates in UMMs and the presence of abundant large voids which often show angular outlines (e.g., Kurat et al., 1992). Apparently, carbonates were destroyed either in the upper atmosphere by sulfuric aerosols (e.g., Lazrus and Gandrud, 1977) or, more likely, by the melt ice water forming during their storage in the ice or during their recovery from it. This mechanism could also be responsible for the partial depletion of Mn in UMMs, because Mn strongly partitiones into dolomite coexisting with phyllosilicates (e.g., Fredriksson and Kerridge, 1988; Brandst„tter er al., 1987, 1992). The variable but common depletion of Na in UMMs could be due to terrestrial leaching of a soluble Na-bearing salt such as NaCl or Na-bearing sulfate, phases which are common in CI and CM chondrites (Bostroem and Fredriksson, 1966; Fredriksson and Kerridge, 1988) but which are usually not present in UMMs (we have encountered only two UMMs containing rare NaCl grains but none which contained sulfates). The depletion of Na in CSs is much more pronounced than that in UMMs - obviously the result of volatilization during melting. The correlation of Na content with the number of bubble-like voids in CSs strongly supports this interpretation. Trace element contents of MMs as determined by INAA (Koeberl et al., 1992; Kurat et al., 1992a) support the conclusions drawn from bulk major and minor element data. Selected major and trace element data for UMMs and SMMs are given in Table 4. The refractory lithophile trace element contents of unmelted and scoriaceous micrometeorites (Fig.6) are very similar to those of CM chondrites. For the UMMs (Fig.6a), the abundances of moderately volatile and volatile elements fit those of CM chondrites with the possible exception of K, which is enriched in UMMs. The SMMs (Fig.6b) are generally depleted in volatile elements (Na, Zn, Br) to different degrees as compared to the UMMs. Here too, the K contents are anomalously high, which can be taken as an indication that K has a terrestrial source. Not all SMMs are depleted in all volatile elements. Particle M5 (Fig.6b) is depleted in Na (as are all other SMMs) but is enriched in Zn and Br. Enrichments in Br (and Cl) are common among IDPs (van der Stap et al., 1986; Wallenwein et al., 1989; Flynn and Sutton, 1987, 1990,1991, 1992c; Sutton and Flynn, 1988, 1989) and have mostly been interpreted as being of extraterrestrial origin. However, Jessberger et al., (1992) could convincingly show that Br and Cl enrichments of IDPs are very likely of terrestrial origin, a contamination from within the atmospheric E-layer which is rich in meteoroid vapor (e.g., Hunten et al., 1980; Steinweg et al., 1992). Many of the small micrometeorites (50-150 æm) seem to be contaminated in a way similar to IDPs (Flynn et al., 1992). The majority of large MMs (>150 æm) as analyzed by us does not show such enrichments (Fig.6). The refractory siderophile elements in both UMMs and SMMs have abundances similar to that of CI chondrites (Fig.7). Nickel and Co are in general depleted as compared to Ir with Ni usually more strongly depleted than Co. The Ni, Co depletion is surprisingly more severe in UMMs than in the SMMs which may be an indication that the Ni, Co depletion has mainly terrestrial causes. One possibility could be the leaching of Ni, Co-bearing sulfates which are common in CM and CI chondrites (Fredriksson et al., 1980). Such a mechanism could also account for the general S depletion of UMMs (Kurat et al., 1992a). An alternative might be terrestrial weathering of sulfides (Kl”ck et al., 1992). The Fe contents of UMMs and SMMs are about chondritic with a slight tendency towards super-chondritic abundances. This tendency is apparently stronger for the SMMs as compared to the phyllosilicate-dominated UMMs. Such an enrichment could be due to increased deposition of magnetite envelopes on SMMs as compared to UMMs (Fig.2). Gold and As contents are almost always above CI and CM levels. Selenium is commonly depleted compared to CI and CM abundances; however, this depletion is much less severe than that for S. Clearly, the melted MMs are more strongly depleted in Se than the unmelted ones - obviously a consequence of the melting event. Micrometeorites and larger meteoroids Extraterrestrial matter accretes onto the Earth in a variety of different sizes and chemical compositions. There appears to be a correlation between size and composition. The largest objects (>1018g, 10 km diameter) thought to be responsible for worldwide mass extinctions like that at the Cretaceous-Tertiary boundary are believed to be mainly comets (e.g., Davis et al., 1984; Hut et al., 1987) although asteroids are favoured by others (e.g., Shoemaker and Izett, 1992). Less massive crater-forming objects (1012-1015g, 102-103m diameter) tend to be dominated by differentiated meteorites (e.g., Grieve, 1982; Palme, 1982). Meteorite-producing meteoroids (102-109g, 10-2-10m diameter) are dominated by matter of ordinary chondrite composition (e.g., Dodd, 1981). Meteor-producing meteoroids (mass range about 10-5-10g) are not very well characterized but seem to contain a high proportion of low density, carbonaceous matter-like material (e.g., Harvey, 1973; Millman, 1979). Micrometeorites (MMs) with a mass between 10-5 and 10-8g are dominated by carbonaceous chondrite matter resembling CM chondrites. CM-type matter is also present among the interplanetary dust particles (IDPs) collected in the stratosphere. About 50% of IDPs consist of CM-type matter, the other 50% of very fine-grained anhydrous olivine aggregates which are believed to represent typical cometary dust (e.g., Bradley et al., 1988; Thomas et al., 1992; Zolensky and Lindstrom, 1991). In this compilation it is surprising to find that comets are probably contributing to the largest and the smallest objects accreted by the Earth. Also surprising is the fact that the most common meteorites, the ordinary chondrites, appear not to be represented among the interplanetary dust and that the most common dust bears mineralogical and chemical similarities to a very rare chondrite class, the CM chondrites. The apparent correlation between mass and chemical/mineralogical composition of small solar system bodies cannot be fortuitous but rather must reflect primary formation conditions. Conclusions Micrometeorites are the most common matter accreting on the Earth today. In their mineralogical and chemical composition, the micrometeorites resemble CM (and occasionally CI) chondrites. This is surprising because CM chondrites comprise just 2% of the total meteorite flux (Dodd, 1981). A surprisingly large fraction of large micrometeoroids is decelerated in the Earth's atmosphere in such a gentle way as to reach the Earth's surface almost unaltered. The mineralogy, mineral chemistry, texture, and bulk major and trace element contents are similar to CM (and sometimes CI) chondrites. However, some differences exist, which are due to solar nebula processes and to particle-terrestrial environment interaction. The high abundance of low-Ca pyroxene in micrometeorites as compared to CM chondrites (with the exception of Kaidun - see, e.g. Brandst„tter et al., 1992) is certainly a primary feature which distinguishes micrometeorites from chondrites. The lack of very FeO-poor, Ca-, Al-rich olivines in micrometeorites is probably also a primary feature. All other deviations of MM properties from those of CM chondrites appear to be of terrestrial heritage. The unmelted MMs are generally depleted in Na, Ni, Co, and S which presumably is due to leaching of NaCl and Na, Ni, and Co-bearing sulfates in the terrestrial environment. High-grade depletions in Na and other volatile elements (Zn, Br, Se) appears to be due to the melting event . Probably all enrichments over CI, CM levels observed in unmelted MMs (K, Au, As) and UMMs (K, Au, As, and occasionally Zn, Br, and Se) are due to precipitation of these elements onto the particles in the terrestrial atmosphere, mainly in the E-layer which is enriched in all these elements from meteoroid evaporation. This process has previously been suggested to be responsible for the omnipresent Br and Cl enrichments among IDPs (Jessberger et al., 1992). Our data indicate also a general enrichment of Fe in micrometeorites (see Figs. 4 and 7). Since many micrometeorites are enveloped by a magnetite cover, particularly the scoriaceous ones (e.g., Maurette et al., 1991) and many show an enrichment in Fe from the surface into the interior, the Fe-enrichment is not surprising. The magnetite envelop of micrometeorites (Fig.2a) is obviously a precipitate very likely of an Fe-enriched vapor in the atmospheric E-layer (e.g., Hunten et al., 1980). Although we have to be very careful in interpreting micrometeorite data, there seems to be now little doubt that some compositional features of micrometeorites are primordial. These features set them apart from the early solar system matter represented by meteorites. Thus, micrometeorites are not only the dominating extraterrestrial matter accumulated by the Earth today, but they also represent a new and previously unknown type of solar system matter. We are far from fully understanding micrometeorite genesis but some mist seems to clear. This new, exciting, and important extraterrestrial matter deserves proper (if possible, concerted) investigation by all methods available today. Acknowledgements This work was supported by the Fonds zur F”rderung der wissenschaftlichen Forschung (project P8125-GEO) in Austria and the European Economic Community SCIENCE (Twinning and Operations) Program (Contract Mo.SCI*-CT91-0618,SMM) and IN2P3 in France. Figure captions: Figure 1: Scanning electron microscope images of polished micrometeorites from Cap Prudhomme, Antarctica. a) SE image of a phyllosilicate-dominated UMM (sample Ku3-6) containig abundant magnetite of characteristic shapes (plaquettes, framboids, and isolated small spheres - all bright grey or white), and some sulfides (light grey, irregular or rectangular shape) resembling CI chondrites (see also Kurat et al., 1992b). The surface is thermally altered and degassed in places. The particle is 120 æm long. b) BSE image of a coarse-grained crystalline UMM (sample M1), consisting of Fe-bearing olivine and pyroxene (grey), calcic plagioclase (dark grey), and opaques (white). The latter are chromite, magnetite, sulfide, and metal. Almost no Fe-rich rim is developed. c) SE image of a scoriaceous micrometeorite (sample M2) which consists mainly of a foamy, glass-rich matrix. Relictic anhydrous minerals are present (esp. at the left). These relics are Fe-poor olivines and low-Ca pyroxenes. A magnetite rim is well developed in places. d) BSE image of a cosmic sphere (sample 91/3-91) consisting of olivine (mostly dark grey), magnetite (white), and interstitial glass (grey). Figure 2: Scanning electron microscope images of magnetite covers on micrometeorites. a) Magnetite covering the surface of micrometeorite M2; several generations appear to have been precipitated; SE image. b) Magnetite rim on phyllosilicate-dominated UMM (sample 91/1-119); magnetite is white; on top of the magnetite abundant COPS, an iron oxide rich in C, O, P, S, and a variety of minor elements, precipitate (grey) is present (Maurette et al., 1989c; Perreau et al., 1992). Note also the large Fe-rich pyroxene at upper right; BSE image. Figure 3: Chemical composition of olivines (a) and low-Ca pyroxenes (b) from micrometeorites from Antarctic ice (samples 91/1 and 91/2) in the MnO vs.FeO (wt.%) diagram. Note the low Fe/Mn ratios of the Fe-poor silicates. Figure 4: Bulk chemical compositions of Antarctic UMMs (sample 89A) obtained by EMPA are normalized to CI composition and compared to CM, CO, and CV chondrite compositions (Wasson and Kallemeyn, 1988; Palme et al., 1981). CM chondrites are most similar to UMMs despite the large compositional variation of the latter. Note the depletions in Ca, Mn, Na, and Ni in UMMs as compared to chondrites. Fe has a tendency towards being enriched over chondritic compositions. Figure 5: Bulk chemical compositions of Antarctic cosmic spherules from sample 89B obtained by EMPA are normalized to CI composition (Palme et al., 1981). The elements are arranged in order of increasing volatility from left to right. Siderophile elements Fe and Ni are shown separately. The refractory lithophile elements are generally enriched in CSs, the volatile element Na is strongly depleted. However, a small group of CSs is also depleted in Ca. Note the variability in Cr and Mn contents. The spheres containing Na are all foamy (transitional from scoriaceous MMs). The depletion in Ni is omnipresent and extends to very low levels (apparently due to separation of metal during melting). Figure 6: Lithophile trace element contents of micrometeorites (Tab.4) normalized to CI abundances (Palme et al., 1981). Elements are arranged in order of increasing volatility from left to right, except for the rare earth elements. a) Unmelted, phyllosilicate-dominated micrometeorites and CM chondrite composition (from Wasson and Kallemeyn, 1988). b) Scoriaceous micrometeorites. Figure 7: Siderophile trace element contents of micrometeorites (Tab.4) normalized to CI abundances (Palme et al., 1981). Elements are arranged in order of increasing volatility from left to right. a) Phyllosilicate-dominated UMMs compared CM bulk composition (from Wasson and Kallemeyn, 1988, and Mason, 1971). b) Scoriaceous micrometeorites. 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