Chondritic micrometeorites from the Transantarctic Mountains

Authors

  • Matthias Van GINNEKEN,

    Corresponding author
    1. Museo Nazionale dell’Antartide, Università di Siena, Via Laterina 8, 53100 Siena, Italy
    2. Present address: Korea Polar Research Institute, Get-pearl Tower, 12 Gaetbeol-ro, Yeonsu-gu, Incheon 406-840, Korea
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  • Luigi FOLCO,

    1. Museo Nazionale dell’Antartide, Università di Siena, Via Laterina 8, 53100 Siena, Italy
    2. Present address: Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria, 52126 Pisa, Italy
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  • Carole CORDIER,

    1. Museo Nazionale dell’Antartide, Università di Siena, Via Laterina 8, 53100 Siena, Italy
    2. Present address: ISTerre, CNRS, Université Joseph Fourier de Grenoble, BP 53, 38041 Grenoble Cedex 09, France
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  • Pierre ROCHETTE

    1. CEREGE, CNRS Aix-Marseille Université, PB80 13545, Aix en Provence, Cedex 4, France
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Corresponding author. E-mail: vanginneken@kopri.re.kr

Abstract

Abstract– On the basis of morphological and petrographic characteristics, eight “giant” unmelted micrometeorites in the 300–1100 μm size range were selected from the Transantarctic Mountain micrometeorite collection, Victoria Land, Antarctica. Mineralogical and geochemical data obtained by means of scanning electron microscopy, electron probe microanalyses, and synchrotron X-ray diffraction allow their classification as chondritic micrometeorites. The large size of the micrometeorites increases considerably the amount of mineralogical and geochemical information compared to micrometeorites in smaller size fractions, therefore allowing a better definition of their parent material. A large variety of material is observed: five micrometeorites are related to unequilibrated and equilibrated ordinary chondrite, one to CV chondrite, one to CM chondrite, and one to CI chondrite parent materials. Besides reporting the first occurrence of a CV-like micrometeorite, our study shows that the abundance of chondritic material supports observations from recent studies on cosmic spherules that a large part of the micrometeorite flux in this size range is of asteroidal origin.

Introduction

Micrometeorites are extraterrestrial particles 10 μm to 2 mm in size which represent the most important part of the flux of extraterrestrial material to accrete onto Earth’s surface (Yada et al. 2004; Rubin and Grossman 2010). Compared to other micrometeorite collections from Antarctica (Koeberl and Hagen 1989; Maurette et al. 1991; Taylor et al. 2000; Duprat et al. 2007), the Transantarctic Mountain (TAM) micrometeorite traps have been collecting particles for a much longer period, i.e., over approximately the last 1 Ma (Folco et al. 2008, 2011; Rochette et al. 2008; Suavet et al. 2010). Such an extended collection time, in an environment which is particularly favorable for the preservation of geological materials like Antarctica, allows the observation of a wider range of extraterrestrial material, including a large number of “giant” unmelted micrometeorites from 400 μm to approximately 2000 μm in size (Rochette et al. 2008) which are underrepresented, or even missing, in other collections (e.g., Taylor et al. 2005).

Unmelted micrometeorites have suffered little alteration during atmospheric entry heating, compared to the more abundant cosmic spherules which experienced total melting. Compared to cosmic spherules, unmelted micrometeorites allow us to define more accurately the nature of their parent material in space, and are thus of great interest for exploring the inventory of the matter present in the solar system. Several attempts to establish their association with other classes of meteorites in the solar system have been made by using mainly the mineralogical and petrographic properties of relatively small unmelted micrometeorites less than 300 μm in size (Kurat et al. 1994; Genge et al. 1997, 2005; Genge 2008a).

Herein we report a petrographic study of eight unmelted micrometeorites recovered from the TAM micrometeorite traps ranging in size from 300 to 1100 μm. These “giant” micrometeorites represent a new material for planetary science and an opportunity to explore micrometeorite diversity. Furthermore, the large amount of petrographic information available from these large particles permits a better definition of their parent bodies. Finally, the occurrence of unmelted chondritic micrometeorites in the above size range supports observations derived from micrometeoroid entry-heating models (e.g., Love and Brownlee 1991).

Samples and Methods

The micrometeorites studied in this work were found in micrometeorite traps at the top surface of Miller Butte (72°42.078′ S, 160°14.333′ E) and Frontier Mountain (72°59.282′ S, 160°20.166′ E), Victoria Land TAM during the 2006 Programma Nazionale di Ricerche in Antartide (PNRA) expedition for the search for micrometeorites and microtektites. A detailed description of the geological setting of the Frontier Mountain and Miller Butte micrometeorite traps is given in Rochette et al. (2008) and Van Ginneken et al. (2010), respectively. The micrometeorite traps consist of eroded joints and fractures partially filled with disaggregated, fine-grained bedrock detritus. Five detritus samples having a total mass of 37.9 kg (namely samples #2, #4, #5, #19 and #20c) were collected using a small shovel, a spatula, a paintbrush, and sieves. The collected detritus samples were washed in water, dried using vacuum pumping, sieved in the 100, 200, 400, 800 μm size fractions, and inspected under a stereomicroscope. Six unmelted micrometeorites from the greater than 400 μm size fraction, namely #4.14, #5.11, #5.22, #5.31, #19.11, and #20c.343, and two from the greater than 200 μm size fraction, namely #2.1i and #2.1c, were selected on the basis of morphological and petrographic characteristics for this study.

The micrometeorites were first mounted onto a conductive tape, C-coated, and observed under a Scanning Electron Microscope (SEM-EDS) Philips XL30 at Siena University to gather information on morphology and overall structure.

Subsequently, the fine-grained particle #2.1c was mounted on a glass capillary and analyzed using synchrotron X-ray diffraction (XRD) at the BM8 GILDA beamline at ESRF, Grenoble. XRD patterns were recorded in the 2–49° 2θ range, using an exposure time of 900 s, with a Fuji Imaging-Plate detector using a monochromatic beam (λ = 6.8780 nm), calibrated against X-ray absorption of pure metal foils. The sample-to-detector distance and the image plate tilt were calibrated with X-ray powder diffraction of standard LaB6 (NIST-SRM 660a). Data were reduced with the Fit2D software. The synchrotron XRD spectra were examined for phase identification through the EVA (Bruker-AXS) software. A Rietveld study using the TOPAS-Academic software was performed to determine the relative abundances of crystalline phases in particle #2.1c (Coelho 2007). For all modeled phases, structural data were taken from the literature, and held fixed during the refinement, with cell parameters refined for clinoenstatite, magnetite, and troilite.

All the particles were then embedded in epoxy, sectioned, and polished for petrographic study using the SEM-EDS at Siena University and a JEOL 6500F Field Emission Gun Scanning Electron Microscope (FEG-SEM) at Istituto Nazionale di Geofisica e Vulcanologia (INGV) in Rome.

The major element bulk compositions of particles #4.14 and #2.1c were obtained by averaging multiple (typically >10) electron probe microanalyses (EPMA) using a Cameca SX50 electron microprobe at the Consiglio Nazionale delle Ricerche (CNR) in Padova. Running conditions were 15 kV accelerating voltage, 15 nA beam current, and 10 μm beam spot. In particular the bulk composition of particle #4.14 was obtained through 184 spot analyses on a rectangular grid over a chemically representative area of about 7.104 μm2.

The mineral chemistry of all particles was determined by EPMA using a JEOL JXA 8200 Superprobe at INGV in Rome. Oxides, metals, and silicates were analyzed using a 15 kV accelerating voltage, a 7 nA beam current and a nominal beam spot of 1 μm. For all major elements analysis, the ZAF procedure was used for raw data reduction. A number of synthetic and natural standards were used for instrumental calibration.

Structure and Morphology

The eight particles studied in this work range in size from 300 μm (#2.1c) to 1100 μm (#5.31) and exhibit various morphologies and structures (Fig. 1). Four particles show angular morphologies (#5.11, #20c.343, #2.1c, and #4.14), one is subangular (#5.31), one is subrounded (#19.11) and two are rounded (#2.1i and #5.22). Particle #2.1i shows a globular morphology and particle #5.22 has a prominent scoriaceous structure. All particles except #5.11 show continuous morphologies (i.e., devoid of major broken surfaces like particles #20c.343, #2.1i, and #5.22) or almost continuous morphologies (like particles #2.1c, #4.14, and #5.31) as observed in complete individuals. Likewise, five particles, namely #20c.343, #4.14, #5.31, #2.1i, and #5.22, exhibit shells of microscopic magnetite crystals which almost continuously surround them (Figs. 2a and 2b).

Figure 1.

 Backscattered electron images of the studied micrometeorites. The samples are ordered according to morphology, namely from particles with angular shapes to particles with rounded shapes. a) Particle #5.11. b) Particle #20c.343. c) Particle #2.1c. d) Particle #4.14. e) Particle #5.31. f) Particle #19.11. g) Particle #2.1i. h) Particle #5.22.

Figure 2.

 Close-up backscattered electron images of the surfaces of particles #4.14 (a) and #5.22 (b) showing magnetite shells.

Petrographic Observations

Particle #4.14

In section, particle #4.14 shows chondritic structure with two whole well-defined type-I porphyritic olivine and pyroxene (POP) chondrules and many chondrule fragments up to 240 μm in size set in a fine-grained matrix (Fig. 3a). The matrix to chondrule volume ratio is about 1.2. An almost continuous array of micron-sized magnetite crystals, about 5 μm thick, occurs at the edge of the studied polished section and forms the magnetite shell described in the previous section.

Figure 3.

 a) Backscattered electron image of sectioned particle #4.14, consisting of abundant POP chondrules set in a fine-grained matrix. A clast of FeO-rich olivine and a subhedral chromite grain occur in the matrix (FeOl and Chr). The particle is surrounded by a thin magnetite rim (arrowed). b) Close-up view of the core of a type-I POP chondrule consisting mainly of microphenocrysts of olivine and enstatite set in a glassy mesostases with abundant dendrites Fe-rich opaque phase. Minor augite occurs associated or intergrown with enstatite. c) A magnetite grain close to the edge of the particle showing decomposition texture of a formerly associated mineral phase likely lost during atmospheric entry. d) Close-up view of the matrix consisting mainly of euhedral to subhedral micron-sized grains of FeO-rich olivine, locally showing normal zoning (arrowed), and lesser Ni-bearing iron-sulfide grains with dominant interstitial setting.

Type-I POP chondrules consist mainly of microphenocrysts of Mg-rich olivine Fa3.2 ± 0.8 and enstatite Fs1.5 ± 0.2 Wo1.0 ± 0.1 (Tables 1 and 2) set in a glassy mesostasis with abundant dendrites of Fe-rich opaque phase. Minor augite Fs1.9 ± 0.6 Wo40.7 ± 1.9 occurs associated or intergrown with low-Ca pyroxene (Fig. 3b). Iron oxide grains up to 30 μm in size frequently occur within chondrules (Fig. 3c). Their overall outline is generally subhedral; crystal boundaries are, however, finely embayed and contain or are surrounded by voids, as typically observed in decomposition textures. Fe-oxides consist of wüstite at the core of the particle, whereas two grains at its edge are magnetite (Fig. 3c; Table 3). Micrometer-sized Fe-Ni metal blebs rarely occur enclosed in few olivine microphenocrysts.

Table 1.   Electron microprobe analyses composition of olivine (oxide wt%) from the studied micrometeorites.
SampleSettingNSiO2TiO2Al2O3Cr2O3FeOMnOMgOCaONa2OK2ONiOS (el wt%)TotalFa
  1. Notes: n.d. = not determined; N = number of analyzed crystals; Avg = average composition; S.D.  = standard deviation.

4.14Chondrule5Avg42.7<0.03<0.020.233.160.1054.10.20n.d.n.d.<0.08n.d.100.53.2
S.D.0.30.020.740.000.70.000.50.8
4.14Matrix5Avg31.60.143.510.4239.30.3517.80.520.570.090.311.6398.755.3
S.D.1.30.070.760.083.10.071.10.100.210.040.080.397.93.4
4.14Clast in matrix1 36.3<0.030.120.2034.50.5130.30.330.050.07n.d.n.d.102.439.0
2.1i7Avg42.8<0.03<0.020.511.200.1955.50.27n.d.n.d.<0.08n.d.100.61.2
S.D.0.90.120.100.140.80.090.80.1
2.1iIgneous rim1 32.10.081.760.3847.40.5015.91.960.190.020.370.28101.462.5
5.22Coarse-grained4Avg39.90.05<0.020.4012.00.6646.50.18n.d.n.d.<0.08n.d.99.712.6
S.D.0.30.040.191.60.041.40.020.91.8
5.11Chondrule and matrix20Avg39.80.05<0.020.0717.80.4542.90.05n.d.n.d.<0.08n.d.101.218.8
S.D.0.40.060.080.50.050.60.060.50.6
5.31Chondrule and matrix9Avg38.4<0.03<0.02<0.0622.30.4637.9<0.03n.d.n.d.<0.08n.d.99.524.9
S.D.1.20.80.061.20.90.9
19.1110Avg40.4<0.03<0.02<0.0616.50.5143.5<0.03n.d.n.d.<0.08n.d.101.017.4
S.D.0.50.40.041.51.00.6
20c.3439Avg39.0<0.03<0.02<0.0622.60.4738.9<0.03n.d.n.d.<0.08n.d.101.024.6
S.D.0.60.30.050.50.70.3
Table 2.   Electron microprobe analyses composition of low-Ca and high-Ca pyroxene (oxide wt%) from the studied micrometeorites.
SampleSettingNSiO2TiO2Al2O3Cr2O3FeOMnOMgOCaONa2ONiOTotalFsWo
  1. Notes: n.d.  = not determined; N = number of analyzed crystals; Avg = average composition; S.D.  = standard deviation.

4.14Chondrule7Avg58.80.311.220.361.060.0438.00.55n.d.<0.08100.21.51.0
S.D.0.90.060.190.030.160.020.20.051.00.20.1
4.14Chondrule2Avg54.90.632.020.481.230.2220.820.5<0.02<0.08100.91.940.7
S.D.2.10.321.440.130.310.101.70.21.30.61.9
4.14Clast in matrix153.20.331.760.684.340.5821.017.250.05<0.0899.26.834.6
2.1i5Avg57.90.262.160.800.910.2036.42.11n.d.<0.08100.71.34.0
S.D.0.60.081.080.170.080.071.00.570.50.11.1
5.227Avg56.80.151.461.016.500.6332.21.55n.d.<0.08100.49.93.0
S.D.0.70.080.390.140.200.10.60.360.40.30.7
5.11Chondrule and matrix23Avg56.80.100.460.3611.00.4930.80.90n.d.<0.08101.016.41.7
S.D.1.00.070.480.310.90.071.40.090.51.42.0
5.31Chondrule and matrix27Avg55.60.080.240.2613.90.4729.40.67n.d.<0.08100.820.71.3
S.D.0.80.050.150.150.90.060.80.551.31.31.0
19.116Avg56.00.150.160.1510.80.531.20.80n.d.<0.0899.816.01.5
S.D.0.90.050.030.030.60.030.90.090.91.00.2
19.11155.00.510.510.783.680.2616.822.050.62n.d.100.36.045.6
20c.3437Avg56.70.210.150.1313.60.4928.90.87n.d.<0.08101.220.51.7
S.D.0.70.030.020.070.10.050.40.061.00.30.1
Table 3.   Electron microprobe analyses composition of wüstite, magnetite, Cr spinel, spinel, and chromite (oxide wt%) from the studied micrometeorites.
ParticleMineralSettingSiO2TiO2Al2O3Cr2O3FeOMnOMgOCaONa2OK2ONiOS (el wt%)ClSumCr/(Cr + Al)Fe/(Fe + Mg)
  1. Note: n.d.  = not determined.

4.14WüstiteInner particle core<0.04<0.050.150.0899.20.100.78<0.03n.d.n.d.<0.050.04n.d.100.4
4.14MagnetiteParticle edge0.15<0.05<0.030.5592.40.080.340.15n.d.n.d.0.320.18n.d.94.4
4.14MagnetiteMagnetite rim<0.04<0.050.090.0691.00.100.980.14n.d.n.d.0.430.02n.d.92.9
4.14ChromiteMatrix0.350.7315.947.826.90.287.81<0.03n.d.<0.01<0.050.02n.d.99.90.670.66
2.1iSpinelOlivine inclusion0.130.1868.201.040.63<0.0528.2<0.03<0.02<0.01<0.05n.d.n.d.98.4
5.11Iron oxideMatrix2.60<0.05<0.03<0.0482.5<0.05<0.02<0.03n.d.n.d.4.401.611.5195.0
5.11ChromiteMatrix0.241.464.9759.328.90.813.20<0.03n.d.<0.010.07n.d.n.d.99.00.890.84
5.31ChromiteMatrix0.580.865.3457.030.90.662.510.090.09<0.010.140.49n.d.99.50.880.88
20c.343ChromiteMatrix0.173.175.3255.928.30.634.26<0.030.04<0.01<0.05n.d.n.d.97.80.890.84
19.11ChromiteMatrix0.101.426.2856.829.51.402.650.10<0.02<0.01<0.05n.d.n.d.98.30.860.86

The major constituents of the matrix are subhedral, rounded micron-sized grains of FeO-rich olivine, locally showing normal zoning from approximately Fa53 to about Fa62 (Fig. 3d; Table 1) and lesser Ni-bearing iron-sulfide grains, typically less than 1 μm in size, with dominant interstitial setting. Ni-bearing iron-sulfides also occur as inclusions in matrix olivine grains. Angular grains of Mg-rich olivine, enstatite and augite up to 60 μm in size are interspersed within the matrix. Their composition is similar to that of the chondrule silicates (Tables 1 and 2) and they are likely chondrule fragments. A zoned olivine grain (core composition Fa39, Table 1) approximately 30 μm in size appears to be unrelated to chondrule mineralogy. A subhedral crystal of Mg-Al-rich chromite 15 μm in size was also observed in the matrix (MgO = 7.8 wt%; Al2O3 = 15.9 wt%; Table 3).

Jarosite is a common mineral in the fractures and voids, mainly due to the loss of olivine crystals by weathering. These features conform to the terrestrial weathering style commonly observed in the micrometeorites from the TAM (Rochette et al. 2008; Van Ginneken et al. 2010).

Particle #4.14 has an overall chondritic bulk composition except for important K enrichment and Ni and S depletion (Fig. 4). The occurrence of jarosite would explain the anomalous K enrichment of the bulk composition.

Figure 4.

 Elemental abundance pattern of particles #4.14 and #2.1c, normalized to CI chondrite composition (Anders and Grevesse 1989). See the Sample and Methods section for details on the analytical procedure.

Particle #2.1i

The section of particle #2.1i consists mainly of two almost circular mineral assemblages with apparent mean diameter of 155 μm dominated by olivine and enstatite crystals 10–100 μm in size with rounded contours (Fig. 5a). A scoriaceous igneous layer up to 50 μm thick continuously surrounds the two circular mineral assemblages and fills in the interstices between their mineral constituents. The edge of the particle is continuously decorated by a μm-thick magnetite rim.

Figure 5.

 a) Backscattered electron image of sectioned particle #2.1i, consisting of two almost circular mineral assemblages dominated by olivine and enstatite crystals. A large vesicle is present in the core of a mineral assemblage (arrowed). b) An euhedral Mg,Al spinel enclosed in an olivine crystal. c) The igneous rim material, consisting of glass with abundant crystallites of FeO-rich olivine. The two rounded bright crystals are Ni-bearing sulfide. d) Two rounded bright crystals of Ni-bearing sulfide in the igneous material filling the interstices.

Olivine and low-Ca pyroxene in the two circular objects have composition Fa1.2 ± 0.1 and Fs1.3 ± 0.1 Wo4.0 ± 1.1, respectively (Tables 1 and 2). One olivine 100 μm in size poikilitically encloses a euhedral Mg-Al spinel 20 μm in size (Fig. 5b; Table 3). Fe-Ni metal blebs ranging in size from 2 to 5 μm are occasionally enclosed in olivine.

The scoriaceous igneous material consists of glass (Figs. 5c and 5d) with abundant crystallites of FeO-rich olivine Fa62.5 (Table 1). Two rounded Ni-bearing sulfide crystals up to 5 μm in size are set in the scoriaceous material filling the interstices of the two circular mineral assemblages. Microscopic crystallites of Fe-oxides (likely magnetite) are abundant toward the edge of the particle and are not observed at its core. Likewise vesicularity increases toward the edge of the particle, except for an irregularly shaped vesicle approximately 50 μm in size at the center of the particle (Fig. 5a).

Particle #2.1c

Particle #2.1c (Fig. 6a) consists of fine-grained and porous Si-rich material, speckled with magnetite framboids and platelets up to approximately 10 μm in size, and minor Ni-sulfide in the form of elongated crystals about 4 μm in size (Fig. 6b). Abundant cracks, a few μm wide and up to 60 μm long, are observed in the particle. The particle is devoid of a magnetite rim.

Figure 6.

 a) Backscattered electron image of section of particle #2.1c, consisting of fine-grained and porous Si-rich material, speckled with magnetite framboids and plaquettes. Dehydration cracks are visible throughout. b) Close-up on elongated troilite crystals. c) Close-up of the Si-rich material showing that it is devoid of structure.

SEM observations reveal that the silicate material appears devoid of obvious crystalline structure down at the μm-scale (Fig. 6c). Crystalline phases and their relative abundances, detected using synchrotron XRD and Rietveld analyses, are clinoenstatite 67.5 ± 1.7 vol%, magnetite 30.4% ± 1.1 vol%, troilite 2.1 ± 0.7 vol%, and traces of Mg-Fe carbonates and goethite as a likely product of terrestrial weathering (Fig. 7). Agreement factors for the Rietveld analyses were Rp = 7.9% and wRp = 6.1%. The high background level in the XRD spectra is consistent with the occurrence of abundant amorphous material. The synchrotron XRD spectra (Fig. 7) also shows that enstatite diffraction peaks are rather large, suggesting that enstatite is characterized by poor crystallinity. Furthermore, enstatite crystals were not detected during SEM survey, suggesting that crystals are less than 1 μm in size.

Figure 7.

 Close-up view of the XRD spectra of particle #2.1c (background subtracted). Major phases include magnetite (M) and enstatite (E). Ni-sulfide (S) is a minor phase.

Particle #5.22

In section, particle #5.22 consists mainly of a 360 μm diameter, almost circular microporphyritic assemblage, dominated by microphenocrysts of olivine and enstatite crystals 40–150 μm in size, with rounded contours, set in a glassy mesostasis (Fig. 8a). A scoriaceous igneous layer up to 30 μm thick continuously surrounds the microporphyritic assemblage. The edge of the particle is discontinuously decorated by a μm-thick magnetite rim.

Figure 8.

 a) Backscattered electron image of sectioned particle #5.22, which consists of a rounded assemblage of olivine and enstatite crystals, set in a glassy mesostasis. A scoriaceous igneous layer continuously surrounds the microporphyritic assemblage. A thin magnetite rim is present in areas lacking the scoriaceous layer (arrowed). b) A subrounded bytownite crystal in the glassy mesostasis. c) The scoriaceous igneous layer contains abundant euhedral crystals of FeO-rich olivine and crystallites of likely magnetite. Olivine crystals embedded in the igneous layer show subrounded morphologies and FeO-rich olivine coronas.

Olivine and enstatite have composition Fa12.6 ± 1.8 and Fs9.9 ± 0.3 Wo3.0 ± 0.7, respectively (Tables 1 and 2). A subrounded microphenocryst of bytownite (Ab17.5, An82.4) 25 μm in size is also observed in the glassy mesostasis (Fig. 8b; Table 5). Some Fe-Ni blebs (Ni = 5.84 wt%, Co = 0.25 wt%) ranging from 5 to 15 μm in size are set in the glassy mesostasis or are enclosed in enstatite microphenocrysts (Table 4).

Table 4.   Electron microprobe analyses composition of Fe-Ni metal and troilite (element wt%) from micrometeorites #5.11, #5.31, #19.11, #2.1i and #5.22.
 SiSTiCrFeMnMgNiCoPSum
  1. Note: n.d. = not determined.

Fe-Ni metal
5.220.10<0.03<0.03<0.0490.6<0.04<0.035.840.25n.d.96.9
2.1i0.21<0.03<0.030.3281.0<0.040.4412.50.410.7695.7
2.1i0.68<0.030.010.2484.70.010.667.440.310.4894.5
5.31<0.05<0.03<0.03<0.0491.40.06<0.035.69n.d.n.d.97.2
Troilite
5.11<0.0536.4n.d.0.0962.9<0.04n.d.0.050.04n.d.99.5
5.310.0436.6<0.030.1458.50.05<0.030.82n.d.n.d.96.2
19.110.4234.2<0.030.1058.30.06<0.030.11n.d.n.d.93.2

The scoriaceous igneous layer contains abundant euhedral crystals of FeO-rich olivine less than 10 μm in size and crystallites of magnetite (Fig. 8c). The latter are often concentrated at the very edge of the particle where the igneous rim is absent. Microphenocrysts of olivine in contact or embedded within the igneous layer are surrounded by FeO-rich coronas (Fig. 8c).

Particle #5.11

In section, particle #5.11 consists of three readily delineated porphyritic chondrules with apparent diameter ranging from 200 to 250 μm, set in a relatively coarse-grained matrix (Fig. 9a). One side of the particle shows a scoriaceous igneous layer up to 120 μm thick. Weathering products encrust this side of the particle which obviously represents the external surface.

Figure 9.

 a) Backscattered electron image of sectioned particle #5.11, consisting of three readily delineated porphyritic chondrules set in a relatively coarse-grained matrix. A microporphyritic scoriaceous igneous layer is observed on one side of the particle. Weathering products encrust this side of the particle. b) The glass present in the particle contains abundant subrounded to rounded vesicles. c) A close-up view of the igneous layer that consists of euhedral microcrystals of olivine and magnetite dendrites.

Two chondrules are porphyritic olivine (PO) chondrules, the third one is a porphyritic pyroxene (PP) chondrule. The matrix is clastic with grains up to 120 μm in size. Olivine and enstatite from chondrule and matrix have nearly homogeneous compositions Fa18.8 ± 0.6 and Fs16.5 ± 1.5 Wo1.7 ± 2.0, respectively. Vesicular glass is an important phase in the chondrules and in the matrix and fills the interstices between the silicates inside the whole particle (Fig. 9b). Vesicles are less than 10 μm in size. Opaque minerals include troilite, Ni- and Cl-bearing iron oxide masses as likely weathering products of Fe-Ni metal and lesser subhedral chromite up to 30 μm in size with Cr/(Cr + Al) = 0.88–0.90 and Fe/(Fe + Mg) = 0.82–0.86 (Table 3).

The scoriaceous igneous layer shows a microporphyritic texture and consists of euhedral grains of olivine up to 20 μm in size and dendrites of magnetite set in a Na-bearing glass (Fig. 9c) (Na2O = 0.8 wt%). Olivine crystals in this layer are Ni-bearing, with Ni content up to 2.12 wt%.

Particle #5.31

Particle #5.31 shows chondritic texture, consisting of poorly delineated chondrules (Fig. 10b), up to 500 μm in size, set in a relatively coarse-grained matrix with typical grain size of 100–150 μm (Fig. 10a). The edge of the particle is decorated with a magnetite rim (Fig. 10c) up to 5 μm thick partially substituted by weathering products.

Figure 10.

 a) Backscattered electron image of sectioned particle #5.31, consisting of poorly delineated chondrules, up to 500 μm in size, set in a relatively coarse-grained matrix. The edge of the particle is decorated with a magnetite rim, partially replaced by weathering products. b) Sketch showing the location of the poorly delineated chondrules. c) Close-up view of the magnetite rim. d) Close-up view of an area showing the relatively coarse-grained texture. Clean feldspar grains (dark phase) occur between the silicate crystals. A subhedral chromite grain (lower left) is observed.

The chondrules belong to the POP type and consist mainly of olivine and enstatite with interstitial oligoclase (Ab68.2; An22.9, Table 5) speckled with abundant inclusions of high-Ca pyroxene grains up to 20 μm in size. The composition of olivine and enstatite in the chondrules and matrix is homogeneous Fa24.7 ± 0.7 and Fs20.7 ± 1.3 Wo1.26 ± 1.03, respectively (Tables 1 and 2). Clean feldspar grains 20 ± 8 μm in size of albitic composition Ab85.3 An4.1 fill the interstices between silicates in the matrix (Fig. 10d). Opaque minerals include an assemblage of Fe-Ni metal and troilite, and a subhedral chromite 15 μm in size with Cr/(Cr + Al) = 0.88 and Fe/(Fe + Mg) = 0.88 (Tables 3 and 5).

Table 5.   Electron microprobe analyses composition of feldspar (oxide wt%) from micrometeorites #5.22, #5.31, #20.343, and #19.11.
ParticleNSetting SiO2TiO2Al2O3Cr2O3FeOMnOMgOCaONa2OK2OTotalAbAn
  1. Notes: N = number of analyzed crystals; Avg = average composition; S.D. = standard deviation.

5.221Enclosed in glass 47.9<0.0329.9<0.060.480.070.5116.81.87<0.0497.517.582.4
5.313ChondruleAvg60.40.2823.10.761.250.010.224.537.471.4999.668.222.9
S.D.0.00.230.50.400.520.010.250.580.140.491.00.43.2
5.315MatrixAvg68.20.1519.6<0.060.82<0.060.180.869.761.84101.485.34.1
S.D.0.80.140.40.090.160.150.320.330.51.40.8
20.3436InterstitialAvg65.7<0.0320.0<0.060.72<0.060.142.139.551.1199.382.89.8
S.D.1.10.40.340.300.170.350.131.01.10.8
19.115InterstitialAvg62.7<0.0320.9<0.060.51<0.06<0.032.1510.111.0297.384.59.9
S.D.0.70.30.050.160.240.120.71.20.6

Particle #20c.343

Particle #20c.343 mainly consists of a granoblastic assemblage of coarse-grained crystals of olivine and enstatite up to 650 μm and minor oligoclase plus augite (Fig. 11a). The edge of the particle is decorated with a magnetite rim approximately 2 μm in thickness (Fig. 11b).

Figure 11.

 a) Backscattered electron image of sectioned particle #20c.343, consisting of a granoblastic assemblage of coarse-grained crystals of olivine and enstatite plus minor oligoclase and augite. Chromite (Chr) inclusions are observed in enstatite. b) The edge of the particle is decorated with a thin magnetite rim.

Olivine and enstatite have homogeneous composition Fa24.6 ± 0.3 and Fs20.5 ± 0.3 Wo1.96 ± 0.14 (Tables 1 and 2). Oligoclase crystals are up to 100 μm in size and have homogeneous composition (Ab82.8 ± 1.1; An9.8 ± 0.8, Table 5). Accessory minerals include subhedral chromite up to 10 μm in size with Cr/(Cr + Al) = 0.89 and Fe/(Fe + Mg) = 0.84 (Table 3) generally in the form of inclusions in low-Ca pyroxene crystals.

Olivine crystals are affected by important dissolution, particularly at the margins of the particle as a result of terrestrial weathering. The weathering style has been described in TAM micrometeorites by Rochette et al. (2008) and in ablation debris by Van Ginneken et al. (2010), and it is due to saline waters occasionally produced by the melting of the snow in the trap.

Particle #19.11

In section, particle 19.11 consists of an assemblage of coarse-grained crystals of olivine and low-Ca pyroxene (Fig. 12a) with interstitial feldspar. One side of the particle bears a continuous igneous layer up to 100 μm thick. The same igneous material is present in places also on the other side of the particle.

Figure 12.

 a) Backscattered electron image of sectioned particle #19.11, consisting of an assemblage of coarse-grained crystals of olivine and enstatite with interstitial feldspar. b) One side of the particle bears a scoriaceous igneous layer, composed mainly by skeletal crystals of olivine about 10 μm in size set in glass. The microporphyritic igneous material is also observed in places at the opposite particle edge (arrowed).

Grain size ranges from 10 μm up to 400 μm. Olivine and enstatite have homogeneous compositions Fa19.1 ± 0.5 and Fs15.5 ± 0.3 Wo1.5 ± 0.2 (Tables 1 and 2). Augite crystals up to 100 μm in size have composition Fs6.0 Wo45.6 (Table 2). Feldspar grains up to 60 μm in size have oligoclase composition Ab84.5 An9.9 (Table 5). Opaque minerals include subrounded troilite grains ranging from 10 to 15 μm (Table 4) and subhedral chromite up to 15 μm in size with Cr/(Cr + Al) = 0.86 and Fe/(Fe + Mg) = 0.86 (Table 3). Two large voids up to 85 μm size are encrusted with iron oxide as a likely weathering substitution of Fe-Ni metal.

The scoriaceous igneous layer mainly consists of skeletal crystals of olivine about 10 μm in size and microscopic magnetite dendrites set in glass (Fig. 12b). Vesicles in this layer are up to 30 μm in size. The composition of the igneous layer is (in wt%) SiO2 = 42.5; Al2O3 = 2.38; Cr2O3 = 0.40; FeO = 17.1; MnO = 0.35; MgO = 32.8; CaO = 2.26; Na2O = 0.72; K2O = 0.10; NiO = 0.10, and SO3 = 0.16.

Classification

Micrometeorites or Meteorite Fragments?

One of the major issues to address is to determine whether the particles studied herein are actually micrometeorites, i.e., small objects before entering the Earth’s atmosphere (Genge et al. 2008) and not fragments of larger meteorites, namely meteorite ablations debris that detached from the main body during atmospheric entry (Harvey et al. 1998; Van Ginneken et al. 2010), or meteorite fragments produced upon impact with the Earth’s crust or subsequent weathering.

The complete or partial magnetite rims a few μm thick around particles #4.14, #2.1i, #5.22, #5.31, and #20c.343 are typical of unmelted micrometeorites and they are thought to form during atmospheric entry heating and surface melting (Toppani et al. 2001; Toppani and Libourel 2003). Therefore, the origin as ablation debris of a larger meteorite can be ruled out for these particles. The magnetite rims in particles #2.1i and #5.22 are associated with a scoriaceous igneous layer. This layer at the margin of these particles is similar to the igneous rim described around unmelted micrometeorites (Genge 2006) attesting to a more pervasive surface melting during atmospheric entry heating. Particles #5.11 and #19.11 are devoid of magnetite rims but have thicker (between tens of μm up to 120 μm) igneous rims, testifying to an even larger surface melting. As expected, the increasing amount of surface melting is evidenced by the shape of the particles, which become progressively more rounded (e.g., Fig. 1).

Particle 2.1c has an angular shape (Figs. 1 and 5a) and is devoid of a magnetite rim, as well as an igneous rim. The pervasive cracks are the result of the dehydration of phyllosilicates during limited atmospheric entry heating (Genge et al. 2008). They are observed in phyllosilicate-rich unmelted micrometeorites and in the substrate beneath the fusion crust of hydrous chondritic material (e.g., CI chondrites). Hence, it cannot be ruled out that this particle is a fragment of a larger object.

Particle #5.11 is angular and shows broken surfaces. An igneous rim and a weathering rind of laminated Cl-bearing iron oxide encrusting it occur only on one side of the particle; therefore it is obviously a fragment of a larger object. The composition of the igneous layer of particle #19.11, especially in volatile elements such as Na and S, is similar to that of fusion crusts of chondrites and meteoritic ablation debris (Genge and Grady 1999; Van Ginneken et al. 2010). This suggests that particle #19.11 is also a fragment of a larger object. The occurrence of igneous material throughout the studied sections indicates atmospheric entry heating. However, we cannot establish with certainty if these particles are fragments of larger micrometeorites or portions of meteorites close to their external surface. Indeed, the large dimensions of particles #5.11 and #19.11, which were studied for the first time here, could allow them to have properties between micrometeorites and meteorites.

In summary, particles #4.14, #2.1i, #5.22, #5.31, and 20c.343 are micrometeorites, whereas particle #2.1c could be a fragment of a larger object. On the other hand, particles #5.11 and #19.11 are fragments of larger objects, but it is not clear if they are fragments of larger micrometeorites or of meteorites.

Particle #4.14: A CV-Like Micrometeorite

The presence of a 5 μm thick magnetite rim surrounding particle #4.14 (Fig. 3a) suggests that melting during atmospheric entry was limited to the very surface of the particle (Toppani and Libourel 2003). The fine grain size of the matrix, the good definition of the chondrules, the variable mineral composition of olivine in the chondrules (Fa3.2) and matrix (Fa55.3), and the presence of glass in the chondrule mesostasis indicate an unequilibrated chondritic parentage (e.g., Brearley and Jones 1998). The occurrence of abundant type-I POP chondrules with frequent low- and high-Ca pyroxene intergrowths (Fig. 3b) (Brearley and Jones 1998) also suggests an affinity with CV and CO-like material. Furthermore, the minor element composition of olivine and low-Ca pyroxene in the chondrules (Fig. 13) is consistent with this inference. The paucity of chondrules in the polished section does not allow the use of their size as an efficient criterion for classification. Clasts of forsterite, enstatite, and high-Ca pyroxenes are also frequently observed in the matrix of unequilibrated chondrites and are considered as chondrule fragments (Brearley and Jones 1998). The mineralogy of the matrix, characterized by a large abundance of relatively fayalitic olivine grains (Fa55.3), is similar to that of the oxidized CV3 or CO3 chemical classes (Scott and Krot 2005). The subhedral, rounded morphology and the grain size of Fe-rich olivine grains in the matrix (Fig. 3d) are similar to those observed in some CV3 carbonaceous chondrites like Bali, Grosnaja, and Efremovka (Weisberg and Prinz 1998; Nakamura et al. 2000; Jogo et al. 2009). The occurrence of Ni-bearing sulfide in the matrix and as inclusions in matrix olivine is also observed in CV3 chondrites and absent in CO3 chondrites (Brearley and Jones 1998; Brearley 1999; Weisberg et al. 2006). Figure 14 shows that the composition of the chromite crystal in the matrix does not fall in the compositional field of the CV chondrites. This could be the result of a bias in the literature data due to the small amount of compositional data available for chromite in CV chondrites compared to the other groups represented in Fig. 14.

Figure 13.

 Minor element concentrations in olivine (left column) and enstatite (right column) of the studied micrometeorites. Compositional fields for the equilibrated ordinary chondrites (EOC), unequilibrated ordinary chondrites (UOC), CM chondrites (CM), CV chondrites (CV), and CO chondrites (CO) are reported for comparison (data from Genge [2008a] and Noguchi [1989]).

Figure 14.

 Fe/(Fe + Mg) versus Cr/(Cr + Al) ratios in chromite from the studied micrometeorites. Data from literature are shown for comparison (Bunch et al. 1967, 1972; Lange et al. 1974; Fudali and Noonan 1975; Jaques et al. 1975; Klob et al. 1981; Johnson and Prinz 1991; Wlotzka 2005).

The above petrographic features suggest that particle #4.14 has an overall affinity with CV3 and CO3 meteoritic material; however, matrix mineralogy favors a CV3 chondrite parentage. The lack of calcium-aluminum-rich inclusions and amoeboid olivine aggregates, which account for 10 vol% in CV3 chondrites, could be accidental and due to their large size (mm to cm-sized) relative to the microscopic size of particle #4.14 (Scott and Krot 2006).

The occurrence of wüstite nodules with decomposition textures in the interior of the particle and magnetite nodules at the margin of the particle raises a classification issue. The major iron phase in CV chondrites is in fact magnetite or Fe-Ni metal in oxidized or reduced CV chondrites, respectively (Weisberg et al. 2006). One possible scenario that explains the distribution of Fe-oxides in particle #4.14 is the reduction of magnetite into wüstite inside the particle during atmospheric entry heating. Such a reaction implies temperatures greater than 570 °C and reducing conditions. Such reducing conditions can be achieved inside the particle as a result of troilite decomposition which is typically associated with magnetite (decomposition temperature 300–600 °C) and/or pyrolysis of carbonaceous material. This scenario was proposed by Kim et al. (2009) to explain the presence of wüstite close to the fusion crust of the CO3 chondrite Dominion Range 03238. The lack of detectable Ni in the Fe-oxides of particle #4.14 argues against the alternative scenario envisaging increasing oxidation of metal toward the surface of the particle during atmospheric entry.

Particle #2.1i: A CM Micrometeorite

The igneous rim at the margin of particle #2.1i associated with a well-developed external magnetite shell (Fig. 5c) shows all the mineralogical and textural characteristics of the micrometeorite igneous rims (Genge 2006). The scoriaceous igneous material observed between the coarse-grained olivine and enstatite crystals at the core of the particle indicates, however, pervasive heating and melting in particle #2.1i. Furthermore, the 50 μm-sized vesicle (Fig. 5a) at the center of the particle indicates important degassing during atmospheric entry. The observation of abundant submicrometer-sized olivine crystals (Figs. 5c and 5d), associated with high vesicularity suggests thermal decomposition of phyllosilicates at 600 °C during atmospheric entry (Greshake et al. 1998).

The homogeneous coarse-grained MgO-rich olivine (Fa1.2) and enstatite (Fs1.3 Wo4.0) crystals with rounded contours at the core of the particle are interpreted as primary minerals. Consequently, the two mineral assemblages are interpreted as two porphyritic chondrule relicts. The rounded contours of the olivine and pyroxene grains indicate some amount of digestion/dissolution by the surrounding melt during atmospheric entry heating and melting. Chondrule relicts with thermally altered phyllosilicate groundmass have been observed near the fusion crust of the Murchison CM chondrite (Inoue and Nakamura 1996). Consistently, minor elements composition of olivine and of enstatite falls in the composition fields of ferromagnesian chondrules in chondrites of type 2 and type 3 (Fig. 13). Fe-Ni metal blebs and the Mg,Al spinel crystal enclosed in olivine have been protected from thermal alteration and are therefore primary. The presence of P, Si, and Cr in the small kamacite inclusions in olivine (Table 4) is consistent with a low petrographic type (Zanda et al. 1994). Mg,Al spinel commonly occurs in CM2 or CO3 chondrites (Jones 1992; Kimura et al. 2006).

All the evidences presented above suggest that particle #2.1i has CM chondrite parentage.

Particle #2.1c: A CI-Like Micrometeorite

The chondritic nature of particle #2.1c is evidenced by a broadly chondritic bulk composition (Fig. 4). The absence of igneous rim or magnetite rim suggests that the particle did not suffer surface melting. The occurrence of a large amount of amorphous material, poorly crystalline enstatite, and dehydration cracks inside the particle is commonly attributed to thermal alteration of phyllosilicates in micrometeorites during atmospheric entry (Genge et al. 2008). Furthermore, heating experiments on phyllosilicate-bearing chondrite matrices (e.g., C1 and C2 chondrites) showed that enstatite is a common thermal decomposition product of dehydrated phyllosilicates (Greshake et al. 1998; Nozaki et al. 2006).

Magnetite framboids and platelets are commonly observed in CI chondrites, in Tagish Lake, CM2 and CR2 chondrites. Ni-sulfide is a common accessory phase in matrices of chondrites of petrologic type 1 (hereafter C1), that are CI chondrites, and chondrites of petrologic type 2 (hereafter C2) (Brearley and Jones 1998). An important feature to differentiate between C1 and C2 micrometeorites is the chemical heterogeneity throughout the particle, especially Fe/Mg ratio, which is easily observable using SEM backscattered electron (BSE) imaging. C2 material is heterogeneous on a 10 μm scale, whereas C1 material is homogeneous as a result of aqueous alteration on the parent body (Genge et al. 2008). Figure 6a shows that particle #2.1c is homogeneous throughout. Also, relatively abundant anhydrous olivine and pyroxene more than 4 μm are often observed in the fine-grained component of C2 material but are rare in C1 material. Such anhydrous silicate crystals have not been detected in particle #2.1c.

Collectively, these lines of evidence suggest that particle #2.1c has a CI chondrite parentage.

Particle #5.22: A Type 3 Ordinary Chondrite-Like Micrometeorite

The scoriaceous igneous rim testifies to surface melting during atmospheric entry (Fig. 8).

The coarse-grained olivine and enstatite crystals and the glassy mesostasis associated with a bytownite grain at the core of the particle are interpreted as primary minerals. Overall, they form a rounded microporphyritic mineral assemblage whose morphology and mineralogy indicate that it is a remnant of a POP chondrule. Bytownite occasionally occurs in the mesostasis of type 3 chondrites and is rarer in type 4 chondrites (Grossman and Rubin 1986; Kovach and Jones 2010). Minor element composition of olivine and enstatite fall in the compositional field of ferromagnesian chondrules of type 3 ordinary chondrites. Ni and Co contents in the Fe-Ni metal are also consistent with a type 3 ordinary chondrite parentage (Afiattalab and Wasson 1980).

All evidence presented above suggests that particle #5.22 is an ablated chondrule of type 3 ordinary chondrite parentage.

Particle #5.11: a Type 4 Ordinary Chondrite-Like Micrometeorite

The chondritic nature of particle #5.11 is evidenced by the presence of three readily delineated chondrules (Fig. 9a) and by its overall mineralogy. The igneous layer surrounding the particle testifies to surface melting during atmospheric entry. The occurrence of vesicular glass within the matrix suggests that incipient melting due to atmospheric entry heating occurred throughout the observed section of this chondritic particle.

The overall texture of the particle, characterized by readily delineated chondrules set in a relatively coarse-grained matrix suggests petrologic type 4 (e.g., Brearley and Jones 1998). Consistently, the composition of olivine and enstatite in chondrules and matrix is homogeneous (Tables 1 and 2). Olivine (Fa18.8 ± 0.6) and low-Ca pyroxene (Fs16.5 ± 1.5) compositions plot in the compositional field of the H-type ordinary chondrites (Fig. 15). Figure 13 shows that minor element compositions of olivine and enstatite are consistent with equilibrated ordinary chondrite parentage. Likewise, chromite composition (Fig. 14) falls in the compositional field of the type 4–6 ordinary chondrites (Bunch et al. 1967; Wlotzka 2005).

Figure 15.

 Fayalite content in olivine versus ferrosilite content in enstatite in all particles except #2.1c, compared to equilibrated ordinary chondrites (data from Brearley and Jones 1998).

All the lines of evidence mentioned above suggest that particle #5.11 can be classified as an H4-ordinary chondritic micrometeorite.

Particles #5.31, #20.343, and #19.11: Three Type 6 Ordinary Chondrite-Like Micrometeorites

Particles #5.31, #20c.343, and #19.11 share a number of petrographic features typical of equilibrated chondrites and will be discussed together. Particles #5.31 and #20c.343 show magnetite rims and particle #19.11 shows an igneous rim, suggesting that surface melting affected the latter more extensively than the former.

The mineralogy of the three particles, dominated by olivine, enstatite, oligoclase, Fe-Ni metal, troilite, and accessory chromite is chondritic (Brearley and Jones 1998). The poorly defined chondrules observed in particle #5.31, the homogeneous composition of olivine and pyroxene (Tables 1 and 2), and the relatively coarse-grained textures in particles #5.31, #20c.343, and #19.11 (Figs. 10a, 11a, and 12a) indicate high petrographic type 6 (e.g., Brearley and Jones 1998). Figure 15 shows that the major element composition of olivine and enstatite falls in the compositional field of the H-type ordinary chondrites for particle #19.11, whereas it falls in the L-type ordinary chondrites for particles #5.31 and #20c.343. Figure 13 shows that minor element compositions of olivine and enstatite are homogeneous throughout the particles and plot in the equilibrated ordinary chondrite field. The plagioclase (Table 5) and chromite (Fig. 14) (Bunch et al. 1967; Wlotzka 2005) compositions in these particles match that of type 4–6 ordinary chondrites.

The presence of an augite crystal greater than 50 μm in size in particle #19.11 with composition Fs6.0 and Wo45.6 is also typical of type 6 ordinary chondrites (Huss et al. 2006). The presence of albitic plagioclase crystals greater than 10 μm in size for particle #5.31 and greater than 50 μm in particles #20c.343 and #19.11 also suggests a petrologic type 6 (Huss et al. 2006).

All the elements mentioned above suggest that particle #5.31 can be classified as an H6 ordinary chondritic micrometeorite, and that particles #20.343 and #19.11 can be classified as L6 ordinary chondritic micrometeorites.

Discussion

The present selection of eight “giant” unmelted micrometeorites in the 300–1100 μm size range is constituted by five ordinary chondrite-like micrometeorites, one CV-like micrometeorite, one CM-like micrometeorite, and one CI-like micrometeorite. One important feature of this study is the discovery of a CV-like micrometeorite. CV material was detected among cosmic spherules greater than 500 μm in size by high precision oxygen isotope analysis (Suavet et al. 2010). The occurrence of CV-like material among unmelted micrometeorites was suspected since CV-like material can be captured by Earth’s gravitational field, even if it is at a lower degree compared to other extraterrestrial material (Genge et al. 2008). This discovery confirms that micrometeoroids deriving from CV parent bodies can be captured by Earth’s gravitational field.

Nesvorný et al. (2010) proposed a zodiacal cloud model to explain why unmelted micrometeorites in the smaller size fractions (typically <300 μm) that are related to primitive carbonaceous chondrites with a cometary origin are more abundant than micrometeorites related to asteroids. Their model suggests that most of the zodiacal cloud is mainly composed of Jupiter-family cometary particles that are scattered by Jupiter before decoupling from the planet and drifting down to 1 AU. This observation is consistent with the study of unmelted micrometeorites less than 300 μm in size related to primitive carbonaceous chondrites recovered from the Antarctic ice, which are thought to be related to comets (Engrand and Maurette 1998; Dobrica et al. 2010). But it is not clear whether these micrometeorites related to primitive carbonaceous chondrites have a cometary or an asteroidal origin. For example, this larger abundance of micrometeorites related to primitive carbonaceous chondrites in the smaller size fraction less than 300 μm has also been explained by Flynn et al. (2009) by the disruption, in the main asteroid belt, of hydrous asteroids (i.e., composed by primitive carbonaceous chondritic material) that will overproduce particles about 100 μm in size, compared to anhydrous asteroids (i.e., composed by ordinary chondritic material). The large abundance of ordinary chondritic material and the presence of the CM-like and CV-like material clearly suggest that most of the micrometeorites from the larger size fraction (>400 μm) have asteroidal origin. On the other hand, it is still not clear if the CI-like material in the present study has a cometary or asteroidal origin. Recent studies of the population of cosmic spherules from the TAM collection established that 30% of those greater than 500 μm (Suavet et al. 2010) and 70% of those greater than 800 μm (Suavet et al. 2011) are made of ordinary chondritic material. A similar observation was made, based on a study of particles less than 300 μm, by Genge (2008a), who suggested that ∼20% of all micrometeorites could be related to ordinary chondrites. Our study tends to confirm this order of abundance for unmelted micrometeorites greater than 300 μm. This suggests that unmelted micrometeorites resulting from the disruption of asteroids are abundant in the larger size fractions.

The fact that most of the “giant” unmelted micrometeorites (i.e., in the >400 μm size fraction, which are particles #4.14, #5.22, #5.11, #5.31, #20c.343, and #19.11) studied herein are coarse-grained could be accounted for by the fact that entirely fine-grained ones, that were mainly derived from phyllosilicate-rich material are more easily fragmented during atmospheric entry. One possible explanation for this is the formation of dehydration cracks which would favor particle fragmentation (Genge 2008b). Such dehydration cracks are observed only in the fine-grained particle #2.1c, which is the smallest one (<400 μm in size) and the only one lacking any sign of partial melting on its surface. This observation supports the idea that in size fractions greater than 400 μm, phyllosilicate-rich fine-grained micrometeorites will be less likely to survive atmospheric entry without being fragmented.

This study confirms observations by Rochette et al. (2008) that a non-negligible amount of micrometeorites greater than 300 μm in size are unmelted, and therefore survived atmospheric entry without suffering strong heating. Dynamical models of atmospheric entry of micrometeoroids predict that particles greater than 100 μm in size may survive atmospheric entry heating if entering at low entry velocities, i.e., close to the Earth’s escape velocity (e.g., Love and Brownlee 1991). In particular, Flynn (1992) suggested that due to the systematically higher entry velocity of cometary dust compared to asteroidal dust, particles greater than 100 μm will be likely of asteroidal origin. Our study shows that “giant” micrometeorites with mineralogy and textures typical of asteroidal material are abundant in the TAM collection and supports this theory.

Conclusion

  • 1 Our detailed mineralogical and geochemical study of eight micrometeorites from the TAM collection shows that unmelted micrometeorites deriving from chondritic material are frequent in the 300–1100 μm size range.
  • 2 A large variety of chondritic material is observed, with four EOC-like, one CI-like, one CV-like, one UOC-like, and one CM-like micrometeorite (note, however, that the EOC-particles #5.11 and #19.11 are fragments of larger objects and may not be micrometeorites). Remarkably, the discovery of a CV-like unmelted micrometeorite further expands the inventory of extraterrestrial material among micrometeorites.
  • 3 Our inventory confirms that a large part of the micrometeorite flux in the 300–1100 μm size fraction is of asteroidal origin. In particular, the significant number of ordinary chondritic unmelted micrometeorites found here parallels results from oxygen isotope analyses of cosmic spherules from the literature (Suavet et al. 2010, 2011).
  • 4 The abundance of “giant” (i.e., >300 μm) unmelted micrometeorites of asteroidal origin in the TAM collection is consistent with dynamical model of atmospheric entry of micrometeorites, which predicts that micrometeorites greater than 100 μm may survive atmospheric entry heating and melting if entering at asteroidal velocities (Flynn 1992).

Acknowledgments–– This work was supported by the PNRA. M. V. G., L. F., and C. C. are also supported by the European Commission through the Marie Curie Actions-RTN ORIGINS project (ID: 35519). This work is part of M. V. G.’s Ph.D. thesis (Scuola di Dottorato in Scienze Polari) supervised by L. F. Natale Perchiazzi (Università di Pisa) who is thanked for assistance in synchrotron XRD analyses. A. Brearley is acknowledged for editorial assistance, and Matt Genge and an anonymous reviewer for their constructive comments.

Editorial Handling–– Dr. Adrian Brearley

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