STUDIES OF BRAZILIAN METEORITES XIV. MINERALOGY,PETROLOGY, AND CHEMISTRY OF THE CONQUISTA,MINAS GERAIS,CHONDRITE

The Conquista chondrite consists of major olivine, low-Ca pyroxene (both ortho- and twinned clino-), troilite and metallic nickel-iron; minor glassy to microcrystalline material and pigeonite; and accessory chromite, high-Ca clinopyroxene and hydrous ferric oxides that formed by terrestrial weathering of metallic nickel-iron. Results of microscopic, electron microprobe, and bulk chemical studies, particularly the compositions of olivine (Fa_17.2) and low-Ca pyroxene (Fs_15.4); the contents of metallic nickel-iron (18.5%) and total iron (25.83%); and the ratios of Fe°/Fe_total (0.64), Fe°/Ni° (9.59) and Fe_total/SiO₂ (0.69) indicate H-group classification. The pronounced, well-developed chondritic texture; the slight compositional variations in constituent phases; the high Ca contents of pyroxene and the presence of pigeonite, glassy to microcrystalline interstitial material rich in alkalis and SiO₂, and of twinned low-Ca clinopyroxene suggest that Conquista is of petrologic type 4.     

THE INMAN,McPHERSON COUNTY,KANSAS METEORITE

Inman (find, 1966) is a single, relatively unweathered stone of 7.25 kg that contains fresh metal and only few weathering products away from fractures. It has a pronounced chondritic texture, with 38 vol % of the meteorite being made up of chondrules of virtually all textural types. The recalculated bulk analysis, particularly the ratios of Fe_total/SiO₂ (0.46), Fe°/Fe_total (0.35), and Fe°/Ni° (6.67) and the contents of Fe_total (19.45%) and metallic nickel-iron (7.94%), indicate that Inman is an L-group chondrite. The pronounced chondritic texture; the compositional variabilities of olivine, pyroxene, chromite, and ilmenite; the presence of a fine-grained, nearly opaque matrix, glass and twinned monoclinic low-Ca pyroxene indicate that the chondrite belongs to petrologic type 3.     

STUDIES OF BRAZILIAN METEORITES XIII. MINERALOGY,PETROLOGY, AND CHEMISTRY OF THE PUTINGA,RIO GRANDE DO SUL,CHONDRITE

The Putinga, Rio Grande do Sul, chondrite (fall, August 16, 1937), consists of major olivine (Fa_24.8), orthopyroxene (Fs_21.3), and metallic nickel-iron (kamacite, taenite, and plessite); minor maskelynite (Ab_81.0An_12.4Or_6.6) and troilite; and accessory chromite (Cm_79.0Uv_8.2Pc_1.8Sp_11.0) and whitlockite. Mineral compositions, particularly of olivine and orthorhombic pyroxene, as well as the bulk chemical composition, particularly the ratios of Fe°/Ni° (5.24), Fe_total/SiO₂ (0.58), and Fe°/Fe_total (0.27), and the contents of Fe_total (22.42%) and total metallic nickel-iron (7.25%) classify the meteorite as an L-group chondrite. The highly recrystallized texture of the stone, with well-indurated, poorly discernible chondrules; xenomorphic, well-crystallized groundmass olivine and pyroxene; and the occurrence of poikilitic intergrowth of olivine in orthopyroxene suggest that Putinga belongs to petrologic type 6. Maskelynite of oligoclase composition was formed by solid state shock transformation of previously existing well-crystallized plagioclase at estimated shock pressures of about 250–350 kbar. Thus, recrystallization (i.e., formation of well-crystallized oligoclase) must have preceded shock transformation into maskelynite.     

TWO NEW CHONDRITE-FALLS IN JAPAN

In the summer of 1984, two meteorites fell in the northern part of Honshu, Japan; Aomori, at 1:50 p.m. on June 30, and Tomiya, at 1:35 p.m. on August 22. Coordinates of the falls of the Aomori and the Tomiya are at 140°47.1'E., 40°48.6'N., and 140°51.9'E., 38°22.0'N., respectively. Results of chemical analyses of major elements, ratios of Fe_total/SiO₂ (0.546 and 0.803) and Fe_metal/Fe_total (0.332 and 0.581), and molar compositions of olivines (Fa₂₅ and Fa₁₉) indicate that the Aomori and the Tomiya are typical L- and H-group ordinary chondrites, respectively. In the Aomori, chondrules are present as relicts in the well-recrystallized matrix. Olivine and pyroxene are homogeneous in composition, and coarse clear feldspar, up to 100 micrometers in size, is well developed in the chondrules and matrix. Though the Aomori is a petrologic type 6 based on its texture and mineralogy, it includes a few grains of multiple twinned clinobronzite which is rarely observed in highly equilibrated ordinary chondrites. In the Tomiya, chondrules possess a fine-grained mesostasis, and both orthopyroxene and clinobronzite are noticeable in thin sections. Plagioclase is mostly microcrystalline, but is also sparsely present as tiny, visible grains. Thus, the Tomiya was classified to be petrologic type between 4 and 5. The deformation texture of olivine, pyroxene and plagioclase indicates that both meteorites were shocked by 0.2-0.25 Mb. In conjunction with the discussion of the frequency of meteorite-falls, all observed falls of meteorites in Japan are tabulated in this paper.     

CHEMICAL COMPOSITION,PETROGRAPHY AND MINERALOGY OF THE SHIBAYAMA CHONDRITE FOUND IN SHIBAYAMA-MACHI,SANBU-GUN,CHIBA-KEN,JAPAN

In April 1969, the chondrite was accidentally found in the side wall of the vegetable storage excavated at Shibayama-machi, Sanbu-gun, Chiba-ken, Japan, by Mr. A. Ishii and his grandson, Mr. S. Ito. The chondrite named Shibayama has been weathered thoroughly for a long period of burial underground. The bulk chemical composition, especially total Fe (21.41%) and ratios of Fe_total/SiO₂(0.557), SiO₂/MgO (1.59) and molar composition of olivine (Fa₂₃) and pyroxene (Fs₂₂) as well as mineral composition, indicate that Shibayama is a typical olivine-hypersthene chondrite. If the oxidized Fe is assumed only from metallic Fe, the original metallic Fe (7.75%) and Fe_metal/Fe_total(0.361) also support the above conclusion. From the well-recrystallized texture, indistinct and obliterated chondrule-matrix boundary, homogeneous composition of olivine and pyroxene, absence of igneous glass, and interstitial and well-developed plagioclase, this chondrite could be classified in petrologic type 6. Mosaic texture, kink bands, undulatory extinction of silicate grains and maskelynitization of plagioclase indicate that Shibayama suffered from a heavy shock effect, as is seen in other L-6 group chondrites.     

THE CHEMISTRY AND MINERALOGY OF THE HOMEWOOD,MANITOBA, METEORITE

The Homewood meteorite is a slightly weathered find of 325 grams discovered in 1970 about 64 km southwest of Winnipeg, Manitoba. It consists of olivine (Fa_25.4; 43.8 normative wt. percent), orthopyroxene (Fs_23.3; 28.5 percent), kamacite and taenite (7.5 percent), troilite (5.6 percent), maskelynite (8.3 percent), chromite (1.0 percent), whitlockite (0.7 percent) and minor patchy Ca pyroxene. Bulk chemical analysis yielded Fe_total 21.60 wt. percent, Fe/SiO₂0.55, SiO₂/MgO 1.53 and Fe^O/Fe_total 0.29. Barred olivine, radiating pyroxene and porphyritic chondrules, all with ill-defined outlines, occur in the meteorite. Most chemical and mineralogical features characterize the Homewood meteorite as an L6 (hypersthene) chondrite. The presence of maskelynite, the undulatory extinction, extensive fracturing and pervasive mosaicism of olivine, and the poor definition of chondrule outlines suggest that the Homewood meteorite has been shocked in the range of 300–350 kbar.     

MINERALOGY,PETROGRAPHY AND CHEMISTRY OF THE CHONDRITE,KAMIOMI, SASHIMA-GUN,IBARAKI-KEN,JAPAN

The Kamiomi, Sashima-gun (Iwai-shi), Ibaraki-ken, Japan, chondrite (observed to fall in spring, during the period 1913–6), consists of olivine, orthopyroxene, nickel-iron and troilite with minor amount of plagioclase, clinopyroxene, apatite and chromite. The average molar composition of olivine (Fa₁₉) and orthopyroxene (Fs₁₇) indicates that Kamiomi is a typical olivine bronzite chondrite. From the well-recrystallized texture, the presence of poorly-definable chondrules, homogeneous composition of olivine and absence of glass, this chondrite could be classified in petrologic type 5. The bulk chemical composition, especially, total Fe (27.33%) and metallic Fe (17.00%) as well as Fe_total/SiO₂(0.72), Fe_metal/Fe_total (0–633) and SiO₂/MgO (1.59) support the above conclusion. Coexistence of heavily-shocked olivine grains in the matrix composed of olivines and pyroxenes which suffered from light to moderate shock effect suggest that impacting phenomena, small-scaled but locally strong, occurred on the Kamiomi parent body.     

THE KRAMER CREEK,COLORADO METEORITE: A NEW L4 CHONDRITE

The Kramer Creek, Colorado, chondrite was found in 1966 and identified as a meteorite in 1972. Bulk chemical analysis, particularly the total iron content (20.36%) and the ratio of Fe_total/SiO₂ (0.52), as well as the compositions of olivine (Fa_21.7) and orthopyroxene (Fs_18.3) place the meteorite into the L-group of chondrites. The well-defined chondritic texture of the meteorite, the presence of igneous glass in the chondrules and of low-Ca clinopyroxene, as well as the slight variations in FeO contents of olivine (2.4% MD) and orthopyroxene (5.6% MD) indicate that the chondrite belongs to the type 4 petrologic class.     

CLAST-LADEN MELT-ROCK FRAGMENT IN THE ADAMS COUNTY,COLORADO, H5 CHONDRITE

The Adams County, Colorado, H5 chondrite contains a lithic fragment, 1 cm in size, that is texturally and mineralogically quite different from the chondritic host. It is composed of: a groundmass of fine-grained euhedral to subhedral olivine (3–15 μm) and interstitial glass enclosing larger olivine and pyroxene grains (0.15-0.5 mm; about 15 vol %); an assemblage of enstatite grains (subfragment within) and an assemblage of olivine plus orthopyroxene (a second subfragment); and about 11 vol % grains of mixed troilite and nickel-iron metal. Analyses yielded these results: (i) olivine grains of the fragment groundmass have a compositional range (Fa_12–45) and most grains contain substantial CaO and Cr₂O₃ (～ 0.20 and 0.30 avg. wt%, respectively); interstitial glass has ～ 55 wt% SiO₂; (ii) larger olivine grains of the fragment are similarly high in CaO and Cr₂O₃ and also have a wide FeO/MgO range; one unusual pyroxene is an Mg-rich pigeonite; (iii) the metal is martensite in composition (11–14 wt% Ni); and (iv) major and trace element analyses by INAA indicate an H-group bulk composition for the entire 1 cm lithic fragment. On the basis of its texture and bulk and mineral compositions, the fragment is interpreted to represent unequilibrated H-group material that was partly melted by impact. The Ca- and Cr-enriched groundmass olivine and interstitial glass resulted from rapid crystallization of the chondritic melt. The Ca- and Cr-enriched larger silicate grains, including the enstatite sub-fragment and the pigeonite grain, are residual, unmelted clasts from the target material (this is supported by the presence of similar material in actual H3 chondrites). Further impact brecciation of the clast-laden melt material, and resultant impact-splashing accounts for the presence of the fragment in the H-group Adams County host and documents the coexistence of unequilibrated and equilibrated H-group material as surface regolith on one parent body.     

The Paris CM chondrite: Secondary minerals and asteroidal processing

We report a petrographic and mineralogical survey of Paris, a new CM chondrite considered to be the least‐altered CM identified so far (Hewins et al. 2014 ). Compared to other CMs, Paris exhibits (1) a higher concentration of Fe‐Ni metal beads, with nickel contents in the range 4.1–8.1 wt%; (2) the systematic presence of thin lamellae and tiny blebs of pentlandite in pyrrhotite grains; and (3) ubiquitous tochilinite/cronstedtite associations with higher FeO/SiO₂ and S/SiO₂ ratios. In addition, Paris shows the highest concentration of trapped ³⁶Ar reported so far for a CM chondrite (Hewins et al. 2014 ). In combination with the findings of previous studies, our data confirm the reliability of (1) the alteration sequence based on the chemical composition of tochilinite/cronstedtite associations to quantify the fluid alteration processes and (2) the use of Cr content variability in type II ferroan chondrule olivine as a proxy of thermal metamorphism. In contrast, the scales based on (1) the Fe³⁺ content of serpentine in the matrix to estimate the degree of aqueous alteration and (2) the chemical composition of Fe‐Ni metal beads for quantifying the intensity of the thermal metamorphism are not supported by the characteristics of Paris. It also appears that the amount of trapped ³⁶Ar is a sensitive indicator of the secondary alteration modifications experienced by chondrites, for both aqueous alteration and thermal metamorphism. Considering Paris, our data suggest that this chondrite should be classified as type 2.7 as it suffered limited but significant fluid alteration and only mild thermal metamorphism. These results point out that two separated scales should be used to quantify the degree of the respective role of aqueous alteration and thermal metamorphism in establishing the characteristics of CM chondrites.     

A nondestructive analytical method for stone meteorites—and a controversial discrepancy

Abstract— A method is described for whole rock analyses of major elements in stone meteorites using the electron microprobe and requiring only powdering of the sample, most of which can be retrieved after analysis for additional analytical studies, such as instrument neutron activation analysis (INAA), radiochemical neutron activation analysis (RNAA) and O-isotopic analysis. Whole individual chondrules of > 1 mg can be analyzed. The method is especially attractive for meteorites in short supply or of great rarity. A total of 398 meteorites were analyzed by this method. The results compare favorably with wet chemical analyses. A study was made of seventeen ordinary chondrites to compare their whole rock (metal free) compositions with the averaged compositions of eleven to thirty-eight of their respective individual chondrules (a total of 374 chondrules). The oxide ratio Al₂O₃/CaO is generally lower in chondrules than in their respective chondrites, the disparity being larger for petrographic grade 5 than for grade 3. Ordinary chondrites are not simply the sum of their respective chondrules. Furthermore, correlations between CaO, Al₂O₃ and TiO₂ are strong for chondrules in unequilibrated chondrites and nonexistent in equilibrated chondrites. Also H, L and LL chondrite groups have similar bulk compositions within their respective groups, in spite of the different proportions of chondrules, kinds of chondrules, chondrule debris, and matrix. All this brings into question the metamorphic classification in which high petrographic grades are the metamorphosed equivalents of low petrographic grades.     

Petrographic classification of Middle Ordovician fossil meteorites from Sweden

Abstract— The maximum diameter of chromite (FeCr₂O₄) grains within L chondrites reflects the petrographic type of the sample. On the basis of our measurements of nine recent L chondrites, L3 chromite D_max = 34–50 μm, L4 = 87–150 μm, L5 = 76–158 μm, and L6 = 253–638 μm. This variation reflects the crystallization of the chromite grains during parent body thermal metamorphism. We use this calibration to classify six fossil meteorites from the Middle Ordovician in Sweden as type 3 (or 4) to 6. The high flux of L chondrites at 470 Ma contained a range of petrographic types and may have had a higher proportion of lower petrographic type meteorites than are found in recent L chondrite falls. The fossil meteorites have in places preserved recognizable chondrule textures, including porphyritic olivine, barred olivine, and radiating pyroxene. A large relict clast and fusion crust have also been tentatively identified in one fossil meteorite. Apart from chromite, all of the original meteorite minerals have been replaced by carbonate (and sheet silicate and sulfate) during diagenesis within the limestone host. The preservation of chondrule definition has allowed us to measure the mean diameters of relict chondrules. The range (0.4–0.6 mm) is consistent with measurements made in the same way on recent L chondrites.     

Reclassification of Hart and Northwest Africa 6047: Criteria for distinguishing between CV and CK3 chondrites

The single parent body model for the CV and CK chondrites (Greenwood et al. 2010 ) was challenged by Dunn et al. ( 2016a ), who argued that magnetite compositions could not be reconciled by a single metamorphic sequence (i.e., CV3 → CK3 → CK4–6). Cr isotopic compositions, which are distinguishable between the CV and CK chondrites, also support two different parent bodies (Yin et al. 2017 ). Despite this, there are many petrographic and mineralogical similarities between the unequilibrated (petrologic type 3) CK chondrites and the CV chondrites (also type 3), which may result in misclassification of samples. Hart and Northwest Africa 6047 (NWA 6047) are an excellent example of this. In this study, we revisit the classification of Hart and NWA 6047 using magnetite compositions, petrography, and compositions of olivine, the most ubiquitous mineral in both CV and CK chondrites. Not only do our results suggest that NWA 6047 and Hart were misclassified, but our assessment of CV and CK3 chondrites has also led to the development of criteria that can be used to distinguish between CV and CK3 chondrites. These criteria include: abundances of Cr₂O₃, TiO₂, NiO, and Al₂O₃ in magnetite; Fa content and NiO abundance of matrix olivine; FeO content of chondrules; and the chondrule:matrix ratio. Classification as a CV chondrite is also supported by the presence of igneous chondrule rims, calcium‐aluminum‐rich inclusions, and an elongated petrofabric. However, none of these petrographic characteristics can be used conclusively to distinguish between CV and CK3 chondrites.     

THE IJOPEGA CHONDRITE: A NEW H6 FALL

The Ijopega (Papua New Guinea) meteorite is a new H6 group chondrite fall which contains olivine (Fa 19.9 mole %), bronzite (Fs 17.8 mole %), plagioclase (An 12.1 Or 6.3 Ab 81.6 mole %), diopside, kamacite, taenite, troilite, chromite and whitlockite. The meteorite is extensively recrystallized and brecciated, and shows evidence of moderate shock deformation. Examination of Fe²⁺ and Mg partitioning between ortho- and clinopyroxene indicates a high equilibration temperature (940° or 880 °C). Chemical analysis shows the meteorite to be rich in S, containing about twice the average H-group abundance. Trace elements, including REE, are in accord with established H-group chondrite abundances.     

The Binningup H5 Chondrite: A New Fall from Western Australia

Abstract— Following a brilliant daylight fireball at 10:10 a.m. (local time) on 30 September 1984, a single stone weighing 488.1 grams was recovered from Binningup beach (33°09′23″S, 115°40′35″E), Western Australia. Data from 23 reported sightings of the fireball indicate an angle of trajectory 20–40° from the horizontal, a flight-path bearing N210°E and an end-point (ca. 32°39′S, 115°54.5′E) at a height of ～20–30 km. A recrystallized chondritic texture and the presence of olivine and low-Ca orthopyroxene with compositions of Fa_18.4 (PMD 1.1)and Fs_16.1 (PMD 1.1), respectively, show that Binningup is a typical member of the H-group of ordinary chondrites. Uniform mineral compositions and the presence of generally microcrystalline plagioclase feldspar indicate that the meteorite belongs to petrologic type 5. Pervasive fracturing of silicates suggests mild pre-terrestrial shock loading. Measurements (dpm kg^?1) of cosmogenic radionuclides including ²²Na (61 ± 5), ²⁶Al (49 ± 3) and ⁵⁴Mn (66 ± 10) indicate a normal history of irradiation.     

The iron record of asteroidal processes in carbonaceous chondrites

The valence of iron has been used in terrestrial studies to trace the hydrolysis of primary silicate rocks. Here, we use a similar approach to characterize the secondary processes, namely thermal metamorphism and aqueous alteration, that have affected carbonaceous chondrites. X‐ray absorption near‐edge structure spectroscopy at the Fe‐K‐edge was performed on a series of 36 CM, 9 CR, 10 CV, and 2 CI chondrites. While previous studies have focused on the relative distribution of Fe⁰ with respect to oxidized iron (Fe_ox = Fe²⁺ + Fe³⁺) or the iron distribution in some specific phases (e.g., Urey–Craig diagram; Urey and Craig 1953), our measurements enable us to assess the fractions of iron in each of its three oxidation states: Fe⁰, Fe²⁺, and Fe³⁺. Among the four carbonaceous chondrites groups studied, a correlation between the iron oxidation index (IOI = [2(Fe²⁺) + 3(Fe³⁺)]/[Fe_TOT]) and the hydrogen content is observed. However, within the CM group, for which a progressive alteration sequence has been defined, a conversion of Fe³⁺ to Fe²⁺ is observed with increasing degree of aqueous alteration. This reduction of iron can be explained by an evolution in the mineralogy of the secondary phases. In the case of the few CM chondrites that experienced some thermal metamorphism, in addition to aqueous alteration, a redox memory of the aqueous alteration is present: a significant fraction of Fe³⁺ is present, together with Fe²⁺ and sometimes Fe⁰. From our data set, the CR chondrites show a wider range of IOI from 1.5 to 2.5. In all considered CR chondrites, the three oxidation states of iron coexist. Even in the least‐altered CR chondrites, the fraction of Fe³⁺ can be high (30% for MET 00426). This observation confirms that oxidized iron has been integrated during formation of fine‐grained amorphous material in the matrix (Le Guillou and Brearley 2014; Le Guillou et al. 2015; Hopp and Vollmer 2018). Last, the IOI of CV chondrites does not reflect the reduced/oxidized classification based on metal and magnetite proportions, but is strongly correlated with petrographic types. The valence of iron in CV chondrites therefore appears to be most closely related to thermal history, rather than aqueous alteration, even if these processes can occur together (Krot et al. 2004; Brearley and Krot 2013).     

Effect of hot desert weathering on the bulk‐rock iron isotope composition of L6 and H5 ordinary chondrites

Abstract– Although iron isotopes are increasingly used for meteorites studies, no attempt has been made to evaluate the effect of terrestrial weathering on this isotopic tracer. We have thus conducted a petrographic, chemical, and iron isotopic study of equilibrated ordinary chondrites (OC) recovered from hot Moroccan and Algerian Saharan deserts environment. As previously noticed, we observe that terrestrial desertic weathering is characterized by the oxidation of Fe‐Ni metal (Fe⁰), sulfide and Fe²⁺ occurring in olivine and pyroxene. It produces Fe‐oxides and oxyhydroxides that partially replace metal, sulfide grains and also fill fractures. The bulk chemical compositions of the ordinary chondrites studied show a strong Sr and Ba enrichment and a S depletion during weathering. Bulk meteoritic iron isotope compositions are well correlated with the degree of weathering and S, Sr, and Ba contents. Most weathered chondrites display the heaviest isotopic composition, by up to 0.1‰, which is of similar magnitude to the isotopic variations resulting from meteorite parent bodies’ formation and evolution. This is probably due to the release of isotopically light Fe²⁺ to waters on the Earth’s surface. Hence, when subtle Fe isotopic effects have to be studied in chondrites, meteorites with weathering grade above W2 should be avoided.     

The activity of chromite in multicomponent spinels: Implications for T‐fO2 conditions of equilibrated H chondrites

Abstract— Activities of chromite in multicomponent spinels with compositions similar to those of H chondrites were experimentally determined by equilibrating Pt‐alloys with spinel at known temperature and fO₂. Our results are consistent with predictions based on the spinel solid solution model incorporated into the MELTS program. Therefore, we combined literature formulations for the activities of components in spinel, the ferromagnesian silicates, and alloys with measured and literature (bulk alloy) compositions of the meteoritic phases to constrain T‐fO₂ conditions for the H‐group chondrites Avanhandava (H4), Allegan (H5), and Guareña (H6). Log₁₀fO₂ values based on the assemblage of olivine + orthopyroxene + metal are 2.19–2.56 log units below the iron‐wüstite (IW) buffer for any equilibration temperature between 740 and 990 °C, regardless of petrographic type. Only lower limits on fO₂ could be determined from spinel + metal equilibria because of the extremely low concentrations of Cr in the alloys of equilibrated H chondrites (≤3 ppb). Log₁₀fO₂ values required by spinel + metal equilibria are inconsistent with those for olivine + orthopyroxene + metal if equilibration temperatures were at or above those inferred from olivine‐spinel thermometry. This probably indicates that the closure for spinel + metal equilibria occurred under retrograde conditions at temperatures below ～625 °C for Allegan and Guareña and below ～660 °C for Avanhandava.     

Perceptive of the pyroxene-bearing micrometeorites and their relation to chondrites

We studied 149 pyroxenes from 69 pyroxene-bearing micrometeorites collected from deep-sea sediments of the Indian Ocean and South Pole Water Well at Antarctica, Amundsen-Scott South Pole station. The minor elements in pyroxenes from micrometeorites are present in the ranges as follows: MnO ~0.0–0.4 wt%, Al₂O₃ ~0.0–1.5 wt%, CaO ~0.0–1.0 wt%, Cr₂O₃ ~0.3–0.9 wt%, and FeO ~0.5–4 wt%. Their chemical compositions suggest that pyroxene-bearing micrometeorites are mostly related to precursors from carbonaceous chondrites rather than ordinary chondrites. The Fe/(Fe+Mg) ratio of the pyroxenes and olivines in micrometeorites shows similarities to carbonaceous chondrites with values lying between 0 and 0.2, and those with values beyond this range are dominated by ordinary chondrites. Atmospheric entry of the pyroxene-bearing micrometeorites is expected to have a relatively low entry velocity of <16 km s⁻¹ and high zenith angle (70–90°) to preserve their chemical compositions. In addition, similarities in the pyroxene and olivine mineralogical compositions between carbonaceous chondrites and cometary particles suggest that dust in the solar system is populated by materials from different sources that are chemically similar to each other. Our results on pyroxene chemical compositions reveal significant differences with those from ordinary chondrites. The narrow range in olivine and pyroxene chemical compositions are similar to those from carbonaceous chondrites, and a small proportion to ordinary chondrites indicates that dust is largely sourced from carbonaceous chondrite-type bodies.