Fall,classification and cosmogenic records of the Sabrum (LL6) chondrite

Abstract— The petrographic and chemical characteristics of a fresh Indian meteorite fall at Sabrum are described. Its mean mineral composition is defined by olivine (Fa_31.4), orthopyroxene (Fs_25.1,Wo_2.0), clinopyroxene (Wo₄₅En_45.6Fs_9.4) and plagioclase (An_10.6Ab_83.6Or_5.8). The meteorite shows moderate shock features, which indicate that it belongs to the S4 category. Based on mineralogical and chemical criteria the meteorite is classified as an LL6 brecciated veined chondrite. Several cosmogenic radioisotopes (⁴⁶Sc, ⁷Be, ⁵⁴Mn, ²²Na and ²⁶Al), noble gas (He, Ne, Ar, Kr and Xe), nitrogen isotopes, and particle tracks density have been measured. Concentrations of cosmogenic ²¹Ne and ³⁸Ar indicate that its cosmic‐ray exposure age is 24.8 Ma. Small amounts of trapped Kr and Xe, consistent with petrologic class 5/6, are present. The track density in olivines is found to be (1.3 ± 0.3) × 10⁶/cm². Activities of most of the short‐lived isotopes are lower than those expected from solar cycle variation. ²²Na/²⁶Al (1.12 ± 0.02) is found to be significantly anomalous, being ?25% lower than expected from the Climax neutron monitor data. These results indicate that the cosmic‐ray flux during the terminal segment of the meteoroid orbit was low. The activities of ²⁶Al and ⁶⁰Co and the track density indicate small meteoroid size with a radius ?15 cm.     

Itawa Bhopji (L3–5) chondrite regolith breccia: Fall,classification, and cosmogenic records

Abstract— A stony meteorite fell at Itawa Bhopji, Rajasthan, India on 2000 May 30. This is the fifth recorded fall in a small area of Rajasthan during the past decade. The meteorite is an ordinary chondrite with light clasts in a dark matrix, consisting of a mixture of equilibrated (mainly type 5) and unequilibrated components. Olivine is Fa_24–26 and pyroxene Fs_20–22 but, within the unequilibrated components, olivine (Fa_5–29) and low calcium pyroxene (Fs_5–37) are highly variable. Based on petrographic studies and chemical analyses, it is classified as L(3–5) regolith breccia. Studies of various cosmogenic records, including several gamma‐emitting radionuclides varying in half‐life from 5.6 day ⁵²Mn to 0.73 Ma ²⁶Al, tracks and rare gases have been carried out. The exposure age of the meteorite is estimated from cosmogenic components of rare gases to be 19.6 Ma. The track density varies by a factor of ?3 (from 4 to 12 times 10⁶/cm²) within the meteorite, indicating a preatmospheric body of ?9 cm radius (corresponding to a meteoroid mass of ?11 kg) and small ablation (1.5 to 3.6 cm). Trapped components in various rare gases are high and the solar component is present in the dark portion of the meteorite. Large excess of neutron‐produced ⁸²Kr and ¹²⁸Xe in both the light and the dark lithology but very low ⁶⁰Co, indicating low neutron fluxes received by the meteoroid in the interplanetary space, are clear signatures of an additional irradiation on the parent body.     

Petrography,classification, oxygen isotopes,noble gases,and cosmogenic records of Kamargaon (L6) meteorite: The latest fall in India

A single piece of meteorite fell on Kamargaon village in the state of Assam in India on November 13, 2015. Based on mineralogical, chemical, and oxygen isotope data, Kamargaon is classified as an L‐chondrite. Homogeneous olivine (Fa: 25 ± 0.7) and low‐Ca pyroxene (Fs: 21 ± 0.4) compositions with percent mean deviation of <2, further suggest that Kamargaon is a coarsely equilibrated, petrologic type 6 chondrite. Kamargaon is thermally metamorphosed with an estimated peak metamorphic temperature of ~800 °C as determined by two‐pyroxene thermometry. Shock metamorphism studies suggest that this meteorite include portions of different shock stages, e.g., S3 and S4 (Stöffler et al. 1991 ); however, local presence of quenched metal‐sulfide melt within shock veins/pockets suggest disequilibrium melting and relatively higher shock stage of up to S5 (Bennett and McSween 1996 ). Based on noble gas isotopes, the cosmic‐ray exposure age is estimated as 7.03 ± 1.60 Ma and nitrogen isotope composition (δ¹⁵N = 18‰) also correspond well with the L‐chondrite group. The He‐U, Th, and K‐Ar yield younger ages (170 ± 25 Ma 684 ± 93, respectively) and are discordant. A loss of He during the resetting event is implied by the lower He‐U and Th age. Elemental ratios of trapped Ar, Kr, and Xe can be explained through the presence of a normal Q noble gas component. Relatively low activity of ²⁶Al (39 dpm/kg) and the absence of ⁶⁰Co activity suggest a likely low shielding depth and envisage a small preatmospheric size of the meteoroid (<10 cm in radius). The Kr isotopic ratios (⁸²Kr/⁸⁴Kr) further argue that the meteorite was derived from a shallow depth.     

Fall,classification, and exposure history of the Mifflin L5 chondrite

The Mifflin meteorite fell on the night of April 14, 2010, in southwestern Wisconsin. A bright fireball was observed throughout a wide area of the midwestern United States. The petrography, mineral compositions, and oxygen isotope ratios indicate that the meteorite is a L5 chondrite fragmental breccia with light/dark structure. The meteorite shows a low shock stage of S2, although some shock‐melted veins are present. The U,Th‐He age is 0.7 Ga, and the K‐Ar age is 1.8 Ga, indicating that Mifflin might have been heated at the time of the 470 Ma L‐chondrite parent body breakup and that U, Th‐He, and K‐Ar ages were partially reset. The cosmogenic radionuclide data indicate that Mifflin was exposed to cosmic rays while its radius was 30–65 cm. Assuming this exposure geometry, a cosmic‐ray exposure age of 25 ± 3 Ma is calculated from cosmogenic noble gas concentrations. The low ²²Ne/²¹Ne ratio may, however, indicate a two‐stage exposure with a longer first‐stage exposure at high shielding. Mifflin is unusual in having a low radiogenic gas content combined with a low shock stage and no evidence of late stage annealing; this inconsistency remains unexplained.     

Bhawad LL6 chondrite: Chemistry,petrology, noble gases,nuclear tracks,and cosmogenic radionuclides

Abstract— Chemical and mineral analysis of the Bhawad chondrite, which fell in Rajasthan in 2002, suggest that this stone belongs to LL6 group of chondrites. Based on helium, neon, and argon isotopes, it has a cosmic ray exposure age of 16.3 Ma. The track density in the olivines shows a narrow range of 1.7–6.8 times 10⁶/cm². The ²²Na/²⁶Al ratio of 1.13 is about 25% lower than the solar cycle average value of about 1.5, but is consistent with irradiation of the meteoroid to modulated galactic cosmic ray fluxes as expected for a fall around the solar maximum. The cosmogenic records indicate a pre‐atmospheric radius of about 7.5 cm. Based on U/Th‐⁴He and K‐⁴⁰Ar, the gas retention ages are low (about 1.1 Ga), indicating a major thermal event or shock event that lead to the complete loss of radiogenic ⁴He and ⁴⁰Ar and the partial loss of radiogenic ¹²⁹Xe and fission Xe from ²⁴⁴Pu.     

Chemical and isotopic characteristics of the Didwana‐Rajod (H5) chondrite

Abstract— The mineralogical and chemical characteristics of the Didwana‐Rajod chondrite are described. The mean mineral composition is found to be olivine (Fo_83.2) and pyroxene (En_83.5Wo_0.7Fs_15.8), and feldspar is mainly oligoclase. Oxygen isotopic analysis shows δ¹⁸O = +3.8%0 and δ¹⁷O = +2.59%0. The nitrogen content of Didwana‐Rajod is ～2 ppm with δ¹⁵N ? 3.4%0. Based on microscopic, chemical, isotopic and electron probe microanalysis, the meteorite is classified as an H5 chondrite. Cosmogenic tracks, radionuclides and the isotopic composition of rare gases were also measured in this meteorite. The track density in olivines varies in a narrow range with an average value of (6.5 ± 0.5) × 10⁵/cm² for four spot samples taken at the four corners of the stone. The cosmic‐ray exposure age based on neon and argon is 9.8 Ma. ²²Na/²⁶Al ? 0.94 is lower than the solar‐cycle average value of ～1.5 and is consistent with irradiation of the meteoroid to lower galactic cosmic‐ray fluxes as expected at the solar maximum. The track density, rare gas isotopic ratios, ⁶⁰Co activity and other radionuclide data are consistent with a preatmospheric radius of ～15 cm, corresponding to a mass of ～50 kg. The cosmogenic properties are consistent with a simple exposure history in interplanetary space.     

Fall and recovery of the Noblesville (Indiana,U.S.A.) H chondrite

Abstract We describe the circumstances of fall of a 0.5-kg meteorite that landed in Noblesville, Indiana, USA, 3.6 m in front of two witnesses on 1991 August 31. Provisional assessment of Noblesville as an H chondrite is confirmed by data reported elsewhere which demonstrate that it is a regolith breccia with unusual properties.     

Solar and galactic cosmic‐ray records of the Fermo (H) chondrite regolith breccia

Abstract— ‐We demonstrate the presence of solar flare as well as neutron capture effects in the isotopic composition of rare gases in the Fermo regolith breccia acquired on its parent body based on the measurements of tracks, rare gases and radionuclides. The track density along a 3.2 cm long core decreases by a factor of about 6 and by more than a factor of 13 within the meteorite, indicating small (2–9 cm) and asymmetrical ablation. Rare gases show a large trapped component; the isotopic ratios, particularly ²⁰Ne/²²Ne ? 11 and ²⁰Ne/³⁶Ar = 10 are indicative of a solar component. The galactic cosmic‐ray exposure age is determined to be 8.8 Ma. Activities of a dozen radionuclides ranging in half‐life from 16 day ⁴⁸V to 0.73 Ma ²⁶Al are consistent with their expected production rates. Track, rare gas and radionuclide data show that the meteoroid was a small body (≤ 120 kg) and had a simple, one‐stage exposure history to cosmic rays in the interplanetary space. However, ⁸²Kr and ¹²⁸Xe show an excess due to neutron irradiation on the parent body of the meteorite. The presence of solar gases and the neutron capture effects indicate several stages of irradiation on the parent asteroid. The chemical composition of Fermo confirms that it belongs to the H group of ordinary chondrites with lithic clasts having varying compositions. δ¹⁵N is found to be 8.3 ± 1.2%0, close to the typical values observed in H chondrites.     

The Wilder H5 chondrite: A 1982 find from Idaho

Abstract— The Wilder chondrite, a single stone of 1970 g, was found in southwestern Idaho in 1982. A classification of H5 is indicated by the mean compositions of olivines (Fa_18.4) and orthopyroxenes (Fs_16.3), and the narrow range in mineral compositions, recrystallized nature of the matrix, and mean Wo content of the orthopyroxenes (Wo_1.34).     

Complex brecciation and shock effects in the Buck Mountain Wash (H3–5) chondrite

Abstract Buck Mountain Wash (BMW) is a new genomict breccia (H3‐5) found in the Franconia (H5) strewn field in Arizona that shows complex brecciation and shock effects. It contains three distinct chondritic lithologies in sharp contact: a) a main lithology that consists primarily of petrographic type 5 material but which has finely intermixed type 3 and 4 material, b) a shock‐blackened (shock stage S5) type 3 lithology (lithology A), and c) a shock‐blackened type 3/4 lithology (lithology B). Buck Mountain Wash was lithified after impact‐mixing and impact‐melting of weakly and strongly metamorphosed materials, possibly at depth in the regolith of the parent body. Shock effects included brecciation on a fine scale, localized impact‐melting of silicates, partial melting, and mobilization of metal‐sulfide, and chemical fractionations that produced non‐H‐group composition kamacite by two disequilibrium mechanisms. Shock heating did not cause significant thermal metamorphism in the shock‐blackened lithologies of BMW, except possibly in areas adjacent to whole‐rock shock melt. During lithification, cooling must have been rapid at high temperatures to preserve glass and inhomogeneous silicate compositions, but not so fast at lower temperatures as to produce dendritic metal‐sulfide globules or martensite.     

The Lohawat howardite: Mineralogy,chemistry and cosmogenic effects

Abstract— The Lohawat meteorite is a texturally heterogeneous breccia having a variety of mineral and lithic fragments. Among mineral fragments, pyroxenes show a wide range of composition (Wo_0.011–0.17En_0.37–0.78Fs_0.21–0.60) whereas plagioclase is anorthitic (An_0.92Ab_0.07Or_0.007). Abundant rounded “chondrule‐like” objects ranging in size up to ～7 mm, some with concentric layering, have been observed. Petrographic features, trace element composition and rare earth element patterns show the presence of eucritic and diogenitic components confirming that it is a typical howardite. Cosmogenic tracks, rare gases (He, Ne, and Ar) and radionuclides (²²Na and ²⁶Al) were measured. Track density in olivine and plagioclase varies between 0.7 to 6 times 10⁶/cm². ³⁸Ar exposure age is estimated to be ～110 Ma, being the highest among howardites. The track production rates correspond to ablation of 9 to 15 cm, implying a radius for its preatmospheric size of ～27 cm. ²²Na/²⁶Al ～ 1, as expected from the production models and solar modulation of galactic cosmic‐ray fluxes before its fall, suggesting that the meteoroid did not undergo any fragmentation during the past ～2 Ma in interplanetary space. The radiogenic age based on K‐Ar method is 4.3 Ga while the U‐Th‐⁴He age is 3.3 Ga indicating partial loss of He.     

Lea County 001, an H5 chondrite,and Lea County 002, an ungrouped type 3 chondrite

Abstract— A search of active deflation basins near Jal, Lea County, New Mexico resulted in the discovery of two meteorites, Lea County 001 and 002. Lea County 001 has mean olivine and low-Ca pyroxene compositions of Fa₁₉ and Fs₁₇, respectively. These and all other mineralogical and petrological data collected indicate a classification of H5 for this stone. Lea County 002 has mean olivine and low-Ca pyroxene compositions of Fa₂ and Fs₄, and is unequilibrated. Although it is mineralogically most similar to Kakangari and chondritic clasts within Cumberland Falls, the high modal amount of forsterite makes Lea County a unique type 3 chondrite. Oxygen isotope data for Lea County 002 fall on an ¹⁶O-mixing line through those of the enstatite meteorites and IAB irons, a feature shared by Kakangari.     

The new polymict eucrite Dar al Gani 983: Petrography,chemical composition,noble gas record,and evolution

Abstract— Mineralogical and chemical studies of Dar al Gani 983 show that this meteorite is a eucrite. Its texture is that of an impact breccia. It contains cumulate pyroxene and feldspar megacrysts, a variety of recrystallized melt clasts, clasts of subophitic basalt, and mesostasis. These components are embedded in a matrix of fragmental pyroxene and plagioclase. In addition, the entire rock is penetrated by glassy melt veins and patches, and displays features of strong shock. The mineralogical and chemical evidence obtained for DaG 983 indicates that this meteorite experienced a complex evolutionary history. The presence of cumulate silicate crystals implies substantial, large scale cratering events on the HED asteroid. As a result of these impacts, rocks from different intrusive bodies to extrusive surface layers were laterally and vertically transported to form a thoroughly mixed megaregolith. DaG 983 represents a sample of this megabreccia.     

The Leedey,Oklahoma, chondrite: Fall,petrology, chemistry and an unusual Fe,Ni-FeS inclusion

Abstract— The Leedey, Oklahoma, meteorite shower fell on 1943 November 25, following a fireball which was visible across much of southwestern Oklahoma and northcentral Texas. The shower produced 24 stones with a total mass of ～51.5 kg. The stones formed a strewnfield ～18 km in length in the same direction as the observed path of the meteor (N50°W). Leedey is classified as an L6(S3) ordinary chondrite. We report bulk major element chemical analyses from four separate laboratories. Leedey contains an unusual 6 by 8 mm composite Fe,Ni-FeS grain, which is composed of a 3 mm kamacite grain adjacent to a 5 mm troilite grain. A 50–100 μm rim of high-Ni (45–55 wt%) taenite (tetrataenite) occurs at the boundary between kamacite and troilite. A single, zoned pyrophanite grain is observed at the boundary between the inclusion troilite and host silicates. An origin as a foreign particle incorporated after metamorphism or during impact melting appears unlikely. This particle likely formed by a complex set of processes, including melting in the nebula, parent body metamorphism and reheating by later shock, mirroring the history of the host chondrite.     

New mineralogical and chemical data on the Machinga (L6) chondrite,Malawi

Abstract— The Machinga, southern Malawi, Africa, L6 chondrite (observed fall, 22 January 1981) contains accessory phases of metal, troilite, chromite, and native Cu (which is associated with limonite and found in zones of aqueous alteration). Rare accessory phases are apatite and pentlandite, which are uncommon in L6 chondrites. Major mineral constituents (olivine, orthopyroxene, and plagioclase) indicate shock effects at a level of about 15–20 GPa shock pressure. The meteorite is thus classified to be of L6d type. Melt pockets of widely variable composition are abundant.     

The thermal and physical characteristics of the Gao-Guenie (H5) meteorite

Measurements of the bulk density, grain density, porosity, and magnetic susceptibility of 19 Gao-Guenie H5 chondrite meteorite samples are presented. We find average values of bulk density 〈ρ_bulk〉=3.46±0.07 g/cm³, grain density 〈ρ_grain〉=3.53±0.08 g/cm³, porosity 〈P(%)〉=2.46±1.39, and bulk mass magnetic susceptibility 〈log χ〉=5.23±0.11. Measurements of the specific heat capacity for a 3.01-g Gao-Guenie sample, a 61.37-g Gao-Guenie sample, a 62.35-g Jilin H5 chondrite meteorite sample, and a 51.37-g Sikhote-Alin IIAB Iron meteorite sample are also presented. Temperature interpolation formula are further provided for the specific heat capacity, thermal conductivity, and thermal diffusivity of the 3.01-g Gao-Guenie sample in the temperature range 300<T (K)<800. We briefly review the possible effects of the newly deduced specific heat and thermal conductivity values on the ablation of meteoroids within the Earth's atmosphere, the modeling of asteroid interiors and the orbital evolution of meteoroids through the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect.     

The Burnwell,Kentucky, low iron oxide chondrite fall: Description,classification and origin

Abstract— The Burnwell, Kentucky, meteorite fell as a single stone on 1990 September 4. The Burnwell meteorite has lower Fa in olivine (15.8 mol%), Fs in orthopyroxene (13.4 mol%), Co in kamacite (0.36 wt%), FeO from bulk chemical analysis (9.43 wt%), and Δ¹⁷O (0.51 ± 0.02%), and higher Fe, Ni, Co metal (19.75 wt% from bulk wet chemical analysis) than observed in H chondrites. The Burnwell meteorite plots on extensions of H-L-LL chondrite trends for each of these properties towards more reducing compositions than in H chondrites. Extensions of this trend have been previously suggested in the case of other low-FeO chondrites or silicate inclusions in the HE iron Netschaëvo, but interpretation of the evidence in these meteorites is complicated by terrestrial weathering, chemical disequilibrium or reduction. In contrast, the Burn-well meteorite is an equilibrated fall that exhibits no evidence for reduction. As such, it provides the first definitive evidence for extension of the H-L-LL ordinary chondrite trend beyond typical H values towards more reducing compositions.     

Devgaon (H3) chondrite: Classification and complex cosmic ray exposure history

Abstract— The Devgaon meteorite fell in India on February 12, 2001 and was immediately collected. It is an ordinary chondrite having a number of SiO₂‐rich objects and some Ca, Al‐rich inclusions. Olivines (Fa_17–19) are fairly equilibrated, while pyroxenes (Fs_4–20) are unequilibrated. Occasionally, shock veins are visible, but the bulk rock sample is very weakly shocked (S2). Chondrules and chondrule fragments are abundant. Based on chemical and petrological features, Devgaon is classified as an H3.8 group chondrite. Several cosmogenic radionuclides ranging in half‐lives from 5.6 d (⁵²Mn) to 7.3 times 10⁵ yr (²⁶Al), noble gases (He, Ne, Ar, Kr, and Xe), and particle track density have been measured. The track density in olivines from five spot samples varies between (4.6 to 9) × 10⁶ cm^?2 showing a small gradient within the meteorite. The light noble gases are dominated by cosmogenic and radiogenic components. Large amounts of trapped gases (Ar, Kr, and Xe) are present. In addition, (n, γ) products from Br and I are found in Kr and Xe, respectively. The average cosmic ray exposure age of 101 ± 8 Ma is derived based on cosmogenic ³⁸Ar, ⁸³Kr, and ¹²⁶Xe. The track production rates correspond to shielding depths of about 4.9 to 7.8 cm, indicating that the stone suffered type IV ablation. Low ⁶⁰Co, high (²²Ne/²¹Ne)_c, and large neutron produced excesses at ⁸⁰Kr, ⁸²Kr, and ¹²⁸Xe indicate a complex exposure history of the meteoroid. In the first stage, a meter‐sized body was exposed for nearly 10⁸ yr in the interplanetary space that broke up in ?50 cm‐sized fragments about a million years ago (stage 2), before it was captured by the Earth.     

The Iguaraçu H5 chondrite: a fall from Paraná State,Brazil

Abstract— The Iguaraçu meteorite, a single stone of 1200 g, fell in Paraná State, Brazil in October 1977. It is classified as an H5 ordinary chondrite with olivine of composition Fa 18.7.