Tabbita: An L6c chondrite from New South Wales,Australia

Abstract— A crusted stone weighing 3.10 kg was found in 1983 near Tabbita in south central New South Wales (ca. 34°03′S, 145°50′E), Australia. Compositions of the ferro-magnesian silicates (olivine Fa_24.6; orthopyroxene Fs_20.9) show that the meteorite belongs to the L-group of chondrites. Uniformity of silicate compositions and the presence of abundant crystalline plagioclase feldspar (An_10.8Ab_81.7Or_7.5) show that the meteorite belongs to petrologic type 6. Silicates that display undulose extinction, and the absence of any thermal effects induced by shock indicate that Tabbita is shock facies c. Tabbita is distinct from several other L6 chondrites found in the same general area.     

PETROLOGY AND CLASSIFICATION OF THE GARRAF,SPAIN CHONDRITE

Microscopic and electron microprobe studies indicate that the Garraf meteorite is a highly-recrystallized chondrite of petrologic type 6. Olivine (Fa24.7; PMD 1.1) and low-Ca pyroxene (Fs20.9; PMD 1.1) compositions indicate that it belongs to the L-group. Based on contents of noble gases, pervasive fracturing of silicates, common undulose extinction of olivine and plagioclase, and the lack of melt pockets and maskelynite, we place Garraf into shock facies b. We conclude that Garraf is a highly recrystallized L6b chondrite that, after recrystallization, was cataclased and comminuted by shock.     

THE BEAVER-HARRISON,BEAVER COUNTY,UTAH L6 CHONDRITE

The Beaver-Harrison, Utah chondrite (find July 24, 1979), a single, shock-veined stone of 925 grams, consists of major olivine (Fa_25.0), low-Ca pyroxene (En_77.3Fs_21.1Wo_1.6) and metallic nickel-iron; minor troilite and plagioclase (Ab_82.6An_11.1Or_6.3), accessory high-Ca pyroxene (En_47.0Fs_8.5Wo_44.5), chromite (Cm_8.7Sp_10.6Uv_9.4Pc_0.6Hc_0.7), chlorapatite and whitlockite; and hydrous ferric oxide of terrestrial weathering origin. Mineral compositions indicate L-group classification, and homogeneity of minerals, highly recrystallized texture and presence of clear plagioclase suggest that the meteorite belongs to petrologic type 6.     

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.     

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.     

PETROLOGICAL FEATURES OF SHOCK METAMORPHISM IN CHONDRITES: ALFIANELLO

The Alfianello meteorite was inspected by optical microscopy, both by transmitted and reflected light, in order to look for evidence of shock metamorphism. The stone is a well-recrystallized olivine-hypersthene chondrite that shows unambiguous features of dynamic deformation. Some examples of planar deformation structures, which are notable in olivine and plagioclase feldspar crystals, are reported. The presence of several shock-transformed components is described in some detail. Alfianello appears to belong to a class of chondrites lightly-to-moderately shocked.     

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.     

Complex origins of silicate veinlets in HED meteorites: A case study of Northwest Africa 1109

We report on the petrography and mineralogy of three types of silicate veinlets in the brecciated eucrite Northwest Africa (NWA) 1109. These include Fe‐rich olivine, Mg‐rich olivine, and pyroxene veinlets. The Fe‐rich olivine veinlets mainly infill fractures in pyroxene and also occur along grain boundaries between pyroxene and plagioclase crystals, in both nonequilibrated and equilibrated lithic clasts. The host pyroxene of Fe‐rich olivine veinlets shows large chemical variations between and within grains. The Fe‐rich olivine veinlets also contain fine‐grained Fe³⁺‐bearing chromite, highly calcic plagioclase, merrillite, apatite, and troilite. Based on texture and mineral chemistry, we argue that the formation of Fe‐rich olivine was related to fluid deposition at relatively high temperatures. However, the source of Fe‐rich olivine in the veinlets remains unclear. Magnesium‐rich olivine veinlets were found in three diogenitic lithic clasts. In one of these, the Mg‐rich olivine veinlets only occur in one of the fine‐grained interstitial regions and extend into fractures within surrounding coarse‐grained orthopyroxene. Based on the texture of the interstitial materials, we suggest that the Mg‐rich olivine veinlets formed by shock‐induced localized melting and recrystallization. Pyroxene veinlets were only observed in one clast where they infill fractures within large plagioclase grains and are associated with fine‐grained pyroxene surrounding coarse‐grained pyroxene. The large chemical variations in pyroxene and the fracture‐filling texture indicate that the pyroxene veinlets might also have formed by shock‐induced localized melting and rapid crystallization. Our study demonstrates that silicate veinlets formed by a range of different surface processes on the surface of Vesta.     

Shock‐induced melting,recrystallization, and exsolution in plagioclase from the Martian lherzolitic shergottite GRV 99027

Abstract— Plagioclase in the Martian lherzolitic shergottite Grove Mountains (GRV) 99027 was shocked, melted, and recrystallized. The recrystallized plagioclase contains lamellae of pyroxene, olivine, and minor ilmenite (<1 μm wide). Both the pyroxene and the olivine inclusions enclosed in plagioclase and grains neighboring the plagioclase were partially melted into plagioclase melt pools. The formation of these lamellar inclusions in plagioclase is attributed to exsolution from recrystallizing melt. Distinct from other Martian meteorites, GRV 99027 contains no maskelynite but does contain recrystallized plagioclase. This shows that the meteorite experienced a slower cooling than maskelynite‐bearing meteorites. We suggest that the parent rock of GRV 99027 could have been embedded in hot rocks, which facilitated a more protracted cooling history.     

Detailed mineralogy and petrology of highly shocked poikilitic shergottite Northwest Africa 6342

Northwest Africa (NWA) 6342 is an intermediate, poikilitic shergottite, found in Algeria in 2010. It is comprised of two distinct petrographic areas; poikilitic domains with rounded Mg‐rich olivine chadacrysts enclosed by large low‐Ca pyroxene oikocrysts, and a nonpoikilitic domain mainly comprised of subhedral olivine and vesicular recrystallized plagioclase. Oxygen fugacity conditions become more oxidizing during crystallization from the poikilitic to the nonpoikilitic domain (QFM?3.0 to QFM?2.2). As such, it is likely that NWA 6342 experienced a two‐stage (polybaric) crystallization history similar to that of the enriched poikilitic shergottites. NWA 6342 also experienced relatively high levels of shock metamorphism in comparison to most other poikilitic shergottites as evidenced by the fine‐grained recrystallization texture in olivine, as well as melting and subsequent crystallization of plagioclase. The recrystallization of plagioclase requires an extended period of postshock thermal metamorphism for NWA 6342 and similarly shocked intermediate poikilitic shergottites NWA 4797 and Grove Mountains 99027 most likely due to launch from Mars. The similarities in petrology, chemistry, and shock features between these three meteorites indicate that they have similar crystallization and shock histories; possibly originating from the same source area on Mars.     

Redistribution of elements in the heavily shocked Yanzhuang chondrite

Abstract— Compositions of metal, sulfide, olivine, pyroxene, and plagioclase/plagioclase glass were studied for the melted and unmelted parts of the heavily shocked H6(S6) chondrite‐Yanzhuang. We found that the partitioning of some trace elements significantly changed between the 2 parts; compared with the corresponding minerals in the unmelted part, Ga is enriched in the metal, Co, Cr, and Zn are enriched in the sulfide, Cr is enriched in olivine and pyroxene, and Ti is enriched in the plagioclase glass of the melt pocket. These detailed studies of the mineral phases put constraints on 3 important parameters (temperature, pressure, and duration) associated with the post‐shock melting process. The coexistence of melted and unmelted olivine in the melt pocket of Yanzhuang implies a peak temperature after shock that approaches the melting point of olivine. The lack of Ni in the olivine crystallized from a melt suggests crystallization of olivine at pressures below 10 kbar. The resetting of Ga partitioning between metal and silicate in the melt pocket indicates that the interval from the peak temperature after shock to the crystallization of metal‐sulfide and plagioclase glass in the melted part of Yanzhuang is longer than 500 sec.     

Analysis of ordinary chondrites using powder X‐ray diffraction: 2. Applications to ordinary chondrite parent‐body processes

Abstract– We evaluate the chemical and physical conditions of metamorphism in ordinary chondrite parent bodies using X‐ray diffraction (XRD)‐measured modal mineral abundances and geochemical analyses of 48 type 4–6 ordinary chondrites. Several observations indicate that oxidation may have occurred during progressive metamorphism of equilibrated chondrites, including systematic changes with petrologic type in XRD‐derived olivine and low‐Ca pyroxene abundances, increasing ratios of MgO/(MgO+FeO) in olivine and pyroxene, mean Ni/Fe and Co/Fe ratios in bulk metal with increasing metamorphic grade, and linear Fe addition trends in molar Fe/Mn and Fe/Mg plots. An aqueous fluid, likely incorporated as hydrous silicates and distributed homogeneously throughout the parent body, was responsible for oxidation. Based on mass balance calculations, a minimum of 0.3–0.4 wt% H₂O reacted with metal to produce oxidized Fe. Prior to oxidation the parent body underwent a period of reduction, as evidenced by the unequilibrated chondrites. Unlike olivine and pyroxene, average plagioclase abundances do not show any systematic changes with increasing petrologic type. Based on this observation and a comparison of modal and normative plagioclase abundances, we suggest that plagioclase completely crystallized from glass by type 4 temperature conditions in the H and L chondrites and by type 5 in the LL chondrites. Because the validity of using the plagioclase thermometer to determine peak temperatures rests on the assumption that plagioclase continued to crystallize through type 6 conditions, we suggest that temperatures calculated using pyroxene goethermometry provide more accurate estimates of the peak temperatures reached in ordinary chondrite parent bodies.     

THE MARYVILLE METEORITE: A 1983 FALL OF AN L6 CHONDRITE

The Maryville chondrite fell on January 28, 1983 in eastern Tennessee. Compositions of olivine (Fa 24.5), orthopyroxene (Fs 20.8), plagioclase (An 10.6), along with the bulk composition and siderophile concentrations, indicate L-group classification. The presence of highly equilibrated minerals, strongly recrystallized matrix, and the development of large, clear plagioclase grains suggest petrologic type 6 classification. Subsequent to metamorphism the meteorite was subjected to high transient pressures that converted some feldspar to glass, deformed the silicates, and caused small amounts of melting to occur. The effects of this shock event correspond to shock facies “d” or “e”.     

RAGLAND,AN LL3.4 CHONDRITE FIND FROM NEW MEXICO

The Ragland, New Mexico chondrite was found in 1978, and consists of a single stone of 12.16 kg that broke into three pieces. The stone is moderately weathered and has a pronounced chondritic texture. Bulk composition favors an LL classification, and modal analysis and oxygen isotopic composition are consistent with this. The thermoluminescence sensitivity of 0.056 ± 0.020 normalized to Dhajala, compositional variability of olivine (mean Fa 18.3, σ = 10.1) and low-Ca pyroxene (mean Fs 14.6, σ = 6.7), and Ca concentrations in olivine indicate metamorphic subtype 3.4 ± 0.1. The isotopically heavy oxygen composition, which is characteristic of subtypes 3.0–3.1, may be a primary characteristic and not a result of weathering. Low concentrations of radiogenic ⁴⁰Ar and planetary ³⁶Ar suggest noble gas loss.     

THE SEONI CHONDRITE

The Seoni (India) chondrite is an H6 group ordinary chondrite that contains olivine (Fa, 19.7 mole%), orthopyroxene (Fs, 15.9 mole%), clinopyroxene, plagioclase (An, 10.3; Or, 5.6 mole%), together with chromite, troilite, kamacite, taenite, chlorapatite, and whitlockite. Recrystallization has been quite extensive as indicated by the presence of few remnant chondrules, low abundance of clinopyroxene and relatively high abundance of well formed plagioclase. Treatment of Fe²⁺ and Mg partitioning between clinopyroxene and orthopyroxene and between olivine and chromite indicates equilibration temperatures of between 875–920 °C.     

Estimating the modal mineralogy of eucrite and diogenite meteorites using visible–near infrared reflectance spectroscopy

Reliable quantitative mapping of minerals exposed on Vesta's surface is crucial for understanding the crustal composition, petrologic evolution, and surface modification of the howardite, eucrite, and diogenite (HED) parent body. However, mineral abundance estimates derived from visible–near infrared (VIS–NIR) reflectance spectra are complicated by multiple scattering, particle size, and nonlinear mixing effects. Radiative transfer models can be employed to accommodate these issues, and here we assess the utility of such models to accurately and efficiently determine modal mineralogy for a suite of eucrite and olivine‐bearing (harzburgitic) diogenite meteorites. Hapke and Shkuratov radiative transfer models were implemented to simultaneously estimate mineral abundances and particle size from VIS–NIR reflectance spectra of these samples. The models were tested and compared for laboratory‐made binary (pyroxene–plagioclase) and ternary mixtures (pyroxene–olivine–plagioclase) as well as eucrite and diogenite meteorite samples. Results for both models show that the derived mineral abundances are commonly within 5–10% of modal values and the estimated particle sizes are within the expected ranges. Results for the Hapke model suggest a lower detection limit for olivine in HEDs when compared with the Shkuratov model (5% versus 15%). Our current implementation yields lower uncertainties in mineral abundance (commonly <5%) for the Hapke model, though both models have an advantage over typically used parameters such as band depth, position, and shape in that they provide quantitative information on mineral abundance and particle size. These results indicate that both the Hapke and Shkuratov models may be applied to Dawn VIR data in a computationally efficient manner to quantify the spatial distribution of pyroxene, plagioclase, and olivine on the surface of Vesta.     

Automated energy dispersive spectrometer modal analysis applied to the diogenites

Abstract— We have analyzed the modal abundances of 23 of the known 24 diogenites in 31 thin sections using an energy dispersive spectrometer (EDS) and automated phase distribution analysis software. Orthopyroxene is predictably the most abundant phase, ranging from 27.7 vol% to 99.8 vol% in these samples. The grand average mode of all the analyzed diogenites includes the “olivine diogenites” but not ALH 85015, a probable howardite, and ALHA81208, a sample with an abundant silica phase. The grand average of these 21 diogenites is: orthopyroxene 92.2 vol%, olivine 4.2 vol%, clinopyroxene 1.2 vol%, chromite 0.9 vol%, plagioclase 0.4 vol%, FeNi metal 0.1 vol%, troilite 0.6 vol%, and silica phase 0.4 vol%. Plagioclase feldspar is extremely depleted in all samples, with modal abundance from none detected to 4.6 vol% in range. Such a low volume of plagioclase may indicate that the diogenite parental melts originated in a source region depleted in Al (Warren, 1985; Stolper, 1975), which is consistent with crystallization from a melt derived from material that had previously experienced extraction of a eucrite-type melt.     

A NOTE ON THE MINERALOGY AND CLASSIFICATION OF THE VILNA METEORITE

Electron microprobe analyses of olivine, orthopyroxene, clinopyroxene, plagioclase feldspar, chromite, whitlockite, apatite, troilite and metals indicate that the Vilna meteorite is a hypersthene chondrite belonging to the L5 or L6 subgroups.     

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.     

MINERALOGICAL AND CHEMICAL RESEARCHES ON L-CHONDRITES: GIRGENTI

A mineralogical and chemical analysis has been performed on the largest mass (8750 g) of the Girgenti, Italy, meteorite, from the collection of the Smithsonian Institution, Washington, D.C. The mineralogical composition is olivine, Fa_24–25; hypersthene, Fs_{21 –22}; plagioclase, An₁₂; maskelynite; whitlockite; nickel-iron; troilite; chromite and ilmenite. Girgenti is a severly metamorphosed stone, whose total iron content (23.5%) is somewhat higher than the average for hypersthene chondrites.