Chromite-plagioclase assemblages as a new shock indicator; implications for the shock and thermal histories of ordinary chondrites

Chromite in ordinary chondrites (OC) can be used as a shock indicator. A survey of 76 equilibrated H, L and LL chondrites shows that unshocked chromite grains occur in equant, subhedral and rounded morphologies surrounded by silicate or intergrown with metallic Fe-Ni and/or troilite. Some unmelted chromite grains are fractured or crushed during whole-rock brecciation. Others are transected by opaque veins; the veins form when impacts cause localized heating of metal-troilite intergrowths above the Fe-FeS eutectic (988°C), mobilization of metal-troilite melts, and penetration of the melt into fractures in chromite grains. Chromite-plagioclase assemblages occur in nearly every shock-stage S3-S6 OC; the assemblages range in size from 20-300 μm and consist of 0.2-20-μm-size euhedral, subhedral, anhedral and rounded chromite grains surrounded by plagioclase or glass of plagioclase composition. Plagioclase has a low impedance to shock compression. Heat from shock-melted plagioclase caused adjacent chromite grains to melt; chromite grains crystallized from this melt. Those chromite grains in the assemblages that are completely surrounded by plagioclase are generally richer in Al₂O₃ than unmelted, matrix chromite grains in the same meteorite. Chromite veinlets (typically 0.5-2 μm thick and 10-300 μm long) occur typically in the vicinity of chromite-plagioclase assemblages. The veinlets formed from chromite-plagioclase melts that were injected into fractures in neighboring silicate grains; chromite crystallized in the fractures and the residual plagioclase-rich melt continued to flow, eventually pooling to form plagioclase-rich melt pockets. Chromite-rich “chondrules” (consisting mainly of olivine, plagioclase-normative mesostasis, and 5-15 vol.% chromite) occur in many shocked OC and OC regolith breccias but they are absent from primitive type-3 OC. They may have formed by impact melting chromite, plagioclase and adjacent mafic silicates during higher-energy shock events. The melt was jetted from the impact site and formed droplets due to surface tension. Crystallization of these droplets may have commenced in flight, prior to landing on the parent-body surface.Chromite-plagioclase assemblages and chromite veinlets occur in 25 out of 25 shock-stage S1 OC of petrologic type 5 and 6 that I examined. Although these rocks contain unstrained olivine with sharp optical extinction, most possess other shock indicators such as extensive silicate darkening, numerous occurrences of metallic Cu, polycrystalline troilite, and opaque veins. It seems likely that these rocks were shocked to levels at least as high as shock-stage S3 and then annealed by heat generated during the shock event. During annealing, the olivine crystal lattices healed but other shock indicators survived. Published Ar-Ar age data for some S1 OC indicate that many shock and annealing events occurred very early in the history of the parent asteroids. The common occurrence of shocked and annealed OC is consistent with collisions being a major mechanism responsible for metamorphosing OC.     

Metal-troilite-magnetite assemblage in shock veins of Sixiangkou meteorite

The first natural occurrence of metal-troilite-magnetite assemblage composed of Fe-Ni metal and magnetite dendrites and a groundmass of troilite was identified in the shock veins of the Sixiangkou L6 chondrite, which contain abundant high-pressure minerals. This assemblage suggests a liquid miscibility among metal, FeS, and iron oxide, and subsequently quenching under pressure. Components of magnetite could be, in origin, related to chromite that was embedded and dissolved in an Fe-Ni-S liquid. Cr₂O₃ dissociated from chromite was mainly incorporated into garnet and magnesiowüstite in the fine-grained matrix of shock veins, in which chromium behaves as a lithophile element at the P-T conditions experienced by the shock veins. The occurrence of metal-troilite-magnetite assemblage suggests that the shock veins were still under pressure at temperatures from 900 to 950°C during solidification of Fe-Ni-S or Fe-Ni-S-O liquid, hence indicating a long duration of high pressure in the shock veins.     

Matrices of type 3 ordinary chondrites—primitive nebular records

Petrologic studies were made on the fine-grained matrices of type 3 ordinary chondrites of the lowest petrologic subtype. The matrix minerals, in order of abundance, are olivine (Fo₉₉ to Fo₉), enstatite or bronzite, augite or subcalcic augite, albite, Fe-Ni metal, troilite, magnetite, spinel (MgAl₂O₄), chromite, and calcite. Fe- and Mg-rich fluffy particles and albite-like particles are also major constituents. The chemical compositions of olivine and pyroxenes vary within and among the chondrites and are in gross disequilibrium, showing that the matrix materials were hardly heated after their formation. Textural relationships indicate that magnesian olivine was formed after Ca-pyroxene, followed by intermediate to iron-rich olivine. Intermediate olivine was formed from enstatite and metallic iron under relatively oxidizing conditions. The observations indicate that matrices of chondrites are neither the fragments of chondrules nor the precursors of chondrules. They were mostly the products of condensation and reaction among solids and/or between solids and the ambient gas mostly at low temperatures, and thus they contain records of primitive processes in the nebula. In order to explain the presence of olivines more iron-rich than Fo₅₀, the presence of free SiO₂ or a high activity of SiO₂ in the gas is necessary, which was not shown in previous thermochemical calculations. Mineral assemblages of matrix minerals of chondrites of different chemical groups differ systematically according to oxidation state of the parental meteorites, indicating that they were formed at different oxygen fugacities. The rims of chondrules, and surrounding matrix materials, must have accreted onto chondrules during turbulent movements of the nebula.     

Petrogenesis of acapulcoites and lodranites: A shock-melting model

Acapulcoites are modeled as having formed by shock melting CR-like carbonaceous chondrite precursors; the degree of melting of some acapulcoites was low enough to allow the preservation of 3-6 vol % relict chondrules. Shock effects in acapulcoites include veins of metallic Fe-Ni and troilite, polycrystalline kamacite, fine-grained metal-troilite assemblages, metallic Cu, and irregularly shaped troilite grains within metallic Fe-Ni. While at elevated temperatures, acapulcoites experienced appreciable reduction. Because graphite is present in some acapulcoites and lodranites, it seems likely that carbon was the principal reducing agent. Reduction is responsible for the low contents of olivine Fa (4-14 mol %) and low-Ca pyroxene Fs (3-13 mol %) in the acapulcoites, the observation that, in more than two-thirds of the acapulcoites, the Fa value is lower than the Fs value (in contrast to the case for equilibrated ordinary chondrites), the low FeO/MnO ratios in acapulcoite olivine (16-18, compared to 32-38 in equilibrated H chondrites), the relatively high modal orthopyroxene/olivine ratios (e.g., 1.7 in Monument Draw compared to 0.74 in H chondrites), and reverse zoning in some mafic silicate grains. Lodranites formed in a similar manner to acapulcoites but suffered more extensive heating, loss of plagioclase, and loss of an Fe-Ni-S melt.Acapulcoites and lodranites experienced moderate post-shock annealing, presumably resulting from burial beneath material of low thermal diffusivity. The annealing process repaired damaged olivine crystal lattices, lending acapulcoites and lodranites the appearance of unshocked (i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed during initial shock reverted to their low-pressure polymorphs during annealing. Some samples were subsequently shocked again; several acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the lodranite MAC 88177 reached S5.     

Microchondrule-bearing clast in the Piancaldoli LL3 meteorite: a new kind of type 3 chondrite and its relevance to the history of chondrules

In the Piancaldoli LL3 chondrite, we found a mm-sized clast containing ~100 chondrules 0.2–64 μm in apparent diameter (much smaller than any previously reported) that are all of the same textural type (radial pyroxene; FS_1–17). This clast, like other type 3 chondrites, has a fine-grained Ferich opaque silicate matrix, sharply defined chondrules, abundant low-Ca clinopyroxene and minor troilite and Si- and Cr-bearing metallic Fe,Ni. However, the very high modal matrix abundance (63 ± 8 vol. %), unique characteristics of the chondrules, and absence of microscopically-observable olivine indicate that the clast is a new kind of type 3 chondrite. Most chondrules have FeO-rich edges, and chondrule size is inversely correlated with chondrule-core FeO concentration (the first reported correlation of chondrule size and composition). Chondrules acquired Fe by diffusion from Fe-rich matrix material during mild metamorphism, possibly before final consolidation of the rock. Microchondrules (those chondrules ? 100 μm in diameter) are also abundant in another new kind of type 3 chondrite clast in the Rio Negro L chondrite regolith breccia. In other type 3 chondrite groups, microchondrule abundance appears to be anticorrelated with mean chondrule size, viz. 0.02–0.04 vol. % in H and CO chondrites and ?0.006 vol. % in L, LL, and CV chondrites.Microchondrules probably formed by the same process that formed normal-sized droplet chondrules: melting of pre-existing dustballs. Because most compound chondrules in the clast and other type 3 chondrites formed by collisions between chondrules of the same textural type, we suggest that dust grains were mineralogically sorted in the nebula before aggregating into dustballs. The sizes of compound chondrules and chondrule craters, which resulted from collisions of similarly-sized chondrules while they were plastic, indicate that size-sorting (of dustballs) occurred before chondrule formation, probably by aerodynamic processes in the nebula. We predict that other kinds of type 3 chondrites exist which contain chondrule abundances, size-ranges and proportions of textural types different from known chondrite groups.     

Chromiferous spinels of the elephant's head dike

Analysis by optical, X-ray diffraction and microprobe methods, of essentially unzoned, disseminated spinels within cumulus picrites and olivine gabbros reveals an unbroken range of compositions from aluminian chromite (34% Cr₂O₃) to chromian magnetite (4% Cr₂O₃). TiO₂ contents vary between 0.5 and 7.7%. Exsolution of ilmenite indicates originally higher TiO₂ contents. Quenched contact-facies rocks with abundant olivine phenocrysts contain strongly zoned spinels in which a titaniferous chromian magnetite rim (16% Cr₂O₃, 10% TiO₂) encloses cores of weakly titanian chromite (40% Cr₂O₃). Platy dendrites of exsolved spinel occur in abundance within cumulus olivines. The evidence suggests that crystallization of the disseminated spinels occurred under the influence of an increase in oxygen fugacity towards the interior of the intrusion, and that the compositional diversity has stemmed from the homogenization of originally zoned grains mantled to varying degrees by high-Ti, low-Cr rims.     

Origin of zoned spinel by coupled dissolution–precipitation and inter-crystalline diffusion: evidence from serpentinized wehrlite, Bangriposi, Eastern India

Textural and mineral–chemical characteristics in the Bangriposi wehrlites (Eastern India) provide insight into metamorphic processes that morphologically and chemically modified magmatic spinel during serpentinization of wehrlite. Aluminous chromite included in unaltered magmatic olivine is chemically homogenous. In sub-cm to 10s-of-micron-wide veins, magnetite associated with antigorite and clinochlore comprising the serpentine matrix is near-stoichiometric. But Al–Cr–Fe³⁺ spinels in the chlorite–magnetite veins are invariably zoned, e.g., chemically homogenous Al-rich chromite interior successively mantled by ferritchromite/Cr-rich magnetite zone and magnetite continuous with vein magnetite in the serpentine matrix. In aluminous chromite, ferritchromite/Cr-rich magnetite zones are symmetrically disposed adjacent to fracture-controlled magnetite veins that are physically continuous with magnetite rim. The morphology of ferritchromite–Cr-rich magnetite mimics the morphology of aluminous chromite interior but is incongruous with the exterior margin of magnetite mantle. Micropores are abundant in magnetite veins, but are fewer in and do not appear to be integral to the adjacent ferritchromite–Cr-rich magnetite zones. Sandwiched between chemically homogenous aluminous chromite interior and magnetite mantle, ferritchromite–Cr-rich magnetite zones show rim-ward decrease in Cr₂O₃, Al₂O₃ and MgO and complementary increase in Fe₂O₃ at constant FeO. In diffusion profiles, Fe₂O₃–Cr₂O₃ crossover coincides with Al₂O₃ decrease to values <0.5 wt% in ferritchromite zone, with Cr₂O₃ continuing to decrease within magnetite mantle. Following fluid-mediated (hydrous) dissolution of magmatic olivine and olivine + Al–chromite aggregates, antigorite + magnetite and chlorite + magnetite were transported in 10s-of-microns to sub-cm-wide veins and precipitated along porosity networks during serpentinization (T: 550–600 °C, f(O₂): ?19 to ?22 log units). These veins acted as conduits for precipitation of magnetite as mantles and veins apophytic in chemically/morphologically modified magmatic Al-rich chromite. Inter-crystalline diffusion induced by chemical gradient at interfaces separating aluminous chromite interiors and magnetite mantles/veins led to the growth of ferritchromite/Cr-rich magnetite zones, mimicking the morphology of chemically modified Al–Cr–Fe–Mg spinel interiors. Inter-crystalline diffusion outlasted fluid-mediated aluminous chromite dissolution, mass transfer and magnetite precipitation.     

Ilmenite, Magnetite and Chromite Beach Placers from South Maharashtra, Central West Coast of India

The heavy mineral placer deposits of the coastal sediments in south Maharashtra stretch for 12.5 km from Pirwadi in the north to Talashil in the south. The area is a sand bar represented by a narrow submergent coastal plain lying between the Achara and Gad Rivers. The sediments in the area are mainly sands which are moderately well sorted to well sorted. The heavy mineral concentration in the surficial sediments ranges between 0.69 and 98.32 wt % (28.73 wt % in average). The heavy mineral concentration shows an increasing trend from north to south. The heavy mineral suite consists predominantly of opaque minerals (ilmenite, magnetite and chromite), garnet, pyroxene, amphibole, zircon, tourmaline, rutile, staurolite, etc. Ilmenite grains are fresh whereas magnetite grains show the effect of weathering and alteration. The chromite grains are rounded to sub-rounded with alteration at the margin of the grains. The surficial textures of the opaque minerals show mechanical breaking that indicates limited distance of transportation. Ilmenite has TiO₂ in the range between 40.04 and 46.6 wt %. Based on ore microscopy studies, the magnetite grains appear to be of two types: pure magnetite and titano-magnetite. Compositionally, the total magnetite fractions have Fe₂O₃ between 32 and 46 wt %, FeO between 19.0 and 25 wt % and TiO₂ between 14.3 and 23.9 wt %. The chromite grains are an admixture of two varieties, ferro-chromite and magnesio-chromite. The chromite grains have 32.06–47.5 wt % of Cr₂O₃ with total iron between 23.86 wt % (4.73% Fe₂O₃ and 19.13% FeO) and 27.89 wt % (4.36% Fe₂O₃ and 23.53% FeO) and MgO between 12 and 40 wt %. The observed variations in the distribution of heavy minerals in the area are due to differences in the sediment supply, their specific gravity and oceanographic processes all of which result in a selective sorting of the sediments. The observed mineral assemblages of transparent heavy minerals (pyroxene, amphibole, tourmaline, kyanite, garnet, zircon and olivine) are suggestive of their derivation from a heterogeneous provenance comprising of igneous rocks, high grade metamorphic rocks and reworked Kaladgi sediments. The chromite grains appear to have been derived from ultrabasic rocks present in the upper reaches of the Gad River. The inferred reserves of ilmenite, magnetite and chromite are 0.175, 0.395 and 0.032 million tons, respectively.     

Mineralogy,petrology, and oxygen isotopic composition of Northwest Africa 12379, metal-rich chondrite with affinity to ordinary chondrites

Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼ 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa_18.1–28.3, average Fa_24.9±3.2, PMD = 12.8; Cr₂O₃ = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs_3.2–18.7Wo_0.2–4.5; average Fs_14.7±3.7Wo_1.4±1.3; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ¹⁷O ∼ 0.2–1.4‰, average ∼ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa₂₇) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ¹⁷O of the ferroan olivine (∼ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ¹⁸O is significantly higher (15–19‰, average = 17‰ vs. 3―12‰, average = 8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.     

Exsolutions of Diopside and Magnetite in Olivine from Mantle Dunite, Luobusa Ophiolite, Tibet, China

The exsolutious of diopside and magnetite occur as intergrowth and orient within olivine from the mantle dunite, Luobusa ophiolite, Tibet. The dunite is very fresh with a mineral assemblage of olivine （〉95%） ＋ chromite （1%-4%） ＋ diopside （〈1%）. Two types of olivine are found in thin sections： one （Fo = 94） is coarse-grained, elongated with development of kink bands, wavy extinction and irregular margins; and the other （Fo = 96） is fine-grained and poly-angied. Some of the olivine grains contain minor Ca, Cr and Ni. Besides the exsolutions in olivine, three micron-size inclusions are also discovered. Analyzed through energy dispersive system （EDS） with unitary analytical method, the average compositions of the inclusions are： Na20, 3.12%-3.84%; MgO, 19.51%-23.79%; Al2O3, 9.33%-11.31%; SiO2, 44.89%-46.29%; CaO, 11.46%-12.90%; Cr2O3, 0.74%-2.29%; FeO, 4.26%- 5.27%, which is quite similar to those of amphibole. Diopside is anhedral f＇dling between olivines, or as micro-inclusions oriented in olivines. Chromite appears euhedral distributed between olivines, sometimes with apparent compositional zone. From core to rim of the chromite, Fe content increases and Cr decreases; and A! and Mg drop greatly on the rim. There is always incomplete magnetite zone around the chromite. Compared with the nodular chromite in the same section, the euhedral chromite has higher Fe3O4 and lower MgCr2O4 and MgAI2O4 end member contents, which means it formed under higher oxygen fugacity environment. With a geothermometer estimation, the equilibrium crystalline temperature is 820℃-960℃ for olivine and nodular chromite, 630℃-770℃ for olivine and euhedral chromite, and 350℃-550℃ for olivine and exsoluted magnetite, showing that the exsolutions occurred late at low temperature. Thus we propose that previously depleted mantle harzburgite reacted with the melt containing Na, Al and Ca, and produced an olivine solid solution added with Na^＋, Al^3＋, Ca^2＋, Fe^3＋, Cr^3＋. With temperature d     

Exsolution Lamellae in Olivine Grains of Dunite Units from Different Types of Mafic‐Ultramafic Complexes

Exsolution microstructures in olivine grains from dunite units in a few selected tectonic environments are reported here. They include lamellae of clinopyroxene and clinopyroxene-magnetite intergrowth in the Gaositai and Yellow Hill Alaskan-type complexes, clinopyroxene-magnetite intergrowth in the K?z?lda? ophiolite, and chromite lamellae in the Hongshishan mafic-ultramafic intrusive complex. These lamellae commonly occur as needle-or rod-like features and are oriented in olivine grains. The host olivine grains have Fo contents of 92.5–92.6 in the Gaositai complex, 86.5–90.1 in the Yellow Hill complex, 93.2–93.4 in the K?z?lda? ophiolite and 86.9–88.3 in the Hongshishan complex. Clinopyroxene in the rod-like intergrowth exsolved in olivine grains in the Gaositai and Yellow Hill is diopside with similar major element compositions of Ca O(23.6–24.3 wt%), SiO_2(52.2–54.0 wt%), Al_2O_3(0.67–2.15 wt%), Cr_2O_3(0.10–0.42 wt%) and Na_2O(0.14–0.26 wt%). It falls into the compositional field of hydrothermal clinopyroxene and its origin is thus probably related to reaction between dunite and fluids. The enrichment of the fluids in Ca~(2+), Fe~(3+), Cr~(3+) and Na+, resulted in elevated concentrations of these cations in olivine solid solutions via the reaction. With decreasing temperature, the olivine solid solutions altered to an intergrowth of magnetite and clinopyroxene. The Fe~(3+) and Cr~(3+) preferentially partitioned into magnetite, while Ca~(2+) and Na+ entered clinopyroxene. Since the studied Alaskan-type complexes and ophiolite formed in a subduction environment, the fluids were probably released from the subducted slab. In contrast, the exsolved chromite in olivine grains from the Hongshishan complex that formed in post-orogenic extension setting can be related to olivine equilibrated with Cr-bearing liquid. Similarly, these lamellae have all been observed in serpentine surrounding olivine grains, indicating genetic relations with serpentinization.     

Melt impregnation phases in the mantle section of the Ślęża ophiolite (SW Poland)

The ultramafic member of the Variscan Ślęża Ophiolite (SW Poland) consists of heavily serpentinised, refractory harzburgites. Those located down to 1.5 km below paleo-Moho contain scarce grains or aggregates of olivine, clinopyroxene and spinel. Non-serpentine phases occur in various assemblages: M1—olivine (Fo 90.2–91.0%, NiO 0.38–0.47 wt.%) and rounded or amaeboidal aluminous chromite, rimmed by Al poor chromite and magnetite; M2—olivine (Fo 90.5–91.5, NiO 0.32–0.44 wt.%), olivine with magnetite inclusions (Fo 87.1–92.5, NiO 0.01–0.68 wt.%), rounded, cleavaged clinopyroxene I (Mg# 91.1–93.2, Al₂O₃ 3.00–4.00 wt.%, Cr₂O₃ 1.00–1.40 wt.%) and elongated clinopyroxene II and clinopyroxene from symplectites with magnetite (Mg# = 92.2–94.1, Al₂O₃ 2.20–3.20 wt.% and Cr₂O₃ 0.80–1.20 wt.%). Clinopyroxene is depleted in REEs relative to chondrite. The M3 assemblage consists of intergrown olivine (Fo 90.8–92.7, NiO 0.20–0.38 wt.%) and clinopyroxene (Mg# = 96.0–98.1, Al₂O₃ 0.00–1.00 wt.% and Cr₂O₃ 0.20–0.60 wt.%).The M1 assemblage contains chromite which records greenschist-facies metamorphism. Textural relationships and chemical composition of clinopyroxene occurring in the M2 assemblage are similar to those formed in oceanic spreading centres by LREE depleted basaltic melt percolation. Olivine occurring in M1 assemblage and part of that from M2 have composition typical of residual olivine from the abyssal harzburgites and of olivine formed in those rocks by melt percolation. The olivine with magnetite inclusions (M2 assemblage) and that from M3 record later deserpentinization event, which supposedly produced also M3 clinopyroxene. The non-serpentine phases from the Ślęża ophiolite mantle member, albeit very poorly preserved, document depleted basaltic melt percolation in the Variscan oceanic spreading centre.     

Smyer H-chondrite impact-melt breccia and evidence for sulfur vaporization

Smyer is an H-chondrite impact-melt breccia containing ∼20 vol% 0.5- to 13-mm-thick silicate-rich melt veins surrounding unmelted subrounded chondritic clasts up to 7 cm in maximum dimension. At the interface between some of the melt veins and chondritic clasts, there are troilite-rich regions consisting of unmelted, crushed 0.2- to 140-μm-size angular silicate grains and chondrule fragments surrounded by troilite and transected by thin troilite veins. Troilite fills every available fracture in the silicates, including some as thin as 0.1 μm. Little metallic Fe-Ni is present in these regions: the FeS/Fe modal ratio ranges from ∼25:1 to ∼500:1, far higher than the eutectic weight ratio of 7.5:1. The texture of these regions indicates that the sulfide formed from a fluid of very low viscosity. The moderately high viscosity (0.2 poise) and large surface tension of liquid FeS, its inability to wet silicate grain surfaces at low oxygen fugacities, and the supereutectic FeS/Fe ratios in the troilite-rich regions indicate that the fluid was a vapor. It seems likely that during the shock event that melted Smyer, many silicates adjacent to the melt veins were crushed. Upon release of shock pressure, some of the troilite evaporated and dissociated. Molecules of S₂ were transported and condensed into fractures and around tiny silicate grains; there, they combined with Fe from small adjacent metallic Fe-Ni grains to form troilite. The Ni content at the edges of some of these metal grains increased significantly; Co from these Ni-rich grains diffused into nearby kamacite. Impact-induced S volatilization may have played a major role in depleting the surface of 433 Eros (and other chondritic asteroids) in S.     

Opaque phases in type-II chondrules from CR2 chondrites: Implications for CR parent body formation

We report the results of a detailed study of sulfide-bearing opaque assemblages from the MAC 87320, EET 92011, and Renazzo CR carbonaceous chondrites. The objectives of this study are to (1) characterize sulfide and associated phases within CR2 chondrites; (2) determine the petrographic relationship between sulfides, metals, and chondrules; (3) constrain the history of type-II chondrules; (4) ascertain the environments in which type-II chondrules formed and were altered; and (5) unravel the formation and alteration history of the CR parent body as recorded in sulfide-bearing assemblages. Sulfide-bearing opaque assemblages occur primarily within type-II (FeO-rich) chondrules. The sulfide assemblages are concentrated near the chondrule edges. Assemblages in MAC 87320 are composed of troilite, phosphate, and Ni-rich metal. EET 92011 contains assemblages composed of pentlandite, troilite, and Ni-rich metal. The assemblages in Renazzo contain tochilinite, magnetite, troilite, pentlandite, and phosphate. In all of the assemblages in Renazzo the tochilinite is fine grained and intimately mixed with troilite, pentlandite, or magnetite. Opaque assemblages in CR chondrites record a complex history that includes both high- and low-temperature processes. The morphology and composition of sulfides in CR2 chondrites suggests that the sulfide-bearing assemblages originally formed in gas-solid reactions in the nebula at temperatures above the Fe-FeS eutectic (988 °C). Many of the assemblages were subsequently aqueously altered on the CR-chondrite parent body to various degrees at temperatures from ∼50 to 200 °C. We combine these observations and interpretations to provide a detailed model of the history of the CR parent body.     

Postshock annealing and postannealing shock in equilibrated ordinary chondrites: implications for the thermal and shock histories of chondritic asteroids

In addition to shock effects in olivine, plagioclase, orthopyroxene and Ca-pyroxene, petrographic shock indicators in equilibrated ordinary chondrites (OC) include chromite veinlets, chromite-plagioclase assemblages, polycrystalline troilite, metallic Cu, irregularly shaped troilite grains within metallic Fe-Ni, rapidly solidified metal-sulfide intergrowths, martensite and various types of plessite, metal-sulfide veins, large metal and/or sulfide nodules, silicate melt veins, silicate darkening, low-Ca clinopyroxene, silicate melt pockets, and large regions of silicate melt. The presence of some of these indicators in every petrologic type-4 to -6 ordinary chondrite demonstrates that collisional events caused all equilibrated OC to reach shock stages S3-S6. Those type-4 to -6 OC that are classified as shock-stage S1 (on the basis of sharp optical extinction in olivine) underwent postshock annealing due to burial beneath materials heated by the impact event. Those type-4 to -6 OC that are classified S2 (on the basis of undulose extinction and lack of planar fractures in olivine) were shocked to stage S3-S6, annealed to stage S1 and then shocked again to stage S2. Some OC were probably shocked to stage ≥ S3 after annealing. It seems likely that many OC experienced multiple episodes of shock and annealing.Because ⁴⁰Ar-³⁹Ar chronological data indicate that MIL 99301 (LL6, S1) was annealed ∼ 4.26 Ga ago, presumably as a consequence of a major impact, it seems reasonable to suggest that other equilibrated S1 and S2 OC (which contain relict shock features) were also annealed by impacts. Because some type-6 S1 OC (e.g., Guareña, Kernouvé, Portales Valley, all of which contain relict shock features) were annealed 4.44- 4.45 Ga ago (during a period when impacts were prevalent and most OC were thermally metamorphosed), it follows that impact-induced annealing could have contributed significantly to OC thermal metamorphism.     

Petrology and magnetic properties of the Chiang Khan,Thailand, meteorite

Summary The Chiang Khan meteorite fell on 18th November, 1981 at Chiang Khan, Thailand. It consists of olivine, orthopyroxene, clinopyroxene, Fe-Ni metal, troilite, chromite, plagioclase, glass, and phosphate in order of abundance. Olivine forms barred or porphyritic chondrules, and its composition is uniform (average Fo_80.2), close to the average composition of olivine in equilibrated H chondrites. Orthopyroxene and clinopyroxene also have compositions similar to those in equilibrated H chondrites. Both well-defined chondrules and their broken fragments are present in the recrystallized matrix. Microcrystalline plagioclase and clinopyroxene often occur in the groundmass of chondrules, but clear interstitial plagioclase is absent. Chemical composition of chromite plots in the field of chromites in H chondrites. Chiang Khan meteorite is thus classified as an equilibrated H 5 type chondrite. The equilibrium temperatures estimated by using mineral pairs are as follows: Opx-Cpx 800–900°C; Ol-Chromite 510°C.Water content is 0.24 wt %, and the hydrogen isotopic composition (D) is –89.5In the thermal demagnetization experiment magnetization steadily decreased from 0 to 500°C, whereas the remanent magnetization obtained in the A.C. demagnetization experiment is very unstable, probably owing to the large grain size of the Fe-Ni metal.With 9 Figures     

The substance of the Chelyabinsk meteorite: Results of geochemical and thermomagnetic studies

The fragments of the Chelyabinsk meteorite studied are represented by light-gray granular rock of chondritic structure. The chondrules and their cementing matter are mainly constituted by olivine and orthopyroxene. The matrix consists of a pyroxene-olivine aggregate with plagioclase, apatite, melted glass, and the inclusions of ore minerals: taenite, kamacite, troilite, pyrrhotite and pentlandite (more rarely), and individual grains of chromite and ilmenite. The comparison of the composition of the Chelyabinsk meteorite to the average composition of LL chondrites had shown their complete convergence. The concentrations of sidero- and chalcophile rare elements in the meteorite, normalized to CI chondrites, are much close to the values for LL chondrites and almost reproduce the character of their distribution in the spider diagram. However, some high-charged and lithophile elements (Nb, Zr, Hf, Sr, Ba, Th, and U) not belonging to the mentioned groups are characterized by somewhat increased contents. The enrichment of the samples of the Chelyabinsk meteorite in rare-earth elements compared to LL chondrite (5.18 against 3.58 ppm) is also revealed. This is related to the higher concentrations of light lanthanides in the meteorite samples, which is seen from the increased La/Yb ratio compared to the value for LL chondrite (1.9–2.3 and 1.4, respectively). Iron-nickel alloys are the main magnetism carriers in the Chelyabinsk meteorite. The compositions of kamacite, taenite, chromite, and Fe-sulfides are not much different. The optical and microprobe data are confirmed by the thermomagnetic parameters as well: (1) The specific magnetization of 4–6 Am²/kg points to small variations in the concentrations of magnetic minerals. (2) The M(T) curves for all the samples nearly repeat each other, and the Curie temperatures of 490–520 and 740–770°C are registered in the curves of the first and second heating, hence, these curves correspond to kamacite of various composition, right up to pure iron. (3) The monocline ferrimagnetic pyrrhotite of T_C = 320–340°C is registered in the treated fragments in both the M(T) curves of heating and cooling. (4) The concentrations by thermomagnetic analysis amount to 0.6–1.6% (0.9% average) for kamacite, 0.7–1.5% (1.1% average) for taenite, and 0–1.5% (0.4% average) for monocline pyrrhotite. (5) No magnetite was found in the M(T) curve during the first heating of the samples. Hence, the content of magnetite is much below 0.1.     

Post-shock annealing of Miller Range 99301 (LL6): Implications for impact heating of ordinary chondrites

MIL 99301 is an LL chondrite that has experienced successive episodes of thermal metamorphism, shock metamorphism and annealing. The first recognizable petrogenetic episode resulted in thermal metamorphism of the rock to petrologic type 6 (as indicated by homogeneous olivine compositions, significant textural recrystallization, and the presence of coarse grains of plagioclase, metallic Fe-Ni and troilite). The source of heat for this thermal episode is not identified. The rock also experienced shock metamorphism to shock stage ∼S4 as indicated by extensive silicate darkening (caused by the dispersion within silicate grains of thin chromite melt veins and trails of metallic Fe-Ni and troilite blebs), polycrystalline troilite, myrmekitic plessite, a relatively high occurrence abundance (OA) of metallic Cu (3.6), the presence of numerous chromite-plagioclase assemblages, and coarse grains of low-Ca clinopyroxene with polysynthetic twinning. The shock event responsible for these effects must have occurred after the epoch of thermal metamorphism to type-6 levels; otherwise the polycrystallinity of the troilite would have disappeared and the low-Ca clinopyroxene would have transformed into orthopyroxene. Despite abundant evidence of strong shock, olivine and plagioclase in MIL 99301 exhibit sharp optical extinction, consistent with shock stage S1 and characteristic of an unshocked rock. This implies that an episode of post-shock annealing healed the damaged olivine and plagioclase crystal lattices and thereby changed undulose extinction into sharp extinction. The rock was probably annealed to metamorphic levels approximating petrologic type 4; more significant heating would have transformed the low-Ca clinopyroxene into orthopyroxene. It is not plausible that an episode of annealing occurring after the epoch of thermal metamorphism could have been caused by the decay of ²⁶Al because this isotope would have decayed away by that time. Impact heating is a more plausible source of post-metamorphic annealing of rocks in the vicinity of impact craters on low-density, high-porosity asteroids with rubble-pile structures.     

Brachinite meteorites: Partial melt residues from an FeO-rich asteroid

Brachinites are ultramafic, dunitic to wherlitic, unbrecciated and essentially unshocked rocks that are low in SiO₂ (∼36–39 wt.%), high in MgO (∼27–30 wt.%) and notably high in FeO (∼26–37 wt.%), and low in Al₂O₃ (∼0.2–2.5 wt.%) and combined alkalis Na₂O and K₂O (∼0–0.7 wt.%). They consist mostly of olivine (∼71–96 vol.%; ∼Fo_64–73), major clinopyroxene (minor to ∼15 vol.%; ∼En_40–63Wo_36–48), with variable small amounts of plagioclase (0 to ∼10 vol.%; ∼An_15–33), and minor to trace amounts of orthopyroxene (none to ∼20 vol.%; En_69–73Wo_2–4), Fe-sulfides (trace to ∼7 vol.%), chromite (none to ∼5 vol.%), phosphates (none to ∼3 vol.%) and metallic Fe,Ni (trace to ∼2 vol.%). Minerals tend to be homogeneous, and textures are medium to coarse-grained (∼0.1–1.5 mm), with olivine commonly displaying triple junctions. Brachina has near-chondritic lithophile element abundances, whereas other brachinites show variable depletions in Al, Ca, Rb, K, Na, and LREE. Siderophile element abundance patterns vary and range from ∼0.01 to ∼0.9 CI. Oxygen isotope composition (Δ¹⁷O) ranges from ∼−0.09 to −0.39‰, with the mean = −0.23 ± 0.14‰. Brachinites are ancient rocks, as was recognized early by the detection, in some brachinites, of excess ¹²⁹Xe from the decay of short-lived ¹²⁹I (half-life 17 Ma) and of fission tracks from the decay of ²⁴⁴Pu (half-life 82 Ma) in phosphate, high-Ca clinopyroxene and olivine. The first precise crystallization age was determined for Brachina using ⁵³Mn–⁵³Cr systematics, relative to the Pb–Pb age of the angrite LEW 86010, and yielded an age of 4563.7 ± 0.9 Ma. Thus, Brachina is at most ∼4 Ma younger that the CAIs whose age is 4567.2 ± 0.6 Ma. There is no consensus on the origin of brachinites, but they most likely are primitive achondrites, i.e., ultra-mafic residues from various low degrees of partial melting. Partial melting experiments suggest that they possibly formed from a parent lithology chemically similar but not identical to the Rumuruti (R) chondrites, although the different oxygen isotopic compositions of the R chondrites and the brachinites put a serious constraint on this hypothesis. The apparent lack of abundant rocks representing the partial melts suggests that brachinites may have formed on a parent body <∼100 km in radius, where early partial melts were removed from the parent body by explosive pyroclastic volcanism. Graves Nunataks 06128 and 06129 are felsic, andesitic basalts which have properties that suggest a relationship to brachinites and thus, may be samples of the elusive partial melts.     

Mineralogy and texture of Fe-Ni sulfides in CI1 chondrites: Clues to the extent of aqueous alteration on the CI1 parent body

To better understand the role of aqueous alteration on the CI1 parent body, we have analyzed the texture, composition and mineral associations of iron nickel sulfides in four of the five known CI1 chondrites.The most commonly-occurring sulfide present in the CI1 chondrites is the iron-deficient Fe,Ni sulfide pyrrhotite ([Fe,Ni]_1−xS), that has a composition close to that of stoichiometric troilite (FeS). Three of the CI1s (Alais, Ivuna and Tonk) also contain pentlandite ([Fe,Ni]₉S₈), although pentlandite is a rare phase in Ivuna. Cubanite (CuFe₂S₃) was found in both Alais and Ivuna in this study, although it has also been reported in Orgueil (MacDougall and Kerridge, 1977). The pyrrhotite grains in all four chondrites form hexagonal, rectangular or irregular shapes, and show no evidence of Ni or Co zoning. The pyrrhotite grains in Orgueil and Ivuna are, in general, smaller, and show more “corrosions,” or “embayments,” than those in Alais or Tonk.We suggest that the precursor sulfide present in the CI1 chondrites was troilite which, during brecciation and oxidation on the parent body at a temperature of 100°C or less, converted the troilite to magnetite and pyrrhotite with pentlandite inclusions. Subsequently, continued alteration on the parent body removed pentlandite—partially from Alais, Tonk and Ivuna, completely from Orgueil—leaving behind pyrrhotite with spaces (“corrosions”) where the pentlandite had been. Ni derived from the pentlandite was incorporated into ferrihydrite, onto the surface of which the Ni,Na sulfate Ni-bloedite formed.Based on the size and abundant “corrosions” within pyrrhotite grains, combined with observations from other authors, we conclude that Orgueil and Ivuna have undergone a greater degree of alteration than Alais and Tonk. Further work is needed to assess the conditions under which pentlandite would be dissolved preferentially to pyrrhotite, as the study of terrestrial literature indicates that the latter mineral is preferentially removed.