Accretion, dispersal, and reaccumulation of the Bishunpur (LL3.1) brecciated chondrite: Evidence from troilite-silicate-metal inclusions and chondrule rims

A set of troilite-silicate-metal (TSM) inclusions and chondrule rims in the Bishunpur (LL3.1) chondrite provide information regarding impact brecciation of small bodies in the early solar system. The TSM inclusions and chondrule rims consist of numerous angular to subrounded silicate grains that are individually enclosed by fine networks of troilite. FeNi metal also occurs in the troilite matrix. The silicates include olivine (Fo_55-98), low-Ca pyroxene (En_78-98), and high-Ca pyroxene (En_48-68Wo_11-32). Al- and Si-rich glass coexists with the silicates. Relatively coarse silicate grains are apparently fragments of chondrules typical of petrologic type-3 chondrites. Troilite fills all available cracks and pores in the silicate grains. Some of the TSM inclusions and rims are themselves surrounded by fine-grained silicate-rich rims (FGR).The TSM inclusions and rims texturally resemble the troilite-rich regions in the Smyer H-chondrite breccia. They probably formed by shock-induced mobilization of troilite during an impact event on a primitive asteroidal body. Because silicates in the TSM inclusions and rims have highly unequilibrated compositions, their precursor was presumably type-3 chondritic material like Bishunpur itself. The TSM inclusions and the chondrules with the TSM rims were fragmented and dispersed after the impact-induced compaction, then reaccreted onto the Bishunpur parent body. FGR probably formed around the TSM inclusions and rims, as well as around some chondrules, during the reaccumulation process. Components of most type-2 and 3 chondrites probably experienced similar processing, i.e., dispersal of unconsolidated materials and subsequent reaccumulation.     

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.     

Origin of rapidly solidified metal-troilite grains in chondrites and iron meteorites

Inclusions of troilite and metallic Fe,Ni 0.2–4 mm in size with a dendritic or cellular texture were observed in 12 ordinary chondrites. Cooling rates in the interval 1400?950°C calculated from the spacing of secondary dendrite arms or cell widths and published experimental data range from 10^?7 to 10⁴°C/sec. In 8 of these chondrites, which are breccias containing some normal slow-cooled metal grains, the inclusions solidified before they were incorporated into the breccias. Their cooling rates of 1–300 °C/sec indicate cooling by radiation, or by conduction in contact with cold silicate or hot silicate volumes only 6–40 mm in size. This is quantitative evidence that these inclusions and their associated clasts were melted on the surface of a parent body (by impact), and were not formed at depth from an internally derived melt. In Ramsdorf, Rose City and Shaw, which show extensive reheating to ? 1000°C, Fe-FeS textures in melted areas are coarser and indicate cooling rates of 10^?1 to 10^?4°C/sec during solidification. This metal may have solidified inside hot silicate volumes that were 10–300 cm in size. As Shaw and Rose City are breccias of unmelted and melted material, their melted metal did not necessarily cool through 1000°C within a few m of the surface. Shock-melted, fine-grained, irregular intergrowths of metal and troilite formed in situ in many irons and some chondrites by rapid solidification at cooling rates of ? 10⁵°C/sec. Their kamacite and taenite compositions may result from annealing at ~250°C of metallic glass or exceedingly fine-grained quench products.     

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.     

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.     

A AB-complex iron meteorite containing low-Ca clinopyroxene: northwest Africa 468 and its relationship to lodranites and formation by impact melting

Northwest Africa 468 (NWA 468) is a new ungrouped, silicate-rich member of the IAB complex of nonmagmatic iron meteorites. The silicates contain relatively coarse (∼300 μm) grains of low-Ca clinopyroxene with polysynthetic twinning and inclined extinction. Low-Ca clinopyroxene is indicative of quenching from high temperatures (either from protoenstatite in a few seconds or high-temperature clinoenstatite in a few hours). It seems likely that NWA 468 formed by impact melting followed by rapid cooling to ≤660°C. After the loss of a metal-sulfide melt from the silicates, sulfide was reintroduced, either from impact-mobilized FeS or as an S₂ vapor that combined with metallic Fe to produce FeS. The O isotopic composition (Δ¹⁷O = −1.39‰) indicates that the precursor material of NWA 468 was a metal-rich (e.g., CR) carbonaceous chondrite. Lodranites are similar in bulk chemical and O isotopic composition to the silicates in NWA 468; the MAC 88177 lodranite (which also contains low-Ca clinopyroxene) is close in bulk chemical composition. Both NWA 468 and MAC 88177 have relatively low abundances of rare earth and plagiophile elements. Siderophiles in the metal-rich areas of NWA 468 are similar to those in the MAC 88177 whole rock; both samples contain low Ir and relatively high Fe, Cu, and Se. Most unweathered lodranites contain ∼20 to 38 wt.% metallic Fe-Ni. These rocks may have formed in an analogous manner to NWA 468 (i.e., by impact melting of metal-rich carbonaceous chondrite precursors) but with less separation of metal-rich melts from silicates.     

车里雅宾斯克(Chelyabinsk)陨石的岩石矿物特征与冲击变质作用

2013年2月15日,俄罗斯车里雅宾斯克(Chelyabinsk)发生了伴随罕见的空中爆炸的大规模陨石雨事件。本文对3块代表不同冲击变质程度的车里雅宾斯克陨石碎块进行了研究。它们都具有部分熔壳,其中1块仅出现碎裂,1块含有冲击熔融细脉,1块基本由冲击熔融囊和冲击熔脉组成。冲击变质程度最低的样品,代表了该陨石母体小行星的原始岩石矿物学特征:即具有粗粒的岩石结构和均一的矿物化学组成,但仍保留一些残余球粒,表明受到了明显的热变质作用,其岩石类型可划分为5型。铁镁质硅酸盐高的Fe O含量(橄榄石Fa:27.9mol%~28.2mol%,辉石Fs值:23.3mol%~23.7mol%)、以及较低的Fe-Ni金属含量,表明其化学群属于低铁低金属的LL群。我们所分析的样品与前人报导的结果相似,未发现不同岩性的岩屑,表明车里雅宾斯克陨石的原始岩矿特征较为均一。3块陨石碎块中,随着冲击程度的增强,其冲击变质特征依次表现为硅酸盐矿物的破碎、熔长石化更为普遍、陨硫铁与铁镍合金共熔、硅酸盐熔脉的形成、铬铁矿与长石共熔、以及大量熔融囊的发育等。但是,在冲击熔融囊和熔脉中,以及相邻围岩中均未发现高压矿物相。熔脉中的橄榄石晶屑和相邻围岩的橄榄石颗粒表现为化学成分的不均一,在背散射电子图像中呈不同灰度的结构。这与其他强烈冲击变质陨石中橄榄石的林伍德石或瓦茨利石相变相似。该陨石中林伍德石或瓦茨利石的缺失很可能是由于强烈撞击后高温产生的退变质。这也表明车里雅宾斯克陨石的母体小行星可能遭受了非常强烈的撞击事件。     

Petrology, Mineralogy and Geochemisty of Antarctic Mesosiderite GRV 020175: Implications for Its Complex Formation History

<正>GRV 020175 is an Antarctic mesosiderite,containing about 43 vol%silicates and 57 vol% metal.Metal occurs in a variety of textures from irregular large masses,to veins penetrating silicates, and to matrix fine grains.The metallic portion contains kamacite,troilite and minor taenite.Terrestrial weathering is evident as partial replacement of the metal and troilite veins by Fe oxides.Silicate phases exhibit a porphyritic texture with pyroxene,plagioclase,minor silica and rare olivine phenocrysts embedded in a fine-grained groundmass.The matrix is ophitic and consists mainly of pyroxene and plagioclase grains.Some orthopyroxene phenocrysts occur as euhedral crystals with chemical zoning from a magnesian core to a ferroan overgrowth;others are characterized by many fine inclusions of plagioclase composition.Pigeonite has almost inverted to its orthopyroxene host with augite lamellae, enclosed by more magnesian rims.Olivine occurs as subhedral crystals,surrounded by a necklace of tiny chromite grains(about 2-3μm).Plagioclase has a heterogeneous composition without zoning. Pyroxene geothermometry of GRV 020175 gives a peak metamorphic temperature(～1000℃) and a closure temperature(～875℃).Molar Fe/Mn ratios(19-32) of pyroxenes are consistent with mesosiderite pyroxenes(16-35) and most plagioclase compositions(An_(87.5_96.6)) are within the range of mesosiderite plagioclase grains(An_(88-95)).Olivine composition(Fo_(53.8)) is only slightly lower than the range of olivine compositions in mesosiderites(Fo_(55-90)).All petrographic characteristics and chemical compositions of GRV 020175 are consistent with those of mesosiderite and based on its matrix texture and relatively abundant plagioclase,it can be further classified as a type 3A mesosiderite.Mineralogical, penological,and geochemical studies of GRV 020175 imply a complex formation history starting as rapid crystallization from a magma in a lava flow on the surface or as a shallow intrusion.Following primary igneous crystallization,the silicate underwent varying degrees of reheating.It was reheated to 1000℃,followed by rapid cooling to 875℃.Subsequently,metal mixed with silicate,during or after which,reduction of silicates occurred;the reducing agent is likely to have been sulfur.After redox reaction,the sample underwent thermal metamorphism,which produced the corona on the olivine, rims on the inverted pigeonite phenocrysts and overgrowths on the orthopyroxene phenocrysts,and homogenized matrix pyroxenes.Nevertheless,metamorphism was not extensive enough to completely reequilibrate the GRV 020175 materials.     

Redistribution of Trace Elements During Shock-inducd Melting and Phase Tranzition of Minerals in the Suizhou l6 Chondrite

The Suizhou meteorite is an L6 chondrite. This meteorite is consisted of olivine, low-Ca pyroxene, plagioclase, FeNi metal, troilite, whitlockite, chlorapatite, chromite and ilmenite. Olivine and pyroxene grains display shock-induced mosaic texture, and most plagioclase grains were melted and transformed to maskelynite. This meteorite contains a few very thin shock-produced melt veins ranging from 20 to 100 μm in width. They are chondritic in composition and contain abundant high-pressure minerals in two assemblages. One is the coarse-grained assemblage of ringwoodite, majorite, lingunite with minor amount of tuite, xieite, the CF-phase, akimotoite and amorphized perovskite, and the fine-grained assemblage (the melt vein matrix) composed of majorite-pyrope garnet, magnesiowüstite. FeNi metal and troilite in the Suizhou shock veins were molten and occur as small intergrowth grains or veinlets filling the interstices of garnet crystals or cracks in the vein matrix. It was revealed that olivine, pyroxene and plagioclase in the Suizhou shock veins have transformed in solid state to their high-pressure polymorphs ringwoodite, majorite, and lingunite, respectively, without change in their chemical compositions.     

Nitrogen and water bubbles, oxygen isotopes, shock effects: Deciphering the history of the Bencubbin meteorite breccia

We have observed vesicles filled with heavy nitrogen gas and water vapor in three settings in the Bencubbin CB chondrite: in the mesostasis of the silicate clasts, in the mesostasis of the chondrules of an ordinary chondrite (OC) xenolith, and in grains we refer to as bubble grains, and interpret as remelted OC chondrule mesostasis. In our view, these bubbles are a local phenomenon and formed as a consequence of the impact of the OC fragment onto the Bencubbin parent body. The heavy nitrogen in the bubbles came from one or several of its carrier phases in Bencubbin, and the water came from hydrous silicates. As formulated by Meibom et al. (Meibom A., Righter K., Chabot N., Dehn G., Antignano A., McCoy T. J., Krot A. N., Zolensky M. E., Petaev M. I. and Keil K. (2005) Shock melts in QUE 94411, Hammadah al Hamra 237, and Bencubbin: remains of the missing matrix? Meteorit. Planet. Sci.40, 1377-1391) these hydrous phases, similar to the hydrated clasts now found in CH and CB_b chondrites, were probably common in the Bencubbin parent body at that time. They were later almost totally destroyed by a large scale shock event, and contributed to form the impact melt that now fills space in between the large clasts of Bencubbin. Our observations indirectly confirm this hypothesis. From our composition measurements, we infer that the silicate part of the impact melt was made in roughly equal proportions of melted phyllosilicates and melted anhydrous silicates. The oxygen isotopic composition of the impact melt is much heavier than that of the silicate clasts, probably reflecting the composition of the water at the origin of the phyllosilicates. The O isotope measurements of the OC inclusion chondrules present some features that seem to be common in OCs: a composition of the chondrule crystals slightly lighter than that of whole chondrules, and one olivine crystal with a very light composition.     

I-Xe dating of silicate and troilite from IAB iron meteorites

The IAB iron meteorites may be related to the chondrites: siderophile elements in the metal matrix have chondritic abundances, and the abundant silicate inclusions are chondritic both in mineralogy and in chemical composition. Silicate and troilite (FeS) from IAB irons were analyzed by the I-Xe technique. Four IAB silicate samples gave well-defined I-Xe ages [in millions of years relative to Bjurböle; the monitor error (± 2.5 Myr) is not included]: ?3.7 ± 0.3 for Woodbine, ?0.7 ± 0. 6 for Mundrabilla, +1.4 ± 0.7 for Copiapo, and +2.6 ± 0.6 for Landes. The (¹²⁹Xe/¹³²Xe)_trapped ratios are consistent with previous values for chondrites, with the exception of Landes which has an extraordinary trapped ratio of 3.5 ± 0.2. Both analyses of silicate from Pitts gave anomalous I-Xe patterns.Troilite samples were also analyzed: Pitts troilite gave a complex I-Xe pattern, which suggests an age of +17 Myr; Mundrabilla troilite defined a good I-Xe correlation, which after correction for neutron capture on ¹²⁸Te gave an age of ?10.8 ± 0.7 Myr. Thus, surprisingly, low-melting troilite substantially predates high-melting silicate in Mundrabilla.Abundances of Ga, Ge, and Ni in metal from these meteorites are correlated with I-Xe ages of the silicate; meteorites with older silicates have greater Ni contents. No model easily accounts for this result as well as other properties of IAB irons; nevertheless, these results, taken at face value, overall favor a nebular formation model (e.g. Wasson, 1970, Icarus 12, 407–423). The great age of troilite from Mundrabilla suggests that this troilite formed in a different nebular region from the silicate and metal, and was later mechanically mixed with these other phases.The correlation between the trace elements in the metal and the I-Xe ages of the silicate provides one of the first known instances in which another well-defined meteoritic property correlates with I-Xe ages. In addition, almost all the ¹²⁹Xe in Mundrabilla silicate (etched in acid) was correlated with ¹²⁸Xe. These two results further support the validity of the I-Xe dating method.     

Rare earth element geochemistry and petrogenesis of miles (IIE) silicate inclusions

An ion probe study of rare earth element (REE) geochemistry of silicate inclusions in the Miles IIE iron meteorite was carried out. Individual mineral phases among inclusions have distinct REE patterns and abundances. Most silicate grains have homogeneous REE abundances but show considerable intergrain variations between inclusions. A few pyroxene grains display normal igneous REE zoning. Phosphates (whitlockite and apatite) are highly enriched in REEs (50 to 2000 × CI) with a relatively light rare earth element (LREE)-enriched REE pattern. They usually occurred near the interfaces between inclusions and Fe host. In Miles, albitic glasses exhibit two distinctive REE patterns: a highly fractionated LREE-enriched (CI normalized La/Sm ∼15) pattern with a large positive Eu anomaly and a relatively heavy rare earth element (HREE)-enriched pattern (CI-normalized Lu/Gd ∼4) with a positive Eu anomaly and a negative Yb anomaly. The glass is generally depleted in REEs relative to CI chondrites.The bulk REE abundances for each inclusion, calculated from modal abundances, vary widely, from relatively depleted in REEs (0.1 to 3 × CI) with a fractionated HREE-enriched pattern to highly enriched in REEs (10 to 100 × CI) with a relatively LREE-enriched pattern. The estimated whole rock REE abundances for Miles are at ∼ 10 × CI with a relatively LREE-enriched pattern. This implies that Miles silicates could represent the product of a low degree (∼10%) partial melting of a chondritic source. Phenocrysts of pyroxene in pyroxene-glassy inclusions were not in equilibrium with coexisting albitic glass and they could have crystallized from a parental melt with REEs of ∼ 10 × CI. Albitic glass appears to have formed by remelting of preexisting feldspar + pyroxene + tridymite assemblage. Yb anomaly played an important role in differentiation processes of Miles silicate inclusions; however, its origin remains unsolved.The REE data from this study suggest that Miles, like Colomera and Weekeroo Station, formed when a molten Fe ball collided on a differentiated silicate regolith near the surface of an asteroid. Silicate fragments were mixed with molten Fe by the impact. Heat from molten Fe caused localized melting of feldspar + pyroxene + tridymite assemblage. The inclusions remained isolated from one another during subsequent rapid cooling.     

Impact features of enstatite-rich meteorites

Enstatite-rich meteorites include EH and EL chondrites, rare ungrouped enstatite chondrites, aubrites, a few metal-rich meteorites (possibly derived from the mantle of the aubrite parent body), various impact-melt breccias and impact-melt rocks, and a few samples that may be partial-melt residues ultimately derived from enstatite chondrites. Members of these sets of rocks exhibit a wide range of impact features including mineral-lattice deformation, whole-rock brecciation, petrofabrics, opaque veins, rare high-pressure phases, silicate darkening, silicate-rich melt veins and melt pockets, shock-produced diamonds, euhedral enstatite grains, nucleation of enstatite on relict grains and chondrules, low MnO in enstatite, high Mn in troilite and oldhamite, grains of keilite, abundant silica, euhedral graphite, euhedral sinoite, F-rich amphibole and mica, and impact-melt globules and spherules. No single meteorite possesses all of these features, although many possess several. Impacts can also cause bulk REE fractionations due to melting and loss of oldhamite (CaS) – the main REE carrier in enstatite meteorites. The Shallowater aubrite can be modeled as an impact-melt rock derived from a large cratering event on a porous enstatite chondritic asteroid; it may have been shock melted at depth, slowly cooled and then excavated and quenched. Mount Egerton may share a broadly similar shock and thermal history; it could be from the same parent body as Shallowater. Many aubrites contain large pyroxene grains that exhibit weak mosaic extinction, consistent with shock-stage S4; in contrast, small olivine grains in some of these same aubrites have sharp or undulose extinction, consistent with shock stage S1 to S2. Because elemental diffusion is much faster in olivine than pyroxene, it seems likely that these aubrites experienced mild post-shock annealing, perhaps due to relatively shallow burial after an energetic impact event. There are correlations among EH and EL chondrites between petrologic type and the degree of shock, consistent with the hypothesis that collisional heating is mainly responsible for enstatite-chondrite thermal metamorphism. Nevertheless, the apparent shock stages of EL6 and EH6 chondrites tend to be lower than EL3-5 and EH3-5 chondrites, suggesting that the type-6 enstatite chondrites (many of which possess impact-produced features) were shocked and annealed. The relatively young Ar–Ar ages of enstatite chondrites record heating events that occurred long after any ²⁶Al that may have been present initially had decayed away. Impacts remain the only plausible heat source at these late dates. Some enstatite meteorites accreted to other celestial bodies: Hadley Rille (EH) was partly melted when it struck the Moon; Galim (b), also an EH chondrite, was shocked and partly oxidized when it accreted to the LL parent asteroid. EH, EL and aubrite-like clasts also occur in the polymict breccias Kaidun (a carbonaceous chondrite) and Almahata Sitta (an anomalous ureilite). The EH and EL clasts in Kaidun appear unshocked; some clasts in Almahata Sitta may have been extensively shocked on their parent bodies prior to being incorporated into the Almahata Sitta host.     

Silicate-bearing iron meteorites and their implications for the evolution of asteroidal parent bodies

Silicate-bearing iron meteorites differ from other iron meteorites in containing variable amounts of silicates, ranging from minor to stony-iron proportions (∼50%). These irons provide important constraints on the evolution of planetesimals and asteroids, especially with regard to the nature of metal–silicate separation and mixing. I present a review and synthesis of available data, including a compilation and interpretation of host metal trace-element compositions, oxygen-isotope compositions, textures, mineralogy, phase chemistries, and bulk compositions of silicate portions, ages of silicate and metal portions, and thermal histories. Case studies for the petrogeneses of igneous silicate lithologies from different groups are provided. Silicate-bearing irons were formed on multiple parent bodies under different conditions. The IAB/IIICD irons have silicates that are mainly chondritic in composition, but include some igneous lithologies, and were derived from a volatile-rich asteroid that underwent small amounts of silicate partial melting but larger amounts of metallic melting. A large proportion of IIE irons contain fractionated alkali-silica-rich inclusions formed as partial melts of chondrite, although other IIE irons have silicates of chondritic composition. The IIEs were derived from an H-chondrite-like asteroid that experienced more significant melting than the IAB asteroid. The two stony-iron IVAs were derived from an extensively melted and apparently chemically processed L or LL-like asteroid that also produced a metallic core. Ungrouped silicate-bearing irons were derived from seven additional asteroids. Hf–W age data imply that metal–silicate separation occurred within 0–10 Ma of CAI formation for these irons, suggesting internal heating by ²⁶Al. Chronometers were partly re-set at later times, mainly earlier for the IABs and later for the IIEs, including one late (3.60 ± 0.15 Ga) strong impact that affected the “young silicate” IIEs Watson (unfractionated silicate, and probable impact melt), Netschaëvo (unfractionated, and metamorphosed), and Kodaikanal (fractionated). Kodaikanal probably did not undergo differentiation in this late impact, but the similar ages of the “young silicate” IIEs imply that relatively undifferentiated and differentiated materials co-existed on the same asteroid. The thermal histories and petrogeneses of fractionated IIE irons and IVA stony irons are best accommodated by a model of disruption and reassembly of partly molten asteroids.     

山东省鄄城陨石中的金属矿物及组构特征

1997年降落在山东省鄄城县的陨石雨,是橄榄石-古铜辉石球粒陨石。该陨石中的金属矿物主要为铁纹石和陨硫铁,其次为镍纹石,金属矿物呈填隙状分布于以橄榄石和古铜辉石为主的硅酸盐矿物粒间及球粒周围。陨石中可见由铁纹石和镍纹石组成的显微蠕虫状连晶,是陨石中金属矿物在降温冷却过程中发生固溶体分离作用而成。陨石中金属矿物的分布特征表明,金属Fe-Ni和硫化物（FeS）应该是星云凝聚不同阶段的产物。陨石中金属矿物的成分和组构特征及陨石中出现的球粒结构、橄榄石的炉条结构等特征表明,该球粒陨石是星云物质快速冷却的产物。     

The formation and chronology of the PAT 91501 impact-melt L chondrite with vesicle-metal-sulfide assemblages

The L chondrite Patuxent Range (PAT) 91501 is an 8.5-kg unshocked, homogeneous, igneous-textured impact melt that cooled slowly compared to other meteoritic impact melts in a crater floor melt sheet or sub-crater dike [Mittlefehldt D. W. and Lindstrom M. M. (2001) Petrology and geochemistry of Patuxent Range 91501 and Lewis Cliff 88663. Meteoritics Planet. Sci. 36, 439-457]. We conducted mineralogical and tomographic studies of previously unstudied mm- to cm-sized metal-sulfide-vesicle assemblages and chronologic studies of the silicate host. Metal-sulfide clasts constitute about 1 vol.%, comprise zoned taenite, troilite, and pentlandite, and exhibit a consistent orientation between metal and sulfide and of metal-sulfide contacts. Vesicles make up ∼2 vol.% and exhibit a similar orientation of long axes. ³⁹Ar-⁴⁰Ar measurements probably date the time of impact at 4.461 ± 0.008 Gyr B.P. Cosmogenic noble gases and ¹⁰Be and ²⁶Al activities suggest a pre-atmospheric radius of 40-60 cm and a cosmic ray exposure age of 25-29 Myr, similar to ages of a cluster of L chondrites. PAT 91501 dates the oldest known impact on the L chondrite parent body. The dominant vesicle-forming gas was S₂ (∼15-20 ppm), which formed in equilibrium with impact-melted sulfides. The meteorite formed in an impact melt dike beneath a crater, as did other impact melted L chondrites, such as Chico. Cooling and solidification occurred over ∼2 h. During this time, ∼90% of metal and sulfide segregated from the local melt. Remaining metal and sulfide grains oriented themselves in the local gravitational field, a feature nearly unique among meteorites. Many of these metal-sulfide grains adhered to vesicles to form aggregates that may have been close to neutrally buoyant. These aggregates would have been carried upward with the residual melt, inhibiting further buoyancy-driven segregation. Although similar processes operated individually in other chondritic impact melts, their interaction produced the unique assemblage observed in PAT 91501.     

Mass-dependent fractionation of Mg, Si, and Fe isotopes in five stony cosmic spherules

We have measured with an electron microprobe the Mg, Al, Si, Ca, Ti, Mn, and Fe contents of five strongly heated stony cosmic spherules (sCS) from the South Pole water well. We have also measured the isotopic compositions of Si, and when possible of Mg and of Fe in these objects by ion microprobe. Except for iron, the measured elemental compositions are chondritic within a factor of 2. In four samples, the ratio of ⁵⁷Fe/⁵⁶Fe exceeds the terrestrial value by 3.5‰ to 48‰. Mass-dependent fractionation of the isotopes of Si ranges from ∼2 to ∼8 ‰/AMU in three samples. Mg is clearly fractionated in only one sample, for which δ²⁵Mg = ∼8 ‰. The extent of mass-dependent fractionation of the isotopes and, by implication, of evaporative loss generally follows a trend Mg < Si < Fe. The trend is similar to that found in laboratory heating experiments of charges with solar composition. Although the observed isotopic inhomogeneities within some samples call into question the strict validity of the Rayleigh equation for the sCS, its approximate application to our new and to previously published results for Mg suggests that evaporative losses of greater than 40 wt.% occur rarely from sCS, and that the precursor grains of the sCS had a CM-carbonaceous-chondrite-like complement of Mg, Si, Ca, and Al. Low Fe contents relative to CM abundances could reflect an unusual precursor composition, or, more probably, losses by processes that did not fractionate isotopes, i.e., ejection of immiscible FeS and FeNi beads from the melt or rapid, complete separation and decomposition of FeS at the surface.     

Transmission electron microscopic study of the fine-grained vein matrix in the Suizhou L6 meteorite

The mineralogy of shock vein matrix in the Suizhou meteorite has been investigated by optical and transmission electron microscopy. It was revealed that the vein matrix is composed of majorite-pyrope garnet, magnesiowüstite, and ringwoodite, with FeNi–FeS intergrowths. The observation and character of ring-like selected electron diffraction (SAED) patterns indicate that the idiomorphic garnet crystals in the vein matrix have different orientations. The polycrystalline nature of magnesiowüstite is also confirmed by a ring-like SAED pattern. Both garnet and magnesiowüstite crystals showed sharp diffraction spots, signifying the good crystallinity of these two minerals. The SAED pattern of cryptocrystalline ringwoodite shows only diffuse concentric diffraction rings. FeNi metal and troilite (FeS), which were molten during the shock event, occur in the matrix as fine eutectic FeNi–FeS intergrowths filling the interstices between garnet and magnesiowüstite grains. Based on the phase diagram of the Allende chondrite and the results of this TEM study, it is inferred that majorite-pyrope garnet first crystallized from the Suizhou chondritic melt at 22–26 GPa, followed by crystallization of magnesiowüstite at 20–24 GPa, and then ringwoodite at 18–20 GPa. The eutectic intergrowths of FeNi-metal and troilite are proposed to have crystallized during meteorite cooling and solidified at the last stage of vein formation.     

Silica and pyroxene in IVA irons; possible formation of the IVA magma by impact melting and reduction of L-LL-chondrite materials followed by crystallization and cooling

Group IVA is a large magmatic group of iron meteorites. The mean Δ¹⁷O (=δ¹⁷O − 0.52·δ¹⁸O) of the silicates is ∼+1.2‰, similar to the highest values in L chondrites and the lowest values in LL chondrites; δ¹⁸O values are also in the L/LL range. This strongly suggests that IVA irons formed by melting L-LL parental material, but the mean Ni content of IVA irons (83 mg/g) is much lower than that of a presumed L-LL parent (∼170 mg/g) and the low-Ca pyroxene present in two IVA meteorites is Fs13, much lower than the Fs20-29 values in L and LL chondrites. Thus, formation from L-LL precursors requires extensive addition of metallic Fe, probably produced by reduction of FeS and FeO. Group IVA also has S/Ni, Ga/Ni, and Ge/Ni ratios that are much lower than those in L-LL chondrites or any chondrite group that preserves nebular compositions, implying loss of these volatile elements during asteroidal processing. We suggest that these reduction and loss processes occurred near the surface of the asteroid during impact heating, and resulted partly from reduction by C, and partly from the thermal dissociation of FeS and FeO with loss of O and S. The hot (∼1770 K) low-viscosity melt quickly moved through channels in the porous asteroid to form a core. Two members of the IVA group, São João Nepomuceno (hereafter, SJN) and Steinbach, contain moderate amounts of orthopyroxene and silica, and minor amounts of low-Ca clinopyroxene. Even though SJN formed after ∼26% crystallization and Steinbach formed after ∼77% crystallization of the IVA core, both could have originated within several tens of meters of the core-mantle interface if 99% of the crystallization occurred from the center outwards. Two other members of the group (Gibeon and Bishop Canyon) contain tabular tridymite, which we infer to have initially formed as veins deposited from a cooling SiO-rich vapor. The silicates were clearly introduced into IVA irons after the initial magma crystallized. Because the γ-iron crystals in SJN are typically about 5 cm across, an order of magnitude smaller than in IVA irons that do not contain massive silicates, we infer that the metal was in the γ-iron field when the silicates were injected. The SJN and Steinbach silicate compositions are near the low-Ca-pyroxene/silica eutectic compositions. We suggest that a tectonic event produced a eutectic-like liquid and injected it together with unmelted pyroxene grains into fissures in the solid metal core. Published estimates of IVA metallographic cooling rates range from 20 to 3000 K/Ma, leading to a hypothesized breakup of the core during a major impact followed by scrambling of the core and mantle debris [Haack, H., Scott, E.R.D., Love, S.G., Brearley, A. 1996. Thermal histories of IVA stony-iron and iron meteorites: evidence for asteroid fragmentation and reaccretion. Geochim. Cosmochim. Acta60, 3103-3113]. This scrambling model is physically implausible and cannot explain the strong correlation of estimated cooling rates with metal composition. Previous workers concluded that the low-Ca clinopyroxene in SJN and Steinbach formed from protopyroxene by quenching at a cooling rate of 10¹² K/Ma, and suggested that this also supported an impact-scrambling model. This implausible spike in cooling rate by a factor of 10¹⁰ can be avoided if the low-Ca clinopyroxene were formed by a late shock event that converted orthopyroxene to clinopyroxene followed by minimal growth in the clinopyroxene field, probably because melt was also produced. We suggest that metallographic cooling-rate estimates (e.g., based on island taenite) giving similar values throughout the metal compositional range are more plausible, and that the IVA parent asteroid can be modeled by monotonic cooling followed by a high-temperature impact event that introduced silicates into the metal and a low-temperature impact event that partially converted orthopyroxene into low-Ca clinopyroxene.     

Sulfur isotope composition of putative primary troilite in chondrules from Bishunpur and Semarkona

The sulfur isotopic compositions of putative primary troilite grains within 15 ferromagnesian chondrules (10 FeO-poor and 5 FeO-rich chondrules) in the least metamorphosed ordinary chondrites, Bishunpur and Semarkona, have been measured by ion microprobe. Some troilite grains are located inside metal spherules within chondrules. Since such an occurrence is unlikely to be formed by secondary sulfidization processes in the solar nebula or on parent bodies, those troilites are most likely primary, having survived chondrule-forming high-temperature events. If they are primary, they may be the residues of evaporation at high temperatures during chondrule formation and may have recorded mass-dependent isotopic fractionations. However, the supposed primary troilites measured in this study do not show any significant sulfur isotopic fractionations (<1 ‰/amu) relative to large troilite grains in matrix. Among other chondrule troilites that we measured, only one (BI-CH22) apparently has a small excess of heavy isotopes (2.7 ± 1.4 ‰/amu) consistent with isotopic fractionation during evaporation. All other grains have isotopic fractionations of <1 ‰/amu. Because sulfur is so volatile that evaporation during chondrule formation is probably inevitable, non-Rayleigh evaporation most likely explains the lack of isotopic fractionation in putative primary troilite inside chondrules. Evaporation through the surrounding silicate melt would have suppressed the isotopic fractionation after silicate dust grains melted. At lower temperatures below extensive melting of silicates, a heating rate of >10⁴-10⁶ K/h would be required to avoid a large degree of sulfur isotopic fractionation in the chondrule precursors. This heating rate may provide a new constraint on the chondrule formation processes.