Variations in impact effects among IIIE iron meteorites

Group‐IIIE iron meteorites can be ordered into four categories reflecting increasing degrees of shock alteration. Weakly shocked samples (Armanty, Colonia Obrera, Coopertown, Porto Alegre, Rhine Villa, Staunton, and Tanokami Mountain) have haxonite within plessite, unrecrystallized kamacite grains containing Neumann lines or possessing the ? structure, and sulfide inclusions typically consisting of polycrystalline troilite with daubréelite exsolution lamellae. The only moderately shocked sample is NWA 4704, in which haxonite has been partially decomposed to graphite; the majority of the kamacite in NWA 4704 is recrystallized, and its sulfide inclusions were partly melted. Strongly shocked samples (Cachiyuyal, Kokstad, and Paloduro) contain graphite and no haxonite, suggesting that pre‐existing haxonite fully decomposed. Also present in these rocks are recrystallized kamacite and melted troilite. Residual heat from the impact caused annealing and recrystallization of kamacite as well as the decomposition of haxonite into graphite. Severely shocked samples (Aliskerovo and Willow Creek) have sulfide‐rich assemblages consisting of fragmental and subhedral daubréelite crystals, 1–4 vol% spidery troilite filaments, and 30–50 vol% low‐Ni kamacite grains, some of which contain up to 6.0 wt% Co; haxonite in these inclusions has fully decomposed to graphite. The wide range of impact effects in IIIE irons is attributed to one or more major collision(s) on the parent asteroid that affected different group members to different extents depending on their proximity to the impact point.     

On the possible origin of troilite‐metal nodules in the Katol chondrite (L6‐7)

Microtextural study of a single troilite‐metal nodule (TMN) from the Katol L6‐7 chondrite, a recent fall (May, 2012) in India suggests that the TMN is primarily an aggregate of submicron‐scale intergrowth of troilite and kamacite (mean Ni: 6.18 wt%) juxtaposed with intensely fractured silicates, mainly olivine (Fa: 25 mole%), low‐Ca pyroxene (Fs: 21.2 mole%), and a large volume of maskelynite. Evidence of shock textures in the TMN indicates a high degree of shock metamorphism that involves plagioclase‐maskelynite and olivine‐wadsleyite/ringwoodite transformations and formation of quenched metal‐sulfide melt textures due to localized shear‐induced frictional melting. It is inferred that the TMN formation is an independent, localized event by a high energy impact and its subsequent incorporation in the ejected chondritic fragment of the parent body. Katol chondrite has been calibrated with a peak shock pressure of S5 (~45 GPa) after Stöffler et al. (1991), whereas peak shock pressure within the TMN exceeds the shock facies S6 (>45 GPa) following Bennett and McSween (1996) and Stöffler et al. (1991). Overall, the shock‐thermal history of the Katol TMN is dissimilar as compared to the host chondrite.     

Shock features in iron-nickel metal and troilite of L-group ordinary chondrites

Abstract— Shock metamorphic features in opaque minerals (FeNi metal and troilite) of 22 L chondrites have been studied petrographically and geochemically in an attempt to establish a connection between the present silicate-based shock classification scheme (Stöffler et al., 1991) and the peak-shock and postshock thermal history recorded in these minerals. Unshocked to weakly shocked (S1–S3) L chondrites contain FeNi metal and troilite that display textures related to normal, slow cooling. They may also contain rare disequilibrium shock features, which suggest localized departures from equilibrium shock conditions. Above shock stage S3, selected melting of FeNi metal and troilite produces melt droplets whose composition and abundance correspond to the maximum equilibrium shock state achieved by the sample. At these higher shock levels, the abundance of other shock-induced features, such as polycrystalline kamacite, sheared and fizzed troilite, coarse-grained pearlitic plessite, polycrystalline troilite, and polymineralic melt veins serve as textural criteria that can be used to establish peak-shock conditions. Minimum postshock temperatures obtained from analyses of plessite components show a systematic increase in temperature with an increase in shock stage, thereby providing additional information about the postshock thermal histories of L chondrites. At the highest shock levels recorded in L chondrites (S6 and above), melting and chemical homogenization of FeNi metal produces flattened Ni profiles that may partially to completely obscure any evidence for an earlier, slow-cooling history. All of these features serve as aids for shock classifying L chondrites as well as for quantifying minimum peak temperatures that resulted during shock metamorphism.     

Contribution of early impact events to metal‐silicate separation,thermal annealing,and volatile redistribution: Evidence in the Pułtusk H chondrite

Three‐dimensional X‐ray tomographic reconstructions and petrologic studies reveal voluminous accumulations of metal in Pu?tusk H chondrite. At the contact of these accumulations, the chondritic rock is enriched in troilite. The rock contains plagioclase‐rich bands, with textures suggesting crystallization from melt. Unusually large phosphates are associated with the plagioclase and consist of assemblages of merrillite, and fluorapatite and chlorapatite. The metal accumulations were formed by impact melting, rapid segregation of metal‐sulfide melt and the incorporation of this melt into the fractured crater basement. The impact most likely occurred in the early evolution of the H chondrite parent body, when post‐impact heat overlapped with radiogenic heat. This enabled slow cooling and separation of the metallic melt into metal‐rich and sulfide‐rich fractions. This led to recrystallization of chondritic rock in contact with the metal accumulations and the crystallization of shock melts. Phosphorus was liberated from the metal and subsumed by the silicate shock melt, owing to oxidative conditions upon slow cooling. The melt was also a host for volatiles. Upon further cooling, phosphorus reacted with silicates leading to the formation of merrillite, while volatiles partitioned into the residual halogen‐rich, dry fluid. In the late stages, the fluid altered merrillite to patchy Cl/F‐apatite. The above sequence of alterations demonstrates that impact during the early evolution of chondritic parent bodies might have contributed to local metal segregation and silicate melting. In addition, postshock conditions supported secondary processes: compositional/textural equilibration, redistribution of volatiles, and fluid alterations.     

What metal‐troilite textures can tell us about post‐impact metamorphism in chondrite meteorites

Abstract— Metal‐troilite textures are examined in metamorphosed and impact‐affected ordinary chondrites to examine the response of these phases to rapid changes in temperature. Complexly intergrown metal‐troilite textures are shown to form in response to three different impact‐related processes. (1) During impacts, immiscible melt emulsions form in response to spatially focused heating. (2) Immediately after impact events, re‐equilibration of heterogeneously distributed heat promotes metamorphism adjacent to zones of maximum impact heating. Where temperatures exceed ～850 ° C, this post‐impact metamorphism results in melting of conjoined metal‐troilite grains in chondrites that were previously equilibrated through radiogenic metamorphism. When the resulting Fe‐Ni‐S melt domains crystallize, a finely intergrown mixture of troilite and metal forms, which can be zoned with kamacite‐rich margins and taenite‐rich cores. (3) At lower temperatures, post‐impact metamorphism can also cause liberation of sulfur from troilite, which migrates into adjacent Fe‐Ni metal, allowing formation of troilite and occasionally copper within the metal during cooling. Because impact events cause heating within a small volume, post‐impact metamorphism is a short duration event (days to years) compared with radiogenic metamorphism (>10⁶ years). The fast kinetics of metal‐sulfide reactions allows widespread textural changes in conjoined metal‐troilite grains during post‐impact metamorphism, whereas the slow rate of silicate reactions causes these to be either unaffected or only partially annealed, except in the largest impact events. Utilizing this knowledge, information can be gleaned as to whether a given meteorite has suffered a post‐impact thermal overprint, and some constraints can be placed on the temperatures reached and duration of heating.     

Compositions and microstructures of CB sulfides: Implications for the thermal history of the CB chondrite parent body

We studied textures and compositions of sulfide inclusions in unzoned Fe,Ni metal particles within CBa Gujba, CBa Weatherford, CBb HH 237, and CBb QUE 94411 in order to constrain formation conditions and secondary thermal histories on the CB parent body. Unzoned metal particles in all four chondrites have very similar metal and sulfide compositions. Metal particles contain different types of sulfides, which we categorize as: homogeneous low‐Cr sulfides composed of troilite, troilite‐containing exsolved daubreelite lamellae, arcuate sulfides that occur along metal grain boundaries, and shock‐melted sulfides composed of a mixture of troilite and Fe, Ni metal. Our model for formation proposes that the unzoned metal particles were initially metal droplets that formed from splashing by a partially molten impacting body. Sulfide inclusions later formed as a result of precipitation of excess S from solid metal at low temperatures, either during single stage cooling or during a reheating event by impacts. Sulfides containing exsolution lamellae record temperatures of ?600 °C, and irregular Fe‐FeS intergrowth textures suggest localized shock melting, both of which are indicative of heterogeneous heating by impact processes on the CB parent body. Our study shows that CBa and CBb chondrites formed in a similar environment, and also experienced similar secondary impact processing.     

Impact melting of the largest known enstatite meteorite: Al Haggounia 001, a fossil EL chondrite

Al Haggounia 001 and paired specimens (including Northwest Africa [NWA] 2828 and 7401) are part of a vesicular, incompletely melted, EL chondrite impact melt rock with a mass of ~3 metric tons. The meteorite exhibits numerous shock effects including (1) development of undulose to weak mosaic extinction in low‐Ca pyroxene; (2) dispersion of metal‐sulfide blebs within silicates causing “darkening”; (3) incomplete impact melting wherein some relict chondrules survived; (4) vaporization of troilite, resulting in S₂ bubbles that infused the melt; (5) formation of immiscible silicate and metal‐sulfide melts; (6) shock‐induced transportation of the metal‐sulfide melt to distances >10 cm; (7) partial resorption of relict chondrules and coarse silicate grains by the surrounding silicate melt; (8) crystallization of enstatite in the matrix and as overgrowths on relict silicate grains and relict chondrules; (9) crystallization of plagioclase from the melt; and (10) quenching of the vesicular silicate melt. The vesicular samples lost almost all of their metal during the shock event and were less susceptible to terrestrial weathering; in contrast, the samples in which the metal melt accumulated became severely weathered. Literature data indicate the meteorite fell ~23,000 yr ago; numerous secondary phases formed during weathering. Both impact melting and weathering altered the meteorite's bulk chemical composition: e.g., impact melting and loss of a metal‐sulfide melt from NWA 2828 is responsible for bulk depletions in common siderophile elements and in Mn (from alabandite); weathering of oldhamite caused depletions in many rare earth elements; the growth of secondary phases caused enrichments in alkalis, Ga, As, Se, and Au.     

Formation of feldspathic and metallic melts by shock in enstatite chondrite Reckling Peak A80259

Abstract— The enstatite chondrite reckling peak (rkp) a80259 contains feldspathic glass, kamacite, troilite, and unusual sets of parallel fine‐grained enstatite prisms that formed by rapid cooling of shock melts. Metallic Fe,Ni and troilite occur as spherical inclusions in feldspathic glass, reflecting the immiscible Fe‐Ni‐S and feldspathic melts generated during the impact. The Fe‐Ni‐S and feldspathic liquids were injected into fractures in coarse‐grained enstatite and cooled rapidly, resulting in thin (≤ 10 μm) semicontinuous to discontinuous veins and inclusion trails in host enstatite. Whole‐rock melt veins characteristic of heavily shocked ordinary chondrites are conspicuously absent. Raman spectroscopy shows that the feldspathic material is a glass. Elevated MgO and SiO₂ contents of the glass indicate that some enstatite and silica were incorporated in the feldspathic melt. Metallic Fe,Ni globules are enclosed by sulfide and exhibit Nienrichment along their margins characteristic of rapid crystallization from a Fe‐Ni‐S liquid. Metal enclosed by sulfide is higher in Si and P than metal in feldspathic glass and enstatite, possibly indicating lower O fugacities in metal/sulfide than in silicate domains. Fine‐grained, elongate enstatite prisms in troilite or feldspathic glass crystallized from local pyroxene melts that formed along precursor grain boundaries, but most of the enstatite in the target rock remained solid during the impact and occurs as deformed, coarsegrained crystals with lower CaO, Al₂O₃, and FeO than the fine‐grained enstatite. Reckling Peak A80259 represents an intermediate stage of shock melting between unmelted E chondrites and whole‐rock shock melts and melt breccias documented by previous workers. The shock petrogenesis of RKPA80259 reflects the extensive impact processing of the enstatite chondrite parent bodies relative to those of other chondrite types.     

GALLIUM-BEARING SPHALERITE IN A METAL-SULFIDE NODULE OF THE QINGZHEN (EH3) CHONDRITE

The Qingzhen (EH3) chondrite contains a population of spheroidal metal-sulfide nodules, which display textural evidence of reheating and melting. Evidence of metal sulfuration is also present, suggesting replacement of metal by sulfide during melting. This process has led to the nucleation of perryite along metal-sulfide interfaces. Gallium-bearing sphalerite and a Cu-sulfide of composition intermediate between chalcopyrite and cubanite occur as inclusions within the metal of some nodules. Other phases present are: kamacite, troilite, Ga-free sphalerite, niningerite, perryite, schreibersite, oldhamite, Cr-sulfide (minerals A and B), djerfisherite, SiO₂, albite and enstatite. The Ga-bearing sphalerite may have formed by injection of molten sulfide droplets into the metal followed by subsolidus diffusion of Ga from the metal into the sulfide. The latter may occur because of Ga supersaturation in the metal during progressive sulfuration and its decreased affinity for the metal phase during cooling below the taenite-kamacite transition point.     

Northwest Africa 2526: A partial melt residue of enstatite chondrite parentage

Abstract— NWA 2526 is a coarse‐grained, achondritic rock dominated by equigranular grains of polysynthetically twinned enstatite (?85 vol%) with frequent 120° triple junctions and ?10–15 vol% of kamacite + terrestrial weathering products. All other phases including troilite, daubreelite, schreibersite, and silica‐normative melt areas make up 相似文献   

Northwest Africa 428: Impact‐induced annealing of an L6 chondrite breccia

Abstract— Northwest Africa (NWA) 428 is an L chondrite that was successively thermally metamorphosed to petrologic type‐6, shocked to stage S4–S5, brecciated, and annealed to approximately petrologic type‐4. Its thermal and shock history resembles that of the previously studied LL6 chondrite, Miller Range (MIL) 99301, which formed on a different asteroid. The petrologic type‐6 classification of NWA 428 is based on its highly recrystallized texture, coarse metal (150 ± 150 μm), troilite (100 ± 170 μm), and plagioclase (20–60 μm) grains, and relatively homogeneous olivine (Fa_{24.4 ± 0.6}), low‐Ca pyroxene (Fs_{20.5 ± 0.4}), and plagioclase (Ab_{84.2 ± 0.4}) compositions. The petrographic criteria that indicate shock stage S4–S5 include the presence of chromite veinlets, chromite‐plagioclase assemblages, numerous occurrences of metallic Cu, irregular troilite grains within metallic Fe‐Ni, polycrystalline troilite, duplex plessite, metal and troilite veins, large troilite nodules, and low‐Ca clinopyroxene with polysynthetic twins. If the rock had been shocked before thermal metamorphism, low‐Ca clinopyroxene produced by the shock event would have transformed into orthopyroxene. Post‐shock brecciation is indicated by the presence of recrystallized clasts and highly shocked clasts that form sharp boundaries with the host. Post‐shock annealing is indicated by the sharp optical extinction of the olivine grains; during annealing, the damaged olivine crystal lattices healed. If temperatures exceeded those approximating petrologic type‐4 (?600–700°C) during annealing, the low‐Ca clinopyroxene would have transformed into orthopyroxene. The other shock indicators, likewise, survived the mild annealing. An impact event is the most plausible source of post‐metamorphic, post‐shock annealing because any ²⁶Al that may have been present when the asteroid accreted would have decayed away by the time NWA 428 was annealed. The similar inferred histories of NWA 428 (L6) and MIL 99301 (LL6) indicate that impact heating affected more than 1 ordinary chondrite parent body.     

Origin and age of metal veins in Canyon Diablo graphite nodules

Previous studies attributed the origin of metal veins penetrating graphite nodules in the Canyon Diablo IAB main group iron meteorite to condensation from vapor or melting of host metal. Abundances of 16 siderophile elements measured in kamacite within vein and host meteorite are most consistent with an origin by melting of the host metal followed by fractional crystallization of the liquid. The presence of the veins within graphite nodules may be explained by impact, as peak shock temperatures, and thus the most likely areas to undergo metal melting are at metal–graphite interfaces. The origin of the veins is constrained by Re‐Os chronometry to have occurred early (>4 Ga) in solar system history.     

Shock‐thermal history of the Agoudal (IIAB) iron meteorite from microstructural studies

The Agoudal IIAB iron meteorite exhibits only kamacite grains (~6 mm across) without any taenite. The kamacite is homogeneously enriched with numerous rhabdite inclusions of different size, shape, and composition. In some kamacite domains, this appears frosty due to micron‐scale rhabdite inclusions (~5 to 100 μm) of moderate to high Ni content (~26 to 40 wt%). In addition, all the kamacite grains in matrix are marked with a prominent linear crack formed during an atmospheric break‐up event and subsequently oxidized. This feature, also defined by trails of lowest Ni‐bearing (mean Ni: 23 wt%) mm‐scale rhabdite plates (fractured and oxidized) could be a trace of a pre‐existing γ–α interface. Agoudal experienced a very slow rate of primary cooling ~4 °C Ma^?1 estimated from the binary plots of true rhabdite width against corresponding Ni wt% and the computed cooling rate curves after Randich and Goldstein (1978). Chemically, Agoudal iron (Ga: 54 ppm; Ge: 140 ppm; Ir: 0.03 ppm) resembles the Ainsworth iron, the coarsest octahedrite of the IIAB group. Agoudal contains multiple sets of Neumann bands that are formed in space and time at different scales and densities due to multiple impacts with shock magnitude up to 130 kb. Signatures of recrystallization due to postshock low temperature mild reheating at about 400 °C are also locally present.     

New findings for the equilibrated enstatite chondrite Grein 002

Abstract— We report new petrographic and chemical data for the equilibrated EL chondrite Grein 002, including the occurrence of osbornite, metallic copper, abundant taenite, and abundant diopside. As inferred from low Si concentrations in kamacite, the presence of ferroan alabandite, textural deformation, chemical equilibration of mafic silicates, and a subsolar noble gas component, we concur with Grein 002's previous classification as an EL4‐5 chondrite. Furthermore, the existence of pockets consisting of relatively coarse, euhedral enstatite crystals protruding large patches of Fe‐Ni alloys suggests to us that this EL4‐5 chondrite has been locally melted. We suspect impact induced shock to have triggered the formation of the melt pockets. Mineralogical evidence indicates that the localized melting of metal and adjacent enstatite must have happened relatively late in the meteorite's history. The deformation of chondrules, equilibration of mafic silicates, and generation of normal zoning in Fe, Zn‐sulfides took place during thermal alteration before the melting event. Following parent body metamorphism, daubreelite was exsolved from troilite in response to a period of slow cooling at subsolidus temperatures. Exsolution of schreibersite from the coarse metal patches probably occurred during a similar period of slow cooling subsequent to the event that induced the formation of the melt pockets. Overall shock features other than localized melting correspond to stage S2 and were likely established by the final impact that excavated the Grein 002 meteoroid.     

A comparative study of opaque phases in Qingzhen (EH3) and MacAlpine Hills 88136 (EL3): Representatives of EH and EL parent bodies

Abstract— Opaque minerals in the Qingzhen (EH3) and MacAlpine Hills (MAC) 88136 (EL3) enstatite chondrites were studied and compared with other EH and EL chondrites. All opaque minerals usually occur in multi‐sulfide‐metal clasts and nodules in the matrix between chondrules (El Goresy et al., 1988). The higher abundance of opaque minerals, the occurrence of niningerite and various alkali‐sulfides (e.g., caswellsilverite, phases A and B, djerfisherite) are diagnostic criteria for EH chondrites, while alabandite is characteristic for EL chondrites. In addition, EH chondrites are characterized by enrichments of Si in both kamacite and perryite, and alkali elements in sphalerite and chalcopyrite. The Mn contents of daubreelite and sphalerite are lower in EH than in EL chondrites. These are consistent with lower oxygen fugacity and higher H₂S fugacity of EH than EL chondrites. In contrast, the discovery of sphalerite and Zn‐rich daubreelite in MAC 88136 indicates that their absence in EL6 chondrites is probably related to thermal metamorphism in the parent body. Schreibersite microspherules are commonly enclosed in most sulfides in Qingzhen, but are absent in MAC 88136. They were once molten, and probably predated all sulfide host phases. The petrographic setting and chemical compositions of the sulfide hosts of the schreibersite microspherules in EH3 chondrites are consistent with formation by condensation. The earliest sulfide condensates oldhamite and niningerite occupy the interiors of the clasts and nodules, whereas the rims consist of troilite and djerfisherite. In addition, in Qingzhen, some other troilite, djerfisherite and sphalerite assemblages coexist with perryite. They were produced by sulfurization of metallic Fe‐Ni in the nebula. In MAC 88136, sulfurization of Si‐bearing Fe‐Ni metal is less pronounced, and it produced troilite, schreibersite and less abundant perryite. Two kinds of normal zoning and a reverse zoning trends of niningerite, and both normal and reverse zoning of sphalerite were found in clasts and nodules in Qingzhen. The coexistence of normal and reverse zoning profiles in niningerite grains in the same meteorite strongly suggests that they formed before accretion in the parent body, because an asteroidal metamorphic or an impact event in the parent body would have erased these contrasting profiles and destroyed the textural settings. In contrast, alabandite in MAC 88136 shows only normal zoning, with the FeS content decreasing to 9.3 mol% toward troilite, indicating very slow cooling at low temperature.     

Secondary‐volatiles linked metallic iron in eucrites: The dual‐origin metals of Camel Donga

The unique occurrence of abundant (~1 vol%) near‐pure‐Fe metal in the Camel Donga eucrite is more complicated than previously believed. In addition to that component of groundmass metal, scattered within the meteorite are discrete nodules of much higher kamacite abundance. We have studied the petrology and composition of two of these nodules in the form of samples we call CD2 and CD3. The nodules are ovoids 11 (CD2) to 15 (CD3) mm across, with metal, or inferred preweathering metal, abundances of 12–17 vol% (CD2 is unfortunately quite weathered). The CD3 nodule also includes at its center a 5 mm ovoid clumping (6 vol%) of F‐apatite. Both nodules are fine‐grained, so the high Fe metal and apatite contents are clearly not flukes of inadequate sampling. The metals within the nodules are distinctly Ni‐rich (0.3–0.6 wt%) compared to the pure‐Fe (Ni generally 0.01 wt%) groundmass metals. Bulk analyses of three pieces of the CD2 nodule show that trace siderophile elements Ir, Os, and Co are commensurately enriched; Au is enriched to a lesser degree. The siderophile evidence shows the nodules did not form by in situ reduction of pyroxene FeO. Moreover, the nodules do not show features such as silica‐phase enrichment or pyroxene with reduced FeO (as constrained by FeO/MgO and especially FeO/MnO) predicted by the in situ reduction model. The oxide minerals, even in groundmass samples well away from the nodules, also show little evidence of reduction. Although the nodule boundaries are generally sharp, groundmass‐metal Ni content is anti‐correlated with distance from the CD3 nodule. We infer that the nodules represent materials that originated within impactors into the Camel Donga portion of the eucrite crust, but probably were profoundly altered during later metamorphism/metasomatism. Origin of the pure‐Fe groundmass metal remains enigmatic. In situ reduction probably played an important role, and association in the same meteorite of the Fe‐nodules is probably significant. But the fluid during alteration was probably not (as previously modeled) purely S and O, of simple heat‐driven internal derivation. We conjecture a two‐stage metasomatism, as fluids passed through Camel Donga after impact heating of volatile‐rich chondritic masses (survivors of gentle accretionary impacts) within the nearby crust. First, reduction to form troilite may have been triggered by fluids rich in S₂ and CO (derived from the protonodules?), and then in a distinct later stage, fluids were (comparatively) H₂O‐rich, and thus reacted with troilite to form pure‐Fe metal along with H₂S and SO₂. The early eucrite crust was in places a dynamic fluid‐bearing environment that hosted complex chemical processes, including some that engendered significant diversity among metal+sulfide alterations.     

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

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

Origin of kamacite,schreibersite, and perryite in metal‐sulfide nodules of the enstatite chondrite Sahara 97072 (EH3)

Abstract– Perryite [(Fe,Ni)_x(Si,P)_y], schreibersite [(Fe,Ni)₃P], and kamacite (αFeNi) are constituent minerals of the metal‐sulfide nodules in the Sahara 97072 (EH3) enstatite chondrite meteorite. We have measured concentrations of Ni, Cu, Ga, Au, Ir, Ru, and Pd in these minerals with laser ablation, inductively coupled plasma mass spectrometry (ICP‐MS). We also measured their Fe, Ni, P, Si, and Co concentrations with electron microprobe. In kamacite, ratios of Ru/Ir, Pd/Ir, and Pd/Ru cluster around their respective CI values and all elements analyzed plot near the intersection of the equilibrium condensation trajectory versus Ni and the respective CI ratios. In schreibersite, the Pd/Ru ratio is near the CI value and perryite contains significant Cu, Ga, and Pd. We propose that schreibersite and perryite formed separately near the condensation temperatures of P and Si in a reduced gas and were incorporated into Fe‐Ni alloy. Upon further cooling, sulfidation of Fe in kamacite resulted in the formation of additional perryite at the sulfide interface. Still later, transient heating re‐melted this perryite near the Fe‐FeS eutectic temperature during partial melting of the metal‐sulfide nodules. The metal‐sulfide nodules are pre‐accretionary objects that retain CI ratios of most siderophile elements, although they have experienced transient heating events.     

The Gao‐Guenie impact melt breccia—Sampling a rapidly cooled impact melt dike on an H chondrite asteroid?

The Gao‐Guenie H5 chondrite that fell on Burkina Faso (March 1960) has portions that were impact‐melted on an H chondrite asteroid at ~300 Ma and, through later impact events in space, sent into an Earth‐crossing orbit. This article presents a petrographic and electron microprobe analysis of a representative sample of the Gao‐Guenie impact melt breccia consisting of a chondritic clast domain, quenched melt in contact with chondritic clasts, and an igneous‐textured impact melt domain. Olivine is predominantly Fo_80–82. The clast domain contains low‐Ca pyroxene. Impact melt‐grown pyroxene is commonly zoned from low‐Ca pyroxene in cores to pigeonite and augite in rims. Metal–troilite orbs in the impact melt domain measure up to ~2 mm across. The cores of metal orbs in the impact melt domain contain ~7.9 wt% of Ni and are typically surrounded by taenite and Ni‐rich troilite. The metallography of metal–troilite droplets suggest a stage I cooling rate of order 10 °C s^?1 for the superheated impact melt. The subsolidus stage II cooling rate for the impact melt breccia could not be determined directly, but was presumably fast. An analogy between the Ni rim gradients in metal of the Gao‐Guenie impact melt breccia and the impact‐melted H6 chondrite Orvinio suggests similar cooling rates, probably on the order of ~5000–40,000 °C yr^?1. A simple model of conductive heat transfer shows that the Gao‐Guenie impact melt breccia may have formed in a melt injection dike ~0.5–5 m in width, generated during a sizeable impact event on the H chondrite parent asteroid.     

Shock experiments with the H6 chondrite Kernouvé: Pressure calibration of microscopic shock effects

Abstract— Shock‐recovery experiments were carried out on samples of the H6 chondrite Kernouvé at shock pressures of 10, 15, 20, 25, 30, 35, 45, and 60 GPa and preheating temperatures of 293 K (low‐temperature experiments) and 920 K (high‐temperature experiments). Using a calculated equation of state of Kernouvé, pressure‐pulse durations of 0.3 to 1.2 μs were estimated. The shocked samples were investigated by optical microscopy to calibrate the various shock effects in olivine, orthopyroxene, oligoclase, and troilite. The following pressure calibration is proposed for silicates: (1) undulatory extinction of olivine <GPa; (2) weak mosaicism of olivine from 10–15 GPa to 20–25 GPa; (3) onset of strong mosaicism of olivine at 20–25 GPa; (4) transformation of oligoclase to diaplectic glass completed at 25–30 GPa (low‐temperature experiments) and at 20–25 GPa (high‐temperature experiments); (5) onset of weak mosaicism in orthopyroxene at 30–35 GPa (low‐temperature experiments) and at 25–30 GPa (high‐temperature experiments); and (6) recrystallization or melting of olivine starting at 45–60 GPa (low‐temperature experiments) and at 35–45 GPa (high‐temperature experiments), and completed above 45–60 GPa in the high‐temperature experiments. Troilite displays distinct differences between the samples shocked at low and high temperatures. In the low‐temperature experiments, the following effects can be observed in troilite: (1) undulatory extinction up to 25 GPa, (2) twinning up to 45 GPa, (3) partial recrystallization from 30 to 60 GPa, and (4) complete recrystallization >35 GPa; whereas in the high‐temperature experiments, troilite shows (1) complete recrystallization from 10 up to 45 GPa and (2) melting and crystallization above 45 GPa. Localized shock‐induced melting is observed in samples shocked to pressures >15 GPa in the high‐temperature experiments and >30 GPa for the low‐temperature experiments in the form of FeNi metal and troilite melt injections and intergrowths and as pockets and veins of whole‐rock melt. Obviously, the onset and abundance of shock‐induced localized melting strongly depends on the initial temperature of the sample.