STUDIES OF KAMACITE,PERRYITE AND SCHREIBERSITE IN E-CHONDRITES AND AUBRITES

The bulk composition of metal (kamacite plus perryite) was determined in eleven E-chondrites and eight aubrites. The data are compatible with the subdivision of the E-chondrites into two groups (Yavnel, 1963; Anders, 1964), St Mark's and St Sauveur belonging to type I (Easton, 1985). The Ni content of kamacite plus perryite in Kota Kota (5.49%) is within the range covered by the remaining E-chondrites. Normative perryite, (Fe, Ni)_x(Si,P)_y constitutes 2.1% of Kota Kota and 2.7% of South Oman. The Ni content in the bulk metal of Aubres, Bishopville, Norton County and Peña Blanca Spring is about half the average Ni content in the metal of E-chondrites or the remaining aubrites (Bustee, Khor Temiki, Mayo Belwa and Shallowater). High Ga/Ni and Ge/Ni ratios distinguish the metal in E-chondrites and aubrites from that in ordinary chondrites. In a large metal grain from Aubres perryite formed on reheating, whereas in one from Khor Temiki there is evidence of shock and displacement of fragmented schreibersite (rhabdite). Thirty-eight metal grains (< 1.5 mm diameter) from Khor Temiki have a wide compositional range like that in Mayo Belwa (Graham, 1978). In Shallowater the distribution of Ni in the metal is bimodal (5.2 and 11.6%) and there is evidence of rapid cooling. The composition of both bulk metal and individual grains in aubrites makes it unlikely that they represent residual metal trapped during magmatic differentiation and/or fractional crystallization of E6 material. Compositional differences between metal grains strongly indicate that the aubrites are polymict breccias.     

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.     

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.     

Geochemical zoning and magnetic mineralogy at Fe,Ni‐alloy–troilite interfaces of three iron meteorites from Morasko,Coahuila II,and Mundrabilla

We combined high‐resolution and space‐resolved elemental distribution with investigations of magnetic minerals across Fe,Ni‐alloy and troilite interfaces for two nonmagmatic (Morasko and Mundrabilla) IAB group iron meteorites and an octahedrite found in 1993 in Coahuila/Mexico (Coahuila II) preliminarily classified on Ir and Au content as IIAB group. The aim of this study was to elucidate the crystallization and thermal history using gradients of the siderophile elements Ni, Co, Ge, and Ga and the chalcophile elements Cr, Cu, and Se with a focus on magnetic minerals. The Morasko and Coahuila II meteorite show a several mm‐thick carbon‐ and phosphorous‐rich transition zone between Fe,Ni‐alloy and troilite, which is characterized by magnetic cohenite and nonmagnetic or magnetic schreibersite. At Morasko, these phases have a characteristic trace element composition with Mo enriched in cohenite. In both Morasko and Coahuila II, Ni is enriched in schreibersite. The minerals have crystallized from immiscible melts, either by fractional crystallization and C‐ and P‐enrichment in the melt, or by partial melting at temperatures slightly above the eutectic point. During crystallization of Mundrabilla, the field of immiscibility was not reached. Independent of meteorite group and cooling history, the magnetic mineralogy (daubreelite, cohenite and/or schreibersite, magnetite) is very similar to the troilite (and transition zone) for all three investigated iron meteorites. If these minerals can be separated from the metal, they might provide important information about the early solar system magnetic field. Magnetite is interpreted as a partial melting or a terrestrial weathering product of the Fe,Ni‐alloy under oxidizing conditions.     

Aqueous alteration of kamacite in CM chondrites

Abstract– The study of aqueous alteration of kamacite in CM chondrites provides insight on the conditions, products, and relative timing of aqueous alteration. We studied unaltered, partially altered, and fully altered kamacite grains from Murray, Murchison, Cold Bokkeveld, and Nogoya using optical microscopy, electron microprobe analysis, scanning electron microscopy, and Raman spectroscopy. From textual evidence and chemical analysis, we established three separate microchemical environments. 1) In a microchemical environment with a high S activity, kamacite alters to form tochilinite, P‐bearing sulfides, eskolaite, and schreibersite. Mass balance calculations show that 81% of the Fe in kamacite is removed from the alteration region, making it available for the formation of other minerals or Fe‐rich aureoles. The release of Fe can alter the mesostasis of type‐I chondrules forming cronstedtite. 2) In a microchemical environment with a high Si activity and a low S activity, kamacite alters to form cronstedtite with small accessory sulfide inclusions. 3) A microchemical environment with limited S and Si activity results in kamacite alteration forming magnetite. The resulting magnetite retains associated Ni that can distinguish it from precipitated magnetite. In addition, the accessory phases of pentlandite and apatite can be formed if S or Ca are present. Finally, we note that small tochilinite grains in the matrix lack typical Ni, P, and Co levels, suggesting that they did not form from kamacite but possibly by sulfidization of magnetite.     

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.     

Opaque assemblages in CR2 Graves Nunataks (GRA) 06100 as indicators of shock‐driven hydrothermal alteration in the CR chondrite parent body

We have studied the petrologic characteristics of sulfide‐metal lodes, polymineralic Fe‐Ni nodules, and opaque assemblages in the CR2 chondrite Graves Nunataks (GRA) 06100, one of the most altered CR chondrites. Unlike low petrologic type CR chondrites, alteration of metal appears to have played a central role in the formation of secondary minerals in GRA 06100. Differences in the mineralogy and chemical compositions of materials in GRA 06100 suggest that it experienced higher temperatures than other CR2 chondrites. Mineralogic features indicative of high temperature include: (1) exsolution of Ni‐poor and Ni‐rich metal from nebular kamacite; (2) formation of sulfides, oxides, and phosphates; (3) changes in the Co/Ni ratios; and (4) carbidization of Fe‐Ni metal. The conspicuous absence of pentlandite may indicate that peak temperatures exceeded 600 °C. Opaques appear to have been affected by the action of aqueous fluids that resulted in the formation of abundant oxides, Fe‐rich carbonates, including endmember ankerite, and the sulfide‐silicate‐phosphate scorzalite. We suggest that these materials formed via impact‐driven metamorphism. Mineralogic features indicative of impact metamorphism include (1) the presence of sulfide‐metal lodes; (2) the abundance of polymineralic opaque assemblages with mosaic‐like textures; and (3) the presence of suessite. Initial shock metamorphism probably resulted in replacement of nebular Fe‐Ni metal in chondrules and in matrix by Ni‐rich, Co‐rich Fe metal, Al‐Ti‐Cr‐rich alloys, and Fe sulfides, while subsequent hydrothermal alteration produced accessory oxides, phosphates, and Fe carbonates. An extensive network of sulfide‐metal veins permitted effective exchange of siderophile elements from pre‐existing metal nodules with adjacent chondrules and matrix, resulting in unusually high Fe contents in these objects.     

Differentiation of metal‐rich meteoritic parent bodies: I. Measurements of PGEs,Re, Mo,W, and Au in meteoritic Fe‐Ni metal

Abstract— We describe an analytical technique for measurements of Fe, Ni, Co, Mo, Ru, Rh, W, Re, Os, Ir, Pt, and Au in bulk samples of iron meteorites. The technique involves EPMA (Fe, Ni, Co) and LA‐ICP‐MS analyses of individual phases of iron meteorites, followed by calculation of bulk compositions based on the abundances of these phases. We report, for the first time, a consistent set of concentrations of Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in the iron meteorites Arispe, Bennett County, Grant, Cape of Good Hope, Cape York, Carbo, Chinga, Coahuila, Duchesne, Gibeon, Henbury, Mundrabilla, Negrillos, Odessa, Sikhote‐Alin, and Toluca and the Divnoe primitive achondrite. The comparison of our LA‐ICP‐MS data for a number of iron meteorites with high‐precision isotope dilution and INAA data demonstrates the good precision and accuracy of our technique. The narrow ranges of variations of Mo and Pd concentrations within individual groups of iron meteorites suggest that these elements can provide important insights into the evolution of parent bodies of iron meteorites. Under certain assumptions, the Mo concentrations can be used to estimate mass fractions of the metal‐sulfide cores in the parent bodies of iron meteorites. It appears that a range of Pd variations within a group of iron meteorites can serve as a useful indicator of S content in the core of its parent body.     

Reevaluation of formation of metal nodules in ordinary chondrites

Abstract— Compositions of four metal nodules from two ordinary chondrites, WIS91627 (H3.7) and Juin (H5), were determined by instrumental and radiochemical neutron activation analyses. Compared with bulk metal fractions, the metal nodules are characterized by strong and variable depletion of the refractory siderophile elements Re, Os, Ir, Ru, Pt and Rh but normal W and Mo, some fractionation of Co from Ni, and low Cu concentrations. These characteristics are difficult to explain by shock-induced vaporization followed by fractional condensation, a mechanism suggested by Widom et al. (1986). We propose formation of metal nodules during metamorphism in the parent body. Refractory siderophile elements, such as Ir, Os, Rh, etc., are partly locked up in noble metal nuggets and cannot participate in kamacite formation. The occurrence of metal nodules in both equilibrated and unequilibrated ordinary chondrites suggests that diffusion along grain boundaries was important in the development of kamacite and taenite in ordinary chondrites.     

Trace Element Partitioning between Taenite and Kamacite; Relationship to the Cooling Rates of Iron Meteorites

Abstract— Instrumental neutron activation analysis (INAA) was used to determine Ni, Co, Cu, Ga, As, Au, W, Re and Ir in taenite lamellae isolated by acid dissolution from eight iron meteorites from groups IA, IIIAB and IVA. Taenite is enriched in Ni, Cu, Ga, As, Au, W, Re and Ir relative to kamacite, whereas taenite is depleted in Co. Taenite/kamacite partition ratios in slowly cooled IAB meteorites are farther from unity than those in rapidly cooled IVA meteorites. Taenite/kamacite partition ratios for Cu, Ir, Au and Co may be sensitive cooling rate indicators.     

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.     

Compositions of unzoned and zoned metal in the CBb chondrites Hammadah al Hamra 237 and Queen Alexandra Range 94627

Abstract— The CB_b chondrites are rare, primitive, metal‐rich meteorites that contain several features, including zoned metal, that have previously been interpreted as evidence for origins in the solar nebula. We have measured concentrations of Ni, Cu, Ga, Ru, Pd, Ir, and Au within both zoned and unzoned metal grains in the CB_b chondrites Hammadah al Hamra (HaH) 237 and Queen Alexandra Range (QUE) 94627 using laser ablation inductively coupled plasma mass spectrometry. The refractory elements Ni, Ru, and Ir are enriched in the grain cores, relative to the rims, in the zoned metal. All refractory elements are uniform across the unzoned metal grains, at concentrations that are highly variable between grains. The volatile elements Cu, Ga, and Au are usually depleted relative to chondritic abundances and are most often uniform within the grains but are sometimes slightly elevated at the outermost rim. The Pd abundances are nearly uniform, at close to chondritic abundances, in all of the metal grains. A condensation origin is inferred for both types of metal. The data support a model in which the zoned metal formed at high temperatures, in a relatively rapidly cooling nebular gas, and the unzoned metal formed at lower temperatures and at a lower cooling rate. The CB_b metal appears to have formed by a process very similar to that of the CH chondrites, but the CB_b meteorite components experienced even less thermal alteration following their formation and are among the most primitive materials known to have formed in the solar nebula.     

Minor and trace element concentrations in adjacent kamacite and taenite in the Krymka chondrite

We report in situ NanoSIMS siderophile minor and trace element abundances in individual Fe‐Ni metal grains in the unequilibrated chondrite Krymka (LL3.2). Associated kamacite and taenite of 10 metal grains in four chondrules and one matrix metal were analyzed for elemental concentrations of Fe, Ni, Co, Cu, Rh, Ir, and Pt. The results show large elemental variations among the metal grains. However, complementary and correlative variations exist between adjacent kamacite‐taenite. This is consistent with the unequilibrated character of the chondrite and corroborates an attainment of chemical equilibrium between the metal phases. The calculated equilibrium temperature is 446 ± 9 °C. This is concordant with the range given by Kimura et al. (2008) for the Krymka postaccretion thermal metamorphism. Based on Ni diffusivity in taenite, a slow cooling rate is estimated of the Krymka parent body that does not exceed ~1K Myr^?1, which is consistent with cooling rates inferred by other workers for unequilibrated ordinary chondrites. Elemental ionic radii might have played a role in controlling elemental partitioning between kamacite and taenite. The bulk compositions of the Krymka metal grains have nonsolar (mostly subsolar) element/Ni ratios suggesting the Fe‐Ni grains could have formed from distinct precursors of nonsolar compositions or had their compositions modified subsequent to chondrule formation events.     

Metal‐saturated sulfide assemblages in NWA 2737: Evidence for impact‐related sulfur devolatilization in Martian meteorites

NWA 2737, a Martian meteorite from the Chassignite subclass, contains minute amounts (0.010 ± 0.005 vol%) of metal‐saturated Fe‐Ni sulfides. These latter bear evidence of the strong shock effects documented by abundant Fe nanoparticles and planar defects in Northwest Africa (NWA) 2737 olivine. A Ni‐poor troilite (Fe/S = 1.0 ± 0.01), sometimes Cr‐bearing (up to 1 wt%), coexists with micrometer‐sized taenite/tetrataenite‐type native Ni‐Fe alloys (Ni/Fe = 1) and Fe‐Os‐Ir‐(Ru) alloys a few hundreds of nanometers across. The troilite has exsolved flame‐like pentlandite (Fe/Fe + Ni = 0.5–0.6). Chalcopyrite is almost lacking, and no pyrite has been found. As a hot desert find, NWA 2737 shows astonishingly fresh sulfides. The composition of troilite coexisting with Ni‐Fe alloys is completely at odds with Chassigny and Nahkla sulfides (pyrite + metal‐deficient monoclinic‐type pyrrhotite). It indicates strongly reducing crystallization conditions (close to IW), several log units below the fO₂ conditions inferred from chromites compositions and accepted for Chassignites (FMQ‐1 log unit). It is proposed that reduction in sulfides into base and precious metal alloys is operated via sulfur degassing, which is supported by the highly resorbed and denticulated shape of sulfide blebs and their spongy textures. Shock‐related S degassing may be responsible for considerable damages in magmatic sulfide structures and sulfide assemblages, with concomitant loss of magnetic properties as documented in some other Martian meteorites.     

Clues to the nature of the impacting bodies from platinum-group elements (rhenium and gold) in borehole samples from the Clearwater East crater (Canada) and the Boltysh impact crater (Ukraine)

Abstract— Seven large (10 g) impact melt rock samples from boreholes from the Boltysh impact crater (Ukraine) and six samples from the East Clearwater crater (Canada) were analyzed for Os, Ir, Ru, Rh, Pd, Re and Au by the nickel sulfide technique in combination with neutron activation. Earlier analyses of Clearwater East impact melt rocks have shown that they are strongly enriched in Ir, Os, Pd and Re. In this work, I confirm earlier findings and demonstrate similarly high enrichments of Rh and Ru. The average Os/Ir, Ru/Ir, Pd/Ir, Rh/Ir and Ru/Rh ratios of the melt rock samples from Clearwater East are CI-chondritic and yield an average Ir content of 25.2 ± 6.5 ng/g relative to an average upper crust concentration of 0.03 ± 0.02 ng/g Ir. The amount of meteoritic component corresponds to 4 to 7% of a nominal CI component for Clearwater East. The impact melt rock samples from a bore hole from Boltysh are low in Ir with an average of 0.2 ± 0.1 ng/g. The CI-normalized abundances increase from the refractory to the more volatile siderophile elements (Os < Ir < Ru < Rh ～ Pd ～ Au ～ Ni ～ Co). Because of the low Ir anomaly and uncertainties in making corrections (correlations are weak) for indigenous siderophile elements, no clear projectile assignment can be made.     

New insights into the mineralogy and weathering of the Meridiani Planum meteorite,Mars

Meridiani Planum is the first officially recognized meteorite find on the surface of Mars. It was discovered at and named after the landing site of the Mars Exploration Rover Opportunity. Based on its composition, it was classified as a IAB complex iron meteorite. Mössbauer spectra obtained by Opportunity are dominated by kamacite (α‐Fe‐Ni) and exhibit a small contribution of ferric oxide. Several small features in the spectra have been neglected to date. To shed more light on these features, five iron meteorite specimens were investigated as analogs to Meridiani Planum with a laboratory Mössbauer setup. Measurements were performed on (1) their metallic bulk, (2) troilite (FeS) inclusions, (3) cohenite ((Fe,Ni,Co)₃C) and schreibersite ((Fe,Ni)₃P), and (4) corroded rims. In addition to these room‐temperature measurements, a specimen from the Mundrabilla IAB‐ungrouped meteorite was measured at Mars‐equivalent temperatures. Based on these measurements, the features in Meridiani Planum spectra can be explained with the presence of small amounts of schreibersite and/or cohenite and iron oxides. The iron oxides can be attributed to a previously reported coating on Meridiani Planum. Their presence indicates weathering through the interaction of the meteorite with small amounts of water.     

Thermal and shock history of Almahata Sitta meteorite inferred from structure refinement of pyroxene and Mössbauer spectroscopy of Fe-Ni metal

The crystal structures of orthopyroxene (En_86.3Fs_8.6Wo_5.1, space group Pbca) and pigeonite (En_81.7Fs_8.8Wo_9.5, space group P2₁/c) from the Almahata Sitta ureilite (fragment#051) have been refined to R1 indices of 3.10% and 2.53%, respectively, using single-crystal X-ray diffraction data. The unit formulas were calculated from electron microprobe analysis, and the occupancies at the M1 and M2 sites were refined for both pyroxenes from the single-crystal diffraction data. The results indicate a rather disordered intracrystalline Fe²⁺-Mg cation distribution over the M1 and M2 sites, with a closure temperature of 726(±55)°C for orthopyroxene and 704(±110)°C for pigeonite, suggesting fast cooling of these pyroxenes. The Mössbauer spectrum of the Fe-Ni metal particles of Almahata Sitta ureilite (fragment#051) is dominated by two overlapping magnetic sextets that are assigned to Fe atoms in Si-bearing kamacite, and arise from two different nearest-neighbor configurations of Fe* (=Fe+Ni) and Si atoms in the bcc structure of kamacite; (8F*, 0Si) and (7Fe*, 1Si). In addition, the spectrum shows weak absorption peaks that are attributed to the presence of small amounts of cohenite [(Fe,Ni)₃C], schreibersite [(Fe,Ni)₃P], and an Fe-oxide/hydroxide phase. The fast cooling of pyroxene to the closure temperature (after equilibration at ~1200°C) and the incorporation of Si in kamacite can be interpreted as due to a shock event that took place on the meteorite parent body, consistent with the proposed formation history of ureilites parent body where a fast cooling has occurred at a later stage of its formation.     

Clastic matrix in EH3 chondrites

Abstract— Patches of clastic matrix (15 to 730 μm in size) constitute 4.9 vol% of EH3 Yamato (Y‐) 691 and 11.7 vol% of EH3 Allan Hills (ALH) 81189. Individual patches in Y‐691 consist of 1) ?25 vol% relatively coarse opaque grain fragments and polycrystalline assemblages of kamacite, schreibersite, perryite, troilite (some grains with daubréelite exsolution lamellae), niningerite, oldhamite, and caswellsilverite; 2) ?30 vol% relatively coarse silicate grains including enstatite, albitic plagioclase, silica and diopside; and 3) an inferred fine nebular component (?45 vol%) comprised of submicrometer‐size grains. Clastic matrix patches in ALH 81189 contain relatively coarse grains of opaques (?20 vol%; kamacite, schreibersite, perryite and troilite) and silicates (?30 vol%; enstatite, silica and forsterite) as well as an inferred fine nebular component (?50 vol%). The O‐isotopic composition of clastic matrix in Y‐691 is indistinguishable from that of olivine and pyroxene grains in adjacent chondrules; both sets of objects lie on the terrestrial mass‐fractionation line on the standard three‐isotope graph. Some patches of fine‐grained matrix in Y‐691 have distinguishable bulk concentrations of Na and K, inferred to be inherited from the solar nebula. Some patches in ALH 81189 differ in their bulk concentrations of Ca, Cr, Mn, and Ni. The average compositions of matrix material in Y‐691 and ALH 81189 are similar but not identical‐matrix in ALH 81189 is much richer in Mn (0.23 ± 0.05 versus 0.07 ± 0.02 wt%) and appreciably richer in Ni (0.36 ± 0.10 versus 0.18 ± 0.05 wt%) than matrix in Y‐691. Each of the two whole‐rocks exhibits a petrofabric, probably produced by shock processes on their parent asteroid.     

Ordinary chondrite metallography: Part 1. Fe‐Ni taenite cooling experiments

Abstract— Cooling rate experiments were performed on P‐free Fe‐Ni alloys that are compositionally similar to ordinary chondrite metal to study the taenite ? taenite + kamacite reaction. The role of taenite grain boundaries and the effect of adding Co and S to Fe‐Ni alloys were investigated. In P‐free alloys, kamacite nucleates at taenite/taenite grain boundaries, taenite triple junctions, and taenite grain corners. Grain boundary diffusion enables growth of kamacite grain boundary precipitates into one of the parent taenite grains. Likely, grain boundary nucleation and grain boundary diffusion are the applicable mechanisms for the development of the microstructure of much of the metal in ordinary chondrites. No intragranular (matrix) kamacite precipitates are observed in P‐free Fe‐Ni alloys. The absence of intragranular kamacite indicates that P‐free, monocrystalline taenite particles will transform to martensite upon cooling. This transformation process could explain the metallography of zoneless plessite particles observed in H and L chondrites. In P‐bearing Fe‐Ni alloys and iron meteorites, kamacite precipitates can nucleate both on taenite grain boundaries and intragranularly as Widmanstätten kamacite plates. Therefore, P‐free chondritic metal and P‐bearing iron meteorite/pallasite metal are controlled by different chemical systems and different types of taenite transformation processes.     

Fe‐Ni metal and sulfide minerals in CM chondrites: An indicator for thermal history

Abstract– CM chondrites were subjected to aqueous alteration and, in some cases, to secondary metamorphic heating. The effects of these processes vary widely, and have mainly been documented in silicate phases. Herein, we report the characteristic features of Fe‐Ni metal and sulfide phases in 13 CM and 2 CM‐related chondrites to explore the thermal history of these chondrites. The texture and compositional distribution of the metal in CM are different from those in unequilibrated ordinary and CO chondrites, but most have similarities to those in highly primitive chondrites, such as CH, CR, and Acfer 094. We classified the CM samples into three categories based on metal composition and sulfide texture. Fe‐Ni metal in category A is kamacite to martensite. Category B is characterized by pyrrhotite grains always containing blebs or lamellae of pentlandite. Opaque mineral assemblages of category C are typically kamacite, Ni‐Co‐rich metal, and pyrrhotite. These categories are closely related to the degree of secondary heating and are not related to degree of the aqueous alteration. The characteristic features of the opaque minerals can be explained by secondary heating processes after aqueous alteration. Category A CM chondrites are unheated, whereas those in category B experienced small degrees of secondary heating. CMs in category C were subjected to the most severe secondary heating process. Thus, opaque minerals can provide constraints on the thermal history for CM chondrites.