Mineralogical and Geochemical Study of Zhaoping,Xifu and Hami Iron Meteorites

The mineralogy and bulk chemical compositions of three iron meteorites (Zhaoping, Xifu and Hami) recently found in China are reported here and are classified on the basis of their bulk chemical compositions. Zhaoping contains 93.4 mg/g Ni, 85.9 μg/g Ga, 418 μg/g Ge, 5.24 mg/g Co, 1.94 μg/g Ir, 0.774 μg/g W, and 1.62 μg/g Au and belongs to the low-Ni, low-Au subgroup of IAB. It is a coarse octahedrite and consists of kamacite, taenite, troilite, schreibersite and cohenite. The cohenite has entirely decomposed to graphite and low-Ni kamacite in our samples. Zhaoping contains some inclusions of Mn-free sarcopside which were rarely reported in IAB iron meteorites. Xifu has 74.1 mg/g Ni, 58.8gμg/g Ga, 150 μg/g Ge, and 0.913 μg/g W. Xifu is a member of group IIICD iron meteorite. Like most of IIICD irons, Xifu is a coarsest octahedrite with kamacite bandwidth larger than 3mm, and contains kamacite, taenite and schreibersite. Carbides and graphite are not found in the sample because of its being heterogeneous. Hami has 106 mg/g Ni, 5.36 mg/g Co and 0.922 μg/g Ir. We did not obtain the Ga and Ge contents in Hami because of their low concentrations and the limited precision of the INAA technique. Hami is an unclassified iron meteorite on the basis of the contents of other trace elements, structure and mineralogy. On mineralogy and structure, Hami resembles Rafruti, another unclassified iron meteorite.     

Meteoritic iron from the Libyan glass area,southwestern Egypt

Abstract— A fragment of a weathered iron meteorite was collected from the Libyan Desert glass area of southwestern Egypt in 1991 May. The specimen is of irregular shape, measuring 5 × 3.5 × 2 cm and weighing 110 g. It is covered by a shiny black layer of magnetite ～1 mm thick. The interior is brownish-black in color owing to terrestrial oxidation. An unetched polished surface shows some areas still having remnants of kamacite and taenite. Remnants of the lamellar octahedral structure have been detected. The kamacite bandwidth ranges from 0.2 mm to 0.8 mm. Chemical analysis shows that the meteorite contains 3.4% Ni, 0.24% Co, 85 ppm Cu, 4 ppm As, 132 ppm Au and 2530 ppm Ir. The meteorite was discovered after the discovery of two different chondritic meteorites in the same area; so, it will be named Great Sand Sea 003.     

THE COMPOSITION OF IRON METEORITES: A STUDY BY FACTOR ANALYSIS

A factor analysis has been performed on nickel and trace element data for iron meteorites. The technique shows that the present distribution of these elements is the result of three processes. These can be identified from the elements involved:

• 1 Ga, Ge, Sb and Zn (condensation and accretion).
• 2 Ni, Pd, Co and Cu (oxidation and sulphuration).
• 3 Ir, Au, As, Re, Pt, Os, Ru and Cr (an igneous event).

The distribution of Mo, however, is not readily explicable in terms of these processes. Within the groups IAB and IIAB only one process is required for all elements, but in groups IIIAB and IVA the situation for Ga, Ge and Sb is more complex.     

Ion probe measurements of carbon and nitrogen in iron meteorites

Abstract— Carbon and nitrogen distributions in iron meteorites, their concentrations in various phases, and their isotopic compositions in certain phases were measured by secondary ion mass spectrometry (SIMS). Taenite (and its decomposition products) is the main carrier of C, except for IAB iron meteorites, where graphite and/or carbide (cohenite) may be the main carrier. Taenite is also the main carrier of N in most iron meteorites unless nitrides (carlsbergite CrN or roaldite (Fe, Ni)₄N) are present. Carbon and N distributions in taenite are well correlated unless carbides and/or nitrides are exsolved. There seem to be three types of C and N distributions within taenite. (1) These elements are enriched at the center of taenite (convex type). (2) They are enriched at the edge of taenite (concave type). (3) They are enriched near but some distance away from the edge of taenite (complex type). The first case (1) is explained as equilibrium distribution of C and N in Fe-Ni alloy with M-shape Ni concentration profile. The second case (2) seems to be best explained as diffusion controlled C and N distributions. In the third case (3), the interior of taenite has been transformed to the α phase (kamacite or martensite). Carbon and N were expelled from the α phase and enriched near the inner border of the remaining γ phase. Such differences in the C and N distributions in taenite may reflect different cooling rates of iron meteorites. Nitrogen concentrations in taenite are quite high approaching 1 wt% in some iron meteorites. Nitride (carlsbergite and roaldite) is present in meteorites with high N concentrations in taenite, which suggests that the nitride was formed due to supersaturation of the metallic phases with N. The same tendency is generally observed for C (i.e., high C concentrations in taenite correlate with the presence of carbide and/or graphite). Concentrations of C and N in kamacite are generally below detection limits. Isotopic compositions of C and N in taenite can be measured with a precision of several permil. Isotopic analysis in kamacite in most iron meteorites is not possible because of the low concentrations. The C isotopic compositions seem to be somewhat fractionated among various phases, reflecting closure of C transport at low temperatures. A remarkable isotopic anomaly was observed for the Mundrabilla (IIICD anomalous) meteorite. Nitrogen isotopic compositions of taenite measured by SIMS agree very well with those of the bulk samples measured by conventional mass spectrometry.     

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.     

Thermal and impact histories of reheated group IVA,IVB, and ungrouped iron meteorites and their parent asteroids

Abstract– The microstructures of six reheated iron meteorites—two IVA irons, Maria Elena (1935), Fuzzy Creek; one IVB iron, Ternera; and three ungrouped irons, Hammond, Babb’s Mill (Blake’s Iron), and Babb’s Mill (Troost’s Iron)—were characterized using scanning and transmission electron microscopy, electron‐probe microanalysis, and electron backscatter diffraction techniques to determine their thermal and shock history and that of their parent asteroids. Maria Elena and Hammond were heated below approximately 700–750 °C, so that kamacite was recrystallized and taenite was exsolved in kamacite and was spheroidized in plessite. Both meteorites retained a record of the original Widmanstätten pattern. The other four, which show no trace of their original microstructure, were heated above 600–700 °C and recrystallized to form 10–20 μm wide homogeneous taenite grains. On cooling, kamacite formed on taenite grain boundaries with their close‐packed planes aligned. Formation of homogeneous 20 μm wide taenite grains with diverse orientations would have required as long as approximately 800 yr at 600 °C or approximately 1 h at 1300 °C. All six irons contain approximately 5–10 μm wide taenite grains with internal microprecipitates of kamacite and nanometer‐scale M‐shaped Ni profiles that reach approximately 40% Ni indicating cooling over 100–10,000 yr. Un‐decomposed high‐Ni martensite (α₂) in taenite—the first occurrence in irons—appears to be a characteristic of strongly reheated irons. From our studies and published work, we identified four progressive stages of shock and reheating in IVA irons using these criteria: cloudy taenite, M‐shaped Ni profiles in taenite, Neumann twin lamellae, martensite, shock‐hatched kamacite, recrystallization, microprecipitates of taenite, and shock‐melted troilite. Maria Elena and Fuzzy Creek represent stages 3 and 4, respectively. Although not all reheated irons contain evidence for shock, it was probably the main cause of reheating. Cooling over years rather than hours precludes shock during the impacts that exposed the irons to cosmic rays. If the reheated irons that we studied are representative, the IVA irons may have been shocked soon after they cooled below 200 °C at 4.5 Gyr in an impact that created a rubblepile asteroid with fragments from diverse depths. The primary cooling rates of the IVA irons and the proposed early history are remarkably consistent with the Pb‐Pb ages of troilite inclusions in two IVA irons including the oldest known differentiated meteorite ( Blichert‐Toft et al. 2010 ).     

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.     

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.     

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.     

Mineral and chemical composition of the new iron meteorite Javorje from Slovenia

Abstract– The single‐piece iron meteorite Javorje, with a mass of 4920 g, is the heaviest and largest meteorite found in the territory of Slovenia. The meteorite Javorje is a medium octahedrite with kamacite bandwidth of 0.85 ± 0.26 mm. The bulk composition of Ni (7.83 wt%), Co (0.48 wt%) and trace elements Ga (25 μg/g), Ge (47 μg/g), Ir (7.6 μg/g), As (5.8 μg/g), Au (0.47 μg/g), and Pt (13.4 μg/g) indicates that the meteorite Javorje belongs to the chemical group IIIAB. Mineral and bulk chemical compositions are consistent with other reported group IIIAB meteorites. The presence of numerous rhabdites, carlsbergite, sparse troilite, and chromite and abundance of daubréelites are in accordance with low‐Ni and low‐P IIIAB iron meteorites. The severely weathered surface and secondary weathering products in the interior of the meteorite suggest its high terrestrial age.     

COMPOSITION AND ORIGIN OF THE UNUSUAL OKTIBBEHA COUNTY IRON METEORITE

We have analyzed Oktibbeha County, the most Ni-rich iron meteorite, for Ni, Co, Cu, Ga, Ge, As, Sb, Ir, and Au. Cu and Sb are higher than in any other iron, but other trace elements are within the ranges typically found in iron meteorites. Extrapolation of trace element trends in group IAB indicates that Oktibbeha County is a member of this group. This sheds light on the origin of groups IAB and IIICD, which are thought to be derived from impact melts on parent bodies of chondritic composition. Lafayette (iron), another sample reported in the literature to have a similarly high Ni content, is probably a pseudometeorite.     

An experimental test of Henry's Law in solid metal‐liquid metal systems with implications for iron meteorites

Abstract— Experimental solid metal‐liquid metal partition coefficients have been used to model the crystallization of magmatic iron meteorites and understand the evolution of asteroid cores. However, the majority of the partitioning experiments have been conducted with trace elements doped at levels that are orders of magnitude higher than measured in iron meteorites. Concern about Henry's Law and the unnatural doping levels have been cited as one reason that two recent iron meteorite studies have dismissed the experimental partition coefficients in their modeling. Using laser ablation ICP‐MS analysis, this study reports experimentally determined solid metal‐liquid metal trace element partition coefficients from runs doped down to the levels occurring in iron meteorites. The analyses for 12 trace elements (As, Co, Cr, Cu, Ga, Ge, Ir, Os, Pd, Pt, Re, and W) show no deviations from Henry's Law, and these results support decades of experimental work in which the partition coefficients were assumed to be independent of trace element concentration. Further, since our experiments are doped with natural levels of trace elements, the partitioning results are directly applicable to iron meteorites and should be used when modeling their crystallization. In contrast, our new Ag data are inconsistent with previous studies, suggesting the high Ag‐content in previous studies may have influenced the measured Ag partitioning behavior.     

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.     

Atom‐probe tomography and transmission electron microscopy of the kamacite–taenite interface in the fast‐cooled Bristol IVA iron meteorite

We report the first combined atom‐probe tomography (APT) and transmission electron microscopy (TEM) study of a kamacite–tetrataenite (K–T) interface region within an iron meteorite, Bristol (IVA). Ten APT nanotips were prepared from the K–T interface with focused ion beam scanning electron microscopy (FIB‐SEM) and then studied using TEM followed by APT. Near the K‐T interface, we found 3.8 ± 0.5 wt% Ni in kamacite and 53.4 ± 0.5 wt% Ni in tetrataenite. High‐Ni precipitate regions of the cloudy zone (CZ) have 50.4 ± 0.8 wt% Ni. A region near the CZ and martensite interface has <10 nm sized Ni‐rich precipitates with 38.4 ± 0.7 wt% Ni present within a low‐Ni matrix having 25.5 ± 0.6 wt% Ni. We found that Cu is predominantly concentrated in tetrataenite, whereas Co, P, and Cr are concentrated in kamacite. Phosphorus is preferentially concentrated along the K‐T interface. This study is the first precise measurement of the phase composition at high spatial resolution and in 3‐D of the K‐T interface region in a IVA iron meteorite and furthers our knowledge of the phase composition changes in a fast‐cooled iron meteorite below 400 °C. We demonstrate that APT in conjunction with TEM is a useful approach to study the major, minor, and trace elemental composition of nanoscale features within fast‐cooled iron meteorites.     

An Iron Meteorite from Akyumak

Abstract— The microstructure of an iron meteorite which fell near Akyumak, East Anatolia, Turkey on 2 August 1981 has been examined and its composition determined. The Ni content is 7.7% and kamacite bandwidth is 0.32 ± 0.06 mm. The kamacite contains Neumann bands and some daubreelite inclusions. Taenite and plessite account for about 45 to 50% of the metal; finger, cellular and net plessite are observed. Gallium (1.9 ppm), Ge (< 40 ppm) and Ir (1.81 ppm) were determined by neutron activation. The microstructural observations and chemical data show Akyumak to be a fine octahedrite and a member of group IVA.     

Metallographic cooling rates of group IIF iron meteorites

Abstract— Metallographic cooling rates have been calculated for all five members of the iron meteorites group IIF using two different techniques. We have determined cooling rates of ～5 °C/Ma based on Ni profiles through the taenite rim enclosing kamacite spindles. Ni profiles through the kamacite phase are less precise cooling rate indicators, but suggest a cooling rate of ～1 °C/Ma within an order of magnitude at lower temperatures (360–400 °C). Based on the kamacite bandwidth and the Ni profiles through the taenite, we estimate that the kamacite nucleated 130–200 °C below the temperature predicted from the phase diagram. The size of and the distance between the large kamacite spindles is found to be consistent with the thermal history that we have determined on the basis of Ni profiles in kamacite and taenite. We find that previously published kamacite bandwidth cooling rates for the five group IIF members are most likely in error because of the presence of large schreibersite spindles in some kamacite spindles and because undercooling of kamacite was ignored. Contrary to previous workers we find that the metallographic cooling rates are consistent with cooling in a common core.     

Mass‐dependent fractionation of nickel isotopes in meteoritic metal

Abstract— We measured nickel isotopes via multicollector inductively coupled plasma mass spectrometry (MC‐ICPMS) in the bulk metal from 36 meteorites, including chondrites, pallasites, and irons (magmatic and non‐magmatic). The Ni isotopes in these meteorites are mass fractionated; the fractionation spans an overall range of ～0.4‰ amu^?1. The ranges of Ni isotopic compositions (relative to the SRM 986 Ni isotopic standard) in metal from iron meteorites (～0.0 to ～0.3‰ amu^?1) and chondrites (～0.0 to ～0.2‰ amu^?1) are similar, whereas the range in pallasite metal (～–0.1 to 0.0‰ amu^?1) appears distinct. The fractionation of Ni isotopes within a suite of fourteen IIIAB irons (～0.0 to ～0.3‰ amu^?1) spans the entire range measured in all magmatic irons. However, the degree of Ni isotopic fractionation in these samples does not correlate with their Ni content, suggesting that core crystallization did not fractionate Ni isotopes in a systematic way. We also measured the Ni and Fe isotopes in adjacent kamacite and taenite from the Toluca IAB iron meteorite. Nickel isotopes show clearly resolvable fractionation between these two phases; kamacite is heavier relative to taenite by ～0.4‰ amu^?1. In contrast, the Fe isotopes do not show a resolvable fractionation between kamacite and taenite. The observed isotopic compositions of kamacite and taenite can be understood in terms of kinetic fractionation due to diffusion of Ni during cooling of the Fe‐Ni alloy and the development of the Widmanstätten pattern.     

Effect of silicon on trace element partitioning in iron‐bearing metallic melts

Abstract– Despite the fact that Si is considered a potentially important metalloid in planetary systems, little is known about the effect of Si in metallic melts on trace element partitioning behavior. Previous studies have established the effects of S, C, and P, nonmetals, through solid metal/liquid metal experiments in the corresponding Fe binary systems, but the Fe‐Si system is not appropriate for similar experiments because of the high solubility of Si in solid metal. In this work, we present the results from 0.1 MPa experiments with two coexisting immiscible metallic liquids in the Fe‐S‐Si system. By leveraging the extensive available knowledge about the effect of S on trace element partitioning behavior, we explore the effect of Si. Results for 22 trace elements are presented. Strong Si avoidance behavior is demonstrated by As, Au, Ga, Ge, Sb, Sn, and Zn. Iridium, Os, Pt, Re, Ru, and W exhibit weak Si avoidance tendencies. Silicon appears to have no significant effect on the partitioning behaviors of Ag, Co, Cu, Cr, Ni, Pd, and V, all of which had similar partition coefficients over a wide range of Si liquid concentrations from Si‐free to 13 wt%. The only elements in our experiments to show evidence of a potentially weak attraction to Si were Mo and Rh. Applications of the newly determined effects of Si to problems in planetary science indicate that (1) The elements Ni, Co, Mo, and W, which are commonly used in planetary differentiation models, are minimally affected by the presence of Si in the metal, especially in comparison to other effects such as from oxygen fugacity. 2) Reduced enstatite‐rich meteorites may record a chemical signature due to Si in the metallic melts during partial melting, and if so, elements identified by this study as having strong Si avoidance may offer unique insight into unraveling the history of these meteorites.     

Procrustean science: Indigenous siderophiles in the lunar highlands,according to Delano and Ringwood

The best estimate of indigenous lunar siderophiles comes from 29 pristine lunar rocks, characterized by low siderophile abundances, plutonic textures, and high age. Delano and Ringwood's blanket rejection of these rocks, on the contention that they are impact melts, is not justified by the petrologic evidence. Contrary to their claims, gold in highland breccias is largely meteoritic and is unaffected by fumarolic volcanism, as shown by its correlation with Ir and noncorrelation with fumarolic T1 (r=0.896 and 0.272). Delano and Ringwood's approach, involving subtraction of an H-chondrite meteoritic component from highland breccias, ignores the variation of Ir/Au ratios in modern and ancient meteorites, and hence leads to spurious excesses of Au, Ni, and volatiles, and in some cases to physically meaningless, negative residuals. Their excess volatiles in highland crust relative to mare basalts disappear when the highland composition is based on pristine lunar rocks rather than under-corrected breccias. Contrary to claims by Delano and Ringwood, the Ni/Co trend in Apollo 16 samples cannot be explained by an indigenous component rich in Ni (150–200 ppm) and Co (30–45ppm); mixing lines show that much lower Ni and Co contents are required (e.g., 7 ppm each).Chondrites and lunar highland breccias show essentially parallel fractionation trends for the siderophile-element ratios Re/Ir, Au/Ir, Ni/Ir, Ni/Pd, and Os/Ir. Because the chondritic ratios were established in the solar nebula, it appears that the lunar ratios also reflect nebular processes, and have not been modified by planetary processes.Properly derived abundances for the lunar highlands show large, systematic depletions relative to terrestrial oceanic tholeiites, by the following factors: Ge 270, Re 230, Sb170, Zn150, Au60, Tl 50, Ag 48, Ni 42, Se 12. It would seem that the resemblance to the Earth's mantle is not quite as striking as claimed by Delano and Ringwood.     

Thermal histories of IVA iron meteorites from transmission electron microscopy of the cloudy zone microstructure

Abstract— We have measured the size of the high‐Ni particles in the cloudy zone and the width of the outer taenite rim in eight low shocked and eight moderately to heavily shocked IVA irons using a transmission electron microscope (TEM). Thin sections for TEM analysis were produced by a focused ion beam instrument. Use of the TEM allowed us to avoid potential artifacts which may be introduced during specimen preparation for SEM analysis of high Ni particles <30 nm in size and to identify microchemical and microstructural changes due to the effects of shock induced reheating. No cloudy zone was observed in five of the eight moderately to highly shocked (>13 GPa) IVA irons that were examined in the TEM. Shock induced reheating has allowed for diffusion from 20 nm to 400 nm across kamacite/taenite boundaries, recrystallization of kamacite, and the formation, in Jamestown, of taenite grain boundaries. In the eleven IVA irons with cloudy zone microstructures, the size of the high‐Ni particles in the cloudy zone increases directly with increasing bulk Ni content. Our data and the inverse correlation between cooling rate and high‐Ni particle size for irons and stony‐irons show that IVA cooling rates at 350‐200 °C are inversely correlated with bulk Ni concentration and vary by a factor of about 15. This cooling rate variation is incompatible with cooling in a metallic core that was insulated with a silicate mantle, but is compatible with cooling in a metallic body of radius 150 ± 50 km. The widths of the tetrataenite regions next to the cloudy zone correlate directly with high‐Ni particle size providing another method to measure low temperature cooling rates.