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Siderophile element systematics of IAB complex iron meteorites: New insights into the formation of an enigmatic group.

ABSTRACT: Siderophile trace element abundances and the 187Re-187Os isotopic systematics of the metal phases of 58 IAB complex iron meteorites were determined in order to investigate formation processes and how meteorites within chemical subgroups may be related. Close adherence of 187Re-187Os isotopic data of most IAB iron meteorites to a primordial isochron indicates that the siderophile elements of most members of the complex remained closed to elemental disturbance soon after formation. Minor, presumably late-stage open-system behavior, however, is observed in some members of the sLM, sLH, sHL, and sHH subgroups. The new siderophile element abundance data are consistent with the findings of prior studies suggesting that the IAB subgroups cannot be related to one another by any known crystallization process. Equilibrium crystallization, coupled with crystal segregation, solid-liquid mixing, and subsequent fractional crystallization can account for the siderophile element variations among meteorites within the IAB main group (MG). The data for the sLM subgroup are consistent with equilibrium crystallization, combined with crystal segregation and mixing. By contrast, the limited fractionation of siderophile elements within the sLL subgroup is consistent with metal extraction from a chondritic source with little subsequent processing. The limited data for the other subgroups were insufficient to draw robust conclusions about crystallization processes involved in their formation. Collectively, multiple formational processes are represented in the IAB complex, and modeling results suggest that fractional crystallization within the MG may have been a more significant process than has been previously recognized.

SUBMITTER: Worsham EA

PROVIDER: S-EPMC6457471 | biostudies-literature | 2016 Aug

REPOSITORIES: biostudies-literature

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Siderophile element systematics of IAB complex iron meteorites: New insights into the formation of an enigmatic group.

Worsham Emily A EA Bermingham Katherine R KR Walker Richard J RJ

Geochimica et cosmochimica acta 20160521

Siderophile trace element abundances and the 187Re-187Os isotopic systematics of the metal phases of 58 IAB complex iron meteorites were determined in order to investigate formation processes and how meteorites within chemical subgroups may be related. Close adherence of 187Re-187Os isotopic data of most IAB iron meteorites to a primordial isochron indicates that the siderophile elements of most members of the complex remained closed to elemental distu ...[more]

PMID: 30983598

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Project description:The eight iron meteorites currently classified as belonging to the IIC group were characterized with respect to the compositions of 21 siderophile elements. Several of these meteorites were also characterized for mass independent isotopic compositions of Mo, Ru and W. Chemical and isotopic data for one, Wiley, indicate that it is not a IIC iron meteorite and should be reclassified as ungrouped. The remaining seven IIC iron meteorites exhibit broadly similar bulk chemical and isotopic characteristics, consistent with an origin from a common parent body. Variations in highly siderophile element (HSE) abundances among the members of the group can be well accounted for by a fractional crystallization model with all the meteorites crystallizing between ~10 and ~26% of the original melt, assuming initial S and P concentrations of 8 wt.% and 2 wt.%, respectively. Abundances of HSE estimated for the parental melt suggest a composition with chondritic relative abundances of HSE ~6 times higher than in bulk carbonaceous chondrites, consistent with the IIC irons sampling a parent body core comprising ~17% of the mass of the body. Radiogenic 182W abundances of two group IIC irons, corrected for a nucleosynthetic component, indicate a metal-silicate segregation age of 3.2 ± 0.5 Myr subsequent to the formation of Calcium-Aluminum-rich Inclusions (CAI). When this age is coupled with thermal modeling, and assumptions about the Hf/W of precursor materials, a parent body accretion age of 1.4 ± 0.5 Myr (post-CAI) is obtained. The IIC irons and Wiley have 100Ru mass independent "genetic" isotopic compositions that are identical to other irons with so-called carbonaceous chondrite (CC) type genetic affinities, but enrichments in 94,95,97Mo and 183W that indicate greater s-process deficits relative to most known CC iron meteorites. If the IIC irons and Wiley are of the CC type, this indicates variable s-process deficits within the CC reservoir, similar to the s-process variability within the NC reservoir observed for iron meteorites. Nucleosynthetic models indicate that Mo and 183W s-process variability should correlate with Ru isotopic variability, which is not observed. This may indicate the IIC irons and Wiley experienced selective thermal processing of nucleosynthetic carriers, or are genetically distinct from the CC and NC precursor materials.

Project description:Coupled 187Os/188Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data are reported for Apollo 12 (12005, 12009, 12019, 12022, 12038, 12039, 12040), Apollo 15 (15555) and Apollo 17 (70135) mare basalts, along with mare basalt meteorites La Paz icefield (LAP) 04841 and Miller Range (MIL) 05035. The most magnesian samples have chondrite-relative HSE abundances and chondritic measured and calculated initial 187Os/188Os, with mare basalts having consistently low HSE abundances at ~2 ×10-5 to 2 ×10-7 the chondritic abundance. The lower and more fractionated HSE compositions of evolved mare basalts can be reproduced with bulk-partition coefficients of ~2 for Os, Ir, Ru, Pt and Pd and ~1.5 for Re. Lunar mare basalt bulk-partition coefficients are probably higher than for terrestrial melts as a result of more reducing conditions, leading to increased HSE compatibility. The chondritic-relative abundances and chondritic 187Os/188Os of the most primitive high-MgO mare basalts cannot be explained through regolith contamination during emplacement at the lunar surface. Instead, mare basalt compositions can be modelled as representing ~5-11% partial melting of metal-free sources with low Os, Ir, Ru, Pd (~0.1 ng g-1), Pt (~0.2 ng g-1) Re (~0.01 ng g-1) and S, with sulphide-melt partitioning between 1000 and 10000. Apollo 12 olivine-, pigeonite- and ilmenite normative mare basalts define an imprecise 187Re-187Os age of 3.0 ±0.6 Ga. This age is within uncertainty of 147Sm-143Nd ages for the same samples and the isochron yields an initial 187Os/188Os of 0.109 ±0.008. The Os isotopic composition of the Apollo 12 source indicates that the lunar mantle source of these rocks evolved with Re/Os within ~10% of chondrite meteorites from the time that the mantle source became a system closed to siderophile additions to the time that the basalts erupted. The similarity in absolute HSE abundances between mare basalts from the Apollo 12, 15 and 17 sites, and from unknown regions of the Moon (La Paz mare basalts, MIL 05035) indicates relatively homogeneous and low HSE abundances within the lunar interior. Low absolute HSE abundances and chondritic Re/Os of mare basalts are consistent with ~0.02% late accretion addition that was added prior to the formation of the lunar crust and significantly prior to cessation of lunar mantle differentiation (>4.4 Ga) to enable efficient mixing and homogenization. The HSE abundances are also consistent with the observed, small 182W excess (20 ppm) in the bulk silicate Moon relative to the bulk silicate Earth.

Project description:The abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Pd), as well as 187Re-187Os and 182Hf-182W isotopic systematics were determined for separated metal, slightly magnetic, and nonmagnetic fractions from seven H4 to H6 ordinary chondrites. The HSE are too abundant in nonmagnetic fractions to reflect metal-silicate equilibration. The disequilibrium was likely a primary feature, as 187Re-187Os data indicate only minor open-system behavior of the HSE in the slightly and non-magnetic fractions. 182Hf-182W data for slightly magnetic and nonmagnetic fractions define precise isochrons for most meteorites that range from 5.2 ± 1.6 Ma to 15.2 ± 1.0 Ma after calcium aluminum inclusion (CAI) formation. By contrast, 182W model ages for the metal fractions are typically 2-5 Ma older than the slope-derived isochron ages for their respective, slightly magnetic and nonmagnetic fractions, with model ages ranging from 1.4 ± 0.8 Ma to 12.6 ± 0.9 Ma after CAI formation. This indicates that the W present in the silicates and oxides was not fully equilibrated with the metal when diffusive transport among components ceased, consistent with the HSE data. Further, the W isotopic compositions of size-sorted metal fractions from some of the H chondrites also differ, indicating disequilibrium among some metal grains. The chemical/isotopic disequilibrium of siderophile elements among H chondrite components is likely the result of inefficient diffusion of siderophile elements from silicates and oxides to some metal and/or localized equilibration as H chondrites cooled towards their respective Hf-W closure temperatures. The tendency of 182Hf-182W isochron ages to young from H5 to H6 chondrites may indicate derivation of these meteorites from a slowly cooled, undisturbed, concentrically-zoned parent body, consistent with models that have been commonly invoked for H chondrites. Overlap of isochron ages for H4 and H5 chondrites, by contrast, appear to be more consistent with shallow impact disruption models. The W isotopic composition of metal from one CR chondrite was examined to compare with H chondrite metals. In contrast to the H chondrites, the CR chondrite metal is characterized by an enrichment in 183W that is consistent with nucleosynthetic s-process depletion. Once corrected for the correlative nucleosynthetic effect on 182W, the 182W model age for this meteorite of 7.0 ± 3.6 Ma is within the range of model ages of most metal fractions from H chondrites. The metal is therefore too young to be a direct nebular condensate, as proposed by some prior studies.

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Dataset Information

Siderophile element systematics of IAB complex iron meteorites: New insights into the formation of an enigmatic group.

Publications

Siderophile element systematics of IAB complex iron meteorites: New insights into the formation of an enigmatic group.

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