Pt-Re-Os systematics of group IIAB and IIIAB iron meteorites 1

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Abstract

The Pt-Re-Os isotopic and elemental systematics of 13 group IIAB and 23 group IIIAB iron meteorites are examined. As has been noted previously for iron meteorite groups and experimental systems, solid metal-liquid metal bulk distribution coefficients (D values) for both IIAB and IIIAB systems show DOs>DRe>>DPt>1 during the initial stages of core crystallization. Assuming closed-system crystallization, the latter stages of crystallization for each core are generally characterized by DPt>DRe>DOs. The processes governing the concentrations of these elements are much more complex in the IIIAB core relative to the IIAB core. Several crystallization models utilizing different starting parameters and bulk distribution coefficients are considered for the Re-Os pair. Each model has flaws, but in general, the results suggest that the concentrations of these elements were dominated by equilibrium crystallization and subsequent interactions between solid metal and both equilibrium and evolved melts. Late additions of primitive metal to either core were likely minor or nonexistent.

The 187Re-187Os systematics of the IIAB and IIIAB groups are consistent with generally closed-system behavior for both elements since the first several tens of Ma of the formation of the solar system, consistent with short-lived chronometers. The Re-Os isochron ages for the complete suites of IIAB and IIIAB irons are 4530 ± 50 Ma and 4517 ± 32 Ma, respectively, and are similar to previously reported Re-Os ages for the lower-Ni endmembers of these two groups. Both isochrons are consistent with, but do not require crystallization of the entire groups within 10–30 Ma of the initiation of crystallization.

The first high-precision 190Pt-186Os isochrons for IIAB and IIIAB irons are presented. The Pt-Os isochron ages for the IIAB and IIIAB irons, calculated using the current best estimate of the λ for 190Pt, are 4323 ± 80 Ma and 4325 ± 26 Ma respectively. The Re-Os and Pt-Os ages do not overlap within the uncertainties. The younger apparent ages recorded by the Pt-Os system likely reflect error in the 190Pt decay constant. The slope from the Pt-Os isochron is combined with the age from the Re-Os isochron for the IIIAB irons to calculate a revised λ of 1.415 × 10−12 a−1 for 190Pt, although additional study of this decay constant is still needed.

Introduction

Iron meteorites are pieces of Fe-Ni metal that segregated from chondritic silicates during the early stages of solar system evolution. Most iron meteorites fall into one of about 13 groups and are classified on the basis of their structures and trace element contents Wasson 1974, Buchwald 1975, Wasson and Kallemeyn 2002. Each group presumably formed in a distinct parent body (Kelly and Larimer, 1977). Iron meteorites belonging to 10 of the 13 groups most likely represent the cores of different parent bodies and are known as “magmatic irons” (Wasson, 1974). The other groups are termed “nonmagmatic irons” and likely derive from parent bodies in an arrested state of differentiation and do not represent core materials Wasson and Wang 1986, Choi et al 1995, Stewart et al 1996, Wasson and Kallemeyn 2002. Here we examine the Pt-Re-Os elemental and isotopic systematics of the two largest magmatic iron groups, IIAB and IIIAB. Utilizing the distributions of these elements, the ratios of solid metal-liquid metal bulk distribution coefficients (D values) are determined. Rhenium and Os D values for both high- and low-S systems are combined with two sets of starting compositions to model crystal-liquid fractionation processes potentially involved in the generation of the IIIAB iron system. Finally, the 187Re-187Os and 190Pt-186Os isotopic systematics of these meteorites are examined to constrain the duration of their crystallization.

Constraining the timing of late-stage crystallization is essential for understanding the complete cooling histories of the parent bodies of iron meteorites. Tungsten isotope compositions indicate that the metal comprising at least some iron meteorite groups segregated from silicates within ∼0–20 Ma of solar system formation Harper and Jacobsen 1996, Lee and Halliday 1996, Horan et al 1998, Schoenberg et al 2002. Because of the essentially complete isolation of W from Hf during core formation, this system cannot be used to determine metal-crystallization ages. The long-lived Re-Os system, coupled with the short-lived Pd-Ag and Mn-Cr systems, suggest that the cores of iron meteorite parent bodies began to crystallize within the first 40 Ma Luck et al 1980, Smoliar et al 1996, Shen et al 1996, Chen and Wasserburg 1996, Birck and Allegre 1998, Carlson and Hauri 2001, Chen et al 2002. The duration of core crystallization, however, remains poorly constrained for the larger, more complex cores. It is plausible that crystallization of group IIAB and IIIAB irons may have occurred over tens of Ma because crystal-liquid fractionation led to increases in the S and P contents of fractionated melts. This, in turn, would have led to large decreases in liquidus temperatures and long crystallization periods, especially for insulated pockets of trapped liquid (Wasson, 1999).

Crystal-liquid fractionation of metallic magmas typically produces large variations in Re/Os and Pt/Os ratios during core solidification, making both isotope systems viable for isochron chronometry Hirt et al 1963, Luck et al 1980, Pernicka and Wasson 1987. Extensive crystal-liquid fractionation of IIAB and IIIAB cores produced decreases of several orders of magnitude in the concentrations of Re and Os, but only modest decreases in Pt concentrations, leading to much larger variations in Pt/Os relative to Re/Os for both groups. Despite this advantage for isochron dating, the Pt-Os system is not without shortcomings relative to the Re-Os system. The long half-life (ca. 470 b.y.) of 190Pt coupled with its miniscule isotopic abundance (0.01292 atomic %; Morgan et al., 2002) has produced only modest enrichments of radiogenic 186Os in most geologic materials and a correspondingly narrow range in 186Os/188Os ratios. In general, these variations are much less than those of 187Os/188Os ratios produced by the decay of 187Re. Thus, the detection and resolution of variations in 186Os/188Os ratios requires that the precision of the measurements exceed those of 187Os/188Os by at least an order of magnitude (Walker et al., 1997).

Section snippets

Sample preparation and chemical processing

Iron meteorite samples were cut into appropriately sized, rectangular pieces using a Leco Vari-cut saw with a 5.0-inch-diameter diamond wafering blade. The saw’s cooling system was modified to use high-purity water rather than oil. To avoid cross-contamination between samples, the blade and blade assembly were cleaned with water after each sample was cut, and the cooling water was discarded and replaced. Blades were further cleaned by cutting a piece of carborundum after washing the blade and

Group IIAB irons

The group IIAB irons display one of the clearest cases of fractional crystallization of metallic magma during core formation (Scott, 1972). The well-established Ni-Ir trend for the IIAB group defines a very steep negative slope (Fig. 1), indicating a high Ir distribution coefficient and a Ni distribution coefficient near unity. The trend is also consistent with moderately high S content in the parental metallic magma as suggested by Jones and Drake (1983). Some or even most of the scatter of

Results

Isotopic and concentration data for the IIAB and IIIAB iron meteorites are summarized in Table 1, Table 2. The reported 187Re/188Os and 190Pt/188Os ratio 2σ uncertainties include the combined uncertainty in the isotopic measurements and blank corrections. Not included is the uncertainty in the calibration of the Os spike, which is due to uncertainties in the stoichiometry of the Os standard used for spike calibration Morgan et al 1995, Yin et al 2001. The Os and Re blank corrections introduced

Fractionation trends in group IIAB and IIIAB irons

Variations in the abundances of Re, Os, Pt, and other highly siderophile elements (HSE), for both the IIAB and IIIAB systems, were dominated by solid metal-liquid metal partitioning as crystal-liquid fractionation proceeded. Nonmetal phases, such as sulfides and phosphides, that crystallized from pockets of trapped melt, contain much lower abundances of Re and Os than metal phases, so their crystallization likely had only very minor impacts on the fractionation and possibly the fractionation of

Conclusions

The Pt-Re-Os elemental systematics of the IIAB and IIIAB irons are dominated by the effects of fractional crystallization. For the IIAB irons, solid metal-liquid metal bulk distribution coefficients (D values) are DOs>DRe>>DPt>1 during the initial stages of core crystallization. As has been previously noted, there is a major change in DRe/DOs approximately at the structural divide between hexahedrites and octahedrites. The latter stages of IIAB crystallization are characterized by D values for

Acknowledgements

This work was partially supported by NSF CSEDI grant 0001921 (to R.J.W.), and NASA Cosmochemistry grants NAGW 3625 and NAG 54769 (to R.J.W.) and NAGW 5–12887 (to J.T.W.). Samples were obtained from the Smithsonian Institution’s National Museum of Natural History, and the Geological Museum, University of Copenhagen. Provision of these materials is greatly appreciated. We especially thank Roy Clarke for his long-term assistance on this project. We thank Michael Smoliar for Figure 5a–c, which

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    Associate editor: G. Herzog

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