Pt-Re-Os systematics of group IIAB and IIIAB iron meteorites 1
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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2022, Geochimica et Cosmochimica ActaCitation Excerpt :These values range from −11 ± 2 to +40 ± 2, with most plotting within uncertainties of the reference isochron. The abundances of HSE in the IIAB, IID, IIIAB, and IIIF iron meteorites examined are typically within ±20% of values reported by Schaudy and Wasson (1972), Scott and Wasson (1976), Pernicka and Wasson (1987), Wasson (1999), Grossman (2000), Cook et al. (2004), Wasson and Huber (2006), Wasson et al. (2007), and Hopp et al. (2018). The concentrations reported here are compared to literature values in Table S1 of the supplemental materials.
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Associate editor: G. Herzog
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