Elsevier

Geochimica et Cosmochimica Acta

Volume 333, 15 September 2022, Pages 1-21
Geochimica et Cosmochimica Acta

Nickel-rich, volatile depleted iron meteorites: Relationships and formation processes

https://doi.org/10.1016/j.gca.2022.06.008Get rights and content

Abstract

Ungrouped iron meteorites Tishomingo, Willow Grove, and Chinga, and group IVB iron meteorites, are Ni-rich. Similarities include enrichments of 10–100 × CI for some refractory siderophile elements, and equivalent depletions in more volatile siderophile elements. Superimposed on the overall enrichment/depletion trend, certain siderophile elements (P, W, Fe, Mo) are depleted relative to elements of similar volatility. All three ungrouped irons derive from parent bodies formed in the early Solar System. Willow Grove and Chinga are characterized by cosmic ray exposure corrected 182W/184W consistent with metal-silicate segregation on their parent bodies within 1–3 Myr of Solar System formation, within the age range determined for segregation of magmatic iron meteorite parent bodies, including group IVB irons. Tishomingo is characterized by a younger model age 4–5 Myr subsequent to Solar System formation, reflecting either late stage melting resulting from 26Al decay, or an impact resetting. The discovery of stishovite in Tishomingo, indicating exposure to a minimum shock pressure of 8–9 GPa, is consistent with the latter.

Stishovite in Tishomingo and chromite included in troilite-daubréelite in IVB irons allows oxygen isotopic composition comparison between these meteorites. Different mass independent oxygen isotopic compositions of IVB irons and Tishomingo indicate genetically distinct parent bodies. By contrast, mass independent Mo isotopic compositions overlap within analytical uncertainties, indicating a similar, carbonaceous chondrite (CC) type genetic heritage. Molybdenum and 183W isotopic data for Chinga and Willow Grove indicate derivation from CC type parent bodies. Willow Grove shares Mo and 183W isotopic compositions with the Ni-rich South Byron Trio (SBT) grouplet and the Milton pallasite. These Ni-rich meteorites likely formed in the same general nebular environment as other CC planetesimals, likely the outer Solar System.

Highly siderophile element (HSE) abundances of Willow Grove and Tishomingo are similar to some IVB meteorites, consistent with formation by moderate degrees of fractional crystallization from initial metallic melts with low S and P, and modestly fractionated HSE. The comparable HSE abundances of Tishomingo and Willow Grove to some IVB irons, yet substantially higher Ni concentrations, indicate formation on parent bodies with lower bulk HSE abundances or HSE concentration in proportionally smaller volumes of metal. HSE abundances in Chinga are considerably lower than in IVB irons, highly fractionated, and processes responsible for these remain elusive.

For IVB irons and these ungrouped irons, high temperature condensation likely dominated the enrichment and depletion of the refractory and volatile siderophile elements, respectively. Parent body degassing may have also played a role. Relative depletion of volatile siderophile elements is not, however, a universal feature of high-Ni meteorites. The SBT and Milton pallasite are Ni-rich, but less depleted in the more volatile siderophile elements. Nickel enrichment was likely driven by oxidation of Fe metal during parent body accretion or core segregation. Oxidation of the Tishomingo and Willow Grove parent bodies may have occurred at ∼ IW + 1, indicated by relative Mo and W depletions due to metal/water reaction during differentiation. Late-stage reduction, indicated by the presence of Cr-bearing sulfides in Tishomingo and IVB irons, may have resulted from exhaustion of the oxidant.

Introduction

Of the > 1300 recognized iron meteorites, ∼90% fall into compositional clusters thought to represent ∼ 13 individual parent bodies. These clusters are used to define the well-known iron meteorite groups (e.g., IIAB, IIIAB, IVA, IVB). The remaining ∼ 10% of irons do not fit into compositional clusters consistent with the recognized groups, and are termed “ungrouped” (Wasson et al., 1989). The origin of most individual ungrouped irons is unclear, with possible formation mechanisms including metals segregated from silicates in impact melts on chondritic bodies, or as products of fractional crystallization or liquid immiscibility in planetesimal cores. Most ungrouped irons probably represent the only samples of their > 100 individual parent bodies currently present in the world’s meteorite collections (Burbine et al., 2002). Thus, ungrouped iron meteorites must sample more parent bodies than all of the other meteorite types combined, and serve as a resource of currently untapped information regarding the genetic, chemical and chronologic diversity of meteorite parent bodies.

In this study the processes that generated the Ni-rich, comparatively volatile element depleted, ungrouped iron meteorites Tishomingo, Willow Grove, and Chinga, are explored. Possible linkages with the similarly Ni-rich IVB irons, the South Byron Trio (SBT) iron meteorite grouplet, and the Milton pallasite are also considered. This work is spurred by the possibility that these ungrouped irons, characterized by unusual textures and compositions, could potentially have been generated on parent bodies that underwent similar processes to the IVB iron group or the SBT grouplet. Such processes include oxidation, reduction, volatilization, fractional crystallization and shock.

A key component of this study is the “genetic” comparison of meteorites. Genetic comparisons are made based on mass independent isotopic compositions of various elements. Meteorites with genetic commonality can potentially be related to the same nebular formational environment, or even the same parent body. By contrast, genetic differences in most instances imply different parent bodies. Mass independent oxygen isotopic variations (Δ17O values), most likely caused by self-shielding effects in the solar nebula (e.g., Thiemens, 1999), have historically been the most widely used genetic tracer for meteorites since first observed in meteorites (e.g., Clayton et al., 1973). Because of the paucity of oxygen-bearing phases in iron meteorites, however, only silicate-bearing iron and stony iron meteorites have previously been studied for mass independent oxygen isotopic compositions.

Heck et al., 2010, Caplan et al., 2015, Caplan et al., 2016) have developed techniques and standards for in situ analyses of oxygen isotopes in chromite grains from fossil meteorites in sedimentary rocks. The application of these techniques allows the first measurements of oxygen isotopic compositions for group IVB irons to be made here. In addition, a method for analyzing a single crystal of SiO2 via laser fluorination was developed as part of this study in order to obtain oxygen isotope data for iron meteorites bearing small silicate inclusions.

Oxygen isotopes are no longer the sole tool for assessing genetic relationships. Lithophile elements, such as Ti and Cr, are also characterized by mass independent isotopic variations among planetary materials (e.g., Leya et al., 2008, Trinquier et al., 2008). These variations have been determined to reflect variable contributions of diverse nucleosynthetic materials to the parent bodies. Further, during the past 20 years, the mass independent isotopic compositions of certain siderophile elements (Ni, Mo, Ru, W, Pd) have also been shown to be valuable for genetic comparisons, particularly among iron meteorites (e.g., Dauphas et al., 2004, Burkhardt et al., 2011, Spitzer et al., 2020). Genetic isotopic compositions of meteorites using lithophile and/or siderophile element tracers have been used to distinguish between the so-called non-carbonaceous (NC) type meteorites from carbonaceous chondrite (CC) type meteorites, which have been suggested to reflect formation in the inner and outer Solar System, respectively (e.g., Warren, 2011). Subsequent studies have sought to advance these concepts by integrating chemical, isotopic and dynamical modelling of early solar system processes (e.g., Kruijer et al., 2017, Ek et al., 2020, Lichtenberg et al., 2021). Here the advances in genetic comparisons allow us to examine possible genetic linkages between IVB irons and the three ungrouped irons with high Ni concentrations, as well as the SBT iron meteorite grouplet and the Milton pallasite.

In addition to genetic comparisons, the 182Hf-182W short-lived isotopic system allows an assessment of the timing of metal-silicate segregation on the parent body of iron meteorites (e.g., Kruijer et al., 2014a). As with genetic isotopes, 182W model ages can be used to ascertain whether different meteorites could be related by the same process at the same time, and are applied here.

Section snippets

IVB irons

The primary comparison for the ungrouped iron meteorites of this study is the IVB iron meteorite group. The IVB irons are the fifth largest group of iron meteorites, with 18 members, and are the most Ni-rich of the iron meteorite groups. The IVB irons display an ataxitic structure, with kamacite present as spindles and grains in plessite. Sulfides, graphite, carbides, and silicates are rare or absent in IVB irons (Hutchison, 2004). Compositionally, IVB irons are volatile poor, with low Ga, Ge,

Inclusion Mineralogy

Given the report of oxygen bearing phases in IVB irons (Buchwald, 1975, Teshima and Larimer, 1983), and lack of previous oxygen isotopic analyses, we examined a number of IVB irons to find oxygen bearing phases. Our examination of two sections of Santa Clara failed to reveal silicate, chromite, or any other oxygen bearing phases, despite reports of the presence of these minerals by Teshima and Larimer (1983). Two grains of chromite, however, were found within larger inclusions in a section of

Genetics and age

The application of genetic tracing has been extended to iron meteorites, primarily through the isotopic examination of siderophile elements, such as Ni, Mo, Ru and W (183W) (e.g., Dauphas et al., 2002, Regelous et al., 2008, Fischer-Gödde et al., 2015, Kruijer et al., 2017). Molybdenum nucleosynthetic isotopic anomalies, in particular, have proven particularly useful for distinguishing among early Solar System reservoirs sampled by iron meteorites (e.g., Burkhardt et al., 2011, Kruijer et al.,

Conclusions

This study of Tishomingo, Chinga, Willow Grove and the IVB irons Hoba and Warburton Range provides new insights into the formation of these Ni-rich meteorites, including:

  • The isotopic characteristics of the ungrouped irons indicate the parent bodies to these meteorites all formed within the carbonaceous chondrite (CC) type region of the solar nebula, with metal-silicate segregation occurring at approximately 1–5 Myr following solar system formation.

  • The collective application of chemical

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank the three reviewers and Associate Editor Thorsten Kleine for their helpful reviews. Their comments and suggestions made this a better paper. The authors also thank Jijin Yang, Nancy Chabot, and our late colleagues Joe Goldstein, Ed Scott and John Wasson for productive discussions regarding this work. C. Corrigan acknowledges funding from the Smithsonian Institution’s Henderson Fund for Meteorites. This study was also partially supported by NASA Emerging Worlds grants

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