Elsevier

Geochemistry

Volume 79, Issue 4, December 2019, 125528
Geochemistry

Formation and destruction of magnetite in CO3 chondrites and other chondrite groups

https://doi.org/10.1016/j.chemer.2019.07.009Get rights and content

Abstract

Primitive CO3.00–3.1 chondrites contain ∼2-8 vol.% magnetite, minor troilite and accessory carbide and chromite; some CO3.1 chondrites have fayalite-rich veins, chondrule rims and euhedral matrix grains. All CO3.00–3.1 chondrites contain little metallic Fe-Ni (0.4–1.2 vol.%). CO3.2–3.7 chondrites contain 1–5 vol.% metallic Fe-Ni, minor troilite, accessory chromite and 0-0.6 vol.% magnetite. Magnetite is formed in primitive CO3 chondrites from metallic Fe by parent-body aqueous alteration, resulting in decreased metallic Fe-Ni and an increase in the proportion of high-Ni metal grains. The paucity or absence of magnetite in CO chondrites of subtype ≥3.2 suggests that magnetite is destroyed during thermal metamorphism; thermochemical calculations from the literature suggest that magnetite is reduced by H2 and reacts with SiO2 to form fayalite and secondary kamacite. Analogous processes of magnetite formation and destruction occur in other chondrite groups: (1) Primitive type-3 OC have opaque assemblages containing magnetite, carbide, Ni-rich metal and Ni-rich sulfide, but OC of subtype >3.4 contain little or no magnetite. (2) Primitive R3 chondrites and clasts (subtype ≲3.5) contain up to 6 vol.% magnetite, but most R chondrites contain no magnetite. The principal exception is magnetite with 9–20 wt.% Cr2O3 in a few R4-6 chondrites. Magnetite grains with high Cr2O3 behave like chromite and are more stable under reducing conditions. (3) CK chondrites average ∼4 vol.% magnetite with substantial Cr2O3 (up to ∼15 wt.%); these magnetite grains also are stable against reduction during metamorphism. (4) The modal abundance of magnetite decreases with metamorphic grade in CV3 chondrites. (5) Chromite occurs instead of magnetite in those rare samples classified CR6, CR7 and CV7.

Introduction

CO3 chondrites form a metamorphic sequence that exhibits systematic changes in texture, mineralogy, thermoluminescence (TL) sensitivity, mineral chemistry, bulk chemistry and bulk O-isotopic composition. With increasing metamorphism, CO chondrites exhibit: (1) increases in the degree of textural recrystallization, mafic mineral homogeneity, the FeO contents of mafic phases, and the development and thickening of ferroan rims on olivine grains in chondrules and amoeboid olivine inclusions (AOIs) [a.k.a. amoeboid olivine aggregates (AOAs)] (McSween, 1977a, b; Chizmadia et al., 2002), (2) increasingly positive valence states for Ti and Cr in olivine at the early stages of metamorphism (Simon et al., 2018, 2019), (3) decreases in the abundances of chondrule glass, metallic Fe-Ni grains, presolar diamonds, presolar SiC, and bulk volatile elements (including noble gases) (McSween, 1977a, b; Huss, 1990; Scott and Jones, 1990; Davidson et al., 2014a), (4) decreases in the abundance of presolar silicates in the matrix (e.g., Nittler et al., 2013, 2018), (5) increases in the abundances of straight-chain amino acids (which are produced during thermal metamorphism; Burton et al., 2012), (6) increasingly heavy bulk O isotopes (Clayton and Mayeda, 1984; Table 4 of Rubin, 1998), (7) decreases in bulk C, D/H ratio and 15N/14N ratio as insoluble organic matter (IOM) becomes increasingly graphitized (e.g., Alexander et al., 2007, 2018; Bonal et al., 2007), (8) alteration and conversion of CAI minerals (i.e., FeO-enrichment of spinel, conversion of perovskite to ilmenite, and depletion of melilite) (Russell et al., 1988; Rubin, 1998), (9) increasing TL sensitivity (Keck and Sears, 1987; Sears, 2013, 2016), (10) increasing replacement of fine-grained plessite by coarser Ni-rich metal (taenite and tetrataenite) and associated kamacite (Kimura et al., 2008), (11) changes in the shape of chromite grains from euhedral to subhedral or anhedral (e.g., Johnson and Prinz, 1991; Davidson et al., 2014a), and (12) changes in the Cr2O3 content of ferroan olivine grains. This final change involves steady decreases in olivine Cr2O3 while the standard deviation of the olivine Cr2O3 content increases at the earliest stages of metamorphism (subtypes 3.00–3.05) and then decreases gradually through subtype 3.2 (Grossman and Brearley, 2005; Davidson et al., 2014a). This process is accompanied by the development of narrow (<0.3-μm-thick) chromite veins within the olivine grains (Brearley and Simon, 2019).

The opaque mineralogy of CO chondrites shows variations that are plausibly attributable to fluid-assisted thermal metamorphism. In addition to sulfide, DOM 08006 (CO3.00) contains carbide-magnetite veins as well as grains of metallic Fe-Ni that have been partially replaced by magnetite (Simon et al., 2019). Magnetite also occurs in Y-81020 (CO3.05), DOM 08004 (CO3.1), EET 90043 (CO3.1), DOM 03238 (CO3.1) and Ornans (CO3.4) (e.g., Krot et al., 2019; Grossman and Rubin, 2006; Rubin and Wasson, 1988), but is rare to absent in Kainsaz (CO3.2), Felix (CO3.4), Lancé (CO3.4), Warrenton (CO3.6) and Isna (CO3.7) (e.g., McSween, 1977a). The paucity or absence of magnetite in higher CO subtypes suggests that magnetite disappears with increasing thermal metamorphism. The same process likely operated in other chondrite groups. This study seeks a better understanding of the effects of thermal metamorphism on the opaque assemblages in ordinary, carbonaceous and R chondrites.

Section snippets

Analytical procedures

Thin sections were examined microscopically in transmitted and reflected light. Grain sizes were measured with a calibrated reticle. Modal abundances were determined microscopically in reflected light using an automated point counter. Back-scattered electron (BSE) images were made at UCLA with a JEOL JXA-8200 electron microprobe using an acceleration voltage of 15 kV and a working distance of ∼11 mm. Mineral compositions were determined with the microprobe using natural and synthetic standards,

Classification of CO3 chondrites

McSween (1977a) ordered CO chondrites into a metamorphic sequence using the Roman numerals I, II, III. After Sears et al. (1980) had defined subtypes 3.0–3.9 for ordinary chondrites (OC), Scott and Jones (1990) assigned analogous subtypes to CO3 chondrites on the basis of petrographic and mineralogical similarities to LL3 chondrites. The CO3 subtypes were later slightly modified based on the petrographic characteristics of their amoeboid olivine inclusions (Chizmadia et al., 2002) and bulk TL

Formation of magnetite during parent-body aqueous alteration

The O-isotopic composition of magnetite grains in CV and ordinary chondrites shows that magnetite is not in isotopic equilibrium with chondrule olivine in these meteorites. Magnetite therefore formed after chondrule phenocrysts crystallized (Choi et al., 1997, 1998; Doyle et al., 2015), presumably during aqueous alteration of metallic Fe on the parent chondritic asteroids. In type-3 OC, magnetite occurs in association with carbide (cohenite - (Fe,Ni)3C and haxonite - (Fe,Ni)23C6), kamacite,

Conclusions

When ordinary, carbonaceous and R chondrites agglomerated, they were probably magnetite free. Heating of ice or dehydration of phyllosilicates during subsequent parent-body heating caused mobilization of aqueous fluids and the formation of magnetite mainly from metallic Fe. The concomitant formation of carbides in CO and ordinary chondrites indicates that the fluids contained C, possibly derived from organic compounds. Subsequent thermal metamorphism likely produced high H2/H2O ratios in the

Acknowledgments

We thank A. N. Krot, S. B. Simon and M. Yu. Zolotov for helpful discussions and R. Esposito and F. T. Kyte for technical assistance on EPMA analyses. We are grateful for helpful reviews from A. N. Krot and an anonymous referee. This work was supported by NASA grant NNG06GF95G as well as the National Natural Science Foundation of China (Grant No.41803051), the Minor Planet Foundation of China, and the Natural Science Foundation of Jiangsu Province (Grant No.BK20161098).

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