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Occurrence, Formation and Destruction of Magnetite in Chondritic Meteorites

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Occurrence, Formation and Destruction of Magnetite in Chondritic Meteorites 51st Lunar and Planetary Science Conference (2020) 1092.pdf OCCURRENCE, FORMATION AND DESTRUCTION OF MAGNETITE IN CHONDRITIC METEORITES. Alan E. Rubin1 and Ye Li2, 1Dept. Earth, Planet. Space Sci., UCLA, Los Angeles, CA 90095-1567, USA. 2Key Lab. Planet. Sci., Purple Mount. Observ., Chinese Academy of Sciences, Nanjing, 210034, China. Occurrence: Magnetite with low Cr2O3 is present in chondritic asteroids. CO3.1 chondrites also contain aqueously altered carbonaceous chondrites (CI1, veins of fayalite as well as fayalitic overgrowths on CM2.0-2.2, CR1, CV3OxA, CV3OxB, CO3.00-3.1) [1-6], magnesian olivine grains; these occurrences of ferroan type 3.00-3.4 ordinary chondrites [7,8] and type 3.5 R olivine are alteration products (as is magnetite). chondrites [9]. Magnetite in CI Orgueil averages 0.04 Carbide-magnetite assemblages may have formed wt.% Cr2O3 [10]; magnetite in CO3 chondrites averages from metallic Fe by a C-O-H-bearing fluid in a process 0.27 wt.% Cr2O3 [11]; magnetite in LL3.00 Semarkona involving carbidization by CO(g) and oxidation by is nearly pure Fe3O4 (with ~0.1 wt.% Co) [7]. Magnetite H2O(g) [8]. Textural evidence indicates that magnetite is rare to absent in less-altered, more-metamorphosed also replaces iron carbide and troilite in type-3 OC [8]. CM, CR, CO, OC and R chondrites; in addition, mag- This is consistent with the apparent replacement of pyr- netite is rare in CV3Red chondrites and less abundant in rhotite by framboidal magnetite in CM chondrites [22] type-3.6 Allende (CVOxA) than in type-3.1 Kaba and in the Kaidun ungrouped polymict carbonaceous (CVOxB). In those groups of equilibrated chondrites that chondrite [23]. contain magnetite, the magnetite grains are rich in Destruction: The essential absence of magnetite in Cr2O3: e.g., magnetite in individual R chondrites aver- type-6 OC [16] and CO3.2-3.8 chondrites [11] and the ages ~9-20 wt.% Cr2O3 [e.g., 12]; magnetite in CK decreased abundance of magnetite in the more-meta- chondrites averages ~4 wt.% Cr2O3 [13]. morphosed CV3Ox chondrites suggest that magnetite is Where present in CO3 chondrites, chromite occurs destroyed in these rocks during thermal metamorphism. commonly as 2-30-µm-size euhedral to subhedral grains Reduction of magnetite during parent-body heating pro- within Type-II chondrules or in association with ferroan duces secondary metallic Fe-Ni (evident in metal-rich olivine grains in the matrix. In CO3.1 chondrites, chro- CO≥3.2 chondrites) and is likely responsible for the rel- mite is present as small (<4-µm-size) grains associated atively high modal abundance ratio of kamacite to Ni- with magnetite within chondrules and in the matrix. rich metal in CO3 Kainsaz [11]. Our data show that the Chromite occurs instead of magnetite in the rare mete- kamacite/Ni-rich-metal ratio in CO3.1 Dominion Range orites classified as CR6 and CR7 [14]. Chromite consti- (DOM) 08351 (~2-3) is far lower than that in CO3.2 tutes 0.37±0.11 wt.% (n=35) of type-6 OC [15], Kainsaz (~10). whereas magnetite in these rocks is essentially absent There is also textural evidence that some chromite [16]. Enstatite chondrites completely lack such oxide formed by replacement of magnetite in CO3.1 Domin- phases [17]. ion Range (DOM) 10104: Cr2O3 from olivine may have In type-3.00-3.4 OC, magnetite occurs in association reacted with magnetite to produce chromite while add- with cohenite, haxonite, kamacite, troilite, pentlandite ing FeO to olivine. A possible reaction is: and Ni-rich metal [7,8]. In CO3.00-3.1 samples, Type- Cr2O3 + Fe3O4 = FeCr2O4 +2FeO + ½O2↑ I chondrules typically contain 10-25-µm-size spheroidal Thermochemical equilibrium calculations [24] show or irregular masses of metallic-Fe-Ni+troilite and/or that, during metamorphism of ordinary and carbona- magnetite+pentlandite [6,18,19]. Many opaque masses ceous chondrites, the H2/H2O ratio in the gas within as- in CO3.00-3.1 chondrites have cores of metallic Fe-Ni teroids increased, leading to reduction of magnetite and (kamacite, taenite, tetrataenite) mantled by magnetite. the formation of fayalite and secondary kamacite. Chro- Formation: The O-isotopic compositions of CV3 mite is more stable than magnetite under reducing con- and type-3 OC magnetite grains show that magnetite is ditions [24]; the magnetite/chromite ratio decreases as not in isotopic equilibrium with chondrule olivine. Mag- the f(H2/H2O) ratio increases. This is consistent with the netite must therefore have formed after chondrule phe- ubiquitous occurrence of chromite [15] and essential ab- nocrysts crystallized [20,21]. There is an inverse corre- sence of magnetite [16] in equilibrated OC. Magnetite lation in CO3 chondrites between the abundances of with appreciable Cr2O3 is also more stable than Cr2O3- metallic Fe-Ni and magnetite: magnetite-rich CO chon- free magnetite under reducing conditions; i.e., Cr2O3- drites contain 1.2 vol.% metallic Fe-Ni, whereas me- rich magnetite behaves similarly to chromite. This is tallic-Fe-Ni-rich CO chondrites contain 0.6 vol.% consistent with the presence of magnetite with high magnetite. It is probable that magnetite generally Cr2O3 contents in equilibrated R and CK chondrites formed from metallic Fe during aqueous alteration on [12,13]. 51st Lunar and Planetary Science Conference (2020) 1092.pdf References: [1] Hyman M. and Rowe M. W. (1983) LPS, XIII, A736-A740. [2] Rubin A. E. et al. (2007) GCA, 71, 2361-2382. [3] Weisberg M. K. and Huber H. (2007) MPS, 42, 1495-1503. [4] McSween H. Y. (1997) GCA, 41, 1777-1790. [5] Weisberg M. K. et al. (1997) MPS, 32, 138-139. [6] Simon S. B. et al. (2019) LPS, L, abst. #1444. [7] Taylor G. J. et al. (1981) LPS, XII, 1076-1078. [8] Krot A. N. et al. (1997) GCA, 61, 219- 237. [9] Rubin A. E. and Kallemeyn G. W. (1994) Me- teoritics, 29, 255-264. [10] Hua X. and Buseck P. R. (1998) MPS, 33, A215-A220. [11] Rubin A. E. and Li Y. (2019) Chemie der Erde - Geochemistry, 79, 125528. [12] Mikouchi T. et al. (2007) LPS, XXXVIII, abst. #1928. [13] Geiger T. and Bischoff A. (1995) Planet. Space Sci., 43, 485-498. [14] Meteoritical Bulletin Da- tabase. [15] Keil K. (1962) JGR, 67, 4055-4061. [16] Rubin A. E. et al. (1988) In Meteorites and the Early Solar System (ed. Kerridge J. F. and Matthews M. S.), 488-511. [17] Keil K. (1968) JGR, 73, 6945-6976. [18] Doyle P. M. et al. (2015) Nat. Commun., 6, 74444 DOI: 10.1038. [19] Davidson J. et al. (2014) LPS, XLV, abst. # 1384. [20] Choi B.-G. et al. (1997) EPSL, 146, 337- 349. [21] Choi B.-G. et al. (1998) Nature, 392, 577-579. [22] Zolensky M. E. et al. (1997) GCA, 61, 5099-5115. [23] Zolensky M. E. and Ivanov A. (2003) Chemie der Erde, 63, 183-246. [24] Zolotov M. Yu. et al. (2006) MPS, 41, 1775-1796. .
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