51st Lunar and Planetary Science Conference (2020) 1092.pdf

OCCURRENCE, FORMATION AND DESTRUCTION OF IN CHONDRITIC . 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 . CO3.1 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 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 [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 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 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 , haxonite, kamacite, troilite, 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, , 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 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

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