Heavily metamorphosed clasts from the CV chondrite breccias Mokoia and Yamato-86009

Abstract

Metamorphosed clasts in the CV carbonaceous chondrite breccias Mokoia and Yamato-86009 (Y-86009) are coarse-grained, granular, polymineralic rocks composed of Ca-bearing (up to 0.6 wt% CaO) ferroan olivine (Fa34-39), ferroan Al-diopside (Fs9-13Wo47-50, approximately 2-7 wt% Al2O3), plagioclase (An37-84Ab63-17), Cr-spinel (Cr/(Cr+Al)= 0.19-0.45, Fe/(Fe+Mg)=0.60-0.79), nepheline, pyrrhotite, pentlandite, Ca-phosphate, and rare grains of Ni-rich taenite; low-Ca pyroxene is absent. Most clasts have triple junctions between silicate grains, indicative of prolonged thermal annealing. Based on the olivine-spinel and pyroxene thermometry, the estimated metamorphic temperature recorded by the clasts is approximately 1100K. Few clasts experienced thermal metamorphism to a lower degree and preserved chondrule-like textures. The Mokoia and Y-86009 clasts are mineralogically unique and different from metamorphosed chondrites of known groups (H, L, LL, R, EH, EL, CO, CK) and primitive achondrites (acapulcoites, brachinites, lodranites). On a three-isotope oxygen diagram, compositions of olivine in the clasts plot along carbonaceous chondrite anhydrous mineral line and the Allende mass-fractionation line, and overlap with those of the CV chondrule olivines; the Δ17O values of the clasts range from about -4.3‰ to -3.0‰. We suggest that the clasts represent fragments of the CV-like material that experienced metasomatic alteration, high-temperature metamorphism, and possibly melting in the interior of the CV parent asteroid. The lack of low-Ca pyroxene in the clasts could be due to its replacement by ferroan olivine during iron-alkali metasomatic alteration or by high-Ca ferroan pyroxene during melting under oxidizing conditions. © 2013 The Meteoritical Society.