Mineralogical characterization of primitive, type-3 lithologies in Rumuruti chondrites

Rumuruti (R) chondrites constitute a new, well-established chondrite group different from the carbonaceous, ordinary, and enstatite chondrites. Many of these samples are gas-rich regolith breccias showing the typical light-dark structure and consist of abundant fragments of various parent-body lithologies embedded in a fine-grained olivine-rich matrix. Unequilibrated type-3 lithologies among these fragments have frequently been mentioned in various publications. In this study, detailed mineralogical data on seven primitive fragments from the R-chondrites Dar al Gani 013 and Hughes 030 are presented. The fragments range from similar to 300 mu m in size up to several millimeters. Generally, the main characteristics can be summarized as follows: (1) Unequilibrated type-3 fragments have a well-preserved chondritic texture with a chondrule-to-matrix ratio of similar to 1:1. Chondrules and chondrule fragments are embedded in a fine-grained olivine-rich matrix. Thus, the texture is quite similar to that of type-3 carbonaceous chondrites. (2) In all cases, matrix olivines in type-3 fragments have a significantly higher Fa content (44-57 mol%) than olivines in other (equilibrated) lithologies (38-40 mol% Fa). (3) Olivines and pyroxenes occurring within chondrules or as fragments are highly variable in composition (Fa(0-65) and Fs(0-33), respectively) and, generally, more magnesian than those found in equilibrated R chondrites. Agglomerated material of the R-chondrite parent body (or bodies) was highly unequilibrated. It is suggested that the material that accreted to form the parent body consisted of chondrules and chondrule fragments, mainly having Mg-rich silicate constituents, and Fe-rich highly oxidized fine-grained materials. The dominating phase of this fine-grained material may have been Fa-rich olivine from the beginning. The brecciated whole rocks, the R-chondrite regolith breccias, were not significantly reheated subsequent to brecciation or during lithification, as indicated by negligible degree of equilibration between matrix components and Mg-rich olivines and pyroxenes in primitive type-3 fragments.