Lunar Meteorite Dhofar 026: a Second-generation Impact Melt

Introduction: The meteorite Dhofar 026 (Dh26) was found in Oman in January, 2000 [1]. It is a clastpoor, anorthositic, crystalline melt breccia with no fusion crust and little terrestrial weathering. Solar wind abundance measurements show that it is unpaired with the lunar regolith breccia Dhofar 025 [2]. The Dh26 oxygen isotopic composition indicates its lunar origin [3], while its majorand trace-element bulk composition [3] shows that a FAN component is a major contributor (bulk An98 and Mg#=63), producing a prominent positive Eu anomaly (Sm/Eu = 1.04). We have obtained petrography and EMP analyses to characterize this meteorite. The main mass represents a crystalline impact melt, derived from a mixture of FAN and a component similar to the crystalline lunar spherules (CLS). The spherule-like objects, in turn, are probably impact melts of a mixture containing a highly fractionated, basaltic precursor. Petrography: The groundmass consists of blebs of olivine up to 10's of μm's in diameter set into multicrystalline plagioclase. "Spherules," made up of acicular plagioclase with interstitial pyroxene and olivine, have well-defined borders with the groundmass. These spherules are usually ~100 μm in diameter and ovoid, but the largest exceed 250 μm in diameter. Spherules make up <0.5% of the meteorite by volume. Blotchy areas seem to be intermediate between the groundmass and spherules. Blotches are composed of olivine and pyroxene with a similar, though degraded, texture as the inside of the spherules. These blotchy areas have no characteristic size and grade into the groundmass with no definite borders. Approximately 50% of Dh26 by volume has this blotchy texture, though this is difficult to estimate because of the indefinite borders. Minor chromite, ilmenite, troilite, and apatite occur as small (<20 μm), isolated blebs within the poikilitic groundmass. Mineralogy: Poikilitic groundmass. Olivines are unzoned and vary little among blebs within the narrow range Fo61-64 (Fig. 2b). Groundmass feldspar is An95-99; embedded single-crystal clasts of feldspar are slightly more calcic at An96-99 (Fig. 2c). Ilmenites have extremely low Mg# (6-8). Spinels plot along the chromite-ulvöspinel join though they are too small to determine possible chemical zonation. Troilite is common and often associated with FeNi metal. The largest metal nuggets are Ni-rich (15-27 wt% Ni; 1.0-1.3 wt% Co). Minute high-Ni metal blebs are often included in troilite and olivine grains but are too small for analyses. Apatite is chlorine-rich. Spherules. Spherules are composed of acicular plagioclase with a lower An content (An93-95) than their surroundings (Fig. 3). Olivines (Fo52-81; Fig 2) in the spherules vary in composition within a single spherule and show a bimodal distribution. Pyroxenes within a single spherule can range from augite to very iron-rich compositions (Fig. 2). Spherules are surrounded by a thin rim of "clean" plagioclase, separating them from blotchy areas. Most spherules have tiny void spaces but some have much larger spaces as well (Fig. 1b shows both small and large spaces). Void space is usually filled with Si. It is unclear at this point whether these Si-filled spaces are artifacts of preparation. Blotchy areas. The plagioclase (An95-98) in these areas is indistinguishable from the poikilitic groundmass (Fig 3). The composition and distribution of olivines (Fo70-80) and pyroxenes, on the other hand, is indistinguishable from the spherules (Fig. 2)