Dark Inclusions in the Reduced CV3 Chondrite Efremovka: Evidence for Various Degrees of Aqueous Alteration and Thermal Metamorphism

We studied the petrography, mineralogy, and bulk oxygen isotopic compositions of three Efremovka dark inclusions. These DIs consist of chondrules, CAIs and mineral grains embedded in opaque matrices rich in metal; magnetite is absent and sulfides are either absent or rare. All coarse-grained components are replaced to various degrees by an Fe-rich phase with olivine composition, but rich in Al, Ca, Na, Cr, Mn, Na, and S. TEM studies show that the Fe-rich phase consists of abundant fine-grained (below 0.2 micron) fayalitic olivine with minor interstitial poorly-crystalline Si-Al-rich material and chlorite. The fayalitic olivines are heavily strained and contain abundant voids. We infer that Efremovka DIs experienced various degrees of aqueous alteration and were subsequently metamorphosed resulting in incomplete dehydration, growth of fayalitic olivine and secondary Ca-rich phases. These observations confirm the model of aqueous alteration and dehydration of the oxidized CV3s and their DIs.