The Chaunskij anomalous mesosiderite: Petrology, chemistry, oxygen isotopes, classification, and origin

The Chaunskij meteorite, which was found in 1985 in northeastern Russia, co nsists of similar to 90 vol % metal and similar to 10% phosphate-silicate i nclusions and sulfide nodules. Metallographic structures of the metal are consistent with a typical mesosi derite cooling history. The metal is unusually rich in Ni and depleted in W , Ir and Pt, and has been subjected to higher shock levels than metal in ot her mesosiderites. The phosphate-silicate inclusions contain three distinct lithologies that t end to replace one another. The primary igneous lithology: preserved only i n the largest inclusions, consists of a fine-grained groundmass enclosing l arger mineral and rock fragments and metal-sulfide droplets. The groundmass is an ophitic-to-xenomorphic aggregate of orthopyroxene (Fs(29.5)Wo(0.4)) and plagioclase (An(90.3)) With subordinate amounts of silica and minor or accessory whitlockite. chromite, ilmenite, rutile, troilite, and metal. The groundmass also contains numerous granoblastic zones enriched in troilite and quartz and depleted in pyroxene, which we treat as a separate granoblas tic lithology. Abundant relict grains of pyroxene in both quartz and troili te indicate that granoblastic lithology has replaced the primary igneous li thology via the I reaction: Fe-rich Opx + S-2 --> Fe-poor Opx + FeS + Q + O-2. Truly metamorphic lithology which typically consists of a fine-grained horn felsic-to-granoblastic aggregate of cordierite (mg# = 85), quartz, pyroxene , troilite, and whitlockite, occurs as small inclusions among the primary i gneous lithology and among the host metal in larger inclusions. Tn the larg est inclusion studied, cordierite sometimes forms millimeter-sized poikilob lasts enclosing small euhedral-to-subhedral troilite and quartz crystals, p yroxene relicts, and, rarely, intergrowths of pyroxene and plagioclase. Ver y close to the interface with the primary pyroxene-plagioclase rock, cordie rite contains relicts of pyroxene and plagioclase, indicative of the substi tution of the metamorphic lithology for the igneous one according to the re action: Opx + Plag + P[ss in metal] + O-2 --> Cord + Qz + Whit. Thermodynamic analysis of the metamorphic reactions and mineral thermobarom etry indicate that these reactions have proceeded simultaneously at similar to6 kbar total pressure and temperatures of 600-950 degreesC in the deep i nterior of a large parent body. Bulk chemical compositions of phosphate-silicate inclusions ale close to eu crites and mesosiderites, except for a large enrichment in P and volatile c halcophiles in Chaunskij. The oxygen isotope composition of the primary ign eous lithology plots in the HED-Mes-Pal cluster; leaving no doubt as to the mesosideritic affinity of Chaunskij. The Chaunskij meteorite is classified as a type A mesosiderite, and is the most highly metamorphosed, shock-modified, and metal-rich example known.