Petrology and oxygen-isotope geochemistry of the Yamba Lake kimberlite rocks, NWT

The Torrie, Sputnik, and Eddie kimberlite rocks, located near Yamba Lake, c entral Slave province, N.W.T., are volcaniclastic, macrocrystic, heterolith ic, olivine-rich tuff, and olivine-rich tuff breccia. Torrie and Sputnik ki mberlite rocks contain pyroxene and garnet xenocrysts and megacrysts with m ajor-element compositions consistent with derivation mostly from disaggrega ted garnet lherzolite, with subordinate contributions from eclogite, spinel lherzolite, garnet harzburgite, and websterite. The presence of primary gr oundmass phlogopite and compositionally evolved spinel, and the absence of mantle xenocrysts, xenoliths, and megacrystic ilmenite distinguish the Eddi e kimberlite pipe from the other two kimberlite pipes. Large variations in delta(18)O of garnet and clinopyroxene in xenocrysts and xenoliths (+3.98 t o +6.36 parts per thousand), nonequilibrium intermineral isotopic fractiona tion, and major-element heterogeneity are interpreted as resulting from inf iltration of fluids or melts produced by dehydration or melting of subducte d oceanic crust into overlying peridotite. Although the timing is unconstra ined for the xenocysts, the xenolith must have experienced this metasomatic interaction shortly before entrainment in the kimberlite. Variable delta(1 8)O values for magnesian ilmenite are also interpreted to result indirectly from such metasomatic activity in the mantle as well. The Torrie and Sputn ik kimberlite rocks have low concentrations of diamond indicator minerals c onsistent with their low-diamond grades. These kimberlite rocks did not sam ple a significant amount of garnet harzburgite, the rock type commonly asso ciated with high-diamond grades in other kimberlite rocks. Furthermore, met asomatism just prior to kimberlite eruption may have caused the resorption of any diamond present.