The origin of the “eclogites” from Salt Lake Crater, Hawaii

Abstract

The chemistry and petrography of garnet pyroxenite (“eclogite”) inclusions in olivine nephelinite tuff at Salt Lake Crater, Hawaii, are examined in relation to recent experimental studies of melting and subsolidus relations in basaltic rocks at high pressure. It is concluded that the rocks originated as accumulates of liquidus or near-liquidus sub-calcic clinopyroxene derived from alkali olivine basalt or basanite at a pressure of approximately 13–18 kb. The original accumulate cooled from temperatures of 1350–1400°C to about 1000°C at constant pressure causing exsolution of garnet from the original highly aluminous pyroxene and reaction of spinel and pyroxenes to yield garnet and olivine. The P, T conditions of initial accumulation and later recrystallization of the garnet pyroxenites diverge widely from the P, T conditions responsible for the formation of natural eclogites occurring in metamorphic terranes or as inclusions in kimberlite pipes.