Combustion-metamorphic glasses

Abstract

During spontaneous subsurface combustion of sediments rich in organic matter, such as oil shales, temperatures high enough to cause partial or total melting of the parent rocks can be reached. Total rock melts crystallize usually during cooling to form pseudo-magnetic rocks. An exception are those melts formed from phosphorites at temperatures above 1650°C which cool to phosphatic glasses. Partial melts, however, because of their smaller volume and lower initial temperature of about 1000°C, generally cool quickly enough to be preserved as glasses; their high silica content also helps to prevent crystallization. When only small amounts of these melts are formed, they cool in situ and form irregular pods within combustion-metamorphosed unmelted parent rocks; in larger volumes they behave like magmas, and intrude the country rocks to form sills or dikes. In almost all physical, chemical and textural properties these low-temperature combustion glasses are indistinguishable from obsidians. They are black, more rarely brown or red, splintery, have a high glassy (more rarely waxy) luster and frequently exhibit flow textures. The chemical composition of the best studied group of combustion glasses, those of the Grimes Canyon area in Southern California, is that of rhyolites and rhyodacites, from which they differ only in their oxygen isotope composition which is that of the sedimentary parent rocks. All combustion glasses so far studied are of Pleistocene age.