A Geologic Si‐O‐C Pathway to Incorporate Carbon in Silicates

Abstract

Geologic and planetary processes are punctuated by sudden cataclysmic events, and planetary evolution is irrevocably changed by impacts and intense seismic and magmatic/volcanic activity. Such events often are associated with or generate high temperature, high pressure, and low oxygen fugacity. Their traces in the accessible geologic record are not pristine but altered by subsequent petrologic reactions. Evidence from the thermochemistry of synthetic materials, largely studied in a materials science context, in Si-O-C and M-Si-O-C-H systems under reducing conditions can be used to propose some possible rare but significant reactions, together called a geologic Si-O-C pathway, involving carbon-containing silicate melts, glasses, and amorphous materials. The substitution of carbon for oxygen in the first coordination shell of silicon provides a reducing local environment for the formation of metals, carbides, and silicides. Grains of these refractory compounds may persist long after the main carbon-containing silicate phase has transformed and disappeared. Such relict refractory materials may be markers of impact events and unusual volcanism. Anomalies in minor phases, trace elements, and textures in settings ranging from ultra-high pressure metamorphic rocks to impact craters to carbonado diamonds may be linked to the transient presence of carbon-rich silicate phases generated under reducing conditions from initially carbon-rich target rocks and/or impactors.