Evaluation of Volume of Deep Mantle Outgassing during Rifting. Final Report,September 1987-February 1989

摘要

It has been proposed that the major source of natural gas accumulations isinorganic methane outgassed from the mantle. However, for mantle methane to be incorporated into explorable hydrocarbon reservoirs, it must be transported upwards to the near-surface. Volatiles are strongly partitioned into melt phases and can be carried to the near surface by melts. Extensional rifts are sites of mantle partial melting, and thus perspective sites for outflow of mantle methane. The large-scale breaching of the lithosphere that occurs during rifting is a primary conduit for flow of deep mantle volatiles. Non-extension structures that do not breach the lithosphere and are non-volcanic, such as the Siljan ring, are unlikely to contain significant methane. However, not all continental extension leads to mantle melting. Rifts, such as the hydrocarbon-rich Gulf of Suez, that have had no significant mantle melting are unlikely to contain abiogenic hydrocarbons. To get significant mantle melt and volatile transport requires large extensional strain in localized areas, creating asthenospheric upwelling sufficient to induce decompression melting. There could be and probably are large volumes of methane liberated from some continental rifts where there has been melting.