Exploratory Studies on Deep Earth Sources of Methane: Hg as a Diagnostic and Prospecting Tool. Annual Report April-September 1985

摘要

The report attempts to establish whether primordial volatiles are presently being vented to the surface of the earth. The objective is to examine the proposition that the mantle is a major repository for juvenile (abiotic) methane. The author seeks to complement data on excess 3He, the only volatile generally accepted as being of mantle origin. The gases (fluids) that serve as carriers for 3He might also be expected to transport volcanogenic trace volatiles such as Hg, Br, As, Sb, Se, Zn and Cu. These elements, representing a range of volatilities and geochemistries, are being measured in tholeiitic glass (quenched lavas) samples from different ocean ridge and ocean island mantle sources, for which 3He data are available. A number of possibly significant concentration trends and interelement correlations are observed. As examples, tholeiitic magmas from diverse sources contain unfractionated Hg and Sb and covariant Zn and Cu (the latter pair relative to cosmic abundance). A primitive source is inferred for all the volatiles and their interrelation may complement the 3He mantle signature. Preliminary data on Hg associated with hydrocarbon-containing igneous rock formations appear to support the thesis that Hg may accompany hydrocarbons from non-biogenic sources and thus may be used for prospecting.