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Abstract
On both the Canadian and the Fennoscandian Shields, large discharges of CH$\sb4$, N$\sb2$, higher hydrocarbons and He have been documented in association with saline fluids and brines. The range of compositional and isotopic variation on both Shields is very similar. CH$\sb4$ and N$\sb2$ are the two major components, accounting for 80-90% of the total gas volume. Isotopically, the majority of the Shield methanes fall outside the isotopic ranges developed for conventional bacterial and thermogenic gas deposits.
While a bacterial origin is implied for CH$\sb4$ from one mine in Finland, and a component of such a bacterial gas is possible at a number of other Shield sites, attempts to interpret the majority of the Shield gases with respect to conventional theories of origin are unsuccessful. The inability of conventional bacterial or thermogenic theories of origin to account for the distinct isotopic and compositional features of these Shield gases has led to an examination of possible mechanisms of abiogenic gas synthesis in the crystalline environment.
Thermodynamic studies predict a wide variety of possible inorganic CH$\sb4$-producing reactions from a number of potential carbon- and hydrogen-bearing species under typical crustal conditions. There is no conclusive evidence of carbon isotope equilibrium between CH$\sb4$ and any identified carbon-bearing species (DIC, fracture minerals), or conjectured carbon source (mantle CO$\sb2)$ on the Shields. However, the participation of any of these species in kinetically-controlled processes of CH$\sb4$ formation cannot be ruled out.
Potential CH$\sb4$ and higher hydrocarbon formation mechanisms include Fischer-Tropson synthesis, metamorphism of carbonates, and CH$\sb4$ formation from graphite. However, low temperature serpentinization processes in particular may play an important role in CH$\sb4$ and H$\sb2$ production at a number of sites.
A correlation of $\delta\sp{15}$N$\sb{\rm (N2)}$ and $\delta$D$\sb{\rm (CH4)}$ values on both Shields lends further support to the theory of crystalline gas production via inorganic temperature-dependent processes. The observed trend is consistent with production of the most $\delta$D-enriched methanes ($-$200 to $-$350$\perthous),$ and the most $\delta\sp{15}$-depleted N$\sb2$ (+1$-$2$\perthous)$ under relatively low temperature conditions, possibly related to serpentinization processes. Progressive $\delta$D$\sb{\rm (CH4)}$ depletion and $\delta\sp{15}$N$\sb{\rm (N2)}$ enrichment appears to occur with increasing metamorphic grade and temperature. The progressive $\delta$D$\sb{\rm (CH4)}$ depletion is consistent with higher temperature re-equilibration of CH$\sb4$ with the characteristically more $\delta$D-depleted H$\sb2$ gas. (Abstract shortened by UMI.)
