More on the Use of Fluid Inclusion Gaseous Species as Tracers in Geothermal Systems

Abstract

This paper continues previous efforts to develop the science of fluid inclusion gas analysis. The principal objective is to generate and refine interpretations of fluid source and hydrothermal process from analyses of fluid inclusions votatiles.

A good knowledge of magmatic gas compositions is necessary in order to identify magmatic volatiles in fluid inclusions. Ratios N2, Ar, and He in Magma were measured in magmatic glass inclusions. Heating inclusions to glass melting temperatures yields reproducible measurements that cluster tightly on ratio diagrams. Analyses by cold crushing were not reproducible.

Measured ratios of N2 to Ar in some inclusions approximate that of air and show a larger range of ratios from analysis to analysis than is possible to obtain from meteoric water. Contamination by air was suspected. However, modeling shows that these values are to be expected in vapor and liquid evolved from boiling meteroric waters. Tiwi inclusions serve as an example. They are mixtures of vapor dominated and liquid dominated inclusions trapped during fluid boiling. The N2/Ar ratios for most inclusions range from 30 to 105, which are the ratios expected in vapor and residual liquid when meteoric water is boiled. Overall most Tiwi fluid inclusion gas compositions are consistent with a boiling meteoric fluid that has reacted with country rock accumulating organic compounds.