Joseph N. Moore1 and David I. Norman2
The gases were released from the fluid inclusions by either thermal decrepitation at 300 to 500oC or by crushing. Gases found in the inclusions include N2, He, H2, Ar, Ne, CO2, CO, CH4, H2S, SO2, and hydrocarbons (C2-C7). A capacitance manometer was used to measure the gas contents. Water was frozen in a capillary tube and determined by weighing or by pressure measurement. Details of the methodology are described by Norman and Sawkins (1987).
The Broadlands-Ohaaki geothermal system is located in the Taupo Volcanic Zone on New Zealand's North Island. Within the geothermal field, a thick sequence of rhyolitic pyroclasitic deposits overlies a faulted basement dominated by marine graywacke and argillite.
The compositions of fluid-inclusion gases from a single quartz crystal from well BR-12 were determined and are shown in Figure 1. The analyses are characterized by high total gas contents (to about 23 mole percent), a relatively wide range of N2/Ar ratios that exceed values found in air-saturated water, and variable N2/CH4 and CO2/CH4 ratios (Fig. 1a). A strong correlation exists between CH4 and C2-7 organic species but there is little correlation between N2 and the organic compounds. Thus, the analyses suggest that the inclusion fluids were deeply circulating waters that interacted with carbonaceous basement rocks and accumulated upward-fluxing magmatic volatiles. Helium isotope compositions of the modern geothermal fluids provide independent evidence for the presence of a magmatic component (Giggenbach, 1986).
Fig. 1. Fluid-inclusion gas compositions from a single crystal of quartz from Broadlands-Ohaaki, New Zealand. The gases were liberated by crushing. a) a N2- Ar-CH4 diagram. Gas ratios of air, air-saturated water, and magmatic and crustal fluids are also shown. b) a N2-CO2-CH4 diagram. The solid line shows the compositions of coexisting liquid and vapor produced by a boiling fluid at 260oC.
The gas contents of some of the analyses are unreasonably high if it is assumed that only a single phase liquid was present when the inclusion fluids were trapped. Thus, these analyses must reflect a contribution from vapor-rich inclusions. The effects of boiling at 260oC on the N2-CH4-CO2 gas ratios of coexisting liquid and vapor are shown in Figure 1b. The results indicate that boiling can account for only a small proportion of the variation in these gas species.
Tiwi is a large geothermal system associated with an andesitic volcano on Luzon, Philippines. Fluid inclusions were studied from Matalibong-25, a 2439 m core hole drilled within the upwelling center of the system. The hole is dominated by basalt and basaltic andesite lavas, flow breccias, and volcaniclastic rocks deposited in a subaerial environment above a depth of 2012 m, and marine andesitic sandstones at greater depths. Homogenization temperatures of fluid inclusions in vein quartz, calcite, and anhydrite ranged from 195 to 325oC, although most were above 240 to 250oC at depths below 1100 m. Salinities are less than 1.7 weight percent NaCl equivalent within the volcanic sequence, but are similar to sea-water (greater than 2.5 weight percent NaCl equivalent) in the underlying marine sediments.
Figure 2 illustrates the N2-Ar-CH4 ratios of fluid inclusions from depths between 1113 and 2428 m. In contrast to analyses from Broadlands-Ohakki, Tiwi samples have N2/Ar ratios similar to air-saturated water. These data suggest that the Tiwi fluids are dominantly meteoric waters that have reacted to varying degrees with the basement rocks. Only a few of the analyses are characterized by enrichments in N2. These samples may contain magmatic fluids that preferentially utilized some of the fracture zones cut by the well.
Fig. 2. N2-Ar-CH4 diagram of fluid-inclusion gases from Tiwi, Philippines.
The Geysers provides an example of a long-lived hydrothermal system intimately associated with the intrusion of a hypabyssal granitic pluton. Fluid-inclusion, mineralogic, and isotope data show that the present vapor-dominated conditions evolved from a larger, liquid-dominated system that developed mainly in Franciscan metagraywacke and in the underlying intrusives. Pressure-corrected homogenization temperatures and apparent salinities of liquid-rich fluid inclusions range from 440oC and 44 weight percent NaCl equivalent at distances of <600 m from the pluton to 325oC and 5 weight percent NaCl equivalent at distances of approximately 1500 m from the intrusion. The apparent salinities of the shallow fluids suggest that the bulk of the hydrothermal system was dominated by connate waters modified by water-rock interactions while the high-salinity fluids are interpreted as magmatic brines. Meteoric waters appear to be represented by low-salinity inclusion fluids with homogenization temperatures of less than 200oC. Widespread boiling apparently occurred as temperatures within the initial hydrothermal system dropped to about 265oC. Present-day temperatures are close to 240oC in the main steam reservoir, but can reach as high as 342oC in the highest-temperature zones in the northwest portion of the field.
The N2-Ar-CH4 ratios of fluid-inclusion gases and present-day steam from The Geysers are shown in Figure 3. For comparison, analyses from several metamorphic veins located outside the thermal system are also shown. The gases were released by thermal decrepitation at temperatures of 400 to 500oC. Inclusions from The Geysers and metamorphic veins, like those from Tiwi, are dominated by gases derived from meteoric and crustal sources. Similar gas ratios characterize the modern steam.
Fig. 3. Fluid-inclusion analyses from The Geysers, California.
Two of the samples containing inclusions with hypersaline brines yielded high N2/Ar ratios, consistent with a magmatic origin, but variable CH4 contents. These differences could be due to either variations in the CH4 composition of the initial magmatic fluid or to the subsequent trapping of crustal-derived volatiles. The third sample was enriched in Ar, reflecting the incursion and trapping of meteoric waters. Thus, the fluid-inclusion data document the early development of a hydrothermal system dominated by crustal and magmatic fluids and the subsequent influx of meteoric waters. Boiling and the formation of the present vapor-dominated conditions may be reflected in the high gas contents of the analyses.
The results of these investigations demonstrate that fluid-inclusion gas analyses are powerful tools for tracing fluid sources and evaluating the paleohydrology of geothermal systems. Ratios of N2-Ar-CH4 allow the distinction between magmatic, crustal, and meteoric sources of gas. Gas distributions and the presence of excess gas can provide direct evidence of boiling, while variations in gas ratios not related to boiling may indicate mixing or the heterogeneous trapping of gas species from different sources.