New Interpretations of Geothermal Fluid Inclusion Volatiles: Ar/he and N2/ar Ratios - a Better Indicator of Magmatic Volatiles, and Equilibrium Gas Geothermometry

N2/Ar and Ar/He relationships: Fluid inclusion magmatic gases are measured using quadrupole mass spectrometers and the data is compared to measurements of volcanic gas analyses. Data reported by Giggenbach from active volcanoes give N2/Ar ratios in the general range of 100 to 1000 whereas corresponding Ar/He ratios vary from 100 to 2. Plotted on a Ar/He vs. N2/Ar diagram the data defines a field that has a negative slope. Fluid inclusion gases from Mt. Erebus anorthoclase and analyses of magmatic gas-filled inclusions provided by Jake Lowenstern also plot within the same field. Thus we considered the Ar/He-N2/Ar field outline by volcanic gases and magmatic inclusion analyses to represent magmatic gases. The trend in the data is attributed to the stronger partitioning of Ar into the gas phase and as a magmatic system evolves; thus Ar becomes more rapidly depleted from the melt than N2 and He. The advantage of Ar/He vs. N2/Ar plots is that they discriminate between fluids that have magmatic N2/Ar ratios from admixed organic N2. Hansonburg MVT (Mississippi Valley Type) deposit fluid inclusions have Ar/He ratios of 0.12 to 2.69 and N2/Ar ratios ranging from 69 to 182, and the ratios >100 might suggest a magmatic component. However, MVT deposits generally have some inclusions filled with hydrocarbon compounds, as does the Hansonberg deposit (ref), and MVT ore fluids are considered to be amagmatic and sedimentary in origin (ref). When plotted on a Ar/He-N2/Ar plot Hansonberg gas analyses plot outside the magmatic box. The same is true for Coso analyses that have some N2/Ar greater than 1000. One third of the gas data analyses from Broadlands plot within the magmatic box. The Geysers analyses plot both in and below the magmatic box and can be explained in terms of mixing between magmatic and a crustal component. Geothermometry: With the improved capability to make accurate fluid inclusion H2 analyses we have applied the following equilibrium gas geothermometers to our fluid inclusion gas analyses. C + 2H2O = 2H2 + CO2 ..........(1) 2H2O + CH4 = CO2 + 4H2 ..........(2) The assumptions for fluid inclusion gas geothermometry are equilibrium conditions at the time of trapping, a single fluid, no boiling occurred, and that species have not reacted nor were lost after trapping. Gas geothermometry based on equation 1 applied to Hansonburg fluorite gives temperatures of 157 to 308 oC average 245±47 oC 1σ whereas the fluid inclusion Th’s range from 147 to 229 oC and average 182±20 oC 1σ. Hansonburg inclusions show no evidence of boiling, however, necking is common and gas data suggest two fluids. The Th distribution is attributed in part to inclusion necking. Snowbird pegmatite quartz inclusions show no evidence of boiling. Equilibrium gas geothermometry values ranging from 384 to 463 oC (2) and average 437±25 oC 1σ. Similarly, values range from 350 to 421 oC (1) and average 395±23 oC 1σ, whereas Tt is estimated at 420 oC from microthermometry. A Karaha quartz analysis from drill hole T-8, 794.7m give temperatures of 270 (1) and 293 (2). There is no thermometric data on this sample, however Th values for other quartz samples range from 280 to 330 oC. In conclusion fluid inclusion gas geothermometry can be applied with caution provided that H2 analyses are accurate, fluid inclusion necking is absent, and there is no indication of fluid boiling or trapping multiple fluids. This paper is subdivided into two parts, one that addresses the use of N2-Ar-He gas ratios in identifying a field common to magmatic sources. The second aspect deals with the application of gas geothermometry to fluid inclusion gas analysis. Both subjects being addressed require data collected by analyzing the gases trapped within fluid inclusions.