PACROFI VI - Electronic Program

Fluid inclusions in calcite from the Yucca Mountain exploratory tunnel

Dublyansky, Yuri, Vadim Reutsky and Nina Shugurova

Institute of Mineralogy and Petrology, Russian Academy of Sciences, Siberian Division, 3, University Avenue, Novosibirsk, 630090, RUSSIA

The calcite filling the fractures within unsaturated zone of Yucca Mountain is often interpreted as being precipitated from rainwaters that have descended along interconnected fractures carrying dissolved carbonate from the overlying soil environment (e.g., Roedder et al., 1994). This interpretation, however, is at odds with a large body of the data: fluid inclusions, ESR, 18O in calcite-opal pairs, gradients 18O/ z in calcites from drill holes (Hill et al., 1995) implying elevated temperatures and geothermal gradients in the Yucca Mountain subsurface in the past.

Previous fluid inclusion studies at Yucca Mountains. Only 7 reliable homogenization temperatures have been obtained by calcite from drill holes (YMP, 1993; depths from 178 to 386 m). The data imply paleothermal gradients of 150-160oC/km. Inclusions, suitable for instrumental studies were observed by Roedder et al. (1994); no numerical data have been reported, however.

Fluid inclusions in calcite from exploratory tunnel
We have observed four major types of inclusions: (1) all-liquid; (2) gas-liquid with low vapor-to-liquid ratio (~0.1 and less); (3) gas-rich with variable, but generally high gas-to-liquid ratio (~0.4 to 0.8); (4) all-gas (they may contain some water, though it is not always distinguishable optically).

All-liquid and gas-liquid inclusions. Morphologically similar, often occur as clusters, and may represent the same fluid. Normally, only one out of 150-200 inclusions contains bubble. Most homogenization temperatures measured in these inclusions fell in the range from 35 to 55oC (Fig. 1). Extremely low and consistent homogenization temperatures provide strong evidence against stretching or leakage.

All but three inclusions subjected to the freezing experiments (39) contained diluted water, NaCl, Na2SO4, and MgSO4 were identified in several inclusions. It should be noted, that the term 'diluted water' depicts water with concentration of less than approximately 0.5-1.0 wt% (at such concentrations freezing rarely yields reliable data due to metastability problems). These concentrations, however, lie well within the "hydrogeological" range of salinities (1 wt% = 10 g/l).

Three all-liquid inclusions from one sample yielded concentration of 11 wt% of MgCl2. The origin of these fluids is unclear; more data are to be acquired.

Fig. 1 Homogenization temperatures measured in calcites from exploratory tunnel.

Gas-rich and all-gas inclusions. Such inclusions are more rare. They are larger than typical all-liquid and gas-liquid inclusions and restricted to the earliest generations of calcite (near the contact with tuff). On crushing, the bubbles of the three gas-rich inclusions decreased in volume by factor of 30 to 120; hence, they could not have been trapped at atmospheric pressure characteristic of the vadose setting. Neither the gas composition of the bubbles: 80-84 vol% hydrocarbons (CH4?), 16-18 vol. % N2 + noble gases, traces of CO2, and nil O2 may be attributed to the vadose-zone underground atmosphere (the semi-quantitative data were obtained by selective absorption volumetric analysis on individual bubbles). Four all-gas inclusions decreased by factor of 20 to 27 on crushing. Their composition was similar to those of "collapsing" bubbles from gas-rich inclusions, except three of them contained more CO2 (10 to 15 vol.%). Another one all-gas inclusion showed slight (1.8) increase on crushing maintaining gas chemistry typical of the gas-rich inclusions (CH4 > N2). Presence of CH4 (1 to 5 mg per kg of the mineral), as well as traces of heavy and light hydrocarbons was confined by means of bulk chromatographic analysis.

The gas-rich and all-gas inclusions were trapped from a fluid carrying both dissolved and gaseous methane. An inclusion trapped as one-phase liquid followed the isochore and, once it was cooled enough, the "normal" shrinkage bubble appeared (as the fluid sealed in the inclusion contained dissolved methane, the latter appeared in shrinkage bubble). The pressure in such inclusion is less than 1 atmosphere; thus, the bubble decreases on crushing. An inclusion trapped as two-phase gas + liquid fluid (heterogeneous entrapment) will pressure corresponding to the entrapment pressure. Increase of volume of bubbles on crushing indicates that the latter was higher than 1 atmosphere.

Environment of formation evidenced by fluid inclusion data
The fluid inclusion homogenization temperatures obtained in this study imply that calcites from exploratory tunnel were deposited from epithermal fluids. The new data are in good agreement with paleogeothermal gradient inferred from earlier fluid inclusion results (Fig. 2).

Fig. 2 Fluid inclusion and ambient temperatures in the Yucca Mountain subsurface. Circles - by YMP, 1993; diamonds - by Sass et al., 1987

Large bubbles, occupying 40-80% of vacuoles have been interpreted earlier as the "air-water-CO2 phrase" (Roedder et al., 1994) trapped during the growth of calcite from films of percolating rainwater in vadose zone. Our results do not corroborate this interpretation. Instead, they imply heterogeneous entrapment of methane-rich water fluids in saturated zone.

The data obtained in this study are not compatible with the vadose-zone setting. Calcite encountered in association with zeolite, quartz, and opal in the Yucca Mountain exploration tunnel was formed within a low-temperature hydrothermal (epithermal) system.