PACROFI VI - Electronic Program


Paleobarometry Using Low-Temperature Heterogeneously Entrapped Fluid Inclusions in Quartz Cement, Upper Ordovician Viola Formation, Kansas, USA

K.D. Newell1 and Robert H. Goldstein2

  1. Kansas Geological Survey, University of Kansas, Lawrence, KS 66047
  2. University of Kansas, Department of Geology, 120 Lindley Hall, Lawrence KS 66045


Coarsely crystalline euhedral quartz represents first-stage mineralization in a vug (1187 m depth) in shelfal carbonate strata of the Upper Ordovician Viola Formation in the Sedgwick basin of Kansas (Fig. 1). The vug containing the quartz occurred after an episode of regional dolomitization because dolomite rhombs in the rock matrix along the side of the vug are truncated and corroded (Newell, 1996). Succeeding minerals filling the vug include coarsely crystalline saddle dolomite and calcite.

The quartz contains primary fluid inclusions having characteristics of heterogeneous entrapment (i.e., highly variable liquid-to-vapor ratios, all-liquid inclusions, and numerous vapor-rich inclusions). The variable liquid-to-vapor ratios could not have resulted from necking-down of high-temperature fluid inclusions because many fluid inclusions contain greater than 15 volume % gas. The heterogeneously entrapped fluid inclusions are concentrated in four separate growth bands (fluid-inclusion assemblages) in the quartz. The first and third growth bands are relatively thick and contain abundant fluid inclusions and solid inclusions of acicular anhydrite. The second and fourth growth bands are relatively thin and free of anhydrite, but have fluid inclusions in thin zones oriented parallel to the crystal faces. Secondary fluid inclusions, mostly elongate ones in healed microfractures, cut across primary growth bands.

Chips of the quartz were removed from the thin-section glass and placed in a crushing stage. Crushing runs used glycerine or kerosene as immersion media. Two types of behavior were observed, with bubbles either expanding or contracting. The contracting bubbles are associated with the late-stage secondary fluid inclusions, and probably represent entrapment of a gas-deficient liquid at elevated temperature. The inclusions with expanding bubbles are from the primary fluid inclusions. Using Boyle's Law, the calculated pressures range from 1.3 to 11.4 atm., with a dominant mode at 1.5 to 2 atm. and a minor mode at 6.5 to 7 atm.. Released bubbles dissolve within seconds in kerosene, thus methane or other organic gases are indicated.

Because all-liquid inclusions are present within fluid-inclusion assemblages, entrapment was at low temperature. Thus, the pressure determined from the primary fluid inclusions is approximately equal to entrapment pressure with a minor correction for amount of dissolved gas (see Goldstein and Reynolds, 1994). The water column necessary for the highest pressure is approximately 110 m. The mode at 1.5 to 2 atm. corresponds to water-column depths of 10 to 20 m; the mode at 6.5 to 7 atm. corresponds to water-column depths of 60 to 70 m, depending on water salinity.

The paleobarometric data (Fig. 2) from the heterogeneously entrapped fluid inclusions indicate precipitation of the quartz occurred at shallow depths. Regional stratigraphic thicknesses of overlying strata indicate that the Viola Formation was buried to an approximate depth of 100 m by the end of Silurian time. Burial did not exceed 225 m before the end of Mississippian time. In Late Mississippian-Early Pennsylvanian time, the Viola was buried to its approximate present-day depth. Shallow hydrostatic depths associated with the mode at 1.5 to 2 atm. may represent diagenetic conditions when the water table was relatively deep, perhaps associated with arid periods during the Silurian. Evaporitic conditions are supported by the presence of the anhydrite in the quartz and fluid inclusions registering salinities up to 23 wt.% NaCl equivalent. Occasional fluxes of nearly fresh water (i.e., freezing-point depression of 0.1oC) are recorded by the second growth band within the quartz and may reflect fluids associated with subaerial exposure and unconformity development up-section.

The vug formation, its early quartz mineralization, and probably the dolomitization prior to the vug formation, occurred within influence of the near-surface during arid periods of the Silurian and during fresh-water influx during development of unconformities.




References