PACROFI VI - Electronic Program

Permian Paleoclimate Data from Fluid Inclusions in Halite

Benison, K. C. and Robert H. Goldstein

University of Kansas, Department of Geology, 120 Lindley Hall, Lawrence, KS 66045

Temperatures of Permian surface waters have been determined by homogenization of originally one-phase primary fluid inclusions in depositional halite. This method requires nucleation of vapor bubbles in all-liquid inclusions by cooling. After bubble nucleation, temperatures of homogenization to liquid can be measured. This study has yielded surface water paleotemperatures from primary fluid inclusions in mid Permian Nippewalla Group halite from western Kansas.

Petrographic study indicates that this bedded halite formed in a shallow (centimeter-scale) subaqueous setting. Centimeter-scale vertically elongated chevron crystals represent halite precipitation at the bottom of a body of water. Millimeter-scale equant cornet crystals grew at the water-atmosphere interface and accumulated on the bottom. Thin beds of mudcracked microcrystalline salt crusts suggest desiccation. Thin mud partings, dissolution surfaces, and dissolution pipes filled with halite cement record flooding events. This halite was formed in shallow saline lakes and salt pans subjected to flooding, evaporative concentration, and desiccation.

Primary fluid inclusions are abundant and well-preserved in Nippewalla Group halite. Chevron and cornet growth bands are rich in cubic, negative crystal-shaped fluid inclusions, ranging in size from 1 to 100 microns in diameter. The majority of primary fluid inclusions are one phase aqueous inclusions at room temperature. Some inclusions also contain "accidental" anhydrite crystals. Rare primary fluid inclusions dominated by a gas phase may have trapped Permian air as halite crystals grew at the water-atmosphere interface.

Halite chevron and cornet crystals from nine stratigraphic levels in two cores were used. Both thin section fragments and uncut, unpolished cleavage chips were cooled to -10oC in order to nucleate vapor bubbles. Approximately 10% of the inclusions produced vapor bubbles upon cooling. Halite samples were warmed slowly on a heating/freezing stage from approximately -10oC until homogenization was observed. Heating runs were repeated in order to test reproducibility.

Homogenization temperatures measured in over 200 fluid inclusions range from 21 to 49oC (Fig. 1). Homogenization temperatures are relatively consistent (within approximately 5oC) within individual fluid inclusion assemblages (the finest, high-frequency fluid inclusion growth bands). These homogenization temperatures range up to 26oC from base to top of millimeter-thick chevron bands. Homogenization temperatures were consistent and reproducible in both smaller and larger fluid inclusions within single fluid inclusion assemblages. This indicates that fluid inclusions have not been altered by thermal reequilibration (Goldstein and Reynolds, 1994). Studies have shown that large halite inclusions tend to undergo alteration much more than small inclusions (Petrichenko, 1979; Roedder, 1984; Roedder and Belkin, 1980). The small range of homogenization temperatures from various sized fluid inclusions within the same fluid inclusion assemblage is strong evidence that this method is valid and that inclusions are unaltered.

The abundant primary fluid inclusions that remained one phase after cooling suggest that they were entrapped at even lower temperatures than those that nucleated bubbles. A less likely possibility is that these inclusions resist vapor bubble nucleation because of minor compositional variation.

We interpret the homogenization temperatures measured in this study to represent surface water paleotemperatures. The range in temperatures from base to top of thick chevron bands may reflect daily (or perhaps seasonal) temperature variations. These Permian surface water temperatures fall within the same range as some modern evaporative surface waters, suggesting that this Permian environment may have been relatively similar to its modern counterparts.

Surface water paleotemperatures are direct measurements of paleoclimate. Studies of modern evaporative environments show that surface water temperatures correlate closely with average air temperatures (Fig. 2; Eubank and Brough, 1980; Roberts and Spencer, 1995). A method identical to the one employed herein, using homogenization temperatures of originally all-liquid inclusions in halite, has recently been used to interpret paleoclimate from the Pleistocene of Death Valley (Roberts and Spencer, 1995). Further application of this method to other ancient halites may result in a wealth of quantitative local paleoclimate data.