FLUID INCLUSIONS IN QUARTZ OVERGROWTHS: IMPLICATIONS FOR PALEOTHERMOMETRY AND QUARTZ AUTHIGENESIS, PENNSYLVANIAN SANDSTONES, CENTRAL APPALACHIAN BASIN

Fluid inclusions in core-sampled Pennsylvanian sandstones from the central Appalachian basin of West Virginia and Maryland, containing greater than 8% syntaxial quartz overgrowths, were studied to constrain thermal evolution and mineral paragenesis. Sample depths for the quartz arenites range from 170 to 400 meters.

Fluid inclusions within most of the authigenic quartz overgrowths are rare and relatively small (2-5 µm). Most occur as primary inclusions that are isolated well within the quartz overgrowth that formed during authigenic quartz precipitation. Fewer secondary inclusions occur along healed fractures and may have formed during secondary alteration of the quartz cement.

Primary inclusions display anomalously high homogenization temperatures. Temperatures greater than 150°C were common for many of the smallest fluid inclusions. These temperatures are inconsistent with other paleothermometers used in the Appalachian basin, including vitrinite reflectance and apatite fission track analysis.

First melting of inclusions occurred between –42 and –40°C, suggesting a calcium and/or magnesium-rich aqueous solution in the inclusions. Final melting occurred between –13 and –10°C, indicating salinities of about 15 weight percent.

The reason for the discrepancy between fluid inclusion-derived paleotemperatures and those obtained from other geothermometers is not clear. One possible explanation is that, because fluid inclusions provide a record of instantaneous or short-lived (in a geologic sense) temperature fluctuations, whereas other geothermometers used are time-dependent, the inclusions are recording the passage of pulses of high temperature fluids being expelled from deeper in the basin. The composition (calcium-bearing) and salinity of the fluids are consistent with an origin deeper in a sedimentary basin. The inclusions may thus provide evidence of short duration thermal events that are not recorded by time-dependent geothermometers such as vitrinite reflectance or fission-tracks.