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 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 microns). 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.

Fluid inclusions in the Upper Pennsylvanian display anomalously high homogenization temperatures. Temperatures greater than 170°C were recorded. These temperatures are somewhat inconsistent with other paleothermometers used in the Appalachian basin, including vitrinite reflectance and apatite fission track analysis. Lower Pennsylvanian inclusions yielded homogenization temperatures less than 145°C. These values are more consistent with heating incurred under normal burial conditions.

First melting of inclusions occurred between -42 and -40°C, suggesting a calcium and/or magnesium-rich aqueous solution in the inclusions. Final melting for the Upper Pennsylvanian sample occurred between -9.8 and -9.3°C and -2 and -1°C for the Lower Pennsylvanian sandstone, indicating salinities of about 13.5 and 2.5 weight percent NaCl equivalent, respectively.

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.