A new technique for surface and shallow subsurface paleobarometry using fluid inclusions: an example from the Upper Ordovician Viola Formation, Kansas, USA

This research illustrates a new approach for paleobarometry employing heter ogeneously entrapped fluid inclusions to determine timing and depth of diag enesis. Heterogeneously entrapped fluid inclusions (gas + water) in vug-fil ling quartz from the Upper Ordovician Viola Formation in the Midcontinent o f the United States were analyzed for their internal pressure with a fluid- inclusion crushing stage. The free gas in fluid inclusions was entrapped at near-surface temperature, as indicated by the presence of all-liquid fluid inclusions and fluid inclusions with low homogenization temperatures (< 40 degrees C). Crushing the crystal and measuring the change in bubble size d etermines the pressure of entrapment directly. Heterogeneous trapping is in dicated by widely varying L:V ratios, from all-liquid to vapor-rich. Gas bu bbles in most fluid inclusions analyzed expanded upon release to atmospheri c pressure, but some collapsed. A mode of 1.5 to 2.0 atm internal pressure was indicated by the crushing runs, but pressures up to 42.9 atm were recor ded. Quartz precipitation and associated fluid-inclusion entrapment therefo re occurred over a wide depth-range, but principally at depths of approxima tely 10 m, Crushing runs done in kerosene confirmed the presence of hydroca rbon gases in most of these inclusions, and bulk analyses of gases in the q uartz by quadrupole mass spectrometer revealed methane, ethane, and atmosph eric gases. The hydrocarbon gases may have originated in deeper thermogenic ally mature sedimentary strata, and then leaked to the near-surface where t hey were entrapped in the precipitating quartz cement. Freezing data indica te an event of quartz precipitation from fluids of marine-fresh water inter mediate salinity and other events of precipitation from more saline fluids. Considering the determined pressures, the precipitating fluids probably or iginated at surfaces of subaerial exposure (unconformities) and surfaces of evaporite precipitation in the overlying Silurian strata. Thus, saline inc lusions most likely originated from sinking of saline surface waters during Silurian time. Lower-salinity fluids record fluxes of meteoric water durin g development of unconformities in the Silurian. This type of paleobarometr ic study may have application in many other sedimentary systems, provided l ow-temperature and heterogeneous entrapment of an immiscible gas phase can be demonstrated for the fluid-inclusion assemblages studied. (C) 1999 Elsev ier Science B.V. All rights reserved.