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Johnson, Aaron W.1, Kevin L. Shelton2, Jay M. Gregg3, Ian D. Somerville4, Wayne R. Wright5 
(1) University of Virginia's College at Wise, Wise, VA 
(2) University of Missouri-Columbia, Columbia, MO 
(3) University of Missouri-Rolla, Rolla, MO 
(4) University College Dublin, Dublin, Ireland 
(5) Robertson Research International, North Wales, United Kingdom

ABSTRACT: Dolomite Cements and Their Included Fluids in the Irish Midlands: Coupling Solid (13C, 18O and 87Sr/86Sr) and Fluid (Halogens and Trace Elements) Geochemistry to Understand Multiple Fluid Interactions During Brine Migration

Lower Carboniferous carbonate rocks of the Irish Midlands and Dublin Basin record a complex history of multiple fluid interactions during dolomitization. Strontium-halogen-temperature systematics of carbonate cements and their included fluids define three end-member compositions: (1) a low-temperature, high-salinity fluid that is enriched in 18O, has variable Sr concentrations (10 to 550 ppm), and 87Sr/86Sr ratios that are slightly higher than Lower Carboniferous seawater; (2) a low-temperature, moderate-salinity fluid that is enriched in 18O, has very high Sr concentrations (500-650 ppm), and high 87Sr/86Sr ratios; and, (3) a high-temperature, moderate-salinity fluid that is enriched in 18O, has low Sr concentrations (10-60 ppm), and 87Sr/86Sr ratios that are slightly higher than Lower Carboniferous seawater. Halogen analyses indicate that fluid type 1 was generated from seawater evaporated beyond the point of halite precipitation whereas fluid types 2 and 3 were generated from seawater evaporated to the point of gypsum precipitation. 
End-member fluid type 1 represents a regional migration of highly evaporated seawater whereas fluids 2 and 3 are present locally. Mixing trends between fluids 1 & 2 and 1 and 3 may represent the presence of convection cells associated with thermohaline circulation of moderately evaporated seawater along local fault and fracture systems. Analyses of the chemistry of dolomite cements and their included fluids can provide powerful insight into the fluid histories of complicated diagenetic systems like that of the Irish Midlands as the former provides information concerning temperature and fluid-rock interactions, whereas the latter reflects salinity (fluid) source areas. 

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.