HR: 14:30h
AN: V33F-03 INVITED     [Abstracts]
TI: Fluid Inclusion Evidence for Brines in the Earth's Crust
AU: * Bodnar, R J
EM: rjb@vt.edu
AF: Virginia Tech, Dept. of Geosciences, Blacksburg, VA 14061 United States
AB: Over the past few decades it has become clear that brines of variable origin, salinity and chemistry are common in many crustal environments. Most of our current understanding of the distribution and chemistry of brines has come from studies of fluid inclusions trapped in minerals. Previously it was only possible to determine the bulk salinity (in terms of weight percent NaCl equivalent) and the major salts present in the inclusions. However, over the past two decades significant advances have been made in our ability to analyze individual fluid inclusions using techniques such as Raman and FTIR spectroscopies, synchrotron XRF, PIXE, PIGE and laser ablation ICP-MS. Today, it is possible to determine not only the major element chemistry but also the trace element chemistry and volatile contents of individual inclusions as small as about 10 microns. In deep sedimentary basins, brines with salinities of 30-40 wt.percent TDS (total dissolved solid) are common and are often associated with hydrocarbon reservoirs. These basinal brines usually contain Na or Ca as the dominant cation and Cl as the major anion and originate through hydration reactions and interaction with evaporites during basin evolution. Methane is a common component of these fluids. Such brines are thought to be the main source of metals to form Mississippi Valley-type Pb-Zn-Cu deposits. Brines in silicic magmatic hydrothermal systems often achieve salinities in excess of 50 wt.percent TDS. The chemistry is dominated by Na and K chlorides, and include significant amounts of Fe and other transition metals in ore-forming systems. In these magmatic systems, the high salinities are often the result of aqueous fluid immiscibility that preferentially partitions the less-volatile components into a highly saline liquid phase, while more volatile components are partitioned into a low salinity vapor. In other magmatic systems, high salinity brines are exsolved directly from the crystallizing melt - in some cases the salinities are so high that the fluids are more correctly referred to as salt melts rather than aqueous solutions. In submarine hydrothermal systems brines achieve their salinities through sub-seafloor boiling of seawater. Such brines are dominated by calcium salts and may contain significant amounts of methane. Brines are less common in metamorphic environments compared to deep sedimentary basins or magmatic systems. Metamorphic brines are thought to originate mainly as a result fluid immiscibility (as in magmatic systems) or by metamorphism of evaporates.
DE: 8424 Hydrothermal systems (8135)
DE: 4832 Hydrothermal systems
DE: 3665 Mineral occurrences and deposits
DE: 3035 Midocean ridge processes
DE: 1020 Composition of the crust
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting