Intracrystalline Boron Isotope Fractionation in Illite/Smectite: A Potential Geothermometer and Paleo-Fluid indicator r

Arizona State University, Scottsdale AZ

Investigators

Abstract

Intra-crystalline Boron Isotope Fractionation in Illite/Smectite Boron is found in two different crystallographic sites of the 2:1 clay minerals illite/smectite. Boron is present in the interlayers of these clay minerals and also substitutes in the tetrahedral layers. Our pilot study (NSF SGER #0108852) developed a technique to analyze the boron isotopic ratio in these two sites, and we found that in natural samples the difference in the B-isotope ratios between each crystallographic site can be as high as 40. The present work is to study the factors that affect intra-crystalline B isotope variations. These results have the potential to provide a major advance in our understanding of clay mineral diagenesis and may also be a useful low temperature geothermometer. The research combines two different approaches: the synthesis of illite/smectite under carefully controlled conditions of temperature, pressure, and fluid composition and the analysis of natural samples from well-characterized localities. Our experimental work will react a variety of smectites to illite in the presence of fluids with different aqueous d11B starting compositions. In order to test the B selectivity for tetrahedral sites, we will examine beidellite and montmorillonite compositions and di- and tri-octahedral clays to determine the effect of octahedral cation exchange relative to tetrahedral cation exchange on the d11Bint compositions. Isotope equilibration of I/S phases can be tested in experimental samples by changing the fluid d11B during reaction progress. It is critical to compare the experimental work to natural I/S undergoing illitization in different hydrothermal regimes. The mechanisms of illitization may be different for closed (mudstone or bentonite) versus open (sandstone or geothermal) systems. We need to test the dependence of the intracrystalline d11B variations in different thermal regimes and varying the fluid/rock ratio. To do this we propose to study samples from a bentonite (closed system), a rapidly buried shale versus slowly buried mudstones and geothermal samples from 180-400C (open system). The geothermal system holds the most promise for finding fluids and natural I/S in equilibrium. The combined experiments and analysis of natural samples will allow us to assess the role of fluid composition, mineral composition, temperature, temperature gradient, and fluid/rock ratios on the composition of B in the interlayers and tetrahedral layers of I/S. This new research will produce important information relating to the illitization process and may prove the potential for a novel low-temperature geothermometer.