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

伊利石/蒙皂石中的晶内硼同位素分馏硼存在于2：1粘土矿物伊利石/蒙皂石的两个不同的晶体学位置。硼存在于这些粘土矿物的夹层中，并且也在四面体层中替代。我们的初步研究（NSF SGER #0108852）开发了一种技术来分析这两个位点的硼同位素比，我们发现，在天然样品中，每个晶体位点之间的硼同位素比差异可高达40。本文研究了影响晶体内B同位素变化的因素。这些结果有可能为我们理解粘土矿物成岩作用提供重大进展，也可能是一个有用的低温地质温度计。该研究结合了两种不同的方法：在精心控制的温度，压力和流体组成的条件下合成伊利石/蒙皂石，并分析来自特征良好的地方的天然样品。我们的实验工作将在具有不同水性d11 B起始组合物的流体存在下，使各种蒙皂石反应为伊利石。为了测试的B四面体网站的选择性，我们将检查贝得石和蒙脱石组合物和二和三八面体粘土，以确定八面体阳离子交换相对于四面体阳离子交换的d11 Bint组合物的效果。通过改变反应过程中的流体d11 B，可以测试实验样品中I/S相的同位素平衡，并将实验工作与不同热液体系中经历伊利化的天然I/S相进行比较。封闭系统（泥岩或膨润土）与开放系统（砂岩或地热）的伊利化机制可能不同。我们需要测试在不同的热制度和不同的流体/岩石比的晶内d11 B的变化的依赖性。为此，我们建议研究膨润土（封闭系统）的样品，快速埋藏的页岩与缓慢埋藏的泥岩和180- 400 C（开放系统）的地热样品。地热系统最有希望找到平衡的流体和天然I/S，结合天然样品的实验和分析，将使我们能够评估流体成分、矿物成分、温度、温度梯度和流体/岩石比对I/S夹层和四面体层中B成分的作用。这项新的研究将产生有关伊利石化过程的重要信息，并可能证明一种新型低温地质温度计的潜力。

Intra-crystalline Boron Isotope Fractionation in Illite/SmectiteBoron 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.