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在2：1粘土矿物伊利石/蒙皂石的两个不同晶位中发现了硼。硼存在于这些粘土矿物的夹层中，也存在于四面体的层中。我们的初步研究(NSF SGER#0108852)开发了一种技术来分析这两个地点的硼同位素比率，我们发现在自然样品中，每个结晶地点之间的B同位素比率的差异可以高达40%。目前的工作是研究影响晶内B同位素变化的因素。这些结果有可能为我们对粘土矿物成岩作用的理解提供重大进展，也可能成为有用的低温地质温度计。这项研究结合了两种不同的方法：在严格控制的温度、压力和流体组成条件下合成伊利石/蒙脱石，以及从表征良好的地点分析天然样品。我们的实验工作将在具有不同初始水相组成的流体存在下，使各种蒙脱石与伊利石反应。为了测试B对四面体位置的选择性，我们将检查贝氏晶石和蒙脱石的组成以及二和三八面体粘土，以确定相对于四面体阳离子交换的八面体阳离子交换对d11Bint组成的影响。通过改变反应过程中的流体d11B，可以在实验样品中测试I/S相的同位素平衡，关键是要将实验工作与自然的I/S在不同的水热条件下进行伊利化进行比较。封闭(泥岩或膨润土)和开放(砂岩或地热)系统的泥石化机制可能不同。我们需要测试晶内d11B在不同热区和不同流体/岩石比下变化的相关性。为此，我们建议研究来自膨润土(封闭系统)、快速埋藏页岩和缓慢埋藏泥岩的样品，以及180-400C(开放系统)的地热样品。地热系统在寻找流体和天然I/S平衡方面最有希望。通过对自然样品的联合实验和分析，我们可以评估流体组成、矿物组成、温度、温度梯度和流体/岩石比对I/S夹层和四面体地层中B的组成的影响。这项新的研究将产生与伊利石化过程有关的重要信息，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.