Mechanism of Smectite Illitization from Mineralogical Analysis of a Hydrothermally Altered Bentonite, Ponza Island, Italy

[9417758]在许多沉积盆地和热液环境中都发生蒙脱石与伊利石的矿物反应。该反应被称为蒙脱石伊利化，通过混合层伊利石/蒙脱石（I/S）中间体进行，其中伊利石中间层的百分比和有序度通常随着温度和时间的增加而增加。此外，伊利石多型从1Md到1M到2M1发生变化。因此，I/S和伊利石多型的矿物学和地球化学分析可以为解读一个地区的地质历史提供基础信息。此外，由于I/S的体积丰度和蒙脱石伊利化过程中所涉及的化学变化，该反应在许多沉积盆地过程（如高孔隙水压力的产生、石油的成熟和运移、生长断层的形成、卤水的化学演化和砂岩的胶结作用）中起着重要作用。伊利石和I/S的辐射年龄可以确定热液流体幕式迁移、石油迁移、天然气侵位、区域逆冲和断层泥形成的时间。尽管具有广泛的意义，但蒙脱石伊利化和伊利石多型转化的机制尚不完全清楚。反应机制的大类包括固态转变和溶解结晶，其中子晶体保持母体晶体的大致大小和形状；溶解结晶是指在反应过程中失去母体晶体的形态特征。在以岩石为主的体系（如膨润土）中，固态机制似乎最适合模拟非化作用，而在砂岩和热液环境等以流体为主的体系中，溶解/结晶机制似乎最适合模拟非化作用。本萨岛（意大利）含有一大片热液蚀变的上新世膨润土。虽然蒙脱石是我们许多样品中主要的蚀变阶段，但在相对较浅的钻芯间隔（65m）以及Ponza岛北部和邻近的Gavi岛的露头中发生了显著的非石化作用。提出了一种综合分析方法，对露头和两个50m深岩心蓬扎膨脱土中块状样品的矿物学和化学组成，块状样品的矿物学、粒度、形状、绝对年龄、化学组成以及蒙脱石、I/S和伊利石的矿物学、粒度、形状、绝对年龄、化学组成和稳定同位素组成进行了表征。收集的样品将使用x射线衍射、扫描和透射电子显微镜、放射成因和稳定同位素、核磁共振波谱、红外波谱、x射线荧光、电感耦合等离子体、中子活化分析、流体包裹体和光学显微镜进行研究。为了充分了解庞扎的热液历史和复杂矿物反应的性质，需要多种分析方法。由于矿物杂质的存在、样品数量少、I/S矿物学范围有限或地质历史信息有限，以往对非石化和多型转化机制的研究受到阻碍。Ponza岛提供了一个理想的机会，可以深入了解illiization机制，因为在较小的横向和垂直距离上，I/S矿物学变化很大。此外，岩石含有丰富的I/S（这将允许对选定的样品进行多种分析），铁含量相对较低（这将提高重要的核磁共振分析的分辨率）。研究结果将极大地提高对旁扎的地质历史的认识，包括热液蚀变的温度和时间以及热液流体的组成和起源。本扎热液蚀变性质的预期信息有望提高我们对矿物学反应的整体理解。

9417758 Altaner The mineralogical reaction of smectite to illite occurs in many sedimentary basins and hydrothermal environments. The reaction, termed smectite illitization, proceeds through mixed-layer illite/smectite (I/S) intermediates in which the percentage and ordering if illite interlayers typically increases with temperature and time. In addition, illite polytype changes from 1Md to 1M to 2M1. Therefore, mineralogical and geochemical analysis of I/S and illite polytypes can provide fundamental information in deciphering the geological history of an area. In addition, because of the volumetric abundance of I/S and the chemical changes involved during smectite illitization, the reaction plays an important role in many sedimentary basinal processes (e.g., creation of high porewater pressures, maturation and migration of oil, formation of growth faults, chemical evolution of brines, and cementation of sandstone). Radiometric ages of illite and I/S can date the timing of episodic migrations of hydrothermal fluids, oil migrations, emplacement of natural gas, regional overthrusting, and fault gouge formation. Despite such broad significance, mechanisms of smectite illitization and illite polytype conversion are incompletely understood. The broad categories of reaction mechanisms are solid-state transformation, in which the daughter crystal maintains the approximate size and shape of the parent, and dissolution and crystallization, in which the morphological characteristics of the parent crystal are lost during reaction. A solid-state mechanism seems to best model illitization in rock-dominated systems such as bentonite and a dissolution/crystallization mechanism seems to best model illitization in fluid-dominated systems such as sandstone and hydrothermal environments. Ponza island (Italy) contains an extensive area of hydrothermally altered Pliocene bentonite. Although smectite is the dominant alteration phase in many ourcrop samples, significant illitization occurs at relative ly shallow intervals (65m) of drillcore and in outcrops at the northern part of Ponza island and at the neighboring island of Gavi. An integrated analytical approach is proposed to characterize the mineralogy and chemical composition of bulk samples as well as the mineralogy, size, shape, absolute age, chemical composition of bulk samples as well as the mineralogy, size, shape, absolute age, chemical composition, and stable isotopic composition of smectite, I/S, and illite in the Ponza bentonite from outcrops and two 50m deep drillcores. Collected samples will be studied using X-ray diffraction, scanning and transmission electron microscopy, radiogenic and stable isotopes, nuclear magnetic resonance spectroscopy, infrared spectroscopy, X-ray flurescence, inductively coupled plasma, neutron activation analysis, fluid inclusions, and optical microscopy. A variety of analytical methods are needed to fully understand the hydrothermal history of Ponza and the nature of the complex mineralogical reactions. Previous studies of mechanisms of illitization and polytype conversion have been hindered by presence of mineral impurities, small sample amounts, a limited range in I/S mineralogy, or limited information on geologic history. Ponza island represents an ideal opportunity to gain considerable insight in mechanisms of illitization because there is a large variation in I/S mineralogy over small lateral and vertical distances. In addition, the rocks contain abundant I/S ( which will allow for many kinds of analyses of selected samples) with a relatively low iron content (which will enhance resolution of important NMR analyses). Study results should greatly improve understanding of the geologic history of Ponza, including temperature and timing of hydrothermal alteration as well as the composition and origin of hydrothermal fluids. Anticipated information ol the nature of hydrothermal alteration of Ponza is expected to improve our overall understanding of mineralogical reactions.