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

小行星9417758 蒙皂石向伊利石的矿物学反应发生在许多沉积盆地和热液环境中。 该反应称为蒙脱石伊利石化，通过混合层伊利石/蒙脱石（I/S）中间体进行，其中伊利石夹层的百分比和排序通常随温度和时间而增加。 此外，伊利石多型体从1 Md到1 M再到2 M1。 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释一个地区的地质历史提供基础信息。 此外，由于I/S的体积丰度和蒙皂石伊利石化过程中涉及的化学变化，该反应在许多沉积盆地过程中起着重要作用（例如，高孔隙水压力的产生、石油的成熟和运移、生长断层的形成、盐水的化学演化和砂岩的胶结作用）。 伊利石和I/S放射性年龄可以确定热液流体幕式运移、石油运移、天然气侵位、区域逆冲推覆和断层泥形成的时间。 尽管具有如此广泛的意义，蒙皂石伊利石化和伊利石多型转化的机制还不完全清楚。 反应机制的大类是固态转化，其中子晶体保持母体的近似尺寸和形状，以及溶解和结晶，其中母体晶体的形态特征在反应期间丢失。 固态机制似乎最好的模型伊利化在岩石为主的系统，如膨润土和溶解/结晶机制似乎最好的模型伊利化在流体为主的系统，如砂岩和热液环境。 蓬扎岛（意大利）含有大面积的热液蚀变上新世膨润土。 尽管蒙皂石是许多露头样品中的主要蚀变相，但在相对较浅的岩芯间隔（65 m）处以及Ponza岛北方部分和邻近的Gavi岛的露头中，均发生了显著的伊利石化。 提出了一种综合分析方法来表征块状样品的矿物学和化学成分，以及块状样品的矿物学、尺寸、形状、绝对年龄、化学成分，以及来自露头和两个50 m深的钻孔岩心的庞扎膨润土中的蒙皂石、I/S和伊利石的矿物学、尺寸、形状、绝对年龄、化学成分和稳定同位素组成。 将使用X射线衍射、扫描和透射电子显微镜、放射性同位素和稳定同位素、核磁共振光谱、红外光谱、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对收集的样品进行研究。 需要各种分析方法来充分了解庞扎的热液历史和复杂矿物学反应的性质。 以前的伊利石化和多型转化机制的研究受到矿物杂质的存在，样品量小，I/S矿物学范围有限，或地质历史信息有限的阻碍。 庞扎岛是一个理想的机会，以获得相当大的洞察力伊利石化机制，因为有一个很大的变化，在I/S矿物在小的横向和垂直距离。 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NMR分析的分辨率）。 研究结果将大大提高对Ponza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成分和起源。 对庞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认识。

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.