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.
