Fluid Geochemistry as Indicator of Tectonically-Related, Deep Water Circulations in the Sardinian Rift-Campidano Graben (Italy): New Insights from Environmental Isotopes

Abstract

The EC funded Geochemical Seismic Zonation program (EEC GSZ Project 1996-1998 chose Sardinia as a low-seismicity site, in which the relationships between fluid geochemistry and seismo-tectonics had to be investigated and results compared with outcomes from other selected high-seismicity sites. A first article, examining the role of fault segmentation and seismic quiescence on the geochemical composition of groundwaters and gases, has already been presented (Angelone et al. 2005). This article deals with environmental isotopes which, together with selected hydrochemical data, give hints on tectonically-related fluid circulations. Four water-dominated hydrothermal systems were considered, all located along regional fault systems and discharging groundwaters belonging to the Na-HCO 3 and Na-Cl facies. In the considered systems, groundwater circulation takes place, principally, in the Palaeozoic Crystalline Basement (PCB), with the exception of the Logudoro system, where hydrological circuits develop in the Mesozoic Carbonate Platform (MCP). The high CO 2 contents, the non-attainment of fluid-rock equilibrium and the large lithological variability prevent the construction of a unique hydrogeological-geochemical conceptual model. In this case, stable isotopes provide a useful tool to describe the origin of fluids and their subterranean movements. Stable isotopes of water, integrated with hydrochemical data, indicate that fluids are derived from three main end members. The dominant component is a relatively recent local meteoric water; the second one is marine water; and the third one is a fossil freshwater, depleted in heavy isotopes with respect to modern rains. The latter end member entered the aquifer system in the past, when climatic conditions were greatly different from today. At least two circulation systems can be recognised, namely a shallow cold system and a deep hydrothermal system, as well as two distinct hydrological processes: (1) gravity-controlled descent of cold water towards greater depths and (2) convection linked to a thermal gradient, causing deep fluids to rise up from the hydrothermal reservoir towards the surface. The highly variable d 13 C TDIC values suggest the presence of two distinct CO 2 sources, namely, a biogenic one and a thermogenic one. The relation between the isotopic compositions of CO 2 and He indicates an increased mantle signature in uprising CO 2 -rich fluids.

Geofluids Assessment of the Ayub and Shafa Hot Springs in Kopet-Dagh Zone (NE Iran): An Isotopic Geochemistry Approach

“…Based on 18 O and 2 H values of a geothermal water, the heat source of the water may be determined [14]. The 18 O and 2 H values of AP and SH hot springs are plotted within the range of geothermal and meteoric waters (Figure 8).…”

“…Understanding the origin of hydrothermal fluids and the reason for the elevation of temperature is a very important step in revealing the temperature and pressure conditions of the underground reservoirs [13]. To determine the reason and origin of the temperature/heat in a geothermal field, several methods such as remote sensing, geological investigation, geophysical survey, thermal anomalies measurements, and hydrochemical and isotopic method can be applied [14]. Today oxygen and hydrogen isotopes are widely used in geothermal studies to determine the origin and history of geothermal fluids and often serve as natural tracers for the…”