Project

Discipline

Mineralogy; Chlorite; Thermometry; Oxygen; Isotope; Chemical Composition

Lead
Lay summary
Chlorite is an important mineral in many rocks influenced by diagenesis, low- up to high-grade metamorphism, as well as in many hydrothermal ore deposits as gangue mineral. In many cases its presence is directly related to the transport of fluids and fluid-rock interactions involved during this transport in metamorphic rocks or rocks influenced by diagenesis. Hence, its conditions of formation are not only of interest from an academic-scientific point of view, but also for a better understanding of and hence exploration of a wide range of ore deposits and oil/gas resources. Mineralogical-geochemical studies of chlorite may thus help geologists to interpret the thermal conditions of formation and evaluate the associated importance of fluids and their extent of migration. In this respect the hydrogen and oxygen stable isotope composition of chlorite and/or the thermodynamic models on the Fe-Mg and Al substitutions within chlorite can be powerful tools for studying the geological processes that occur during metamorphism and/or diagenesis. The aim of this project is to determine the oxygen isotope fractionation factors between different Fe-Mg chlorites and quartz and/or calcite using an empirical, field-based, and mineralogical approach. Natural samples from different places where temperature is well-constrained by independent methods will be selected. In addition, results will be compared to theoretical determinations using bond strength and incremental methods. This research program will emphasize calibrations of isotopic fractionation for low temperature geological processes (<600°C). Indeed, one of the most important aspects of stable isotope geochemistry is probably its application to fluid-rock interactions, process tracers and evaluations of open versus closed system processes, and geothermometry. Chlorite, often occurring together with quartz and calcite as gangue minerals in veins or the matrix of the rocks of interest, may hence play a key role as its O-isotope fractionation relative to co-existing quartz and calcite, can provide a potentially important geothermometer for a wide range of rock types, notably also rocks of basic composition. Yet, even though some experimental calibrations have been made, there is little consensus on the calibrations obtained and it is the aim of this project to help resolve some of the existing controversies.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

Chlorite is an important mineral in many rocks influenced by diagenesis, low- up to high-grade metamorphism, as well as in many hydrothermal ore deposits as gangue mineral. In many cases its presence is directly related to the transport of fluids and fluid-rock interactions involved during this transport in metamorphic rocks or rocks influenced by diagenesis. Hence, its conditions of formation are not only of interest from an academic-scientific point of view, but also for a better understanding of and hence exploration of a wide range of ore deposits and oil/gas resources. Mineralogical-geochemical studies of chlorite may thus help geologists to interpret the thermal conditions of formation and evaluate the associated importance of fluids and their extent of migration. In this respect the hydrogen and oxygen stable isotope composition of chlorite and/or the thermodynamic models on the Fe-Mg and Al substitutions within chlorite can be powerful tools for studying the geological processes that occur during metamorphism and/or diagenesis. In the past half century stable isotope geochemistry has been used to address a wide range of questions on fluid-rock interactions, petrological reconstructions, geothermobarometry and paleoclimate. Indeed, one of the most important aspects of stable isotope geochemistry is probably its application to fluid-rock interactions, process tracers and evaluations of open versus closed system processes, and geothermometry. Chlorite, often occurring together with quartz and calcite as gangue minerals in veins or the matrix of the rocks of interest, may hence play a key role here as its O-isotope fractionation relative to co-existing quartz and calcite, may provide a potentially important geothermometer for a wide range of rock types, notably also rocks of basic composition. Experimental calibrations on oxygen isotope fractionations between chlorite, quartz, calcite and water have been made at high temperatures, but it is questionable if these can also be extrapolated to low temperatures (e.g., Bottinga and Javoy, 1975; Savin and Lee, 1988; Clayton et al., 1989; Zheng, 1993; 1995; Sharp, 2009; Chacko et al., 2001). In addition the differences between the existing calibrations may exceed 1.5 or more permil at any given temperature, which is thought to be related to compositional differences of the chlorite (Figure 1). Compositional differences will, in turn, have an effect on the thermodynamic models for elemental substitutions within chlorite and hence would also influence the temperatures calculated on this basis.The aim of this proposal is to determine the fractionation factor between different Fe-Mg chlorites and quartz and/or calcite using an empirical, field-based, and mineralogical approach. Natural samples from different places where temperature is well-constrained by independent methods will be selected. In addition, results will be compared to theoretical determinations using bond strength and incremental methods (e.g., Zheng, 1993, 1995; and Kim et O’Neil, 1997). This research program will emphasize calibrations of isotopic fractionation for low temperature geological processes (<600°C).