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Hydrothermal systems from plutonic to volcanic: fundamental insights and fundamental unknowns

Thomas Driesner

Abstract

Hydrothermal systems related to magmatism are key actors determining rates of magma cooling, mass transfer via fluid convection and/or fumarolic degassing, and the formation of ore deposits and geothermal resources. I will review a spectrum of fundamental insights that have been gained mostly via numerical simulation of such systems. Geologically, these are short-lived at typically 103 - 105 years and vary greatly in time and space as well as between different systems types. Systems in which magmatic intrusions mainly act as heat source without providing much fluid input are liquid-dominated and the extent of boiling as well as their contribution to magma cooling mainly depend on the host rocks’ permeability. Depending on the latter, intrusive and hydrothermal activity may even be separated in time at a given site. Magmatic-hydrothermal systems (i.e., those in which a major part of the fluid is released from the magma itself) are much more energetic and can display different modes of operation, which reflect self-organizing feedbacks between magmatic degassing rate, host rock mechanical behavior, fluid enthalpy etc. The shallow hydrothermal systems suggested for the interior of volcanoes are rather poorly understood as topography and complex fluid compositions with significant volatile contents add further feedbacks. I will highlight how our current lack of accurate knowledge of the phase relations and fluid properties in even relatively simple fluid systems such as H2O-CO2-NaCl prevents further progress in understanding volcanic hydrothermal systems and may render interpretation of field observations ambiguous.