Conceptual Model for Emplacement Depth of Magma Chambers and Genesis of Geothermal Systems

摘要

Genesis of high temperature hydrothcrmal systems is ultimately constrained by the emplacement depth of magma chambers. A conceptual model for the emplacement depth of high-level magma chambers is developed. Density-depth profiles of crust are generalized in the oceanic regions by MORB and in the continental crust by normal sediments. Methods to estimate magma density from petrochemical data are synthesized using simple approximations for the original water content, temperature and compressibility. Model calculations show that alkaline magmas arc the most buoyant for a given silica content and they may prefer to entirely extrude, particularly in the oceanic crust regions. Tholeiitic magmas are the least buoyant for a given silica content. Their emplacement depths are around 10 km in the continental crust regions which can not effectively contribute to hydrothermal heat sources, but are 2 -I km in the oceanic regions which makes them potential hydrothermal heat sources. Calc-alkaline magmas have an intermediate buoyancy. Their emplacement depths are 8 - 2 km in the continental crust and they arc the most effective hydrothermal heat sources in the continental crust regions. Most of the high temperature hydrothermal systems in the Japanese Islands arc well explained by this model and vice versa it demonstrates the validity of this model. Thus, the model may predict which types of magmas could generate high temperature hydrothermal systems.