Is water responsible for geophysical anomalies in the deep continental crust? A petrological perspective

Abstract

It is common to ascribe conductivity and seismic anomalies in the lower crust to the presence of fluids. We note that fluids cannot be stored long in the lower crust for both mechanical reasons and because at temperatures above ca. 250°C reaction rates are so fast that fluids must be rapidly consumed by hydration reactions. To maintain a “wet” lower crust, therefore, fluid must be continually supplied. There are five sources for such fluids. Gravity-fed meteoric water may percolate many kilometers into the crust, but it cannot move to regions that are hotter than lowermost greenschist conditions because at these temperatures reactions consume water faster that water can be transported. Passive mantle degassing can only take place in areas of active magmatism, because in localities where the lower crust and upper mantle are cold, fluids are incompatible with known mantle mineralogy. Metamorphic devolatilization, evolution of igneous fluids, and tectonic introduction of fluids into the crust are all restricted to areas of active tectonism and are likely to proceed episodically. The fluid flow in the lower crust will vary according to the tectonic environment. In stable cratons fluids would be gravitationally driven but would be able to gain access only to the upper 10 km or so of the crust. Below this the crust would be “dry”. In extensional regimes the fluids would be thermally driven. Carbonic fluids of mantle origin may be present in the lower crust near underplated mantle melts, whereas fluids of meteoric or igneous origin will occur at shallow depths. In compressional regimes fluids may be driven by tectonic or thermal processes and may be of mantle, metamorphic, igneous, or meteoric origin. When considering the causes for lower crustal geophysical anomalies, therefore, one must consider the tectonic regime. Only in areas of active metamorphism or magmatism are fluids likely to play an important role. In cratonal regions the lower crust is “dry”. In such regions enhanced conductivity is likely to be caused by mineral films (graphite, magnetite, or sulfides) and reflectivity by lithologic variations due to either mylonitization or magmatic underplating.