Element mobility and volumetric strain in brittle and brittle–viscous shear zones of the superdeep well KTB (Germany)

Mass-balance studies of brittle and brittle–viscous shear zones of the superdeep well KTB (Germany) show that element mobility and associated volumetric strain is markedly different in metabasites and paragneisses. Shear zones in metabasites show thickening and volume increase due to mineralization of prehnite (±epidote±calcite) within open fractures and pore space. Gains in Al2O3, CaO and SiO2 are compatible with these observations. Shear zones in paragneisses show either constant volume or volume loss. Volume loss of the paragneiss shear zones can be explained by pressure solution of quartz and by retrograde mica-forming reactions associated with significant gains in potassium-group elements (K, Rb, Ba) and losses in SiO2. The differences in volumetric strain between paragneiss and metabasite shear zones can be explained by the different deformation mechanisms and rheology of paragneiss and metabasite at T=ca. 250–350°C. The rigid metabasites supported long-lived open fractures and pores where new minerals could precipitate from a fluid phase. The paragneisses, on the other hand, were weak because of increasing amounts of crystal plasticity and pressure solution of quartz which is typical for the brittle–viscous transition of quartz-bearing rocks. Open fractures and pores were rapidly closed hampering the deposition of new minerals. Graphite enrichment has been found in both paragneiss and metabasite shear zones. There is clear evidence that graphite enrichment results from fluid phases which carried C into the shear zones from external sources suggesting that C was largely mobile during the shearing processes.