FRACTURE PATTERNS DURING SERPENTINIZATION IN OCEANIC LOWER CRUST: INSIGHTS FROM THE OMAN OPHIOLITE AND NUMERICAL SIMULATIONS

Serpentinization in the oceanic lithosphere provide significant influences on water budget and dynamic processes within the earth’s interiors. The progress of serpentinization is controlled by the way of fluid infiltration through lower crust and Moho. However, although several models have been proposed, it is still controversial how fluid pathways develops through the layered gabbros.

In this study, we analyzed textures (extent of serpentinization and fracture patterns) of the variously serpentinized samples (olivine gabbros, troctolite and hartzburgite ) which are consisted by lower crust and upper mantle section of the Oman ophiolite, taken from Oman Drilling Project drill hole CM1A (Wadi Zeeb, Northern Sharqiyah, Oman). The lower crust section (gabbros and troctolites) has layered structure composed of olivine-rich part and olivine-poor part. We found that serpentinized olivine grains have mesh texture with radial cracks in surrounding plagioclase and clinopyroxene, and that the crack density within serpentinized olivine grains increases with increasing the extent of serpentinization.

We conducted the numerical simulations based on two-dimensional coupled hydrological-mechanical-chemical discrete element method (DEM), as well as fracturing induced by thermal and tectonic stresses. The numerical simulations revealed that mesh-like texture is formed in the case of thermal cracking during cooling and in case of volume expansion (serpentinization), and the radial cracks develops only in the latter case.

The integration of the petrographic observations and numerical simulations suggests that (1) the thermal cracking of olivine grains occurred prior to serpentinization during cooling, and (2) extensive fracture networks developed induced by serpentinization, which enhanced fluid infiltration through layered lower crust. These thermal-chemical-mechanical coupled processes may important for the fluid infiltration within the lower crust and control of upper mantle serpentinization.