Frictional properties of oceanic olivine-bearing basalt under hydrothermal conditions
Frictional properties of basalt as the principal component of the oceanic crust provide a basis for a deeper understanding of the mechanical behaviors including large earthquakes on the boundary of the subducting plate.To investigate whether a trace amount of olivine mineral affects the frictional properties of basalt,an olivine-bearing basalt sample obtained from the International Ocean Discovery Program(IODP)voyage 349 drill were selected.In our shearing experiments,temperatures were applied in a range from 100 C to 600℃,with effective normal pressure and pore water pressure of 150 MPa and 100 MPa,respectively.In this study,we found that the basalt sample presents unstable oscillation at temperature of 300～400℃.At higher temperature(T＞400 ℃),shear of basalt is characterized by significant slip weakening phenomenon of shear strength,which is manifested as a continuous decrease of frictional strength coefficient with the progress of shear displacement.With the shear displacement up to 3 mm,frictional coefficient can decrease from 0.7 to 0.55.Based on preliminary microscopic observations,we speculate that olivine catalyzes the retrograde reaction process to produce weak mineral,but the content of weak mineral seems not enough to significantly weaken the frictional strength of the gouge.Microstructural analysis also indicates that the shear structure of gouge layer changed from localized brittle shear to pervasive distributed shear deformation as temperature increased.This change was accompanied by a significant decrease in porosity,consequently,the slip weakening of shear strength may be related to the progressively enhanced intergranular pressure solution process with the presence of pore water.
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