Rheology Of Dry K-Feldspar Aggregates At High Temperature And High Pressure: An Experimental Study

Quartz and feldspar are the two most abundant minerals in the continental crust. The rheology of quartz and feldspar has been used to approximate the rheology of the upper and the lower crust, respectively. However, compared with quartz and plagioclase, the rheology of K-feldspar (Kfs) is much less characterized. We present here systematic axial compression deformation experiments on a relatively dry (22 +/- 2 wt ppm) synthetic Kfs aggregates (Or(81)Ab(19)) at strain rates of 10(-4)-10(-5) s(-1), temperatures of 950-1100 degrees C and a pressure of 1.5 GPa, using a 5 GPa modified Griggs-type deformation apparatus. The rheology of Kfs aggregates can be best described by the constitutive equation of (epsilon)over dot = 10(2.4 +/- 3)(.1)sigma(3.0 +/- 0.6)exp(-368 +/- 70/RT) EBSD mapping and TEM analyses reveal clear intracrystalline deformation and pronounced crystallographic preferred orientations resulted from activation of dislocation slip along the [100] (010) and [100] (001) slip systems at elevated temperatures. Extrapolation of the Kfs flow law to a natural strain rate of 10(-14)s(-1) suggests a weaker strength than that of anorthite of similar water concentrations deformed in the dislocation creep regime. The higher creep strength of relatively dry Kfs and anorthite aggregates comparing to wet quartzite provides an explanation for feldspar forming porphyroclasts in crustal rocks. However, Kfs may also exhibit ductile deformation at high temperatures under lower crustal and deep continental subduction conditions.