Strain-Enhanced Diffusion in Feldspar: A Strain Speedometer?

English Abstract:

Rocks in Earth’s crust deform over many different timescales. The rate at which they deform (strain rate) cannot be directly measured in the rock record with present tools. Strain rate can be constrained in specific cases of dateable features such as syn-kinematic mineral growth or intrusions, but a generally applicable tool has not yet been developed. Previous studies on tourmaline show deformation-enhanced element mobility that could, in theory, be used to calculate the duration of deformation. Unfortunately, the required diffusion parameters are lacking for tourmaline and it is not a rock-forming mineral, rendering it unusable as a tool to infer bulk rock strain rate. Feldspars dominate the rheology of Earth’s crust, diffusion parameters are known across a range of pressure-temperature conditions, and feldspars can grow with compositional zoning. Compositional zoning serves as a physical, and potentially chemical, strain marker. I combined strain measurements along a 500-meter-long strain gradient transect with element mobilities measured as a function of strain in zoned plagioclase phenocrysts. Major, minor, and trace element analyses on electron microprobe and laser ablation inductively coupled plasma mass spectrometry provide the necessary resolution for precise diffusion modeling at the low strains observed. Thermobarometry indicates that conditions of feldspar formation were 700 ± 30ºC and 190 ± 30 MPa, with no indication of later fluid overprint during deformation. Any divergent element mobilities in zoned plagioclase phenocrysts from this study are most likely a result of strain, and because fluids did not significantly mobilize elements, modeling changes in element mobilities can be used to yield an estimated duration of deformation. These results will be combined to inform a paleo-strain speedometer for the middle crust.