Oxygen bulk diffusion measurements and TEM characterization of a natural ultramylonite: implications for fluid transport in mica-bearing rocks |
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| Authors: | Farver & Yund |
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| Institution: | Department of Geology and Center for Materials Science, Bowling Green State University, Bowling Green, OH 43403, USA (;), Department of Geological Sciences, Brown University, Providence, RI 02912, USA |
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| Abstract: | Oxygen bulk diffusion rates were experimentally determined in a natural ultramylonite sample ( c . 5 μ m grain size; 15–20% biotite, 20% quartz, 60–65% feldspars, and minor Fe-oxides) from the Gerrish Island shear zone, SE Maine, USA. The diffusion experiments were performed at 250–550 °C and 100 MPa water pressure. Oxygen bulk diffusion rates were determined both parallel and perpendicular to the strong foliation of the sample. The Arrhenius parameters for transport parallel to the foliation are: D bulk0=2.0×10?11 m2 s?1 and Q =30±6 kJ mol?1. The bulk diffusivity perpendicular to the foliation is about a factor of 3.5 less than that parallel to the foliation with the same activation energy. The values of bulk diffusivity and activation energy obtained are consistent with ionic diffusion through a static aqueous fluid, suggesting that an interconnected fluid exists in the ultramylonite even under hydrostatic conditions. The microstructure of the ultramylonite was characterized using transmission electron microscopy (TEM). The nature and distribution of the interconnected fluid cannot be completely resolved from the TEM analysis; however, the low percentage of three-grain channels and open grain/interphase boundaries suggests that the fluid resides as a thin film on the grain surfaces. The results of this study have direct applications in many important geological settings and provide valuable insights into the observed rapid diffusion rates, strong lithological control and pervasive nature of fluid transport in mica-bearing rocks. |
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| Keywords: | fluid distribution oxygen bulk diffusion TEM characterization ultramylonite |
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