The Electrical Conductivity of Dehydrating Lawsonite and Implications for Cascadia Subduction Zone Structure
Abstract
Electromagnetic data have been increasingly used to image subduction zones, and electrical models commonly show features related to the release of fluids at several depths, consistent with thermal and petrologic models of dehydration of the downgoing slab. There are, however, significant variabilities in structure, even within a single subduction system, that possibly relate to differences in fluid input into the subduction system from the incoming plate. Anomalous seafloors (hosting features such as fracture zones, ridges, seamounts) represent one vehicle for high fluid flux by allowing a greater addition of fluid penetration into the mantle with subsequent higher degrees of formation of fluid rich alteration minerals.
In Cascadia, examination of the seafloor geology shows evidence for a region of damage that could have contributed to enrich the subduction system in volatiles. A suite of hydrous phases is thermodynamically stable at specific depth and temperature ranges in the incoming hydrated oceanic crust and upper mantle. These phases contribute to the H2O flux in the subduction zone and in particular, lawsonite represents an efficient water reservoir as it can carry about 12 wt.% water and is stable over a wide pressure range. We report an experimental investigation of the electrical properties of natural polycrystalline lawsonite from Reed Station, CA. Experiments were performed up to 1325°C and under pressure from 1 to 10 GPa using a multi-anvil apparatus. We observe that temperature increases lawsonite conductivity until fluids escape the cell after dehydration occurs. The effect of pressure on conductivity is not linear and at 500°C, conductivity measurements during compression indicate electrical transitions at about 4.0 and 9.7 GPa that are consistent with crystallographic transitions. During dehydration, the electrical variation corresponds to a diffusion-limited process; at 8 GPa, a diffusion coefficient of 3x10-10m2/s is obtained at 975°C and is consistent with water diffusion. We suggest that lawsonite dehydration could contribute to high conductivity anomalies observed in the Cascades by releasing aqueous fluid at a depth ( 50 km) consistent with the basalt-eclogite transition.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T24B..07P
- Keywords:
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- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICSDE: 8170 Subduction zone processes;
- TECTONOPHYSICSDE: 8180 Tomography;
- TECTONOPHYSICSDE: 8185 Volcanic arcs;
- TECTONOPHYSICS