Constraining kinetics of metastable olivine in the Marianas slab from seismic observations and dynamic models

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• 作者：Javier Quinteros^a ; ^b ; ^{javier@gfz-potsdam.de} ; Stephan V. Sobolev^a ; ^c
• 关键词：Metastable olivine ; Numerical modeling ; Marianas ; Subduction
• 刊名：Tectonophysics
• 出版年：2012
• 期刊代码：82_00401951
• 类别：ear
• 出版时间：10 March, 2012
• 卷：526-529
• 期：Complete
• 页码：48-55
• 文件大小：1206 K

摘要

Transformation kinetics associated with the presence of a metastable olivine wedge in old and fast subducting slabs has been the subject of many studies in the last years. Even with improvements in kinetics models, many of the parameters are still not well constrained. In particular, there is no consensus on the blocking temperature that could inhibit the transformation from olivine to spinel.

Recently, based on anomalous later phases in the P wave coda and differential P wave slowness, a wedge of metastable olivine was detected in the Marianas subduction zone, that is approximately 25 km wide, at a depth of 590 km and is truncated at 630 km.

In this work, a thermomechanical model was used to mimic the subduction in the Marianas and try different blocking temperatures for the olivine/spinel transformation. The model includes, among other features, non-linear elasto-visco-plastic rheology based on laboratory data, phase transformations, latent heat, proper coupling between stress and thermal state of the slab and force balance of the system.

The results show a positive correlation between the blocking temperature, depth of the wedge and its distance from the trench (or subduction angle). We compare these results to the situation in the Marianas and suggest that a blocking temperature for the olivine/spinel transformation of approximately 725 掳C would be the most likely.

The volume of the wedge presents some oscillations that we relate to a runaway effect of the transformation kinetics in the mantle transition zone. Namely, the interaction between latent heat release and the advection of the isotherms due to the subduction velocity.

The inclusion of shear heating in the model was fundamental to modeling such a subduction zone. Without shear heating, the slab shows a higher level of internal stress and the necessary bending to mimic the Marianas subduction zone cannot be reached.