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[SIT20-01] Water content in olivine and its polymorphs coexisting with hydrous phases under water undersaturated conditions
Keywords:subducting slab, water cycle, mantle, olivine, hydrous phase, deep-focus earthquake
Olivine is the major mineral in the upper mantle, which transforms to wadsleyite and ringwoodite at 410 km and 520 km depths, respectively, along mantle geotherm. On the other hand, dry olivine can metastably exist even over the phase boundaries in cold subducting slabs due to the delayed transformation kinetics, which has been seismically observed as low-velocity zone in cold slab core. The transformation of metastable olivine is the widely accepted explanation for deep-focus earthquakes. However, many geochemical and geophysical observations and mineral physics data indicate that subducting slabs are hydrated and transport water as hydroxyl groups in both hydrous and nominally anhydrous minerals, into the deep mantle. The presence of metastable olivine in wet slabs is therefore paradoxical, and the hydration state of the slabs remains an open question.
In this study, we conducted water-partitioning experiments between olivine or its high-pressure polymorphs of wadsleyite and ringwoodite and dense hydrous magnesium silicates under water-undersaturated conditions using Kawai-type multi-anvil apparatus. Water contents in olivine and its polymorphs coexisting with hydrous phases were determined by Fourier transform infrared spectroscopy.
We show that olivine and wadsleyite coexisting with hydrous phase A contain less than 1 ppm and ~300 ppm water, respectively, and ringwoodite coexisting with super-hydrous phase B contains ~5 ppm water, which are kinetically dry. Our results imply that olivine and its high-pressure polymorphs show dry transformation kinetics even in wet slabs, resolving the paradox of metastable olivine in wet slabs. Therefore, deep-focus earthquakes and large slab deformation creating stagnant slabs caused by the olivine transformation could occur in the water-undersaturated wet slabs. These processes could be caused jointly by dehydration of hydrous minerals and the subsequent rapid phase transformation when the dehydration starts at lower temperatures than the phase transformation.
In this study, we conducted water-partitioning experiments between olivine or its high-pressure polymorphs of wadsleyite and ringwoodite and dense hydrous magnesium silicates under water-undersaturated conditions using Kawai-type multi-anvil apparatus. Water contents in olivine and its polymorphs coexisting with hydrous phases were determined by Fourier transform infrared spectroscopy.
We show that olivine and wadsleyite coexisting with hydrous phase A contain less than 1 ppm and ~300 ppm water, respectively, and ringwoodite coexisting with super-hydrous phase B contains ~5 ppm water, which are kinetically dry. Our results imply that olivine and its high-pressure polymorphs show dry transformation kinetics even in wet slabs, resolving the paradox of metastable olivine in wet slabs. Therefore, deep-focus earthquakes and large slab deformation creating stagnant slabs caused by the olivine transformation could occur in the water-undersaturated wet slabs. These processes could be caused jointly by dehydration of hydrous minerals and the subsequent rapid phase transformation when the dehydration starts at lower temperatures than the phase transformation.