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[SIT20-P02] Role of water in subducting slabs: dry phase transformation kinetics in wet slabs
Keywords:Hydrous mineral, Nominally hydrous mineral, Subducting slab, Water content, Seismicity, Phase transforamation kinetics
We have investigated the water partitioning between nominally hydrous minerals and hydrous minerals under the water-undersaturated conditions. Our experiments on partitioning of water between olivine and hydrous phase A, wasleyite and hydrous phase A, bridgmanite and phase D/phase d revealed that water is strongly preferred to hydrous phases resulting in formation of nearly dry nominally anhydrous minerals such as olivine (<1 ppm), wadsleyite (<300ppm), and bridgmanite (<1ppm) under the water- undersaturated conditions.
Oceanic lithosphere generated at the ridge originally contains very small amount of water about 100-200 ppm [1]. However, after formation of the oceanic plates, various processes provided hydration of the oceanic plates such as ascending plumes across the lithosphere [2], Putit spot volcanisms recently discovered in the back-arc regions [3], fracturing of the plate bending at the shallow depths of the slab subduction estimated from the seismicity near the trenches [4, 5]. Seismic tomography studies in subducting slabs [6, 7] and distributions of the intermediate earthquakes such as double seismic zones reveal that wet regions in the slabs extend into exceeding 50 km inward from the slab surface [8, 9]. We estimated the average water contents in the slabs based on the analyses of geophysical observations and mineral physics data on hydrous and nominally hydrous minerals by Hacker et al. [10] to be in the range from 1600-3500 ppm. These water contents in the slabs together with our experiments reveal that water in the slabs is stored in hydrous minerals, i.e., the subducting slabs are under the wet conditions, whereas coexisting nominally anhydrous minerals contain very small amount of water, resulting in dry conditions.
Rheology and deformation of dry olivine and its phase transformation kinetics have been applied for the large deformation of subducting slabs in the mantle transition zone [11] and origin of the deep seismicity [12, 13]. Geophysical and mineral physics data together with our experiments revealed that subducting slabs are under the wet conditions, and water in the slabs is stored only in hydrous minerals, and coexisting nominally anhydrous minerals are essentially dry. Therefore, the dry transformation kinetics of olivine and its high-pressure polymorphs can be applied even for the wet slabs under water-undersaturated conditions. Our results also revealed that hydrous minerals with dry anhydrous minerals could exist in wet slabs at the top of the lower mantle. Therefore, some of the deep earthquakes occurred at the depths exceeding 660 km may be caused by dehydration of hydrous minerals such as superhydrous phase B or phase D which is stable at the top of the lower mantle [14].
References:
[1] Peslier, A. H. et al., Space Science Reviews, 212, 743-810 (2017). [2] Seno, T. & Yamanaka, Geophys. Monogr. Ser., vol. 96, edited by G. E. Bebout et al., pp. 347–355, AGU, Washington, D.C. (1996). [3] Hirano et al. Science, 313, 1426-1428. [4] Peacock, S. M. Geology, 29, 299-302 (2001). [5] Kanamori, H. Physics of the Earth and Planetary Interiors, 4, 289-300 (1971). [6] Nakajima, J. et al., Gondwana Research, 16, 470-481 (2009). [7] Bloch, W. et al., Geochemistry, Geophysics, Geosystems, 19, 3189–3207 (2018). [8] Wei, S. S. et al., Science advances, 3, e1601755(2017). [9] Kita, S. & Ferrand, T.P., Scientific reports, 8, 1-11 (2018). [10] Hacker, B. R. et al., Journal of Geophysical Research: Solid Earth, 108, 2030, (2003). [11] Mohiuddin, A. et al., Nature Geosci. 13, 170–174 (2020). [12] Green II, H.W. & Houston, H. Annu. Rev. Earth Planet. Sci. 23, 169–213 (1995). [13] Kirby, S.H. et al., Rev. Geophys. 34, 261–306 (1996). [14] Zahradnik, J. et al., Sci. Rep., 7, 1-11 (2017).
Oceanic lithosphere generated at the ridge originally contains very small amount of water about 100-200 ppm [1]. However, after formation of the oceanic plates, various processes provided hydration of the oceanic plates such as ascending plumes across the lithosphere [2], Putit spot volcanisms recently discovered in the back-arc regions [3], fracturing of the plate bending at the shallow depths of the slab subduction estimated from the seismicity near the trenches [4, 5]. Seismic tomography studies in subducting slabs [6, 7] and distributions of the intermediate earthquakes such as double seismic zones reveal that wet regions in the slabs extend into exceeding 50 km inward from the slab surface [8, 9]. We estimated the average water contents in the slabs based on the analyses of geophysical observations and mineral physics data on hydrous and nominally hydrous minerals by Hacker et al. [10] to be in the range from 1600-3500 ppm. These water contents in the slabs together with our experiments reveal that water in the slabs is stored in hydrous minerals, i.e., the subducting slabs are under the wet conditions, whereas coexisting nominally anhydrous minerals contain very small amount of water, resulting in dry conditions.
Rheology and deformation of dry olivine and its phase transformation kinetics have been applied for the large deformation of subducting slabs in the mantle transition zone [11] and origin of the deep seismicity [12, 13]. Geophysical and mineral physics data together with our experiments revealed that subducting slabs are under the wet conditions, and water in the slabs is stored only in hydrous minerals, and coexisting nominally anhydrous minerals are essentially dry. Therefore, the dry transformation kinetics of olivine and its high-pressure polymorphs can be applied even for the wet slabs under water-undersaturated conditions. Our results also revealed that hydrous minerals with dry anhydrous minerals could exist in wet slabs at the top of the lower mantle. Therefore, some of the deep earthquakes occurred at the depths exceeding 660 km may be caused by dehydration of hydrous minerals such as superhydrous phase B or phase D which is stable at the top of the lower mantle [14].
References:
[1] Peslier, A. H. et al., Space Science Reviews, 212, 743-810 (2017). [2] Seno, T. & Yamanaka, Geophys. Monogr. Ser., vol. 96, edited by G. E. Bebout et al., pp. 347–355, AGU, Washington, D.C. (1996). [3] Hirano et al. Science, 313, 1426-1428. [4] Peacock, S. M. Geology, 29, 299-302 (2001). [5] Kanamori, H. Physics of the Earth and Planetary Interiors, 4, 289-300 (1971). [6] Nakajima, J. et al., Gondwana Research, 16, 470-481 (2009). [7] Bloch, W. et al., Geochemistry, Geophysics, Geosystems, 19, 3189–3207 (2018). [8] Wei, S. S. et al., Science advances, 3, e1601755(2017). [9] Kita, S. & Ferrand, T.P., Scientific reports, 8, 1-11 (2018). [10] Hacker, B. R. et al., Journal of Geophysical Research: Solid Earth, 108, 2030, (2003). [11] Mohiuddin, A. et al., Nature Geosci. 13, 170–174 (2020). [12] Green II, H.W. & Houston, H. Annu. Rev. Earth Planet. Sci. 23, 169–213 (1995). [13] Kirby, S.H. et al., Rev. Geophys. 34, 261–306 (1996). [14] Zahradnik, J. et al., Sci. Rep., 7, 1-11 (2017).