Hydrous silicate melts and the deep mantle H2O cycle

James W.E Drewitt,Michael J Walter,John P Brodholt,Joshua M.R Muir,Oliver T Lord

doi:10.1016/j.epsl.2022.117408

Abstract

We report ab initio atomistic simulations of hydrous silicate melts under deep upper mantle to shallow lower mantle conditions and use them to parameterise density and viscosity across the ternary system MgO-SiO2-H2O (MSH). On the basis of phase relations in the MSH system, primary hydrous partial melts of the mantle have 40-50 mol% H2O. Our results show that these melts will be positively buoyant at the upper and lower boundaries of the mantle transition zone except in very iron-rich compositions, where ≳ 75% Mg is substituted by Fe. Hydrous partial melts will also be highly inviscid. Our results indicate that if melting occurs when wadsleyite transforms to olivine at 410 km, melts will be buoyant and ponding of melts is unexpected. Box models of mantle circulation incorporating the upward mobility of partial melts above and below the transition zone suggest that the upper mantle becomes efficiently hydrated at the expense of the transition zone such that large differences in H2O concentration between the upper mantle, transition zone and lower mantle are difficult to maintain on timescales of mantle recycling. The MORB source mantle with ∼0.02-0.04 wt% H2O may be indicative of the H2O content of the transition zone and lower mantle, resulting in a bulk mantle H2O content of the order 0.5 to 1 ocean mass, which is consistent with geochemical constraints and estimates of subduction ingassing.

Highlights

H2O exerts a powerful control on the location and composition of partial melts in Earth’s deep interior by dramatically reducing the mantle solidus (Hirschmann, 2006)
Stalling and stagnation of slabs in the transition zone may lead to dehydration of serpentinized mantle in cooler slabs providing a mechanism for deep mantle hydration (Iwamori, 2004; Komabayashi and Omori, 2006), a process implicated in deep focus earthquakes and consistent with the petrology of some sublithospheric diamonds from the deep transition zone or shallow lower mantle (Omori et al, 2004; Pearson et al, 2014; Shirey et al, 2021)
All simulated hydrous melts are positively buoyant, inviscid and highly mobile at depths above, throughout and below the transition zone, this behaviour may be modified with the addition of further components such as Fe

Summary

Introduction

H2O exerts a powerful control on the location and composition of partial melts in Earth’s deep interior by dramatically reducing the mantle solidus (Hirschmann, 2006). Relatively insoluble in upper and lower mantle minerals, H2O has a high solubility in wadsleyite and ringwoodite in the transition zone (410-670 km) providing the capacity for storing ∼3 ocean masses of H2O in this reservoir alone (Bolfan-Casanova et al, 2006; Fei and Katsura, 2020; Hirschmann, 2006; Hirschmann et al, 2005; Ohtani, 2015) ( hydrogen substitutes typically as H+ or OH−, hereafter we refer to H2O for simplicity). Stalling and stagnation of slabs in the transition zone may lead to dehydration of serpentinized mantle in cooler slabs providing a mechanism for deep mantle hydration (Iwamori, 2004; Komabayashi and Omori, 2006), a process implicated in deep focus earthquakes and consistent with the petrology of some sublithospheric diamonds from the deep transition zone or shallow lower mantle (Omori et al, 2004; Pearson et al, 2014; Shirey et al, 2021).

Methods

Results

Discussion

Conclusion