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We study the interplay of the thermal structure within subducted oceanic slabs together with the stability fields of hydrous phases that control slab dehydration and the amount of water transported into the deeper mantle. We implement different published thermodynamic data for phase A and Mg‐sursassite into a model of 56 subduction zones and evaluate the effect of these phases on the global water budget. We modeled vertical fluid fluxes within the slab such that dehydration‐derived fluid was allowed to react with fluid‐undersaturated rocks in other parts of the slab. The effect of Mg‐sursassite on the global water budget is limited, because the Clapeyron slope of this dehydration reaction is steeper than most pressure‐temperature trajectories in subduction zones. Two sets of published thermodynamic data for phase A yield significantly different values for the amount of deeply subducted water, ranging between 8 × 108 Tg/Ma and 1.4 × 109 Tg/Ma. In some subduction zones, the differences span several orders of magnitude. The absolute modeled amount of deeply subducted water strongly depends on the depth and intensity of slab mantle hydration, but a comparison of modeled and experimental data indicates that the thermodynamic dataset that yielded higher values is more reliable and should be implemented in future thermodynamic models. Our results show that the stable phases around the choke point as well as the slope and position of the phase A‐out reaction influence the deep water release from the slab, but the slope and position of the phase A dehydration reaction mainly control the recycling of water into the deep mantle. Plain Language Summary: Water on our planet is constantly exchanged between the oceans and the interior of the Earth. This happens at subduction zones where hydrated oceanic crust is sinking into the Earth's mantle, thus bringing water to depths exceeding 300 km. We quantify this amount of water that is globally recycled into the deeper Earth in subduction zones through computer models that combine the temperature structure of the subducted rocks with the stability of hydrous phases. Knowledge about the global water budget is important to quantify the Earth's total water amount, to understand the water release from the subducting oceanic plate, and to interpret sea level changes in Earth's history. We compared thermodynamic datasets for two minerals, namely Mg‐sursassite and phase A, in the oceanic plate that can transfer large amounts of water into the deeper Earth. Our thermodynamic models show that the stability of phase A in the down going oceanic plate controls much of the recycled water in most subduction zones. Key Points: Mg‐sursassite has a limited influence on the global amount of subducted waterMg‐sursassite can transfer water to depths of up to 250 km and lead to deep dehydration and water release from the slabThe Clapeyron slope of the phase A dehydration reaction is sub‐parallel to many slab P‐T paths and mainly controls the deep water subduction [ABSTRACT FROM AUTHOR]

Abstract We study the interplay of the thermal structure within subducted oceanic slabs together with the stability fields of hydrous phases that control slab dehydration and the amount of water transported into the deeper mantle. We implement different published thermodynamic data for phase A and Mg‐sursassite into a model of 56 subduction zones and evaluate the effect of these phases on the global water budget. We modeled vertical fluid fluxes within the slab such that dehydration‐derived fluid was allowed to react with fluid‐undersaturated rocks in other parts of the slab. The effect of Mg‐sursassite on the global water budget is limited, because the Clapeyron slope of this dehydration reaction is steeper than most pressure‐temperature trajectories in subduction zones. Two sets of published thermodynamic data for phase A yield significantly different values for the amount of deeply subducted water, ranging between 8 × 108 Tg/Ma and 1.4 × 109 Tg/Ma. In some subduction zones, the differences span several orders of magnitude. The absolute modeled amount of deeply subducted water strongly depends on the depth and intensity of slab mantle hydration, but a comparison of modeled and experimental data indicates that the thermodynamic dataset that yielded higher values is more reliable and should be implemented in future thermodynamic models. Our results show that the stable phases around the choke point as well as the slope and position of the phase A‐out reaction influence the deep water release from the slab, but the slope and position of the phase A dehydration reaction mainly control the recycling of water into the deep mantle.

We report the synthesis, at 7 GPa and 923 K, and the thermoelastic characterization, up to 16 GPa and 850 K, of a single crystal of Mg-sursassite, Mg5Al5Si6O21(OH)7. In situ high-pressure and high-temperature single-crystal diffraction allowed the study of structural variation at non-ambient conditions and the determination of bulk elastic properties. The refined parameters of a second-order Birch-Murnaghan equation of state (BM-II EoS) are V0 = 446.02(1) Å3 and KT0 = 135.6(7) GPa. The thermal expansion coefficients of a Berman-type EoS are α0 = 3.14 (5) × 10−5 K−1, α1 = 2.50(16) × 10−8 K−2, and V0 = 445.94(3). For comparison, the P-V EoS is determined for a natural sursassite sample, ideally Mn4Al6Si6O22(OH)6. The refined parameters of BM-II EoS [V0 = 470.2(3) Å3, KT0 = 128(4) GPa] indicate that composition has a minimal effect on elastic properties. The similarity of density and bulk properties of Mg-sursassite if compared to olivine and other anhydrous mantle minerals suggests that this phase could be overseen by geophysical methods. [ABSTRACT FROM AUTHOR]

The crystal structure of a new high-pressure hydrous phase, Si-rich Mg-sursassite, of ideal composition Mg4Al5Si7O23(OH)5, that was produced by sub-solidus reaction at 24 GPa and 1400 °C in an experiment using a model sedimentary bulk composition, has been determined by single-crystal X‑ray diffraction. The phase was found to be topologically identical to Mg-sursassite, Mg5Al5Si6O21(OH)7, and has space group P21/m and lattice parameters a = 8.4222(7), b = 5.5812(3), c = 9.4055(9) Å, β = 106.793(8)°, V = 423.26(6) Å3, and Z = 1. The empirical formula determined by electron microprobe analysis of the same crystal as was used in the X‑ray experiment is [Mg3.93(3)Fe0.03(1)]S3.96[Al4.98(3)Cr0.04(1)]S5.02 Si7.02(4)O23(OH)5, with hydroxyl content implied by the crystal-structure analysis. The most significant aspect of the structure of Si-rich Mg-sursassite is the presence of octahedrally coordinated Si. Its structural formula is M 1 , VII M g 2 M 2 VI Mg 2 2 + M 3 ,VI ( Al 0.5 Si 0.5 ) 2 M 4 ,VI Al 2 M 5 ,VI Al 2 T 1 ,IV Si 2 T 2 ,IV Si 2 T 3 ,IV Si 2 O 23 (OH) 5. $^{M1,\text{VII}}M{{g}_{2}}{{\,}^{{{M}_{2}}\text{VI}}}\text{Mg}{{_{2}^{2+}}^{\,M3\text{,VI}}}{{\left(\text{A}{{\text{l}}_{\text{0}\text{.5}}}\text{S}{{\text{i}}_{\text{0}\text{.5}}} \right)}_{2}}{{\,}^{M4\text{,VI}}}\text{A}{{\text{l}}_{2}}{{\,}^{{{M}_{5}}\text{,VI}}}\text{A}{{\text{l}}_{2}}^{T1\text{,IV}}\text{S}{{\text{i}}_{\text{2}}}{{\,}^{T2\text{,IV}}}\text{S}{{\text{i}}_{2}}{{\,}^{T3\text{,IV}}}\text{S}{{\text{i}}_{2}}\,\,{{\text{O}}_{\text{23}}}{{\left(\text{OH} \right)}_{\text{5}}}.$ Si-rich Mg-sursassite joins the group of hydrous ultrahigh-pressure phases with octahedrally coordinated Si that have been discovered by experiment, and that may play a significant role in the distribution and hosting of water in the deep mantle at subduction zones. The reactions defining the stability of Si-rich Mg-sursassite are unknown, but are likely to be fundamentally diferent from those of Mg-sursassite, and involve other ultrahigh-pressure dense structures such as phase D, rather than phase A. [ABSTRACT FROM AUTHOR]