Water Solubility in Fe‐Bearing Wadsleyite at Mantle Transition Zone Temperatures

Abstract

H o w e ve r, th e so lu b ility o f H 2O in Fe -b e arin g w ad sleyite is p o o rly co n strain e d co m p are d w ith rin gw o o d ite an d Fe-fre e w ad sleyite , w h ich h ave b e en exte n sive ly investigated . T h e exact H 2O sto rage cap acity of th e m an tle tran sitio n zo n e th e refo re re m ain s u n kn o w n . H e re w e inve stigate d th e so lu b ility of H 2O in Fe -b e arin g w ad sleyite as a fu n ction o f te m p e ratu re . T h e re su lts in d icate th at w ad sleyite can store ~1 .0 w t.% H 2O at tran sitio n zo n e te m p e ratu re s an d ~0 .6 5 w t.% alon g a p lu m e ge o th erm . H 2O so lu b ility in w ad sleyite is lo w e r th an th at in rin gw o o d ite in m antle p lu m e s. A d e hyd ration m e ltin g layer at th e 52 0 -km d isco n tin u ity n e ar p lu m e s can th erefore fo rm via th e p h ase tran sitio n fro m rin gw o o d ite to w ad sleyite u n d er H 2O -satu rated co n d itio n s d rive n b y u p w e llin g flo w in m antle p lu m e s. K e y w o r d s : M an tle tran sition zo n e; H 2O so lu b ility; Fe-b e arin g w ad sle yite K e y p o i n t s : H 2O so lu b ility in Fe-b e arin g w ad sle yite d e cre ase s w ith in creasin g te m p e ratu re . Fe -b e arin g w ad sle yite can co n tain ~1 .0 w t.% H 2O at m an tle tran sitio n zon e te m p e ratu re . D e h yd ration m e ltin g co u ld o ccu r at th e 52 0 -km d isco n tin u ity b y u p w e llin g flo w n e ar p lu m e s. 1 . I n t r o d u c t i o n W ad sleyite, w ith a ch e m ical fo rm u la o f (M g ,Fe )2SiO 4, is th e d o m in an t m in e ral in th e u p p e r p art of th e m antle tran sition zon e fro m 41 0 to 5 20 km d ep th . B e cau se w ad sleyite can co ntain u p to ~3 .0 w t.% H 2O as hyd roxyl in its crystal stru ctu re (Ko h lste d t et al., 1 9 96 ; Sm yth et al., 19 8 7 ; 1 99 4 ), th e tran sitio n zo n e is co n sid e red to b e a sp on ge in th e Earth 's in terio r. Th e p re se n ce o f H 2O can Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

Water storage capacity of the martian mantle through time

“…2b), which is due to inconsistencies in water measurements in the available literature data on wd. Fei and Katsura (2021) show that at similar temperatures, the water storage capacity in Fe-bearing wd is indeed higher than that in Fe-free wd, but they also noted that there is no experimentally-resolvable effect of iron within the Fe-bearing samples with Mg# > 88, consistent with the regression results in Dong et al (2021). This relatively high variation in the extrapolation of the iron effect for wd is well characterized in the 10 4 regressions of bootstrapped datasets (Fig.…”

Water Enhancement of Si Self‐Diffusion in Wadsleyite

“…Because the viscosity of silicate minerals is considered to be inversely proportional to the diffusion coefficient of Si, the H 2 O enhancement of D Si will induce the reduction of viscosity. Using the C H2O exponent of 0.8, the viscosity of H 2 O‐rich wadsleyite ( C H2O = ∼1.0 wt.%, H 2 O‐saturated wadsleyite at mantle transition zone temperature [Fei & Katsura, 2021]) is by 1.6 orders of magnitude lower than H 2 O‐poor wadsleyite ( C H2O = ∼0.01 wt.%) (Figure 8). On the other hand, although the C H2O exponent for D Si in ringwoodite is unknown, it can be estimated to be r ≈ 1.1 based on the H 2 O dependence of dislocation‐annihilation rate in ringwoodite (Fei et al., 2017).…”

“…Protons are incorporated into wadsleyite primarily by substituting the Mg sites, where one Mg ion is replaced by two protons (e.g., Demouchy et al., 2005; Druzhbin et al., 2021; Fei & Katsura, 2021; Litasov et al., 2011; Purevjav et al., 2016), leading to much higher defect concentrations on the Mg sites than the Si sites. The primary hydration reaction is, $$${{\mathrm{Mg}}_{\text{Mg}}}^{\times }+2{{\mathrm{O}}_{\mathrm{O}}}^{\times }+{\mathrm{H}}_{2}\mathrm{O}={\left\{{\mathrm{V}}_{\text{Mg}}''-2{{(\text{OH})}_{\mathrm{O}}}^{{\bullet}}\right\}}^{\times }+\,\text{MgO}$$$ where the defect association {V Mg ’’−2(OH) O • } × is equivalent with (2H) Mg × .…”

“…A vital feature of the mantle transition zone is that its dominant minerals, wadsleyite and ringwoodite, can dissolve more than 1 wt.% H 2 O in their crystal structures as hydroxyl (e.g., Demouchy et al., 2005; Druzhbin et al., 2021; Fei & Katsura, 2020a, 2021; Kohlstedt et al., 1996; Litasov et al., 2011; Sun et al., 2018). Therefore, the transition zone is expected to be H 2 O‐enriched in contrast to the relatively dry upper and lower mantle, as demonstrated by the H 2 O‐rich ringwoodite inclusion (Pearson et al., 2014), mineral viscosity (Fei et al., 2017), and electrical conductivity analysis (e.g., Dai & Karato, 2009; Huang et al., 2005; Karato, 2011; Kelbert et al., 2009; Manthilake et al., 2009).…”

Stability of H2O‐Rich Fluid in the Deep Mantle Indicated by the MgO‐SiO2‐H2O Phase Relations at 23 GPa and 2,000 K

“…The fundamental issue about water transportation and circulation is how water is stored in the deep mantle. It is known that water in the mantle is primarily held as bonded hydroxyl in the crystal structures of hydrous (Ghosh & Schmidt, 2014; Ohtani et al., 2001; Pamato et al., 2015) and nominally anhydrous minerals (e.g., olivine, wadsleyite, and ringwoodite) (Bell & Rossman, 1992; Bolfan‐Casanova, 2005; Druzhbin et al., 2021; Fei & Katsura, 2020, 2021; Kohlstedt et al., 1996) and as hydroxyl species in water‐induced hydrous silicate melts (Fei, 2021; Ghosh & Schmidt, 2014; Hirth & Kohlstedt, 1996; Kushiro, 1972; Melekhova et al., 2007; Nakajima et al., 2019; O'Hara, 1965; Schmandt et al., 2014). In contrast, the pure or nearly pure H 2 O fluid phase is thought to be present only in the shallow mantle (e.g., less than about 100 km depth) (Bureau & Keppler, 1999; Shen & Keppler, 1997; Wang et al., 2021).…”