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REVIEW 4 major objections 5 minor 75 references

High solubility of water in post-perovskite and bridgmanite in the Earth's deep lower mantle

T0 review · 4 major / 5 minor · reviewed 2026-08-04 · deepseek-v4-flash

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Pith's one-line read Bridgmanite and post-perovskite can hold far more water than previously thought—up to 5,500 ppm and 1.2 wt% respectively—meaning subducting slabs carry water deep into the lower mantle.

desk verdict New Bdg/PPv water solubility data to CMB conditions are a solid advance, but the D/H fractionation evidence is not yet quantitative and the deep low-D/H reservoir story leans on it. read the letter →

arxiv 2608.00039 v1 pith:XOAA4MDT submitted 2026-07-23 physics.geo-ph astro-ph.EPcond-mat.mtrl-sci

Megumi Semba , Naoya Sakamoto , Shuhei Mita , Kei Hirose , Yutaro Tsutsumi , Keisuke Ozawa , John W. Hernlund , Shiro Miwa This is my paper · ORCID
classification physics.geo-phastro-ph.EPcond-mat.mtrl-sci
keywords bridgmanitepost-perovskitewatersolubilitylowermantlecore-mantleboundaryhydrogenisotopesD/Hfractionationbasalmagmaocean
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper reports laser-heated diamond-anvil-cell melting experiments on a hydrous, natural mantle composition covering the entire pressure range of the lower mantle (36–141 GPa). Cryogenic secondary-ion mass spectrometry on the recovered samples shows that bridgmanite, the main lower-mantle mineral, dissolves up to 5,510 ppm water, and post-perovskite, the mineral of the lowermost mantle, dissolves more than 1 wt% (up to 1.22 wt%). The crystals also strongly prefer deuterium over hydrogen relative to coexisting melt, with a fractionation factor of 1.15–1.30, which the authors take as proof that the water sits in the mineral lattice rather than in melt inclusions. From this they argue that subducting slabs do not dehydrate when they reach the core-mantle boundary, and that water-rich bridgmanite and post-perovskite cumulates in large low-shear-velocity provinces and ultralow-velocity zones could hold a substantial, D-depleted deep water reservoir inherited from a basal magma ocean. If these findings hold, the lowermost mantle may be one of Earth's largest water reservoirs.

What carries the argument

The argument is carried by the combination of laser-heated diamond-anvil-cell melting experiments (36–141 GPa, 3,340–4,860 K) with fully cryogenic sample recovery and Cryo-SIMS imaging of 1H+ and 2D+ secondary ions, which lets the authors measure water concentrations and D/H ratios in individual crystal and melt regions while avoiding hydrogen contamination and loss. The key physical quantity is the hydrogen isotope fractionation factor α=(D/H)_crystal/(D/H)_melt=1.15–1.30, which both proves the water is lattice-bound and (via the temperature dependence α≈1+a/T² with a=2.95×10^6) seeds the prediction of a deep, D-depleted water reservoir. Water incorporation is attributed mainly to the coupl

What would settle it

Synthesize a bridgmanite crystal with a known D/H ratio, measure it by the same Cryo-SIMS method alongside a melt glass of identical D/H; if the measured D/H ratio differs between the two phases, the reported fractionation is a matrix artifact rather than a real effect.

Watch

Extended reading notes

Core claim

The paper's central claim is that water solubility in the two dominant minerals of the deep lower mantle is far higher than previously recognized—bridgmanite hosts up to 5,510 ppm (H2O+D2O) and post-perovskite up to 1.22 wt% at core-mantle-boundary pressures and temperatures. The authors further report strong hydrogen isotope fractionation, α=(D/H)_Bdg/(D/H)_melt=1.15–1.30, with crystals enriched in deuterium relative to coexisting melt, which they use to show the water is truly in the crystal lattice. They conclude that dehydration of subducting slabs is not a viable source of melt or hydrogen at the base of the mantle, and that the large low-shear-velocity provinces and ultralow-velocity z

Load-bearing premise

The deep low-D/H reservoir story depends on the measured deuterium enrichment in bridgmanite and post-perovskite relative to coexisting melt being a real chemical effect, not an artifact of measuring two different materials (crystalline mineral versus glassy melt) with the same ion microprobe—no matrix-matched D/H standard is available to rule that out.

Editorial extensions

If this is right

  • Subducting slabs that reach the core-mantle boundary will retain their water rather than release it; dehydration melting cannot explain ULVZ-like seismic structures or a hydrogen-rich layer at the top of the outer core.
  • Hydrous post-perovskite and bridgmanite forming in a crystallizing basal magma ocean can store roughly a third of the water present in the initial magma ocean, sequestering it in LLSVPs and ULVZs.
  • Water stabilizes post-perovskite relative to bridgmanite, so hydrous cumulates could explain post-perovskite-like seismic discontinuities inside LLSVPs.
  • Plumes entraining this material could deliver D-depleted water (δD as low as –400 to –510‰ at 2,000 K), matching negative D/H anomalies in some ocean island basalts.
  • Combined reservoirs imply the core may contain at least 0.3 wt% hydrogen.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If deep mantle minerals really hold percent-level water, other volatile elements such as carbon or nitrogen may be similarly hosted in Bdg/PPv, implying a much larger deep-mantle volatile cycle than current models assume.
  • The strong D/H fractionation between crystals and melt means mantle convection and partial melting should produce measurable isotopic heterogeneity in the deep mantle—plume lavas with low D/H may be a tracer of unfractionated, ancient mantle rather than recycled surface water.
  • The measured positive pressure dependence of water solubility suggests that with slightly higher temperatures at the CMB, the capacity could exceed the observed values, making a fully hydrated D″ layer plausible if enough water is transported there.
  • A direct experimental extension would be to dope the starting material with variable Al2O3 to test whether PPv water capacity scales with Al content as the authors assume for natural pyrolite.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

Summary. The paper reports laser-heated diamond-anvil cell melting experiments on a hydrous pyrolite composition across 36–141 GPa and 3,340–4,860 K, using synchrotron XRD to identify liquidus phases and Cryo-SIMS to measure H2O/D2O abundances and D/H ratios in coexisting Bdg/PPv and melt. It reports high water solubilities in bridgmanite (up to 5,510 ppm) and post-perovskite (up to 1.22 wt%), a positive pressure dependence of the Bdg/melt H2O partition coefficient (Eq. 1), and D/H fractionation between crystals and melt (α = 1.15–1.30, Eq. 3). From these data the paper argues that subducting slabs do not release water at the CMB, and that hydrous Bdg/PPv cumulates in LLSVPs/ULVZs could constitute a deep low-D/H reservoir that may explain negative δD values reported in some plume-derived lavas.

Significance. If the solubility measurements hold, this is a field-changing result: it challenges the prevailing view that Bdg and PPv are nearly dry at deep lower-mantle conditions and implies that the lowermost mantle can store water at the wt% level, with direct consequences for slab dehydration, ULVZ/LLSVP origins, and the deep volatile cycle. The cryogenic sample handling, two-dimensional SIMS imaging of H and D in crystal layers away from melt pools, and the explicit reporting of raw data are clear strengths. However, the absolute concentration calibration and the D/H fractionation factor rest on assumptions that are not fully validated, and the BMO/geochemical 'predictions' are scenario calculations that depend on the paper's own fitted parameters. The solubility data and the geochemical narrative should be evaluated separately.

major comments (4)
  1. [Methods: SIMS analyses] Absolute H2O and D2O concentrations in Bdg/PPv are derived from a calibration curve (1H+/28Si+ versus H/Si mass ratio) built on three silicate glasses (Extended Data Fig. 8a). No Bdg/PPv standard of known water content is used. The relative sensitivity factor for H/Si in a (Mg,Fe,Al)SiO3 perovskite or post-perovskite matrix may differ from that in rhyolitic glass; if so, the reported solubilities (e.g., 1.22 wt% in PPv) could be systematically biased. At minimum, the authors should justify the transfer of the glass calibration to crystalline Bdg/PPv, or provide a cross-check with an independent method (e.g., FTIR or atom-probe quantification).
  2. [Methods: Matrix effect in hydrogen isotope analysis by SIMS] The argument that α = (D/H)Bdg/(D/H)melt is free of matrix effects is incomplete. Cancellation of the D/H relative sensitivity requires the relative sensitivity factor R = (D/H)measured/(D/H)true to be identical in Bdg and melt; identical ion energy does not guarantee this. The calibration was performed on a single rhyolitic glass, not on Bdg/PPv. Therefore α = 1.15–1.30 may be an artifact of differing ionization efficiencies between crystalline silicate and melt. This affects the lattice-residence argument (main text, 'The difference in the D/H ratio...') and all deuterium modeling in Fig. 4. A crystalline Bdg/PPv D/H standard with independently known D/H is needed.
  3. [Crystallization of hydrous Bdg and PPv in a basal magma ocean; Fig. 4] The deep low-D/H reservoir story is not an independent prediction: the δD values in Fig. 4 are computed using the paper's own fitted α (Eq. 3) and D(H2O) (Eq. 1), combined with assumed model choices (F=0.5, Bdg:Fp=6:4, PPv-in at ~114 GPa, and an initial H2O of 1,075 ppm derived by mass balance with an assumed complement reservoir). The apparent match to Koolau/Baffin Island δD values is therefore a consequence of the assumptions, not a test. The authors should provide a sensitivity analysis to these choices or reframe the results as a scenario calculation rather than a prediction.
  4. [Fate of water transported by subducting slabs into the deep lower mantle] The conclusion that subducting slabs do not release water at the CMB extrapolates the experiments to conditions not covered by the data: the present runs are at 3,340–4,860 K with 2–3 wt% Al2O3 in Bdg/PPv, whereas slabs are cold and Al-poor. The estimate of ~1,000 ppm H2O transported, and the assumed Al3+:H+ = 1:1 capacity limit for Al-poor Bdg, are reasonable but unverified. This should be stated explicitly as an extrapolation; the abstract's phrasing ('subducting slabs do not release water') is stronger than the evidence supports.
minor comments (5)
  1. [Fig. 3 caption / main text] The caption notes that low-temperature uncertainty may be larger than illustrated, yet the main text uses Eq. (3) at 2,000 K (α = 1.7) to give 'as low as δD = –400 to –510‰' without propagating this uncertainty. Please quantify or soften the statement.
  2. [Methods, 'Electron microprobe analyses'] Typo: 'Extended Datra Fig. 3' should read 'Extended Data Fig. 3'.
  3. [Methods, 'Modelling the evolution of a basal magma ocean', Eq. (5)] The notation in Eq. (5) is hard to follow: the integration limits and the meaning of m are not fully defined. Please rewrite with explicit definitions.
  4. [Extended Data Table 1] Many melt D2O cells are blank (e.g., runs 2, 4–8, 10–13). Please add placeholders or a note explaining which samples were excluded from D2O analysis and why, to avoid the appearance of missing data.
  5. [Abstract / main text] The phrase 'subducting slabs do not release water when they reach the bottom of the mantle' is a strong conclusion. Consider 'are unlikely to release water' or 'may retain water' to match the extrapolative nature of the argument.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: solubility and partition coefficients are direct Cryo-SIMS measurements; the BMO δD outputs are explicit model extrapolations using those measured parameters, not inputs disguised as predictions.

full rationale

The paper's central claims rest on primary experimental measurements: water contents in Bdg (1,850–5,510 ppm) and PPv (0.65–1.22 wt%) and D/H fractionation factors α = 1.15–1.30 were directly measured by Cryo-SIMS with calibration against standard glasses and compared with external data (e.g., Lu et al. 2025). Equations (1) and (3) are empirical fits to these measured partition coefficients and fractionation factors; they are not definitions of the quantities they later feed into the BMO model. The modeled BMO evolution and δD values are explicitly described as calculations using Eq. (1), Eq. (3), and Monte Carlo error propagation, so the low-δD outputs are model extrapolations (notably α at 2,000 K) rather than circular 'predictions' that are equivalent to the inputs. The inference that Bdg/PPv water is lattice-bound follows from the observed D/H difference between crystal and coexisting melt plus textural observations; while this inference would be invalid if the D/H difference were a SIMS matrix artifact, that is a measurement-validity concern, not a circularity of the derivation chain. Self-citations (Cryo-SIMS method, BMO solidification model) supply methodology and background, but the load-bearing solubility results are not justified by those citations and are independently supported by comparison with earlier experiments. Thus no circular step is present.

Assumptions & free parameters 7 free parameters · 8 assumptions · 1 invented entities

The experimental solubility data rest on standard petrology assumptions (equilibrium, cryogenic preservation, SIMS calibration), while the BMO/geochemical narrative adds several unverified choices: F = 0.5, Bdg:Fp = 6:4, 114 GPa PPv-in, anhydrous Fp, and a model-internal back-calculation of the initial magma-ocean water content. The D/H reservoir prediction follows from the paper's own fitted α and is not independently constrained.

free parameters (7)
  • log10 DH2O(Bdg/melt) Eq. 1 coefficients = -0.06(44), -6.2(14)e3 K, 20.6(87) K/GPa
    Fit to Bdg/melt H2O partition data from this study and Lu et al. (2025); used to extrapolate D at CMB conditions and in the BMO model.
  • a in α = 1 + a/T^2 = a = 2.95(26)e6 K^2
    Fit to cryo-SIMS D/H fractionation data at 36–77 GPa; extrapolated to 2,000 K in the BMO model to predict D-depleted plume water.
  • Residual melt fraction at rheological transition F = 0.5
    Assumed from Caracas et al. 2019; sets the initial BMO mass and controls the 35.2% H2O sequestration estimate.
  • Crystallizing solid ratio Bdg:Fp = 6:4 by weight
    Assumed for fractional crystallization of the BMO; ferropericlase is treated as completely anhydrous.
  • Bdg-PPv transition pressure in hydrous BMO = ~114 GPa
    Inferred from run #9 (Bdg+PPv+melt at 117 GPa, 4,240 K) and used to switch the crystallizing phase in the model.
  • Initial magma-ocean H2O and δD = 1,075 ppm; δD = -67.6‰
    Back-calculated from the model's 35.2% sequestration fraction and the assumed complementary reservoir (710 ppm, δD = -38‰); not directly measured.
  • Al3+:H+ = 1:1 maximum for Al-poor Bdg = ~2,000 ppm H2O at ~1 wt% Al2O3
    Assumed crystal-chemical bound used to argue that Al-poor slab Bdg will not dehydrate at the CMB.
assumptions (8)
  • domain assumption Water partitioning reached chemical equilibrium between melt and Bdg/PPv during 3–10 s laser heating.
    Methods cite a 1 s equilibrium time for SiO2-water (Tsutsumi et al. 2024) and apply it to Bdg/PPv.
  • domain assumption Cryo-SIMS preserves original H/D distributions and avoids ambient hydrogen contamination.
    Relies on Sakamoto et al. 2026 (ref 25); run #2 shows the method can fail when cryo conditions are interrupted.
  • domain assumption SIMS D/H matrix effects between crystalline Bdg/PPv and silicate melt are negligible or smaller than observed α.
    The Methods 'Matrix effect' section asserts this without a phase-matched calibration standard.
  • domain assumption H2O and D2O solubility and partitioning differ only by the single isotopic fractionation factor α.
    The BMO model sets DD = DH × α; no independent test verifies that D2O partitioning equals H2O partitioning across all conditions.
  • domain assumption The BMO solidified by batch crystallization until F = 0.5, then fractional crystallization of Bdg:Fp = 6:4 at its top.
    Adopted from Ozawa et al. 2024 and Caracas et al. 2019; load-bearing for the 35.2% sequestration estimate.
  • domain assumption Hydrous PPv is stable above ~114 GPa in the BMO.
    Based on run #9 three-phase coexistence and first-principles calculations (ref 12); used to produce PPv cumulates in LLSVPs/ULVZs.
  • domain assumption The complementary reservoir has 710 ppm H2O and δD = -38‰.
    From Hirschmann 2018, Lécuyer et al. 1998, and Loewen et al. 2019; used to calibrate the initial magma-ocean water content.
  • domain assumption Si4+ = Al3+ + H+ is the main water incorporation mechanism, so Al content scales water capacity.
    Extended Data Fig. 6 supports this for the measured Al-rich crystals; extrapolation to Al-poor slab Bdg/PPv is assumed.
invented entities (1)
  • Sequestrated deep water reservoir in hydrous LLSVPs/ULVZs (PPv/Bdg cumulates of a basal magma ocean) independent evidence
    purpose: Explain low-δD plume lavas and store ~35% of Earth's initial H2O
    Falsifiable handles: predicted >0.4–1 wt% H2O in PPv/Bdg cumulates, positive δD in LLSVPs, and D-depleted water (δD −400 to −510‰) released by plumes. These can be tested by geochemistry and seismology, though no direct observation is reported.

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Pith. "Pith review of High solubility of water in post-perovskite and bridgmanite in the Earth's deep lower mantle." pith.science (2026). https://pith.science/paper/XOAA4MDT

@misc{pith2026260800039,
  author       = {Pith},
  title        = {Pith review of: High solubility of water in post-perovskite and bridgmanite in the Earth's deep lower mantle},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/XOAA4MDT}},
  note         = {Machine review of arXiv:2608.00039}
}
abstract

Water in the Earth's mantle induces melting, giving rise to strong chemical heterogeneity, and alters its rheological and transport properties, which are keys to understanding seismic-wave speeds anomalies, convective motions and electrical conductivity. However, the abundance of water and its role in the deep mantle remain uncertain and controversial, particularly at conditions of the core-mantle boundary (CMB) region. Here we carry out melting experiments on a hydrous, natural mantle composition including both $H_2O$ and $D_2O$ over the entire pressure range of the Earth's lower mantle, in which melt coexists with bridgmanite (Bdg) and/or post-perovskite (PPv), the primary constituents of the respective lower and lowermost mantle. High-resolution secondary-ion mass spectroscopy (Cryo-SIMS) measurements reveal high water concentrations in Bdg (up to 5,500 ppm by weight) and PPv (up to 1.22 wt%) with strong enrichment in deuterium/hydrogen (D/H) relative to coexisting melt. The high solubilities of water in Bdg and PPv at CMB conditions suggest that subducting slabs do not release water when they reach the bottom of the mantle, and such a process cannot account for strong chemical heterogeneities inferred in the lowermost mantle and the topmost outer core, nor for ultralow-velocity zone (ULVZ)-like structures in seismically fast regions. Water-rich Bdg and PPv may be present in the large low shear velocity provinces and ULVZs, which may be the residue of a basal magma ocean and are predicted to host a deep low D/H water reservoir inherited from the early Earth that is occasionally sampled by plumes of a lowermost-mantle origin.

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    && " *' && - - - - -

    considering Bdg dehydration at the middle/shallow lower mantle and also from that of the BMO residuum (black) when it solidifies and releases water to surrounding PPv/Bdg in upwellings, which dehydrates at shallower depths accompanying isotopic fractionation. See the main text...

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Pith tools

Reviewed August 4, 2026 · model on record in the stance chip above.