Iron disproportionation in peridotite fragments from the mantle transition zone

Domain: earth_science
Published: 2025-07-01
Editorial outcome: major_revision

Abstract
Previous high-temperature-pressure experiments predicted metallic iron’s potential presence in the deep mantle below 250 km, arising from ferrous disproportionation in silicates, which could profoundly impact the redox environment and physicochemical properties. However, direct natural petrological evidence has been lacking, except scant clues like Fe-alloy inclusions in ultradeep diamonds. Here we present peridotite fragments, found in Cenozoic basalts from eastern China, containing decomposed Na-rich majoritic garnets (from depths of 410-550 km) and olivine with Fe0-spinel-bearing inclusions, likely originated from retrograded wadsleyite/ringwoodite. Enriched Zn-Sr isotopic compositions of the decomposed garnet indicate an origin associated with the stagnant Pacific slab in the mantle transition zone. Disproportionation of iron is evidenced by widely distributed submicron-sized spherical Fe-Ni alloys and Fe3+-rich (Fe3+/ΣFe = 0.35-0.40) olivine. These findings provide compelling evidence for recycling of stagnant slab components in the eastern Asia big mantle wedge (BMW), and iron disproportionation in the deep mantle.

Introduction
Iron is the most abundant redox-sensitive element in the deep mantle, and the relative concentrations of Fe3+, Fe2+, and Fe0 can well record the variations in oxygen fugacity1. These variations play a crucial role in the recycling of volatile elements like carbon, hydrogen, and sulfur between Earth’s interior and surface2,3. A prevailing notion is that considerable portions of the mantle below 250 km are saturated with an iron–nickel rich metal phase formed by iron disproportionation in silicates, with the redox state controlled by the iron–wüstite (IW) buffer4–6, as oxygen fugacity decreases with depth in Earth’s interior7. This pressure-driven auto redox reaction is evidenced in samples from both Earth and Moon’s surfaces, particularly in meteorite impact zones8,9. Some ultradeep diamonds (>300 km) contain native iron-alloy inclusions attributed to this deep mantle process10. Although deep upper mantle and uppermost lower mantle silicate minerals often exhibit high ferric iron content (Fe3+/ΣFe = 0.15–0.87)11–13, native iron-bearing phases have not been concurrently found. This discrepancy is likely due to oxidation caused by infiltration of oxidized carbonated fluids/melts, rather than disproportionation of iron in a reducing setting11. Thus, the underlying mechanism governing iron’s redox state variation in the deep mantle remains enigmatic, lacking direct petrological evidence.

Here, we present five “garnet” lherzolite xenoliths extracted from Cenozoic alkaline basalts located in Zhejiang Province, Southeast China (Supplementary Fig. 1). In this area, Oligocene (27–23 Ma) basanite-tephrite rocks are distributed in the interior of the continent, while Miocene to Pliocene (17–2 Ma) basanite, alkaline basalt, and olivine tholeiite are found along the coast. The eruption ages of these rocks decrease oceanward, with an ~100-km spatial gap, correlating temporally and spatially with repeated retreats and rollbacks of the westward subducting Pacific plate during the Tertiary period14. The host rock for the studied xenoliths is nephelinite, with SiO2 ranging from 39.99 to 40.76 wt.%, Al2O3 from 9.55 to 10.30 wt.%, MgO from 12.12 to 14.17 wt.% (Mg# = 64–67), CaO from 10.17 to 10.76 wt.%, Na2O from 4.39 to 5.02 wt.%, and a Na2O/K2O ratio of 2.3 to 2.5 (Supplementary Data 1). The ferric iron contents (Fe3+/ΣFe) of the nephelinite range from 0.25 to 0.31, which is consistent with the Cenozoic highly oxidized intraplate basalts in eastern China15. The contemporaneous nephelinite in eastern China have been suggested to originate from the mantle transition zone (MTZ)16. The “garnet” lherzolite xenoliths (Fig. 1a) exhibit minimal hydrothermal alteration, appearing fresh with a porphyroclastic texture. They consist mainly olivine (Ol1, ~45 vol.%), orthopyroxene (~25 vol.%), clinopyroxene (~20 vol.%), and kelyphitized garnet (K-Grt, ~10 vol.%).

Results
The “garnets” in the lherzolites are completely kelyphitized, displaying a distinct core-rim structure (Fig. 1a). The rim is composed of anhedral orthopyroxene, clinopyroxene, spinel, and plagioclase (Fig. 1b and Supplementary Fig. 2), with variable, out of equilibrium chemical compositions (Supplementary Data 2). These minerals result from rapid exhumation via the reaction between garnet and olivine (Reaction 1: Grt + Ol1 = Opx + Cpx + Sp/Pl), with plagioclase representing the terminal phase of this retrograde reaction. Notably, the presence of plagioclase suggests sodium and potassium loss from the K-Grt core (Supplementary Fig. 2).

The K-Grt core dominantly consists of orthopyroxene, olivine (Ol2), spinel, Fe-Ni alloys, and a Na-rich matrix (Fig. 1c-d and Supplementary Fig. 3). Fe-Ni alloys are widespread in the core but tend to diminish near the rim (Fig. 1b). Two domains are discernible in the core: Opx + Sp + Na-rich matrix and Ol2 + Sp + Na-rich matrix. The Na-rich matrix in the decomposed garnet is dominantly composed of plagioclase crystallites with variable Na2O, K2O, and CaO contents (Supplementary Fig. 4). Complex multiphase symplectite-bearing coronae around breakdown garnets are often found in exhumed mantle xenoliths formed through rapid decompression reaction17,18. Previous studies on garnets from Zinst have interpreted sodium enrichment in the kelyphite garnet as a result of partial metasomatism by external Na-rich carbonate-bearing melts or fluids in the uppermost mantle (<60 km)17. However, this is inconsistent with the formation of the K-Grt core in this study, despite the similar microstructure to the isochemical breakdown of garnet observed in those studies. All K-Grt cores in this study contain visible Fe-Ni spherules, indicating highly reduced conditions, which contrasts with the highly oxidized host nephelinite and precludes a reaction mechanism involving external Na-rich carbonate-bearing melts or fluids. Instead, the K-Grt core is likely the breakdown product of the parental garnet itself (Reaction 2: Grt = Opx + Sp + An ± Cpx)18. In addition, the host nephelinite shows significant enrichment in lithophile elements, consistent with the Cenozoic OIB-type intraplate basalts in eastern China (Supplementary Fig. 5a). However, in-situ trace element analyses of the core show typical lithospheric garnet features, i.e., very low lithophile element concentrations and enrichment of heavy rare earth elements (Supplementary Fig. 5 and Supplementary Data 3), indicating a relatively closed isostoichiometric system and minimal exchange with external fluids/melts. Similar isochemical breakdown of natural garnet has been previously observed in orogenic garnet peridotites19. The widespread occurrence of olivine and the Na-rich matrix in the decomposed garnet likely formed by decompression-induced incongruent melting18,20.

Seven K-Grt cores from three xenoliths were selected for major element analysis of the parental garnet. To mitigate the challenge of inhomogeneous composition caused by kelyphitization, a 25-μm diameter electronic beam was used for electron microprobe measurement, taking the average of more than fifteen different areas in each K-Grt core as the representative composition of the parental garnet (Supplementary Data 4). The parental garnets show compositions of SiO2 (44.42–45.00 wt.%), Al2O3 (22.33–23.05 wt.%), MgO (18.62–20.41 wt.%), Cr2O3 (1.60–1.78 wt.%), CaO (4.84–5.69 wt.%), FeO (3.39–3.90 wt.%), TiO2 (0.18-0.24 wt.%), along with variable Na2O (0.75–2.41 wt.%) and K2O (0.11–0.73 wt.%) contents. The variations in sodium and potassium contents are attributed to partial migration into the K-Grt rim (Fig. 1b and Supplementary Fig. 2). Standard formula calculation shows high Si+Ti (3.13–3.15 per formula unit (p.f.u.)), along with low Mg+Fe+Ca+Mn+Na+K (2.83–2.96 p.f.u.) contents in the parentalgarnets. Given that a substantial portion (10-30%) of iron in majoritic garnets11 should exist as Fe3+, this imbalance would be exacerbated under the conditions of the deep mantle. This suggests a loss of highly mobile cations, likely Na and K, during kelyphitization. Evidence for this loss of Na and K from the K-Grt core is apparent in the elemental maps of the K-Grt rim (Supplementary Fig. 2). As these components were not included in the garnet reconstruction process, the proportions of Na-rich majorite represent the lowest conceivable contents for the studied garnets.

The formation of majorite (Maj) at high pressure relies on two substitution mechanisms21: Mg-Maj substitution in a peridotitic composition (Reaction 3: 2Al3+ = M2+ + Si4+) and Na-Maj substitution in an eclogitic composition (Reaction 4: Al3+ + M2+ = X+ + Si4+), where M2+ is for Mg2+, Fe2+, Ca2+, Mn2+ and X+ is for Na+ and K+, respectively. High-temperature and high-pressure experiments have demonstrated that the composition of majoritic garnet is pressure-dependent13,21–23, making it a reliable and widely accepted pressure indicator for deep mantle materials (at depths of 300–660 km). The parental garnet reconstructed in this study belongs to the group of eclogite-derived majoritic garnet (Supplementary Fig. 6a, b). Phase variations of synthetic eclogite were performed in a simplified diopside (Di)–hedenbergite (Hd)–jadeite (Jd) system over a pressure range of 7–24 GPa, yielding a linear correlation between the Na content in majoritic garnet and pressure (PB08)23. Using this empirical barometer, we obtained an equilibrium pressure of 14.5-19.0 GPa (Supplementary Data 4). Additionally, a new barometer (PB17) based on high-temperature and high-pressure experiments of basaltic and pyroxenitic compositions21, yielded a comparable equilibrium pressure of 13.3–18.9 GPa for the parental majoritic garnet (Fig. 2a). Therefore, the high Na-K contents of the parental majoritic garnet imply an origin in the MTZ.

Seismic tomography shows the subducting Pacific slab is stagnant in the MTZ beneath eastern Asia24–26. The Na-rich majoritic garnet lherzolite in this study is hosted within Cenozoic basaltic volcanics located above the stagnant slab. Previous studies on the isotopic compositions of the basalts have speculated on the possible recycling of materials from the stagnant slab14,27. To trace the origin of the decomposed garnets, we analyzed four powdered samples (300 mg for each) for whole-rock Sr, Nd, and Zn isotopic compositions, given the notably low Sr (<5 ppm) and Nd (<1 ppm) concentrations (Supplementary Data 3). The results (Supplementary Data 5) reveal depleted Nd isotopic compositions (143Nd/144Nd = 0.5131–0.5132) and slightly enriched Sr isotopic compositions (87Sr/86Sr = 0.7039–0.7042). Besides, these decomposed garnets show elevated zinc isotopic compositions (δ66Zn = 0.32–0.33‰), compared to those (δ66Zn = 0.2–0.3‰) of the mantle and oceanic crust28,29. Their zinc isotopic compositions are consistent with those of the Cenozoic intraplate basalts in eastern China, which are proposed to have significant carbonated eclogite (subducted Pacific MORB residuum) contributions in their mantle source28,29. The Sr and Nd isotopic feature differs from the typical Sr-Nd isotopic evolution of the depleted MORB mantle and the enriched mantles influenced by subducted continental crust (Fig. 2b). Instead, the Sr–Nd–Zn isotopic characteristics of these majoritic garnets are similar to those of the carbonated MORB (Fig. 2b and Supplementary Fig. 6c), suggesting that they likely form via reactions with components from the stagnant Pacific slab within the MTZ24–26. Interaction with peridotitic components during these reactions may have led to enrichment in Mg and Cr, and depletion in Ca and Ti, along with elevated Na and K contents, resulting in the observed transitional geochemical features21.

A third type of olivine (Ol3) is found intergrown with relatively large kelyphitized garnets (Fig. 3a and Supplementary Fig. 7). These olivine inclusions are black, ellipsoidal or irregular in shape, ranging in size from 0.5 to 3.0 mm. Backscattered electron micrographs show widespread tiny Fe-Ni alloy inclusions within these olivine grains (Fig. 3b). High-angle annular dark-field scanning transmission electron micrographs also reveal many elliptic inclusions in olivine-Ol3, ranging from nearly 10 to 100 nm (Fig. 3c and Supplementary Figs. 8a–f). Additionally, one octahedral inclusion (a typical shape for spinel) was discovered coexisting with an elliptical inclusion and two Fe-Ni alloy inclusions in the same focus ion beam (FIB) foil (Fig. 3c and Supplementary Figs. 8g–l). Their compositions are detailed in Supplementary Data 6. High-resolution transmission electron microscope (HRTEM) images and corresponding fast Fourier transform (FFT)/nano-beam electron diffraction (NBD) reveal comparable Mioré Patterns between the octahedral inclusion and elliptic inclusions (Supplementary Fig. 8m–o). Their NBD patterns can be indexed as a mixture of olivine (Ol4) and spinel. Amorphous silicon dioxide (A-SiO2) is found adjacent to both the octahedral inclusion and an Fe-Ni alloy inclusion (Fig. 3c).

The valence states of iron in olivine (Ol3), elliptic inclusion and Fe-Ni alloy inclusion were measured using electron energy loss spectroscopy (EELS). Fe3+ content was assessed using a modified integral intensity ratio of the Fe L2 and L3 white lines, yielding Fe3+/ΣFe ratios of 0.35 for olivine (Ol3) and 0.40 for the elliptic inclusion (Fig. 3d). Molecular formula calculations indicate total cations of the olivine (Ol3) and elliptic inclusions within the range of 3.10-3.29 (Supplementary Data 6), higher than those of standard olivine and spinel (A2BO4 with total cations of 3), implying the likely presence of minor nano-Fe-Ni alloys. These nano-Fe0 dots were sometimes observed in HRTEM images. In addition, the elliptic inclusion presents a more pronounced shoulder peak at 708.4 eV compared to the olivine (Ol3) (Fig. 3d), demonstrating the presence of more nano-Fe0. Analysis of the relative contents of Si, Al, and Cr indicates spinel proportions ranging from 7% to 27% in the elliptic inclusions and nearly 20% olivine (Ol4) in the octahedral inclusion (Supplementary Data 6).

High-temperature-pressure experiments in natural pyrolite lherzite lherzite lherzite lherzolite show that the high-pressure polyform olivine (HP-Ol) (i.e. wadsleyite/ringwoodite), dominant in the MTZ, often contains significant aluminum (up to 1 wt.%) in their modified-spinel/spinel crystal structures for a high-pressure AB2O4 stoichiometry30,31. In the upwelling asthenosphere, these aluminum-rich HP-Ol minerals become unstable and tend to decompose into olivine ((Mg,Fe)2SiO4) and spinel ((Mg,Fe)Al2O4). Thus, both the elliptic inclusions (Ol4+Sp) and the octahedral inclusion (Sp+Ol4) with comparable Mioré Patterns in the studied olivine (Ol3) are attributed to be the residual phases of retrograded HP-Ol under descending pressure. Totally, as the Na-rich majoritic garnet host originated from depth of 410–550 km, all the olivine (Ol3) crystals were considered to be the final phase of retrograded HP-Ol originally captured in the MTZ.

Super-reduced mineral assemblages, such as native elements, Fe-carbides, and silicides, have been documented worldwide in various upper mantle lithologies, often formed by reactions between reducing C–H–O fluids and surrounding silicates32–35. The reaction between olivine and highly reducing C–H–O fluids releases significant iron, leading to the high Mg# (up to 99) (Mg# = 100 × Mg/(Mg+Fe)) in residual olivine/wadsleyite/ringwoodite36. However, contrary to this trend, olivine-Ol3 and elliptic inclusions show relatively low Mg# (89-92). The release of CO2 and H2O fluids during these redox reactions would typically lower the melting temperature and induce partial melting of surrounding silicate materials. Nevertheless, the high alkali contents of the decomposed garnet, low Mg# olivine, and fertile clinopyroxene of the “garnet” lherzolite xenoliths, preclude widespread partial melting. Additionally, Fe–Ni spherules are exclusively found in olivine inclusions (Ol3) and decomposed majoritic garnets, absent in the host lherzolite, indicating a distinct formation mechanism from typical redox reactions between reducing fluids and surrounding silicates. Fe3+/ΣFe in olivine is typically low (<0.01) in the shallow upper mantle37,38. However, under high pressure and oxidizing environments, the presence of water in olivine can elevate concentrations of Fe3+ (Fe3+/ΣFe up to 0.15)37,38. This ratio can escalate further in wadsleyite, reaching up to 0.44 in high-pressure hydrous experimental systems37. High-pressure and high-temperature experiments suggest that iron disproportionation reaction can lead to significant ferric iron content in wadsleyite and ringwoodite, enhanced by water dissolution and Al incorporation4,37,39. The Fe3+/ΣFe values in this study resemble those observed in hydrous experimental wadsleyite under the MTZ conditions37. Thus, the coexistence of Fe0, Fe2+, and Fe3+ in olivine inclusions (Ol3) and elliptic inclusions confirms the formation of metallic and ferric iron via iron disproportionation in the deep mantle. Presence of amorphous silicon dioxide next to the octahedral inclusion and Fe-Ni alloy inclusion (Fig. 3c) is likely the byproduct of the disproportionation reaction: 3Fe2SiO4 = 2(Fe3+, vacancy) SiO4 + 4Fe0 + SiO2 + O2 (Reaction 5).

Decomposed majoritic garnets show clearly low Fe contents (0.20–0.23 p.f.u.) and high Mg# of 90–91, contrasting with Na-rich majorite inclusions (Fe: 0.6–1.1 p.f.u., Mg#: 30–70) in ultradeep diamonds40. The significant lower Fe contents in observed decomposed majoritic garnets relative to other eclogite-derived majorites suggest Fe release through iron disproportionation, with Fe3+ retained while Fe0 exsolves as metallic phase. These Fe–Ni alloys are believed to remain as small, isolated intergranular grains within their host in the upper mantle41. This hypothesis is supported by the presence of native Fe–Ni alloy inclusions in both the olivine inclusions (Ol3) and decomposed majoritic garnets observed in this study. These metallic Fe-Ni alloys have high stability in the deep mantle characterized by a low oxygen fugacity environment. Our observations show that they tend to aggregate and persist within their hosts, rather than undergoing neutralization reactions and disappearing with ferric iron during the retrogression of wadsleyite/ringwoodite and majoritic garnet in the upwelling asthenosphere. These petrological features strongly support the widespread of metallic Fe-Ni phases and iron disproportionation in the deep mantle.

Evidence from previous experiments and ultradeep diamond inclusions indicates that carbonatite melts associated with a subducting slab can form in the deep upper mantle and subsequently subject to redox freezing in ambient peridotite2,3,42. The upwelling of metasomatized mantle initiates partial melt extraction around 250–300 km2,3,43, leading to chemical and isotopic heterogeneity in the upper mantle. These processes emphasize the vital role of redox melting in the recycling of subducted slabs. Here we propose a schematic model to illustrate the recycling of Na-rich majoritic garnets in the big mantle wedge of eastern Asia25,26 (Fig. 4). The envisioned recycling process is facilitated by small-scale upwellings (‘return flow’44,45) originating from the MTZ in response to the arrival of the subducting Pacific slab. In the subducting oceanic crust, one major driving force for this process is the increased negative buoyancy in response to the mafic rocks-eclogite transformation in the crust46. As depth increases, the volume fraction of the garnet component rises due to the dissolution clinopyroxene into garnet, eventually forming a monomineralic majoritic garnetite crust at MTZ depths (pressure > 14 GPa)47. This monomineralic majoritic garnetite could be captured by surrounding peridotite composed of Mg-Majorite and retrograded high-pressure polymorph of wadsleyite/ringwoodite (RHP-Ol) in the MTZ. The upwellings initially carry majoritic garnets up to nearly 300 km depth, where they are inherited by the asthenosphere lherzolite. Subsequently, the eruption of basaltic volcanics, derived from the asthenosphere lherzolite, rapidly transport these materials to the surface. Slab subduction and asthenosphere upwelling drive vertical mantle convection, promoting mechanical and chemical mixing in the mantle.

Oceanic slab subduction has traditionally been seen as a mechanism for introducing oxidized crustal materials back into the Earth’s interior, leading to significant oxidization in the deep mantle48–50. However, the vigorous disproportionation of ferrous iron to native iron and ferric iron preserved in the studied Na-rich majoritic garnet and RHP-Ol indicates that the redox state of the MTZ remains highly reduced. The Fe and Ni contents in the Fe-Ni alloys were measured by STEM-EDS (Supplementary Data 6). The Fe/Ni ratios of the Fe-Ni alloys in Na-rich majortic garnets range from 247–280 in the high-Na grains to 20–25 in the low-Na grains, indicating progressively increasing oxygen fugacity relative to the ΔIW buffer7. In contrast, the Fe-Ni alloys in the RHP-Ol are enriched in nickel, with Fe/Ni ratios of 0.44-0.53 (Supplementary Data 6). This enrichment of metallic Ni in Fe–Ni phases within the RHP-Ol indicates that the oxygen fugacity in their originating MTZ is only slightly below the IW buffer7. The ferric iron content (Fe3+/ΣFe = 0.35–0.40) in the RHP-Ol is comparable to reported values in wadsleyite37, implying that disproportionation of iron in the MTZ is probably independent of the arrival of subducting materials. Our discovery provides compelling evidence supporting the widespread presence of metallic Fe-Ni phases and iron disproportionation in the deep mantle.

Methods
Electron microscope observation
The back-scatter electron (BSE) images were obtained using a FEI Apero S scanning electron microscope at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR) in China University of Geosciences (Wuhan). The images were acquired with an accelerating voltage of 10 kV, a spot size of 10, an emission current of about 135 μA, and a working distance of 10 mm. The transmission electron microscopy (TEM) specimens were prepared utilizing the Focus Ion Beam (FIB) system (Helios G4 CX, Thermo Fisher Scientific) equipped with a Transmission Kikuchi Diffraction (TKD) system at GPMR. Initially, a predefined area of approximately 20 µm2 was coated with platinum (Pt), followed by precise cuttings of its surroundings to a depth of around 10 µm using a gallium (Ga) ion beam. Subsequently, the resulting foil was delicately retrieved using EasyLift, an in-situ tungsten (W) probe inside the FIB, and mounted onto a TEM copper (Cu) grid (Omniprobe, Oxford Instrument). The extracted samples were thinned to 150 nm using a Ga ion beam at 30 kV, with beam currents ranging from 9.3 nA to 80 pA during processing, followed by additional steps at 5 kV with a beam current of 43 pA and 2 kV with a beam current of 23 pA for final processing. The analyses of the K-Grt cores involved using the TKD map dataset, generating band contrast (BC) images, phase maps, Euler images, and elemental maps.

Major and trace elements analyses
Whole-rock nephelinite samples were analyzed for major elements, using conventional X-ray Fluorescence (XRF) method at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. Loss on ignition (LOI) was determined by measuring the weight loss after drying the samples at 1000 °C. The analytical uncertainty for major element analysis is generally <5%. The ferric iron contents (Fe3+/ΣFe) in the nephelinite were measured using wet chemistry techniques. Trace elements, including rare earth elements (REE), were measured using an Agilent 7500a inductively coupled plasma mass spectrometer (ICP–MS) at GPMR. The relative error for most trace elements is below 5%, with the exception of Cr, Ni, Pb, and Zn, which have a relative error of approximately 10%.

Major element compositions of minerals and elemental mapping of the K-Grt rims were determined at GPMR using a JEOL JXA-8230 Electron Probe Micro-Analyzer (EPMA) equipped with five wavelength-dispersive spectrometers (WDS). Prior to analysis, the samples were coated with a thin conductive carbon film. Typically, an accelerating voltage of 15 kV, a beam current of 20 nA, and a spot size of 1–5 µm were used for analyzing minerals in the host peridotites and the K-Grt rims. Data were corrected online using a ZAF (atomic number, absorption, fluorescence) correction procedure to ensure accuracy. The peak counting time for elements such as Na, Mg, Al, Si, K, Ca, Fe, P, and Cr was set at 10 s, while for Mn, Ti, it was extended to 20 s. Background counting time was maintained at one-half of the peak counting time on both high- and low-energy background positions. The following standards were used for element-specific calibrations: Jadeite (Na), Almandine (Si, Mg, Fe in K-Grt cores), Olivine (Si, Mg, Fe in other minerals), Corundum (Al), Diopside (Ca), Sanidine (K), Rutile (Ti), Rhodonite (Mn), Apatite (P), Chromium Oxide (Cr), Pentlandite (Ni). The detection limit for major elements is generally less than 160 ppm, while the analytical uncertainty of the total amount of major elements is generally better than 2 wt.%.

Seven K-Grt cores, denoted M1-M7, were selected for major and trace element analyses in this study. To mitigate the challenge of inhomogeneous composition caused by kelyphitization, a meticulous approach was employed. Electron microprobe measurements were conducted using a substantial (25-μm) diameter electronic beam, averaging data from over fifteen different areas in each K-Grt core to represent the composition of the parental garnet. It is worth noting that all other measuring conditions remained consistent with those applied to the K-Grt rims. Analysis of the K-Grt cores revealed a depletion in Mg+Fe+Ca+Mn+Na+K content, ranging from 2.83 to 2.96 p.f.u., and an excessive Si+Ti content, ranging from 3.13 to 3.15 p.f.u., according to the garnet/majorite formula.

The trace element compositions of the K-Grt cores were analyzed on polished thick sections using LA-ICP-MS (GeoLas 2005+ Agilent 7500a) at GPMR. Laser sampling was performed using a spot size of 44 μm and a laser energy of 60 mJ. Each analysis included approximately 20–30 s of background acquisition (from a gas blank), followed by 50 s of data acquisition from the sample. The element contents were calibrated against USGS reference glasses (BCR-2G, BHVO-2G and BIR-1G), and normalization of the sum of all metal oxides to 100 wt% was applied. Off-line selection and integration of background and analyzed signals with time-drift correction and quantitative calibration were performed using ICPMSDataCal51. The smooth signal intensity observed during the analysis of the K-Grt cores indicates no significant contamination by inclusions or other mineral phases. Detailed data from major and trace element analyses are listed in Supplementary Data 1–4.

Strontium, neodymium and zinc isotopic compositions
The K-Grt cores were carefully chosen under a microscope from four garnet lherzolite samples and subsequently crushed in an agate mortar to ensure representative sampling. Due to the low Sr (<5 ppm) and Nd (<1 ppm) concentrations (Supplementary Data 3) of these decomposed garnet samples, 300 mg powder was prepared for each sample to conduct whole-rock Sr and Nd isotopic analyses. The Sr and Nd isotopic ratios were measured using a Triton thermal ionization mass spectrometer at GPMR. To facilitate comparison and interpretation, the Sr and Nd isotopic ratios were normalized to 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. During the analysis, the NBS 987 standard yielded an average 87Sr/86Sr value of 0.710249±10 (2σ), and the BCR-2 standard gave an average 143Nd/144Nd value of 0.512630±2 (2σ). Details of Sr and Nd isotopic analytical procedures can be found in Li et al.52 and Wei et al.53. Zn isotopic analyses were performed on a Nu Plasma 1700 MC-ICP-MS instrument, at GPMR, China University of Geosciences (Wuhan), China. Due to the small sample size, Zn isotopic analysis for sample SD53 was not performed. The data are reported in δ-notation in per mil relative to the JMC3-0749L: δ66Zn or δ68Zn = ((66Zn/64Zn)sample / (66Zn/64Zn)JMC3-0749L − 1) × 1000. The external reproducibility for δ66Zn measurement is better than ±0.05‰ (2σ) based on long-term analyses of the international basalt standard BHVO-2 (0.28±0.07‰) and standard solution NIST 682 (−2.45±0.05‰). The hole-rock Sr, Nd and Zn isotopic data are presented in Supplementary Data 5.

Transmission electron microscope (TEM) analyses
The internal structure of the olivine inclusions (Ol3) was studied using an FEI Talos F200S transmission electron microscope at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. A combination of various techniques, including scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDS), was used for comprehensive morphological, structural, and compositional investigations. Major elements of the Ol3, along with its elliptic and octahedral inclusions, were analyzed via STEM-EDS and are presented in Supplementary Data 6. Additionally, to investigate the valence and potential coordination of Fe in the samples, spatially resolved electron energy-loss spectroscopy (EELS) was utilized at high energy resolution in the STEM mode (STEM-EELS). The EELS analyses were performed using a Gatan 1077 EELS spectrometer, with a pixel step of 2 nm for acquiring spectroscopic images. All data were acquired in a dual EELS mode with zero-peak locking. Subsequent to data acquisition, all EELS data processing tasks, including background subduction, signal integration, data fitting, and mapping, were conducted using the Gatan Microscope Suite (GMS) software (version 3.50). The assessment of Fe3+ content in the elliptic inclusion and olivine (Ol3) was conducted using a modified integral intensity ratio of the Fe L2 and L3 white lines with two 2.1-eV-wide integration windows ranging from 708.85 to 710.95 eV and from 719.65 to 721.75 eV, respectively (Fig. 3d), with absolute errors estimated at approximately ±0.04 for Fe3+/ΣFe (ΣFe = Fe3++Fe2+) ratios. For a comprehensive understanding of the Fe3+/ΣFe calculation procedure in EELS analysis, as well as analytical precision and accuracy, readers are referred to Xian et al.9.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Recommendation: Major Revision\nThis manuscript presents a study on peridotite xenoliths from Cenozoic basalts in eastern China, proposing evidence for\niron disproportionation in the mantle transition zone (MTZ). The authors link Na-rich majoritic garnets, Fe-Ni inclusions, and\nthe stagnant Pacific slab to redox processes in the deep mantle. While the topic is novel and potentially impactful, the\nmanuscript falls short of the evidentiary rigor and comprehensive analysis expected for Nature Communications.\nMy primary concern lies in the robustness of the central claim that the peridotite xenoliths originate from the MTZ and provide\ndirect evidence of iron disproportionation. This claim requires significant reinforcement due to the following points:\n(1) The overwhelming majority of peridotite xenoliths worldwide are derived from the lithospheric mantle or the lithosphere-\nasthenosphere boundary, not the MTZ. The manuscript does not convincingly address why the studied xenoliths would be\nan exception, nor does it provide unequivocal evidence supporting their MTZ origin.\n(2) Prior research on peridotite xenoliths from Cenozoic basalts in eastern China predominantly attributes their origin to\nfragments of the subcontinental lithospheric mantle. This view contradicts the MTZ origin proposed here.\n(3) If these xenoliths originated in the MTZ, the host basalt must source from greater or equivalent depths to have capacity to\nentrain material from the MTZ. However, eastern China basalts are widely recognized as products of asthenospheric mantle\nmelting at depths shallower than 300 km. This is further supported by the authors’ earlier work (Fig 12 of Pan et al., 2021),\nwhich concluded that mantle melting responsible for the host basalts of the xenoliths occurred\nwithin the asthenosphere rather than the MTZ.\nSpecific Points to Address\n(1) Host Basalt Provenance: Basalts derived from shallower depths (<300 km) are unlikely to transport xenoliths from the\nMTZ. The depth of the host basalt's origin should be well re-constrained to ensure compatibility with the capture and\ntransport of MTZ fragments.\n(2) Melting in the MTZ: The manuscript posits that melting occurs within the MTZ but does not address the lack of\nconventional melting triggers such as decompression, heating, or water addition in this environment. It should also clarify\nhow MTZ-derived melts might migrate to shallower depths while entraining xenoliths.\n(3) MTZ Origin of Peridotite Xenoliths: The reliance on Na-rich majoritic garnet compositions and equilibrium pressure\ncalculations is insufficient to conclusively establish an MTZ origin. The evidence for retrograded wadsleyite/ringwoodite\nderived from olivine inclusions is unconvincing. Additional evidence is required. I recommend that (1) conduct detailed\ncomparisons with garnets from the lithospheric and asthenospheric mantle to definitively rule out shallower origins; (2)\npresent additional structural and spectroscopic evidence to differentiate retrograded high-pressure phases from\ndecompression-induced features.\nAdditional Concerns\nThe isotopic similarity of garnets to carbonated MORBs is interesting but insufficient to confirm slab-derived carbonate\ncontributions. Sr-Nd isotopes alone lack the sensitivity required for such interpretations. The association of garnet isotopes\nwith the stagnant Pacific slab remains speculative without broader comparative data. Incorporate other geochemical\nproxies to corroborate the proposed linkage to subducted slab components is necessary.\nThe manuscript does not address potential interactions between the peridotite xenoliths and the host basalt, which could\nsignificantly alter the mineralogical and geochemical signatures.\nPerhaps high-pressure signatures alone do not unequivocally confirm MTZ origin. Alternative explanations, such as\nlocalized subduction-related anomalies, should be considered.\nCertain technical terms, such as \"kelyphitized garnet,\" require clearer definitions to ensure accessibility to a broader\nscientific audience.", "reviewer_2": "The manuscript „Iron disproportionation in peridotite fragments from the mantle transition zone” by Pan et al. is concerned\nwith the discovery of (Fe,Ni) metal in olivine and kelyphitized majoritic garnet from xenoliths brought to the surface by\nCenozoic basalts from eastern China. The story is that these rock fragments represent former basaltic rocks from the ocean\nfloor that were subducted to transition zone depth (down to ~550 km depth) and later entrained in mantle upwellings bringing\nthem back to the surface. The metal inclusions may then serve as (first) natural evidence for reducing conditions in the\ntransition zone predicted by high P-T experiments.\nThe study is based on two interpretations, firstly that the garnets represent earlier oceanic crust and secondly that the\ngarnets were subducted very deeply into the transition zone. Both interpretations should, in my opinion, be better\nsubstantiated (major revisions) before the manuscript can be published (see comments below). Nevertheless, the overall\nwork is of very good quality, the samples are spectacular and the story is timely and certainly of interest for a broad scientific\naudience.\n(1) Pressure derived from Garnet compositions. I agree that the high Na contents in garnet are most likely caused by high P\nconditions. However, I don’t understand why the authors use Bobrov et al. to derive the pressure of garnet\nformation/equilibration (Fig. 2a). Bobrov describes his approach such that it “may be the basis for a majorite geobarometer”,\nmeaning that it is not calibrated as a geobarometer and the relationship I shown in Fig. 2a neither is. Problematic is, that in\nBobrov’s chemical system Na and Si contents of the resulting majorites are strongly correlated and both Na and Si contents\ndepend on pressure. This is not the case in the present study as Na contents vary significantly at constant Si contents\nimplying that the mechanism of Na incorporation in majoritic garnet is likely different and the P estimate thus in error. I’d\nrecommend to use the Beyer et al. (2017) geobarometer (also in Fig. 2) to derive pressures as it is actually calibrated for Na-\nrich compositions and involves Na and Si.\nImportantly, even if you use the Beyer et al. calibration (perhaps giving similar pressures), it remains puzzling and requires\nfurther explanation that Na and Si concentrations of the garnets are obviously decoupled in the samples presented here. To\nmy knowledge this was never observed before. The data of Bobrov, Gasparik, Kiseeva and Beyer clearly show, that Na and\nSi must be correlated in Na majorites as Na is incorporated likely as Na2M(Si2)Si3O12 sodium-majorite (Na-Maj)\ncomponent via the substitution (Mg2+ + Al3+ = Na+ + Si4+).\n(2) Origin of the garnets. The authors claim that the garnets stem from subducted oceanic crust and belong thus to the\neclogitic suite. However, the xenoliths in which they are found are lherzolithic/peridotitic. Does this imply that the garnets are\neclogitic xenocrysts within the peridotitic xenoliths? How is it possible to disintegrate the eclogite and incorporate only\ngarnet into the peridotite? Please elaborate a bit on the process you envisage for this uncommon finding.\nNot sure I understand the Sr isotope argument: The garnets show high 87Sr/86Sr at constant 143Nd/144Nd. This is\ncommonly attributed to seawater alteration (87Sr/86Sr = 0.709) of the subducted MORB which was transformed to eclogite\nduring subduction. Is that what is meant with “carbonated MORB” in Fig. 2b? Why should it be carbonate instead of fluid and\nwhy is this signature preserved in the garnet despite dehydration/decarbonation during subduction?\nLine 32: maybe delete vigorous\nLine 49: maybe give a real number of samples?\nLine 51: “… of the continent…”\nLine 50-55: unclear, please consider rephrasing (rocks migrate oceanward?)\nLine 66: What is the composition of the Fe,Ni alloy? It would be very interesting to know this composition to constrain the fO2\nfurther and to correlate the alloy composition with the model of Frost and McCammon 2008 (Ni-rich close to the Ni-\nprecipitation curve and becoming progressively more Fe rich with increasing pressure). The alloys were measured by EELS\nbut not by TEM-EDX or EPMA?\nLine 79: What is meant by Na-rich matrix and why wasn’t it characterized further using e.g., TEM? It seems to be a\nsubstantial part of the former garnet core so any breakdown reaction should involve this Na-rich matrix, I guess.\nLine 83: lithophile elements? Please explain a bit more detailed what Ext. Data Fig 4 tells about the garnets. 4b reflects the\npartitioning behavior for REE but what is the implication of Fig 4a is and why is it indicative for “…minimal exchange with\nexternal fluids/melts…” (line 84-85).\nLine 86: This is important so I’d suggest to present this in greater detail and also cite some relevant literature here, for\nexample the work of Obata et al. regarding the textures.\nLine 113: please put references here.\nLine 117: synthetic eclogite?\nLines 185-187: Any reference for the method? Van Aken 1998 I guess or is this implemented in the Gatan software?\nLines 192: maybe add Schmidt et al. Progress in Earth and Planetary Science 2014, 1:27 as reference for SiC originating\nfrom reduced fluids\nLine 195: I'd suggest to consider only Fe2+ for calculating the #Mg.\nLines 203ff: I do not really understand the arguments here. The olivine structure has no position for Fe3+, that’s why Fe3+ is\nso low in olivine in all geologic settings. Even if high-P wadsleyite contained Fe3+, the retrograde back transformation to\nolivine should exclude all Fe3+ from this olivine. There seems no way that olivine hosts 40% (3-4 wt.%!) structural Fe2O3.\nPlease elaborate how this should be possible. Further, I don’t understand why hydrous experimental wadsleyite is\nconsidered for comparison as interaction with fluids is excluded as likely scenario.\nLine 257: what is meant by RHP? Was the Ni-content measured in the Fe,Ni phases?\nLine 405: kV\nLine 463: chosen\nLine 494: ∑\nOverall very nice work! With best wishes,"}, "author_response": "Responses to the comments from the reviewers (in blue color)\nReviewer #1 (Remarks to the Author):\nRecommendation: Major Revision\nThis manuscript presents a study on peridotite xenoliths from Cenozoic basalts in eastern China,\nproposing evidence for iron disproportionation in the mantle transition zone (MTZ). The authors\nlink Na-rich majoritic garnets, Fe-Ni inclusions, and the stagnant Pacific slab to redox processes\nin the deep mantle. While the topic is novel and potentially impactful, the manuscript falls short\nof the evidentiary rigor and comprehensive analysis expected for Nature Communications.\nMy primary concern lies in the robustness of the central claim that the peridotite xenoliths\noriginate from the MTZ and provide direct evidence of iron disproportionation. This claim\nrequires significant reinforcement due to the following points:\n(1) The overwhelming majority of peridotite xenoliths worldwide are derived from the\nlithospheric mantle or the lithosphere-asthenosphere boundary, not the MTZ. The manuscript\ndoes not convincingly address why the studied xenoliths would be an exception, nor does it\nprovide unequivocal evidence supporting their MTZ origin.\n(2) Prior research on peridotite xenoliths from Cenozoic basalts in eastern China predominantly\nattributes their origin to fragments of the subcontinental lithospheric mantle. This view\ncontradicts the MTZ origin proposed here.\n(3) If these xenoliths originated in the MTZ, the host basalt must source from greater or\nequivalent depths to have capacity to entrain material from the MTZ. However, eastern China\nbasalts are widely recognized as products of asthenospheric mantle melting at depths shallower\nthan 300 km. This is further supported by the authors’ earlier work (Fig 12 of Pan et al., 2021),\nwhich concluded that mantle melting responsible for the host basalts of the xenoliths occurred\nwithin the asthenosphere rather than the MTZ.\nResponse: Thank you for your thoughtful comments. Tracing the depth origin of mantle\nxenoliths is indeed a challenging task due to the decomposition of high-pressure deep mantle\nminerals during their ascent to the surface. Most mineralogical and geochemical evidence that\ncould help trace their origin tends to be either erased or significantly altered during upwelling.\nWhile it is true that the majority of peridotite xenoliths worldwide are thought to derive from the\nlithospheric mantle or the lithosphere-asthenosphere boundary, our xenoliths are unique in\nseveral ways that support the MTZ origin.\n1. Garnet vs. Spinel Peridotites: The majority of previous studies on peridotite xenoliths from\neastern China have reported spinel peridotites. In contrast, our study reports garnet\nperidotites, which exhibit distinctly different mineral associations and geochemical\ncompositions. This is a clear indication that these xenoliths are not typical lithospheric\nperidotites.\n2. Geochemical Evidence of the host rock. The mantle source of the Cenozoic intraplate\nOIB-like basalts in eastern China has been shown to involve recycled carbonated slab\ncomponents from the MTZ. This interpretation is supported by accumulating evidence from\nmajor- and trace-elements geochemistry, Sr-Nd-Pb-Zn-Mg-Ca-O isotopic compositions (e.g.,\nDong et al., 2024; Li et al., 2017; Pan et al., 2021; Wang et al., 2015; Zeng et al., 2021).\nMoreover, recent studies have suggested that the contemporaneous nephelinites in eastern\nChina, which resemble the host rock of our studied garnet peridotites, also originate from the\nMTZ (Zeng et al., 2021).\n3. Direct Evidence for MTZ Origin. We provided three key pieces of evidence supporting the\nMTZ origin of these garnet peridotite xenoliths. 1) Pressure-induced Fe2+  Fe0 + Fe3+\ndisproportionation in silicates. This phenomenon, which is typical of deep mantle below 250\nkm (Forst et al., 2004; Rohrbach et al., 2007), was observed in kelyphitized Na-rich\nmajoritic garnet and olivine. 2) Na-rich majoritic garnet. Known to be a reliable pressure\nindicator for deep mantle materials at depths of 300-660 km (Beyer and Frost, 2017; Bobrov\net al., 2008), we determined formation pressures (18.9–13.3 GPa) based on the sodium and\npotassium contents in the decomposed garnet. 3) Mioré Patterns of spinel + olivine + Fe\nnano-inclusions. These patterns in olivine point to a high-pressure origin, consistent with\nminerals found in the MTZ, such as wadsleyite/ringwoodite.\nThese three lines of evidence are self-consistent and provide robust support for the MTZ\norigin of the xenoliths, a finding that has not been reported previously in mantle xenoliths from\neastern China.\n1. Host Basalt Provenance: Basalts derived from shallower depths (<300 km) are unlikely to\ntransport xenoliths from the MTZ. The depth of the host basalt's origin should be well\nre-constrained to ensure compatibility with the capture and transport of MTZ fragments.\nResponse: The host rock for the studied xenoliths is a nephelinite, whose major and trace\nelements are provided in Supplementary Table 1. The contemporaneous nephelinites in eastern\nChina, which closely resemble the host rock of our garnet peridotite samples, have also been\nsuggested to originate from the MTZ (Zeng et al., 2021). To address your concern, we have\nrephrased the text to include information on the composition of the host rock (lines 52-58).\nAdditionally, considerable portions of the mantle below 250 km are saturated with an\niron–nickel rich metal phase formed by the ferrous disproportionation of iron-bearing silicates,\nwith the redox state controlled by the iron–wüstite (IW) buffer (Forst et al., 2004; Rohrbach et al.,\n2007; Kuwahara et al., 2023). As oxygen fugacity in Earth’s interior decreases with increasing\ndepth, the ferrous disproportionation found in this study provides direct mineralogical evidence\nof a deep origin (more than 250 km depth at least) of the host nephelinites.\n2. Melting in the MTZ: The manuscript posits that melting occurs within the MTZ but does not\naddress the lack of conventional melting triggers such as decompression, heating, or water\naddition in this environment. It should also clarify how MTZ-derived melts might migrate to\nshallower depths while entraining xenoliths.\nResponse: An intermittent layer composed of low seismic wave velocity zones (LVZs) has been\nidentified globally just above the top of the mantle transition zone (e.g., Revenaugh et al., 1994;\nTauzin and Debayle, 2010). These LVZs are commonly attributed to a small amount of\nsilicate-rich melts in the mantle (Revenaugh et al., 1994; Tauzin and Debayle, 2010) or grain-size\nsensitive anelastic relaxation near the mantle solidus (Karato, 2012; Takei, 2017).\nHigh-temperature and high-pressure partial melting experiments have found that carbon redox\nmelting plays an important role in deep mantle melting (Dasgupta et al., 2013; Rohrbach and\nSchmidt, 2011), which is regulated by the mantle oxygen fugacity depended on the ferric iron\ncontent relative to total iron (Fe3+/ΣFe) (Moussallam et al., 2019; Stagno et al., 2013; Rohrbach\nand Schmidt, 2011). For a typical asthenosphere source, a Fe3+/ΣFe ratio of nearly 3-4% leads to\ncarbonated silicate melt generation at 150-250 km depth (Dasgupta et al., 2013; Stagno et al.,\n2013). However, when the whole-rock ferric iron content exceeds 12% of total Fe, carbonated\nmelting would commence at depths >300 km (Moussallam et al., 2019; Stagno et al., 2013).\nLVZs are widely found atop the mantle transition zone (300-400 km) in eastern China, with a\nthickness ranging from 50-80 km in North China to 20-60 km in South China (Tauzin et al., 2010;\nMa et al., 2020; Han et al., 2021). The widespread Cenozoic intraplate volcanisms in eastern\nChina are spatially distributed above the LVZs, indicating a potential petrogenesis contribution\nfrom the MTZ, as suggested by geophysical modeling results (Yang and Faccenda, 2020).\nGeochemical data on these intraplate basalts also speculated that some of them may derive from\nthe MTZ (Wang et al., 2015; Zeng et al., 2021). The metal inclusions found in this study\nprovided first direct natural evidence for reducing conditions in the transition zone.\nThe ferrous disproportionation observed could also alter the relative concentrations of Fe3+,\nFe2+, and Fe0, potentially regulating oxygen fugacity and inducing partial melting in the deep\nmantle. We have identified a garnet pyroxenite (Figure 1) that may serve as a highly oxidized\nmagma source for deep (> 300 km) upper mantle melting. This pyroxenite xenolith contains a\nsignificant proportion (~15%) of Na-rich majoritic garnet. Olivine clinopyroxenite (Region A)\nand garnet olivine websterite (Region C) show an interbedded structure (Figure 1a). Fe-Ni alloys\nare present in some kelyphitized Na-rich majoritic garnets, and Fe-Cu-Ni-rich sulfides are\npresent in the nepheline veins of Region C (Figures 1f and 1g), indicating a highly reduced\ncondition. Sulfate veins, sulfide grains and graphite (Figures 1d and 1e) are found at the contact\ninterface between olivine clinopyroxenite and garnet olivine websterite (Region B). The olivine\nclinopyroxenite part was sampled for whole-rock major element analysis, revealing the following\ncomposition: SiO of 50.53 wt.%, Al O of 7.43 wt.%, CaO of 13.70 wt.%, MgO of 19.26 wt.%,\n2 2 3\nand TFe O of 4.65 wt.% with Mg# value of 89. This olivine clinopyroxenite is also\n2 3\ncharacterized by a high ferric iron content (Fe3+/ΣFe = 0.12). The distinct oxygen fugacity\nsignatures preserved in the garnet pyroxenite, and their potential effects on partial melting of the\nupwelling asthenosphere, will be discussed in our subsequent papers. In this study, we aim to\nprovide mineralogical evidence (such as ferrous disproportionation in olivine or its high pressure\npolymorphs) for the deep origin of these unique mantle xenoliths.\nFigure 1. Inverse oxygen fugacity recorded in a pyroxenite xenolith. (a) The pyroxenite\nconsists of three distinct regions: garnet-free pyroxenite (Region A), garnet-rich pyroxenite\n(Region C), and the interaction zone between them (Region B). (b) and (c) Region A is\ncomposed of coarse-grained anhedral orthopyroxene (Opx) and clinopyroxene (Cpx I), as well as\nfine-grained clinopyroxene (Cpx II), olivine (Ol), and nepheline (Nph). The fine-grained Ol and\nCpx II are interpreted to have formed through reactions between Opx and carbonatitic melts. (d)\nand (e) Region B shows the coexistence of oxidizing sulfate (Sft) and reducing sulfide and\ngraphite (Gp), indicative of different oxygen fugacity conditions. (f) and (g) Region C is\ncharacterized by coarse-grained garnet (Grt), Opx, and Cpx I. Reducing sulfides and nepheline\nare common in the mineral assemblage, and Fe-Ni alloys are observed in the garnet cores.\n3. MTZ Origin of Peridotite Xenoliths: The reliance on Na-rich majoritic garnet compositions\nand equilibrium pressure calculations is insufficient to conclusively establish an MTZ origin.\nThe evidence for retrograded wadsleyite/ringwoodite derived from olivine inclusions is\nunconvincing. Additional evidence is required. I recommend that (1) conduct detailed\ncomparisons with garnets from the lithospheric and asthenospheric mantle to definitively rule\nout shallower origins; (2) present additional structural and spectroscopic evidence to\ndifferentiate retrograded high-pressure phases from decompression-induced features.\nResponse: The reviewer didn’t provide specific reasons for dismissing the evidence we\npresented. The majorite component can only exist in garnets that equilibrated at depth > 150 km,\nas a result of the dissolution of coexisting orthopyroxene and clinopyroxene into garnet\n(Ringwood and Major, 1971; Beyer et al., 2017). The Na-rich majorite component is in\nequilibrium at even higher pressures, above 7 GPa (Bobrov et al., 2008a, b; Dymshits et al.,\n2013). Thus, the high-pressure crystal structures of most majoritic garnets are typically preserved\nas inclusions in diamonds (Kiseeva et al., 2013 and references therein).\nWadsleyite and ringwoodite are dominant minerals in the mantle transition zone (MTZ). These\nhigh-pressure polymorphs of olivine are rarely preserved in deep mantle diamonds (Pearson et al.,\n2014; Gu et al., 2022). Our peridotite xenolith samples were captured in an open igneous system,\nwhich makes it extremely difficult to preserve the original phases of these ultra-high-pressure\nminerals. The Na-rich matrix in the decomposed garnet is dominantly composed of plagioclase\ncrystallites with varying Na O, K O, and CaO contents (Supplementary Figs. 3 and 4). This\n2 2\nsubmicron structure precludes an exotic origin for the alkali contents. Therefore, the high Na O\n2\n(0.75–2.41 wt.%) and K O (0.11–0.73 wt.%) signatures observed in the studied Na-rich majoritic\n2\ngarnets strongly support a retrograded high-pressure phase origin.\nThe presence of Fe3+-rich (Fe3+/ΣFe = 0.35-0.40) olivine and widespread spherical Fe-Ni\nalloys further confirms pressure-driven disproportionation of iron in silicates in the deep mantle.\nIron-saturation due to pressure-driven ferrous disproportionation is widely accepted to occur only\nat depths > 250 km (Rohrbach et al., 2007; Frost et al., 2008; Beyer et al., 2021; Armstrong et al.,\n2019; Kuwahara et al., 2023), which rules out a shallow origin for the Na-rich kelyphitized\ngarnets and the olivine inclusions we studied.\nHigh-resolution transmission electron microscope (HRTEM) images and corresponding fast\nFourier transform (FFT)\\nano-beam electron diffraction (NBD) reveal comparable Mioré Patterns\nbetween the octahedral inclusion and elliptic inclusions (Extended Data Fig. 7m-o). We also\ndetermined their compositions using STEM-EDS. Based on their structural characteristics and\ncompositions, we conclude that these inclusions are a mixture of olivine (Ol ) and spinel.\nSupplementary Figure 4. Transmission electron microscopy analyses of the kelyphitized\ngarnet core. Panel (a) is a HAADF-STEM micrograph, while panels (b-k) are EDS elemental\nmaps.\n4. The isotopic similarity of garnets to carbonated MORBs is interesting but insufficient to\nconfirm slab-derived carbonate contributions. Sr-Nd isotopes alone lack the sensitivity\nrequired for such interpretations. The association of garnet isotopes with the stagnant Pacific\nslab remains speculative without broader comparative data. Incorporate other geochemical\nproxies to corroborate the proposed linkage to subducted slab components is necessary.\nResponse: Carbonated signatures from recycled subducted slab components have been\nconsistently proposed as contributing to the mantle source of the Cenozoic intraplate OIB-like\nbasalts in eastern China, based on major- and trace-elements geochemistry, as well as\nSr-Nd-Pb-Zn-Mg-Ca-O isotopic compositions (i.e. Dong et al., 2024; Li et al., 2017; Pan et al.,\n2021; Wang et al., 2015; Zeng et al., 2021). The Sr-Nd isotopic compositions of the Na-rich\nmajoritic garnet are compared with those of asthenosphere-derived magmas, such as MORBs,\nHIUM, EM-1, EM-2 (Fig. 2b).\nFollowing the reviewer’s suggestion, we have also measured Zn isotopes in the Na-rich\nmajoritic garnets and compared them with those of the Cenozoic intraplate basalts in eastern\nChina (supplementary Fig. 6c). The isotopic similarity between the garnets and the basalts\nfurther supports the hypothesis of an association with the stagnant Pacific slab in the MTZ.\nThese new data have been included in the revised text (lines 155-160).\nSupplementary Figure 6. Characteristic compositions and the Zn-Sr isotopes of the\ndecomposed Na-rich majoritic garnets. (a) Plot of the sum of (Mg²⁺ + Ca²⁺ + Fe²⁺ + Mn²⁺)\np.f.u. versus Si + Ti (p.f.u). (b) Plot of the sum of (Al³⁺ + Cr³⁺) p.f.u. versus Si + Ti (p.f.u).\nMajoritic garnet inclusions from natural diamonds are sourced from Kiseeva et al. (2013). The\nexperimental data (Jd) are from Bobrov et al. (2008). All Fe in majoritic garnet is treated as Fe²⁺\nfor comparison. (c) Zn and Sr isotopic compositions of the decomposed Na-rich majoritic garnets\nand Cenozoic intraplate basalts from our study area (grey rhombus). The Zn-Sr isotopic\ncompositions of the decomposed garnets are consistent with those of carbonated eclogite. Both\nthe Zn-Sr isotopes of the Cenozoic intraplate basalts and the mixing model are according to Xu\net al. (2022).\n5. The manuscript does not address potential interactions between the peridotite xenoliths and\nthe host basalt, which could significantly alter the mineralogical and geochemical signatures.\nResponse: Thank you for this excellent point. The host rock for the studied xenoliths is\nnephelinite, and the major and trace elements of this rock have been provided in Supplementary\nTable 1. The ferric iron contents (Fe3+/ΣFe) of the nephelinite, analyzed by wet chemistry\nmeasurements, range from 0.25 to 0.31, which is consistent with the highly oxidized Cenozoic\nintraplate basalts in eastern China (Dong et al., 2024). Additionally, Fe-Ni alloys are widespread\nin both the garnet core and olivine (Ol ), which precludes significant interactions between the\n3\nperidotite xenoliths and the highly oxidized host nephelinite. Furthermore, the decomposed\nNa-rich majoritic garnet shows typical garnet geochemical signatures that clearly differ from\nthose of the host nephelinite (Supplementary Figure 5 in the Extended Data). These observations\nstrongly suggest that interactions between the xenoliths and the host nephelinite were negligible.\nWe have rephrased the text for clarity (lines 82-84 and 94-99).\n6. Perhaps high-pressure signatures alone do not unequivocally confirm MTZ origin.\nAlternative explanations, such as localized subduction-related anomalies, should be\nconsidered.\nResponse: This is precisely the point we are making in the manuscript. As shown by the\ngeophysical observations (Fig. 4 & Wei et al., 2012), the most significant subduction-related\nanomalies in the mantle region are the stagnant slab in the MTZ. To our best knowledge, there\nare currently no reports of other subduction-related anomalies in the upper mantle beneath\neastern China.\n7. Certain technical terms, such as \"kelyphitized garnet,\" require clearer definitions to ensure\naccessibility to a broader scientific audience.\nResponse: Thank you for the suggestion. We have clarified the two kelyphitization mechanisms\nin the revised manuscript (lines 72-102). The kelyphitized garnet rim forms due to rapid\nexhumation via the reaction between garnet and olivine (Grt + Ol = Opx + Cpx + Sp/Pl), with\n1\nplagioclase representing the terminal phase of this retrograde reaction. Meanwhile, the K-Grt\ncore is likely the isostoichiometric breakdown product of the parental garnet itself (Grt = Opx +\nSp + An ± Cpx).\nReviewer #2 Evaluations:\nThe manuscript “Iron disproportionation in peridotite fragments from the mantle transition\nzone” by Pan et al. is concerned with the discovery of (Fe,Ni) metal in olivine and kelyphitized\nmajoritic garnet from xenoliths brought to the surface by Cenozoic basalts from eastern China.\nThe story is that these rock fragments represent former basaltic rocks from the ocean floor that\nwere subducted to transition zone depth (down to ~550 km depth) and later entrained in mantle\nupwellings bringing them back to the surface. The metal inclusions may then serve as (first)\nnatural evidence for reducing conditions in the transition zone predicted by high P-T\nexperiments.\nThe study is based on two interpretations, firstly that the garnets represent earlier oceanic crust\nand secondly that the garnets were subducted very deeply into the transition zone. Both\ninterpretations should, in my opinion, be better substantiated (major revisions) before the\nmanuscript can be published (see comments below). Nevertheless, the overall work is of very\ngood quality, the samples are spectacular and the story is timely and certainly of interest for a\nbroad scientific audience.\n1. Pressure derived from Garnet compositions. I agree that the high Na contents in garnet are\nmost likely caused by high P conditions. However, I don’t understand why the authors use\nBobrov et al. to derive the pressure of garnet formation/equilibration (Fig. 2a). Bobrov\ndescribes his approach such that it “may be the basis for a majorite geobarometer”, meaning\nthat it is not calibrated as a geobarometer and the relationship I shown in Fig. 2a neither is.\nProblematic is, that in Bobrov’s chemical system Na and Si contents of the resulting\nmajorites are strongly correlated and both Na and Si contents depend on pressure. This is not\nthe case in the present study as Na contents vary significantly at constant Si contents\nimplying that the mechanism of Na incorporation in majoritic garnet is likely different and\nthe P estimate thus in error. I’d recommend to use the Beyer et al. (2017) geobarometer (also\nin Fig. 2) to derive pressures as it is actually calibrated for Na-rich compositions and\ninvolves Na and Si.\nImportantly, even if you use the Beyer et al. calibration (perhaps giving similar pressures), it\nremains puzzling and requires further explanation that Na and Si concentrations of the\ngarnets are obviously decoupled in the samples presented here. To my knowledge this was\nnever observed before. The data of Bobrov, Gasparik, Kiseeva and Beyer clearly show, that\nNa and Si must be correlated in Na majorites as Na is incorporated likely as\nNa2M(Si2)Si3O12 sodium-majorite (Na-Maj) component via the substitution (Mg2+ + Al3+\n= Na+ + Si4+).\nResponse: Thank you for the constructive feedback. We agree with your point regarding the use\nof the Bobrov et al. (2008) approach to derive the pressure of garnet formation/equilibration. We\nhave taken your suggestion and revised our manuscript to use the Beyer et al. (2017)\ngeobarometer to estimate the pressures, as it is specifically calibrated for Na-rich compositions\nand accounts for both Na and Si contents. We have updated Figure 2 accordingly and discussed\nthe application of the Beyer et al. geobarometer in the revised text.\nRegarding the decoupling of Na and Si concentrations in our garnets, we hypothesize that\nthis could be related to the kelyphitization process, which occurred in a relatively closed\nisostoichiometric system with minimal exchange with external fluids/melts. However, the\npresence of plagioclase in the garnet rim suggests that sodium and potassium were lost from the\ngarnet core (K-Grt) to the rim, leading to the observed decoupling. We originally discussed this\npotential influence in the \"Analytical Methods\" section, but we have now moved this discussion\nto the main text (lines 114-119) for clarity.\nWe appreciate your valuable input and hope these revisions address your concerns\neffectively.\n2. Origin of the garnets. The authors claim that the garnets stem from subducted oceanic crust\nand belong thus to the eclogitic suite. However, the xenoliths in which they are found are\nlherzolithic/peridotitic. Does this imply that the garnets are eclogitic xenocrysts within the\nperidotitic xenoliths? How is it possible to disintegrate the eclogite and incorporate only\ngarnet into the peridotite? Please elaborate a bit on the process you envisage for this\nuncommon finding.\nNot sure I understand the Sr isotope argument: The garnets show high 87Sr/86Sr at constant\n143Nd/144Nd. This is commonly attributed to seawater alteration (87Sr/86Sr = 0.709) of the\nsubducted MORB which was transformed to eclogite during subduction. Is that what is\nmeant with “carbonated MORB” in Fig. 2b? Why should it be carbonate instead of fluid and\nwhy is this signature preserved in the garnet despite dehydration/decarbonation during\nsubduction?\nResponse: Thank you for your thoughtful comments and for raising important questions\nregarding the origin of the garnets. You are correct that the xenoliths in which the garnets are\nfound are peridotitic, specifically lherzolithic in composition. The presence of eclogitic garnet in\nthese peridotitic xenoliths, which are typically dominated by olivine and pyroxenes, indeed raises\nan interesting question. Our interpretation is that these garnets likely represent xenocrysts from a\nsubducted oceanic crust that were captured by the surrounding peridotites during mantle\nupwelling processes (lines 276-283).\nAs oceanic crust subducts and descends into the mantle, the volume fraction of garnet\nincreases with depth as clinopyroxene is dissolved in to garnet, leading to the formation of\nmonomineralic majoritic garnetite at pressures greater than 14 GPa in the mantle transition zone\n(MTZ). These majoritic garnetites may then become part of the xenoliths captured by upwelling\nperidotites in the MTZ, forming a mixture of wadsleyite/ringwoodite/olivine and garnetite. It is\npossible for these garnetites to remain as inclusions or xenocrysts within the surrounding\nperidotitic material after exhumation. We have rephrased the manuscript to clarify this\ninterpretation.\nRegarding the Sr isotopic compositions, we agree that high 87Sr/86Sr ratios at constant\n143Nd/144Nd are typically attributed to seawater alteration of subducted oceanic crust. The\n“carbonated MORB” signature in Fig. 2b refers to the alteration of the subducted oceanic crust\nby carbon-rich fluids, likely originating from carbonate-rich sediments or altered oceanic crust\nitself. This process could explain the elevated Sr isotopic ratio, and we propose that it reflects the\nincorporation of carbonated material into the garnet during subduction.\nYou raise an excellent point about the preservation of this signature despite the dehydration\nand decarbonation that occurs during subduction. We suggest that the carbonation signature may\nhave been retained in the garnet during its formation at high pressures in the MTZ, possibly due\nto the stability of carbonate in the deeper parts of the mantle. Additionally, we have analyzed the\nZn isotopic compositions of the decomposed Na-rich majoritic garnet, and the Zn isotopic data\nshow a similar pattern to that observed in Cenozoic intraplate basalts from eastern China, further\nsupporting the hypothesis of carbonate alteration in the subducted oceanic crust.\nWe have updated the manuscript to incorporate these clarifications and additional data (Zn\nisotopes) to strengthen the argument for a carbonated origin of the garnet.\n3. Line 32: maybe delete vigorous\nResponse: We have removed it as suggested.\n4. Line 49: maybe give a real number of samples?\nResponse: We have now specified the number of samples in the revised text. We collected more\nthan twenty “garnet” lherzolite xenoliths, but only five samples (SD07, SD50, SD51, SD52, and\nSD53) are presented in this study.\n5. Line 51: “… of the continent…”\nResponse: We have corrected it.\n6. Line 50-55: unclear, please consider rephrasing (rocks migrate oceanward?)\nResponse: This sentence has been rephrased as follows: “The eruption ages of these rocks\ndecrease oceanward, with an ~100-km spatial gap,”\n7. Line 66: What is the composition of the Fe,Ni alloy? It would be very interesting to know\nthis composition to constrain the fO2 further and to correlate the alloy composition with the\nmodel of Frost and McCammon 2008 (Ni-rich close to the Ni-precipitation curve and\nbecoming progressively more Fe rich with increasing pressure). The alloys were measured by\nEELS but not by TEM-EDX or EPMA?\nResponse: The compositions of the Fe-Ni alloys have been measured by STEM-EDS\n(Supplementary Table 6), and we have provided additional information in revised text (lines\n292-297).\n8. Line 79: What is meant by Na-rich matrix and why wasn’t it characterized further using e.g.,\nTEM? It seems to be a substantial part of the former garnet core so any breakdown reaction\nshould involve this Na-rich matrix, I guess.\nResponse: Following your suggestion, we have characterized the Na-rich matrix in more detail\nusing TEM. The results show that the Na-rich matrix in the decomposed garnet consists\nprimarily of plagioclase crystallites with variable Na O, K O, and CaO contents (Supplementary\n2 2\nFig. 4). We have included this new information in the revised text (lines 82-84) for better clarity.\n9. Line 83: lithophile elements? Please explain a bit more detailed what Ext. Data Fig 4 tells\nabout the garnets. 4b reflects the partitioning behavior for REE but what is the implication of\nFig 4a is and why is it indicative for “…minimal exchange with external fluids/melts…”\n(line 84-85).\nResponse: Regarding your question on lithophile elements and the trace element data, we have\nclarified this point in the revised manuscript. The conclusion of minimal exchange with external\nfluids/melts is based on a comparison of the trace element compositions between the\nkelyphitized garnet core and their nephelinite host. As shown in Supplementary Fig. 5a and\nSupplementary Table 3, the nephelinite host shows a significant enrichment in large-ion\nlithophile elements (LILE), typical of Cenozoic OIB-type intraplate basalts from eastern China.\nIn contrast, the kelyphitized garnet cores exhibit extremely low concentrations of lithophile\nelements and are enriched in heavy rare earth elements (HREE), which is characteristic of\nlithospheric garnets. This significant difference in trace element signatures suggests that the\ngarnets have undergone minimal exchange with external fluids or melts. We have rephrased the\ntext to highlight this conclusion more clearly (lines 94-99).\n10. Line 86: This is important so I’d suggest to present this in greater detail and also cite some\nrelevant literature here, for example the work of Obata et al. regarding the textures.\nResponse: Following your suggestion, we have expanded the discussion (lines 84-102) to\ninclude more details and relevant references of Špaček et al. (2013) and Obata et al. (2013),\nwhich address textures related to high-pressure garnets and their decomposition.\n11. Line 113: please put references here.\nResponse: We have cited Beyer & Frost (2017) here.\n12. Line 117: synthetic eclogite?\nResponse: We have changed it.\n13. Lines 185-187: Any reference for the method? Van Aken 1998 I guess or is this implemented\nin the Gatan software?\nResponse: van Aken et al. (1998) has been cited in the revised text as the reference for the\nmethod.\n14. Lines 192: maybe add Schmidt et al. Progress in Earth and Planetary Science 2014, 1:27 as\nreference for SiC originating from reduced fluids\nResponse: This reference has been cited in the revised text.\n15. Line 195: I'd suggest to consider only Fe2+ for calculating the #Mg.\nResponse: We have considered the lattice substitution of ferric iron and its implications for\ncalculating the Mg#. Ferric iron is associated with crystal defects in olivine structures (see our\nresponse to Comment #16). Therefore, we have calculated the Mg# using total iron as the\nminimum value for olivine and its high-pressure polymorphs in the deep mantle.\n16. Lines 203ff: I do not really understand the arguments here. The olivine structure has no\nposition for Fe3+, that’s why Fe3+ is so low in olivine in all geologic settings. Even if high-P\nwadsleyite contained Fe3+, the retrograde back transformation to olivine should exclude all\nFe3+ from this olivine. There seems no way that olivine hosts 40% (3-4 wt.%!) structural\nFe2O3. Please elaborate how this should be possible. Further, I don’t understand why\nhydrous experimental wadsleyite is considered for comparison as interaction with fluids is\nexcluded as likely scenario.\nResponse: We appreciate your insights. The argument for Fe3+ presence in olivine under deep\nmantle conditions is based on its disequilibrium in the crystal structure, where it accumulates\nprimarily around Fe-Ni alloys, which contain numerous crystal defects (see Figure 2). These\ndefects are likely the result of ferrous disproportionation. While ferric iron in olivine is unusual\nunder low pressure conditions and tends to migrate out during the ascent, we hypothesize that\nFe3+ is trapped around crystal defects associated with the Fe-Ni alloys.\nAs for the RHP-Ol (retrograded high-pressure polymorphs of wadsleyite/ringwoodite), these\ngrains were initially captured in the mantle transition zone (MTZ) and may have contained\nsignificant hydrogen concentrations (up to 1.5 wt.% H O, as reported by Fei et al., 2020, 2021;\nwadsleyite/ringwoodite (McCammon et al., 2004; Muir et al., 2023). We also note that no\nevidence suggests interaction with external Na-rich melts or fluids, as previously explained.\nFigure 2. Fe-Ni alloys and associated crystal defects in olivine (Ol₃). (a) and (b) show crystal\ndefects surrounding the Fe-Ni alloys in the olivine.\n17. Line 257: what is meant by RHP? Was the Ni-content measured in the Fe,Ni phases?\nResponse: Thank you for the clarification request. RHP refers to retrograded high-pressure\npolymorph of wadsleyite/ringwoodite. We have defined this term clearly in the revised text (lines\n282).\nAdditionally, the Ni contents of the Fe-Ni phases were measured by STEM-EDS for both the\nNa-rich majoritic garnets and the RHP-Olivine (RHP-Ol) phases. These measurements are\npresented in the revised text (lines 292-297) and in Supplementary Table 6.\n18. Line 405: kV\nResponse: We have corrected it as suggested.\n19. Line 463: chosen\nResponse: We have corrected it as suggested.\n20. Line 494: ∑\nResponse: We have corrected it as suggested.\nCited references\nBeyer, C. & Frost, D. J. The depth of sub-lithospheric diamond formation and the redistribution\nof carbon in the deep mantle. Earth and Planetary Science Letters 461, 30-39,\ndoi:https://doi.org/10.1016/j.epsl.2016.12.017 (2017).\nBobrov, A. V., Kojitani, H., Akaogi, M. & Litvin, Y. A. Phase relations on the diopside–jadeite\n–hedenbergite join up to 24 GPa and stability of Na-bearing majoritic garnet. Geochimica\net Cosmochimica Acta 72, 2392-2408, doi:https://doi.org/10.1016/j.gca.2008.03.003 (2008)\nDong, X.-H. et al. Highly oxidized intraplate basalts and deep carbon storage. Science Advances\n10, eadm8138 (2024).\nEzad, I. S. et al. Kelyphite textures experimentally reproduced through garnet breakdown in the\npresence of a melt phase. Journal of Petrology 63, doi:10.1093/petrology/egac110 (2022).\nFei, H. & Katsura, T. High water solubility of ringwoodite at mantle transition zone temperature.\nEarth and Planetary Science Letters 531, 115987 (2020).\nFei, H. & Katsura, T. Water solubility in Fe‐bearing wadsleyite at mantle transition zone\ntemperatures. Geophysical Research Letters 48, e2021GL092836 (2021).\nFrost, D. J. & McCammon, C. A. The redox state of Earth's mantle. Annual Review of Earth and\nPlanetary Sciences 36, 389-420, doi:10.1146/annurev.earth.36.031207.124322 (2008).\nHan, G. et al. Pervasive low-velocity layer atop the 410-km discontinuity beneath the northwest\nPacific subduction zone: Implications for rheology and geodynamics. Earth and Planetary\nScience Letters 554, 116642, doi:https://doi.org/10.1016/j.epsl.2020.116642 (2021).\nKarato, S.-i. On the origin of the asthenosphere. Earth and Planetary Science Letters 321,\n95-103 (2012).\nKiseeva, E. S. et al. Metapyroxenite in the mantle transition zone revealed from majorite\ninclusions in diamonds. Geology 41, 883-886, doi:10.1130/g34311.1 (2013)\nKiseeva, E. S. et al. Oxidized iron in garnets from the mantle transition zone. Nature Geoscience\n11, 144-147, doi:10.1038/s41561-017-0055-7 (2018).\nKuwahara, H., Nakada, R., Kadoya, S., Yoshino, T. & Irifune, T. Hadean mantle oxidation\ninferred from melting of peridotite under lower-mantle conditions. Nature Geoscience 16,\n461-465, doi:10.1038/s41561-023-01169-4 (2023).\nMa, M. et al. The melt content of the low velocity layer in the eastern South China: Implications\nfor the subduction process of the Western Pacific plate. Physics of the Earth and Planetary\nInteriors 298, 106321, doi:https://doi.org/10.1016/j.pepi.2019.106321 (2020).\nMcCammon, C. et al. Oxidation state of iron in hydrous mantle phases: implications for\nsubduction and mantle oxygen fugacity. Physics of the Earth and Planetary interiors 143,\n157-169 (2004).\nMuir, J. M. R., Jollands, M. & Zhang, F. The oxidation states of Iron in dry and wet olivine: a\nthermodynamic model. Journal of Geophysical Research: Solid Earth 128, e2023JB026840,\ndoi:https://doi.org/10.1029/2023JB026840 (2023).\nMoussallam, Y. et al. Mantle plumes are oxidised. Earth and Planetary Science Letters 527,\n115798, doi:https://doi.org/10.1016/j.epsl.2019.115798 (2019).\nObata, M., Ozawa, K., Naemura, K. & Miyake, A. Isochemical breakdown of garnet in orogenic\ngarnet peridotite and its implication to reaction kinetics. Mineralogy and Petrology 107,\n881-895, doi:10.1007/s00710-012-0260-4 (2013).\nPan, F.-B., Jin, C., He, X., Tao, L. & Jia, B.-J. A plate-mantle convection system in the west\nPacific revealed by Tertiary ultramafic-mafic volcanic rocks in Southeast China. Earth and\nSpace Science 8, e2020EA001324, doi:https://doi.org/10.1029/2020EA001324 (2021).\nPearson, D. et al. Hydrous mantle transition zone indicated by ringwoodite included within\ndiamond. Nature 507, 221-224 (2014).\nRohrbach, A. et al. Metal saturation in the upper mantle. Nature 449, 456-458,\ndoi:10.1038/nature06183 (2007).\nRohrbach, A. & Schmidt, M. W. Redox freezing and melting in the Earth’s deep mantle resulting\nfrom carbon–iron redox coupling. Nature 472, 209-212 (2011).\nRevenaugh, J. & Sipkin, S. Seismic evidence for silicate melt atop the 410-km mantle\ndiscontinuity. Nature 369, 474-476 (1994).\nStagno, V., Ojwang, D. O., McCammon, C. A. & Frost, D. J. The oxidation state of the mantle\nand the extraction of carbon from Earth’s interior. Nature 493, 84-88,\ndoi:10.1038/nature11679 (2013).\nŠpaček, P., Ackerman, L., Habler, G., Abart, R. & Ulrych, J. Garnet Breakdown, Symplectite\nFormation and Melting in Basanite-hosted Peridotite Xenoliths from Zinst (Bavaria,\nBohemian Massif). Journal of Petrology 54, 1691-1723, doi:10.1093/petrology/egt028\n(2013).\nTauzin, B., Debayle, E. & Wittlinger, G. Seismic evidence for a global low-velocity layer within\nthe Earth’s upper mantle. Nature Geoscience 3, 718-721, doi:10.1038/ngeo969 (2010).\nTakei, Y. Effects of Partial Melting on Seismic Velocity and Attenuation: A New Insight from\nExperiments. Annual Review of Earth and Planetary Sciences 45, 447-470,\ndoi:https://doi.org/10.1146/annurev-earth-063016-015820 (2017).\nvan Aken, P. A., Liebscher, B. & Styrsa, V. J. Quantitative determination of iron oxidation states\nin minerals using Fe L2,3-edge electron energy-loss near-edge structure spectroscopy.\nPhysics and Chemistry of Minerals 25, 323-327, doi:10.1007/s002690050122 (1998).\nWang, X.-C., Wilde, S. A., Li, Q.-L. & Yang, Y.-N. Continental flood basalts derived from the\nhydrous mantle transition zone. Nature Communications 6, 7700 (2015).\nWang, Z.-Z., Liu, S.-A., Chen, L.-H., Li, S.-G. & Zeng, G. Compositional transition in natural\nalkaline lavas through silica-undersaturated melt–lithosphere interaction. Geology 46,\n771-774, doi:10.1130/g45145.1 (2018).\nWei, W., Xu, J. D., Zhao, D. P. & Shi, Y. L. East Asia mantle tomography: New insight into plate\nsubduction and intraplate volcanism. Journal of Asian Earth Sciences 60, 88-103,\ndoi:10.1016/j.jseaes.2012.08.001 (2012).\nXu, R. et al. Decoupled Zn-Sr-Nd isotopic composition of continental intraplate basalts caused\nby two-stage melting process. Geochimica et Cosmochimica Acta 326, 234-252,\ndoi:https://doi.org/10.1016/j.gca.2022.03.014 (2022).\nYang, J. & Faccenda, M. Intraplate volcanism originating from upwelling hydrous mantle\ntransition zone. Nature 579, 88-91 (2020).\nZeng, G. et al. Nephelinites in eastern China originating from the mantle transition zone.\nChemical Geology 576, 120276, doi:https://doi.org/10.1016/j.chemgeo.2021.120276 (2021)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all my concerns and provided comprehensive responses to my comments. The manuscript has\nimproved significantly in terms of clarity and quality. However, I still believe that the first issue I raised (MTZ origin for the\nxenoliths) is worth further investigation. Providing additional evidence, such as other compositional information from MTZ\nsources, would strengthen the argument and enhance the manuscript’s persuasiveness.\nThat said, the paper is internally consistent and well-structured. I recommend publication either in its current form or after\nminor revisions to further refine the discussion.", "reviewer_2": "The revised version of “Iron disproportionation in peridotite fragments from the mantle transition zone” is more to the point\nand addresses the questions/aspects I raised in the review properly, either in the main text or in the rebuttal. Specifically, the\nauthors use now the Beyer geobarometer to calculate pressures, provide an explanation for the decoupling of Na and Si in\nthe garnet, clarify how eclogitic garnets are incorporated in lherzolithic xenoliths provide an explanation how Fe3+ is\nincorporated in olivine. Also, in my opinion, the concerns of reviewer 1 are adequately addressed but I’d leave that decision\nto the other reviewer or the editors, of course. Overall, I’d be happy to see this manuscript being published in Nature\nCommunications, to me it is high quality work on an exciting topic. I have some additional minor comments the authors might\nadopt.\n1) Maybe elaborate a bit on the uncertainties of the pressure calculation. Given the rather large variation for the Na\nmeasurements in garnet, the reader might be interested how this translates into a 2 sigma uncertainty of P.\n2) Looking again at the garnet compositions and on the criteria to distinguish eclogitic from peridotitic garnet (Fig. 7 of Beyer\nand Frost), it seems that garnets M1-M7 are rather (besides the Na content) transitional in Cr content and Ca# and belong\nclearly to the peridotitic suite in terms of Mg# and Ti content. Maybe it would be interesting for the reader to address this\napparent contradiction.\n3) Fig. 2 in the rebuttal letter is very nice, maybe consider to put it into the Supplement.\n4) Line 103 (file with track changes) lithophile\n5) Line 135 (file with track changes) I’d rather say “calculated” and give uncertainties"}, "author_response": "Response: Thank you for your constructive suggestions and recognition of our work. We will\ncontinue to study these interesting samples and aim to provide additional information in\nfollow-up work.\n\nResponse: Thank you for your thoughtful comments. In response to your suggestion, we have\nadded 2 sigma uncertainties for the major element compositions in revised supplementary Table\n4. Additionally, and exhibited the 2 sigma uncertainties for the calculated pressures are now\nshown in the revised Figure 2a.\n\nResponse: We agree that the garnets show some transitional features in composition. As noted in\nthe text (lines 146-149), these Na-rich majoritic garnets are likely reaction products of\ncarbonated MORB within the MTZ, which could modify their chemical compositions. We have\nadded this possible interpretation in the revised manuscript (lines 149-151).\n\nResponse: We appreciate your valuable insights. Crystal defects in olivine (Ol ) may play an\n3\nimportant role in the storage of both Fe3+ and H+ (McCammon et al., 2004; Muir et al., 2023), as\nwell as in the rheological softening of deep mantle rocks (Cline et al., 2018). We believe these\nimportant geological processes warrant a more in-depth discussion, which we plan to address in\nfuture work rather than in the current supplemental material.\n\nResponse: We have corrected it as suggested.\n\nResponse: We have corrected it as suggested.\n\nCited references\nBeyer, C. & Frost, D. J. The depth of sub-lithospheric diamond formation and the redistribution\nof carbon in the deep mantle. Earth and Planetary Science Letters 461, 30-39,\ndoi:https://doi.org/10.1016/j.epsl.2016.12.017 (2017).\nCline II, C., Faul, U., David, E., Berry, A. & Jackson, I. Redox-influenced seismic properties of\nupper-mantle olivine. Nature 555, 355-358 (2018).\nMcCammon, C. et al. Oxidation state of iron in hydrous mantle phases: implications for\nsubduction and mantle oxygen fugacity. Physics of the Earth and Planetary interiors 143,\n157-169 (2004).\nMuir, J. M. R., Jollands, M. & Zhang, F. The oxidation states of Iron in dry and wet olivine: a\nthermodynamic model. Journal of Geophysical Research: Solid Earth 128, e2023JB026840,\ndoi:https://doi.org/10.1029/2023JB026840 (2023)."}]

Decision letter