The role of C-O-H-F-Cl fluids in the making of Earth’s continental roots

Domain: earth_science
Published: 2025-08-22
Editorial outcome: major_revision

Abstract
The cratonic ‘roots’ of Earth’s major continents extend to depths of over 160 km and have remained stable for more than 2.5 billion years due to buoyant, refractory harzburgites formed by Archean mantle melting. However, mantle harzburgites from some global cratons (e.g., Kaapvaal, Siberia, Slave, Rae and Tanzania) show unusual orthopyroxene and silica enrichment, alongside titanium depletion, which cannot be explained by simple melting processes. The origins of the orthopyroxene-rich harzburgites are debated and include high-pressure melting residues, with komatiite melt interaction, or subduction-related silicic melts and fluids. To further investigate this we analysed volatile (H2O, F, Cl) contents in Kaapvaal craton peridotites. Orthopyroxene-rich harzburgites, including a diamond-bearing sample, show elevated volatile contents, suggesting infiltration by supercritical C-O-H fluids—rich in silica, fluorine and chlorine and depleted in Ti—fluxed from subducted oceanic lithosphere (carbonated pelites, eclogites and serpentinites). These findings highlight the role of C-O-H-F-Cl bearing fluids in shaping cratonic lithosphere and offer a new framework for understanding craton evolution, mantle metasomatism and diamond genesis in early Earth.

Introduction
Over 43% of the surface area of Earth’s major continents is composed of ancient cratons1 (Fig. 1). These are regions where the lithosphere extends to depths >160 km and which have been tectonically stable for the last 2.5 Ga. The long-term resistance of these thick roots to tectonic activity2 has been linked to the presence of low-density, refractory peridotites known as harzburgites, which constitute 35 to 60% of the cargo of mantle xenoliths transported to Earth’s surface by kimberlites3. Cratonic harzburgites are depleted in ‘magmaphile’ elements (e.g., Fe, Ca and Al) and are believed to represent low-density residues from extensive adiabatic decompression melting (30–40%) of the convecting mantle during the Archaean4,5. As they cooled, these buoyant residues aggregated to form the overlying rigid continental (lithospheric) mantle. Subsequent re-fertilisation of the continental mantle by melts and fluids, derived from either the convecting mantle or down-going slabs6,7, has modified the mineralogy or caused chemical (cryptic) changes8.

The cratonic roots of the Kaapvaal, Siberia, Slave, Rae and Tanzania cratons (Fig. 1) are notably enriched in orthopyroxene (>25%) and garnet (up to ~10 modal%)5,9–17. Figure 2a highlights this relationship for the Kaapvaal craton, where some of the most extreme enrichments in orthopyroxene are observed. These orthopyroxene-rich garnet harzburgites typically equilibrated at temperatures below 1150 °C, and are rich in SiO2 (>45 wt.%) but depleted in FeO (<7.0 wt.%) and Ti (<200 µg/g) (data from this work and refs. 14,16,18,19). They exhibit a wide variation in Mg# (Mg/Mg+Fe×100), which is expressed in the forsterite content of olivines (91.6–93.9) and Mg# of orthopyroxenes (93.7–95.2)9,10,16,18. Notably, orthopyroxene-rich garnet harzburgites exhibit lower bulk MgO/SiO2 ratios (Fig. 2b) and higher Mg# compared to lherzolites and other harzburgite varieties.

Despite long-standing knowledge that orthopyroxene-rich garnet harzburgites found in many global cratonic xenolith suites deviate from the low-pressure melting (<3 GPa) trend observed in modern oceanic mantle residues (Fig. 3a)14,20, their origin remains a topic of considerable debate5,15,16,18,19,21–26. The combined high Mg# and low modal % of olivine in orthopyroxene-rich garnet harzburgites relative to other types of cratonic mantle peridotites was initially interpreted as evidence of high-pressure melting (4 to 8 GPa)27 but this requires physically implausible amounts of fusion (40 to 70%; Fig. 3a), even at the high mantle potential temperatures (TP) that existed in the Archaean28. A single-stage mode of formation for the orthopyroxene-rich harzburgites is also not supported by the presence of veins or segregations of orthopyroxene (Supplementary Fig. 1) and overgrowths of orthopyroxene on olivine19 (Supplementary Fig. 2).

In addition to large enrichments in orthopyroxene, harzburgites from the Kaapvaal craton of southern Africa contain olivines with the highest Fo contents5,9–16,18 (Figs. 1, 3). This cratonic mantle initially formed as residues from melting events between 2.5 and 3 billion years ago, which also coincides with the maximum age of formation of excess orthopyroxene (2.6 to 3.0 Ga)16,19,23,29. It has been proposed that the excess silica required to form orthopyroxene originated from reactions between fertile peridotite and melts of recycled eclogite30, or repeated interaction of harzburgites in the continental mantle with komatiite melts formed at high pressures within hot upwelling mantle plumes16,18,21,31. These open-system melt-rock reaction processes require high-temperature thermal anomalies (TP = 1700–1800 °C) in the Archaean convecting mantle and contrast with theories that involve interactions between depleted continental mantle peridotite initially with subduction-derived silicic melts5,9,22,25,32 or fluids19,26, or hydrous within-plate silicic melts23. Another model suggests that the excess orthopyroxene formed through the hydration and serpentinisation of abyssal harzburgites prior to their subduction and incorporation into cratonic roots33,34.

The diverse prevailing hypotheses for the formation of the excess orthopyroxene have important implications for our fundamental understanding of the tectonic processes involved in stabilising Earth’s continental cores. While the various models that have been proposed appear to satisfactorily explain the excess orthopyroxene and bulk major element characteristics (e.g., low MgO/SiO2)5,18,19,21,24,26,30,31,33,34 of orthopyroxene-rich garnet harzburgites, existing datasets cannot readily be used to discriminate between them. Hence the ongoing controversy. The motivation behind our work is that each tectonic scenario that has been proposed for the origin of the excess orthopyroxene will imprint unique trace and volatile element (including C, H, F, Cl and S) signatures on the mantle. This is because of the variable compatibility of these elements during mantle melting and also different partitioning into melts and fluids. For example, large amounts of adiabatic decompression melting in the convecting mantle (~40%) during the Archaean would leave residues devoid of strongly incompatible elements (including C, H and F35) and more depleted than the present-day upper mantle MORB source36 (see Supplementary Information), whereas multiple phases of enrichment would generate elevated, but variable, trace and volatile element concentrations37,38. Furthermore, peridotite and eclogite melts might be expected to have different volatile and trace element signatures to metasomatic melts or fluids fluxed from sediments, altered oceanic crust and serpentinites in down-going slabs of oceanic lithosphere.

Some orthopyroxene-rich garnet harzburgites are diamondiferous39–41, which is important because lithospheric mantle diamonds are widely believed to form from metasomatic fluids42,43, and it has been proposed that the formation of the excess orthopyroxene (>25%) and introduction of carbon are linked23. Here, we offer a new perspective on the formation of excess orthopyroxene and diamond formation in the roots of cratons by presenting high-precision in situ secondary-ion mass spectrometry (SIMS) analyses of multiple volatiles (H2O, F and Cl), together with major and trace elements of the most abundant phases in mantle peridotites predominantly from the Kaapvaal and also Tanzanian cratons (Supplementary Fig. 3), where some of the most extreme enrichments of orthopyroxene and silica occur (Fig. 3a)5,12,14,15,18.

Results
Major and trace element chemistry
The bulk compositions of Kaapvaal orthopyroxene-rich garnet harzburgites in our study are relatively silica-rich, with MgO/SiO2 ratios between 0.89 and 0.94, compared to the global range of 0.85 to 1.0618. Kaapvaal sample BD2125 is notably more magnesian than the other orthopyroxene-rich garnet harzburgites, with olivine at Fo93 and orthopyroxene Mg# at 93.9 (Supplementary Data 1, Supplementary Fig. 4) but, despite this, has the lowest bulk MgO/SiO2 ratio (0.89) in our entire analysed xenolith suite (Supplementary Data 2 and Fig. 2a). This is primarily because of its high orthopyroxene content. Additionally, phlogopite in this orthopyroxene-rich garnet harzburgite has higher Cr2O3 (0.82 wt.%) but lower TiO2 contents (0.12 wt.%) than found in other lithologies Kaapvaal lithologies (e.g., this work, ref. 44) (Supplementary Data 4).

The garnets in Kaapvaal orthopyroxene-rich harzburgites are subcalcic and differ from those found in other mantle lithologies by their low contents of Ti and Y, together with sinusoidal rare-earth element (REE) patterns that peak in the middle REEs (MREEs; Supplementary Fig. 7)10,45. Both the garnets and clinopyroxenes in these harzburgites are distinguished from those in other lithologies by their positive heavy-REE (HREE) slopes, i.e., between Ho and Lu10. Since garnet and clinopyroxene are key hosts of incompatible trace elements in the mantle, they influence the bulk compositions of these and other peridotite xenoliths (Fig. 4). On MORB-source36 normalised multi-element plots, the reconstructed bulk compositions of orthopyroxene-rich garnet harzburgites exhibit depletions at high-field strength elements (i.e., Nb, Ta and Ti) together with Th and U but have high Pb concentrations (Fig. 5). In comparison to other lithologies in the cratonic mantle, the orthopyroxene-rich garnet harzburgites are depleted in Ti and Sr together with the light rare-earth elements (LREEs) and HREEs. They also have the lowest Ce/Pb ratios, which are less than 1 (0.03–0.73) times the MORB source. The diamondiferous orthopyroxene-rich garnet harzburgite (BD2125) exhibits the greatest bulk-rock depletion in LREEs and Sr, yet it has the highest contents of MREEs and Pb (Figs. 4, 5).

Volatile element (H, F and Cl) chemistry
The volatile contents of the most abundant mineral phases (olivine and pyroxenes) in peridotites from the Kaapvaal and Tanzania cratons show considerable variability (Supplementary Fig. 5 and Supplementary Data 1). We use a broad definition of water that includes H incorporated into the crystal lattice of silicate minerals, reporting concentrations as µg/g H2O46,47. Mantle garnets have relatively low contents of water (mean = 11 µg/g48) and so were not analysed here.

Statistical analysis of our new and published data from the Kaapvaal craton10,48–50 (Supplementary Data 5) shows that olivines in the orthopyroxene-rich garnet harzburgites have similar contents of H2O (40 ± 26 µg/g) to those in garnet-bearing lherzolites (47 ± 49 µg/g) and garnet harzburgites (52 ± 43 µg/g) but lower than those in the dunite (81 µg/g) and much higher than olivine in the spinel harzburgites (7 ± 2 µg/g). The orthopyroxenes in the orthopyroxene-rich garnet harzburgites have similar high contents of H2O (174 ± 70 µg/g) to the garnet harzburgites (180 ± 73 µg/g) and dunite (203 µg/g) but are more hydrated than those in the garnet lherzolites (158 ± 72 µg/g) and spinel harzburgites (118 ± 97 µg/g). Clinopyroxenes in the orthopyroxene-rich garnet harzburgites also have higher H2O contents (280 ± 73 µg/g) than those in the garnet lherzolites (206 ± 97 µg/g), dunite (195 µg/g) and spinel harzburgite (74 ± 5 ug/g). Notably, the diamondiferous orthopyroxene-rich garnet harzburgite (BD2125) has high H2O in olivine (92 µg/g) compared to other orthopyroxene-rich garnet harzburgites (Supplementary Fig. 6). Of the most abundant phases in the cratonic mantle, olivine is the main host of fluorine10. In the orthopyroxene-rich garnet harzburgites, mean contents of F in olivine are lower (65 ± 50 µg/g) than the dunite (164 µg/g) but similar to the garnet lherzolites (47 ± 54 µg/g), garnet harzburgites (57 ± 40 µg/g), and spinel harzburgites (54 µg/g) (Supplementary Fig. 56 and Supplementary Data 5). Notable is the exceptionally high F content of olivine (135 µg/g) in the diamondiferous orthopyroxene-rich garnet harzburgite (BD2125; Supplementary Figs. 5, 6).

Because of the strong partitioning of F into olivine, the mean contents of F in orthopyroxene are low and similar in the orthopyroxene-rich garnet harzburgites (17 ± 7 µg/g), garnet lherzolites (20 ± 5 µg/g), garnet harzburgites (22 ± 10 µg/g), dunite (26 µg/g) and spinel harzburgites (9 ± 6 µg/g; Supplementary Fig. 6). The F contents of clinopyroxene in the orthopyroxene-rich garnet harzburgites (23 ± 8 µg/g) are intermediate between those of the garnet harzburgites (29 ± 7 µg/g), garnet lherzolites (38 ± 14 µg/g), dunite (46 µg/g) and spinel harzburgite (20 ± 6 µg/g). The most F enriched pyroxenes in the orthopyroxene-rich garnet harzburgites are found in BD2125 (Supplementary Fig. 6).

Cl contents are low in olivine and pyroxenes from all peridotites in the Kaapvaal and Tanzania cratons and most of our analyses are close to background levels (Supplementary Data 3). We note, however, that phlogopite in diamondiferous orthopyroxene-rich garnet harzburgite BD2125 contains higher Cl (0.17 µg/g; Supplementary Data 4) than found in other lithologies Kaapvaal mantle lithologies (<0.05 µg/g) (e.g., this work, ref. 44).

Bulk volatile contents
The partitioning of volatiles (such as H, F and Cl) into the crystal lattice of mantle phases is complex and controlled by a wide variety of factors, including mineral chemistry, temperature, pressure, oxygen fugacity, melt or fluid composition and bulk assemblage10,46,47,49,51–53. Over the course of geological time, these variables will influence subsolidus re-equilibration between co-existing phases and—given the age of samples in our study—it is crucial to examine bulk-rock as well as mineral volatile contents. Since olivine is typically the most abundant phase in mantle peridotites, its volatile content generally influences the overall bulk composition.

Statistical analysis of both our new and published reconstructed bulk volatile data for the Kaapvaal craton (Supplementary Data 6) reveals that the mean bulk H2O content of the orthopyroxene-rich harzburgites (83 ± 38 µg/g) is similar to the garnet lherzolites (71 ± 32 µg/g) and dunite (90 µg/g) but higher than the garnet harzburgites (59 ± 29) and spinel harzburgites (37 ± 25 ug/g; Fig. 6). Since the orthopyroxene-rich harzburgites have low Ce this gives rise to the highest bulk H2O/Ce (Fig. 5). The diamondiferous sample (BD2125) has exceptionally high bulk H2O (119 µg/g) and H2O/Ce.

Akin to H2O, the mean bulk F contents of the orthopyroxene-rich garnet harzburgites (51 ± 26 µg/g) are similar to the garnet lherzolites (36 ± 44 µg/g) and dunite (153 µg/g) but higher than the garnet harzburgites (48 ± 30 µg/g) and spinel harzburgites (23 µg/g; Fig. 6). Within the orthopyroxene-rich garnet harzburgites there is a positive correlation between bulk F content, modal % of orthopyroxene (Fig. 7) and bulk contents of SiO2 and Cr2O3 but negative correlations with NiO and MgO. Moreover, bulk F contents increase as bulk LREE abundances decrease and the peak in the normalised REE pattern migrates from Nd to Sm, reaching the most extreme in BD2125 (Fig. 4). This diamondiferous orthopyroxene-rich garnet harzburgite contains exceptionally high bulk F (70 µg/g; Fig. 6).

Methods
Samples
Our micro-analytical study has focused on olivine and pyroxenes, which are known to contain variable but potentially important concentrations (up to 100’s of µg/g) of H49,95,96 and F, and low amounts of Cl1,10,50,52,53. These are from well-characterised peridotite xenoliths from the Archaean Kaapvaal (Mothae and Bultfontein; Fig. S1) and Tanzania cratons (Lashaine) and, for comparison, from post-Archaean continental off-craton locations (SW USA, Mongolia, Antarctic Peninsula and the Eifel, W. Germany). The thin sections were digitally scanned, and modal abundances of mineral phases were determined using the JMicroVision image analysis toolbox (www.jmicrovision.com/index.htm, version 1.2.7). A minimum of 500 points was undertaken for each scan. The harzburgites, lherzolites and dunite analysed in our study represent the full spectrum of peridotite lithologies found in global on- and off-craton settings, cover a large pressure (2 to 6 GPa) and temperature range (800–1270 °C) and include a rare diamondiferous harzburgite (BD2125, Supplementary Data 1). This coarse-grained harzburgite was the first mantle peridotite in which in situ diamonds were reported in the published literature39 and exhibits the most extreme enrichment in orthopyroxene (39%) and phlogopite (1.2 modal%). The compositions of minerals in the orthopyroxene-rich garnet harzburgites analysed in our study span the full range observed in this lithology in global cratons, with olivines varying from Fo91.8-93 (Fig. S2) and orthopyroxenes with Mg# between 92.9 and 93.99,18.

Analytical methods
Prior to analysis, all of the mantle xenoliths described in this study were first examined in thin section to ensure that they were petrographically fresh and free of serpentinisation. Individual mineral grains were carefully hand-picked from the crushate. Only grains free from melt and mineral inclusions were selected for cleaning, mounting with crystal bond, and then polished to a 1 µm finish in the clean laboratory at the Department of Earth Sciences, University of Cambridge. Both samples and polishing pads were regularly cleaned in an ultrasonic bath to minimise contamination. The polished mounts were examined using an SEM under low vacuum conditions to verify the selection of the correct mineral phases. Once verified, the polished crystals were removed from the temporary crystal bond, cleaned with acetone and mounted in indium within aluminium disks.

Mineral major and minor element chemistry
In situ analysis of major and minor elements in olivine, clinopyroxene, orthopyroxene, garnet and phlogopite was conducted using the Cameca SX-100 electron microprobe analyser (EMPA) at the Department of Earth Sciences, University of Cambridge. The instrument is equipped with five WDS detectors and one EDS detector. The analysis was performed using an accelerating voltage of 15 keV, a beam current of 20 nA, and a beam size of 1 µm. The counting times varied, with 10 s for Na and K (alkali elements), 20–30 s for major elements, and up to 140 s for minor elements to minimise error. A higher current (20 nA) was employed to enhance the number of X-rays detected and lower the detection limits. Calibration standards included Na on jadeite, K on K-feldspar, Fe on fayalite, Mn, Ni, and Cr on pure metals, Si and Ca on diopside, Al on corundum, Mg on St Johns olivine, and Ti on rutile. Secondary standards, including San Carlos olivine, augite, diopside, and basaltic glass, were analysed before, during, and after each analytical session. The ZAF correction method was used to adjust for matrix effects.

Mineral trace element chemistry
In situ trace element analysis was undertaken using an ESI UP193NC laser ablation system attached to a Nexion 350D ICP-MS system (LA-ICP-MS) in the Department of Earth Sciences at the University of Cambridge. The NWR193 is equipped with a TwoVol2 sample chamber and small volume sample cup to quickly and efficiently transfer samples and washout performance. The analysis setup involved a 100 µm diameter laser beam, laser repetition rate of 10 Hz for clinopyroxene and garnet and 20 Hz for olivine and orthopyroxene, and laser fluence of 8 J cm−2 set at 50% power in order to ensure optimum signal intensity while minimising downhole fractionation. The LA-ICP-MS data acquisition settings were 1 sweep per reading, 80 readings, 1 replicate, and the total data acquisition lasted 60 s (approximately 1 data point for each element per second). The laser was programmed to run for 40 s on clinopyroxene and garnet, and 35 s on olivine, with a laser warm-up time of 20 s for each spot analysis in both instances.

During LA-ICP-MS analysis, the spots were carefully chosen to avoid contamination from nearby cracks, inclusions, or any overlapping neighbouring phases. Ablated sample material was carried in pure helium gas at 700 mL min−1, which was joined with the argon nebulised gas line from the ICP-MS using a signal homogenised device at 0.8 L min−1 before the ICP-MS torch. The ICP-MS dwell time for each mass was dependent on the isotope and concentration of the element in the samples, but was typically 20 milliseconds (ms) for most trace elements in clinopyroxene and garnet, and this was increased up to 100 ms for some low concentration elements in olivine. Clinopyroxene and garnets were analysed using CaO and SiO2 from EMPA analysis as internal standard normalisation. NIST612 was used to calibrate element concentrations. Accuracy and drift were assessed using NIST614, while BCR-2G, BIR-1, BHVO-2, GOR132-G and in-house clinopyroxene and garnet standards were also analysed. The results for these standards are reported in Supplementary Data 2 and Jackson and Gibson10. Clinopyroxenes were re-analysed in October 2023 so that the dataset included Ba, Rb, Th and U—these are absent in the dataset of ref. 10. Data was processed using Glitter software (GEMOC, Australia). The data were filtered for background and melt/mineral inclusions after each analysis to optimise the data. Replicate spots were then averaged. Accuracy is good for data, with results typically between 80 and 110%.

Analysis of H, F and Cl by SIMS
Concentrations of H, F and Cl in olivine and pyroxenes were determined by Secondary-Ion Mass Spectrometry (SIMS) at the NERC Ion Microprobe Facility at the University of Edinburgh. SIMS analysis was undertaken before EMPA and LA-ICP-MS analysis to avoid contamination and elemental migration under the electron beam. A Cameca IMS4f was used for Kaapvaal samples analysed in September 2018 and May 2019, and a Cameca IMS-7f GEO for samples from Tanzania and off-craton locations analysed in November 2019 and May/June 2021.

Crystal mounts were gold-coated before being loaded into the vacuum chamber of the SIMS instrument for over 48 h (often in excess of 60 h) before the first analysis. An O− primary beam was used with a 10 kV primary accelerating voltage and 5 nA current. Prior to each analysis, an area of 50 µm (Cameca IMS4f) or 15 µm (Cameca IMS-7f GEO) was chosen to raster for 4 min. The acquisition went through 10 (Cameca IMS4f) or 20 cycles (Cameca IMS-7f GEO), but only the last 50% were used. An energy offset of 75 was chosen to reduce the interference without losing signal. Prior to each analysis, the peaks for Si, Mg, Ca, H, F and Cl were aligned. Si was used as the reference and in ratios to calculate other concentrations. Mg and Ca were analysed for cross-referencing.

At the start of each day, background values were measured by analysing the anhydrous olivine reference standards OL DC212, OL 116610-5, Ol-Koh and Ol-Fo. The concentrations of H2O and F in the mineral separates were calculated using a calibration line based on the ratio of H/Si counts for standards with known H2O and F concentrations. Clinopyroxene and orthopyroxene reference standards with known H2O and F concentrations (CPX-SC-J1, CPX-KH03-4, CPX-SMC31139-1, CPX SMC31011, OPX KH03-4, OPX 116610-3; Ref. 97) were used for the calibration. Concentrations of H and F in olivine were calculated using the basaltic glass standard ALV-519-4-1 (Supplementary Data 3) and then pyroxene standards because the matrix effect is established to be minimal (C.J. De Hoog, pers. comm). San Carlos olivine was repeatedly analysed throughout each run, as it has low concentrations of H2O (e.g., Aubaud et al.98) and serves as an internal reference to track any analytical drift (Supplementary Data 3).

Chlorine concentrations were not available for any pyroxene or olivine reference standards during the analysis of mineral grains from the Kaapvaal craton (Mothae and Bultfontein) in September 2018. The raw data were reprocessed through JCION5 using a basaltic glass, ST8-1A9, with known Cl concentration (2025 ppm; Supplementary Data 3). The counts were then normalised to the Si concentrations from prior EMPA and Cl concentrations calculated using the ST8-1A9 composition and the conversion factor established using Si counts. For mineral grains from off-craton peridotites and also for Lashaine in Tanzania concentrations of Cl were calculated using ST8.1.A92-2. The Cl acquisition technique was later refined to include optical glass herasil, containing 0.4 µg/g Cl, for background monitoring, which was generously provided by B. Urann. This consistently showed low Cl backgrounds, ranging from 0.3 to 0.6 µg/g.

The raw data were initially processed by C Talavera Rodriguez using the JCION5 software, followed by manual background correction on Microsoft Excel. H2O and F concentrations of the mineral separates were calculated from a calibration line of the ratio of H/Si counts for standards with known H2O and F concentration (Supplementary Data 1). The error for all H2O, F and Cl concentrations is calculated by summing the instrument error and the standard deviation in point analyses averaged for each crystal (Supplementary Data 3). The instrument error is calculated by taking the square root of the sum of the square of the % counter error in H and Si, F and Si or Cl and Si.
% instrument error = sqrt((% error Si)^2 + (% error H)^2)

Mineral grains in two samples of Kaapvaal peridotites were subsequently analysed for H2O by FTIR99 and revealed comparable results (Supplementary Data 3).

Data availability
The authors declare that the data supporting the findings of this study are available within the paper and its supplementary information files.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Gibson et al. presented the volatile (H₂O, F, Cl) contents in Kaapvaal Craton peridotites to decode the origin of\northopyroxene (Opx) enrichment in the cratonic lithospheric mantle. Their results indicate that the mean bulk H₂O content of\nthe orthopyroxene-rich harzburgites is similar to that of garnet harzburgites and spinel harzburgites, but higher than that of\ngarnet lherzolites. In contrast, the mean bulk F contents are lower in the orthopyroxene-rich garnet harzburgites than in both\nthe garnet harzburgites and spinel harzburgites, but higher than in the garnet lherzolites. These variations are interpreted to\nresult from reactions with supercritical C-O-H fluids, rich in silica, fluorine, and chlorine, that infiltrated from subducted\noceanic lithosphere during the Archean. Given the new and important volatile data presented for these peridotites, I\nrecommend publication after revisions, considering the following suggestions and comments:\nThe supercritical C-O-H fluids from serpentinites, which infiltrated through carbonated pelites, should also be enriched in Na,\nK, and P, not just Si. Did the authors examine major element variations in relation to the Opx enrichments? Moreover,\nsignificant Opx enrichment would require modal metasomatism, which implies that whole rock trace elements might have\nbeen reset by supercritical fluids, especially given the refractory nature of harzburgites with low trace element contents.\nThese Opx-rich harzburgites do not show LREE enrichments, which would be expected from the supercritical fluids\ninfiltrating sediments (see D from Kessel et al. 2005, Nature). A mixing calculation between refractory harzburgites and\nsupercritical C-O-H fluids equilibrated with sediments should be performed.\nGenerally, F compatibility in mantle silicate minerals follows the order: garnet > pyroxene > olivine. Interestingly, the\nKaapvaal Craton Opx-rich harzburgites show the opposite trend, with olivine exhibiting much higher F contents than\npyroxenes. The authors need offer an explanation for this observation.\nFluids are generally enriched in Cl relative to F, as also suggested by Fig. S7 in the manuscript. Is it possible to reconstruct\nthe bulk Cl contents to further investigate the role of supercritical C-O-H fluids? I also noticed some discussion about\nalkalinity addition through diamondiferous orthopyroxene-rich garnet harzburgites, but more details are more helpful.\nThe methods section provides detailed information on volatile measurements using SIMS, but it does not address the\nreliability of the results. Some standard references as blind samples should be used to estimate the reliability of their results.\nI hope these suggestions help improve the manuscript.\nMinor comments\nLine 823. elements to element\nLine 853. % to wt.%\nLine 872. What are the residues predicts? Melting residues?\nLine 888. MORB source normalized?\nLine 913. is from Ref. 100", "reviewer_2": "The role of C-O-H-F-Cl fluids in the making of Earth's continental roots\nThis manuscript by Gibson et al. addresses a longstanding and significant open question in mantle geochemistry regarding\nthe origin of Si enrichment observed in some of Earth's cratonic 'roots.' Since Boyd's initial observations in 1989, this\nphenomenon has been the focus of numerous studies, leading to two primary schools of thought: 1. orthopyroxene\nenrichment is attributed to be a key characteristic inherited from a nonpyrolitic mantle, or 2. Secondarily, silicon enrichment\nin the form of access orthopyroxene (opx) occurs in preexisting olivine-rich melting residues, such as metasomatism in\nsubduction zones or plume settings. Overall, the entire manuscript is a pleasure to read and very clear.\nThe authors tackle this question by analyzing a suite of mantle xenoliths and their minerals for volatile contents, including\nH2O, Cl, and F, with a particular focus on the opx-rich xenoliths from the Kaapvaal Craton, including one diamond-bearing\nopx-rich harzburgite. The authors argue that based on the observed volatile enrichment of olivines within the opx-rich\nxenoliths, the most likely cause of the opx enrichment was the infiltration of a small volume of supercritical C-O-H-F-Cl-\nbearing fluids sourced from subducted oceanic lithosphere ± sediment and its reaction with a previously depleted mantle to\nform the observed Si enrichment. The above is in contrast to recent thermodynamic studies, which propose a reaction\nbetween \"dry\" komatiitic melts and a melt-depleted lithospheric mantle at high pressure, forming opx in a peritectic reaction\n(e.g., Tomlinson and Kamber 2021).\nThe Cambridge group is one of the pioneering teams in the field and is an expert in the analysis of volatiles in mantle\nminerals and their storage at lithospheric conditions. This large body of work on the volatile concentrations (F, H2O, and Cl)\non 22 xenoliths and 65 mineral separates coupled with petrographic major and trace element analysis represents a very\nimpressive analytical campaign and achievement. In my opinion, this contribution is vital to the community and keeps the\nlong-lasting debate alive.\nAfter some moderate to significant revisions, this study should be published in Nature Communications.\nMajor remark\nIt appears that the entire model is heavily reliant on a single diamond-bearing, opx-rich harzburgite with approximately 120\nppm F, along with two additional xenoliths, all from the Bultfontein locality. While the scarcity of suitable samples is\nunderstandable, drawing robust correlations from only three data points is challenging. Additionally, making a global\nassessment based on such limited data, which primarily pertains to the Kaapvaal Craton, seems to be overreaching. Given\nthe long residence time of these xenoliths in the mantle, a more detailed explanation is needed to clarify how the samples\nretained such high F contents over geological timescales.\nSpecific remarks in the text\nAbstract\nThe abstract is well written very lays out the question very elegantly, and the novelty of the study.\nLine 13: Why mention Siberia and Rae and Tasmania cratons here? It is a bit misleading in my mind.\nLine 14: What is the significance of the Ti depletion? Maybe highlight this observation a bit more here?\nLine 20: A bit vague, in my opinion.\nIntroduction\nLine 48: Mg# notation is explained in line 50. I would move it here.\nResults\nMajor and trace element chemistry\nLine 110: What is the significance of phlogiopite? Is this phase related to kimberlite metasomatism, or is it more ancient?\nVolatile elements\nt would be helpful to visualize the different populations and how they relate to one another, perhaps using a box-and-whisker\ndiagram similar to the one in the supplemental file. Additionally, many of the measurement error bars appear to overlap. If\nthis is typical in the field of volatiles in the mantle, an explanation of why the results remain robust would be beneficial. For\nexample, in lines 144–146, there is a clear population difference, and this should be represented in a figure to strengthen the\nargument.\nBulk volatile content\nLine 185: How robust are the correlations that are not shown? This seems like a critical point to address. Additionally, are\nthere correlations with other minor or trace elements in olivine and orthopyroxene? It might be interesting to examine first-\nrow transition elements such as Zn, Sc, V, and Mn.\nThe current projection in Figure 5, illustrating the relationship between bulk LREE versus F and the Sm/Nd peak position,\ncould be improved, as it appears somewhat misleading. Alternative projections, such as plotting ∑LREE (ppm) versus F\n(ppm), Si, Mg, or Nd/La versus F, Si, Mg, might provide clearer insights. It might also be worth considering a more integrated\npresentation of panels A and B to better highlight the differences between the opx-rich and opx-poor samples.\nMantle Melting Models\nI think this and subsequent sections are part of the discussion, but it is not noted in the text.\nLine 193: I believe you meant 2b and not 2a? Additionally, I found the use of the color gradient a bit excessive. If there is a\nstrong correlation, it might be worth dedicating its own panel—just a suggestion. Line 192-193: Fo. Molar proportions, yes?\nReactive infiltration of Al-enriched komatiites\nMy main comment here is that the proposed fluid-associated opx addition is comparable to melt addition, except for one\nsample that is a bit strange and needs to be addressed. The Ti and HREE mismatch is very interesting.\nInfiltration of subduction-related melts and fluids\nLine 249-252 I think a citation is needed for this statement.\nWhy did you choose the fluid composition of Sokol et al.? ~1100°C at ~8 GPa seems a bit cold for Archean tectonics. What\ngeotherm does this plot on? It might be worth looking at the Timmerman et al. (2022) EPSL paper on Mesoarchean\ndiamonds to see if their thermal modeling could explain the cold and transient conditions relevant to your proposed fluids.\nIs there additional evidence supporting monazite dissolution? Including references to radiogenic isotope studies could\nstrengthen this argument. The explanation for subaerial weathering seems a bit too convenient.\nIt might also be valuable to explore natural examples of silicic fluids trapped in diamonds as potential candidates for the\nrequired supercritical fluid. The recent compilation by the HUJI group on fluid compositions in diamonds could be relevant.\nWhile there is no F content data for these inclusions due to hydrofluoric acid use during sample prep, H2O, Cl, Si, Al, and\nother trace elements are well-characterized and are known metasomatic agents.\nRegarding diamond-forming fluids, to my knowledge, there are no fibrous diamonds from the Kaapvaal of this age that are\nSiO2-rich. However, their absence in the diamond record does not necessarily exclude their involvement. The oldest\nobserved fluids in the Kaapvaal are carbonatitic, which cannot account for Si enrichment. You might consider looking at\nWeiss et al. (2021, Nature Communications) on how fibrous diamonds track the metasomatic history of the Kaapvaal using\nhelium isotope systematics.\nThe \"smoking gun,\" in my view, would be high-Mg olivine in diamonds with high F, Cl, and H2O. While this might be beyond\nthe scope of this study, it is worth considering. On that note, olivine inclusions from the De-Beers pool kimberlites have a\ndistinct high-Mg number and could provide valuable insights (see Stachel et al., 2022, RIMG review, Figure 6).\nI'm not sure that the segway to the next section is warranted. Maybe lines 279-300 should be incorporated here, and\nsections Infiltration of subduction-related melts and fluids & Carbonation, hydration, fluorination, and chlorination of cratonic\nmantle should be renamed and restructured.\nLine 289: The major obstacle is not just oxygen isotopes but also mass balance considerations and the cold geotherm that\nyou just noted above.\nLine 329: The Slave and Kaapvaal cratons have very different geological histories. Before reaching this conclusion, Slave\nxenoliths should be added.\nLine 344: Why is this seen in Fig S3? Maybe you mean S4? If so, I think this (or a version of it) should be moved to the main\ntext.\nLine 344-347: This claim warrants a figure in my mind; also, it is worth noting that the K and (F) in K-rich phases are likely\nlost during subsequent melting.\nLines 364-376: The link to wider implications is very interesting but a bit of an overpromise and\nunderdeliver, especially for the North China Craton. Is there any insight as to why the opx enrichment is restricted to the\nKaapvaal Slave and Siberian Cratons?\nSome other remarks\nThe experimental results of Gao et al. (2021) might be worth considering aside from the introduction as they are similar to\nwhat the authors propose.\nWhat about using the ΔMg/Si notation of Canil and Lee 2009? You did something similar in supp Fig. S4 but did not add\nvolatiles to the color bar. It seems from Fig. 2 that there are no robust correlations. Don't you expect that the more the xenolith\ndeparts from the Boyd trend, the more fluid reacts, and thus, the more F in the olivine? Here, some diffusion modeling might\ncome in handy.\nWhy does BD1153 (dunite) have so much F in it?\nFigures and supplementary materials remarks\nThe Figures overall are very engaging. I have a couple of remarks and possible suggestions.\nFig. 1: Since the focus is primarily on the Kaapvaal Craton, could you include a panel showing the location of the retrieved\nxenoliths? Or provide it in the supplement? Is there any observed spatial variation?\nFig. 2 Panel A might be redundant; I think that Panel B is the star of this figure.\nFig. 3 See comment above. Plenty of projections might be better utilized to illustrate the differences you are discussing.\nWhy normalize to MORB source and not Primitive mantle? From the way I read the manuscript, it is not clear why you chose\nto show this normalization.\nFig. 5. Are there any inclusions in the diamond to plot on this diagram? Why don't you show the olivine from the opx-rich\nsample in panel a? It is a bit misleading. In panel b, the diamond-bearing opx-rich sample BD2125 has 120 ppm F in the\nsupplementary table and is plotted as ~138 ppm in the figure. Am I missing something? Is there any meaning to the\ncorrelations?\nFigure 8 is great, but it has been recycled from other publications. If you want to show a conceptual model, I believe it merits\na new figure.\nThe supplemental tables need a bit more work to make them clear to the reader. missing descriptions\nIn the supplementary table of the bulk trace element compositions, there is no table description indicating which samples are\nfrom the Kaapvaal, Tanzania, or off-craton. For those who are not familiar with the local names, it is hard to follow. Also, can\nyou group them in a systematic order?\nSupp table Bulk trace element compositions There is data in columns O and P that should be deleted.\nThe clinopyroxene table needs a description.\nI have no remarks on the Methods section\nBest,\nOded Elazar", "reviewer_3": "Review of the manuscript “The role of C-O-H-F-Cl fluids in the making of Earth’s continental roots” by Gibson, Jackson,\nCrosby, and Day, submitted to Nature Communications\nThe focus of this manuscript is the existence of opx-rich garnet harzburgites in the SCML.\nThe authors examine 3 scenarios for the formation of excess opx in these xenoliths: high-pressure melting residues,\nkomatiite interactions, or subduction-related silicic melts and fluids. The authors analyze olivine, pyroxene and garnet in\ndifferent xenoliths for H2O, F and Cl content and conclude that the best model is the interaction of the harzburgitic mantle\nwith C-H2O-F-Cl fluids derived from dehydration of the subducting slab.\nThe formation of opx-rich harzburgites is a long-standing question. The suggestion that these xenoliths were formed due to\nthe interaction of depleted harzburgite with slab-derived silica-rich fluids or melts is not new. However, this manuscript\nattempts to provide additional support for this model via the analyses of volatiles in the peridotite minerals themselves, and\nprovides unique information for the type of fluid/melt interacting with the harzburgite (volatile-rich). This manuscript is\nimportant and very interesting.\nThe manuscript is well written and easy to follow.\nMajor comments and thought\n1. Opx-rich harzburgites are mainly found in Slave, Rae, Kaapvaal and Siberia cratons. Does that mean that fluids interacted\nwith the SCML only in these regions and no other SCML?\n2. There is no discussion on carbon in these potential fluids.\n3. Saline HDF (found in fluid inclusions in diamonds) were identified in many cratons around the world (see review by Weiss\net al. 2022). Not just were there is excess opx. Also, Weiss and others suggested that silicic HDFs originate from the\ndehydration of the subducting slab. It will be interesting to use the trace element compositions of either the saline HDF or\nsilicic HDF instead of experimental supercritical fluid in Figure 7.\n4. Elazar and Kessel (2024) interacted harzburgite with eclogite in a hydrous environment (CHO fluid) at the conditions\nabove the eclogitic wet solidus, but below the wet peridotite solidus. At the experimental conditions, eclogitic supercritical\nfluid was interacting with the harzburgite. During this interaction, the amount of ol decreases while the amount of opx\nincreases. Their work may apply to the suggested model.\nMinor comments\n5. Line 90 - The authors use Kessel et al. (2005) (ref. 35) for trace elements profiles during enrichments. Kessel et al. (2005)\nfocuses on trace elements in eclogite+H2O system. It is better to refer to Kessel et al. (2015): The behaviour of incompatible\nelements during hydrous melting of metasomatized peridotite at 4–6 GPa and 1000 °C–1200 °C.\n6. Line 116 – REE profiles are in Figure S5 (not S4).\n7. Lines 131 - Please check the calculated averages for the H2O and F in the various minerals and in the bulk peridotites.\nFollowing the data presented in Table S1, I calculate different values. For example, The H2O content in olivine in opx-rich\ngarnet harzburgite in Table S1 are 37, 69 and 69 ppm. The average is 58±18 ppm (and not 49.5±68 as states in the text).\nSame for opx etc.\n8. Line 191-193 – Figure 2a does not show bulk H2O+F content. Figures 2b does. The authors declare that the phanerozoic\noceanic melt residues show a decrease in bulk H2O+F with increasing Fo content and model abundance of ol.\n9. Line 236-237 – Ti seems to fit well with the infiltration of Al-enriched komatiites model. Ba and Rb do not fit.\n10. Figure 7 – In the legend, the reference is Sokol et al (2024). In the figure caption, it is Sokol et al. (2023).\n11. The fluid taken from this reference is a supercritical fluid in equilibrium with a pelite. Please explain this choice. Why not\nchoose a supercritical fluid in equilibrium with eclogite\n12. Check the symbol of the supercritical fluid from Sokol in Fig. 7b. (Triangle in the legend, circles in the figure)."}, "author_response": "RESPONSE TO REVIEWERS’ COMMENTS\nReviewer #1 (Remarks to the Author):\nGibson et al. presented the volatile (H2O, F,Cl) contents in Kaapvaal Craton peridotites to decode\nthe origin of orthopyroxene (Opx) enrichment in the cratonic lithospheric mantle. Their results\nindicate that the mean bulk H 0 content of the orthopyroxene-rich harzburgites is similar to that\n2\nof garnet harzburgites and spinel harzburgites, but higher than that of garnet lherzolites.In\ncontrast, the mean bulk F contents are lower in the orthopyroxene-rich garnet harzburgites than in\nboth the garnet harzburgites and spinel harzburgites, but higher than in the garnet lherzolites.\nThese variations are interpreted to result from reactions with supercritical C-0-H fluids, rich in silica,\nfluorine, and chlorine, that infiltrated from subducted oceanic lithosphere during the Archean.\nGiven the new and important volatile data presented for these peridotites, I\nrecommend publication after revisions, considering the following suggestions and\ncomments:\n1) The supercritical C-O-H fluids from serpentinites, which infiltrated through carbonated\npelites, should also be enriched in Na, K, and P, not just Si. Did the authors examine major\nelement variations in relation to the Opx enrichments? We observed an enrichment in K (but\nnot Na) and do not have data for P. As mentioned in the text, the K enrichment is linked to\nphlogopite.\n2) Moreover,significant Opx enrichment would require modal metasomatism, which implies\nthat whole rock trace elements might have been reset by supercritical fluids,especially\ngiven the refractory nature of harzburgites with low trace element contents. Yes, we fully\nagree with this statement in our manuscript.\n3) These Opx-rich harzburgites do not show LREE enrichments,which would be expected from\nthe supercritical fluids infiltrating sediments (see D from Kessel et al. 2005,Nature). A mixing\ncalculation between refractory harzburgites and supercritical C-0-H fluids equilibrated with\nsediments should be performed. We did show the results of this mixing calculation in\nFigure 9b (previously 7b) but have replaced it with a comparison of the predicted\nmetasomatic agent in equilibrium with garnets in the orthopyroxene-rich harzburgites.\nThis shows that they are remarkably similar to mixing of refractory harzburgite with\nsupercritical fluids equilibrated with sediments (based on the study of Sokol et al., 2023 &\n2024).\n4) Generally, F compatibility in mantle silicate minerals follows the order:garnet > pyroxene >\nolivine. Interestingly, the Kaapvaal Craton Opx-rich harzburgites show the opposite\ntrend,with olivine exhibiting much higher F contents than pyroxenes.The authors need offer\nan explanation for this observation. There are very few data on F in mantle minerals and we\nare one of the first groups to generate this data for cratonic mantle. Our work shows that the\nmain host of F is olivine followed by the pyroxenes (see lines 147-159).\n5) Fluids are generally enriched in Cl relative to F, as also suggested by Fig. 57 in the\nmanuscript. Is it possible to reconstruct the bulk Cl contents to further investigate the role\n1\nof supercritical C-0-H fluids? Chlorine is not compatible in mantle olivine and the pyroxenes\nand so it is not possible to reconstruct bulk Cl compositions for most of the peridotite xenoliths\nin our study. Chlorine does, however, partition into phlogopite and so we cite the Cl data where\nthis phase is present (Lines 164 & 331).\n6) I also noticed some discussion about alkalinity addition through diamondiferous\northopyroxene-rich garnet harzburgites, but more details are more helpful.\nEvidence of K addition is present from the phlogopite in the diamondiferous orthopyroxene-\nrich garnet harzburgite. The phlogopite also has relatively high Na and Cl contents as\nmentioned in the text (Line 164). I agree this is important and have added the following\nsentence in Line 337 of the main text: ‘The presence of phlogopite with relatively high Na\ncontents in the most orthopyroxene-rich garnet harzburgites is evidence of the alkaline\nnature of the percolating silicic fluids.’\n7) The methods section provides detailed information on volatile measurements using SIMS,\nbut it does not address the reliability of the results. Some standard references as blind\nsamples should be used to estimate the reliability of their results. Unfortunately, there are\nno sets of SIMS standards for olivine. At the time of analysis the most appropriate standard\nwas ALV-519-4-1 (https://georem.mpch-mainz.gwdg.de). This is a basaltic glass from the\nFAMOUS area of the Mid Atlantic Ridge (Langmuir et al., 1977). For H O backgrounds, we\n2\nalso analysed San Carlos olivine (see Aubaud et al., 2015) and OL-116610-5. Analyses of\nthese standards have been added to Supplementary Table 3.\nAll of the data presented in our paper were run at the Edinburgh Ion Microprobe Facility\n(EIMF) using the same method and so internally consistent. Mantle olivines and pyroxenes\nseparated from some of the same Kaapvaal peridotite xenoliths have been analysed by\nSIMS at EIMF and also by FTIR at Bayreuth (using the Withers 2012 calibration; Heckel,\n2023). The table below shows the data are comparable. This has now forms Table S2 in the\nSupplementary material.\nOlivine Orthopyroxene\nThis work Heckel (2023)30 This work Heckel (2023)30\nSIMS (Edinburgh) FTIR (Bayreuth) SIMS (Edinburgh) FTIR (Bayreuth)\nBD2128 22 30 191 155\nBD2170 44 183 180\nI hope these suggestions help improve the\nmanuscript. Minor comments\nLine 823. elements to element. This typo has been corrected\nLine 853.% to wt.% This has been corrected to volume %\nLine 888. MORB source normalized? This typo has been corrected\nLine 913. is from Ref. 100. ‘Ref. 100’ has been added\nReviewer #2 (Remarks to the Author):\nThe role of C-0-H-F-CI fluids in the making of Earth's continental roots\nThis manuscript by Gibson et al. addresses a longstanding and significant open question in\nmantle geochemistry regarding the origin of Si enrichment observed in some of Earth's cratonic\n'roots.' Since Boyd's initial observations in 1989, this phenomenon has been the focus of\nnumerous\nstudies, leading to two primary schools of thought: 1. orthopyroxene enrichment is attributed to\nbe a key characteristic inherited from a nonpyrolitic mantle, or 2. Secondarily, silicon enrichment in\nthe form of access orthopyroxene (opx) occurs in preexisting olivine-rich melting residues, such as\nmetasomatism in subduction zones or plume settings. Overall, the entire manuscript is a pleasure\nto read and very clear.\nThe authors tackle this question by analyzing a suite of mantle xenoliths and their minerals for\nvolatile contents, including H20, Cl, and F, with a particular focus on the opx-rich xenoliths from\nthe Kaapvaal Craton, including one diamond-bearing opx-rich harzburgite. The authors argue\nthat\nbased on the observed volatile enrichment of olivines within the opx-rich xenoliths, the most likely\ncause of the opx enrichment was the infiltration of a small volume of supercritical C-0-H-F-CI\nbearing fluids sourced from subducted oceanic lithosphere ± sediment and its reaction with a\npreviously depleted mantle to form the observed Si enrichment. The above is in contrast to\nrecent thermodynamic studies, which propose a reaction between \"dry\" komatiitic melts and a\nmelt depleted lithospheric mantle at high pressure, forming opx in a peritectic reaction (e.g.,\nTomlinson and Kamber 2021).\nThe Cambridge group is one of the pioneering teams in the field and is an expert in the analysis\nof volatiles in mantle minerals and their storage at lithospheric conditions. This large body of work\non the volatile concentrations (F, H20, and Cl) on 22 xenoliths and 65 mineral separates coupled\nwith petrographic major and trace element analysis represents a very impressive analytical\ncampaign and achievement. In my opinion, this contribution is vital to the community and keeps\nthe long lasting debate alive.\nAfter some moderate to significant revisions, this study should be published in Nature\nCommunication\ns. Major remark\n1) It appears that the entire model is heavily reliant on a single diamond-bearing,\nopx-rich harzburgite with approximately 120 ppm F, along with two additional\nxenoliths, all from the Bultfontein locality. While the scarcity of suitable samples is\nunderstandable, drawing robust correlations from only three data points is\nchallenging. This is not strictly true. The manuscript focuses on the volatile\n3\ncontents of a suite of mantle peridotites from the Kaapvaal craton. The\ndiamond-bearing peridotite (BD2125) represents an end-member in the\nxenoliths that have excess orthopyroxene. We believe that the rarity of\ndiamond bearing peridotites (there are probably <10 that have been reported\nin the published literature) makes this sample especially important and our\nfindings novel.\n2) Additionally, making a global assessment based on such limited data, which\nprimarily pertains to the Kaapvaal Craton, seems to be overreaching. While our work\nfocuses on the origin of excess orthopyroxene in the Kaapvaal (and to a lesser\nextent the Tanzania craton) it has long been known that such enrichments are\npresent in numerous other global cratons. Although our multiple volatile dataset is\nunique in addressing the cause of the excess orthopyroxene, we hope that our work\nwill now drive forward similar analyses in xenolth suites from other cratons.\n3) Given the long residence time of these xenoliths in the mantle, a more detailed\nexplanation is needed to clarify how the samples retained such high F contents\nover geological timescales. This is beyond the scope of the current manuscript but\nsomething we aim to address in a long format journal. In essence, the diffusion of F\nin olivine is very slow and similar to Ti.\nSpecific remarks in the text\nAbstract\nThe abstract is well written very lays out the question very elegantly,and the novelty of the study.\nLine 13: Why mention Siberia and Rae and Tasmania cratons here? It is a bit misleading in my\nmind. These cratons are mentioned to highlight that the enrichment of the lithospheric mantle in\northopyroxene is not restricted to the Kaapvaal craton, i.e. this is a global phenomenon. We\nhave now emphasized the global occurrence of this mantle lithology in the Discussion by\nadding:\n‘While our study has primarily focused on the association of volatiles and orthopyroxene-\nenrichment in the Kaapvaal craton mantle, where some of the greatest excess orthopyroxene\nand silica has been observed, similar mantle peridotites have been widely reported from the\nSiberian, Slave, Rae and Tanzanian cratons e.g. 5,9–17. There are, however, no F or Cl data for\nolivine and pyroxenes in well-characterised peridotites from these other cratons and systematic\nanalytical work is now required to firmly establish if the enrichment in multiple volatiles and\northopyroxene is a global phenomenon.’\nLine 14: What is the significance of the Ti depletion? Maybe highlight this observation a bit more\nhere? Yes, this is a good point. I have modified this sentence to read ‘This suggests infiltration by\nsuper-critical C-O-H fluids -- rich in silica, fluorine and chlorine and depleted in Ti -- fluxed from\nsubducted oceanic lithosphere (carbonated pelites, eclogites and serpentinites) was important in\ndriving olivine-to-orthopyroxene transformation and diamond formation under reduced conditions\nduring the Archean.’\nfurther investigate the role of melts and fluids, we analysed volatile (H₂O, F, Cl) contents in\nKaapvaal craton peridotites’\nIntroduction\nLine 48: Mg# notation is explained in line 50. I would move it here. Agreed. This definition has\nbeen moved.\nResults\nMajor and trace element chemistry\nLine 110: What is the significance of phlogiopite? Is this phase related to kimberlite\nmetasomatism, or is it more ancient? There is no evidence of kimberlite-related metasomatism in\nthis sample and we believe that the phlogopite formed at the same time as the excess\northopyroxene (see experiments of Gao et al. (2021).\nVolatile elements\nIt would be helpful to visualize the different populations and how they relate to one another,\nperhaps using a box-and-whisker diagram similar to the one in the supplemental file.\nAdditionally, many of the measurement error bars appear to overlap. If this is typical in the field\nof volatiles in the mantle, an explanation of why the results remain robust would be beneficial.\nFor example,in lines 144-146, there is a clear population difference, and this should be\nrepresented in a figure to strengthen the argument. Thank you for this suggestion. A new set of\nbox and whisker plots has been included in the main text (new Figure 6) to show the variations\nin bulk volatile contents of different Kaapvaal lithologies. There is also a new set of box and\nwhisker plots in the Supplementary material (Figure S6) to show variations of volatiles in olivine\nand pyroxenes. Both help visualize the variations and show how the standard deviations vary\nbetween lithologies. The ‘boxes’ show where 75% of the data fall and that there are systematic\nvariations between the reconstructed volatile contents of the different mantle peridotite\nlithologies.\nBulk volatile content\nLine 185: How robust are the correlations that are not shown? This seems like a critical point to\naddress. Additionally, are there correlations with other minor or trace elements in olivine and\northopyroxene? It might be interesting to examine first-row transition elements such as Zn, Sc,\nV, and Mn. Because of the relatively small number of samples it is not possible to\nundertake a rigorous statistical analysis of bulk compositions, so these are qualitative\nobservations. The correlations of volatiles with minor elements in olivine and orthopyroxene\n(where the dataset is larger) is beyond the scope of the current study and something that\nwe are actively working on.\nThe current projection in Figure 5, illustrating the relationship between bulk LREE versus F and the\nSm/Nd peak position, could be improved, as it appears somewhat misleading. Alternative\nprojections, such as plotting :LLREE (ppm) versus F (ppm), Si, Mg, or Nd/La versus F, Si, Mg,\nmight provide clearer insights. It might also be worth considering a more integrated\npresentation of panels A and B to better highlight the differences between the opx-rich and\n5\nopx-poor samples.\nI think the reviewer is referring to Figure 3 rather than Figure 5 in the original version of the\nmanuscript? This plot was chosen to show the variability in bulk rare-earth element\nabundances. We have removed the text ‘Si, Mg, C & H+F’ from the upper plot as we agree this\nis misleading.\nMantle Melting Models\nI think this and subsequent sections are part of the discussion, but it is not noted in the text.\nAgreed. ‘Discussion’ has now been added as a sub-heading to make this clearer.\nLine 193: I believe you meant 2b and not 2a? Thank you – this typo has been corrected\nAdditionally, I found the use of the color gradient a bit excessive. If there is a strong correlation,\nit might be worth dedicating its own panel-just a suggestion.\nThe color gradient is used in Figure 2b to highlight: (i) high Mg samples have the highest\ncontents of Bulk H2O + F; and (ii) how this differs from xenoliths that follow the Phanerozoic melt\nextraction trend. We would like to keep the figure and colourbar as it is.\nLine 192-193: Fo. Molar proportions, yes? Yes, thank you for spotting this typo.\nReactive infiltration of Al-enriched komatiites\nMy main comment here is that the proposed fluid-associated opx addition is comparable to\nmelt addition, except for one sample that is a bit strange and needs to be addressed. The Ti and\nHREE mismatch is very interesting. I have re-modelled the trace element data so that it now\nfocuses on metasomatic agents in equilibrium with garnet in the diamondiferous\northopyroxene-rich harzburgite. This is a more appropriate approach but does not permit us to\nmodel the volatiles because of a lack of partitioning data and analyses for garnet.\nInfiltration of subduction-related melts and fluids\nLine 249-252 I think a citation is needed for this statement. Agreed, I’ ve added examples of\nrelevant publications, i.e. Kessel (2005), Meltzer & Kessel (2023) and Chen et al. (2023).\nWhy did you choose the fluid composition of Sokol et al.? -1100oC at -8 GPa seems a bit cold\nfor Archean tectonics. What geotherm does this plot on? It might be worth looking at the\nTimmerman et al. (2022) EPSL paper on Mesoarchean diamonds to see if their thermal modeling\ncould explain the cold and transient conditions relevant to your proposed fluids. There are very\nfew experiments that have looked at the trace and volatile (H, F & Cl) contents of supercritical\nmelts and fluids in subduction zones. The results of Sokol et al. (2024) are appropriate for the\ntemperature and pressure in the down-going slab rather than the cool conductive geotherm of the\nKaapvaal cratonic root. Sokol’s experiments were conducted over 2 GPa intervals but we chose\nthe high pressure (7.8 GPa) and temperature (>940 oC) results because monazite and phengite\nwere absent in the run products, i.e. had broken down and contributed to the trace element\nbudgets of the super-critical fluids.\nweathering seems a bit too convenient. This section has been removed from the revised text.\nIt might also be valuable to explore natural examples of silicic fluids trapped in diamonds as\npotential candidates for the required supercritical fluid. The recent compilation by the HUJI group\non fluid compositions in diamonds could be relevant. While there is no F content data for these\ninclusions due to hydrofluoric acid use during sample prep, H20, Cl, Si,AI,and other trace\nelements are well-characterized and are known metasomatic agents. This is a helpful suggestion,\nthank you. I have looked into the available data and undertaken some new modelling related to the\ncompositions of the diamond inclusion fluids published by the HUJI group (see Figure 9c).\nRegarding diamond-forming fluids, to my knowledge, there are no fibrous diamonds from the\nKaapvaal of this age that are Si02-rich. However, their absence in the diamond record does not\nnecessarily exclude their involvement. The oldest observed fluids in the Kaapvaal are carbonatitic,\nwhich cannot account for Si enrichment. You might consider looking at Weiss et al. (2021, Nature\nCommunications) on how fibrous diamonds track the metasomatic history of the Kaapvaal using\nhelium isotope systematics. This is a very interesting paper, which I have re-read, and note that\nthe oldest observed fluids reported by Weiss et al. (2021) are younger (Proterozoic) than those\nthat we interpret from the petrology and geochemistry of the orthopyroxene-rich garnet\nharzburgites. As the reviewer indicates, a lack of data does not preclude the presence of SiO rich\n2\nfluids in the Archean.\nThe \"smoking gun,\" in my view, would be high-Mg olivine in diamonds with high F, Cl, and H20.\nWhile this might be beyond the scope of this study, it is worth considering.\nYes, agreed. There are some olivines in diamonds with high F contents but the data is presented in\nan unpublished PhD thesis and Edinburgh ion probe report.\nOn that note, olivine inclusions from the De-Beers pool kimberlites have a distinct high-Mg\nnumber and could provide valuable insights (see Stachel et al., 2022, RIMG review, Figure 6). Yes,\nthis is interesting, and we have now included a sentence related to the De-Beers pool diamond\ninclusions in the ‘Discussion’.\nI'm not sure that the segway to the next section is warranted. Maybe lines 279-300 should be\nincorporated here, and sections Infiltration of subduction-related melts and fluids & Carbonation,\nhydration, fluorination, and chlorination of cratonic mantle should be renamed and restructured.\nThis now forms a sub-section in the ‘Discussion’ section.\nLine 289: The major obstacle is not just oxygen isotopes but also mass balance considerations and\nthe cold geotherm that you just noted above. We now show that the mass balance considerations\nare not a problem with the revised trace-element modelling.\nLine 329: The Slave and Kaapvaal cratons have very different geological histories. Before reaching\nthis conclusion, Slave xenoliths should be added. Agreed. I have replaced this sentence with\n‘Further support for the idea of recycling of sediment in the Archean beneath the Kaapvaal\ncraton is provided by sulfur isotope ratios of sulfide inclusions in diamond’.\nIf so, I think this (or a version of it) should be moved to the main text. The original Fig. S3 has\nbeen moved to the main text.\nLine 344-347: This claim warrants a figure in my mind. I think this is clear without a figure, given the\njournal restrictions and that we have already added several new figures.\nAlso,it is worth noting that the K and (F) in K-rich phases are likely lost during subsequent melting.\nWe do not believe that there has been any subsequent melting of the mantle preserved in\nthese xenoliths.\nLines 364-376: The link to wider implications is very interesting but a bit of an overpromise and\nunderdeliver, especially for the North China Craton. Is there any insight as to why the opx\nenrichment is restricted to the Kaapvaal Slave and Siberian Cratons?\nIt is also apparent in the Tanzania craton. It may be that it is even more widespread but has not\nbeen observed in mantle xenolith suites from other, less well-studied cratons.\nSome other remarks\nThe experimental results of Gao et al. (2021) might be worth considering aside from the\nintroduction as they are similar to what the authors propose. The experiments of Gao\net al. (2021) are interesting but were undertaken at low pressures (1.5 GPa) and\ninconsistent with the presence of diamond in our sample suite. On the basis of these,\nGao et al. (2021) suggest that orthopyroxene enrichment in the Kaapvaal craton\nmantle may be explained by a two-stage ‘metasomatism-depletion’ model. The second\nmelting event is required to generate the high Mg# and quite different to our model.\nThe presence of phlogopite in experimental run products generated by Gao et al.\n(2021) at <1100 oC and its absence at >1200 oC is, however, consistent with previous\nexperiments on phlogopite stability and the temperature threshold that we invoke in our\nmodel. A reference to this has now been included in the main text.\nWhat about using the delta Mg/Si notation of Canil and Lee 2009? We preferred to use Bulk\nMgO/SiO rather than the delta Mg/Si notation.\nFig. 2 that there are no robust correlations. The new box and whisker plot (Figure 6) shows that\nthere are correlations between lithological type and reconstructed bulk H + F content. This\nsupports the information on Figure 3b.\nDon't you expect that the more the xenolith departs from the Boyd trend, the more fluid reacts,\nand thus, the more Fin the olivine? Here, some diffusion modeling might come in handy.\nThis would be an excellent calculation to undertake, and something that we have considered\npreviously but unfortunately there are no F diffusion data for olivine.\nWhy does BD1153 (dunite) have so much F in it? Fluorine strongly partitions into olivine and this is a\nmetasomatised dunite.\nThe Figures overall are very engaging. I have a couple of remarks and possible suggestions.\nFig. 1: Since the focus is primarily on the Kaapvaal Craton, could you include a panel showing the\nlocation of the retrieved xenoliths? Or provide it in the supplement? Is there any observed spatial\nvariation? This is a good idea. A map showing the locations of samples in the Kaapvaal craton has\nbeen included in the Supplementary material (Fig. S3). There is no spatial variation in volatile\ncontents.\nFig. 2 Panel A might be redundant; I think that Panel B is the star of this figure.\nWe included Panel A because this shows the original concept proposed by Boyd (1989), which\nthe broader readership of Nature may not be familiar with.\nFig. 3 See comment above. Plenty of projections might be better utilized to illustrate the\ndifferences you are discussing. Please see our response above.\nWhy normalize to MORB source and not Primitive mantle? From the way I read the manuscript,it is\nnot clear why you chose to show this normalization.This normalization is used because cratonic\nmantle is widely believed to represent residues of large amounts of melting. We believe that this\nis more appropriate to model subsequent depletion and enrichment events than primitive mantle.\nThis is now explained in the caption for Figure 4 as follows ‘A MORB-source normalization is\nused because cratonic mantle is widely believed to represent residues of large amounts of\nmelting. MORB source normalisation factors are from Salters & Stracke 34.’\nFig. 5. Are there any inclusions in the diamond to plot on this diagram? Not to our knowledge\nWhy don't you show the olivine from the opx-rich sample in panel a? It is a bit misleading. If the\ntwo plots are combined the figure becomes overwhelming and the impact of the various trends is\nlost, hence our decision to have two figures.\nIn panel b, the diamond-bearing opx-rich sample BD2125 has 120 ppm F in the supplementary\ntable and is plotted as -138 ppm in the figure. Am I missing something? Sincere apologies. The incorrect\ntable was uploaded to the Supplementary Material and has now been corrected.\nFigure 8 is great, but it has been recycled from other publications. If you want to show a conceptual\nmodel,I believe it merits a new figure. This diagram fully supports our findings and we would like\nto keep it as it is.\nThe supplemental tables need a bit more work to make them clear to the reader.\nmissing descriptions.In the supplementary table of the bulk trace element compositions,\nthere is no table description indicating which samples are from the Kaapvaal, Tanzania,\nor off-craton. For those who are not familiar with the local names, it is hard to follow.\nAgreed. This information has now been added.\nAlso, can you group them in a systematic order? They are grouped according to location. This\nwasn’t apparent before but should be now (please see response to comment above).\ndeleted. Thank you. This typo has been corrected.\nThe clinopyroxene table needs a description. Agreed. A caption to this dataset has now been\nadded.\nI have no remarks on the Methods section\nBest,\nOded Elazar\nReview of the manuscript \"The role of C-O-H-F-CI fluids in the making of Earth's continental\nroots\" by Gibson,Jackson, Crosby, and Day, submitted to Nature Communications\nThe focus of this manuscript is the existence of opx-rich garnet harzburgites in the SCML.\nThe authors examine 3 scenarios for the formation of excess opx in these xenoliths: high-pressure\nmelting residues, komatiite interactions, or subduction-related silicic melts and fluids. The authors\nanalyze olivine, pyroxene and garnet in different xenoliths for H2O, F and Cl content and\nconclude that the best model is the interaction of the harzburgitic mantle with C-H2O-F-CI fluids\nderived from dehydration of the subducting slab.\nThe formation of opx-rich harzburgites is a long-standing question. The suggestion that these\nxenoliths were formed due to the interaction of depleted harzburgite with slab-derived silica-rich\nfluids or melts is not new. However, this manuscript attempts to provide additional support for this\nmodel via the analyses of volatiles in the peridotite minerals themselves,and provides unique\ninformation for the type of fluid/melt interacting with the harzburgite (volatile-rich). This\nmanuscript is important and very interesting.\nThe manuscript is well written and easy to follow.\nMajor comments and thought\n1. Opx-rich harzburgites are mainly found in Slave, Rae, Kaapvaal and Siberia cratons. Does that\nmean that fluids interacted with the SCML only in these regions and no other SCML? This is\nan interesting point. Mantle xenoliths are localized in their distribution and it is interesting that\nthe orthopyroxene-rich ones are found in cratons where the largest numbers of mantle xenoliths\nhave been found and analysed to date. It doesn’t, however, negate orthopyroxene-rich mantle\nbeing present elsewhere. They may well just not have been found.\n2. There is no discussion on carbon in these potential fluids.\nMore information on C in these fluids has been added to the Discussion section.\n3. Saline HDF (found in fluid inclusions in diamonds) were identified in many cratons around the\nworld (see review by Weiss et al. 2022). Not just were there is excess opx. Also, Weiss and others\nsuggested that silicic HDFs originate from the dehydration of the subducting slab. It will be\ninteresting to use the trace element compositions of either the saline HDF or silicic HDF instead of\nexperimental supercritical fluid in Figure 7. Thank you, this is a helpful suggestion and we have\nnow explored these datasets. Silicic and saline HDFs have variable concentrations of trace\nelements. There are no data for volatiles but variations in normalized [Zr/Hf] and [Sr/Nd] suggest\nthat the fluids associated with the orthopyroxene-rich harzburgites may be comparable to silicic\nlow Mg carbonatitic HDFs. Some text and a figure illustrating this is now included in the\nSupplementary Material.\n4. Elazar and Kessel (2024) interacted harzburgite with eclogite in a hydrous environment (CHO\nfluid) at the conditions above the eclogitic wet solidus, but below the wet peridotite solidus. At the\nexperimental conditions, eclogitic supercritical fluid was interacting with the harzburgite. During\n11\nthis interaction, the amount of ol decreases while the amount of opx increases. Their work may\napply to the suggested model. Agreed, this work is relevant to our study and we have cited\nit at various places in the manuscript. Unfortunately, only a few trace elements were\nanalysed and so it is not possible to use this data in our numerical models. We have also\nlooked at the data for eclogitic fluids in Tsay et al. (2017) and Rustioni et al. (2021) but\nthey have positive anomalies at Sr and so inconsistent with our observations.\nMinor comments\n5. Line 90- The authors use Kessel et al. (2005) (ref. 35) for trace elements profiles during\nenrichments. Kessel et al. (2005) focuses on trace elements in eclogite+H20 system. It is\nbetter to refer to Kessel et al. (2015): The behaviour of incompatible elements during\nhydrous melting of metasomatized peridotite at 4-6 GPa and 1000 DC-1200 oc. Thank you.\nThis citation has been changed\n6. Line 116 - REE profiles are in Figure S5 (not S4). This typo has been changed to S5.\n7. Lines 131 - Please check the calculated averages for the H20 and F in the various minerals\nand in the bulk peridotites. Following the data presented in Table 51, I calculate different\nvalues. For example, The H20 content in olivine in opx-rich garnet harzburgite in Table 51 are\n37,69 and 69 ppm. The average is 58±18 ppm (and not 49.5±68 as states in the text). Same for\nopx etc. Mea culpa! I uploaded the incorrect table to the Supplementary Material. The\ndiscrepancy arose because Table S1 shows our new data whereas the averages stated in\nthe text included published analyses for the Kaapvaal. Thank you for noticing this typo, which\nhas now been corrected. I have also included two additional spreadsheets (Supplementary\nTables 5 & 6) that show the means, standard deviations, median values and number of\ncounts of the mineral volatile contents and bulk xenolith volatile contents cited in the\nmanuscript.\n8. Line 191-193 - Figure 2a does not show bulk H2O+F content. Figures 2b does. The authors\ndeclare that the phanerozoic oceanic melt residues show a decrease in bulk H20+F with\nincreasing Fo content and model abundance of ol. Thank you. This has been changed.\n9. Line 236-237 - Ti seems to fit well with the infiltration of Al-enriched komatiites model. Ba\nand\nRb do not fit. This has been changed.\n10. Figure 7 - In the legend, the reference is Sokol et al (2024). In the figure caption, it is Sokol\net al. (2023). Thank you. This has been changed.\n11. The fluid taken from this reference is a supercritical fluid in equilibrium with a pelite.\nPlease explain this choice. Why not choose a supercritical fluid in equilibrium with\neclogite\nUnfortunately, we have not been able to find appropriate trace element and volatile\ndata for a supercritical fluid in equilibrium with eclogite. This is despite an extensive\nliterature search and reaching out to others in this field.\n12. Check the symbol of the supercritical fluid from Sokol in Fig. 7b. (Triangle in the legend,\ncircles in the figure). Thank you. This symbol has been changed.\n13"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thank you to the authors for addressing my concerns. I am happy to recommend this version for publication, subject to the\nfollowing minor suggestions:\nI suggest changing \"continental roots\" in the title to \"cratonic roots\" for greater precision.\nLine 23: Please specify the type of volatiles being referred to.\nLine 135: Please note the lowest Ce/Pb ratios observed.", "reviewer_2": "The authors have addressed all of my previous remarks, and the revised manuscript looks excellent!\nJust one thought that came to mind during my second read (and apologies if I missed this)\nDo you have any nitrogen aggregation data on the diamonds hosted in the diamondiferous orthopyroxene-rich garnet\nharzburgite (BD2125)?\nAlthough previous work has clearly shown that these diamonds are old, it would be a nice addition if you have nitrogen\naggregation systematics—especially if the diamonds have high nitrogen contents and show significant aggregation. This\ncould further strengthen the interpretation of an ancient subduction-related origin.\nI also took a moment to look for any remaining typos, and everything looks good to me. I trust the editorial team will give it\none final check before publication.\nOther than that, the paper looks great!\nAll the best,\nOded Elazar", "reviewer_3": "see attached file"}, "author_response": "AUTHORS RESPONSES TO REVIEWERS' COMMENTS\nIn green Authors replies to comments in comments in black (Reviewers 1 & 2) or red\n(Reviewer #3)\nReviewer #1 (Remarks to the Author)\nThank you to the authors for addressing my concerns. I am happy to recommend\nthis version for publication, subject to the following minor suggestions:\nI suggest changing \"continental roots\" in the title to \"cratonic roots\" for greater\nprecision.\nWe would like to keep the original title\nLine 23: Please specify the type of volatiles being referred to.\nThis information has been added\nLine 135: Please note the lowest Ce/Pb ratios observed.\nThis information has been added\nReviewer #2 (Remarks to the Author)\nThe authors have addressed all of my previous remarks, and the revised manuscript\nlooks excellent!\nJust one thought that came to mind during my second read (and apologies if I\nmissed this)\nDo you have any nitrogen aggregation data on the diamonds hosted in the\ndiamondiferous orthopyroxene-rich garnet harzburgite (BD2125)?\nAlthough previous work has clearly shown that these diamonds are old, it would be a\nnice addition if you have nitrogen aggregation systematics—especially if the\ndiamonds have high nitrogen contents and show significant aggregation. This could\nfurther strengthen the interpretation of an ancient subduction-related origin.\nI’m afraid that there is no nitrogen data available for the diamond hosted in BD2125.\nIt would be fabulous if this type of study could be undertaken on other diamonds\nfound in orthopyroxene-rich harzburgites.\nI also took a moment to look for any remaining typos, and everything looks good to\nme. I trust the editorial team will give it one final check before publication.\nOther than that, the paper looks great!\nAll the best,\nOded Elazar\nReviewer #3 (Remarks to the Author)\nI accept all the changes and replies made by the authors, except a few small\ncorrections, as detailed below.\nIn black – the original comment\nIn Blue – the authors reply\nIn Red – my reply to their reply\n4. Elazar and Kessel (2024) interacted harzburgite with eclogite in a hydrous\nenvironment (CHO fluid) at the conditions above the eclogitic wet solidus, but\nbelow the wet peridotite solidus. At the experimental conditions, eclogitic\nsupercritical fluid was interacting with the harzburgite. During this interaction, the\namount of ol decreases while the amount of opx increases. Their work may apply\nto the suggested model.\nAgreed, this work is relevant to our study and we have cited it at various places in\nthe manuscript. Unfortunately, only a few trace elements were analysed and so it\nis not possible to use this data in our numerical models. We have also looked at\nthe data for eclogitic fluids in Tsay et al. (2017) and Rustioni et al. (2021) but they\nhave positive anomalies at Sr and so inconsistent with our observations.\nAlthough the authors agreed as to the relevance of Elazar and Kessel (2024) work\nand wrote that it is cited and discussed in the manuscript, I do not see it is cited.\nInstead, the study of Elazar et al. (2019) is cited. This is also a relevant study on the\nformation of silicic HDFs, however, it focuses on the interaction of H2O-CO2 fluids\nwith eclogites while the study of Elazar and Kessel (2024) discusses the enrichment\nof OPX due to the interaction of harzburgite with eclogite in a hydrous environment.\nI believe Elazar and Kessel (2024) is more relevant to the model suggested in this\nstudy.\nThank you, this reference has been changed to Elazar & Kessel (2024)\n5. Line 90- The authors use Kessel et al. (2005) (ref. 35) for trace elements\nprofiles during enrichments. Kessel et al. (2005) focuses on trace elements in\neclogite+H20 system. It is better to refer to Kessel et al. (2015): The behaviour of\nincompatible elements during hydrous melting of metasomatized peridotite at 4-6\nGPa and 1000\nDC-1200 oc.\nThank you. This citation has been changed\nThe authors cited the wrong Kessel et al. (2015) paper. They cited the work titled\n[Melting of metasomatized peridotite at 4–6 GPa and up to 1200 °C: an\nexperimental approach. Contrib Mineral Petrol 169, 37 (2015)] which does not deal\nwith trace elements. Instead, the relevant Kessel et al. (2015) study is titled [The\nbehaviour of incompatible elements during hydrous melting of metasomatized\nperidotite at 4-6\nGPa and 1000 DC-1200oC. Lithos, 236-237, 141-155]. This study focuses on trace\nelements, relevant to the current manuscript.\nThank you, this reference has been corrected\n11. The fluid taken from this reference is a supercritical fluid in equilibrium with a\npelite. Please explain this choice. Why not choose a supercritical fluid in equilibrium\nwith eclogite /\nUnfortunately, we have not been able to find appropriate trace element and volatile\ndata for a supercritical fluid in equilibrium with eclogite. This is despite an extensive\nliterature search and reaching out to others in this field.\nThe changes made to Figure 9 (originally 7) as well as the relevant text are a\nwelcome improvement. The only small comment is that in the revised caption there\nis now an unclear sentence (a part of it is missing? which was provided in the\noriginal caption of this figure):\nThank you, the figure caption has been revised to remove the typo.\nSally A Gibson\n22nd July 2025\nComments to comments:\nI accept all the changes and replies made by the authors, except a few small\ncorrections, as detailed below.\nIn black – the original comment\nIn Blue – the authors reply\nIn Red – my reply to their reply\n4. Elazar and Kessel (2024) interacted harzburgite with eclogite in a hydrous\nenvironment (CHO fluid) at the conditions above the eclogitic wet solidus, but\nbelow the wet peridotite solidus. At the experimental conditions, eclogitic supercritical fluid\nwas interacting with the harzburgite. During this interaction, the amount of ol\ndecreases while the amount of opx increases. Their work may apply to the\nsuggested model.\nAgreed, this work is relevant to our study and we have cited it at various places in\nthe manuscript. Unfortunately, only a few trace elements were analysed and so it is\nnot possible to use this data in our numerical models. We have also looked at the\ndata for eclogitic fluids in Tsay et al. (2017) and Rustioni et al. (2021) but they have\npositive anomalies at Sr and so inconsistent with our observations.\nAlthough the authors agreed as to the relevance of Elazar and Kessel (2024) work\nand wrote that it is cited and discussed in the manuscript, I do not see it is cited.\nInstead, the study of Elazar et al. (2019) is cited. This is also a relevant study on the\nformation of silicic HDFs, however, it focuses on the interaction of H2O-CO2 fluids\nwith eclogites while the study of Elazar and Kessel (2024) discusses the enrichment\nof OPX due to the interaction of harzburgite with eclogite in a hydrous environment. I\nbelieve Elazar and Kessel (2024) is more relevant to the model suggested in this\nstudy.\n5. Line 90- The authors use Kessel et al. (2005) (ref. 35) for trace elements profiles\nduring enrichments. Kessel et al. (2005) focuses on trace elements in eclogite+H20\nsystem. It is better to refer to Kessel et al. (2015): The behaviour of incompatible\nelements during hydrous melting of metasomatized peridotite at 4-6 GPa and 1000\nDC-1200 oc.\nThank you. This citation has been changed\nThe authors cited the wrong Kessel et al. (2015) paper. They cited the work titled\n[Melting of metasomatized peridotite at 4–6 GPa and up to 1200 °C: an experimental\napproach. Contrib Mineral Petrol 169, 37 (2015)] which does not deal with trace\nelements. Instead, the relevant Kessel et al. (2015) study is titled [The behaviour of\nincompatible elements during hydrous melting of metasomatized peridotite at 4-6\nGPa and 1000 DC-1200oC. Lithos, 236-237, 141-155]. This study focuses on trace\nelements, relevant to the current manuscript.\n11. The fluid taken from this reference is a supercritical fluid in equilibrium with a\npelite. Please explain this choice. Why not choose a supercritical fluid in equilibrium\nwith eclogite /\nUnfortunately, we have not been able to find appropriate trace element and volatile\ndata for a supercritical fluid in equilibrium with eclogite. This is despite an extensive\nliterature search and reaching out to others in this field.\nThe changes made to Figure 9 (originally 7) as well as the relevant text are a\nwelcome improvement. The only small comment is that in the revised caption there\nis now an unclear sentence (a part of it is missing? which was provided in the\noriginal caption of this figure):"}]

Decision letter