Copper isotopes track the Neoproterozoic oxidation of cratonic mantle roots

Domain: earth_science
Published: 2024-05-21
Editorial outcome: major_revision

Abstract
The oxygen fugacity (fO ) of the lower cratonic lithosphere influences diamond formation, melting mechanisms, and lithospheric evolution, but its redox evolution over time is unclear. We apply Cu isotopes (δ65Cu) of ~1.4 Ga lamproites and <0.59 Ga silica-undersaturated alkaline rocks from the lithosphere-asthenosphere boundary (LAB) of the North Atlantic Craton to characterize fO and volatile speciation in their sources. The lamproites’ low δ65Cu (−0.19 to −0.12‰) show that the LAB was metal-saturated with CH + H O as the dominant volatiles during the Mesoproterozoic. The mantle-like δ65Cu of the <0.59 Ga alkaline rocks (0.03 to 0.15‰) indicate that the LAB was more oxidized, stabilizing CO + H O and destabilizing metals. The Neoproterozoic oxidation resulted in an increase of at least 2.5 log units in fO at the LAB. Combined with previously reported high fO in peridotites from the Slave, Kaapvaal, and Siberia cratonic roots, this oxidation might occur in cratonic roots globally.

Introduction
The continental lithosphere–asthenosphere boundary (LAB) is a physically and chemically unique layer1 and controls and/or modifies the chemical compositions (including volatile abundances) of continental magmas2. The oxygen fugacity (fO ) at the cratonic LAB influences diamond formation, melting mechanisms, and lithospheric evolution2,3. Generally, the fO of the lithospheric mantle decreases with depth at a rate of about 0.4–0.6 log units per GPa because of the increasing stabilization and solubility of Fe3+ in garnet with pressure4,5. Therefore, the lower reaches of the cratonic lithosphere (>150 km) are reduced with fO ≤ ΔFMQ−4 (FMQ: fayalite–magnetite–quartz oxygen buffer)5. However, the lower lithosphere has experienced extensive later metasomatism by various types of fluid and/or melt from the asthenosphere throughout the history of the lithosphere mantle6, which may modify its fO . Several studies of metasomatized peridotite xenoliths have shown that melt infiltration causes oxidation of the lower cratonic mantle7–9, but these events are all thought to have occurred shortly before the eruption of their host kimberlites in the Phanerozoic. The oxidation state and redox evolution at the LAB further back in time remain unclear.

Alkaline rocks are generally produced by volatile-triggered melting close to the LAB in continental settings and are more abundant in rifts, where the passage of melts to the surface is easier2. The speciation of C–O–H volatiles (H O, CO , CH , and H ) is strongly dependent on the fO of the mantle10,11. Under reducing conditions, melting is caused by the increased activity of water (a ) during the oxidation of CH 10 and produces melts with high H O contents but little carbon (<0.2 wt%) because the solubility of CH is much lower than that of H O and CO 12. Under more oxidized conditions, melting occurs because of further depression of the melting point when CO becomes stable10,13. The melt compositions and rock types produced depend on the melting conditions and mechanisms (Supplementary Fig. S1). Lamproites are generally silica-saturated mantle magmas formed in reducing conditions in which C–O–H volatiles are dominated by CH and H O14, as shown by major element and mineral compositions as well as phase stabilities in high-pressure experiments15,16. This conclusion is supported by their low carbon contents (generally <0.12 wt%; see the “Methods” section), which is explained by the very low solubility of reduced carbon in melts. In contrast, carbonate-rich and/or silica-undersaturated alkaline rocks, including carbonatites, aillikites, kimberlites, and nephelinites are produced in oxidized conditions where C–O–H volatiles are present as CO and H O13.

The Labrador coast on the North Atlantic Craton (NAC) is a representative occurrence where a sequence of alkaline rocks was emplaced over more than 1200 Ma: lamproites were emplaced in the Mesoproterozoic and carbonate-bearing rocks since the Late Neoproterozoic (Fig. 1a)17. The rise of carbonate-bearing magmas since the Late Neoproterozoic in Labrador is consistent with a significant increase in the abundance of carbonatites and kimberlites globally over the past 700 Myr18,19 (Fig. 1d). This Aillik Bay locality in Labrador experienced three magmatic pulses: lamproites at ca. 1.4 Ga, CO -rich ultramafic lamprophyres and carbonatites at 590–555 Ma, and silica-undersaturated nephelinites and melilitites at 142 Ma (Fig. 1a). They are believed to be formed by melting of fusible components at the lithosphere–asthenosphere boundary (see the “Methods” section): phlogopite pyroxenites are required in the source of the lamproites at 5–7 GPa, whereas the later ultramafic lamprophyre (and carbonatite) and nephelinite (and melilitite) periods require phlogopite–carbonate veins at 5–7 GPa, and amphibole wehrlite metasomes at 2–3 GPa, respectively. The lithosphere thinned under Aillik Bay over time from >180 km for the lamproites and ultramafic lamprophyres/carbonatites to 75–90 km for the nephelinites17,20,21 (see Supplementary Fig. S2 and see the “Methods” section for geological setting, petrogenesis, and the estimated pressure–temperature of melting).

Cu isotopes (63Cu and 65Cu) have been widely applied to trace redox reaction processes because significant Cu isotopic variation can occur during oxidation–reduction reactions22. They have great potential to characterize the redox state of the mantle sources of these alkaline rocks. In a reduced and metal-saturated mantle (fO ≤ the iron–wüstite buffer, IW), Cu is mainly sequestered in Fe–Ni alloy as Cu0 and in sulfide as Cu1+23. Cu0 in Fe–Ni alloy is more enriched in 65Cu than Cu1+ in silicate melt and sulfide (Fig. 2a)24,25. In a metal-free mantle at intermediate oxidation states (IW < fO ≤ FMQ), Cu is mainly sequestered in Ni-bearing sulfide, whereas in an oxidized mantle (fO ≥ ΔFMQ+0.5), Cu is mainly hosted by sulfate as Cu2+26. Therefore, if a melt is produced with a metal alloy in the source, the copper isotope composition of that melt would be lighter than that of the Bulk Silicate Earth (BSE). Compared to sulfides, the oxidized Cu2+ in sulfate is generally enriched in 65Cu (Fig. 2a)22.

To delve into the redox evolution at the base of the cratonic lithosphere, we determine the redox state and volatile speciation in the mantle sources of these alkaline rocks from Aillik Bay by employing a novel proxy that copper isotopes (Supplementary Table S1) and compare our results with the redox state recorded in the peridotites from the Slave, Kaapvaal, and Siberia cratons.

Results
The Cu isotope compositions of the Mesoproterozoic lamproites from Aillik Bay are homogeneous (δ65Cu = −0.12‰ to −0.19‰) and consistently lower than those of modern mid-ocean ridge basalts (MORBs, 0.07 ± 0.1‰)24 (Fig. 1b). In contrast, the Late Proterozoic ultramafic lamprophyres (aillikites and mela-aillikites) and the Mesozoic nephelinites show MORB-like δ65Cu values (0.03–0.15‰ and 0.04–0.11‰, respectively). Evolved examples of the Late Proterozoic ultramafic lamprophyres (damtjernites) and carbonatites have slightly higher δ65Cu values than the primitive aillikites (Supplementary Fig. S3).

Firstly, it is necessary to consider whether Cu isotopes would fractionate during sulfide segregation from Cu1+-bearing silicate melt or during partial melting of a sulfide-bearing mantle in which Cu remains monovalent. High-pressure experiments on Cu isotope fractionation between silicate melt and Ni-free and Ni-bearing sulfides (<1.3 and 21.4–29.0 wt% Ni, respectively)24,25 show enrichment of 63Cu in Ni-free sulfides but no resolvable Cu isotopic fractionation between Ni-bearing sulfides and silicate melt (Fig. 2a). Sulfides in natural peridotite and pyroxenite xenoliths have enriched and highly variable Ni contents (5–54 wt%)27,28 (Supplementary Fig. S4). Although the effect of such a large variation of Ni contents in sulfide on the Cu isotope fractionation factor has not been investigated, the homogeneous δ65Cu values of sulfide-saturated MORBs, sulfide-undersaturated komatiites, and the bulk silicate Earth24,29 (Supplementary Fig. S5), and the lack of Cu isotope fractionation during the evolution of sulfide-saturated arc magmas30,31 indicate that most of the sulfide-dominated magmatic processes in the mantle would not cause significant Cu isotope variation in the melts. Highly variable δ65Cu values of some peridotite and pyroxenite xenoliths have been attributed to kinetic Cu isotope fractionation on the scale of individual xenoliths32, but these effects have not been observed on a large magmatic scale. We model Cu isotope fractionation during magmatic processes using Cu isotope fractionation factors between silicate melt and Ni-free and Ni-bearing sulfides (Fig. 2b, c, see the “Methods” section, Supplementary Figs. S6, S7, and Supplementary Data 2, 3). The results indicate that Ni-bearing sulfide-dominated partial melting and magmatic differentiation would not fractionate Cu isotopes of melts, whereas partial melting and magmatic differentiation involving Ni-free sulfides would result in heavy Cu isotope compositions in the melts (Fig. 2b, c).

Both Mesoproterozoic lamproites and Mesozoic nephelinites show nearly constant Cu contents at 40–80 ppm, independent of MgO content (Fig. 3). This indicates sulfide-saturated evolution, in contrast to the sulfide-undersaturated evolution of komatiites and picrites33 or oxidized arc magmas30 with increased Cu contents (Fig. 3). Their Cu isotopes are homogeneous regardless of varying MgO contents, indicating that the effect of magmatic differentiation on δ65Cu is negligible (Fig. 2c). Furthermore, the lamproites and nephelinites have higher Ni contents than MORBs (Supplementary Fig. S4c), suggesting that sulfides participating in partial melting and magmatic differentiation are Ni-bearing rather than Ni-free. This is consistent with the observation that their Cu isotope compositions plot away from the modeling results of Cu isotope fractionation during Ni-free sulfide-dominated magmatic processes (Fig. 2b, c). Although their enriched trace element and radiogenic isotope compositions have been interpreted to result from metasomatism at the base of the lithosphere mantle caused by melts infiltrating from the asthenosphere17, sulfide-dominated magmatic processes would not modify their Cu isotope compositions. The primitive Late Proterozoic ultramafic lamprophyres (aillikites and mela-aillikites) show sulfide-undersaturated evolution with increasing Cu contents during melt evolution, consistent with the trend of komatiites and picrites (Fig. 3). Copper behaves as an incompatible element during magmatic differentiation in sulfide-undersaturated magmas, and so Cu isotopes would not be fractionated. The most evolved mela-aillikites show δ65Cu values similar to the aillikites (Fig. 1b), suggesting that no significant Cu isotope fractionation occurred during magmatic differentiation of the sulfide-undersaturated magmas. This is in agreement with the homogeneous MORB-like δ65Cu values in sulfide-undersaturated komatiites, picrites24, and arc magmas30 (Supplementary Fig. S5). The negligible Cu isotope fractionation during magmatic differentiation of the alkaline magmas discussed above is attributed to the monovalent state of Cu. The evolved damtjernites and carbonatites are thought to have formed by liquid immiscibility from aillikite magma at a shallow crustal depth20. The evolved damtjernites and carbonatites show higher and more variable V/Sc ratios than their primary aillikite magmas (Supplementary Fig. S3), which may be a result of variation of magma fO during magmatic differentiation. Oxidized V4+ (or V5+) has a greater affinity for melts than Sc3+, and the positive correlation between V/Sc and δ65Cu values in the evolved damtjernites and carbonatites (Supplementary Fig. S3) reveals oxidation of some Cu1+ to Cu2+ in the magmas, which is supported by the occurrence of sulfate crystals in carbonatites20.

Fluids or melts derived from oxidized subducting slabs have higher Cu contents and δ65Cu values than the mantle34,35, so the addition of recycled crustal materials could appreciably elevate the Cu content and change the Cu isotopic composition of some mantle rocks26 (Supplementary Fig. S5). All primitive alkaline rocks have similar or lower Cu content than MORBs (Fig. 3). Furthermore, the addition of high-δ65Cu crustal materials to the mantle cannot account for the low δ65Cu values of the lamproites (Supplementary Fig. S5). In summary, the effect of magmatic differentiation and crustal recycling on Cu isotopes is negligible, so the δ65Cu of primitive alkaline rocks can be confidently used to decode the redox state of their sources.

Copper isotopes reveal the oxidation of the LAB under the North Atlantic Craton

The Mesoproterozoic lamproites were produced by the melting of a mantle source involving phlogopite pyroxenite. The low δ65Cu values of the Mesoproterozoic lamproites could be attributed to either a metasomatic pyroxenite with low δ65Cu values or isotope fractionation during melting. Melting of a low-δ65Cu pyroxenite vein would cause melting of and reaction with the surrounding peridotite wall, which could produce a melt with a mixed isotope signature blending characteristics between mantle peridotite with BSE-like δ65Cu and high 143Nd/144Nd, and metasomatic pyroxenite with low δ65Cu and low 143Nd/144Nd. The lack of correlations between δ65Cu and Sr-Nd isotopes in the Mesoproterozoic lamproites precludes a low-δ65Cu pyroxenite source for these lamproites (Supplementary Fig. S4d). As discussed above, Ni-bearing sulfide-dominated magmatic processes could not cause Cu isotope fractionation, also suggested by the BSE-like δ65Cu values of mantle pyroxenite xenoliths (average 65Cu 0.068‰)36. Therefore, we suggest that the metasomatic pyroxenites in the source of the lamproites had a BSE-like source δ65Cu value, and their homogeneous and low δ65Cu values are attributed to the melting of a metal-saturated source as presented below and in Fig. 2b.

Fe–Ni alloy is thought to be stable in the deeper upper mantle37. Based on Cu partitioning systematics between Fe–Ni alloy and sulfide at 8 GPa23, Fe–Ni alloy, along with sulfide, should be the major host of Cu in the metal-saturated mantle. During partial melting of the metal-saturated mantle, sulfide is consumed preferentially relative to Fe–Ni alloy23. Consequently, any silicate melts produced are enriched in 63Cu and the residue in 65Cu due to the high δ65Cu values of Fe–Ni alloy24,25. The low δ65Cu values of the Mesoproterozoic lamproites (−0.19‰ to −0.12‰) appear to require a Cu0-bearing mantle source. We modeled Cu isotope fractionation during partial melting of metal-saturated peridotite with a BSE-like δ65Cu value of 0.07‰24 at 6 GPa (details see the “Methods” section, Supplementary Fig. S8, and Supplementary Data 4). The results suggest that melting of peridotite with 1000–2000 ppm Fe–Ni alloy (reasonable Fe–Ni alloy content in metal-saturated peridotite38) would produce silicate melt with a δ65Cu value of −0.1‰ to −0.13‰ (Fig. 2b and Supplementary Fig. S8), which is similar to that of the lamproites. Furthermore, the δ65Cu value of melt would not vary significantly with melting degree during partial melting of a metal-saturated peridotite. The low and homogenous δ65Cu values of the lamproites, therefore, indicate that their source (at ≈200 km depth) was metal-saturated at low fO , which is in keeping with conclusions from geochemical and experimental results16. The redox state of the metal-saturated mantle is controlled by the IW buffer reaction (ΔFMQ-5)39, indicating that the oxygen fugacity of the LAB during Mesoproterozoic was at least as low as IW with CH –H O as the major volatile species at about 6–7 GPa (Fig. 4a).

The MORB-like δ65Cu values of the Late Proterozoic aillikites and Mesozoic nephelinites suggest that they originated from a sulfide-dominated mantle with neither Cu0–metal nor Cu2+–sulfate (IW < fO ≤ FMQ). Accordingly, a CO -rich, oxidized mantle source is needed to produce both the carbonate-rich aillikites at about 5–7 GPa and the nephelinites at 2–3 GPa10,17. The mole fraction of CO 2− in the carbonated melt varies as a function of pressure, temperature, and mantle fO 40. At a depth of about 200 km, pure carbonatite melt may exist at ambient mantle fO around ΔFMQ-1, with decreasing carbonate components in the melt towards lower fO 4,40 (Fig. 4a). Using the method of Stagno et al.40, we estimated the fO of the Late Proterozoic primary aillikites equilibrated with the mantle to be between ΔFMQ-2.5 and ΔFMQ-2.2. Given the similar melting depths for the aillikites and lamproites, the LAB under the NAC during the Late Proterozoic was about 2.5 log units more oxidized than during the generation of the lamproites 700–800 Myr earlier in the Mesoproterozoic (Fig. 4a), indicating a significant oxidation event. This is also supported by the fact that the fO for CO –H O fluids in the mantle must be 2–4 log units higher than for CH –H O fluid10.

Therefore, Cu isotopes of the alkaline rocks of the NAC sampled at the three stages of mantle evolution reveal the redox evolution of the LAB. Before the Neoproterozoic, the LAB under the NAC was metal-saturated and reducing, and CH and H O were the dominant C–O–H species (Figs. 4 and 5). Since the Neoproterozoic, the LAB became more oxidized with metal saturation pushed to greater depth (from ≈200 to >250 km)37 so that CO and H O came to dominate as the dominant C–O–H species (Fig. 5b), and melts infiltrating the lower lithosphere from this time were more oxidized and carbonated. We suggest that the transition from CH + H O-dominated and metal-saturated to CO + H O-dominated and oxidized conditions at the base of the NAC took place in the mantle in Neoproterozoic times prior to 600 Ma (Fig. 4a).

Neoproterozoic oxidation at the base of cratonic lithosphere

We compare the redox evolution at the root of the NAC with that of other cratons (Slave, Kaapvaal, and Siberia) as documented by cratonic peridotite xenoliths (Fig. 6). These cratonic peridotite xenoliths have been divided into two groups based on their petrogenesis and equilibrated pressure8: (1) reduced and depleted peridotites at low pressures (mostly at <5 GPa) defining the redox state of cratonic lithosphere mantle without modification by mantle metasomatism (gray region in Fig. 6); and (2) relatively oxidized and enriched peridotite xenoliths at pressures >5 GPa (light red region in Fig. 6). Based on the redox evolution with depth recorded in the reduced and un-metasomatized peridotites, the lithosphere mantle would be metal-saturated at the pressure of >5.5–7 GPa if they had not experienced mantle metasomatism by the oxidized agents (Fig. 6). This is consistent with our results on the redox state of the mantle source of the lamproites from the NAC, which in our case is anchored by metal-saturated lithosphere mantle at about 6–7 GPa during the Mesoproterozoic. The extent of the oxidation event is about 2–3 log units. The increase in fO beneath the NAC is similar in magnitude to that documented by the peridotite xenoliths from other cratons. The relatively oxidized and enriched peridotites at >5 GPa from the Slave and Kaapvaal cratons were suggested to result from mantle metasomatism by carbonate-rich melt or fluid8,41. Our estimated fO for the NAC aillikites is consistent with that of the metasomatic and oxidized peridotite xenoliths from Slave, Kaapvaal, and Siberia (Fig. 6). These results highlight that redox evolution at the root of the NAC is robust and the oxidation of the lithospheric base caused by infiltration of asthenospheric carbonate-rich melts was widespread beneath global cratons.

Our results suggest that the oxidization of the cratonic root of the NAC took place between 1350 and 600 Ma. Peridotite xenoliths from the Premier kimberlite (1150 Ma) in the Kaapvaal craton show that oxidation of the Kaapvaal cratonic root first started prior to 1150 Ma41. However, the exact timing of the oxidation of the cratonic roots is poorly constrained because it is difficult to date the carbonatite metasomatism event seen in these peridotite xenoliths. We compiled the global age distribution of nonorogenic lamproites (Supplementary Data 1), kimberlites18, and carbonatites42 to decode carbonatite activity in the mantle (Fig. 1c, d). Carbonatites (defined as containing >50% carbonate minerals) and kimberlites show a marked crescendo from the Neoproterozoic onwards42. Detrital zircons in sandstones from Antarctica also indicate an onset of widespread carbonatite and silica-undersaturated alkaline magmatism during the Late Neoproterozoic43 (Fig. 1d), suggesting that the age distribution of carbonatites cannot be simply attributed to preservation bias (see the “Methods” section), as also suggested for kimberlites by Tappe et al.18. For the lamproites, we restrict our attention to nonorogenic lamproites because they occur in stable continental regions and are thought to form by melting in reduced conditions14. Our compilation shows that the nonorogenic lamproites mainly occurred in the early-middle Proterozoic and Mesozoic–Cenozoic periods, with a conspicuous gap between 1000 and 300 Ma (Fig. 1c).

Based on the distribution of kimberlites and carbonatites, we suggest that the oxidation of the global cratonic roots has continually increased since the Neoproterozoic. The well-known association of carbonate-rich magmatic rocks with rifts since the Late Neoproterozoic2,44 is thought to be the result of either the remobilization and oxidation of carbon that was stored in the lower lithosphere as diamond2,6 or by melting of carbonate-bearing metasomatic veins at the LAB with the aid of asthenosphere-lithosphere interaction, indicating that the LAB has been oxidized since the late Neoproterozoic. This is also suggested by a lack of lamproites between 300 and 1000 Ma (Fig. 1a, b). Note that although there has been a resurgence in nonorogenic lamproite activity since 300 Ma (Fig. 1a), isotope data for occurrences on different continents, including Australia45, North America46, China47, and Antarctica48 indicate that the source components of these lamproites in the lithosphere date from the Mesoproterozoic or earlier. This suggests that these young lamproites are products of the melting of residual and regional Mesoproterozoic veins that pre-date the oxidation of the LAB. We suggest that the oxidation of the global cratonic roots started prior to 1150 Ma, but the large-scale oxidation of the global cratonic roots began in Neoproterozoic times prior to 600 Ma (Fig. 4).

The origin of carbonate-rich melts as metasomatic agents of cratonic roots is the key to understanding the driver of the oxidation of cratonic roots. The continuous increase of carbonatites and kimberlites since 2.0 Ga has been interpreted to be the consequence of secular mantle cooling18,19, an increasing oxidation state of the asthenosphere through time49, or increased subduction of oxidized crustal materials into the deep mantle50. Secular mantle cooling is not the only factor in controlling the formation of carbonate-rich melts in the deep mantle because oxidized conditions are also required to produce carbonate-rich melts. The fO of the asthenosphere was suggested to have increased by 0.93–1.3 log units at 2.4–1.9 Ga51,52. One alternative interpretation is the more effective recycling of oxidized crustal materials (Fe3+-rich altered basalt, sulfate, and sedimentary carbonate) into the mantle by colder subduction53,54 (Fig. 4b). The effective subduction of oxidized crustal materials caused increased redox melting and freezing reactions to produce carbonate-rich melt in the deep asthenosphere55.

In summary, this study underscores the capability of Cu isotopes to serve as an indicator of melting in the presence of metal alloys, presenting a reliable marker for reduced melts. This could be significant not only for mantle petrology but also for researching reduced bodies within the solar system.

Methods
Analytical methods

Copper isotopes. Chemical purification and measurement of copper isotopes were performed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China (SKLGPMR-CUG). The analytical procedures, including sample dissolution, column chemistry, and multi-collector inductively coupled plasma mass spectrometry (MC-ICP–MS) measurements, followed established methods30,31,56,57. Aliquots of powdered materials (14-60 mg) were digested by a mixture of concentrated hydrofluoric and nitric acids (HF or HNO ) in Teflon vessels on a hot plate (∼120 °C) for 48 h. After digestion, the samples were dried down and re-dissolved in 1 ml HCl and 1 ml HNO on a hot plate at 120 °C for 48 h. They were then dried and converted into chloride form by the addition and evaporation of concentrated HCl twice. Finally, the samples were dissolved in 1 ml of 8 mol/l HCl + 0.001% H O for column chemistry.

The chemical purification method used in this study is modified from Maréchal et al.58. Copper purification was performed in a Biorad column containing 2 ml pre-cleaned AG-MP-1M resin. The matrix was removed by elution using 8 ml 8 mol/l HCl + 0.001% H O , after which 28 ml 8 mol/l HCl + 0.001% H O was used to collect Cu. The column separation procedure was repeated to guarantee the purity of the Cu solution. The Cu fractions were dried down, converted to nitrides, and re-dissolved in 2% HNO for isotope analysis. Copper was efficiently separated from the matrix such as Na, K, Ca, Fe, Mg, Si, Ti, Ni, and Mn; for details, see the elution curves for international rock standard BHVO-2 basalt and AGV-2 andesite from Zhu et al.56 and Liu et al.57. High recovery of Cu (>99%) and a low total procedural blank of 3–5 ng were achieved.

Copper isotope ratios were measured with a Nu Plasma 1700 MC-ICP–MS instrument. Isotope measurements were performed using standard-sample bracketing to correct for instrumental mass bias over time. The Cu concentrations of the samples were checked and adjusted to match the Cu standard solution within 10%. The measurements were performed in wet plasma and low-resolution mode. About 300 ppm Cu solution was used for routine analyses and achieved about 4–5 V for 63Cu. For each measurement, data were collected over three blocks of 25 cycles with 8 s integration. Copper isotope data are reported in standard δ-notation in per mil relative to the standard reference material (SRM) NIST 976.

The geological reference materials BHVO-2 and BIR-1 were digested and analyzed along with samples; the long-term external uncertainty for δ65Cu (reflected by these reference materials) is better than ±0.05‰ (2sd). The δ65Cu values obtained for BHVO-2 and BIR-1 are 0.11 ± 0.02‰ (2sd, n=3) and 0.05 ± 0.01‰ (2sd, n=3), respectively, in agreement with previously published data (Supplementary Table S2). All replicates of samples, digested from different aliquots of sample powder, show identical δ65Cu results with an uncertainty of about 0.05‰ (2sd, Supplementary Table S1).

Major and trace elements. No Cu contents have been reported for these alkaline rocks previously, so we have remeasured major and trace elements using solution ICP–MS of the sample rocks (Supplementary Table S1). The sample powders were digested by HF + HNO in Teflon bombs and analyzed using an Agilent 7500a ICP–MS at the SKLGPMR-CUG. The detailed sample digestion procedure for ICP–MS analysis is given in Liu et al.59. Analyses of rock standards (BCR-2, BHVO-2, and AGV-2) and sample replicates indicate both accuracies and reproducibilities are better than 5% for major elements and 10% for trace elements.

Geological setting and petrogenesis of the alkaline rocks from the Aillik Bay

Geological setting and samples. The NAC was split during the Mesozoic–Cenozoic into two Archean blocks—the Nain Province of Labrador and the Archean terranes of West Greenland, preserving tonalitic crust as old as 3.8 Ga60 (Supplementary Fig. S1a). The NAC experienced one of the longest craton splitting histories known17,20: the first stage corresponds to ca. 1370 Ma olivine lamproite magmas; the second stage corresponds to widespread emplacement of ultramafic lamprophyres and carbonatites in the late Neoproterozoic; the third episode led to successful rifting and the production of new oceanic crust with the eruption of kimberlites, ultramafic lamprophyres and carbonatites, and eventually nephelinite and melilitite magmas along the rift flanks at ca. 200–100 Ma (Supplementary Fig. S1a). The samples selected for Cu isotope analysis are from the Aillik Bay locality, which is the only known area where alkaline magmatic rocks of all three rifting stages occur. They include four 1.37 Ga old lamproites, 19 ultramafic lamprophyres and carbonatites emplaced between 590 and 555 Ma, and five nephelinites dated at ca. 142 Ma17,20,21.

Melting temperature and pressure of the alkaline rocks. The melting temperature and pressure of the alkaline rocks at Aillik Bay were estimated by Chen et al.21 using the method of Sun and Dasgupta61. This thermobarometer treats the Mg-number (Mg#) of olivine in the mantle source and the H O content of primary magmas as key parameters61. Chen et al.21 compiled the Mg# of olivine of carbonated peridotites and wehrlites to calculate the melting temperature and pressure. Chen et al.21 assumed 80 for Mg# of olivine in the source of the lamproites, and here, we assume 80, 85, and 90 to evaluate the effect of Mg# of olivine on the calculation of melting temperature and pressure (Supplementary Fig. S2). We found that Mg# of olivine has a relatively small effect on the estimation of melting pressure but has a significant effect on the estimation of melting temperature. As shown in Supplementary Fig. S2, the melting pressure was about 5–7 GPa for the lamproites and aillikites, and about 2–3 GPa for the nephelinites. The lithosphere thinned over time from >150 km (5–7 GPa) for the lamproites at ca. 1.4 Ga and ultramafic lamprophyres/carbonatites at 590–555 Ma to 75–90 km (2–3 GPa) for the nephelinites at 142 Ma.

Metasomatic LAB sources for the alkaline rocks. The petrogenesis of these alkaline rocks should be considered before decoding the redox state of their sources using Cu isotopes. Cratonic alkaline rocks have been widely thought to be derived from the melting of metasomatized regions of the lower lithospheric mantle (such as the LAB)17. However, recent geochemical studies on olivine geochemistry have claimed that cratonic lamproites were formed by the interaction between asthenospheric carbonate-bearing melt and phlogopite-rich wall rocks in the lithosphere mantle, meaning that lamproites would share a similar origin with kimberlites62,63. However, this interpretation was based on mineral chemistry alone and overlooks the petrogenesis of many cratonic lamproites, e.g., their low carbon contents (average values calculated as CO are 0.05 wt% for Gaussberg, 0.43 wt% for West Kimberley, and 0.5 wt% for Leucite Hills64,65). The “lamproites” studied by Sarkar et al.62,63 were previously known as orangeites and have much higher CO contents (average 5.1 wt%65) and distinct origins.

According to the major and trace elements and melting depth estimation of the alkaline rocks at Aillik Bay17,20,21, we argue that the lamproites originated from melting of the metasomatic lithosphere mantle at the LAB and have no genetic similarity with kimberlites (Supplementary Fig. S1b). The major element compositions of the lamproites closely resemble experimental melts of melting of CO -free phlogopite pyroxenites. These lamproites are silica-rich, in contrast to the silica-deficient for the aillikites, nephelinites, and kimberlites, as shown in the diagram of CaTs-En join of O’Hara66 (Supplementary Fig. S1b), which is caused by CO in the source.

The estimated melting pressure of the lamproites and aillikites is about 5–7 GPa, similar to those of the cratonic LAB (Supplementary Fig. S2). Therefore, these alkaline rocks at the three stages were formed by the destruction and melting of the metasomatic LAB caused by the convective instabilities in the asthenosphere during passive rifting of the NAC (Supplementary Fig. S2) and can be used to decode the redox state of the metasomatic LAB.

Effects of low-temperature alteration on Cu isotopes of the studied samples

The lamproite, ultramafic lamprophyre, and nephelinite samples used in this study are fresh with little or no signs of hydrothermal overprinting20. For magmatic silicate rocks, homogeneous Cu isotope compositions of both lamproites and nephelinites indicate negligible effects of alteration. The primary magmatic carbonate-bearing aillikites have overlapping mantle-like δ13C and δ18O values, and δ13C and δ18O values of the evolved damtjernites and carbonatites show a trend of high-temperature fractionation at >600 °C, in contrast to strongly fractionated or hydrothermally overprinted carbonatites20. Importantly, the aillikites and mela-aillikites show MORB-like and homogeneous Cu isotope compositions. These observations suggest a negligible effect of alteration on the Cu isotopic compositions of these alkaline rocks.

Global age distribution of nonorogenic lamproites and carbonatites

We compile the global age distribution of nonorogenic lamproites (Supplementary Data 1 and Fig. 1c, d). The compiled nonorogenic lamproites occur in stable craton regions (Supplementary Data 1) and are much younger than the formation of cratons, indicating that they have no relationship with subduction and collision. These nonorogenic lamproites originated from the lower lithosphere mantle and were formed by melting in reduced conditions14. In contrast, orogenic lamproites occur in mobile belts and are generated at shallow depths in post-collisional environments, so they are not relevant for the lower cratonic mantle67. The nonorogenic lamproites mainly occurred in the early-middle Proterozoic and Mesozoic–Cenozoic periods, with a conspicuous gap between 1000 and 300 Ma (Fig. 1b).

The age distribution of carbonatites and kimberlites are from Woolley and Kjarsgaard42 and Tappe et al.18, respectively. The abundance of the carbonatites and kimberlites show a general increase since ~2 Ga with a significant increase over the past 700 Ma. Previous studies suggested that no relationship exists between the erosion-modified surface areas of kimberlite bodies and their emplacement ages18, and the distribution of carbonatites exhibits greater variability than the current surface area age distribution19, suggesting that the global scale ‘delayed’ appearance of kimberlites and carbonatites is genuine and cannot be simply attributed to preservation bias18,19. Furthermore, the detrital zircons in Neoproterozoic–Triassic sandstones from Antarctica also show an onset of widespread carbonatite and silica-undersaturated alkaline magmatism during the Late Neoproterozoic43 (Fig. 1c), suggesting that the age distribution of carbonatites cannot be simply attributed to preservation bias.

Modeling of Cu isotope fractionation during magmatic processes

Partial melting of peridotite at 1.5 GPa. We quantitatively modeled copper isotope fractionation during the partial melting of peridotite at 1.5 GPa using an incremental batch melting model in which the fractionation factors between melt and residues were calculated at each step (Supplementary Data 2). The starting mineral abundances and melting modes of silicate phases are the same as those in Chen et al.68. The starting mineral abundance of sulfide and partition coefficient values (D) of Cu between mineral and melt during partial melting are from Lee et al.69. The melting of sulfide is dependent of the solubility of sulfur which is a function of temperature, pressure, oxygen fugacity, and FeS activity. The calculation of the solubility of sulfur follows Lee et al.69. In detail, the δ65Cu ratio of starting peridotite is 0.07‰24. The oxygen fugacity during partial melting of metal-free peridotite is set at ΔFMQ-1.5. The Cu isotope fractionation factor between sulfide and silicate melt (α Sulfide-Silicate melt ) is 1 for Ni-bearing sulfide constrained by Xia et al.25 and is 0.9990 and 0.9975 for Ni-free sulfide from high-pressure experiments of Savage et al.24 and Xia et al.25, respectively. The modeling processes and results are presented in Fig. 2b, Supplementary Fig. S6, and Supplementary Data 2.

Magmatic differentiation of a MORB melt at 0.2 GPa. We quantitively modeled the behavior of Cu isotopes during magmatic differentiation of a MORB melt at 0.2 GPa using an incremental fractional crystallization calculation in which the fractionation factors between melt and cumulates were recalculated at each step70. The crystallization sequence of silicate phases and compositions of melt are calculated using the PETROLOGY software71 and are the same as those in Chen et al.70. The crystallization of sulfide depends on the sulfur content of melt and sulfur concentration at sulfide saturation of the melt. The partition coefficient values of Cu and sulfur concentration at sulfide saturation of the melt are calculated following Lee et al.69. The Cu isotope fractionation factors α Sulfide-Silicate melt are the same as those for modeling partial melting of a peridotite as presented above. The modeling processes and results are presented in Fig. 2c, Supplementary Fig. S7, and Supplementary Data 3.

Partial melting of a metal-saturated pyroxenite at 6 GPa. We quantitatively modeled Cu isotope fractionation during partial melting of a metal-saturated pyroxenite at 6 GPa using an incremental batch melting model. The abundance of starting minerals and melting of silicate phases are the same as those in Chen et al.21. The starting mineral abundance of sulfide, partition coefficient values of Cu, and calculation of the melting abundance of sulfide are the same as those of modeling partial melting of a peridotite as presented above. The melting abundance of sulfide is dependent on the solubility of sulfur in the melt, which is a function of temperature, pressure, oxygen fugacity, and FeS activity. The oxygen fugacity during partial melting is set at ΔFMQ-4. The abundance of Fe–Ni alloy in a metal-saturated peridotite is assumed to be 1000 and 2000 ppm, respectively. The chosen concentrations of 1000-2000 ppm are reasonable for Fe–Ni alloy in metal-saturated peridotite38. The partition coefficient of Cu between Fe–Ni alloy and silicate melt is similar to that between sulfide and silicate melt based on experimental data at 8 GPa23. As discussed in the main text, the Cu isotope fractionation factor α Sulfide-Silicate melt is 1. The Cu isotope fractionation factor between the metal-saturated source and melt (α FeNi-Silicate melt ) is assumed to be 1.00025 within the range of experimental values of 1.00025–1.0000624,25. The modeling processes and results are presented in Fig. 2b, Supplementary Fig. S8, and Supplementary Data 4.

Copper contents in these silicate minerals are too low to affect the behavior of Cu isotopes69. In other words, the behavior of Cu isotope fractionation depends on the sulfide and metal regardless of the silicate mineralogy of the source (peridotite or pyroxenite)69. With increasing degree of melting, the consumption of sulfide during partial melting of the metal-saturated pyroxenite at 6 GPa is much slower than that of melting of the peridotite at 1.5 GPa (Supplementary Figs. S6 and S8) due to the much lower solubility of sulfur in the melt at 6 GPa. Furthermore, besides sulfide, the metal is also an important carrier of Cu in the source of the metal-saturated source. Therefore, the Cu isotope fractionation factor between the metal-saturated source and melt would not significantly change with melting degree (Supplementary Data 4), resulting in a small variation of melt δ65Cu as the increase of melting degree (Fig. 2b and Supplementary Fig. S8).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper reports new geochemical and Cu isotopic data for a suite of alkaline magmas from Aillik Bay (Labrador), which serves as a natural laboratory to investigate the thinning of deep cratonic lithospheric mantle, possibly attended by oxidation, through time. The authors find significantly lower d65Cu values for the older lamproites than for younger, CO2-richer magmas and, using modelling in support of their argument, suggest that this reflects that the inferred deep lithospheric sources of these rocks became oxidised by Neoproterozoic time.\nI have read this article with great interest and I think it uses a smart and novel approach to gauge ƒO2 conditions in the sources of alkaline magmas through time, which has great potential. However, I also see much scope for improvement and would recommend major revisions.\n- The authors suggest reduced sources for lamproites and oxidised sources for other alkaline rock types, and the introduction and discussion reveal that this is not something that is unveiled by using Cu isotopes, but which has been suggested before. In this respect I would suggest they change their title to “Cu isotope track the Neoproterozoic oxidation of cratonic mantle roots” or similar, which will take nothing away from the thrust of the paper.\n- It has recently been shown that lamproites have a common source in the convecting mantle, but they have stronger interaction with/ contribution from the SCLM (Sarkar et al. 2022 Geology, 2023 JPet), as true for other alkaline melts like kimberlites (Woodhead et al. 2019 Nature; Giuliani et al. 2020 SciAdv). In fact, the elephant in the room may be why kimberlites are not included in the analysis? Is it because their composition is dominantly determined in the convecting mantle (e.g., Becker and Le Roex 2006 JPet and many others)? A few words would be welcome, as other readers may similarly wonder. Anyways, I feel it should be much more stringently argued, why the analysis presented in this paper is relevant to redox conditions near the LAB, which supposedly (even plausibly) became more oxidised with time. How certain is it that the magma types used for comparison reflect conditions at the LAB, rather than those in the convecting mantle plus some contribution from the SCLM? This needs a much more in-depth analysis, in my opinion, if need be in a supplement.\n- Is it sound to take geochemical and Cu isotope results from one region (Aillik Bay) and simply correlate these to all lamproites vs. aillikites/damtjernites/carbonatites/nephelinites globally, as done in Fig. 1 and insinuated by statements in the title and throughout the manuscript, which suggest the authors are proposing a global scenario of deep cratonic SCLM oxidation?\n- Fig. 1 could be enhanced – and the authors’ findings supported – by plotting CO2 contents in the various alkaline magmas of interest as a function of time. This is all the more important as the table reveals that there is quite some overlap between CO2 contents of Aillik Bay lamproites and younger magma types, which seems inconsistent with a simple reduced vs. oxidised sources scenario.\n- This paper aims to investigate oxidation of deep cratonic mantle roots. What is missing is a validation of the model from actual cratonic mantle samples, which have been entrained prior to the inferred Neoproterozoic oxidation (e.g. Premier/Cullinan in Kaapvaal, Kyle Lake in Superior, and Wajrakarur in Dharwar), and again after this transition (localities world-wide). Even if direct ƒO2 estimates are not available, are the xenoliths from older magmas sampling less refertilised (and presumably oxidised) sources than the younger ones, and do the deep xenoliths contain metal-rich “sulphides” that are consistent with the inferred low ƒO2? Do the xenoliths from younger magmas record ƒO2 near the LAB that are permissive of the degree of oxidation shown in Fig. 5B?\n- In order to explain the low d65Cu, the authors call for fractionation involving silicate melt and Fe-Ni alloy-bearing mantle that has lost sulphide to melt extraction. There are several possible problems with this scenario: (1) You need to extract some 15-25% of melt to exhaust the sulpide component (depending on initial S content and fO2; Aulbach et al. 2016 RIMG). Given that it is widely agreed that cratonic lithosphere forms by decompression melting (e.g. Herzberg 2004 JPet), this level of depletion seems well plausible for mid-lithospheres but ambitious for SCLM near the LAB, the suggested source of the alklaline melts considered in this paper. The deep SCLM would have experienced the lowest level of depletion. (2) Even so, on sulphide exhaustion, an IPGE-rich alloy will saturate (Mungall and Brenan 2014 GCA) that might also affect Cu isotope partitioning. (3) More importantly, I don't think that alkaline rocks can be formed at melt fractions that are high enough to exhaust sulphide. Is there another scenario that does not require a sulphide-free source, that can be invoked to explain low d65Cu?\n- The explanation for the reappearance of lamproites after the Proterozoic cratonic root oxidation, that they sample older metasomatised sources, is not entirely satisfactory. It requires that these sources, which have low solidi, escaped earlier reactivation, and that they also were not affected by the subsequent deep LAB oxidation event. It is not impossible, but amounts to special pleading.\n- Statements in the manuscript are often unnecessarily imprecise and sometimes inaccurate, and require more attention to detail. I have indicated these instances directly in the text. A spellcheck and/or attentive reading would have caught many of the typos. Reviewers should not have to fix this many language lapses. There are a number of smaller comments and edits of typos (although I am sure there are more) etc. throughout the word documents of both the main text and the supplementary file, which I have taken the liberty to annotate directly.\nIn summary, I think a major overhaul is required, but if the authors make the effort to interpret their intriguing Cu isotope data within a solid interpretative framework, it would certainly be of wide interest and appropriate for the journal. I’d be happy to discuss further with the authors and provide clarifications if they so wished. In the meantime, I hope that my comments are useful and that the authors can easily rebut those that are not.", "reviewer_2": "Review of “Oxidation of cratonic roots during the Neoproterozoic”\nThis paper utilises a novel isotopic proxy, that of Cu isotopes, to elucidate the oxidation state of the lithosphere-asthenosphere boundary over time, from the Neoproterozoic to modern. By analysing alkali volcanic rocks whose petrogenesis started in the mantle, likely at the LAB, they suggest that increasingly heavy Cu isotope ratios implies that before the Neoproterozoic the lithospheric mantle was reducing (and metallic alloys were stable), and that post-600Ma the lithospheric mantle and the roots of cratons had reached their modern oxidation state. The authors go further to suggest that this is likely linked to the start of “cold subduction”, that is the onset of modern plate tectonics during the Neoproterozoic.\nThis was a really interesting paper to read – it is well-written and easy to follow (bar the odd spelling mistake) and approaches this topic of the timing of oxidation of cratonic roots from a really novel perspective. I like the idea that Cu isotopes might be useful in this regard, and comment mainly on the authors application of this isotope system as a new proxy for mantle oxidation state. I should state that I'm not an expert in the other evidence for oxidation and timing of oxidation of the cratons, but based on the literature cited in this work, the authors' new results seem to line up well with these previous studies. All in all, I would say that after some adjustments, particularly to the discussion of the application of the Cu isotope proxy and the ruling out of other processes, as well as the modelling, I would be happy to recommend publication of this work.\nBefore that, though, I would say that one swallow does not make a summer; that is, the anomalously light Cu isotope composition of the Proterozoic lamproite suite measured here is, really, the crux of the argument of this paper. I think a stronger argument might be made if a number of lamproites were analysed, sourced globally, which date from across the Proterozoic - which were all isotopically light wrt MORB/modern carbonatites etc. I don’t think it’s a reason to reject the paper, but I think the authors might be wary that they might start an arms race for the next people to analyse ancient ultrapotassic samples!\nMoving on to the Cu isotope aspect – I have no major reason to question the accuracy of the methods – I note the authors don’t seem to cite Marechal et al 1999, which is the original paper that most modern (including this study’s) Cu isotope purification methods are based on. One small points is, on line 447, the authors make a point of saying that the method does a good job of separating Ca – why is this particular element picked out? Is it specifically for the carbonatites? Otherwise, normally you would worry about elements like Na, Mg, Ti, Fe, where they have been shown to cause either serious matrix effects during analysis, or generate molecular interferences on the two Cu isotope beams. Because these are weird rocks, with potentially odd matrices that aren’t completely comparable to basalts (which are used as the external standards), perhaps explain that matrix contamination in the samples was checked for (I assume it was!).\nFinally, the authors mention that their geostandard Cu isotope data match well with previous studies, but do not cite said studies – I agree that their geostandards do match, but only because I analyse Cu isotopes so know the numbers well! Others might not be so familiar. Also, in relation to the precisions quoted for their geostandards, these seem very good compared to the error bars associated with each sample - I guess these errors are from a pooling of a number of measurement data - but then could the authors make this clear?\nFor the isotopically light lamproite data – I’m not sure, but I don’t think the authors spend too much time ruling out mantle metasomatism perhaps alterating the Cu isotope composition of the source of these more ancient rocks. I may have missed it, but in that case could the authors make it more clear that the Cu isotope composition of these rocks reflects melting of an “unaltered” mantle composition that has Fe-alloy stable. Could there have been a refertilisation event between 1.4 Ga and 0.6 Ga that affected the mantle in this region?\nFor the Cu isotope model, which predicts isotopically light melts from a mantle source only when buffered by an Fe-alloy (containing isotopically heavy Cu), I have a couple of questions. The first concerns the assertion that there is no Cu isotope fractionation between sulphides and silicate melt; this is based on the experimental data that show that Ni-rich sulphides and silicates have a negligible Cu isotope fractionation between them. This assertion is used to rule out any Cu isotope fractionation during igneous processes such as sulphide fractionation; although the extant data, which show that MORB and komatiite and also arc lavas all seem to have very predictable Cu isotope compositions seem to suggest that minimal Cu isotope fractionation takes place during sulphide precipitation, I think the authors could have been a bit more careful here when completely ruling this sort of process out.\nThe first thing to say is that this seems to rely on all sulphides being Ni-rich. This is surely not the case – as the authors know, sulphide petrology and composition can be quite variable, and so simply suggesting that every sulphide involved in the petrogenesis of these rocks is “Ni-rich” is a little difficult to believe based on the evidence provided in this paper; equally, I would ask what “Ni-rich” means – these fractionation factors were taking from a study looking at the crystallisation of the lunar core – are these fractionation factors applicable to Earth’s mantle? This criticism may seem harsh, but all I am saying is that Ni-poor sulphides can fractionate Cu isotopes when equilibrating with a silicate melt – hence I would recommend the authors at least mention this, or take this into account, in their models. I’m pretty sure, when I've modelled mantle melting and sulphide precipitation, you don’t get a huge different in Cu isotope composition in melts, as the maximum sulphide-silicate fractionation factors are pretty small, especially at mantle temperatures.\nIn relation to the authors' metal-sulphide fractionation models, these are a little difficult to follow, and they aren’t described in much detail – the authors mainly refer to their supplementary information database. I think a little more detail on the models in the written component of the supplementary material, plus perhaps show how [Cu] varies in the melts and restite in the models – as this can help the reader in following the model graphically (as it is, the figure displaying the model is not intuitive because of the buffering of the sulphide in the melt by the metal phase).\nFinally, could the authors comment on how not all sulphides in mantle rocks are available to melt as well as being less susceptible to metasomatism, i.e. if these are present as inclusions in olivine – is there a potential for a “hidden reservoir” of Cu isotopes in mantle rocks here.\nMinor comments\nOn line 80, the authors state that the Methods contain sample descriptions and petrology – but really this is limited to the thermobarometry. I think it would be good to include at least a brief description of the rocks utilised in this work, in terms of where and when they were collected, what their mineralogies are, etc.\nThere seems to be a lot of assumptions/generalisations used in the thermobarometric calculations – whilst I’m sure these are all strong assumptions, it might be worth the authors including some statements to explain how sensitive their P and T estimates are to these assumptions.\nIn figure 4, that authors have a label that states that global plate tectonics starts at 3.2 Ga. Whilst this is one of the periods that is commonly accepted that plate tectonics started globally, perhaps they authors might just take this off, as this is still a date that is up for debate.\nI’m not sure that figure 5 in the main text really is adding much to the piece – I think showing an updated Figure S5, which includes [Cu] in the melt and restite, would be more instructive.", "reviewer_3": "The study of Chen et al. (2023) seeks to constrain the redox evolution of the LAB from the Proterozoic to the present-day using Cu isotopic data from a suite of alkaline rocks located in the NAC of Labrador, Canada. They find a temporal shift in Cu isotopic data that demonstrates that the LAB at this locality has oxidized in the mid-Proterozoic, possibly as a result of the rise of cold, modern style subduction. The premise and topic of this study are both interesting and clever, and for that reason it deserves to be published in this journal, as mantle redox evolution in all its permutations has strong effects on atmospheric evolution. The use of Cu isotopes to quantify mantle redox at the LAB is both innovative and interesting.\nHowever, currently the paper is bogged down by a variety of ideas and exercises that prevent a clear story about Cu isotopes from being imparted to the reader. Chiefly among these is the use of a compilation of a very low number (31 by my counting) of nonorogenic lamproites and a much larger number of carbonatites to postulate a switch from predominant lamproites to predominant carbonatite production. Given the very general title of the paper, the reader is confused and initially thinks that the paper doesn’t focus on Cu isotopes of a single locality, and is instead concerned with compiling ages of alkali rocks. The discussion of the age distributions of alkali rocks is treated like a headliner of the paper but it is not, and it should be relegated to the discussion section where the authors try to link the Aiilak locality with global change. This brings me to my second overarching problem with the paper: the generalization of a single locality to the whole Earth and the lack of discussion of the NAC specifically in favor of assuming that it reflect global trends. The mantle lithosphere shows immense amounts of redox heterogeneity relating to metasomatism, and a global secular trend is not demonstrated by a trend in a single 20x20 km area. If the authors were to alter the title and discussion to instead discuss how their data may be indicative of a global change as opposed to definitively stating that these data show global change, the manuscript would be much stronger. The method used here (Cu isotopes of alkali rocks) can be concluded to show promise for quantifying global trends once more localities are analyzed, and that in my opinion is the most important conclusions of the paper.\nTwo more general points I have with the study: how easy is it to distinguish orogenic from nonorogenic lamproites in tectonically complex Proterozoic terranes? Is it possible that misidentification could have lead to the distribution in nonorogenic lamproites observed? Finally, much is made of the globally rare nature of carbonatites in the Precambrian, but the low preservation potential of carbonatites in the rock record is never once mentioned. This is an oversight and should be corrected, as given the low amount of crust available from the early Earth and the rarity of carbonatites in general throughout Earth history, the exponential increase curve of carbonatites shown must be placed in proper context.\nOverall, the paper is well written and the figures are thorough and highly professional. Re-focusing the paper on Cu isotopic data for NAC lavas and backing away from global generalizations and the age compilation data will serve this excellent dataset better.\nSpecific Comments\n-In this section I detail specific in-line comments. I have also provided numerous grammatical and usage edits in the “tracked changes” file returned with this review.\n-In Line 135, V/Sc is used as a proxy for oxygen fugacity of low melt fraction, enriched lavas. V/Sc is only a useful oxybarometer if three conditions are met: 1) melting does not leave garnet in the residue or fractionate garnet, due to the strongly compatible nature of Sc in garnet (much like the HREE) relative to V, 2) the degree of partial melting is well constrained and similar between the two types of rocks being compared. At low degrees of partial melting (<10%) like those examined here, small changes in the degree of partial melting can strongly influence V/Sc at constant fO2 (see Figure 1 of Li and Lee, 2004), 3) Finally and most importantly, V/Sc is a forward model for fO2 and assumes a constant V/Sc in the mantle source regions of the rocks examined. This is almost certainly untrue in this case as the mantle source regions are highly metasomatized lithosphere. I would not use V/Sc as a redox proxy at all, as your Cu isotope redox story is compelling enough without it.\n-In Line 145 you use Cu/Sc to argue that mantle source regions do not show prior Cu enrichments. You use the observation that Cu/Sc is elevated in OIB relative to MORB to invoke Cu enrichment in the OIB source mantle, but similar to my previous comment, this is almost certainly the result of the compatibility of Sc in garnet relative to Cu and unrelated to Cu enrichment, as garnet is found in the residue of most OIB but not MORB. If you want a trivalent cation that is less influenced by garnet, try using Ga, as V/Ga has been suggested for use instead of V/Sc for this very reason (Laubier XXX). Alternatively, all of the V/Sc and Cu/Sc discussion can be cut and the manuscript focused better on the main point: Cu isotopes.\n-Line 159: How did you chose your starting isotopic value? Is it the accepted BSE value? If so cite it.\n-Line 178: What is NAC? Define terms appropriately.\n-Line 182: What about the youngest rocks and their fO2? Tell the three stages of the story here, not just the first two.\nLine 191: What about preservation bias of carbonatite rocks? This is never mentioned or addressed. Carbonatite rocks can weather much more readily than silicates and therefore their preservation from the Archean to present day is perhaps unlikely.\n-Line 222: This almost certainly testably by examining initial Nd and Sr isotopic signatures of global carbonatites and lamproites, as subducted recycled materials should impart a strongly enriched signature. I am not suggesting that you go through the exercise of compiling these data and seeing if such a shift is evident, but I am saying you should state that this would be a good way to test your model in a future study. Besides it is always good practice to end a paper in an open-ended manner, with an eye towards what sort of future data or studies could support the model presented.\n-Figure 4: Take out “vast” from the carbonatite-in line\n-Figure 4: Why is the “onset of plate tectonics” marked on this figure? This is never discussed in the text and remains a hotly debated topic. This figure is busy, and this label adds little to it. As you only focus on the Proterozoic to Phanerozoic, why not start the figure at 2500 Ma?\n-Figure 5: this is an excellent figure for illustrating your model for LAB, good job.\n-Line 532: Which is it? 1000 or 2000 ppm or did you vary the concentration between these two values?"}, "author_response": "Response to referees´ comments, manuscript NCOMMS-23-32963\nChunfei Chen, Stephen F. Foley, Svyatoslav S. Shcheka, Yongsheng Liu\n“Copper isotopes track the Neoproterozoic oxidation of cratonic mantle roots”\nReferees’ comments in black\nReplies in blue\nWe thank the referees for the constructive comments and suggestions that significantly improved the manuscript.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nThis paper reports new geochemical and Cu isotopic data for a suite of alkaline magmas from Aillik Bay (Labrador), which serves as a natural laboratory to investigate the thinning of deep cratonic lithospheric mantle, possibly attended by oxidation, through time. The authors find significantly lower d65Cu values for the older lamproites than for younger, CO2-richer magmas and, using modelling in support of their argument, suggest that this reflects that the inferred deep lithospheric sources of these rocks became oxidised by Neoproterozoic time.\nI have read this article with great interest and I think it uses a smart and novel approach to gauge ƒO2 conditions in the sources of alkaline magmas through time, which has great potential. However, I also see much scope for improvement and would recommend major revisions.\n- The authors suggest reduced sources for lamproites and oxidised sources for other alkaline rock types, and the introduction and discussion reveal that this is not something that is unveiled by using Cu isotopes, but which has been suggested before. In this respect I would suggest they change their title to “Cu isotope track the Neoproterozoic oxidation of cratonic mantle roots” or similar, which will take nothing away from the thrust of the paper.\nReply: We have modified the title to “Copper isotopes track the Neoproterozoic oxidation of cratonic mantle roots”.\n- It has recently been shown that lamproites have a common source in the convecting mantle, but they have stronger interaction with/ contribution from the SCLM (Sarkar et al. 2022 Geology, 2023 JPet), as true for other alkaline melts like kimberlites (Woodhead et al. 2019 Nature; Giuliani et al. 2020 SciAdv). In fact, the elephant in the room may be why kimberlites are not included in the analysis? Is it because their composition is dominantly determined in the convecting mantle (e.g., Becker and Le Roex 2006 JPet and many others)? A few words would be welcome, as other readers may similarly wonder. Anyways, I feel it should be much more stringently argued, why the analysis presented in this paper is relevant to redox conditions near the LAB, which supposedly (even plausibly) became more oxidised with time. How certain is it that the magma types used for comparison reflect conditions at the LAB, rather than those in the convecting mantle plus some contribution from the SCLM? This needs a much more in-depth analysis, in my opinion, if need be in a supplement.\nReply: We have added a section (Geological setting and petrogenesis of the alkaline rocks from the Aillik Bay) in the “Methods” (Lines 621-646) and a figure of the CaTs-En join of O’Hara1 (Supplementary Fig. S1b) to discuss the petrogenesis of the alkaline rocks from Aillik Bay.\nThe recent geochemical studies on olivine geochemistry by Sarkar and others interpreted cratonic lamproites to be formed by the interaction between asthenospheric carbonate-bearing melt and phlogopite-rich wall rocks in the lithosphere mantle, and therefore that they are related to kimberlites2,3. The rocks they consider used to be known as orangeites, but have been renamed by various geochemists in the last few years as “carbonate-rich lamproites” or “lamproites var. Kaapvaal”. These conclusions appeal to mineral chemistry but ignore the petrogenesis. Most cratonic lamproites are extremely poor in carbon (see Figure below, noting log scale for CO ), which is consistent with a reduced source, because the solubility of reduced carbon in 2 melts is very low. An asthenospheric carbonate-rich melt could never achieve the exceptionally low CO 2 contents of most lamproites. Our interpretation is that the re-assignment of orangeites as lamproites is a mistake and results only in confusion.\nAccording to the major and trace elements and melting depth estimation of the Aillik Bay alkaline rocks4-6, we suggest that the lamproites originated from the melting of the metasomatic lithosphere mantle at LAB and do not have any genetic relationship with kimberlites. Firstly, the major element compositions of the lamproites highly resemble the experimental melts of melting of CO -free phlogopite pyroxenites. 2 These lamproites are silica-saturated (Supplementary Fig. S1b), in contrast to the silica-deficiency (caused by CO ) for the aillikites, nephelinites, and kimberlites. Secondly, the estimated melting pressure of the 2 lamproites and aillikites is about 5-7 GPa, similar to those of the cratonic LAB (Supplementary Fig. S2). Therefore, these alkaline rocks at the three stages were formed by the destruction and melting of the metasomatic LAB during passive rifting of the NAC (Supplementary Fig. S2), and can be used to decode the redox state of the metasomatic LAB.\nFigure caption: SiO versus CO contents in lamproites and orangeites.\n2 2\n- Is it sound to take geochemical and Cu isotope results from one region (Aillik Bay) and simply correlate these to all lamproites vs. aillikites/damtjernites/carbonatites/nephelinites globally, as done in Fig. 1 and insinuated by statements in the title and throughout the manuscript, which suggest the authors are proposing a global scenario of deep cratonic SCLM oxidation?\nReply: We have reworked the section on global extrapolation to add more explanation (Lines 209-228). We compare the redox evolution at the root of the NAC with those of other cratons (Slave, Kaapvaal, and Siberia) as documented by cratonic peridotite xenoliths (Fig. 6). These cratonic peridotite xenoliths have been divided into two groups based on their petrogenesis and equilibrated pressure7: (1) reduced and depleted peridotites at low pressures (mostly at <5 GPa) defining the redox state of a craton lithosphere mantle without modification from mantle metasomatism (grey region in Fig. 6); and (2) relatively oxidized and enriched peridotite xenoliths at the pressure of >5 GPa (light red region in Fig. 6). Based on the redox evolution with depth recorded in the reduced and un-metasomatic peridotites, the lithosphere mantle would be metal-saturated at the pressure of >5.5-7 GPa if they did not experience mantle metasomatism by the oxidized agents (Fig. 6). This is consistent with our result on the redox state of the mantle source of the lamproites from the NAC, indicating a metal-saturated lithosphere mantle at about 6-7 GPa during the Mesoproterozoic.\nThe relatively oxidized and enriched peridotites at > 5 GPa from the Slave and Kaapvaal cratons were suggested to result from mantle metasomatism by carbonate-rich melt/fluid7,8. Our estimated fO of the 2 aillikites from the NAC in this study is consistent with these of the metasomatic and oxidized peridotite xenoliths from Slave, Kaapvaal, and Siberia (Fig. 6). These highlight that our results of redox evolution at the root of the NAC are robust and the oxidization of the lithospheric base caused by infiltration of asthenospheric carbonate-rich melts occurred beneath the global cratons.\n- Fig. 1 could be enhanced – and the authors’ findings supported – by plotting CO2 contents in the various alkaline magmas of interest as a function of time. This is all the more important as the table reveals that there is quite some overlap between CO2 contents of Aillik Bay lamproites and younger magma types, which seems inconsistent with a simple reduced vs. oxidised sources scenario.\nReply: The CO values in the lamproites do not represent CO in the magma because these values only 2 2 represent carbon contents and we cannot measure the redox state of carbon in the samples. We have changed the reported data from CO to carbon content.\n2\n- This paper aims to investigate oxidation of deep cratonic mantle roots. What is missing is a validation of the model from actual cratonic mantle samples, which have been entrained prior to the inferred Neoproterozoic oxidation (e.g. Premier/Cullinan in Kaapvaal, Kyle Lake in Superior, and Wajrakarur in Dharwar), and again after this transition (localities world-wide). Even if direct ƒO2 estimates are not available, are the xenoliths from older magmas sampling less refertilised (and presumably oxidised) sources than the younger ones, and do the deep xenoliths contain metal-rich “sulphides” that are consistent with the inferred low ƒO2? Do the xenoliths from younger magmas record ƒO2 near the LAB that are permissive of the degree of oxidation shown in Fig. 5B?\nReply: The answers to this are the same as for the above comment. We have compared our results of redox evolution at the root of the NAC with those of other cratons (Slave, Kaapvaal, and Siberia) as revealed by the cratonic peridotite xenoliths. Our results are consistent with those from peridotite xenoliths from Slave, Kaapvaal, and Siberia.\nThe peridotite xenoliths from old kimberlite magmas also experienced metasomatism by carbonate-bearing melts or kimberlites8. Based on the evidence recorded in the peridotites, the lithosphere would be metal-saturated at > 5.5-7 GPa if they did not experience mantle metasomatism by oxidized agents (Fig. 6). This is consistent with our result on the redox state of the mantle source of the lamproites from the NAC, indicating a metal-saturated lithosphere mantle at about 6-7 GPa during the Mesoproterozoic.\n- In order to explain the low d65Cu, the authors call for fractionation involving silicate melt and Fe-Ni alloy-bearing mantle that has lost sulphide to melt extraction. There are several possible problems with this scenario: (1) You need to extract some 15-25% of melt to exhaust the sulpide component (depending on initial S content and fO2; Aulbach et al. 2016 RIMG). Given that it is widely agreed that cratonic lithosphere forms by decompression melting (e.g. Herzberg 2004 JPet), this level of depletion seems well plausible for mid-lithospheres but ambitious for SCLM near the LAB, the suggested source of the alklaline melts considered in this paper. The deep SCLM would have experienced the lowest level of depletion. (2) Even so, on sulphide exhaustion, an IPGE-rich alloy will saturate (Mungall and Brenan 2014 GCA) that might also affect Cu isotope partitioning. (3) More importantly, I don't think that alkaline rocks can be formed at melt fractions that are high enough to exhaust sulphide. Is there another scenario that does not require a sulphide-free source, that can be invoked to explain low d65Cu?\nReply: We present the details of modelling Cu isotope fractionation during partial melting of a metal-saturated mantle at 6 GPa and compare the results with modelling Cu isotope fractionation during partial melting of a peridotite at 1.5 GPa. The details can be found in the section “Modelling of Cu isotope fractionation during various magmatic processes” of “Methods’ (Lines 678-731), Fig. 2b-c, Supplementary Figs. S6 and S8, and Supplementary Data S2 and S4.\nOur results indicate low δ65Cu values for the melts once the melting of a metal-saturated mantle is triggered. Copper isotope compositions of the melt depend on the Cu isotope fractionation factor between the metal-saturated source and melt (α ). Due to heavy isotopes in the metal of the mantle source, the Source-Melt\nCu isotope fractionation factor α >1, and the produced melts have low δ65Cu value.\nSource-Melt\nWith increasing melting degree, the consumption of sulfide during partial melting of the metal-saturated pyroxenite at 6 GPa is much slower than that of melting of the peridotite at 1.5 GPa (Supplementary Figs. S6 and S8) due to the much lower solubility of sulfur of melt at 6 GPa. Furthermore, besides sulfide, the metal is also an important carrier of Cu in the source of the metal-saturated source. Therefore, the Cu isotope fractionation factor between the metal-saturated source and melt would not significantly change with melting degree (Supplementary Data S4), resulting in a small variation of melt δ65Cu as the increase of melting degree (Fig.2B and Supplementary Fig. S8).\nThe low δ65Cu values of the lamproites do not require sulfide exhaustion during melting. Although we do not know the melting degree for the lamproites, it is possible to melting of the 15 wt.% of a mixed mantle source with fusible metasomatic component (not peridotite) at the LAB9.\n- The explanation for the reappearance of lamproites after the Proterozoic cratonic root oxidation, that they sample older metasomatised sources, is not entirely satisfactory. It requires that these sources, which have low solidi, escaped earlier reactivation, and that they also were not affected by the subsequent deep LAB oxidation event. It is not impossible, but amounts to special pleading.\nReply: This is only the case if oxidation events are uniform over the whole area of the craton base, whereas they are localised influxes of melts. Nevertheless, we have weakened the reliance on evidence from the distribution of lamproites.\nWe have emphasized the distribution of carbonatite and kimberlite activities to roughly constrain the time of oxidization of cratonic root.\n- Statements in the manuscript are often unnecessarily imprecise and sometimes inaccurate, and require more attention to detail. I have indicated these instances directly in the text. A spellcheck and/or attentive reading would have caught many of the typos. Reviewers should not have to fix this many language lapses. There are a number of smaller comments and edits of typos (although I am sure there are more) etc. throughout the word documents of both the main text and the supplementary file, which I have taken the liberty to annotate directly.\nReply: We have thoroughly revised the manuscript.\nIn summary, I think a major overhaul is required, but if the authors make the effort to interpret their intriguing Cu isotope data within a solid interpretative framework, it would certainly be of wide interest and appropriate for the journal. I’d be happy to discuss further with the authors and provide clarifications if they so wished. In the meantime, I hope that my comments are useful and that the authors can easily rebut those that are not.\nReply: Thanks for your constructive suggestions and we have made a major overhaul on the manuscript.\nHopefully, these modifications have clarified these issues.\nReviewer #2 (Remarks to the Author):\nReview of “Oxidation of cratonic roots during the Neoproterozoic”\nThis paper utilises a novel isotopic proxy, that of Cu isotopes, to elucidate the oxidation state of the lithosphere-asthenosphere boundary over time, from the Neoproterozoic to modern. By analysing alkali volcanic rocks whose petrogenesis started in the mantle, likely at the LAB, they suggest that increasingly heavy Cu isotope ratios implies that before the Neoproterozoic the lithospheric mantle was reducing (and metallic alloys were stable), and that post-600Ma the lithospheric mantle and the roots of cratons had reached their modern oxidation state. The authors go further to suggest that this is likely linked to the start of “cold subduction”, that is the onset of modern plate tectonics during the Neoproterozoic.\nThis was a really interesting paper to read – it is well-written and easy to follow (bar the odd spelling mistake) and approaches this topic of the timing of oxidation of cratonic roots from a really novel perspective. I like the idea that Cu isotopes might be useful in this regard, and comment mainly on the authors application of this isotope system as a new proxy for mantle oxidation state. I should state that I'm not an expert in the other evidence for oxidation and timing of oxidation of the cratons, but based on the literature cited in this work, the authors' new results seem to line up well with these previous studies. All in all, I would say that after some adjustments, particularly to the discussion of the application of the Cu isotope proxy and the ruling out of other processes, as well as the modelling, I would be happy to recommend publication of this work.\nReply: Thanks for your constructive comments and suggestions.\nBefore that, though, I would say that one swallow does not make a summer; that is, the anomalously light Cu isotope composition of the Proterozoic lamproite suite measured here is, really, the crux of the argument of this paper. I think a stronger argument might be made if a number of lamproites were analysed, sourced globally, which date from across the Proterozoic - which were all isotopically light wrt MORB/modern carbonatites etc. I don’t think it’s a reason to reject the paper, but I think the authors might be wary that they might start an arms race for the next people to analyse ancient ultrapotassic samples!\nReply: We have weakened the reliance on evidence from the distribution of lamproites. We have compared our results with the redox state of cratonic peridotite xenoliths (Lines 209-228). See responses to Reviewer 1 on this topic.\nMoving on to the Cu isotope aspect – I have no major reason to question the accuracy of the methods – I note the authors don’t seem to cite Marechal et al 1999, which is the original paper that most modern (including this study’s) Cu isotope purification methods are based on. One small points is, on line 447, the authors make a point of saying that the method does a good job of separating Ca – why is this particular element picked out? Is it specifically for the carbonatites? Otherwise, normally you would worry about elements like Na, Mg, Ti, Fe, where they have been shown to cause either serious matrix effects during analysis, or generate molecular interferences on the two Cu isotope beams. Because these are weird rocks, with potentially odd matrices that aren’t completely comparable to basalts (which are used as the external standards), perhaps explain that matrix contamination in the samples was checked for (I assume it was!).\nReply: We have added more details and references to describe the chemical purification method. The chemical purification method in this study is modified from Maréchal et al.10. Copper was efficiently separated from the matrix such as Na, K, Ca, Fe, Mg, Si, Ti, Ni, Mn, and the details see the elution curves for international rock standard BHVO-2 basalt and AGV-2 andesite from Zhu et al.11 and Liu et al.12. High recovery of Cu (>99%) and a low total procedural blank of 3-5 ng were achieved.\nFinally, the authors mention that their geostandard Cu isotope data match well with previous studies, but do not cite said studies – I agree that their geostandards do match, but only because I analyse Cu isotopes so know the numbers well! Others might not be so familiar. Also, in relation to the precisions quoted for their geostandards, these seem very good compared to the error bars associated with each sample - I guess these errors are from a pooling of a number of measurement data - but then could the authors make this clear?\nReply: We have added a supplementary table (Supplementary Table S2) to compare our results of reference standards with those in previous studies.\nFor the isotopically light lamproite data – I’m not sure, but I don’t think the authors spend too much time ruling out mantle metasomatism perhaps alterating the Cu isotope composition of the source of these more ancient rocks. I may have missed it, but in that case could the authors make it more clear that the Cu isotope composition of these rocks reflects melting of an “unaltered” mantle composition that has Fe-alloy stable. Could there have been a refertilisation event between 1.4 Ga and 0.6 Ga that affected the mantle in this region?\nReply: The Mesoproterozoic lamproites were produced by melting of a mantle source involving phlogopite pyroxenite. The low δ65Cu values of the Mesoproterozoic lamproites could be conceived as being due to either a metasomatic pyroxenite with low δ65Cu values or isotope fractionation during melting. Melting of a mantle source involving δ65Cu-low pyroxenite could produce a melt with mixed isotopes between mantle peridotite and pyroxenite. No correlation between δ65Cu and Sr-Nd isotopes in the Mesoproterozoic lamproites precludes a δ65Cu-low pyroxenite source for these lamproites (Supplementary Fig. S4d). As discussed above, magmatic processes dominated by Ni-bearing sulfide could not cause Cu isotope fractionation, consistent with the BSE-like δ65Cu values of mantle pyroxenite xenoliths13. Therefore, we suggest that the metasomatic pyroxenites in the source of the lamproites has the BSE-like source δ65Cu value and their homogenous and low δ65Cu values are attributed to melting process of a metal-saturated mantle source as presented below and in Fig. 2b. We have revised the manuscript in Lines 154-165.\nFor the Cu isotope model, which predicts isotopically light melts from a mantle source only when buffered by an Fe-alloy (containing isotopically heavy Cu), I have a couple of questions. The first concerns the assertion that there is no Cu isotope fractionation between sulphides and silicate melt; this is based on the experimental data that show that Ni-rich sulphides and silicates have a negligible Cu isotope fractionation between them. This assertion is used to rule out any Cu isotope fractionation during igneous processes such as sulphide fractionation; although the extant data, which show that MORB and komatiite and also arc lavas all seem to have very predictable Cu isotope compositions seem to suggest that minimal Cu isotope fractionation takes place during sulphide precipitation, I think the authors could have been a bit more careful here when completely ruling this sort of process out.\nThe first thing to say is that this seems to rely on all sulphides being Ni-rich. This is surely not the case – as the authors know, sulphide petrology and composition can be quite variable, and so simply suggesting that every sulphide involved in the petrogenesis of these rocks is “Ni-rich” is a little difficult to believe based on the evidence provided in this paper; equally, I would ask what “Ni-rich” means – these fractionation factors were taking from a study looking at the crystallisation of the lunar core – are these fractionation factors applicable to Earth’s mantle? This criticism may seem harsh, but all I am saying is that Ni-poor sulphides can fractionate Cu isotopes when equilibrating with a silicate melt – hence I would recommend the authors at least mention this, or take this into account, in their models. I’m pretty sure, when I've modelled mantle melting and sulphide precipitation, you don’t get a huge different in Cu isotope composition in melts, as the maximum sulphide-silicate fractionation factors are pretty small, especially at mantle temperatures.\nReply: We have added Cu isotope fractionation models for magmatic processes using Cu isotope fractionation factors between silicate melt and Ni-free, Ni-bearing sulfides (Lines 678-731). The results indicate that partial melting and magmatic differentiation would not fractionate Cu isotopes of the melts using isotope fractionation factor of Ni-bearing sulfide but would result in heavy Cu isotopes in the melts using isotope fractionation factor of Ni-free sulfide (Fig. 2).\nSulfides in natural peridotite and pyroxenite xenoliths have enriched and highly variable Ni content (5- 54 wt.%)14,15 (Supplementary Fig. S4). Although the effect of such a large variation of Ni content in sulfide on Cu isotope fractionation factor has not been investigated, the homogeneous δ65Cu values of sulfide-saturated MORBs, sulfide-undersaturated komatiites, the bulk silicate Earth16,17 (Supplementary Fig. S5), and the lack of Cu isotope fractionation during the evolution of sulfide-saturated arc magmas18,19 indicate that most of magmatic processes involving sulfide in the mantle would not cause significant equilibrium Cu isotope fractionation.\nFurthermore, the lamproites and nephelinites have higher Ni contents than the MORBs (Supplementary Fig. S4c), indicating that sulfides in participating partial melting and magmatic differentiation are Ni-bearing rather than Ni-free. This is consistent with that their Cu isotope compositions plot away from the modelling results of Cu isotope fractionation during Ni-free sulfide-dominated magmatic processes (Fig. 2b-c).\nIn relation to the authors' metal-sulphide fractionation models, these are a little difficult to follow, and they aren’t described in much detail – the authors mainly refer to their supplementary information database. I think a little more detail on the models in the written component of the supplementary material, plus perhaps show how [Cu] varies in the melts and restite in the models – as this can help the reader in following the model graphically (as it is, the figure displaying the model is not intuitive because of the buffering of the sulphide in the melt by the metal phase).\nReply: We have added a section “Modelling of Cu isotope fractionation during various magmatic processes” in Methods. We have presented all details of modelling Cu isotope fractionation during partial melting of a peridotite at 1.5 GPa, magmatic differentiation of a MORB melt at 0.2 GPa, and partial melting of a metal-bearing pyroxenite at 6 GPa. See “Methods”, Fig. 2, Supplementary Figs. S6-8, and Supplementary Data S2-4. The Cu contents and isotopes of melts and residues are shown in Supplementary Figs. S6-8.\nFinally, could the authors comment on how not all sulphides in mantle rocks are available to melt as well as being less susceptible to metasomatism, i.e. if these are present as inclusions in olivine – is there a potential for a “hidden reservoir” of Cu isotopes in mantle rocks here.\nReply: Most of sulfides in the metasomatic mantle source appear as interstitial crystals between silicate minerals. Furthermore, the mantle source of the lamproites involves an olivine-poor pyroxenite, so that the known sulfide inclusions in olivine (in peridotite) do not apply. Therefore, we will not consider the storage of Cu inclusions in olivine as an important contribution of Cu to the magma.\nMinor comments\nOn line 80, the authors state that the Methods contain sample descriptions and petrology – but really this is limited to the thermobarometry. I think it would be good to include at least a brief description of the rocks utilised in this work, in terms of where and when they were collected, what their mineralogies are, etc.\nReply: We have added a section in “Methods” to describe the geological setting and petrogenesis of these alkaline rocks (Lines 594-646). Furthermore, our recent EPSL paper (https://doi.org/10.1016/j.epsl.2023.118489) described these samples in detail.\nThere seems to be a lot of assumptions/generalisations used in the thermobarometric calculations – whilst I’m sure these are all strong assumptions, it might be worth the authors including some statements to explain how sensitive their P and T estimates are to these assumptions.\nReply: The estimation of the melting temperature and pressure of the alkaline rocks at Aillik Bay was given in detail by Chen et al.4 using the method of Sun and Dasgupta20. This thermobarometer treats the Mg-number (Mg#) of olivine in the mantle source and the HO content of primary magmas as key parameters20. 2 Chen et al.4 compiled the Mg# of olivine of carbonated peridotites and wehrlites to calculate the melting temperature and pressure. However, the Mg# of olivine in the source of phlogopite pyroxenites was not constrained because there is no olivine in the phlogopite pyroxenites (MARID). Chen et al.4 assumed 80 for Mg# of olivine in the source of the lamproites, and here, we assume 80, 85, and 90 to evaluate the effect of Mg# of olivine on the calculation of melting temperature and pressure (Supplementary Fig. S2). We found that Mg# of olivine has a small effect on the estimation of melting pressure but has a significant on the estimation of melting temperature. As shown in Supplementary Fig. S2, the melting pressure was about 5-7 GPa for the lamproites and aillikites, and about 2-3 GPa for the nephelinites. The lithosphere thinned over time from >150km (5-7 GPa) for the lamproites at ca. 1.4 Ga and ultramafic lamprophyres/carbonatites at 590-555 Ma to 75-90 km (2-3 GPa) for the nephelinites at 142 Ma.\nIn figure 4, that authors have a label that states that global plate tectonics starts at 3.2 Ga. Whilst this is one of the periods that is commonly accepted that plate tectonics started globally, perhaps they authors might just take this off, as this is still a date that is up for debate.\nReply: We have removed it.\nI’m not sure that figure 5 in the main text really is adding much to the piece – I think showing an updated Figure S5, which includes [Cu] in the melt and restite, would be more instructive.\nReply: We have added Supplementary Fig. S6-8 to show the modelling details.\nReviewer #3 (Remarks to the Author):\nThe study of Chen et al. (2023) seeks to constrain the redox evolution of the LAB from the Proterozoic to the present-day using Cu isotopic data from a suite of alkaline rocks located in the NAC of Labrador, Canada. They find a temporal shift in Cu isotopic data that demonstrates that the LAB at this locality has oxidized in the mid-Proterozoic, possibly as a result of the rise of cold, modern style subduction. The premise and topic of this study are both interesting and clever, and for that reason it deserves to be published in this journal, as mantle redox evolution in all its permutations has strong effects on atmospheric evolution. The use of Cu isotopes to quantify mantle redox at the LAB is both innovative and interesting.\nHowever, currently the paper is bogged down by a variety of ideas and exercises that prevent a clear story about Cu isotopes from being imparted to the reader. Chiefly among these is the use of a compilation of a very low number (31 by my counting) of nonorogenic lamproites and a much larger number of carbonatites to postulate a switch from predominant lamproites to predominant carbonatite production. Given the very general title of the paper, the reader is confused and initially thinks that the paper doesn’t focus on Cu isotopes of a single locality, and is instead concerned with compiling ages of alkali rocks. The discussion of the age distributions of alkali rocks is treated like a headliner of the paper but it is not, and it should be relegated to the discussion section where the authors try to link the Aiilak locality with global change. This brings me to my second overarching problem with the paper: the generalization of a single locality to the whole Earth and the lack of discussion of the NAC specifically in favor of assuming that it reflect global trends. The mantle lithosphere shows immense amounts of redox heterogeneity relating to metasomatism, and a global secular trend is not demonstrated by a trend in a single 20x20 km area. If the authors were to alter the title and discussion to instead discuss how their data may be indicative of a global change as opposed to definitively stating that these data show global change, the manuscript would be much stronger. The method used here (Cu isotopes of alkali rocks) can be concluded to show promise for quantifying global trends once more localities are analyzed, and that in my opinion is the most important conclusions of the paper.\nReply: We have weakened the reliance on evidence from the distribution of lamproites. We have compared our results with the redox state of cratonic peridotite xenoliths (Lines 209-228). See responses to Reviewer 1 concerning the use of peridotite data from other cratons.\nTwo more general points I have with the study: how easy is it to distinguish orogenic from nonorogenic lamproites in tectonically complex Proterozoic terranes? Is it possible that misidentification could have lead to the distribution in nonorogenic lamproites observed? Finally, much is made of the globally rare nature of carbonatites in the Precambrian, but the low preservation potential of carbonatites in the rock record is never once mentioned. This is an oversight and should be corrected, as given the low amount of crust available from the early Earth and the rarity of carbonatites in general throughout Earth history, the exponential increase curve of carbonatites shown must be placed in proper context.\nReply: We compile the global age distribution of nonorogenic lamproites (Supplementary Data S1 and Fig. 1c-d). The compiled nonorogenic lamproites occur in stable craton regions (Supplementary Data S1), and are much younger than formation of cratons, indicating that they have no relationship with subduction and collision. These nonorogenic lamproites originated from the lower lithosphere mantle and were formed by melting in reduced conditions21. In contrast, orogenic lamproites occur in mobile belts being generated at shallow depths in post-collisional environments, and so are not relevant for the lower cratonic mantle22. The result shows that the nonorogenic lamproites mainly occurred in the early-middle Proterozoic and Mesozoic-Cenozoic period, with a conspicuous gap between 1000 and 300 Ma (Fig. 1b).\nThe age distribution of carbonatites and kimberlites are from Woolley and Kjarsgaard23 and Tappe et al.24, respectively. The abundance of the carbonatites and kimberlites show a general increase since ~2 Ga with a significant increase over the past 700 Ma. We have added text based on these two review papers which shows that preservation bias is not a major problem. We have revised the manuscript in Lines 657-677.\nOverall, the paper is well written and the figures are thorough and highly professional. Re-focusing the paper on Cu isotopic data for NAC lavas and backing away from global generalizations and the age compilation data will serve this excellent dataset better.\nReply: Thanks for your constructive suggestions.\nThanks for an interesting read,\nSpecific Comments\n-In this section I detail specific in-line comments. I have also provided numerous grammatical and usage edits in the “tracked changes” file returned with this review.\n-In Line 135, V/Sc is used as a proxy for oxygen fugacity of low melt fraction, enriched lavas. V/Sc is only a useful oxybarometer if three conditions are met: 1) melting does not leave garnet in the residue or fractionate garnet, due to the strongly compatible nature of Sc in garnet (much like the HREE) relative to V, 2) the degree of partial melting is well constrained and similar between the two types of rocks being compared. At low degrees of partial melting (<10%) like those examined here, small changes in the degree of partial melting can strongly influence V/Sc at constant fO2 (see Figure 1 of Li and Lee, 2004), 3) Finally and most importantly, V/Sc is a forward model for fO2 and assumes a constant V/Sc in the mantle source regions of the rocks examined. This is almost certainly untrue in this case as the mantle source regions are highly metasomatized lithosphere. I would not use V/Sc as a redox proxy at all, as your Cu isotope redox story is compelling enough without it.\nReply: V/Sc ratio is not used to reflect the source but represent the fO in magma evolution. The evolved 2 damtjernites and carbonatites are thought to have formed by liquid immiscibility from aillikite magma at a shallow crustal depth6. The evolved damtjernites and carbonatites show higher and highly variable V/Sc ratios than primary aillikite magmas (Supplementary Fig. S3) which may be the result of variable fO during 2 magmatic differentiation. The positive correlation between V/Sc and δ65Cu values in the evolved damtjernites and carbonatites (Supplementary Fig. S3) reveals oxidization of some Cu1+ to Cu2+ in the magmas during liquid immiscibility, which is supported by the occurrence of sulfate crystals in carbonatites6.\n-In Line 145 you use Cu/Sc to argue that mantle source regions do not show prior Cu enrichments. You use the observation that Cu/Sc is elevated in OIB relative to MORB to invoke Cu enrichment in the OIB source mantle, but similar to my previous comment, this is almost certainly the result of the compatibility of Sc in garnet relative to Cu and unrelated to Cu enrichment, as garnet is found in the residue of most OIB but not MORB. If you want a trivalent cation that is less influenced by garnet, try using Ga, as V/Ga has been suggested for use instead of V/Sc for this very reason (Laubier XXX). Alternatively, all of the V/Sc and Cu/Sc discussion can be cut and the manuscript focused better on the main point: Cu isotopes.\nReply: We have deleted the discussion of Cu/Sc.\n-Line 159: How did you chose your starting isotopic value? Is it the accepted BSE value? If so cite it.\nReply: We have cited it.\n-Line 178: What is NAC? Define terms appropriately.\nReply: We have defined NAC in Line 57.\n-Line 182: What about the youngest rocks and their fO2? Tell the three stages of the story here, not just the first two.\nReply: The fO calculation using the method of Stagno et al.25 requires primary melt compositions, especially 2 CO content. The 142 Ma nephelinites experienced significant fractional crystallization4. Furthermore, they 2 are CO -bearing magmas and may experience degassing during eruption. Therefore, it is difficult to calculate 2 the fO of the nephelinites.\nReply: We have now discussed this point in “Methods” and show, based on the compilations of Woolley and Kjarsgaard23 and Tappe et al.24, that preservation bias is not a major problem.\n-Line 222: This almost certainly testably by examining initial Nd and Sr isotopic signatures of global carbonatites and lamproites, as subducted recycled materials should impart a strongly enriched signature. I am not suggesting that you go through the exercise of compiling these data and seeing if such a shift is evident, but I am saying you should state that this would be a good way to test your model in a future study. Besides it is always good practice to end a paper in an open-ended manner, with an eye towards what sort of future data or studies could support the model presented.\nReply: We have discussed this point in our revised manuscript. We agree that it is difficult to decode the oxidation mechanism from our data.\n-Figure 4: Take out “vast” from the carbonatite-in line\nReply: We have deleted it.\n-Figure 4: Why is the “onset of plate tectonics” marked on this figure? This is never discussed in the text and remains a hotly debated topic. This figure is busy, and this label adds little to it. As you only focus on the Proterozoic to Phanerozoic, why not start the figure at 2500 Ma?\nReply: We have deleted it.\n-Figure 5: this is an excellent figure for illustrating your model for LAB, good job.\nReply: Thanks a lot.\n-Line 532: Which is it? 1000 or 2000 ppm or did you vary the concentration between these two values?\nReply: We have rephrased it.\nReferences:"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have read the authors' replies to all reviewer comments and find them to be reasonably satisfactory, although I am not entirely convinced of all the arguments presented, in particular regarding the nature of sulphides, in particular of metasomatic origin, in the cratonic lithospheric mantle. However, this can be fixed by putting an appropriate caveat and replacing a few \"indicates\" with \"may indicate\", and is not a reason to hold up publication of this manuscript.\nThe authors made significant changes in response to the reviewer comments and provide welcome clarifications in the supplement. They may wish to consider the following very minor issues, which came up when I read the version without tracked changes (numbers refer to lines in this clean document) with a \"fresh eye\":\n12 the abstract really should mention the actual Cu isotope values prior and post oxidation to do justice to the title and the novelty of the approach to estimating fO2\n57 could simply say “is a representative\" to avoid the repetition\n80 use the umlaut “wüstite”\n98 perhaps more accurate to say “no resolvable Cu isotopic fractionation”\n122 “suggesting” might be a better word. Lorand and Gregoire 2002 CMP show that sulphide are not in equilibrium with peridotites with respect to Ni/Fe, having lower Ni/Fe\n143 \"oxidation\", not \"oxidisation\" - multiple instances in manuscript\n153 fix the the\n160 \"low-d65Cu\"\n186 \"with neither…. nor\"\n232 delete \"to\"\n258 perhaps refer to “isolated\" or \"regional\" oxidation or similar, to provide a contrast with “large-scale” mentioned in the second part of the sentence?\nI think this is now in a fine state, and with very moderate additional effort will be ready for publication.", "reviewer_2": "Review of “Copper isotopes track the Neoproterozoic oxidation of cratonic mantle roots”\nI thank the authors for their detailed rebuttal document to the previous round of reviews, as well as providing a tracked-changes document. Broadly, this is a much improved manuscript that I would be happy to recommend for publication, perhaps after a couple of minor alterations/clarifications as noted below.\nI think changing the focus of the paper to suggesting Cu isotopes as a proxy for mantle melting fO2, and then moving on to a wider discussion of the oxidation of the NAC from other proxies makes the work much more holistic and less reliant on the smallish database of Cu isotope data, which is excellent. I do agree with another of the reviewers that the authors could lean even more into the novelty of using Cu isotopes here to show that their lamproites formed with a metal alloy in the source region. This is (I think) the first study to suggest or even attempt to use copper isotopes in this way, and I think more should be made of this – particularly given what seems to be a set of data that really do agree with the authors hypothesis.\nFor instance, Line 73: In this paragraph, I would highlight that although Cu isotopes are redox sensitive they have not really been applied to investigating the redox conditions of mantle melting – this is the authors’ new and novel idea. At the moment, in the paragraph, it sounds like Cu isotopes are a routine tracer – but they are not really (there have been attempts but not, I think, as successful as this work). This should be the hook that gets people interested in the paper. Furthermore, at the end of this section, in Line 85: perhaps a summary sentence of the proxy here – i.e. “Hence, if a melt is generated with metal allow in the source, the copper isotope composition of that melt would be lighter than BSE”. This novelty could be further highlighted in the abstract, and also in a summary sentence at the end of the main text, e.g.: perhaps again point to the potential for Cu isotopes to apparently be diagnostic of melting in the presence of metal alloys – a robust proxy for reduced melts – could be interesting not just for mantle petrology but for studies of reduced bodies in the solar system.\nI have one or two questions that have dawned on my based on my second reading of this work, which I think will be easy to address:\nThe first (Line 176): is there any other chemical evidence in the Mesoproterozoic lamproites that there was a metal alloy in the melt source? How about other siderophile elements which might show depletions given they would be strongly enriched in this alloy relative to the silicate minerals that are fusing to form the melt? Again, this would be another line of evidence to support the interpretation of the Cu isotope data.\nThe second again refers to the argument that metasomatic alteration of the source is not the reason for the isotopically light Cu of the lamproites; (Line 160) Could the authors explain in a bit more detail as to why this lack of correlation between Nd isotopes and Cu isotopes rules out the idea that a metasomatism could have affected the Cu isotope composition of the source rock? I don’t really follow this logic (I’m probably being stupid!).\nDoes ruling out a wholly pyroxenitic source strictly rule out the potential for cryptic metasomatism in the source which may (or may not) have altered its Cu isotope composition? The only reason I ask is that biotite mica can display mineral-melt D values >1 in some systems; I’m not sure there’s much in the way of phlogopite Cu partitioning data unfortunately, but if the lamproites are forming from the breakdown of phlogopite, could this mica have enriched Cu and then released its cargo (which could be isotopically light, who knows?) to the melt?\nAnyway, I thank the authors again for their conscientious approach to the first round of reviews. My main suggestions would be to make the novelty of the Cu isotope approach more overt in the work, and then perhaps spell out more clearly the arguments against metasomatic alteration of the source as the cause of the isotopically light Cu in the lamproites (and potentially highlight any other chemical evidence for metal alloy in the source). Other than that I would be happy to recommend publication.\nSome minor points:\nLine 57: repetition: “…represents a representative…”\nLine 90: Perhaps a reference to the MORB reference values here?\nLine 105: Here perhaps qualify this new statement to limit it to (mantle) partial melting processes – there’s fairly good evidence that crystallisation of sulphides in e.g. magma chambers or subarc mantle can affect the Cu isotope compositions of melts – even if basalts (sensu lato) don’t seem to reflect this globally. N.B. any effect of fractional crystallisation of sulphides in MORB melt is likely buffered by continuous refreshing of the magma system by ongoing melting.\nLine 131: I’m not sure Figure 3 best illustrates Cu behaving as an incompatible element in komatiites, ultramafic lamprophyres etc. Nevertheless, this section could be clarified with a simple statement that states that without sulphides on the liquidus, Cu will behave as a strongly incompatible element and there should be no a priori reason to suggest that magmatic differentiation will affect the Cu isotope composition of a sulphide-undersaturated melt.\nLine 147: Perhaps also point the readers to the compilation of arc basalts some of which are pushed to heavier Cu isotope compositions, so show this effect of dehydration and release of isotopically heavy Cu.\nLine 222: sp. “peridotite”\nLine 607: Perhaps point to references here where readers may find more information about the rocks in question.\nFigure 2 – perhaps include the refs for the metal-silicate fractionation factors too, if you are doing it for the sulphide-silicate factors.\nFig 6. Spelling mistake in lamproites (in the key). Also spelling mistake in the caption (curce instead of “curve”).", "reviewer_3": "Please see attached review.\nThank you for your extensive re-writes, and especially for removing much of the global focus of the paper to focus instead specifically on the rocks you analyzed. I think the paper is much stronger as a result of the edits. I have gone through again and have a few additional minor comments, but overall I think the paper is ready to go once these issues are fixed.\nSpecific Comments\nLine 57: is a representative\nLine 114: what do you mean by “residual” melts? Do you mean residues?\nLine 140: add a line here saying why you know higher V/Sc in carbonatites means higher fO2: i.e. oxidized V+4 has a greater affinity for carbonatite melts than Sc+3.\nLine 149: our modeling shows that addition of crustal materials…\nLine 197: that the fO of…\nLine 221: Use “log units” instead of “orders of magnitude” for oxygen fugacity\nLine 222: spelling typo\nLine 232: delete “to”\nLine 246: oxidation state has continually increased since…\nLine 261: as measomatic agents…\nLine 262: delete “the”\nLine 270: just use “colder” as opposed to “cold and warm”"}, "author_response": "Response to referees´ comments, manuscript NCOMMS-23-32963A\nChunfei Chen, Stephen F. Foley, Svyatoslav S. Shcheka, Yongsheng Liu\n“Copper isotopes track the Neoproterozoic oxidation of cratonic mantle roots”\nReferees’ comments in black\nReplies in blue\nWe thank the referees for the constructive comments and suggestions that significantly improved the manuscript.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nI have read the authors' replies to all reviewer comments and find them to be reasonably satisfactory, although I am not entirely convinced of all the arguments presented, in particular regarding the nature of sulphides, in particular of metasomatic origin, in the cratonic lithospheric mantle. However, this can be fixed by putting an appropriate caveat and replacing a few \"indicates\" with \"may indicate\", and is not a reason to hold up publication of this manuscript.\nThe authors made significant changes in response to the reviewer comments and provide welcome clarifications in the supplement. They may wish to consider the following very minor issues, which came up when I read the version without tracked changes (numbers refer to lines in this clean document) with a \"fresh eye\":\n12 the abstract really should mention the actual Cu isotope values prior and post oxidation to do justice to the title and the novelty of the approach to estimating fO2\nReply: We have revised the abstract with actual Cu isotope values of the lamproites and <0.59 Ga silica-undersaturated alkaline rocks (Lines 18-20).\n57 could simply say “is a representative\" to avoid the repetition\nReply: We have revised it (Line 56).\n80 use the umlaut “wüstite”\nReply: Done (Line 79).\n98 perhaps more accurate to say “no resolvable Cu isotopic fractionation”\nReply: We have revised it (Line 99).\n122 “suggesting” might be a better word. Lorand and Gregoire 2002 CMP show that sulphide are not in equilibrium with peridotites with respect to Ni/Fe, having lower Ni/Fe\nReply: We have revised it (Line 122).\n143 \"oxidation\", not \"oxidisation\" - multiple instances in manuscript\nReply: We have revised it all through the manuscript (Line 145).\n153 fix the the\nReply: We have deleted the repetitive “the”.\n160 \"low-d65Cu\"\nReply: We have revised it (Line 163).\n186 \"with neither…. nor\"\nReply: We have revised it (Line 190).\n232 delete \"to\"\nReply: Done.\n258 perhaps refer to “isolated\" or \"regional\" oxidation or similar, to provide a contrast with “large-scale” mentioned in the second part of the sentence?\nReply: We have revised it (Line 261).\nReply: We have revised it (Line 272).\nReply: We have revised it (Line 350).\nI think this is now in a fine state, and with very moderate additional effort will be ready for publication.\nThanks for your constructive suggestions.\nReviewer #2 (Remarks to the Author):\nReview of “Copper isotopes track the Neoproterozoic oxidation of cratonic mantle roots”\nI thank the authors for their detailed rebuttal document to the previous round of reviews, as well as providing a tracked-changes document. Broadly, this is a much improved manuscript that I would be happy to recommend for publication, perhaps after a couple of minor alterations/clarifications as noted below.\nI think changing the focus of the paper to suggesting Cu isotopes as a proxy for mantle melting fO2, and then moving on to a wider discussion of the oxidation of the NAC from other proxies makes the work much more holistic and less reliant on the smallish database of Cu isotope data, which is excellent. I do agree with another of the reviewers that the authors could lean even more into the novelty of using Cu isotopes here to show that their lamproites formed with a metal alloy in the source region. This is (I think) the first study to suggest or even attempt to use copper isotopes in this way, and I think more should be made of this – particularly given what seems to be a set of data that really do agree with the authors hypothesis.\nFor instance, Line 73: In this paragraph, I would highlight that although Cu isotopes are redox sensitive they have not really been applied to investigating the redox conditions of mantle melting – this is the authors’ new and novel idea. At the moment, in the paragraph, it sounds like Cu isotopes are a routine tracer – but they are not really (there have been attempts but not, I think, as successful as this work). This should be the hook that gets people interested in the paper. Furthermore, at the end of this section, in Line 85: perhaps a summary sentence of the proxy here – i.e. “Hence, if a melt is generated with metal allow in the source, the copper isotope composition of that melt would be lighter than BSE”. This novelty could be further highlighted in the abstract, and also in a summary sentence at the end of the main text, e.g.: perhaps again point to the potential for Cu isotopes to apparently be diagnostic of melting in the presence of metal alloys – a robust proxy for reduced melts – could be interesting not just for mantle petrology but for studies of reduced bodies in the solar system.\nReply: Thanks for your suggestion. We have reinforced the statement on the novelty of using copper isotopes to trace redox processes in the mantle (Lines 74 and 83-85), and highlighted this further at the end of the main text (Lines 277-280).\nI have one or two questions that have dawned on my based on my second reading of this work, which I think will be easy to address:\nThe first (Line 176): is there any other chemical evidence in the Mesoproterozoic lamproites that there was a metal alloy in the melt source? How about other siderophile elements which might show depletions given they would be strongly enriched in this alloy relative to the silicate minerals that are fusing to form the melt? Again, this would be another line of evidence to support the interpretation of the Cu isotope data.\nReply: Unfortunately, the PGE contents of these samples have not been reported. But it would be good to embark on a new project about the PGE contents of these samples.\nThe second again refers to the argument that metasomatic alteration of the source is not the reason for the isotopically light Cu of the lamproites; (Line 160) Could the authors explain in a bit more detail as to why this lack of correlation between Nd isotopes and Cu isotopes rules out the idea that a metasomatism could have affected the Cu isotope composition of the source rock? I don’t really follow this logic (I’m probably being stupid!).\nReply: We have clarified how the lack of correlations between δ65Cu and Sr-Nd isotopes in the Mesoproterozoic lamproites preclude a low-δ65Cu pyroxenite source for these lamproites (Lines 159-162). Melting of a low-δ65Cu pyroxenite vein would cause melting of and reaction with the surrounding peridotite wall, which could produce a melt with a mixed isotope signature blending characteristics between mantle peridotite with BSE-like δ65Cu and high 143Nd/144Nd, and metasomatic pyroxenite with low δ65Cu and low 143Nd/144Nd.\nDoes ruling out a wholly pyroxenitic source strictly rule out the potential for cryptic metasomatism in the source which may (or may not) have altered its Cu isotope composition? The only reason I ask is that biotite mica can display mineral-melt D values >1 in some systems; I’m not sure there’s much in the way of phlogopite Cu partitioning data unfortunately, but if the lamproites are forming from the breakdown of phlogopite, could this mica have enriched Cu and then released its cargo (which could be isotopically light, who knows?) to the melt?\nReply: Regrettably, our search found no data on Cu partitioning between phlogopite and silicate melts in the mantle. Nonetheless, given its silicate mineral structure, phlogopite is unlikely to exhibit high levels of Cu enrichment. Historically, silicate minerals have not been regarded as Cu-rich in the majority of existing literature and unpublished data in our research group show that phlogopite may be enriched in Ni and Cr, but not Cu. Additionally, the lamproites we examined display characteristics of sulfide saturation along with low Cu content, contradicting the notion of melting Cu-rich phlogopite.\nAnyway, I thank the authors again for their conscientious approach to the first round of reviews. My main suggestions would be to make the novelty of the Cu isotope approach more overt in the work, and then perhaps spell out more clearly the arguments against metasomatic alteration of the source as the cause of the isotopically light Cu in the lamproites (and potentially highlight any other chemical evidence for metal alloy in the source). Other than that I would be happy to recommend publication.\nReply: Thanks.\nSome minor points:\nLine 57: repetition: “…represents a representative…”\nReply: We have revised it (Line 56).\nLine 90: Perhaps a reference to the MORB reference values here?\nReply: The reference of Savage et al.1 is already there.\nLine 105: Here perhaps qualify this new statement to limit it to (mantle) partial melting processes – there’s fairly good evidence that crystallisation of sulphides in e.g. magma chambers or subarc mantle can affect the Cu isotope compositions of melts – even if basalts (sensu lato) don’t seem to reflect this globally. N.B. any effect of fractional crystallisation of sulphides in MORB melt is likely buffered by continuous refreshing of the magma system by ongoing melting.\nReply: Cu isotopic fractionation in mantle-derived rocks has been observed in certain cumulate rocks, like gabbroic rocks2,3, but not in their corresponding magmas. Yet, these Cu isotopic variations are not believed to arise from magmatic processes occurring during the cumulus stage3. Rather, the significant Cu isotopic variation in gabbroic rocks is ascribed to post-cumulus melt evolution in the lower crust, which involves fractionation between sulfide and inter-cumulus melts at relatively low temperatures at the hand specimen scale3.\nLine 131: I’m not sure Figure 3 best illustrates Cu behaving as an incompatible element in komatiites, ultramafic lamprophyres etc. Nevertheless, this section could be clarified with a simple statement that states that without sulphides on the liquidus, Cu will behave as a strongly incompatible element and there should be no a priori reason to suggest that magmatic differentiation will affect the Cu isotope composition of a sulphide-undersaturated melt.\nReply: We have added a statement for this point (Lines 131-133).\nLine 147: Perhaps also point the readers to the compilation of arc basalts some of which are pushed to heavier Cu isotope compositions, so show this effect of dehydration and release of isotopically heavy Cu.\nReply: We propose that the magmatic differentiation of damtjernites and carbonatites in the rifting setting diverges from that of arc basalts. We therefore refrain from discussing the processes and Cu isotopes specific to arc basalts.\nLine 222: sp. “peridotite”\nReply: We have revised it.\nLine 607: Perhaps point to references here where readers may find more information about the rocks in question.\nReply: We have added the references (Line 615).\nFigure 2 – perhaps include the refs for the metal-silicate fractionation factors too, if you are doing it for the sulphide-silicate factors.\nReply: We have added the references in Figure 2.\nFig 6. Spelling mistake in lamproites (in the key). Also spelling mistake in the caption (curce instead of “curve”).\nReply: We have revised these typos.\nReviewer #3 (Remarks to the Author):\nThank you for your extensive re-writes, and especially for removing much of the global focus of the paper to focus instead specifically on the rocks you analyzed. I think the paper is much stronger as a result of the edits. I have gone through again and have a few additional minor comments, but overall I think the paper is ready to go once these issues are fixed.\n-Robert Nicklas\nBoston College\nThank you very much for the constructive comments.\nSpecific Comments\nLine 57: is a representative\nReply: We have revised it (Line 56).\nLine 114: what do you mean by “residual” melts? Do you mean residues?\nReply: We have revised it (Line 115).\nLine 140: add a line here saying why you know higher V/Sc in carbonatites means higher fO2: i.e. oxidized V+4 has a greater affinity for carbonatite melts than Sc+3.\nReply: We have added it (Line 143).\nLine 149: our modeling shows that addition of crustal materials…\nReply: We have clarified it (Line 151).\nLine 197: that the fO2 of…\nReply: We have revised it (Line 201).\nLine 208: Please proof-read this section once more, there are several grammatical errors.\nReply: We have revised it.\nLine 221: Use “log units” instead of “orders of magnitude” for oxygen fugacity\nReply: We have revised it (Line 225).\nLine 222: spelling typo\nReply: We have revised it.\nLine 232: delete “to”\nReply: We have deleted it.\nLine 246: oxidation state has continually increased since…\nReply: We have revised it (Line 250-251).\nLine 261: as measomatic agents…\nReply: We have revised it (Line 265).\nLine 262: delete “the”\nReply: We have deleted it.\nLine 270: just use “colder” as opposed to “cold and warm”\nReply: We have revised it (Line 274).\nReferences:"}]

Decision letter