Pristine helium from the Karoo mantle plume within the shallow asthenosphere beneath Patagonia

Domain: earth_science
Published: 2024-07-30
Editorial outcome: major_revision

Abstract
Mantle xenoliths usually represent fragments derived from the depleted and degassed lithospheric mantle with 3He/4He isotope ratios (6 ± 1 RA) lower than those of mid-ocean ridge basalts (8 ± 1 RA). Otherwise, basalts from oceanic islands related to hotspots often have high 3He/4He ratios (>10 RA), suggesting a deep and pristine undegassed mantle source. Here we present a striking high-3He/4He component (up to 27.68 RA) recorded by spinel-facies mantle xenoliths from Patagonia. Remarkably, the highest ratios were found in a long-lived trans-lithospheric suture zone related to the Carboniferous-Permian collision of two continental blocks: the Deseado and the North Patagonian massifs. The mantle xenoliths with notably high-3He/4He ratios are inferred to be fragments of the shallow asthenosphere rising through the eroded and rejuvenated thin lithosphere. The pristine helium component is derived from the western margin of the Karoo mantle plume, related to the initial stages of the Gondwana fragmentation.

Introduction
The Karoo mantle plume has been invoked to explain the origin of the Karoo, Ferrar, and Chon-Aike large igneous provinces (LIPs), as well as volcanic sequences of the Malvinas/Falklands Islands and Ellsworth-Whitmore Terrane1–12. The Karoo LIP is essentially widespread in South Africa, East Antarctica, and Malvinas/Falklands Islands, whereas the Ferrar LIP is mainly located in East Antarctica, and the Chon-Aike LIP extends throughout Patagonia, Antarctic Peninsula, and Ellsworth-Whitmore Terrane.

Mantle xenoliths derived from the subcontinental lithospheric mantle (SCLM) of Patagonia have demonstrated mineralogical and compositional heterogeneity related to its complex geodynamic history13–22. These processes include the lithospheric mantle formation by variable degrees of partial melt extraction mainly during the Paleo- to Mesoproterozoic (2.1–1.0 Ga), the fragmentation of the supercontinent Rodinia, and the subsequent collision of continental blocks assembled at southwestern Gondwana13. The similar compositions and T model ages based on Os isotopes between the SCLM of southern Patagonia and southern South Africa support their common origin and connection since the Mesoproterozoic13. Later, the Mesozoic and Cenozoic geological evolution includes the formation of an active subduction zone at the western margin of South America (Andean Cordillera), where the collisions of the Farallon–Aluk spreading ridge (Paleocene-Eocene) and Chile Ridge (since Miocene) promoted the opening of successive slab-window segments beneath the southern Patagonia.

Noble gas isotopes, particularly helium, are important tools for the definition of mantle reservoirs, however, very limited noble gas isotope data are available for mantle xenoliths from Patagonia. It is well-constrained that, in general, the SCLM is characterized by radiogenic 3He/4He ratios (6 ± 1 RA 17,23) (where RA corresponds to atmospheric ratio of 1.4 × 10−6 24) compared to mid-ocean ridge basalts (MORB = 8 ± 1 RA 25,26) and primordial/undegassed source of ocean island basalts (OIB usually >10 RA 27). In Patagonia, the existence of an archetypical SCLM was well constrained by Pali-Aike xenoliths, which have uniform 3He/4He composition (total crushing of 6.91 ± 0.04 RA, n=60 17). Mantle xenoliths from Gobernador Gregores are characterized by typical MORB-like component related to the Chile Ridge subduction (total crushing of 7.34 ± 0.02 RA, n=35 17). Differently, mantle xenoliths from Coyhaique evidence the effective recycling of subduction-related melt/fluids in the Patagonian SCLM (3He/4He = 0.24–1.74 RA 20).

In this paper, we report the presence of a deep, undegassed, and pristine mantle component with high-3He/4He isotope ratios (21.65–27.68 RA) preserved by spinel-bearing mantle xenoliths stable in the unusual shallow asthenosphere beneath Patagonia. Also, we discuss the most plausible geodynamic processes responsible for the upwelling of pristine volatiles possibly associated to the Karoo mantle plume and its apparent relation with a long-lived trans-lithospheric structure related to the collision of the Deseado and North Patagonian continental blocks at the southwestern edge of Gondwana.

Results
Mantle xenoliths were collected from the Central-North Patagonia region (40’46”–44’52” °S), in the domains of the North Patagonian Massif [Cerro del Mojón (PM4); Estancia Alvarez (PM7); and Prahuaniyeu (PM8)], as well as close to an inferred collision zone [Cerro Chenque (PM10) and Cerro de los Chenques (PM12)] (Fig. 1). This trans-lithospheric suture zone would have been generated by the northeasterly collision between the Deseado and the North Patagonian massifs during the Carboniferous-Permian 28.

Most of the lavas hosting mantle xenoliths have Pliocene to Pleistocene K-Ar ages (3.76–0.73 Ma; Cerro del Mojón, Prahuaniyeu, and Cerro de los Chenques) (Supplementary Data 1). A lava flow from Estancia Alvarez yielded Miocene K-Ar age (22.06 ± 1.18 Ma) (Supplementary Data 1). Cerro Chenque is interpreted as an ancient volcanic conduit composed by two distinct dikes. The oldest is formed by a porphyritic strongly weathered rock containing large mantle xenoliths (up to 20 cm). It was not possible to determine the eruption age of this rock using geochronological methods, but it is estimated to be Lower Eocene (~52 Ma) according to its geochemical, textural, and structural similarities with the adjacent volcanoes 14. Differently, the younger dike is composed by fresh lava with aphanitic texture, Pliocene K-Ar age (4.36 ± 0.24 Ma) and hosts small mantle xenoliths (up to 5 cm) (Supplementary Data 1). Only mantle xenoliths from the older lava flow could be analyzed in this study.

Helium, neon, and argon isotope ratios and concentrations of mantle xenoliths are presented in Supplementary Data 2. In general, helium isotope ratios (in RA) discussed here are assumed as mantle-derived because they were extracted by both stepwise and single crushing methods under high vacuum, which avoid the cosmogenic or radiogenic contributions from the mineral matrix. However, even analyzed by crushing, due to the lack of high-quality neon data to better constrain the existence of pristine high-3He component, we have decided to disregard doubtful results possible containing minor contribution from the mineral matrix (Supplementary Information and Supplementary Data 2). Previous measurements aiming to estimate the abundance of gas present in studied mantle xenoliths were performed by single heating extraction method. In order to constrain whether the crystal-lattice-hosted component is primordial or cosmogenic in samples PM10-Q77 and PM10-Q109, we analyzed their rock powders (after stepwise crushing) by heating extraction.

The 3He/4He isotopic ratios of truly trapped fluid inclusions extracted by crushing method reveal the existence of intrinsic SCLM beneath Central-North Patagonia (5.33–6.99 RA), MORB-like (7.26–7.60 RA), and primordial (23.02–27.68 RA) components (Fig. 2). The pristine component observed in mantle xenoliths from Cerro Chenque (PM10) is confirmed by the 3He/4He ratio obtained by heating the rock powder after stepwise crushing (21.65 ± 0.32 RA). This value is equivalent (slightly lower) to the previous results obtained by stepwise crushing, indicating negligible effect of cosmic ray irradiation and small addition of a radiogenic 4He from U and Th (Supplementary Fig. 1). Based on these results, we conclude that the high-3He/4He ratio (single crushing = 23.02 ± 6.84 RA) revealed by sample PM10-B2Ol also reflect a true pristine component. It is justified by its identical 3He/4He ratio compared to the result revealed by the sample PM10-Q77, although the analytical uncertainty is higher. Conversely, the high-3He/4He isotope ratios obtained by heating extraction (10.50–188.08 RA) are essentially cosmogenic (18–140 %) (see Supplementary Information and Supplementary Fig. 2).

Most neon isotopes contain strong contributions of blank (>10%), as well as of 40Ar++ on 20Ne+ and CO2++ on 22Ne+ (both >10%), defining insufficient quality to be considered in the discussion. The data containing enough analytical quality do not differ from air composition (20Ne/22Ne = 9.80; 21Ne/22Ne = 0.02924) within 2σ analytical uncertainties. The 40Ar/36Ar ratios between 304 and 2692 vary from an near-atmospheric ratio (296 24) toward a small contribution of a mantle endmember exemplified by the increasing isotope ratio values.

Notably, mantle xenoliths with high-3He/4He isotopic ratios are found in a region characterized by low values of elastic thickness estimations (Te < 20 km, Fig. 1), suggesting the presence of thin and weak (low viscosity) continental lithosphere 13,29. The highest 3He/4He isotopic ratios (21.65–27.68 RA) were obtained in mantle xenoliths from Cerro Chenque (PM10), located in the southern area of the North Patagonian Massif (Fig. 1). It is worthy of note that the zone characterized by the lowest values of Te estimations is displaced to the north with respect to the inferred collision zone proposed by ref. 28. (Fig. 1).

Seismic tomographic data (SA2019 30) reveal a strong low-velocity anomaly in the mantle wedge beneath Central-North Patagonia (~50–180 km), immediately overlying the top of the Nazca oceanic plate 31 (Fig. 3). This anomaly possibly reflects thermal evidence of the connection between a ~N-S slab tearing related to the Farallon–Aluk mid-ocean ridge during the Paleocene-Eocene (north of 46°30’S) and the slab window produced by the Miocene to Present collision of the Chile Ridge, which covers a significant part of Santa Cruz province (south of 46°30’S 32,33). Thus, this region indicates a lithospheric thinning and an asthenospheric rising. This observation is consistent with the Te values presented in Fig. 1, as well as with the shallow lithosphere-asthenosphere boundary 34 observed in the inferred collision zone (~50 km, Fig. 3). In this context, the crust-mantle boundary (Moho) has ca. 40 km, revealing an extremely thin lithospheric mantle with ca. 10 km thick 35 (Fig. 3).

Based on these results, we exclude the presence of an active mantle plume (thermal anomaly) in this region due to the absence of continuity of the low-velocity anomaly toward the deeper mantle column (>200 km) (Fig. 3). Additional evidence is given by mantle xenoliths containing pristine helium composition (high-3He/4He), which have low temperature estimates than those necessary to justify the existence of a hot mantle plume (838°C; sample PM10-Q77 14). Also, we discard any influence of the Chile Ridge into the high-3He/4He ratios of Patagonian mantle xenoliths essentially due to its homogeneous and slightly radiogenic MORB composition recorded by basaltic glasses (total heating = 7.84 ± 0.08 RA 36, Fig. 2). In the mantle transition zone (410–660 km), there is a high-velocity anomaly that probably represents the stagnant Farallon Plate 37 (Fig. 3).

Aiming to confirm the quality of helium data determined by crushing extraction, we evaluated the contribution of atmospheric, post-eruptive radiogenic 4He, and cosmogenic 3He components into the 3He/4He isotope ratios of Patagonian mantle xenoliths (see detailed calculations in the Supplementary Information and Supplementary Data 2). In summary, our results show a negligible contribution of atmospheric (usually <1%) and post-eruptive radiogenic production of 4He (usually <0.3%). Conversely, considering the xenoliths from the inferred collision zone (Cerro Chenque) that have truly high-3He/4He ratios, the effect of cosmic ray irradiation reaches 15%. Although this percentage seems high, its influence on the isotopic ratios is not significant and does not change the interpretation of the data.

Helium isotopes allowed the identification of three reservoirs in the upper mantle at the spinel stability field beneath Central-North Patagonia. Most mantle xenoliths from the North Patagonian Massif preserved their SCLM component (3He/4He = 5.33–6.45 RA, most > 6.0 RA), which was partially overprinted by a MORB-like component (3He/4He = 7.31–7.60 RA) (Fig. 2). Contrary, mantle xenoliths outcropping through the inferred collision zone are characterized by the mixing between primordial component (3He/4He = 21.65–27.68 RA) with subordinated SCLM and MORB-like components (3He/4He = 6.89–7.26 RA) (Fig. 2).

Notably, our samples have strong compositional similarity with mantle xenoliths from Ferrar LIP (Antarctica 38,39), which mainly preserved the SCLM reservoir (3He/4He = 5.86–6.98 RA) superimposed by MORB-type (3He/4He = 7.00–9.05 RA) and OIB-like (3He/4He = 10.60–26.80 RA) components (Fig. 2). It is important to observe that although the high-3He/4He identified beneath the Ferrar LIP was not discussed as primordial by the authors mentioned above, there is no reason to rule out this source. For example, ref. 38 have decided to ignore 3He/4He ratios higher than 10 RA (up to 26.80 RA) due to the mass fractionation revealed by neon and argon results. In this case, we agree that samples within the Solar and/or mass fractionation lines coupled with 38Ar/36Ar ratios lower than the atmosphere (0.188 24) must be disregarded (see Fig. 4 of ref. 38). However, considering the analytical uncertainties (2σ), five samples show neon isotopes compatible with the atmospheric, MORB, and OIB components (Supplementary Fig. 3 40,41), plotting within the mantle array in the 20Ne/22Ne versus 38Ar/36Ar (see Fig. 4 of ref. 38). Remarkably, ref. 39 found a high-3He/4He ratio (20.18 RA) accompanied by atmospheric neon isotopic ratios, which coincides with the result obtained for Cerro Chenque (total crushing of 26.28 RA) as well as with the highest ratio reported by ref. 38. (26.80 RA) (Fig. 2).

The South Atlantic hotspots Shona (beneath the Weddell Sea), Bouvet (beneath the Antarctica), and Discovery (beneath the Karoo province) are assumed as the expression of the Jurassic Karoo-Ferrar mantle plume 12,42. These hotspot lavas are characterized by pristine and high-3He/4He isotopic ratios (12–15 RA 43–46) (Fig. 2). Regarding the lower isotopic ratios recorded by these samples compared to our results, it can be explained by the less frequent occurrence of hotspots with high 3He/4He ratios in the Atlantic large low-shear velocity province (LLSVP 47). According to these authors, Shona and Bouvet hotspots are connected to (supplied by) a mantle plume (in this case, Karoo mantle plume) while Discovery is defined as disconnected. Alternatively, the lower 3He/4He ratios observed in these hotspot lavas can be a consequence of the plume-ridge interaction in which a deeper and undegassed component (OIB) interacted with a shallower and degassed (MORB) mantle source (Fig. 2). Similarly, olivine-rich magmas from the Paraná-Etendeka LIP (Namibia) show similar high-3He/4He ratios (18.80–25.85 RA 48) compared to the Karoo mantle plume. These results revealed a complex mixing between different sources, such as radiogenic SCLM, MORB-type, and plume material (Fig. 2).

Based on the information above, we conclude that the primordial helium (>20 RA) found in the upper mantle beneath Central-North Patagonia represents remnants of a deep and common component related to the initial stages of the Gondwana breakup. In addition, the helium mixed-source observed beneath Patagonia, South Atlantic hotspots, and Antarctica is likely to be the norm rather than the exception in which the deep-seated material interacted with the shallow SCLM and MORB components.

The high-3He/4He of mantle xenoliths from Central-North Patagonia indicates the existence of a pristine and deep helium component beneath southern South America. The rising of this component was most likely channelized throughout an inferred collision zone, which is a weakness zone related to a trans-lithospheric suture formed since the establishment of Pangea (~320–300 Ma) (Fig. 4).

Considering the geodynamic evolution of Patagonia, we infer the combination of two mechanisms to explain the rejuvenation and thinning of the base of the continental lithosphere beneath this collision zone (Fig. 4). The development of a flat subduction during the Carboniferous-Triassic continental collision promoted the dehydration of the oceanic slab and the subsequent hydration of the lithospheric mantle, decreasing its rheological strength 13,49–51 (Fig. 4a). Posteriorly, the subduction angle changes from shallow to steep, inducing inflow of the asthenosphere through the opening of a slab-window as a response to the slab break-off 13 (Fig. 4b). Thus, this event promoted the lithosphere erosion beneath the southern edge of the North Patagonian Massif 13. The inferred collision zone seems to be related to the region with lowest Te (Fig. 1), as well as with a shallow lithosphere-asthenosphere boundary (~50 km; Fig. 3).

Thereafter, the continental surface of this collision zone was covered by the thick Late Triassic-Jurassic sedimentary successions of the Golfo San Jorge Rift Basin during the continental subsidence 52 (Fig. 4c). The dominant extensional tectonic regime comprised by deep normal faults that enhanced the lithospheric thinning is associated to the opening of this rift basin (Fig. 4c). This event is coeval with the early stages of the fragmentation of southwestern Gondwana, which was triggered by the Karoo mantle plume 4,5,8. In this context, during the Late Triassic to Lower Jurassic, the consolidation of another flat subduction was followed by a slab steepening-induced (rollback) mantle flow that resulted in a southwestward asthenospheric advection 4,5,8 (Fig. 4d,e). Thus, the mantle flow may have exerted a suction effect of a component enriched in primordial high-3He from the Karoo mantle plume towards Patagonia (Fig. 4e). In addition, the influence of hotspots from the South Atlantic Ocean (Shona, Bouvet, and Discovery) has been invoked to explain the compositional heterogeneity evidenced by lavas from the Patagonian back-arc 53 and Chile Ridge 54. Therefore, the development of flat-slabs and the thermal erosion promoted by a mantle plume replaced the base of weak lithosphere by uprising asthenosphere reducing its viscosity and thickness. Consequently, we conclude that the mantle xenoliths from Cerro Chenque represent asthenospheric fragments while the mantle xenoliths from other Patagonian localities are derived from the SCLM.

Expanding the perspective beyond the Patagonian back-arc, we propose that the complexity involved in the long-lived tectonic evolution of intraplate continental settings favors the conformation of upper mantle reservoirs with strong geochemical and isotopic compositional heterogeneity, that can include deep and pristine asthenospheric mantle.

Methods
40K/40Ar chronology
K-Ar ages of mantle xenolith host lavas were determined using the unspiked sensitivity method (Supplementary Data 1). Ar analyses were performed using a noble gas mass spectrometer MS-III (modified VG5400) at the Geochemical Research Center, Graduate School of Science, University of Tokyo. The crushed and sieved whole-rock samples (~0.5 g) were wrapped in aluminum foil 10-μm thick and loaded into a sample holder made of Pyrex-glass, which is connected to a vacuum line at an extraction oven were the sample is completely melted at 1700°C. The gas released was purified and separated before its introduction in the mass spectrometer. The Ar isotope analyses were made on a relatively small amount of Ar gas (<2 × 10−7 cm3 STP). When the amount of Ar gas extracted from the sample exceeded this limit, it was reduced using the purification line. Uncertainties on 40Ar sensitivity and 40Ar/36Ar ratio are estimated to be 5 and 0.2%, respectively, based on repeated measurements of the atmospheric standard containing 1.5 × 10−7 cm3 STP of 40Ar. The K concentration of each sample was determined for an aliquot of the crushed and sieved whole-rock fractions used for Ar analysis by the X-ray fluorescence (XRF) method (Phillips PW2400). The analytical precision was verified by analysis of the rock standard JB-1a, issued by Geological Survey of Japan.

Noble gas isotopes
He-Ne-Ar isotopic ratios of mantle xenoliths from Central-North Patagonia were performed at the Research Center for Advanced Science and Technology, University of Tokyo (Supplementary Data 2). Here we applied different extraction methods to obtain the noble gas isotopic ratios. In order to release the noble gases trapped in fluid inclusions from whole-rock (WR), olivine (Ol), clinopyroxene (Cpx), and olivine plus orthopyroxene (Opx) samples, in-vacuum stepwise and single crushing measurements were performed. Twelve samples (0.21–1.33 g) were crushed in a stainless-steel tube with a single (2000 strokes) or sequential number of strokes (usually applying 100, 500, 1000, and 2000 strokes) from a nickel piston driven from outside the vacuum by a solenoid magnet 55, implying 25 measurements. Single step heating extraction method was previously performed (before crushing measurements) for 20 disaggregated minerals representing whole-rock samples (0.52–0.68 g). Aiming to verify whether the high-3He/4He ratios displayed by samples PM10-Q77 WR and PM10-Q109 WR have cosmogenic contribution or whether they represent a pristine component, additionally single step heating measurements were carried out using their rock powders.

Based on the reproducibility of measurements of a Japanese helium standard (HESJ) and those of a calibrated air standard for other noble gases, the estimated experimental uncertainties for noble gas concentrations were 5 % for helium and argon, and 10 % for neon. Uncertainties assigned to the observed isotopic ratios were one standard deviation (1σ), including uncertainties of blank and mass discrimination corrections determined by the measurements of HESJ (3He/4He = 20.415 ± 0.029 RA 56) and atmospheric gas. Helium concentrations [4He] corrected to the air range from 0.39 to 53.39 × 10−9 cm3 STP/g for crushing measurements, and from 1.38 to 265.04 × 10−9 cm3 STP/g for heating. Blanks were run by using the same procedure as that used for the samples.

The blank contribution for 4He in crushing measurements is low (<2.5%; n=23 of 25), except for samples PM7-B1 WR (6.1% in stepwise 10 strokes) and PM10-B2 Ol (4.1% in single 2000 strokes). Comparatively, the blank contribution for 4He during heating extraction is high, with 10 of 22 results containing >2.0% (up to 16.62%).

Neon results obtained by crushing and heating extraction methods were corrected for 40Ar++ on 20Ne+ and CO2++ on 22Ne+ following the method described by ref. 57. They vary, respectively, from 0.1 to 29 % and 0.8 to 74 %. Considering both extraction methods, the blank contribution for 20Ne vary from 0.39 to 198.00 %, where 38 of 48 measurements have less than 15 %. However, considering 2σ uncertainty and the contributions of 40Ar++ on 20Ne+ and CO2++ on 22Ne+ as less than 10%, none on data has enough quality to be discussed.

For 40Ar, the blank contribution is <21 % considering crushing measurements, but <10% eliminating the 100 strokes of Cerro Chenque (PM10-Q77 and PM10-Q109). Regarding heating measurements, the blank contribution for 40Ar usually is lower than 15% (n=16 of 22), reaching up to 56.82%.

Inductively coupled plasma mass spectrometer (ICP-MS)
The thorium (Th) and uranium (U) concentrations (in ppm) of whole-rock mantle xenoliths were measured at the LMTG (Observatoire Midi-Pyrénées) of the University Paul Sabatier (Toulouse III). These results were used to calculate the 4He radiogenic contribution into Patagonian mantle xenoliths (Supplementary Information). The measurements were carried out using a quadrupole inductively coupled plasma mass spectrometer (ICP-MS) Elan 6000 Perkin Elmer. Calibrations, internal standard, and interferences corrections were done following the procedure described in ref. 58. Data quality was controlled by running BCR-2 standard. Relative standard deviations are generally ≤5%.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Thank you for giving me the opportunity to review this manuscript. This study continues the effort to identify subducted oceanic crust and hotspot signatures in mafic lavas. The concept of mantle re-fertilization from subducted hotspots has been shown in Central American basalts, but not in Central-North Patagonia during the Cenozoic. The authors put forth a novel sequence of events that integrates radiogenic isotopes, trace elements, noble gas isotopes, geochronology, and seismic tomography to trace the source of magmatism underneath Somuncura and Canquel to a fossil hotspot trail. These methods are appropriate for assessing a range of processes. Constraining the transfer and cycling of elements and volatiles via metasomatism is important for understanding the origin of enriched mafic lavas and their fate during subduction.\nThe results draw upon previous work from Central Patagonia and several SE Pacific hotspots. The authors take care to choose representative geochemical data for their regional setting and relevant localities. Overall, the analytical procedures and methods are suitable and consistent with previous studies of the region. I am unable to comment on the specifics of Patagonian regional tectonics or seismic tomography, but I have some concerns about the underlying assumptions used to interpret Helium isotopes and clarity of the geologic model. I believe this work is publishable, and would be highly impactful if the following points are addressed:\n1. The fossil(ized) hotspot trail is a major focus of the title and abstract. It would be an extremely novel discovery– however, the existence is mostly supported by Helium isotopes, extracted from crushing. Is the Helium from the same source as the SE Pacific hotspots? Is Helium decoupled from Sr-Nd-Pb? Is it possible to quantify the relative contributions of mantle, radiogenic, and atmosphere? Can there be more explicit characterization of the 3He/4He of the hotspot and preservation of this signature? Such estimates or discussion of the limitations should be included.\n2. The sequence of events, beginning with fossil hotspot volcanism and leading up to Andean back-arc volcanism, was introduced in the abstract and further illustrated in the discussion. While highly detailed, it is unclear which findings were supported directly by the new data, and/or previous studies. A numbered list with approximate ages and separation of events would be useful. Considering the multidisciplinary breadth of the journal, the manuscript would benefit significantly with the addition of a figure showing the proposed sequence. This is a very dynamic setting with distinct periods of activity over millions of years. A clear diagram would further communicate the impact of the findings.\nIn summary, this work is publishable, and the information will be more impactful and accessible with minor revisions. Specific comments are attached, and these include suggestions for figures.\nBest Regards,\nThi Truong", "reviewer_2": "The article entitled Recycling of a fossil hotspot trail beneath Central-North Patagonia during Cenozoic presents new geochemical, isotopic and geochronological data on the Cenozoic magmatism of Patagonia. Although these data are valuable, the interpretations made from them in this article must be fully revised and corrected. The article has numerous problems, which have been explained in detail in the attached file. First, the authors indicate that the low-velocity seismic anomaly beneath Patagonia is produced by the hot-spot track subducted material, in order to explain the peculiarities of the Patagonian Cenozoic magmatism. The back-arc magmatism of Patagonia began ~50 Ma ago, so the hot spot track must have been subducted during the Late Cretaceous-Early Paleocene, and today is still exactly below the Cenozoic intraplate basalts. There was not westward continental drift?\nThere are multiple papers showing the opening a slab window by the Chile Ridge subduction at ~46° S. Excactly the same location. It is widely known that a slab window induces partial melting of the sub-slab portion of the upper mantle at ~100 km depth (e.g., Breitsprecher and Thorkelson, 2009; Guillaume et al., 2010; Georgieva et al., 2019), so the low seismic velocity anomaly under a sector of Patagonia is, and has been attributed to the subduction of the Chilean ridge. The authors have mentioned this slab window in this work, so it is striking that they do not consider it to explain this seismic anomaly. The chile ridge began to be subducted beneath Patagonia at 16-14 Ma (e.g., Muller et al., 2016 between many others) and with it, the beggining of the most modern intraplate magmatism of Patagonia.\nLikewise, the seismic tomography of Patagonian region has also shown an N-S slab tearing at western Patagonia (e.g., Aragón et al., 2011; Navarrete et al., 2020), so, the northward elongation of the seismic anomaly can also be explained by this gap.\nThe authors should take into account these previous data from Patagonia.\nAlso, it is obvious that both the Chile ridge and the Juan Fernández hot spot show similar low velocity seismic anomalies at ~100 km depth, since both the ridge and the \"plume\" generate decompression-induced magmatism at those depths. The seismic resemblance cannot be used as evidence of a subducted hot spot.\nMoreover, it is widely known that the subduction of a hot spot track induces modifications on the subducted slab such as its shallowing (i.e., Juan Fernández hot spot track). It was not considered by the authors.\nFinally, the authors do not consider any other possibility to explain MORB mantle metasomatism, such as the mantle transition zone that is currently being studied worldwide since much of the intraplate magmatism has recently been attributed to recycled subducted materials in this mantle layer.", "reviewer_3": "First, I want to acknowledge that the data itself is outstanding and that the discovery merits a high-impact publication. That said, the interpretation is overcomplicating the results. The authors discovered a clear connection to a primordial, deep Earth source (3/4He up to 27 RA!) for these lavas. There is no need to invoke recycling of hotspot tracks where the signal is so unambiguous. This interpretation is also not supported by modeling of physicochemical mechanisms that will allow 3/4 He to be recycled from subducted material that conceptually will be degassed. I think the authors need to carefully evaluate the alternative, and more logical explanation these these Patagonia lavas sampled a mantle plume."}, "author_response": "Revision notes\nWe would like to thank the three reviewers for their constructive comments and suggestions. They provided different improvements according with their expertise’s.\nBased on these comments, we carefully revised all results and the calculations involved in the treatment of data. We will address all the major and minor comments of the reviewers as follows. A word file containing the track changes also is resubmitted. The numbers of lines indicate the modifications made on the updated file considering the track changes.\n(Text highlighted in italic and blue color = Reviewer’s comments)\nReviewer #1 (Remarks to the Author):\nIt appears that the authors have addressed the reviewers' suggestions and clarified the interpretation in the context of the data and previous understanding of regional tectonic events for the given time period. The geodynamic interpretation in the discussion and addition of Figure 3 was much needed in the first version of this manuscript, and is a beneficial addition. There are a few figures that would benefit from additional edits as previously suggested. However, based on the changes I would recommend the revised manuscript for publication.\nResponse to Reviewer #1\nWe appreciate very much the positive feedback given by Reviewer #1 and we would like to apologize for does not properly respond to the first round of reviewers. It was a consequence of our decision of completely reformulate the manuscript. However, here we will address the answers for all comments that are related to the data and/or interpretation maintained after the reformulation of our manuscript. All modifications made in response to the comments given by Reviewer #1 were applied in the revised manuscript.\nProofreading of grammar and corrected case is needed in a few instances (for example, in the abstract). Both “Juan Fernandez” and “Juan Fernández” appear in the text and figure captions. “Somuncura” and “Sumuncura” are also present in different places. Perhaps these are interchangeable in this case but I am unsure.\nResponse. We carefully adjusted all occurrences along the text, as well as in the figure captions.\nConsider revising word choice or provide evidence when using particular phrases. For example, in the abstract:\n“undoubtful” is perhaps not an effective word here.\nResponse. This word was deleted.\nIncluding some detail about FOZO or “C” may be useful here. Arguably, Helium isotopes are not straightforward tracers for mantle endmembers in some instances. This limitation should be discussed briefly.\nResponse. As suggested, we detailed the concept of FOZO, PREMA, and C components between the lines 268-286 and 406-415. Regarding the assumption about the limitation of helium isotopes to constrain mantle endmembers, we believe that they in fact are a powerful toll to this purpose. We understand as a limitation the situation in which there is a mixing between endmembers, but this complexity is usually solved through neon isotopes (and sometimes using argon). Of course, radiogenic isotopes and other geochemical proxies also are useful to make sure about mantle endmembers. An example of this limitation to our work is the high-3He/4He ratios that would be primordial or cosmogenic in origin. This possibility was deeply explored in the main text (lines 120-136, 224-251), as well as in the Supplementary Information (lines 56-59, 134-206) based on the considerations given by Reviewer #4.\nDo errors vary for samples or for the type of material? Is there an explanation for the sample with ±7 RA?\nResponse. In terms of noble gas isotopes, the mantle endmembers are especially well-defined for the oceanic basalts (MORB and OIB) because the volcanic glasses and phenocrysts usually are gas-rich samples compared to the SCLM, which is represented by mantle xenoliths (gas-poor xenocrysts). The subduction-related reservoir has mainly been defined based on phenocrysts and/or volcanic gas samples. Therefore, the representative samples of each tectonic environment define the compositional variation among them. In this case, the type of material (i.e., glass, phenocryst, xenocryst, gas) will determine the amount of gas. A sample with 3He/4He isotopic ratio of 7 R can be defined as belongs to MORB or SCLM reservoirs and, in this case, it is necessary additional evidence to elucidate this result. For instance, neon isotopic ratios are essential solve this doubt. Alternatively, additional measurements are necessary to verify if the predominance of results indicates MORB or SCLM.\nAssuming these are used to indicate the pristine and deep component, are there any characteristics about the samples with 26-27.74 RA samples that are noteworthy? Phase, location, etc. Is it significant that the highest 3He/4He is from a websterite?\nResponse. This question was properly discussed in the second version of our manuscript, where we associated this notable high-3He/4He ratio to a hypothetical trans-lithospheric suture zone related to the Carboniferous-Permian collision of two continental blocks (Deseado and the North Patagonian massifs). This suture zone coincides with a region characterized by low values of elastic thickness estimations, as well as by a shallow lithosphere-asthenosphere boundary (~50 km). This discussion was added to the abstract (lines 32-35), as well as to the main text (lines 170-186, 256-259, 290-293, 315-318, 367-376, 410-415). Furthermore, based on this study, we consider that there is an association between pyroxene-rich lithologies and the high-3He/4He ratios (lines 388-391).\nAre there any further constraints on this component, e.g. noble gas signature and depth? Could He-Ar results be used to assess mixing or contributions for air, upper mantle, and deep mantle for samples with the highest 3He/4He?\nResponse. We invoked the existence of a shallow asthenosphere beneath the inferred collision zone, which favored the upwelling of primordial 3He from the superdeep mantle (lines 170-186, 317-318, 367-376). Concerning the argon results, unfortunately they are not useful to our discussion due to the significant contribution from the air.\nRegarding “the original helium composition of the SCLM was superimposed”. How much Helium was transferred, what concentrations are needed to reflect MORB and/or OIB contributions?\nResponse. It is important to consider that the noble gases are chemically inert, being modified by physical (mechanical) processes. Hence, here we assume that the original helium composition of most SCLM mantle xenoliths investigated in this study was replaced by subsequent events during to the long-lived tectonic evolution recorded by the SCLM beneath Patagonia. These events potentially include the influence of subduction-related components (radiogenic 3He/4He ratios) and the events associated to the consumption of oceanic slabs throughout the geological time. For example, the development of slab-windows that favors the asthenospheric upwelling to the mantle wedge and induces the overprint of the SCLM by MORB-like signature. Finally, the helium ratios containing OIB-like composition are a challenge to explain in the context of Patagonian back-arc. However, it is our main objective in this work. With respect to how much helium was transferred to justify the replacement of the SCLM composition by other endmember (i.e., MORB, radiogenic, OIB), it is quite difficult to estimate considering that the concentration of helium varies significantly depending on the analyzed material (i.e., volcanic glass, volcanic gas, phenocryst, and xenocryst), which can exhibit any composition.\nWhere do the thermometry estimates come from?\nResponse. I am sorry, but probably I does not understand the question. However, I will try to answer. The temperature estimate mentioned here (line 213) was taken from Bertotto et al. (2021). Considering the suggestions given by Reviewer #4, this part of the main text was reformulated, and the estimates previously mentioned were disregarded.\nHow can you tell it is stagnant? And is there a strongly high-velocity anomaly between 80-100 km depth that is east of Canquel?\nResponse. In this case, we are considering the paper published by Navarrete et al. (2020) as reference to affirm that the high-velocity anomaly observed between 300 and 600 km represent the stagnated (fossil) Farallon oceanic plate. I am sorry because I didn’t indicate this reference in the first version of our manuscript.\nUnfortunately, due to the limited number of seismographs acquiring data in this region, we have a limited resolution that unfeasible the certainty of the existence of tomographic anomalies with less than 100 km. However, the high-velocity anomaly observed between 80 and 100 km east of Canquel represent the oceanic crust (in the Atlantic Ocean).\nNot sure if I would say I see the similar anomalies if profiles at the other latitudes are not shown. Perhaps the profiles can be included in a supplementary figure.\nResponse. I completely understand your point-of-view. Here we were comparing the low-velocity anomaly between 50 and 180 km based on the image (a), which represent the tomographic result in 100 km depth. For the second version of our manuscript, we don’t stablish this comparison. Hence, we decided don’t modify this figure including the vertical section related to the region of Juan Fernandez and San Félix.\nI find the 3He/4He error bars are light colored and difficult to see.\nResponse. We modified the intensity of color in the error bars.\nIt is difficult to discern if the cluster of samples with the highest 3He/4He are PM10-B2 OL or PM10-Q77 WR. These may be confused easily with PM-7 B1 WR. Perhaps include labels for the most important samples.\nResponse. Considering the comments given by Reviewer #4, we have decided to assume as primordial only the samples PM10-Q77 WR and PM10-B2 Ol. For this reason, we moved the other samples with high-3He/4He ratios to the Supplementary Figure S1, which includes the high-3He/4He ratios related to the cosmogenic contribution.\nI suggest including a San Felix value here. Sample 99654 has an olivine crushing measurement reported in Jackson and Steinberger (2021) in G3, Table S2. 10.1029/2020GC009525\nResponse. We appreciate very much for this recommendation, but we consider that that this result (3He/4He = 6.97  0.21 R ) isn’t essential to our discussion. Also, it would represent an additional citation in the reference list due to only one isotopic ratio.\nIt may be misleading to show the arrow trending towards “Pristine component” starting at >10 RA, as this suggests 10 RA qualifies as pristine, even though that is somewhat in the realm of MORB. A pristine component would be better portrayed if the arrow points off the diagram to much higher values. As is, perhaps “OIB source”, or “Less degassed mantle” is more appropriate.\nResponse. We agree with this comment, although usually it is assumed that a primordial component has >10 R . Thus, based on this comment, we adjusted the information related to the arrow as “less degassed mantle”.\nAre SE Pacific hotspot data all from lavas, i.e., whole rock? Are you distinguishing between mineral separates and whole rock? Juan Fernandez only has Helium isotope measurements on olivine and clinopyroxene.\nResponse. In our opinion, there is no difference between the type of material employed to noble gas measurements. Thus, we didn’t select a specific type of sample to plot in the 3He/4He vs. 4He diagram. However, the helium isotopic data from Farley et al. (1993) are from olivine separates, except for the sample PF-20 CPX (clinopyroxene). Differently, Poreda et al. (1993) discussed the helium results of basaltic glasses. The results reported by Yamamoto et al. (2020) for petit-spots also are from basaltic glasses. On the other hand, the paper of Kurz et al. (2009) includes helium isotopic data from basaltic glasses and olivine separates.\nI am not a seismologist so I cannot comment on the use of the tomographic slices given for the region. This figure would benefit from annotations that could connect the proposed sequence of events and components with the view from seismic tomography. Some suggestions below:\n● Label y-axes on a) and b)\nResponse. As suggested by Reviewer #1, the labels were inserted.\n● Index contour should be specified in the caption and perhaps labeled in diagram\nResponse. Aiming to facilitate the identification of the variation of tomographic anomalies, the indices for each contour were included, labeling their respective values.\n● In a) it could be useful to superimpose island/seamount outlines with present day hotspot for San Felix and Juan Fernandez\nResponse. The contours of the islands and the main seamounts were included in Figure 3a, representing the current positions of the hotspots of São Felix and Juan Fernandez.\n● Instead of a vertical solid black line on b), label Canquel at the surface (0 km) with the same yellow diamond symbols used in a)\nResponse. This suggestion was done.\n● Show convergent boundary, lithosphere-asthenosphere boundary, trench, or other important features of the subduction zone in a) or b).\nResponse. Based on the model proposed by Priestley et al. (2019), we included the depth of the lithosphere-asthenosphere boundary (LAB) along the profile (dashed black line) in Figure 3b. Based on the model proposed by Rivadeneyra-Vera et al. (2019), we also included the depth of the crust-mantle boundary (Moho) along the profile (white dashed line). Finally, based on the model proposed by Portner et al. (2020), we included the outline of the top of the Nazca Plate (green dashed line). It was not possible to obtain this contour along the profile because the slab model does not reach the latitude of Cerro Chenque (-43.8), and therefore the information from the closest latitude (-40) was used. We understand that the contour used is representative for the top of the slab for the region of Cerro Chenque.\nPortner, D.E. (2020). https://doi.org/10.1029/2019JB017884\nPriestley, K. (2018). https://doi.org/10.1002/9781119249740.ch6\nRivadeneyra‐Vera, C. (2019). https://doi.org/10.1029/2018JB016811\nPM12-15 Ol+Opx 2000x 1 - 1.07g, is there a reason as to why olivine and opx were measured together? How can you discern if the trapped Helium originates from the same stage?\nResponse. This sample (PM12-15 Ol+Opx) is composed by olivine and orthopyroxene because the amount and size of clinopyroxene are too small. For this reason, we decided to verify the consistency of the noble gas results from this sample by comparing its composition with whole-rock samples. Considering the analytical uncertainties, the 3He/4He ratios of samples PM12-15 Ol+Opx (6.98  0.19), PM12-26 WR (6.88  0.69), and PM12-48 WR (7.21  0.18) are identical. Thus, we conclude that there is no significant compositional variation between the mineral phases (olivine, orthopyroxene, and clinopyroxene) from this locality (Cerro de los Chenques).\nReviewer #2 (Remarks to the Author):\nMy comments, corrections, and suggestions are indicated in the attached PDF file.\nGood luck!\nResponse to Reviewer #2\nWe would like to thank the suggestions provided by Reviewer #2. They surely contributed to the substantial improvement of our work, especially regarding the tectonic evolution of Patagonia. All modifications made in response to the comments given by Reviewer #2 in the pdf file were applied.\nLine 30. undergassed?\nResponse. Here the correct word is really “undegassed” because we are referring to a mantle source that selectively retained the volatiles (in this case, noble gases) in the deep mantle. This work is widely used in works focusing on noble gas isotopes to mentioning less depleted (and less degassed) lower mantle.\nLine 33. hypothetical\nResponse. As suggested, we inserted this word before “trans-lithospheric suture zone”.\nLine 41. the mantle transition zone is not a superdeep mantle region\nResponse. Here we are assuming as a superdeep mantle that deeper than 300 km. It is a consensus between several publications involving superdeep diamonds (e.g., Anzolini et al., 2016, 2018, 2019, 2020; Brenker et al., 2021; Smith and Nestola, 2022; Nestola et al., 2023). Therefore, we understand that the mantle transition zone (410-660 km) represents a superdeep mantle.\nAnzolini et al. (2016). http://dx.doi.org/10.1016/j.lithos.2016.09.025\nAnzolini et al. (2018). http://dx.doi.org/10.2138/am-2018-6184\nAnzolini et al. (2019). https:// doi .org /10 .1130/G45605.1\nAnzolini et al. (2020). www.pnas.org/cgi/doi/10.1073/pnas.2004269117\nBrenker et al. (2021). https://doi.org/10.2138/am-2020-7513\nSmith and Nestola (2022). https://doi.org/10.1002/9781119528609.ch7\nNestola et al. (2023). https://doi.org/10.1038/s41586-022-05392-8\nLine 79. undergassed\nResponse. Please, consider the answer above.\nLine 91. hypothetical suture\nResponse. The proposed changed was applied.\nLine 91. would have possibly been\nResponse. The proposed changed was applied as “would have been”.\nLine 133. Zaffarana et al. (2017) have established that the Gastre Fault is not a transcontinental fault. The authors should read in detail this paper.\nResponse. In fact, the Reviewer #2 is correct. We carefully read the work of Zaffarana et al. (2017), as well as the publications of Zaffarana et al. (2012) and von Gosen and Loske (2004). We are sorry for this misunderstood related to the Gastre Fault System. Due to this information, we removed the Gastre Fault from Figure 1, as well as adjusted the main text of our manuscript (lines 174, 176, 291, 317, 458). As a consequence, the reference number 25 (Zaffarana et al., 2017) was deleted.\nGosen and Loske (2004). https://doi:10.1016/j.jsames.2004.08.007\nZaffarana et al. (2012). https://revista.geologica.org.ar/raga/article/view/560\nLine 143-146. According to multiple previous studies, the slab window related to the subduction of the Chile ridge is currently located (opened) in the Santa Cruz province (southern Patagonia). The low-velocity anomaly beneath the central and northern Patagonia was interpreted as a ~N-S slab tearing since there is a slab gap identified in seismic tomographies exactly beneath this anomaly (see Navarrete et al., 2020). This slab tearing would be connected with the slab window of the Chile ridge. Authors should mention this previous interpretation.\nResponse. We appreciate very much for this information. As suggested, we verified the paper published by Navarrete et al. (2020) and improved our discussion (lines 180-183).\nLine 157-160. As was interpreted by Navarrete et al. (2020).\nResponse. As suggested, we added this information (line 218).\nLine 201-202. Authors should carefully read the work by Zaffarana et al. (2017), since they are assuming that the Gastre fault is linked to the hypothetical suture.\nResponse. Based on the previous comment given by Reviewer #2, as well as in the contribution of Zaffarana et al. (2017) and references therein, we have decided to remove any discussion about the Gastre Fault System from the manuscript.\nLine 217-218. A Permian (and Triassic) slab break-off was proposed by Pankhurst et al. (2006) and Gianni and Navarrete (2022).\nResponse. Yes, it is true that the authors mentioned above also proposed a slab break-off for the studied region. For this reason, we included these references in the text (line 315).\nLine 223-227. It is worthwhile for the authors to know that there is no consensus on the origin of the Golfo San Jorge basin. There are proposals that indicate that during the Cretaceous, this basin was a foreland basin (see Gianni et al., 2015. Synorogenic foreland rifts and...), which would explain, among other things, why there was no magmatism in this basin during the Cretaceous.\nIt is true that during the Jurassic there were multiple extensional-related depocenters that allow indicating a lithospheric thinning.\nResponse. We would like to thank the opportunity to have additional knowledge related to the understanding of the Golfo San Jorge Basin through the contribution of Gianni et al. (2015). We didn’t know this publication. Based on it, we included additional information to our discussion (lines 325-327).\nLine 294. undergassed?\nResponse. Please, consider the answer above.\nReviewer #4 (Remarks to the Author):\nReview by Jens Hopp of manuscript “Shallow asthenosphere with pristine helium from the superdeep mantle beneath Patagonia” by Jalowitzki et al., submitted to Nature Communications.\nThe thematic issue of this manuscript is identifying a superdeep (>300 km) mantle component by its elevated 3He/4He-ratio in mantle-derived rocks from Patagonia. For this purpose high 3He/4He-ratios found in mantle xenoliths from various locations of various age (but predominantly Pliocene) in Patagonia are interpreted within a geotectonic reference frame encompassing the last hundreds of millions of years. If the postulated scenario is true, this would be the first evidence for a superdeep wet plume.\nResponse to Reviewer #4 (Prof. Jens Hopp)\nWe really want to thank very much the contribution given by Prof. Hopp. Apparently, he was the specialist in noble gas isotopes involved in the reviewer process of our manuscript. Although we are confident with respect to the obtention of our data due to the method employed to the extraction of the noble gases from Patagonian mantle xenoliths, the aspects questioned by Prof. Hopp were fundamental to ensure that we really found the first primordial component beneath Patagonia. Therefore, his contribution allowed the substantial improvement of our work. For instance, we verified if the high-3He/4He ratios of samples PM10-Q77 and PM10-Q109 (both analyzed by stepwise crushing extraction) are in fact primordial or if they have cosmogenic contribution by applying the heating extraction of their rock powders. Based on these new results, we are convinced that sample PM10-Q77 is primordial (see details below), whereas sample PM10-Q109 showed cosmogenic contribution (it is unexpected for us). Unfortunately, due to the doubt generated by this controversial result, we decided to discard the other high-3He/4He ratios of samples “PM7-B1 Ol”, “PM12-01 WR”, and “PM12-17 WR”. It is important to clarify that it was a preventive decision because we don’t have these rock powders anymore to apply the same verification. Concerning the general and detailed comments from Prof. Hopp, they will be answered as follow:\nIn order to link a superdeep component to the Patagonian magmatism in time the authors propose (1) a geodynamic model for the geologic evolution of the respective crust-mantle evolution, (2) the presence of water in the transition zone that is responsible for formation of hydrous minerals observed in the mantle xenoliths and (3) a link of the helium isotope record to the postulated upwelling of superdeep material, which in turn link the water with the transition zone. To manage this the authors have to propose a quite controversial topic, namely the presence of significant volumes of water within the transition zone. But this cannot be proven by the data (a sponge might be dry, no matter of its storage capabilities). The only what the authors can prove is that water plays a role in formation of hydrous minerals without any inference of its origin. The helium could have other sources as well. Other explanations might also work, in particular the introduction of water by dehydration of an oceanic slab. This situation – hydrous minerals and subduction – is frequent and even might lead to a coexistence of primary minerals (from a “dry” mantle region and “wet” slab fluids).\nResponse. We would like to elucidate that differently of indicated by Prof. Hopp, we are attributing the formation of hydrated minerals (phlogopite and pargasite) in the SCLM beneath Patagonia to two main processes. (1) the small-scale edge-drive convection, which favored the mantle hybridization and the preferential generation of pyroxene-rich lithologies due to the thermo-mechanical erosion in the base of the lithosphere (please, see lines 382-391). (2) the opening of slab-windows and the consequent asthenosphere upwelling throughout the Phanerozoic. An example of the latter process can be observed in a new suite of mantle xenoliths from Patagonia that contains well-developed glass veins with adakite composition associated to the crystallization of phlogopite (manuscript in preparation by our research group). In this case, the partial melting associated to the dehydration of the metamorphized subducted slab (eclogite or garnet amphibolite) plus overlying sediments provided the components necessary to the generation of phlogopite in the mantle wedge. Therefore, both processes occur in the lithosphere/asthenosphere instead in the mantle transition zone (please, see Fig. 3c for additional information).\nWe still would like to clarify that the tectonic model proposed to explain the origin of the high-3He/4He (up to 27.74 R ) observed in the mantle xenolith PM10-Q77 essentially includes the contribution from the thermomechanical erosion promoted by the small-scale mantle convection in a rifting system (Fig. 3c), but it also would be related to the development of slab-windows (Figs. b and d), as well as to the mantle transition zone (Fig. 3c). In this case, the wet plume as a response to the mechanical perturbation promoted by the reaching of the Farallon oceanic slab into the mantle transition zone would be a possible candidate only to the mantle xenoliths brought to the surface from Miocene to Present (it is not our case).\nHere we consider crucial clarify that we are not correlating the high-3He/4He ratios to the influence of water, but to volatiles (helium) containing deep origin (probably from the asthenosphere, but also from the mantle transition zone). This assumption is based on the existence of primordial helium component in mantle xenoliths from Cerro Chenque (PM10), which is located in the inferred collision zone between the Deseado and North Patagonian massifs. Thus, we conclude that the primordial helium was preferentially channelized throughout this suture zone. In this context, it is expected that this component be rare (limited) compared to the influence of other slab-derived sources (i.e., radiogenic and MORB-like), or even the preservation of the original SCLM component of mantle xenoliths. Actually, it is exemplified by our data that essentially record SLCM and MORB-like helium signatures (Supplementary Table S1), with only two samples recording true primordial composition.\nRegarding the presence of significant volume of water in the mantle transition zone, it is well-constrained through mineral inclusions from superdeep diamonds, as well as based on experimental studies, that there is a substantial amount of water in this Earth’s layer. For example, ringwoodite and wadsleyite, both capable to incorporate up to ~3 wt.% of water in their structures, represent the high-pressure polymorphs of forsteritic olivine in the mantle transition zone and lower mantle, that is, they are the most abundant phases in this part of the Earth. (Chen et al., 2004; Chamorro Pérez et al., 2006; Pearson et al., 2014; Chang et al., 2015; Thomas et al., 2015; Bolfan-Casanova et al., 2018). Furthermore, currently our research group is investigating the existence of water in iron oxides hosted by diamonds from the superdeep mantle.\nChen et al. (2004). https://doi.org/10.1073/pnas.0405048101\nChamorro Pérez et al. (2006). https://doi.org/10.1007/s00269-006-0096-8\nPearson et al. (2014). https://doi.org/10.1038/nature13080\nChang et al. (2015). https://doi.org/10.1002/2015JB012123\nThomas et al. (2015). https://doi.org/10.3389/feart.2014.00038\nBolfan-Casanova et al. (2018). https://doi.org/10.3389/feart.2018.00075\nAnother objection here is the expected lateral extension of this upwelling component – this is not visible in a 2D figure. From the authors geodynamic model a line of volcanic units might be characterized by this superdeep signature (its not a “hotspot” sensu strictu) – is there any evidence for this?\nResponse. We consider the existence of a volcanic line with superdeep signature unlikely in Patagonia because in our study we verified that the rising of a pristine helium component is conditioned to the suture zone (inferred collision zone) that probably is more effective in the deeper samples (or source) than in shallower samples (e.g., spinel-bearing xenoliths). In Patagonian back-arc, it is a fact that all mechanisms involved in the generation of magmatism has been attributed to the development of the subduction systems, with no conclusive evidence about the role of a hotspot sensu stricto. The progressive increase of ages, as well as chemical heterogeneity, is only observed in localities that record the change of the dip angle of the oceanic plates. It is important to observe that Navarrete et al. (2020) provided a comprehensive revision about several aspects related to the geological evolution of Patagonia, where they improved the models proposed by Honda et al. (2006) and Orihashi et al. (2006). These authors associated the eruption of the large volume of lava from Somuncura and Canquel plateaus, the most important basaltic plateaus of Patagonia, to the existence of a wet plume triggered by the mechanical perturbation promoted by the Farallon oceanic plate when it reached the mantle transition zone during the Oligocene-Miocene. Even in this context, there is no evidence of a line with a hotspot-like signature.\nMoreover, there is a scarce number of studies about mantle xenoliths from Patagonia (Jalowitzki et al., 2016; Novais-Rodrigues et al., 2021), whereas still there is no publication about alkaline basalts. We are sure that with the advance of noble gas investigations of mantle xenoliths and alkaline basalts from Patagonia, this pristine component will be better characterized and understood.\nHonda et al. (2006). https://doi.org/10.1186/BF03352614\nOrihashi et al. (2006). https://doi.org/10.1016/j.gca.2006.06.932\nJalowitzki et al. (2016). http://dx.doi.org/10.1016/j.epsl.2016.06.034\nNavarrete et al. (2020). https://doi.org/10.1016/j.earscirev.2020.103379\nNovais-Rodrigues et al. (2021). https://doi.org/10.1016/j.gr.2021.06.003\nMy main objection of this topic is not the water itself but the presented He data, which are the basement of their conclusions – no high 3He/4He, no superdeep component. The authors made a lot of analyses, also of heated complementary samples to their crushed specimens. A lot of work, I appreciate that. Their heating He data show a clear presence of cosmogenic He in the mineral lattices – if not, the signature is typical lithospheric helium. There are some studies showing that such cosmogenic He is not relevant if a stepwise crushing method is applied for gas extraction. But this is not the entire truth. Clearly, He diffusion might redistribute He from lattice sites into fluid inclusions, in particular in old samples like the Cerro Chenque xenoliths, and in addition there is a certain probability to redistribute the helium via recoil into fluid inclusions. Commonly, small amounts of cosmogenic He are hardly recognized within an “ocean” of mantle helium. But the mantle xenoliths investigated for this study are quite low in gas concentrations. A given range of 4He of <10-9 ccm STP/g is equivalent of about E-14 ccm STP/g 3He. To account for the reported high 3He/4He-ratios requires not really much cosmogenic He. For example, the highest values observed during crushing of samples from the PM12 suite (single strokes) from Cerro de los Chenques (is this the same as Cerro Chenques?) are associated with lowest 4He concentrations, i.e. these samples are most prone to contamination with cosmogenic helium. I would not build a quite substantial model of geochemistry upon samples which are suspect of containg a minor, but in the end significant, cosmogenic helium component. Other examples of stepwise crushing show increasing 3He/4He with increasing number of strokes, e.g., both series of PM10. Sample PM 7 shows within errors a constant ratio of about 6.2 Ra, i.e. as soon as we enter the field of lithospheric signatures no systematic increase is apparent. But for PM7 4He concentrations are one order of magnitude larger, as is the case for PM4 with asthenospheric 3He/4He. From all samples in this study only one extraction is a potential candidate for a mantle helium signature, PM7-B1 Ol 2000x (single crush). In this case 4He concentrations are higher and 3He/4He is about 14. Which is peculiar given the non-existence of such a component in the stepwise crushing experiment. And sorry, one single – only possibly – correct mantle value is not enough to constrain a superdeep wet plume model. Note, that in the latter sample no clear evidence of cosmogenic neon is visible (I would not necessarily expect that anyway).\nProvided the He is truely of mantle origin (which I doubt): It could be also possible that the Eocene Cerro Chenque volcanics might be related to an enrichment in 3He within the local lithosphere during the earlier rifting process, documented in Fig 3c, but unrelated to formation of hydrous phases from subduction related fluids.\nAs probably is clear from above I unfortunately cannot really support publication in its current form though I like the general geodynamic model. But the data appear not robust enough to support the conclusions presented by the authors.\nResponse. We would like to initiate the response to this concern explaining that the heating extraction measurements (from separated minerals) were performed before the crushing in order to verify which samples are gas-rich. Based on this information, we selected the gas-rich samples to apply the stepwise crushing. Also, it is important to clarify that in this work we are integrating two moments of generation of data. In a first moment, the single measurements applying 2000 strokes (produced by Prof. Conceição), whereas the heating as well as the stepwise crushing belong to a second stage (produced by the first author). It is undoubtable that unfortunately our heating measurements show strong influence of a cosmogenic component.\nHowever, we definitely believe that our high 3He/4He isotopic ratios obtained by stepwise crushing represent a primordial rather than a cosmogenic component. In support of our interpretation of the relevant samples, we would like to emphasize the following points:\n1. It is well-defined that helium is preferentially extracted by mineral crushing under high-vacuum conditions while cosmogenic and/or radiogenic helium isotopes are released by mineral melting (heating) as they are produced and remain within the mineral’s matrix (e.g., Scarsi, 2000; Sumino et al., 2006, 2011; Yokochi et al., 2005). Since radiogenic/cosmogenic helium should be homogeneously distributed in the sample, and the pristine component preserved in fluid inclusions should be released in the earlier steps during crushing, we would expect to observe the increase in 3He/4He with the progress of crushing if the high 3He/4He is derived from insignificant cosmogenic 3He. However, it should be noted that there is no constant progressive increase of 3He/4He ratios in both stepwise crushing extractions for samples PM10-Q77 and PM10-Q109.\nReferences\nScarsi, P. (2000). Fractional extraction of helium by crushing of olivine and clinopyroxene phenocrysts: Effects on the 3He/4He measured ratio. Geochim. Cosmochim. Acta. 64, 3751-3762. https://doi.org/10.1016/S0016-7037(00)00419-1\nSumino, H., Kaneoka, I., Matsufuji, K. and Sobolev, A. V. (2006). Deep mantle origin of kimberlite magmas revealed by neon isotopes. Geophys. Res. Lett. 33, L16318, https://doi:16310.11029/12006GL027144.\nSumino, H., Dobrzhinetskaya, L. F., Burgess, R. and Kagi, H. (2011). Deep-mantle-derived noble gases in metamorphic diamonds from the Kokchetav massif, Kazakhstan. Earth Planet. Sci. Lett. 307, 439-449. https://doi.org/10.1016/j.epsl.2011.05.018\nYokochi, R., Marty, B., Pik, R. and Burnard, P. (2005). High 3He/4He ratios in peridotite xenoliths from SW Japan revisited: Evidence for cosmogenic 3He released by vacuum crushing. Geochemistry Geophysics Geosystems. 6. https://doi.org/10.1029/2004GC000836\n------------------\n2. Additionally, if the high 3He/4He ratios were of cosmogenic origin, its contribution should inevitably be detected in neon isotopes, resulting in high 21Ne/22Ne ratios (21Ne enrichment) coupled with 20Ne/22Ne ratios lower than the air. However, excluding the samples with high blank, 40Ar++ on 20Ne+, and CO ++ on 22Ne+ contributions (>10 %), we essentially observe the air component in our samples. The only exception is the sample PM7-B1, which plot within the MORB-air mixing line. Nevertheless, it is essential to note that we have decided to disregard this specific result due to its high analytical uncertainty considering 2. As demonstrated by Sumino et al. (2000), we also estimated the potential cosmogenic neon contribution associated with possible cosmogenic helium. The results are presented in the Supplementary Table 2, as well as the calculations are thoroughly described in the Supplementary Information. Based on the information above, we confidently conclude that the helium component of our xenoliths is primordial, rather than cosmogenic.\nReference\nSumino, H., Nakai, S., Nagao, K. and Notsu, K. (2000). High 3He/4He ratio in xenoliths from Takashima: evidence for plume type volcanism in southwestern Japan. Geophys. Res. Lett. 27, 1211-1214. https://doi.org/10.1029/1999GL008438\n-------------------\n3. Even considering a hypothetical scenario in which the selective release of cosmogenic helium occurs without a significant release of cosmogenic neon (Yokochi et al., 2005), we refute this possibility based on findings by Blard et al. (2008). These authors demonstrated that the crushers used in the former publication were too much effective in releasing only helium due to heat generated during the crushing extraction. To strengthen our argument that our crushers are not effective in releasing matrix-sited cosmogenic/radiogenic helium, we generated unpublished data showing that they didn’t release cosmogenic helium during stepwise crushing by analyzing the same samples of Yokochi et al. (2005).\nReferences\nBlard, P.-H., Puchol, N. and Farley, K. A. (2008). Constraints on the loss of matrix-sited helium during vacuum crushing of mafic phenocrysts. Geochim. Cosmochim. Acta. 72, 3788-3803. https://doi.org/10.1016/j.gca.2008.05.044\n----------------\n4. After Prof. Neely communicate the decision regarding our manuscript, we tried to attest the quality of our data. Thus, we analyzed the rock powders (after stepwise crushing) by heating extraction at the University of Tokyo to determine whether the crystal-lattice-hosted component is primordial or cosmogenic in samples PM10-Q77 and PM10-Q109. The new results confirmed that the high-3He/4He ratios revealed by sample PM10-Q77 is truly pristine. The 3He/4He ratio obtained by heating (using the rock powder) for this sample is 21.64  0.32 R , which is equivalent (slightly lower) to the previous results obtained by stepwise crushing (23.39  0.59 R to 27.74  0.21 R ). Moreover, neon results do not differ from the atmospheric ratio considering the analytical uncertainties. Therefore, we conclude that there is no doubt about the primordial component of this sample, as well as about the insignificant addition of a radiogenic 4He from U and Th.\nBased on these results, we assume that the high-3He/4He ratio of sample PM10-B2 Ol 2000x (23.02  6.84 R ) also reflect a true pristine component. This value is identical to those obtained by stepwise crushing for sample PM10-Q77, but the analytical uncertainty is high. However, sample PM10-Q109 surprisingly shows the effect of cosmic ray irradiation as evidenced by a higher 3He/4He ratio (50.29  0.71 R ) compared to the stepwise crushing (14.32-19.25 R ). Yet, neon data corroborate this finding because this sample has high 21Ne/22Ne (0.0494) coupled with low 20Ne/22Ne (9.55). We don’t have an explanation to this unexpected result at this moment, but probably it would be related to the mechanism mentioned by Prof. Hopp, in which the “He diffusion might redistribute He from lattice sites into fluid inclusions”. Also, it is evident that during the heating experiments of whole-rock samples (separated minerals), sample PM10-Q77 presented the lower cosmogenic effect (15 %; 3He/4He = 30.97  0.93 R ) among all analyzed samples from Cerro Chenque (where 3He/4He ratio of PM10-Q109 is 87.98  1.61 R , which imply 70 % of cosmogenic contribution). In any case, we reformulated the manuscript based on this new information, which led us to the decision of ignore the other results with high-3He/4He ratios (samples PM7-B1 Ol 2000x, PM12-01 WR 2000x, PM12-17 WR 2000x, and PM12-26 Ol 2000x). We prefer to abandon these suspected results to guarantee the quality of our data, although it is important to clarify that this decision is preventive because we don’t have the rock powders of single crushing extraction (2000 strokes) anymore to apply the same verification.\nRegarding the following argument “the mantle xenoliths investigated for this study are quite low in gas concentrations”, we would like to consider that it is quite common to mantle xenoliths display low amount of gas (xenocrysts are gas-poor samples), especially compared to volcanic glasses but also to phenocrysts.\nWith respect to the number of samples (or results) to justify the publication, we consider that our discovery is impactful (relevant) to the tectonic evolution of Patagonia because there are few noble gas data available, as well as it is the first report of a pristine component to this region. Therefore, we believe that the six measurements (including stepwise crushing, single crushing, and powder heating) presented here are enough to justify our findings.\nAs expressed by Prof. Hopp, our preferred scenario to the origin of a primordial helium also is the rifting process exemplified by the opening of the Golfo San Jorge Basin during the Jurassic-Cretaceous (Fig. 3c), although we cannot exclude the possible contribution from the other mechanisms (i.e., opening of slab-windows and the perturbation of the mantle transition zone).\nDetailed comments:\nL. 150: “… a pristine helium composition …” or “…pristine helium compositions…”\nSame paragraph: I don’t think that the existence of low T xenoliths excludes a high T hotspot. If an ascending melt transports helium upwards this He (together with CO2 and water) can be incorporated en passant in colder host rocks.\nResponse. This modification was applied (line 189). Actually, we agree with this comment, but our objective in this sentence is reinforce that there are no high-T mantle xenoliths in this study.\nL. 230ff: This sentence is too long and should be shortened in two pieces at least.\nResponse. This suggestion was accepted, and the sentence was shortened (lines 329-333).\nL. 233: “ allow for”\nResponse. We are sorry, but we disagree with this suggestion. For this reason, we maintained this sentence without alteration.\nL. 237: “… geometry of the subducting slab…”\nResponse. This suggestion was applied (line 335).\nL. 243: “… peak of magmatism …”\nResponse. This suggestion was applied (line 350).\nL. 289: It is difficult to judge from the outside, but websterite could be a former magmatic vein or, alternatively, have a cumulate origin. To conclude, its presence supports a hybridization model, is at least ambivalent.\nResponse. Yes, in fact veins or cumulates are plausive explanations to the occurrence of pyroxenites, such as to our websterite (PM10-Q77). According with our interpretation, this lithology is a product of mantle hybridization related to the mixing between SiO -2 rich melts (i.e., adakite melts), depleted mantle, and sediments. This reaction (mixing) favors the selective consumption of clinopyroxene to form orthopyroxene-rich lithologies, which can be represented by a layer (or vein) in the SCLM. Of course, this process is valid to pyroxenes in general, which will result in pyroxenitic lithologies. Based on our tectonic model (Fig. 3c), this hybridization probably is coeval with the Gondwana fragmentation (Jurassic to Cretaceous).\nFigure 1: Write “ Antarctic Plate”, “Inferred Collision Zone”\nResponse. These adjusts were done. Thank you for indicating these errors!\nFigure 3: Might it be possible to mark the positions of the xenoliths in time by a number or “x” (or something similar) in c), d) and e) ?\nResponse. It would be extremely useful, but the indication of where the xenoliths from Cerro Cenque (PM10) were in time would be totally speculative. For this reason, we prefer does not indicate its position.\nSupplementary information:\nCorrections: Correction for atmospheric He is commonly done with the air He/Ne ratio, like here. This is of course a possible way to do it, but since the trapped atmospheric component not necessarily is equivalent to the air element ratio this is a bit random. It gives an order of magnitude of a possible interaction with air, but I am not sure if this is a well-defined correction. Nevermind, the extent of correction is negligible in these samples.\nResponse. In our opinion, some details always will confer some uncertainty to noble gas data. However, as mentioned by Prof. Hopp, we assume that the atmospheric contribution to helium ratios is negligible. Moreover, we would like to make it clear that our objective in this work is guarantee the quality of our results. For this reason, we verified the contribution of different components to our samples.\nMore complicated is the correction for cosmogenic 3He. The authors use the crush results as representative of a true mantle 3He/4He and then calculate the cosmogenic 3He for the heated samples. But this requires that the crushed helium data are not corrupted by cosmogenic helium in itself… Note that during crushing cosmogenic He is likely overrepresented in the fluid inclusions relative to Ne (see above), so I have doubt that a correction will correctly ever reproduce the real amount of cosmogenic helium in crushed samples. This is in this study crucial.\nResponse. As mentioned above, we understand that due to limitations imposed by the samples or even by the results, sometimes it is complicated to have the perfect condition to define the data. However, here we used the available tools to ensure that they are reliable. In our study, the effect of cosmogenic contribution was verified in stepwise crushing results because it is widely assumed that they undoubtably release the noble gases from fluid inclusions with minor contributions from the mineral matrix. In any case, we have decided to exclude from the discussion all high-3He/4He results that were not well-defined as pristine. Therefore, based on this significant reformulation, we will consider the estimation of cosmogenic contribution especially for those samples from Cerro Chenque that truly are pristine (PM10-Q77 WR and PM10-B2 Ol). Yet, it is assumed that the heating extraction imply the release of noble gases from both fluid inclusions and the mineral matrix. Although the helium data of sample PM10-Q77 WR shows the effect of cosmogenic contribution when analyzed by heating (3He/4He = 30.97  0.93 R ), subtracting its effect in our sample (15 %) and considering the 1 analytical uncertainties, we achieve a value of 25.50 R , which overlaps the result of the total crushing (26.28  0.19 R ). Regarding the heating experiments, note that in this case we are discussing the disaggregated (separated) minerals, not to rock powder after stepwise crushing.\nIn the supplementary information, paragraph L. 146ff, the authors calculate cosmogenic 3He concentrations. I can’t follow this. Their given 3He(cos) is about 2 x 10E-10 ccm STP in PM10-B2ol, but total measured 3He is about 6 x E-13 ccm STP/g. The weight is about 280 mg, so a factor of ca. 4 is still needed to make the numbers comparable (sample 3He is ca. 1.5 x E-13 ccm STP), but the discrepancy is three orders of magnitude… In turn, to get a high 3He/4He-value (from an asthenospheric base value of 7 Ra) requires only addition of an absolute 3He(cos) concentration of ca. 8 x 10E-14. Which is not really much. Looking at Ne is not really helpful, because the 3He/21Ne-ratio in inclusions might differ from production ratio. And their given value of 0.001 seems to support this (however, I am not sure if this value has any relevance if the 3He(cos) is already not correct). Note, that in their given example Ne blank correction is reported to be 198%., which doesn’t facilitate a comparison. Is it possible that the authors forgot to include an exponent in their calculations (in Suppl Table 2 at least 3He is given in E-6 in the value fields, but with E-8 for overall columns, which gives E-14…). Looking at their 3He/21Ne(cos) ratios in heated samples with a denoted 70-100% cosmogenic component provides values of 8-12, whereas in the crushing samples the ratio is quite small. I guess, most Ne is in fact non-cosmogenic… which would fit my expectations and explains the ca. air composition in most samples.\nResponse. We completely understand the arguments of Prof. Hopp, but we would like to clarify some mistakes from our part that leading to an erroneous interpretation. For instance, the expressed results in Supplementary Table 2, as well as in the Supplementary Information are incorrect (confused) because we indicated exponent values, but the results were not adjusted. We corrected this mistake in the updated version of both files (text and table), and we would like to apologize for it. Using the example above, the correct value of [3He ] for PM10-B2 Ol is 5.9510-14, whereas the [3He ] is sample cosmogenic 1.9010-18. Therefore, the trapped 3He concentration in this sample is four orders of magnitude higher than in the cosmogenic. In this case, the calculated cosmogenic contribution is 0.0032 %. The [3He ] for each crushing measurement of sample PM10-Q77 WR varies from 4.7910-14 to 3.2910-14 while the [3He ] varies from cosmogenic 6.1210-16 to 1.4210-15. Consequently, the estimated cosmogenic contribution is from 2.04 to 4.43 %, where the first extraction (100 strokes) does not demonstrate the effect of cosmic ray irradiation. For calculations, we are assuming the single crushing 3He/4He value (23.02 R ) to sample PM10-B2 Ol, and the total crushing 3He/4He (26.42 R ) value A A to sample PM10-Q77 WR as representative of mantle. In our opinion, this assumption is reliable due to the several arguments presented above.\nConcerning the application of cosmogenic corrections to neon isotopes, we agree that there is no reason to use this approach, especially because these results are strongly affected by the blank or mass interference (40Ar++ on 20Ne+ and CO ++ on 22Ne+). The few 2 samples that contain analytical quality enough to be considered usually do not differ from the atmospheric composition.\nConsidering the suggestions given by the three reviewers, it was necessary to modify some parts of the main text, as well as the Supplementary Information. These adjusts are indicated as follow:\nMain text\nLine 33. We replaced “Cenozoic” by “Eocene” due to the comments given by Reviewer #4. After them, we can ensure the existence of a pristine component with high helium only for samples from Cerro Chenque (PM10).\nLines 37-41. Based on the suggestion given by Reviewer #4, we decided to emphasize the influence of the thermomechanical erosion promoted by the small-scale mantle convection to the pristine 3He observed in mantle xenoliths from Cerro Chenque (PM10).\nLine 90. Based on the exclusion of the samples with some suspicious pristine 3He/4He ratios, we modified the information about the variation of these ratios.\nLine 110. The information related to the high 3He/4He ratio of Estancia Alvarez (PM7) was deleted based on the considerations made by Reviewer #4.\nLines 120-139. The text regarding the extraction methods employed in this work was adjusted based on the considerations made by Reviewer #4.\nLines 142-150. We added the information needed to corroborate the existence of a pristine component recorded in mantle xenoliths PM10-Q77 WR and PM10-B2 Ol 2000x.\nLines 153-156. The order of these sentences was inverted to make the text clearer.\nLine 172. It was necessary to correct the interval of values of high-3He/4He ratios.\nLine 174. The information about mantle xenoliths with high-3He/4He ratios from Cerro de los Chenques (PM12) and Estancia Alvarez (PM7) were removed due to the considerations made by Reviewer #4.\nLines 184-186. As suggested by Reviewer #1, we inserted the lithosphere-asthenosphere boundary (LAB) in the Figure 3. The results reported by Priestley et al. (2019) reinforce the existence of a shallow asthenosphere beneath the inferred collision zone. In this case, also beneath Cerro Chenque (PM10).\nLines 210-211. As a consequence of the modifications suggested by Reviewer #4, we kept only the temperature estimate of sample PM10-Q77. As a consequence, we removed the other estimates, as well as their references.\nLines 227-230. As mentioned by Reviewer #4, we informed that our relevant samples (with high 3He/4He) have low helium concentrations and based on this characteristic, we decided to perform additional measurements to verify the possible influence of a cosmogenic component.\nLine 240. We removed the information regarding the high-3He/4He ratio of sample PM7-B1 (from Estancia Alvarez).\nLine 242-243. Here we updated the interval of values from samples with high-3He/4He ratios.\nLine 270. We decided to delete the word “some”.\nLines 297-298. The information about the thickness of the lithosphere was added to the text, as suggested by Reviewer #1.\nLines 343-344. We decided to invert the order of the tectonic events based on the opinion of Reviewer #4.\nLine 374. The information regarding samples PM10-Q109 and PM12-17 were deleted due to the comments given by Reviewer #4.\nCaption of Figure 1. The interval of values of high-3He/4He ratios was updated.\nCaption of Figure 2. We adjusted the text to be clearer.\nCaption of Figure 3. We realized that the indication of (a), (b), (c), (d) and (e) was incorrect. Therefore, our mistake was corrected.\nCaption of Supplementary Fig. 1. We inserted the information about the plot of samples analyzed by single and stepwise crushing experiments containing doubtable high-3He/4He.\nCaption of Supplementary Fig. 2. As suggested by Reviewer #2, we inserted the information about the low-velocity anomaly, which represent the connection between a slab tearing widespread along Central-North Patagonia and the slab-window generated through the collision and subduction of the Chile Ridge. Moreover, we updated this figure, as well as this caption according with the suggestions given by Reviewer #1.\nSupplementary Information\nLine 45. We removed the indication about the number of samples studied here due to the ambiguity between measurements and samples.\nLines 54-59. It was necessary to adjust this part due to the suggestions given by Reviewer #4.\nLine 76. We corrected this value after the careful verification of data.\nLine 100. We corrected the values after the careful verification of data.\nLine 103. We corrected this value after the careful verification of data.\nLine 133-212. The topic “Cosmogenic component” was completely reformulated based on the suggestions given by Reviewer #4. Furthermore, the citation about the publication of Staudacher et al. (1993) was deleted, whereas Jalowitzki et al. (2016) was added.\nIn summary, based on the compelling evidence presented above, we cordially request that our manuscript be considered for publication in the Nature Communications. We definitely believe that the results obtained from samples PM10-Q77 WR and PM10-B2 Ol genuinely reflect a pristine component and that our interpretation is well-supported by them.\nPlease let us know if you require any further clarification or additional information. We believe that addressing these crucial points will strengthen the significance of our findings.\nThank you for considering our work for publication.\nSincerely,\nTiago"}, {"round": 2, "reviewer_comments": {"reviewer_1": "It appears that the authors have addressed the reviewers' suggestions and clarified the interpretation in the context of the data and previous understanding of regional tectonic events for the given time period. The geodynamic interpretation in the discussion and addition of Figure 3 was much needed in the first version of this manuscript, and is a beneficial addition. There are a few figures that would benefit from additional edits as previously suggested. However, based on the changes I would recommend the revised manuscript for publication.", "reviewer_2": "My comments, corrections, and suggestions are indicated in the attached PDF file.\nGood luck!", "reviewer_4": "Review by Jens Hopp of manuscript “Shallow asthenosphere with pristine helium from the superdeep mantle beneath Patagonia” by Jalowitzki et al., submitted to Nature Communications.\nThe thematic issue of this manuscript is identifying a superdeep (>300 km) mantle component by its elevated 3He/4He-ratio in mantle-derived rocks from Patagonia. For this purpose high 3He/4He-ratios found in mantle xenoliths from various locations of various age (but predominantly Pliocene) in Patagonia are interpreted within a geotectonic reference frame encompassing the last hundreds of millions of years. If the postulated scenario is true, this would be the first evidence for a superdeep wet plume.\nIn order to link a superdeep component to the Patagonian magmatism in time the authors propose (1) a geodynamic model for the geologic evolution of the respective crust-mantle evolution, (2) the presence of water in the transition zone that is responsible for formation of hydrous minerals observed in the mantle xenoliths and (3) a link of the helium isotope record to the postulated upwelling of superdeep material, which in turn link the water with the transition zone. To manage this the authors have to propose a quite controversial topic, namely the presence of significant volumes of water within the transition zone. But this cannot be proven by the data (a sponge might be dry, no matter of its storage capabilities). The only what the authors can prove is that water plays a role in formation of hydrous minerals without any inference of its origin. The helium could have other sources as well. Other explanations might also work, in particular the introduction of water by dehydration of an oceanic slab. This situation – hydrous minerals and subduction – is frequent and even might lead to a coexistence of primary minerals (from a “dry” mantle region and “wet” slab fluids). Another objection here is the expected lateral extension of this upwelling component – this is not visible in a 2D figure. From the authors geodynamic model a line of volcanic units might be characterized by this superdeep signature (its not a “hotspot” sensu strictu) – is there any evidence for this?\nMy main objection of this topic is not the water itself but the presented He data, which are the basement of their conclusions – no high 3He/4He, no superdeep component. The authors made a lot of analyses, also of heated complementary samples to their crushed specimens. A lot of work, I appreciate that. Their heating He data show a clear presence of cosmogenic He in the mineral lattices – if not, the signature is typical lithospheric helium. There are some studies showing that such cosmogenic He is not relevant if a stepwise crushing method is applied for gas extraction. But this is not the entire truth. Clearly, He diffusion might redistribute He from lattice sites into fluid inclusions, in particular in old samples like the Cerro Chenque xenoliths, and in addition there is a certain probability to redistribute the helium via recoil into fluid inclusions. Commonly, small amounts of cosmogenic He are hardly recognized within an “ocean” of mantle helium. But the mantle xenoliths investigated for this study are quite low in gas concentrations. A given range of 4He of <10-9 ccm STP/g is equivalent of about E-14 ccm STP/g 3He. To account for the reported high 3He/4He-ratios requires not really much cosmogenic He. For example, the highest values observed during crushing of samples from the PM12 suite (single strokes) from Cerro de los Chenques (is this the same as Cerro Chenques?) are associated with lowest 4He concentrations, i.e. these samples are most prone to contamination with cosmogenic helium. I would not build a quite substantial model of geochemistry upon samples which are suspect of containg a minor, but in the end significant, cosmogenic helium component. Other examples of stepwise crushing show increasing 3He/4He with increasing number of strokes, e.g., both series of PM10. Sample PM 7 shows within errors a constant ratio of about 6.2 Ra, i.e. as soon as we enter the field of lithospheric signatures no systematic increase is apparent. But for PM7 4He concentrations are one order of magnitude larger, as is the case for PM4 with asthenospheric 3He/4He. From all samples in this study only one extraction is a potential candidate for a mantle helium signature, PM7-B1 Ol 2000x (single crush). In this case 4He concentrations are higher and 3He/4He is about 14. Which is peculiar given the non-existence of such a component in the stepwise crushing experiment. And sorry, one single – only possibly – correct mantle value is not enough to constrain a superdeep wet plume model. Note, that in the latter sample no clear evidence of cosmogenic neon is visible (I would not necessarily expect that anyway).\nProvided the He is truely of mantle origin (which I doubt): It could be also possible that the Eocene Cerro Chenque volcanics might be related to an enrichment in 3He within the local lithosphere during the earlier rifting process, documented in Fig 3c, but unrelated to formation of hydrous phases from subduction related fluids.\nAs probably is clear from above I unfortunately cannot really support publication in its current form though I like the general geodynamic model. But the data appear not robust enough to support the conclusions presented by the authors.\nDetailed comments:\nL. 150: “… a pristine helium composition …” or “…pristine helium compositions…”\nSame paragraph: I don’t think that the existence of low T xenoliths excludes a high T hotspot. If an ascending melt transports helium upwards this He (together with CO2 and water) can be incorporated en passant in colder host rocks.\nL. 230ff: This sentence is too long and should be shortened in two pieces at least.\nL. 233: “ allow for”\nL. 237: “… geometry of the subducting slab…”\nL. 243: “… peak of magmatism …”\nL. 289: It is difficult to judge from the outside, but websterite could be a former magmatic vein or, alternatively, have a cumulate origin. To conclude, its presence supports a hybridization model, is at least ambivalent.\nFigure 1: Write “ Antarctic Plate”, “Inferred Collision Zone”\nFigure 3: Might it be possible to mark the positions of the xenoliths in time by a number or “x” (or something similar) in c), d) and e) ?\nSupplementary information:\nCorrections: Correction for atmospheric He is commonly done with the air He/Ne ratio, like here. This is of course a possible way to do it, but since the trapped atmospheric component not necessarily is equivalent to the air element ratio this is a bit random. It gives an order of magnitude of a possible interaction with air, but I am not sure if this is a well-defined correction. Nevermind, the extent of correction is negligible in these samples.\nMore complicated is the correction for cosmogenic 3He. The authors use the crush results as representative of a true mantle 3He/4He and then calculate the cosmogenic 3He for the heated samples. But this requires that the crushed helium data are not corrupted by cosmogenic helium in itself… Note that during crushing cosmogenic He is likely overrepresented in the fluid inclusions relative to Ne (see above), so I have doubt that a correction will correctly ever reproduce the real amount of cosmogenic helium in crushed samples. This is in this study crucial.\nIn the supplementary information, paragraph L. 146ff, the authors calculate cosmogenic 3He concentrations. I can’t follow this. Their given 3He(cos) is about 2 x 10E-10 ccm STP in PM10-B2ol, but total measured 3He is about 6 x E-13 ccm STP/g. The weight is about 280 mg, so a factor of ca. 4 is still needed to make the numbers comparable (sample 3He is ca. 1.5 x E-13 ccm STP), but the discrepancy is three orders of magnitude… In turn, to get a high 3He/4He-value (from an asthenospheric base value of 7 Ra) requires only addition of an absolute 3He(cos) concentration of ca. 8 x 10E-14. Which is not really much. Looking at Ne is not really helpful, because the 3He/21Ne-ratio in inclusions might differ from production ratio. And their given value of 0.001 seems to support this (however, I am not sure if this value has any relevance if the 3He(cos) is already not correct). Note, that in their given example Ne blank correction is reported to be 198%., which doesn’t facilitate a comparison. Is it possible that the authors forgot to include an exponent in their calculations (in Suppl Table 2 at least 3He is given in E-6 in the value fields, but with E-8 for overall columns, which gives E-14…). Looking at their 3He/21Ne(cos) ratios in heated samples with a denoted 70-100% cosmogenic component provides values of 8-12, whereas in the crushing samples the ratio is quite small. I guess, most Ne is in fact non-cosmogenic… which would fit my expectations and explains the ca. air composition in most samples."}, "author_response": "Re-review by Jens Hopp of manuscript “Shallow asthenosphere with pristine helium from the superdeep mantle beneath Patagonia” by Jalowitzki et al., submitted to Nature Communications.\nAs this is a re-review I will keep my comments rather short. The authors have made substantial changes in detail, which covers most of my concerns listed in my first review. Two critical issues remain, one is the interpretation of the helium data, the second is related to the source of the pristine helium component within a tectonic frame. See details in the comments further down.\n1) New data added by the authors and a calculation by myself shows that in case of sample PM-Q77 a pristine helium signature might have be retained, though I still have some skepticism on this topic. There is surely some cosmogenic helium admixed, the question is how much. In case of doubt (I cannot prove that either) I would leave this issue to the reader and accept this interpretation.\n2) Establishing of such a local pristine component of presumably deep mantle origin then leads to the question within which tectonic frame this helium was transported to the surface. The age precludes their straightforward interpretation of a wet plume transporting pristine helium through a slab window, because the age is younger then the inferred Eocene age. On the other hand, an alternative wet plume further back in time (Mesozoic) is not really convincing to me as it is a different tectonic situation, with a large-scale introduction of volatiles into the asthenosheric mantle. And some Nd-Sr isotopic data are also not in favor of a deep component present in Eocene magmatism. However, I am no expert in paleotectonics. It seems rather difficult to explain the findings within wet plume scenarios, maybe other alternatives (simple hotspot source?) could fit as well? Because of the complex situation I would suggest the wet plume scenario as a suggestion, which might be of broader interest and could induce further discussion. Therefore, I would recommend publication. But the authors should make some minor changes in describing their wet plume hypothesis with more emphasis on how in Eocene times this could have been possible, and discuss the pitfalls of their hypothesis.\nDetailed comments:\nI still have some reservations in interpretation of the high 3He/4He as pristine mantle component. But I also have to admit, that neither prove or disprove is successful here, as long as neon isotopes of sufficient quality are missing to clarify this – and this is something outside the influence of the authors, they did their best. I made a rough calculation of cosmogenic 3He/21Ne-ratios based on their existing neon isotope data. The heated samples listed at the end of their suppl. table 2 all have significant contributions of cosmogenic gases, with 3He/21Ne(cos) in a range between 2-17 (mostly below 10). This is a rather expected range, but suffers from precision of 21Ne(cos). Taking the most clearly extractions showing elevated 21Ne/22Ne and lowered 20Ne/22Ne gives a 3He/21Ne(cos) of about 8. Now, in turn, with this value I tried to calculate a potential corrected 3He/4He ratio of the most important sample PM10-Q77 and find lowered 3He/4He-ratios (14-22 RA), but still elevated in respect to MORB. As such, a pristine component in this specific xenolith cannot be ruled out definitely. The authors also added some more information by analyzing the powder residue by heating of two samples including Q77, with the latter showing no difference to the crushing extractions before. This may in fact indicate a trapped component (or a ubiquitous cosmogenic component…). Nonetheless, the authors should state explicitly in their manuscript that in this particular xenolith cosmogenic helium could still account for some, but not the whole 3He budget, making a pristine component still more likely, to make clear that they have cared for that.\nIn this context I would like to point out some errors again in their Suppl. Table 2. Their columns AO and AP are identical though telling the reader it should be 22Ne/21Ne and 21Ne/20Ne. In addition, their calculation of cosmogenic 21Ne in column AR is wrong. They calculated the excess 21Ne as 20Ne*(21Ne/22Ne – air 21Ne/20Ne). For simplicity, you simply could calculate 21Ne – 20Ne*air 21Ne/20Ne. Please check all your columns again for some more possibly undetected errors (I haven`t found more, but who knows). I also find their method to calculate cosmogenic 3He in part questionable, as they partly define the trapped component by their crushing result. This would be true if it is proven that during crushing NEVER cosmogenic 3He can be released – but this is not necessarily the case. Hence, a potential crushing extraction with notable cosmogenic 3He would never be characterized as such. Commonly, I would try to assess a cosmogenic contribution as worst case scenario, e.g., taking the local SCLM-values as they did for other samples.\nL. 398-400: I am lost here … You argue here that upwelling of a wet plume with FOZO signature occurred during the Oligocene-Miocene. On the other hand, the FOZO type signature is observed in older Eocene xenoliths and actually you were mentioning this explicitly before, in L. 265-269, but not in the Fig. 4c. This is a crucial gap in your hypothesis. If a wet plume is responsible for high 3He/4He (supply of deep component) and this happened earlier, you need to adjust your model in Fig. 4. Otherwise, the wet plume is not responsible for high 3He/4He here. Then we have to face the question, which other possibilities are plausible? After your statement in L. 269 you added “Thus, the origin of the pristine helium must be older, probably during Paleozoic and Mesozoic. In this context, …” and now follows a short introduction to the capability of Wadsleyite / Ringwoodite for carrying water. But this is not related to the fundamental question, how this pristine helium arrived at Eocene age on the surface? In your Fig. 4c, a broad upward cycling of volatiles during edge driven convection could be the major source of helium but this refers to a Mesozoic situation. Do you believe, He is stored somewhere in the asthenosphere for about 100 Ma years, mixed with MORB-type helium and then very locally is remobilized in a new tectonic situation? Seems rather unrealistic, and in particular this FOZO component would have distributed in the asthensphere quite on a large scale, if looking at your Fig 4c. After Navarrete et al. 2020 the Paleocene-Eocene magmatic rocks have Nd-Sr isotope ratios more similar to depleted mantle (asthenospheric) which would fit an asthenospheric upwelling in a slab window during break-up of the Aluk slab. I am not an expert in paleotectonics, but I think an interested reader might find this interpretation contradictory.\nOther points:\nL.158-159: “… crust-mantle boundary has ca. 40 km, revealing extremely thin lithospheric mantle with ca. 10 km thick …” I don’t understand this. Is 40km the depth of MOHO or a thickness? Similarly, you likely mean 10 km thickness?\nL. 212: “primordial components” Be aware that your 3He in SCLM and MORB is also primordial… Since you have defined FOZO as having >10Ra you may choose this term instead of primordial here.\nL. 232: You may delete “Similarly” here.\nL. 252: “…also was allowing the rise of …”\nL. 264: Instead “plausive” write “plausible”.\nL. 279: “ …in an eroded …”\nL. 317-322: This sentence is too long and complicated. Make more sentences out of it."}, {"round": 3, "reviewer_comments": {"reviewer_4": "This is now my third review of this manuscript and apparently the authors optimized their manuscript quite satisfactory according to my previous suggestions. With these changes this improved manuscript seems ready for publication, with very minor issues mentioned below.\nI found no errors in the main text now, but some minor issues in caption/legend to Fig 4a-e: In Fig 4c your header shows \"Late Triassic-Early Jurassic\" but in the caption you write \"...triggered by the opening of the Golfo San Jorge Rift Basin during the Jurassic-Cretaceeous\". Please, adjust accordingly. By the way, the header gives an age range 200-170 Ma, whereas the next two Figs d and e have ages of 190 Ma and 180 Ma. This does also not fit really. Maybe only state 200 Ma in Fig 4c? Check the ages carefully.\nIn Line 343: Write \"flat-slab\" instead \"flat-lab\".\nLine 346: write \"steep\" instead of \"step\", and \"sink\" instead of \"sinking\".\nI have recognized that you have removed wadsleyite and ringwoodite from the manuscript (also the term \"wet plume\"), but still you have included the phases in Fig.4. Though these minerals may stay there (it is not wrong) there is of course no connection to the text anymore - maybe you could remove them (I leave that to the authors)?\nIn general, Fig 4 is now much better displaying the authors basic ideas as before."}, "author_response": "REVIEWERS' COMMENTS\nReviewer #4 (Remarks to the Author):\nThis is now my third review of this manuscript and apparently the authors optimized their manuscript quite satisfactory according to my previous suggestions. With these changes this improved manuscript seems ready for publication, with very minor issues mentioned below.\nResponse. After three rounds of revision, we are very grateful to Prof. Hopp due to his dedication and commitment to improve our work. This final version certainly is a product of a great joint effort between the authors and reviewers!\nI found no errors in the main text now, but some minor issues in caption/legend to Fig 4a-e. In Fig 4c your header shows \"Late Triassic-Early Jurassic\" but in the caption you write \"...triggered by the opening of the Golfo San Jorge Rift Basin during the Jurassic-Cretaceeous\". Please, adjust accordingly. By the way, the header gives an age range 200-170 Ma, whereas the next two Figs d and e have ages of 190 Ma and 180 Ma. This does also not fit really. Maybe only state 200 Ma in Fig 4c? Check the ages carefully.\nResponse. We have mentioned Jurassic-Cretaceous in the caption of Figure 4c because it is the interval of time defined to the opening of the Golfo San Jorge Rift Basin (Renda et al., 2019). Concerning the information inserted in the header of this figure, we assumed a shorter interval (Late Triassic-Early Jurassic) to the lithospheric hybridization promoted by the edge-driving convection mechanism (SCLM erosion). Regarding the sequence of ages, we have tried to detail it as accurately as possible. However, in order to avoid misinterpretations for the readers, we have decided to adjust it maintaining only Early Jurassic (~200 Ma) in both caption and header. Thank you for this suggestion!\nRenda et al. (2019) - https://doi.org/10.1016/j.tecto.2019.228232\nIn Line 343. Write \"flat-slab\" instead \"flat-lab\".\nResponse. Thank you for pointing out this error, which has been corrected!\nLine 346. write \"steep\" instead of \"step\", and \"sink\" instead of \"sinking\".\nResponse. These modifications were applied.\nI have recognized that you have removed wadsleyite and ringwoodite from the manuscript (also the term \"wet plume\"), but still you have included the phases in Fig.4. Though these minerals may stay there (it is not wrong) there is of course no connection to the text anymore - maybe you could remove them (I leave that to the authors)?\nResponse. We agree with this consideration and, therefore, we have decided to remove “ringwoodite” and “wadsleyite” from Figure 4.\nIn general, Fig 4 is now much better displaying the authors basic ideas as before. Response. Thank you!"}]

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