Synchronous motion of the Easter mantle plume and the East Pacific Rise

Domain: earth_science
Published: 2024-11-17
Editorial outcome: minor_revision

Abstract
The Easter mantle plume has produced one of the longest hotspot tracks in the Pacific Ocean. While previous studies have focused on the eastern side extending across the Nazca Plate, we use 40Ar/39Ar isotopic and geochemical data to investigate the less explored western side around the Easter Microplate. We propose a dynamic model in which a deeper (600 km-depth), less buoyant mantle exerts a westward force on the East Pacific Rise (EPR), while a more buoyant plume region drives Easter hotspot volcanism and a localised acceleration in seafloor spreading. Our findings suggest that the Easter hotspot is the more focused surface expression of the most buoyant region of a vast, deep-seated mantle plume extending from the Pacific Large Low Shear Velocity Province (LLSVP). This challenges the traditional view of hotspots as isolated phenomena and suggests they are part of broader LLSVP-related mantle structures. Our results imply a more intricate, large-scale relationship between hotspots, mantle plumes, spreading ridges, and mantle dynamics.

Introduction
The numerous volcanic ridges and seamounts scattered across the Pacific seafloor offer valuable insights into the interactions between hotspots and the lithosphere-asthenosphere system (e.g. 1–3). Furthermore, volcanism associated with near-ridge hotspots provides a useful framework for examining the dynamics between mantle plumes and spreading ridges.

Notably, the Easter Island (aka Rapa Nui) region, situated east of the Easter Microplate on the Nazca Plate and adjacent to the East Pacific Rise (EPR) (Figs. 1, 2), is linked to a deep, lower mantle plume origin (e.g. 1, 4–7). Easter stands out as a rare high buoyancy, high helium (3He/4He), PREvalent MAntle (PREMA) type plume that closely adheres to the classic plume model, similar to counterparts such as Louisville, Kerguelen, Reunion, and Tristan 1, 8. The hotspot track generated by the Easter plume spans thousands of kilometres across the Nazca and Pacific plates, comparable in scale only to the Louisville Seamount Chain in the southeast Pacific.

The Easter hotspot generated the Salas y Gómez Ridge (SYGR), which extends eastward across the Nazca Plate (Fig. 1). To the west of the Easter hotspot, the Easter microplate evolved in two tectonic stages 9–11. From ~5 Ma and until about 2.5 Ma the East Rift propagated northward across the Easter-Salas hotspot, transferring and rotating lithosphere of the Nazca Plate clockwise. The second stage, beginning around 2.5 Ma, involved a slowdown in the northward propagation and continued curved opening of the East Rift, a southeastward propagation of the SW Rift, and the rigid rotation of transferred Nazca lithosphere 9, 10.

Published 40Ar/39Ar dates for basaltic rock samples dredged from the SYGR and along the Nazca Ridge are consistent with age-progressive volcanism currently originating near Salas y Gómez Island (~106°W) 12, 13. This agrees with the observed higher Pb and Sr and lower Nd isotopic ratios (i.e., ‘enrichment’) in basalts from Salas y Gómez Island and adjacent seamounts, compared to lavas from volcanoes further west, including Easter Island, and the East Rift of the Easter microplate 14–16. The decreasing Sr-Nd-Pb ‘enrichment’ closer to the East Rift is attributed to the deflection of plume mantle toward the East Rift and progressive melt extraction of enriched mantle components 14–18. Alternatively, it may result from plume-ridge mixing and the effects of decreasing lithosphere thickness 19–22.

Numerous multidisciplinary studies have investigated the Easter hotspot track and its association with the EPR. While these studies have generated considerable findings, developing cohesive models that integrates surface observations with deep mantle processes remains a significant challenge. Previous research predominantly focused on the Nazca side of the EPR. Our study bridges this gap by exploring volcanic ridges and seamounts extending from the Pacific plate across the southwest margin of the Easter microplate (Fig. 2). Our primary goal is to provide new isotopic dates and geochemical analyses for this complex network, aiming to clarify the influence of the Easter plume on microplate development, the SYGR, and the asymmetric spreading history of the slowly migrating EPR (as expressed by the uniquely low ratio of lateral ridge migration relative to the very fast total spreading of the EPR) 23, 24. By synthesising findings from various studies, we aim to construct a model that enhances our understanding of the interactions between plumes from the Large Low-Shear-Velocity Province (LLSVP), surface intraplate volcanism, and spreading ridge migration.

Results
During the RV SONNE 80a-Midplate III expedition (Valparaiso-Easter Island 25), basaltic rocks were dredged from seamounts and volcanic elongate ridges (VERs) located on the Pacific Plate and the southwestern side of the Easter Microplate (Fig. 2) 26, 27. The VERs range from 70 to 200 km in length, in width from 20 to 35 km, and have elevations ranging from 1100 to 2900 meters. Their morphological characteristics resemble the ‘cross-grain’ type ridges observed in other Pacific regions, which have been associated with plate-wide tensile stress 3, 26, 28, 29. In this study, we present primarily new isotopic dates and geochemical analyses from these VERs (black dots in Fig. 2) and combine these with existing data (open circles).

Predominantly trending in an ENE direction, the VERs display a right-stepping en echelon pattern with an E-W regional trend, resulting in an ~400-kilometre offset of local magnetic anomalies 9, 10, 26–28, 30. This study focuses on four of the previously described VERs. The southwestern ends of the VERs are marked by seamounts, notably Crough and Thomas, located at the ends of VER 5 and VER 4, respectively 26. The northeastern ends intersect with the lithosphere formed at the SW Rift of the microplate (Fig. 2). Further details on these volcanic edifices can be found in refs. 25, 26, 28.

Isotopic Dating
Analytical methods and a discussion of the data quality are detailed in the “methods” section. Summary and completed data files can be found in Supplementary Data 1 and Supplementary Data 2, respectively. Supplementary Data 3 provides a summary the 40Ar/39Ar incremental heating isotopic dates. The calculations for 40Ar/39Ar dates are presented in Supplementary Data 4. The age-distance trends defined by new dates for the southwest margin of the Easter microplate and published isotopic dates for the SYGR are shown in Fig. 3. Notably, samples from volcanic edifices on the Pacific Plate and the microplate lithosphere (SW Rift) exhibit contrasting age progressions. The ‘Pacific trend’ (red symbols in Fig. 3) is characterized by dates obtained from the southern ends of the VERs. This trend corresponds to a propagation rate of ~65 mm/yr. In contrast, the ‘microplate trend’ (blue symbols in Fig. 3) is constrained by dates for samples from the northern end of VER 1, inner and outer pseudofaults of the SW Rift, and from a small ridge or large seamount associated with the ambiguously defined triple junction region in the southern part of the East Rift 9. This trend indicates a propagation rate of ~192 mm/year, likely reflecting the fast propagation of the SW Rift, and a start of volcanic activity on this ridge, as evidenced by samples from VER 1 prior to 3.5 Ma and continuing at its southern end until about 2.7 Ma.

Trace Element and Isotope Composition
Analytical methods are in the “methods” section. Trace element, and Sr-Nd-Pb isotopic ratios are provided in Supplementary Data 5. Lavas from the ‘Pacific’ trend exhibits more ‘enriched’ trace element compositions, indicated by higher Nb/Zr and La/Sm ratios compared to NMORB (Fig. 4), and higher 87Sr/86Sr ratios (Fig. 5). In contrast, ‘Microplate’ trend lavas are less enriched, showing lower Nb/Zr and La/Sm ratios compared to normal MORB (NMORB) (Fig. 4). Furthermore, trace element ratios span a spectrum from depleted MORB (D-MORB) to enriched MORB (E-MORB) compositions. However, the Pb isotope ratios do not show a marked difference between the ‘Pacific’ and ‘Microplate’ trends (Fig. 5), except that samples from Crough Seamount (DS67, DS68), located at the southern end of VER 5, display the highest 206Pb/204Pb ratios.

A comparison of 40Ar/39Ar isotopic dates with seafloor anomaly ages (Fig. 6) indicates that the ‘Pacific’ trend lavas erupted ~170 km west of the nearest spreading axis, with the underlying crust being around 2 Ma at the time of eruption. Conversely, at the northeastern end of VER 1, associated with the ‘microplate’ trend, lavas erupted near the closest spreading axis, i.e., the eventual microplate SW Rift. The relatively depleted composition of these ‘microplate’ lavas correlates with their formation closer to or at the SW Rift of the microplate. As such, the microplate trend reflects reduced pressure and increased melting beneath thinner lithosphere, resulting in lower La/Sm and Nb/Zr ratios. In contrast, beneath the somewhat older and thicker Pacific lithosphere, the average extent of melting is lower, which may have induced preferential melting of more fertile, enriched mantle lithologies (e.g. 31, 32) that is resulting in higher La/Sm and Nb/Zr ratios. This distinction is particularly evident when comparing the enriched and depleted characteristics of the south and north ends of VER 1, respectively.

Lavas from the SYGR exhibit a distinct and tightly correlated dataset within the Pb-isotope space. Additionally, they demonstrate a linear Pb-Sr isotope relationship, consistent with a two-component mixing model 16. These Salas-type lavas display the highest radiogenic Pb isotope compositions, evidenced by 206Pb/204Pb ratios exceeding 19.6 (Fig. 5). The Easter-type composition (206Pb/204Pb > 19.0) extends from Easter Island westward to the microplate, where it overlaps in composition with MORB erupted at the East Rift at the same latitude as Easter Island. This alignment is also consistent with a spike in 206Pb/204Pb and 3He/4He ratios (RA > 11) observed in the East Rift from 28°S to 26°S (33; Fig. 5B).

This indicates that the ‘Pacific’ and ‘Microplate’ age-distance trends also have distinct melting processes, beneath the thicker lithosphere of the Pacific Plate and the thinner lithosphere of the SW Rift, as is evident in the data from VER 1. While Sr-Nd-Pb isotope ratios show minimal variation, they reveal a clear distinction between the more depleted hotspot lavas on the western (Pacific) side of the microplate/EPR and the more enriched lavas on the Nazca side, consistent with the well-documented two-component mixing process between the Easter plume and the microplate’s East Rift (e.g. 19).

Methods
40Ar/39Ar dating
As previously described in 2019 89 and 90 the groundmass samples were prepared following the methods of ref. 91. The 200–180 μm samples measured at Oregon State were cleaned in a series of hour-long acid baths, progressing from 1N HCl to 6N HCl to 1N HNO3 to 3N HNO3, followed by a final milli-Q water bath. Each separate was picked by hand under a binocular microscope to ensure the removal of alteration, and to confirm the purity of the separate. Plagioclase separates were further leached using 5% HF for 15 min.

Groundmass samples were irradiated for 6 h in the CLICIT position at the Oregon State University TRIGA reactor. Incremental heating experiments were conducted for each sample. Irradiated samples were loaded into copper planchettes for analysis using a Thermo Scientific ARGUS-VI multi-collector mass spectrometer at the OSU Argon Geochronology Laboratory following the procedure described in ref. 92.

All ages are calculated relative to Fish Canyon Tuff (FCT) sanidine with an age of 28.201 ± 0.023 Ma, 1σ 93 and using the decay constants after ref. 94 The correction factors for neutron interference reactions at the TRIGA are (2.70 ± 0.17% 1σ) × 10−4 for (36Ar/37Ar)Ca, (6.43 ± 0.92% 1σ) × 10-4 for (39Ar/37Ar)Ca, (1.21 ± 0.09% 1σ) × 10−2 for (38Ar/39Ar)K and (6.07 ± 9.65% 1σ) × 10−4 for (40Ar/39Ar)K. Ages were calculated using the ArArCALC v2.7.0 software of Koppers 95, with errors including uncertainties on the blank corrections, irradiation constants, J-curve, collector calibrations, mass fractionation, and the decay of 37Ar and 39Ar.

All ages are calculated using the corrected value for the original Steiger and Jäger’s 96 constant for total 41K decay to 41Ar with a new value of 5.463 ± 0.107 × 10−10/yr (2σ) as reported by ref. 94 Air standard used is 40Ar/36Ar = 298.56 ± 0.31 (0.104% SD) as reported in ref. 97 For all other constants used in the age calculations, we refer to Table 2 in ref. 98 Individual J-values for each sample are calculated by parabolic extrapolation of the measured flux gradient against irradiation height and typically give 0.1–0.3% uncertainties (1σ). Incremental heating plateau ages and isochron ages are calculated as either plateau or weighted mean with 1/σ2 as the weighting factor 99 and as YORK2 least-square fits with correlated errors 100 using the ArArCALC v2.7.0 software from Koppers 95 available from the following website http://earthref.org/ArArCALC/. The quality of a 40Ar/39Ar step-heating experiment is assessed based on the following criteria: an acceptable age plateau (1) includes at least 50% of the gas released, (2) a mean square weighted deviation (MSWD) of ~1.0 and within the statistically allowed upper limit, (3) shows an inverse isochron with a 40Ar/36Ar intercept of 298.56 ± 0.31 (0.104% SD, ref. 97), and (5) a p-value > 5.

The most robust dates 16 unique samples based on single or replicated analyses are indicated in Supplementary Data 3 and Supplementary Data 4. One of these dates (64DS-3) has less than 50% 39Ar (41%) so cannot be considered robust but rather an approximate age estimate. Dates for three plagioclase samples that are systematically lower than the corresponding groundmass age are rejected as these samples likely contain hydrothermally altered phenocrysts (ref. 25) consistent with plateau age suppression due to sericite alteration 101.

Trace elements
As previously described in 2019 89 ~0.05 g of sample was accurately weighed into a Teflon beaker and digested in 1 ml 15M HNO3 and 3 ml 12M HF for 12 h in sealed beakers on a hot plate at 80 °C. After cooling, 0.2 ml of HClO4 was added to the sample, and the solution evaporated to incipient dryness at 120 °C. 2 ml of 15M HNO3 was added to the sample, and evaporated to near dryness, and this step was repeated twice before increasing the hot plate temperature to 150 °C and fuming off excess HClO4. The sample was then redissolved in 4 ml 15M HNO3 and 4 ml H2O, 2 drops of 12M HF were added, and the sealed beakers were left on a hot plate at 80 °C for 12 h. The samples were then placed in an ultrasonic bath for 30 minutes, before heating at 80 °C for another 12 h. At this stage, all samples were completely in solution. The sample solutions were then quantitatively transferred to 250 ml HDPE bottles and diluted to 200 g with MQ water to obtain a final solution of 2% HNO3 + 0.002M HF with a sample dilution factor of about 4000 and total dissolved solids of 250 μg/ml. All reagents used were distilled in Teflon stills and diluted with MQ 18.2 Ohm water.

Trace element measurements were carried out at the GeoZentrum Nordbayern using a Thermo Scientific X-Series 2 quadrupole inductively coupled plasma mass spectrometer. Samples were introduced into the instrument through a Cetac Aridus 2 desolvating nebuliser system to reduce molecular interferences. An ESI SC-2DX FAST autosampler was used to reduce washout times between samples. The instrument was tuned using a 5 ppb solution of Be, In and U; the typical sensitivity for 238U was 2 × 106 counts per second for a sample uptake rate of 50 μl/min. The Ce/CeO ratio was typically >4500, and thus corrections for the interference of oxides of Ba and the light rare-earth elements on Eu and Gd were unnecessary. Before each measurement session, the instrument was calibrated using multielement solutions covering the relevant concentration range. A mixed Be, Rh, In and Bi solution (30, 10, 10, 5 ppb) was mixed with the sample online and these elements were used as internal standards to correct for instrumental drift. Procedural blanks analysed during this work were negligible for all elements measured. Trace element data for the rock standard BHVO-2 measured as an unknown are given in Supplementary Data 5.

Nd, Sr and Pb isotopic compositions
All samples (either glasses or aphyric basalts) were washed in 2N HCl and double quartz distilled water. Clean pieces of glass or basalts were handpicked and crushed in a boron carbide mortar. The powder was leached with 0.1N HCl for 10 minutes in an ultrasonic bath and rinsed in deionised water. Separations and analyses of Sr, Nd and Pb were performed from the same sample dissolution, according to the procedure described in ref. 102. Sr and Nd ratios were measured in dynamic mode on a TIMS Finnigan MAT 261. Results were corrected for mass fractionation to a value of 8.375209 for 88Sr/86Sr and 0.721903 for 146Nd/144Nd. NBS 987 and La Jolla standards during the period when the analyses were carried out had values of 0.71026 +/- 3 (n = 68) and 0.511852 +/- 10 (n = 34). Replicate analyses of the Pb isotope standard NBS 981 give an average of 16.908 +/- 3, 15.451 +/- 4, 36.579 +/- 15 (n = 34) for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb. Pb isotope data were corrected to the values given in ref. 103. with a fractionation factor of 0.0011 per mass unit.

Data availability
All the original 40Ar/39Ar and geochemical data generated in this study are available in the article Supplementary files.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I have now completed my review of the ms “Synchronous motion of the Easter mantle plume and the\nEast Pacific Rise”NCOMMS-23-61651.\nThis work is the result of extensive mapping, sampling and laboratory analysis of an important plate\nboundary in the eastern Pacific ocean by the PI’s. Their goal is to “see through” the filter of the entire\noceanic crustal magmatic system to discern movements of long term reservoirs in the underlying mantle.\nThis admixture of different mantle sources is the direct result of mantle convection, the history of the\nindividual portions of the mantle involved and the motions of the overlying plates.\nThis work devotes considerable effort to coupling plate motions with the isotopic and trace element\nevidence for the manifestation of different mantle sources. Their goal is to document the location of the\nplume over time and its isotopic evolution. Their key findings are that the Easter plume is linked in\nconvective space to the overlying East Pacific Rise spreading center, having caused a westward migration\nof the spreaidng center, while the plume itself has been fixed in space by the Pacific LLSVP underlying\nthe mantle in that region.\nAdditionally the authors deconstruct the detailed spreading history of of the EPR, noting that the\ntransfer of entire microplates from the pacific to the Nazca plate has forced a westward migration of the\nEPR, in concert with the changes in geochemistry over the last 20Ma.\nThe study is a sweeping reinterpretation of the tectonic history of the eastern Pacific, incorporating\nisotopic, trace element and geochronologic evidence. As such it is certainly of sufficient impact to justify\npublication in Nature Communications. I feel that the data presented support the conclusions regarding\nthe tectonic and geochemical evolution and I recommend its publication with changes outlined below:\nI am confused by the structure of the paper. I’m used to seeing a conclusion section at the end of the\npaper that integrates the themes of the discussion and discusses their broader implications. Instead,\neach part of the discussion section seems to have its own summary paragraph. Lines 276, 383, 455, 532\nand 658 each begin “In summary”, but I get to the end of the substantive part of the paper and find… a\nmethods section? And no conclusions? This is a big paper, with a lot of different kinds of very in-depth\ndata and analysis, but it feels like five (or more) separate papers. Restructuring to integrate the whole\nwith its parts is badly needed here.\nI recall a debate some years ago about ridge centered hotspots: whether ridges capture plumes or\nplumes capture ridges. It would seems from their evidence that here it’s the latter – isn’t this the sort of\nglobal process they were setting out to figure out? I couldn’t find where this was discussed in the\nimmensity of the document.\nFigures 6 and 7 are missing axis labels and units.\nFigure 8 should have a north arrow, or some other indication that these data are in map view.", "reviewer_2": "There is really a lot in this paper! There is a lot of data and interpretations that took me some very\ndetailed reading to properly digest. The different data threads however are drawn together in a logical\nfashion that together to form a cohesive model for the evolution of this entire region. I really enjoyed\nreading this manuscript and feel that it makes very valuable contributions.\nIn particular, I really like the detailed plate tectonic reconstruction and seismic tomography analysis that\nare used to clearly explain the formation and relationship among all the seafloor features in this region,\nincluding the apparent discrepancy in Nazca plate motions, in terms of the interaction between the\nplume and the evolution of the spreading ridge and microplates.\nI have very few comments on the science presented in this paper and would be very happy to see it\npublished with the text largely as is. My only real comments relate to the details of the figures, which\ncould do with some tidying up around font sizes, labelling, colours, and consistency between the figure\nand the caption. These are detailed below along with a couple of minor points on the text.\n----------------------------------------------\nAll figures: many/most labels are all too small to read easily, even when zooming in on the pdf.\nAdmittedly I’m getting on and likely need reading glasses but I could not read the figure labels in the\nversion I printed and had to zoom a lot on screen to solve the problem.\nFigure 3: what are the values ~64.6 (in red) and 191.6 (in blue) referring to? Are these also mm/yr? The\ncaption text refers to small open symbols but I cannot see these. Are the small orange squares referred\nto as small red boxes in the caption? If yes, then there’s a mismatch between part A where these are red\ndots and part B where they are orange squares. There are three different red symbols used, might be\neasier with additional colours? Also, might be good to have exactly the same colours and symbols for all\nthe features between part A and B of this figure?\nFigure 4: Is the grey rectangle shading the VER1 sites? If yes, maybe move the label? (better in Figure 5,\nwhich is where I figured it out).\nFigure 6: I find it disconcerting that part B above part A. Missing axes labels on part B.\nFigure 9: cannot see red disks? Orange maybe?\nFigure 10: Would be great to have the green dots that are on the cut-through views also on the map\nview. Could the plate boundaries shown on the map view seismic velocity maps also be shown on the\nmap view for easy reference. 19-12°S mentioned in text but latitudes unreadable on map view even very\nzoomed in.\nLine 50-51: Cannot see Easter Island on Figure 1. Add reference to Figure 2 also. Or mark on Fig 1.\nLine 50 vs Line 64: Two slightly different suggested locations for the Easter plume. Might be good to\nclarify where these come from.\nLine 92: briefly detail the SO voyage?\nLine 258: shifting position of Easter Island. I think it’s the shifting position of Easter Island relative to the\nEaster plume. Maybe remind readers here that Easter Island has ages from 3Ma?\nLine 261: it would be good to see this working out somewhere.\nLine 266: Where did the 170 km come from?\nLine 430 – maybe this could be a separate section?\nLine 547: Title of this section is a bit confusing as I can’t find the two-stage model being already\nintroduced explicitly. It is briefly mentioned on line 451, but maybe a quick (re-) introduction here would\nbe handy."}, "author_response": "Point-by-point response to the reviewers’ comments, reproduced verbatim\nResponses to reviewers\nReviewer #1 (Remarks to the Author):\nThis work is the result of extensive mapping, sampling and laboratory analysis of an important\nplate boundary in the eastern Pacific ocean by the PI’s. Their goal is to “see through” the filter\nof the entire oceanic crustal magmatic system to discern movements of long term reservoirs in\nthe underlying mantle. This admixture of different mantle sources is the direct result of mantle\nconvection, the history of the individual portions of the mantle involved and the motions of the\noverlying plates.\nThis work devotes considerable effort to coupling plate motions with the isotopic and trace\nelement evidence for the manifestation of different mantle sources. Their goal is to document\nthe location of the plume over time and its isotopic evolution. Their key findings are that the\nEaster plume is linked in convective space to the overlying East Pacific Rise spreading center,\nhaving caused a westward migration of the spreading center, while the plume itself has been\nfixed in space by the Pacific LLSVP underlying the mantle in that region.\nAdditionally the authors deconstruct the detailed spreading history of the EPR, noting that the\ntransfer of entire microplates from the pacific to the Nazca plate has forced a westward\nmigration of the EPR, in concert with the changes in geochemistry over the last 20Ma.\nThe study is a sweeping reinterpretation of the tectonic history of the eastern Pacific,\nincorporating isotopic, trace element and geochronologic evidence. As such it is certainly of\nsufficient impact to justify publication in Nature Communications. I feel that the data presented\nsupport the conclusions regarding the tectonic and geochemical evolution and I recommend its\npublication with changes outlined below:\nWe greatly appreciate the reviewer's insightful summary of our work. We have carefully\nincorporated his feedback into our revised manuscript to enhance both clarity and depth. This\nintegration has not only enhanced our discussion but also significantly strengthened our\nconclusions.\nI am confused by the structure of the paper. I’m used to seeing a conclusion section at the end\nof the paper that integrates the themes of the discussion and discusses their broader\nimplications. Instead, each part of the discussion section seems to have its own summary\nparagraph. Lines 276, 383, 455, 532 and 658 each begin “In summary”, but I get to the end of\nthe substantive part of the paper and find… a methods section? And no conclusions? This is a\nbig paper, with a lot of different kinds of very in-depth data and analysis, but it feels like five (or\nmore) separate papers. Restructuring to integrate the whole with its parts is badly needed here.\nWe strongly agree with the reviewer’s suggestion regarding the need for better integration of the\nvarious sections of our manuscript. Recognizing the complexity of the content, we have\nundertaken substantial revisions to enhance the coherence and flow throughout.\nSpecifically, we have revised the abstract and made significant additions across the manuscript,\nparticularly in the introduction, to ensure a cohesive presentation of our research. Additionally, we\nhave introduced a new 'Summary & Conclusions' section. These revisions aim to seamlessly\nintegrate the various parts of the manuscript and provide a clear, continuous narrative that is\nclosely linked to the data presented.\nWe believe these revisions much improve the manuscript’s readability and effectively convey the\nsignificance of our findings.\nI recall a debate some years ago about ridge centered hotspots: whether ridges capture plumes\nor plumes capture ridges. It would seems from their evidence that here it’s the latter – isn’t this\nthe sort of global process they were setting out to figure out? I couldn’t find where this was\ndiscussed in the immensity of the document.\nWe appreciate the reviewer highlighting this crucial aspect of our study. In response, we have\nexpanded on this point in the final paragraph of the revised manuscript to ensure that it is\nprominently addressed within the broader implications of our work. We also mention now in the\nabstract that the plume controls the location of the EPR.\nFigures 6 and 7 are missing axis labels and units.\nFixed\nFigure 8 should have a north arrow, or some other indication that these data are in map view.\nDone\nReviewer #2 (Remarks to the Author):\nThere is really a lot in this paper! There is a lot of data and interpretations that took me some\nvery detailed reading to properly digest. The different data threads however are drawn together\nin a logical fashion that together to form a cohesive model for the evolution of this entire region.\nI really enjoyed reading this manuscript and feel that it makes very valuable contributions.\nIn particular, I really like the detailed plate tectonic reconstruction and seismic tomography\nanalysis that are used to clearly explain the formation and relationship among all the seafloor\nfeatures in this region, including the apparent discrepancy in Nazca plate motions, in terms of\nthe interaction between the plume and the evolution of the spreading ridge and microplates.\nWe are very grateful for this reviewer’s insightful summary of our work, which has helped us to\nclarify and enhance our presentation.\nI have very few comments on the science presented in this paper and would be very happy to see\nit published with the text largely as is. My only real comments relate to the details of the figures,\nwhich could do with some tidying up around font sizes, labelling, colours, and consistency\nbetween the figure and the caption. These are detailed below along with a couple of minor points\non the text.\nThe reviewer’s comments have proven invaluable in enhancing the clarity and effectiveness of our\nfigures, which now better support the arguments underpinning our conclusions. We have made\nsignificant modifications to font sizes, labeling, color schemes, and consistency between the\nfigures and their captions along with adding more labels. Additionally, we have incorporated\nchanges to the text as suggested.\nAll figures: many/most labels are all too small to read easily, even when zooming in on the\npdf. Admittedly I’m getting on and likely need reading glasses but I could not read the figure\nlabels in the version I printed and had to zoom a lot on screen to solve the problem.\nWe have enlarged the figure labels as much as possible to enhance readability. Additionally,\nshould our manuscript be accepted, we will ask during the production phase that the final figures\nare as large as possible to further optimize their impact and clarity for readers.\nFigure 3:\nwhat are the values ~64.6 (in red) and 191.6 (in blue) referring to? Are these also mm/yr?\nWe have added mm/yr to these numbers on the figure and revised the caption as follows:\n“The Pacific trend (64.6 mm/yr; solid red line and diamonds….”\n“The microplate trend (191.6 mm/yr; diagonal solid blue line)…”\nThe caption text refers to small open symbols but I cannot see these.\nThank you for identifying this error. We have removed the sentence in question from the revised\ntext.\nAre the small orange squares referred to as small red boxes in the caption? If yes, there is\na mismatch between part A where these are red dots and part B where they are orange\nsquares.\nThank you for your observation regarding the small orange squares. In the revised version, we\nhave changed these indicators to yellow, which also aligns better with the Easter-type isotopic\ncompositions shown in Figure 5.\nThere are three different red symbols used, might be easier with additional colours? Also,\nmight be good to have exactly the same colours and symbols for all the features between part A\nand B of this figure?\nVery good point. Here is the revised section of the relevant section of the caption.\n\"The small yellow boxes represent published 40Ar/39Ar dates for submarine volcanism at Umu,\nMoai, and Easter Island (O'Connor et al., 1995).\"\nFigure 4: Is the grey rectangle shading the VER1 sites? If yes, maybe move the label? (better\nin Figure 5, which is where I figured it out).\nDone. We have also added and arrow for further clarity\nFigure 6: I find it disconcerting that part B above part A. Missing axes labels on part B.\nPart A is now above part B and labels have been added to the axes labels (also in Figure 7).We\nhave also added location labels to the map view to improve clarity.\nFigure 9: cannot see red disks? Orange maybe?\nWe’ve clarified in the caption that the large red disks are show with 42% transparency to show\nunderlying seafloor structure.\nFigure 10:\nWould be great to have the green dots that are on the cut-through views also on the map view.\nDone\nCould the plate boundaries shown on the map view seismic velocity maps also be shown on the\nmap view for easy reference.\nDone\n19-12°S mentioned in text but latitudes unreadable on map view even very zoomed in.\nFixed\nLine 50-51: Cannot see Easter Island on Figure 1. Add reference to Figure 2 also. Or mark\non Fig 1.\nFixed\nLine 50 vs Line 64: Two slightly different suggested locations for the Easter plume. Might be\ngood to clarify where these come from.\nVery good point. We have revised the relevant text to read as follows:\n“Notably, the Easter Island (aka Rapa Nui) region, situated east of the Easter Microplate on the\nNazca Plate and adjacent to the East Pacific Rise (EPR) (Figs. 1 & 2), is linked to a deep, lower\nmantle plume origin (e.g., Courtillot et al. 2003; Montelli et al., 2006; French & Romanowicz,\n2015; Hosseini et al., 2020; Koppers et al. 2021).”\n“Published 40Ar/39Ar dates for basaltic rock samples dredged from the Salas y Gómez Ridge\n(SYGR) and the Nazca Ridge are consistent with age-progressive volcanism currently originating\nfrom near Salas y Gómez Island (approximately 106°W) (Ray et al., 2012; O’Connor et al., 1995).”\nLine 92: briefly detail the SO voyage?\nWe have added the following to the revised text:\n“During the RV SONNE 80a-Midplate III expedition (Valparaiso - Easter Island, Stoffers et al.,\n1992)… ”\nStoffers et al, 1992 refers to the cruise report, which is available online.\nLine 258: shifting position of Easter Island. I think it’s the shifting position of Easter Island\nrelative to the Easter plume.\nThe revised sentence now reads as follows:\n“We have also employed magnetic anomaly data for the Easter microplate (Fig. 7) to establish a\ncorrelation between the region-wide synchronous westward (~92 mm/yr) movement of the EPR\nand the Easter plume and the evolution of the Easter microplate (Fig. 7).”\nMaybe remind readers here that Easter Island has ages from 3Ma?\nLooking again at the Vezzoli & Acocella (2009) compilation of K-Ar ages for Easter Island we\nhave added the following text to the Figure 3 caption:\n\" Additionally, a compilation of published K-Ar ages indicates that the three shield volcanoes on\nEaster Island—Poike, Rano Kau, and Terevaka—underwent two phases of activity between 0.78\nand 0.3 Ma and 0.24 and 0.11 Ma (Vezzoli & Acocella (2009). These phases are marked by vertical\nblack lines. Three K-Ar ages ranging from 3 to 1.89 Ma are considered unreliable, as discussed in\nVezzoli & Acocella (2009).\"\nLine 261: it would be good to see this working out somewhere.\nLine 266: Where did the 170 km come from?\nWe have address both these issues by revising the main text as follows:\n“A comparison of 40Ar/39Ar isotopic dates with seafloor anomaly ages (Fig. 6) indicates that the\n‘Pacific’ trend lavas erupted approximately 170 km west of the nearest spreading axis, with the\nunderlying crust being around 2 Ma at the time of eruption.”\nFurthermore, we have added text to the Figure 6 caption as follows:\n“ The 40Ar/39Ar dates defining the ‘Pacific’ trend (red diamonds and black dashed line) align with\nthe migration of the EPR (West Rift) crust (circle symbols and black regression line). The distance\nbetween these trends implies that the ‘Pacific’ trend volcanism erupted about 170 km west of the\nEPR (West Rift).”\nLine 430 – maybe this could be a separate section?\nWe appreciate the valuable suggestion. We have introduced a new and expanded section titled\n\"Absolute Plate Motion\" to further clarify and focus on this important aspect of our study.\nLine 547: Title of this section is a bit confusing as I can’t find the two-stage model being already\nintroduced explicitly. It is briefly mentioned on line 451, but maybe a quick (re-) introduction\nhere would be handy.\nThank you for the suggestion. We have incorporated a brief reintroduction at the beginning of the\nsection to provide readers with a concise overview, enhancing their understanding of the\nsubsequent content."}, {"round": 2, "reviewer_comments": {"reviewer_2": "I raised some concerns about the presentation of some figures and working out in the previous version\nof this manuscript. I feel that these concerns have been addressed in this revised version of the\nmanuscript and have no concerns about the the data and ability to support conclusions.\nIn addition, the text changes undertaken on the advice of the other reviewer has also improved the\npaper.\nVery happy to see this published."}, "author_response": "Point-by-point response to the reviewer 2’s comments on Version 2\nI raised some concerns about the presentation of some figures and working out in the previous\nversion of this manuscript. I feel that these concerns have been addressed in this revised\nversion of the manuscript and have no concerns about the data and ability to support\nconclusions.\nIn addition, the text changes undertaken on the advice of the other reviewer has also improved\nthe paper.\nVery happy to see this published.\nWe greatly appreciate the reviewer’s help in improving the presentation of the figures and\nrefining this revised version of the manuscript."}]

Decision letter