Boninite-like intraplate magmas from Manihiki Plateau require ultra-depleted and enriched source components

The Ontong Java and Manihiki oceanic plateaus are believed to have formed through high-degree melting of a mantle plume head. Boninite-like, low-Ti basement rocks at Manihiki, however, imply a more complex magma genesis compared with Ontong Java basement lavas that can be generated by ∼30% melting of a primitive mantle source. Here we show that the trace element and isotope compositions of low-Ti Manihiki rocks can best be explained by re-melting of an ultra-depleted source (possibly a common mantle component in the Ontong Java and Manihiki plume sources) re-enriched by ≤1% of an ocean-island-basalt-like melt component. Unlike boninites formed via hydrous flux melting of refractory mantle at subduction zones, these boninite-like intraplate rocks formed through adiabatic decompression melting of refractory plume material that has been metasomatized by ocean-island-basalt-like melts. Our results suggest that caution is required before assuming all Archaean boninites were formed in association with subduction processes.

L arge igneous provinces are formed over a relatively short time period (several million years) through massive volcanism related to pressure-release melting of upwelling mantle plume heads [1][2][3] . Some basement lavas from the Ontong Java (OJP), Manihiki and Hikurangi Plateaus, located in the Southwest Pacific Ocean (Fig. 1), have similar chemical compositions and ages (B118-125 Myr), consistent with their derivation from a common source through high degrees (B30%) of mantle melting [4][5][6][7] . It has been proposed that they represent fragments of a single 'super' plateau 8,9 and that this magmatic event, the largest in the Phanerozoic, covered B1% of Earth's surface 5 .
The primitive Kroenke and more evolved but isotopically identical and dominant Kwaimbaita basement lavas from OJP 6,10,11 are thought to be derived from a primitive, ancient, lower-mantle reservoir 12 . These lavas have flat, primitive-mantlenormalized incompatible-element patterns. The stratigraphically younger and minor Singgalo lavas 10,11 at OJP have geochemical characteristics similar to Manihiki high-Ti basement lavas 4 , for example, drilled at DSDP Site 317 (ref. 5). Owing to their possibly younger stratigraphic age, we do not consider these lavas further in this paper.
In contrast to OJP, a major part of the Manihiki Plateau is composed of relatively low-Ti, high-MgO rocks, dated at 118-125 Myr (refs 4,13), and related to the basement (main) phase of plateau formation similar to OJP Kroenke/Kwaimbaita lavas. These low-Ti rocks have variable contents of the most incompatible elements, positive relative Nb-Ta anomalies, and most of them have U-shaped patterns on primitive mantle-and mid-ocean-ridge basalt (MORB-)-normalized incompatible element diagrams 4,13 . The boninite-like, U-shaped patterns are unique and enigmatic for plume-derived intraplate magmas, indicating depletion and re-enrichment processes involved in their genesis. In previous studies, the origin of the low-Ti rocks was explained by high-degree melting of a mixed source containing depleted mantle wedge material and small amounts of subducted volcaniclastic sediments 13 and by extensive melting of a hybrid mantle source comprising FOZO-type peridotite and recycled oceanic crust 4 .
Low-Ti rocks/glasses (Timm et al., 2011)   Here we present geochemical data from rare, fresh glass samples and additional major element data from bulk rock samples from the Manihiki Plateau obtained by dredging during the R/V Sonne SO193 (ref. 4) and SO225 expeditions (Fig. 1). Glass samples from the SO193 cruise yielded ages of 122.9 ± 1.6 and 124.5 ± 1.5 Myr (ref. 4). The fresh glasses provide a unique opportunity for precise geochemical characterization of the parental Manihiki melts, not possible with phenocryst-bearing whole rock samples variably affected by seawater alteration. We show that the origin of the low-Ti Manihiki rocks is related to re-melting of an isotopically distinctive, highly depleted plume source beneath Manihiki and possibly the OJP, comprising small and variable amounts of a recycled HIMU (high time-integrated 238 U/ 204 Pb ratio)-like component.

Results
Composition of low-Ti glasses. Relics of fresh glasses were found in strongly palagonitized hyaloclastites and pillow-lava margins. The glasses contain very rare vesicles and fresh olivine microphenocrysts with inclusions of chromium-rich spinel. The glasses are tholeiitic (SiO 2 ¼ 51.1-52.4 wt%; Na 2 O þ K 2 O ¼ 1.5-1.8 wt%), characterized by very low TiO 2 (0.27-0.44 wt%) and relatively high MgO (9.0-9.8 wt%) contents ( Fig. 2a 16,19,20 . However, Cl and H 2 O in the Manihiki glasses exhibit strong positive correlations with incompatible trace elements, whose concentrations are very low in seawater (for example, Th, Nb, rare earth elements (REE)). Such correlations preclude significant H 2 O and Cl degassing from magmas before and during eruption and are unexpected if magmas assimilated variable amounts of seawater-derived components, typically brine 21 . Instead, the relative Cl and H 2 O enrichment in low-Ti glasses may reflect their source composition, which might be similar to the incompatible-element depleted sources of komatiites from the Abitibi Belt, Canada and Gorgona Island, eastern Pacific 22,23 .
The low-Ti Manihiki glass samples exhibit variable contents of highly incompatible elements and display U-shaped patterns on incompatible-element diagrams with concentrations normalized to N-MORB (Fig. 2c). U-shaped patterns are also particularly well expressed by the REE normalized to chondrites (Fig. 2d). The U-shaped patterns in Manihiki glasses are similar to those in high-Ca boninites from the Troodos ophiolite on Cyprus and the Tonga Arc, whereas Kroenke-type OJP rocks have relatively flat trace element patterns (Fig. 2c). The low-Ti Manihiki glasses are more depleted in moderately incompatible elements (Lu to Zr) than primitive Kroenke lavas, whereas more incompatible elements (for example, light rare earth elements (LREE), Nb, Ta, Th, U, Ba, Rb) in low-Ti Manihiki glasses range from being more depleted to more enriched (Fig. 2c). Both, low-Ti Manihiki and Kroenke/Kwaimbaita lavas show strong relative depletion in K relative to LREE and relative enrichment in Nb-Ta in the normalized incompatible-element patterns that distinguishes them from subduction-zone-related boninites from Troodos and Tonga, as does the lack of strong relative Pb enrichment of the low-Ti Manihiki glasses (Fig. 2c).
Isotope compositions of the low-Ti glass samples fall into two clusters forming positive correlations on the 87 Sr/ 86 Sr versus 143 Nd/ 144 Nd and the Pb isotope diagrams, and negative correlations on 206 Pb/ 204 Pb versus 143 Nd/ 144 Nd and 87 Sr/ 86 Sr isotope diagrams (Fig. 3, Supplementary Fig. 1). The most incompatible-element-depleted low-Ti tholeiites have isotopic compositions similar to enriched mid-ocean-ridge basalts (E-MORB; Fig. 3, Supplementary Fig. 1). Glasses with the strongest enrichment in the most incompatible elements have the most radiogenic Pb (Supplementary Table 1) and the least radiogenic Sr and Nd isotope signatures, and trend towards the compositions of some HIMU-like ocean-island basalts (OIBs).

Discussion
Manihiki low-Ti tholeiites share a number of characteristics with boninites from subduction-related settings, which are defined as rocks with 452 wt% SiO 2 , 48 wt% MgO and o0.5 wt% TiO 2 (ref. 14). Two glass samples, as well as a significant number of whole rock samples, fulfil the major element criteria for boninites in terms of SiO 2 , TiO 2 and MgO content (Fig. 2a,b, Supplementary Table 1, 2). In addition, both the low-Ti Manihiki rocks and boninites have U-shaped incompatible element patterns with low Sm/Yb but high La/Sm (Fig. 2c,d), and low Ti/V (o12) ratios. Based on high CaO/Al 2 O 3 ratios (0.83-0.88), the mafic low-Ti Manihiki tholeiites are similar to high-Caboninites 24 , such as, for example, those from the Tonga Arc 25 and the Troodos upper pillow lavas on Cyprus 26 , indicating their origin from a clinopyroxene-bearing, depleted lherzolite or harzburgite source 27 .
A major process for generating the characteristic compositional features of boninites is melting of highly depleted (through previous melt extraction) peridotitic mantle, triggered through addition of a hydrous slab-derived fluid or melt, enriched in incompatible elements 25 . The hydrous fluid/melt lowers the solidus temperature of the depleted peridotite, causing it to melt 24,28 . Several geochemical characteristics of the low-Ti Manihiki magmas, however, are not consistent with a subduction origin. There is no evidence of strong, selective enrichment in highly fluid-mobile elements (for example, Rb, Sr, U, Pb). Further, the strong enrichment in Nb and Ta compared with other incompatible elements is unusual for subductionrelated magmas (Fig. 2c). In addition, the Manihiki glasses have low H 2 O contents (0.13-0.25 wt%, see Supplementary Table 1 and  ref. 4). Finally, palaeo-reconstruction studies indicate that the plateau basement formed in an oceanic intraplate setting, possibly near a mid-ocean-ridge spreading centre, but far away from a subduction zone 8,29 .
In contrast to the enriched fluid or melt component in boninites, which is derived from the subducting slab, we show in the following discussion that the re-enrichment of the ultradepleted mantle source at Manihiki most likely occurred through addition of an enriched HIMU-like mantle plume component to the source. Unlike hydrous fluids involved in boninite petrogenesis, the small amounts (r1%, see below) of this HIMU mantle component probably did not have a major affect on the degree of mantle melting, and instead were mainly responsible for the variability in the concentrations of highly incompatible elements and Sr-Nd-Pb isotope ratios in low-Ti Manihiki magmas.
Variations of radiogenic isotope ratios ( Before we can determine the compositions of the depleted and enriched end members, we need to calculate the compositions of primary Manihiki magmas from the composition of the low-Ti glasses. We calculated the primary magma compositions using the PRIMELT2 model 31 Table 3).
Concentrations of incompatible elements in the primary magmas were then calculated by dilution of the trace element concentrations in glasses proportional to the amount of olivine added to the glass compositions.
To estimate the composition of the depleted source component, we first carried out forward modelling of pooled fractional melting, using a primitive mantle composition 32 and bulk partition coefficients for peridotite melting 33 to derive melts with similar concentrations of moderately incompatible elements, for example, heavy rare earth elements (HREE), to those in the calculated low-Ti primary melts (Fig. 4, Supplementary Fig. 4). The results of the forward modelling show that up to 15% fractional melting of fertile or moderately depleted mantle peridotite cannot explain the strong depletion of the Manihiki melts in moderately incompatible elements (Fig. 4a). The required depletion can only be achieved with multi-stage melting, when a significantly depleted source (through previous melting) is re-melted, and the magmas produced during the second stage of melting do not mix with magmas of the initial stage of melting, implying a time gap between these two melting events. The best-fit parameters of the two-stage melting scenario obtained by the least-squares fitting of the HREE concentrations in the modelled melts to the Manihiki primary magmas are B11% melting of fertile peridotite during the first stage and B9% melting of the residual mantle during the second stage. Modelling with more depleted sources than primitive mantle (for example, Depleted MORB Mantle (DMM) type source 33 ) results in decreasing the estimate for the total extent of melting. Taking into account the uncertainties in the physical parameters of melting and the initial source composition, the estimates in Fig. 4a only provide a possible (but not unique) origin for the depleted end member of the low-Ti Manihiki primary melts. Although the degrees of melting of the individual melting stages cannot be fully constrained, the conclusion about the necessity of two (or multi)-stage melting to explain the concentrations of moderately incompatible elements in primary Manihiki magmas is robust. In conclusion, the modelling results show that the depleted end member involved in the origin of the low-Ti Manihiki magmas could be highly depleted peridotite (that is, ultra-depleted mantle, UDM; Supplementary table 4) residual after B11% fractional melting and melt removal.
Re-melting of residual mantle peridotite (and subsequent re-enrichment in incompatible elements) points to a similar type of origin for the low-Ti intraplate rocks and subduction-related boninites 24 . Since the low-Ti rocks do not show any evidence for a subduction-related origin as discussed above, elevated temperature and further decompression melting related to the emplacement of a hot mantle plume head are likely to have driven the melting under the Manihiki Plateau rather than a hydrous fluid/melt. The overall lower SiO 2 content of the low-Ti tholeiites compared with boninites can also be explained through dry refractory lherzolite melting, since hydrous melting of refractory lherzolite would increase the contribution of orthopyroxene to the melt resulting in a higher SiO 2 melt content 28 .
A recent study described ultra-depleted melt inclusions in olivines from a Kroenke-type lava (Fig. 2a,b,d), which were proposed to originate from a previously unrecognized ultradepleted component in the OJP mantle source 16 Supplementary Fig. 1). Higher 208 Pb/ 204 Pb and more radiogenic 87 Sr/ 86 Sr compared with Pacific MORB (Fig. 3, Supplementary Fig. 1 The primitive-mantle-normalized incompatible element patterns of the most enriched low-Ti glasses show enrichment in the moderately to highly incompatible elements, relative enrichments in Nb and Ta and relative depletions in K, Pb and Zr-Hf, general characteristics of HIMU-type lavas (Fig. 4). Below we will test the source and melt mixing models to see which can best explain the compositions of the low-Ti melts.
First, we test the source-mixing model (Fig. 4b) using the composition of primitive (MgO410 wt%) HIMU-like lavas from the Rurutu Island, Austral archipelago, Central Pacific 34 , which have isotope compositions similar to the most LREE-enriched low-Ti rocks, to serve as the enriched end member. The model reproduces the general shape of the incompatible-element patterns and abundances of U, Nb, Ta and LREE in the primitive low-Ti Manihiki magmas well (Fig. 4b). Small discrepancies are however evident, primarily for fluid-mobile incompatible elements (Rb, Ba, U and Sr). A slightly better fit for the incompatible elements in the low-Ti Manihiki primary magmas can be achieved if the most enriched Macquarie Island ophiolite glasses 35 (Supplementary Fig. 4a), which also have HIMU-like incompatible element compositions but distinct (Pacific-MORB-like) isotopic compositions, are assumed for the enriched end member. The addition of B0.06 to 1% of the Rurutu or Macquarie enriched end members to the UDM source can reproduce the full range of low-Ti Manihiki incompatible-  Fig. 4b,c). The melt-mixing model, however, requires an order of magnitude more enriched melt (1-10%) to contribute to the Manihiki primary magmas than the sourcemixing model. The melt-mixing model was therefore rejected as less probable due to the absence of correlations between major and trace elements in Manihiki glasses, expected from the relatively large mass fraction of the enriched melt required to fit the trace-element patterns. The origin of the magmas can thus be qualitatively attributed to melting ultra-depleted mantle metasomatized by the enriched melt component, present either cryptically or as a different lithology. The possible isotopic composition of the enriched component was calculated by solving the general mixing equations for isotope ratios 30 to fit the estimated mass fraction of the enriched component in the UDM obtained from trace element modelling and assuming Sr, Nd and Pb concentrations in the enriched component equal to the Rurutu melt used for the mixing model (see Fig. 4b The enriched Manihiki end member has Sr-Nd-Pb isotopic ratios similar to lavas from sources with HIMU-type Ocean Island Basalt lavas from the Cook-Austral chain, such as those from Rurutu (young lavas) and Tubuai 34,36 at 120 Myr ago ( Fig. 3; Supplementary Fig. 1). Following the prevailing point of view, the HIMU-like enriched component may originate from recycled oceanic crust 37 .
On the basis of our new and previously published data, OJP and Manihiki Plateau, as well as possibly Hikurangi Plateau, were likely formed by melting of a compositionally heterogeneous plume source. The dominant plume component beneath the OJP and possibly the Hikurangi part of the original super-plateau (before breakup into individual plateau fragments) was the source of Kroenke/Kwaimbaita basement lavas 5,10,11 (Fig. 5), which possibly represented a nearly primitive mantle source from the lower mantle 12 . The UDM component might also be present in the OJP source, but thus far has never been recognized in whole rock samples, but only as melt inclusions in olivine from one OJP Kroenke-type rock 16 . The dominant plume component beneath Manihiki appears to be the UDM metasomatized by HIMU-like melts. Together the UDM (mantle from which melt has been previously extracted) and enriched HIMU-like (recycled ocean crust) end members may represent relatively young, subducted Indian-MORB-like oceanic lithosphere, which stalled in the transition zone. Such oceanic lithosphere could have been entrained and recycled by the OJP/Manihiki/Hikurangi plume head as it upwelled from the lower mantle. Incorporation of the dense subducted ocean lithosphere could also have served to reduce the buoyancy of the upwelling plume head, such that it did not cause major uplift of the seafloor 38 . This could explain why the OJP, Manihiki and Hikurangi Plateaus remained largely submarine throughout their history on the seafloor. A piece of this entrained oceanic crust (eclogite) may still be located beneath OJP as proposed in a recent study 39 to explain the fast shear wave velocity of 4.75 km s À 1 observed beneath OJP. Regardless of the origin of the UDM and HIMU-like components, the composition of the mantle source below OJP (Kroenke/Kwaimbaita with minor UDM) and Manihiki (mainly UDM with some HIMU-like component) was largely different (Fig. 5). In conclusion, our results indicate that melting of incompatible-element ultra-depleted and subsequently re-enriched mantle is possible on a large-scale in intraplate settings during the formation of large igneous provinces, especially under young, thin oceanic lithosphere.
The presence of boninites in Archaean rock formations has been cited as evidence that subduction, and thus plate tectonics, initiated in the Archaean 40 . Recently, comparison of rock sequences including boninites from a modern arc system (Izu-Bonin-Mariana) and an Archaean greenstone belt in Canada suggested that subduction of oceanic crust has been operating since the Hadean 40 . High degrees of alteration, however, are common in Archaean rocks, affecting in particular major and mobile trace elements. The results of our study show that rocks with affinities to boninites in terms of major and trace elements (Fig. 2) can also originate in intraplate settings. Such melts may have been more abundant in earlier Earth history due to a hotter geotherm or enhanced plume-related magmatism. Thus, it is important to distinguish Archaean boninites that formed in subduction versus intraplate settings. The principal difference between intraplate boninite-like and most subduction-related boninites may be whether the Nb-Ta anomaly is positive or negative and if Nb/U normalized to N-MORB is greater or less than one. For example, in the Abitibi greenstone belt of Canada, some low-Ti rocks 41 have trace element patterns very similar to those of the low-Ti Manihiki tholeiites ( Supplementary  Fig. 5). Thus, these Archaean sequences could possibly be plumerelated unlike greenstones from the Nuvvuagittuq supracrustal belt in northern Quebec 42 or Isua, Greenland 43 , which have arc-like boninite signatures with low Nb/La 40,42 . As a consequence, subduction-related Archaean boninites may not be as abundant as previously thought, placing some questions as to the exact initiation and on the extent of subduction, and thus of large-scale plate tectonics, in the early Earth. Future appraisal of Archaean mafic rocks may be able to exploit this distinction. X-ray fluorescence. Whole-rock samples (powders) were analysed for major elements by X-ray fluorescence spectrometry (XRF) on fused pellets using a Magix Pro PW 2540 XRF at the Institute of Mineralogy and Petrography at the University of Hamburg, Germany. International reference samples JGB-1, JB-3, JB-2, JA-3 and JG-3 provided by the Geological Survey of Japan (https:// gbank.gsj.jp/geostandards/gsj1mainj.html) 48 were measured along with the samples (Supplementary Table 6b).
Laser ablation inductively coupled plasma mass spectrometry. Trace element concentrations were determined by LA-ICP-MS using an Agilent 7500s quadrupole instrument coupled with a GeoLas Pro 193 nm excimer laser system at the Institute of Geosciences (IfG), Kiel University. Sample mounts were mounted in a twovolume ablation cell 49 , and the ablated sample aerosol was transported with helium as the carrier gas and mixed with argon prior to introduction into the spectrometer. Glass shards were analysed with 60-80 mm crater size, 10 J cm À 2 laser energy and 10 Hz laser frequency. The total analysis time for each single analysis was set to 80 s (20 s background and 60 s counting time during laser ablation). The measured intensities were converted to concentrations in the Glitter software by using 43 Ca as internal standard and Ca concentrations from EMPA. The SRM NIST612 standard 50 was used for initial calibration. Matrix correction was applied by using data for KL2-G reference glass 51 . Reference glasses GOR128-G and BCR-2G were analysed as unknown, and the data are listed in Supplementary Table 6c. Consistency between EMPA and LA-ICP-MS data was checked using Ti contents, which agreed within 10% rel. between the two techniques. Analytical precision for multiple analyses was typically better than 3% rel. for most elements. Further details of the setup and calibration strategy can be found in ref. 52. The LA-ICP-MS data in Supplementary Table 1 are average trace element concentrations obtained from three to five spots for every sample. In addition, whole-rock powder of the sample SO225DR24-10 was analysed for trace elements using an Agilent 7500cs ICP-MS instrument following acid digestion as described in ref. 53. The trace element data generated by laser ablation ICP-MS on glasses and solution ICP-MS on the bulk sample show excellent agreement on p.p.m. to sub-p.p.m. concentrations, except for elements easily affected in bulk rocks by seawater alteration (for example, Cs, Sb, Rb, U). Elements concentrated in olivine and chromium spinel (Ni, Cr) have systematically higher concentrations in the whole-rock solution ICP-MS data.
Secondary ion mass spectrometry. Concentrations of H 2 O were determined using the CAMECA IMS 1280 HR2 instrument at CRPG (Nancy, France), following the approach taken by ref. 54. The samples (previously analysed for major element contents by EMPA) were carefully re-polished to remove any residual carbon from the carbon-coating process (finishing with the 0.25 mm grain size Al 2 O 3 suspension), ultrasonically cleaned, removed from the epoxy mounts and then remounted by pressing them into indium metal. The samples were sputtered with a 10 kV 133 Cs þ primary beam with a beam current of 0.8-1.5 nA focused to 5-10 mm spots and rastered to 20 Â 20 mm. A normal-incidence electron flood gun was used to compensate for sample charge. A field aperture of B1,000 mm was used to eliminate secondary ion signal that may come from spot margins due to residual carbon surviving the cleaning process. A mass resolving power (MRP) of B5,000, enough to resolve 17 O from 16 OH and 29 SiH from 30 Si peaks, was applied. The 12 C À (6 s), 16 OH À (4), 27 Al À (2 s) and 30 Si À (2 s) ions were counted after 300 s pre-sputtering during 12 cycles. To establish the calibration curves, reference glasses (experimental glasses M9, M15 and M47 from the Institut für Mineralogie, Leibniz Universität Hannover 55 , as well as natural standard reference glasses ETNA II-6, ETNA II-7 and ALV 981 R23 (ref. 22 and references therein)) with CO 2 ¼ 0 À 365 p.p.m., H 2 O ¼ 0.0 À 1.5 wt.% and SiO 2 ¼ 48 À 50 wt.% were analysed. Olivine was also analysed together with the glasses to account for H 2 O and CO 2 background.
Thermal ionization mass spectrometry. Sr-Nd-Pb isotope analyses were carried out at GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany by thermal ionization mass spectrometry (TIMS) following the methods outlined in ref. 56. In brief, before sample dissolution in hot HF-HNO 3 , approximately 100-250 mg hand-picked glass (125-250 mm) and whole rock (250-500 mm) chips were leached in warm 2 N HCl at 70°C for 1 h and subsequently triple-rinsed in 18 MO water to minimize the effects of alteration and sample handling. Ion chromatography followed established standard procedures 57,58 [60][61][62][63][64] . Nd and Sr isotope ratios were measured on a Thermo Fisher TRITON TIMS. Within-run mass bias correction used 146 Nd/ 144 Nd ¼ 0.7219 and 86 Sr/ 88 Sr ¼ 0.1194. NBS987 and La Jolla reference material was measured five to six times each turret to obtain a normalization value relative to our preferred values of 87 Sr/ 86 Sr ¼ 0.710250 and 143 Nd/ 144 Nd ¼ 0.511850, respectively. The normalization value was then applied to the sample data of each turret. Over the course of the study, the long-term machine drift compensated external 2s uncertainties (2SD) are ± 0.000011 (n ¼ 25) for NBS987 and ± 0.000007 (n ¼ 33) for La Jolla. Within-run 2 s.e. errors are equal or smaller than 2 s.d. of the reference materials. Total chemistry blanks were 10 to 30 pg for Pb and below 150 pg for Sr and Nd and thus negligible. Replicate analyses by means of a second sample digest were carried out for SO225 ROV-3-6 whole-rock chips. While Sr and Nd isotope ratios are reproduced within 2 s.d. of the standards, only 207 Pb/ 204 Pb of the Pb isotope ratios matches this criteria. The limited reproducibility of Pb isotope ratios involving 206 Pb and 208 Pb is ascribed to the combined effects of (i) seawater interaction during low temperature alteration that can lead to a heterogeneous enrichment of uranium causing variable degrees of 206 Pb ingrowth over time and (ii) Pb removal at high temperatures leading to variable U/Pb and Th/Pb and thus 206 Pb and 208 Pb ingrowth. The slight variations caused by alteration, however, do not affect the overall scientific interpretations and conclusions derived from the Pb isotope data.
Data availability. The authors declare that all data supporting the findings of this study are available within the paper and its Supplementary Information files.