# The hottest lavas of the Phanerozoic and the survival of deep Archaean reservoirs

Jarek Trela, Esteban Gazel, Alexander Sobolev, Lowell Moore, Michael

Bizimis, Brian Jicha, Valentina Batanova

► To cite this version:

Jarek Trela, Esteban Gazel, Alexander Sobolev, Lowell Moore, Michael Bizimis, et al.. The hottest lavas of the Phanerozoic and the survival of deep Archaean reservoirs. Nature Geoscience, 2017, 10 (6), pp.451-456. 10.1038/NGEO2954. hal-04991425

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# The hottest lavas of the Phanerozoic and the survival of deep Archaean reservoirs

Jarek Trela<fnref n="1" />, Esteban Gazel<fnref n="1" />*, Alexander V. Sobolev²,³, Lowell Moore<fnref n="1" />, Michael Bizimis⁴, Brian Jicha⁵ and Valentina G. Batanova²,³

Large igneous provinces and some hotspot volcanoes are thought to form above thermochemical anomalies known as mantle plumes. Petrologic investigations that support this model suggest that plume-derived melts originated at high mantle temperatures (greater than 1,500 °C) relative to those generated at ambient mid-ocean ridge conditions (about 1,350 °C). Earth's mantle has also cooled appreciably during its history and the temperatures of modern mantle derived melts are substantially lower than those produced during the Archaean (2.5 to 4.0 billion years ago), as recorded by komatiites (greater than 1,700 °C). Here we use geochemical analyses of the Tortugal lava suite to show that these Galapagos-Plume-related lavas, which formed 89 million years ago, record mantle temperatures as high as Archaean komatiites and about 400 °C hotter than the modern ambient mantle. These results are also supported by highly magnesian olivine phenocrysts and Al-in-olivine crystallization temperatures of 1,570 ± 20 °C. As mantle plumes are chemically and thermally heterogeneous, we interpret these rocks as the result of melting the hot core of the plume head that produced the Caribbean large igneous province. Our results imply that a mantle reservoir as hot as those responsible for some Archaean lavas has survived eons of convection in the deep Earth and is still being tapped by mantle plumes.

Geochemical evidence from hotspot-related lavas offer crucial information about deep planetary recycling processes<fnref n="1" /> and petrological studies indicate that these melts are hotter than ambient mid-ocean ridge basalts (MORBs)², supporting a hot mantle plume origin. Recent tomographic³ studies advocate that mantle plumes that melted to form large igneous provinces (LIPs) and some ocean island basalts (OIBs) are closely associated with large low shear velocity provinces (LLSVPs), linking hotspots with the deep mantle and provide evidence of the geochemical and lithological heterogeneity of the lower mantle. Mantle plumes originating from the core–mantle boundary can preserve primitive geochemical compositions related to the early stages of planetary accretion that survived eons of crustal recycling, mantle degassing, and mixing of heterogeneous reservoirs⁴, thus providing the only ‘windows’ into the deep Earth’s composition and evolution.

Primary melts from plume-related lavas provide key information about melting conditions of their mantle sources. Particularly useful are the MgO and FeO contents of a peridotite-derived primary melt (a melt that has not crystallized any mineral phases), which can be used to estimate the thermobarometric conditions during melting⁵,⁶. The MgO content of a primary peridotite-derived melt positively correlates to initial melting temperature⁵. For example, primary MORBs contain about ~10–13.5 wt% MgO⁶,⁷ whereas LIPs are derived from primary melts with higher MgO (~12–20 wt%) contents⁵. Melting at extreme pressure and temperature (P–T; hot, deep) conditions is thought to have occurred frequently during the Archaean (2.5–4.0 billion years ago) when most ultramafic peridotite-derived lavas (known as komatiites) were produced. Petrological modelling yields high initial melting temperatures

(>1,700 °C, ~400 °C hotter than modern day upper ambient mantle temperatures) for samples from the classic komatiite localities (Fig. 1a). Although komatiites have been studied for decades, the melting processes responsible for their formation remain contested. The most widely accepted petrological model suggests that komatiites form via adiabatic decompression melting of relatively anhydrous, anomalously hot mantle plumes. The Gorgona Island case links komatiites with LIPs⁸⁻¹¹. Alternative hypotheses for the genesis of komatiites include fluxed hydrous melting in Archaean subduction zones (for example, refs 12,13), although these melts do not typically have trace element signatures indicative of a subduction origin. Nevertheless, independent of their origin, komatiites represent remnants of a younger Earth, when the temperatures of the ambient mantle and deep Earth reservoirs were substantially hotter¹⁴.

The lava flows on Gorgona Island remain the only true example of Phanerozoic (younger than 540 Ma) macroscopically spinifex-textured komatiites. Gorgona komatiites are interpreted to be related to the initial melting stages of the Galapagos mantle plume head at ~89 Ma that formed the Caribbean large igneous province (CLIP)¹⁵⁻¹⁷. Other occurrences of high-MgO (~27 wt%) microspinifex-textured lavas petrogenetically related to the initial melting of the Galapagos Plume (Supplementary Table 1) also exist in other locations; for example, the Curacao Lava Formation on the island of Curacao and the Tortugal Suite in Costa Rica¹⁸ (Fig. 1a). Tortugal olivines are unique relative to other plume-related terranes with respect to their high Mg-number (Mg#) (molar Mg/(Mg + Fe) × 100), which reach values up to 94.2 and systematically overlap those of Archaean komatiites (Fig. 1b). These olivines are phenocrysts, as evidenced by their high Ca,

<footnote n="1">¹Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA. ²Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, 38000 Grenoble, France. ³Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 ul. Kosygina, Moscow 119991, Russia. ⁴School of Earth, Ocean, and Environment, University of South Carolina, Columbia, South Carolina 29208, USA. ⁵Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA. *e-mail: egazel@vt.edu</footnote>
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**Figure 1** | Locations of classic komatiites, hotspot lavas, and the frequency distribution of their olivine Mg-numbers. **a.** Map highlighting key komatiite localities including Barberton, Abitibi, Pilbara, and Gorgona. Other high-MgO lavas discussed in this study include the Siberian Traps, Emeishan LIP, North Atlantic igneous province (NAIP), Baffin Island, and the Karoo LIP, as well as other LIPs and OIBs. A red star denotes the Tortugal Suite. EAR, East African rift; OJP, Ontong Java Plateau. **b.** Histogram comparing olivine Mg# for Archaean komatiites, high-MgO Phanerozoic lavas, and the Tortugal Suite. Note that Tortugal olivines have extremely high Mg# and their population overlaps that of Archaean komatiites. Olivine data from refs 21,22,24,25,40-42 and the Georoc database (http://georoc.mpch-mainz.gwdg.de/georoc).

Cr, Al, and Ti contents (relative to residual/lithospheric olivine), abundance of melt inclusions even in the most forsteritic olivine, and normal magmatic zoning from core to rim (more details in Supplementary Information). Petrologic modelling indicates that these phenocrysts crystallized from a hot, primary peridotite melt, and their Ni contents are consistent with crystallization from a melt with ~27 wt% MgO (Fig. 2a), indicative of high initial melting pressure and temperature¹⁹,²⁰.

## Estimating temperatures using olivine as a thermometer

Although the Mg# of an olivine crystal is related to the temperature at which it crystallized, diffusion and re-equilibration of Fe and Mg (in slowly cooled rocks) within the crystal lattice will alter the original crystallization temperature and skew results²¹. Therefore, a more robust way to estimate crystallization temperatures is via olivine-spinel thermometry and analysis of the Al and Cr contents in

each respective mineral phase²¹⁻²³. We used this method to estimate minimum crystallization temperatures for Tortugal and Curacao olivines in order to compare our results to other high-MgO terranes and LIPs (Fig. 2a,b). This thermometer is advantageous for two main reasons: the slow diffusion of Al and Cr preserves initial olivine-spinel crystallization temperatures; and the independence of this thermometer from source lithology composition. It is important to note that although Tortugal spinels have chromium contents higher than the calibrated range for the Aluminium (Al)-in-olivine thermometer, there is a strong correlation (using the available global data) between $K_d$ (Al₂O₃olivine/Al₂O₃spinel) and spinel chromium number (molar proportion of Cr/Cr+Al). Because the partition coefficient ($K_d$) increases proportionally (and with a statistical significance) with increasing spinel Cr#, then we can conclude that olivines in equilibrium with the most Cr-rich spinels also provide meaningful temperature results²⁴. The application of this
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**Figure 2** | Global olivine systematics and Al-in-olivine thermometry for komatiites and the hottest Phanerozoic olivines. **a**, Graph showing the relationship between Ni (ppm) and Mg#. The green curves are olivine liquid lines of descent from primary magmas having initial MgO contents between 20 and 30%. Tortugal olivine plot within the Barberton komatiite olivine field and are consistent with derivation from a primary liquid with 26–30 wt% MgO. MORB olivines are from the East Pacific Rise⁴¹. LLD, liquid lines of descent. **b**, Results from Al-in-olivine thermometry plotted against host olivine Mg#. Tortugal data filtered for the low-Ti, high-temperature group of olivines.

thermometer to olivine-spinel pairs of komatiites and low-calcium boninites also outside the calibrated range yield temperatures within the errors of calculated olivine-melt temperatures²⁵. Finally, a new experimental calibration of the Al-in-olivine-spinel thermometer up to 1550 °C and Cr# of spinel up to 0.95 (ref. 26) yields temperature estimates similar to ref. 21 within a ±20 °C uncertainty for Tortugal olivines. Tortugal high-Mg olivines contain abundant micro-inclusions (from 20 µm down to a few micrometres) of spinel, melt, and combined spinel and crystalized melt inclusions (images in Supplementary Information). This suggests that small spinel crystals already coexisted with melt during crystallization of high-Mg olivines, and further justifies the application of the Al-in-olivine geothermometer.

Tortugal olivine phenocrysts indicate the highest Mg# and highest olivine-spinel equilibration temperatures relative to modern Phanerozoic high-MgO intraplate occurrences (Figs 2b and 3). We estimated minimum crystallization temperatures of 1,570 ± 20 °C for the most primitive Tortugal olivine phenocrysts as an average (±2σ) for the hottest group of points in the high-temperature/low-Ti data group. Although spinel typically crystallizes at lower temperatures than olivine, our petrologic modelling suggests that the spinel liquidus shifts to higher temperatures with increasing pressure (more details in Supplementary Information). Even though our

models suggest cotectic olivine-spinel crystallization at lithospheric pressure (about 0.6 GPa), this pressure effect is insignificant in modifying the estimated Al-in-olivine temperatures²². The temperature range (up to ~150 °C) for the same olivine Mg# (~93) could be due to partial re-equilibration of olivine-spinel on cooling. For comparison with the new Tortugal data, the most primitive Gorgona and Curacao olivine from the CLIP (Mg# 88–92) record minimum crystallization temperatures between 1,200–1,450 °C and 1,200–1,350 °C, respectively. Other mantle plume and hotspot-related terranes (for example, Karoo, Emeishan, and Iceland) also record anomalously high crystallization temperatures relative to MORB olivine, yet no locality records temperatures as high as the Tortugal Suite (Fig. 3).

Also for comparison, the Abitibi komatiites record a maximum Al-in-olivine crystallization temperature of only 1,450 °C, possibly due to late spinel crystallization²⁵ (Figs 2b and 3). Fractional crystallization of olivine from a melt will result in progressively lower olivine-spinel equilibration temperatures; however, even in a narrow window of olivine Mg# (for example, 91–93) we observe a large temperature variation, ranging from 1,100 °C in MORB to 1,600 °C in our Tortugal samples. Thus, based on both the high Mg# and high olivine-spinel crystallization temperatures, we suggest that Tortugal melts derived via melting of a hot mantle plume. We propose that melting of the core of Galapagos Plume during the LIP event as the most viable geodynamic interpretation for the origin of the Tortugal ultramafic melts.

## Modelling primary magmas and melting conditions

To model the melting conditions that generated Tortugal lavas, we computed primary melt compositions by adding or subtracting olivine from whole rock compositions until reaching equilibrium with a mantle with an Mg# equal to 94.2 (maximum Mg# observed in the olivine from Tortugal). We determined a dry and reduced source (based on volatiles in the melt inclusions and on the stoichiometry of spinel compositions²⁷,²⁸, respectively) and estimated the average $Fe^{2+}/Fe^{total}$ of the melt in equilibrium with spinel to be 0.95. Sulfur concentrations measured in melt inclusions are also low (max 995 ppm S at the max measured H₂O content, see below in the discussion on volatiles) which also confirms a relatively reduced primary melt. Additionally, the V/Sc ratio of Tortugal picrites (5.42 ± 0.57, 1σ) are on the lower range relative to modern MORBs (6.74 ± 1.11, 1σ) and Archaean basalts (6.34 ± 0.62, 1σ), also supporting a reduced mantle source for these melts²⁹.

Our models yielded primary magma solutions with ~10.5 wt% FeO and ~27 wt% MgO (Fig. 4a), which suggests that these primary melts originated at depths >10 GPa and record mantle potential temperatures between 1,700 and 1,800 °C (Fig. 4b). These P–T estimates for Tortugal melts are well within the range for Archaean komatiites, and indicate melting pressures and temperatures that surpass those for the other Phanerozoic plume-related terranes (Fig. 4b). One important influence on the P–T melting conditions is the amount of volatiles present in the mantle source. To estimate the pre-eruptive volatile content of melts from Tortugal, we analysed olivine-hosted melt inclusions in phenocrysts with Mg#s up to 93. The maximum H₂O content from the suite of melt inclusions analysed is 0.24 wt% (Supplementary Information). Because olivine-hosted melt inclusions have the potential to lose water by diffusion of H⁺ through the host crystal, this represents a minimum estimate for the water content of the primary melt. Furthermore, we can also use the H₂O/Ce (ref. 30) and CO₂/Nb (ref. 31) ratios of Pacific intraplate lavas (200 and 1,029 ppm, respectively) with the Ce and Nb values from the hottest Tortugal sample (TO-080514-1; Supplementary Table 2A) as another proxy for volatile concentrations. Using this approach, we find that Tortugal melts could be consistent with a maximum of 0.4 wt% H₂O and 1.6 wt% CO₂. Addition
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**Figure 3** | Tortugal olivines record the hottest crystallization temperatures relative to high-MgO Phanerozoic melts and Archaean komatiites. One-way comparison of the results from the Al-in-olivine thermometer²¹. Olivines from the Tortugal terrane record the highest Al-in-olivine crystallization temperatures to date. Circles on the right side of the graph denote the result of analysis of variance (ANOVA) using a Student's t-test (STT) at the 95% confidence interval for each terrane/location, represented by the error bars. The radii of the circles are proportional to the standard error of the mean. Olivine trace element data is from refs 21,22,24,25,40–42 and this study. Emei., Emeeshan; Ice., Iceland.

of these volatiles would result in a ~50 °C decrease³² in the Tₚ calculation, lowering the Tₚ maxima from ~1,800 to 1,750 °C (still within the actual Tₚ uncertainty of ±50 °C; ref. 5). Therefore, our mantle potential temperatures, along with the high Al-in-olivine crystallization temperatures, support extremely hot, reduced, and relatively dry conditions for the generation of these lavas. An additional assessment of volatile effects on petrological modelling is presented in Supplementary Information.

## The preservation of deep primitive mantle reservoirs

When compared globally, there is internal consistency between estimated Al-in-olivine temperatures and petrologically modelled mantle potential temperatures for Phanerozoic primitive melts (Fig. 5a). We suggest that the anomalously hot Tortugal melts most likely originated via melting of the hot Galapagos Plume axis while Curacao, Gorgona and, other CLIP lavas probably originated in the adjacent areas of the plume at lower pressures due to plume capture by a mid-ocean ridge³³ (Fig. 5b). Therefore, it is likely that the source of Tortugal melts was significantly hotter and, due to this, the range of depths at which melting occurred was deeper relative to other high-MgO Phanerozoic localities (Fig. 5a,b).

High degrees of partial melting are expected at such extreme P–T conditions, producing melts depleted in trace element concentrations and resembling the composition of the mantle source. Interestingly, Tortugal melts display enriched trace element concentrations as well as radiogenic isotopic ratios. These melts also display a negative correlation between ¹⁴³Nd/¹⁴⁴Nd and TiO₂ and a positive correlation between ¹⁴³Nd/¹⁴⁴Nd and Gd/Yb that suggest mixing between two compositionally distinct melts. We used the correlations between TiO₂ concentrations and other rare earth elements to model the trace element concentrations of Tortugal primary melts as a function of TiO₂ (Supplementary Fig. 6). This modelling essentially provides possible solutions to the trace element concentrations of endmember melt compositions that mixed to produce the resultant Tortugal Suite. These relationships, paired with the trace element chemistry of Tortugal olivines, provide crucial information regarding mantle source compositions and the magmatic plumbing system of Tortugal. We propose that Tortugal olivines crystallized from two compositionally distinct parental melts: one low-Ti/high-temperature melt, from which most of the magnesium-rich olivines (Mg#>92) crystallized, and a

second high-Ti melt enriched in light rare earth elements (LREE) with a moderate temperature. It is possible that the former melt derived from high degrees of melting in the hottest part of the Galapagos plume and the latter by low degrees of melting in the less hot part of the plume. Most of the olivines crystallized from the low-Ti depleted melts (with TiO₂ < 0.5 wt%) at high temperatures and were subsequently mixed and transported along with the high-Ti/LREE-enriched melt. A fertile recycled oceanic crustal component¹ could be a plausible explanation for the enriched trace element characteristics of Tortugal melts; however, the minor and trace element signatures in Tortugal olivine phenocrysts, as well as the whole rock major element compositions, indicate crystallization from a dominant peridotite source (Fig. 2a). Melted recycled crust embedded within partially melted peridotite may explain the trace element and radiogenic isotope mixing lines (Supplementary Information). Thus, we suggest that this enriched component could be a separate entity within the peridotite matrix. The exact composition and lithology of the enriched component remains unclear, and can be further clarified by future geochemical investigations: for example, helium and osmium radiogenic isotopes, and non-traditional stable isotope systematics. What is evident is that Galapagos-related lavas record heterogeneous melt productivity in a secularly cooling plume³⁴ and may result from deep planetary crustal recycling processes and plume entrainment during ascent.

The Tortugal Suite represents an exceptional ultramafic Phanerozoic location. The extreme P–T melting conditions, high olivine crystallization temperatures, and enriched trace element and radiogenic isotope systematics stand as a tribute to the immense compositional and thermal variability of the deep Earth (for example, the LLSVPs) as sampled by mantle plumes. These regions may represent laterally heterogeneous primordial thermochemical piles mixed with recycled oceanic lithosphere³⁵,³⁶ that do not actively mix into the overlying lower mantle³⁶,³⁷. Geochemical investigations of Phanerozoic basalts from Baffin Island and the Ontong Java Plateau also show elevated ¹⁸²W/¹⁸⁴W ratios (µ¹⁸²W up to 50 ppm), implying that mantle plumes in the recent geologic past tapped ancient primordial reservoirs that escaped eons of convective mantle mixing⁴,³⁸. Considering that Tortugal melts preserve an Archaean mantle temperature, this suite offers a unique opportunity to study the reservoirs sampled by mantle plumes to
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**Figure 4** | Primary melt petrologic modelling of hot Phanerozoic lavas.

*a*, FeO–MgO projection showing primary melt compositions for high-MgO Phanerozoic melts and Archaean komatiites formed by accumulated fractional melting¹⁸. Grey diagonal lines represent equilibrium olivine compositions and the range of Archaean komatiite whole rock data is shown for comparison. *b*, Estimated temperatures and pressures of final melting. The coloured curves represent the maximum temperatures at which orthopyroxene (Opx), clinopyroxene (Cpx), and garnet (Gt) are stable during equilibrium melting of a fertile peridotite¹⁹. Tortugal primary melts fall within the estimated P–T range of Archaean komatiites¹⁴. We assume that melting ceased for Tortugal when all Cpx and Opx were exhausted. Primary melts for other terranes were estimated using whole rock samples from studies listed in Supplementary Table 2C.

answer key questions regarding the interconnectivity between deep mantle convection, the nature of LLVPs, and the thermochemical evolution of the early mantle.

A recent numerical study³⁹ suggested a 4.3-billion-year steady-state thermal scenario for the lowermost mantle. If plumes do in fact sample primordial LLSVP material that has not appreciably cooled over 4.3 billion years, then it stands to reason that even modern plumes can produce basaltic melts at similar P–T conditions as inferred during the Archaean if this reservoir can reach the surface without sufficiently interacting with cooler ambient upper mantle. Initially hot Phanerozoic plumes captured in spreading ridges during ascent may also mix with cooler upper ambient MORB mantle that would result in a significant decrease in the plumes temperature³³. The results from Tortugal (and other anomalously hot Phanerozoic locations) imply that if the mantle is still producing melts as hot as during Archaean time, then there must exist isolated reservoirs in the deep Earth that preserve Archaean temperatures (Fig. 5c). These hot, Archaean-like temperatures preserved at the base of the lower mantle may be sustained over time if the temperature conditions at the core–mantle boundary are buffered by the crystallization of the Earth’s core.

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**Figure 5** | Correlation between two independent thermometers suggests the preservation of deep, hot Archaean mantle reservoirs. *a*, Graph showing our estimated maximum mantle potential temperatures plotted against maximum Al-in-olivine crystallization temperatures. Error bars represent the uncertainty of both thermometers⁵,²¹. The Tortugal terrane records the highest T<sub>p</sub> and Al-in-olivine crystallization temperatures to date. Al-in-olivine temperature estimates include data from this study and refs 21,22,24,25,40,42. The North Atlantic igneous province (NAIP) has been separated into three locations in this figure, including Greenland, Baffin Island, and the British Palaeogene igneous province (BPIP).

*b*, Schematic representation for the genesis of Tortugal melts. Tortugal liquids originated via melting the hot, axial core of the plume under high-pressure conditions. Gorgona and Curacao probably melted at lower pressures due to plume capture by a mid-ocean ridge. *c*, Schematic representation showing the excess temperature of the deep mantle relative to modern ambient MORB upper mantle inspired by ref 36.

**Methods**

Methods, including statements of data availability and any associated accession codes and references, are available in the online version of this paper.
Received 9 December 2016; accepted 19 April 2017;
published online 22 May 2017

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Acknowledgements

This project was supported by NSF awards EAR-12019033 and EAR-1565614 to E.G. The participation of A.V.S. and V.G.B. in the experimental heating of melt inclusions and analyses of olivine and spinel was funded by the Russian Science Foundation grant number 14-17-00491 and Institut Universitaire de France (to A.V.S.).

S. Krasheninnikov and E. Asafov from Vernadsky Institute of Geochemistry performed heating and quenching of melt inclusions. We thank J. Heinonen and K. Putirka for their thorough review of our manuscript. Discussions with N. Arndt, C. Herzberg and P. Asimow improved this project and the final manuscript.

Author contributions

E.G. planned the project and led field and analytical efforts. E.G. and J.T. produced the new data, compiled and modelled the geochemical data, developed the ideas, and wrote the manuscript. L.M. prepared melt inclusion samples, collected volatile data, performed mass balance calculations, and contributed to the writing of the paper. B.J. performed Ar–Ar age dating and data analysis. A.S., M.B. and V.B., collaborated on the project with data collection, analysis, and the development of ideas.

Additional information

Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Correspondence and requests for materials should be addressed to E.G.

Competing financial interests

The authors declare no competing financial interests.
## Methods

At the University of Wisconsin-Madison Rare Gas Geochronology Laboratory, ⁴⁰Ar/³⁹Ar incremental heating experiments were conducted on groundmass and mineral separates following the procedures of ref 43. Isotope data were reduced using ArArCalc software version 2.5 (http://earthref.org/ArArCALC/). We report ages with 2σ uncertainties (includes the J uncertainty) which are calculated relative to a Fish Canyon standard age of 28.201 ± 0.046 Ma (ref. 44) and a value for λ⁴⁰K of 5.463 ± 0.107 × 10⁻¹⁰ yr⁻¹ (ref. 45; Supplementary Table 1A). We do not prefer the values for the 40 K decay constants or the standard age calibration proposed in ref. 46. In fact, numerous Ar/Ar laboratories also do not recognize the values from ref. 46 for reasons that are well beyond the scope of this paper. Thus, the ages used in this manuscript do not need to be recalculated⁴⁷.

Alteration-free rock chips were selected under stereoscopic microscope and powdered in an alumina mill at the Department of Geosciences at Virginia Tech. From these powders, whole rock major and trace element compositions were collected at Virginia Tech by X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS)⁴⁸ and are reported in Supplementary Table 2A. Curacao picrite samples were processed and analysed in the geochemistry laboratory at the Department of Geosciences at Virginia Tech following the procedures reported in ref. 49. XRF was used to determine major element concentrations. The Relative Standard Deviation for ten replicates of BHVO-2g was <2.70% for all major elements and accuracy better than 1.74%, with the exception of P₂O₅. Trace elements as well as P₂O₅ were collected from the same fluxed glasses with an Agilent 7500ce ICP-MS coupled with a Geolas laser ablation system, following the protocols reported in ref. 49. The data were calibrated against USGS standards BCR-2, BHVO-2, G-2, and BIR-1a, using Ti from XRF as an internal standard and the standard element values reported in ref. 50. The accuracy for four replicates of BHVO-2g was better than 5% for all elements except for Ba, Hf, and Ge (7–9%). The accuracy for four replicates of BIR-1a was better than 5% for most elements, except for Ga, Ge, Ce, Pr, Eu, Tb, Dy, Ho, Tm, Er, Yb, and Lu (5–9%).

Isotope analyses were performed at the Center for Elemental Mass Spectrometry, University of South Carolina. Sample powders from the same aliquots used for major and trace elements were digested in sub-boiling Teflon-distilled 3:1 HF:HNO₃ (v/v) mixture, and the isotopes were analysed on aliquots of a single digestion. Pb was separated on an anion resin in HBr, HNO₃ acids. Sr and REE were separated from the bulk rock washes of the Pb chemistry on a cation resin in HCl. Sr was further purified on a Sr-spec resin and Nd on an Ln-resin (both from EICHROM). The radiogenic isotope ratios were determined on a Thermo Neptune multi collector ICP-MS with the PLUS upgrade. Sample introduction was with an APEX enhanced sensitivity spray chamber and high-sensitivity cones (JET and X-skimmer configuration) resulting in Nd sensitivities on the order of 1,400–1,500 V ppm⁻¹. The NIST SRM-987 Sr standard was determined at ⁸⁷Sr/⁸⁶Sr = 0.710320 ± 0.000012 (n = 12) using ⁸⁶Sr/⁸⁷Sr = 0.1194 for instrumental fractionation correction. All Sr-isotope ratios are reported relative to the ⁸⁷Sr/⁸⁶Sr = 0.710250 for the NIST standard to correct for instrument bias. The LaJolla Nd standard was determined at ¹⁴⁴Nd/¹⁴⁴Nd of 0.511858 ± 0.000007 (n = 10) using ¹⁴⁴Nd/¹⁴⁶Nd = 0.7219 for fractionation correction. Pb isotope ratios were determined by the Tl-addition technique⁵¹ using ²⁰⁵Tl/²⁰⁵Tl = 0.418922 and assuming identical fractionation factors between Tl and Pb. The Pb/Tl ratio (²⁰⁸Pb/²⁰⁵Tl ~ 7) of the samples was kept near identical to the NBS-981 standard by first performing dip checks in the samples and spiking Tl to the appropriate level. The standard NBS-981 was determined at ²⁰⁶Pb/²⁰⁴Pb = 16.936 ± 0.001, ²⁰⁷Pb/²⁰⁴Pb = 15.940 ± 0.01, ²⁰⁸Pb/²⁰⁴Pb = 36.694 ± 0.003 (n = 13). Measured, as well as age-corrected, radiogenic isotopes and errors are reported in Supplementary Table 2A.

Measured radiogenic isotope ratios of new Tortugal and Curacao samples were age-corrected to initial eruptive values at 90 Ma using parent/daughter values reported in Supplementary Table 2A. We included initial Nd–Pb (age-corrected to 90 Ma) ratios of previously studied Quepos samples³⁴ and CLIP basalts and glasses for comparison.

Melt inclusions in olivine from all Tortugal lava samples were completely recrystallized, so it was necessary to experimentally reheat them. Separated olivine grains were heated to 1,300 °C in a vertical heating stage at oxygen fugacity corresponding to the quartz-fayalite-magnetite (QFM) buffer at Vernadsky Institute of Geochemistry, Russian Academy of Science, Moscow. Olivine grains were quenched after exposure of 5 min at top temperature. The total exposure time of inclusions at temperatures over 800 °C was less than 10 min, which precludes any significant loss of H⁺ from the melt inclusions. Grains were ground and polished to expose inclusions on the surface. Exposed inclusions were then measured by EPMA, SIMS, and Raman spectroscopy. The effects of reheating on H₂O content in the melt inclusions are discussed in Supplementary Information.

We used the high-precision method of ref. 52 to analyse olivine phenocrysts from Tortugal samples using only the most primitive, unfractionated (samples in olivine control) basalts. Data was collected at the University of Joseph Fourier, Grenoble France on a JEOL JXA-8230 Superprobe using a focused beam (~1 µm in diameter) of 25 kV and 900 nA. We analysed the San Carlos olivine

standard at regular intervals during each analysis to correct for instrumental drift. Primary standards, statistics for the secondary standards and statistics for the olivine analyses are given in Supplementary Table 2B. The average relative 2σ error for major and trace elements on the San Carlos olivine standard (n = 45) were: Si ~ 0.14, Mg ~ 0.14, Fe ~ 0.036 and Ni ~ 0.002, Mn ~ 0.001, Ca 0.001, Na ~ 0.002, Al ~ 0.002, Co ~ 0.002, P ~ 0.002, and Ti ~ 0.0005. An internal house standard (xen-7) was measured at regular intervals to correct for instrumental drift in Zn. The 2σ error for Zn on xen-7 was ~0.002. The relative accuracy was better than 2% for all oxides except Co (2.8%), P (4.6%), and Zn (17%). Major elements of olivine-hosted melt inclusions and clinopyroxene crystals were analysed at Virginia Tech with a Cameca SX 50 electron microprobe using a focused beam (~1 µm in diameter) of 15 kV and 25 nA.

Olivine-hosted melt inclusions and clinopyroxenes were analysed at Woods Hole Oceanographic Institution. We determined H₂O, CO₂, F, S, and Cl concentrations from ratios of ¹⁶O¹H, (other isotopes of C, F, S, and Cl) to ³⁰Si using a Cs beam and a 1 µm spot size within 50 µm × 50 µm rastered pits in each phase. Calibrations and statistics may be found in Supplementary Table 2SF.

We performed petrologic modelling on Tortugal samples following the approach reported in ref. 5 to constrain melt generation P–T conditions. We incorporated data from other komatiite and high-MgO terranes (that is, Barberton, Kambalda, Munro, Emeishan, Baffin Island, and Gorgona) using precompiled datasets found on the GEOROC Database. PRIMELT3 MEGA.xlsm software is a user-friendly program in which the operator needs only to import whole rock data composition and specify desired source oxidation state. Melting is assumed to be due to accumulated fractional melting. The software first identifies a range of primary magma composition through olivine addition or subtraction. Olivine is added or subtracted until the software identifies a common melt fraction in multicomponent (CMAS) and FeO–MgO space. In the case of high-MgO basalts, we add/subtract olivine until a primary magma solution in equilibrium with the highest olivine Mg# is reached (Mg# = 94 for Tortugal melts). We then apply the experimentally determined equations to estimate Tₚ, T⁽⁰⁾Tₚ, and initial melting pressure (Pᵣ). Additionally, we recalculated primary melt compositions in equilibrium with the mode of olivine Mg-numbers (Mg# ~ 93), because it is possible that the most magnesian olivine crystallized form melts that did not mix efficiently with the rest of the accumulated fractional melts, and hence contained substantially lower FeO. Using the modal Mg# yields primary melt solutions that record an average Tₚ of 1,691 °C and a maximum Tₚ of 1,776 °C (~85 °C and 52 °C, receptively lower than when in equilibrium with olivine having an Mg# of 94.2). It is important to note that these potential temperatures are still within the range for many Archaean komatiites. These melting equations are not calibrated for sources that have undergone such extensive melting, and therefore restrain us from calculating final melting pressures (Pᵣ), because komatiites most likely left behind residues of olivine+clinopyroxene+garnet. Therefore, we used the approach of ref. 3 to qualitatively estimate Pᵣ. We assume that the final melting temperatures and pressures coincide with the intersection of the adiabat with the maximum pyroxene stability curve¹⁴ (Fig. 4b). Melt productivity essentially stops once opx is melted out of the source and leaves behind a refractory dunite residue.

**Data availability.** The authors declare that the data supporting the findings of this study are available within the article, supplementary files, and in the Supplementary Information section.

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