GoKawiilGeological context of the samples
The Comoros archipelago is the surface manifestation of a deep plume. The main islands that make up this archipelago are Grande Comore, Mohéli and Anjouan, as well as the islands that constitute Mayotte, namely Grande-Terre and Petite-Terre, from west to east53. Of these, Mayotte corresponds to the oldest volcanic activity, with the first subaerial eruption occurring about 11 million years ago (Ma). However, its volcanic activity has alternated between periods of active and quiescent phases, shaping its two islands of Grande-Terre and Petite-Terre54. The most recent volcanic activity was observed on Petite-Terre and is Holocene54,55 in age. Between June 2018 and January 2021, a new submarine volcano called Fani Maoré erupted approximately 55 km east of Mayotte, representing the most recent volcanic expression of the plume (see Fig. 1 in ref. 56 and Extended Data Fig. 1). The 2018–2021 crisis that led to the formation of Fani Maoré is associated with a volcanic ridge that extends westwards towards Petite-Terre56,57. In this context, we analysed 13 samples from the new submarine volcano Fani Maoré and eight samples from the eastern flank of Petite-Terre, Mayotte (Extended Data Fig. 1). These samples were collected during a series of oceanographic cruises between 2018 and 2021 (MAYOBS1, MAYOBS2, MAYOBS4 and MAYOBS15 (refs. 57,58)) as part of several dredging operations targeting both the Fani Maoré volcanic edifices and the eastern flank of Petite-Terre. All Fani Maoré samples are basanites, whereas Petite-Terre samples include three basanites and five phonolites. Detailed petrological descriptions can be found in ref. 56 and chemical and isotope compositions are reported in ref. 51.
Sample preparation and Nd isotope measurements
All 21 OIB samples were processed in the clean laboratory of the Institut de Physique du Globe de Paris (IPGP) and their Nd isotope compositions were measured on a Nu Instruments TIMS, following procedures described in ref. 22. Depending on the REE concentration and the amount of powder available, aliquots of 50–300 mg of homogenized bulk rock powder were digested in a 3:1 mixture of distilled 28 M HF and 15 M HNO 3 at 75 °C for 48 h on a hotplate and then evaporated to incipient dryness. Fluorides were decomposed with repeated dissolution and evaporation cycles using alternating 6 M HCl and a 1:1 mixture of 6 M HCl and 15 M HNO 3 , until the solution was clear of any precipitate after centrifugation. Neodymium was chemically isolated using a four-step chemical separation procedure. REE were initially separated from the matrix by cation exchange chromatography using AG50W-X8 resin (200–400 mesh; 2 ml for a typical sample of 35–50 mg). Samples with digested mass exceeding 50 mg were divided into several columns to avoid saturation of the resin. The resulting REE fractions were subsequently recombined. In the second step, Ce was separated from the other REE using a redox technique and LN resin (50–100 µm; 0.5 ml). Sodium bromate (NaBrO 3 ) was used to oxidize Ce from Ce3+ to Ce4+, allowing it to be retained in the LN resin while the other REE3+ were eluted. This step was completed twice to ensure complete Ce removal. Residual Na and Br were removed from the Ce-free REE fraction during the third step using AG50W-X8 resin (200–400 mesh; 1 ml). Finally, in the fourth step, Nd was isolated from the other REE using a thin LN resin (25–50 mesh; 0.82 ml). Total procedural Nd yields were ≥90% and the procedural blank for Nd was less than 40 pg (n = 3), which is negligible relative to the 1–9 µg of Nd collected from the samples. Residual Sm and Ce remaining in the final Nd fraction were consistently negligible (<0.5 ng Sm and <0.2 ng Ce) relative to the mass of Nd.
High-precision multi-dynamic Nd isotope analyses were performed on a Nu Instruments TIMS at IPGP following a five-line acquisition method thoroughly detailed in ref. 22 (cup configuration in their Fig. 1). For both standards and samples, about 800 ng of Nd per analysis was loaded onto zone-refined 99.999% Re filaments used in a double-filament configuration. The five-line acquisition method in positive mode allows the simultaneous measurement of all Nd isotopes during all five acquisition lines, each separated by a one-mass-unit jump. This technique allows the combination of two to three acquisition lines to dynamically measure (in the same Faraday cup) three ratios per Nd isotope, the average of which gives a multi-dynamic ratio essentially free from biases owing to the Faraday cups. A typical run consists of 40 blocks of 20 cycles with about 6 V of 142Nd+ measured for 18 h. Mass discrimination was corrected using 146Nd/144Nd = 0.7219 and the exponential law. Time drift was corrected using 11-cycles interpolations. Data were systematically corrected22 for Sm and Ce isobaric interferences in case 147Sm/144Nd was higher than 7 × 10−7 or 140Ce/144Nd was higher than 9 × 10−6. This allows reliable correction and prevents overcorrection that might increase errors. The detection threshold to consistently distinguish Sm and Ce signal from the background noise during our analyses is established at 147Sm/144Nd > 5 × 10−7 and 140Ce/144Nd > 5 × 10−6 (for I142Nd+ about 5 V; see Fig. 9a,c in ref. 22).
External precision was evaluated using two pure Nd solution standards measured over two analytical sessions, alongside the samples. We analysed AMES Rennes Nd five times during the first session (March 2022 to May 2022) and we analysed AMES Rennes Nd and JNdi-1 15 and 23 times, respectively, during the second session (July 2022 to January 2024). They gave similar isotope ratios within error, with typical reproducibility (2 s.d.) between 1.8 and 3.9 ppm for 142Nd/144Nd, 2.0–3.8 ppm for 143Nd/144Nd, 1.8–3.1 ppm for 145Nd/144Nd, 4.2–5.6 ppm for 148Nd/144Nd and 11.4–11.9 for 150Nd/144Nd. JNdi-1 was the most frequently measured standard during this study and its reproducibility (2 s.d. of 3.1 for 142Nd/144Nd) is taken as representative of the external error for all samples. The short-term reproducibility, which is usually reported in the literature as being equivalent to uninterrupted series of samples and standard measurements, is similar. We identify six series (March–May 2022; July 2022; December 2022 to January 2023; April–June 2023; August–September 2023; November 2023–January 2024), the reproducibility of which on the 142Nd/144Nd ratio ranges from 1.8 to 4.1 ppm (n = 4–9, excluding n ≤ 3), giving an average reproducibility of 3.1 ppm. Details on the individual measurements of AMES Rennes Nd and JNdi-1 are given in Supplementary Data Table 1b and are shown in Extended Data Figs. 2 and 3.
Rock reference materials were measured during the same analytical sessions as our samples. The results were extensively described in ref. 22 and show similar reproducibility.
The 143Nd/144Nd ratios are given in the epsilon notation following the equation ε143Nd = ((143Nd/144Nd sample /143Nd/144Nd CHUR ) −1) × 104, in which CHUR is the CHondritic Uniform Reservoir with 143Nd/144Nd = 0.512630 (ref. 59). All other Nd isotope ratios are given in the mu notation following the equation µxNd = ((xNd/144Nd sample /xNd/144Nd terrestrial reference ) −1) × 106, in which the terrestrial reference composition for this study is the widely used JNdi-1 pure Nd solution standard. All of the samples analysed during session 2 were measured concurrently with JNdi-1 and their mu values were calculated directly. However, three samples were measured during session 1. To ensure proper comparison of these data with the rest of the dataset, their measured isotope ratios were first normalized to session 2 using the AMES Rennes Nd measured during both sessions, following the equation xNd/144Nd corrected sample = xNd/144Nd measured sample × (xNd/144Nd AMES,session 2 /xNd/144Nd AMES,session 1 ). The mu values for these corrected samples were then calculated similarly to the rest of the dataset. All Nd isotope measurements of samples and pure Nd reference materials acquired during the course of this study are reported in Supplementary Data Table 1 and Extended Data Figs. 2 and 3.
Critical evaluation of the data obtained on natural samples
Under terrestrial conditions, excesses and deficits in 142Nd reflect the decay of 146Sm. They are subtle and therefore more prone to analytical bias and misinterpretation. To detect these, Sm and Ce interferences as well as 145Nd/144Nd, 148Nd/144Nd and 150Nd/144Nd ratios were precisely measured and monitored.
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