Both thermal ionisation mass spectrometer and ICPMS/MS were installed in early 2020. While it was demonstrated that the required performances were reached by the end of February, full installation of the ICPMS/MS was disturbed by the sanitary crisis in March 2020. The entire course of the project was significantly slowed down during a period of over a year. Nevertheless, we were able to set up analytical procedures to run high-precision strontium and neodymium with an external precision better than 5 ppm. The first data were presented in 2021 followed by a publication in Chemical Geology.
The ICPMS/MS was intensively used to ensure that all the chemical procedures necessary for high-precision isotopic analyses were effective. It was also used to determine the trace element contents of rock samples, set up the measurement of strontium isotopic ratios using the separating power of the reaction cell and coupled to a femtosecond laser ablation system to perform in-situ analyses.
The first volcanic samples that we decided to focus on are the samples collected on the new volcano located next to Mayotte in the Indian ocean. This scientific target was not originally planned in the SHRED project but the opportunity of getting the first new geochemical results for a brand-new and unknown submarine volcano could not be missed. The first results were presented in 2021.
In 2023, we acquired a femtosecond laser ablation system and coupled it to both the ICP-MS/MS and a Neptune MC-ICPMS available at IPGP. The results are extremely positive because, as hoped, there is little to no matrix effect on the measurements. We were able to obtain excellent trace element data when coupled to the ICP-MS/MS; when coupled to the Neptune MC-ICPMS, we obtained excellent results for sulphur isotopes.
Using the TIMS, we were able to produce extremely reproducible 142Nd isotopic ratios with an external error of only 3 ppm over long periods. Such precision had never been achieved. Using this method, we measured the Nd isotopic composition of the newly formed volcano (Fani Maoré) and discovered that, for the first time in literature, a volcano had a positive 142Nd anomaly. We interpret the result as due to the involvement, in the source, of bridgmanite that crystallised in the deep mantle about 4.4 Ga ago. The manuscript is presently under revision at Nature. We also obtained data for a range of Polynesian basalts but the work is not finished. However, up to now, no significant positive (or negative) anomaly was detected in those rocks suggesting that if Hadean material is present in their source, its proportion is too low to be detected. We also obtained 142Nd isotopic data for another location where 182W isotopic data were published. The main finding is that the Nd and W isotopic systems are disconnected suggesting that the 182W isotopic anomalies are most probably due to interaction of the plume source with the core and not to sampling of an enriched deep basal magma ocean.
We also demonstrate that Mass Independent Fractionation of sulphur as published by the PI and other scientists are not reproduced when analysing other OIB or the same basalts but using a different technique. We also discovered that the negative d34S values reported when measured by ion probe, might represent an analytical artefact when analysing small sulphides. We quantified mixing times in the early magma ocean and demonstrate that it is short but that, due to the rotation of the planet, large zones of the magma ocean do not necessarily mix creating potential heterogeneities prior to crystallisation. We traced back the origin of the material involved in several southern hemisphere ocean islands (including Fani Maoré) and demonstrate that their source is most probably unrelated to the African LLSVP. The exact location in the mantle, of the Hadean component present in Fani Maoré remains to be determined.
