DOI: 10.18715/ipgp.2023.lq6ndlzz
title: Location, physio-chemistry, geochemical composition of Piton de la Fournaise cold waters, and index probability (HInorm) of the presence of a hydrothermal component.

description: This dataset accompanies the article "Use of cold waters geochemistry as a geothermal prospecting tool for hidden hydrothermal systems in Réunion Island" published in the journal Communications Earth and Environment. Here are given the location, physio-chemistry and geochemical composition of Piton de la Fournaise cold waters, which were used to investigate the presence of a hydrothermal system. The resulting relative probability of the presence of a hydrothermal component (normalized hydrothermal index HInorm) computed from the geochemistry is also given in the table. The groundwaters and surface waters were sampled during the dry season in 2020 and 2021. The 82 water samples (from 80 sampling points) were analyzed for major and trace elements concentrations and δ13C and δ11B isotopic ratios. The pH, temperature, and electrical conductivity (EC, expressed at 25°C) of each sample were measured in situ. All water samples were filtered at 0.2 µm on site and the containers were rinsed three times with filtered water. Samples for the analysis of major, trace elements and B isotopes were collected in LDPE bottles. Samples for the analysis of major cations and trace elements were acidified with nitric acid (HNO3 0.6 N) on site. Samples for the analysis of total dissolved inorganic carbon content (TDIC) and C isotopes were collected in Labco 12 ml Exetainer vials with no headspace. All samples were stored at a temperature of 6°C before shipment. All analyses were performed at the Institut de Physique du Globe de Paris (IPGP). Major anions, fluoride and nitrate concentrations were determined by ion exchange chromatography. Major cations and trace element concentrations were determined using inductively coupled plasma mass spectrometry (ICP-MS). The accuracy for major cations and anions concentrations is &lt;5%, and &lt;20% for trace elements, as inferred from replicate measurement of solutions of known compositions. The TDIC and carbon isotope ratios of the TDIC were determined together by gas chromatography and isotope ratio mass spectrometry (GC-IRMS), after releasing the TDIC as CO2 by H3PO4 acidification in a vial from an aliquot of 5 mL 81. For the analysis with the GC-IRMS, a finite amount of gas is pumped inside the vial and sent into the analysis circuit. This step is repeated several times and the TDIC and δ13C are computed from the results of several runs. This method proved not adapted for the analysis of our samples due to a too low content of TDIC. With an aliquot of just 5 mL, the available gaseous CO2 in the vial sampled by the GC-IRMS was too low and the analysis did not yield results for several runs and for several samples. We obtained reliable results for only 51 out of 82 samples. For these samples, HCO3 concentrations were computed from TDIC, pH and temperature, using the temperature dependent acid dissociation constants from Mook and Koene, (1975). For the other samples, HCO3 concentrations were estimated for an ionic balance equal to 0 %. In the dataset, the HCO3 concentrations values obtained for these samples are marked by a * symbol. δ13C are reported relative to PDB standard with a 1SD reproducibility, ± 0.05 ‰. Boron isotope ratios were determined by MC-ICP-MS with a direct injection nebulizer (d-DIHEN, 83), after boron extraction through ion chromatography 84. δ11B are reported relative to NIST SRM951 standard with a 2SD reproducibility, between 0.1 and 0.4 ‰.