CO2 and H2O SIMS measurements of clinopyroxene-hosted silicate melt inclusions from Conical Seamount, Papua New Guinea.
Description
Conical Seamount, in Papua New Guinea, is a shoshonitic seamount that hosts epithermal-style mineralization (Petersen et al., 2002). A shallow crustal magma chamber appears to be the main difference between Conical Seamount and the other, barren seamounts nearby. It is characterized by dynamic convection, extensive fractional crystallization and frequent magmatic replenishment (Gautreau et al., 2024). This magma chamber appears to be critical for the transfer of metals and volatiles into the exsolving fluid phase and is thus of key importance for epithermal ore formation (Gautreau et al., 2024). In this study, we aim at presenting the first robust constraints on volatile contents and saturation as well as the timing of magmatic degassing underneath Conical Seamount. Thus, we quantified CO2 and H2O concentrations in silicate melt inclusions contained in clinopyroxene crystals to determine the different magma stagnation levels below Conical Seamount. We used the CAMECA 1280-HR Secondary Ion Mass Spectrometer (SIMS) at the GFZ facility in Potsdam. These data will be used to model pressure and temperature conditions of the silicate melt (e.g., Iacovino et al., 2021). At the end of the analytical session, all SIMS pits were imaged with an optical microscope in reflected light mode to check the exact location of each analysis and whether other minerals, inclusions or cracks were hit. If SIMS pits touched cracks or inclusions, the corresponding analyses were considered as outliers. If the central part of the SIMS craters (5 µm in diameter) touched the pyroxene crystal hosting the silicate melt inclusions, then the calculated CO2 and H2O concentrations were considered as minimum values. Visible crystals in the melt inclusions are mentioned for each measurement point in the notes. Finally, we exclude results for volatile contents outside of the range of the basaltic glass reference materials (i.e., > 5,900 µg/g for CO2 and > 8.8 wt.% for H2O; Scicchitano et al., 2024). Gautreau, L., Hansteen, T.H., Portnyagin, M., Beier, C., Frische, M., Brandl, P.A., 2024. Understanding the links between volcanic systems and epithermal ore formation : A case study from Conical Seamount , Papua New Guinea. LITHOS 482–483, 1–18. https://doi.org/10.1016/j.lithos.2024.107695 Iacovino, K., Matthews, S., Wieser, P.E., Moore, G.M., Bégué, F., 2021. VESIcal Part I: An Open-Source Thermodynamic Model Engine for Mixed Volatile (H2O-CO2) Solubility in Silicate Melts. Earth Sp. Sci. 8, 1–55. https://doi.org/10.1029/2020EA001584 Petersen, S., Herzig, P.M., Hannington, M.D., Jonasson, I.R., 2002. Submarine Gold Mineralization Near Lihir Island , New Ireland Fore-Arc , Papua New Guinea 97, 1795–1813. Scicchitano, M.R., Shishkina, T.A., Wilke, F.D.H., Wilke, M., Botcharnikov, R.E., Almeev, R.R., 2024. Basaltic glasses for quantification of CO2 and H2O content by Secondary Ion Mass Spectrometry (SIMS). GFZ Data Services. https://doi.org/10.5880/GFZ.3.1.2024.009
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Polished pyroxene grains containing silicate melt inclusions (MIs) were pressed in the centre of four indium discs having a diameter of 2.54 cm, along with grains of basaltic glass internal reference materials (RMs) available at GEOMAR with published CO2 and H2O contents (ALV-519-4-1, Helo et al., 2011; HW-G1, sample named 6K495-5 in Shimizu et al., 2019) and San Carlos Olivine (SCO). SCO is assumed to contain no measurable amount of CO2 (<1 µg/g, Keppler et al., 2003) and H2O. The quantification of CO2 and H2O contents in MIs was done with 10 basaltic glasses RMs in the GFZ SIMS lab. The RMs were analysed at the beginning and at the end of the session and a full characterization is provided in Scicchitano et al. (2024). Prior to SIMS analysis, indium and epoxy discs were cleaned with high–purity ethanol and coated with 35 nm high–purity gold, to assure electrical conductivity during the analyses. Positive relief was < 5 μm as confirmed by white–light profilometry. Indium and epoxy discs were stored in the ultra-high-vacuum sample storage chamber of the SIMS two weeks prior the analyses, to reduce the amount of water released in the sample chamber. CO2 and H2O concentrations in MIs were measured with the GFZ CAMECA IMS 1280–HR ion microprobe. Detailed analytical settings and method description are given in the Table_S1 and Supplement_method_S2. The uncertainty of calculated CO2 values is 12-25% at concentrations <1,000 µg/g, and uncertainties on calculated H2O values is ca. 13% at concentrations <1 wt.%. At higher concentrations (CO2 > 3,000 µg/g and H2O > 1 wt.%), uncertainties are lower (2-5% for CO2 and < 6% for H2O). References Helo C., Longpré M.-A., Shimizu N., Clague D.A., Stix J. (2011), Explosive eruptions at mid-ocean ridges driven by CO2-rich magmas. Nature Geoscience, 4, 260-263. doi: 10.1038/ngeo1104 Keppler H., Wiedenbeck M., Shcheka S.S. (2003), Carbon solubility in olivine and the mode of carbon storage in the Earth´s mantle. Nature, 24, 414-416. doi.org/10.1038/nature01828 Scicchitano, M.R., Shishkina, T.A., Wilke, F.D.H., Wilke, M., Botcharnikov, R.E., Almeev, R.R., 2024. Basaltic glasses for quantification of CO2 and H2O content by Secondary Ion Mass Spectrometry (SIMS). GFZ Data Services. https://doi.org/10.5880/GFZ.3.1.2024.009 Shimizu K., Ito M., Chang Q., Miyazaki T., Ueki K., Toyama C., Senda R., Vaglarov B., Ishikawa T., Kimura J.-I. (2019), Identifying volatile mantle trend with the water-fluorine-cerium systematics of basaltic glass. Chemical Geology, 522, 283-294. doi.org/10.1016/j.chemgeo.2019.06.014 Shishkina, T.A., Botcharnikov R.E., Holtz F., Almeev R.R., Portnyagin M.V. (2010), Solubility of H2O- and CO2-bearing fluids in tholeiitic basalts at pressures up to 500 MPa. Chemical Geology, 277, 115-125. doi:10.1016/j.chemgeo.2010.07.014 Wetzel D.T., Hauri E.H., Saal A.E., Rutherford M.J. (2015), Carbon content and degassing history oft he lunar volcanic glasses. Nature Geoscience, 8, 755-758. doi: 10.1038/NGEO2511