GOLDSCHMIDT CONFERENCE TOULOUSE 1998 Evolution of magmatic volatiles during Miocene seamount to shield stage of Gran Canaria (Canary Islands): Evidence from OI- and Cpx-hosted melt and fluid inclusions A. A. Gurenko T. H. Hansteen H.-U. Schmincke GEOMAR Research Center, Wischhofstrasse 1-3, D-24148 Kiel, Germany Magmatic volatiles (1) play an important role for melt generation in the mantle, (2) affect physical properties of melts and, therefore, the character of volcanic eruptions, and (3) have an impact on global climate. We have studied volatile components (H20, CO2, F, C1, S) and S6+/Stot ratios in fluid and naturally quenched glass inclusions in O1 and Cpx phenocrysts from Miocene basaltic hyaloclastites drilled during ODP Leg 157. Our goal is to understand the behaviour of magmatic volatiles from magma generation to eruption during the seamount to shield stages of Gran Canaria - a well studied oceanic intraplate volcano. Methods Concentrations of major elements, F, C1 and S, and S6+/atot ratios in glass inclusions were analysed with a Cameca SX50 electron microprobe (GEOMAR, Kiel, Germany), and H20 concentrations - with ion microprobes Cameca IMS 3f (CRPG, Nancy, France) and IMS 4f (Microelectronics Inst., Yaroslavl, Russia). Microthermometry of fluid inclusions was performed using a Fluid Inc. | heating-cooling stage at GEOMAR Research Center. Results Microthermometry of fluid inclusions trapped in olivine Foss-ss showed that fluid coexisting with melt was composed of >95 mol% CO2 (triple point -56.6+_0.2 ~ C), whereas H20 and other volatiles (i.e. CO, H2S, SO2, N2) can only be present in minor or trace concentrations. Inclusions homogenize in the vapour at +27.7 ~ to +29.7~ (n = 14) and in the liquid phases at +29.6 ~ to 30.4~ (n = 14) that corresponds to CO2 densities of 0.28-0.61 g/cm3 and yields a lithostatic pressure of 1-3 kbar calculated for the temperature range of 1100-1300~ Based on existing CO2 solubility models (e.g. Dixon et al., 1995), we expect 0.04-0.1 wt.% CO2 dissolved in the melt. A range of 0.14-1.2 wt.% H20 was obtained for O1- and Cpx-hosted glass inclusions by ion microprobe. H20 does not correlate with K20, and only the highest H20 concentrations (H20/K20 = 0.8-1.2) correspond to those of MORB and OIB magmas (Carroll and Holloway, 1994) (Fig. 1). Both CO2 and H20 concentrations are in good agreement with the results of FTIR studies of glass inclusions from the same or similar samples (0.01-0.1 wt.% CO2 and 0.06-1.4 wt.% H20; Wallace, 1998). Sulphur shows considerable scatter and two groups of glass compositions can be recognized: (1) inclusions with relatively high S concentrations (-0.1 to -0.5 wt.% S) correlating positively with FeO and likely representing S-saturated, undegassed magmas; and (2) inclusions having a spread from -0.15 to ~0.01 wt.% S at a given FeO, suggesting progressive magma degassing (Fig. 2a). The S6+/Stot 10 0.1 0.01 0.1 1 wt% H20 FI~. 1. H20 vs K:O in O1- and Cpx-hosted glass inclusions. Here and in Fig. 2, shaded area represents submarine MORB and OIB tholeiitic and alkali glasses (Carroll and Holloway, 1994: Dixon et al., 1997 and refs. therein. 551