SIMS analysis of volatiles in silicate glasses, 2: isotopes and abundances in Hawaiian melt inclusions

摘要

Ion microprobe measurements of the concentrations of H2O, CO2, F, S and Cl and the isotopic composition of hydrogen are reported for populations of olivine-hosted melt inclusions from five lava samples from the Hawaiian volcanoes Loihi, Kilauea, Mauna Loa and Koolau. After reheating of the melt inclusions and correction for the effects of post-entrapment modification, the melt inclusions have MgO contents ranging from 8.9% to 15.1% and averaging 11%, significantly higher in MgO than most submarine glasses. The melt inclusions show large ranges in H2O (0.03-0.84), CO2 (5-862 ppm), F (308-1000 ppm), S (156-3330 ppm) and Cl (8 ppm to 1.11 wt.%), accompanied by large ranges in deltaD ( - 165parts per thousand to + 40parts per thousand). Laboratory reheating experiments on Loihi inclusions show that diffusive loss of hydrogen can occur from olivine-hosted melt inclusions on hour- to day-long time scales via proton diffusion through olivine, with consequent positive shifts in the D/H ratios of the residual hydrogen in the melt inclusion. Most melt inclusions from subaerial Kilauea and Mauna Loa samples have signatures of low H2O (0,05 - 0.2 wt.%) and high deltaD (up to + 40%) compared to published analyses of submarine glasses, suggesting diffusive H loss during slow cooling of inclusions shortly after eruption, Koolau melt inclusions have the lowest deltaD values yet measured in oceanic basalts ( - 61parts per thousand to - 165parts per thousand). Shallow-level degassing produces a H2O-deltaD relationship in most Koolau melt inclusions which can be explained by open-system (Rayleigh) degassing with a vapor-melt D/H fractionation factor of 1.024, similar to previous estimates. Shallow degassing is also indicated in some inclusions by parallel depletions in H2O and S, but degassed melt inclusions from all volcanoes display a wide range in CO2 concentrations, indicating kilometer-scale vertical delta(13)C convection of melts within Hawaiian magma reservoirs. The measured of three CO2-bearing melt inclusions from Koolau volcano are depleted ( - 12parts per thousand to - 29parts per thousand) and correlated with deltaD, possibly consistent with open-system degassing Of CO2-rich magmas and subsequent mixing with less-degassed magmas. Assimilation of seawater-derived components is indicated in a small number of melt inclusions which exhibit high Cl and Cl/K ratios, with an extreme example from Loihi (I. 11 wt.% Cl, 0.48 - wt.% H2O, deltaD = - 118parts per thousand). A subset of melt inclusions have escaped the confounding effects of H diffusion, shallow degassing and crustal contamination, and provide evidence for heterogeneity of D/H ratios in the Hawaiian mantle, which appear to correlate with published radiogenic (Sr, Nd, Pb, Os) and oxygen isotope data. If the apparent Hawaiian deltaD variability can be confirmed to be a source signature, then the Hawaiian D/H data indicate that heterogeneities within the Hawaiian plume are required to be large in scale (tens of kilometers) and/or young in age (< 1 Ga) in order to preserve hydrogen isotopic variability against the extremely rapid diffusivity of hydrogen in the mantle. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 82]