Investigation of thallium fluxes from subaerial volcanism-Implications for the present and past mass balance of thallium in the oceans

摘要

A suite of 34 volcanic gas condensates and particulates from Kilauea (Hawaii), Mt. Etna and Vulcano (Italy), Mt. Merapi(Indonesia), White Island and Mt. Nguaruhoe (New Zealand) were analysed for both Tl isotope compositions and Tl/Pbratios. When considered together with published Tl—Pb abundance data, the measurements provide globally representativebest estimates of Tl/Pb = 0.46 ± 0.25 and ε205Tl = -1.7 ± 2.0 for the emissions of subaerial volcanism to the atmosphereand oceans (ε205Tl is the deviation of the205Tl/203Tlisotope ratio from NIST SRM 997 isotope standard in parts per10,000). Compared to igneous rocks of the crust and mantle, volcanic gases were found to have (i) Tl/Pb ratios that are typ-ically about an order of magnitude higher, and (ii) significantly more variable T1 isotope compositions but a mean ε205Tl valuethat is indistinguishable from estimates for the Earth's mantle and continental crust. The first observation can be explained bythe more volatile nature of Tl compared to Pb during the production of volcanic gases, whilst the second reflects the contrast-ing and approximately balanced isotope fractionation effects that are generated by partial evaporation of T1 during magmadegassing and partial Tl condensation as a result of the cooling and differentiation of volcanic gases. Mass balance calculations, based on results from this and other recent Tl isotope studies, were carried out to investigatewhether temporal changes in the volcanic Tl fluxes could be responsible for the dramatic shift in the ε205Tl value of the oceansat ～55 Ma, which has been inferred from TI isotope time series data for ferromanganese crusts. The calculations demonstratethat even large changes in the marine Tl input fluxes from volcanism and other sources are unable to significantly alter the Tlisotope composition of the oceans. Based on modelling, it is shown that the large inferred change in the ε205T1 value ofseawater is best explained if the oceans of the early Cenozoic featured significantly larger Tl output fluxes to oxic pelagic sed-iments, whilst the sink fluxes to altered ocean crust remained approximately constant.