The degassing behavior of volatile heavy metals in subaerially erupted magmas and their chemical diffusion in silicate melts

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dc.contributor.author Johnson, Angela D.
dc.date.accessioned 2009-12-22T16:39:23Z
dc.date.available 2009-12-22T16:39:23Z
dc.date.copyright 2009 en
dc.date.issued 2009-12-22T16:39:23Z
dc.identifier.uri http://hdl.handle.net/1828/2014
dc.description.abstract Volatile heavy metals are liberated from magmas during eruptive and passively degassing volcanic activity. Volcanic emanations have been estimated to contribute 20-40% of volatile elements such as Bi, Pb, As or Sb, and up to 40-50 % of Cd and Hg annually (Nriagu, 1989). Some workers, however, believe these ranges are too high (Hinkley, 1999) or too low (Zreda-Gostynska and Kyle, 1997) leading to considerable differences in global inventory budgets of these metals and the degree to which they load the atmosphere. The objective of this work is to investigate the behavior of volatile heavy metals such as Au, Tl, As, Pb etc. in subaerially erupted magmas and experimentally in silicate melts. Analysis of natural pumice samples confirm the futile, sporadic nature of Hg and associated heavy metals, suggesting these metals are fully degassed prior to deposition. Diffusion experiments were conducted in natural basalt, dacite and synthetic rhyolite (Ab-Or-Qz minimum eutectic) over a range of temperatures (1200 – 1430 °C) at 0.1 MPa. Starting compositions were doped with a heavy metal cocktail (Bi, Pb, Tl, Au, Re, Sb, Sn, Cd, Mo, As, Cu) and loaded into open top Pt capsules. One set of experiments examined the effect of melt composition (polymerization) on element diffusion, and the second investigated the effects of ligands on diffusion by adding known concentrations of Cl and S. During experiments of varying duration, concentration gradients arose in the volatile trace metals due to their varying volatility, as measured (normal to the melt/gas interface) by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in quenched glasses. Diffusion profiles followed an Arrhenius relationship from which diffusion coefficients (D) and activation energies (Ea) were obtained for Au, Tl, As, Cd, Re, Pb and Bi (in decreasing order of volatility). Results show Au and Tl are the most volatile in dacite and rhyolite yielding LogDDac Au = -10.7 ± 0.1 m2/s and LogDDac Tl = -10.9 ± 0.1 m2/s in dacite, and LogDRhy Au = -10.9 ± 0.1 m2/s and LogDRhy Tl = -11.3 ± 0.3 m2/s in rhyolite respectively. The D for Au could not be measured in basalt but Tl was the fastest diffusing species LogDBas Tl = -10.8 ± 0.2 m2/s. Ligands Cl and S were shown to increase the volatilities of all metals, with S having a more profound effect. Diffusivities were applied to a simple 1D bubble growth model (Smith 1955). Model results indicate diffusion coefficients play a major role in metal fractionation processes occurring at depths that ultimately dictate what metal ratios are measured at the surface of volcanoes. en
dc.language English eng
dc.language.iso en en
dc.rights Available to the World Wide Web en
dc.subject volcanic degassing en
dc.subject diffusion en
dc.subject trace metals en
dc.subject volcano en
dc.subject magmas en
dc.subject basalt en
dc.subject silicate melt en
dc.subject heavy metals en
dc.subject.lcsh UVic Subject Index::Sciences and Engineering::Earth and Ocean Sciences en
dc.title The degassing behavior of volatile heavy metals in subaerially erupted magmas and their chemical diffusion in silicate melts en
dc.type Thesis en
dc.contributor.supervisor Canil, Dante
dc.degree.department School of Earth and Ocean Sciences en
dc.degree.level Master of Science M.Sc. en

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