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Thallium Isotope Fractionation During Magma Degassing: Evidence From Experiments and Kamchatka Arc Lavas
Author(s) -
Nielsen Sune G.,
Shu Yunchao,
Wood Bernard J.,
Blusztajn Jerzy,
Auro Maureen,
Norris C. Ashley,
Wörner Gerhard
Publication year - 2021
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2020gc009608
Subject(s) - geology , subaerial , isotope fractionation , mass independent fractionation , geochemistry , basalt , magma , isotope , equilibrium fractionation , stable isotope ratio , isotopic signature , radiogenic nuclide , mineralogy , fractionation , volcano , mantle (geology) , chemistry , physics , organic chemistry , quantum mechanics
Thallium (Tl) isotope ratios are an emerging tool that can be used to trace crustal recycling processes in arc lavas and ocean island basalts (OIBs). Thallium is a highly volatile metal that is enriched in volcanic fumaroles, but it is unknown whether degassing of Tl from subaerial lavas has a significant effect on their residual Tl isotope compositions. Here, we present Tl isotope and concentration data from degassing experiments that are best explained by Rayleigh kinetic isotope fractionation during Tl loss. Our data closely follow predicted isotope fractionation models in which TlCl is the primary degassed species and where Tl loss is controlled by diffusion and natural convection, consistent with the slow gas advection velocity utilized during our experiments. We calculate that degassing into air should be associated with a net Tl isotope fractionation factor of α net = 0.99969 for diffusion and natural gas convection (low gas velocities) and α net = 0.99955 for diffusion and forced gas convection (high gas velocities). We also show that lavas from three volcanoes in the Kamchatka arc exhibit Tl isotope and concentration patterns that plot in between the two different gas convection regimes, implying that degassing played an important role in controlling the observed Tl isotope compositions in these three volcanoes. Literature inspection of Tl isotope data for subaerial lavas reveals that the majority of these appear only minorly affected by degassing, although a few samples from both OIBs and arc volcanoes can be identified that likely experienced some Tl degassing.