The Geochemical Behavior of Thallium in Mantle-Derived Basalts


DOEI Funded Research: 2011


We seek funds to perform the first measurements of in-situ thallium concentrations in melt inclusions and quenched lavas with an ion probe.  The samples targeted are selected with the aim of determining the geochemical affinity of thallium in different primitive igneous environments.  Thallium is generally believed to follow sulphur (also termed chalcophile behavior) in igneous systems (e.g.  McDonough and Sun, 1995), but several recent studies of magmatic Tl (Baker et al., 2010; Jochum and Verma, 1996) appear to indicate that it is more similar to the alkali metals (potassium, rubidium and cesium).  In addition, Tl abundances could also be partially controlled by volcanic de-gassing processes because the melting point is only 304°C.  However, due to a scarcity of high quality Tl data in the literature it is currently not possible to perform a detailed assessment of the geochemical affinity of Tl.

We will complement the Tl concentration measurements with simultaneous collection of sulfur (S), potassium (K) and chlorine (Cl) abundances, which will allow us to determine is Tl is mainly chalcophile (like S), lithophile (like K) or volatile (like Cl) in igneous systems.   The information obtained in this study will be central to future studies of Tl isotope ratios in igneous systems.  Co-PI Nielsen has recently had an NSF EAR grant funded to study Tl isotopes in arcs and it is anticipated that the data provided by this ion probe study will form an ideal platform for further expansion of Tl isotope studies in igneous settings such as mantle plume volcanism, subduction zones and mid ocean ridge basalts.