Behavior of the Mo, Tl, and U isotope systems during differentiation in the Kilauea Iki lava lake

Thallium Isotopic Compositions in Hawaiian Lavas: Evidence for Recycled Materials on the Kea Side of the Hawaiian Mantle Plume

“…The samples from West Maui, which include two samples heavy in ε 205 Tl, form the opposite trend, where Ce/Tl decreases as ε 205 Tl increases (Figure 3a). A similar trend was noted in a study of samples from the Kilauea Iki lava lake, where samples with the lowest Tl concentrations had a wide range of ε 205 Tl values and were not associated exclusively with heavy ε 205 Tl (Gaschnig et al, 2021). If higher Ce/Tl values are indicative of degassing, this is either decoupled from Tl isotope systematics in Hawaiian shield lavas, or the samples with highest Ce/Tl are additionally altered with respect to Ce and/or Tl.…”

“…Although this is an estimate for Kīlauea, it is likely representative of other Hawaiian tholeiites as Mauna Loa, Mauna Kea, Kīlauea, and Lō'ihi all have similar sulfur contents (Davis et al, 2003). The samples for which we have thin sections and hand samples do not have sulphide abundances close to the required estimate of Gaschnig et al (2021) and as such it is unlikely that sulphide formation is the cause of the heavy Tl isotopic values.…”

“…There are other factors that can yield high Ce/Tl in OIB, including a lower sulphide to pyroxene ratio in the source (<1,500 ppm S) and overall heterogeneity of Ce/Tl in the mantle (Nielsen et al, 2014). With regards to the first factor, Tl concentrations between groundmass and groundmass sulphide crystals in Kīlauea Iki basalts were found to be heterogeneous, and fractionation of Tl towards heavier isotopic values from sulphide formation in the melt is estimated to require an average modal abundance of 6% sulphide blebs (Gaschnig et al, 2021). This modal abundance is not reached in the Kīlauea Iki samples given their low bulk S content (∼700-1,300 ppm; Gaschnig et al, 2021;Gerlach & Graeber, 1985).…”

“…With regards to the first factor, Tl concentrations between groundmass and groundmass sulphide crystals in Kīlauea Iki basalts were found to be heterogeneous, and fractionation of Tl towards heavier isotopic values from sulphide formation in the melt is estimated to require an average modal abundance of 6% sulphide blebs (Gaschnig et al, 2021). This modal abundance is not reached in the Kīlauea Iki samples given their low bulk S content (∼700-1,300 ppm; Gaschnig et al, 2021;Gerlach & Graeber, 1985). Although this is an estimate for Kīlauea, it is likely representative of other Hawaiian tholeiites as Mauna Loa, Mauna Kea, Kīlauea, and Lō'ihi all have similar sulfur contents (Davis et al, 2003).…”

“…Conversely, low-temperature altered oceanic crust (AOC) is isotopically light, with an ε 205 Tl as low as −15, due to the addition of light Tl from seawater that permeates into the upper crust (Nielsen, Rehkämper, Teagle, et al, 2006). The potential utility of Tl as a mantle source tracer is enhanced by the fact that Tl isotopes are not typically fractionated during magmatic processes, such as fractional crystallization (Gaschnig et al, 2021;Prytulak et al, 2017) and partial melting, as assessed by comparison of MORB and mantle samples (Fitzpayne et al, 2020;Nielsen et al, 2015;Nielsen, Rehkämper, Norman, et al, 2006), and thus should retain a primary source composition.…”