Geochemical Behavior of Selenium in Igneous Systems

Selenium (Se) is generally assumed to behave much like sulfur (S) in igneous systems. However, it is unclear how valid this assumption is considering that so little is known about the geochemical behaviour of Se. Constraining the range in Se and S concentrations of mantle-derived magmas is important to studies of: core segregation; the composition of late-accreted material; collisional erosion models; processes of mantle melting in various tectonic environments; and recycling of lithospheric components into the mantle, to mention only a few. Previous estimates of the S/Se of primitive and depleted mantle assume that S-Se are similar to Zr-Hf and Nb-Ta in their geochemical coherence, and that S/Se of the Earth's mantle is chondritic (Palme and O"Neill 2003 and references therein). Due to the low abundances of Se in mantle-derived rocks and the lack of calibration materials for routine analysis (i.e. NIST 612), this assumption remains unchallenged. Using a combination of SHRIMP, electron probe, and LA-ICP-MS techniques the concentration of Se in NIST 612 and BCR-2G was obtained to permit rapid in situ LA-ICP-MS analysis of Se concentrations in volcanic glasses. We have obtained Se, S, major and comprehensive trace element data for volcanic glasses from a global range of tectonic settings (mid-ocean ridges, ocean island, island arc and back-arc basins) to improve understanding of the behaviour of Se during igneous processes (e.g. partial melting, fractional crystallisation, and volatile phase separation). Analysis of a range of mid-ocean ridge glasses shows that Se behaves as an incompatible element, but is decoupled from S as the S/Se extends to values well in excess of the chondritic estimate (i.e. 2528; Palme and O"Neill 2003). During back-arc basin and island-arc magmatism, the abundance, systematics and S/Se are diverse. For example, at a specific MgO content, the absolute abundance of Se varies with depletion of the mantle source. Within back-arc suites derived from relatively fertile mantle sources (e.g., Coriolis Troughs, New Hebrides), Se is negatively correlated with MgO abundances in the range from basalt to dacitic compositions. In contrast, Se abundances in back-arc suites derived from relatively depleted sources (e.g., Manus and Lau Basin) show less incompatible behaviour. In detail, these systematics are complex and in the case of the Pacmanus site in the Manus Basin and Valu Fa Ridge of the Lau Basin, may be related to timing of sulfide and magnetite saturation and fractionation. Palme, H. and O"Neill, H.St.C. (2003) Cosmochemical Estimates of Mantle Composition, Treatise on Geochemistry, Vol. 2: The Mantle and Core. Ed. R.W. Carlson, Elsevier, pp.1-38.