TRACING CONTRIBUTIONS TO CASCADE MAGMATISM WITH THALLIUM ISOTOPES

The cause and contributions to volcanism in the Cascadia hot subduction zone of Western North America is the subject of extensive and ongoing research [e.g., 1]. Debate centers around the nature of compositionally distinct endmembers along the ~1300km strike of the arc, and whether radiogenic isotope compositions reflect subduction components such as sediments and/or slab melts, or if underlying, long-lived mantle heterogeneity has a more dominant role [1]. Stable thallium (Tl) isotopes are arguably the most sensitive tracer of sediment addition to a mantle source [2] and thus are an ideal tracer to apply to investigation of subduction components to Cascades volcanism. Thallium isotopes have been used to distinguish not only the presence of an exotic input to arc magma sources, but also the nature of this input, due to two main vectors of fractionation: pelagic clays and/or ferromanganese components are isotopically 'heavy', whereas oceanic crust altered at low temperatures is isotopically 'light' [2]. The utility of Tl is enhanced by the lack of isotope fractionation during normal magmatic processes [3]. We investigate sediment samples from ODP Sites 888B, 1027B and 1027C outboard the British Columbia coast [4] and well-characterised arc lavas from volcanic centres spanning the length of the Cascade Range [1], including the Canadian Garibaldi segment (GVB) in British Columbia and the High Cascades of the American Pacific Northwest for their Tl stable isotope composition. Preliminary data show sedimentary Ɛ²⁰⁵Tl at Site 888B dominated by continental input, within error of upper mantle and continental crust estimates (Ɛ²⁰⁵Tl ≈ -2; [3]). Thallium concentrations in GVB lavas are extremely low, notionally consistent with the suggestion that they have negligible sedimentary contribution [1]. We will present further stable Tl isotope data from the length of the arc to investigate the contributions to Cascades volcanism and provide tight constraints on their petrogenesis by combined stable and radiogenic isotope systematics.

[1] Mullen et al. 2017. Chem. Geol. 448, 43-70.

[2] Nielsen et al. 2016. Rev. Min. Geochem, 82, 759-798.

[3] Prytulak et al. 2017, Chem. Geol. 448, 71.83.

[4] Carpentier et al. 2014, Chem. Geol. 382, 67-82.