The lower crust as a volatile reservoir: new constraints from the Talkeetna arc (Alaska)

Volatiles play a fundamental role in arc magmatism and ore-forming processes (Wilkinson, 2013; Zellmer et al., 2015). However, their origin and distribution within the lower crust of arcs remain poorly constrained, despite the potential of the lower crust to act as a significant volatile reservoir. Well-preserved fossil arc sections such as the Talkeetna arc (Alaska) provide rare opportunities to investigate volatile behavior across near-complete profiles of lower arc crust (Clift et al., 2005; Hacker et al., 2008).

In this study, we aim to combine petrography, thermobarometry, volatile concentrations, major and trace element geochemistry to constrain the distribution and origin of volatiles within the lower crust of the Talkeetna arc. We selected a sample suite that consists of 14 mafic to ultramafic lower crustal cumulates, including pyroxenites and gabbroic lithologies, representative of different levels of the arc crust.

Volatile content (H₂O, C, Cl, S, F), together with major and trace elements concentrations are measured in Nominally Anhydrous Minerals (NAMs), including clinopyroxene (cpx) and orthopyroxene (opx) using SIMS, EPMA and LA-ICP-MS, respectively. Volatile concentrations show significant variability among mineral phases and lithologies. Clinopyroxene systematically contains at least 200 ppm H₂O across all lithologies with an average concentration of 650 ppm H₂O. Orthopyroxene generally contains lower H₂O contents, below 400 ppm H₂O, with an average of 300 ppm H₂O.

These non-negligible H₂O concentrations in NAMs have important implications for volatile recycling and storage in the lower crust (Wallace, 2005; Bekaert et al., 2021). Although NAMs contain lower H₂O concentrations than hydrous phases, their volumetric dominance may make them a major volatile reservoir on Earth. By integrating volatile concentrations with estimates of crustal thicknesses and lithological proportions, we provide constraints on the volatile budget of the Talkeetna lower crust, and assess its volatile storage capacity. This study provides a point of comparison of volatile budgets to other fossil arcs, such as Kohistan (Urann et al., 2022), and investigates the role of the lower crust as a long-term volatile reservoir.