title: Volatile Cycling and Metasomatism in Flat-Slab Subduction Zones of the Central Andes
Creators: name: Hiett, Coleman
Publisher: name: Utah State University
Year: 2023

description: Flat-slab subduction, where an oceanic plate subducts horizontally below a continental margin for hundreds of kilometers, is an enigmatic but prevalent tectonic configuration in which chemical cycling and alteration of the continental plate is poorly constrained. Geochemical investigations in regions of modern and ancient flat-slab subduction in the Central Andes afford an opportunity to study this process. Certain elements naturally occur with varying number of neutrons in their nuclei (isotopes), and measurements of isotope ratios within geologic materials inform on chemical sources and geologic processes. This research leverages stable isotope analyses and other geochemical tools to investigate volatiles and other material transferred from a flat slab to the overlying continental plate and how this chemically modifies the roots of continental mountain belts. I employ geochemical investigation of gasses emitted from thermal springs above a modern flat-slab subduction zone in central Peru, as well as investigations of volcanic rocks that entrained fragments of the crust and mantle during ascent to the surface (xenoliths) in a region affected by shallow subduction > 25 million years ago.
Geochemical analyses of thermal springs across the Peruvian Andes, including helium, carbon, and nitrogen stable isotope ratios, relative gas abundances, and aqueous geochemistry, reveal mantle and crustal degassing in response to active flat slab-fluid transfer and continental hydration. Mantle volatile fluxes require continuous fluid transfer and permeable pathways through chemically modified continental crust and mantle. Variability in deep volatile signatures reflect mantle-to-surface transit times and mixing between mantle and crustal volatile sources that are locally influenced by tectonic processes in the oceanic and continental plates.
Xenoliths recovered from Quaternary lava flows in the northern Altiplano Plateau include a suite of mica-rich alkaline igneous lithologies that yield hydrogen isotope ratios and trace element characteristics indicating formation by melting of a chemically modified mantle source enriched by fluids or melts during flat-slab subduction. A second suite of carbonate- and mica-rich mantle xenoliths are samples of the modified mantle. Carbon and oxygen isotope ratios of calcite and trace element characteristics of these xenoliths, coupled with observations from xenolith hosting alkaline lava flows, reveal that chemical modification of the mantle was facilitated by melts of subducted sediments accreted to the base of the continental plate during flat-slab subduction. The collective results of this dissertation highlight novel insight into chemical cycling and modification of the continental mantle in flat-slab systems, including plate-scale volatile transport and the role of subducted sediments in mantle alteration. More broadly, this work represents profound advances in our understanding of the coupled tectonic and chemical evolution of the Andean Cordillera and continental margins globally.