• Causes and consequences of coupled crystallization and vesiculation in ascending mafic magmas

      Lindoo, Amanda N.; Larsen, Jessica F.; Freymueller, Jeffrey; Izbekov, Pavel; Trainor, Tom (2017-08)
      Transitions in eruptive style and eruption intensity in mafic magmas are poorly understood. While silicic systems are the most researched and publicized due to their explosive character, mafic volcanoes remain the dominant form of volcanism on the earth. Eruptions are typically effusive, but changes in flow behavior can result in explosive, ash generating episodes. The efficiency of volatiles to degas from an ascending magma greatly influences eruption style. It is well known that volatile exsolution in magmas is a primary driving force for volcanic eruptions, however the roles vesicles and syn-eruptive crystallization play in eruption dynamics are poorly understood. Permeability development, which occurs when gas bubbles within a rising magma form connected pathways, has been suspected to influence eruption style and intensity. Numerous investigations on natural eruptive products, experimental samples, and analog experiments have extended the understanding of permeability development and fragmentation processes. However, these studies have focused on silicic, high viscosity, crystal-poor magmas. Little progress has been made in understanding fragmentation mechanisms in mafic or alkali magmas. Mafic systems involve lower viscosity magmas that often form small crystals, also known as microlites, during ascent. Because the merging of bubbles in magma is mitigated by melt viscosity, it is predicted that permeability development in mafic magma will occur at lower bubble volume fractions than in silicic magma. However, no study has been performed on experimental samples to provide evidence for this hypothesis. Furthermore, it is unknown how microlites affect the degassing process in terms of facilitating or hindering permeability development. This thesis employs experimental petrology to: 1) experimentally observe how melt viscosity alone affects permeability development, 2) Understand the effects of syn-eruptive crystallization in vesiculating mafic magmas and synergizes these results to 3) relate experimental findings to the 2008 eruption of Kasatochi volcano.
    • Cenozoic tectono-thermal history of the southern Talkeetna Mountains, Alaska: multiple topographic development drivers through time

      Terhune, Patrick J.; Benowitz, Jeffrey; Freymueller, Jeffrey; Gillis, Robert (2018-08)
      Intraplate mountain ranges can have polyphase topographic development histories reflecting diverse plate boundary conditions. We apply ⁴⁰Ar/³⁹Ar, apatite fission track (AFT) and apatite (U-Th)/He (AHe) geochronology-thermochronology to plutonic and volcanic rocks in the southern Talkeetna Mountains of Alaska to document regional magmatism, rock cooling and inferred exhumation patterns as proxies for the deformation history of this long-lived intraplate mountain range. High-temperature ⁴⁰Ar/³⁹Ar geochronology on muscovite, biotite and K-feldspar from Jurassic granitoids indicates post-emplacement (~158-125 Ma) cooling and Paleocene (~61 Ma) thermal resetting. ⁴⁰Ar/³⁹Ar whole rock volcanic ages and AFT cooling ages in the southern Talkeetna Mountains are predominantly Paleocene-Eocene, suggesting that the Range is partially paleotopography that formed during an earlier tectonic setting. Miocene AHe cooling ages within ~10 km of the Castle Mountain Fault suggest ~2-3 km of vertical displacement that also contributed to mountain building, likely in response to the inboard progression of the subducted Yakutat microplate. Paleocene-Eocene volcanic and exhumation ages across interior southern Alaska north of the Border Ranges Fault System are similar and show no N-S or W-E progressions, suggesting a broadly synchronous and widespread volcanic and exhumation event that conflicts with the proposed diachronous subduction of an active west-east sweeping spreading ridge beneath south-central Alaska. To reconcile this, we propose a new model for the Cenozoic tectonic evolution of southern Alaska. We infer that slab breakoff sub-parallel to the trench and subsequent mantle upwelling drove magmatism, exhumation and rock cooling synchronously across south-central Alaska and played a primary role in the development of the southern Talkeetna Mountains.