Aspects of the petrology and geochemistry of the Huckleberry Ridge Tuff, Yellowstone
Authors:   , Swallow Elliot 
Published: November 2021

<p>Silicic (i.e. dacitic-rhyolitic) magmatic systems have the potential to generate large, explosive caldera-forming eruptions which have global effects and consequences. How, and over what timescale, magma accumulates and is stored in the upper crust are key aspects in understanding such systems and their associated hazards. The absence of such eruptions in the historical record, however, has forced understanding of these systems to be developed through numerical models or the study of the deposits in the geological record. Numerical models primarily focus on the long-term generation but instantaneous eruption of single magma (i.e. melt-dominant) bodies. In contrast, the stratigraphic and geochemical nature of eruption deposits often show features more consistent with complex magmatic systems comprising multiple melt-dominant bodies that may have formed rapidly but erupted episodically. Further studies of past eruption deposits are valuable, therefore, in reconstructing silicic magmatic systems and highlighting the nature of melt-dominant body generation and storage.  To this end, this thesis examines the 2.08 Ma, ∼2,500 km³ Huckleberry Ridge Tuff (HRT), Yellowstone Plateau volcanic field (YPVF), U.S.A, the deposit of the first and largest of three caldera-forming eruptions in the YPVF. The HRT comprises an initial fall deposit followed by three ignimbrite members (A, B and C) with a second fall deposit between members B and C. Despite emanating from an archetypal silicic volcanic field, minimal previous work has been undertaken on the geochemical nature of the HRT but it is thought to conform to traditional, unitary magma body ideas. A revised stratigraphic framework, detailing an episodic and prolonged initial fall deposit, identification of a weeks-months time gap between members A and B, and a similar but longer years-decades hiatus in activity between members B and C provides the context for this geochemical investigation. A large sample suite representative of the diverse range of physical characteristics of clasts and material found in the HRT was analysed. In situ micro-analysis of matrix glass (major and trace elements) and crystals (major elements) in the initial fall deposit are coupled with major and trace element, and isotopic compositions of single silicic clasts (i.e. pumice/fiamme) from all three ignimbrite members, supplemented by in situ analysis of their crystals and groundmass glass. These data are used to reconstruct the silicic magmatic system. Furthermore, major and trace element, andisotopic compositions of rare mafic (i.e. basaltic to andesitic) material found in members A and B provide an insight into the thermal and chemical drivers of HRT silicic volcanism.  This macro- and micro-analytical investigation using multiple techniques reveals remarkable complexity within the large-scale HRT magmatic complex. Four geochemically distinct magmatic systems are differentiated on single clast elemental and isotopic characteristics that are further reflected in crystal and glass compositions. Two of these systems (1 and 2) were active in the initial fall deposit and member A. Magmatic system 1 is volumetrically dominant in the HRT and is characterised by moderate-high Ba single clast (450-3540 ppm) and glass (100-3360 ppm) compositions, in contrast to the distinctly low-Ba (≤250 ppm single clast, <65 ppm glass Ba contents) magmatic system 2. Both these magmatic systems exhibit clustered glass compositions, indicating multiple, laterally-adjacent melt-dominant bodies were present, and shared moderate isotopic compositions (e.g. ⁸⁷Sr/⁸⁶SrAC = 0.70950-0.71191) are explicable by a multi-stage partial melting-fractional crystallisation petrogenesis. The time break between members A and B is associated with mixing and mingling within magmatic system 1, related to a renewed influx of mafic material, and a cessation of activity of system 2, which is absent from member B. The time break between members B and C reflects significant changes within the magmatic complex. Magmatic system 2 is rejuvenated and melt-dominant bodies associated with two new magmatic systems (3 and 4) are formed, with at least system 3 comprising multiple bodies. These latter two magmatic systems strongly differ in their elemental characteristics (system 3: high SiO₂ [75-78 wt% SiO₂]; system 4: dacite-rhyolite [66-75 wt% SiO₂]). Despite this, they have similar and highly radiogenic (e.g. ⁸⁷Sr/⁸⁶SrAC = 0.72462-0.72962) isotopic compositions indicating shared extensive incorporation of Archean crust. They also contrast in their relation to mafic compositions, with system 4 associated with olivine tholeiitic compositions erupted prior to and following the HRT in the YPVF. In contrast, system 3, like systems 1 and 2, is associated with high-Ba, high-Zr mafic compositions found co-erupted in HRT members A and B. These compositions are similar to lava flows erupted further west at the Craters of the Moon field, and are interpreted as representing partial melts from regions in the lithospheric mantle enriched by high-T, P fluids emanating from the subducted Farallon slab.  Overall, the HRT magmatic complex was remarkably heterogeneous. Two contemporaneous mafic root zones drove four silicic magmatic systems, episodically active throughout the eruption. At least three of these systems comprised multiple laterally-adjacent melt-dominant bodies. Intra-eruption time breaks are associated with broad-scale reorganisation of the magmatic complex. This complexity highlights the utility of a detailed, systematic, multi-technique geochemical investigation, within a stratigraphic framework, of the deposits of large silicic caldera-forming eruptions, and breaks new ground in the understanding of such systems.</p>