Browsing by Author "Zellmer GF"

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    Deposits, character and timing of recent eruptions and gravitational collapses in Tatun Volcanic Group, Northern Taiwan: Hazard-related issues
    (ELSEVIER SCIENCE BV, 2010) Belousov A; Belousova M; Chen C-H; Zellmer GF
    Taipei City, with a population of around 8 million, as well as two nuclear power plants is located in close proximity to the Quaternary, dominantly andesitic Tatun Volcanic Group (TVG) of Northern Taiwan. We have investigated the stratigraphy of the youngest volcaniclastic deposits, as well as the morphology of lava flows and domes of the TVG in order to reconstruct the character and timing of the most recent eruptions and related hazardous events in the area. Our data indicate that recent eruptions of the group were dominated by long-term, voluminous extrusions of crystal-rich, very viscous lavas. These eruptions formed closely spaced monogenetic domes and lava flows. Based on morphological parameters of the lava flows (thicknesses 80–150 m, lengths up to 5.6 km, and volumes up to 0.6 km3), average rates of magma effusion ranged from 1 to 10 m3/s, eruption durations from 500 to 1800 days, and lava front speeds from 0.5 to 6 m/h. Explosive activity of TVG was diverse, ranging from weak phreatic to highly explosive (VEI 4) Plinian eruptions; vulcanian activity with deposition of lithic ashes was most common. Interaction of rising magma with ground water frequently occurred during the eruptions. This study presents the first radiocarbon dates of various volcaniclastic deposits of the TVG, which indicate that Cising, Siaoguanyin, and possibly Huangzuei volcanoes had magmatic eruptions in the period 13,000–23,000 years ago. In addition, Mt. Cising had a phreatic eruption 6000 years ago, and possibly an effusive eruption just before that. Gravitational collapses of volcanic edifices with volumes 0.01–0.1 km3 and H/L 0.16–0.25 were also common. They occurred on intersections with tectonic faults and may have been triggered by seismic activity. The youngest collapses occurred at Mt. Siaoguanyin (23,000 BP) and Mt. Cising (6000 BP). It is concluded that the TVG should be considered volcanically active. The results of this study provide a basis for volcanic hazard assessment and mitigation in the area.
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    EPMA maps unveil the actual chemical variations and crystallization sequence of pyroxenes and plagioclase solidified from a basaltic liquid at variable cooling rates
    (Elsevier B V, 2023-12-05) Gennaro E; Radica F; Iezzi G; Vetere F; Nazzari M; Zellmer GF; Scarlato P; Romano C
    Crystal-chemical variations of pyroxene (px) and plagioclase (plg) have been analysed by X-ray electron microprobe (EPMA) mapping to quantify their actual chemical dispersions. These phases were experimentally crystallised from a basaltic liquid (B100, MORB from Iceland) at cooling rates of 1, 7, 60 and 180 ◦C/h from 1300 ◦C down to 800 ◦C. Experiments were run at ambient conditions applying defined temperature paths mirroring characteristic cooling rates from innermost to outermost portions of metre- to centimetre-thick lavas, dikes and bombs emplaced under submarine to subaerial conditions. As the cooling rate increases from 1 to 180 ◦C/h, the run-products become progressively enriched in pyroxene and depleted in plagioclase, while spinel is invariably low (few area%) and glass is significant only at 180 ◦C/h. An increase of cooling rate generally leads to enrichment of Al2O3 and depletion of MgO in px, while the opposite behaviour is observed for plg; these trends are mirrored by calculated cations (apfu: atom per formula unit) and components. Average variations as a function of cooling rate are similar to those already observed through classical analysis performed by single point EPMA. However, the actual chemical distributions of CaO versus MgO, Al2O3 and FeOtot oxides unveil the presence of a wider range in pyroxene chemistry. In particular, one px (px-1, CaO-rich, diopsidic type) is present at all the applied cooling rates; a very low CaO-px (px-2, pigeonite or orthopyroxene type) is detected at 1 ◦C/h; and, finally, once more population of px (px-3, CaO-poor diopsidic type) appears at 60 and 180 ◦C/h. By contrast, plg analyses yield invariably identical compositions. Textural variations as a function of cooling rate and geo thermometric estimations indicate that px-1 crystallised at high-T (or low ΔT), while plg mainly grew in the residual melt produced by the saturation of px. If only textures were evaluated, this order of segregation would like remain unrecognised since px at low cooling rates is smaller than plg. The abundance of phases, their crystal chemical features, and their order of segregation can be regarded through a theoretical framework of a time temperature-transformation (TTT) diagram. The most significant chemical variations are displayed by MgO and Al2O3 for both px and plg, which faithfully capture the evolution of cooling conditions. The chemical compositions of px-1 is close to the thermodynamic equilibrium only at 1 ◦C/h. As the cooling rates increase, the px chemistry indicates disequilibrium conditions. Finally, this study shows that as ΔT/Δt increases, the most abundant px (px-1) and plg are forced towards compositions that become progressively closer to those of the parental liquid.
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    Exploring intrusive processes through the crystal cargo of volcanic rocks: The case of lava flows from Taranaki volcano, New Zealand
    (Elsevier B V, 2024-11) D'Mello NG; Zellmer GF; Ubide T; Caulfield J; Usuki M; Iizuka Y; Kereszturi G; Procter JN; Stewart RB; Romano C
    The present-day edifice of Taranaki volcano, New Zealand, is largely made up of lava flows extruded over approximately the last 8 kyr. The crystal cargo of plagioclase, pyroxene and amphibole in these lavas displays varied major, minor, and trace element zoning patterns, pointing to long and complex crystal growth histories. Crystal zoning patterns do not vary systematically between stratigraphic units, and multiple patterns are seen within the same sample over very short length scales. Intracrystalline elemental variations reveal mineral-melt interactions, which result in repeated resorption and recrystallisation in varied environments. Variable degrees of undercooling are evidenced by clinopyroxenes, with most crystals displaying sector zoning (ΔT < 50 K), while others only show concentric zoning, which suggests very low ΔT. The common occurrence of resorbed cores within the crystals and the prevalence of glomerocrysts indicate antecrystic and/or xenocrystic origins and crystal aggregation processes. We hypothesise that the repeated intrusion of melts into the crustal basement of Taranaki volcano has resulted in the formation of a heterogeneous subsolidus plutonic to supersolidus mushy (∼15–55 vol% crystals) system that interacts with intruding melts from the mantle. These interactions result in disaggregation of crystal clots from the plutonic intrusives and remobilization of the crystals through various sub-environments of small ephemeral mush pockets. Eruption-triggering injections of melt then pick up these crystals with varied growth histories to be extruded
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    MushPEC: Correcting Post-entrapment Processes Affecting Melt Inclusions Hosted in Olivine Antecrysts
    (Frontiers Media S A, Switzerland, 2021-01-29) Brahm R; Zellmer GF; Kuritani T; Coulthard Jr. D; Nakagawa M; Sakamoto N; Yurimoto H; Sato E; Petrone CM
    Olivine-hosted melt inclusions (MIs) are widely used as a tool to study the early stages of magmatic evolution. There are a series of processes that affect MI compositions after trapping, including post-entrapment crystallization (PEC) of the host mineral at the MI boundaries, exsolution of volatile phases into a “shrinkage bubble” and diffusive exchange between a MI and its host. Classical correction schemes applied to olivine-hosted MIs include PEC correction through addition of olivine back to the melt until it reaches equilibrium with the host composition and “Fe-loss” correction due to Fe-Mg diffusive exchange. These corrections rely on the assumption that the original host composition is preserved. However, for many volcanic samples the crystal cargo is thought to be antecrystic, and the olivine composition may thus have been completely re-equilibrated during long crystal storage times. Here, we develop a novel MI correction scheme that is applicable when the original host crystal composition has not been preserved and the initial MI composition variability can be represented by simple fractional crystallization (FC). The new scheme allows correction of MI compositions in antecrystic hosts with long and varied temperature histories. The correction fits a set of MI compositions to modelled liquid lines of descent generated by FC. A MATLAB® script (called MushPEC) iterates FC simulations using the rhyolite-MELTS algorithm. In addition to obtaining the corrected MI compositions, the application of this methodology provides estimations of magmatic conditions during MI entrapment. A set of MIs hosted in olivine crystals of homogeneous composition (Fo77–78) from a basaltic tephra of Akita-Komagatake volcano was used to test the methodology. The tephra sample shows evidence of re-equilibration of the MIs to a narrow Mg# range equivalent to the carrier melt composition. The correction shows that olivine hosts were stored in the upper crust (c. 125 – 150 MPa) at undersaturated H2O contents of c. 1 – 2 wt% H2O)