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J W Cole - One of the best experts on this subject based on the ideXlab platform.

  • extraction of crystal poor Rhyolite from a hornblende bearing intermediate mush a case study of the caldera forming matahina eruption okataina volcanic complex
    Contributions to Mineralogy and Petrology, 2011
    Co-Authors: C D Deering, J W Cole, Thomas A Vogel
    Abstract:

    The Matahina Ignimbrite (~160 km3 Rhyolite magma, 330 ka) was deposited during a caldera-forming eruption from the Okataina Volcanic Centre, Taupo Volcanic Zone (TVZ), New Zealand. Juvenile clasts are divided into three groups: Group (1) the dominant crystal-poor Rhyolite type, Group (2) a minor coarse-grained, mingled/mixed intermediate type, and Group (3) a rare fine-grained basalt. The ignimbrite consists of the Group 1 type and is divided into three members: a lower and middle member, which is high-silica, crystal-poor (<10 vol.%) Rhyolite, and the upper member, which is low-silica and slightly more crystal-rich (up to 21 vol.%). Cognate, crystal-rich (up to 50 vol.%) basalt to intermediate pumice occurs on top of lag breccias and within lithic-rich pyroclastic density current deposits along the caldera margin (Groups 2 and 3). Several lines of evidence indicate that the intermediate clasts represent the cumulate complement to the melt-rich Rhyolite: (1) continuity in the compositions of plagioclase, orthopyroxene, hornblende, and oxides and normal zoning of individual phenocrysts; (2) the silicic glass from the intermediate magma (interstitial melt) overlaps compositionally with the bulk rock Rhyolite and glass; (3) high Zr and a slight positive Eu anomaly in the intermediate magma relative to quenched enclaves from other intermediate TVZ eruptions indicates zircon and plagioclase accumulation, respectively; (4) an increase in the Cl contents in glass from the least evolved to most evolved is consistent with the concentration of volatiles during magma evolution. Most of the compositional variations in the low- to high-silica Rhyolites can be accounted for by continued Rayleigh fractionation (up to 15%), following melt extraction from the underlying mush, under varying fO2–fH2O conditions to form a slightly compositionally zoned rhyolitic cap. This link to the varying fO2–fH2O conditions is evidenced by the strong correlation between key geochemical parameters (e.g. Dy, Y), that qualitatively reflect fH2O conditions (presence or absence of hornblende/biotite), and fO2 estimated from Fe–Ti oxide equilibrium. Magma mingling/mixing between the basalt–andesite and the main slightly compositionally zoned rhyolitic magma occurred during caldera-collapse, modifying the least-evolved Rhyolite at the lower portion of the reservoir and effectively destroying any pre-eruptive gradients.

  • a Rhyolite compositional continuum governed by lower crustal source conditions in the taupo volcanic zone new zealand
    Journal of Petrology, 2008
    Co-Authors: C D Deering, J W Cole, Thomas A Vogel
    Abstract:

    Rhyolites generated in the modern Taupo Volcanic Zone (TVZ), New Zealand, have previously been interpreted as having evolved by a combination of extensive fractional crystallization of mantle-derived mafic magmas and limited crustal assimilation (up to 25%). Polytopic vector analysis (PVA), a form of multivariate statistical analysis, of the major-element compositions of over 475 basaltic to rhyolitic bulk-rock samples, representing over 600 kyr of volcanism within the TVZ, has provided a robust platform for Rhyolite characterization and new insights into Rhyolite petrogenesis. There is a continuum of compositions between two Rhyolite end-member magma types (EM1 and EM2), which have been identified on the basis of the PVA and which have distinct petrological and geochemical characteristics, as follows. EM1: crystal-rich (up to 45%), hydrous phases (± hornblende ± biotite ± cummingtonite), high Aluminum Saturation Index [ASI; molar Al 2 O 3 /(CaO + Na 2 O + K 2 O)], low FeO*/MgO (calc-alkaline series), depleted abundances of middle rare earth elements (MREE) and Y, and high Sr; EM2: crystal-poor (<10%), anhydrous phases (orthopyroxene ± clinopyroxene), high FeO*/MgO (tholeiitic series), low ASI, less depleted MREE and Y, and low Sr. The range of ASI values, and relative depletion in MREE and Y in the Rhyolites is consistent with the results of experiments to constrain the partial melting behaviour of amphibolite at crustal pressures. The major- and trace-element data are also consistent with 50-60% equilibrium crystallization of a crustally contaminated, hornblende-bearing andesite to produce the TVZ Rhyolites. Distinct major- and trace-element variations along the continuum between the two Rhyolite end-member types can be effectively modelled by simulating changes in the temperature-fO 2 -fH 2 O conditions in the lower crust where mantle-derived mafic magmas are stored and differentiate. Low T and high fO 2 and fH 2 O in the crustal magma storage zone promote abundant hornblende crystallization and suppress plagioclase crystallization, which produces the EM1 type Rhyolite. By increasing the temperature and/or lowering fO 2 and fH 2 O in the magma storage region, plagioclase becomes more dominant and hornblende crystallization is suppressed, producing more EM2-like rhyolitic magma types.

  • silicic recharge of multiple Rhyolite magmas by basaltic intrusion during the 22 6 ka okareka eruption episode new zealand
    Lithos, 2008
    Co-Authors: Phil Shane, Ian A Nairn, Victoria C Smith, Miles Darragh, Kate Beggs, J W Cole
    Abstract:

    Abstract Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete Rhyolite magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption (∼ 0.01 km3 magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing Rhyolite magma (T1) with a volume of ∼ 0.3 km3. Hybrid basalt/Rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other Rhyolite magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende Rhyolite magma (T2) (∼ 0.3 km3), and a hotter (780 °C), crystal-poor, pyroxene–hornblende Rhyolite magma (T3) (∼ 4.5 km3). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km3 of T3 magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the Rhyolite magmas. The eruption sequence and the restricted direct basaltic intrusion into only one magma (T1) is consistent with the Rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of

  • multiple Rhyolite magmas and basalt injection in the 17 7 ka rerewhakaaitu eruption episode from tarawera volcanic complex new zealand
    Journal of Volcanology and Geothermal Research, 2007
    Co-Authors: Phil Shane, J W Cole, Victoria C Smith, Miles Darragh, Kate Beggs, S B Martin, Ian A Nairn
    Abstract:

    Abstract The 17.7 ka Rerewhakaaitu eruption episode (volume ∼ 5 km3 DRE Rhyolite magma) was the second of five major episodes that have built the Tarawera volcanic complex in the Okataina Volcanic Centre over the past 22 kyr. The Rerewhakaaitu episode produced a widespread tephra fall deposit, associated proximal pyroclastic flow deposits, and voluminous Rhyolite lava extrusions. Two different Rhyolite magmas (T1 and T2) were simultaneously erupted from the main vent area throughout much of the eruption episode. T1 magma was a crystal-poor orthopyroxene-hornblende Rhyolite that is highly evolved (whole rock SiO2 = 77 wt.%), with a moderate temperature (∼ 760 °C, based on Fe–Ti oxides). T2 is a crystal-rich biotite-hornblende Rhyolite that is less evolved (SiO2 = 75 wt.%), with a Fe–Ti oxide temperature of ∼ 700 °C. Ejecta from the simultaneous and sequential eruption of these two magmas include some pumice clasts with mixed (hybrid) and mingled glass compositions and crystal populations. A third Rhyolite magma (T3) was extruded from another vent 3 km distant to form an apparently contemporaneous lava dome. T3 was the least evolved (SiO2 = 74 wt.%) and hottest (∼ 820 °C) of the three magmas. Saturation pressures calculated using dissolved H2O and CO2 contents of melt inclusions in quartz crystals indicate that T2 magma stagnated and crystallised at about 12 km depth, while small quartz crystals in T1 magma grew during ascent through ∼ 8 km depths. Some T1 and T2 Rhyolite clasts contain vesicular brown blebs with widely variable (andesite to Rhyolite) glass compositions, accompanied by olivine, clinopyroxene and calcic plagioclase crystals that are interpreted as xenocrysts derived from injected basalt. Temperatures over 1000 °C estimated from pyroxene phase equilibria in these clasts reflect intrusion of the more mafic magma, which is now identified as the priming and triggering mechanism for three of the four post-22 ka Tarawera Rhyolite eruption episodes. However, the Rhyolite magma bodies and conduits modelled for each episode have considerable differences in characteristics and geometry. Our preferred model for the Rerewhakaaitu episode is that eruptions occurred from three laterally and vertically isolated Rhyolite magma bodies that were initially primed and triggered by basalt intrusion during a regional rifting event. The ascending hotter and less viscous T1 Rhyolite magma intersected and further invigorated a stagnant pond of cooler, denser and more viscous T2 magma, and lubricated its transport to the surface.

  • Rhyolite magma processes of the ad 1315 kaharoa eruption episode tarawera volcano new zealand
    Journal of Volcanology and Geothermal Research, 2004
    Co-Authors: Ian A Nairn, J W Cole, Phil Shane, G J Leonard, Stephen Self, N Pearson
    Abstract:

    Abstract Products of the ∼5 km3 (DRE), ∼5-yr duration Kaharoa eruption episode display two main high-silica Rhyolite compositions; T1 erupted early (as plinian pyroclastics), and T2 erupted late (mostly as lavas). The T1 and T2 eruptive types are defined by crystal contents and compositional variations in whole rock, glass, plagioclase and biotite. Stratigraphically intermediate pyroclastic deposits have an intermediate composition (T1+2). A small volume of rhyodacite pyroclastics, mingled with injected basalt, was also erupted. The Kaharoa Rhyolites were erupted from multiple sources spread along an 8-km linear vent zone, but the changes in eruptive compositions were largely controlled by position in the eruption sequence and magma discharge rates, rather than vent locations. Data from the Kaharoa eruptive types, vent locations, eruption sequence and discharge rates can be combined with concepts of magma chamber evacuation processes to produce a preliminary dimensional model of the pre-eruption Rhyolite magma body. Our model magma body is sill-like, ∼8 km long, 1 km wide, 1.4 km thick, and located at ∼6–7 km depth in the upper crust. T1 magma overlay T2 magma in the upper levels of the chamber, with each magma layer internally mixed to a homogeneous composition along an axial extent defined by the vent locations. An underlying third Rhyolite magma (T3) is recognised as the silicic end-member that was modified by basalt to form the rhyodacite eruptives. The Rhyolite magma stratification survived multiple injections of basalt magma, which primed and finally triggered the Kaharoa eruptions. The T1+2 eruptives resulted from syn-eruption mingling in the conduit of the two main Rhyolite magma types. Thickness of the T1 layer in the model can be estimated at 0.25 km; the T2 layer was somewhat thicker. Thicknesses of the underlying T3 and basalt layers are uncertain. Post-eruption geothermal heat flow indicates a residual magma volume of ≥6 km3, suggesting that the pre-eruption magma volume was ≥11 km3.

Axel K. Schmitt - One of the best experts on this subject based on the ideXlab platform.

  • Acigöl Rhyolite field, central Anatolia (part II): geochemical and isotopic (Sr–Nd–Pb, δ18O) constraints on volcanism involving two high-silica Rhyolite suites
    Contributions to Mineralogy and Petrology, 2011
    Co-Authors: Wolfgang Siebel, Axel K. Schmitt, Elena Kiemele, Martin Danišík, Faruk Aydin
    Abstract:

    The Acigol Rhyolite field erupted the most recent high-silica Rhyolites within the Cappadocian Volcanic Province of central Anatolia, Turkey. It comprises two sequences of domes and pyroclastic rocks with eruption ages of ~150–200 ka (eastern group) and ~20–25 ka (western group). Compositionally, the eastern Rhyolite group lavas are less evolved (SiO2 = 74–76 wt%), whereas the western group has higher silica abundance (SiO2 = ~77 wt%) with extremely depleted feldspar-compatible trace elements. Within each group, compositional variability is small and 143Nd/144Nd (0.51257–0.51265) and Pb isotope compositions (206Pb/204Pb = 18.87–18.88, 207Pb/204Pb = 15.65–15.67 and 208Pb/204Pb = 38.94–38.98) are homogeneous. The western group Rhyolites have δ18O(zircon) overlapping mantle values (5.7 ± 0.2‰), whereas eastern group Rhyolites are enriched in δ18O by ~0.5‰, consistent with a tendency to lower eNd values. By contrast, western group Rhyolites have markedly more radiogenic 87Sr/86Sr ratios (0.7065–0.7091) compared to those of the eastern group (0.7059–0.7065). The presence of angular granitic xenoliths and a correlation between hydration (based on loss on ignition data) and 87Sr/86Sr in the western lavas, however, indicates that Sr was added during the eruption or post-eruption alteration. Isotope constraints preclude the possibility that the Rhyolite magmas formed by partial melting of any known regional crystalline basement rocks. Basalts and andesites erupted in the periphery of the Acigol field are characterised by 87Sr/86Sr ratios between 0.7040 and 0.7053, 143Nd/144Nd = 0.51259–0.51300, 206Pb/204Pb = 18.85–18.87, 207Pb/204Pb = 15.646–15.655, 208Pb/204Pb = 38.90–38.97. The isotopic and trace element data favour an origin of the Rhyolites by mixing of basaltic/andesitic magmas with minor amounts of crustal melts and followed by extensive fractional crystallization.

  • acigol Rhyolite field central anatolia part 1 high resolution dating of eruption episodes and zircon growth rates
    Contributions to Mineralogy and Petrology, 2011
    Co-Authors: Axel K. Schmitt, Wolfgang Siebel, Elena Kiemele, Martin Danišík, Faruk Aydin, Noreen J Evans, Janet C Harvey
    Abstract:

    Protracted pre-eruptive zircon residence is frequently detected in continental Rhyolites and can conflict with thermal models, indicating briefer magma cooling durations if scaled to erupted volumes. Here, we present combined U-Th and (U-Th)/He zircon ages from the Acigol Rhyolite field (Central Anatolia, Turkey), which is part of a Quaternary bimodal volcanic complex. Unlike other geochronometers, this approach dates crystallization and eruption on the same crystals, allowing for internal consistency testing. Despite the overall longevity of Acigol Rhyolite volcanism and systematic trends of progressive depletion in compatible trace elements and decreasing zircon saturation temperatures, we find that zircon crystallized in two brief pulses corresponding to eruptions in the eastern and western part of the field during Middle and Late Pleistocene times, respectively. For Late Pleistocene zircon, resolvable differences exist between interior (average: 30.7 ± 0.9 ka; 1σ error) and rim (21.9 ± 1.3 ka) crystallization ages. These translate into radial crystal growth rates of ~10−13 to 10−14 cm/s, broadly consistent with those constrained by diffusion experiments. Rim crystallization and (U-Th)/He eruption ages (24.2 ± 0.4 ka) overlap within uncertainty. Evidence for brief zircon residence at Acigol contrasts with many other Rhyolite fields, suggesting that protracted zircon crystallization in, or recycling from, long-lived crystal mushes is not ubiquitous in continental silicic magma systems. Instead, the span of pre-eruptive zircon ages is consistent with autochthonous crystallization in individual small-volume magma batches that originated from basaltic precursors.

  • large volume Rhyolite genesis in caldera complexes of the snake river plain insights from the kilgore tuff of the heise volcanic field idaho with comparison to yellowstone and bruneau jarbidge Rhyolites
    Journal of Petrology, 2011
    Co-Authors: Kathryn E Watts, Ilya N Bindeman, Axel K. Schmitt
    Abstract:

    Generation of large-volume Rhyolites in the shallow crust is an important, yet enigmatic, process in the Snake River Plain and worldwide. Here, we present data for voluminous Rhyolites from the 6·6–4·5 Ma Heise volcanic field in eastern Idaho. Heise is arguably the best site to evaluate shallow Rhyolite genesis in the Snake River Plain; it is the youngest complete record of caldera cluster volcanism along the Yellowstone hotspot track and it culminated with the eruption of the most voluminous low-δ 18 O Rhyolite known on Earth: the 1800 km 3 Kilgore Tuff (δ 18 O = 3·4‰). Such low-δ 18 O values fingerprint meteoric waters, and thus the shallow crust. New oxygen isotope data for phenocrysts, obtained by laser fluorination, correspond to a low-δ 18 O magma value of 3·4 ± 0·1‰ (2 standard error) for Kilgore Tuff samples erupted >100 km apart; however, ion microprobe data for single zircon crystals show significant diversity, with δ 18 O values that range from –1·3‰ to 6·1‰. U–Pb zircon ages, mineral chemistry, whole-rock major and trace element geochemistry, Sr and Nd isotope data, and magmatic (liquidus) temperatures are similar and/or overlapping for all studied samples of the Kilgore Tuff. Normal-δ 18 O Heise tuff units that preceded the Kilgore Tuff define a temporal compositional trend in trace element concentrations, trace element ratios, and Sr and Nd isotope ratios that is consistent with fractional crystallization from a common reservoir, whereas low-δ 18 O Kilgore cycle units have compositions that define a sharp reversal in the temporal trend back towards the composition of the first normal-δ 18 O Heise tuff (6·62 Ma Blacktail Creek Tuff). The data support derivation of the voluminous low-δ 18 O Kilgore Tuff from remelting of hydrothermally altered ( 18 O depleted) intracaldera and subvolcanic portions of the Blacktail Creek Tuff. Single pockets of melt with variable low-δ 18 O values were assembled and homogenized on a caldera-wide scale prior to the climactic Kilgore Tuff eruption, and the best record of this process is provided by the δ 18 O diversity in Kilgore Tuff zircons. Temporal trends of oxygen isotopic depletion and recovery in Rhyolite eruptions of the Heise volcanic field are clearly linked to caldera collapse events, and remarkably consistent with trends in the Yellowstone Plateau volcanic field. At Heise and Yellowstone, magmatic δ 18 O values can be predicted on the basis of cumulative eruptive volumes, with a decrease in δ 18 O by ∼1‰ for every ∼1000 km 3 of erupted Rhyolite. The Kilgore Tuff of the Heise volcanic field has the same timing, magnitude of δ 18 O depletion, and cumulative eruptive volume as the youngest phase of voluminous rhyolitic eruptions in the Yellowstone Plateau volcanic field, indicating that the Kilgore Tuff may serve as a useful analog for these and perhaps other large-volume low-δ 18 O Rhyolites on Earth.

  • Accessory mineral U–Th–Pb ages and ^40Ar/^39Ar eruption chronology, and their bearing on rhyolitic magma evolution in the Pleistocene Coso volcanic field, California
    Contributions to Mineralogy and Petrology, 2009
    Co-Authors: Justin I. Simon, Jorge A. Vazquez, Axel K. Schmitt, Paul R. Renne, Charles R. Bacon, Mary R. Reid
    Abstract:

    We determined Ar/Ar eruption ages of eight extrusions from the Pleistocene Coso volcanic field, a long-lived series of small volume rhyolitic domes in eastern California. Combined with ion-microprobe dating of crystal ages of zircon and allanite from these lavas and from granophyre geothermal well cuttings, we were able to track the range of magma-production rates over the past 650 ka at Coso. In ≤230 ka Rhyolites we find no evidence of protracted magma residence or recycled zircon (or allanite) from Pleistocene predecessors. A significant subset of zircon in the ~85 ka Rhyolites yielded ages between ~100 and 200 Ma, requiring that generation of at least some Rhyolites involves material from Mesozoic basement. Similar zircon xenocrysts are found in an ~200 ka granophyre. The new age constraints imply that magma evolution at Coso can occur rapidly as demonstrated by significant changes in Rhyolite composition over short time intervals (≤10’s to 100’s ka). In conjunction with radioisotopic age constraints from other young silicic volcanic fields, dating of Coso Rhyolites highlights the fact that at least some (and often the more voluminous) Rhyolites are produced relatively rapidly, but that many small-volume Rhyolites likely represent separation from long-lived mushy magma bodies.

  • alteration and remelting of nascent oceanic crust during continental rupture evidence from zircon geochemistry of Rhyolites and xenoliths from the salton trough california
    Earth and Planetary Science Letters, 2006
    Co-Authors: Axel K. Schmitt, Jorge A. Vazquez
    Abstract:

    Abstract Rhyolite lavas and xenoliths from the Salton Sea geothermal field (Southern California) provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust. Salton Buttes Rhyolite lavas contain xenoliths that include granophyres, fine-grained altered Rhyolites (“felsite”), and amphibole-bearing basalts. Zircon is present in lavas and xenoliths, surprisingly even in the basaltic xenoliths, where it occurs in plagioclase-rich regions interpreted as pockets of crystallized partial melt. Zircons in the xenoliths are exclusively Late Pleistocene–Holocene in age and lack evidence for inheritance. U–Th isochron ages are: 20.5− 1.2+ 1.2 ka (granophyres), 18.3− 3.5+ 3.6 ka (felsite), 30.1− 12.4+ 14.1 ka and 9.2− 6.6+ 7.0 ka (basalts; all errors 1σ). The dominant zircon population in the Rhyolite lavas yielded U–Th ages between ∼ 18 and 10 ka, with few pre-Quaternary xenocrysts present. δ18Ozircon values are lower than typical crustal basement values, thus ruling out Rhyolite genesis by melting of continental crust. Moreover, δ18Ozircon values are ∼ 0.5–1.0‰ lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid-ocean ridge basalt, suggesting that basaltic crust and silicic plutons in the subsurface of the Salton Sea geothermal field isotopically exchanged with meteoric waters. This is evidence for deep-reaching hydrothermal circulation and indicates Rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma.

Ian A Nairn - One of the best experts on this subject based on the ideXlab platform.

  • silicic recharge of multiple Rhyolite magmas by basaltic intrusion during the 22 6 ka okareka eruption episode new zealand
    Lithos, 2008
    Co-Authors: Phil Shane, Ian A Nairn, Victoria C Smith, Miles Darragh, Kate Beggs, J W Cole
    Abstract:

    Abstract Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete Rhyolite magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption (∼ 0.01 km3 magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing Rhyolite magma (T1) with a volume of ∼ 0.3 km3. Hybrid basalt/Rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other Rhyolite magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende Rhyolite magma (T2) (∼ 0.3 km3), and a hotter (780 °C), crystal-poor, pyroxene–hornblende Rhyolite magma (T3) (∼ 4.5 km3). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km3 of T3 magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the Rhyolite magmas. The eruption sequence and the restricted direct basaltic intrusion into only one magma (T1) is consistent with the Rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of

  • multiple Rhyolite magmas and basalt injection in the 17 7 ka rerewhakaaitu eruption episode from tarawera volcanic complex new zealand
    Journal of Volcanology and Geothermal Research, 2007
    Co-Authors: Phil Shane, J W Cole, Victoria C Smith, Miles Darragh, Kate Beggs, S B Martin, Ian A Nairn
    Abstract:

    Abstract The 17.7 ka Rerewhakaaitu eruption episode (volume ∼ 5 km3 DRE Rhyolite magma) was the second of five major episodes that have built the Tarawera volcanic complex in the Okataina Volcanic Centre over the past 22 kyr. The Rerewhakaaitu episode produced a widespread tephra fall deposit, associated proximal pyroclastic flow deposits, and voluminous Rhyolite lava extrusions. Two different Rhyolite magmas (T1 and T2) were simultaneously erupted from the main vent area throughout much of the eruption episode. T1 magma was a crystal-poor orthopyroxene-hornblende Rhyolite that is highly evolved (whole rock SiO2 = 77 wt.%), with a moderate temperature (∼ 760 °C, based on Fe–Ti oxides). T2 is a crystal-rich biotite-hornblende Rhyolite that is less evolved (SiO2 = 75 wt.%), with a Fe–Ti oxide temperature of ∼ 700 °C. Ejecta from the simultaneous and sequential eruption of these two magmas include some pumice clasts with mixed (hybrid) and mingled glass compositions and crystal populations. A third Rhyolite magma (T3) was extruded from another vent 3 km distant to form an apparently contemporaneous lava dome. T3 was the least evolved (SiO2 = 74 wt.%) and hottest (∼ 820 °C) of the three magmas. Saturation pressures calculated using dissolved H2O and CO2 contents of melt inclusions in quartz crystals indicate that T2 magma stagnated and crystallised at about 12 km depth, while small quartz crystals in T1 magma grew during ascent through ∼ 8 km depths. Some T1 and T2 Rhyolite clasts contain vesicular brown blebs with widely variable (andesite to Rhyolite) glass compositions, accompanied by olivine, clinopyroxene and calcic plagioclase crystals that are interpreted as xenocrysts derived from injected basalt. Temperatures over 1000 °C estimated from pyroxene phase equilibria in these clasts reflect intrusion of the more mafic magma, which is now identified as the priming and triggering mechanism for three of the four post-22 ka Tarawera Rhyolite eruption episodes. However, the Rhyolite magma bodies and conduits modelled for each episode have considerable differences in characteristics and geometry. Our preferred model for the Rerewhakaaitu episode is that eruptions occurred from three laterally and vertically isolated Rhyolite magma bodies that were initially primed and triggered by basalt intrusion during a regional rifting event. The ascending hotter and less viscous T1 Rhyolite magma intersected and further invigorated a stagnant pond of cooler, denser and more viscous T2 magma, and lubricated its transport to the surface.

  • Rhyolite magma processes of the ad 1315 kaharoa eruption episode tarawera volcano new zealand
    Journal of Volcanology and Geothermal Research, 2004
    Co-Authors: Ian A Nairn, J W Cole, Phil Shane, G J Leonard, Stephen Self, N Pearson
    Abstract:

    Abstract Products of the ∼5 km3 (DRE), ∼5-yr duration Kaharoa eruption episode display two main high-silica Rhyolite compositions; T1 erupted early (as plinian pyroclastics), and T2 erupted late (mostly as lavas). The T1 and T2 eruptive types are defined by crystal contents and compositional variations in whole rock, glass, plagioclase and biotite. Stratigraphically intermediate pyroclastic deposits have an intermediate composition (T1+2). A small volume of rhyodacite pyroclastics, mingled with injected basalt, was also erupted. The Kaharoa Rhyolites were erupted from multiple sources spread along an 8-km linear vent zone, but the changes in eruptive compositions were largely controlled by position in the eruption sequence and magma discharge rates, rather than vent locations. Data from the Kaharoa eruptive types, vent locations, eruption sequence and discharge rates can be combined with concepts of magma chamber evacuation processes to produce a preliminary dimensional model of the pre-eruption Rhyolite magma body. Our model magma body is sill-like, ∼8 km long, 1 km wide, 1.4 km thick, and located at ∼6–7 km depth in the upper crust. T1 magma overlay T2 magma in the upper levels of the chamber, with each magma layer internally mixed to a homogeneous composition along an axial extent defined by the vent locations. An underlying third Rhyolite magma (T3) is recognised as the silicic end-member that was modified by basalt to form the rhyodacite eruptives. The Rhyolite magma stratification survived multiple injections of basalt magma, which primed and finally triggered the Kaharoa eruptions. The T1+2 eruptives resulted from syn-eruption mingling in the conduit of the two main Rhyolite magma types. Thickness of the T1 layer in the model can be estimated at 0.25 km; the T2 layer was somewhat thicker. Thicknesses of the underlying T3 and basalt layers are uncertain. Post-eruption geothermal heat flow indicates a residual magma volume of ≥6 km3, suggesting that the pre-eruption magma volume was ≥11 km3.

C D Deering - One of the best experts on this subject based on the ideXlab platform.

  • phase equilibrium geobarometers for silicic rocks based on Rhyolite melts part 2 application to taupo volcanic zone Rhyolites
    Contributions to Mineralogy and Petrology, 2014
    Co-Authors: Florence Begue, C D Deering, Guilherme A R Gualda, Mark S Ghiorso, Ayla S Pamukcu, Ben Kennedy, D M Gravley, Isabelle Chambefort
    Abstract:

    Constraining the pressure of crystallisation of large silicic magma bodies gives important insight into the depth and vertical extent of magmatic plumbing systems; however, it is notably difficult to constrain pressure at the level of detail necessary to understand shallow magmatic systems. In this study, we use the recently developed Rhyolite-MELTS geobarometer to constrain the crystallisation pressures of Rhyolites from the Taupo Volcanic Zone (TVZ). As sanidine is absent from the studied deposits, we calculate the pressures at which quartz and feldspar are found to be in equilibrium with melt now preserved as glass (the quartz +1 feldspar constraint of Gualda and Ghiorso, Contrib Mineral Petrol 168:1033. doi: 10.1007/s00410-014-1033-3. 2014). We use glass compositions (matrix glass and melt inclusions) from seven eruptive deposits dated between ~320 and 0.7 ka from four distinct calderas in the central TVZ, and we discuss advantages and limitations of the Rhyolite-MELTS geobarometer in comparison with other geobarometers applied to the same eruptive deposits. Overall, there is good agreement with other pressure estimates from the literature (amphibole geobarometry and H2O–CO2 solubility models). One of the main advantages of this new geobarometer is that it can be applied to both matrix glass and melt inclusions—regardless of volatile saturation. The examples presented also emphasise the utility of this method to filter out spurious glass compositions. Pressure estimates obtained with the new Rhyolite-MELTS geobarometer range between ~250 to ~50 MPa, with a large majority at ~100 MPa. These results confirm that the TVZ hosts some of the shallowest rhyolitic magma bodies on the planet, resulting from the extensional tectonic regime and thinning of the crust. Distinct populations with different equilibration pressures are also recognised, which is consistent with the idea that multiple batches of eruptible magma can be present in the crust at the same time and can be tapped simultaneously by large eruptive events.

  • extraction of crystal poor Rhyolite from a hornblende bearing intermediate mush a case study of the caldera forming matahina eruption okataina volcanic complex
    Contributions to Mineralogy and Petrology, 2011
    Co-Authors: C D Deering, J W Cole, Thomas A Vogel
    Abstract:

    The Matahina Ignimbrite (~160 km3 Rhyolite magma, 330 ka) was deposited during a caldera-forming eruption from the Okataina Volcanic Centre, Taupo Volcanic Zone (TVZ), New Zealand. Juvenile clasts are divided into three groups: Group (1) the dominant crystal-poor Rhyolite type, Group (2) a minor coarse-grained, mingled/mixed intermediate type, and Group (3) a rare fine-grained basalt. The ignimbrite consists of the Group 1 type and is divided into three members: a lower and middle member, which is high-silica, crystal-poor (<10 vol.%) Rhyolite, and the upper member, which is low-silica and slightly more crystal-rich (up to 21 vol.%). Cognate, crystal-rich (up to 50 vol.%) basalt to intermediate pumice occurs on top of lag breccias and within lithic-rich pyroclastic density current deposits along the caldera margin (Groups 2 and 3). Several lines of evidence indicate that the intermediate clasts represent the cumulate complement to the melt-rich Rhyolite: (1) continuity in the compositions of plagioclase, orthopyroxene, hornblende, and oxides and normal zoning of individual phenocrysts; (2) the silicic glass from the intermediate magma (interstitial melt) overlaps compositionally with the bulk rock Rhyolite and glass; (3) high Zr and a slight positive Eu anomaly in the intermediate magma relative to quenched enclaves from other intermediate TVZ eruptions indicates zircon and plagioclase accumulation, respectively; (4) an increase in the Cl contents in glass from the least evolved to most evolved is consistent with the concentration of volatiles during magma evolution. Most of the compositional variations in the low- to high-silica Rhyolites can be accounted for by continued Rayleigh fractionation (up to 15%), following melt extraction from the underlying mush, under varying fO2–fH2O conditions to form a slightly compositionally zoned rhyolitic cap. This link to the varying fO2–fH2O conditions is evidenced by the strong correlation between key geochemical parameters (e.g. Dy, Y), that qualitatively reflect fH2O conditions (presence or absence of hornblende/biotite), and fO2 estimated from Fe–Ti oxide equilibrium. Magma mingling/mixing between the basalt–andesite and the main slightly compositionally zoned rhyolitic magma occurred during caldera-collapse, modifying the least-evolved Rhyolite at the lower portion of the reservoir and effectively destroying any pre-eruptive gradients.

  • a Rhyolite compositional continuum governed by lower crustal source conditions in the taupo volcanic zone new zealand
    Journal of Petrology, 2008
    Co-Authors: C D Deering, J W Cole, Thomas A Vogel
    Abstract:

    Rhyolites generated in the modern Taupo Volcanic Zone (TVZ), New Zealand, have previously been interpreted as having evolved by a combination of extensive fractional crystallization of mantle-derived mafic magmas and limited crustal assimilation (up to 25%). Polytopic vector analysis (PVA), a form of multivariate statistical analysis, of the major-element compositions of over 475 basaltic to rhyolitic bulk-rock samples, representing over 600 kyr of volcanism within the TVZ, has provided a robust platform for Rhyolite characterization and new insights into Rhyolite petrogenesis. There is a continuum of compositions between two Rhyolite end-member magma types (EM1 and EM2), which have been identified on the basis of the PVA and which have distinct petrological and geochemical characteristics, as follows. EM1: crystal-rich (up to 45%), hydrous phases (± hornblende ± biotite ± cummingtonite), high Aluminum Saturation Index [ASI; molar Al 2 O 3 /(CaO + Na 2 O + K 2 O)], low FeO*/MgO (calc-alkaline series), depleted abundances of middle rare earth elements (MREE) and Y, and high Sr; EM2: crystal-poor (<10%), anhydrous phases (orthopyroxene ± clinopyroxene), high FeO*/MgO (tholeiitic series), low ASI, less depleted MREE and Y, and low Sr. The range of ASI values, and relative depletion in MREE and Y in the Rhyolites is consistent with the results of experiments to constrain the partial melting behaviour of amphibolite at crustal pressures. The major- and trace-element data are also consistent with 50-60% equilibrium crystallization of a crustally contaminated, hornblende-bearing andesite to produce the TVZ Rhyolites. Distinct major- and trace-element variations along the continuum between the two Rhyolite end-member types can be effectively modelled by simulating changes in the temperature-fO 2 -fH 2 O conditions in the lower crust where mantle-derived mafic magmas are stored and differentiate. Low T and high fO 2 and fH 2 O in the crustal magma storage zone promote abundant hornblende crystallization and suppress plagioclase crystallization, which produces the EM1 type Rhyolite. By increasing the temperature and/or lowering fO 2 and fH 2 O in the magma storage region, plagioclase becomes more dominant and hornblende crystallization is suppressed, producing more EM2-like rhyolitic magma types.

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  • genesis of post hotspot a type Rhyolite of the eastern snake river plain volcanic field by extreme fractional crystallization of olivine tholeiite
    Bulletin of Volcanology, 2008
    Co-Authors: Michael Mccurry, Karl P Hayden, Lee H Morse, Stan Mertzman
    Abstract:

    Rhyolites occur as a subordinate component of the basalt-dominated Eastern Snake River Plain volcanic field. The basalt-dominated volcanic field spatially overlaps and post-dates voluminous late Miocene to Pliocene Rhyolites of the Yellowstone–Snake River Plain hotspot track. In some areas the basalt lavas are intruded, interlayered or overlain by ~15 km3 of cryptodomes, domes and flows of high-silica Rhyolite. These post-hotspot Rhyolites have distinctive A-type geochemical signatures including high whole-rock FeOtot/(FeOtot+MgO), high Rb/Sr, low Sr (0.5–10 ppm) and are either aphyric, or contain an anhydrous phenocryst assemblage of sodic sanidine ± plagioclase + quartz > fayalite + ferroaugite > magnetite > ilmenite + accessory zircon + apatite + chevkinite. Nd- and Sr-isotopic compositions overlap with coeval olivine tholeiites (ɛNd = −4 to −6; 87Sr/86Sri = 0.7080–0.7102) and contrast markedly with isotopically evolved Archean country rocks. In at least two cases, the Rhyolite lavas occur as cogenetic parts of compositionally zoned (~55–75% SiO2) shield volcanoes. Both consist dominantly of intermediate composition lavas and have cumulative volumes of several 10’s of km3 each. They exhibit two distinct, systematic and continuous types of compositional trends: (1) At Cedar Butte (0.4 Ma) the volcanic rocks are characterized by prominent curvilinear patterns of whole-rock chemical covariation. Whole-rock compositions correlate systematically with changes in phenocryst compositions and assemblages. (2) At Unnamed Butte (1.4 Ma) the lavas are dominated by linear patterns of whole-rock chemical covariation, disequilibrium phenocryst assemblages, and magmatic enclaves. Intermediate compositions in this group resulted from variable amounts of mixing and hybridization of olivine tholeiite and Rhyolite parent magmas. Interestingly, models of Rhyolite genesis that involve large degrees of melting of Archean crust or previously consolidated mafic or silicic Tertiary intrusions do not produce observed ranges of Nd- and Sr-isotopes, extreme depletions in Sr-concentration, and cogenetic spectra of intermediate rock compositions for both groups. Instead, least-squares mass-balance, energy-constrained assimilation and fractional crystallization modeling, and mineral thermobarometry can explain Rhyolite production by 77% low-pressure fractional crystallization of a basaltic trachyandesite parent magma (~55% SiO2), accompanied by minor (0.03–7%) assimilation of Archean upper crust. We present a physical model that links the Rhyolites and parental intermediate magmas to primitive olivine tholeiite by fractional crystallization. Assimilation, recharge, mixing and fractional melting occur to limited degrees, but are not essential parts of the Rhyolite formation process.

  • Contrasting origins of Cenozoic silicic volcanic rocks from the western Cordillera of the United States
    Bulletin of Volcanology, 2008
    Co-Authors: Eric H. Christiansen, Michael Mccurry
    Abstract:

    Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and Rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin and contrast with Rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and Rhyolites are generally calc-alkaline, magnesian, oxidized, wet, cool (

  • contrasting origins of cenozoic silicic volcanic rocks from the western cordillera of the united states
    Bulletin of Volcanology, 2008
    Co-Authors: Eric H. Christiansen, Michael Mccurry
    Abstract:

    Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and Rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin and contrast with Rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and Rhyolites are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and polybaric crystal fractionation were all important processes in gener- ating this Oligocene suite. In contrast, most of the Rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830-1,050°C), Sr-and Al-poor, and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely imposed by their derivation from parental basalt (with low f H2O and low fO2) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation of plagioclase and retarded crystallization of Fe-Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment. Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation, and minor assimilation of older crust led to the array of Rhyolite compositions. Only very small volumes of distinctive Rhyolite were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte trend.