Phenocryst

About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.

Caldera Volcanoes of the Taupo Volcanic Zone, New Zealand

Abstract: The Taupo volcanic zone (TVZ) has been active since 2 Ma and has erupted >104 km3 of dominantly rhyolitic magma during the last 1 m.y. Most of the volcanism is concentrated in a 125×60 km area forming the central TVZ and is expressed largely as six major caldera volcanoes, Rotorua, Okataina, Kapenga, Mangakino, Maroa, and Taupo, marked by localized collapse of the underlying basement and clustering of known or inferred vent sites. These centers have activity spans from 150 to 600 ka and have each erupted at least 300 to 1000 km3 of magma. All centers except Rotorua are known or inferred to have had complex histories of multiple caldera collapse, which have occurred alongside general basement collapse within the TVZ accompanying regional extension. Deep-seated NE trending basement lineations and/or faults have influenced vent sites at Okataina, Maroa, and Taupo. Welded ignimbrites are prominent in the pre-140 ka record; their absence since then is attributed to the effects of surface water on eruption styles rather than to a change in eruptive behavior. Volcanism from the centers has been overwhelmingly rhyolitic (>97% SiO2 69–77 wt%) with minor high-A1 basalt and dacite and traces of andesite, mostly as lithic fragments in ignimbrites from Okataina and Mangakino. Although insignificant in volume, the basalt is important as a low-Si end-member in mixing relationships with the rhyolite (at one extreme generating the dacites) and occasionally as a trigger for the rhyolitic eruptions. The current average rhyolite magma eruption rate from the central TVZ is ∼0.27 m3 s−1, equally divided between Okataina and Taupo, a figure close to the long-term average for the last 1.1 Ma. However, geothermal heat flow data imply that a further 1.4–1.8 m3 s−1 of magma may be intruded within the crust. The ratio of inferred intruded material to erupted material is higher at centers where lava extrusions are volumetrically significant (Okataina, Maroa), and this is correlated with lower phenocryst equilibration temperatures in the eruptives. Evidence for resurgent doming and long-term (>105 years) magma cycles documented at similarly sized rhyolitic calderas in the western United States is absent from the TVZ centers; this is attributed to the young faulted crust of the region, preventing the formation of sufficiently large high-level magma chambers. In overall terms, the central TVZ is comparable in size and longevity to the Yellowstone system, but its individual eruptions have very much shorter recurrence intervals and smaller volumes.

Gradients in H2O, CO2, and exsolved gas in a large-volume silicic magma system: Interpreting the record preserved in melt inclusions from the Bishop Tuff

Abstract: Infrared spectroscopic analyses of ∼140 melt inclusions in quartz phenocrysts from the zoned Bishop rhyolitic tuff demonstrate that systematic gradients in dissolved magmatic H2O and CO2 concentrations were present during preemptive crystallization of the magma body. Melt inclusions from the earliest erupted samples contain lower H2O (5.3±0.4 wt %) and CO2 (62±37 ppm) than inclusions from the middle of the eruption (5.7±0.2 wt % H2O; 120±60 ppm CO2). Melt inclusions from late erupted samples have much lower H2O (4.1±0.3 wt %) and higher and variable CO2 (150–1085 ppm). Trace element analyses of melt inclusions by ion microprobe show that inclusions within single pumice clasts from the early and middle Bishop Tuff have an inverse correlation between CO2 and incompatible elements. This pattern indicates that the magma was gas-saturated during crystallization, with CO2 partitioning into a coexisting gas phase. Quantitative modeling using H2O-CO2 solubility relations reveals a preeruptive gradient in exsolved gas, with gas contents varying from ∼1 wt % in the deeper regions of the magma body to nearly 6 wt % near the top. Dissolved Cl, B, Li, and Be in melt inclusions correlate negatively with CO2. Mass balance modeling of Cl loss to exsolving H2O-rich gas during crystallization provides strong corroborating evidence for the mass fractions of exsolved gas estimated from H2O, CO2, and trace element data. Pressures of quartz crystallization and melt inclusion entrapment calculated from inclusion H2O-CO2 data are consistent with progressive downward tapping of a zoned magma body during the eruption. Melt inclusion gas saturation pressures, magma volume estimates, and time-stratigraphic-compositional relations suggest that early erupted magma was stored at the top of a downward widening magma body. Melt inclusion data and the inferred gradients in dissolved H2O, CO2 and exsolved gas in the Bishop magma body suggest that gas saturation plays an important role in the formation and subsequent preservation of compositional gradients in silicic magma reservoirs.

Composition and age of Lau Basin and Ridge volcanic rocks: Implications for evolution of an interarc basin and remnant arc

Abstract: The Tonga-Kermadec Ridge, Lau Basin, and Lau-Colville Ridge are, respectively, a frontal arc, interarc basin, and remnant arc at the Australian-Pacific plate boundary. Basement rocks of the Lau-Colville Ridge (Lau Volcanics) are 9- to 6-m.y.-old basaltic andesites to dacites with 55 to 66 percent SiO 2 , K 60 = 1.0 to 1.5 percent, little Fe enrichment, Sr 87 /Sr 86 = 0.7030 to 0.7034, and enrichment in light rare-earth elements. Westward increases in K, Rb, Th, and U suggest that subducted lithosphere was un-derthrust from the east. Variations in rock composition are consistent qualitatively with derivation from basalt by low-pressure crystal-liquid fractionation involving removal of phenocryst phases: plagioclase + clinopyroxene + orthopyroxene + magnetite. Volcaniclastic turbidites of the same age and derived from western sources are found in Tonga. These andesitic vol-canogenic recks are overlain on both ridges by Pliocene limestones, which are capped on the Lau-Colville Ridge by 3.9- to 3.5-m.y.-old olivine + hypersthene normative tholeiites (Korombasanga Volcanics) having minor 56 to 60 percent SiO 2 andesitic differentiates. Lau Basin basalts are transitional between ocean-floor and island-arc tholeiites, sharing with the latter their higher Rb, Ba, light rare-earth element, and Sr 87 contents and lower Ti, Zr, and Hf contents. These data support Karig9s idea that the Lau and Tonga Ridges represent a once-united island arc now dismembered by rifting, which has formed the intervening Lau Basin. This rifting began about 5 m.y. B.P. The change in volcanism on the Lau-Colville Ridge reflects its removal from a subduction site.

H2O, CO2, CI, and gas in Plinian and ash-flow Bishop rhyolite

Abstract: Infrared spectroscopic (H_2O, CO_2 and electron microprobe (CI) analyses of glass inclusions in Plinian and ash-flow quartz phenocrysts from the Bishop Tuff reveal the preeruption concentrations of volatiles in separate parts of the body of magma. There is an inverse relation between H_2O, CO_2 that can be explained (1) by closed-system, gas-saturated crystallization of parent magma to yield Plinian magma + (lost) crystals or (2) by the rise of CO_2-rich bubbles through water-rich magma. Our estimated pressures of gas-saturation range from about 1.6 kbar (Plinian) to 2.3 kbar (ash flow) and accord with geologic evidence for 3 km of magma withdrawal and caldera subsidence.