The Oruanui Eruption: Insights into the Generation and Dynamics of the World's Youngest Supereruption

<p>This work investigates the pre- and syn-eruptive magmatic processes that culminated in the world’s youngest supereruption – the ~25.4 ka, 530 km³ Oruanui eruption from Taupo volcano, New Zealand – from the perspective of crystals contained in single parcels of frozen magma (pumice). The eruption is unusual in its variety of magmatic compositions. About 98-99 % by mass of the juvenile material is high-SiO₂ rhyolite (HSR; >74 wt% SiO₂), with lesser volumes of tholeiitic and calc-alkaline mafic magmas (total 3-5 km³; basaltic andesite to andesite: 53-63 % SiO₂), low-silica rhyolite (LSR: 0.1-0.5 km³; <74 wt% SiO₂) and a ‘foreign’ biotite-bearing rhyolite from an adjacent magma source (0.03 km³; ~74 wt% SiO₂). Detailed textural and chemical data from amphibole, plagioclase, and orthopyroxene are placed within the context of an established time-stratigraphic, volcanological and petrographic framework, of unrivalled detail globally for an eruption of this age and magnitude. Other previously published information from zircon and quartz is also incorporated. This unique contextual information is used to constrain observations and inferences regarding the processes that moved the Oruanui magma from a largely uneruptible crystal-rich progenitor at depth (where an eruption was possible), to a highly eruptible melt-rich magma at shallow crustal levels (where eruption was inevitable). A thermally and compositionally stratified crystal mush body, with an upper SiO₂-saturated and quartz bearing cap at ~3.5 km depth and quartz-free roots extended down to at least ~10 km. This inference is made on three bases. 1) That the quartz cores contain trapped melt that is more evolved than the melt component of the immediately pre-eruptive magma body, indicating their growth within mush from a more evolved interstitial melt. 2) The majority of plagioclase, amphibole, and orthopyroxene cores, in contrast to quartz have compositions that indicate growth from less evolved melts than that encountered in the final melt-dominant magma body. 3) Barometric estimates from amphibole core compositions indicate derivation from a range of depths (~3.5 to 10 km). The spatial and temporal transitions from mush to melt-dominant magma body are recorded in the textural and compositional zonations within the crystal phases. Crystals from all levels of the zoned mush body were entrained during the melt extraction process resulting in a diversity of crystal compositions being brought together in the melt-dominant magma body. Textural disequilibrium features in the cores of orthopyroxene and plagioclase crystals reflect their temporary departure from stability during the accompanying significant decompression (recorded in the amphibole model pressures). Counterpart chemical signatures, reflecting this partial orthopyroxene and plagioclase dissolution, are recorded in the amphiboles which show no textural evidence for destabilisation during ascent. Crystal chemical and textural zonation in the rim growths of the plagioclase, orthopyroxene, and amphibole record further crystallisation in the accumulating melt-dominant magma body, and reflect cooling and compositional evolution of the body towards its final pre-eruptive conditions. The timing of growth of the melt dominant magma body is constrained by Fe-Mg diffusion modelling of key boundaries in orthopyroxene crystals. Accumulation of this body began only ~1600 years and peaked at 230 years prior to the eruption, as vast volumes of melt and entrained crystals were drained from the mush body and began to accumulate at shallower levels (~3.5 to 6.0 km depth). Within the thin, sill-like melt-dominant magma body, significant heat loss drove vigorous convection. Textural and chemical zonation patterns within the rim-zones of plagioclase, orthopyroxene and amphibole, inferred to have grown solely in the melt-dominant magma body, depict a secular cooling and melt evolution trends towards final uniform thermal (~770 °C) and compositional conditions inferred for the HSR magma. Despite the rapid accumulation of a vast volume of crystal-poor HSR magma at shallow crustal levels, the apparent gas-saturated nature of that magma, and vigorous convection within the melt-dominant magma body itself, the chronologies from HSR orthopyroxene imply that the magma underwent a period of stasis of about 60 years. The presence of 3-16 wt% of ‘foreign’ biotite-bearing juvenile pumices in the early Oruanui fall deposits (phases 1 and 2) show that coincident with the onset of the Oruanui eruption, magma was transported laterally in a dike from an adjacent independent magma system 10-15 km to the NNE to intersect the active Oruanui conduit. Consideration of the tectonic stress orientations associated with this lateral transport imply that an external tectonic influence through a major rifting event was a critical factor in the initiation of the Oruanui eruption. Only the presence of the foreign magma, and linkages to detailed field-based and geochemical constraints enables the tectonic influence to be identified. During the eruption itself, minor quantities of Oruanui LSR magma were erupted , and with a crystal cargo, reflecting derivation from deeper (mostly >6 km), hotter (~820 °C) sources in the crystal mush roots to the system. Comparisons of LSR crystal compositions with cores to many HSR crystals for plagioclase, orthopyroxene and amphibole imply that the LSR magma was derived from pockets in the mush zone ruptured during escalation of the eruption vigour during phase 3. The LSR and its crystals are inferred to be closely similar in their characteristics to the feedstock magma that generated the melt-dominant body and evolved through subsequent cooling and fractionation to form the HSR. In overall terms, the evidence from the crystal phases demonstrates that a super-sized rhyolite magma body can be physically created in a geologically very short period of time. The compositional textures and data for all the mineral phases, both previously published and newly presented in this work, yield a consistent story of extraordinarily rapid extraction of LSR melt and entrained crystals into a rapidly evolving and cooling HSR body. When coupled with field constraints these data establish a central role for extensional tectonics in regulating the pre-and syn-eruptive processes and their timings in the Oruanui system.</p>

The Oruanui Eruption: Insights into the Generation and Dynamics of the World's Youngest Supereruption

<p>This work investigates the pre- and syn-eruptive magmatic processes that culminated in the world’s youngest supereruption – the ~25.4 ka, 530 km³ Oruanui eruption from Taupo volcano, New Zealand – from the perspective of crystals contained in single parcels of frozen magma (pumice). The eruption is unusual in its variety of magmatic compositions. About 98-99 % by mass of the juvenile material is high-SiO₂ rhyolite (HSR; >74 wt% SiO₂), with lesser volumes of tholeiitic and calc-alkaline mafic magmas (total 3-5 km³; basaltic andesite to andesite: 53-63 % SiO₂), low-silica rhyolite (LSR: 0.1-0.5 km³; <74 wt% SiO₂) and a ‘foreign’ biotite-bearing rhyolite from an adjacent magma source (0.03 km³; ~74 wt% SiO₂). Detailed textural and chemical data from amphibole, plagioclase, and orthopyroxene are placed within the context of an established time-stratigraphic, volcanological and petrographic framework, of unrivalled detail globally for an eruption of this age and magnitude. Other previously published information from zircon and quartz is also incorporated. This unique contextual information is used to constrain observations and inferences regarding the processes that moved the Oruanui magma from a largely uneruptible crystal-rich progenitor at depth (where an eruption was possible), to a highly eruptible melt-rich magma at shallow crustal levels (where eruption was inevitable). A thermally and compositionally stratified crystal mush body, with an upper SiO₂-saturated and quartz bearing cap at ~3.5 km depth and quartz-free roots extended down to at least ~10 km. This inference is made on three bases. 1) That the quartz cores contain trapped melt that is more evolved than the melt component of the immediately pre-eruptive magma body, indicating their growth within mush from a more evolved interstitial melt. 2) The majority of plagioclase, amphibole, and orthopyroxene cores, in contrast to quartz have compositions that indicate growth from less evolved melts than that encountered in the final melt-dominant magma body. 3) Barometric estimates from amphibole core compositions indicate derivation from a range of depths (~3.5 to 10 km). The spatial and temporal transitions from mush to melt-dominant magma body are recorded in the textural and compositional zonations within the crystal phases. Crystals from all levels of the zoned mush body were entrained during the melt extraction process resulting in a diversity of crystal compositions being brought together in the melt-dominant magma body. Textural disequilibrium features in the cores of orthopyroxene and plagioclase crystals reflect their temporary departure from stability during the accompanying significant decompression (recorded in the amphibole model pressures). Counterpart chemical signatures, reflecting this partial orthopyroxene and plagioclase dissolution, are recorded in the amphiboles which show no textural evidence for destabilisation during ascent. Crystal chemical and textural zonation in the rim growths of the plagioclase, orthopyroxene, and amphibole record further crystallisation in the accumulating melt-dominant magma body, and reflect cooling and compositional evolution of the body towards its final pre-eruptive conditions. The timing of growth of the melt dominant magma body is constrained by Fe-Mg diffusion modelling of key boundaries in orthopyroxene crystals. Accumulation of this body began only ~1600 years and peaked at 230 years prior to the eruption, as vast volumes of melt and entrained crystals were drained from the mush body and began to accumulate at shallower levels (~3.5 to 6.0 km depth). Within the thin, sill-like melt-dominant magma body, significant heat loss drove vigorous convection. Textural and chemical zonation patterns within the rim-zones of plagioclase, orthopyroxene and amphibole, inferred to have grown solely in the melt-dominant magma body, depict a secular cooling and melt evolution trends towards final uniform thermal (~770 °C) and compositional conditions inferred for the HSR magma. Despite the rapid accumulation of a vast volume of crystal-poor HSR magma at shallow crustal levels, the apparent gas-saturated nature of that magma, and vigorous convection within the melt-dominant magma body itself, the chronologies from HSR orthopyroxene imply that the magma underwent a period of stasis of about 60 years. The presence of 3-16 wt% of ‘foreign’ biotite-bearing juvenile pumices in the early Oruanui fall deposits (phases 1 and 2) show that coincident with the onset of the Oruanui eruption, magma was transported laterally in a dike from an adjacent independent magma system 10-15 km to the NNE to intersect the active Oruanui conduit. Consideration of the tectonic stress orientations associated with this lateral transport imply that an external tectonic influence through a major rifting event was a critical factor in the initiation of the Oruanui eruption. Only the presence of the foreign magma, and linkages to detailed field-based and geochemical constraints enables the tectonic influence to be identified. During the eruption itself, minor quantities of Oruanui LSR magma were erupted , and with a crystal cargo, reflecting derivation from deeper (mostly >6 km), hotter (~820 °C) sources in the crystal mush roots to the system. Comparisons of LSR crystal compositions with cores to many HSR crystals for plagioclase, orthopyroxene and amphibole imply that the LSR magma was derived from pockets in the mush zone ruptured during escalation of the eruption vigour during phase 3. The LSR and its crystals are inferred to be closely similar in their characteristics to the feedstock magma that generated the melt-dominant body and evolved through subsequent cooling and fractionation to form the HSR. In overall terms, the evidence from the crystal phases demonstrates that a super-sized rhyolite magma body can be physically created in a geologically very short period of time. The compositional textures and data for all the mineral phases, both previously published and newly presented in this work, yield a consistent story of extraordinarily rapid extraction of LSR melt and entrained crystals into a rapidly evolving and cooling HSR body. When coupled with field constraints these data establish a central role for extensional tectonics in regulating the pre-and syn-eruptive processes and their timings in the Oruanui system.</p>

Micro-Analytical Studies of the Petrogenesis of Silicic Arc Magmas in the Taupo Volcanic Zone and Southern Kermadec Arc, New Zealand

<p>The petrogenesis of silicic arc magmas is controversial with end-member models of fractional crystallisation and crustal anatexis having been invoked. A prime example of this is the archetypical continental Taupo Volcanic Zone and the adjacent oceanic Kermadec Arc. Insights into the genesis and timescales of magmatic processes of four continental rhyolitic magmas (Whakamaru, Oruanui, Taupo and Rotorua eruptives) and an oceanic (Healy seamount) rhyodacitic magma are documented through micro-analytical chemical studies of melt inclusions and crystal zonation of plagioclase and quartz. Electron probe microanalysis, laser ablation inductively coupled plasma mass spectrometry and Fourier transform infrared spectroscopy have been used to measure major, trace and volatile element concentrations, respectively, of melt inclusions and crystals. Melt inclusions are high silica (e.g. 74 - 79 wt%) irrespective of arc setting and display a wide range of trace element compositions (e.g. Sr = 17 - 180 ppm). Taupo Volcanic Zone melt inclusions exhibit higher K2O and Ce/Yb relative to Healy melt inclusions reflecting the assimilation of continental lithosphere. Quantitative trace element modelling of melt inclusion compositions: (a) demonstrates that magma genesis occurred through 62 - 76% fractional crystallisation at Healy whereas assimilation of continental lithosphere (greywacke) in addition to 60 - 80% fractional crystallisation is required for the Taupo Volcanic Zone magmas; and (b) suggests the presence of crystal mush bodies beneath silicic magma chambers in both continental and oceanic arc environments. Water concentrations of melt inclusions ranged between 1.4 - 5.1 wt% for the Whakamaru, Taupo and Healy samples. However, the inconsistency in the measured molecular water to hydroxyl concentrations of melt inclusions relative to those determined experimentally for groundmass rhyolitic glasses provide evidence for the degassing of inclusions prior to quenching, by diffusion of hydroxyl groups through the crystal host. Thus, partial pressures of water estimated from the inclusions and inferred depths of the crystallising magma bodies are underestimated. Chemical profiles of mineral zonation, however, indicate a more complex origin of silicic melts than simple fractionation and assimilation. For example, trace element modelling of Whakamaru plagioclase suggests that the three distinct textural plagioclase populations present in Whakamaru samples crystallised from four physiochemically discrete silicic melts. This modelling indicates a strong petrogenetic link between andesitic and silicic magmas from the chemical variation of selected Whakamaru plagioclase crystals possessing high anorthite (45-60 mol %) cores and low anorthite (~ 30 mol %) rim compositions and the interaction of greywacke partial melts. Furthermore, Sr diffusion modelling of core-rim interfaces of the same plagioclase crystals indicate the amalgamation of the magma chamber occurred continuously over the 15,000 years preceding the climactic eruption. Conversely, the major element zonation of Taupo plagioclases implies magma genesis occurred solely through assimilation and fractional crystallisation without the incorporation of evolved crystal mush magmas, indicating a spectrum of magmatic processes are occurring beneath the Taupo Volcanic Zone with each eruption providing only a snapshot of the petrogenesis of the Taupo Volcanic Zone.</p>

Micro-Analytical Studies of the Petrogenesis of Silicic Arc Magmas in the Taupo Volcanic Zone and Southern Kermadec Arc, New Zealand

<p>The petrogenesis of silicic arc magmas is controversial with end-member models of fractional crystallisation and crustal anatexis having been invoked. A prime example of this is the archetypical continental Taupo Volcanic Zone and the adjacent oceanic Kermadec Arc. Insights into the genesis and timescales of magmatic processes of four continental rhyolitic magmas (Whakamaru, Oruanui, Taupo and Rotorua eruptives) and an oceanic (Healy seamount) rhyodacitic magma are documented through micro-analytical chemical studies of melt inclusions and crystal zonation of plagioclase and quartz. Electron probe microanalysis, laser ablation inductively coupled plasma mass spectrometry and Fourier transform infrared spectroscopy have been used to measure major, trace and volatile element concentrations, respectively, of melt inclusions and crystals. Melt inclusions are high silica (e.g. 74 - 79 wt%) irrespective of arc setting and display a wide range of trace element compositions (e.g. Sr = 17 - 180 ppm). Taupo Volcanic Zone melt inclusions exhibit higher K2O and Ce/Yb relative to Healy melt inclusions reflecting the assimilation of continental lithosphere. Quantitative trace element modelling of melt inclusion compositions: (a) demonstrates that magma genesis occurred through 62 - 76% fractional crystallisation at Healy whereas assimilation of continental lithosphere (greywacke) in addition to 60 - 80% fractional crystallisation is required for the Taupo Volcanic Zone magmas; and (b) suggests the presence of crystal mush bodies beneath silicic magma chambers in both continental and oceanic arc environments. Water concentrations of melt inclusions ranged between 1.4 - 5.1 wt% for the Whakamaru, Taupo and Healy samples. However, the inconsistency in the measured molecular water to hydroxyl concentrations of melt inclusions relative to those determined experimentally for groundmass rhyolitic glasses provide evidence for the degassing of inclusions prior to quenching, by diffusion of hydroxyl groups through the crystal host. Thus, partial pressures of water estimated from the inclusions and inferred depths of the crystallising magma bodies are underestimated. Chemical profiles of mineral zonation, however, indicate a more complex origin of silicic melts than simple fractionation and assimilation. For example, trace element modelling of Whakamaru plagioclase suggests that the three distinct textural plagioclase populations present in Whakamaru samples crystallised from four physiochemically discrete silicic melts. This modelling indicates a strong petrogenetic link between andesitic and silicic magmas from the chemical variation of selected Whakamaru plagioclase crystals possessing high anorthite (45-60 mol %) cores and low anorthite (~ 30 mol %) rim compositions and the interaction of greywacke partial melts. Furthermore, Sr diffusion modelling of core-rim interfaces of the same plagioclase crystals indicate the amalgamation of the magma chamber occurred continuously over the 15,000 years preceding the climactic eruption. Conversely, the major element zonation of Taupo plagioclases implies magma genesis occurred solely through assimilation and fractional crystallisation without the incorporation of evolved crystal mush magmas, indicating a spectrum of magmatic processes are occurring beneath the Taupo Volcanic Zone with each eruption providing only a snapshot of the petrogenesis of the Taupo Volcanic Zone.</p>

Petrogenesis of Triassic Caojian A-type rhyolites and associated I-type granites in the southeastern Tibetan Plateau: rejuvenation of crystal mush

The orogenic process and crustal growth of the Changning–Menglian Palaeo-Tethys orogenic belt in the southeastern Tibetan Plateau is not fully understood. Triassic Caojian rhyolites and granites occur extensively in this orogenic belt and represent important constraints for this issue. This study aims to examine the relationships between the Triassic Caojian rhyolites and granites and to gain a better understanding of their possible petrogenesis. The study used zircon U–Pb geochronology, trace element analyses and Sr–Nd–Hf isotope data to better understand the relationships and possible origin of the rhyolites and granites. Recent zircon U–Pb ages indicated that the Caojian rhyolites were emplaced at 227.2 Ma, whereas age estimates for Caojian granites were slightly older (233.4–236.9 Ma). The Caojian rhyolites are enriched in large-ion lithophile elements and high-field-strength elements, with elevated FeOtot/MgO and Ga/Al ratios. However, they are significantly depleted in Ba, Sr, Eu, P and Ti. These geochemical characteristics indicate that they have an A-type affinity. Furthermore, the Caojian granites comprise biotite monzogranites and granodiorites and show unfractionated composition. Mineralogically, the Caojian granites were found to contain diagnostic I-type minerals such as hornblende. Geochemical data suggest that the petrogenesis of the Triassic Caojian rhyolites is characterized by rejuvenation of crystal mush represented by the Triassic Caojian granites. The necessary thermal input was supplied by mafic magma. This magmatic evolution was likely related to lithospheric delamination and upwelling of the asthenosphere during the Mid- to Late Triassic, forming post-collisional I-type granites and A-type volcanics in the Changning–Menglian Palaeo-Tethys orogenic belt.

A Crystal Mush Perspective Explains Magma Variability at La Fossa Volcano (Vulcano, Italy)

The eruptive products of the last 1000 years at La Fossa volcano on the island of Vulcano (Italy) are characterized by abrupt changes of chemical composition that span from latite to rhyolite. The wide variety of textural features of these products has given rise to several petrological models dealing with the mingling/mixing processes involving mafic-intermediate and rhyolitic magmas. In this paper, we use published whole-rock data for the erupted products of La Fossa and combine them in geochemical and thermodynamic modelling in order to provide new constrains for the interpretations of the dynamics of the active magmatic system. The obtained results allow us to picture a polybaric plumbing system characterized by multiple magma reservoirs and related crystal mushes, formed from time to time during the differentiation of shoshonitic magmas, to produce latites, trachytes and rhyolites. The residing crystal mushes are periodically perturbated by new, fresh magma injections that, on one hand, induce the partial melting of the mush and, on the other hand, favor the extraction of highly differentiated interstitial melts. The subsequent mixing and mingling of mush-derived melts ultimately determine the formation of magmas erupted at La Fossa, whose textural and chemical features are otherwise not explained by simple assimilation and fractional crystallization models. In such a system, the compositional variability of the erupted products reflects the complexity of the physical and chemical interactions among recharging magmas and the crystal mushes.

Crystal mush dykes as conduits for mineralising fluids in the Yerington porphyry copper district, Nevada

Porphyry-type deposits are the world’s main source of copper and molybdenum and provide a large proportion of gold and other metals. However, the mechanism by which mineralising fluids are extracted from source magmas and transported upwards into the ore-forming environment is not clearly understood. Here we use field, micro-textural and geochemical techniques to investigate field relationships and samples from a circa 8 km deep cross-section through the archetypal Yerington porphyry district, Nevada. We identify an interconnected network of relatively low-temperature hydrothermal quartz that is connected to mineralised miarolitic cavities within aplite dykes. We propose that porphyry-deposit-forming fluids migrated from evolved, more water-rich internal regions of the underlying Luhr Hill granite via these aplite dykes which contained a permeable magmatic crystal mush of feldspar and quartz. The textures we describe provide petrographic evidence for the transport of fluids through crystal mush dykes. We suggest that this process should be considered in future models for the formation of porphyry- and similar-type deposits.

Zooming in on crystal mush: recent advances in volcano tomography

&lt;p&gt;The lack of direct seismological evidence for large molten magma chambers is considered to be one of the most important arguments in support of the mush paradigm. However, most published melt fraction estimates based on interpretation of seismological data are associated with large uncertainties because of two limitations: i) inherent limits to resolution of seismic tomography and, ii) trade-offs in the constitutive relationships that tie seismic properties to melt fraction. Low-velocity volumes associated with magma storage are particularly difficult to image with conventional travel-time tomography due to limited resolution and wavefront healing, resulting in blurred images and a high velocity bias. We tackle these limitation by applying full waveform inversion to active source seismic data collected over the Kolumbo submarine volcano (Greece). We recover a previously undetected Vp anomaly of &amp;#8211;50% beneath the volcano and interpret this as a shallow magmatic intrusion. Extension of this approach to the wider Santorini volcanic system is ongoing. Concurrently, we are tackling the second limitation, which is the result of the dependence of elastic properties on the microgeometry of the melt. Seismological melt estimates rely on the assumption that the melt pore space can be represented by simple geometrical shapes, usually ellipsoids, with a given aspect ratio. Since the aspect ratio is poorly constrained, this results in a trade-off between melt fraction and melt geometry. We have adapted a method for the homogenisation of the elastic properties of multi-phase composites and applied it to calculating the elastic properties of partially molten rocks starting from the melt microstructure determined by X-ray CT scanning. The microgeometry of the mush can be inferred from the study of glomerocrysts: crystal mush inclusions with quenched interstitial melt that are carried to the surface by erupted lava. After the sample is digitized and segmented into its constitutive phases (crystals, melt, vesicles), the average elastic properties are determined by numerical homogenisation which consists of numerically simulating the deformation of the sample under load and predicting its elastic response. The results are compared to a semi analytical solution for ellipsoidal inclusions. We apply this approach to a plutonic nodule from St Kitts and show that the melt microstructure leads to an elastic response equivalent to that of ellipsoidal melt inclusions with an aspect ratio of 0.1 (oblate spheroids). This equivalent aspect ratio is used to refine melt estimates for Montserrat, Santorini and Kolumbo volcano.&lt;/p&gt;