Chad D. Deering

Changes in magma storage conditions following caldera collapse at Okataina Volcanic Center, New Zealand

Large silicic volcanic centers produce both small rhyolitic eruptions and catastrophic caldera-forming eruptions. Although changes in trace element and isotopic compositions within eruptions following caldera collapse have been observed at rhyolitic volcanic centers such as Yellowstone and Long Valley, much still remains unknown about the ways in which magma reservoirs are affected by caldera collapse. We present 238U–230Th age, trace element, and Hf isotopic data from individual zircon crystals from four eruptions from the Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand, in order to assess changes in trace element and isotopic composition of the reservoir following the 45-ka caldera-forming Rotoiti eruption. Our data indicate that (1) mixing of magmas derived from crustal melts and mantle melts takes place within the shallow reservoir; (2) while the basic processes of melt generation likely did not change significantly between pre- and post-caldera rhyolites, post-caldera zircons show increased trace element and isotopic heterogeneity that suggests a decrease in the degree of interconnectedness of the liquid within the reservoir following collapse; and (3) post-caldera eruptions from different vents indicate different storage times of the amalgamated melt prior to eruption. These data further suggest that the timescales needed to generate large volumes of eruptible melt may depend on the timescales needed to increase interconnectedness and achieve widespread homogenization throughout the reservoir.

Rapid cooling and cold storage in a silicic magma reservoir recorded in individual crystals

Taupo Volcanic Zone magma spent more than 90% of its life deep and crystalline before rapid shallow accumulation and eruption. Quick eruption after a long bake Minerals such as zircon can record the storage conditions of magma before volcanic eruption. Rubin et al. combined traditional 238U-230Th dating with lithium concentration profiles in seven zircons from the Taupo supervolcanic complex in New Zealand to determine magma storage conditions. The zircons spent more than 90% of their lifetime in an uneruptible, mostly crystalline, and deep magmatic reservoir. The zircons were eventually transported to hotter, shallower, and eruptible magma bodies, where they spent only decades to hundreds of years before eruption. The result suggests a two-stage model for magmatic systems with large thermal variations. Science, this issue p. 1154 Silicic volcanic eruptions pose considerable hazards, yet the processes leading to these eruptions remain poorly known. A missing link is knowledge of the thermal history of magma feeding such eruptions, which largely controls crystallinity and therefore eruptability. We have determined the thermal history of individual zircon crystals from an eruption of the Taupo Volcanic Zone, New Zealand. Results show that although zircons resided in the magmatic system for 103 to 105 years, they experienced temperatures >650° to 750°C for only years to centuries. This implies near-solidus long-term crystal storage, punctuated by rapid heating and cooling. Reconciling these data with existing models of magma storage requires considering multiple small intrusions and multiple spatial scales, and our approach can help to quantify heat input to and output from magma reservoirs.

Conditions of melt generation beneath the Taupo Volcanic Zone: The influence of heterogeneous mantle inputs on large-volume silicic systems

Many arc silicic igneous provinces exhibit compositional variability defined by oscillation between dry and wet rhyolites. The origins of this variability are often uncertain due to the poor constraints on the compositions of the mantle-derived inputs to the lower crustal hybridization zones. The Taupo Volcanic Zone (TVZ) in New Zealand, the most productive of modern silicic igneous provinces, exhibits variability of rhyolite compositions, but small-volume coeval basaltic eruptions also occur, making it an ideal location to study the mantle contributions to these distinctive types of rhyolite. Here we present major and trace element data from 12 magmatic centers that reveal that mafic units from the northern portion of the central TVZ are enriched in large-ion lithophile elements (LILE) in comparison to the southern TVZ. The results of models quantifying slab-derived contributions to the mantle suggest that the observed heterogeneity in LILE concentrations, and volatile fugacities, can be explained by variable amounts of subduction-derived fluid within the melting region of the basalts. These new data and modeling results provide the first direct evidence that the spatial diversity in the flux of mantle-derived basalts and associated volatile elements into the lower crustal differentiation system of the TVZ are coincident with wet to dry rhyolite compositional variability.

Zircon record of the plutonic-volcanic connection and protracted rhyolite melt evolution

The potential petrogenetic link between a crystal-poor rhyolite (the Rhyolite Canyon Tuff) and its associated subvolcanic intrusion and crystal-rich post-caldera lavas from Turkey Creek, Arizona (USA), is examined using zircon chemical abrasion–thermal ionization mass spectrometry U-Pb geochronology and inductively coupled plasma mass spectrometry trace element analyses. U-Pb ages indicate that zircon growth within the rhyolite and the dacite-monzonite porphyry magmas was coeval over ∼300 k.y. prior to the large eruptive event. Trends in zircon trace elements (Hf, Y/Dy, Sm/Yb, Eu/Eu*) through time in the dacitic-monzonitic units and rhyolite reflect melt evolution dominated by crystal fractionation. Importantly, the Y/Dy ratio in zircons in both units remains mostly similar for the first ∼150 k.y. of the system’s evolution, but the dominant population in the rhyolitic unit diverges from that of the dacite-monzonite porphyry ∼150 k.y. before eruption. We interpret this divergence in trace element composition to record the assembly time of the melt-rich cap within its intermediate mush zone in the upper crustal reservoir. These results are consistent with (1) a connection between plutonic and volcanic realms in the upper crust, (2) a protracted time scale for constructing an intermediate mush large enough to hold 500 km 3 of rhyolite, and (3) the prolonged extraction of that melt prior to eruption.

Formation of rhyolite at the Okataina Volcanic Complex, New Zealand: New insights from analysis of quartz clusters in plutonic lithics

Abstract Granitoid lithic clasts from the 0.7 ka Kaharoa eruption at the Tarawera volcano (Okataina Volcanic Complex, Taupo Volcanic Zone, New Zealand) provide insight into the processes of rhyolite formation. The plutonic lithic clasts of the Kaharoa eruption consist of (1) quartz phenocrysts, which are often grouped into clusters of two to eight quartz grains, (2) plagioclase phenocrysts (mostly ~An40 with up to An60 cores), and (3) interstitial alkali feldspar. Quartz orientations obtained through electron backscatter diffraction (EBSD) methods show that 78% of the 82 analyzed clusters have at least one pair of quartz grains with the dominant dipyramidal faces matched. Variations in cathodoluminescence (CL) zoning patterns of the quartz suggest that quartz clusters came together after initial crystal growth and that many quartz crystals were subject to one or more resorption events. The process of quartz crystals with different magmatic histories coming together into common relative orientations to form clusters is indicative of oriented quartz synneusis and suggests a history of crystal accumulation. The quartz clusters are interpreted to have formed as part of a crystal cumulate mush within a shallow magma chamber where quartz crystals rotated into contact along their dominant dipyramidal faces during hindered settling and/or compaction. The preservation of oriented quartz clusters from the Kaharoa plutonic lithics thus provides evidence for synchronous, shallow pluton formation from a cumulate mush during active silicic volcanism. This result is consistent with models whereby meltrich, high-silica rhyolite formation occurs via interstitial melt extraction from a low-silica rhyolite mush in the shallow crust.

Magmatic volatile distribution as recorded by rhyolitic melt inclusions in the Taupo Volcanic Zone, New Zealand

Abstract The central Taupo Volcanic Zone (TVZ) is an actively rifting continental arc and is well known for its exceptionally high rate of rhyolitic magma generation and frequent caldera-forming eruptions. Two end-member types of rhyolites (R1 and R2) have been previously identified based on differences in their bulk-rock chemistry and mineral assemblage with hydrous phases crystallizing in the R1 type, which are not present or only rare in R2 rhyolites. Here we present new trace element and volatile data from rhyolitic melt inclusions measured in several representative eruptive deposits (R1 and R2 rhyolites) from the central TVZ to examine their volatile concentrations and origin. R1 and R2 show very distinct Cl concentrations, with R2 rhyolites being enriched in Cl by c. 1000 ppm. H2O is slightly higher in the R1 rhyolites, whereas CO2 concentrations are similar between the two end-member types. The origin of these volatile disparities between R1 and R2 melts is assigned to differences in the initial bulk volatile content of the parental magma, possibly associated with distinct input of fluids from the subduction zone. These disparities in bulk volatile concentrations can lead to variations in relative timing of exsolution of volatile phase(s) prior to melt inclusion entrapment. Supplementary material: Major, trace and volatile composition for the analysed central TVZ rhyolites, and comparison of H2O data between the transmission and reflectance FTIR are available at http://www.geolsoc.org.uk/SUP18767.

Mineralogical, geochemical, and textural indicators of crystal accumulation in the Adamello Batholith (Northern Italy)

Abstract In this study, we quantitatively investigate crystal-melt segregation processes in two upper-crustal, intermediate-to-silicic plutons from the Tertiary Adamello Batholith, Italian Alps, by combining (1) an estimation of the amount of crystallized interstitial liquid using cathodoluminescence images, phase maps, and mass-balance calculations with (2) quantification of crystal preferred orientation using electron backscatter diffraction. Cathodoluminescence images, phase maps, and plagioclase profiles are used together to distinguish early grown primocrysts from overgrowths formed after the rheological “lock-up” of the magma bodies. Mass-balance calculations, taking into account mineral compositions and bulk-rock chemistry, are used as an additional means to quantify the amount of trapped melt. The following features are indicative of crystal accumulation (or melt loss) in some parts of the batholith: (1) The amount of crystallized interstitial liquid can be low and negatively correlated with crystal (and shape) preferred orientations. Locally, up to ca. 27% melt may have been lost. (2) Significant intracrystalline deformation in plagioclase (up to ca. 13° of lattice distortion) is present in strongly foliated samples, resulting from compaction in a highly crystalline mush. These mineralogical and textural features indicative of variability in the degree of crystal accumulation in some areas of the Adamello batholith may explain the highly scattered bulk-rock geochemical patterns (particularly in trace elements). However, the precise quantification of the amount of melt loss remains challenging in felsic plutons, because of the compositional deviation from liquid lines of descent due to multi-scale variations in the degree of crystal-melt segregation and the fact that magmatic textures indicative of crystal accumulation can be subtle.

Magma storage, differentiation, and interaction at Lake City caldera, Colorado, USA

Rocks from the 23 Ma Lake City caldera show diverse chemical affinities attesting to a complex magmatic system beneath the caldera. Field and geochemical data from ignimbrites and intrusions constrain magma storage and magma interactions during the formation of the caldera. Two geochemically distinct magma batches erupted during caldera formation: batch A, consisting of rhyolites and trachytes, and batch B, consisting of dacites and trachyandesites. The ignimbrites of the Lower, Middle, and Upper Sunshine Peak Tuff represent the bulk of erupted batch A magma, with an increasing proportion of trachyte to rhyolite as the eruption progressed. Overall, the observed trends of major and trace elements are consistent with the sequential eruption of a magmatic system with a rhyolitic upper portion and trachytic lower portion. The Middle Sunshine Peak Tuff contains two distinct types of pumice clast, while the Upper Sunshine Peak Tuff contains four distinct pumice clast types, with one type chemically related to batch B magma. The link between the rhyolite and trachyte of batch A is supported by major- and trace-element geochemical modeling of an initially trachytic magma that fractionated and was subjected to crystal/melt segregation following 50%–60% crystallization. Compositional gaps and chemical heterogeneity in the bulk ignimbrite composition show that the proportions of these different magma types varied significantly during eruption. We propose that the fractionating batch A and B magmas formed distinct magma pods, some containing residual magma mush, that were tapped during different phases of caldera formation. After collapse, dacite lavas of batch B were erupted concurrent with resurgent uplift from shallow intrusion of both residual mingled batch A and batch B magma. In summary, our observations suggest (1) a complex magma chamber geometry from two fractionating magma batches, and (2) magma replenishment and accelerated periods of magma reorganization in the shallow magma plumbing system during a single caldera cycle at Lake City.

Second boiling effects on the Al-content of hornblende rims from an exhumed Cretaceous arc pluton, Stewart Island, New Zealand

Abstract High-resolution transects across amphiboles from the layered mafic-felsic Halfmoon Pluton suggest that amphibole compositions in the outermost ~40 μm of grains analyzed fluctuate in response to P-T-PH₂O conditions during the final stages of pluton emplacement and crystallization. Detailed transects (~10-15 μm point spacing) across 15 crystals from six representative rock types within the compositionally stratified Halfmoon Pluton revealed that in almost all crystals Altot increased over at least the outer 40 μm, coupled with a decrease in temperature and Ti-content. P-T estimates for Halfmoon Pluton amphiboles using the Al-in-hornblende geobarometer and the hornblende-plagioclase geothermometer are consistent with upper crustal emplacement depths. Amphibole Altot within the core and midsections is controlled by the temperature-dependent edenite and Ti-Tschermak exchanges, and is interpreted to reflect the influence of repeated injections of hot mafic magma into the magma reservoir during crystallization. Increases in Altot at all amphibole rims arise from a notable increase in VIAl and small increases in IVAl, indicating that, unlike the core and midsections that exhibit a strong temperature dependence, rim-Altot is a function of the pressure-dependent Al-Tschermak exchange. Pressure estimates calculated for amphibole rims record a rim-ward increase in P equivalent to between 0.2 and 1.9 kbar, which corresponds to increases in depth of crystallization of 0.7 and 6.7 km, respectively. This observed increase in pressure at the rims of all amphibole grains analyzed is not consistent with increasing lithostatic pressure as a result of pluton burial, but to an increase in PH₂O in response to second boiling, a common process during late-stage crystallization of H2O-rich intermediate-silicic magmas. The presence of water escape structures throughout the pluton, and the predominance of biotite-rich mafic enclaves that commonly contain amygdules toward the inferred top of the magmatic “stratigraphy” provide further evidence to support the process of second boiling. These results indicate that caution is required in choosing suitable analytical sites when applying the Al-in-hornblende geobarometer, particularly within plutonic bodies where there is evidence for vesiculation and water escape structures, as the outer rim compositions of amphiboles may not be a function of lithostatic pressure alone.

Evidence for a spike in mantle carbon outgassing during the Ediacaran period

Long-term cycles in Earth’s climate are thought to be primarily controlled by changes in atmospheric CO2 concentrations. Changes in carbon emissions from volcanic activity can create an imbalance in the carbon cycle. Large-scale changes in volcanic activity have been inferred from proxies such as the age abundance of detrital zircons, but the magnitude of carbon emissions depends on the style of volcanism as well as the amount. Here we analyse U–Pb age and trace element data of detrital zircons from Antarctica and compare the results with the global rock record. We identify a spike in CO2-rich carbonatite and alkaline magmatism during the Ediacaran period. Before the Ediacaran, secular cooling of the mantle and the advent of cooler subduction regimes promoted the sequestration of carbon derived from decarbonation of subducting oceanic slabs in the mantle. We infer that subsequent magmatism led to the extensive release of carbon that may at least in part be recorded in the Shuram–Wonoka carbon isotope excursion. We therefore suggest that this pulse of alkaline volcanism reflects a profound reorganization of the Neoproterozoic deep and surface carbon cycles and promoted planetary warming before the Cambrian radiation.A spike of carbon-rich volcanism during the Ediacaran period identified in detrital zircon data may reflect a reorganization of the Neoproterozoic deep carbon cycle.