B. L. A. Charlier

Zircon crystallization and recycling in the magma chamber of the rhyolitic Kos Plateau Tuff (Aegean arc)

In contrast to most large-volume silicic magmas in continental arcs, which are thought to evolve as open systems with significant assimilation of preexisting crust, the Kos Plateau Tuff magma formed dominantly by crystal fractionation of mafic parents. Deposits from this ∼60 km3 pyroclastic eruption (the largest known in the Aegean arc) lack xenocrystic zircons [secondary ion mass spectrometry (SIMS) U-Pb ages on zircon cores never older than 500 ka] and display Sr-Nd whole-rock isotopic ratios within the range of European mantle in an area with exposed Paleozoic and Tertiary continental crust; this evidence implies a nearly closed-system chemical differentiation. Consequently, the age range provided by zircon SIMS U-Th-Pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus. The age distribution from 160 ka (age of eruption by sanidine 40Ar/39Ar dating; [Smith et al., 1996][1]) to ca. 500 ka combined with textural characteristics (high crystal content, corrosion of most anhydrous phenocrysts, but stability of hydrous phases) suggest (1) a protracted residence in the crust as a crystal mush and (2) rejuvenation (reduced crystallization and even partial resorption of minerals) prior to eruption probably induced by new influx of heat (and volatiles). This extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned Aegean microplate acts as a refractory container for magmas in the dying Aegean subduction zone (continent-continent subduction). [1]: #ref-32

Late Quaternary evolution of a hyperactive rhyolite magmatic system: Taupo volcanic centre, New Zealand

Taupo volcanic centre has been active for c. 300 000 years. Since the c. 26.5 ka caldera‐forming Oruanui eruption 28 eruptions have occurred from Taupo, varying between 0.1 and >45 km3 in minimum bulk volume, and with repose periods ranging from c. 20 to 6000 years. All magma erupted post‐26.5 ka is compositionally and mineralogically distinct from pre‐Oruanui and Oruanui eruptives, and is inferred to have formed at or after 26.5 ka. Four post‐Oruanui magma types are identified on the basis of whole rock and mineral compositions: one dacitic, forming three eruptions between 20.5 ka and 17 ka, and three subtly distinct rhyolite compositions erupted in discrete periods from 11.8 to 9.95, 7.05 to 2.75 and 2.15 to 1.74 ka. Stepwise compositional variations between, and limited variation within, rhyolite groups suggest emplacement of three petrogenetically separate batches of magma within only 10 000 years. The 15–35 km3 of magma erupted at 1.77 ka evidently appeared in <103 years; this short residence time may have contributed to the lack of zonation within this magma chamber. Taupo is unusual amongst large rhyolite volcanoes in terms of the high frequency of activity since 26.5 ka, rapid stepwise changes in rhyolite compositions, and insignificant differentiation within individual subgroups. These traits are attributed to the combined effects of the extensional arc setting, thermal energy from mafic magma, and the shallow slope of the plagioclase‐saturated rhyolite liquidus.

Some remarks on U–Th mineral ages from igneous rocks with prolonged crystallisation histories

Abstract Mineral isochron dating is a frequently used geochronological tool. One of its assumptions is that the minerals grow over a time period that is small compared to the half-life of the radiogenic isotope system used. In recent years, increasing analytical precision has promoted the use of the short-lived U-series isotope system in order to date young crystallisation events. Three whole-rock zircon U–Th isochrons from the 26.5 ka Oruanui eruption in the Taupo Volcanic Zone, New Zealand, yield pre-eruptive model ages of 5.5±0.8 ka, 9.7±1.7 ka and 12.3±0.8 ka for the sub-63 μm, 63–125 μm and 125–250 μm zircon size fractions, respectively. This suggests that in this case the assumption of instantaneous crystal growth breaks down. Instead, the U–Th data may be explained by continuous zircon growth over a period of ∼90 ka. However, cathodoluminescence shows that crystals are typically composed of an euhedral core surrounded by a sector-zoned euhedral rim, and the U–Th data can also be modelled by mixing an older (∼27 ka model age) population of zircon crystals with a young zircon rim that formed shortly prior to eruption of the Oruanui rhyolite. This indicates that detailed petrographic studies are critical for deciphering the histories of prolonged crystallisation in the magmatic environment. It is concluded that conventional U-series mineral isochrons may underestimate the age of the onset of crystallisation by more than an order of magnitude. In future, microanalytical techniques will lead to significant advances in the understanding of crystallisation processes and timescales.

First field evidence of southward ductile flow of Asian crust beneath southern Tibet

There is lively debate on whether Asian plate material was involved in southward flow of mid-lower crust in a ductile channel beneath southern Tibet. One argument against such involvement is the apparent absence of material derived from Asian lithosphere within the High Himalayan Series (Indian plate) that could represent the putative channel. A north-south–trending mid-Miocene dike swarm that intrudes the Tethyan sedimentary cover of the Sakya gneiss dome (Indian plate) yields new Sr-Nd isotopic data (87Sr/86Sr = 0.7071–0.7079; ϵNd −4 to −6) indicating that these melts share the same source as Miocene dacitic dikes from north of the Indus-Tsangpo suture. Moreover, dikes on both sides of this suture represent crustal melts derived largely from mid-lower crust of the Asian plate, exposed today as the Nyainqentanglha gneisses that underlie the Gangdese batholith. We infer that melting of the Asian lithosphere extended south of the surface trace of the suture, requiring southward propagation of anatectic Asian middle crustal material during the Miocene. The emplacement ages of the southern dike swarm (12–9 Ma) thus delimit the timing of active southward ductile flow of Asian material.

Evidence from zircon U-Pb age spectra for crustal structure and felsic magma genesis at Taupo volcano, New Zealand

U-Pb zircon age spectra from a metasedimentary xenolith and a young dacite provide evidence for rapid episodes of crustal melting and eruption at Taupo volcano, as well as unexpected crustal structure beneath the Taupo Volcanic Zone. Ages from the xenolith in a 28 ka rhyolite dome match those from Early Cretaceous Pahau Terrane graywackes that crop out >75 km east of the volcano and were underthrust and partially melted during the penecontemporaneous accretion process. Boundaries between graywacke terranes in the North Island are thus more complex than implied from map views. In contrast, ages from zircons in the 20 ka Omega dacite match the nearby surficial graywacke of the Jurassic Kaweka Terrane (55% of grains), Quaternary plutonic sources (39%), and Pahau Terrane graywacke (6%). Five of 72 grains analyzed contain graywacke cores and Pleistocene rims. Survival of euhedral xenocrystic zircons through the generation processes for Omega dacite magma reflects wholesale melting of crustal protoliths (including varieties not previously considered for petrogenetic modeling), open-system behavior, and crustal melts being generated and erupted within ∼1–10 yr.

Mineral-scale Sr isotope variation in plutonic rocks - a tool for unravelling the evolution of magma systems.

Isotope ratios of elements such as Sr, Nd, Pb and Hf can be used as tracers of magmatic sources and processes. Analytical capabilities have evolved so that isotope ratios can now be analysed in situ, and isotopic tracers can therefore be used within single minerals to track the changing magmatic environment in which a given mineral grew. This contribution shows that Sr isotope ratios in feldspars that make up plutonic rocks will typically preserve initial isotopic variations, provided precise and accurate age corrections can be applied. Variations in initial isotope ratio can give a core-to-rim record of magmatic evolution and can be used to diagnose open system events such as contamination and magma recharge and mixing. New single grain Sr isotope data are presented from the Dais Intrusion, Antarctica, which reflect an open system origin for the crystals. The crystal cargo appears to be aggregated and assembled during transport and emplacement. This model, as opposed to a magma body crystallising post emplacement, may be more applicable to plutonic rocks in general, and is testable using the in situ isotopic determination methods described here.

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

Rubin et al. (Reports, 16 June 2017, p. 1154) proposed that gradients in lithium abundance in zircons from a rhyolitic eruption in New Zealand reflected short-lived residence at magmatic temperatures interleaved with long-term “cold” (<650°C) storage. Important issues arise with the interpretation of these lithium gradients and consequent crystal thermal histories that raise concerns about the validity of this conclusion.

Mafic inputs into the rhyolitic magmatic system of the 2.08 Ma Huckleberry Ridge eruption, Yellowstone

Abstract The silicic (broadly dacitic to rhyolitic) magmatic systems that feed supereruptions show great diversity, but have in common a role for mafic (broadly basaltic to andesitic) magmas as drivers of the systems. Here we document the mafic component in the rhyolitic magmatic system of the 2.08 Ma Huckleberry Ridge Tuff (HRT), Yellowstone, and compare it to mafic materials erupted prior to and following the HRT eruption in the area within and immediately around its associated caldera. The HRT eruption generated initial fall deposits, then three ignimbrite members A, B, and C, with further fall deposits locally separating B and C. A “scoria” component was previously known from the upper B ignimbrite, but we additionally recognize juvenile mafic material as a sparse component in early A, locally abundant in upper A and sparsely in lower B. It has not been found in the C ignimbrite. In upper B the mafic material is vesicular, black to oxidized red-brown scoria, but at other sites is overwhelmingly non-vesicular, and sparsely porphyritic to aphyric. Despite their contrasting appearances and occurrences, the mafic components form a coherent compositional suite from 49.3–63.3 wt% SiO2, with high alkalis (Na2O+K2O = 4.5–7.3 wt%), high P2O5 (0.52–1.80 wt%), and notably high concentrations of both high field strength and large-ion lithophile elements (e.g., Zr = 790–1830 ppm; Ba = 2650–3800 ppm). Coupled with the trace-element data, Sr-Nd-Pb isotopic systematics show influences from Archean age lower crust and lithospheric mantle modified by metasomatism during the late Cretaceous to Eocene, as previously proposed for extensive Eocene magmatism/volcanism around the Yellowstone area. The HRT mafic compositions contrast markedly with the Snake River Plain olivine tholeiites erupted before and after the HRT eruption, but are broadly similar in several respects to the generally small-volume Craters of the Moon-type mafic to intermediate lavas erupted recently just west of the HRT caldera, as well as farther west in their type area. The combination of trace element and isotopic data on the HRT mafics are only consistent with an origin for their parental magma as melts from mantle enriched by high temperature and pressure melts, most likely from the underlying Farallon slab. Subsequent interaction of the HRT mafic magmas occurred with the Archean lower crust and lithospheric mantle, but not the highly radiogenic upper crust in this area. The close temporal and spatial relationships of the HRT mafic compositions and the preceding Snake River Plain olivine tholeiite eruptives suggest a high degree of spatial heterogeneity in the mantle beneath the Yellowstone area during the early (and subsequent) development of its modern magmatic system.