ALBERT

Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Three volcanic arcs have been the source of New Zealand's volcanic activity since the Neogene: Northland arc, Coromandel Volcanic Zone (CVZ) and Taupō Volcanic Zone (TVZ). The eruption chronology for the Quaternary, sourced by the TVZ, is well studied and established, whereas the volcanic evolution of the precursor arc systems, like the CVZ (central activity c. 18 to 2 Ma), is poorly known due to limited accessibility to, or identification of, onshore volcanic deposits and their sources. Here, we investigate the marine tephra record of the Neogene, mostly sourced by the CVZ, of cores from IODP Exp. 375 (Sites U1520 and U1526), ODP Leg 181 (Sites 1123, 1124 and 1125), IODP Leg 329 (Site U1371) and DSDP Leg 90 (Site 594) offshore of New Zealand. In total, we identify 306 primary tephra layers in the marine sediments. Multi‐approach age models (e.g. biostratigraphy, zircon ages) are used in combination with geochemical fingerprinting (major and trace element compositions) and the stratigraphic context of each marine tephra layer to establish 168 tie‐lines between marine tephra layers from different holes and sites. Following this approach, we identify 208 explosive volcanic events in the Neogene between c. 17.5 and 2.6 Ma. This is the first comprehensive study of New Zealand's Neogene explosive volcanism established from tephrochronostratigraphic studies, which reveals continuous volcanic activity between c. 12 and 2.6 Ma with an abrupt compositional change at c. 4.5 Ma, potentially associated with the transition from CVZ to TVZ.〈/p〉

Description: Plain Language Summary: Since 18 Ma, volcanic activity in New Zealand is dominantly sourced by the Coromandel Volcanic Zone (CVZ). Most caldera systems of the CVZ identified so far are located on Coromandel Peninsula in the NW of North Island, New Zealand, but studies of the CVZ are rare mainly due to the limited accessibility of its volcanic deposits, as well as missing stratigraphic continuity between different outcrops and the volcanic source. Here, our ocean drilling tephra record—mainly volcanic ash from explosive eruptions, distributed and falling out over the ocean—has a great potential to reveal the eruption history of the CVZ because it is preserved in marine sediments in a nearly undisturbed stratigraphic context. We analyzed ∼400 marine tephra layers from multiple ocean sediment cores off the coast of New Zealand for their geochemical glass compositions and identified 306 as largely undisturbed ash deposits. These primary ash deposits correspond to a total number of 208 Neogene volcanic events. Different dating methods result in a continuous marine tephra record for the last 12 Ma, equivalent to a unique and most complete eruptive history for the CVZ. This enables us to further unravel changes in the composition of the associated magmas with time.〈/p〉

Description: Key Points: 〈list list-type="bullet"〉 〈list-item〉 〈p xml:lang="en"〉New Zealand's Neogene explosive volcanism based on the marine tephra record〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Geochemical fingerprinting of marine tephra layers across the study area to establish volcanic events〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Insights into geochemical variations with time, repose times and spatiotemporal distribution〈/p〉〈/list-item〉 〈/list〉 〈/p〉

Description: DFG

Description: Marsden project

Description: https://doi.org/10.14379/iodp.proc.372B375.210.2023

Description: Five late Pleistocene lava domes with a combined eruptive volume of ~40 km 3 distributed over an area of ~2000 km 2 represent the waning stages of the 10–1 Ma ignimbrite flare-up in the Altiplano Puna Volcanic Complex (APVC) of the Central Andes. Zircon crystal face (on unsectioned rims) and interior (on sectioned crystals) ages (U-Th and U-Pb, respectively) for a total of 252 crystals indicate remarkably consistent zircon crystallization histories: the youngest zircon surface ages (ca. 104–83 ka) are near 40 Ar/ 39 Ar eruption ages from sanidine and biotite (ca. 120–87 ka), but a significant population of surface ages predates eruption, ranging to secular equilibrium (with U-Pb interior ages to 3.5 Ma). The essentially continuous zircon crystallization history implies protracted magma presence, which agrees with temporally invariant Ti-in-zircon model temperatures, backed by the homogeneity of indirectly temperature-dependent compositional parameters. Zircon age spectra modeled using a finite-difference thermal and mass-balance model for open-system magma evolution indicate protracted zircon production in the magma reservoirs that require time-integrated recharge rates of ~1 x 10 –3 km 3 /yr, corresponding to high intrusive to extrusive ratios of 75: 1. This rate is below the ~5 x 10 –3 km 3 /yr threshold proposed in the literature for incubating the supereruptions defining the flare-up. When accounting for the shorter durations of high versus low recharge episodes over the ~10 m.y. lifetime of the APVC flare-up, the contributions to composite batholith formation in the shallow crust of the APVC remained broadly constant during peaks and lulls in eruptive activity. This connotes that eruptive fluxes are a poor measure for intrusive fluxes. A corollary of this interpretation is that commonly applied intrusive to extrusive ratios will severely underestimate pluton formation rates during periods of low eruptive flux.

Description: Zircons from 15 crystal-rich monotonous intermediate ignimbrites and 1 crystal-poor rhyolite ignimbrite erupted during the 11–1 Ma Altiplano-Puna Volcanic Complex (APVC) ignimbrite flare-up record multiscale episodicity in the magmatic history of the shallowest levels (5–10 km beneath the surface) of the Altiplano-Puna Magma Body (APMB). This record reveals the construction of a subvolcanic batholith and its magmatic and eruptive tempo. More than 750 U-Pb ages of zircon rims and interiors of polished grains determined by secondary ion mass spectrometry define complex age spectra for each ignimbrite with a dominant peak of autocrysts and subsidiary antecryst peaks. Xenocrysts are rare. Weighted averages obtained by pooling the youngest analytically indistinguishable zircon ages mostly correspond to the dominant crystallization ages for zircons in the magma. These magmatic ages are consistent with eruptive stratigraphy, and fall into four groups defining distinct pulses (from older to younger, pulses 1 through 4) of magmatism that correlate with eruptive pulses, but indicate that magmatic construction in each pulse initiated at least 1 m.y. before eruptions began. Magmatism was initially distributed diffusely on the eastern and western flanks of the APVC, but spread out over much of the APVC as activity waxed before focusing in the central part during the peak of the flare-up. Each pulse consists of spatially distinct but temporally sequenced subpulses of magma that represent the construction of pre-eruptive magma reservoirs. Three nested calderas were the main eruptive loci during the peak of the flare-up from ca. 6 to 2.5 Ma. These show broadly synchronous magmatic development but some discordance in their later eruptive histories. These relations are interpreted to indicate that eruptive tempo is controlled locally from the top down, while magmatic tempo is a more systemic, deeper, bottom-up feature. Synchroneity in magmatic history at distinct upper crustal magmatic foci implicates a shared connection deeper within the APMB. Each ignimbrite records the development of a discrete magma. Zircon age distributions of individual ignimbrites become more complex with time, reflecting the carryover of antecrysts in successively younger magmas and attesting to upper crustal assimilation in the APVC. Although present, xenocrysts are rare, suggesting that inheritance is limited. This is attributed to basement assimilation under zircon-undersaturated conditions deeper in the APMB than the pre-eruptive levels, where antecrysts were incorporated in zircon-saturated conditions. Magmatic ages for individual ignimbrites are older than the 40 Ar/ 39 Ar eruption ages. This difference is interpreted as the average minimum Zr-saturated melt-present lifetime for APVC magmas, the magmatic duration or age. The average age of ca. 0.4 Ma indicates that thermochemical conditions for zircon saturation were maintained for several hundreds of thousands of years prior to eruption of APVC magmas. This is consistent with a narrow range of zircon saturation temperatures of 730–815 °C that record upper crustal conditions and Zr/Hf, Th/U, Eu/Eu*, and Ti that reveal protracted magma differentiation under secular cooling rates an order of magnitude slower than typical pluton cooling rates. In concert, these data all suggest that the pre-eruptive magma reservoirs were perched in a thermally and chemically buffered state during their long pre-eruptive lifetimes. Trace element variations suggest subtle differences in crystallinity, melt fraction, and melt composition within different zones of individual magma reservoirs. Significant volumes of plutonic rocks associated with ignimbrites are supported by geophysical data, the limited compositional range over 10 m.y., the thermal inertia of the magmatic systems, and the evidence of resurgent magmatism and uplift at the calderas and eruptive centers, the distribution of which defines a composite, episodically constructed subvolcanic batholith. The multiscale episodicity revealed by the zircon U-Pb ages of the APVC flare-up can be interpreted in the context of continental arc magmatic systems in general. The APVC ignimbrite flare-up as a whole is a secondary pulse of ~10 m.y., with magmatic pulses 1 through 4 reflecting tertiary pulses of ~2 m.y., and the individual ignimbrite zircon spectra defining quaternary pulses of 〈1 m.y. This hierarchy of pulses is thought to reflect how a magmatic front, driven by the primary mantle power input, propagates through the crust with individual magmatic events occurring over sequentially smaller spatial and faster temporal scales in the upper crust of the Central Andes from ~30 km to the surface.

Description: Finely acicular rutile intergrown with host quartz (rutilated quartz) is commonly found in hydrothermal veins, including the renown cleft mineral locations of the Swiss Alps. These Alpine cleft mineralizations reportedly formed between ~13.5 and 15.2 Ma (based on ages of rare hydrothermal monazite and titanite) at temperatures ( T ) of ~150–450 °C (based on fluid inclusions and bulk quartz-mineral oxygen isotope exchange equilibria), and pressures ( P ) of 0.5–2.5 kbar (estimated from a geothermal gradient of 30 °C/km). The potential of rutilated quartz as a thermochronometer, however, has not been harnessed previously. Here, we present the first results of age and T determinations for rutilated quartz from six locations in the Swiss Alps with vein country rocks that cover peak-metamorphic conditions between ~600 and 〈350 °C. Samples were cut and mounted in epoxy disks to expose rutile (~30 to 1400 μm in diameter) and its host quartz. Cathodoluminescence (CL) and backscattered electron (BSE) imaging of host quartz and rutile inclusions, respectively, shows internal zonations, which are nevertheless isotopically homogeneous. Newly developed secondary ionization mass spectrometry (SIMS) oxygen isotopic analysis protocols for rutile were combined with those established for trace elements (including Zr) and U-Pb ages in rutile, and Ti abundances in the host quartz. U-Pb rutile ages average 15.1 ± 1.7 Ma (2), in excellent agreement with previous accessory mineral geochronometers. Pressure-independent T estimates, calibrated for low-temperature conditions, from oxygen isotope fractionation between rutile and quartz in touching pairs are 310–576 °C. Individual rutile needles vary in Zr abundances beyond analytical uncertainties, but average Zr-in-rutile inversely correlates with oxygen isotopic fractionation between quartz and rutile. Linear regression of the data yields: \[ T(^\circ \hbox{ C })=\frac{26(\pm 9)}{0.07(\pm 0.01)-\hbox{ R\hspace{0.17em}ln\hspace{0.17em} }x}-273 \] with x = Zr ppm and R = 0.008314 (uncertainties scaled by the square root of the mean square of weighted deviates MSWD = 11; n = 9). This relationship supports previously recognized temperature-dependent Zr uptake in rutile, although widely used Zr-in-rutile thermometer calibrations based on high- T experiments are at variance with oxygen isotope exchange temperatures. By contrast, Ti-in-quartz lacks systematic relations with oxygen isotope temperatures. The discrepancy between low- T Ti-in-quartz thermometry on one side, and oxygen isotope and Zr-in-rutile thermometry on the other, suggests that Ti-in-quartz thermometry should be applied with caution for low- T (〈500 °C) rocks.

Description: Few terrestrial localities preserve more than a trace lithic record prior to ca. 3.8 Ga greatly limiting our understanding of the first 700 Ma of Earth history, a period inferred to have included a spike in the bolide flux to the inner solar system at ca. 3.85–3.95 Ga (the Late Heavy Bombardment, LHB)....

Description: Baddeleyite is a frequently found accessory mineral in silica undersaturated lavas. Because it is typically enriched in uranium, while having low initial lead, baddeleyite has long been a prime target for U-Pb geochronology of mafic rocks. The difficulties in retrieving small baddeleyite grains from volcanic samples and the lack of a detailed understanding of baddeleyite occurrence, however, have limited baddeleyite chronology largely to coarse-grained mafic intrusive rocks. Here, the development of U-Th in situ baddeleyite analysis using secondary ionization mass spectrometry (SIMS) is presented together with an assessment of baddeleyite occurrence in Quaternary silica-undersaturated lavas from Campi Flegrei (Naples, Italy). Samples studied comprise the pre- and post Campanian Ignimbrite (ca. 40 ka) lava domes of Cuma and Punta Marmolite, and Astroni and Accademia, respectively. The in situ sample preparation required initial identification of baddeleyite crystals from sawed and polished rock billets using scanning electron microscope (SEM) backscatter imaging and energy-dispersive X-ray analysis. U-Th baddeleyite isochron ages for intra-caldera Astroni and Accademia lava domes are 5.01 +2.61 –2.55 ka (MSWD = 2.0; n = 17) and 4.36 +1.13 –1.12 ka (MSWD = 2.9; n = 24), respectively. The ages for Punta Marmolite (62.4 +3.9 –3.8 ka; MSWD = 1.2; n = 11) and Cuma (45.9 +3.6 –3.5 ka; MSWD = 2.2; n = 11) predate the Campanian Ignimbrite. Rapid baddeleyite crystallization at the time of emplacement is supported by petrologic observations that 〉50% of the baddeleyite crystals documented in this study occur either in vesicles or in vesicle-rich regions of the host lavas whose textures developed over timescales of a few years to a few decades based on microlite crystal size distribution (CSD) analysis. Radiometric U-Th baddeleyite ages are mostly in agreement with previously determined K-Ar eruption ages, except for the Punta Marmolite lava dome whose U-Th baddeleyite age predates the K-Ar age by ca. 15 ka. Baddeleyite thus records eruptive emplacement with little evidence for significant pre-eruptive crystal residence, and has potential as an eruption chronometer for Quaternary silica-undersaturated volcanic rocks.

Description: 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- 18 O rhyolite known on Earth: the 1800 km 3 Kilgore Tuff ( 18 O = 3·4). Such low- 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- 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 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- 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- 18 O Kilgore cycle units have compositions that define a sharp reversal in the temporal trend back towards the composition of the first normal- 18 O Heise tuff (6·62 Ma Blacktail Creek Tuff). The data support derivation of the voluminous low- 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- 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 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 18 O values can be predicted on the basis of cumulative eruptive volumes, with a decrease in 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 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- 18 O rhyolites on Earth.

Description: The 1.85 Ga Belomorian Belt, Karelia, Russia, hosts ultralow 18 O and D (as low as –27.3 and –235 standard mean ocean water [SMOW], respectively), high-Al gneisses and amphibolites that we attribute to the Paleoproterozoic "Slushball Earth" glaciation. They now occur in at least 11 localities spanning 450 km. To constrain distribution of 18 O-depleted rocks, we performed detailed field mapping in Khitostrov, where 18 O values are the lowest. Using 430 new and previously published laser fluorination isotope analyses, we show that the elongated, concentrically zoned area of 18 O depletion is greater than 6 x 2 km in areal extent, ~10 times larger than previously thought. Relationships between 17 O versus 18 O strictly adhere to the equilibrium terrestrial mass-dependent fractionation with a slope of 0.527. We also report the results of ion microprobe U-Pb geochronology of zircons coupled with co-registered oxygen isotope spot analyses for mafic intrusions and host gneisses in six localities. The 2.9–2.7 Ga gneiss zircon cores are normal in 18 O (5–7). They show truncated oscillatory cathodoluminescence (CL) patterns and rounded shape indicative of original igneous crystallization with subsequent detrital overprinting. A younger 2.6–2.55 Ga metamorphic zircon domain with normal 18 O, low Th/U, dark cathodoluminescence, and also with rounded crystal morphology is commonly preserved. Cores are surrounded by ubiquitous rims highly depleted in 18 O (re-)crystallized with Svecofennian (1.85–1.89 Ga) ages. Rims are interpreted as metamorphic due to bright and uniform CL and Th/U 〈0.05. Mafic intrusions preserve few igneous zircon crystals between ca. 2.23 and 2.4 Ga in age, but neoblastic zircon in these intrusions originated mostly during 1.85 Ga Svecofennian metamorphism. The 18 O-age relationship for metamorphic rims in zircon and corundum grains suggests that 18 O values of fluids were subtly increasing with time during metamorphism. Large metamorphic corundum grains have ~3 intracrystalline 18 O isotope zonation from –24 to –21, which likely developed during interaction with metamorphic fluids. The Zr-in-rutile geothermometer temperatures are in the range of 760 to 720 °C, in accordance with mineral assemblages and amphibolite metamorphic grade. High and irregular rare-earth element (REE) abundance in cores and rims of many zircons correlates with high phosphorus content and is explained by nanometer-scale xenotime and monazite inclusions, likely in metamict zones during 1.85 Ga Svecofennian metamorphism. A survey of oxygen isotopes in ultrahigh-pressure diamond and coesite-bearing metamorphic terrains around the world reveals normal to high- 18 O values, suggesting that the low 18 O in metamorphic rocks of Dabie Shan, Kokchetav, and in Karelia, are genetically unrelated to metamorphism. We discuss alternative ways to achieve extreme 18 O depletion by kinetic, Rayleigh, and thermal diffusion processes, and by metamorphism. We prefer an interpretation where the low- 18 O and high-Al signature of the rocks predates metamorphism, and is caused by shallow hydrothermal alteration and partial dissolution of the protolith surrounding shallow mafic intrusions by glacial meltwaters during pan-global Paleoproterozoic "Slushball Earth" glaciations between ca. 2.4 and ca. 2.23 Ga.