ALBERT

Description: The geochemistry of pyroclasts sampled from four volcanoes along the Kermadec arc in the SW Pacific is used to investigate the genesis of silicic magmas in a young (〈2 Myr), archetypical intra-oceanic arc setting. Raoul, Macauley and Raoul SW volcanoes in the northern Kermadec arc, and Healy volcano in the southern Kermadec arc have all recently erupted dacitic to rhyolitic crystal-poor pumice. In addition to whole-rock analyses, we present a detailed study of mineral and glass chemistries to highlight the complex structure of the Kermadec magmatic systems. Major and trace element bulk-rock compositions mostly fall into relatively narrow compositional ranges, forming discrete groups by eruption for Raoul, and varying with relative crystal contents for Healy. In contrast, pumices from Macauley cover a wide range of compositions, between 66 and 72·5 wt % SiO 2 . At all four volcanoes the trace element patterns of pumice are subparallel to both those of previously erupted basalts and/or whole mafic blebs found both as discrete pyroclasts and as inclusions within pumices. Pb and Sr isotopic compositions have limited ranges within single volcanoes, but vary considerably along the arc, being more radiogenic in the southern volcanoes. Distinctive crystal populations and zonation patterns in pumices, mafic blebs and plutonic xenoliths indicate that many crystals did not grow in the evolved magmas, but are instead mixed from other sources including gabbros and hydrothermally altered tonalites. Such open-system mixing is ubiquitous at the four volcanoes. Oxygen isotope compositions of both phenocrysts (silicic origin) and xenocrysts or antecrysts (mafic origin) are typical for mantle-derived melts. Whole-rock, glass and mineral chemistries are consistent with evolved magmas being generated at each volcano through ~70–80% crystal fractionation of a basaltic parent. Our results are not consistent with silicic magma generation via crustal anatexis, as previously suggested for these Kermadec arc volcanoes. Although crystallization is the dominant process driving melt evolution in the Kermadec volcanoes, we show that the magmatic systems are open to contributions from both newly arriving melts and wholly crystalline plutonic bodies. Such processes occur in variable proportions between magma batches, and are largely reflected in small-scale chemical variations between eruption units.

Description: Explosive eruptions create a transient bridge between the solid Earth and atmosphere, frequently injecting volcanic aerosols to stratospheric levels. Although known to disrupt terrestrial and aquatic ecosystems at the surface, the role of explosive volcanism in airborne transport of microscopic organisms has never been characterized. This study documents abundant freshwater diatoms (microskeletons of siliceous algae) in widespread tephra from the 25.4 ka Oruanui eruption of Taupo volcano, New Zealand. By matching the tephra-hosted species assemblages to those in coerupted clasts of lacustrine sediment, we demonstrate that ~0.6 km 3 of diatom remains were incorporated during magma-water interaction with a lake system overlying the vents, and were dispersed along with fine ash particles hundreds of kilometers downwind. One of the dominant species, Cyclostephanos novaezeelandiae , is endemic to New Zealand’s North Island and serves as a unique identifier of the eruptive source region. Our results suggest that dispersal of microorganisms may be an overlooked feature of a number of ancient and modern eruptions, and indicate a novel pathway of microbe transport in airborne volcanic plumes. We conclude that the biogenic signatures contained within distal tephras have potential application in the characterization of eruption dynamics, location, and environmental settings of volcanic source areas.

Description: The late Mesozoic Yanshanian volcanic arc affected an extensive region of SE China, but the conclusion of magmatism and later evolution are not fully understood. Widespread Yanshanian ignimbrites and their contemporaneous granites exposed in Hong Kong represent a microcosm of this magmatic arc. To constrain the post-magmatic thermal history of the region, we present zircon and apatite fission-track analyses from these rocks. Double dating using laser ablation inductively coupled plasma mass spectrometry U–Pb and fission-track techniques on detrital zircons from post-volcanic Cretaceous sediments is used to further constrain the tectonothermal evolution. The resulting dataset and thermal modelling suggest that the igneous rocks and Cretaceous sediments together experienced post-emplacement or post-depositional heating to 〉250 °C, subsequently cooling through 120–60 °C after c . 80 Ma. The heating reflects the combined effects of an enhanced geothermal gradient and burial. We interpret the enhanced gradient to represent continuing Yanshanian magmatic activity until c . 100–80 Ma, much later than previously considered. Our data also indicate a long-term, slow cooling ( c . 1 °C myr –1 ) since the early Cenozoic, linked to c . 2–3 km of erosion-driven exhumation. The thermotectonic history of Hong Kong reflects the mid-Cretaceous transition of SE China from an active to a passive margin bordered by marginal basins that formed in the early Cenozoic. Supplementary material: Descriptions of samples, operating conditions of the laser ablation inductively coupled plasma mass spectrometry system and the full dataset of U–Pb dating of detrital zircons are available at www.geolsoc.org.uk/SUP18750 .

Description: We here explore the temporal and spatial relationships between the contrasting sources for two eruptive episodes that collectively represent the Whakamaru Group, the largest ignimbrite-forming sequence in the ~2 m.y. history of the Taupo Volcanic Zone in New Zealand. At 349 ± 4 ka (weighted mean at 2), the 〉1500 km 3 widespread Whakamaru Group ignimbrites and ~700 km 3 Rangitawa Tephra fallout were erupted in association with collapse of the 40 km long by 25 km wide rectilinear Whakamaru caldera. New 40 Ar/ 39 Ar age data presented here show that the co-magmatic 〉110 km 3 Paeroa Subgroup ignimbrites, previously included as part of the Whakamaru Group, are slightly younger and were erupted at 339 ± 5 ka (weighted mean at 2). New field evidence also presented here demonstrates that the Paeroa Subgroup ignimbrites came from a source geographically separated from vents for the widespread Whakamaru Group ignimbrites. The presence of co-ignimbrite lag breccias, sizes of vent-derived lithic clasts, thicknesses of exposed and subsurface deposits, and morphologies of deposits imply that eruptions of the Paeroa Subgroup occurred from a linear source (the Paeroa linear vent zone), coinciding with the present-day northeast-striking Paeroa fault, and outside (northeast) of the earlier Whakamaru caldera collapse area. No separate caldera has been recognized, although three nearby areas may have undergone eruption-related subsidence. Residual magma from the Whakamaru or adjacent Kapenga caldera areas may have migrated toward the Paeroa linear vent zone during eruptive episodes, resulting in subsidence in either, or both, of these areas. Shallow plutons are also inferred to lie beneath near source fault blocks (Paeroa and Te Weta) on each side of the fault, and eruption-related subsidence may have been expressed as movement across the Paeroa fault and localized subsidence in the southern Paeroa fault block. Subsequent secular, rift-related displacement along the Paeroa fault has obscured the Paeroa linear vent zone.

Description: New zircon U–Th model-age and trace element datasets are presented from Taupo volcano (New Zealand), which are used to investigate the timescales and broad-scale magmatic processes involving zircon crystallization after the caldera-forming 25·4 ka Oruanui supereruption. Detailed 14 C-based chronologies and controls on vent locations allow the timing and location of post-caldera eruptions to be spatially and temporally constrained to an extent not possible for any other supervolcano. After ~5 kyr of post-Oruanui quiescence, Taupo erupted three dacitic units, followed by another ~5 kyr break, and then a sequence of rhyolitic units in three subgroups (SG1–SG3) from 12 ka onwards. Despite overlapping vent sites and crustal source domains between the Oruanui and post-Oruanui eruptions, U–Th zircon model ages in Taupo SG1 rhyolites (erupted from 12 to 10 ka) indicate only minor inheritance of crystals from the Oruanui magma source. Post-Oruanui model-age spectra are instead typically centred close to eruption ages with subordinate older pre-300 ka equiline grains in some units. U–Pb dating of these older grains shows that both 300–450 ka plutonic-derived and pre-100 Ma greywacke basement-derived zircons are present. The former largely coincide in age with zircons from the 350 ka Whakamaru eruption products, and are dominant over greywacke in young units that were vented within the outline of the Whakamaru caldera. Despite multiple ages and vent sites, trace element compositions are broadly similar in zircons, regardless of their ages. However, a small subset of zircons analysed from SG1 rhyolite (Units B and C) have notably high concentrations of U, Th, P, Y + (REE) 3+ and Nb but with only minor variations in Hf and Ti. SG2 zircons typically have higher Sc contents, reflecting large-scale changes in melt chemistry and crystallizing mineral phases with time. The age spectra indicate that most Oruanui zircons were removed by thermally induced dissolution immediately following the supereruption. U–Th ages from single post-Oruanui eruptions show consistent inheritance of post-Oruanui grains with model ages that centre between the temporally separated but geographically overlapping eruption groups, generating model-age modes. Within the statistical limitations of the isotopic measurements, we interpret these repeated modes to be significant, resulting from incorporation of crystal populations from cyclic post-Oruanui periods of magmatic cooling and crystallization, acting within a crustal protolith chemically independent of that which was dominant in the Oruanui system. These periods of cooling and crystallization alternate with times of rejuvenation and eruption, sometimes demonstrably accompanying syn-eruptive regional rifting and mafic magma injection. Not only were the processes that developed the supersized Oruanui magma body rapid, but this huge magma system was effectively reset and rebuilt on a comparably short timescale.

Description: Deciphering the ultimate source of chalcophile metals (e.g., Cu, Zn, Pb) in volcanogenic massive sulfide (VMS) deposits and the volatiles that help drive their formation is critical for understanding where, how, and why VMS deposits form. The southern Kermadec arc volcanic front is known to be highly hydrothermally active and host to at least three VMS deposits, whereas the associated back -arc system is apparently hydrothermally inactive, although this may simply be due to a lack of exploration in this region. We have analyzed major, trace, and volatile element concentrations in a suite of basaltic glasses and olivine-hosted melt inclusions from volcanoes, ridges, and rifts of the southern Kermadec arc volcanic front and Havre Trough back-arc basin. These data indicate clear compositional differences in the mantle beneath the arc front and the back arc, with the arc front having higher extents of prior melt extraction and enrichment in volatile and metal elements from slab-derived aqueous fluids. The magmatic budget of Pb is supplied to the mantle source by these slab-derived fluids, whereas magmatic concentrations of Zn and Cu are primarily controlled by the degree of partial melting, with a Cu-bearing residual phase required in the mantle. Consequently, the relative enrichment of chalcophile metals in the mantle-derived melts is Pb 〉〉 Cu 〉 Zn. The magma volatile history recorded by the glasses and melt inclusions indicates that degassing during crystal fractionation is significant and, notably, leads to the reduction of sulfur in the evolving magma as oxidized sulfur degasses preferentially. The apparent absence (or retention) of volatiles (notably CO 2 and SO 3 ) in magma chambers, lack of magmatic activity, and absence of hot, mafic dikes beneath volcanoes may be important factors in inhibiting the formation of active hydrothermal venting, and by extension the potential for VMS deposition.

Description: We present zircon textural, trace element and U–Pb age data obtained by secondary ion mass spectrometry (SIMS) (SHRIMP-RG: sensitive high-resolution ion microprobe, reverse geometry) from 15 stratigraphically controlled Bishop Tuff samples and two Glass Mountain (GM) lava samples (domes OD and YA). Bishop zircon textures divide into four suites: (a) dominant sector-zoned grains, with (b) subordinate grains showing bright rims [lower U, Th, rare earth elements (REE)] in CL imaging, (c) sparse GM-type grains (texturally similar to zircons from GM dome YA) and (d) sparse Mesozoic xenocrysts from Sierran granitoid country rocks. All Bishop zircons from suites (a)–(c) combined have a weighted mean age of 777·9 ± 2·2 ka (95% confidence) and a tail back to ~845 ka. Our eruption age estimate using the weighted mean of 166 rim ages of 766·6 ± 3·1 ka (95% confidence) is identical within uncertainty to published estimates from isotope-dilution thermal ionization mass spectrometry (ID-TIMS) (767·1 ± 0·9 ka, 2) and 40 Ar/ 39 Ar (767·4 ± 2·2 ka, 2) techniques, the latter using the 28·172 Ma age for the Fish Canyon sanidine standard. We estimate also an eruption age for GM dome YA of 862 ± 23 ka (95% confidence), significantly older than the currently accepted 790 ± 20 ka K–Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean age of 838·5 ± 8·8 ka (95% confidence), implying that the Bishop Tuff system was active for only ~80 kyr, and had effectively no temporal overlap with the GM system. Trace element variations in Bishop zircons are influenced strongly for many elements by sector zoning, producing up to 3 x concentration differences between sides and tips within the same growth zone. Contrasting trends in molar (Sc + Y + REE 3+ )/P ratios between sides and tips indicate contrasting mechanisms of substitution in different sectors of the same crystal. Concentrations of Ti in tips are double those in the sides of crystals, hindering applicability of the Ti-in-zircon thermometer, in addition to variations inherent to the 0·15–0·67 range in values proposed for aTiO 2 . The bright-rim portions of grains are inferred to have crystallized from the same magma as that which generated the bright rims seen under cathodoluminescence or back-scattered electron imaging on quartz and feldspar, respectively. This less evolved, slightly hotter magma invaded the deeper parts of the chamber represented in the late-erupted northern units possibly up to ~10 kyr prior to eruption, but invaded shallower levels only very shortly before eruption, as shown by our textural information and previously proposed from the sharp delineation of quartz bright rims. By obtaining a large number of analyses from zircon separates that systematically cover the entire Bishop Tuff eruption sequence we can produce an eruption age estimate using SIMS to the same precision and accuracy as ID-TIMS and 40 Ar/ 39 Ar techniques.

Description: The spatial and temporal distributions of volcaniclastic deposits in arc-related basins reflect a complex interplay between tectonic, volcanic, and magmatic processes that is typically difficult to unravel. We take advantage of comprehensive geothermal drill hole stratigraphic records within the Taupo-Reporoa Basin (TRB), and integrate them with new 40 Ar/ 39 Ar age determinations, existing age data, and new mapping to develop a four-dimensional model of basin evolution in the central Taupo Volcanic Zone (TVZ), New Zealand. Here, exceptional rhyolitic productivity and high rates of extensional tectonism have resulted in the formation of at least eight calderas and two subparallel, northeast-trending rift basins, each of which is currently subsiding at 3 to 4 mm/yr: the Taupo fault belt (TFB) to the northwest and the TRB to the southeast (the main subject of this paper). The basins are separated in the northeast by a high-standing, fault-controlled range termed the Paeroa block, which is the focus of mapping for this study, and in the southwest by an along strike alignment of smaller scale faults and an associated region of lower relief. Stratigraphic age constraints within the Paeroa block indicate that a single basin (~120 km long by 60 km wide) existed within the central TVZ until 339 ± 5 ka (Paeroa Subgroup eruption age), and it is inferred to have drained to the west through a narrow and deep constriction, the present-day Ongaroto Gorge. Stratigraphic evidence and field relationships imply that development of the Paeroa block occurred within 58 ± 26 k.y. of Paeroa Subgroup emplacement, but in two stages. The northern Paeroa block underwent uplift and associated tilting first, followed by the southern Paeroa block. Elevations (〉500 m above sea level) of lacustrine sediments within the southern Paeroa block are consistent with elevations of rhyolite lavas in the Ongaroto Gorge, the outlet to the paleolake in which these sediments were deposited, and indicate that the Paeroa block has remained relatively stable since development. East of the Paeroa block, stratigraphic relationships show that movement along the Kaingaroa Fault zone, the eastern boundary of the central TVZ, is associated with volcano-tectonic events. Stratigraphic and age data are consistent with rapid formation of the paired TRB and TFB at 339 ± 5 ka, and indicate that gradual, secular rifting is punctuated by volcano-tectonic episodes from time to time. Both processes influence basin evolution.