Showing papers by "Colin J. N. Wilson published in 2013"

From mush to eruption in centuries: assembly of the super-sized Oruanui magma body

Abstract: The magmatic systems that give rise to voluminous crystal-poor rhyolite magma bodies can be considered to operate on two contrasting timescales: Those governed by longer-term processes by which a magma acquires its chemical and isotopic characteristics (e.g., fractional crystallisation and assimilation), and those operating at shorter timescales during the physical accumulation of the melt-dominant magma body that finally erupts. We explore the compositional and textural relationships between amphibole and orthopyroxene crystals from the 25.4 ka, 530 km3 (magma) Oruanui eruption products (Taupo volcano, New Zealand) to investigate how processes related to the physical assembly of the pre-eruptive magma body are represented in the crystal record. Over 90 % of orthopyroxenes from the volumetrically dominant high-SiO2 (>74 wt%) rhyolite pumices record textural evidence for a significant disequilibrium event (partial dissolution ± resorption of cores and interiors) prior to the growth of 40–500 μm thick rim zones. This dissolution/regrowth history of orthopyroxene is recorded in the chemistry of co-crystallising amphiboles as a prominent inflection in the concentrations of Mn and Zn, two elements notably enriched in orthopyroxene relative to amphibole. Textural and chemical features, linked with in situ thermobarometric estimates, indicate that a major decompression event preceded the formation of the melt-dominant body. The decompression event is inferred to represent the extraction of large volumes of melt plus crystals from the Oruanui crystal mush/source zone at pressures of 140–300 MPa (~6–12 km depth). Orthopyroxene underwent partial dissolution during ascent before reestablishing in the melt-dominant magma body at pressures of 90–140 MPa (~3.5–6 km). We model Fe–Mg diffusion across the core-rim boundaries along the crystallographic a or b-axes to constrain the timing of this decompression event, which marked establishment of the melt-dominant magma body. Maximum modelled ages indicate that this event did not begin until ~1,600 years before eruption, consistent with constraints from zircon model-age spectra. Once extraction began, it underwent runaway acceleration with a peak extraction age of ~230 years, followed by an apparent period of stasis of ~60 years prior to eruption. The rapidity of the extraction and accumulation processes implies the involvement of a dynamic driving force which, in the rifted continental arc setting of the Taupo Volcanic Zone, seems likely to be represented by magma-assisted extensional tectonic processes.

Geochemistry and Petrogenesis of Silicic Magmas in the Intra-Oceanic Kermadec Arc

Abstract: 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 % SiO2. 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.

High-flying diatoms: Widespread dispersal of microorganisms in an explosive volcanic eruption

Abstract: 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 fi ne 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 identifi er 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.

Highly vesicular pumice generated by buoyant detachment of magma in subaqueous volcanism

Abstract: Many submarine caldera volcanoes are blanketed with deposits of highly vesicular pumice, typically attributed to vigorous explosive activity1, 2, 3, 4 However, it is challenging to relate volcanic products to specific eruptive styles in submarine volcanism5, 6 Here we document vesicularity and textural characteristics of pumice clasts dredged from the submarine Macauley volcano in the Kermadec arc, southwest Pacific Ocean We find that clasts show a bimodal distribution, with corresponding differences in vesicle abundances and shapes Specifically, we find a sharp mode at 91% vesicularity and a broad mode at 65–80% Subordinate clasts show gradients in vesicularity We attribute the bimodality to a previously undocumented eruptive style that is neither effusive nor explosive The eruption rate is insufficient to cause magma to fragment explosively, yet too high to passively feed a lava dome Instead, the magma foam buoyantly detaches at the vent and rises as discrete magma parcels, or blebs, while continuing to vesiculate internally The blebs are widely distributed by ocean currents before they disintegrate or become waterlogged This disintegration creates individual clasts from interior and rim fragments, yielding the bimodal vesicularity characteristics We conclude that the generation and widespread dispersal of highly vesicular pumice in the marine environment does not require highly explosive activity

Large‐Scale Interaction of Lake Water and Rhyolitic Magma During the 1.8 Ka Taupo Eruption, New Zealand

Abstract: The 1.8 ka Taupo eruption was one of the largest and most powerful of eruptions world wide in the last 5000 years, and notable for its uniform chemical composition, yet diversity of eruptive styles. Three phreatomagmatic phases (phases 1,3,4) involved extensive interaction with a pre-existing caldera lake. The eruption shifted abruptly between "wet" and "dry" eruptive conditions. These shifts accompanied changes in vent position that led to discharge of melt that had undergone quite different ascent and degassing histories. Vesicle size distributions and morphologies in the 1.8 ka juvenile ejecta supply constraints on these processes. Pumice clasts in units 1 and 3 have microtextures reflecting vesicle growth and coalescence during steady ascent of magma, with slight contrasts probably due to contrasting faster (phase 3) and slower (phase 1) rates of rise. There is no unambiguous evidence for magma:water interaction triggering, or contributing to fragmentation during these phases and we suspect that the principal role of external water was after fragmentation. Unit 4 clasts have microtextures reflecting bubble collapse. It is clear that this melt was disrupted after its peak of vesiculation; a scenario most compatible with a staged ascent of the melt. Low vesicularities and vesicle number densities and ample evidence for bubble collapse suggest that magma:water interaction played the dominant role in fragmentation for this phase. It is clear from the deposits of the Taupo eruption that interaction between surface water and silicic magma took place under widely varying conditions and that the physical state of the melt (in turn reflecting the ascent and degassing histories) was the determinant variable. At Taupo, extensive interaction of the rising magma with the caldera lake only occurred when slow final magma ascent permitted egress of lake water to the shallow conduit.