Whakaari/White Island is a partially submerged, offshore andesite island volcano, located at the northern end of the Taupo Volcanic Zone. Since the late 1960s, volcanic activity has alternated between quiescence, unrest, and eruption on short timescales. For this review we compiled extensive observational records, examined the rich scientific literature, and use newly acquired data, to understand the broad volcanic history and system dynamics. Based on recent bathymetry data, we propose a distinction exists between the Whakaari edifice and Te Paepae o Aotea/Volkner Rocks, which were previously considered to be part of the same edifice. Geochemical analyses of scoria samples from the island have been used to build a magma system model where dominantly andesitic-dacitic magma is periodically intruded by basalt. More dynamic processes are recorded in the hydrothermal system, where the location and activity of fumarolic features have been ephemeral and the crater lake has varied in scale over short time intervals. Eruptions of the dominantly andesitic magma have historically been small and range from phreatomagmatic through to magmatic, largely depositing ash and scoria to a restricted distance that is confined to the main crater floor. Phreatic eruptions are the most common eruption style, based on recently observed and monitored activity.

Chemcial fingerprinting of crystal populations from Sekincau, Marapi and Sinabung volcanoes, Sumatra

Recent eruptions from Mt. Ruapehu have been difficult to predict, despite the presence of a multi-parametric monitoring network. As a result, it is necessary to assess precursory signals prior to an eruption and align those to magmatic processes at depth. Fortuitously, scoria from all historical Ruapehu eruptions contains pyroxene crystals that are strongly reversely zoned in the form of a thin (2 to 3μm), outermost rim. These crystals therefore preserved changes in the magmatic system soon before their eruption. We used experimentally determined diffusion coefficients to assess the timescales of magma–magma interaction, and compared those to the monitoring record. Four of the five eruptions analysed (1969, 1971, 1977, 1995) gave diffusion timescales ~3 to 5months before their eruption, with an increased number of crystals recording timescales within 1month of eruption. Pyroxene crystals from the 1996 eruption record events that occurred prior to and during the 1995 eruption suggesting that the bulk of the 1996 crystals was derived from the 1995 magma. These diffusion timescales do not compare well to a change in any monitoring signal before historical eruptions. However, an examination of recent seismicity (2005–2013) since a significant upgrade (both in number of stations and type of seismometers) showed that two phreatic eruptions in 2006 and 2007 were preceded by a seismic swarm from ~5 to 15km depth, ~3 to 5months before each eruption — consistent with the diffusion timescales. Based on this correlation, deep seismic swarms likely indicate a period of pressurisation in the magmatic system, which may lead to gas-rich, phreatic eruptions.

Advances in analytical techniques are fundamental to the enhanced understandings of many geological processes. Zoned volcanic crystals have been analysed by low (5) kV field emission gun electron probe micro-analyser (FEG-EPMA) and NanoSIMS to obtain sub-micrometre chemical profiles and compared to time-of-flight SIMS (TOF-SIMS) and high (15–20) kV EPMA profiles. Plagioclase and orthopyroxene crystals have been analysed by FEG-EPMA, at accelerating voltages of 5kV providing a spatial resolution (step size) of ≤350nm (the resolution of the lowest energy X-ray) for orthopyroxene crystals using a 30nm beam and ca. 750nm for plagioclase crystals which at low voltages are unstable and require a 500nm defocused beam. Step sizes are comparable in size to interaction volumes. Analytical protocols are detailed that permit quantitative major and minor element compositions to be acquired at similar precision and accuracy as traditional EPMA analyses at 15–20kV. NanoSIMS analysis of the same crystals provides a greater spatial resolution of up to 200nm and allows the measurement of Li also. The NanoSIMS profiles, however, cannot currently be quantified. The ability to analyse crystals at sub-micrometre scales is demonstrated by the good agreement between NanoSIMS, FEG-EPMA, conventional EPMA and TOF-SIMS data. FEG-EPMA, NanoSIMS and TOF-SIMS techniques have broad applications within the earth sciences. In petrologic studies for example, these methods have the ability to analyse small crystals in experimental charges and provide chemical profiles of crystal zoning at a spatial resolution of ca. 200–300nm. Such profiles are important in crystal forensics and diffusion chronometry studies. The implications for the latter application are that timescales of volcanic processes that occur in the days–years immediately prior to the eruption can now be studied.

Zoned phenocrysts in volcanic rocks potentially provide an archive of magmatic processes. As a crystal grows and comes into contact with different melt batches, the chemical and textural signature of this journey is recorded within its crystal lattice. The timescale of some magmatic processes can be investigated through the relaxation of chemical gradients across crystal growth zones through the application of diffusion modeling techniques. One of the current limitations to diffusion modeling is the spatial and analytical resolution of the chemical profile that conventional techniques such as electron probe microanalyzer (EPMA), dynamic secondary ion mass spectrometry (SIMS), and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) can achieve. Here, for the first time, we present time-of-flight (TOF) SIMS (TOF-SIMS) data for zoning of orthopyroxene crystals from the May 1982 eruption of Mount St. Helens volcano, U.S.A., and cross-calibrate these data between backscattered electron images and EPMA. TOF-SLMS has the advantage of being able to achieve micrometer to nanoscale spatial resolution of major elements as well as analyses of light elements, such as Li, and trace and minor elements (Na, K, and Ni) at concentrations that cannot be achieved by EPMA, provided that convolution (overlap) effects and polyatomic mass inferences are carefully considered. With TOF-SIMS analyses we identified zoning of Li on a spatial scale (ca. 5-10 mu m) that would be inaccessible to most other conventional analytical techniques. Preliminary results indicate that Li, a fast-diffusing element, may be introduced to the crystals in the minutes, hours, or days prior to eruption and may provide insights into pre-eruptive magmatic processes. Thus, TOF-SIMS has the potential to be a powerful tool for obtaining minor and trace element profiles across compositional interfaces within crystals at high-spatial resolution.

Many active volcanoes exhibit changes in seismicity, ground deformation, and gas emissions, which in some instances arise from magma movement in the crust before eruption. An enduring challenge in volcano monitoring is interpreting signs of unrest in terms of the causal subterranean magmatic processes. We examined over 300 zoned orthopyroxene crystals from the 1980-1986 eruption of Mount St. Helens that record pulsatory intrusions of new magma and volatiles into an existing larger reservoir before the eruption occurred. Diffusion chronometry applied to orthopyroxene crystal rims shows that episodes of magma intrusion correlate temporally with recorded seismicity, providing evidence that some seismic events are related to magma intrusion. These time scales are commensurate with monitoring signals at restless volcanoes, thus improving our ability to forecast volcanic eruptions by using petrology.

Erta Ale volcano, Ethiopia, erupted in November 2010, emplacing new lava flows on the main crater floor, the first such eruption from the southern pit into the main crater since 1973, and the first eruption at this remote volcano in the modern satellite age. For many decades, Erta Ale has contained a persistently active lava lake which is ordinarily confined, several tens of metres below the level of the main crater, within the southern pit. We combine on-the-ground field observations with multispectral imaging from the SEVIRI satellite to reconstruct the entire eruptive episode beginning on 11 November and ending prior to 14 December 2010. A period of quiescence occurred between 14 and 19 November. The main eruptive activity developed between 19 and 22 November, finally subsiding to pre-eruptive levels between 8 and 15 December. The estimated total volume of lava erupted is ∼0.006 km 3 . The mineralogy of the 2010 lava is plagioclase + clinopyroxene + olivine. Geochemically, the lava is slightly more mafic than previously erupted lava lining the caldera floor, but lies within the range of historical lavas from Erta Ale. SIMS analysis of olivine-hosted melt inclusions shows the Erta Ale lavas to be relatively volatile-poor, with H 2 O contents ≤1,300 ppm and CO 2 contents of ≤200 ppm. Incompatible trace and volatile element systematics of melt inclusions show, however, that the November 2010 lavas were volatile-saturated, and that degassing and crystallisation occurred concomitantly. Volatile saturation pressures are in the range 7–42 MPa, indicating shallow crystallisation. Calculated pre-eruption and melt inclusion entrapment temperatures from mineral/liquid thermometers are ∼1,150 °C, consistent with previously published field measurements.

The Whakamaru eruption is the largest-volume eruption known to have originated from the hyper-productive Taupo Volcanic Zone, New Zealand. Major, minor and trace element concentrations of plagioclase crystals and cathodoluminescence images, used as a proxy for Ti concentrations in quartz crystals, have been used to explore their chemical zonation. Three plagioclase populations are identified. Group 1 crystals are characterized by inherited cores of composition An(45-60), Ba 115-650 ppm and La 3-9 ppm, rims of c. An(30), Ba 450-800 ppm and La 7-10 ppm and the presence of a thin overgrowth rim on several crystals cores. Group 2 crystals are oscillatory-zoned plagioclases of composition An(30-40), Ba 450-730 ppm and La 8 center dot 5-9 center dot 5 ppm. Group 3 plagioclase crystals have cores of An(25-35) and rims of An(20-25) and low Sr contents (280-480 ppm). From the chemical composition of these plagioclase crystals, four physicochemically distinct rhyolitic melts are identified: (1) an andesitic progenitor melt in which the cores of Group 1 crystals crystallized; (2) a greywacke melt or greywacke protolith melt responsible for narrow overgrowth rims on Group 1 crystal cores; (3) melt derived from the rejuvenation of a mature crystal mush body from which Group 3 plagioclase crystals crystallized; (4) a final, rhyolitic melt created by the amalgamation of varying proportions of the andesitic, greywacke-derived and rejuvenated melts with subsequent, open-system fractional crystallization of a plagioclase-dominant crystal assemblage. Cathodoluminescence imaging of quartz crystals reveals complex zonation, the result of a dynamic crystallization history from potentially polygenetic sources. Diffusion modelling of the greyscale intensity of cathodoluminescence images (as a proxy for Ti content) for a selection of bright core-rim interfaces of quartz crystals suggests that renewed quartz growth at the rim zones occurred < 300 years (peak likelihood 50-70 years) prior to and continued towards the climactic eruption. This is consistent with timescales of < 280 years determined from core-rim interfaces of Group 1 plagioclase crystals, suggesting that the magma chamber was ephemeral, derived from mixing of magmas from multiple sources shortly prior to eruption. This study adds to a growing body of evidence for the ephemeral nature and geologically rapid mixing and mobilization of liquid silicic magma bodies leading to supereruptions, compared with the timescales of hundreds of thousands of years required to accumulate the precursor magma and crystals.

The petrogenesis of large volume silicic arc magmas are investigated through microanalysis of four major eruptives of variable size from the continental Taupo Volcanic Zone and one from the oceanic Kermadec Arc. Orthopyroxene, plagioclase and quartz-hosted melt inclusions and groundmass glass display a range in major element (e.g. SiO2=74–79wt.%, CaO=0.2–2.5wt.% and FeO=0–3wt.%) and trace element compositions (e.g. Sr=17–180ppm and Ba=140–1500ppm). Healy melt inclusions are lower in K2O and Ce/Yb relative to Taupo Volcanic Zone melt inclusions reflecting the lack of continental lithosphere in the oceanic setting. Quantitative trace element modelling of Healy melt inclusions indicates fractional crystallisation is the dominant process responsible for the generation of silicic magma at Healy seamount, although remobilisation of a crystal mush body and/or crustal anatexis cannot be discounted. The chemical heterogeneity of melt inclusions in the continental Taupo Volcanic Zone indicates that the observed crystal populations present in silicic magmas are composed of both phenocrysts and antecrysts and thus derived from polygenetic sources, including mature crystal mush zones that are amalgamated together prior to eruption. This is further enforced through cathodoluminescence imaging of a selection of quartz hosts from the Whakamaru Ignimbrite that allows the location of melt inclusions to be established. This reveals that the most evolved Whakamaru melt inclusions are located in the core of quartz crystals and the least evolved Whakamaru melt inclusions are co-genetic with the groundmass glass in the rim of the crystals. In contrast, Taupo melt inclusions and groundmass glasses are all co-genetic indicating that all crystals grew from this final magma batch prior to eruption. A general trend of increasing magma evolution (to higher SiO2, Rb/Sr) with erupted volume suggests that the larger magma bodies present in continental crust incorporate more crustal material through assimilation and/or as partial melting of crust.