High temperature experiments reveal previous eruptions were characterized by shallow magma storage, a scenario incompatible with the depth of the current anomaly
Cerru Uturuncu, Bolivia, is a continental arc volcano located in the Central Andes. Recent satellite observations of ground deformation in the area have measured uplift of 1 – 2cm per year. This has been accompanied by persistent seismic activity and indications are strong that Uturuncu may be entering a period of unrest and possible magma build up. Inverse modelling of the deformation has indicated a large diameter anomaly at 11 – 17km beneath the volcano.
A new study by Muir and co-workers at the University of Bristol conducted high temperature experiments on the two types of lava primarily from the volcano: rhyolite and dacite. The aim of their work to determine if previous episodes of magma storage are consistent with the depth of the anomaly causing the current deformation, and whether future eruptions would likely be effusive (in continuation of past activity at Uturuncu) or larger-scale explosive events.
The natural mineral assemblages in both types of lava were replicated by experiments at 870ºC at pressures equivalent to 2 – 6 km depth, a similar crustal level to the location of recent earthquakes recorded at the volcano. This experimental evidence precludes the role of dacites and rhyolites in producing the observed anomaly beneath Uturuncu. Instead, the authors propose a model where dacitic magmas are formed from fractional crystallisation in an underlying, deeper magma body before stalling in the shallow crust prior to their effusive eruption.
Muir DD, Blundy JD, Rust AC, & Hickey J (2014) ‘Experimental constraints on dacite pre-eruptive magma storage conditions beneath Uturuncu volcano’. Journal of Petrology, 55(4), 749-767. http://dx.doi.org/10.1093/petrology/egu005
High pressure experiments on a high-Mg basalt indicate parental magmas beneath Grenada are oxidised, and resolve the origin of two distinct lavas series
Experimental petrologists at the University of Bristol conducted experiments on lavas from Grenada using a range of experimental apparata to simulate to pressures and temperatures found beneath the island arc volcano. The redox conditions of the experimental runs were measured using the Diamond Light Source synchrotron, UK, and spanned a wide range of oxygen fugacities. Synthetic replicas of natural rocks produced at moderately oxidising conditions were found to be comparable to the most primitive lavas erupted on Grenada.
Stamper and co-workers were able to use the composition of olivine crystals produced in experiments to calibrate a novel oxybarometer, which uses the partitioning of Fe and Mg between liquid and crystals to measure the oxygen fugacity of an olivine-bearing basalt.
Experiments from this study also resolve the origin of the geochemically and petrographically distinct M- and C-series lavas, the latter type being unique to Grenada. At high pressures, experimental liquids are able to track the geochemical evolution of the highly magnesian M-series. In contrast, at lower pressures, clinopyroxene saturation is displaced to lower temperatures, relative to olivine, and so residual melts generated at these conditions become enriched in calcium, replicating the characteristic feature of the C-series.
Stamper CC, Melekhova E, Blundy JD, Arculus, RJ, Humphreys, MCS & Brooker, RA (2014) ‘Oxidised phase relations of a primitive basalt from Grenada, Lesser Antilles’, Contibutions to Mineralogy and Petrology, 167:954. http://dx.doi.org/10.1007/s00410-013-0954-6
Numerical modelling shows that magma buoyancy is the most important factor in determining the frequency and magnitude of the Earth’s most destructive volcanic phenomena
A new paper by a collaboration from the Universities of Geneva, Bristol and Savoie quantifies the relative contributions of magma supply, mechanical properties of the crust and magma, and tectonic regime in controlling the frequency and magnitude of volcanic eruptions. The team, led by Professor Luca Caricchi, coupled over 1.2 million simulations of a thermomechanical numerical model of magma injection into Earth’s crust with complex statistical analysis to try and replicate the behaviour of melt beneath a volcano.
This work reveals a dichotomy in the causes of volcanic eruptions, which is related to their size. It is known that small, frequent eruptions are triggered by magma replenishment, which imparts stress on the magma chamber walls; eruptions occur when this stress exceeds the strength of the surrounding rock. In contrast, Caricchi et al. demonstrate that bigger, less frequent eruptions are instead driven by the intrinsic buoyancy associated with large magma bodies, a consequence of the slow accumulation of low-density magma beneath a volcano.
These findings are particularly important because this is the first time a physical link between the frequency and magnitude of volcanic eruptions has been established. The findings allow the predictions of the scale of the largest possible volcanic eruption on Earth; the work suggests magma chamber can contain a maximum of 35,000 km3 of eruptible magma, translating to an eruption spewing out approximately 3,500 km3 of rock. This is three times the volume released during the supereruption of Yellowstone around 640,000 years ago.
Caricchi, L, Annen, CJ, Blundy, JD, Simpson, G & Pinel, V (2014) ‘Frequency and magnitude of volcanic eruptions controlled by magma injection and buoyancy’, Nature Geoscience. http://dx.doi.org/10.1038/ngeo2041.
High resolution imaging of crystal zoning reveals events occurring in the months and days before an eruptions
Development of new techniques enables quantification of major and trace element zoning in lava phenocysts to nanometre scale
Whilst the long-term evolution of a magmatic system may occur over many thousands of years, changes immediately preceding volcanic eruptions may occur on the timescale of months, days and minutes. This kind of resolution is not obtainable using classical radiometric dating, being is constrained by the half-lives of elements. Within the last decade, petrologists have increasingly turned to the technique of diffusion chronometry; here, the blurring in chemical zoning within lava phenocrysts is used to estimate the duration between the last perturbation of the magmatic system and the eruption. The better the resolution of the electron microscope image, the smaller the timescale that can be calculated from a crystal.
Saunders and co-workers interrogated plagioclase and orthopyroxene crystals from the 1980 eruption of Mt St Helens using the FEG-EPMA at the University of Bristol, a high-resolution electron microprobe purchased as part of the CRITMAG grant. The FEG-EPMA has a beam size of ~ 30 nm, two orders of magnitude smaller than is common in conventional microprobe analysis. This allows collection of both detailed back scattered electron (BSE) images and major element composition. The latter has a quantitative resolution of 750 nm (0.00075 mm), making FEG-EPMA ideally suited for application to diffusion chronometry. The authors also look at two other methods of microanalysis, NanoSIMS and TOF-SIMS, which cannot image samples but have the advantage of being able to measure a large range of major and trace elements at ≥50 nm resolution.
Using one, or a combination of the three techniques described in the paper, the authors demonstrate is it possible to obtain chemical profiles of zoned minerals with nanoscale precision. Such detail facilitates characterisation of events that occurred in the months and days before historic eruptions.
Saunders K, Buse B, Kilburn MR, Kearns S, & Blundy J (2014) ‘Nanoscale characterisation of crystal zoning’, Chemical Geology, 364, 20-32. http://dx.doi.org/10.1016/j.chemgeo.2013.11.019