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Can experimental oxygen fugacity be controlled?

More precise data is required to determine whether a novel experimental technique designed to control experimental fO2 is effective under H2O-undersaturated conditions.

Piston cylinder experiments typically employ noble metals as sample containers due to their low reactivity and high melting temperature. Many years of experimental research has demonstrated that choice of capsule metal often involves a payoff between melting temperature and the ability to control important compositional parameters (e.g., loss of Fe and H2O).

Oxygen fugacity (ƒO2) is intrinsically linked to these variables and is a key property of an experiment because it controls the valence of multivalent elements. In turn, this alters phase relations and mineral compositions, and affects speciation of other volatiles elements such as sulphur.

Earlier work by Jakobsson (2012) presented a novel experimental technique for controlling ƒO2 by physically separating a redox buffer from an Au-Pd alloy inner capsule with a hydrogen-permeable barrier. This technique relies on hydrogen fugacity being equal in both in the outer and inner capsule as fixed by the solid buffer; however, the original study failed to take into account the H2O-undersaturated nature of the experimental melts, which actually act to reduce the ƒO2 imposed on the inner capsule.

Jakobsson 12 capsule setup

Diagram of the Jakobsson (2012) capsule setup

This amendment acknowledges this oversight, and corrects the measured ƒO2 in the original study for H2O-undersaturation. The authors conclude that whilst this sample assembly is capable of controlling fO2 in H2O-saturated runs, more precise analysis of other parameters (such as the activity of Fe and H2O in the melt) are needed to assess whether the same holds true for H2O-undersaturated variants.

Jakobsson, S., Blundy, J., & Moore, G. (2014). Oxygen fugacity control in piston-cylinder experiments: a re-evaluation. Contributions to Mineralogy and Petrology, 167(6), 1-4. http://dx.doi.org/10.1007/s00410-014-1007-5

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Dacites not responsible for rapid uplift at Uturuncu volcano

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.

BSE image of an experimentally synthesized dacitic lava.

BSE image of an experimentally synthesized dacitic lava.

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

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Experimental tracking of primitive magmas beneath Grenada, Lesser Antilles

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.

Piston cylinder experiment from Stamper et al. 2014

A synthetic replica of a Grenadan magma produced during a high pressure experiment, as seen through a scanning electron microscope (gl: glass, ol: olivine, qu: quench, spl: splinel)

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

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Compositional distribution of erupted lavas is controlled by phase relations of primitive basalts

High pressure experiments demonstrate that variations in water content and depth of differentiation can produce a wide variety of erupted lavas from a single primitive source

Lava suites erupted from individual volcanic centres commonly exhibit a compositional ‘gap’ between basaltic and rhyolitic compositions, where the volume of intermediate eruptives is less than mafic and acidic equivalents.  A study by Melekhova and co-workers explores the distribution of lava compositions erupted from crustal volcanoes, focusing on a case study from the volcanic island of St Vincent in the Lesser Antilles. The crystallisation of cooling basaltic magmas was simulated using high pressure experiments, with synthetic run products analysed using a variety of microanalytical techniques.  The authors discovered that variation in melt fraction (the amount of molten rock remaining in the model system) and melt composition with temperature is controlled by the composition of minerals crystallising from the parent magma. For example, a rapid decrease in melt fraction, and increase in melt SiO2, occurs when the minerals and melt have similar (eutectic-like) compositions, which is the case when little water is present.

Summit of Soufriere St Vincent, Lesser Antilles,

View of lava dome in the summit caldera of Soufriere St Vincent, Lesser Antilles. Credit: Richard Arculus

The experimentally determined phase relations were incorporated into a numerical model, which allowed enabled the team to explore the evolution of a magmatic system over time by simulating the incremental emplacement of small batches of magma beneath a volcano. When model results were compared with natural rocks from St Vincent, the best fit is produced by theoretical runs with water contents mirroring data from recently analysed melt inclusions, and heat content correlating well with the age of the island (~0.4 – 2.0 Ma). Furthermore, calculations show that the observed bimodality in erupted compositions is a natural consequence of the ‘damp’ nature of sub-arc melts.

Although Melekhova et al.’s approach focused on an oceanic island arc volcano, it offers insights into other types of volcanic system; because magmas produced from a given basalt exhibit tractable changes in composition with time, they can be compared to lavas from any igneous terrains where there are good temporal constraints on changing magma (or melt inclusion) chemistry.

Melekhova, E, Annen, CJ & Blundy, JD (2013) ‘Compositional gaps in igneous rock suites controlled by magma system heat and water content’ Nature Geoscience, vol 6, pp. 385-390. http://dx.doi.org/10.1038/ngeo1781

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