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Hidden record of mafic enclaves tracks pluton assembly over time

Thermomechanical modelling of enclave deformation demonstrates that plutons grow in response to repeated injections of small pulses of magma

Plutons are large igneous bodies formed from the slow cooling of molten rock in the subsurface. Their construction reflects how magma is produced and transferred from depth, though whether this happens through sudden episodes of magma injection or small pulses of growth is a matter of active research.

Granitic plutons commonly contain mafic enclaves (fragments of less chemically evolved magma suspended in a more evolved host), produced from the intrusion and disaggregation of hotter, more mafic melts into cooler, more felsic magma.  Distortion of these roughly spherical enclaves reflects the strain experienced by different areas of the pluton. The study, from Caricchi and co-authors from the University of Bristol, harnesses this record of deformation to probe the rheological, and hence thermal, evolution of a pluton during its accretion.

Caricchi et al. found that the repeated injection of magmatic pulses into a pluton resulted in expansion of the body, but that enclaves were only deformed in a two narrow temperature windows in which both the host and enclave had a similar viscosity. Knowing this, the team developed a thermomechanical model to simulate how the strain trajectories of enclaves vary as a function of time and distance from the magma injection point. Application of this model to the Lago Della Vacca Complex (LDVC), a 4.5 by 4.7 km section of the Adamello Pluton in Italy, shows that the magma body underwent radial expansion in response to multi-stage growth over 50,000- 150,000 years. Their findings are in agreement with recent geochronological estimates from zircon dating of the structure, and supply evidence for the ‘piecemeal’ nature of pluton assembly.

Caricchi, L, Annen, CJ, Rust, AC & Blundy, JD (2012) ‘Insights into the mechanisms and timescales of pluton assembly from deformation patterns of mafic enclaves’ Journal of Geophysical Research, vol 117, no. B11206. http://dx.doi.org/10.1029/2012JB009325

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Element maps: A leap forward for microtextural analysis

Quantitative mapping of elements in a lava sample provides insights into the kinetics of explosively-erupting magmatic systems

As magmas ascend from depth towards the surface they undergo decompression and cooling, the former being responsible for the release of dissolved volatiles in the form of volcanic gases; all three factors induce crystals to form. The speed at which magma travels towards the surface affects the rate of crystallisation, and as such, the study of the textures of explosively erupted lavas can reveal quantitative information about magmatic ascent rates and crystallisation history.

Muir and co-workers from the University of Bristol present a novel method of analysing lava microtextures. They harness a powerful new technique (EDS element mapping) whereby a rock sample is bombarded with electrons. Sample interaction with electrons produces a variety of emissions, some of which are X-rays with wavelengths characteristic to the elemental composition of the target. The combination of different elements translates into an energy spectrum which is analysed to determine the abundance of specific elements. This is repeated many thousands of times over a small area to build up a ‘map’ for each element, where the intensity of colour is proportional to the elemental concentration. Individual mineral phases can be identified and isolated; calculations are then performed to obtain the relative proportions, sizes and distributions of glass, minerals and bubbles.

SH154_AlK copy

Aluminium element map of a lava sample erupted from the Mt St Helens Dome on 29th Mar 1984. The image, taken with an electron microscope, reveals zoning in plagioclase crystals.

The technique of EDS element mapping is applied to lavas from Mt St Helens erupted bewteen 1980 – 1986. The observed trends in microtextures are similar to those previously published, with groundmass crystallinity displaying a sharp increase after the catastrophic eruption in the summer of 1980, before increasing more gradually during the next dome-building phase of activity.

EDS element mapping presents significant advantages over the previous method of manually extracting data from greyscale backscattered electron images through faster data processing, reduction in operator time and accurate identification of all textural components. The authors also highlight the potential for coupling of this technique with developing technologies, such as field emission gun (FEG) sources, which would radically reduce acquisition time and enable better spatial resolution at small crystal sizes.

Muir, DD, Blundy, JD & Rust, AC 2012, ‘Multiphase petrography of volcanic rocks using element maps: a method applied to Mount St. Helens, 1980–2005′ Bulletin of Volcanology. http://dx.doi.org/10.1007/s00445-012-0586-0

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Plutonic xenoliths from St Vincent, Lesser Antilles, act as ‘windows into the deep’

Coarse-grained igneous rocks, sourced directly from a sub-volcanic magma chamber, provide glimpses into the prevailing conditions beneath an active volcano.

Tollan and co-workers used several different techniques to analyse the major and isotopic composition of mineral phases in cumulates sourced from the active volcanic island of St Vincent in the Lesser Antilles. Cumulates are igneous rocks comprise the first fractionating minerals that form from a crystallising melt. The combinations of minerals and their composition are modulated by the conditions imposed upon the magma when it is cooling, and thus each rock represents a unique ‘snapshots’ of magmatic evolution.

The study revealed that the rounded cumulate nodules are distinctively rich in anorthitic plagioclase and pargasitic hornblende, accompanied by fresh olivine and pyroxenes. The composition of the minerals indicates the cumulates formed at ~970 – 1150°C at 5-6 km below the Earth’s surface.

Cumulate xenoliths on the island of Bequia, Lesser Antilles.

Cumulate xenoliths on the island of Bequia, Lesser Antilles.

The paper concludes that all the cumulates collected from St Vincent formed from relatively evolved melts rich in calcium, aluminium and water. These types of magmas are produced from early crystallisation of mafic phases (olivine, clinopyroxene and Cr-rich spinel) before their low density facilitates ascent through the crust, where they stall and deposit the observed cumulus minerals in shallow magma chambers. Evidence from oxygen isotopes suggests the cumulates have a residence time of ~50,000 years before being entrained and fragmented by newly injected magmas, and transported to the surface during explosive volcanic eruptions.

Full reference: Tollan, PME, Bindeman, I & Blundy, JD (2012) ‘Oxygen and hydrogen isotope compositions of plutonic xenoliths from St. Vincent, Lesser Antilles Island Arc.’ Contributions to Minerology and Petrology, no. 163, pp. 189-208. http://dx.doi.org/10.1007/s00410-011-0665-9

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Lava microtextures reflect sub-volcanic geometry

Analysis of microtextures in lava samples can be used to calculate key eruption characteristics, such as magma chamber depth and discharge rate

Microscopic imaging of erupted lavas reveals such rocks comprise a mix of glass (rapidly quenched molten rock), bubbles (originally filled with gases), and crystals. The arrangement of these components is described as the rock’s ‘texture’ and is highly variable between different eruptions and volcanoes. Lavas from explosive eruptions, where lava is rapidly erupted to the surface, are dominated by bubbles; in contrast, slowly extruded samples have time to crystallise tiny minerals (microphenocrysts) and degas dissolved volatiles.

MSH lava texture

Back scattered electron (BSE) image of an erupted lava from Mt St Helens showing distribution of vesicles (voids which were originally filled with gas – now black), glass (light grey) and crystals (geometric shapes in glass). Image width is 50 µm. Photo credit: Kathy Cashman

Melnik and co-workers used crystal size distributions (CSD), a way of analysing crystal size and ’roundness’, as a method of quantifying volcanic texture. The study determined the relationship between the volume fraction of crystals in a sample, and parameters such as the cross-sectional area of the volcanic conduit and crystal nucleation rate. CSD can therefore be used as a measure of how magma travels through the sub-volcanic system at different depths and to track the ascent of molten rock to the surface.

The model is applied to a lava sample from the 1980 eruption of Mt St Helens volcano, USA. High resolution imaging allows the study of small (<10 µm) crystals and extends the depth range of the calculations to ~8 km, which agree with other estimates from seismic imaging and petrologic studies. Not only does the new model of Melnik et al. pave the way for more advanced understanding of how lava ascend through volcanic conduits, it also allows estimation of discharge rate and conduit geometry for prehistoric or unmonitored eruptions.

Melnik, OE, Blundy, JD, Rust, AC & Muir, DD (2011) ‘Subvolcanic plumbing systems imaged through crystal size distributions’ Geology, vol 39(4), pp. 403 – 406. http://dx.doi.org/10.1130/g31691.1

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