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The Rum layered intrusion testifies to modification by injection of hot magma and remobilization of pre-existing cumulate rocks.
The Isle of Rum in the Inner Herbrides of Scotland is a classic and much-studied example of an igneous layered intrusion. Dated at 60 Ma, it’s emplacement was related to the development of the proto-Icelandic plume.
New work by Leuthold et al. focusses on a particular layer within the intrusion, ‘Unit 9’, which shows a progression from peridotite (olivine-rich) through troctolite (olivine + plagioclase) to gabbro (plagioclase + clinopyroxene).
By integrating field and geochemical observations, this study challenges the idea that Unit 9 was formed through progressive fractional crystallization of a single parental liquid. Instead, the authors hypothesise that multiple generations of rimmed clinopyroxenes with sharp boundaries in Cr2O3 and REE indicate that Unit 9 underwent two separate episodes of partial melting in response to the intrusion of hot picritic magma.
This upward and lateral migration of melts and the reactive remobilisation of a cumulate pile may be an important process in all layered intrusions and open magma chambers.
Leuthold J, Blundy JD, Holness MB, & Sides R (2014) ‘Successive episodes of reactive liquid flow through a layered intrusion (Unit 9, Rum Eastern Layered Intrusion, Scotland)’. Contributions to Mineralogy and Petrology, 168(1), 1-27. http://dx.doi.org/10.1007/s00410-014-1021-7
A new study shows that substitution of aluminium for silicon in clinopyroxene tetrahedra increases during cooling. The resulting local charge balance favours REE incorporation into the crystal lattice.
A new study by Scarlato and co-workers shines the spotlight on clinopyroxenes in a trachybasaltic dyke in the Valle del Bove depression on Mt Etna, Italy. Previous studies by Mollo et al. were able to quantify how the the cooling rate of the crystals increased from 0.02ºC/min to 1.13ºC/min from the core to rim of the dyke. The fact that the solidification path in this igneous feature was already well-constrained provided a perfect platform for this study, which explores the effect of temperature variations on the partitioning of rare earth elements (REE) between clinopyroxene and melt.
The structure of the clinopyroxene mineral consists of interlocking silicon tetrahedra, and two sites where metal cations can be incorporated (called M1 and M2). The study found that a higher cooling rate favours the substitution of tetrahedral aluminium (AlIV) for silicon. AlIV acts as a local charge balance for REE ions; as a consequence, REE are more easily accommodated into the M2 site of the mineral lattice.
Scarlato et al.’s measurements from this natural laboratory match the theoretical predictions of REE behaviour made using the ‘lattice strain model’ of Blundy & Wood (1994). This demonstrates that the REE partitioning between melt and clinopyroxene in naturally cooled magmas is controlled by local charge balance and cation substitutions, rather than kinetic parameters or diffusion.
Scarlato P, Mollo S, Blundy JD, Iezzi G, & Tiepolo M (2014) ‘The role of natural solidification paths on REE partitioning between clinopyroxene and melt’ Bulletin of Volcanology, 76(3), 1-4. http://dx.doi.org/10.1007/s00445-014-0810-1
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 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.
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