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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

SCIENTIFIC ABSTRACT

The differentiation of basaltic magmas to form more silica-rich magma is a fundamental process in crustal magmatism. However, suites of volcanic rocks erupted from individual volcanic centres rarely exhibit a compositional continuum between basalt and rhyolite. Instead, some rock suites exhibit marked compositional gap . The origin of such gaps has been attributed to partial melting of the crust , the immiscibility of different magma types3, crystallization of specific mineral phases and processes occurring within magma chambers. Here we couple high pressure and temperature experiments on mantle-derived basalt from St Vincent Volcano, Lesser Antilles, with variable water contents, to thermal models of magma differentiation. We show that the compositional distribution of the derivative magma varies as a function of water and heat content of the magmatic system, which is, in turn, related to the flux and duration of magma input. Systems that have relatively low heat content are characterized by compositional gaps, whose extent varies systemically with the water in the parent basalt. Irrespective of water content, compositional gaps diminish with time. Our approach can be used to retrieve information from volcanic rocks on their magmatic heat and water content in the parent basalt and hence explore these parameters as functions of tectonic settings and age.

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