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Pre-eruptive magma mixing in ash-flow deposits of the Tertiary Rum Igneous Centre, Scotland

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Abstract

The Northern Marginal Zone of the Rum Igneous Centre is a remnant of an early caldera and its infill. It is composed of intra-caldera breccias and various small-volume pyroclastic deposits, overlain by prominent rhyodacite ash-flow sheets of up to 100 m thickness. The ash-flows were fed from a feeder system near the caldera ring-fault, and intrusive rhyodacite can locally be seen grading into extrusive deposits. A variety of features suggest that the ash-flows were erupted from a magma chamber that contemporaneously hosted felsic and mafic magmas: (i) chilled basaltic inclusions in rhyodacite; (ii) formerly glassy basaltic to andesitic enclaves with fluid-fluid relationships; (iii) feldspars with thick reaction rims enclosed in the basaltic to andesitic inclusions, yet with cores chemically resembling those of the rhyodacite: (iv) trace element compositions of the rhyodacite and the mafic enclaves form a mixing line between the end-member rhyodacite and basalt compositions. Additionally, textural and chemical features in the rhyodacite feldspar phenocrysts are consistent with magma mixing; (v) feldspars with resorption embayments cutting through internal zonation of the crystals; (vi) complexly zoned crystals with sieve-textured zones that are overgrown with euhedral zones; (vii) oscillatory zonation of feldspar phenocrysts in the rhyodacite, showing sharp increases in anorthite (ΔAn ≥ 10%) followed by gradual decrease in An-content (ΔAn ≤ 4%). This evidence points to eruption of ash-flows from a felsic magma chamber that was periodically replenished by injection of mafic magma. Diffusional mixing between the two magmas was permitted by temperature and compositional differences, but was slow due to the contrast in viscosities and densities. The Fe–Ti–P-enriched basic magma that replenished the chamber was degassing on entering the lower temperature environment and soon equilibrated thermally, followed by chemical exchange between the two end-member magmas. This process formed hybrid andesite enclaves enriched in trace elements beyond that caused by simple mixing, implying trace element diffusion in addition to bulk mixing. Eruption was caused by replenishment with, and degassing of, the basic magma and the chamber partially evacuated while the process of hybridisation was underway. The erupted products record magma mixing by chamber replenishment, blending of two magmas and elemental exchange in the magma chamber, and also physical mingling in the eruptive conduit.

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Acknowledgements

We are grateful to the Scottish Natural Heritage for permission to work on the island and to M. Hendry for field support. K. Atkinson drafted an earlier version of the map and U. Thurow helped with some of the other figures. Thanks are also due to A. Mackie (sample preparation), A. Calder and D. Rau (XRF), D. Herd (EMP), and particularly to F. Hauff for access to the Geomar TIMS facilities. B. Wenskowski, S. Duggen, M. Abratis, and S. Vetter are thanked for assistance and good company during data acquisition at GEOMAR. Discussion with J. Gamble, C. Stillman, W.E. Stephens and J. Reavey is greatly appreciated and B. Upton and A. Kerr are thanked for stimulating journal reviews. CHD and VRT received financial support from the Welsh Bequest of the University of St Andrews.

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Authors and Affiliations

  1. Department of Geology, Trinity College, College Green, Dublin 2, Ireland

    Valentin R. Troll

  2. School of Geosciences, University of St Andrews, The Irvine Building, St Andrews, KY 16 9AL, Scotland

    Colin H. Donaldson

  3. Department of Earth Sciences, University of Durham, South Road, Durham, DH1 3LE, England

    C. Henry. Emeleus

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  1. Valentin R. Troll
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  2. Colin H. Donaldson
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  3. C. Henry. Emeleus
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Correspondence to Valentin R. Troll.

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Troll, V.R., Donaldson, C.H. & Emeleus, C.H. Pre-eruptive magma mixing in ash-flow deposits of the Tertiary Rum Igneous Centre, Scotland. Contrib Mineral Petrol 147, 722–739 (2004). https://doi.org/10.1007/s00410-004-0584-0

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