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dc.contributor.authorNeave, Daviden
dc.contributor.authorMaclennan, Johnen
dc.contributor.authorHartley, Margaret Een
dc.contributor.authorEdmonds, Marieen
dc.contributor.authorThordarson, Thorvalduren
dc.identifier.citationJournal of Petrology 55 (12): 2311-2346. doi: 10.1093/petrology/egu058en
dc.description.abstractMagma mixing and crystal mush disaggregation are important processes in basaltic magma reservoirs. We carried out a detailed petrological and geochemical study on a highly plagioclase-phyric eruption within the Eastern Volcanic Zone of Iceland – the Skuggafjöll eruption – in order to investigate crystal storage and transport processes within a single magmatic system. Crystal content and phase proportions vary between samples: the least phyric samples have phase proportions similar to the low pressure, 3-phase gabbro eutectic (plg:cpx:ol ~ 11:6:3), whereas highly phyric samples are strongly enriched in plagioclase (plg:cpx:ol ~ 8:1:1). Statistically significant geochemical variability in 28 whole-rock samples collected across the eruption can be accounted for by variable accumulation of a troctolitic assemblage containing plagioclase and olivine in an approximately 9:1 ratio. Two macrocryst assemblages are defined using compositional and textural information recorded in QEMSCAN® images: a primitive assemblage of high-anorthite plagioclase (An>83) and high-forsterite olivine (Fo>84), and an evolved assemblage of low-anorthite plagioclase (An<79), low forsterite olivine (Fo<82) and clinopyroxene (Mg# ~ 82). Plagioclase and olivine have strongly bimodal composition distributions whereas the composition distribution of clinopyroxene is unimodal. The mean trace element composition of melt inclusions hosted within high-forsterite olivine and high-anorthite plagioclase macrocrysts is the same (mean Ce/Y ~ 0.47–0.48), confirming that both primitive macrocryst phases crystallised from the same distribution of melts. Clinopyroxene macrocrysts and matrix glasses are in Ce/Yb equilibrium with each other, indicating that the evolved assemblage crystallised from melts with a more incompatible trace element-enriched composition (mean Ce/Y ~ 0.65–71) than the primitive assemblage. Variability in whole-rock, macrocryst and melt inclusion compositions suggests that the Skuggafjöll magma experienced two stages of crystallisation. Primitive macrocrysts crystallised first from incompatible trace element-depleted melts within a shallow crustal magma reservoir. These primitive macrocrysts were subsequently stored in crystal mushes that ultimately disaggregated into an evolved and incompatible trace element-enriched magma from which the evolved assemblage crystallised. On average, ~17% of the erupted magma at Skuggafjöll is composed of accumulated macrocrysts entrained from crystal mushes. The timescale between mush disaggregation and eruption, during which crystal accumulation occurred, was short – of the order of years – according to simple diffusion calculations. Striking petrological similarities between Skuggafjöll and other highly phyric eruptions both in Iceland and along mid-ocean ridges indicate that crystal accumulation by mush disaggregation is likely to be an important mechanism for generating highly phyric magmas in basaltic plumbing systems.
dc.rightsAttribution 2.0 UK: England & Wales
dc.titleCrystal storage and transfer in basaltic systems: the Skuggafjöll eruption, Icelanden
dc.description.versionThis is the final version. It was first published by OUP at
prism.publicationNameJournal of Petrologyen
dc.rioxxterms.funderNatural Environment Research Council
dc.rioxxterms.projectidNE/1528277/1 and IMF484/0513
dc.contributor.orcidMaclennan, John [0000-0001-6857-9600]
dc.contributor.orcidEdmonds, Marie [0000-0003-1243-137X]
rioxxterms.typeJournal Article/Reviewen
pubs.funder-project-idNERC (NE/I012508/1)

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Attribution 2.0 UK: England & Wales
Except where otherwise noted, this item's licence is described as Attribution 2.0 UK: England & Wales