Crystal Storage and Transfer in Basaltic Systems: the Skuggafjoll Eruption, Iceland

• Published in: Journal of Petrology
• Main Authors: Neave, David A., Maclennan, John, Hartley, Margaret E., Edmonds, Marie, Thordarson, Thorvaldur
• Format: Text
• Language: English
• Published: Oxford University Press 2014
• Subjects: Original Papers; Skuggafjöll; Iceland
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/55/12/2311; https://doi.org/10.1093/petrology/egu058

Magma 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—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, three-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 crystallized from the same distribution of melts. Clinopyroxene macrocrysts and matrix glasses are in Ce/Yb equilibrium with each other, indicating that the evolved assemblage crystallized 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 ...