Basaltic melt evolution of the Hengill Volcanic System, SW Iceland, and evidence for clinopyroxene assimilation in primitive tholeiitic magmas

Abstract

The thick oceanic crust of Iceland is formed by tholeiitic central volcanoes arranged in en echelon patterns along the 40–50 km wide rift zones. The Hengill central volcano in the southwestern rift zone has produced 25–30 km3 of hyaloclastites and lava during the last 0.11 m.y., with maximum productivity during the isostatic rebound following the degalciations 0.13 and 0.01 m.y. ago. The eruption units cover a compositional spectrum from picrite to rhyolite, but the volume of andesite, dacite, and rhyolite is small. The petrographic relations of pillow rim and hyaloclastite glass indicate that the basaltic melts were saturated with olivine and plagioclase, except for the most primitive ones that were undersaturated with plagioclase. Saturation with clinopyroxene was reached in some of the intermediate and evolved basaltic melts. Corroded and partly resorbed crystals of clinopyroxene and partly disintegrated gabbro nodules with resorbed clinopyroxene indicate that selective assimilation contributed to the evolution of the most primitive melts. The intermediate and evolved basaltic glass compositions fall along the low‐pressure cotectic for mid‐ocean ridge basalt (MORB) compositions saturated with olivine, plagioclase, and clinopyroxene, but the primitive glasses (9–9.7 wt % MgO) fall well inside the low‐pressure olivine + plagioclase primary phase volume. The primitive Hengill glasses have significantly higher CaO and lower Al2O3 than primitive glasses from oceanic spreading centers. Their low pressure undersaturation with respect to clinopyroxene and the absence of clinopyroxene phenocrysts indicate that they are not parental to the intermediate Hengill basalts, since fractionation modelling requires a large proportion of clinopyroxene in the fractionating assemblage. The most primitive melts could be produced by fractionation of olivine and plagioclase combined with 5–30 % assimilation of clinopyroxene, and the intermediate melts could be derived by mainly olivine and plagioclase fractionation, from melts equilibrated with peridotitic residues at pressures of 1–2 GPa. The further evolution of the basaltic spectrum can be explained by fractionation of olivine, plagioclase, and clinopyroxene combined with minor contamination by anatectic crustal melts. The rate of magma supply from the mantle to the crust is controlled by the isostatic conditions and is very low in periods of glacial loading of the crust. This leads to infrequent and small eruptions of dominantly evolved magmas. The dense picritic magmas (9–9.7 wt % MgO in the glass phase) were driven to the surface by magmatic overpressure in the mantle at an early deglaciation stage characterized by the absence of large, trapping magma chambers in the lower crust. The assimilation of clinopyroxene in these melts could proceed by direct contact with the solidified cumulate sequences and gabbro intrusions. Clinopyroxene assimilation in combination with olivine fractionation may also contribute to the chemical evolution of some of the most primitive MORB magmas.