Háskóli Íslands

Ph.D. student: Paavo Oskari Nikkola

Dissertation title: From partial melting to lava emplacement: the petrogenesis of some Icelandic basalts

Opponent: Dr. Reidar G. Trønnes, Professor at the Natural History Museum, Centre for Earth Evolution and Dynamics University of Oslo

Advisor:  Dr. Þorvaldur Þórðarson, Professor at the Faculty of Earth Sciences, University of Iceland.

Doctoral committee:  Dr. Tapani Rämö, Professor at the Department of Geosciences and Geography, University of Helsinki.
Dr. Enikő Bali, Associate Professor at the Faculty of Earth Sciences, University of Iceland.
Dr. Guðmundur H. Guðfinnsson, Research Scholar at the Institute of Earth Sciences, University of Iceland.

Chair of Ceremony: Dr. Magnús Tumi Guðmundsson, Professor and the Head of the Faculty of Earth Sciences, University of Iceland.

The lecture will take place on Wednesday 27th May at 14:00 and will be streamed on Zoom: https://eu01web.zoom.us/j/67908655217?pwd=QUJzaVRtTmkyQUZDTTFFOTZ6N3BRUT09

This is a joint degree with the University of Helsinki. The defense was delivered in Helsinki on March 20th 2020. 

Abstract:
This thesis provides insights into the petrogenesis of Iceland basalts via three subprojects. The first uses olivine macrocrysts as a proxy for mantle melting conditions below Iceland, the second utilizes petrological thermobarometry to resolve the crustal storage conditions of the most primitive basaltic rocks (ankaramites) of the Eyjafjallajökull volcano, and the third investigates basalt fractionation processes within the Hafnarhraun pāhoehoe lava lobe.

The sub-Icelandic mantle is evidently heterogeneous in composition. Yet olivine major and minor element compositions in Iceland basalts typically concur with common mantle lherzolite as the source of magmas, with the only potential exceptions being the basalts of Eyjafjallajökull and Vestmannaeyjar volcanic systems in South Iceland. These South Iceland basalts have forsterite-rich olivine with relatively high Ni and low Mn contents, together with low Sc and V and high Cr, Ti, Zn, Cu and Li contents. Elevated Ni and low Mn in olivine have been attributed to olivine-free pyroxenitic mantle source; however, the South Iceland olivine compositions are best explained by the effect of comparatively high-pressure (Pfinal>1.4 GPa) and high-temperature melting of somewhat enriched olivine-bearing mantle. I conclude this because (i) elevated Ni and low Mn in olivine can also indicate deep, high-temperature, mantle melting, (ii) the abundances of Sc, V, Ti and Zn in the South Iceland olivine are compatible with low-degree partial melts of olivine-rich mantle, and (iii) melts of olivine-free pyroxenite are, according to recent models, easily consumed in reactions with subsolidus mantle peridotite and thus unlikely to migrate to the crust and crystallize olivine. The identified high-Ni/low-Mn olivine macrocrysts suggest final mantle equilibration depths greater than 45 km for South Iceland magmas, and imply effective mantle-to-surface magma transport.

Two Eyjafjallajökull ankaramite outcrops (Hvammsmúli and Brattaskjól), rich in olivine (Fo81–90) and clinopyroxene (Mg#cpx 78–90) macrocrysts (~30 vol%) in near equal amounts, have a specifically prominent “deep mantle source signature” (high-Ni/low-Mn) in olivine. To investigate the crustal storage conditions of these and other Eyjafjallajökull basaltic magmas, I analyzed olivine, clinopyroxene, spinel and melt inclusion compositions from these volcanic units. These analyzes revealed that the olivine-hosted spinel inclusions have exceptionally high Cr#spl (52–80) and TiO2 (1–3 wt%) and low Al2O3 (8–22 wt%) compared to typical chromian spinels in Iceland, in line with the postulated deep and enriched mantle source of the parental magmas. According to olivine-spinel oxybarometry, these spinels crystallized under a moderate oxygen fugacity (ΔlogFMQ 0–0.5). Furthermore, jadeite-in-clinopyroxene barometry indicates clinopyroxene crystallization at a rather low pressure (1.7–4.2 kbar; external precision ±1.4 kbar), implying a magma storage depth of 10.7±5 km. Additionally, clinopyroxe-liquid, olivine-liquid and liquid only thermometry gives varying crystallization temperatures of 1120–1195 °C, 1136–1213 °C and 1155–1222 °C, respectively, for the compositionally diverse macrocrysts. The scarcity of macrocryst plagioclase and trends in clinopyroxene compositions indicate that the mid-crustal crystallizing assemblage was olivine and clinopyroxene, and plagioclase fractionated later. Diffusive re-equilibration in Brattaskjól olivine grains suggests that this crystal assemblage mobilized and erupted from its storage within a few weeks. To conclude, the Brattaskjól and Hvammsmúli crystal cargoes are agitated wehrlitic or plagioclase-wehrlitic mushes from the mid-crust that ascended to the surface relatively rapidly.

Basaltic lavas are practically never primitive mantle melts owing to fractional crystallization in the crust, which, at low pressure, may be aided by volatile exsolution. Deciphering magma fractionation processes from solidified crustal intrusions is hampered by their often complex emplacement history. The emplacement of pāhoehoe lavas, however, is simpler and well understood, and hence I investigated the mechanisms of basalt fractionation from a differentiated pāhoehoe lava lobe in Hafnarhraun lava flow field. Here, volatile exsolution had facilitated separation of basaltic residual melts to form three types of melt segregations: vesicle cylinders (VC) in the core of the lobe and two types of horizontal vesicle sheets (HVS1 and HVS2) in the upper part of the lobe. Interestingly, the VC do not match chemically with the modelled residual melts of the lobe, and their formation seems to have included two stages: volatile-aided melt separation from crystallizing base of the lobe and later contamination by primitive macro- and microphenocrysts in the lava core. HVS1, which resemble VC, were formed as the ascending VC diapirs accumulated to the upper solidification front of the lava lobe. HVS2, in turn, are distinctly evolved in compositions compared to other units in the lobe and were formed as highly fractionated residual melts seeped to voids in the upper crust of the lobe. Processes analogous to segregation formation at Hafnarhraun may contribute to genesis of evolved basalts and silicic rocks in shallow magmatic systems.

Overall, my work highlights the exceptional nature of South Iceland among other volcanically active regions in Iceland. Furthermore, analyses of the Hafnarhraun pāhoehoe lava reveal the processes of melt segregation formation in pāhoehoe lava lobes. I hope future research will expand on these findings, further resolving the nature of mantle melting below South Iceland and the significance of volatile-aided processes in crustal magma differentiation.

About the doctoral candidate:

Paavo Oskari Nikkola was born and raised in rural Finland.

In 2015, Paavo started a PhD project aiming to a dual doctorate degree from the University of Iceland and the University of Helsinki.

Paavo prefers to spend his free time outdoors as often as possible. Currently, he enjoys a busy family life with his wife and son in Espoo, Finland.

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