Signature of deep mantle melting in South Iceland olivine

Paavo Nikkola,Guðmundur H Guðfinnsson,O Tapani Rämö,Enikő Bali,Tobias Fusswinkel,Thorvaldur Thordarson

doi:10.1007/s00410-019-1580-8

Paavo Nikkola, Guðmundur H Guðfinnsson + Show 4 more

Open Access

https://doi.org/10.1007/s00410-019-1580-8

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Abstract

We present new high-precision major and trace element data on olivine macrocrysts from various volcano-tectonic settings in Iceland and use these data as a proxy for mantle mode and melting conditions. Within individual sampling sites examined (seven lavas and one tephra) olivine-dominated fractional crystallization, magma mixing and diffusive re-equilibration control observed variability in olivine composition. High-pressure fractional crystallization of clinopyroxene may have lowered Ca and increased Fe/Mn in one olivine population and subsolidus diffusion of Ni and Fe–Mg affected the mantle-derived Ni/Fo ratio in some compositionally zoned olivine macrocrysts. Interestingly, magmas erupted at the southern tip of the Eastern Volcanic Zone (SEVZ), South Iceland, have olivines with elevated Ni and low Mn and Ca contents compared to olivines from elsewhere in Iceland, and some of the SEVZ olivines have relatively low Sc and V and high Cr, Ti, Zn, Cu and Li in comparison to depleted Iceland rift tholeiite. In these olivines, the high Ni and low Mn indicate relatively deep melting (Pfinal > 1.4 GPa, ~ 45 km), Sc, Ti and V are compatible with low-degree melts of lherzolite mantle, and elevated Zn may suggest modal (low-olivine) or geochemical (high Zn) enrichment in the source. The SEVZ olivine macrocrysts probably crystallized from magmas derived from olivine-bearing but relatively deep, enriched and fertile parts of the sub-Icelandic mantle, and indicate swift ascent of magma through the SEVZ lithosphere.

Highlights

Icelandic mantle many of the details remain obscure, Earth’s mantle is heterogeneous in mineralogy and chemical composition (Allègre and Turcotte 1986)
Mantle-derived magmas are usually modified in the crust by low-P fractional crystallization, magma mixing and crustal contamination, which may obscure the traits they inherited from the source
We propose that the parental magma of these olivine macrocrysts equilibrated at high temperatures and pressures with a fertile olivine-bearing mantle source

Summary

Introduction

Icelandic mantle many of the details remain obscure, Earth’s mantle is heterogeneous in mineralogy and chemical composition (Allègre and Turcotte 1986). With careful treatment of the data, the quantity of possible endmember mantle source rock-types can be estimated, and the inferred mantle components can be further assessed by comparing their assumed melt productivity to crustal thickness (Shorttle and Maclennan 2011; Brown and Lesher 2014; Shorttle et al 2014; Lambart 2017). Using this approach, it has been established that melting of depleted lherzolite alone is unlikely to produce the compositional variability (especially the high Fe and low Ca) in Icelandic lavas (Shorttle and Maclennan 2011). A minor (4–15%) olivine-free pyroxenite or mixed pyroxenite–peridotite hybrid seem to be required in the mantle source (Shorttle et al 2014)

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