Crystal–Melt Relationships and the Record of Deep Mixing and Crystallization in the ad 1783 Laki Eruption, Iceland

Concurrent mixing and crystallization of magmas has been identified in the plumbing of Icelandic magmatic systems from investigation of their erupted products. Thermobarometric calculations constrain the depth of these processes to the mid- to lower crust. It is currently unknown whether similar processes also occur in the magmas that feed the large fissure eruptions characteristic of the Eastern Volcanic Zone (EVZ). We therefore present the results of an investigation of crystal–melt relationships in the ad 1783 Laki (Skaftár Fires) eruption. The trace element composition of olivine-hosted melt inclusions indicates that concurrent mixing and crystallization of variable mantle melts has occurred in the deep parts of the Laki plumbing system. Magmatic evolution is also recorded in crystal zonation patterns. Zoned plagioclase macrocrysts are made up of three distinct domains: high-anorthite cores, oscillatory zoned mantles and low-anorthite rims. Published mineral–melt equilibrium partition coefficients indicate that macrocryst rims are in equilibrium with the carrier liquid erupted at the surface. High-anorthite cores are more primitive than any other crystal or melt inclusion composition in the magma and are never in equilibrium with melts generated by fractional crystallization models that assume a single liquid line of descent related to the carrier liquid. High-anorthite crystals may have grown from depleted, high-Ca/Na melts of the shallow mantle, which may have been mixed into melts parental to the carrier liquid early in magmatic evolution. The crystal size distribution (CSD) of plagioclase macrocrysts suggests that plagioclase cores represent an assimilated or accumulated crystal population. Cores have acted as nuclei for the growth of oscillatory zoned plagioclase mantles. It is possible to relate the range of plagioclase mantle compositions to the carrier liquid using fractional crystallization models involving eutectic co-crystallization with clinopyroxene and olivine macrocrysts. Negative correlation of titanium and anorthite content in plagioclase mantles confirms that oscillatory zoning is driven by variations in melt composition, most probably the result of repeated recharge and fractionation events. Melt barometry indicates that the melt last equilibrated with olivine, plagioclase and clinopyroxene in the shallow crust at 1–2 kbar, whereas clinopyroxene–melt barometry suggests that the bulk of macrocryst growth occurred within the mid-crust at 2–5·4 kbar. Much of the macrocryst content of the Laki magma occurs as glomerocrysts, of which only rims in contact with the groundmass are in equilibrium with the carrier liquid. This indicates that the formation of glomerocrysts occurred before rim growth, during deposition of crystal mushes in the mid-crust, which subsequently disaggregated upon transport to the shallow crust.