Volatile and light lithophile elements in high-anorthite plagioclase-hosted melt inclusions from Iceland

Geochemcial records of mantle processes are progressively degraded as magmas differentiate and ascend towards the Earth’s surface. This degradation is particularly severe in the case of volatiles (H2O, CO2, F, S and Cl) that decouple from melts upon reaching vapour saturation. Melt inclusions – pools of silicate liquid that are partially insulated from changes in the external magmatic enronment by their host crystals – are thus appealing targets for investigating the behvaiour of magmatic volatiles. Although numerous recent studies have critically evaluated the effects of syn- and post-entrapment modification on olivine-hosted melt inclusion compositions, little comparable information is available for plagioclase-hosted systems, depite plagioclase’s abundance in mafic magmas.

In order to address this imbalance in undertanding between olivine-hosted and plagioclase-hosted systems, we present volatile and light lithophile element analyses from a large number of mainly plagioclase-hosted melt inclusions from the 10 ka Grímsvötn tephra series from Iceland. Major and trace element data have already  been presented in study into the pre-eruptive evolution and storage of the tephra series (Neave et al., 2015).

Volatile-trace element systematics in matrix glasses and melt inclusions used to distinguish between pre-, syn- and post-entrapment signals of variability. Figure from Neave et al. (2017).

The uniformly low CO2 content of melt inclusions cannot be explained by either shallow entrapment or shrinkage bubble formation, suggesting that inclusion CO2 contents were controlled by decrepitation instead. High H2O/Ce values in primitive plagioclase-hosted inclusions (182–823) are most easily accounted for by diffusive H2O gain following the entrainment of primitive macrocrysts into H2O-rich melts a few days before eruption (e.g., Hartley et al., 2015). Extreme F enrichments in primitive plagioclase-hosted inclusions (F/Nd = 51–216 versus 15 in matrix glasses) possibly reflect the entrapment of inclusions from high-Al/(Al+Si) melt pools formed by dissolution-crystallisation processes (as indicated by HFSE depletions in some inclusions), and into which F was concentrated by uphill di ffusion: F is highly soluble in Al-rich melts. The high S/Dy of inclusions (300) indicates that primary melts were rich in S in comparison with most oceanic basalts. Although primitive plagioclase-hosted melt inclusions from the 10 ka Grímsvötn tephra series record few primary signals in their volatile element contents they nevertheless record information about crustal magma processing that is absent from olivine-hosted melt inclusions suites.

Publication

Neave, D.A., Hartley, M.E., Maclennan, J., Edmonds, M. & Thordarson, T. 2017.  Volatile and light lithophile elements in high-anorthite plagioclase-hosted melt inclusions from Iceland. Geochimica et Cosmochimica Acta 205, 110–118. <Open access>