Diffusive over-hydration of olivine-hosted melt inclusions

Abstract

Editor: B. Marty Keywords: melt inclusion melt mixing diffusion water olivine IcelandThe pre-eruptive water content of magma is often estimated using crystal-hosted melt inclusions. However, olivine-hosted melt inclusions are prone to post-entrapment modification by H + diffusion as they re-equilibrate with their external environment. This effect is well established for the case of H + loss from olivine-hosted inclusions that have cooled slowly in degassed magma. Here we present evidence for the opposite effect: the addition of H + into inclusions that are held in melts that are enriched in H 2 O with respect to the trapped melts. The compositional variability in a suite of 211 olivine-hosted inclusions from the Laki and Skuggafjöll eruptions in Iceland's Eastern Volcanic Zone indicates that diffusive H + gain governs the H 2 O content of incompatible trace element depleted inclusions. Individual eruptive units contain olivine-hosted inclusions with widely varying incompatible element concentrations but near-constant H 2 O. Furthermore, over 40% of the inclusions have H 2 O/Ce > 380, significantly higher than the H 2 O/Ce expected in primary Icelandic melts or mid-ocean ridge basalts (150-280). The fact that the highest H 2 O/Ce ratios are found in the most incompatible element depleted inclusions indicates that hydration is a consequence of the concurrent mixing and crystallisation of compositionally diverse primary melts. Hydration occurs when olivines containing depleted inclusions with low H 2 O contents are juxtaposed against more hydrous melts during mixing. Melt inclusions from a single eruption may preserve evidence of both diffusive H + loss and H + gain. Trace element data are therefore vital for determining H 2 O contents of melt inclusions at the time of inclusion trapping and, ultimately, the H 2 O content of the mantle source regions. Crown

Volatile dilution during magma injections and implications for volcano explosivity

“…The apparent absence of H + diffusion may be attributed to minimal H 2 O concentration gradients between inclusions and the host magma during crystal residence, coupled with fast magma ascent prior to and during the 2014 eruption, which would severely limit the time for diffusion under an H 2 O gradient. During magma storage, the activity gradient between the melt inclusions and carrier melt was calculated at -0.07, which is comparable with values of ~0.04 that Hartley et al (2015) argued may have been too small to facilitate H + diffusion through olivine crystals. However, if diffusion occurred under these low activity gradients, calculated diffusive time scales range from 1 week to 5 months.…”

The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)

“…Whilst a number of studies have investigated the postentrapment behaviour of volatiles in olivine-hosted melt inclusions (e.g. Qin et al 1992;Portnyagin et al 2008;Bucholz et al 2013;Hartley et al 2015), volatile behaviour in plagioclase-hosted melt inclusions is less well understood. Whilst the high H 2 O content of melt inclusions precludes CO 2 loss during the final stages of eruption, it is not currently possible to evaluate how CO 2 may be transferred between inclusions and the external melt through plagioclase hosts, though diffusion along lattice defects and cleavage planes is likely to play a role (Viti and Frezzotti 2001;Kress and Ghiorso 2004).…”

Melt inclusion constraints on petrogenesis of the 2014–2015 Holuhraun eruption, Iceland

“…Recent studies of both natural and experimental samples have shown that at magmatic temperatures, the H 2 O contents of crystal-hosted melt inclusions may be modified by several weight percent on timescales of hours to days by diffusive re-equilibration of H + between the melt inclusion and its external environment (e.g., Portnyagin et al 2008;Gaetani et al 2012;Bucholz et al 2013;Hartley et al 2015). Magmatic water contents at the time of melt inclusion trapping can be estimated using Ce as a proxy for H 2 O, if the undegassed H 2 O/Ce of the primary melt is known.…”