Abstract
The 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 H2O 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 H2O content of incompatible trace element depleted inclusions. Individual eruptive units contain olivine-hosted inclusions with widely varying incompatible element concentrations but near-constant H2O. Furthermore, over 40% of the inclusions have H2O/Ce>380, significantly higher than the H2O/Ce expected in primary Icelandic melts or mid-ocean ridge basalts (150–280). The fact that the highest H2O/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 H2O 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 H2O contents of melt inclusions at the time of inclusion trapping and, ultimately, the H2O content of the mantle source regions.
Highlights
The water content of melt inclusions is commonly assumed to be a lower bound on the pre-eruptive H2O content of melts stored in the crust (Métrich and Wallace, 2008)
Relatively dry melt inclusions are in theory susceptible to diffusive H+ gain if they are stored in a water-rich carrier melt (Portnyagin et al, 2008), but this has seldom been demonstrated in nature (Koleszar et al, 2009)
We have used H2O/Ce systematics to reconstruct the history of H+ diffusion in olivine-hosted melt inclusions from Iceland’s Eastern Volcanic Zone (EVZ) (Fig. 9)
Summary
The water content of melt inclusions is commonly assumed to be a lower bound on the pre-eruptive H2O content of melts stored in the crust (Métrich and Wallace, 2008). Experimental studies have demonstrated that olivine-hosted melt inclusions with high initial H2O contents are susceptible to diffusive dehydration on heating under dry external conditions, with diffusive H2O loss occurring within hours to days at magmatic temperatures Relatively dry melt inclusions are in theory susceptible to diffusive H+ gain if they are stored in a water-rich carrier melt (Portnyagin et al, 2008), but this has seldom been demonstrated in nature (Koleszar et al, 2009). This hydration of depleted melt inclusions is an inevitable consequence of concurrent mixing and crystallisation of diverse primary melt compositions This is the first study to illustrate melt inclusion hydration due to water gradients set up by the mixing of melts with heterogeneous mantle parents. We demonstrate that hydration of depleted melt inclusions is not restricted to Iceland, but is a feature of many global melt inclusion datasets
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