Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems

Margaret E. Hartley, Oliver Shorttle, John Maclennan, Yves Moussallam, Marie Edmonds

    Research output: Contribution to journalArticlepeer-review


    The redox state of volcanic products determines their leverage on the oxidation of Earth's oceans and atmosphere, providing a long-term feedback on oxygen accumulation at the planet's surface. An archive of redox conditions in volcanic plumbing systems from a magma's mantle source, through crustal storage, to eruption, is carried in pockets of melt trapped within crystals. While melt inclusions have long been exploited for their capacity to retain information on a magma's history, their permeability to fast-di using elements such as hydrogen is now well documented and their retention of initial oxygen fugacities (fO2) could be similarly diffusion-limited. To test this, we have measured Fe3+/ Fe by micro-XANES spectroscopy in a suite of 65 olivine-hosted melt inclusions and 9 matrix glasses from the AD 1783 Laki eruption, Iceland. This eruption experienced pre-eruptive mixing of chemically diverse magmas, syn-eruptive degassing at the vent, and post-eruptive degassing during lava flow up to 60 km over land, providing an ideal test of whether changes in the fO2 of a magma may be communicated through to its cargo of crystal-hosted melt inclusions. Melt inclusions from rapidly quenched tephra samples have Fe3+/ Fe of 0.206 0.008 enrichment or differentiation. These inclusions preserve the redox conditions of the mixed pre-eruptive Laki magma. When corrected for fractional crystallization to 10 wt.% MgO, these inclusions record a parental magma [Fe3+/ Fe](10) of 0.18 ( QFM of +0.4), significantly more oxidised than the Fe3+/ Fe of 0.10 that is often assumed for Icelandic basalt magmas. Melt inclusions from quenched lava selvages are more reduced than those from the tephra, having Fe3+/ Fe between 0.133 and 0.177 ( QFM from -0.1 to -0.5). These inclusions have approached equilibrium with their carrier lava, which has been reduced by sulphur degassing. The progressive re-equilibration of fO2 between inclusions and carrier melts occurs on timescales of hours to days, causing a drop in the sulfur content at sul de saturation (SCSS) and driving the exsolution of immiscible sul de globules in the inclusions. Our data demonstrate the roles of magma mixing, progressive re-equilibration, and degassing in redox evolution within magmatic systems, and the open-system nature of melt inclusions to fO2 during these processes. Redox heterogeneity present at the time of inclusion trapping may be overprinted by rapid re-equilibration of melt inclusion fO2 with the external environment, both in the magma chamber and during slow cooling in lava at the surface. This can decouple the melt inclusion archives of fO2, major and trace element chemistry, and mask associations between fO2, magmatic differentiation and mantle source heterogeneity unless the assembly of diverse magmas is rapidly followed by eruption. Our tools for understanding the redox conditions of magmas are thus limited; however, careful reconstruction of pre- and post-eruptive magmatic history has enabled us to con rm the relatively oxidised nature of ocean island-type mantle compared to that of mid-ocean ridge mantle.
    Original languageEnglish
    JournalEarth and Planetary Science Letters
    Early online date3 Oct 2017
    Publication statusPublished - 2017


    • oxygen fugacity
    • Fe-XANES
    • melt inclusion
    • Sulfide
    • melt mixing
    • Iceland


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