Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems
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, th...
| Published in: | Earth and Planetary Science Letters |
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| Main Authors: | , , , , |
| Format: | Article in Journal/Newspaper |
| Language: | English |
| Published: |
2017
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| Subjects: | |
| Online Access: | https://research.manchester.ac.uk/en/publications/2d46c8d8-dc43-4a9c-bad2-61e11efed8e1 https://doi.org/10.1016/j.epsl.2017.09.029 |
| Summary: | 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 ... |
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