Melt mixing causes negative correlation of trace element enrichment and CO 2 content prior to an Icelandic eruption

• Published in: Earth and Planetary Science Letters
• Main Authors: Neave, David A., Maclennan, John, Edmonds, Marie, Thordarson, Thorvaldur
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2014
• Subjects: Carbon dioxide; Degassing; Iceland; Melt inclusion; Melt mixing
• Online Access: https://research.manchester.ac.uk/en/publications/bc357260-d07e-4739-abae-fa010e60d104; https://doi.org/10.1016/j.epsl.2014.05.050; http://www.scopus.com/inward/record.url?scp=84902247690&partnerID=8YFLogxK

Major elements, trace elements and volatiles were measured in 110 olivine-hosted melt inclusions from the subglacial Skuggafjöll eruption in the Eastern Volcanic Zone of Iceland. Variations in melt inclusion trace element concentrations can be accounted for by incomplete mixing of diverse mantle parental melts accompanied by variable extents of fractional crystallisation. Binary mixing between an incompatible trace element-enriched and depleted melts provides a good fit to observed variations in trace element ratios such as Ce/Y. Surprisingly, the CO 2 contents of melt inclusions correlate negatively with their degree of trace element enrichment. Depleted, low-Ce/Y inclusions with ~1200 ppm CO 2 have high CO 2 /Nb contents (~400), suggesting that melts experienced little or no CO 2 exsolution before inclusion entrapment. Enriched, high-Ce/Y inclusions contain ~300 ppm CO 2 , have low CO 2 /Nb (contents 50-100) and melts are likely to have exsolved much of their original CO 2 contents prior to inclusion entrapment. The negative correlation between CO 2 content and trace element enrichment may arise either from the more efficient exsolution of CO 2 from enriched melts, or from the intrusion of CO 2 -supersaturated depleted melts into enriched melts that had already exsolved much of their original CO 2 contents. Some inclusions have lower CO 2 contents than predicted from binary mixing models, which suggests that at least some CO 2 exsolution occurred concurrently with mixing. Enriched inclusions record entrapment pressures of ~0.5 kbar. These pressures probably correspond to the depth of mixing. Higher pressures recorded in depleted inclusions may have resulted from the development of CO 2 supersaturation during ascent from storage at ≥1.5 kbar. The presence of CO 2 supersaturation in melt inclusions has the potential to constrain timescales of melt inclusion entrapment.