Volatile Systematics of the Icelandic Mantle from Olivine-Hosted Melt Inclusions

Monday, 15 December 2014
William G R Miller1, John Maclennan1 and Thor Thordarson2, (1)University of Cambridge, Cambridge, United Kingdom, (2)University of Iceland, Faculty of Earth Sciences, Reykjavik, Iceland
The behaviour of volatiles within a volcanic system can tell us about melt storage depths, melt evolution, and degassing processes. It is therefore important to be able to quantify the amount of each volatile species entering the system. Olivine-hosted melt inclusions can provide compositions of the primitive mantle melt entering volcanic systems. In some cases they have been unaffected by processes such as melt mixing, fractional crystallisation and degassing; so chemical variations are source signatures. With the knowledge that volatiles behave as very incompatible elements, and with measured volatile/trace-element ratios, we can estimate volatile concentrations within the Icelandic mantle source.

Abundances of CO2, H2O, Cl, F, S and trace elements have been measured by SIMS for over 100 olivine-hosted (Fo87.8-88.6) melt inclusions from Kistufell, a monogenic subglacial eruption known to have elevated 3He/4He (15.5 R/Ra)[1] and situated above the inferred locus of the Icelandic mantle plume. H2O concentrations in the melt inclusions are near uniform and similar to that of the carrier glass (0.10-0.15 wt%), likely due to diffusive equilibration through the olivine host at low pressure. CO2 shows more variation, ranging from 1200 to 0ppm, probably from a combination of melt heterogeneity and degassing. The majority of melt inclusions are incompatible trace element-depleted (La/Yb = 1.3), indicating that they were probably generated by high fraction melting of a depleted source in the shallow mantle. The CO2/Nb ratio of this source mantle is ~308, very similar to that of Borgarhraun, ~314[2], which has a lower 3He/4He ratio (12.2 R/Ra)[3]. This CO2/Nb ratio, along with H2O/Ce ~200 and F/Nd ~18, are consistent with other measured melt inclusion suites from Iceland, suggesting limited volatile/trace-element ratio variation within the Icelandic depleted melt source. By analysing more eruptions across Iceland, we hope to identify any spatial variations in volatile/trace-element ratios within the Icelandic mantle.

[1] K. Breddam (2002) J. Petrology 43, 2, 345-373; [2] E. H. Hauri (2002) Goldschmidt Abstract; [3] C. Macpherson et al (2005) EPSL 233,411-427.