H_2O-rich melt inclusions in fayalitic olivine from Hekla volcano: Implications for phase relationships in silicic systems and driving forces of explosive volcanism on Iceland

摘要

Silicic Icelandic magmas are widely believed to contain low to moderate H_2O content prior to degassing, and that their high explosivity mostly results from the interaction of the magmas with ice or meteoric water. Here we report the compositions of glass inclusions (SiO_2=57-72wt%, K_2O=1.3-2.6wt%) in Fe-rich olivines (Fo_(2-42)) from the largest Holocene eruptions of Hekla volcano (H3 and H4) on Iceland, which preserved quenched melts with very high primary H_2O contents (3.3-6.2wt%). The silicic Hekla melts originate primarily by extensive (~90%) crystal fractionation of H_2O-poor (~0.6wt%) basalts and represent an end member in the systematics of terrestrial magmas because they originate at low fO2 (ΔQFM ~-0.1 to-0.4) and have as high H_2O contents as significantly more oxidized island-arc magmas (ΔQFM≥1). This demonstrates that H2O and ΔQFM do not correlate in silicic magmas from different tectonic settings, and that fO_2, not H_2O content, shows a major difference between silicic ocean-island (e.g., Icelandic) and island-arc magmas. Analysis of available experimental data suggests that high H_2O activity and low fO_2 expand the field of olivine stability in silicic melts. Low fO_2 and low MgO content could also suppress crystallization of amphibole. On the basis of these results we propose that an anhydrous mineral assemblage bearing Fe-rich olivine in evolved volcanic and Skaergaard-type intrusive rocks does not imply low H_2O in magmas prior to degassing but, in contrast to the commonly held view, is an indicator of H_2O-rich silicic parental magmas crystallized at low fO_2. Finally, the high H_2O content in magma was a major driving force of the largest explosive eruptions of Hekla volcano and must be at least as important for driving silicic explosive volcanism on Iceland as magma-ice interaction.