Open AccessWater‐rich and volatile‐undersaturated magmas at Hekla volcano, Iceland
Author(s) -
Lucic Gregor,
Berg AnneSophie,
Stix John
Publication year - 2016
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1002/2016gc006336
Subject(s) - geology , andesite , melt inclusions , fractional crystallization (geology) , geochemistry , basalt , basaltic andesite , mafic , magma , olivine , rhyolite , saturation (graph theory) , magmatic water , magma chamber , volcano , volcanic rock , fluid inclusions , quartz , paleontology , mathematics , combinatorics
Olivine‐hosted melt inclusions from four eruptions at Hekla volcano in Iceland were analyzed for their dissolved H 2 O, CO 2 , S, and Cl contents. A positive correlation among the repose interval, magmatic evolution, and volatile contents of magmas is revealed. H 2 O is the dominant volatile species; it behaves as an incompatible component, increasing in concentration over time as a result of fractional crystallization in the magma. The full suite of H 2 O contents ranges from a low of 0.80 wt % in basaltic andesites to a maximum of 5.67 wt % in rhyolites. Decreasing H 2 O/K 2 O at fixed major element compositions suggests that syneruptive degassing reduces H 2 O contents significantly. Hekla magmas are CO 2 poor, with very low concentrations present only in the most evolved compositions (∼20–30 ppm or less). The decrease in S content from basaltic andesite to rhyolite demonstrates that sulfide saturation is attained when the melt composition reaches basaltic andesite, resulting in the precipitation of pyrrhotite. Low CO 2 /Nb ratios suggest that vapor saturation is most likely reached during an early period of cooling and solidification in the crust. Fresh injections of mafic magma interact with previously solidified intrusives, producing new melts that are volatile undersaturated. Vapor saturation pressures obtained using the most volatile‐rich melt inclusions suggest the presence of a magma chamber at a minimum depth of ∼7 km. This is in agreement with geophysical observations from recent small‐volume eruptions, but given the possibility of volatile‐undersaturated melts, some of the magmas may reside at greater depths.