Explosive submarine eruptions driven by volatile-coupled degassing at Lō`ihi Seamount, Hawai`i

Details Explosive submarine eruptions driven by volatile-coupled degassing at Lō`ihi Seamount, Hawai`i Explosive submarine eruptions driven by volatile-coupled degassing at Lō`ihi Seamount, Hawai`i

Jul 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010e%26psl.295..497s&link_type=abstract

Earth and Planetary Science Letters, Volume 295, Issue 3-4, p. 497-510.

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Scientific paper

There is a growing body of information on submarine explosive eruptions, but important questions remain about their causes and diversity. Magmatic degassing influences eruption dynamics, but a paucity of appropriate samples from submarine pyroclastic deposits has limited thorough analysis of this process. We present major element and volatile analyses of matrix glasses and olivine-hosted glass inclusions from a submarine scoria cone (the “northern cone”) at ˜ 1060 m below sea level on Lō`ihi Seamount, Hawai`i, to quantitatively constrain ascent and degassing processes, and infer the magmatic plumbing geometries, that permit submarine explosive eruption of basalt in deep water. Olivine crystals form two populations. The first consists of a single crystal (Fo86.8-88.6), bearing inclusions of high-Fe tholeiitic glass (1.18 to 1.28 wt.% H2O; 182 to 505 ppm CO2). Major element concentrations indicate that these high-Fe inclusions represent parental melt that subsequently evolved by crystal fractionation to form the erupted bulk melt. The second population consists of variably zoned crystals bearing low-Fe inclusions with lower volatile contents (0.63 to 0.74 wt.% H2O; 29 to 176 ppm CO2). Low-Fe inclusions are not geochemically related to the bulk melt, and their host crystals are interpreted as xenocrysts entrained from shallow storage. The northern cone magma experienced strongly volatile-coupled (“closed-system,” where bubbles remain in physical contact with the melt from which they have grown) ascent from at least ˜ 1.5 km deep in the conduit. Retention of CO2 in bubbles facilitated strong exsolution of H2O (and S). Exsolution of H2O dominated the vesiculation and acceleration of ascending melt, accounting for the measured 47 to 65% vesicles in lapilli. CO2 played an essential chemical role, but negligible physical role, in driving magma ascent. Our results contradict the common interpretation that high hydrostatic pressures require submarine explosive eruptions to be Strombolian, driven by the accumulation and buoyant decoupling of CO2-rich fluids. Comparison to volatile data from other explosive and effusive eruption products on Lō`ihi indicates that explosivity is controlled primarily by the style of degassing, rather than magma type or initial volatile content. Magmas that ascend directly from source to vent under volatile-coupled conditions retain maximum explosive potential. Conversely, magmas that degas under open-system conditions, or ascend from shallow reservoirs, will have their explosive potential diminished as exsolved volatile fluids are lost to hydrothermal systems. This latter path appears to be the most common at Lō`ihi, causing most magmas to erupt effusively.

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