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Chemical Variations in the 1998, 2011, and 2015 Lava Flows From Axial Seamount, Juan de Fuca Ridge: Cooling During Ascent, Lateral Transport, and Flow
  • Published Date:
    2018
  • Source:
    Geochemistry Geophysics Geosystems, 19(9), 2915-2933.
Filetype[PDF-4.76 MB]


Details:
  • Description:
    Lava flows erupted at Axial Seamount in 1998, 2011, and 2015 are chemically heterogeneous and display decreases in MgO content in their glass rinds with increasing distance from the summit. The trends are consistent with eruption temperature decreases down the rift zones of similar to 0.5 degrees C/km within 20 km of the caldera and similar to 0.9 degrees C/km at greater distance (only observed in the 2011 eruption). Cooling during magma transport in dikes is the likely cause of the temperature trends observed, related to the effects of cooler wall rocks in areas with less frequent dike intrusions. Flows also cooled as they advanced on the seafloor at rates 3-5 times greater than observed at Kilauea volcano in Hawaii for subaerial tube-fed pahoehoe flows. Lavas erupted in and near the caldera in 1998 and 2011 are slightly enriched transitional mid-ocean ridge basalt that are aphyric and have glass MgO content of 7.1-7.6 wt%. The 2015 lavas have similarly enriched incompatible element compositions typical of transitional mid-ocean ridge basalt, but those erupted inside and on the northeast rim of the caldera contain higher glass MgO of 7.8-8.3 wt% and more abundant plagioclase phenocrysts typical of the normal mid-ocean ridge basaltic lavas erupted between 1290 and 1370 CE. The brief recharge period between the 2011 and 2015 eruptions did not allow magma stored in the shallow reservoir to cool and degas as much as between prior eruptions since 1650 CE, suggesting that the most recent recharge period was shorter than the multicentennial average. Plain Language Summary Axial Seamount on the Juan de Fuca mid-ocean ridge erupted multiple lava flows of basaltic lava in 1998, 2011, and 2015. The composition of the flows was determined by analysis of 290 lava samples as well as a single sample of volcanic sand produced during mildly explosive eruptive activity in 2015. The lava flows are all hot spot influenced mid-ocean ridge basalt containing few crystals. Basalts cooled at rates of 0.5-1 degrees C/km and crystallized 15-20% crystals as they were transported away from the summit in dikes. They cooled at rates of similar to 3 degrees C/km as they flowed away from the eruptive fissures. Melt composition from the summit region varies in MgO content and temperature. The hottest lavas, at 1201 degrees C, were those erupted at the summit in 2015 after a 4-year repose period, whereas cooler basalts, at similar to 1186 degrees C, erupted at the summit in 2011 after a 13-year repose period. The 1998 lavas had a similar maximum MgO content and temperature to those erupted in 2011 and so may have had a similar repose period, consistent with the average repose period during the last 350 years of eruptive activity. The 2015 summit lavas were the hottest to erupt at Axial Seamount in the last 350 years.
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