10:00 〜 10:15
[SCG54-05] Superheated magma in Koolau volcano, Hawaii: An alternative hypothesis for the origin of high-silica tholeiite and excess NiO in olivine
キーワード：Koolau volcano, Makapuu stage, Hawaii plume, superheated magma
Koolau volcano stands as an enigmatic outlier among the Hawaiian volcanoes, characterized by high-SiO2 tholeiite and enriched signatures in Sr, Nd and Pd isotopes. The prevailing notion is that Koolau magma originated from a source with the highest proportion of recycled ancient oceanic crust component in the Hawaiian plume (Hauri, 1996). Investigations of the giant Nuuanu landslide blocks (Takahashi et al., eds. 2002; AGU volume) and the rocks in the KSDP drilling project (Haskins & Garcia, 2004) have unveiled that the enriched magma represents solely the final eruption stage (Makapuu stage) of the Koolau volcano, following voluminous activity of Mauna Loa type magma. While prior research predominantly considered the primary Makapuu stage magma as slightly SiO2-rich picrite (SiO2 ~50wt%, MgO=15-18 wt%), we propose a contrasting view: the primary Makapuu stage magma was superheated basaltic andesite (SiO2= 53-54%, MgO=5-7 wt%). Our proposition is founded on petrological and geochemical analyses of the Makapuu stage lavas erupted in the deep ocean, recovered by the Shinkai-6500 submersible during the JAMSTEC cruise in Hawaii. Rocks from the north slope of the Koolau volcano (undisturbed by the giant Nuuanu landslide) obtained by the JAMSTEC submersible dive D500 (4 pieces) were examined by Garcia (2002). We reevaluated all rock specimens (9 pieces) from D500 and discovered that 1) D500-1 to D500-4 (2980-2830m depth) are hyaloclastites formed by the accumulation of glassy basalt fragments with Mauna Loa type chemistry, likely formed by the oceanic entry of subaerial Koolau lavas. 2) D500-5 to D500-9 (2694 to 2602m depth) are submarine-erupted picritic lavas with specific quench textures in the groundmass. Major and trace element compositions of the groundmass of the submarine picrites were examined using SEM-EDS and LA-ICPMS at GIG (Guangzhou Institute of Geochemistry, Chinese Academy of Sciences). It was found that these submarine picrites are identical to Makapuu stage subaerial lavas. They are considered to represent the most primitive magma among the Makapuu stage because they contain only olivine "phenocrysts," whereas most subaerial Makapuu stage lavas contain Pl, Cpx, and/or Opx as phenocrysts as well as olivine. Furthermore, olivine "phenocrysts" in the submarine picrite exhibit thermal erosion (fusion) textures, implying that the basaltic andesite magma was superheated and melted the olivine crystals during ascent (See Fig.1).
Based on high-pressure experimental studies, we demonstrate that the superheated basaltic andesite magma was formed by 30-50% melting of recycled ancient oceanic crust at 3-4 GPa. Due to the steep Clapeyron slope of basaltic andesite, whose liquidus phase is pyroxenes compared with mafic rocks whose liquidus phase is olivine, the basaltic andesite magma may have ascended to the surface with as much as a 100°C superheated condition. This finding aligns with the petrologic features of the submarine picrites (D500-5 to D500-9). We propose a magma genesis model that offers an alternative interpretation for the origin of NiO-enriched olivine found in the Makapuu stage of the Koolau volcano (Sobolev, 2005). Olivine in the Makapuu stage of the Koolau volcano exhibits CaO depletion as well as the NiO enrichment compared to those in other Hawaiian volcanoes (Garcia, 2002). We interpret that NiO-rich and CaO-poor olivine was derived from the conduit zone of the Koolau volcano. Due to the melting reaction (fusion) of olivine with the superheated basaltic andesite magma, they are enriched in NiO and depleted in CaO compared with olivine in normal mantle peridotite. The identification of basaltic-andesite primary magma as an end component of the Hawaiian plume opens new insights for the petrogenesis of Hawaiian shield volcanoes with variable SiO2 content (Frey et al., 1994) and PMT of the Hawaiian plume.
Based on high-pressure experimental studies, we demonstrate that the superheated basaltic andesite magma was formed by 30-50% melting of recycled ancient oceanic crust at 3-4 GPa. Due to the steep Clapeyron slope of basaltic andesite, whose liquidus phase is pyroxenes compared with mafic rocks whose liquidus phase is olivine, the basaltic andesite magma may have ascended to the surface with as much as a 100°C superheated condition. This finding aligns with the petrologic features of the submarine picrites (D500-5 to D500-9). We propose a magma genesis model that offers an alternative interpretation for the origin of NiO-enriched olivine found in the Makapuu stage of the Koolau volcano (Sobolev, 2005). Olivine in the Makapuu stage of the Koolau volcano exhibits CaO depletion as well as the NiO enrichment compared to those in other Hawaiian volcanoes (Garcia, 2002). We interpret that NiO-rich and CaO-poor olivine was derived from the conduit zone of the Koolau volcano. Due to the melting reaction (fusion) of olivine with the superheated basaltic andesite magma, they are enriched in NiO and depleted in CaO compared with olivine in normal mantle peridotite. The identification of basaltic-andesite primary magma as an end component of the Hawaiian plume opens new insights for the petrogenesis of Hawaiian shield volcanoes with variable SiO2 content (Frey et al., 1994) and PMT of the Hawaiian plume.