3:30 PM - 3:45 PM
[SGC45-06] H2O / F ratios in asthenosphere inferred from volatile compositions of petit-spot lavas
Keywords:petit-spot, asthenosphere, water, fluorine
Petit-spot volcano, a newly recognized volcanism into tectonic processes on the Earth, erupts by tectonic force associated with plate-flexure of outer rise. The lavas (foidite to hawaiite) are highly enriched in incompatible elements, likely originated from asthenosphere. Their samples show high vesicularity in spite of the eruption under high hydrostatic pressure at approximately 6000 meters below the sea level, indicating high levels of volatile contents in magma.
We report the volatile compositions in quenched glass rinds of petit-spot lavas from the northwestern Pacific Plate in order to recognize the volatile compositions of asthenospheric mantle below the older part of oceanic lithosphere. They are much higher than most of the previously reported MORBs without seawater alteration. The H2O / F ratio of the studied samples is relatively constant (average: 14.6 ± 1.4, 1 s.d.). It may represent the ratio of asthenosphere below both of young and old lithosphere, because H2O and F in magma behave similar incompatibility during mantle melting and crystallization. The obtained H2O / F ratio is consistent with those of average MORB glasses (~13), but higher than those of HIMU and EM OIBs (H2O/F < 10).