1:45 PM - 3:15 PM
[SGL21-P05] The noble gas isotopic fractionations of volcanic rocks and their effect on ages
Keywords:Noble gas, Argon, Un-spiked K-Ar dating, fractionation, historical lava
Applying the K-Ar dating method to geological samples requires an equilibrium of Ar isotopic composition with the present-day atmosphere except for the radiogenic 40Ar. To date the young volcanic rocks, < 1 million years in typical, the measurement is performed without 38Ar addition as a spike, widely. That manner is called the un-spiked method, or peak comparison method. Even in the case of historical lava, the equilibrium assumption at its solidification is also required; however, it is not always guaranteed in case the sample inherits any deep-source isotopic compositions. In the case of rock dating, the sample having inherited isotopic anomalies, i.e. contributed deep-source isotope compositions resulting in an older than apparent ages, the certain sample is frequently regarded to be containing excess argon.
However, few studies have confirmed this phenomenon considering isotopic fractionations at the eruption. Thus, here we performed a rare gas isotope analysis applying to the lava flows in historical events according as Ozawa et al. (2005) reported under the un-spiked protocols with a GVI-5400He mass spectrometer: manufactured by GV Instruments and installed at JAMSTEC. The samples of various conditions, i.e., distance from the crater/vent, viscosity, and vesicularity, were chosen and analyzed. All the samples showed a certain isotope fractionation tendency in the direction of lighter isotope compositions; all the data were plotted in the area left below the atmospheric composition on the 38Ar/36Ar-40Ar/36Ar three-component isotope plots. Yet to be determined the fractionation processes, subsurface contact with groundwater may be one of the possibilities responsible for such fractionation (e.g. Kaneoka, 1980). Further consideration of the deep-origin components of noble gases will also be reported along with the isotope fractionation of Ar.
However, few studies have confirmed this phenomenon considering isotopic fractionations at the eruption. Thus, here we performed a rare gas isotope analysis applying to the lava flows in historical events according as Ozawa et al. (2005) reported under the un-spiked protocols with a GVI-5400He mass spectrometer: manufactured by GV Instruments and installed at JAMSTEC. The samples of various conditions, i.e., distance from the crater/vent, viscosity, and vesicularity, were chosen and analyzed. All the samples showed a certain isotope fractionation tendency in the direction of lighter isotope compositions; all the data were plotted in the area left below the atmospheric composition on the 38Ar/36Ar-40Ar/36Ar three-component isotope plots. Yet to be determined the fractionation processes, subsurface contact with groundwater may be one of the possibilities responsible for such fractionation (e.g. Kaneoka, 1980). Further consideration of the deep-origin components of noble gases will also be reported along with the isotope fractionation of Ar.