5:15 PM - 7:15 PM
[SGC37-P03] Noble Gas Isotopic Signatures of the Pacific Lithosphere: Insights from Petit-Spot Mantle Xenoliths
Keywords:Noble Gas, Mantle, lithosphere, Petit-spot
Petit-spot volcanoes are intraplate oceanic volcanoes formed without direct influence from mantle plumes (Hirano et al., 2006, Science). Mantle-derived xenoliths carried to the surface by the petit-spot volcanism are expected to be unaffected by mantle plumes and thus provide direct information on the Pacific lithosphere. In this study, noble gas isotopic compositions and abundances of mantle xenoliths, including spinel lherzolite, garnet lherzolite, and garnet harzburgite, collected from a petit-spot on the oceanward slope of the Japan Trench in the northwestern Pacific ("Site A") using the deep-submergence vehicle Shinkai 6500 during the YK24-10S cruise of R/V Yokosuka (JAMSTEC), were analyzed.
The gas extraction was conducted using a furnace connected to a noble gas separation and purification vacuum line and a mass spectrometer (VG-5400/MS-IV, University of Tokyo). After heating the samples, active gases were removed using Ti-Zr getters before noble gas analysis.
The 3He/4He ratios of the samples range from 6.1 to 7.3 times the atmospheric value (Ra). These values are comparable to or slightly lower than those of spinel lherzolite mantle xenoliths collected from the sampling site ("Site A") and surrounding areas (Yamamoto et al., 2009, Chemical Geology).Additionally, they are lower than the values observed in spinel lherzolite mantle xenoliths within petit-spot basalts collected from "Site B", which is ca. 500 km east-southeast of Site A (Yamamoto et al., Chemical Geology, 2009), and the 8 ± 1 Ra observed in MORB. The low 3He/4He values are likely due to the ingrowth of radiogenic 4He, produced by the decay of U and Th, in the Pacific lithosphere. The mantle from which the mid-ocean ridge basaltic magma was extracted had retained MORB-like He at the melt extraction, but over 130 million years, as it was transported to the western edge of the Pacific, radiogenic 4He accumulated in the lithosphere. Since the (U+Th)/3He ratio of the lithosphere is lower than that of the present-day MORB-source mantle due to degassing at the mid-ocean ridge, the 3He/4He ratio would have decreased more rapidly than the MORB-source mantle.
The 40Ar/36Ar ratios range from 430 to 7700. The wide range of 40Ar/36Ar ratios is most likely due to the influence of atmospheric Ar introduced through seawater.
Since there is another possibility that post-eruptive radiogenic ingrowth of 4He is responsible for the low 3He/4He ratios of the samples, future work will focus on gas extraction using a stepwise crushing technique to clarify the intrinsic noble gas feature of these peridotite xenoliths.
The gas extraction was conducted using a furnace connected to a noble gas separation and purification vacuum line and a mass spectrometer (VG-5400/MS-IV, University of Tokyo). After heating the samples, active gases were removed using Ti-Zr getters before noble gas analysis.
The 3He/4He ratios of the samples range from 6.1 to 7.3 times the atmospheric value (Ra). These values are comparable to or slightly lower than those of spinel lherzolite mantle xenoliths collected from the sampling site ("Site A") and surrounding areas (Yamamoto et al., 2009, Chemical Geology).Additionally, they are lower than the values observed in spinel lherzolite mantle xenoliths within petit-spot basalts collected from "Site B", which is ca. 500 km east-southeast of Site A (Yamamoto et al., Chemical Geology, 2009), and the 8 ± 1 Ra observed in MORB. The low 3He/4He values are likely due to the ingrowth of radiogenic 4He, produced by the decay of U and Th, in the Pacific lithosphere. The mantle from which the mid-ocean ridge basaltic magma was extracted had retained MORB-like He at the melt extraction, but over 130 million years, as it was transported to the western edge of the Pacific, radiogenic 4He accumulated in the lithosphere. Since the (U+Th)/3He ratio of the lithosphere is lower than that of the present-day MORB-source mantle due to degassing at the mid-ocean ridge, the 3He/4He ratio would have decreased more rapidly than the MORB-source mantle.
The 40Ar/36Ar ratios range from 430 to 7700. The wide range of 40Ar/36Ar ratios is most likely due to the influence of atmospheric Ar introduced through seawater.
Since there is another possibility that post-eruptive radiogenic ingrowth of 4He is responsible for the low 3He/4He ratios of the samples, future work will focus on gas extraction using a stepwise crushing technique to clarify the intrinsic noble gas feature of these peridotite xenoliths.