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[MZZ44-04] Helium isotopes and beryllium-10 dating of manganese crust in the central Pacific Ocean
Keywords:Manganese crust, Micronesia, Helium isotopes, Be-10 age
There are organic resources such as oil and natural gas, and inorganic ones such as minerals and mud containing useful metals in the deep-sea floor. Submarine mineral resources are classified as manganese nodules, manganese crusts, and hydrothermal deposits. Their occurrence and formation are different, and the metals they contain are also variable. Manganese crusts are iron-manganese oxides similar to manganese nodules and are known to contain nickel, cobalt, and copper as useful metals. They occur to deposit on basalt basement and limestone like asphalt on a road surface, and locate on the slopes and tops of seamounts. Their thickness varies from place to place, but often ranges from 5 cm to 15 cm. Manganese crust, along with manganese nodule, are considered to be sedimentary deposits that are originated by seawater. Therefore, they reflect the chemical composition of seawater at the time of formation and can be considered archives that record the marine environment.
Noble gas elements are chemically inert, and their concentrations and isotopic ratios fluctuate due to physical factors such as diffusion, mixing, partition and radioactive decay. Among them, helium isotope ratio is an important parameter in geochemistry because it is low in the crust and high in the mantle. In fact, it is used not only in global geo-tectonics, such as the Mid-Ocean Ridge and subduction zones [1], but also in a wide range of fields in Earth and Planetary sciences, such as the origin of oil and natural gas [2] and evaluation of volcanic activity [3]. We have measured helium isotope ratios of hydrothermal mineral deposits and manganese crusts for the previously mentioned mineral resources [4,5]. In this study, we began our analysis by focusing on the variation of helium isotope ratios associated with the growth of manganese crusts. Samples were collected by the Hakurei Maru No. 2 as part of a deep-sea floor mineral resource exploration (SOPAC) project at the top of a seamount in the EEZ of the Federated States of Micronesia in the central Pacific Ocean. In addition to helium isotopes, Be-10 ages were measured using an accelerator mass spectrometer to determine the sedimentation rate in the growth direction. In addition, uranium and thorium concentrations were measured by LA-ICPMS to estimate the in situ radiogenic production of helium-4. Together with Be-10 ages, these results may provide a secular variation of helium isotopes at the deep-sea floor.
References [1] Sano & Wakita, 1985, J. Geophys. Res. 90, 8729-8741. [2] Wakita & Sano, 1983, Nature 305, 792-794. [3] Kagoshima et al., 2019, G-cubed 20, 2019GC008544. [4] Ooki et al., 2016, G-cubed 17, 2016GC006360. [5] Sano et al., 1985, Nature 317, 518-520.
Noble gas elements are chemically inert, and their concentrations and isotopic ratios fluctuate due to physical factors such as diffusion, mixing, partition and radioactive decay. Among them, helium isotope ratio is an important parameter in geochemistry because it is low in the crust and high in the mantle. In fact, it is used not only in global geo-tectonics, such as the Mid-Ocean Ridge and subduction zones [1], but also in a wide range of fields in Earth and Planetary sciences, such as the origin of oil and natural gas [2] and evaluation of volcanic activity [3]. We have measured helium isotope ratios of hydrothermal mineral deposits and manganese crusts for the previously mentioned mineral resources [4,5]. In this study, we began our analysis by focusing on the variation of helium isotope ratios associated with the growth of manganese crusts. Samples were collected by the Hakurei Maru No. 2 as part of a deep-sea floor mineral resource exploration (SOPAC) project at the top of a seamount in the EEZ of the Federated States of Micronesia in the central Pacific Ocean. In addition to helium isotopes, Be-10 ages were measured using an accelerator mass spectrometer to determine the sedimentation rate in the growth direction. In addition, uranium and thorium concentrations were measured by LA-ICPMS to estimate the in situ radiogenic production of helium-4. Together with Be-10 ages, these results may provide a secular variation of helium isotopes at the deep-sea floor.
References [1] Sano & Wakita, 1985, J. Geophys. Res. 90, 8729-8741. [2] Wakita & Sano, 1983, Nature 305, 792-794. [3] Kagoshima et al., 2019, G-cubed 20, 2019GC008544. [4] Ooki et al., 2016, G-cubed 17, 2016GC006360. [5] Sano et al., 1985, Nature 317, 518-520.