4:45 PM - 5:00 PM
[SGC35-12] Noble gas isotope characteristics of subcontinental lithospheric mantle as a distinct geochemical reservoir
Keywords:Noble gasses, Subcontinental mantle
Introduction: The noble gas isotopic composition of the SCLM (sub-continental lithospheric mantle) is more obscure than that of the MORB (Mid-Ocean Ridge Basalt) and OIB (Ocean Island Basalt). Ne isotopic compositions from SCLM in Patagonia, South America and Finero show a strong contribution of the nucleogenic component from the spallation reaction compared to MORB. [1], [2] In order to obtain such a composition, it is necessary to be rich in U and Th or to be low in 22Ne, which is a primordial noble gas. Based on the high 3He/22Ne reported, it was proposed that the primordial noble gas in SCLM may be lost in degassing during melt extraction compared to MORB sources. [3] The purpose of this study is to investigate whether similar Ne isotopic compositions can be found in other SCLM, and if so, to confirm the 3He/22Ne composition of these SCLM.
Samples and analysis: We used peridotites of Maracath flow, Lunar Crater volcanic field (Nevada, USA), Toroweap flow (Arizona, USA), Mount Emma (Arizona, USA), San Quintin volcanic field (Mexico) and Beni Bousera peridotite body (Baja California, Morocco). We heated them in the vacuum and extracted and purified noble gases, and analyzed noble gases isotopes and amounts (He~Ar). After that we crushed the samples which were identified with sufficient abundances of noble gases in the heating experiment with a stepwise crushing method, and analyzed noble gases isotopes and amounts (He~Ar) in the same way.
Results and discussion: Helium isotopic ratios (3He/4He) in the samples from northwestern South America exceed the MORB range (8±1 Ra) and the maximum value with OIB (~50 Ra), while those from the Beni Bousera peridotite body in northwestern Africa are below 1 Ra. The samples from the western part of South America are considered to have received a contribution from cosmic ray irradiation based on the correlation between the abundance of 3He and 21Ne, which are presumed to be cosmic ray-producing nuclides. On the other hand, the contribution of 4He produced by radiative decay of U and Th is considered to be large in the samples from northwest Africa. The Ne isotopic compositions of almost all the samples could not be explained only by the mixture of atmospheric composition and cosmic ray-produced nuclides. In addition to atmospheric and cosmic-ray produced nuclides, there may be contributions from nucleogenic components or mantle-derived Ne. However, since the effect of nuclides produced in the crystal lattice of minerals by cosmic ray irradiation is more pronounced in the heating analysis, it is necessary to conduct the analysis using the stepwise crushing method, which is easy to selectively extract noble gases from phases rich in noble gases captured in the mantle, such as fluid inclusions, for more specific investigation. In order to carry out stepwise crushing, it is necessary for the sample to contain sufficient noble gas. Since we were able to identify the sample with sufficient noble gas by heating analysis, we will prioritize the sample for stepwise crushing.
Reference
[1] Jalowitzki et al., EPSL 2016
[2] Fukushima et al., JpGU 2021
[3] Nick et al., EPSL 2018
Samples and analysis: We used peridotites of Maracath flow, Lunar Crater volcanic field (Nevada, USA), Toroweap flow (Arizona, USA), Mount Emma (Arizona, USA), San Quintin volcanic field (Mexico) and Beni Bousera peridotite body (Baja California, Morocco). We heated them in the vacuum and extracted and purified noble gases, and analyzed noble gases isotopes and amounts (He~Ar). After that we crushed the samples which were identified with sufficient abundances of noble gases in the heating experiment with a stepwise crushing method, and analyzed noble gases isotopes and amounts (He~Ar) in the same way.
Results and discussion: Helium isotopic ratios (3He/4He) in the samples from northwestern South America exceed the MORB range (8±1 Ra) and the maximum value with OIB (~50 Ra), while those from the Beni Bousera peridotite body in northwestern Africa are below 1 Ra. The samples from the western part of South America are considered to have received a contribution from cosmic ray irradiation based on the correlation between the abundance of 3He and 21Ne, which are presumed to be cosmic ray-producing nuclides. On the other hand, the contribution of 4He produced by radiative decay of U and Th is considered to be large in the samples from northwest Africa. The Ne isotopic compositions of almost all the samples could not be explained only by the mixture of atmospheric composition and cosmic ray-produced nuclides. In addition to atmospheric and cosmic-ray produced nuclides, there may be contributions from nucleogenic components or mantle-derived Ne. However, since the effect of nuclides produced in the crystal lattice of minerals by cosmic ray irradiation is more pronounced in the heating analysis, it is necessary to conduct the analysis using the stepwise crushing method, which is easy to selectively extract noble gases from phases rich in noble gases captured in the mantle, such as fluid inclusions, for more specific investigation. In order to carry out stepwise crushing, it is necessary for the sample to contain sufficient noble gas. Since we were able to identify the sample with sufficient noble gas by heating analysis, we will prioritize the sample for stepwise crushing.
Reference
[1] Jalowitzki et al., EPSL 2016
[2] Fukushima et al., JpGU 2021
[3] Nick et al., EPSL 2018