15:30 〜 17:00
[SGC37-P09] Noble gas and halogen characteristics of subcontinental lithospheric mantle beneath southwestern North America and northwestern Africa
キーワード：希ガス、ハロゲン、大陸下マントル
The subcontinental lithospheric mantle (SCLM) is generally metasomatized by intra-plate or arc magmatism, making them distinct geochemical reservoirs from mid-ocean ridge basalts (MORBs) and ocean island basalt (OIB) sources. Higher 3He/22Ne is observed in mantle-derived xenoliths from the SCLM in Patagonia, South America (Jalowitzki et al., EPSL 2016) compared to MORBs and OIBs, which reflect the composition of the convecting mantle and plume source, respectively. Based on this 3He/22Ne difference, SCLM is expected to be a distinct geochemical reservoir that has evolved differently from the convecting mantle and plume sources in terms of noble gases (Dygert et al., EPSL 2018). The average 3He/4He of SCLM-derived peridotite samples is about 6 Ra (Gautheron & Moreira, EPSL 2002), where 1 Ra = 1.4 x 10-6 represents atmospheric 3He/4He, suggesting that the contribution of radiogenic 4He derived from the decay of U and Th to SCLM is more significant than that to the convecting mantle with 3He/4He of ca 8 Ra. In addition, Ne isotopic compositions of the SCLM samples from Patagonia in South America have been confirmed to have a more significant contribution of nucleogenic 21Ne from 24Mg[n,α]21Ne and 18O[α,n]21Ne reactions compared to MORBs. If the SCLM is a globally homogeneous reservoir, these features are expected to be observed in SCLMs in other regions. Therefore we analyzed noble gases and halogens in SCLM-derived samples from the Lunar Crater volcanic field, Toroweap flow, the Mendocino Mountains, and Mount Emma in southwestern North America, and Beni Bousera peridotite body in northwestern Africa.
The samples from southwestern North America have 3He/4He that fall within the range of the convecting mantle. On the other hand, the 3He/4He ratios of the Beni Bousera peridotite samples are low, ranging from 0.3 Ra to 0.7 Ra. The Ne isotope ratios of the southwestern North America samples are also accounted for by mixing between air and convecting mantle endmembers. In contrast, those of the Beni Bousera peridotite samples suggest the addition of nucleogenic Ne to the atmospheric compositions. These results indicate that the SCLM is not a homogeneous geochemical reservoir in terms of noble gases.
The MORB-like He and Ne feature of the samples from southwestern North America differs from those of other SCLMs. This result suggests that the SCLMs beneath these regions were incorporated into the mantle convection by delamination, resulting in compositional overwriting with the convecting mantle component.
The contribution of nucleogenic 21Ne correlates with that of radiogenic 4He and is substantial in the Beni Bousera peridotite body samples. The sample was reported to have experienced deformation and dynamic recrystallization, which may have resulted in the loss of mantle-derived noble gases. Based on the U abundance obtained by heating analyses of the neutron-irradiated samples and that of total radiogenic 4He abundance determined by heating analyses of unirradiated samples, the time when the accumulation of radiogenic 4He began was estimated to be approximately 13-17 Ma. This is consistent with the exhumation age of the Beni Bousera peridotite body in the early Miocene (ca. 23-16 Ma).
In contrast to the noble gas features, the Br/Cl and I/Cl ratios of the samples from southwestern North America and northwest Africa share similar characteristics in which Br/Cl and I/Cl range 1.2-7.8 ×10-3 mol/mol and 61-860 ×10-6 mol/mol, respectively, while I/Br showed fewer variations (0.031-0.110 mol/mol). Both samples are from the back-arc regions of the subduction zones, and a similar feature was observed for mantle-derived xenoliths in Takashima, northwestern Kyushu, Japan, which is also in the back-arc region. Therefore, it is possible that this feature is common in the back-arc regions of subduction zones and may result from Cl depletion in the mantle wedge having higher I/Cl due to the addition of iodine-rich subducted halogen component.
The samples from southwestern North America have 3He/4He that fall within the range of the convecting mantle. On the other hand, the 3He/4He ratios of the Beni Bousera peridotite samples are low, ranging from 0.3 Ra to 0.7 Ra. The Ne isotope ratios of the southwestern North America samples are also accounted for by mixing between air and convecting mantle endmembers. In contrast, those of the Beni Bousera peridotite samples suggest the addition of nucleogenic Ne to the atmospheric compositions. These results indicate that the SCLM is not a homogeneous geochemical reservoir in terms of noble gases.
The MORB-like He and Ne feature of the samples from southwestern North America differs from those of other SCLMs. This result suggests that the SCLMs beneath these regions were incorporated into the mantle convection by delamination, resulting in compositional overwriting with the convecting mantle component.
The contribution of nucleogenic 21Ne correlates with that of radiogenic 4He and is substantial in the Beni Bousera peridotite body samples. The sample was reported to have experienced deformation and dynamic recrystallization, which may have resulted in the loss of mantle-derived noble gases. Based on the U abundance obtained by heating analyses of the neutron-irradiated samples and that of total radiogenic 4He abundance determined by heating analyses of unirradiated samples, the time when the accumulation of radiogenic 4He began was estimated to be approximately 13-17 Ma. This is consistent with the exhumation age of the Beni Bousera peridotite body in the early Miocene (ca. 23-16 Ma).
In contrast to the noble gas features, the Br/Cl and I/Cl ratios of the samples from southwestern North America and northwest Africa share similar characteristics in which Br/Cl and I/Cl range 1.2-7.8 ×10-3 mol/mol and 61-860 ×10-6 mol/mol, respectively, while I/Br showed fewer variations (0.031-0.110 mol/mol). Both samples are from the back-arc regions of the subduction zones, and a similar feature was observed for mantle-derived xenoliths in Takashima, northwestern Kyushu, Japan, which is also in the back-arc region. Therefore, it is possible that this feature is common in the back-arc regions of subduction zones and may result from Cl depletion in the mantle wedge having higher I/Cl due to the addition of iodine-rich subducted halogen component.