10:00 AM - 10:15 AM
[SGC33-05] Whole-rock geochemistry of ultramafic rocks from the Kamuikotan Belt, Hokkaido, Japan: Os-Ir depletion as a signature of subduction-zone magmatism
Keywords:Highly siderophile elements, Os isotope ratio, Supra-subduction zones, Kamuikotan metamorphic belt, Ultramafic rocks
Mantle peridotites comprising lithospheres are generally depleted in melt components relative to primitive mantle material. Among these, the peridotites of the Archean lithosphere, obtained as kimberlite-derived xenoliths, are known to be particularly depleted and their genesis is still under debate (Pearson et al., 2021). The occurrence of highly depleted peridotites has been reported in circum-Pacific subduction zones, and they are considered as important modern analogues for investigating the melting conditions of Archean lithosphere (Day & Brown, 2021; Snortum & Day, 2020). The Kamuikotan Belt in Hokkaido is known to host ultramafic bodies originated from highly depleted harzburgites and dunites, where clinopryoxene is absent (Kato and Nakagawa, 1986). They are sporadically outcropping along the north-south trending belt, and the northern bodies are thought to be more depleted than those in the southern part (Tamura et al., 1999). However, its full extent remains unclear because they are strongly serpentinized throughout the area. Here we present the whole-rock geochemical data of ultramafic rocks from the Iwanai-dake, Takadomari, Horokanai, and Teshio areas. The aim of this study was to determine the melting age and conditions by examining the highly siderophile element (HSE) patterns and Os isotope ratios, which are less susceptible to secondary alteration.
The results show that there is little correlation between LOI (Loss on ignition) and HSE abundances in the ultramafic rocks of the Kamuikotan metamorphic belt, and the same is true for Os isotopic ratios. There is a very clear linear correlation between Os and Ir abundances, which are characterized by large variations regardless of sample localities (Os = 0.001-24 ng/g; Ir = 0.004-17 ng/g). Highly depleted samples tend to show larger variations than those in less depleted samples. When Os and Ir abundances are normalized by the primitive upper mantle abundance (Becker et al., 2006), Os/OsPUM and Ir/IrPUM are mostly less than 1. In addition, the 187Os/188Os ratios for the samples with Os/OsPUM greater than 0.01 can be determined between 0.119 and 0.126 with an accuracy of less than ±0.001, which is lower than the PUM (Meisel et al., 1996).
Our results demonstrate that the observed Os-Ir depletion can be related to the melt-depletion event rather than the serpentinization event. Previous studies have proposed that the high degree of melting can be achieved during open system melting in supra-subduction zones (Nishio et al., 2023; Kubo, 2002), which has been associated with the extraction of boninitic melts. Thus, we propose that Os and Ir behave as incompatible elements in a peculiar melting environment leading to the boninite formation. In addition, Re depletion model ages for the samples with Os/OsPUM and Ir/IrPUM greater than 0.01 indicate that they underwent melting between 0.8 to 1.4 Ga. This age is much older than the Early Jurassic, when subduction is thought to have commenced on the eastern margin of the Asian continent, resulting in the formation of the Northeast Japan Arc and the Kamuikotan Belt (Tang et al., 2018). Thus, our model for the history of ultramafic rocks in the Kamuikotan Belt is as follows: they underwent primary melting before being incorporated into the present geological setting, and secondary melting at a subduction zone produced Os-Ir depletions in the residual peridotites during the formation of boninites.
The results show that there is little correlation between LOI (Loss on ignition) and HSE abundances in the ultramafic rocks of the Kamuikotan metamorphic belt, and the same is true for Os isotopic ratios. There is a very clear linear correlation between Os and Ir abundances, which are characterized by large variations regardless of sample localities (Os = 0.001-24 ng/g; Ir = 0.004-17 ng/g). Highly depleted samples tend to show larger variations than those in less depleted samples. When Os and Ir abundances are normalized by the primitive upper mantle abundance (Becker et al., 2006), Os/OsPUM and Ir/IrPUM are mostly less than 1. In addition, the 187Os/188Os ratios for the samples with Os/OsPUM greater than 0.01 can be determined between 0.119 and 0.126 with an accuracy of less than ±0.001, which is lower than the PUM (Meisel et al., 1996).
Our results demonstrate that the observed Os-Ir depletion can be related to the melt-depletion event rather than the serpentinization event. Previous studies have proposed that the high degree of melting can be achieved during open system melting in supra-subduction zones (Nishio et al., 2023; Kubo, 2002), which has been associated with the extraction of boninitic melts. Thus, we propose that Os and Ir behave as incompatible elements in a peculiar melting environment leading to the boninite formation. In addition, Re depletion model ages for the samples with Os/OsPUM and Ir/IrPUM greater than 0.01 indicate that they underwent melting between 0.8 to 1.4 Ga. This age is much older than the Early Jurassic, when subduction is thought to have commenced on the eastern margin of the Asian continent, resulting in the formation of the Northeast Japan Arc and the Kamuikotan Belt (Tang et al., 2018). Thus, our model for the history of ultramafic rocks in the Kamuikotan Belt is as follows: they underwent primary melting before being incorporated into the present geological setting, and secondary melting at a subduction zone produced Os-Ir depletions in the residual peridotites during the formation of boninites.