ORIGINAL ARTICLE
Petrography and Rb-Sr mineral age of mafic dyke rock from Niban Iwa, Lützow-Holm Complex, East Antarctica
2024 年 119 巻 1 号 論文ID: 231207
詳細
2024 年 119 巻 1 号 論文ID: 231207
A WNW-ESE-trending mafic dyke intruding across major structures in high-grade metamorphic rocks was found at Niban Iwa (translated as ‘Number Two Rock’) in the Proterozoic Lützow-Holm Complex of East Antarctica. It is holocrystalline and aphyric, and comprises biotite, hornblende, plagioclase, orthoclase, quartz, apatite, and titanite. Chemically the dyke rock is alkali basalt with high K2O/Na2O and total Fe contents, and low Cr and Ni contents, indicating that it was formed by the differentiation of olivine from a primary alkali basaltic magma derived from the subcontinental mantle. The Rb-Sr mineral isochron age was obtained of 487 ± 15 Ma with SrIR = 0.70486 ± 0.00007. Considering that the metamorphic age of the gneisses at Niban Iwa was estimated to be 532 Ma, the dyke probably intruded after metamorphism as part of the post-orogenic igneous activity following the collision of East and West Gondwana.
Understanding the formation and stabilization of metamorphic complexes is important for comprehension of crustal formation, the evolution of orogenic belts, and amalgamation processes in supercontinents. However, geological phenomena after major metamorphism should also be considered to completely elucidate the stabilization process of the continental crust. The intrusion of mafic igneous rocks has been widely reported in regional metamorphic complexes, and the mafic magma is commonly thought to have originated from the partial melting of the subcontinental mantle related to the stabilization process (e.g., Canning et al., 1996; Murphy et al., 2002). Petrological, geochemical, and chronological investigations of such ‘post-peak’ igneous rocks are therefore necessary to obtain valuable information on the characteristics and condition of the lower crust and mantle beneath the thickened continental crust developed through orogenesis, as well as on the condition of the magma source rocks. Additionally, the orientation of intrusions, especially dykes, can provide information about the stress field in the crust in a late- to post-orogenic settings.
The Lützow-Holm Complex (LHC) of Dronning Maud Land in East Antarctica is a Neoproterozoic-Cambrian metamorphic complex that is a part of the suture zone network between East and West Gondwana (Fitzsimons, 2000; Harley, 2003; Jacobs et al., 2003; Shiraishi et al., 2003). High-grade metamorphism occurred in the complex during the period of 600-520 Ma (e.g., Dunkley et al., 2020, and reference therein; see also Supplementary Fig. S1; Fig. S1 is available online from https://doi.org/10.2465/jmps.231207). A lot research have been carried out in LHC, especially focusing on the metamorphism and structural geology related to the orogeny that occurred during the amalgamation of Gondwana (e.g., Motoyoshi et al., 1989; Shiraishi et al., 1992, 1994; Hokada and Motoyoshi, 2006; Motoyoshi et al., 2006; Shiraishi et al., 2008; Yoshimura et al., 2008). Hiroi et al. (1991) provided an overview of the regional metamorphism, suggesting increasing metamorphic grade from the NE part (amphibolite-facies zone) to the SW part (granulite-facies zone) of the complex. Dunkley et al. (2020) provided an overview of the origin of protoliths in the complex, grouping protoliths of different ages into suites that include an exotic block (Fig. S1). The dominant part of the eastern LHC was grouped into the Akarui (AKR) Suite, in which the Hinode Block (HB) was divided as an isolated mass formed earlier during the Tonian period. However, Baba et al. (2022) recently reported that Tonian metamorphic rocks similar to those in the HB were also present in the Akebono Rock, located east of the HB in the eastern part of the AKR Suite. Additionally, there have been reports that Niban Iwa in the AKR Suite, located southwest of the HB, also records Tonian thermal events (Mori et al., 2023; Kitano et al., 2023; see also Supplementary Table S1; Supplementary Tables S1-S3 are available online from https://doi.org/10.2465/jmps.231207). Petrographic studies with more detailed chronological results are required to understand the evolution of the AKR Suite in the LHC.
Late- to post-tectonic igneous rocks, although limited in volume, have been recognized in most exposures of the LHC. Most of these rocks are granitic emplacements and are common in the NE and central parts of the LHC. Mafic to intermediate dykes intruding across the gneissosity in the host rocks were also observed in a few exposures in all parts of the LHC. Among them, the mafic dykes at Akebono Rock in the AKR Suite comprise weakly metamorphosed basalt to andesite with subalkalic compositions. These dykes intruded with N-S to NW-SE trends across the foliation in the host gneisses and preserved igneous textures (Hiroi et al., 1986). In contrast, the mafic dyke rocks from Cape Hinode are amphibolites, and partly preserve some igneous textures such as blastoporphyritic plagioclase though they are metamorphosed (Yanai and Ishikawa, 1978). The dykes intruded with NE-SW or N-S trends, all cutting across the foliation in the host gneisses. In the western LHC, ultrapotassic mafic dykes were found at Skallevikshalsen, Rundvågshetta, and Innhovde (Arima and Shiraishi, 1993; Miyamoto et al., 2023). These intruded subvertically along a general N-S trend across surrounding gneisses. The mafic igneous activity is considered post-orogenic following the collision of East and West Gondwana (Arima and Shiraishi, 1993; Miyamoto et al., 2023). At Rundvågshetta, holocrystalline clinopyroxene amphibolites also intruded subvertically with a general N-S trend prior to the activity of ultrapotassic mafic dykes (Motoyoshi et al., 1986; Ishikawa et al., 1994). Other small-scale intrusions of mafic rock exist in the western LHC at Skallen (Yoshida et al., 1976; Osanai et al., 2004) and Botnneset (Shiraishi and Yoshida, 1987), but the timing and compositional characteristics of these intrusions are largely unknown. Furthermore, chronological research on post-metamorphic mafic igneous activity in the eastern LHC is scarce.
In this study, we describe the geological and chronological characteristics of a mafic dyke found at Niban Iwa, and evaluate its importance in relation to the tectonic evolution of the LHC.
Niban Iwa is located in the northeastern part of the LHC within the amphibolite-facies zone of Hiroi et al. (1991). According to Dunkley et al. (2020), it is a representative site of the AKR Suite (Fig. S1). It has two principal outcrops, namely, Niban-higasi Iwa and Niban-nisi Iwa (Fig. 1). According to Kizaki et al. (1983), the basement of Niban Iwa is composed of sillimanite-garnet gneiss, biotite gneiss, metabasite, biotite-hornblende gneiss, calc-silicate gneiss, granite and aplite (Fig. 1). The gneisses generally have E-W trending foliation and gneissosity dipping toward the north. A steep synform and sigmoidal fold structure were dominant in Niban-higasi Iwa, whereas a shallow synform and gentle dome structure were found in Niban-nisi Iwa. Therefore, a major structural break may have occurred between the two (Toyoshima, 2023).
Among the metamorphic rocks, biotite- and/or hornblende-bearing felsic orthogneisses were predominant with lesser layers of metasedimentary types including metapelites and calc-silicates. All these rocks have strong gneissosity and strong to weak foliation. Migmatisation was widespread, especially in biotite felsic gneiss with the production of hornblende in association with leucosomes. Metabasites occurred throughout Niban Iwa as thin gneissic layers and discontinuous bodies along the gneissosity in biotite gneiss and migmatitic biotite gneiss. These varieties can be classified as clinopyroxene amphibolite and biotite amphibolite (Kizaki et al., 1983). The biotite amphibolite showed massive appearances but partly developed gneissosity, and was sometimes partially surrounded by thin films of felsic rock (TM11020802, Fig. 2d). Granite and aplite occurred across Niban Iwa as post-metamorphic discordant dykes and sheets. Granite dykes cut gneissosity but were also emplaced along the dextral shear zones, with biotite being aligned parallel to the shearing, indicating syn-shear intrusion.
For the syn- to post-tectonic felsic intrusive rocks of Niban Iwa, Dunkley et al. (2014, 2020) reported sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages for a biotite felsic orthogneiss that cuts gneissosity in the more mafic layered hornblende orthogneiss, with 940 ± 6 Ma for the time of intrusion and ∼ 964 Ma for xenocrysts from the host gneiss (Table S1). An intensely folded and metamorphosed granitic dyke was also dated, with zircon ages of 551 ± 11 Ma for the intrusion and 532 ± 7 Ma for the metamorphism (Dunkley et al., 2020). Recently, Mori et al. (2023) and Kitano et al. (2023) conducted detailed chronological studies following the petrographic analysis of metamorphic rocks from Niban-nisi Iwa. Mori et al. (2023) obtained an electron microprobe U-Th-Pb age of 940.1 ± 9.8 Ma for metamorphic monazite; based on this age, they argued that the main metamorphism took place during the Tonian period. Kitano et al. (2023) reported SHRIMP detrital zircon ages of 1940-1760, 1300, and 1160-1040 Ma for metasedimentary rocks, along with a 994 ± 11 Ma metamorphic age. Since similar Tonian ages for both metamorphism and magmatism have been reported by Shiraishi et al. (1997) and Dunkley et al. (2020) at Cape Hinode, located 20 km ESE of Niban Iwa, these chronological results suggest that Niban Iwa was genetically related to the HB rather than to the AKR Suite. However, evidence of a second high-temperature tectonometamorphic event at ∼ 530 Ma based on the metagranitic dyke at Niban Iwa (Dunkley et al., 2020) has not been observed in zircon dating at Cape Hinode.
Although extensive post-gneissosity emplacement of granitoids was previously known at Niban Iwa, the emplacement of mafic dykes was first recognized during a field survey of the Japanese Antarctic Research Expedition 52 (JARE 52) in the central part of Niban-higasi Iwa (Fig. 1b). The dyke was undeformed, and was 10-30 cm wide and at least 20 m in exposed length (Figs. 2a and 2b), primarily trending in the WNW-ESE direction with a steep dip and with sharp discordant contacts against foliation in the host gneisses. The mafic dyke was penetrated by a quartzo-feldspathic vein (Fig. 2c). The mafic dyke rock was predominantly holocrystalline and aphyric. It mainly comprised biotite, hornblende, plagioclase, orthoclase, quartz, apatite, and titanite, with minor Fe-Ti oxides (Fig. 3a). The biotite had a typical grain size of 0.5-1 mm with some coarser plates being up to 2 mm across. Hornblende was equant to columnar with blue-green axial color, and with a grain size mostly between 0.1 and 0.5 mm, which was rarely up to 2 mm in length. The hornblende is often surrounded by biotite (Figs. 3a and 3c). Plagioclase, orthoclase, and quartz are xenomorphic, with grain sizes between 0.1 and 1 mm and few grains being up to 3 mm across. Apatite was equant and idiomorphic, with a grain size mainly 0.1-1 mm, and rarely up to 3 mm. Titanite grains were equant and subhedral with some rounded edges, and were typically 0.1-0.5 mm across. The predominant phases, particularly biotite and to a lesser extent hornblende, were aligned parallel to the dyke walls (Fig. 3c). Based on the mineral assemblage, the mafic dyke rock resembled lamprophyres such as kersantite or spessartite, and differed from the mineralogy of biotite amphibolite on Niban Iwa (Figs. 3e and 3f).
Whole-rock chemistry of samples of the mafic dyke from Niban Iwa was analyzed following the procedure described by Miyamoto et al. (2023). For geochronology, mafic (mixtures of biotite and hornblende, MF) and felsic (mainly plagioclase and orthoclase, FF) mineral fractions were separated from crushed rock sample TM11020808A. Mineral separation and chemical treatment were conducted according to the methods described by Miyamoto et al. (2023). The Sr isotope compositions as well as total Rb and Sr concentrations were determined by Thermal Ion Mass Spectrometry using the Finnigan MAT-262 instrument at Okayama University. The Sr standard NBS987 yielded 87Sr/86Sr = 0.710233 ± 0.000011 (2σ). The relative analytical errors for the Rb and Sr concentrations were 2% and 1%, respectively. Consequently, the uncertainties in the Rb/Sr ratio were estimated to be 2%. The contamination levels of Rb and Sr were less than 1.0 × 10−10 and 1.0 × 10−10 g per sample, respectively. The decay constant used for the age calculations was 1.42 × 10−11 year−1 for 87Rb (Steiger and Jäger, 1977).
The analytical results of the major and minor element compositions of the mafic dyke rocks and a representative sample of biotite amphibolite from Niban Iwa are presented in Supplementary Table S2. The mafic dyke rocks from Niban Iwa have alkali basaltic compositions and are categorized as tephrite in the total alkalis versus silica (TAS) diagram (Fig. 4a) of Le Maitre et al. (1989). Although the rocks have mafic compositions, they are characterized by a high content of total Fe as Fe2O3 (13.9-14.2 wt%) relative to MgO (5.4-5.5 wt%), and low Ni and Cr contents (Ni = 55-57 ppm, Cr = 35-43 ppm). Such compositional characteristics differ from the whole rock composition of the biotite amphibolite from Niban Iwa (Fig. 4).
The contents of incompatible elements such as K and Rb in the mafic dyke from Niban Iwa were relatively high for mafic magmatic types (Figs. 4 and 5). Normalized trace element patterns generally display a gentle downward trend on a spidergram, with enrichment in Ba and Rb up to 110 and 150 times that of primitive mantle, respectively (Fig. 5a). The chondrite-normalized REE patterns show gentle downward trends with weak negative Eu anomalies (Fig. 5b).
The Rb and Sr contents and Sr isotopic compositions of the mafic and felsic mineral fractions of mafic dyke sample TM11020808A are shown along with the whole-rock composition in Supplementary Table S3. The values for 87Rb/86Sr and 87Sr/86Sr on the isochron diagram (Fig. 6) provide an age estimate of 487 ± 15 Ma (1σ) with an initial Sr ratio (SrIR) of 0.70486 ± 0.00007 (1σ), as calculated from the slope of the line using the error calculation method of IsoplotR (Vermeesch, 2018). This was defined from only three points and the MSWD value was high (13). However, the purity of the selected mafic mineral fraction was high and resulting in a high Rb/Sr value for the MFN fraction (87Rb/86Sr = 10.70); hence, the result is meaningful in terms of the age of intrusion.
The mafic dyke rocks of Niban Iwa were undeformed and cut gneissosity of the host gneisses, in which major structures and the gneissosity were formed during high-grade metamorphism (Kizaki et al., 1983). Dunkley et al. (2014, 2020) reported the time of metamorphism as 532 ± 7 Ma. More recently, older metamorphic ages of 940.1 ± 9.8 and 994 ± 11 Ma were reported for the rocks in Niban-nisi Iwa (Mori et al., 2023; Kitano et al., 2023). These ages are close to the protolith age of the felsic orthogneiss dated at 940 ± 6 Ma (Dunkley et al., 2014, 2020). The sample was obtained from Niban-higasi Iwa, near the boundary between biotite gneiss and the more mafic biotite-hornblende gneiss, where biotite felsic orthogneiss. Here, felsic biotite gneiss intruded across the gneissosity in the biotite-hornblende gneiss and was in turn metamorphosed and deformed into a second gneissosity associated with intense stretching and concentrated in the dextral shear zones. Accordingly, the age of ∼ 940 Ma likely represents intrusion during a Tonian tectonometamorphic event that produced gneissosity in the biotite-hornblende gneiss. A few data consistent with metamorphism at 520-530 Ma were found in the monazite from the metasedimentary gneiss (Mori et al., 2023). These ages match the zircon age for metamorphism in the metagranite dated by Dunkley et al. (2014, 2020). This sample, from a locality that bridges Niban-higasi Iwa and Niban-nisi Iwa, was obtained from a ductile-deformed and metamorphosed granitoid metadyke, which intruded gneissosity in the host metapelitic gneiss at 551 ± 11 Ma, immediately before or during tectonometamorphism that deformed the structures into tight upright folds with NE-trending axial planes. Besides zircon may not be sometimes influenced by metamorphism below amphibolite facies (Rubatto et al., 2001); therefore, the chronological results of Kitano et al. (2023) don’t refute an experience of ∼ 530 Ma metamorphism at Niban-nisi Iwa after the Tonian event. Alternatively, it has been suggested that Niban-higasi Iwa might structurally belong to a different crustal unit (HB) than Niban-nisi Iwa (Toyoshima, 2023). However, no structural evidence suggests that the HB represents a distinct crustal unit; it was only defined as such due to a lack of evidence for the late Proterozoic-Cambrian metamorphism, which defines the LHC on both sides of Cape Hinode. Considering the increasing evidence for Tonian metamorphism in the region, the simplest interpretation is as follows: both Niban-higasi Iwa and Niban-nisi Iwa comprise the Tonian metasedimentary and metaigneous gneisses that were reworked to varying degrees by Cambrian magmatism, metamorphism, and deformation, and indeed that the HB is not a distinct tectonic unit but rather an area of lesser reworking within the LHC.
Consequently, the timing of the undeformed mafic dyke is critical, as it places a lower limit on Cambrian tectonometamorphism. The sample yielded an Rb-Sr isotopic age of 487 ± 15 Ma (Fig. 6). In the dataset regarding the age, the data of the MFN fraction were located at a slightly higher position than the obtained regression line. The MFN fraction comprised biotite and hornblende. The hornblende was sometimes surrounded by biotite in the mafic dyke rocks (Figs. 3a and 3c); the hornblende is presumed to have crystallized before biotite. As a result, the hornblende in the MFN fraction may have had a slightly older isotopic composition; thus, the MFN fraction was plotted at a slightly higher position than the regression line from which it was obtained. Considering that this is a Rb-Sr mineral isochron age and that the sample contains biotite, this age is likely to indicate cooling after the intrusion of the dyke (Spear, 1993). The presence of quartz veins (Fig. 2c) indicates the continuation of hydrothermal activity after intrusion. Therefore, the dyke likely crystallized completely after peak metamorphism and deformation, but when some limited degrees of shearing and hydrothermal activities were still active. This is in contrast to the intense deformation and ∼ 530 Ma metamorphism recorded by the zircon in the ∼ 551 Ma metagranitoid dyke (Dunkley et al., 2014, 2020). Between these periods, a window of waning tectonometamorphism and cooling was observed at Niban Iwa.
Origin of mafic magmaThe mafic dyke rocks have alkali basaltic compositions (Table S2 and Fig. 4), and show a lamprophyre-like appearance owing to the abundance of both biotite and hornblende (Figs. 3a-3c). The REE patterns of the mafic dyke rocks showed slightly negative trends with small negative Eu anomalies (Fig. 5b), expecting poor crustal assimilation and plagioclase fractionation. Although the observation of systematic changes in the compositions of samples is hindered by the small number of samples, the characteristically high Fe (total Fe as Fe2O3/MgO = 2.5-2.6), and low Cr and Ni (Cr = 35-43 ppm, Ni = 55-57 ppm, Figs. 4g and 4h) contents can be attributed to the differentiation of olivine from primary alkali basaltic magma (Kawabata et al., 2011; Taniuchi et al., 2021). Basaltic magmas with high alkalinity, especially K, are widely regarded as the products of deep melting in mantle (e.g., Kuno 1966; Hawkesworth et al., 1994; Taniuchi et al., 2021). The mafic rocks of Niban Iwa show high in K2O contents (3.3-3.7 wt%) and K2O/Na2O (= 1.27-1.45, Fig. 4b) relative to those of other mafic types; therefore, the depth of melting to produce mafic magma is presumed to be deeper than that of common alkali basaltic magma, which usually contains more Na2O than K2O. The residual melt after the fractionation of olivine during or after its ascent into the crust may also have been hydrated during intrusion; consequently, hornblende and biotite were produced rather than olivine or clinopyroxene, which are usually present in alkali basalt, resulting in a lamprophyre-like mineralogy.
Comparison with late-post orogenic dyke rocks of the western LHCThe LHC is a metamorphic complex belonging to the Cambrian suture zones, dividing East and West Gondwana (Fig. S1), with high-grade metamorphism occurring between 600 and 520 Ma (e.g., Dunkley et al., 2020, and references therein). We suggest that the studied mafic dyke at Niban Iwa intruded after peak metamorphism and deformation in a part of the LHC that reworked the Tonian gneisses. Mafic dykes contemporaneous with that at Niban Iwa are scattered throughout the western LHC, and are thought to be related to post-collisional activity following the amalgamations of East and West Gondwana (Arima and Shiraishi, 1993; Miyamoto et al., 2023). The mafic dyke of Niban Iwa is also likely part of a series of igneous activities that occurred following collision. However, the compositions of the mafic dyke rocks of Niban Iwa differ from those of similar ages in the western LHC (Figs. 4 and 5). Whereas the Niban Iwa dyke has an affinity for alkali basalt, others tend to be ultrapotassic (Fig. 4). The REE pattern and spidergram show that Niban Iwa rocks have less HREE depletion relative to LREE than the ultrapotassic mafic dykes of the western LHC (Fig. 5). Thus, the origin of the Niban Iwa dyke can be attributed to deep melting in a typical upper mantle, as opposed to an enriched metasomatized mantle considered to be the origin of ultrapotassic magmas (Miyamoto et al., 2023).
Perhaps it might also be possible to explain the characteristics of trace elements for the Niban Iwa samples by a greater degree of melting of the enriched mantle at shallower depths, although these samples may have been present deeper than the generation depth for common alkali basalt. If the mafic magma had undergone fractionation of olivine and pyroxene from the parent magma, followed by the removal of Mg, Ni, and Cr, the residual melt would have been relatively Fe-rich. However, in this situation, the overall trace element concentrations would have been considerably higher in the residual melt than in this case. Besides, even if clinopyroxene, which tends to contain more trace elements than olivine and orthopyroxene, was differentiated from the magma, incompatible elements, especially large-ion lithophile elements, should have been enriched in the residual melt, resulting in a steep REE pattern and spidergram similar to those observed in ultrapotassic mafic dykes in other regions (Fig. 5). Furthermore, the composition of the residual liquid may have become more felsic if the olivine and pyroxene had been abundantly differentiated. Therefore, it is unlikely that the enriched mantle, which was regarded as the source of ultrapotassic mafic rocks in the western LHC (Miyamoto et al., 2023), is the source rock for mafic magma at Niban Iwa. This may indicate regional differences in the composition of subcontinental mantle. Additionally, significant differences were observed in the ages of crustal materials across the LHC (Dunkley et al., 2020): magmatic protoliths in the AKR Suite of the eastern LHC are exclusively mid to late Proterozoic, and the western LHC contains components that extend in age back to the beginning of the Proterozoic. The heterogeneity of the mantle composition might be related to the different evolutions of the pre-existing crust in different parts of the LHC, and the interactions of such a crust with subduction systems prior to the amalgamation of Gondwana. The orientation of the mafic dykes also differs at Niban Iwa: the ultrapotassic mafic dykes of the western LHC trend in the N-S direction, whereas the mafic dyke of Niban Iwa trends in the WNW-ENE direction (Fig. 1). As the dyke intrusion direction marks out extensional stress fields in the crust, these differences likely indicate different stresses in different parts of the LHC. In the eastern LHC, post-metamorphic dyke rocks also occur at Cape Hinode and Akebono Rock (Yanai and Ishikawa, 1978; Hiroi et al., 1986). Some of them have subalkaline andesitic compositions (Hiroi et al., 1986). The compositional characteristics of the intrusive rocks at Akebono Rock and Niban Iwa are different even though they are separated by a distance of less than 30 km, which is probably related to the different times of intrusion. Future petrological and chronological research on late intrusive rocks from each exposure along the Prince Olav Coast could help elucidate the terminal evolution of this part of the LHC and its relationship to adjacent tectonic domains.
A mafic dyke from Niban Iwa in the eastern part of the LHC was studied. The dyke was undeformed and trended in the WNW-ESE direction, cutting the gneissosity in the host rocks. It was holocrystalline and aphyric, and its main constituent minerals were biotite, hornblende, plagioclase, orthoclase, quartz, apatite, and titanite. A Rb-Sr mineral isochron age was 487 ± 15 Ma with SrIR = 0.70486 ± 0.00007. Considering that the age of metamorphism was 532 Ma at Niban Iwa, where Tonian gneisses were reworked in an unrelated event, and that the dyke was holocrystalline but undeformed, the dyke was presumed to have intruded after high-T metamorphism and deformation at 532 Ma during the end of tectonism associated with the assembly of Gondwana. The composition was alkali basaltic with high K2O/Na2O, rich in Fe2O3, and low in Cr and Ni contents. Such composition can be attributed to the differentiation of olivine from primary alkali basaltic magma derived from the deep mantle. The composition of the mafic dyke was different from those of contemporaneous ultrapotassic mafic dykes in the western LHC, which were derived from the melting of the enriched mantle. Such regional differences are likely to reflect those in the mantle underlying the LHC during the Cambrian period.
We express our sincere thanks to the members of JARE-52 and crew of the icebreaker ‘Shirase’. We would like to thank to the member of West Japan East Antarctic Research Group, especially Profs. H. Ishizuka, T. Kawasaki, Y. Osanai, M. Owada, T. Shimura, and S. Baba, and Drs. N. Nakano and T. Adachi, for their constructive discussions. In them, we acknowledge the useful comments and advice from Prof. M. Owada during data analysis. We thank Prof. M. Satish-Kumar as Chief Editor, Dr. T. Kato for editing of this article, and Dr. Y. Asahara as a reviewer and another anonymous reviewer, who gave us useful advice to revise the text. We would like to thank Editage (www.editage.com) for English language correcting. TM’s research was supported by MEXT KAKENHI Grant Number JP19540484. D.J.Dunkley’s research was supported financially by an OPUS grant UMO2021/43/B/ST10/03161 from the National Science Centre, Poland.
Supplementary Figure S1 and Tables S1-S3 are available online from https://doi.org/10.2465/jmps.231207.
