Official journal of Japan Association of Mineralogical Sciences (JAMS), focusing on mineralogical and petrological sciences and their related fields. Journal of Mineralogical and Petrological Sciences (JMPS) is the successor journal to both “Journal of Mineralogy, Petrology and Economic Geology” and “Mineralogical Journal”.

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Previous reports on Nové Dvory garnet peridotites indicated that they lacked primary spinel, and prograde metamorphism evidence was uncommon. This study revealed the presence of Al– and Cr–rich spinels, clinopyroxenes with a lower content of Na and Fe than what has been previously reported, and chemical heterogeneity in garnet in Nové Dvory garnet peridotites. Cr–poor (0.06–0.12 a.p.f.u.) and Cr–rich (0.10–0.27 a.p.f.u.) garnet populations were identified, and they had contrasting coronas around garnet. The garnet peridotite samples were classified into three types on the basis of the chemical composition of garnet and constituent minerals: type A that includes Cr–rich spinel and Cr–rich garnet; type B that includes Cr–poor garnet and no spinel; and type C that includes both Al– and Cr–rich spinel and both Cr–rich and Cr–poor garnet. The finding of spinel relics {Cr# = [100 Cr/(Cr + Al)] ~ 60–70} in garnet from type A peridotite suggests that the Nové Dvory peridotite body may have been located at relatively shallow depths prior to the ultrahigh–pressure metamorphic stage of >4 GPa. Suitable geothermobarometers were used to establish the P–T history of the rocks. The core compositions of clinopyroxene inclusions in garnet and host garnet yielded 978–1002 °C, 4.87–5.12 GPa (type B) and 1034 °C, 4.93 GPa (type C); stage I. Garnet porphyroblasts in type A peridotite lack clinopyroxene inclusions. The core compositions of garnet and pyroxenes yielded 1005–1072 °C, 4.42–4.46 GPa (type A); stage II. The innermost garnet rims and cores of matrix pyroxenes surrounding garnet yielded 1222–1325 °C, 5.03–5.67 GPa (type B) and 1189–1267 °C, 5.59–6.97 GPa (type C); stage II. Thus, Nové Dvory peridotite has experienced an increase in the P–T conditions, and its chemical heterogeneity (especially in the Cr content of garnet) in type C peridotites may have been created by the mechanical mixing of different rock types (i.e., Cr–rich and Cr–poor types) during the compression and/or decompression stage(s).

Awaruite (Ni_2–3Fe) is a natural occurring Ni–Fe alloy in serpentinite, which represents a better candidate to assess Pb isotope signatures in the mantle wedge since the concentration of Pb in awaruite is almost ten times higher than that in serpentine minerals. Revisiting so–called josephinite from the Josephine Ophiolite confirmed that josephinite is characterized by aggregates of awaruite with minor Ni–arsenide. The Raman spectrum obtained from the josephinite–hosting serpentinite shows diagnostic peaks of antigorite, suggesting josephinite might have formed under stability field of antigorite. Using a stepwise leaching and partial dissolution method, we obtained Pb isotope ratios of josephinite by TIMS. Since all ratios converged to a homogeneous value towards the later steps of the partial dissolution, this allowed to calculate weighted mean values that give precise Pb isotope ratios: ²⁰⁶Pb/²⁰⁴Pb = 18.3283 ± 0.0020 (MSWD = 0.49), ²⁰⁷Pb/²⁰⁴Pb = 15.5645 ± 0.0020 (MSWD = 0.36), and ²⁰⁸Pb/²⁰⁴Pb = 38.0723 ± 0.0061 (MSWD = 0.50); these values can be evaluated as one of the reference Pb isotope ratios in serpentinites from supra–subduction zone ophiolite. The newly obtained Pb isotope ratios of josephinite are consistent with the previous reported isotope ratios, which are characterized by enriched ²⁰⁷Pb/²⁰⁴Pb ratio with MORB–source like ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios. Although these Pb isotope features interpreted as a reflection of arc magmatism in the previous study, the presence of Ni–arsenide and enriched ²⁰⁷Pb/²⁰⁴Pb ratios may indicate an involvement of As–rich fluids derived from slab sediments.

The Hidaka belt in Central Hokkaido, Japan, consists of an early Paleogene subduction complex, referred to as the Hidaka Supergroup, dominated by clastic rocks. The southern area of the Hidaka Supergroup is referred to as the Nakanogawa Group, which gradually leads to the high–temperature Hidaka metamorphic belt in the western part. We collected 17 samples of greenstone from the entire Hidaka belt and examined their whole–rock major and trace element geochemistry. Including those described in previous reports, three distinct types of greenstone exist in the Hidaka belt. Type 1 greenstone is an ocean island basalt–type greenstone. The multi–element and rare earth element (REE) patterns for this type of greenstone show a steep slope up to the left, with Ti/V > 62 and Zr/Nb < 15. Type 2 greenstone is a mid–ocean ridge basalt (MORB)–type greenstone that shows relatively flat chondrite–normalized REE patterns and a gentle slope up to the left on the normal–MORB–normalized multi–element patterns with Ti/V = 26–53 and Zr/Nb = 21–117. Type 3 greenstone shows multi–element and REE patterns similar to those of Type 1, but with a clear relative depletion of Nb, Ta, and Ti. In addition, its Ti–V relations are similar to those of Type 2 greenstone. Type 1 and Type 3 greenstones occur only in the Nakanogawa Group. Type 2 greenstone is mostly distributed in the northern the Hidaka Supergroup. Type 1 and Type 3 greenstones were generated by igneous activity on the Izanagi Plate, which was being subducted during the formation of the subduction complex of the Hidaka belt. Type 2 greenstone is interpreted as a product of a spreading axis that was active during the formation of the same subduction complex. Whereas Type 2 greenstone has been regarded as having a typical MORB–like geochemical signature, our results show slightly different, Indian Ocean MORB–type trace element patterns. These geochemical signatures are different than those of the amphibolites in the Hidaka metamorphic belt. The protolith of the amphibolite is not equivalent to the Type 2 greenstones and is probably an accreted fragment of an older oceanic plate. Type 2 greenstone was presumably generated from upper mantle with an Indian mantle–like geochemical signature during the Izanagi–Pacific ridge subduction on the western margins of the Pacific Ocean around 48 Ma.

The reported discordant and anomalously old K–Ar (⁴⁰Ar/³⁹Ar) phengitic white mica ages from collisional orogenic belts are due to the fact that white micas in continental lithologies are not reset completely during high– to ultrahigh–pressure (HP–UHP) metamorphism because the closure temperature of white mica is much higher than the generally accepted value, approximately 600 °C. On the other hand, phengites in HP–UHP schists experience deformation–induced recrystallization during exhumation of the host lithology. The radiogenic argon is released from the deformed phengite, as documented by comparison of the in situ ⁴⁰Ar/³⁹Ar dating of phengite included in rigid garnet and of stretched phengite in the matrix. These non–resetting and argon–release phenomena give inconsistent phengitic white mica ages in metamorphosed continental lithologies. The heterogeneity in the deformational process due to differences in lithological compositions, local domains or even within single mica crystals results in inconsistent ages, as documented from the in situ ⁴⁰Ar/³⁹Ar dating of the deformed micas. The Sanbagawa HP schist belt and Lago di Cignana HP–UHP units both consist of metamorphosed oceanic lithologies that usually record only a single metamorphic cycle and have phengites without any inherited excess argon. The duration of deformation during exhumation spans from the peak metamorphism to the end of deformation in the crust, making it possible to estimate the exhumation rates of the metamorphic sequences. The low exhumation rates (<6 mm/y) of the Sanbagawa belt suggest a slow strain rate during rock deformation, resulting in a ‘slow schist’ sequence with a recumbent fold structure. The rapid exhumation rates (<26 mm/y) of Lago di Cignana suggest a high strain rate during rock deformations, resulting in a ‘fast schist’ sequence consisting of several units with fault–bounded contacts. The Lago di Cignana UHP unit, which underwent the highest exhumation rate, could indicate a subsequent continental collision event, whereas the Sanbagawa belt did not experience a subsequent continental collision event.

Powder sample of GTS–type sodium titanosilicate (Na–GTS) was prepared using a hydrothermal method. The Er³⁺–exchanged forms [Na_4(1−x)Er_4x/3Ti₄O₄(SiO₄)₃·nH₂O] of Na–GTS with the compositions up to x = 0.96 were obtained by shaking the single–phase sample of Na–GTS in the ErCl₃ aqueous solutions (25 mL, 0.01–0.5 M) at 25 and 60 °C for 6 h. The Er³⁺–exchange experiments revealed that the Er³⁺–exchange amounts (x) increase with increasing the concentration of ErCl₃ aqueous solutions and the higher treatment temperature more effectively promotes ion–exchange. Thermogravimetry–differential thermal analysis (TG–DTA) measurements showed that the exchange of Na⁺ for Er³⁺ decreases the dehydration temperature and the H₂O content. The simulation of powder X–ray diffraction (XRD) patterns suggests that Er³⁺ ions occupy both 4e and 6g sites in the assumed psuedocubic structure.

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