Acknowledgments

The Editorial Board would like to thank all the scientists who served as referees in 2021.

Announcement

(1) Vol.116 (2021) no. 6 is the last issue of the printed version of the Journal.

(2) From Vol. 117 (2022), only electronic version will be published, so please browse freely on the following site:

https://www.jstage.jst.go.jp/browse/jmps/-char/en

Pegmatites occur in the Kontum massif as NE–SW oriented large dykes ranging from a few meters to several tens of meters in width. They consist of 3 stages of formation based on the difference of petrographic characteristics and ages: the first–stage pegmatites are characterized by quartz (40–45 vol%) and K–feldspar (55–60 vol%), the second–stage pegmatites are composed of K–feldspar (70 vol%), quartz (25 vol%), muscovite (<5 vol%), and biotite (<5 vol%), and the third–stage pegmatites are characterized by K–feldspar (60 vol%) and quartz (25 vol%). K–feldspar has common grain sizes of 0.5–1.0 cm with milky white color and shows perthitic texture. The emplacement ages of pegmatites determined by the zircon LA–ICP–MS U–Pb method yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 448.2 ± 6.4 Ma (the first–stage), 269.4 ± 3.2 Ma (the second–stage), and 239 ± 3 Ma (the third–stage). The first–stage pegmatites coincide with the Ordovician–Silurian magmatism probably associated with the closure of an ancient oceanic basin within the Kontum massif. The second– and third–stage pegmatites can be linked with Permian–Triassic collisional magmatic activities. Host granitoids were also collected for petrographic investigations and age dating. Biotite–granite (Ben Giang–Que Son Complex) hosting the first– and second–stage pegmatites showed a magmatic age of 236.9 ± 2.9 Ma. Granites of the Van Canh Complex hosting the third–stage pegmatites yielded a similar age range between 244.2 ± 3.2 and 241.6 ± 1.7 Ma indicating that the third–stage pegmatite intruded shortly after the formation of the host granite. Based on quality assessments, samples from the second–stage pegmatites (~ 269 Ma) in the study area have the most applicable quality for ceramic manufacture and the technological samples fulfill Vietnam’s ceramic standards (VN 6598:2000 standards).

Granitoid magmatism is widespread in the Mongolian Altai, and it provides critical information to understand the crustal formation, evolution, and growth. This study reports newly investigated petrography, whole–rock geochemistry, and zircon U–Pb geochronology of granites and quartz syenites emplaced in the Mongolian Altai to investigate their sources and petrogenesis. Our results allow that granitoids, five petrological groups, and four geochronological stages from this study provide new information to understand the crustal formation and evolution of the Mongolian Altai. Geochemically, the group–I (Bt–Ms granite) and –II (Crd–Bt granite) have high–K calc–alkaline and peraluminous affinity, whereas group–III (Hbl–Bt granite) has calc–alkaline and metaluminous affinity. Group–IV (Bt quartz syenite) and –V (Kfs–porphyritic granite) show shoshonitic– to high–K calc–alkaline and metaluminous characteristics. The zircon U–Pb dating constraints the Devonian magmatic ages of 387–361 Ma for group–I, 369–353 Ma for group–II, and 366–356 Ma for group–III granites, whereas ~ 315 Ma for group–IV quartz syenite and 208–200 Ma for group–V granite. Groups–I and –II peraluminous granites have high–Th/Nb, and low–Ba/Th reflecting sources might be derived from sedimentary rocks, whereas group–III metaluminous granite has low–Th/Nb and high–Ba/Th reflecting a source that might be derived from gabbroic crustal material. Groups–I and –II peraluminous granites have similar geochemical characteristics, but their emplacement ages are different, which suggests they were formed from different magmas. Group–II peraluminous and group–III metaluminous granites were contemporaneous, and their Rb/Ba versus Rb/Sr correlation define a linear trend of the magma mixing process. It demonstrates that group–III metaluminous magma would carry heat to the crust, inducing partial melting of sedimentary rocks to produce group–II peraluminous granite. Finally, geochemical and geochronological data of groups–I, –II, and –III granites demonstrate that they were formed during the main orogenic activity in the Mongolian Altai, which is consistent with the timing of the main metamorphic event at Late Devonian to Early Carboniferous. Subordinated group–IV quartz syenite and group–V granite were probably formed in extensional to post–collisional environments after main orogenic activity in the Mongolian Altai.

The Earth’s core is believed to contain some light elements because it is 10% less dense than pure Fe under the corresponding pressure and temperature conditions. Hydrogen, a promising candidate among light elements, has phase relations and physical properties that have been investigated mainly for the Fe–H system. This study specifically examined an Fe–Si–H system using in–situ neutron diffraction experiments to investigate the site occupancy of deuterium of hcp–Fe_0.95Si_0.05 hydride at 14.7 GPa and 800 K. To date, this pressure condition is the highest for neutron diffraction experiments conducted at high pressure and high temperature, where crystal structure analysis has been conducted. Results of Rietveld refinement indicate hcp–Fe_0.95Si_0.05 hydride as having deuterium (D) occupancy of 0.24(2) exclusively at the interstitial octahedral site in the hcp lattice. The effect on the site occupancy of D by addition of 2.6 wt% Si into Fe (Fe_0.95Si_0.05) was negligible compared to results obtained from an earlier study of an Fe–D system (Machida et al., 2019).

Compositional variations of talc in peridotites and serpentinites could have significant implications for modeling of geochemical cycles involving the upper mantle but have been scarcely studied. We analyzed chemical compositions of prograde and retrograde talc and associated minerals in thermally metamorphosed serpentinites from Southwest Japan. The analyzed talc has variations of Si, Al, Mg, Fe, and Na contents. Most of the Si, Al, Mg, and Fe variations indicate mechanical mixing with serpentine and chlorite at a submicroscopic scale. Spatial distribution of talc–chlorite mixtures suggests their prograde metamorphic origin. Talc–serpentine mixtures could be formed by retrograde decomposition of talc–olivine assemblage and orthopyroxene at conditions of higher temperature and/or higher Si activity than serpentine–brucite mixtures, which are the typical products of serpentinization of olivine. Talc itself, regardless of prograde or retrograde origin, has compositional variations with Na enrichment as a likely result of solid solution or Na–mica mixing. The Na enrichment suggests that talc could be the most capable reservoir of Na in metamorphosed peridotites and serpentinites.

Index to Journal of Mineralogical and Petrological Sciences Vol. 116, No. 1-6

Author Index

Keyword Index

Journal of Mineralogical and Petrological Sciences

Volume 116, Number 1-6

February-December, 2021