Masaya Miyoshi

Volcano-tectonic interactions during rapid plate-boundary evolution in the Kyushu region, SW Japan

Evolution of the local plate tectonic and volcanic system relationship at Kyushu Island is defined by major changes in tectonics and volcanic style at ca. 15, 10, 6, and 2 Ma. Plate reconstructions presented here suggest that prior to 15 Ma, the Pacific plate subduction dominated Kyushu tectonics. From 15 to 6 Ma, the evolving relative plate motions shifted the triple junction between the Pacific plate, Philippine Sea plate, and southwest Japan northwards, so that the Philippine Sea plate was subducted beneath Kyushu. We suggest that a lack of subduction-related volcanism from 10 to 6 Ma is due to shallow subduction of the young Shikoku Basin lithosphere. By 6–5 Ma, changes in the Philippine Sea plate motion led to more rapid, nearly trench-normal, subduction of the Eocene west Philippine Basin crust beneath Kyushu. This model is supported by an increase in arc-like geochemistry of lavas since ca. 6.5 Ma. Subduction of fluid-rich features such as the Kyushu-Palau ridge introduced large volumes of fluids into the Kyushu arc system, leading to voluminous volcanism across Kyushu, focused particularly in areas where the ridge subduction occurs in tandem with local extensional tectonics. Key issues, such as the timing of Izu arc collision with central Japan and the history of motion of the Philippine Sea plate, are reassessed here, resulting in a model that favors Izu arc–central Japan collision at ca. 8–6 Ma, rather than the more widely accepted date of ca. 15 Ma.

Lower crustal H2O controls on the formation of adakitic melts

At volcanic arcs, fluids released from the subducting slab lower the solidus of the mantle wedge and cause melting. Furthermore, slab melts may infiltrate the mantle wedge, and have been suggested to generate adakitic (residual garnet) signatures at some arc volcanoes. However, experimental work indicates that the garnet stability field will expand in the lower overriding crust in the presence of somewhat less hydrous melts, suggesting that such signatures may also develop at crustal levels. Here we use geothermometry and plagioclase hygrometry of mafic eruptives from southwest Japan to demonstrate that the adakitic compositions of associated intermediate magmas are of lower crustal origin due to a decrease in the water content of parental melts, and are not generated by partial melting of the eclogitic subducting slab at elevated temperatures. Lower crustal melt evolution at reduced water contents may represent an important process for generating adakitic signatures in all tectonic settings that have previously been considered to enhance slab melting. Our results demonstrate that magmatic water plays a key role in the differentiation of arc magmas in modern and ancient subduction settings.

Crystal uptake into aphyric arc melts: insights from two-pyroxene pseudo-decompression paths, plagioclase hygrometry, and measurement of hydrogen in olivines from mafic volcanics of SW Japan

Abstract Minerals of mafic rocks from the SW Japan arc have been studied to deduce P–T–XH2O conditions and their variations in mafic arc magmas. Two-pyroxene thermobarometry of magmas from several volcanoes yields constant temperatures and variable pressures. MELTS fractional crystallization modelling is employed to show that such ‘pseudo-decompression paths’ (PDPs) are artefacts that derive from uptake of pyroxene antecrysts formed at a range of crustal levels by isobaric cooling of previously intruded mafic melts. It is shown that PDPs can be used to constrain oxygen fugacities and initial water contents of the intruded magmas. These constraints, and plagioclase hygrometry, indicate that initial melt H2O contents change systematically along the SW Japan arc. Direct determination of hydrogen in olivine by secondary ion mass spectrometry yields consistently low olivine H2O contents of 11±4 ppm (1σ), with little, if any, along-arc variations. MELTS modelling indicates that crystallization of calcic plagioclase and olivine dominantly occurs during upper crustal differentiation of mafic melts. The combined data indicate that aphyric melts are released from the mantle wedge, taking up most if not all crystals from previously intruded plutonic rocks during rapid magma ascent to the surface.

Along-arc geochemical variations in Quaternary magmas of northern Kyushu Island, Japan

Abstract We analysed major and trace elements and Sr–Nd–Pb isotope ratios of Himeshima, Futagoyama and Aso volcanoes, northern Kyushu, Japan. The purpose of the study was to investigate the petrogenesis of Quaternary arc magmas of the northern Kyushu arc, Japan, through along-strike variations. The northern Kyushu arc is underlain by the Philippine Sea Plate, which is divided by the Kyushu–Palau Ridge into the Palaeogene West Philippine Basin segment (60–40 Ma) and the Miocene Shikoku Basin segment (26–15 Ma). The Sr/Y ratios of the arc lavas decrease from north to south along the volcanic front. Mixing relations in Sr–Nd–Pb isotope space suggest recycling of the subducted slab materials from the Philippine Sea Plate to the arc. The 87Sr/86Sr ratios decrease with increasing Sr/Y and SiO2, which argues against a genetic link of fractional crystallization between adakites and basaltic magmas. The observations further suggest that partial melting of the hot and young Shikoku Basin slab produces the high Sr/Y component visible in the arc magmas in the north, whereas dehydration of the older West Philippine Basin slab produces the low Sr/Y arc magmas in the south.

Paleomagnetic investigation of rhyolite lava: Is rhyolite with clearly marked flow structure a high-fidelity geomagnetic field recorder?

Rhyolite is a common volcanic rock; however, few studies have focused on the remanent magnetization of rhyolite lava, and few paleomagnetic studies have successfully investigated rhyolite lavas. We suspect that problems associated with paleomagnetic studies of rhyolite may be due to the nearly ubiquitous flow structure in rhyolite lava. In this study, we examined a thick rhyolite lava flow with clearly marked flow structure to assess its ability to record a consistent paleomagnetic direction, using material penetrated by two drill cores. Progressive thermal demagnetization isolated two magnetization components. A high-temperature component from each of the two cores yields inclinations that differ from each other. The low-temperature component had those that agreed with each other, and were also consistent with the direction expected from a geocentric axial dipole field. The modification of direction of the high-temperature component may be explained by post-magnetization acquisition tilting. The development of flow structure also leads to distortion of directions of the component, which is observed at stratigraphic positions where the volume fraction of light-colored parts of the flow structure >30%. In the case of silicic lava, the low-temperature component may retain directions parallel to the ambient field direction at the time of lava emplacement.

Thermoluminescence age of quartz xenocrysts in basaltic lava from Oninomi monogenetic volcano, northern Kyushu, Japan

We determined the eruption age of basaltic rocks by application of thermoluminescence (TL) method, which is often used for TL dating, to quartz. Mafic magma only rarely includes quartz because of their mutual disequilibration. The basaltic lavas reported herein include quartz as xenocrysts, as corroborated by their rounded or anhedral shape.The basaltic lava used for this study is from the Oninomi monogenetic volcano in northern Kyushu, Japan. The volcano eruption was estimated as occurring 7.3–29 ka because the lava exists between two widespread tephras: Aira-Tanzawa ash (26–29 ka) and Kikai-Akahoya ash (7.3 ka). We succeed-ed in collecting ca. 200 mg of quartz by decomposition of 30 kg of the lava samples. TL measurements for the lava indicate the eruption age as 15.8 ± 2.5 ka, which is fairly consistent with the stratigraphical estimation. Although the TL method has played a considerable part in constraining the timescale of Quaternary events, its application has been limited to silicic samples. The present result demonstrates the availability of quartz for dating even of mafic rock.

Availability and Problem of Environmental Education Using Magma Formation Program; Comparative Analysis for Students Living with or without Volcanic Activity: ― 活動する火山が在る地域と無い地域での実践比較 ―

私たちの生活環境において、火山噴火はそれほ ど身近な現象として捉えられていないと思われ る。しかし、日本の地表のおよそ40%は火成岩 に覆われているため(村田・鹿野 1995)、日本の 環境教育において火山噴火を対象とした教材は不 可欠である。火山噴火が日常生活に与える影響は 正負両面において極めて大きい。正の影響では、 風光明媚な地形である富士山の世界遺産登録や日 本各地のジオパークへの登録、地熱活動による温 泉やエネルギーとしての地熱発電などが挙げられ る。これに対して、負の影響は、雲仙普賢岳や三 宅島、有珠山の火山噴火に見られるように物心両 面に及ぶ甚大な被害が想起される。火山噴火は台 風や豪雨現象などとは違って、数十年から数百年 単位の時間スケールで活動を繰り返すため、これ に備えるには長期的なスケールでの心構えが必要 であり、かつ突発的な活動に対処するためには、 日常的にそして継続的な教育活動が必要と考え る。 火山噴火をテーマとした教育を持続発展可能な 社会の形成のために用いるには、初等・中等の教 育現場における活動がまずは可能であろう。小学 校や中学校のカリキュラムにおける火山噴火に関 する学習としては、6年生理科の単元で「大地の つくりと変化」(文部科学省 2008a)、中学校理科 第2分野で単元「大地の成り立ちと変化」(文部 科学省 2008b)の項目が設けられている。しかし、 これらの授業で火山に関心を持ち、実感を持った 理解を目指した授業展開を可能にする教材は少な い。特に、小学校における「大地のつくりと変化」 単元において、児童が火山噴火を主体的な問題と して理解を深められる授業展開は難しい。火山噴 火を対象とした教材としては、映像によるものや キッチン火山実験などがある。また、廃油を用い 資 料 火山を主眼とする環境学習の有効性と課題