Naoto Hirano

Volcanism on Earth is known to occur in three tectonic settings: divergent plate boundaries (such as mid-ocean ridges), convergent plate boundaries (such as island arcs), and hot spots. We report volcanism on the 135 million-year-old Pacific Plate not belonging to any of these categories. Small alkalic volcanoes form from small percent melts and originate in the asthenosphere, as implied by their trace element geochemistry and noble gas isotopic compositions. We propose that these small volcanoe

ABSTRACT On the western part of the Pacific Plate most seamounts formed during the Cretaceous period in the so‐called West Pacific Seamount Province (WPSP). On the northwestern part of the same plate, the Joban and Japanese Seamount Trail (JJST) are also composed of Early Cretaceous seamounts. However, two new groups of knolls were recently discovered during multibeam surveys on the Pacific Plate along the Japan Trench. One group consists of circular knolls that are flat‐topped in shape and corr

One extremely young volcano (0.05–1 Ma) and other young volcanoes (1.8, 4.2, 6.0, and 8.5 Ma) composed of strongly alkaline magma were recently discovered on the abyssal plain of the Early Cretaceous (135 Ma) Pacific Plate. These volcanoes were dubbed “petit-spots”. The petit-spot volcanic province represents more than 8 Myr of activity over a large area (~600 km along the direction of plate motion), but with a relatively small volume of magma production, thus indicating a small supply of heat i

Alkali pillow basalts were collected from the toe of the oceanward slope of the northern Japan Trench. These alkali‐basalts formed as a result of a low degree of partial melting of Pacific Ocean mantle and rapid rise of the magma (no fractionation in shallow magma chambers). Reconstructing Pacific Plate motion based on 40 Ar‐ 39 Ar age dates of 5.95±0.31 Ma for these basalts indicates that they erupted outboard of outer swell or forebulge of the Japan Trench in the NW Pacific. We suggest that th

In 2009, petit-spot submarine volcanoes were discovered off the oceanward slope of the central Chile trench, offshore from Valparaiso, Chile, at around 33°S. Ar-Ar dating of mugearite and alkali-basalt from the volcanoes yields ages of 10.11 ± 0.22 Ma and 6.69 ± 0.88 Ma, respectively. Back-calculations of plate motion along the present absolute movement direction of the Nazca Plate, conducted using the Ar-Ar age data, indicate that eruption occurred above a zone of plate flexure. The back-calcul

Abstract Petit-spot volcanoes provide a unique opportunity to directly examine the structure and geochemical composition of the lithospheric and asthenospheric mantle beneath the subducting oceanic crust. Currently, petit-spot volcanos—formed in response to the flexure of subducting oceanic plates—are known to exist off the Japan, Java, Chile and Mariana trenches, however, the difficulties associated with detecting them leaves the possibility that they may be more widespread along outer rise of

Abstract The West Pacific Seamount Province (WPSP) represents a series of short‐lived Cretaceous hotspot tracks. However, no intraplate volcanoes in advance of petit‐spot volcanism erupted near a trench have been identified after the formation of the WPSP on the western Pacific Plate. This study reports new ages for Paleogene volcanic edifices within the northern WPSP, specifically the Ogasawara Plateau and related ridges, and Minamitorishima Island. These Paleogene ages are the first reported f

Abstract The Mineoka ophiolite in the southern Boso Peninsula is situated in a unique tectonic setting in the collisional zone between the Izu and Honshu arcs in Japan. The ophiolitic rocks are composed mainly of tholeiitic pillow basalts and dolerites, alkali-basaltic sheet flows, and calc-alkaline dioritic to gabbroic rocks. The tholeiitic basalts show variable trace element compositions ranging from mid-ocean ridge basalt to island-arc basalt, whereas the alkali-basalts have a within-plate af

Abstract. We present geochemical and 40Ar∕39Ar age data for a peridotite xenolith-bearing basalt dredged from the Seifu Seamount (SSM basalt) in the northeast Tsushima Basin, southwest Sea of Japan. An 40Ar∕39Ar plateau age of 8.33±0.15 Ma (2σ) was obtained for the SSM basalt, indicating that it erupted shortly after the termination of back-arc spreading in the Sea of Japan. The SSM basalt is a high-K to shoshonitic alkali basalt that is characterized by light rare earth element enrichment. The

嶺岡帯,鴨川市峠においてアルカリ玄武岩溶岩中に噴出時に取り込まれたと考えられる砂岩を発見した.この砂岩はおもに,斜長石・ダトー石・石英・および珪長質火山岩片から構成され,ダトー石を除けば嶺岡層群神塚層または愛宕山層の砂岩の構成粒子の鉱物組み合わせに似ている.アルカリ玄武岩の活動場は,このような陸源性砂岩が供給されるような場であった事が明らかとなった.アルカリ玄武岩は,その産状と化学組成から,海山を構成していたと考えられるが,アルカリ玄武岩の噴出場と含まれている陸源性砂岩の堆積場は,おそらく海洋プレートと島弧もしくは大陸が会合する収束境界付近であった見込みが高い.アルカリ玄武岩の放射年代は,嶺岡帯のオフィオリティック岩体が本州弧側へ定置したとされる時代の直前であり,本論の結果と調和的である.本研究において嶺岡帯の形成過程を論じるにおいて新たな見地が得られた.

Abstract Areas of old and cold oceanic plate lack conventional volcanism and have been assumed to be devoid of submarine hydrothermal activity. However, petit-spot volcanoes are common in areas of flexure of such oceanic plates. Here, we report hydrothermal ferromanganese oxides dredged from the vicinity of a petit-spot volcano at 5.7 km water depth in an area of oceanic plate flexure east of the Japan Trench. The bulk chemical, lead isotopic and mineralogical compositions of the samples indicat