西南日本,中国地方の新生代アルカリ玄武岩:マグマ進化について

取得学位：博士(理学)，学位授与番号：博甲第325号，学位授与年月日：平成11年9月30日,学位授与年：199

Origin of basaltic magmas of Perşani volcanic field, Romania: A combined whole 6 rock and mineral scale investigation

The Perşani volcanic field is a low-volume flux monogenetic volcanic field in the Carpathian–Pannonian region, 24 eastern-central Europe. Volcanic activity occurred intermittently from1200 ka to 600 ka, forming lava flow fields, 25 scoria cones andmaars. Selected basalts fromthe initial and younger active phaseswere investigated for major and 26 trace element contents and mineral compositions. Bulk compositions are close to those of the primitive magmas; 27 only 5–12% olivine and minor spinel fractionation occurred at 1300–1350 °C, followed by clinopyroxenes at about 28 1250 °C and 0.8–1.2 GPa. Melt generation occurred in the depth range from 85–90 km to 60 km. The estimated 29 mantle potential temperature, 1350–1420 °C, is the lowest in the Pannonian Basin. It suggests that no thermal 30 anomaly exists in the uppermantle beneath the Perşani area and that themaficmagmas were formed by decom- 31 pressionmelting under relatively thin continental lithosphere. Themantle source of themagmas could be slightly 32 heterogeneous, but is dominantly variously depleted MORB-source peridotite, as suggested by the olivine and 33 spinel composition. Based on the Cr-numbers of the spinels, two coherent compositional groups (0.38–0.45 and 34 0.23–0.32, respectively) can be distinguished that correspond to the older and younger volcanic products. This in- 35 dicates a change in themantle source region during the volcanic activity as also inferred from the bulk rockmajor 36 and trace element data. The younger basaltic magmas were generated by lower degree of melting, from a deeper 37 and compositionally slightly different mantle source compared to the older ones. The mantle source character of 38 the Perşanimagmas is akin to that ofmany other alkaline basalt volcanic fields in theMediterranean close to oro- 39 genic areas. The magma ascent rate is estimated based on compositional traverses across olivine xenocrysts using 40 variations of Ca content. Two heating events are recognized; the first one lasted about 1.3 years implying heating 41 of the lower lithosphere by the uprisingmagma,whereas the second one lasted only 4–5 days,whichcorresponds 42 to the time of magma ascent through the continental crust. The alkaline mafic volcanismin the Perşani volcanic 43 field could have occurred as a response to the formation of a narrow rupture in the lower lithosphere, possibly 44 as a far-field effect of the dripping of dense continental lithospheric material beneath the Vrancea zone. Upper 45 crustal extensional stress-field with reactivation of normal faults at the eastern margin of the Transylvanian 46 basin could enhance the rapid ascent of the mafic magmas

Tephrostratigraphy and provenance from IODP Expedition 352, Izu-Bonin arc: tracing tephra sources and volumes from the Oligocene to the Recent

Provenance studies of widely distributed tephras, integrated within a well-defined temporal framework, are important to deduce systematic changes in the source, scale, distribution and changes in regional explosive volcanism. Here, we establish a robust tephro-chronostratigraphy for a total of 157 marine tephra layers collected during IODP Expedition 352. We infer at least three major phases of highly explosive volcanism during Oligocene to Pleistocene time. Provenance analysis based on glass composition assigns 56 of the tephras to a Japan source, including correlations with 12 major and widespread tephra layers resulting from individual eruptions in Kyushu, Central Japan and North Japan between 115 ka and 3.5 Ma. The remaining 101 tephras are assigned to four source regions along the Izu-Bonin arc. One, of exclusively Oligocene age, is proximal to the Bonin Ridge islands; two reflect eruptions within the volcanic front and back-arc of the central Izu-Bonin arc, and a fourth region corresponds to the Northern Izu-Bonin arc source. First-order volume estimates imply eruptive magnitudes ranging from 6.3 to 7.6 for Japan-related eruptions and between 5.5 and 6.5 for IBM eruptions. Our results suggest tephras between 30 and 22 Ma that show a subtly different Izu-Bonin chemical signature compared to the recent arc. After a ∼11 m.y. gap in eruption, tephra supply from the Izu-Bonin arc predominates from 15 to 5 Ma, and finally a subequal mixture of tephra sources from the (palaeo)Honshu and Izu-Bonin arcs occurs within the last ∼5 Ma

Metal-saturated peridotite in the mantle wedge inferred from metal-bearing peridotite xenoliths from Avacha volcano, Kamchatka

Lithospheric mantle is inferred to be more oxidized than the asthenosphere, and mantle-wedge peridotites are characterized by high oxidation state relative to abyssal and continental peridotites due to addition of slab-derived fluids or melts. We found metals (native Ni, Fe silicides, native Fe and possible native Ti) from otherwise oxidized sub-arc mantle peridotite xenoliths from Avacha volcano, Kamchatka. This is contrary to the consensus and experimental results that the metals are stable only in deeper parts of the mantle (> 250 km). The metals from Avacha are different in chemistry and petrography from those in serpentinized peridotites. The Avacha metals are characteristically out of chemical equilibrium between individual grains as well as with surrounding peridotite minerals. This indicates their independent formation from different fluids. Some of the Avacha metals form inclusion trails with fluids and pyroxenes, leading to the inference that very local metal saturation resulted from rapid supply (‘flashing’) of reducing fluids from deeper levels. The fluids, possibly rich in H2, are formed by serpentinization at the cold base of the mantle wedge just above the slab, and they reduce overlying peridotites. We propose a metal-saturated peridotite layer, underlying the main oxidized portion, within the mantle wedge beneath the volcanic front to fore-arc region