SR-ND ISOTOPIC AND CHEMICAL CHARACTERISTICS OF THE SILICIC MAGMA RESERVOIR OF THE AIRA PYROCLASTIC ERUPTION, SOUTHERN KYUSHU, JAPAN

Sr and Nd isotope and geochemical investigations were performed on a r emarkably homogeneous, high-silica rhyolite magma reservoir of the Air a pyroclastic eruption (22,000 years ago), southern Kyushu, Japan. The Aira caldera was formed by this eruption with four flow units (Osumi pumice fall, Tsumaya pryoclastic flow, Kamewarizaka breccia and Ito py roclastic flow). Quite narrow chemical compositions (e.g., 74.0-76.5 w t% of SiO2,) and Sr and Nd isotopic values (Sr-87/Sr-86 = 0.70584-0.70 599 and epsilon(Nd),, = -5.62 to -4.10) were detected for silicic pumi ces from the four units, with the exception of minor amounts of dark p umices in the units, The high Sr isotope ratios (0.7065-0.7076) for th e dark pumices clearly suggest a different origin from the silicic pum ices, Andesite to basalt lavas in pre-caldera (0.37-0.93 Ma) and post- caldera (historical) eruptions show lower Sr-87/Sr-86 (0.70465-0.70540 ) and higher epsilon(Nd) (-1.03 to + 0.96) values than those of the Ai ra silicic and dark pumices. Both andesites of pre-and post-caldera st ages are very similar in major-and trace-element characteristics and i sotope ratios, suggesting that the both andesites had a same source an d experienced the same process of magma generation (magma mixing betwe en basaltic and dacitic magmas). Elemental and isotopic signatures den y direct genetic relationships between the Aira pumices and pre-and po st-caldera lavas. Relatively upper levels of crust (middle-upper crust ) are assumed to have been involved for magma generation for the Aira silicic acid dark pumices, The Aira silicic magma was derived by parti al melting of a separate crust which had homogeneous chemistry and lim ited isotope compositions, while the magma for the Aira dark pumice wa s generated by AFC mixing process between the basement sedimentary roc ks and basaltic parental magma, or by partial melting of crustal mater ials which underlay the basement sediments. The silicic magma did not occupy an upper part of a large magma body with strong compositional z onation, but formed an independent magma body within the crust. The in put and mixing of the magma for dark pumices to the base of the Aira s ilicic magma reservoir might trigger the eruptions in the upper part o f the magma body and could produce a slight Sr isotope gradient in the reservoir, An extremely high thermal structure within the crust, whic h was caused by the uprise and accumulation of the basaltic magma, is presumed to have formed the large volume of silicic magma of the Aira stage. (C) 1998 Elsevier Science B.V.