HR: 1340h
AN: V43D-1643    [Abstracts]
TI: Melt Production History and Thermal Structure of Upwelling Mantle Constrained by the Kita- Matsuura Basalt, Southwestern Japan
AU: * Sakuyama, T
EM: tetsuya-saku@eps.s.u-tokyo.ac.jp
AF: Dept of Earth and Planetary Science, Univ. of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113- 0033, Japan
AU: Nakai, S
EM: snakai@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Sumino, H
EM: sumino@eqchem.s.u-tokyo.ac.jp
AF: Laboratory for Earthquake Chemistry, Univ. of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113- 0033, Japan
AU: Ozawa, K
EM: ozawa@eps.s.u-tokyo.ac.jp
AF: Dept of Earth and Planetary Science, Univ. of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113- 0033, Japan
AB: Temporal change of the amount of melt produced in an upwelling mantle is the most important information relevant to the basalt genesis and the dynamics of melting in the upper mantle. Although this issue has been addressed by experimental and thermodynamic approaches, convincing petrologic constraints from natural samples are still quite few. If an upwelling mantle melts fractionally and quickly separates melt to the Earthfs surface without pondering and aggregation on its way up, basalt magma, particularly less differentiated alkaline basalt, is potentially a good tracer of the melting history. We made systematic geological, petrologic, and geochemical investigations on an intraplate Cenozoic alkaline basaltic volcanism (Kita-Matsuura basalt) in southwestern Japan to reveal the melting history in the upper mantle on the time scale of ~2Myr and the horizontal scale of 35km. The volcanism initiated from mildly alkaline basalt (low- SiO₂ group) followed by sub-alkaline basalt (medium to high- SiO₂ group) in the western and central sections, while the eastern section produced mildly alkaline basalt (low- SiO₂ group) almost all the way up to the uppermost horizon. Each SiO₂ group is clearly distinguished by a specific assemblage of fractionated crystals to produce the major element variation, which can be explained neither by crystal fractionation nor crustal assimilation to each other. Average segregation depths of estimated primary melts for each group were estimated by comparing to anhydrous melting experiments; they are 3.1-2.9, 2.8-2.6, and 2.0-1.8 GPa for low-, medium-, and high-SiO₂ groups respectively. Major element variations such as K₂O, TiO₂, Na₂O, and Al₂O₃/TiO₂ of primary melts suggest increase of melting degree from low- to high-SiO₂ group. A linear relationship between Al₂O₃/TiO₂ and melting degree based on compilation of peridotite melting experiments enables us to estimate differences of melting degree between low- and medium-SiO₂ groups and between medium- and high-SiO₂ groups equally as ~6±3wt%. The chondrite-normalized REE patterns of primary melts show strong enrichment of LREE with almost linear and variable inclination from LREE to HREE. Trace element variations cannot be explained by batch, fractional, accumulate, or stepwise melting of any depleted MORB mantle or primitive mantle, requiring near fractional melting of an enriched mantle in the garnet to spinel stability field. Combination of melting pressure and relative degree of melting of the three lava groups shows that the melt production rate dF/dP is 1.7 ~ 2.4 %/kbar for low -SiO₂ group and 0.7 %/kbar for high-SiO₂ group, respectively. Furthermore, the temporal and spatial variations of melting parameters inferred from both the major and trace elements show that a diapirically upwelling mantle with arched isotherms melted progressively to generate this basaltic volcanism. From these results, we conclude that melt production rate decreased during upwelling of the mantle, which is contrasting to increase of production rate for MORB generation as mantle ascends (e.g., Asimow et al., 1997; Yang et al., 1998). The decrease of melt production rate of a diapirically upwelling mantle shown from natural sample for the first time suggests that melting process is different from an ideal steady state adiabatic melting.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting