第204回 　
日時 平成11年10月27日（水）１7:00より
場所 東京工業大学石川台２号館315号室（地球惑星科学教室会議室）
講 演 者： David H. Green（ANU:オーストラリア国立大学）
講演題目： The Lherzolite Solidus: Magmas from Major Melting and Incipient Melting Regimes.
内容： Solidus phase assemblages and temperatures and variation of liquid compositions and residual phase compositions have been experimentally determined for enriched lherzolite (Hawaiian Pyrolite), fertile lherzolite (MORB Pyrolite) and refractory lherzolite (Tinaquillo Lherzolite). The data can be used to define liquid compositions near the solidus in models of upwelling and compositionally varying mantle lherzolite. The presence of minor H2O and CO2 in mantle derived magmas requires an incipient melting regime at temperatures below the lherzolite solidus. These encompass sthenospheric conditions or deep levels of upwelling in mid-ocean ridge or plume locations.
The compositions of incipient melts have also been defined by experimental studies of intraplate, xenolith bearing basalts, constraining models of migration of such melts within a melt column. At 1 GPa we have determined the compositional range for liquids from the albite + enstatite + olivine (Fo90) cotectic with plagioclase from An18 to An80 (limit of plagioclase stability at the anhydrous solidus at 1 GPa). Liquids are nepheline-normative for sodic and intermediate plagioclase and hypersthene normative for calcic plagioclase at the solidus.
Primitive MORB do not lie on or near the olivine + enstatite + diopside + plagioclase cotectic at 1 GPa, nor can they be derived from pooling of near solidus melts from fertile or enriched sources at deeper levels (in equilibrium with garnet) together with near solidus melts from refractory sources at shallow levels in upwelling mantle. The major element characteristics of primitive MORB are those of batch melts (i.e. high melt retention in upwelling mantle plumes) segregating from residual lherzolite at 1.0 to 2.3 GPa, and mostly at 1.5 to 2.0 GPa. Examination of primitive/parental magmas, and their phenocrysts, from mid-ocean ridge and plume volcanic sequences permits reconstruction of their temperatures and sources. The modern mantle has a potential temperature of 1430 to 1450C and the buoyancy anomaly at >100 km beneath the Hawaiian hot-spot is compositional and not thermal in origin.

Green教授が共同研究者たちとこれまで取り組んできた実験岩石学・地球化学的研究成果に基づいて、玄武岩質マグマおよびカーボナタイト質マグマの生成とそれに関係する上部マントルプロセスを展望する。
特に中央海嶺玄武岩や洪水玄武岩の起源や島弧マグマの深部プロセスについてもこれまでに蓄積されたデータは如何なる制約を与えているのか具体的事例について言及する予定である．

Green 教授について：
オーストラリア国立大学（キャンベラ）のDavid H. Green教授は，A. E. Ringwoodと共にオーストラリアにおける実験岩石学の草分け的役割をにない，「マグマ生成」と「地殻深部〜上部マントルプロセス」に関する数多くの実験を行ってきた．現在，RSES（地球科学研究院）のDirectorをつとめている世界的に著名な科学者である。