蜿238ﾓ旅

第238回

日時平成14年6月２６日（水）１７：００より

場所東京工業大学理学部地球惑星科学教室会議室
石川台２号館３１５号室

講演者臼井　寛裕　（岡山大学固体地球研究センター）

講演題目 Origin and evolution of eclogite xenolith from the Colorado Plateau: Fate
of the subducted Farallon Plate

内容､ Since plate tectonic theory became widely accepted, eclogite xenoliths
brought to the Earth's surface through kimberlite pipes have been studied
intensively as a recycled material subducted deep into the
mantle. However, the eclogite xenoliths have hardly preserved their
initial petrographical and chemical features implanted by subduction
processes, due to their long residence in the mantle. Here we report the
conclusive evidences to support the eclogite xenolith from the Colorado
Plateau being originated in the fragments of subducted Farallon Plate
trailing in upper mantle since Late Cretaceous. We found high pressure
mineral, coesite, in garnet accompanied with lawsonite and omphacite in
eclogite xenoliths, indicating that they formed under significantly low
temperature environment such as inside of subducted oceanic
lithosphere. Moreover, ion probe U-Pb ages of micro zircons provide
discrete ages of 60 Ma and 35 Ma, representing the age of crystallization
during subduction-related metamorphism and the ages of eruption of host
magmas, respectively. These results provide an insight into the
geophysical observations which have focused on the relationship between
geometry of the subducted Farallon Plate and the record of arc magmatism on
the North American plate since Late Cretaceous.

地惑セミナーの写真、講演に対するコメント

やっぱ喉かわくよな（講演者の臼井先生）�

ジルコンがいいんです！

そうか、やっぱジルコンか

今年度三回目の地惑セミナーは、岡山大学固体地球研究センターで現在博士課程2年の臼井君に「Origin and evolution of eclogite xenolith from the Colorado Plateau: Fate of the subducted Farallon Plate」について講演していただきました。実は、彼は東工大出身で学部時代に我々(現D2)と一緒に勉強した仲なのです。

さて、講演は沈み込み帯での地殻-マントル間の物質循環を地学的に解明していこうという内容でした。具体的には、コロラド付近で取った海洋地殻起源と考えられるeclogite xenolithを調べる事によって、コロラドの下にあるFarallon Plateの進化を解明しようとしています。採取したeclogite xenolithにおいてU-Pbを利用した年代測定を行ない、その鉱物がいつ地表に出たか、いつ変成作用を受けだしたかやまた鉱物内をSIMSで分析した結果からその鉱物が辿った温度圧力進化、つまりその鉱物を含んだプレートが地表からどのくらい深い所を通ったのかなどがわかるようです。これらの分析を基にしてFarallon Plateの進化に関するシナリオを提案していました。小さな鉱物から大きなプレートの進化が推定できるということはまさに驚きであるような気がします。

最後に、彼の講演は全体としてコンパクトにまとめられており、全体像の掴みやすいわかりやすいものでした。彼は話しやすく楽しい方です。セミナー後の臼井君を囲んでの飲み会は大いに盛り上がりました。

第238回
日時	平成14年6月２６日（水）１７：００より
場所	東京工業大学理学部地球惑星科学教室会議室石川台２号館３１５号室
講演者	臼井　寛裕　（岡山大学固体地球研究センター）
講演題目	Origin and evolution of eclogite xenolith from the Colorado Plateau: Fate of the subducted Farallon Plate
内容､	Since plate tectonic theory became widely accepted, eclogite xenoliths brought to the Earth's surface through kimberlite pipes have been studied intensively as a recycled material subducted deep into the mantle. However, the eclogite xenoliths have hardly preserved their initial petrographical and chemical features implanted by subduction processes, due to their long residence in the mantle. Here we report the conclusive evidences to support the eclogite xenolith from the Colorado Plateau being originated in the fragments of subducted Farallon Plate trailing in upper mantle since Late Cretaceous. We found high pressure mineral, coesite, in garnet accompanied with lawsonite and omphacite in eclogite xenoliths, indicating that they formed under significantly low temperature environment such as inside of subducted oceanic lithosphere. Moreover, ion probe U-Pb ages of micro zircons provide discrete ages of 60 Ma and 35 Ma, representing the age of crystallization during subduction-related metamorphism and the ages of eruption of host magmas, respectively. These results provide an insight into the geophysical observations which have focused on the relationship between geometry of the subducted Farallon Plate and the record of arc magmatism on the North American plate since Late Cretaceous.