第56回ジオダイナミクスセミナー
Geodynamics Seminar
“Deep crustal recycling and element fractionation
in subduction zones: the case of MORB”
講師：Robert Paul Rapp（GRC客員教授）
主催 ： 愛媛大学地球深部ダイナミクス研究センター
日時 ： ２００２年１２月２０日（金）午後17時〜
場所 ： 愛媛大学理学部講義棟 ２０１教室
要 旨
Subduction transports crustal rocks (MORB and marine and continental
sediments) from Earth's surface to depths into and beyond the mantle transition
zone. The distinctive geochemical fingerprint of deeply recycled crustal
material reappears in ocean island basalts (OIB) associated with mantle
plumes arising from the transition zone, or deeper in the lower mantle.
The "HIMU"-type OIB in particular is believed to form by melting
of a source containing 25% recycled oceanic crust (MORB) and 75% mantle
peridotite. Multi-anvil phase equilibria/melting experiments were conducted
at 3-16 GPa on a hydrothermally-altered, natural MORB, in order to characterize
the geochemical effects associated with slab dehydration and melting. Breakdown
of hydrous minerals such as amphibole, zoisite, phengite, and lawsonite
at 3-11 GPa produces silica-rich "granitoid" liquids at low-degrees
of melting. Ion and laser-ablation ICP-MS microprobe analyses reveal these
melts to be highly-enriched in incompatible elements (including Ba, Th,
U and K), and depleted in other elements (e.g., Ti, Nb, HREEs) that are
compatible in the crystalline residues of melting (garnet eclogite ->
a majoritic eclogite). Mass-balance considerations suggest that the chemically-depleted
residues of MORB dehydration melting cannot comprise the crustal component
in the HIMU source region. A more likely candidate is the mantle overlying
deeply subducted slabs, which would have been metasomatically-enriched
by MORB-derived granitic liquids emanating from the slab.
問い合わせ先：山崎 大輔 TEL （089）927-8408
E-mail yamazaki@sci.ehime-u.ac.jp
