18 th GRC International Frontier Seminar
Nature and Chemical Dynamics of Mantle Heterogeneity: The Role of CO2-rich Melts and Buoyant Residues from the Lower Mantle
Prof. Kenneth D. Collerson
The University of Queensland, Australia
16 May 2008 17:00-18:00
Room 101, Kogi-to Bldg, Faculty of Science, Ehime University
The origin of Earth’s depleted and enriched mantle reservoirs represents
an important chemical geodynamic paradox. Their character has largely been
constrained using radiogenic and rare gas isotopes, and trace element compositions
of mid-ocean ridge basalts (MORB), ocean island basalts (OIBs), kimberlites,
carbonatites, and lamproites. Explanations for these chemistries include:
(1) extraction of basalt to yield depleted MORB mantle (DMM); (2) involvement
of sub-continental lithospheric mantle (SCLM); (3) recycling of subducted
oceanic lithosphere; (4) sediment subduction; or (5) role of an enigmatic
lower mantle component, but except for DMM, no unique multi-isotope explanation
exists for the other reservoirs. By modelling melt formation I show that
147Sm/144Nd, 176Lu/177Hf, 187Re/188Os, and 238U/204Pb isotope ratios required
to yield isotopic compositions and correlations characteristic of HIMU
(high μ - where μ = 238U/204Pb) and EM (enriched mantle) OIBs, kimberlites
and carbonatites are generated in the lower mantle. HIMU is the buoyant
residue of melting, whereas EM is dense silicate melt. This melt would
accumulate at the base of the lower mantle and thus would therefore require
mechanical entrainment in thermochemical plumes to rise to the site of
subsequent magmagenesis. However, if these melts contain > 1% CO2 they
become less dense. Thus, transfer of mantle isotope heterogeneity, with
primitive rare gas isotope compositions in plume-generated HIMU and EM
magmas reflects upwelling of buoyant melt residues and CO2-bearing silicate
melts, as well as the mechanical entrainment of silicate melts from D”.
As the HIMU and EM magma source is less degassed than DMM, this region
must occur below ca. 1700 km, the depth of apparent penetration and entrainment
of subducted oceanic lithosphere. Trace element melt/residue modelling
thus resolves a long-standing conjecture, providing a unique multi-isotope
solution, explaining formation of HIMU and EM as a consequence of carbonate
induced melting of primitive and isolated regions within the lower mantle.
The deep carbon cycle therefore plays an important role in the chemical
evolution of the Earth.
お問い合わせ先： irifune@dpc.ehime-u.ac.jp TEL 089-927-9645
詳細情報：http://www.ehime-u.ac.jp/~grc/
主催：愛媛大学地球深部ダイナミクス研究センター
