The 24th Global-COE International Frontier Seminar
January 11th 2012, 16:30-18:00
The meeting room (#486) 4F, Science Research Bldg 1
ALLUVIAL DIAMONDS FROM NORTH-EAST OF SIBERIAN PLATFORM:
EVIDENCE FOR FORMATION IN SUBDUCTION ENVIRONMENT
Prof. V.S. Shatsky
Institute of Mineralogy and Petrology, Novosibirsk, Russia
Novosibirsk State University,Novosibirsk, Russia
The alluvial diamonds from placers of North-East of Siberian platform was studied throughcharacterization of their external and internal morphologies, impurity defects, mineral andfluid/melt inclusions. Diamond morphologies included dodecahedroids, cuboids, octahedron/dodecahedroids and round forms. The internal structure and nitrogen aggregation variations
testify that significant proportion of round diamonds were formed in two generation, separated
by long time period. More than one hundred diamonds were polished to expose their mineralinclusions. In total 218 inclusions were exposed, among with 139 eclogite (garnet, omphacitic
clinopyroxene, coesite, K-feldspar, rutile, corundum) and 79 peridotite (olivine, pyrope garnet,clinopyroxene, ortopyroxene, chromite) paragenesis. Inclusions of eclogite suite are predominant(64%).A pyropic garnet inclusions plot within the harzburgite (n=5) and lerzholite (n=2) fields.
Elevated Na2O (0.1-0.3 wt%) is typical of eclogitic garnets. Inclusions of majoritic garnet ofultrabasic and eclogite parageneses were identified in two round diamonds. According to theexperimental data, the pressure at which majoritic garnet of harzburgitic paragenesis was formed
should exceed 7.5 GPa, whereas the formation of majoritic garnets of eclogitic paragenesis
required pressure exceeding 13 GPa. Diamonds show a range in carbon isotopic composition from-26 to -1‰ d13C (n=22) with a bimodal distribution. Peridotitic diamonds are restricted to values
between -9.6 and -1,0‰ whereas eclogitic diamonds have wide range of isotopic composition (4.5 to -26‰). Ion microprobe analyses of C isotopes composition reveal large variation withinsingle stone (up to 16‰ in δ13С). The presence of majoritic garnets in the studied diamondssuggests that a part of the placer diamonds was formed under sublithospheric conditions. The
isotopic composition of the studied diamonds containing majoritic garnets provides evidence forthe mantle source of carbon. At the same time, peculiarities in the garnet composition may
provide evidence for crystallization of diamonds in subducted rocks of the Earth’s crust. Multiple
inclusions from diamonds and isotopic composition of carbon are consistent with a mixing modelin which they result from the interaction of slab-derived melt/fluid with surrounding mantle.

Contact: T. Irifune
irifune@dpc.ehime-u.ac.jp
http://www.ehime-u.ac.jp/~grc

The 24th Global-COE International Frontier Seminar
January 11th 2012, 16:30-18:00
The meeting room (#486) 4F, Science Research Bldg 1

EVIDENCE FOR EVOLUTION OF GROWTH MEDIA IN SUPERDEEPDIAMONDS FROM SAO-LUIS (BRASIL)
Dr. D.A. Zedgenizov
Institute of Mineralogy and Petrology, Novosibirsk, Russia
Diamonds from Sao-Luis (Brazil) are known to be originated from the depths oftransition zone and lower mantle. In this study we consider some aspects of thecomposition and evolution of growth media for diamonds from this locality.CL imaging hasrevealed the complex growth history for most diamonds, reflecting their formation inseveral stages. Nitrogen content in an individual diamonds varies from several to 500 ppm.
An apparent tendency for the 3107 cm-1 peak intensity to increase with increasing thenitrogen content gives support to the idea that the conditions favouring the incorporationof nitrogen in these diamonds might also favour the incorporation of hydrogen. Specificfeature of diamonds from Sao-Luis is extremely high nitrogen aggregation state (90100 %B1). The set of luminescence centers N3, H3, H4, 490.7 is typical for all diamonds.
Radiation-induced centers with peaks at 536 and 576 nm are often observed. The total
range of carbon isotope composition in diamonds studied by SIMS makes up from -3.3 to 20.3 ‰ of d13C. Some diamonds show local variations of d13C between different growthzones (up to 7 ‰).The dominant inclusions in studied diamonds are CaSi-perovskite andAlSi-phases. MgSi- and CaTi-perovskites, ferropericlase, native iron, coesite and zircon havealso been found. Raman shift of coesite peak show high residual pressure (>3 GPa atambient temperature). FTIR study of some microinclusion-bearing diamonds showed thatwater and carbonates are not major components of diamond-forming fluids. LA-ICPMS
bulk composition has significant enrichment in Ca, Fe and Al and strong depletion in Mg.
Trace elements show general enrichment in Ti and V and depletion in Sr, LREE and Ni. Most
probable source for such environments might be fluids from deeply subducted rocks ofmetasomatized oceanic lithosphere.

Contact: T. Irifune
irifune@dpc.ehime-u.ac.jp
http://www.ehime-u.ac.jp/~grc