HR: 0800h
AN: U21B-0427    [Abstracts]
TI: In Search of the Continental Mantle End Member
AU: * Richardson, S H
EM: steve.richardson@uct.ac.za
AF: Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa
AU: Shirey, S B
EM: shirey@dtm.ciw.edu
AF: Carnegie Institution of Washington - DTM, 5241 Broad Branch Rd, NW, Washington, DC 20015, United States
AU: Carlson, R W
EM: carlson@dtm.ciw.edu
AF: Carnegie Institution of Washington - DTM, 5241 Broad Branch Rd, NW, Washington, DC 20015, United States
AU: Hart, S R
EM: shart@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AB: The advent of Chemical Geodynamics some 25 years ago spawned a rush of radiogenic isotope tracer studies on a wide variety of direct mantle samples from the lithosphere beneath continents. In particular, Rb-Sr, Sm-Nd, Lu-Hf, and Re-Os isotope studies have been carried out on diverse mantle rocks and minerals including peridotite and eclogite xenoliths, macrocrysts, and silicate and sulfide inclusions in diamonds from kimberlites erupted through Archean cratons. These studies have contributed significantly to current understanding of old thick cold lithospheric mantle keels coupled to and translating with the overlying continental crust. The general consensus is that old continental mantle keels are characterized by unradiogenic Os and Nd, and highly radiogenic Sr isotope signatures as a consequence of early melting and subsequent metasomatism of residual harzburgitic compositions. However, the availability of such material for delamination and eventual incorporation into the source of ocean island volcanics remains controversial. Inclusion bearing diamonds have come to be appreciated as the ultimate time capsules from old continental mantle by virtue of negligible diffusion rates of the relevant elements through diamond stored in the keel on a billion-year timescale. Our current work is focused on harzburgitic diamonds and their potential precursors from the 0.52 Ga Venetia kimberlite, Limpopo Belt, Kaapvaal-Zimbabwe craton. Peridotite xenoliths from this locality include the least radiogenic Os isotope composition of any known terrestrial rock, in particular a garnet lherzolite with an Os model (T_RD) age of 3.7 Ga, effectively a minimum age for the keel to this continental nucleus. Yet, Venetia harzburgitic garnet bearing diamonds yield a diamond crystallization (or recrystallization) age of around 2.0 Ga though with an unradiogenic initial Nd (and radiogenic initial Sr) that requires a >3 Ga lithospheric mantle precursor. A complementary Re-Os study of available sulfide inclusions from Venetia diamonds is currently in progress. These features are reminiscent of the characteristics of lherzolitic diamonds from the 1.2 Ga Premier kimberlite, that mark the major Proterozoic modification of the central Kaapvaal craton in association with emplacement of the massive 2.05 Ga Bushveld complex. They also resemble those of harzburgitic diamonds from the 0.37 Ga Udachnaya kimberlite, Siberian craton, where both ca. 3.5 and 2.0 Ga generations of harzburgitic to lherzolitic diamonds have been identified. A consistent picture is emerging of episodic diamond formation associated with Archean craton formation, stabilization and modification that highlights the continental mantle keel as a potential reservoir of unradiogenic Nd and Os, and radiogenic Sr that would be available for recycling into the convecting mantle if it were to detach from the continent.
DE: 1038 Mantle processes (3621)
SC: Union [U]
MN: 2007 Fall Meeting