HR: 15:10h
AN: V33D-07    [Abstracts]
TI: Formation and Evolution of the Continental Lithospheric Mantle: Perspectives From Radiogenic Isotopes of Silicate and Sulfide Inclusions in Macrodiamonds
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: Richardson, S H
EM: shr@geology.uct.ac.za
AF: Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa
AB: Silicate and sulfide inclusions that occur in diamonds comprise the oldest (>3 Ga), deepest (>140 km) samples of mantle-derived minerals available for study. Their relevance to the evolution of the continental lithosphere is clear because terrestrial macrodiamonds are confined to regions of the Earth with continental lithospheric mantle keels. The goals of analytical work on inclusions in diamond are to obtain paragenesis constraints, radiogenic ages, and initial isotopic compositions. The purpose is to place diamond formation episodes into the broader framework of the geological processes that create and modify the continental lithosphere and to relate the source of the C and N in diamond-forming fluids to understanding the Earth's C and N cycles in the Archean. Although sulfide and silicate inclusions rarely occur in the same diamond, they both can be grouped according to their geochemical similarity with the chief rock types that comprise the mantle keel: peridotite and eclogite. Silicate inclusions are classified as harzburgitic (depleted; olivine > Fo₉₁, garnet Cr₂O₃ > 3 wt% and CaO from 0 to 5 wt%), lherzolitic (fertile), or eclogitic (basaltic; garnet Cr₂O₃ < 2 wt% and CaO from 3 to 15 wt%, clinopyroxene with higher Na₂O, Al₂O₃, and FeO); they are amenable for trace element study by SIMS and for Sm-Nd and Rb-Sr analysis by conventional P-TIMS after grouping by mineralogical similarity. Sulfide inclusions (chiefly FeS with lesser Ni, Cu, and Co) are classified as peridotitic (Ni > 14 wt%; Os > 2 ppm) versus eclogitic (Ni < 10 wt%; Os < 200 ppb); single sulfides are amenable for S isotopic study by SIMS or TIMS, and Re-Os analysis by N-TIMS. Work on inclusions in diamonds depends on the distribution of mined, diamond-bearing kimberlites, and the generosity of mining companies because of the extreme rarity of inclusions in suites of mostly gem-quality diamonds. Most isotopic work has been on the Kaapvaal-Zimbabwe craton with lesser work on the Slave, Siberian, and Australian cratons. Sm-Nd ages on silicate suites and Re-Os ages on sulfide suites confirm diamond formation from the Mesoarchean though the Neoproterozoic. Most important are the systematics across cratons in the context of crustal geology that lead to generalities about craton evolution. Inclusion suites date mantle keels as Mesoarchean and clearly point to subduction as the major process to form the earliest continental nuclei and to amalgamate the cratons in their present form. This is evident from the elevated initial Os isotopic compositions in 3.5 Ga Slave (Panda) and 2.9 Ga Kaapvaal (Kimberley) sulfides, the low Sm/Nd and elevated initial Sr isotopic compositions of 3.4 Ga Kaapvaal (Kimberley) harzburgitic garnets, the preponderance of 2.9 Ga eclogitic sulfides in every western Kaapvaal craton locality, and the occurrence of surficial, volcanogenic S in Kaapvaal (Orapa) sulfides. The continental lithosphere was accessible to melts and fluids from the asthenosphere throughout the Proterozoic as evident from silicate and sulfide inclusion suites of 0.9 to 2.0 Ga age in every locality studied in the Kaapvaal craton. The correspondence of silicate inclusion type with current seismic velocity structure of the Kaapvaal mantle keel shows that its structure is at least Bushveld age (2 Ga) and due to compositional differences. Seismic velocity structures of continental mantle keels may be more a function of their geologic history than current temperature distribution.
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
DE: 8103 Continental cratons
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting