HR: 16:15h
AN: V22G-02    [PDF]
TI: Application of the Rhenium-Osmium Isotopes to the Geochronology of Diamonds
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
AU: Pearson, D G
EM: d.g.pearson@durham.ac.uk
AF: Department of Geological Sciences, Durham University South Road, Durham, DH1 3LE United Kingdom
AU: Harris, J W
EM: jwh@earthsci.gla.ac.uk
AF: Division of Earth Sciences, UNiversity of Glasgow, Glasgow, G12 8QQ United Kingdom
AB: The advent of the modern era of high sensitivity and accuracy measurements of Re and Os isotopic compositions by negative thermal ionization mass spectrometry (N-TIMS; Creaser et al, 1991; Volkening et al, 1991) has led to numerous applications of Re-Os isotopes in tracer studies and geochronology. Recent developments in processing blanks (e.g. Richardson et al, 2001) by miniaturization of chemistry (Re $<$40x10$^{-15}$g; Os $<$2x10$^{-15}$g) permit single sulfide inclusions in minerals such as diamond to be analyzed for their Re-Os isotopic systematics (Pearson et al, 1998; Pearson and Shirey, 1999). Such data on syngenetic inclusions can provide ages on individual macro-diamonds. The microchemistry technique analyses the entire grain, thereby minimizing problems from exsolution. In addition, the low blanks, combined with high sensitivity of N-TIMS allows the analysis of single eclogitic sulfides that are intractable by laser-ICPMS methods This method of diamond geochronology is being applied to diamonds from ancient terranes such as the Kaapvaal-Zimbabwe, Siberian, Slave, and Australian cratons. The work depends on the distribution of mined, diamond-bearing kimberlites, the frequency and size of sulfide inclusions in respective diamond suites and the beneficence of diamond mining companies. A goal of obtaining ages on diamonds is to place diamond formation episodes into the broader framework of the geological processes that create and modify the continental lithosphere. Additionally, diamonds and their inclusions have long held general interest as the most robust containers of ancient minerals from the mantle at depths of 150 km or more. The most detailed application of Re-Os sulfide inclusion ages has been to the evolution of the Kaapvaal-Zimbabwe craton where there exists the widest distribution of mined kimberlites in diverse geologic terrains, the most extensive dataset on silicate inclusion ages and diamond compositions, and recent seismic tomography of the diamond source region in the lithospheric mantle. Diamond ages track the geological evolution of the craton throughout most of its history. Geographically restricted, 3.2-3.3 Ga Sm-Nd ages on harzburgitic garnet inclusions in diamond document early cratonic nuclei development likely by subduction but involving severe mantle depletion and concomitant light REE enrichment. Widely distributed, circa 2.9 Re-Os Ga ages (e.g. Richardson et al, 2001) on eclogitic sulfide inclusions document the subduction accretion that put older cratonic blocks together. Proterozoic ages often unique to each locality and seen in both silicate and sulfide inclusions suites testify to re-fertilization of the cratonic lithospheric mantle by magmatic, metasomatic and subduction-margin processes. This age-framework appears applicable to the more restricted datasets from other cratons and allows us to clearly relate diamond genesis to the dynamics of craton creation and assembly. \\ \noindent Creaser, R., Papanastassiou, D., and Wasserburg, G. (1991) GCA, 55, 397-401. Pearson, D.G., and Shirey, S.B. (1999) D.D. Lambert, and J. Ruiz, Eds. Rev. in Econ. Geol. 12, 143-172. Pearson, D.G., Shirey, S.B., Harris, J.W., and Carlson, R.W. (1998) EPSL, 160, 311-326. Richardson, S.H., Shirey, S.B., Harris, J.W., and Carlson, R.W. (2001) EPSL, 191, 257-266. Volkening, J., Walczyk, T., and Heumann, K. (1991) Int. Jour. of Mass Spectr. \& Ion Proc., 105, 147-159.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1025 Composition of the mantle
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting