HR: 14:25h
AN: V43E-04    [Abstracts]
TI: Peridotite-suite dominated mineral inclusions in diamonds from Kelsey Lake Mine, Colorado U.S.A.
AU: * Schulze, D J
EM: dschulze@utm.utoronto.ca
AF: University of Toronto, Department of Geology, Erindale College, 3359 Mississauga Road, MIssissauga, ON L5L 1C6 Canada
AU: Coopersmith, H G
EM: coopersmith@comcast.net
AF: Coopersmith and Associates, 1205 Steeplechase Ct, Fort Collins, CO 80524 United States
AB: Thirty silicate and oxide inclusions large enough for in situ WDS electron microprobe analysis were exposed by grinding/polishing of 16 diamonds from the Kelsey Lake Mine in the Colorado-Wyoming State Line kimberlite district. Three garnets in two stones belong to the eclogite (E) suite, and 18 olivines, three Mg-chromites and six Cr-pyropes in the other 14 stones belong to the peridotite (P) suite. The peridotite-dominated population is in stark contrast to the other suites studied in the State Line district. The reported inclusion population from George Creek is completely eclogitic and that of the Sloan pipe is overwhelmingly eclogitic, with only a minor, relatively Fe-rich peridotite component. Multiple inclusions are common in single stones, with 12 olivines (of uniform composition) exposed in one example. Kelsey Lake olivine inclusions are magnesian (17 of 18 grains in 9 stones are in the range Fo 92.7-93.1), typical of P-suite stones worldwide, but unlike the more Fe-rich Sloan olivine suite (13 of 14 in the range Fo 91.3-92.2). Mg-chromites (wt percent MgO = 12.8-13.8, wt percent Cr2O3 = 61.4-66.6) are in the lower MgO range of diamond inclusion chromites worldwide. Six Cr-pyropes in four stones have moderately low calcium contents (wt percent CaO = 3.5-4.5) but are very Cr-rich (wt percent Cr2O3 = 10.5-16.7). An olivine-garnet pair in one stone yields a Mg-Fe exchange temperature of 895 degrees C, possibly indicating disequilibrium, whereas an olivine-chromite pair yields an Mg-Fe exchange temperature of 1035 degrees C, cool but reasonable for equilibration within the diamond stability field. Comparison with diamond inclusion minerals worldwide reveals that the Kesley Lake suite is most similar to those from the Slave Craton in Canada, especially in terms of Cr-pyrope compositions. Both suites are somewhat less depleted than suites from southern Africa or Siberian kimberlites. By analogy with the Slave P-suite diamonds of Archean age and a Proterozoic eclogitic component in the Slave mantle, the mixed diamond inclusion populations from the State Line district may support models in which blocks of Archean mantle survive buried beneath Proterozoic continental crust, mixed with eclogitic regimes emplaced by Proterozoic subduction.
DE: 1025 Composition of the mantle
DE: 1065 Major and trace element geochemistry
DE: 3621 Mantle processes (1038)
DE: 8103 Continental cratons
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005