Mineral inclusions in diamonds from the Kelsey Lake Mine, Colorado, USA: Depleted Archean mantle beneath the Proterozoic Yavapai province

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• 作者：Daniel J. Schulze ; Howard G. Coopersmith ; Ben Harte ; Lori-Ann Pizzolato
• 关键词：Athene cunicularia ; Burrowing owl ; Hypoxia ; Hypoxic ventilatory threshold ; Metabolism ; Oxygen convection requirement ; Phylogenetically independent contrasts ; Ventilation
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2008
• 期刊代码：27_00167037
• 类别：ear
• 出版时间：15 March 2008
• 卷：72
• 期：6
• 页码：1685-1695
• 文件大小：569 K

摘要

Thirty-four silicate and oxide inclusions large enough for in situ WDS electron microprobe analysis were exposed by grinding/polishing of 19 diamonds from the Kelsey Lake Mine in the Colorado-Wyoming State Line Kimberlite district. Eighteen olivines, seven Cr-pyropes, four Mg-chromites, and one orthopyroxene in 15 stones belong to the peridotite (P) suite and three garnets and one omphacite in three stones belong to the eclogite (E) suite. The fact that this suite is dominated by the peridotite population is in stark contrast to the other diamond suites studied in the State Line district (Sloan, George Creek), which are overwhelmingly eclogitic. Kelsey Lake olivine inclusions are magnesian (17 of 18 grains in 9 stones are in the range Fo 92.7–93.1), typical of harzburgitic P-suite stones worldwide, but unlike the more Fe-rich (lherzolitic) Sloan olivine suite. Mg-chromites (wt%MgO = 12.8–13.8; wt%Cr₂O₃ = 61.4–66.6) are in the lower MgO range of diamond inclusion chromites worldwide. Seven harzburgitic Cr-pyropes in five stones have moderately low calcium contents (wt%CaO = 3.3–4.3) but are very Cr-rich (wt%Cr₂O₃ = 9.7–16.7). A few stones have been analyzed by SIMS for carbon isotope composition and nitrogen abundance. One peridotitic stone is apparently homogeneous in carbon isotope composition (δ¹³C_PDB = −6.2‰) but with variable nitrogen abundance (1296–2550 ppm). Carbon isotopes in eclogitic stones range from “normal” for the upper mantle (δ¹³C_PDB = −5.5‰) to somewhat low (δ¹³C_PDB = −10.2‰), with little internal variation in individual stones (maximum difference is 3.6‰). Nitrogen contents (2–779 ppm) are lower than in the peridotitic stone, and are lower in cores than in rims. As, worldwide, harzburgite-suite diamonds have been shown to have formed in Archean time, we suggest that the Kelsey Lake diamond population was derived from a block of Archean lithosphere that, at the time of kimberlite eruption, existed beneath the Proterozoic Yavapai province. The mixed diamond inclusion populations from the State Line kimberlites appear to support models in which volumes of Wyoming Craton Archean mantle survive buried beneath Proterozoic continental crust. Such material may be mixed with eclogitic/lherzolitic regimes emplaced beneath or intermingled with the Archean rocks by Proterozoic subduction.