Geochemistry of transition elements in garnet lherzolite nodules in kimberlites |
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| Authors: | N Shimizu C J Allègre |
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| Institution: | (1) Laboratoire de Géochimie et Cosmochimie (LA 196), Institut de Physique du Globe, Université de Paris 6, 75230 Paris Cedex 05, France |
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| Abstract: | The clinopyroxenes and garnets from garnet lherzolite nodules in kimberlites were analyzed for the major and trace elements (Sc, Ti, V, Cr, Mn, Co, Sr, and Zr) with the secondary ion mass spectrometry (SIMS) techniques using an ion-microprobe. The concentration ranges for clinopyroxenes are: 12–90 ppm Sc, 60–2540 ppm Ti, 110–350 ppm V, 2400 ppm-1.68% Cr, 470–1100 ppm Mn, 18–70 ppm Co, 85–710 ppm Sr, and 6.3–120 ppm Zr. Those for garnets are: 71–180 ppm Sc, 140–6200 ppm Ti, 220–450 ppm V, 1.05–6.24% Cr, 1600–5220 ppm Mn, 33–92 ppm Co, and 66–250 ppm Zr.These data were used to assess a possible use of distribution of transition elements between clinopyroxene and garnet for geothermometry/geobarometry. The distribution coefficients of V and Mn show good correlations with (Ca/Ca+Mg+Fe)CPX, suggesting their potential usefulness.The abundances of these elements in minerals (and estimated whole-rocks) suggest that garnet lherzolite nodules in kimberlites can be classified into three groups. It is suggested that one of the groups (Group I) could be close to the  pristine mantle on the basis of Ti and Zr abundances as well as REE systematics. Arguments on Ti/Zr ratios among different nodule types suggest that abundant granular garnet lherzolites could be cumulates crystallized from a residual liquid after ilmenite was extracted.IPG Contribution No. 281 |
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