Popis: |
Eleven eclogite xenoliths from the Bobbejaan and Roberts Victor mines, South Africa have been characterized by a variety of geochemical techniques. The samples include nine bimineralic eclogites and two grospydites that span the range of mantle eclogite major element compositions. Primary phase end-member compositions range from Py 75 Al 16 Gr 09 , Jd 18 Di 82 CaEs 0 to Py 28 Al 21 Gr 51 , Jd 49 Di 36 CaEs 15 . Oxygen isotope analyses of mineral separates were performed for clinopyroxene and garnet. Isotopic values ( δ 18 O) range from +4.0 to +7.8 for clinopyroxene and +3.6 to +7.2 for garnet. Differences in isotopic values between the primary phases ( Δ = clinopyroxene minus garnet) range in value from −0.6 to 0.6. Although oxygen isotopes are not expected to fractionate at mantle temperatures, the anion electrostatic site potential values determined by Smyth (1989) are consistent with small degrees of fractionation in the mantle. Oxygen isotope fractionation between minerals may be calculated by the equation of Smyth and Clayton ( F= A×10 6 T 2 , where A =0.7911 eV). Fractionation values at 1300°K range from 0.6 (Jd-Pyr) to 0.03 (CaTs-Grs), while at 1600°K these values are between 0.39 and 0.02, respectively. The re-equilibration temperatures of the studied eclogite suite are 1050 to 1250°C at 30 kbar, therefore the obtained differences between the mineral phases are consistent with the calculated isotope fractionation values. The data obtained for eclogite oxygen isotopes suggest that 18 O may fractionate to a small extent at mantle PT -conditions. |