Minor Elements in Olivine from Spinel Lherzolite Xenoliths: Implications for Thermobarometry

Suzanne Y. O'Reilly1, D. Chen1, 2, W. L. Griffin1, 3 and C. G. Ryan3
1 National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), School of Earth Sciences, Macquarie University, Sydney, NSW, 2109, Australia
2 Department of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, P.R. China
3 CSIRO Exploration and Mining, Box 136, North Ryde, NSW 2113, Australia

Abstract: The proton microprobe has been used to determine contents of Ca, Ti, Ni, Mn and Zn in the olivine of 54 spinel lherzolite xenoliths from Australian and Chinese basalts. These data are compared with proton-probe data for Ni, Mn and Zn in the olivine of 180 garnet peridotite xenoliths from African and Siberian kimberlites. Fe, Mn, Ni and Zn contents are well-correlated; because the spinel lherzolite olivines have higher mean Fe contents than garnet peridotite olivines (average Fo89.6 vs. Fo90–92) they also have lower Ni and higher Mn contents. Zn and Fe are well-correlated in garnet peridotite olivine, but in spinel peridotites this relationship is perturbed by partitioning of Zn into spinel. None of these elements shows significant correlation with temperature. Consistent differences in trace-element contents of olivines in the two suites is interpreted as reflecting the greater degree of depletion of Archean garnet peridotites as compared to Phanerozoic spinel lherzolites. Ca and Ti contents of spinel-peridotite olivine are well correlated with one another, and with temperature as determined by several types of geothermometer. However, Ca contents are poorly correlated with pressure as determined by the Ca-in-olivine barometer of Köhler and Brey (1990). This reflects the strong T-dependence of this barometer: the uncertainty in pressure (calculated by this method) which is produced by the ±50°C uncertainty expected of any geothermometer is ca ±8 kbar, corresponding to the entire width of the spinel-lherzolite field at 900–1200°C.

Keywords: olivine • minor elements • thermobarometry • mantle • spinel lherzolite • garnet lherzolite

Mineralogical Magazine; April 1997 v. 61; no. 405; p. 257-269; DOI: 10.1180/minmag.1997.061.405.09
© 1997, The Mineralogical Society
Mineralogical Society (www.minersoc.org)