Distribution and Residence of Lithophile Trace Elements in Minerals of Garnet and Spinel Peridotites:
an ICP-MS Study

Dmitri Ionov GEMOC, School of Earth Sciences, Macquarie University, Sydney, 2109 N.S.W., Australia


Whole rocks and coexisting minerals of "anhydrous" spinel and garnet peridotite xenoliths in alkali basalts from southeastern Siberia (Russia) and Mongolia have been analysed for rare earth elements (REE), Ba, Rb, Sr, Zr, Hf, Nb, Ta, Th, U, Pb and Sc by ICP-MS (detection limits 0.2-8 ppb) in order to study subsolidus element partitioning and better define the residence of the trace elements in the upper mantle. Three samples are fertile (2.8-4.3% CaO) garnet lherzolites from the Vitim volcanic field east of Lake Baikal and Tariat, central Mongolia; four samples are fertile (2.9-3.9% CaO) spinel lherzolites from Vitim, Tariat and Dariganga (SE Mongolia) and three others are refractory (0.7-0.9% CaO), LREE-enriched spinel peridotites from Dariganga and Sikhote-Alin (Russian Far East). The minerals analysed are as follows: garnet (gar) from all garnet lherzolites, clinopyroxene (cpx) and orthopyroxene (opx) from all the samples, three olivines (ol) from spinel peridotites and one from a garnet lherzolite. Mineral separates were obtained magnetically, acid leached using cold diluted HF and HCl in an ultrasonic bath, hand picked to optical purity and mildly leached again. Modal abundances of ol, opx, cpx and gar were determined from least squares mixing using whole rock and mineral major element compositions.

The whole rock (wr) compositions of garnet and spinel lherzolites reconstructed from minerals show a good match with the measured wr contents for middle to heavy REE, Sr, Zr, Hf and Sc indicating, consistent with previous studies, that cpx and gar are the dominant hosts of these moderately incompatible elements in lherzolites. Sr/Nd in wr lherzolites and in their cpx's range from 15 to 19. Zr/Hf in wr lherzolites are close to chondritic. In garnet lherzolites, Zr/Hf in gar (60-75) are much higher than in coexisting cpx (22-25). Many wr lherzolites have slight negative Zr and Hf anomalies; in spinel lherzolites the magnitudes of these anomalies are very similar in wr and cpx. Some refractory, LREE-enriched peridotites and their cpx have strong negative Zr-Hf anomalies. Gar, opx and ol from fertile lherzolites show enhanced abundances of Nb, Ta and U relative to LREE; opx is markedly enriched in Zr and Hf relative to medium REE. Most cpx's are strongly depleted in Nb and Ta relative to LREE. For highly incompatible elements (Rb, Ba, Th, U, Nb, Ta) the reconstructed wr compositions of most lherzolites show deficiencies exceeding 20%, these are particularly high (60-95%) for Rb, Ba, Nb and Ta. Significant proportions of these elements are not contained in the major rock-forming minerals and appear to be concentrated at grain boundaries. Many refractory peridotites contain interstitial glass and abundant fluid inclusions in minerals where these elements can reside. However, the highly incompatible elements cannot be fully accounted for from our data in fertile unmetasomatised lherzolites as glass and fluid inclusions are very uncommon in these rocks. The concentrations of Th, U and Pb are low in cpx from unmetasomatised lherzolites. In contrast, the contents of Th, U and Pb are very high (0.1-1 ppm) in cpx from many LREE-enriched peridotites; cpx in such rocks show marked positive anomalies of Th and U relative to LREE and their wr contents can be matched by those reconstructed from minerals. The data obtained can be used to calculate inter-mineral partition coefficients for the trace elements between gar, cpx, opx and ol in mantle peridotites.