Trace-element geochemistry of mantle olivine and application to mantle petrogenesis and geothermobarometry

Abstract

Trace-element compositions of olivine from 75 mantle rocks of diverse origin, including xenoliths from kimberlites, basaltic lavas and orogenic peridotites, were determined by laser ablation ICP-MS to study systematic variations between mantle lithologies, partitioning mechanisms in olivine and their potential for geothermobarometry and unravelling mantle processes. Samples were selected to cover a wide range of forsterite contents (89.1–93.4), equilibration temperatures and pressures (750–1450 °C; 15–80 kbar). Trace elements in olivine can be divided into three groups. Group I elements (Ni, Mn, Co, Cu, Zn and Li) show small concentration ranges and olivine is the major host mineral. These are mostly divalent elements and have ionic radii close to that of Mg. Group II elements (Cr, Al, V, Sc, Ca and Na) show large concentration ranges, which are mainly controlled by the equilibration temperature of the host rock. The elements are strongly concentrated in co-existing mantle minerals (garnet, clinopyroxene and spinel) and show a narrow range of bulk rock concentrations. They fit less comfortably in the olivine lattice than Group I elements because of their charge or size. Differences between garnet and spinel-facies rocks are apparent for Al, Ca and Sc. Group III elements (Ti, Zr, Nb and Y) show large ranges of concentration in olivine as well as in co-existing minerals, and are strongly dependent on bulk rock contents. Concentration differences between olivine from garnet and spinel-facies rocks are apparent for all these elements. They are strongly incompatible in olivine and other rock-forming mantle minerals because of their charge or size. Various mantle lithologies can be discriminated using olivine composition. Spinel, garnet and garnet–spinel peridotites can be distinguished in olivine Sc–Zr and MnO–Al 2O 3 diagrams, whereas volcanic olivine is distinguished by high Ca and Al contents (picritic olivine) or high Nb contents (kimberlitic olivine). Since concentrations of Group III elements in olivine are diagnostic of whole-rock contents they can be used to trace the petrogenetic history of the rock. For instance, Ti contents and Cr# (Cr/(Cr + Al)) of olivine correlate with the amount of melt extracted from a mantle residue, although refertilisation may subsequently have increased Ti contents in high- T peridotites from the base of the lithosphere. The olivine dataset can be used to examine substitution reactions. Uptake of Al and Cr appears to be largely charge-balanced by Na in garnet-facies olivine, and olivine Cr# strongly correlates with that of co-existing minerals, in particular clinopyroxene and spinel. In spinel-facies olivine a large excess of trivalent cations is present in olivine, which can be fully attributed to excess Al. This suggests a Tschermak-style substitution, in which replacement of Mg by Al in the octahedral site is charge-balanced by replacement of Si by Al in the tetrahedral site. Partition coefficients of Group II elements are highly temperature sensitive with most of the element variability being shown by olivine. This allows the definition of simple geothermometers based solely on the concentrations of these elements in olivine. The most widely applicable of these is Al-in-olivine for garnet peridotites, following the expression T Al-ol ( ° C ) = 9423 + 51.4 P + 1860 Cr # ol ( 13.409 − ln [ Al ] ol ) − 273 with P in kbar, Al ol the Al concentration of olivine in ppm, and Cr# ol is Cr/(Cr + Al) in olivine. This thermometer predicts the temperature with a residual of 15 °C based on calibration with two-pyroxene and Al-in-opx geothermobarometers (Brey and Köhler, 1990). Although calibrated using lherzolites only, the thermometer performs well for clinopyroxene-free harzburgites and also spinel peridotites. An alternative thermometer is presented for the case where the presence of Cr 2+ is expected, e.g., for olivine inclusions in diamonds. The geochemical and thermobarometric information recorded by olivine can be a useful tool in studies of the petrogenesis of lithospheric mantle, olivine xenocrysts in mantle-derived magmas, the formation of diamonds and diamond exploration using detrital olivine.