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 laserablation ICP-MS to study systematic variations between mantle lithologies, partitioning mechanisms in olivine and the potential of olivine for geothermobarometry and petrogenic mantle processes. Samples were selected to provide 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, 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, Na) show large concentration ranges, which are mainly controlled by the equilibration temperature of the host rock. The elements are strongly concentrated in coexisting mantle minerals (garnet, clinopyroxene, 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, 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 2 O 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  with P in kbar, Al Ol the concentration of Al in 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 Kohler, 1990). Although calibrated using lherzolites only, the thermometer performs well for clinopyroxenefree harzburgites and even 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.

Neoarchean Mantle-derived Magmatism within the Repulse Bay Block, Melville Peninsula, Nunavut: Implications for Archean Crustal Extraction and Cratonization

“…These features are consistent with derivation from mantle melts (Sato 1977;Sano et al 2002;de Hoog et al 2010). Low Cr contents in olivine (6 ppm) compared to other mantle-derived lherzolite examples (de Hoog et al 2010) is interpreted to reflect sequestering of Cr in phlogopite over olivine. The dunite xenolith also contains a whole rock signature which may be consistent with crustal contamination including LILE and LREE enrichment, and negative Nb, Ti and Y anomalies.…”

The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050 °C and 3 GPa

“…This indicates that Ti and H are being incorporated together as part of the same point defect, and that the formation of this defect is the dominant method of incorporating both Ti and H in the system studied. Note that Ti concentrations overlap the range found in natural mantle olivines (De Hoog et al 2010). Extrapolating the trend of H 2 O concentration vs. Ti concentration to anhydrous conditions gives a Ti [Ti] experiments conducted at variable a H2O .…”

The efficient long-term inhibition of forsterite dissolution by common soil bacteria and fungi at Earth surface conditions

“…A number of studies have previously postulated that microbes preferentially attach themselves to active sites on dissolving mineral surfaces to harvest essential nutrients from the minerals (Hutchens et al, 2006;Bonneville, 2009Bonneville, , 2010Balland et al, 2010). Note that the dissolving San Carlos forsterite contains a large number of the elements required for microbial growth in trace quantities (De Hoog et al, 2010).…”

“…The San Carlos forsterite sample used in this study was originally prepared by Pokrovsky and Schott (2000a and b), who reported that its composition is consistent with Mg 1.82 Fe 0.18 SiO 4 (Fo 91 ). San Carlos Olivine has been reported to contain numerous trace elements including ~40 ppm P (De Hoog et al, 2010). Transparent crystals 0.5 cm in size were handpicked, ground with an agate mortar and pestle, and sieved.…”