TRACE ELEMENT GEOCHEMISTRY OF MANTLE OLIVINE AND ITS APPLICATION TO GEOTHERMOMETRY

A detailed study of the trace-element composition of olivine from a variety of mantle lithologies has been undertaken to (1) constrain the variability within and between various lithologies, (2) to obtain a better understanding of trace element partitioning into olivine and (3) to explore the potential of olivine for thermobarometry of mantle rocks. Results from mantle xenoliths from three localities will be presented, two from kimberlite pipes (Kaalvallei, South- Africa, and Kirkland Lake, Canada) and one from a basaltic volcanic centre (Ray Pic, France). Xenoliths from the latter are all spinel peridotites (ol, opx, cpx, sp), whereas xenoliths from the kimberlites are garnet peridotites (ol, opx, gt, ±cpx, ±sp). Analyses were performed either in situ or on olivine separates by laser ablation ICP-MS at our lab in Gothenburg. Typically five olivines per sample were ablated using 300- µm craters ablated at 10Hz for 120 s, resulting in detection limits between 1 and 10 ppb for most elements. Accuracy was monitored using an in-house standard prepared from olivines separated from a peridotite xenolith from the Kimberley pipe, which has a variability of all trace elements analyzed of less than 3%. Major elements were analyzed by SEM-EDS recording 400s spectra. Accuracy was monitored against San Carlos olivine, of which repeat analysis gave a value of Fo90.13 ± 0.07. Forsterite contents of all olivines lie between 90.1 and 93.3, with the lowest values in the spinel peridotites from France, indicating a lower degree of depletion compared to cratonic xenoliths. The variation in Li, Mn, Co, Ni and Zn is small (less than 25%), in accordance with olivine being the major host for these elements. Manganese and Li correlate positively within each locality, and are higher in spinel than in garnet peridotites. The largest variations are observed in Na, Al, Cr, REE, Zr and are highest for Ti, which varies from 0.4 to 240 ppm, although the variation in much more limited in the spinel peridotites. Similar large variations are observed in Ti contents of minerals co-existing with olivine, which indicates that variation is dominated by bulk-rock concentrations. The extreme Ti variation in cratonic xenoliths reflects the enormous diversity in the compositions of mantle rocks created by multiple events of melt depletion and metasomatic refertilization, and olivine is a sensitive indicator of these processes. Similar considerations may explain variations in Zr and possibly REE. On the other hand, Na, Al and Cr are relatively constant in co-existing minerals, requiring another explanation for the variation of these elements. Sodium and Cr co-variation defines a narrow band of compositions with Na and Cr concentrations being almost equal, independent of lithology. Al and Cr, on the other hand, show significant difference between garnet and spinel peridotites, where spinel peridotites are offset to higher Al values. This is probably related to differences in partitioning behavior between garnet-olivine and spinel-olivine, although currently we cannot rule out a pressure effect on partitioning. A chart of all monovalent cations (Li, Na) against trivalent cations (Al, Cr, Sc) also shows significant differences. Whereas garnet peridotites show a close to 1:1 correlation, which can be expected because of coupled substitution of a tri- and a monovalent cation for two divalent cations (Mg2SiO4 + Na+ + Cr3+ = NaCrSiO4 + 2Mg2+), spinel peridotites show a 1:3 correlation. The excess of trivalent cations can possibly be explained by a Tschermak-type substitution, i.e., Mg2SiO4 +2Al3+ = MgAlVIAlIVO4. + Mg2+ + Si4+. As the size of the Al ion is in-between the size of the octahedral and tetrahedral sites, such partitioning behavior can be expected based on lattice-strain theory [1]. However, in contrast to most other rock-forming silicates, silica tetrahedrons in olivines do not share oxygens, and replacement of Si by Al would therefore significantly distort the crystal lattice, which seems energetically unfavorable [2]. However, the 1:3 correlation in the spinel peridotites presents the interesting possibility that substitution occurs through double coupled substitution, i.e, 2Mg2SiO4 + 3Al3+ + Na+ = NaAlVISiO4 + MgAlVIAlIVO4 + 3Mg2+ + Si4+, in which the two Al-bearing molecules may have a mutually stabilizing effect. Admittedly the proposition is speculative and requires further study. Alternative explanations could be vacancy substitution or the presence of significant amounts of H+, which has not been analyzed for. Why trivalent cation excess occurs in spinel peridotites only is unclear, and the analysis of similar xenolith suites should tell whether it is a general trend. Irrespective of the substitution mechanism, the concentration of Al in olivine from mantle peridotites is strongly temperature dependent, and can therefore be used as a geothermometer. Different calibrations for spinel and garnet peridotite are necessary. Peridotite xenoliths from the Kaalvallei kimberlite (South-Africa) were used for calibration of the garnet-bearing variety of the thermometer, and xenoliths from Ray Pic (France) for the spinel-bearing one. Aluminum contents of the olivines range from 8-140 ppm, with a few anomalous values up to 310 ppm, for the Kaalvallei olivines, and from 49-191 ppm for Ray Pic olivines. Mg# lie between 91.5 and 93.3, and between 90.1 and 91.4, respectively. P-T conditions of the peridotites were estimated with the Al-in-opx geobarometer and cpx-opx geothermometer [3], and plot on conductive geotherms of ca. 40 and 60 mW/m2, respectively. The temperature ranges are 900- 1380°C and 830-1060°C, respectively. Only samples plotting on the local geotherms were considered, as the other samples showed disequilibrium features. The expression for the thermometers are: TGt Al-in-ol(°C) = 11390 / [ 11.88 - ln(ppm Al) ] – 273 (garnet peridotites) TSp Al-in-ol(°C) = 10871 / [ 7.46 - ln(ppm Al) ] – 273 (spinel peridotites) with average residuals of 12°C and 29°C, respectively. As the compositional range of mantle olivine is very small, no correction for major chemical components is necessary. In addition, no correction for Al activity of the system is necessary, as long as an Al-saturated phase such as garnet or spinel is present. Combined with the Ca-in-olivine barometer [4], the new thermometer has the potential to determine P-T conditions of single olivines. Whether olivine is derived from garnet of spinel peridotite can easily be determined from the Cr/Al ratio of the olivine (<0.7 for spinel peridotites). As olivine is an abundant component of heavy mineral separates from kimberlites and glacial till, it could serve as a new tool for diamond exploration. Vanadium and Cr show similar temperature-dependent variations as Al, but to a lesser degree, and would therefore yield less accurate geothermometers. In addition, partitioning of these elements is sensitive to variations in oxidation state. The pressure dependence of the thermometer is the subject of future research and it is recommended that the Alin- olivine thermometer in its current form is applied to rocks derived from comparable geotherms only.

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