Ni-in-garnet geothermometry in mantle rocks: a high pressure experimental recalibration between 1100 and 1325\xa0\xb0C

Abstract

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule left( {{text{lnD}}_{{{text{grt}}/{text{olv}}}}^{{{text{Ni}}}} ; frac{{{text{Ni}}_{{{text{grt}}}} }}{{{text{Ni}}_{{{text{olv}}}} }}} right), the Ca mole fraction in garnet ({mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}}; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet ({mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}}; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as:T left(^circ{rm C} right)=frac{-8254.568}{left(left( {mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}} times 3.023 right)+left({mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}} times 2.307 right)+left({mathrm{lnD}}_{frac{mathrm{grt}}{mathrm{olv}}}^{mathrm{Ni}} - 2.639 right)right)}-273pm 55where {mathrm{D}}_{mathrm{grt}/mathrm{olv}}^{mathrm{Ni}}= frac{{mathrm{Ni}}_{mathrm{grt}}}{{mathrm{Ni}}_{mathrm{olv}}}, Ni is in ppm, {mathrm{X}}_{mathrm{grt}}^{mathrm{Ca}} = Ca/(Ca + Fe + Mg) in garnet, and {mathrm{X}}_{mathrm{grt}}^{mathrm{Cr}}= Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer.