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Experimental calibration of Ni partitioning between garnet and olivine at upper mantle pressures – implications for diamond exploration.

Gregory M. Yaxley 1 and Hugh St.C. O'Neill 1

1 Research School of Earth Sciences, Australian National University , Canberra , ACT 0200, Australia

 

The chemical compositions of lithosphere-derived pyrope garnet, often recovered from heavy mineral concentrates (HMC) obtained during diamond exploration programs, contain a record of the temperature at which the grains equilibrated with olivine in peridotite in the lithosphere.

This information is extremely useful in guiding diamond exploration programs. If the local cratonic geotherm at the time of kimberlite emplacement can be estimated independently (e.g. from studies of multiphase mantle-derived garnet peridotite xenoliths), then temperature information obtained from single-phase thermometry can be fitted to the geotherm allowing estimation of the mantle depth interval sampled by the kimberlite during its ascent to the surface. This is important in assessing the extent of entrainment by kimberlites of material from pressure – temperature conditions within the diamond stability field. This clearly has implications for the diamond prospectivity of individual pipes.

Temperature information is preserved in garnet and olivine major, minor and trace element chemistry. For example, garnet and olivine participate in T dependent exchange equilibria, such as the following for Ni:

3Ni 2+2SiO4 + 2Mg3Al2Si3O12 = 2Ni2+3Al2Si3O12 + 3Mg2SiO4 (1)

olivine garnet garnet olivine

If the temperature and pressure dependence of exchange equilibrium (1) can be calibrated, then in principal the chemical compositions of garnet and olivine coexisting in the upper mantle can be used to determine the temperature at which they equilibrated.

A new empirical calibration of the reaction for Ni-Mg exchange between olivine and garnet at mantle pressures has been conducted at 3.0-4.5 GPa, using piston-cylinder high-pressure experimentation at the Research School of Earth Sciences. This research was kindly funded by de Beers.

An important new finding of this study, is that when our data is combined with an earlier, higher pressure calibration (6-7 GPa) (Canil 1994), the reaction has a resolvable and significant pressure effect of approximately 6-8°C/kbar. This agrees with thermodynamic predictions based on molar volume estimates (Canil 1994). This seriously compromises the validity of a former empirical calibration of Ryan et al. (1996), which does not include pressure effects in the Ni exchange reaction, nor in the Fe-Mg olivine garnet thermometry, which was used in its original formulation. Application of this thermometer to single grain Cr-pyrope garnet may artificially extend the range of temperatures obtained from any given Ni-in-garnet data set, thereby implying a falsely large sampling depth interval for individual kimberlites pipes.

References: Canil D. (1994) An experimental calibration of the nickel in garnet geothermometer with applications. Contributions to Mineralogy and Petrology 117 , 410-420.

Ryan C.G., Griffin W.L. and Pearson N.J. (1996) Garnet geotherms: Pressure-temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks. Journal of Geophysical Research 101 , 5611-5625.