Li as a barometer for bimineralic eclogites: Experiments in natural systems

Abstract

Experiments performed in eclogitic systems show that the partitioning of Li between garnet and clinopyroxene is pressure dependent in natural eclogitic bulk compositions. This supports experimentally the hypothesis, based on the analysis of Li in eclogitic xenoliths and inclusions in diamond, that the partitioning of this particular element between clinopyroxene and garnet is very sensitive to changes in pressure and could be calibrated as a barometer for bimineralic eclogites. In order to calibrate this pressure dependence into a barometer, 16 multianvil experiments were performed from 4 to 13 GPa and 1100–1500 °C using starting materials sourced from a well preserved Mg-rich eclogitic xenolith from the Roberts Victor kimberlite pipe (South Africa). Our findings reinforce the general trend we observed in the CMAS system, that D Li cpx–grt for Li decreases with increasing P, and that at P ≥ 12 GPa, garnet is able to incorporate more Li than clinopyroxene. Multiple linear regression was applied to our experimental results to create the barometer: P = ( 0.000963 * T − ln D Li cpx−grt + 1.581 ) / 0.252 where P is pressure in GPa, T is temperature in °C and D Li cpx–grt is defined as the partitioning coefficient of Li obtained by dividing the concentration of Li in cpx by the concentration of Li in garnet. This barometer reproduces the experimental conditions to ± 0.2 GPa. It is applicable to Mg-rich eclogitic xenoliths, and to peridotitic and eclogitic inclusions in diamond. Application of the barometer to diamond-bearing xenoliths results in pressures in the diamond stability field. Clinopyroxene is easily corrupted in xenoliths and also preferentially takes in Li during short lived metasomatic processes. Care must be taken therefore to analyse primary, unaltered clinopyroxene. Our preliminary application to natural samples shows that the barometer can be applied beyond the experimental range to pressures down to 3.5 GPa. This is higher than the pressures encountered in crustal eclogites and thus the barometer is not applicable to these eclogites.