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Partitioning of Rare Earth Elements between Garnet and Clinopyroxene in Peridotite

Brendan J. Hanger and Greg M. Yaxley

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

Peridotite is the major component present in the Earth’s upper mantle and is comprised of four main minerals at depths of greater than about 100km: olivine, orthopyroxene, clinopyroxene and garnet. The mantle is sampled by volcanic eruptions, such as kimberlites, which sometimes accidentally entrained fragments of mantle peridotite called xenoliths, bringing them rapidly to the surface where they are accessible. Kimberlites also transported most of the Earth’s diamond stocks and as such the study of these peridotite xenoliths is important in helping us understand the processes involved in the creation and transport of diamonds.


In order to understand how fluids and magmas have affected peridotites we generally investigate the concentration of the rare earth elements in each mineral, with particular attention paid to the ratio of the concentration in the mineral compared to a melt phase, this is known as the partition coefficient. Partition coefficients are determined using high pressure experiments where conditions such as temperature, pressure and composition are varied in a systematic manner. However in the past most studies have focussed purely on the partitioning between one mineral and the melt phase, neglecting the interactions of the other minerals. In this study we aim to investigate the interactions between garnet and clinopyroxene in peridotites, as they both host significant concentrations of the rare earth elements and thus record the signature of ancient metasomatism.


We are performing a series of high pressure experiments using piston cylinder apparatus which we will use to determine distribution coefficients between garnet and clinopyroxene over a range of relevant temperatures and Ca in garnet compositions. It is hoped that this will help us understand more about the effects of metasomatism on rare earth elements, including how clinopyroxene can affect REE patterns in garnet. This will also build on our understanding of the systematics involve in the evolution of radiogenic isotopes such as Sm-Nd or Lu-Hf.