The maximum chromium content in harzburgitic garnet: an experimental study at <i>P–T</i> conditions of the Earth’s upper mantle

Abstract Subcalcic Cr-rich pyrope is a typical inclusion in natural diamond and considered as the main indicator mineral in diamond exploration. This article presents the results of experiments using a model garnet–spinel–harzburgite with the objective of determining the maximum Cr content of the garnet. This study supplements the existing CaO vs Cr 2 O 3 diagram with new experimentally obtained data on Cr-rich garnets. A high-pressure apparatus (BARS) was used to conduct experiments at a pressure of 5.5 GPa and temperature of 1300°С, which corresponds to the stability field of both garnet and diamond. A model harzburgite was obtained using natural Mg-serpentine, which decomposes at the pressures and temperatures of this experiment into olivine and orthopyroxene. Natural chromite and carbonatite were used as the sources of Cr and Ca, respectively. The samples formed are composed of forsterite, enstatite, Cr-pyrope and Cr-spinel. The maximum Cr 2 O 3 content, 16.23 wt.%, was detected in grains which grew in contact with chromite. The addition of 1–2 wt.% carbonatite resulted in the crystallisation of garnets with varying Ca contents (2.83–7.49 wt.% CaO). The experiments confirmed the boundary at 16 wt.% Cr 2 O 3 for subcalcic pyrope associated with diamond. It is concluded that the origin of natural samples of Ca-rich lherzolitic/wherlitic and Ca-poor harzburgitic garnets with &gt;16 wt.% Cr 2 O 3 can be attributed to the specific Ca/Cr/Al ratios of the host medium.