Interaction of serpentine and chromite as a possible formation mechanism of subcalcic chromium garnet in the upper mantle: an experimental study

An experimental simulation of serpentine and chromite interaction was conducted at the pressure (P) and temperature (7) conditions of garnet-peridotite stability in order to clarify the potential role of serpentinite as a source for the crystallization of subcalcic garnet in the depleted subcratonic mantle. The experiments were performed at 4 GPa and 1100 degrees C and 5.5 GPa and 1200 degrees C using the high-pressure apparatus BARS. Natural antigorite from ophiolites of the Eastern Sayan (Russia) was used as a starting material. Two groups of chromite grains with different Cr# = 100Cr/(Cr + Al) ratios (from spinel peridotite xenoliths from the Udachnaya kimberlite pipe, Yakutia) were added to the antigorite. Newly formed garnet, spinel, olivine and orthopyroxene were observed as the products in the experiments. Garnet formed only around chromite grains with the lower Cr# value (46.4). Garnet has low CaO contents (<0.05-1.10 wt.%) with chromium contents showing wide intra- and inter-grain variations (Cr# = 0.7-33.5). The Cr content increases from core to rim with the outer zones corresponding most closely to the equilibrium composition of the relevant bulk composition. The garnet total FeO content is in the range 3.4-5.8 wt.%. The experiments demonstrate that serpentinite decomposed at a temperature of 700 degrees C to olivine + orthopyroxene + water. If mingled mechanically with spinel-bearing mantle wedge peridotite upon subduction, it could react to form the range of subcalcic garnet compositions found as inclusions in diamonds.