The high PT stability of apatite and Cl partitioning between apatite and hydrous potassic phases in peridotite: an experimental study to 19 GPa with implications for the transport of P, Cl and K in the upper mantle

High PT experiments were performed in the range 2.5-19 GPa and 800-1,500A degrees C using a synthetic peridotite doped with trace elements and OH-apatite or with Cl-apatite + phlogopite. The aim of the study was (1) to investigate the stability and phase relations of apatite and its high PT breakdown products, (2) to study the compositional evolution with P and T of phosphate and coexisting silicate phases and (3) to measure the Cl-OH partitioning between apatite and coexisting calcic amphibole, phlogopite and K-richterite. Apatite is stable in a garnet-lherzolite assemblage in the range 2.5-8.7 GPa and 800-1,100A degrees C. The high-P breakdown product of apatite is tuite gamma-Ca-3 (PO4)(2), which is stable in the range 8-15 GPa and 1,100-1,300A degrees C. Coexisting apatite and tuite were observed at 8 GPa/1,050A degrees C and 8.7 GPa/1,000A degrees C. MgO in apatite increases with P from 0.8 wt% at 2.5 GPa to 3.2 wt% at 8.7 GPa. Both apatite and tuite may contain significant Na, Sr and REE with a correlation indicating 2 Ca2+=Na+ + REE3+. Tuite has always higher Sr and REE and lower Fe and Mg than apatite. Phosphorus in the peridotite phases decreases in the order P-melt a parts per thousand << P-grt a parts per thousand << P-Mg2SiO4 > P-cpx > P-opx. The phosphate-saturated P2O5 content of garnet increases from 0.07 wt% at 2.5 GPa to 1.5 wt% at 12.8 GPa. Due to the low bulk Na content of the peridotite, (NaPM2+)-Na-[8]-P-[4]-M-[8] (-1) Si-[4](-1) only plays a minor role in controlling the phosphorus content of garnet. Instead, element correlations indicate a major contribution of (M2+[4]PM3+)-M-[6]-M-[6] (-1) Si-[4](-1). Pyroxenes contain similar to 200-500 ppm P and olivine has 0.14-0.23 wt% P2O5 in the P range 4-8.7 GPa without correlation with P, T or X-Mg. At a parts per thousand yen12.7 GPa, all Mg2SiO4 polymorphs have < 200 ppm P. Coexisting olivine and wadsleyite show an equal preference for phosphorus. In case of coexisting wadsleyite and ringwoodite, the latter fractionates phosphorus. Although garnet shows by far the highest phosphorus concentrations of any peridotite silicate phase, olivine is no less important as phosphorus carrier and could store the entire bulk phosphorus budget of primitive mantle. In the Cl-apatite + phlogopite-doped peridotite, apatite contains 0.65-1.35 wt% Cl in the PT range 2.5-8.7 GPa/800-1,000A degrees C. Apatite coexists with calcic amphibole at 2.5 GPa, phlogopite at 2.5-5 GPa and K-richterite at 7 GPa, and all silicates contain between 0.2 and 0.6 wt% Cl. No solid potassic phase is stable between 5 and 8.7 GPa. Cl strongly increases the solubility of K in hydrous fluids. This may lead to the breakdown of phlogopite and give rise to the local presence in the mantle of fluids strongly enriched in K, Cl, P and incompatible trace elements. Such fluids may get trapped as micro-inclusions in diamonds and provide bulk compositions suitable for the formation of unusual phases such as KCl or hypersilicic Cl-rich mica.