Apatite is an important host for halogens and incompatible trace elements, in particular for LILE and REE. The P-T stability of its high-P breakdown product tuite and its potential role as deep-Earth volatile and trace element carrier, however, is still largely unconstrained in both subduction zone and convecting mantle P-T regimes.
To investigate the upper P-T stability limit of tuite and its compositional evolution, multianvil experiments were conducted at 15 to 25 GPa and 1600 to 2000 °C, using a spinel lherzolite doped with β-Ca3(PO4)2, halogens, and a trace element mix containing selected LIL, HFS and REE as starting material. The phases stable in this P-T range include tuite, majoritic garnet, ringwoodite, forsterite, clinoenstatite, bridgmanite, Ca-perovskite, ferropericlase, and melt.
Preliminary SIMS data indicate that Ca-perovskite and tuite are the major REE reservoirs. At 20 GPa/1600°C, REEtuite/REECa-Pv concentration ratios are 0.17 for Ce, 0.28 for Gd and 0.09 for Lu. At 25 GPa/1600°C these ratios are 0.09, 0.09, and 0.04, respectively. Both tuite and Ca-Pv strongly fractionate LREE, resulting in steep chondrite-normalized  REE-patterns. At 20 GPa/1600°C, for example, CeN/LuN ratios are 21.8 and 11.2 for tuite and Ca-Pv, respectively. This partitioning behaviour indicates that Ca-Pv is likely to be the major host for REE in primitive mantle, whereas tuite is an additional important REE-carrier and the most important P-carrier in metasomatically altered mantle.
 Barrat, J.A. et al., (2012) Geochimica et Cosmochimica Acta 83: 79–92.