The stability of Tuite [γ-Ca3(PO4)2] in peridotitic bulk systems and its significance for volatile and trace element transport into the deep Earth

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 [1] 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.

[1] Barrat, J.A. et al., (2012) Geochimica et Cosmochimica Acta 83: 79–92.