Incompatible trace element transport in phosphorus enriched peridotitic mantle across the upper to lower mantle boundary

Increasing pressure and temperature causes progressive dehydration of subducted oceanic lithosphere. This process generates incompatible trace element and halogen-enriched fluids that migrate into the mantle wedge, thereby causing metasomatism across a large depth range. Apatite is a common constituent of metasomatic assemblages in mantle wedge peridotites and melange zones, indicating that phosphorus is a significant component of the trace element flux directed into the mantle wedge. During progressive subduction, tuite [γ-Ca₃(PO₄)₂] forms from apatite at depths of ~220-230 km (7-7.5 GPa) and ~250-280 km (8-9 GPa) in basaltic and peridotitic lithologies, respectively, thereby replacing apatite as phosphorus-saturating phase and major carrier of Y+REE, LILE, U and Th. The significance of Ca-phosphates compared to silicates for phosphorus and incompatible trace element storage and transport is expected to evolve with increasing depth and temperature. Upon crossing the upper-to-lower mantle boundary, major phosphorus and/or LREE carriers such as majoritic garnet and ringwoodite disappear, while new competitors for LILE-LREE-HFSE storage, such as davemaoite, the CAS-phase, and K-hollandite emerge (e.g. Hirose et al., 2004; Suzuki et al., 2012). No experimental data are currently available on the distribution of incompatible trace elements in Ca-phosphate-bearing assemblages at P-T conditions covering this depth interval. This study aims to address the gap in our understanding of upper-to-lower mantle trace element fluxes (1) by determining incompatible trace element concentrations in tuite and its coexisting phases within a peridotite bulk composition at pressures straddling the upper-to-lower mantle transition, and (2) by assessing the role of tuite in trace element storage and transport across this boundary. For this purpose, multi anvil experiments were performed at 15 to 25 GPa and 1600 to 2000°C, using a moderately fertile peridotite doped with 3% synthetic β-Ca₃(PO₄)₂, approximately 2200 µg/g Cl and Br, each, and 1% of a trace element mix containing Y+REE along with selected LILE, HFSE and light elements (Li, B, Be) with concentrations in the range 1-230 µg/g.

In metasomatized peridotites, Ca-phosphates are stable only if the bulk phosphorus concentration exceeds the saturation capacity of the coexisting silicate-(oxide) assemblage. In this case, apatite and tuite can be present throughout the upper and in the uppermost lower mantle and constitute principal hosts of REE, LILE, U, and Th in this depth range. Upon entry of peridotite into the lower mantle, the breakdown of Ca-P-bearing majorite leads to the formation of davemaoite and tuite, both phases becoming the dominant incompatible trace element carriers. In the absence of Ca-phosphates, clinopyroxene, majoritic garnet and davemaoite dominate incompatible trace element storage in the upper and uppermost lower mantle.

Hirose, K. et al., (2004) Phys. Earth Planet. Inter. 146, 249-260.

Suzuki, T. et al., (2012) Phys. Earth Planet. Inter. 208-209, 59-73.