The High P–T Stability of Fluor-Rich Apatite in Peridotitic and Harzburgitic Rocks: Implications for Transport, Storage and Recycling of Phosphorus, Volatiles and Incompatible Trace Elements in the Slab-Mantle Wedge System
Year: 2025
Type: article
Abstract: High P–T experiments were conducted at 3 to 8.7 GPa and 960°C to 1300°C, using a synthetic lherzolite and a harzburgite doped with pure synthetic fluorapatite and incompatible trace elements. The objectives of this study were to (1) constrain the P–T stability field of F-apatite in a representative peridotite and harzburgite and to assess the influence of bulk fertility on apatite stability, (2) determine the compositional evolution of apatite as a function of P, T and bulk composition, and (3) evaluate the significance of apatite and tuite as reservoirs for volatiles and incompatible trace elements in the context of secular variations of subduction zone P–T regimes. In the investigated bulk compositions, pure F-apatite is unstable across all P–T conditions. F contents in apatite are ≤ ~ 2.5 wt % which is attributed to OH⇔F exchange with a COH-carbonatitic fluid, derived from moisture adsorbed onto the starting materials. Additionally, F partitioning into coexisting silicates via F-(OH) defects likely contributed to this instability. In both bulks apatite remained stable to 8.7 GPa at 1000°C to 1200°C, coexisting with olivine + orthopyroxene + clinopyroxene + garnet. In the lherzolite, apatite and tuite coexisted at 8.7 GPa and 1050°C. The upper-T stability limit of apatite in the lherzolite was identified between 1100°C and 1150°C at 3 GPa and between 1150°C and 1200°C at 8.7 GPa. In the harzburgite, apatite disappeared between 1200°C and 1250°C at 3 GPa but remained stable at 1200°C at 8.7 GPa. Apatite breakdown was attributed to its dissolution in a COH-fluid of presumably carbonatitic nature accompanied by transfer of phosphorus from apatite to coexisting silicates. The higher T stability of apatite in harzburgite is likely due to its lower bulk Al content, which results in a reduced modal abundance of garnet. The P–T stability field of F–OH apatite was found to be comparable to that of Cl–OH apatite, suggesting that F does not significantly enhance apatite stability in the same way it stabilizes hydrous silicates. No evidence was found for apatite approaching the pure F-apatite end-member, even when the amount of apatite was minimal. Both apatite and tuite are major potential carriers of large-ion lithophile (LILEs) and rare earth elements (REEs) within their respective P–T stability fields. The role of apatite as a carrier of phosphorus, volatiles, and incompatible trace elements in the mantle and subducting oceanic crust has evolved due to secular cooling of the Earth. Prior to the late Proterozoic, relatively flat subduction P–T paths intersected the upper-T stability limit of apatite at shallow depths (likely 70–140 km), resulting in the transfer of the apatite element inventory to partial melts or fluids rather than to tuite. It was not until the late Proterozoic to early Phanerozoic, that subduction zone P–T paths became sufficiently steep to intersect the upper P stability limit of apatite. This transition enabled the formation of tuite through subduction processes for the first time. The establishment of the apatite–tuite transport system also initiated volatile release at depths of ~220 to 280 km and facilitated the transport of U and Th into the deep mantle.
Source: Journal of Petrology
Institution: Universität Innsbruck
Language: English
Subfield: Geophysics
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $4,353
Funders: University of Bern, Austrian Science Fund
OpenAlex ID: W4409481511