Transport of Pt and Pd in hydrous sodium silicate melt: A study from X-ray absorption spectroscopy

The molecular-level speciation of Platinum (Pt) and Palladium (Pd) in silicate melt remains an essential issue in understanding the geochemical behavior of these elements in planetary magmas. However, this speciation is poorly understood, particularly in hydrous silicate melts. Here, we present experiments to determine the Pt and Pd speciation in H₂O-rich alkali silicate melts using Synchrotron-based X-ray absorption spectroscopy (XAS). All experiments were conducted in a rapid-quench cold seal system at 750 °C, 100 MPa for 72 h. Chlorine (Cl) contents and oxygen fugacity (fO₂) were varied. The fO₂, controlled by either (1) the intrinsic hydrogen fugacity of the autoclave, or (2) a double-capsule assembly, ranged from Ni-NiO (NNO) + 0.5 to Mn₃O₄-Mn₂O₃. The Pt concentrations in the glass were determined by laser ablation ICP-MS and further evaluated by X-ray fluorescence. Quenched glasses contained Pt or Pd nuggets, leading to heterogeneous distributions that were addressed through combining these two methods. The solubility of Pt in the hydrous peralkaline melt varies from ∼220 ppm under the most oxidizing conditions to <10 ppm at NNO + 0.5.By comparing the Pt L₃-edge or Pd K-edge spectra with reference standards, we show that the Pt and Pd oxidation states increase systematically with increasing fO₂. Under the most oxidized condition (Mn₃O₄-Mn₂O₃), Pt is bonded to O with an effective valence near 3+, i.e., a mixed Pt⁴⁺-and Pt²⁺-like environment, with a first-shell Fourier Transform peak occurring at ∼1.70 Å (phase-shift uncorrected). Pd at the same buffer is also O bonded reflecting Pd²⁺and a Fourier Transform peak at ∼1.44 Å. A decrease in both the absorption edge and white line was observed under more reduced conditions, indicating reduction to lower valence states for both elements. At NNO + 0.5, both Pt and Pd spectra closely approach their respective metal signals. Notably, no Pt-Cl or Pd-Cl scattering is detected in any of the measured samples, despite up to 2.0 wt% Cl dissolved in the melt, demonstrating that chloride does not enter the first coordination sphere of Pt or Pd in these hydrous peralkaline melts under present experimental conditions. These results show that for S-free conditions, redox controls dominate PGE mobility in hydrous silicate melts: appreciable dissolved Pt and Pd species occur only for fO₂ > HM; below this, metallic states dominate, promoting nuggeting and limiting long-range melt-mediated transport.