- 1Macquarie University, School of Natural Sciences, Australia (stephen.foley@mq.edu.au)
- 2Research School of Earth Sciences, Australian National University, Canberra, AT 2601, Australia
- 3Centre for Exploration Targeting and ARC Industry Transformational Training Centre in Critical Resources for the Future, School of Earth Sciences, University of Western Australia, Nedlands 6009, Australia
The determination of aqueous fluid-rock partitioning of trace elements has progressed from (1) simple ‘before and after” analysis of solids, assuming that the discrepancy is dissolved in the fluid, through (2) separation of fluid solute from rock in double capsules and (3) separation in diamond traps and consequent analysis of the solute, to (4) cryoablation and analysis of fluid and its solute in diamond traps in the frozen state [1,2]. The last of these is now accepted as the best method currently available, but very few results are available and most of these have concentrated on eclogite in subduction zones and on H2O and Cl fluids.
Appreciable amounts of CO2 and N2 may be present in fluids, particularly in the upper mantle beneath stable continents, but their effect on mobilising trace elements and acting as metasomatic agents has not been quantified. The host rocks through which fluids flow beneath continents include pyroxenites and hydrous assemblages that may differ from those in subduction zones. We present experimentally determined fluid/rock partition coefficients (Df/r) for peridotite and pyroxenite assemblages in equilibrium with a variety of fluid compositions (H2O, H2O+CO2, H2O+CO2+NaCl, H2O+NaCl, and H2O+NH3) for a large range of trace elements at 1.5 GPa pressure and 800 °C. Experimental fluids were separated from the rocks at high pressures using a glassy carbon trap, which has better ablation characteristics than diamond but remains similarly inert. The fluid and solutes in the trap were analysed by cryocell laser ablation ICP-MS in the frozen state.
We show that these mixed aqueous fluids have higher Df/r for pyroxenite than for peridotite. Df/r for the LILE are 10-100 times higher in pyroxenite than peridotite, especially in H2O+NH3 fluids. We confirm earlier conclusions under different conditions that saline fluids dissolve more LILE than pure H2O. Df/r for the HFSE are low and we do not see high Df/r or strong fractionation of LREE from HREE. Cu, Zn and Ni have the highest Df/r amongst the first-row transition elements. Pt and Re have higher Df/r than the HFSE and are most mobile in non-saline aqueous fluids. The fluid composition affects key geochemical ratios: in peridotite > while in pyroxenite ≈ . In pyroxenite assemblages with H2O + CO2 fluids > , whereas in pure H2O, < . The drastic lowering of Df/r for many elements with decreasing pressure probably leads to an optimal pressure-temperature window for fluid-induced metasomatism in the upper mantle. This will vary in depth depending on geodynamic setting (geothermal gradient) and the lithologies present.
[1] Kessel et al. (2005) Nature 437, 724-727.
[2] Rustioni et al. (2019) Geochemical Perspectives Letters 11, 49-54.
How to cite: Foley, S., Phillips, M., and Shcheka, S.: Experimental fluid/rock partition coefficients for H2O with CO2, NaCl and NH3 fluids with peridotite and pyroxenite by cryoablation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15599, https://doi.org/10.5194/egusphere-egu26-15599, 2026.
