Solubility and speciation of iron in hydrothermal fluids

Iron is among the most abundant elements in Earth’s crust and is also a major aqueous solute in a variety of hydrothermal settings, yet major questions remain regarding the solubility and speciation of iron at hydrothermal conditions. Here, we conducted hydrothermal diamond-anvil cell experiments using synchrotron-radiation micro-XRF and XANES analyses as well as Raman spectroscopyin situ at hydrothermal temperatures and pressures, to characterize the solubility and speciation of iron in fluids buffered by a variety of mineral assemblages. Our experiments included the assemblages hematite-magnetite (HM), fayalite-magnetite-quartz (FMQ), and magnetite-pyrite-pyrrhotite (MPP). Our results indicate highest solubilities of HM in HCl solutions. In sodium chloride solutions of similar molalities, FMQ shows higher Fe solubility than MPP. XANES data are interpreted as preponderance of ferrous iron in all experiments. Comparison of XANES spectra of these solutions with calculated XANES spectra from the literature indicates octahedral FeClx(H2O)6−x2−x (x = 0–3) as predominant Fe(II) species at lower Cl-Fe ratios, and suggests additional tetrahedral FeCl42− or FeCl3(H2O) at high Cl-Fe ratios. Raman spectra to 600 °C show that the predominant species in ferric iron solutions is FeCl2(H2O)4+ at temperatures less than 100 °C, which transitions to FeCl4 between 100 and 200 °C. An additional Raman band that occurred in some spectra of a H2O + HCl fluid equilibrated with hematite ± magnetite at temperatures greater than 300 °C may originate from an FeCl3(H2O)x (x = 0–3) species. All Raman spectra of ferrous iron solutions show a fairly broad band at about 280 cm−1, which can be interpreted to stem from Fe(II)–Cl vibrations of FeCl3(H2O)x (x = 0–3) (point group D3h) species or, at low Cl-Fe ratios of about two or less, from octahedral FeClx(H2O)6−x2−x (x = 0–3) species. These results provide important constraints on the hydrothermal mobilization of iron and fluid-rock reactions involving iron- and chloride-bearing fluids.

Scholten, L., Schmidt, C., Lecumberri-Sanchez, P., Newville, M., Lanzirotti, A., Sirbescu, M-L, C., Steele-MacInnis, M., Solubility and speciation of iron in hydrothermal fluids, Geochimica et Cosmochimica Acta, Vol 252 (2019) 126-143, DOI : 10.1016/j.gca.2019.03.001


Schematic images (not to scale) of the experimental setup at the beamline for X-ray fluorescence and XANES analyses. The upper image shows the hydrothermal diamond anvil cell (HDAC); modified after Schmidt et al. (2007). (b) Image of the sample chamber loaded with a hematite crystal, a magnetite crystal, and fluid at 500 °C.