Solubility and speciation of iron in hydrothermal fluids

Abstract 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 cellexperiments using synchrotron-radiation micro-XRF and XANESanalyses as well as Raman spectroscopy, in 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. XANESdata are interpreted as preponderance of ferrous iron in all experiments. Comparison of XANESspectra of these solutions with calculated XANESspectra from the literature indicates octahedral FeCl x (H 2 O) 6−x 2−x (x = 0–3) as predominant Fe(II) species at lower Cl-Fe ratios, and suggests additional tetrahedral FeCl 4 2− or FeCl 3 (H 2 O) − at high Cl-Fe ratios. Raman spectra to 600 °C show that the predominant species in ferric iron solutions is FeCl 2 (H 2 O) 4 + at temperatures less than 100 °C, which transitions to FeCl 4 − between 100 and 200 °C. An additional Raman band that occurred in some spectra of a H 2 O + HCl fluid equilibrated with hematite ± magnetite at temperatures greater than 300 °C may originate from an FeCl 3 (H 2 O) 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 FeCl 3 (H 2 O) x − (x = 0–3) ( point groupD 3h ) species or, at low Cl-Fe ratios of about two or less, from octahedral FeCl x (H 2 O) 6−x 2−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.