Quantitative Measurement of Rare Earth Elements in Brines: Isolation from the Charged Matrix Versus Direct LA‐ICP‐MS Measurements – A Comparative Study

Abstract

The quantification of the rare earth elements (REEs) in natural rocks and fluids is important not only for the sustainable use of geological resources but also for fundamental geological research. Due to significant improvements in experimental, modelling and analytical techniques, it was discovered that the REEs could be efficiently mobilised and transported by hydrothermal fluids. Thus, determining the REEs in fluids, especially in highly saline solutions, is a prerequisite to understand their fate in hydrothermal fluids. An analytical methodology was established to determine the trace quantities of the REEs in aqueous solutions with compositions typical to seawater and brines. The developed protocol was applied for the quantification of REEs in experimental sodium‐rich carbonate‐bearing hydrothermal solutions. For this purpose, we used two analytical approaches: (a) the REEs were chromatographically isolated from the charged matrix and introduced into a single collector quadrupole ICP‐MS and (b) direct determination of the REEs by LA‐ICP‐MS in experimental solutions loaded into glass capillaries without any additional isolation procedure. Both LA‐ICP‐MS and solution nebulisation ICP‐MS analysis after isolation of the REEs provide identical data in terms of REE concentrations in brines and are consistent within analytical uncertainties. Being an express method, LA‐ICP‐MS can be recommended for REE quantification in brines. At the same time, isolation of the REEs from the charged matrix with subsequent ICP‐MS measurement provides higher accuracy and precision, giving additional information for the REEs mass fractions that are below the detection limit of the LA‐ICP‐MS that was estimated at the ~ 10 ng g−1 level.

Carbonate complexation enhances hydrothermal transport of rare earth elements in alkaline fluids

“…The study of trace element behavior in hydrothermal fluids is challenging for a number of reasons, including low concentrations and potential backreactions upon quenching that lead to retrograde precipitation of new mineral phases, possibly obliterating the high-temperature information. Therefore, studies quantifying the solubility of REE minerals, their partitioning between different phases, or the mechanisms that enable their hydrothermal transport have been limited in terms of phases (e.g., the solubility of REECl 3 (s), REEF 3 (s), and, more recently, REEPO 4 (s)) and pH-P-T conditions (pH < 2 and T < 300 °C) 21,24,25,36,[40][41][42] .…”

“…Finally, a technical study of REE quantification methods in fluids also reported 10's to 100's ppm REE dissolved in Na 2 CO 3 /NaHCO 3 solutions to 100-200 °C and 4 MPa 36 , further confirming the potential of alkali-and carbonaterich solutions to carry significant amounts of REE, and potentially scavenge and transport REE in and out of carbonatite intrusions. While the relative HREE enrichments reported by Tsay et al 33 , Anenburg et al 35 , and Kokh et al 36 suggest that the REE complexes formed in alkali-and carbonate-bearing fluids could be HREEselective, the actual speciation of REE in alkaline fluids remains unconstrained from such experiments. The only available constraints on REE speciation under neutral to alkaline conditions are from the early thermodynamic models of Haas et al 37 and Wood et al 38 , which generated thermodynamics properties for REE(OH) 2+ , REE(OH) 2 + , REE(OH) 3 (aq), REE(OH) 4 − and REE(CO 3 ) + to ~1000 °C and 500 MPa via extrapolations from ambient conditions.…”

Accurate determination of rare earth elements in small volumes of porewater from marine sediments by laser ablation solution sampling ICP-MS

“…Although it is mainly applied for solid samples, 32 LA-ICP-MS has also been demonstrated to be suitable for liquid sample analysis. 33–36 For previously reported co-precipitation methods, a final volume of 2–5 mL was obtained to fulfil the requirement of conventional solution nebulization ICP-MS. 13,27 Amazingly, for solution sample introduction by LA, only 1 μL of the liquid is consumed per analysis (∼50 s of laser ablation, 160 μm spot size, and 10 Hz pulse frequency, respectively). 34 In view of the micro sample consumption feature by LA-ICP-MS analysis, it would be completely sufficient even if the volume of the final dissolved sample solution is only a few tens of microliters, which is an unprecedented reduction in the requirement for sample sizes.…”