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Hydrothermal processes that lead to REE fractionation and redistribution are important for understanding water-rock interactions in geothermal energy resources and mineral deposits, and for determining how submarine hydrothermal activity affects the composition of oceanic crust. Much previous work on REE transport and deposition has focused on submarine hydrothermal vents. We report REE concentrations in boiled fluids sampled from five subaerial, basalt-hosted geothermal fields, and explore controls on aqueous REE concentrations by ligand complexation and mineral supersaturation. Samples that boiled at pressures between 0.8 and 2.83 MPa were obtained from the Reykjanes, Svartsengi, Hellisheidi, and Nesjavellir geothermal systems in Iceland, and the Puna geothermal system in Hawaii. For comparison, we also report REE concentrations in hydrothermal fluids from the sediment hosted submarine Middle Valley hydrothermal system, which boiled at >250 MPa. The pH(25°C) values of the sampled subaerial geothermal fluids range from 3.94 to 6.77, and Cl concentrations range from near seawater (502 mmol/kg) to dilute (1.9 mmol/kg). La, Ce and Eu are the only REE present at levels above 5 picomole/kg (pmol/kg) in the boiled geothermal fluids; and there are notable CI chondrite normalized La and Eu anomalies in the saline fluids. REE concentrations in Middle Valley hydrothermal fluids fall within the typical range reported for submarine hydrothermal fluids and have around two orders of magnitude higher REE than the boiled subaerial geothermal fluids. Bulk samples of precipitates in pipes from the Reykjanes geothermal system have detectable REE, confirming that downhole fluids have lost REE during boiling and production of fluids for geothermal energy. Isenthalpic boiling models show that the proportions of La and Eu chloride complexes increase relative to other aqueous species as boiling progresses, attenuating the incorporation of La and Eu into precipitated well scale solids. Fluorapatite is calculated to precipitate on boiling of low pH and saline fluids and calcite is calculated to precipitate from dilute and near-neutral pH fluids, and these minerals likely sequester REE in boiled subaerial fluids. Submarine hydrothermal fluids are constrained to boiling at higher temperatures than subaerial geothermal fluids owing to pressure from overlying cold seawater, therefore secondary minerals and solids that incorporate REE are not extensively precipitated and REE concentrations in the fluids are higher.

Characterizing the geothermal system at Surprise Valley (SV), northeastern California, is important for determining the sustainability of the energy resource, and mitigating hazards associated with hydrothermal eruptions that last occurred in 1951. Previous geochemical studies of the area attempted to reconcile different hot spring compositions on the western and eastern sides of the valley using scenarios of dilution, equilibration at low temperatures, surface evaporation, and differences in rock type along flow paths. These models were primarily supported using classical geothermometry methods, and generally assumed that fluids in the Lake City mud volcano area on the western side of the valley best reflect the composition of a deep geothermal fluid. In this contribution, we address controls on hot spring compositions using a different suite of geochemical tools, including optimized multicomponent geochemistry (GeoT) models, hot spring fluid major and trace element measurements, mineralogical observations, and stable isotope measurements of hot spring fluids and precipitated carbonates. We synthesize the results into a conceptual geochemical model of the Surprise Valley geothermal system, and show that high-temperature (quartz, Na/K, Na/K/Ca) classical geothermometers fail to predict maximum subsurface temperatures because fluids re-equilibrated at progressively lower temperatures during outflow, including in the Lake City area. We propose a model where hot spring fluids originate as a mixture between a deep thermal brine and modern meteoric fluids, with a seasonally variable mixing ratio. The deep brine has deuterium values at least 3 to 4‰ lighter than any known groundwater or high-elevation snow previously measured in and adjacent to SV, suggesting it was recharged during the Pleistocene when meteoric fluids had lower deuterium values. The deuterium values and compositional characteristics of the deep brine have only been identified in thermal springs and groundwater samples collected in proximity to structures that transmit thermal fluids, suggesting the brine may be thermal in nature. On the western side of the valley at the Lake City mud volcano, the deep brine-meteoric water mixture subsequently boils in the shallow subsurface, precipitates calcite, and re-equilibrates at about 130 °C. On the eastern side of the valley, meteoric fluid mixes to a greater extent with the deep brine, cools conductively without boiling, and the composition is modified as dissolved elements are sequestered by secondary minerals that form along the cooling and outflow path at temperatures

Geothermal resource exploration is generally limited to areas with surface expressions of thermal activity (fumaroles and hot springs), or relies on expensive geophysical exploration techniques. In this study, the hidden subsurface distribution of geothermal fluids has been identified using a free and publicly available water quality dataset from agricultural and domestic water wells in Surprise Valley, northeastern California. Thermally evolved waters in Surprise Valley have element ratios that vary in response to Ca carbonate and Mg silicate mineral precipitation, and have elevated total dissolved solids (TDS). The arid climate in Surprise Valley leads to surface water evaporation in a closed basin, producing high TDS Na-Cl-CO3-SO4 brines in three ephemeral alkali lakes and in shallow groundwater under elevated soil CO2 conditions. Evaporated fluids in Surprise Valley follow a chemical divide that leads to Ca carbonate and Mg silicate mineral precipitation. Plots of dissolved element ratios can be used to distinguish groundwater affected by evaporation from water affected by thermal water-rock interaction, however it is challenging to select components for plotting that best illustrate different fluid evolution mechanisms. Here, we use a principal component analysis of centered log-ratio transformed data, coupled with geochemical models of fluid evaporation and thermal mixing pathways, to identify components to plot that distinguish between groundwater samples influenced by evaporation from those influenced by thermal processes. We find that groundwater samples with a thermal signature come from wells that define a coherent, linear geographical distribution that closely matches the location of known and inferred faults. Modification of the general approach employed here provides promise for identifying blind geothermal resources in other locations, by applying low-cost geochemical modeling and statistical techniques to areas where large groundwater quality geochemical datasets are available.

We describe the lithology and present spatially resolved geochemical analyses of samples from the hydrothermally altered Iceland Deep Drilling Project (IDDP) drill core RN-17B. The 9.3m long RN-17B core was collected from the seawater-dominated Reykjanes geothermal system, located on the Reykjanes Peninsula, Iceland. The nature of fluids and the location of the Reykjanes geothermal system make it a useful analog for seafloor hydrothermal processes, although there are important differences. The recovery of drill core from the Reykjanes geothermal system, as opposed to drill cuttings, has provided the opportunity to investigate evolving geothermal conditions by utilizing in-situ geochemical techniques in the context of observed paragenetic and spatial relationships of alteration minerals. The RN-17B core was returned from a vertical depth of ~2560m and an in-situ temperature of ~345°C. The primary lithologies are basaltic in composition and include hyaloclastite breccia, fine-grained volcanic sandstone, lithic breccia, and crystalline basalt. Primary igneous phases have been entirely pseudomorphed by calcic plagioclase+magnesium hornblende+chlorite+titanite+albitized plagioclase+vein epidote and sulfides. Despite the extensive hydrothermal metasomatism, original textures including hyaloclastite glass shards, lithic clasts, chilled margins, and shell-fragment molds are superbly preserved. Multi-collector LA-ICP-MS strontium isotope ratio (87Sr/86Sr) measurements of vein epidote from the core are consistent with seawater as the dominant recharge fluid. Epidote-hosted fluid inclusion homogenization temperature and freezing point depression measurements suggest that the RN-17B core records cooling through the two-phase boundary for seawater over time to current in-situ measured temperatures. Electron microprobe analyses of hydrothermal hornblende and hydrothermal plagioclase confirm that while alteration is of amphibolite-grade, it is in disequilibrium and the extent of alteration is dependent upon protolith type and water/rock ratio. Alteration in the RN-17B core bares many similarities to that of Type II basalts observed in Mid-Atlantic Ridge samples.