Please cite this article as: Timofeev, A., Migdisov, Art.A., Williams-Jones, A.E., An experimental study of the solubility and speciation of niobium in fluoride-bearing aqueous solutions at elevated temperature, Geochimica et Cosmochimica Acta (2015), doi: http://dx. AbstractThe solubility of Nb 2 O 5 and the speciation of niobium in HF-bearing aqueous solutions have been determined at temperatures of 150, 200, and 250 °C and saturated water pressure. At a pH of ~2 and at low HF concentration, niobium is transported primarily as the species Nb(OH) 4 + and at high HF concentration, as the species NbF 2 (OH) 3°. Equilibrium constants for the formation of Nb(OH) 4 + range from -11.23 ± 0.26 to -10.86 ± 0.24 and for the formation of NbF 2 (OH) 3° from -3.84 ± 0.20 to -5.08 ± 0.42, at 150 and 250 °C.The results of this study show that the solubility of Nb 2 O 5 (solid) in aqueous fluids increases with increasing HF concentration, but is not strongly affected by temperature. The influence of pH is variable;at low pH and HF concentration, a decrease in pH increases the solubility of Nb 2 O 5 (solid). At higher pH the reverse may be true. Modeling of the transport and deposition of niobium suggests that simple mixing with a brine is not an effective method for removing niobium from solution. By contrast, interaction of an acidic fluid with carbonate rock results in a rapid reduction of capacity of the fluid to mobilize niobium.

Recent earthquakes have demonstrated that rupture may propagate through geometrically complex networks of faults. Ancient exhumed faults have the potential to reveal the details of complex rupture at seismogenic depths. We present a new set of field observational criteria for determining which of a population of pseudotachylyte fault veins formed in the same earthquake and apply it to map rupture networks representing single earthquakes. An exceptional exposure of an exhumed ancient strand of the Norumbega Shear Zone preserves evidence of multistranded earthquake rupture in the deep seismogenic zone of a continental transform fault. Individual fault strands slipped at least 2–18 cm, so significant slip is represented by each rupture network. Our data show that synchronously slipped faults intersect at angles of 0 to ∼55°, with the opening angles of fault intersections directed toward the dilational quadrants for dextral slip. Multistranded rupture on a fault network instead of rupture of a single fault may result in greater and/or more variable slip and cause slip rake to vary spatially and temporally. Slip on intersecting faults unequivocally means that there will be motion perpendicular to the average fault plane. Modern earthquakes displaying non‐double‐couple components to focal mechanism solutions and spatially varying rake, slip, and anomalous stress drop may be explained by rupture across fault networks that are too close (spatially and temporally) to be resolved seismically as separate events.

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. An experimental study of the solubility and speciation of tantalum in fluoride-bearing aqueous solutions at elevated temperature. Geochimica et Cosmochimica Acta 197, 294-304. http://dx

The behavior of uranium in environments, ranging from those of natural systems responsible for the formation of uranium deposits to those of nuclear reactors providing 11% of the world’s electricity, is governed by processes involving high-temperature aqueous solutions. It has been well documented that uranium is mobile in aqueous solutions in its oxidized, U6+ state, whereas in its reduced, U4+ state, uranium has been assumed to be immobile. Here, we present experimental evidence from high temperature (>100 °C) acidic brines that invalidates this assumption. Our experiments have identified a new uranium chloride species (UCl4°) that is more stable under reducing than oxidized conditions. These results indicate that uranium is mobile under reducing conditions and necessitate a re-evaluation of the mobility of uranium, particularly in ore deposit models involving this metal. Regardless of the scenario considered, reducing conditions can no longer be considered a guarantee of uranium immobility.