Redox evolution and mass transfer during serpentinization: An experimental and theoretical study at 200 °C, 500 bar with implications for ultramafic-hosted hydrothermal systems at Mid-Ocean Ridges

Highly reducing and high-pH vent fluids characterize moderately low temperature ultramafic-hosted hydrothermal systems, such as the recently discovered Lost City hydrothermal field at 30°N Mid-Atlantic Ridge Ridge (MAR). To better understand the role of mineral reaction rates on changes in fluid chemistry and mineralization processes in these and similar systems, we conducted an experimental study involving seawater and peridotite at 200 °C, 500 bar. Time series changes in fluid chemistry were monitored and compared with analogous data predicted using experimental and theoretical data for mineral dissolution rates. Although there was qualitative agreement between predicted and measured changes in the chemical evolution of the fluid for some species, the rate and magnitude of increase in pH, dissolved chloride and H₂ did not agree well with predictions based on theoretical modeling results. Experimental data indicate that dissolved H₂ abruptly and intermittently increased, reaching a value only approximately 20% of that predicted assuming magnetite as the primary Fe-bearing alteration phase. The distribution and valence of Fe in primary and secondary minerals reveal that the most abundant secondary mineral, serpentine, contained significant amounts of both ferric and ferrous Fe, with the less abundant brucite, also being Fe-rich (X_Fe = 0.3). Surprisingly, magnetite was present in only trace amounts, indicating that H₂ generation was largely accommodated by the formation of Fe-chrysotile. Accordingly, the diversity of Fe-bearing secondary minerals together with rates of serpentinization less than theoretically predicted, account best for the relatively low dissolved H₂ concentrations produced. Thus, the experimental data can be used to obtain provisional estimates of thermodynamic data for Fe-bearing minerals, enhancing the application of reaction path models depicting mass transfer processes during serpentinization at mid-ocean ridges. Similarly, the observed differences between theoretically predicted and experimentally measured pH values result from constraints imposed by complex patterns of mass transfer inherent to the experimental system. In particular, the experimental observation of a late stage increase in Na/Cl ratio likely results from the dissolution of a Na₂O component of clinopyroxene, which causes pH to increase sufficiently to induce precipitation of a Ca-bearing phase, perhaps portlandite. As with the redox variability observed during the experiment, this event could not be predicted, underscoring the need to use caution when modeling alteration processes in the chemically complex ultramafic-hosted hydrothermal systems at elevated temperatures and pressures.     

Experimental study of gold-hydrosulphide complexing in aqueous solutions at 350-500°C, 500 and 1000 bars using mineral buffers

The solubility of gold was measured in KCl solutions (0.001-0.1 m) at near-neutral to weakly acidic pH in the presence of the K-feldspar-muscovite-quartz, andalusite-muscovite-quartz, and pyrite-pyrrhotite-magnetite buffers at temperatures 350 to 500°C and pressures 0.5 and 1 kbar. These mineral buffers were used to simultaneously constrain pH, f(S₂), and f(H₂). The experiments were performed using a CORETEST flexible Ti-cell rocking hydrothermal reactor enabling solution sampling at experimental conditions. Measured log m(Au) (mol/kg H₂O) ranges from −7.5 at weakly acid pH to −5.9 in near-neutral solutions, and increases slightly with temperature. Gold solubility in weakly basic and near-neutral solutions decreases with decreasing pH at all temperatures, which implies that Au(HS)₂ is the dominant Au species in solution. In more acidic solutions, solubility is independent of pH. Comparison of the experimentally measured solubilities with literature values for Au hydrolysis constants demonstrates that at 350°C dominates Au aqueous speciation at the weakly acidic pH and f(S₂)/f(H₂) conditions imposed by the pyrite-pyrrhotite-magnetite buffer. In contrast, at temperatures >400°C becomes less important and predominates in weakly acid solutions. Solubility data collected in this study were used to calculate the following equilibrium reaction constants:

     

REE controls in ultramafic hosted MOR hydrothermal systems: An experimental study at elevated temperature and pressure

A hydrothermal experiment involving peridotite and a coexisting aqueous fluid was conducted to assess the role of dissolved Cl⁻ and redox on REE mobility at 400°C, 500 bars. Data show that the onset of reducing conditions enhances the stability of soluble Eu⁺² species. Moreover, Eu⁺² forms strong aqueous complexes with dissolved Cl⁻ at virtually all redox conditions. Thus, high Cl⁻ concentrations and reducing conditions can combine to reinforce Eu mobility. Except for La, trivalent REE are not greatly affected by fluid speciation under the chemical and physical condition considered, suggesting control by secondary mineral-fluid partitioning. LREE enrichment and positive Eu anomalies observed in fluids from the experiment are remarkably similar to patterns of REE mobility in vent fluids issuing from basalt- and peridotite-hosted hydrothermal systems. This suggests that the chondrite normalized REE patterns are influenced greatly by fluid speciation effects and secondary mineral formation processes. Accordingly, caution must be exercised when using REE in hydrothermal vent fluids to infer REE sources in subseafloor reaction zones from which the fluids are derived. Although vent fluid patterns having LREE enrichment and positive Eu anomalies are typically interpreted to suggest plagioclase recrystallization reactions, this need not always be the case.     

Bi-melt formation and gold scavenging from hydrothermal fluids: An experimental study

The role of polymetallic melts in scavenging ore components has recently been highlighted in the context of fluid-poor metamorphosed ore deposits. In contrast, the role of polymetallic melts in systems dominated by hydrothermal fluids remains poorly understood. Using a simple Au-Bi model system, we explored experimentally whether such polymetallic melts can precipitate directly from a hydrothermal fluid, and investigated the ability of these melts to scavenge Au from the solution. The experiments were conducted in custom-built flow-through reactors, designed to reproduce a hydrothermal system where melt components are dissolved at one stage along the flow path (e.g., Bi was dissolved by placing Bi-minerals along the fluid path), whereas melt precipitation was caused further along the flow path by fluid-rock interaction. Bi-rich melts were readily obtained by reaction with pyrrhotite, graphite or amorphous FeS. When Au was added to the system, Bi-Au melts with compositions consistent with the Au-Bi phase diagram were obtained. In the case of fluid reaction with pyrrhotite, epitaxial replacement of pyrrhotite by magnetite was observed, with textures consistent with an interface-coupled dissolution-reprecipitation reaction (ICDRR). In this case, the metallic melt precipitated as blebs that were localized at the replacement front or within the porous magnetite.Direct fractionation of Bi-Au melts from a hydrothermal fluid, or precipitation of a Bi-melt followed by partitioning of Au from ambient fluid, offer new pathways to the enrichment of minor ore components such as Au, without requiring fluid saturation with respect to a Au mineral. This mechanism can explain the strong geochemical affinity recognized between Au and low-melting point chalcophile elements such as Bi in many gold deposits. Examples of deposits where such a model may be applicable include orogenic gold deposits and gold skarns. Contrary to models involving migration of polymetallic melts to explain element remobilization, only small quantities (ppm) of polymetallic melts are required to affect the Au endowment of a deposit via interaction with a hydrothermal fluid. The experiments also show that micro-environments can play a critical role in controlling melt occurrences. For example, reaction fronts developing via ICDR reactions can promote melt formation as observed during the replacement of pyrrhotite by magnetite. The associated transient porosity creates space for the melt and promotes melt-fluid exchanges whereas the reaction front provides local geochemical conditions favorable to melt precipitation (e.g., reduced, low aH₂S(aq), and catalytic surface).     

Rare earth element distribution in >400 °C hot hydrothermal fluids from 5°S, MAR: The role of anhydrite in controlling highly variable distribution patterns

Two submarine hydrothermal vent fields at 5°S, Mid-Atlantic Ridge (MAR) - Turtle Pits and Comfortless Cove - emanate vapor-phase fluids at conditions close to the critical point of seawater (407 °C, 298 bars). In this study, the concentration and distribution of rare earth element (REE) and yttrium (Y) has been investigated. Independent of the major element composition, the fluids display a strong temporal variability of their REE + Y concentrations and relative distributions at different time scales of minutes to years. Chondrite-normalized distributions range from common fluid patterns with light REE enrichment relative to the heavy REE, accompanied by positive Eu anomalies (type I), to strongly REE + Y enriched patterns with a concave-downward distribution with a maximum enrichment of Sm and weakly positive or even negative Eu anomalies (type II). The larger the sum of REE, the smaller Ce_CN/Yb_CN and Eu/Eu∗. We also observed a strong variability in fluid flow and changing fluid temperatures, correlating with the compositional variability.As evident by the positive correlation of total REE, Ca, and Sr concentrations in Turtle Pits and Comfortless Cove fluids, precipitation/dissolution of hydrothermal anhydrite controls the variability in REE concentrations and distributions in these fluids and the transformation of one fluid type to the other. The variable distribution of REE can be explained by the accumulation of particulate anhydrite (with concave-downward REE distribution and negative Eu anomaly) into a fluid with common REE distribution (type I), followed by the modification of the REE fluid signature due to dissolution of incorporated anhydrite. A second model, in which the type II fluids represent a primary REE reaction zone fluid pattern, which is variably modified by precipitation of anhydrite, can also explain the observed correlations of total REE, fractionation of LREE/HREE and size of Eu anomaly as well as Ca, Sr. The emanation of such a fluid may be favored in a young hydrothermal system in its high-activity phase with short migration paths and limited exchange with secondary minerals. However, this model is not as well constrained as the other and requires further investigations.The strongly variable REE fluid signature is restricted to the very hot, actively phase-separating hydrothermal systems Turtle Pits and Comfortless Cove at 5°S and has not been observed at the neighboring Red Lion vent field, which continuously emanates 350 °C hot fluid and displays a stable REE distribution (type I).     

PGE fractionation in seafloor hydrothermal systems: examples from mafic- and ultramafic-hosted hydrothermal fields at the slow-spreading Mid-Atlantic Ridge

The distribution of platinum group elements (PGEs) in massive sulfides and hematite–magnetite±pyrite assemblages from the recently discovered basalt-hosted Turtle Pits hydrothermal field and in massive sulfides from the ultramafic-hosted Logatchev vent field both on the Mid-Atlantic Ridge was studied and compared to that from selected ancient volcanic-hosted massive sulfide (VHMS) deposits. Cu-rich samples from black smoker chimneys of both vent fields are enriched in Pd and Rh (Pd up to 227 ppb and Rh up to 149 ppb) when compared to hematite–magnetite-rich samples from Turtle Pits (Pd up to 10 ppb, Rh up to 1.9 ppb). A significant positive correlation was established between Cu and Rh in sulfide samples from Turtle Pits. PGE chondrite-normalized patterns (with a positive Rh anomaly and Pd and Au enrichment), Pd/Pt and Pd/Au ratios close to global MORB, and high values of Pd/Ir and Pt/Ir ratios indicate mafic source rock and seawater involvement in the hydrothermal system at Turtle Pits. Similarly shaped PGE chondrite-normalized patterns and high values of Pd/Pt and Pd/Ir ratios in Cu-rich sulfides at Logatchev likely reflect a similar mechanism of PGE enrichment but with involvement of ultramafic source rocks.     

Strontium behavior in the aragonite-calcite transformation: An experimental study at 40–98°C

The behavior of strontium during the replacement of aragonite by calcite, in a closed system between 40°C and 98°C, has been experimentally investigated. The experiments were conducted in CaCl₂ solutions, with and without NaCl. The distribution coefficient of strontium in calcite (λ_Sr^2+C) was found to be affected only slightly by temperature changes, and almost insignificantly by the presence of NaCl. λ_Sr^2+C values at 0.01 m_Ca²⁺ (its concentration in normal sea water) are: 0.055 at 40°C and 0.058 at 98°C. These results indicate that the low (around 500 ppm) concentration of strontium in ancient limestones could have been brought about by aragonite-to-calcite transformation in a system open to sea water, and are not necessarily indicative of replacement in fresh waters.     

An experimental study of alteration of oceanic crust and terrigenous sediments at moderate temperatures (51 to 350°C): insights as to chemical processes in near-shore ridge-flank hydrothermal systems

Studies of hydrothermal circulation within partly buried basement on the eastern flank of the Juan de Fuca Ridge (JFR) have shown that ridge-flank geochemical fluxes are potentially important for the global budgets of some elements. There are major uncertainties in these flux calculations, however, because the composition of these basement fluids is strongly dependent on temperature and because they may be modified by interaction with the overlying terrigenous sediments, either by diffusive exchange with basement or during upwelling to the seafloor. To better understand the nature and temperature control of basalt-fluid and sediment-fluid reactions at the JFR flank, we have conducted laboratory experiments between 51 and 350°C and at 400 bars pressure. K, Rb, and Si are leached from basalt between 150 and 351°C, and Sr and U are taken up. The direction of exchange of Li and Ca with basalt varies with temperature. Li and Sr are removed from fluid at 150°C, but isotope studies show that there is simultaneous release of both elements from basalt, indicating that uptake is controlled by the formation of secondary minerals. Moreover, our experiments confirm that Sr isotope exchange with oceanic crust occurs at moderate temperature and is not confined to high-temperature axial hydrothermal systems. Our data and field data from the JDR flank indicate that uptake of U into basalt at moderate temperature could remove between 9.9 and 15 × 10⁶ mol U yr⁻¹ from the oceans. This is higher than a recent estimate based on measurements of U in altered ocean crust (5.7 ± 3.3 × 10⁶ mol yr⁻¹), which concords with arguments that the Δ_element/heat ratios of JDR flank fluids are too large to be representative of average global flank fluids. K, Ca, Sr, Ba, Li, Si, and B are leached from terrigenous sediments between 51 and 350°C, and U is taken up. Cs and Rb are removed from the fluid below 100°C and leached from the sediment at higher temperature. Sr isotope data show that Sr is preferentially mobilised from volcanic components within terrigenous sediments, which may lead to an overestimation of the ridge-flank Sr isotope flux at the JDR if there is exchange of sediment pore fluids with basement.     

A new Fe/Mn geothermometer for hydrothermal systems: Implications for high-salinity fluids at 13°N on the East Pacific Rise

Field and experimental investigations demonstrate the chemistry of mid-ocean ridge hydrothermal vent fluids reflects fluid-mineral reaction at higher temperatures than those typically measured at the seafloor. To account for this and, in turn, be able to better constrain sub-seafloor hydrothermal processes, we have developed an empirical geothermometer based on the dissolved Fe/Mn ratio in high-temperature fluids. Using data from basalt alteration experiments, the relationship; T (°C) = 331.24 + 112.41*log[Fe/Mn] has been calibrated between 350 and 450 °C. The apparent Fe-Mn equilibrium demonstrated by the experimental data is in good agreement with natural vent fluids, suggesting broad applicability. When used in conjunction with constraints imposed by quartz solubility, associated sub-seafloor pressures can be estimated for basalt-hosted systems. As an example, this methodology is used to interpret new data from 13°N on the East Pacific Rise, where high-temperature fluids both enriched and depleted in chloride (339-646 mmol/kg), relative to seawater, are actively venting within a close proximity. Accounting for these variable salinities, active phase separation is clearly taking place at 13°N, yet the fluid Fe/Mn ratios and the silica concentrations suggest equilibration at temperatures less than those coinciding with the two-phase region. These data show the chloride-enriched fluid reflects the highest temperature and pressure (∼432 °C, 400 bars) of equilibration, consistent with circulation near the top of the inferred magma chamber. This is in agreement with the elevated CO₂ concentration relative to the chloride-depleted fluids. The noted temperature derived from the Fe/Mn geothermometer is higher than the critical temperature for a fluid of equivalent salinity. This carries the important implication that, despite being chloride-enriched relative to seawater, these fluids evolved as the vapor component of even higher salinity brine.     

Experimental determination of the activity-composition relations and phase equilibria of H2O-CO2-NaCl fluids at 500°C, 500 bars

An understanding of the activity-composition (a-X) relations and phase equilibria of halite-bearing, mixed-species supercritical fluids is critically important in many geological and industrial applications. We have performed experiments on H₂O-CO₂-NaCl fluids at 500°C, 500 bar, to obtain accurate and precise data on their a-X relations and phase equilibria. Two kinds of experiments were performed. First, H₂O-CO₂-NaCl samples were reacted at fixed activities of H₂O = 0.078, 0.350, 0.425, 0.448, 0.553, 0.560, 0.606, 0.678, 0.798, 0.841, and 0.935 to define the tie lines of known H₂O activity in the halite-vapor and vapor-brine fields. Results indicate that fluids with all but the last of these H₂O activities lie in the vapor-halite two-phase region and that a fluid with a_H2O = 0.841 has a composition close to the three-phase (vapor + brine + halite) field. A second set of experiments was performed to determine the solubility of NaCl in parts of the system in equilibrium with halite. Data from these experiments suggest that the vapor corner of the three-phase field lies at H₂O contents above X_H2O = 0.58 and X_NaCl = 0.06, and below X_H2O = 0.75 and X_NaCl = 0.06, which is a significantly more H₂O-rich composition than indicated by existing thermodynamic models.     

Speciation and thermodynamic properties for cobalt chloride complexes in hydrothermal fluids at 35-440 °C and 600 bar: An in-situ XAS study

Aqueous Co(II) chloride complexes play a crucial role in cobalt transport and deposition in ore-forming hydrothermal systems, ore processing plants, and in the corrosion of special Co-bearing alloys. Reactive transport modelling of cobalt in hydrothermal fluids relies on the availability of thermodynamic properties for Co complexes over a wide range of temperature, pressure and salinity. Synchrotron X-ray absorption spectroscopy was used to determine the speciation of cobalt(II) in 0-6 m chloride solutions at temperatures between 35 and 440 °C at a constant pressure of 600 bar. Qualitative analysis of XANES spectra shows that octahedral species predominate in solution at 35 °C, while tetrahedral species become increasingly important with increasing temperature. Ab initio XANES calculations and EXAFS analyses suggest that in high temperature solutions the main species at high salinity (Cl:Co >> 2) is CoCl₄²⁻, while a lower order tetrahedral complex, most likely CoCl₂(H₂O)_2(aq), predominates at low salinity (Cl:Co ratios ∼2). EXAFS analyses further revealed the bonding distances for the octahedral Co(H₂O)₆²⁺ (^octCo-O = 2.075(19) Å), tetrahedral CoCl₄²⁻ (^tetCo-Cl = 2.252(19) Å) and tetrahedral CoCl₂(H₂O)_2(aq) (^tetCo-O = 2.038(54) Å and ^tetCo-Cl = 2.210(56) Å). An analysis of the Co(II) speciation in sodium bromide solutions shows a similar trend, with tetrahedral bromide complexes becoming predominant at higher temperature/salinity than in the chloride system. EXAFS analysis confirms that the limiting complex at high bromide concentration at high temperature is CoBr₄²⁻. Finally, XANES spectra were used to derive the thermodynamic properties for the CoCl₄²⁻ and CoCl₂(H₂O)_2(aq) complexes, enabling thermodynamic modelling of cobalt transport in hydrothermal fluids. Solubility calculations show that tetrahedral CoCl₄²⁻ is responsible for transport of cobalt in hydrothermal solutions with moderate chloride concentration (∼2 m NaCl) at temperatures of 250 °C and higher, and both cooling and dilution processes can cause deposition of cobalt from hydrothermal fluids.     

Controls on gold solubility in arc magmas: An experimental study at 1000 °C and 4 kbar

In order to (1) explain the worldwide association between epithermal gold-copper-molybdenum deposits and arc magmas and (2) test the hypothesis that adakitic magmas would be Au-specialized, we have determined the solubility of Au at 4 kbar and 1000 °C for three intermediate magmas (two adakites and one calc-alkaline composition) from the Philippines. The experiments were performed over a fO₂ range corresponding to reducing (∼NNO−1), moderately oxidizing (∼NNO+1.5) and strongly oxidizing (∼NNO+3) conditions as measured by solid Ni-Pd-O sensors. They were carried out in gold containers, the latter serving also as the source of gold, in presence of variable amounts of H₂O and, in a few additional experiments, of S. Concentrations of Au in glasses were determined by LA-ICPMS. Gold solubility in melt is very low (30-240 ppb) but increases with fO₂ in a way consistent with the dissolution of gold as both Au¹⁺ and Au³⁺ species. In the S-bearing experiments performed at ∼NNO−1, gold solubility reaches much higher values, from ∼1200 to 4300 ppb, and seems to correlate with melt S content. No systematic difference in gold solubility is observed between the adakitic and the non-adakitic compositions investigated. Oxygen fugacity and the sulfur concentration in melt are the main parameters controlling the incorporation and concentration of gold in magmas. Certain adakitic and non-adakitic magmas have high fO₂ and magmatic S concentrations favorable to the incorporation and transport of gold. Therefore, the cause of a particular association between some arc magmas and Au-Cu-Mo deposits needs to be searched in the origin of those specialized magmas by involvement of Au- and S-rich protoliths. The subducted slab, which contains metal-rich massive sulfides, may constitute a potentially favorable protolith for the genesis of magmas specialized with respect to gold.     

Liquid-vapor fractionation of boron and boron isotopes: Experimental calibration at 400°C/23 MPa to 450°C/42 MPa

We experimentally determined the boron partitioning and boron isotope fractionation between coexisting liquid and vapor in the system H₂O−NaCl−B₂O₃. Experiments were performed along the 400 and 450°C isotherms. Pressure conditions ranged from 23 to 28 MPa at 400°C and from 38 to 42 MPa at 450°C. Boron partitions preferentially into the liquid. Its overall liquid-vapor fractionation is, however, weak: Calculated boron distribution coefficients D_B^liquid-vapor are < 2.5 at all run conditions. With decreasing pressure (i.e. increasing opening of the solvus) D_B^liquid-vapor increases along the individual isotherms. Extrapolation to salt saturated conditions yields maximum boron liquid-vapor fractionations of D_B^liquid-vapor = 1.8 at 450°C and D_B^liquid-vapor = 2.7 at 400°C. ¹¹B preferentially fractionates into the vapor. Calculated Δ¹¹B_vapor-liquid = {[(¹¹B/¹⁰B)_vapor - (¹¹B/¹⁰B)_liquid]/(¹¹B/¹⁰B)_{NBS 951}}*1000 are small and range from 0.2 (± 0.7) to 0.9 (± 0.5) ‰ at 450°C and from 0.1 (± 0.6) to 0.7 (± 0.6) ‰ at 400°C. The data indicate increasing isotopic fractionation with decreasing pressure (i.e. increasing opening of the solvus). Extrapolation to salt saturated conditions yields maximum boron isotope liquid-vapor fractionations of Δ¹¹B_vapor-liquid = 1.5 (± 0.7) ‰ at 450°C and Δ¹¹B_vapor-liquid = 1.3 (± 0.6) ‰ at 400°C. The weak boron isotope fractionation suggests similar trigonal speciation in liquid and vapor. Although the boron and boron isotope fractionation between liquid and vapor is only weak, mass balance calculations indicate that for high degrees of fractionation liquid-vapor phase separation in an open system can significantly alter the boron and boron isotope signature of low-salinity hydrous fluids in hydrothermal systems. Comparing the model calculations with natural oceanic hydrothermal fluids, however, indicate that other processes than fluid phase separation dominate the boron geochemistry in oceanic hydrothermal fluids.     

Geochemistry of high H2 and CH4 vent fluids issuing from ultramafic rocks at the Rainbow hydrothermal field (36°14′N, MAR)

The Flores diving cruise was part of the MAST III-AMORES (1995-1998) program funded by the European Union. One of the major achievements of the Flores cruise was the discovery of the Rainbow hydrothermal field hosted in ultramafic rocks south of the Amar segment on the Mid-Atlantic ridge (MAR). The Rainbow hydrothermal fluids exhibit temperatures of 365 °C, pH of 2.8, high chlorinity (750 mmol/kg), and low silica (6.9 mmol/kg). The uniformity in endmember major, minor, trace element concentrations and gas contents suggests that all Rainbow fluids originate from the same deep source. Although H₂S content is relatively low (1.20 mmol/kg), all vent fluids show extraordinary high H₂ (16 mmol/kg), CH₄ (2.5 mmol/kg) and CO (5 μmol/kg) endmember concentrations compared to fluids collected from other vent sites along the MAR. Hydrogen represents more than 40% of the total gas volume extracted from the fluids. At Rainbow, H₂ production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization. Given that exposures of ultramafic rock may be common, particularly along slow-spreading ridges, the production of H₂ may have important implications for microbial activity at and beneath the seafloor.     

An experimental study of the solubility of molybdenum in H2O and KCl-H2O solutions from 500 °C to 800 °C, and 150 to 300 MPa

The solubility of molybdenum (Mo) was determined at temperatures from 500 °C to 800 °C and 150 to 300 MPa in KCl-H₂O and pure H₂O solutions in cold-seal experiments. The solutions were trapped as synthetic fluid inclusions in quartz at experimental conditions, and analyzed by laser ablation inductively coupled plasma mass spectrometry (LA ICPMS).Mo solubilities of 1.6 wt% in the case of KCl-bearing aqueous solutions and up to 0.8 wt% in pure H₂O were found. Mo solubility is temperature dependent, but not pressure dependent over the investigated range, and correlates positively with salinity (KCl concentration). Molar ratios of ∼1 for Mo/Cl and Mo/K are derived based on our data. In combination with results of synchrotron X-ray absorption spectroscopy of individual fluid inclusions, it is suggested that Mo-oxo-chloride complexes are present at high salinity (>20 wt% KCl) and ion pairs at moderate to low salinity (<11 wt% KCl) in KCl-H₂O aqueous solutions. Similarly, in the pure H₂O experiments molybdic acid is the dominant species in aqueous solution. The results of these hydrothermal Mo experiments fit with earlier studies conducted at lower temperatures and indicate that high Mo concentrations can be transported in aqueous solutions. Therefore, the Mo concentration in aqueous fluids seems not to be the limiting factor for ore formation, whereas precipitation processes and the availability of sulfur appear to be the main controlling factors in the formation of molybdenite (MoS₂).     

An experimental study of spinel clinopyroxenite xenoliths from the Duncansby Ness vent,Caithness, Scotland

Xenoliths of coarse-grained spinel-clinopyroxenite up to 15 cm in size occur in tuff in an isolated Permian vent on the Caithness coast at Duncansby Ness. Highly altered fragments of chrome-spinel lherzolite and wehrlite are also found in the tuff and in a body of monchiquite within the vent. The spinel-clinopyroxenites consist of aluminous augite and aluminous pleonaste spinel (FeO/MgO = 0.9) and their texture suggests the spinel to have exsolved from the augite. Experiments on representative natural xenolith compositions at 18 kb (dry) indicate that all the spinel in the estimated average bulk composition (Sp_4.9Px_95.1) could have exsolved from an original homogeneous pyroxene. Initial fractionation of such a pyroxene from an alkali basaltic magma at P≥18 kb, 1450-1350 °C, would be followed by spinel exsolution at T< 1290 ° C. A similar origin by fractionation of a highly aluminous augite (± aluminous spinel) at high pressure, with subsequent spinel exsolution is proposed for spinel-clinopyroxenites from alkali basalts elsewhere in the world. The similarity of these xenoliths suggests that such a process may form an important stage in the evolution of some undersaturated basaltic rocks.     

Experimental study of arsenic speciation in vapor phase to 500°C: implications for As transport and fractionation in low-density crustal fluids and volcanic gases

The stoichiometry and stability of arsenic gaseous complexes were determined in the system As-H₂O ± NaCl ± HCl ± H₂S at temperatures up to 500°C and pressures up to 600 bar, from both measurements of As^(III) and As^(V) vapor-liquid and vapor-solid partitioning, and X-ray absorption fine structure (XAFS) spectroscopic study of As^(III)-bearing aqueous fluids. Vapor-aqueous solution partitioning for As^(III) was measured from 250 to 450°C at the saturated vapor pressure of the system (P_sat) with a special titanium reactor that allows in situ sampling of the vapor phase. The values of partition coefficients for arsenious acid (H₃AsO₃) between an aqueous solution (pure H₂O) and its saturated vapor (K = mAs_vapor /mAs_liquid) were found to be independent of As^(III) solution concentrations (up to ∼1 to 2 mol As/kg) and equal to 0.012 ± 0.003, 0.063 ± 0.023, and 0.145 ± 0.020 at 250, 300, and 350°C, respectively. These results are interpreted by the formation, in the vapor phase, of As(OH)₃(gas), similar to the aqueous As hydroxide complex dominant in the liquid phase. Arsenic chloride or sulfide gaseous complexes were found to be negligible in the presence of HCl or H₂S (up to ∼0.5 mol/kg of vapor). XAFS spectroscopic measurements carried out on As^(III)-H₂O (±NaCl) solutions up to 500°C demonstrate that the As(OH)₃ complex dominates As speciation both in dense H₂O-NaCl fluids and low-density supercritical vapor. Vapor-liquid partition coefficients for As^(III) measured in the H₂O-NaCl system up to 450°C are consistent with the As speciation derived from these spectroscopic measurements and can be described by a simple relationship as a function of the vapor-to-liquid density ratio and temperature. Arsenic^(III) partitioning between vapor and As-concentrated solutions (>2 mol As/kg) or As₂O₃ solid is consistent with the formation, in the vapor phase, of both As₄O₆ and As(OH)₃. Arsenic^(V) (arsenic acid, H₃AsO₄) vapor-liquid partitioning at 350°C for dilute aqueous solution was interpreted by the formation of AsO(OH)₃ in the vapor phase.The results obtained were combined with the corresponding properties for the aqueous As(III) hydroxide species to generate As(OH)₃(gas) thermodynamic parameters. Equilibrium calculations carried out by using these data indicate that As(OH)₃(gas) is by far the most dominant As complex in both volcanic gases and boiling hydrothermal systems. This species is likely to be responsible for the preferential partition of arsenic into the vapor phase as observed in fluid inclusions from high-temperature (400 to 700°C) Au-Cu (-Sn, -W) magmatic-hydrothermal ore deposits. The results of this study imply that hydrolysis and hydration could be also important for other metals and metalloids in the H₂O-vapor phase. These processes should be taken into account to accurately model element fractionation and chemical equilibria during magma degassing and fluid boiling.     

Quartz solubility in the two-phase and critical region of the NaCl-KCl-H2O system: Implications for submarine hydrothermal vent systems at 9°50′N East Pacific Rise

Experiments were performed to investigate quartz solubility in Cl-bearing aqueous solutions at temperature (365-430 °C) and pressure conditions (219-381 bars) near and within the two-phase region of the NaCl-KCl-H₂O system. Dissolved SiO₂ concentrations increased with pressure along a given isotherm, although the magnitude of this decreased with increasing proximity to the two-phase boundary. Upon intersection of the two-phase boundary, however, significant concentrations of dissolved SiO₂ characterized vapor-rich fluids at both subcritical and supercritical conditions. For these fluids, dissolved silica concentrations ranged from 2.81 to 14.6 mmolal, increasing with dissolved chloride concentration. The experimental data permit regression of a density-based relationship, taking account of non-ideal activity-concentration effects, which can be used to better constrain temperatures and pressures from dissolved SiO₂ and chloride in high temperature vent fluids at mid-ocean ridges. Accordingly, pressure and temperature conditions in subseafloor hydrothermal reaction zones at 9°50′N East Pacific Rise (EPR) were estimated applying data from this experimental study to interval (1991-2002) and new field data (2004). Results indicate reaction zone at conditions ranging from 420 to 430 °C at 600 to 1500 m below seafloor. Thus, conditions predicted for 9°50′N East Pacific Rise (EPR) vent fluids suggest that supercritical phase separation might be more common than previously thought.     

Solubility of tin in (Cl, F)-bearing aqueous fluids at 700 °C, 140 MPa: A LA-ICP-MS study on synthetic fluid inclusions

Synthetic fluid inclusions in quartz were grown from cassiterite-saturated fluids in cold-seal pressure vessels at and subsequently analyzed by laser ablation-ICP-MS. Most inclusions were synthesized using a new technique that allows entrapment of fluid that had no immediate contact to the capsule walls, such that potential disequilibrium effects due to alloying could be avoided. Measured Sn solubilities increase with increasing ligand concentration in the fluid, ranging from 100 to 800 ppm in NaCl-bearing fluids (5-35 wt% NaCl), from 70 to 2000 ppm in HF-bearing fluids (0.5-3.2 m HF), and from 0.8 to 11 wt% in HCl-bearing fluids (0.5-4.4 m HCl). Two runs performed with the in-situ cracking method after 1 week of pre-equilibration demonstrate that the speed of hydrogen diffusion through the capsule wall relative to that of fluid inclusion formation is a critical factor in f_O2-dependent solubility studies. Graphical evaluation of the solubility data suggests that Sn may have been dissolved as Sn(OH)Cl in the NaCl-bearing fluids, as Sn(OH)Cl and SnCl₂ in the HCl-bearing fluids, and as SnF₂ in the HF-bearing fluids. Experiments with NaF-bearing fluids produced an additional melt phase with an approximate composition of 53 wt% SiO₂, 25 wt% H₂O, 14 wt% NaF and 8 wt% SnO, which caused the composition of the coexisting fluid to be buffered at 0.5 wt% NaF and 150 ppm Sn. Fluorine-rich, peralkaline melts may therefore serve as important transport media for Sn in the final crystallization stages of tin granites. Based on the available cassiterite-solubility data in fluids and melts, in natural granite systems is estimated to be in the order of 0.1-4 (depending on their aluminosity), suggesting that Sn is not easily mobilized by magmatic-hydrothermal fluids. This interpretation is in accordance with the high degrees of Sn-enrichment commonly observed in highly fractionated melt inclusions. is primarily controlled by the HCl concentration in the fluid, which in turn is a function of the aluminum saturation index of the magma. Compared to HCl, the effect of fluorine on is subordinate.