Aqueous Fluid Phase Research Articles

Sulfur isotope fractionation between fluid and andesitic melt: An experimental study

Glasses produced from decompression experiments conducted by Fiege et al. (2014a) were used to investigate the fractionation of sulfur isotopes between fluid and andesitic melt upon magma degassing. Starting materials were synthetic glasses with a composition close to a Krakatau dacitic andesite. The glasses contained 4.55–7.95wt% H2O, ∼140 to 2700ppm sulfur (S), and 0–1000ppm chlorine (Cl). The experiments were carried out in internally heated pressure vessels (IHPV) at 1030°C and oxygen fugacities (fO2) ranging from QFM+0.8 log units up to QFM+4.2 log units (QFM: quartz–fayalite–magnetite buffer). The decompression experiments were conducted by releasing pressure (P) continuously from ∼400MPa to final P of 150, 100, 70 and 30MPa. The decompression rate (r) ranged from 0.01 to 0.17MPa/s. The samples were annealed for 0–72h (annealing time, tA) at the final P and quenched rapidly from 1030°C to room temperature (T).The decompression led to the formation of a S-bearing aqueous fluid phase due to the relatively large fluid–melt partitioning coefficients of S. Secondary ion mass spectrometry (SIMS) was used to determine the isotopic composition of the glasses before and after decompression. Mass balance calculations were applied to estimate the gas–melt S isotope fractionation factor αg-m.No detectable effect of r and tA on αg-m was observed. However, SIMS data revealed a remarkable increase of αg-m from ∼0.9985±0.0007 at >QFM+3 to ∼1.0042±0.0042 at ∼QFM+1. Noteworthy, the isotopic fractionation at reducing conditions was about an order of magnitude larger than predicted by previous works. Based on our experimental results and on previous findings for S speciation in fluid and silicate melt a new model predicting the effect of fO2 on αg-m (or Δ34Sg–m) in andesitic systems at 1030°C is proposed. Our experimental results as well as our modeling are of high importance for the interpretation of S isotope signatures in natural samples (e.g., melt inclusions or volcanic gases).

A New typification scheme of postsedimentation transformations of terrigenous deposits of continents and their margins with allowance for geodynamic factors

Previous studies of postsedimentation and deep rock transformations (the diagenetic, catagenetic, and anchimetamorphic stages) by the author and other Russian lithologists are revised by evaluating the different types of zoning of authigenic mineral formation within terrigenous complexes of different ages in definite structural-tectonic units of continents and their margins. It was emphasized that an intrarock aqueous fluid phase played a dual role in authigenic mineral formation: (1) it served as a medium for internal redistribution of sedimentogenic components or (2) as a carrier of external material. Indicator features of its own newly formed (A1) and allogenic (A2) authigenic minerals are given. It was shown that the background subsidence lithogenesis (SL) of the sedimentary sequence provides the formation of A1 varieties in the sequence, whereas the prevalence of A2 marks the superimposed or secondary alterations (SAs) of sedimentary rock. A combination of SL and SA features is demonstrated for different structures: syneclises and anteclises of cratons, rifts, fore troughs, orogens, and others. The SL and SA formation is determined by definite numerous exogenic and endogenic factors, which are often obliterated but integrated by geodynamic settings and geological evolution of sedimentary complexes. According to this conclusion, a new typification scheme of post-sedimentation transformations was proposed on the paleogeodynamic basis.

The Fate of Sulfur During Fluid-Present Melting of Subducting Basaltic Crust at Variable Oxygen Fugacity

To constrain the effect of redox state on sulfur transport from subducting crust to mantle wedge during fluid-present melting and the stability of sulfur-bearing phases in the downgoing ocean crust, here we report high-pressure phase equilibria experiments on a H2O-saturated mid-ocean ridge basalt with 1 wt % S at variable oxygen fugacity (⁠⁠). Double-capsule experiments were conducted at 2·0 and 3·0 GPa and 950–1050°C, using Co–CoO, Ni–NiO, NixPd1–x–NiO, and Fe2O3–Fe3O4 external buffers. Sulfur content at sulfide saturation (SCSS) or sulfur content at sulfate saturation (SCAS) of experimental hydrous partial melts was measured by electron microprobe. All experiments were fluid-saturated and produced either pyrrhotite- or anhydrite-saturated assemblages of silicate glass, clinopyroxene, garnet, and rutile or titanomagnetite, ± amphibole ± quartz ± orthopyroxene. The silicate partial melt composition evolves from rhyolitic at 950°C to trachydacitic and trachyandesitic at 1050°C with increasing ⁠. At pyrrhotite saturation, melt S contents range from ∼30 ppm S at < FMQ – 1 to ∼500 ppm S at FMQ < ≤ FMQ + 1·1, whereas at anhydrite saturation (⁠ ≥ FMQ + 2·5) melt S concentrations range from ∼700 ppm S to 0·3 wt % S. Mass-balance calculations suggest that the aqueous fluid phase at equilibrium may contain as much as ∼15 wt % S at 1050°C at pyrrhotite saturation (≤ FMQ + 1·1), in agreement with previous estimates, and up to 8 wt % S at anhydrite saturation. Our data also show that decreases markedly with increasing at pyrrhotite saturation, from several thousand at < FMQ – 1 to ∼ 200–400 at FMQ < ≤ FMQ + 1·1, owing to the increase of melt S content. At anhydrite saturation, is very low (<100) but increases with decreasing temperature, in an opposite way to previous observations at pyrrhotite saturation. As a consequence, at T ≤ 900°C, might be in the range 200 ± 100, irrespective of ⁠. The present study confirms that slab partial melts saturated with pyrrhotite are unable to efficiently transport S from slab to mantle wedge, and suggests that slab partial melts in equilibrium with anhydrite also have very limited power to enrich the mantle wedge in S. Importantly, slab-derived aqueous fluids appear to be efficient vectors for the transport of sulfur from slab to mantle wedge at all ⁠. Therefore, S transfer from ocean crust to wedge mantle is not dependent and could take place over a range of conditions, and oxidized slab conditions are not necessarily required to enrich the mantle wedge in S. Finally, depending on the initial amount of sulfur in the slab, the proportion of residual anhydrite and pyrrhotite in the dehydrated slab below the region of formation of arc magmas is likely to be significant and may efficiently be recycled into the deep mantle.

HFSE (High Field Strength Elements)-transport and U–Pb–Hf isotope homogenization mediated by Ca-bearing aqueous fluids at 2.04 Ga: Constraints from zircon, monazite, and garnet of the Venetia Klippe, Limpopo Belt, South Africa

Results from laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) and isotope dilution (ID) analyses of minerals and rocks from a single outcrop of the Venetia Klippe of the Limpopo Belt indicate that the U–Pb and Hf isotope system homogenized on the decimetre scale under amphibolite-facies conditions of ⩽645±25°C and ⩽7.0±1.1kbar, i.e. in the presence of an aqueous fluid phase. For a metabasite sample, homogenization is supported by isotope analyses of metamorphic zircon, garnet, and whole rock, which yield a six-point Lu–Hf isochron age of 2039.7±3.4Ma, with initial 176Hf/177Hf of 0.28126±0.00001, and a U–Pb zircon age of 2042±10Ma. The occurrence of a few inherited magmatic zircon cores with ages up to 2705Ma, and with significantly lower initial 176Hf/177Hf of 0.28112, however, indicate that homogenization was incomplete. For a chlorite–biotite–garnet schist isotope homogenization is reflected by within error identical zircon and monazite U–Pb ages of 2045±10Ma and 2041±8Ma, respectively, and by a zircon-garnet-whole rock Lu–Hf isochron age of 2083±63Ma, with an initial 176Hf/177Hf of 0.28140±0.00003. Contemporaneous formation of metamorphic zircon, monazite and garnet in the chlorite schist is not only supported by the isotope data, but also by chlorite inclusions in all three minerals, and by inclusions of metamorphic zircon in garnet. The inclusion textures and the identical initial 176Hf/177Hf support the conclusion that metamorphic zircon grains precipitated from an aqueous fluid phase, after dissolution of zirconium-bearing phases elsewhere, followed by a major HFSE transport, and Hf isotope homogenization. This fluid perhaps was Ca-bearing, as is suggested by the fact that garnet in the schist sample is the only Ca-bearing phase, and that metamorphic monazite, dating the metamorphic peak, is partially replacement by apatite. The fact that the metamorphic zircon rims in the metabasite sample have significantly lower initial 176Hf/177Hf than expected from the Lu–Hf isotope analyses of relic zircon cores and whole rock additionally hint that Ca-HFSE-bearing fluids infiltrated from surrounding quartzo-feldspathic gneisses, which had much lower 176Hf/177Hf at the time of metamorphism.

Re-equilibration of a high-pressure metamorphic fluid: evidence from tourmaline-, apatite- and quartz-hosted fluid inclusions in an Eoalpine eclogite from the Eastern Alps

Fluid inclusions in tourmaline, apatite and quartz from an eclogite of the Polinik Complex, as part of the Koralpe-Wolz high-pressure (H P ) nappe system (Eastern Alps), have been investigated. All three minerals are interpreted as part of the eclogite- to post-eclogite-facies mineral assemblage which formed during Cretaceous Eoalpine metamorphism at ~ 20 kbar and ~ 650°C. Tourmaline, apatite and quartz contain two types of fluid inclusions: (1) a fluid in the system of H 2 O–NaCl–CaCl 2 –CO 2 –CH 4 –N 2 system, and (2) a fluid in the system of H 2 O–NaCl–CaCl 2 ± MgCl 2 system. Type (1) dominates in tourmaline, and type (2) in apatite and quartz. Fluid inclusions of type (1) occur individually and as clusters. In addition to isolated inclusions, type (2)-inclusions also arranged along intragranular/transgranular fluid inclusion planes – but not at grain contacts. The clear distinction in their textural occurrence enables the reconstruction of the H P fluid evolution from a supposed precursor fluid to stages of retrogression accompanied with re-equilibration and preferential loss of the aqueous fluid phase along a proposed exhumation path. Calculated fluid densities of type (1) indicate conditions that reach 14 kbar in tourmaline and 12 kbar in quartz when linked with peak temperatures of ~ 650°C. Conditions are interpreted as minimum conditions during formation of the host minerals. Fluid-inclusion densities from the studied aqueous system type (2) are coherent in the studied host minerals and reach pressures of about 8–10 kbar. A possible reason for the differences in estimated pressure between type (1) and type (2) is, beside textural arguments, the preferred loss of water during recrystallization of the studied minerals at post-peak stages. This hypothesis is supported by the same fluid chemistry of the aqueous phase (H 2 O–NaCl–CaCl 2 ± MgCl 2 ) in type (1)- and type (2)-inclusions. Additionally, different wetting behaviour of fluids containing dominantly polar [type (1)] and non-polar [type (2)] species promoted recrystallization, especially in quartz, and led to widespread decrepitation of large fluid inclusions in quartz during decompression. Hence, the fluid evolution documented in the Polinik eclogites can be reconstructed through effects of physical and chemical changes of the host minerals after peak metamorphism.

Experimental study of trace element release during ultrahigh-pressure serpentinite dehydration

Subduction of serpentinite is envisaged to play a key role in volatile and element recycling at convergent plate margins, but there is currently little known about the composition of the fluid phase(s) released by devolatilisation of deeply subducted serpentinite. We have performed a series of ultrahigh pressure experiments to examine the phase relations and fluid compositions produced by reaction of a natural serpentinite under sub-arc conditions. We employ a novel technique of forming synthetic fluid inclusions in olivine at run conditions to preserve samples of experimental fluids for subsequent analysis. Our experiments confirm that the breakdown of antigorite and chlorite are the most important fluid-producing reactions from serpentinite at sub-arc depths. For our low CaO/Al2O3 peridotitic composition at 3.5 to 4.0 GPa we find that clinopyroxene reacts out below 750 °C and chlorite breaks down progressively between 700 and 800 °C to form garnet harzburgite.Raman analysis of synthetic fluid inclusions indicates that all experiments contained a single aqueous fluid phase, which – together with a lack of textural or mineralogical evidence for hydrous melting – indicates that the water-saturated solidus for our starting composition is above 900 °C at 4.0 GPa. Element concentrations in the fluid for three experiments were determined in situ via laser ablation ICP-MS of individual fluid inclusions. In general, the fluids are enriched in trace elements compared to the bulk starting material, but particularly so for Li, B, LILE, LREE, and U. Chlorite dehydration fluids have high Li/B, LREE/HREE and Ce/Y due to retention of some B in olivine, and retention of Y and HREE in garnet. Our results indicate that fluids produced by serpentinite dehydration at sub-arc depths may carry some of the slab-derived trace elements required for arc magmatism, and may fractionate key trace element ratios in the dehydrated residues, which in turn may ultimately contribute to the geochemical heterogeneity of mantle-derived magmas.

The mobility of W and Mo in subduction zone fluids and the Mo–W–Th–U systematics of island arc magmas

We have studied the solubility of W- and Mo-oxide in aqueous fluids at 2.61GPa, 600 to 800°C at variable controlled oxygen fugacity conditions, relevant to subduction zones. We observed that the solubility of W-oxide is only slightly dependent on fO2 between conditions buffered by Co–CoO and Re–ReO2. In contrast, Mo-oxide solubility is strongly dependent on both oxygen fugacity and fluid salinity under the same conditions. At 2.61GPa, in the fO2 range between Co–CoO and Re–ReO2 their solubility can be described by the following equations:logW=0.07logfO2−4.72×1000/T+4.43andlogMo=0.44logfO2+0.42logNaCl−1.8×1000/T+4.80where W, Mo and NaCl are concentrations in molalities, fO2 is oxygen fugacity and T is temperature in Kelvin. We also studied the solubility of Mo and W in mantle minerals coexisting with Mo- and W-oxide and combined these data with the fluid solubilities to calculate fluid/mineral partition coefficients of W and Mo. Both W and Mo are incompatible in the structure of major eclogite minerals (garnet and clinopyroxene) in the presence of aqueous fluid, but they are compatible in rutile. Thus, the presence or absence of rutile strongly affects the mobility of these elements in subduction zones. From the partition coefficients we calculated the composition of aqueous fluid released from an N-MORB eclogite source. We found that the Mo–Ce–W–Th–U systematics of arc magmas can be modeled by the mixing of depleted mantle with an aqueous fluid phase that was released from a rutile-bearing N-MORB source in fO2-conditions between FMQ-1.6 and FMQ+3 having a maximum salinity of 20wt% NaClequiv. If rutile contents in residual eclogite are independently determined, redox conditions, salinity and amount of fluid added to the source region of melting can be tightly constrained by plotting Mo/Ce, W/Th, and Mo/W vs. U/Th. In particular, the W/Th ratio is a sensitive indicator of the amount of fluid, since W always very strongly partitions into the fluid, nearly independent of oxygen fugacity and salinity. The Mo/W ratio is particularly sensitive to redox conditions, while U/Th reflects both salinity and redox state.

Coexisting carbonate-bearing eclogite and blueschist in SW Tianshan, China: Petrology and phase equilibria

Carbonate-bearing eclogite and blueschist coexist in the same lithological sequence in the Tianshan (ultra-)high-pressure/low-temperature metamorphic belt, northwestern China. Both of them consist of the mineral assemblage garnet+omphacite+dolomite±magnesite+phengite±paragonite+epidote/clinozoisite+glaucophane±barroisite/(Mg-)katophorite+rutile/titanite+pyrite. The eclogite has an omphacite content of ca. 43vol.% and a glaucophane content of <1vol.%. In contrast, the blueschist has an omphacite content of <8vol.% and a glaucophane content of ca. 43vol.%. The blueschist occurs as bands or interlayers in the eclogite. Lawsonite pseudomorphs of epidote/clinozoisite+paragonite assemblages in garnet are commonly observed in the eclogite and blueschist. High carbonate contents in oceanic metabasalts suggest that the precursor basaltic crust has undergone significant hydrothermal alteration prior to subduction. Mineral assemblages and textures show that the omphacite–carbonate-bearing blueschist and the carbonate-bearing eclogite underwent an identical metamorphic evolution. Phase equilibrium modeling in the TiNCaKFMASCHO system further indicates that both the carbonate-bearing eclogite and the omphacite-bearing blueschist equilibrated at peak metamorphic conditions of 540–565°C and 21.8–23.1kbar. Under these conditions calculated XCO2 isopleths reveal the presence of a mainly aqueous fluid phase (XCO2⩽0.02) for the coexisting eclogite and omphacite-bearing blueschist. The stability of the high-pressure mineral assemblages in the eclogite and the omphacite-bearing blueschist is due to differences of the respective bulk-rock composition (the CaO content in particular). This study suggests that three different kinds of blueschist were formed during subduction and exhumation of the South Tianshan oceanic crust, including prograde blueschist formed prior to eclogitization, retrograde blueschist formed by rehydration of eclogite during exhumation in the subduction channel, and coexisting blueschist with eclogite under peak metamorphic conditions.

Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone

Enrichment of the mantle wedge above subduction zones with fluid mobile elements is thought to represent a fundamental process in the origin of arc magmas. This “subduction factory” is typically modeled as a mass balance of inputs (from the subducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the composition of peridotites associated with supra-subduction ophiolites, which form by melt extraction and fluid enrichment in the mantle wedge above nascent subduction zones.The Coast Range ophiolite (CRO), California, is a Jurassic supra-subduction zone ophiolite that preserves mantle lithologies formed in response to hydrous melting. We use high-precision laser ablation ICP-MS analyses of relic pyroxenes from these peridotites to document fluid-mobile element (FME) concentrations, along with a suite of non-fluid mobile elements that includes rare earth and high-field strength elements. In the CRO, fluid-mobile elements are enriched by factors of up to 100× DMM, whereas fluid immobile elements are progressively depleted by melt extraction. The high concentrations of fluid mobile elements in supra-subduction peridotite pyroxene can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. To model this enrichment, we derive a new algorithm that calculates the concentration of fluid mobile elements added to the source:Cwr,add=[Ccpx-obs/[[Dcpx/(Dbulk-PF)]∗[1-(PF/Dbulk)](1/P)]]-[C0,wr]where Cwr,add=concentration of FME added to mantle wedge during a given melt increment, Ccpx-obs=concentration of observed pyroxene, Dcpx and Dbulk=mineral and bulk partition coefficients, P=melt proportion, and F=melt fraction required to model the observed MREE–HREE concentrations. Application of this algorithm to CRO peridotites shows that fluid influx must be continuous with open system melting, which allows us to calculate FME concentrations for small melt increments. Addition of the calculated FME concentrations to depleted MORB mantle (DMM) asthenosphere or refractory arc mantle (RAM) results in pooled magmas that match primitive arc tholeiites and boninites.

Tectonomagmatic origin of some volcanic and sub-volcanic rocks from the Lower Benue rift, Nigeria

Petrological and geochemical studies on some volcanic and sub-volcanic rocks from the Lower Benue rift indicate that they are basalts, basaltic and doleritic sills, trachybasalt and trachyte which generally belong to the alkali basalt series. The alkaline affinity is clearly evident in both their normative and modal mineral compositions, as well as their chemical compositions. The generally high fractionation indices [(La/Yb)N] are 7.06 to 17.65 for the basaltic rocks and 23.59 to 135. 35 for the trachytic rocks, against low values commonly seen in subalkaline (tholeiitic) series, with strong enrichments in the incompatible elements. All this strongly supports their alkaline affinity. The basaltic rocks are generally fine-grained and porphyritic, consisting of phenocrysts of clinopyroxene and olivine in the groundmass of the same minerals together with plagioclase. The clinopyroxene is either diopside or clinoenstatite. The trachyte consists of oligoclase, orthoclase, biotite, quartz and exhibits typical trachytic, flow structure. The basaltic and doleritic sills are commonly altered, with calcite and epidote as common alteration products. This alteration, which is reflected in the erratic behaviour of K2O, MnO and P2O5 on Harker variation diagrams, high values of LOI, strong depletions in the more mobile LILE (Rb, K, Ba and Sr) and high Th/Ta ratios, is attributed to the effects of an aqueous fluid phase and crustal contamination. On the whole, the mineralogical, as well as major-, trace-elements and REE data suggest that the rocks are co-genetic and most likely derived from differentiation of an alkali olivine-basalt magma, generating through variable low degrees of partial melting of probably an enriched lithospheric (upper) mantle following an asthenospheric uplift (mantle plume or intumescence) with HIMU signatures in a within-plate continental rift tectonic setting. This corroborates earlier results obtained for the intrusive rocks in the region.

Chemistry, microstructure, and alpha decay damage of natural brannerite

To investigate the long-term stability of brannerite with respect to alpha decay damage and interaction with aqueous fluids, we have undertaken a study of twelve natural samples from a range of geological environments. Our results indicate that seven of the samples exhibit only minor alteration, usually located within veinlets or around the rim of the sample. The remaining five samples consist of variable amounts of unaltered and altered brannerite. Based on a total of 3 metal cations, the Ti and U contents of unaltered brannerite range from 1.8 to 2.1 and 0.4 to 0.9 atoms per formula unit (apfu), respectively. Maximum amounts of the other major cations on the A-site are 0.48 Ca, 0.22 Th, 0.14 Y, and 0.07 Ln (lanthanide = Ce, Nd, Gd, Sm) atoms pfu. Maximum values of other cations on the B-site are 0.15 Fe, 0.14 Si, 0.09 Al, 0.06 Nb, 0.04 Mn, and 0.04 Ni atoms pfu. Altered regions of brannerite contain significant amounts of Si and other elements incorporated from an aqueous fluid phase, and up to 40–90% of the original amount of U has been lost as a result of alteration. SEM-EDX results also provide evidence for TiO 2 phases, galena, and a thorite-like phase as alteration products. Electron diffraction patterns of all samples typically consist of two broad, diffuse rings that have equivalent d-spacings of 0.31 nm and 0.19 nm, indicating that brannerite is rendered completely amorphous by alpha decay damage. Many of the grains also exhibit weak diffraction spots due to fine-grained inclusions of a uranium oxide phase and galena. Using the available age data, these samples have accumulated alpha decay doses in the range of 0.7–200 × 10 16 α mg −1. Although imprecise, the U–Th–Pb chemical ages determined by microanalysis are generally consistent with the known ages and geological histories of the brannerite host rocks. To a first approximation, it is possible to understand the results of chemical and isotopic dating of brannerite by treating each sample as a complex system composed of nominally unaltered, altered, and recrystallised areas. The dominant effects are U loss from altered areas and Pb loss from unaltered brannerite and, to a lesser extent, altered brannerite, the structure of which approximates to a Ti–Si–O glass-like network wherein Pb is more comfortable as a network modifier.

The influence of Cl concentration and pH on solubility and species of Zn and Pb in water-chloride fluids at T-P parameters of granitoidic magmas degassing

The study of concentrations and species of ore elements in the aqueous fluid phases separating from magmatic melts during their uprising to the surface and crystallization is important for understanding of a role of magmatic fluids in ore-forming systems formation. The purpose of this investigation is determination of solubility and possible species of zinc and lead in water-chloride fluids at Т–Р parameters of granitic magmas degassing. In this report the results of thermodynamic modeling of equilibrium relations of Zn and Pb complexes in water-chloride solutions depending on temperature (600–900С), pressure (0.7–5 kbar), composition and acidity of a fluid (NaCl, KCl, NaCl+HCl), and also initial concentration Zn and Pb in system in the form of ZnO(c), PbO(c) or ZnCl2, PbCl2 are presented. Modeling was conducted by means of program complex HCh [Shvarov, 1999] with use of thermodynamic information received by Shock model [Shock, 1997]. Values of free energy formations of solid phases are borrowed from bank of thermodynamic data of Holland and Powell [Holland, Powell, 1998]. In ZnO-NaCl-H2O system within studied Т–Р range at low chlorine concentrations (CCl 700C Pb(OH)3 is dominant complex within all investigated range of pressure and CCl.

Partitioning of rare earth elements between an aqueous chloride fluid phase and melt during the decompression-driven degassing of granite magmas

This paper reports the results of numerical simulation for the behavior of rare earth elements (REE) during decompression degassing of H2O- and Cl-bearing granite melts at pressures decreasing from 3 to 0.5–0.3 kbar under near isothermal conditions (800 ± 25°C). Fluid phase in equilibrium with the melt contains mainly chloride REE complexes, and their behavior during magma degassing is, therefore, intimately related to the behavior of chlorine. It was shown that the contents and distribution patterns of REE in the aqueous chloride fluid phase formed during decompression vary considerably depending on (1) the contents of volatiles (Cl and H2O) in the initial melt, (2) the redox state of the magma, and (3) the dynamics of fluid phase separation from magmas during their ascent toward the Earth’s surface. During decompressiondriven degassing, the contents of both Cl and REE in the fluid decrease, especially dramatically under opensystem conditions. The REE patterns of the fluid phase compared with those of the melt are characterized by a higher degree of light to heavy REE fractionation. A weak negative Eu anomaly may be present in the REE patterns of Cl-rich fluids formed during the early stages of degassing at relatively high pressures. At a further decrease in pressure and Cl content in the fluid, it is transformed into a positive Eu anomaly increasing during decompression degassing. Such an anomalous behavior of Eu during degassing is related to its occurrence in magmatic melts in two valence states, Eu3+ and Eu2+, whereas the other REE occur in melts mainly as (REE)3+. The Eu3+/Eu2+ ratio of melt is controlled by the redox state of the magmatic system. The higher the degree of melt reduction, the more pronounced the anomalous behavior of Eu during decompression degassing. The amount of REE extracted by fluid from melt during various stages of degassing does not significantly influence the content and distribution patterns of REE in the melt.

日本列島の誕生場―古太平洋の沈み込み開始を示す飛騨外縁帯の５２０Ｍａの熱水活動―

This paper examines the early stage of the geotectonic history of the Japanese Islands on the basis of finding hydrothermal jadeitite including zircons of ca. 520 Ma in serpentinite mélange of the Itoigawa-Omi area of the Hida-Gaien belt, Central Japan. Hydrothermal jadeitite contains euhedral jadeite in natrolite veins and patches, and consists of jadeite-albite and jadeite-natrolite without quartz. These minerals were crystallized from an aqueous fluid phase at the low-pressure and high-temperature side of the reaction boundary of albite = jadeite + quartz in the system NaAlSiO4-SiO2-H2O. The occurrence of rounded relict hornblende mantled by omphacite rimmed by fine-grained aggregates of jadeite in the matrix of jadeite and albite suggests a pervasive hydrothermal fluid flow, through which metabasite was extensively replaced by jadeitite. This rather high-temperature hydrothermal activity of ca. 520 Ma did not occur in an ordinary subduction zone but in a newly-formed mantle wedge suffering severe hydration from a subducting slab. Recently accumulated U-Pb ages of zircon of ca. 450-500 Ma from paleozoic sediments and granitic rocks of the Hida-Gaien belt were due to initiation of subduction followed by subduction zone magmatism. Protolith of serpentinite in the Hida-Gaien belt includes highly depleted harzburgite, thus requiring tectonic setting of a high-temperature-rift zone rather than a low-temperature-slow spreading ridge. Subduction was initiated at ca. 520 Ma along the boundary between low-density harzburgitic rift zone peridotite and lherzolitic spreading ridge peridotite with a slightly higher density, resulting in the common occurrence of harzburgitic serpentinite in the oldest part of the accretionary complex of Southwest Japan. An area including the Japanese Islands was born around the Yangtze block by the breaking up of the Rodinia supercontinent, because the oldest K-Ar age of biotite actinolit rock of 672 Ma (Matsumoto et al., 1981) and the subduction initiation of ca. 520 Ma are in accord with the paleogeographic history of the Yangtze block, and because ca. 300 Ma Renge schists of the Hida-Gaien belt did not suffer the ca. 280-200 Ma collision-type metamorphism of the Hida metamorphic belt that is an eastern extension of the suture between the Sino-Korea and Yangtze blocks.

Experimental constraints on Pt, Pd and Au partitioning and fractionation in silicate melt–sulfide–oxide–aqueous fluid systems at 800 °C, 150 MPa and variable sulfur fugacity

We have performed experiments to constrain the effect of sulfur fugacity ( fS 2) and sulfide saturation on the fractionation and partitioning behavior of Pt, Pd and Au in a silicate melt–sulfide crystal/melt–oxide–supercritical aqueous fluid phase–Pt–Pd–Au system. Experiments were performed at 800 °C, 150 MPa, with oxygen fugacity ( fO 2) fixed at approximately the nickel–nickel oxide buffer (NNO). Sulfur fugacity in the experiments was varied five orders of magnitude from approximately log fS 2 = 0 to log fS 2 = −5 by using two different sulfide phase assemblages. Assemblage one consisted initially of chalcopyrite plus pyrrhotite and assemblage two was loaded with chalcopyrite plus bornite. At run conditions pyrrhotite transformed compositionally to monosulfide solid solution (mss), chalcopyrite to intermediate solid solution (iss), and in assemblage two chalcopyrite and bornite formed a sulfide melt. Run-product silicate glass (i.e., quenched silicate melt) and crystalline materials were analyzed by using both electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. The measured concentrations of Pt, Pd and Au in quenched silicate melt in runs with log fS 2 values ranging from approximately 0.0 to −5.0 do not exhibit any apparent dependence on fS 2. The measured Pt, Pd and Au concentrations in mss do vary as a function of fS 2. The measured Pt, Pd and Au concentrations in iss do not appear dependent on fS 2. The data suggest that fS 2, working in concert with fO 2, via the determinant role that these variables play in controlling the magmatic sulfide phase assemblage and the solubility of Pt, Pd and Au as lattice bound components in magmatic sulfide phases, is a controlling factor on the budgets of Pt, Pd and Au during the evolution of magmatic systems.

Asymmetric Hydrogen Bonding and Orientational Ordering of Water at Hydrophobic and Hydrophilic Surfaces: A Comparison of Water/Vapor, Water/Talc, and Water/Mica Interfaces

Interfaces involving aqueous fluid phases play critical roles in natural and technologically important systems, and the atomic scale differences between interfaces involving hydrophobic and hydrophilic substrates are essential to understanding and manipulating their chemical and physical properties. This paper compares computational molecular dynamics results for the atomic density profiles, H-bonding configurations, and orientational ordering of water molecules at three different and illustrative interfaces. These are the free liquid water surface, which can be considered hydrophobic, and the interfaces of liquid water with talc (001) and muscovite (001) surfaces, which are prototypical hydrophobic and hydrophilic inorganic oxide surfaces, respectively. The results clearly demonstrate the importance of substrate structure and composition in controlling interfacial behavior and illustrate the differences between the vapor interface and those involving solids. The atomic density profiles of water at the so...

Trench-parallel fast axes of seismic anisotropy due to fluid-filled cracks in subducting slabs

Subducting slabs experience deformation and metamorphism as they descend into the upper mantle. The presence of hydrous minerals gained through the interaction with sea water at mid-ocean ridges, transform faults or the outer rise ensures that dehydration reactions will be important at deeper levels. We describe field evidence for brittle hydrofracture in previously subducted rocks from the Western Alps, with a free aqueous fluid phase produced by dehydration reactions in the host blueschists and serpentinites. The protracted history of dehydration reactions, ductile deformation, fluid flow and brittle vein formation in these rocks implies that fluid-filled cracks are continuously produced within the dehydration window. The presence of abundant fluid-filled cracks at these depths has important implications for the seismic anisotropy generated within slabs, which has largely been overlooked. The effects of fluid-filled crack damage on the elastic properties of a blueschist and serpentinite within the slab at depth have been modelled, and show a significant rotation of the fast axes of P and S 1 waves to be trench-parallel for receiving stations in the forearc, above the dehydrating portion of the slab. This model provides an alternative explanation for supra-subduction zone seismic anisotropy that does not require high-stress, high-water conditions, or trench-parallel flow in the supra-subduction zone mantle wedge.

Experimental study on tin partition between granitic silicate melt and coexisting aqueous fluid

The distribution of tin between different granitic silicate melts and coexisting aqueous fluids have been determined at 850°C, 100 MPa and fO2 near NNO. Tin distribution coefficients DSnaq.fl./melt are constrained by the composition of melt. When the starting fluid is 0.1 M HCl, DSnaq.fl./melt decreases rapidly from 0.130 ± 0.090∼0.137 ± 0.016 to 4.22 (±0.47) × 10-3 with the Na2O + K2O mole content and ASI of melt varying from 6.55 to 9.39% and 1.37 to 0.66, respectively, and tin concentrations in aqueous fluids and DSnaq.fl./melt decrease from 9.26 ± 0.56 to 0.220 ± 0.013 μg/g and 3.70 (±0.42) × 10-2 to 1.29 (±0.15) × 10-3, respectively with Na/K mole ratio increasing from 0.43 to 1.57 and nearly constant ASI from 1.04 to 1.10. A series of experiments that use the same K-rich peraluminous haplogranitic melt as starting solid whereas investigate the effects of chlorine and fluorine, the presence of chlorine in aqueous fluids increases DSnaq.fl./melt because of tin complexing, the presence of fluorine in the starting fluid does not significantly influence on DSnaq.fl./melt. For the chlorine experiments, DSnaq.fl./melt shows a positive dependence on the concentration of HCl in the starting fluid with log DSnaq.fl./melt = 2.0247 × log[HCl] + 0.6717 ([HCl] unit is M), and SnCl2 is the dominant tin-bearing complex in aqueous fluid. The results of this experiment showed that the peralkalinity of granitic melt and F-bearing in granite can lead to the enrichment of tin in melt phase, and, therefore, they probably serve as a tin ore reservoir or an important transport medium for tin ore formation, whereas K-rich peraluminous granitic melt and high HCl concentrations in the aqueous fluid phase are favorable for tin partitioning in aqueous fluid.

Production of carbonic fluids during metamorphism of graphitic pelites in a collisional orogen—An assessment from fluid inclusions

Fluid inclusions in quartz veins within Proterozoic metamorphic rocks in the Black Hills, South Dakota, were examined by microthermometry and Raman spectroscopy to assess the evolution of fluid compositions during regional metamorphism of organic-rich shales and late-orogenic magmatism, both of which were related to the collision of the Wyoming and Superior crustal blocks. Fluid inclusions occur in veins that began to be generated before or during regional compression and metamorphism that reached at least garnet-grade conditions, and in veins within the aureole of the Harney Peak Granite (HPG), where temperatures reached second-sillimanite grade conditions. Early veins in the schists have undergone recrystallization during heating and deformation that modified the composition of early CH 4 or CO 2 and N 2-dominated inclusions. These fluids were apparently trapped under conditions of immiscibility with a saline aqueous fluid phase. They are interpreted to represent components generated during maturation of organic matter and dehydration of phyllosilicates during incipient metamorphism at reducing fO 2 conditions. Most inclusions in the quartz veins are, however, secondary CO 2-bearing. They imply a transition to higher fO 2 conditions with increasing temperature of regional metamorphism. The fO 2 conditions may have been controlled by the mineral assemblage in the host metapelites. The prevalence of bimodal distributions of trapped CO 2–N 2 and aqueous endmembers in the biotite and garnet zones also suggests that two immiscible fluid phases existed during the regional metamorphism. In the aureole of the HPG, graphite was evidently consumed by influx of magmatic fluids. CO 2–H 2O fluid inclusions dominate, but they have significantly less N 2 than inclusions at lower metamorphic grades. All inclusions define secondary trails in mostly unstrained quartz. The bimodality of inclusion compositions is not as well defined as at lower grades, with many inclusions containing intermediate CO 2–H 2O compositions. This suggests that a single fluid phase existed at the high temperatures in the granite aureole, but then unmixed during cooling. A set of late quartz veins with graphitized and tourmalinized selvages in the granite aureole contains CH 4-bearing inclusions with little N 2. The existence of CH 4 in these inclusions is attributed to complexing of magmatic B with hydroxyl anions taken from the CO 2–H 2O fluid phase, effectively causing reduction in fO 2 and promoting precipitation of graphite.

Experimental calibration of the vapour–liquid phase relations and lithium isotope fractionation in the system H2O–LiCl at 400°C/20–28 MPa

AbstractWe experimentally determined the vapour–liquid two‐phase field and the vapour–liquid fractionation of the lithium isotopes in the system H2O–LiCl along the 400°C isotherm at 20–28 MPa. The critical point of the 400°C isotherm is at 27.6 MPa/750 mmol kg−1 LiCl, i.e. at slightly lower pressure but higher salt concentration than in the H2O–NaCl system. At a given pressure, the vapour in the H2O–LiCl system has higher salt concentrations compared to the vapour in the H2O–NaCl system whereas salt concentrations in coexisting liquid are comparable in both systems. The vapour–liquid two‐phase field in the H2O–LiCl system therefore opens less with decreasing pressure. The available data indicate that adding LiCl to a predominantly H2O–NaCl fluid may notably increase salt concentrations in the vapour. The lithium isotope fractionation between vapour and liquid is only minor with a slight preference of 7Li for the liquid. The data do not allow quantifying Δ7Livapour‐liquid precisely, but they suggest Δ7Livapour‐liquid ≤ −0.5‰. Even for extreme Rayleigh fractionation in an open system the lithium isotopic signature of vapour, liquid, and bulk system will not be altered by more than 1‰. The lithium isotopes are a conservative tracer during aqueous fluid phase separation and lithium isotope fractionation observed in natural hydrothermal systems reflects the combined effects of source reservoir and fluid–mineral interactions.