Composition Of Fluid Phase Research Articles

Thermodynamic study on the compositions of fluid phases in the C-O-H system under high temperature and pressure

Based on the fluid phase equilibrium of the C-O-H system, the compositions of various fluid phases under high temperatures and pressures have been calculated in terms of the available thermodynamic data and newP-V-T data and on the assumption ofP T= ΣPi in this paper. The results indicate that in this system there are 5 major fluid phases in different proportions at variousT andP. CH4 is the dominant phase (about 70%) under relatively lowerT andP. Its proportion obviously decreases with increasingT, P andfo2. The results provide sufficient theoretical grounds for discussing the possibility of CH4 formation and the physical-chemical conditions of its stable occurrence and proportion in the geological environment.

Contact Metamorphism in Pelitic Rocks on the Island of Kos (Greece, Eastern Aegean Sea): a Test for the Na-in-Cordierite Thermometer

Most of the applicable thermometers involving cordierite require additional Fe–Mg phases and knowledge of pressure and fluid phase composition. These conditions are not necessarily met with natural cordierite-bearing metamorphic rocks. A potential pressure-independent geothermometer for cordierite- and plagioclase-bearing rocks is the experimentally determined inverse linear relationship between temperature and Na contents of Mg-cordierite coexisting with NaOH or albite and NaOH. In this paper, the Na-in-cordierite thermometer is empirically tested on cordierite-bearing metapelites from the island of Kos, contact metamorphosed by a quartz monzonite intrusion. The variety of phase assemblages in the metapelites allows for tight constraints to be placed on temperatures (508–762°C) and pressure (0.2 GPa) during contact metamorphism by calculated and experimentally determined phase equilibria and by Al-in-hornblende barometry. Fluid compositions are crudely estimated from the presence and absence of graphite and calcite and from 2Vx in cordierite. Applying the Na-in-cordierite thermometer to 19 rock samples taken at increasing distances from the intrusion yields realistic peak temperatures. The Na-in-cordierite temperatures are consistent with constraints from petrogenetic grids, with experimentally determined phase equilibria and with temperatures based on Fe–Mg exchange equilibria (garnet–cordierite and garnet–biotite), although the latter tend to give lower temperatures. The Na-in-cordierite temperatures show no systematic variations with Mg values and Al contents of cordierites, Na contents of coexisting plagioclase (in the range An10–An40) and biotite, and H2O/CO2 ratios in coexisting fluids, though the last are only poorly known. Cordierites have either high Na and significant Li contents or display low levels of both elements. Li is probably introduced into cordierite by a coupled substitution involving tetrahedral and octahedral sites, whereas Na may not be charge-balanced by lattice oxygen. The results obtained suggest that the experimentally established relationship between Na contents of cordierite and temperature is a useful thermometer for cordierite- and plagioclase-bearing metapelitic rocks. The nature of Na incorporation in cordierite and its relation to fluid composition, however, must be constrained by further experiments.

Contact Metamorphism in Pelitic Rocks on the Island of Kos (Greece, Eastern Aegean Sea): a Test for the Na-in-Cordierite Thermometer

Most of the applicable thermometers involving cordierite require additional Fe–Mg phases and knowledge of pressure and fluid phase composition. These conditions are not necessarily met with natural cordierite-bearing metamorphic rocks. A potential pressure-independent geothermometer for cordierite- and plagioclase-bearing rocks is the experimentally determined inverse linear relationship between temperature and Na contents of Mg-cordierite coexisting with NaOH or albite and NaOH. In this paper, the Na-in-cordierite thermometer is empirically tested on cordierite-bearing metapelites from the island of Kos, contact metamorphosed by a quartz monzonite intrusion. The variety of phase assemblages in the metapelites allows for tight constraints to be placed on temperatures (508–762°C) and pressure (0.2 GPa) during contact metamorphism by calculated and experimentally determined phase equilibria and by Al-in-hornblende barometry. Fluid compositions are crudely estimated from the presence and absence of graphite and calcite and from 2Vx in cordierite. Applying the Na-in-cordierite thermometer to 19 rock samples taken at increasing distances from the intrusion yields realistic peak temperatures. The Na-in-cordierite temperatures are consistent with constraints from petrogenetic grids, with experimentally determined phase equilibria and with temperatures based on Fe–Mg exchange equilibria (garnet–cordierite and garnet–biotite), although the latter tend to give lower temperatures. The Na-in-cordierite temperatures show no systematic variations with Mg values and Al contents of cordierites, Na contents of coexisting plagioclase (in the range An10–An40) and biotite, and H2O/CO2 ratios in coexisting fluids, though the last are only poorly known. Cordierites have either high Na and significant Li contents or display low levels of both elements. Li is probably introduced into cordierite by a coupled substitution involving tetrahedral and octahedral sites, whereas Na may not be charge-balanced by lattice oxygen. The results obtained suggest that the experimentally established relationship between Na contents of cordierite and temperature is a useful thermometer for cordierite- and plagioclase-bearing metapelitic rocks. The nature of Na incorporation in cordierite and its relation to fluid composition, however, must be constrained by further experiments.

Element transport from slab to volcanic front at the Mariana arc

We present a comprehensive geochemical data set for the most recent volcanics from the Mariana Islands, which provides new constraints on the timing and nature of fluxes from the subducting slab. The lavas display many features typical of island arc volcanics, with all samples showing large negative niobium anomalies and enrichments in alkaline earth elements and lead (e.g., high Ba/La and Pb/Ce). Importantly, many of these key ratios correlate with a large range in 238U excesses, (238U/230Th) = 0.97–1.56. Geochemical features show island to island variations; lavas from Guguan have the largest 238U‐excesses, Pb/Ce and Ba/La ratios, while Agrigan lavas have small 238U excesses, the least radiogenic 143Nd/144Nd, and the largest negative cerium and niobium anomalies. These highly systematic variations enable two discrete slab additions to the subarc mantle to be identified. The geochemical features of the Agrigan lavas are most consistent with a dominant subducted sediment contribution. The added sedimentary component is not identical to bulk subducted sediment and notably shows a marked enrichment of Th relative to Nb. This is most readily explained by melt fractionation of the sediment with residual rutile and transfer of sedimentary material as a melt phase. For most of the highly incompatible elements, the sedimentary contribution dominates the total elemental budgets of the lavas. The characteristics best exemplified by the Guguan lavas are attributed to a slab‐derived aqueous fluid phase, and Pb and Sr isotope compositions point toward the subducted, altered oceanic crust as a source of this fluid. Variable addition of the sedimentary component, but near‐constant aqueous fluid flux along arc strike, can create the compositional trends observed in the Mariana lavas. High field strength element ratios (Ta/Nb and Zr/Nb) of the sediment poor Guguan lavas are higher than those of most mid‐oceanic ridge basalts and suggest a highly depleted subarc mantle prior to any slab additions. The 238U‐230Th systematics indicate >350 kyr between sediment and mantle melting but <30 kyr between slab dehydration and eruption of the lavas. This necessitates rapid magma migration rates and suggests that the aqueous fluid itself may trigger major mantle melting.

Analysis of non-isothermal heterogeneous autocatalytic reactions

This paper presents a general model for autocatalytic heterogeneous reactions in non-isothermal particles. We consider both quadratic and cubic autocatalysis and determine the effects of particle geometry, relative reaction rates, bulk fluid-phase composition, dimensionless temperature differences, and Arrhenius numbers on the catalyst effectiveness factor. In general, diffusion limitations within the heterogeneous phase can lead to effectiveness factors much greater than unity for both exothermic and endothermic autocatalytic reactions. Furthermore, multiple steady states can exist for exothermic reactions in a quadratic autocatalytic system, or for any reaction with a cubic autocatalytic system.

Andalusite‐bearing veins at Vedrette di Ries (eastern Alps, Italy): fluid phase composition based on fluid inclusions

Abstract Andalusite‐bearing veins formed during contact metamorphism in the aureole of the Vedrette di Ries tonalite. In the veins, quartz crystals that are completely armoured by andalusite or that occur in strain shadow areas contain three generations of fluid inclusions: low‐salinity H2O‐CO2‐CH4 mixtures with CH4/(CO2+ CH4) ± 0.35 (type A); low‐salinity aqueous fluids (type B); H2O‐free, CO2‐CH4 fluids with the same carbonic speciation as A (type C). Carbonic types A and C typically have a dark appearance, which is attributed to graphite coatings on inclusion walls. Microstructural analysis of the host quartz and calculated densities indicate that type A inclusions were likely trapped during vein formation. These inclusions underwent strain‐assisted re‐equilibration during cooling that resulted in density increases without change of composition. After the rocks had cooled below about 350 ° C, type C inclusions appear to have formed from one of the immiscible fractions after unmixing of the H2O‐CO2‐CH4 fluid mixtures. Aqueous type B inclusions, apparently trapped between 225 and 350 ° C, could represent an independent fluid, or could be the H2O‐rich fraction of unmixed type A fluids. Taking account of the uncertainties, the composition and density of the complex type A inclusion fluids are in good agreement with the properties of primary fluids calculated from the petrological data. The fluid inclusion data support the model of vein formation by hydrofracturing as a result of dehydration of graphitic metapelites. These new results also demonstrate the importance of considering strain in the interpretation of metamorphic fluid inclusions.

Hyperbaric reservoir fluids: High-pressure phase behavior of asymmetric methane +n-alkane systems

In this paper, experimental three-phase equilibrium (solidn-alkane + liquid + vapor) data for binary methane +n-alkane systems are presented. For the binary system methane + tetracosane, the three-phase curve was determined based on two phase equilibrium measurements in a composition range fromxc24 = 0.0027 toxc24 = 1.0. The second critical endpoint of this system was found atp = (1114.7 ± 0.5) M Pa.T = (322.6 ± 0.25) K, and a mole fraction of tetracosane in the critical fluidphase ofxc24 = 0.0415 ± 0.0015. The second critical endpoint occurs where solid tetracosane is in equilibrium with a critical fluid phase (Sc24 +L =V). For the binary systems of methane with then-alkanes tetradecane, triacontane, tetracontane, and pentacontane, only the coordinates of the second critical endpoints were measured. The second critical endpoint temperature is found close to the atmospheric melting point temperature of then-alkane. The pressures at the second critical endpoints do not exceed 200 MPa. Based on these experimental data and data from the literature, correlations for the pressure. temperature, and fluid phase composition at the second critical endpoint of binary methane +n-alkane systems withn-alkanes between octane and pentacontane were developed.

Investigation of phase equilibrium for ternary systems containing ethanol, water and carbon dioxide at elevated pressures

Abstract Using a two-phase circulation apparatus, phase equilibrium data at elevated pressures for ternary systems consisting of CO 2 , C 2 H 5 OH and H 2 O have been measured. A new method for the calculation of liquid phase compositions from P, T , and supercritical fluid phase compositions by the modified Peng-Robinson equation is proposed. The applicability of this method is examined for our experimental data and literature data. As a result, it is confirmed that the method can represent the experimental data with good accuracy.

Geochemical Types, Petrology, and Genesis of Late Cenozoic Volcanic Rocks from the Kurile-Kamchatka Island-Arc System

The Kurile-Kamchatka Late Cenozoic volcanic rocks can be categorized as magmatic series of island arc and within-plate geochemical types, respectively. These types clearly can be identified on geochemical discriminant diagrams. Across-arc geochemical, mineralogical, and Sr-Nd-isotope zoning is characteristic of island-arc volcanics, but is not found for within-plate volcanics. Such zoning probably is the result of modification of fluid-phase composition, which originates in the underlying subducting plate (as a result of dehydration of water-bearing secondary minerals) and rises into the zone of island-arc magma generation in the mantle wedge. Along-arc Sr, Be, H., and 0 isotopic zoning is noted for island-arc volcanics. Probably it is associated with various levels of island-arc magma contamination. 10Be data for modern island-arc lavas attest to the fact that young pelagic sediments from the subducting plate participate in island-arc magma genesis. Within-plate lavas are found in the region north of Ava...

Phase behavior of reservoir fluids: VI. Cosolvent effects on bitumen fractionation with supercritical CO 2

Yu, J.M., Huang, S.H. and Radosz, M., 1994. Phase behavior of reservoir fluids VI: cosolvent effects on bitumen fractionation with supercritical CO 2. Fluid Phase Equilibria, 93: 353-362. Equilibrium phase compositions ( TPXY) are measured in a flow cell apparatus for a bitumen + decane + CO 2 system. Statistical Associated Fluid Theory (SAFT) [Huang, S.H. and Radosz, M., 1990a. Fluid Phase Equilibria 60: 81-98; 1990b. Ind. Eng. Chem. Res., 29: 2284-2294] and Peng-Robinson equations of state are found to give similar results for the condensed phase compositions. However, SAFT is found superior in predicting the supercritical fluid phase compositions. Hence, the bitumen partitioning coefficients ( K-factors) can be reliably predicted with SAFT but not with PR. Moreover, the SAFT characterization approach based on measurable molecular properties is more appropriate for heavy associating bitumen components than is the PR characterization approach based on hypothetical critical properties. Finally SAFT-predicted K-factors allow for quantifying cosolvent selectivity and capacity in fractionating bitumen with supercritical CO 2.

Halogen geochemistry of fluid during amphibolite-granulite metamorphism as indicated by apatite and hydrous silicates in basic rocks from the Bamble Sector, South Norway

The halogen chemistry of apatites, biotites, amphiboles and titanites from metabasites in the Proterozoic Bamble Sector amphibolite-granulite facies transition zone (Norway) has been studied to gather data on the behaviour of halogens during original magmatism and subsequent metamorphism. Three groups of apatites could be recognized: (1) FOH-apatites with more than 50 mole % F and virtually no Cl. (2) FOH-apatites with up to 15 mole % of Cl. (3) Cl-rich apatites with up to 6.73 wt.% Cl. The latter are the most Cl-rich metamorphic apatites ever reported. The F-contents of coexisting biotite do not follow this division, but their Cl-contents do. Log( z.sfnc;HF z.sfnc;H 2O ) values for the coexisting fluid phase calculated from biotite are slightly lower than those calculated from apatite. Log( z.sfnc;HCl z.sfnc;H 2O ) values in the coexisting fluid phase calculated from biotite are substantially higher than those calculated from apatite. The log( z.sfnc;HF z.sfnc;H 2O ) reflects the composition of the peak metamorphic fluid phase. The apatites show three types of patterns of F-contents: (1) W-shaped, (2) Rimward increasing and (3) Rimward decreasing. Pattern (2) reflects progressive metamorphism. Pattern (3) is thought to reflect growth starting near peak metamorphism and going on during retrograde dilution of the peak metamorphic fluid by a newly introduced aqueous one. Some apatites show local resorption effects along cracks. No regional trend in calculated log( z.sfnc;HF z.sfnc;H 2O ) values is present. As different from other granulite terrains, the Bamble biotites and amphiboles from granulite facies samples do not have higher halogen contents as those from the amphibolite facies area. The samples show that halogen bearing fluids were controlled at a very restricted scale during peak as well as retrograde metamorphism.

Across-arc variation of lava chemistry in the Izu-Bonin Arc: identification of subduction components

In order to understand the role of the subducted lithosphere in producing the geochemical characteristics of arc magmas, major- and trace-element along with Sr- and Nd-isotope compositions have been determined for Quaternary volcanic rocks from the Izu-Bonin intra-oceanic arc. 87Sr/ 86Sr and 143Nd/ 144Nd ratios decrease away from the volcanic front of this arc and lie on mixing lines between the assumed isotopic compositions of fluid phases mainly derived from the basalt layer of the subducted lithosphere and upper-mantle materials in the sub-arc wedge. This across-arc variation can be explained through a simple sequence of processes involving initial release of fluid phases from the subducted oceanic crust to produce hydrous peridotite at the base of the mantle wedge. This hydrous peridotite is dragged downward with the slab and releases a second-stage metasomatizing fluid beneath the volcanic arc. The higher concentrations of both Sr and Nd in the fluid beneath the volcanic front than those beneath the back-arc side may be a possible cause of the observed across-arc variation in Sr-Nd isotopic ratios. The difference in compositions of fluid phases is attributed to the different hydrous phases which decompose in the hydrous peridotite layer; amphibole beneath the volcanic front and phlogopite beneath the back-arc side of the volcanic arc. The mineralogically controlled fluid addition may also be responsible for the across-arc variation in Rb/K and Rb/Zr ratios, increasing away from the volcanic front.

The influence of the fluid phase composition on the solidus temperatures in the haplogranite system NaAlSi3O8-KAlSi3O8-SiO2-H2O-CO2

The solidus temperatures in the haplogranite-system NaAlSi3O8-KAlSi3O8-SiO2-H2O-CO2 have been determined up to 15 kbar for a constant molar ratio of sodium to potassium of 1∶1 and for fluid compositions ranging from pure water to pure carbon dioxide. The data for the water-saturated solidus are virtually identical with those of previous studies. At constant pressure, the solidus curve as a function of the fluid phase composition exhibits a point of inflection in the range of the water-rich compositions. This phenomenon is attributed to chemical interactions between the CO2 and the H2O in the silicate melt. The point of inflection disappears if the CO2 in the gas phase is replaced by molecular nitrogen. The CO2-saturated solidi have been measured at 2 and 5 kbars. The data at 5 kbar indicate a melting point depression in the order of 40° C compared to the dry solidus of Huang and Wyllie (1975). The experimental data can be used to estimate the melting temperatures of common quartz and feldspar bearing crustal rocks under the conditions of granulite facies metamorphism. Since for most fluid phase compositions, the solidus curves are very steep in the P, T-diagram, the beginning of melting is nearly exclusively determined by the fluid composition and almost independent of pressure between about 2 and more than 10 kbar. Therefore, the onset of partial melting in quartz and feldspar containing rocks under granulite facies conditions can be used to estimate the composition of a coexisting H2O-CO2 fluid phase if geothermometric data are available. The temperature range between the beginning of granulite facies metamorphism and the initiation of melting expands with increasing carbon dioxide content in the H2O-CO2 fluid phase. At a CO2 molar fraction of 0.9, this range extends from about 600° C to 900° C and is almost independent of pressure.

U-Pb zircon, Rb-Sr and Sm-Nd geochronology of high- to very-high-pressure meta-acidic rocks from the western Alps

Three meta-acidic rocks from the western Italian Alps, a magnesiochloritoid-bearing “metapelite” from the Monte Rosa massif, a coesite-pyrope-“quartzite” from the Dora Maira massif and the Monte Mucrone granite in the Sesia Zone, have been studied by U-Pb zircon, Rb-Sr on whole-rock, apatite and phengite and Sm-Nd wholerock methods. The mineral parageneses of the investigated rocks indicate high- to very-high-pressure and medium-to-high-temperature metamorphism. This combined isotopic study has enabled us to constrain the ages of magmatic and metamorphic events and also to compare the behaviour of U-Pb zircon systems in three intensely metamorphosed areas of the Pennine domain. The U-Pb zircon data have yielded a magmatic age for the Monte Mucrone granite at 286±2 Ma. This result confirms the occurence of late-Hercynian magmatism in the Sesia Zone, as in other Austro-Alpine units and in other areas of the European crystalline basement. In the Monte Rosa massif, a geologically meaningless lower intercept age of 192±2 Ma has been interpreted as an artefact due to a complex evolution of the U-Pb zircon system. The magmatic shape of the zircons implies a magmatic or volcano-sedimentary protolith for this rock, originally considered as a metasediment. The very-high-pressure metamorphism in the Dora Maira “quartzite” has produced an opening of the U-Pb zircon system at 121+12−29 Ma. The Rb-Sr data support the occurence of high-grade metamorphism during Cretaceous times. Phengites model ages are slightly younger than the U-Pb zircon lower intercept ages due to cooling phenomena or possible response of the phengites to a later deformation. The Nd model ages from the whole-rock samples, as well as the U-Pb upper intercept ages from zircons of all three investigated rocks, indicate the presence of Proterozoic crustal components inherited from the precursors of these meta-acidic rocks. The studied zircon populations and their U-Pb systems apparently showed open-system behaviour only when affected by extreme metamorphic conditions (700–750° C, > 28 kbar), whereas eclogite-facies conditions of 500–550° C and 14–16 kbar were not enough to disturb significantly the U-Pb zircon evolution. It is also probable that the sedimentary or magmatic origin of the protoliths of these meta-acidic rocks, which involved different characteristics such as grain-size and fluid phase concentration and composition, could be another important factor controlling the U-Pb zircon system behaviour during metamorphic events.

Structure sensitivity in heterogeneous catalysis: Activity and yield/selectivity

The relationship between exposed catalytic sites ( a ) and exposed catalyst sites ( A ) was postulated to be in accord with (J. J. Carberry, J. Catal. 107, 248 (1987)) a = D 0( A ̄ ) d + 1 where D 0 is a site discrimination coefficient and d the exponent of structure sensitivity. Recently assembled data of D. Farin and D. Avnir ( J. Amer. Chem. 110, 2039 (1988)) which support the postulated activity-yield/selectivity structure-sensitive relationships are cited. In sum a vast corpus of data supports the law (J. J. Carberry, J. Catal. 107, 248 (1987)) Rate A ̄ = K( A ̄ ) d = N, turnover number for a given site type at a fixed fluid-phase composition. When two or more site types are involved in a complex (multipathed) reaction network, not only is activity (conversion) affected by structure sensitivity but yield/selectivity (the ratio of turnover numbers) can also be influenced. Comments on the significance of the exponent d which suggest a distinction between demanding/facile and structure sensitive/insensitive behaviors are offered. Simple geometric argument predicts d = 0 to 2 while a value of d ≷ 0 can be rationalized if morphological/chemical changes accompany crystallite size variation. A turnover velocity, N 0, independent of fluid-phase composition (conversion x) is suggested, for reactant order n, as N 0 Rate A ̄ C n = N (C 0(1 − x)) n = k vD 0( A ̄ ) d where C 0 is feed concentration and k v the intrinsic reaction velocity per active site per exposed site.

Corundum- and zoisite-bearing marbles in the Rhodope Zone, Xanthi Area (N. Greece): Estimation of the fluid phase pomposition

In the area of Xanthi, N. Greece, regionally metamorphosed marbles which belong to the Upper Tectonic Unit of the Rhodope Zone occur as intercalations within gneisses and amphibolites. In one marble horizon abundant Al-rich minerals like corundum (ruby), spinel, Al-rich pargasite, zoisite, and anorthite were found. In some cases, spinel occurs as a rim around corundum and was formed as a reaction product of corundum + dolomite. Zoisite often coexists in equilibrium with anorthite and calcite. These relationships permit the calculation of the fluid phase composition which is found to be relatively poor in CO2, thus indicating that abundant water was available-at least on a local scale-during the last, Barrow-type metamorphism. This water was probably supplied from the metapelites by dehydration reactions during the amphibolite facies overprint and invaded the rocks through fractures and shear zones and along the contacts between different rock types.

Progressive Charnockitization of a Leptynite-Khondalite Suite in Southern Kerala, India- Evidence for Formation of Charnockites Through Decrease in Fluid Pressure?

Abstract The crustal segment south of the Achankovil zone in southern Kerala is built up by two litho-tectonic units: The Nagerkoil unit is represented by a series of acid to intermediate magmatogenic gneisses and charnockites associated with layered anorthositic to noritic complexes. The Pon Mudi unit consists of interlayered garnet-biotite gneisses (leptynites), garnet-biotite-sil1imanite gneisses (khondalites) and garnet-biotite-sillimanite-cordierite metatexites which represent a series of intensely deformed psammitic and pelitic sediments metamorphosed to upper amphibolite grade (700-750°C; 6-8 k bars). Subsequent to regional metamorphism in the POD Mudi unit, the leptynites have been partly transformed to massive orthopyroxene-garnet-bearing charnockites along a conjugate set of fractures (s1: N30E ilDd s2: S70E) and the foliation planes of the rocks (N30-60W). The geochemical data and the results of geothermobarometry show that charnockitization was essentialJy isochemical and occurred at 750 ± 50°C and 6 ± 1 kbars lithostatic pressure. A complex but conformable development of tbe fluid phase composition during metamorphism is indicated by the occurrence of at least four generations of fluid inclusions (almost pure H2O and CO2 inclusions, mixed CO2-H2O inclusions and CH4-N2 inclusions) in both the leptynites and the charnockites. The common presence of graphite+pyrrhotite+ilmenite furthermore suggests internal buffering of the fluid composition at oxygen fugacities below those defined by the quartz-magnetite-ferrohypersthene assemblage and XCO2>0.5. The results of tbe present study indicate that charnockitization probably was not caused by the influx of CO2-rich fluids of deep-seated origin, tbe presently favoured model of granulite genesis (Newton, 1984), but rather was induced by an isothermal decrease of fluid pressure relative to lithostatic pressure, due to migration of the pore fluid into the network of fractures and ascent into higher crustal levels.

The Origin of Saline Groundwaters in Granitic Rocks: Evidence From Hydrothermal Experiments

AbstractHydrothermal rock-water experiments at 80°, 150° and 250° C have been carried out to investigate the origin of saline groundwaters in the Carnmenellis Granite, Cornwall, UK which have previously been attributed to be derived via dissolution of biotite and plagioclase feldspar by dilute meteoric groundwaters. The nature of product fluid and solid phases in the experiments was dominated by the dissolution of plagioclase and potassium feldspars, coupled with the precipitation of laumontite, calcite and smectite. However, the release of chloride from the rock was minimal, suggesting minor dissolution of the major chloride bearing mineral, biotite. The relative inertness and stability of biotite during the experiments was borne out by direct physical observation of reacted biotites and by consideration of the fluid phase composition in terms of thermodynamic mineral stability diagrams. It is concluded that the experimental and natural systems are buffered under different chemical conditions, which may be due to acidity generated via oxidation of localised sulphide zones within the granite and absent in the experiments. Dissolution of biotite may be an important mechanism for enhancing the salinity of groundwaters in granitic rocks if groundwaters are buffered to be outside the stability of biotite and convective circulation is significant.

Geochemistry of braunite and associated phases in metamorphosed non-calcareous manganese ores of India

In the metamorphosed manganese oxide ores of India, braunite is ubiquitous in all assemblages from chlorite to sillimanite grades. Chemical analyses of braunite from different prograde assemblages confirm the presence of a fixed R2+ (=Mn2++Mg+Ca) SiO3 molecule in the mineral. Element partitioning between coexisting braunite and bixbyite indicates a near-ideal mixing of Fe+3/ -Mn+3 in the phases. This also indicates that braunite became relatively ferrian while equilibrating with associated phases such as bixbyite, hollandite and jacobsite during prograde reactions. Petrogenetic studies show that as a general trend, prograde lower oxide phases appeared by deoxidation of higher oxide phases. But braunite, a more reduced phase than bixbyite, appeared early from deoxidation of pyrolusite in presence of quartz. Bixbyite could appear later from the reacting pyrolusite-braunite-quartz assemblage. Inferred mineral reaction paths and the general trend of pro-grade deoxidation reactions suggest that the composition of ambient fluid phase was internally buffered during metamorphism.

Genesis and mobility of the H 2 O-CO 2 fluid phase during regional greenschist and epidote amphibolite facies metamorphism: a petrological and stable isotope study in the Scottish Dalradian

During high-pressure, low-temperature greenschist and epidote-amphibolite facies metamorphism in Dalradian rocks of the SW Scottish Highlands, mineral assemblages in metabasites and calcareous metasediments were dominantly controlled by infiltration of hydrous fluids; consequently, mineral assemblages capable of buffering the fluid phase composition were rare. Equilibrium prograde H 2 O-CO 2 fluids usually contained less or much less than about 1–2 mol% CO 2 . Three fluid infiltration events are recognized. During prograde greenschist facies metamorphism, metabasic sills were infiltrated by large volumes of CO 2 -bearing hydrous fluid; carbon isotope studies indicate that the CO 2 was locally derived by widespread oxidation of graphite or other organic carbon in adjacent metasediments. This may have occurred under approximately lower greenschist facies conditions as a result of mixing of fluids of varying f O 2 , initiated by thermal expansion of water during heating, decompression and consequent hydraulic fracturing. In the epidote-amphibole facies (garnet zone), dehydration reactions in metabasites generated large quantities of water, which removed carbonate from metabasites on a regional scale and infiltrated calcareous metasediments to produce assemblages containing grossular, diopside, K-feldspar, amphibole, clinozoisite and sphene. A late retrograde infiltration of CO 2 ,-bearing hydrous fluid under lower greenschist facies conditions generated assemblages containing K-feldspar + chlorite + rutile ± dolomite in calcareous rocks and albite prophyroblast schists in zones of intense secondary deformation. Large-scale infiltration of fluid into greenschist-facies metadolerite sills was intimately related to, and possibly controlled by, penetrative deformation and, in the absence of a penetrative deformation, grain-boundary diffusion by itself was an ineffectual mechanism of fluid transport.