Pervasive rehydration of granulites during exhumation – an example from the Pulur complex, Eastern Pontides, Turkey

Summary High-grade gneisses from the Pulur complex in NE Turkey bear evidence for biotite-dehydration melting at 820°C and 0.7–0.8GPa, melt segregation and near-isothermal decompression to 0.4–0.5GPa. During further exhumation, the rocks underwent secondary pervasive rehydration at temperatures between 400 and 230°C and fluid pressures between 0.3 and 0.1GPa. Metamorphic peak conditions are dated at 331–327Ma, while hydrothermal retrogression occurred significantly later at 315–310Ma under static conditions. During the rehydration event, primary high-grade mineral assemblages including garnet, cordierite, sillimanite, spinel, biotite, plagioclase and ilmenite were extensively replaced by muscovite, paragonite, margarite, corundum, diaspore, chlorite, kaolinite, pumpellyite, prehnite, epidote, titanite, anatase, pyrite and chalcopyrite. Secondary mineral assemblages indicate that the infiltrating fluids were characterized by low fO₂, very low X_CO2 (<0.002), variable activities of Ca²⁺, K⁺, Na⁺ and H⁺ and relatively high activities of H₂S and CH₄. Quartz veins that might have acted as pathways for the fluids are rare. Ubiquitous veinlets consisting of (i) albite, (ii) chlorite+calcite+quartz or (iii) K-feldspar+calcite+quartz were formed after the pervasive rehydraton event by precipitation from aqueous solutions that were somewhat richer in CO₂.     

Equation of state,structural behaviour and phase diagram of synthetic MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript>, as a function of pressure and temperature

The behaviour of synthetic Mg-ferrite (MgFe₂O₄) has been investigated at high pressure (in situ high-pressure synchrotron radiation powder diffraction at ESRF) and at high temperature (in situ high-temperature X-ray powder diffraction) conditions. The elastic properties determined by the third-order Birch–Murnaghan equation of state result in K₀=181.5(± 1.3) GPa, K=6.32(± 0.14) and K= –0.0638 GPa^–1. The symmetry-independent coordinate of oxygen does not show significant sensitivity to pressure, and the structure shrinking is mainly attributable to the shortening of the cell edge (homogeneous strain). The lattice parameter thermal expansion is described by _a0+_a1*(T–298)+_a2/(T–298)², where _a0=9.1(1) 10^–6 K^–1, _a1=4.9(2) 10^–9 K^–2 and _a2= 5.1(5) 10^–2 K. The high-temperature cation-ordering reaction which MgFe-spinel undergoes has been interpreted by the ONeill model, whose parameters are = 22.2(± 1.8) kJ mol^–1 and =–17.6(± 1.2) kJ mol^–1. The elastic and thermal properties measured have then been used to model the phase diagram of MgFe₂O₄, which shows that the high-pressure transition from spinel to orthorombic CaMn₂O₄-like structure at T < 1700 K is preceded by a decomposition into MgO and Fe₂O₃.     

Probing the relationship between surface waters and aquifers by <Superscript>18</Superscript>O measurements on the top of the Araripe Plateau/NE Brazil

The Araripe Plateau in northeastern Brazil has an area of about 8,000 km², confined by 39°05E and 40°55E, and 7°10S and 7°50S. Due to high permeability of soils, a surface drainage system is practically inexistent. Water is stored in excavations with clayey soil, the barreiros. Monthly samples were taken for ¹⁸O measurements, from September 1999 to August 2000, from four barreiros, three dug wells and five drilled wells. Results show that (1) groundwaters in the eastern part of the plateau are derived from present-day rainfall (¹⁸O–3.2), whereas groundwaters in the western portion are isotopically different (¹⁸O–5.0); (2) barreiros are strongly marked seasonally by elevated ¹⁸O during the dry period due to elevated evaporation; (3) a dug well at a distance of 30 m from a barreiro exhibits ¹⁸O similar to that of the reservoir, indicating a strong interaction between groundwater and surface water; and (4) a tubular well of 242-m depth, located in a fault, exhibits strong seasonal changes in ¹⁸O and electrical conductivity, revealing downward leakage between aquifers.     

Crystal chemistry of some Mg,Cr, V normal spinels from Sludyanka (Lake Baikal,Russia): the influence of V<Superscript>3+</Superscript> on structural stability

A suite of vanadian magnesiochromites from the Sludyanka metamorphic complex (South Lake Baikal, Russia) were investigated by means of X-ray single-crystal structural refinements and microprobe analyses. Various morphological types of Cr–V-bearing Mg spinels are located in calc–silicate metamorphic rocks, in an assemblage that also contains other Cr–V minerals such as escolaite–karelianite, uvarovite–goldmanite, Cr–V-bearing clinopyroxene, tourmaline, amphibole, mica, etc. Along the suite there is widespread V–Cr substitution (0.14 V³⁺ 0.95 afu, 1.02 Cr³⁺ 1.80 afu), and minor, variable Al contents. The Mg content of slightly lesser than 1 afu, is almost constant. Cell parameters and octahedral bond distances increase with V³⁺. Unexpectedly, the Mg–O tetrahedral bond distance also increases slowly with V³⁺. This weak dragging effect contributes towards maintaining distortion of the oxygen array with respect to the ideal CCP, thus providing a shielding effect, which reduces V³⁺–V³⁺ repulsion. This leads to the energetic stabilization of the structure, in spite of the increase of bond strain with increasing V³⁺ contents.     

Latest precambrian (Cadomian) zircon ages,Nd isotopic systematics and P-T evolution of granitoid orthogneisses of the Erzgebirge,Saxony and Czech Republic

Single zircons from two orthogneiss complexes, the Grey Gneiss and Red Gneiss, the lowermost tectonic units in the Erzgebirge, were dated. The grey Freiberg Gneiss is of igneous origin and has a ²⁰⁷Pb/²⁰⁶Pb emplacement age of 550±7 Ma. A quartz monzonite from Lauenstein contains idiomorphic zircons with a mean ²⁰⁷Pb/²⁰⁶Pb age of 555±7 Ma as well as xenocrysts ranging in age between 850 and 1910 Ma. Red gneisses from the central Erzgebirge contain complex zircon populations, including numerous xenocrysts up to 2464 Ma in age. The youngest, idiomorphic, zircons in all samples yielded uniform ²⁰⁷Pb/²⁰⁶Pb ages between 550±9 and 554±10 Ma. Nd isotopic data support the interpretation of crustal anatexis for the origin of both units. _Nd(t) values for the grey gneisses are –7.5 and –6.0 respectively, (mean crustal residence ages of 1.7–1.8 Ga). The red gneisses have a wider range in _Nd(t) values from –7.7 to –2.8 (T _DM ages of 1.4–1.8 Ga). The zircon ages document a distinct late Proterozoic phase of granitoid magmatism, similar in age to granitoids in the Lusatian block farther north-east. However, Palaeozoic deformation as well as medium pressure metamorphism ( 8 kbar/600–650° C) are identical in both gneiss units and distinguish these rocks from the Lusatian granitoids. The grey and red gneisses were overthrust by units with abundant high-pressure relicts and a contrasting P-T evolution. Zircon xenocryst and Nd model ages in the range 1000–1700 Ma are similar to those in granitoid rocks of Lusatia and the West-Sudetes, and document a pre-Cadomian basement in parts of east-central Europe that, chronologically, has similarities with the Sveconorwegian domain in the Baltic Shield.     

Converging P-T paths of Mesozoic HP-LT metamorphic units (Diego de Almagro Island, Southern Chile): evidence for juxtaposition during late shortening of an active continental margin

Summary On Diego de Almagro Island in Chilean Patagonia (51°30S), a convergent strike slip zone, the Seno Arcabuz shear zone, separates the Diego de Almagro Metamorphic Complex from very low grade metagreywackes in the east, which were intruded by Jurassic granitoids. The Diego de Almagro Metamorphic Complex is composed of a metapsammopelitic sequence containing blueschist intercalations in the west and (garnet) amphibolite lenses in the east. Peak metamorphic conditions (stage I) at 9.5–13.5kbar, 380–450°C in the blueschist and at 11.2–13.2, 460–565°C in the amphibolite indicate subduction and accretion at different positions within the deepest part of the accretionary wedge. A K–Ar age of 117±28Ma of amphibole approximately dates the peak of metamorphism in the amphibolite. The early retrograde stage of metamorphism occurred under static conditions and resulted in localized equilibration (stage II) at 6.3–9.6kbar, 320–385°C in the blueschist and 6.1–8.4kbar, 310–504°C in the amphibolite. Both P-T paths converge within a midcrustal level.In contrast, an orthogneiss of trondhjemitic composition occurring within the Seno Arcabuz shear zone is associated with a garnet mica-schist containing a high temperature/intermediate pressure assemblage formed at 4.9–6.5kbar, 580–690°C. A muscovite K–Ar age of 122.2±4.6Ma dates cooling after this event which is related to a concomitant magmatic arc. These rocks were overprinted by a mylonitic deformation, which is caused by convergent strike slip shearing and ends during formation of a retrograde phengite-chlorite-stilpnomelane assemblage at a minimum pressure of approximately 5.7kbar (at 300°C).Zircon fission track ages from rocks of the Seno Arcabuz shear zone are 64.9±2.7 and 64.9±2.7Ma; they record the end of shearing in the Seno Arcabuz shear zone that juxtaposed all rocks in the middle crust. Zircon fission track ages ranging from 78 to 105Ma in the South Patagonian batholith to the east indicate earlier cooling through 280°C. The rocks of the Diego de Almagro Metamorphic Complex were initially slowly exhumed and resided at a midcrustal level before being emplaced via shearing in the Seno Arcabuz shear zone. Apatite fission track ages (54±8Ma) from the Seno Arcabuz shear zone show that exhumation and cooling rates increased after this event. The incorporation of continental crust within the subduction system was a late process, which modified the Cretaceous accretionary wedge, resulting in considerable shortening of the convergent margin.     

Characteristics and chronology of fracture — fluid infiltration in the Archean,Eye Dashwa Lakes pluton,Superior Province: evidence from H,C, O-isotopes and fluid inclusions

The Archean Eye Dashwa Lakes pluton (2672±24 Ma) has domains of mineralogically fresh isotropic granite, domains that have undergone bulk hydrothermal alteration, and at least eleven sets of sequential fracture arrays, each with distinctive mineral assemblages. Fresh granite is characterized by whole rock ¹⁸O=8.1 to 8.6 and primary magmatic quartz-feldspar (+1.3), quartz-biotite (5.2 to 5.4) and quartz-magnetite (+9.8) fractionations. Magmatic fluids had a calculated isotopic composition of ¹⁸O=7.9±0.5, and D=–80±5. These isotropic volumes of the granite have not experienced significant incursion of external thermal waters. Pegmatites, quartz-molybdenite veins, and phlogopite-muscovite coated fractures are sporadically distributed in the granite, and were precipitated from high-temperature magmatic fluids where ¹⁸O=8.0 to 10.3 and D=–80±5.The most abundant variety of fracture filling assemblage is epidote-quartz-chlorite±muscovite: fractures are bounded by domains of mineralogically similar bulk hydrothermal alteration of the granite. These minerals formed at 160 to 280° C, in the presence of NaCl, and NaCl-MgCl₂ brines (up to 25 wt% NaCl equivalent) of probable evolved marine water origin ( ¹⁸O=+0.4 to +3.8, D=–10 to –35) undergoing transient boiling. Upper plateau ⁴⁰Ar/³⁹Ar ages for the muscovite are 2650±15 Ma. Sequentially in the chronology of fracture-infiltration events, calcite-fluorite veins were deposited from boiling fluids at 340 to 390° C, isotopically characterized by ¹⁸O=4.7 and ¹³C=–5; and rare prehnite-chlorite lined fractures formed at 250 to 290° C. A generation of adularia-bearing veins precipitated at 140 to 230° C, from CaCl₂-NaCl brines, where ¹⁸O=0 to –6.5 and D=–10 to –30. Incremental ⁴⁰Ar/³⁹Ar age spectra on the K-feldspar yield an upper plateau of 1100 Ma. Subsequently, hematite developed during reactivation of earlier fractures, at 140 to 210° C in the presence of fluids characterized by ¹⁸O=–0.4 to –5.4 and D=–15 to –25. Arrays of open fractures partially occupied by gypsum and goethite reflect a fluid infiltration event at temperatures <50° C. Many of the earlier generations of fracture minerals have transgranular fracture infillings which record the presence of low temperature (88–190° C), hypersaline CaCl₂-NaCl brines. Narrow fractures lined with clays±calcite are sites for seepage of modern ground-waters. The isotopic signature of clay ( ¹⁸O=12 to 20, D=–80±5) plots near the line for modern kaolinites, confirming its formation in the presence of recent surface waters. Calcites coexisting with the clay minerals, and in fractured pegmatite show a common isotopic signature ( ¹⁸O=23±0.5, ¹³C=–13.6), indicating precipitation from modern groundwaters, where reactivated fractures have acted as conduits for infiltration of surface waters to depths of 200 m. Intermittent fracture-infiltration has occurred over 2.7 Ga. The early sequences of fracture-related fluid flow are interpreted in terms of devolatilization of the granite, followed by thermal contraction fracturing, incursion of marine water and convective cooling in the Archean. Hematite and adularia fracture fillings correspond to a stage when meteoric water infiltrated the volcanicplutonic terrain during Proterozoic and later times. Episodic fracture-fluid expulsion events may have been driven by seismic pumping, in response to magmatically and tectonically induced stresses within the Shield, with surface waters penetrating to depths of 15 km in the crust.     

Sm−Nd and Pb isotopic study of mafic rocks associated with early Proterozoic continental rifting: the Peröpohja schist belt in northern Finland

The Peräpohja schist belt in northern Finland rests unconformably on Archaean granitoids, and marks the early stages of Proterozoic crustal evolution in the Fennoscandian (Baltic) shield. 2440 Ma old layered mafic intrusions predate the supracrustal , and ca. 2200 Ma old sills of the gabbro-wehrlite association intrude the lowest quartzites and volcanics (Runkaus) of the sequence. The Sm-Nd mineral isochron of the Penikat layered intrusion gives an age of 2410±64 Ma. The initial _Nd-values of the Penikat intrusion (_Nd(2440) = –1.6) and the Runkausvaara sill (_Nd(2200) 0) suggest that these mafic magmas were contaminated by older crustal material. The Sm-Nd and Pb isotopic results on the 2.44–2.2 Ga old Runkaus volcanics indicate mobility of Pb, fractionation of Sm/Nd during late greenschist facies metamorphism, and crustal contamination. The Pb-Pb data provide an age of 1972±80 Ma with a high initial ²⁰⁷Pb/²⁰⁴Pb ratio (₁ = 8.49), while scattered Sm-Nd data result in an imprecise age of 2330±180 Ma, with an initial _Nd-value of about zero. Secondary titanite gives an U-Pb age of ca. 2250 Ma. The Jouttiaapa basalts, in contrast, ascended from the mantle without interaction with older crust. These LREE depleted tholeiites mark a break in continental sedimentation, and yield a Sm-Nd age of 2090±70 Ma. Their initial _Nd = + 4.2 ±0.5 implies that the subcontinental early Proterozoic mantle had been depleted in LREE for a long period of time. The first lava flows are strongly depleted in LREE, suggesting that their source was significantly more depleted than the source of mid-ocean ridge basalts today.     

Fluid-rock interaction between syenites and marbles at Stephen Cross Quarry,Québec,Canada: petrological and stable isotope data

Grenville dolomitic marbles and calc-silicates at Stephen Cross Quarry, Québec, underwent contact metamorphism and metasomatism associated with the intrusion of the Wakefield syenite at ambient pressures of 0.4GPa at 1090–1070Ma. Fluid infiltration produced exoskarns, calcite+periclase+forsterite±diopside±orthoclase assemblages in the marbles, and quartz±calcite±wollastonite±diopside±anorthite assemblages in the calc-silicates. Phase-equilibria in the CaO–MgO–Al₂O₃–SiO₂–H₂O–CO₂ system suggest that fluid infiltration occurred close to the thermal peak of contact metamorphism (715–815°C) and that the fluids hadXCO₂0.15. In the metasediments, ¹⁸O values of calcite (Cc) are as low as 8.6, suggesting that the fluids were in isotopic equilibrium with the syenites (¹⁸O =8.8–10.2). Marble ¹³C(Cc) values are-0.1 to-3.2; the lack of correlation between ¹³C(Cc) and ¹⁸O(Cc) is consistent with the infiltration of water-rich fluids. The resetting of stable isotopes and the mineralogical changes can be explained by time-integrated fluid fluxes of up to 110 m³/m² (4×10⁶ mol/m²), corresponding to actual fluxes of 3×10^-11 to 3×10^-12 m³/m²-s and intrinsic permeabilities of 10^-18 to 10^-20 m² for fluid flow lasting 0.1-1Ma. Marble ¹⁸O(Cc) values do not correlate well with distance from the syenite, and fluids were probably channelled across lithological layering. The correlation between the degree of resetting of marble ¹⁸O(Cc) values with the abundance of submillimetre-wide veins, suggests that fluid focussing may have resulted from variations in fracture density. Late, lower temperature (<500°C), fluid flow formed serpentine (Serp) and brucite (Br) from periclase and forsterite. ¹⁸O(Br) and ¹⁸O(Scrp) values correlate with ¹⁸O(Cc), suggesting that retrogression involved only limited volumes of fluid. The observation that ¹⁸O(Cc-Br) and ¹⁸O(Cc-Serp) values are higher in marbles that have lower ¹⁸O(Cc) values is interpreted as indicating that fluid flow persisted to lower temperatures in those rocks due to higher intrinsic permeabilities. Calcite in the syenite was also formed by the influx of fluids during cooling. Syenite ¹⁸O(Cc) values are approximately in isotopic equilibrium with the high-temperature silicate minerals, suggesting that again only minor volumes of fluid were involved. In detail fluid flow was prolonged and complex, creating problems for the application of quantitative fluid flow models.This paper is a contribution to IGCP 304, Lower Crustal Processes     

Stratiform and vein U,Mo and Cu mineralization in the Novoveská Huta area,CSFR

U-Mo and Cu mineralization occurs in horizons as well as in veins in the Permian formations near Novoveská Huta. Ore mineralization is represented by uraninite, U-Ti oxides, coffinite, molybdenite, chalcopyrite, tennantite and pyrite. The isotopic composition of S and C displays a larger variability in the stratiform ores (³⁴S from –32.7 to +2.7 and ¹³C from –27.1 to — 0.5) These data suggest mixing of meteoric solutions with fluids of volcanic origin and a complex history. There is a narrower range of ³⁴S from –18.8 to –4.6 and ¹³C from –6.3 to –2.5% in quartz-carbonate veins with Cu mineralization suggesting a deep source of ore-bearing solutions. The Permian volcanics were a significant source of ore elements.Their contents of U, Mo, Cu and Y are from two to eight times higher than in sedimentary rocks. Accumulations of ore elements in the horizons were formed by the reduction and adsorption processes 240 ± 30 Ma ago according to U-Pb isotopic dating. Due to Alpine tectonism, these low-grade ores (U<0.1 wt%) were remobilized and higher-grade U-Mo ores (U>0.1 wt%) were formed 130 ± 20 Ma ago at temperatures ranging from 110 to 120° C, according to fluid inclusions. Younger veins with Cu mineralization were formed 115 ± 10 Ma according to the model age of Pb at temperatures ranging from 95 to 190°C.     

The distribution of meteoric <Superscript>36</Superscript>Cl/Cl in the United States: a comparison of models

The natural distribution of ³⁶Cl/Cl in groundwater across the continental United States has recently been reported by Davis et al. (2003). In this paper, the large-scale processes and atmospheric sources of ³⁶Cl and chloride responsible for controlling the observed ³⁶Cl/Cl distribution are discussed.The dominant process that affects ³⁶Cl/Cl in meteoric groundwater at the continental scale is the fallout of stable chloride from the atmosphere, which is mainly derived from oceanic sources. Atmospheric circulation transports marine chloride to the continental interior, where distance from the coast, topography, and wind patterns define the chloride distribution. The only major deviation from this pattern is observed in northern Utah and southern Idaho where it is inferred that a continental source of chloride exists in the Bonneville Salt Flats, Utah.In contrast to previous studies, the atmospheric flux of ³⁶Cl to the land surface was found to be approximately constant over the United States, without a strong correlation between local ³⁶Cl fallout and annual precipitation. However, the correlation between these variables was significantly improved (R ²=0.15 to R ²=0.55) when data from the southeastern USA, which presumably have lower than average atmospheric ³⁶Cl concentrations, were excluded. The total mean flux of ³⁶Cl over the continental United States and total global mean flux of ³⁶Cl are calculated to be 30.5±7.0 and 19.6±4.5 atoms m^–2 s^–1, respectively.The ³⁶Cl/Cl distribution calculated by Bentley et al. (1986) underestimates the magnitude and variability observed for the measured ³⁶Cl/Cl distribution across the continental United States. The model proposed by Hainsworth (1994) provides the best overall fit to the observed ³⁶Cl/Cl distribution in this study. A process-oriented model by Phillips (2000) generally overestimates ³⁶Cl/Cl in most parts of the country and has several significant local departures from the empirical data.

---

Resumen Davis et al. (2003) han informado de la distribución natural de la proporción ³⁶Cl/Cl en las aguas subterráneas de la parte continental de los Estados Unidos de América [EUA]. En este artículo, se discute cuáles son los procesos a gran escala y las fuentes atmosféricas del ³⁶Cl y del cloruro que dan lugar a la distribución observada de ³⁶Cl/Cl.El proceso dominante que afecta a la relación ³⁶Cl/Cl en las aguas subterráneas de origen meteórico a escala continental es el aporte de cloruro estable desde la atmósfera, que procede principalmente de los océanos. La circulación atmosférica transporta el cloruro marino hacia el interior, donde la distancia a la costa, topografía y corrientes del viento definen la distribución del cloruro. La única desviación principal de este esquema tiene lugar al norte de Utah y en el sur de Idaho, donde se deduce que existe una fuente continental de cloruro en los Rellanos Salados de Bonneville (Salt Flats).En contraste con estudios previos (Knies et al. 1994; Phillips 2000), se ha descubierto que el flujo atmosférico de ³⁶Cl hacia la superficie terrestre es aproximadamente constante en todos los estados, sin deducirse una correlación fuerte entre el aporte de ³⁶Cl y la precipitación anual. Sin embargo, la correlación entre estas variables se ve mejorada de forma significativa, con coeficientes de regresión comprendidos entre 0,15 y 0,55, cuando se excluyen los datos recogidos en el sudeste de los EUA, que tienen concentraciones de ³⁶Cl atmosférico presuntamente inferiores a la media. El flujo medio total de ³⁶Cl calculado en la zona continental de los Estados Unidos vale 30,5±7,0 átomos por metro cuadrado y segundo, mientras que el flujo total global de ³⁶Cl es de 19,6±4,5 átomos por metro cuadrado y segundo.La distribución de ³⁶Cl/Cl calculada por Bentley et al. (1986) infravalora la magnitud y variabilidad observada en los valores medidos a lo largo de los Estados Unidos. El modelo propuesto por Hainsworth (1994) proporciona el mejor ajuste conjunto a la distribución observada de ³⁶Cl/Cl en este estudio. El modelo orientado a procesos de Phillips (2000) sobreestima por lo general la distribución de ³⁶Cl/Cl en la mayoría del país y difiere significativamente de algunos valores locales empíricos.

---

Résumé La distribution naturelle du rapport ³⁶Cl/Cl dans les eaux souterraines des États-Unis a été récemment présentée par Davis et al. (2003). Dans ce travail, les processus à grande échelle et les sources atmosphériques de ³⁶Cl et de chlorure responsables du contrôle de la distribution observée du rapport ³⁶Cl/Cl sont discutés. Le processus dominant qui affecte le rapport ³⁶Cl/Cl dans les eaux souterraines dorigine météorique à léchelle continentale est lapport atmosphérique de chlorure stable, qui provient pour lessentiel de sources océaniques. La circulation atmosphérique transporte des chlorures marins vers lintérieur des continents, où la distribution de chlorure est définie par la distance à la côte, la topographie et les régimes des vents. La seule exception majeure à ce schéma est observée dans le nord de lUtah et le sud de lIdaho où lon suppose quil existe une source continentale de chlorure dans les bas-fonds salés de Bonneville. Au contraire de précédentes études (Knies et al. 1994; Phillips 2000), on trouve que le flux atmosphérique de ³⁶Cl vers le sol est approximativement constant sur lensemble des États-Unis, sans forte corrélation entre la retombée locale de ³⁶Cl et les précipitations annuelles. Cependant, la corrélation entre ces variables devient significative (R ²=0.15 à 0.55) lorsquon supprime les données du sud-est des États-Unis, dont on pense quelles présentent des concentrations en ³⁶Cl atmosphérique inférieures à la moyenne. Le flux total moyen de ³⁶Cl sur les États-Unis continentaux et le flux moyen global de ³⁶Cl sont respectivement évalués à 30.5 ± 7.0 et 19.6 ± 4.5 atomes.m^–2.s^–1. La distribution du rapport ³⁶Cl/Cl calculée par Bentley et al. (1986) sous-estime lordre de grandeur et la variabilité observés pour la distribution mesurée du rapport ³⁶Cl/Cl sur les États-Unis continentaux. Le modèle proposé par Hainsworth (1994) fournit le meilleur ajustement densemble à la distribution du rapport ³⁶Cl/Cl observée dans cette étude. Un modèle orienté vers les processus proposé par Phillips (2000) surestime dans lensemble le rapport ³⁶Cl/Cl dans la plupart des régions du pays et présente plusieurs désaccords locaux avec les données empiriques.

     

F-OH exchange equilibria between mica-amphibole mineral pairs

Fluoride-hydroxyl exchange equilibria between phlogopite-pargasite and phlogopite-tremolite mineral pairs were experimentally determined at 1,173K, 500 bars and 1,073–1,173 K, 500 bars respectively. The distribution of fluorine between phlogopite and pargasite was found to favor phlogopite slightly, G _ex ^. (1,173 K)=–1.71 kJ anion^–1, while in the case of phlogopite-tremolite, fluorine was preferentially incorporated in the mica, G _ex ^. (1,073)=– 5.67 kJ anion^–1 and G _ex ^. (1,173K)=–5.84 kJ anion^–1. These results have yielded new values of entropy and Gibbs energy of formation for fluortremolite, S _f =–2,293.4±16.0JK^–1 mol^–1 and G _f = –11,779.3±25.0 kJ mol^–1, respectively. In addition, F-OH mineral exchange equilibria support a recent molten oxide calorimetric value for the Gibbs energy of fluorphlogopite, G _f =–6,014.0±7.0 kJ mol^–1, which is approximately 40 kJ mol^–1 more exothermic than the tabulated value.This work performed in part at Sandia National Laboratories supported by the U.S. Department of Energy, DOE, under contract number DE-AC04-76DP00789     

Die fazielle und paläogeographische Entwicklung im Perm der Karnischen Alpen und in den Randgebieten

Zusammenfassung Im Unterperm bildet sich mit den küstennahen Innenschelf-Sedimenten der Unteren Pseudoschwagerinen-Kalke (UPK) und über der klastischen Fazies der im hochenergetischen Intertidal-Subtidal-Grenzbereich entstandenen Grenzlandbänke (GB) die Karbonatplattform der Oberen Pseudoschwagerinen-Kalke (OPK) heraus. Ökologische Zonenfolgen, die eine zunehmende Verschiebung zu küstenferneren Bereichen anzeigen, weisen die OPK als Ablagerungen des subtidalen Außenschelf-Bereiches aus.Verschieden alte Schelfrandriffe (Karnische Alpen, Sextener Dolomiten) und die Klastischen Trogkofelschichten als Innenschelfentwicklung im E und SE setzen die Entwicklung der Plattform in der Trogkofelstufe fort.Bodenunruhen mit Brekzienbildungen in den Klastischen Trogkofelschichten gipfeln in einem Zerbrechen der Karbonatplattform an der Wende Unter/Mittelperm und führen zur Ablagerung der Tarviser Brekzie in lokalen Senken.Mit den im W zunächst noch kontinentalen, in den Karnischen Alpen und Westkarawanken aber überwiegend marinen Grödener Schichten und den bei fortschreitender Transgression abgelagerten Bellerophonschichten setzt der alpidische Sedimentationszyklus ein.

---

Paleontological and sedimentological data of Permian rocks from the Carnic Alps and its neighbouring areas south of the Periadriatic Lineament could fit to the following paleogeographical patterns:The Untere Pseudoschwagerinen-Kalke (UPK) of the Rattendorfer Schichten were cyclically deposited in the inner-shelf area. During the upper UPK erosive activities and source areas of the clastics were changing. Increasing sedimentation of clastics due to the temporary erosion of metamorphic rocks and acid plutonites characterizes the Grenzland-Bänke (GB), deposited in a near-shore high-energy environment with intertidal to subtidal conditions (mega-ripples). The outer shelf sedimentation pattern of the Obere Pseudoschwagerinen-Kalke (OPK) is indicated by special associations of organisms in subtidal offshore areas.On this carbonate platform some knoll reefs of different age were built up during the Trogkofel stage (Carnic Alps, Sextener Dolomiten). Towards the east and southeast the platform shows again characteristics of the inner shelf (Klastische Trogkofel-Schichten).Synsedimentary tectonic activities during the late Lower Permian are manifested by the carbonate breccias of the Klastische Trogkofel-Schichten (Garnie Alps, Karawanken, Slovenia). The components of these breccias are dated as Middle Carboniferous to late Lower Permian. A further destruction of the platform at the Lower/Middle Permian boundary results in the sedimentation of the Tarviser Brekzie.The alpidic cycle Starts with the sedimentation of the clastic Grödener Schichten which during the first phase of sedimentation were deposited as continental clastics in South Tyrol but as predominately marine sediments in the Carnic Alps and in the Karawanken.The increasing transgression during the Upper Permian results in the deposition of bituminous sediments and basal evaporites of the Bellerophon-Schichten. In the south-western near-coast area the evaporites are dominating (facies fiammaza) whereas in northeastern South Tyrol gypsum with open marine carbonates prevails (facies badiotica). The Bellerophon-Schichten of the Carnic Alps indicate connections with the marine Zazar-Schichten in the Save area and with the limestones and dolomites of the Velebit mountains.

---

Résumé L'étude paléontologique et sédimentologique du Permien des Alpes carniques et des régions septentrionales, voisines du linéament périadriatique, a permis d'en tracér une equisse paléogéographique. - Les Untere Pseudoschwagerinen-Kalke (UPK) qui marquent le début des Rattendorfer Schichten, constituent une sédimentation cyclique de nature littorale, à laquelle succède, à la suite d'une activité érosive intense, les Grenzlandbänke (GB) caractérisés par un facies clastique de haute énergie, avec, dans ce dernier, des intervalles où apparaissent des roches métamorphiques et des roches plutoniques acides. Ce milieu, parcouru par des courants instables, nous indique un domaine mitoyen» subtidal-intertidal «(méga-ripples), à labase des marées. Des faunes et flores, variées et abondantes, sont distribuées dans une succession de zones écologiques de plus en plus distantes de la côte. Les Obere Pseudoschwagerinen-Kalke (OPK), déposés ensuite, se sont donc formés sur le plateau continental extérieur à caractère subtidal.Sur cette plate-forme à formations carbonatées, et au cours de l'étage de Trogkofel, des récifs ont pu s'installer, tandis que l'E et le SW de la plate-forme, plus rapprochés de la côte, ont reçu une sédimentation clastique (Klastische Trogkofelschichten). — Les mouvements de la fin du Permien inférieur se sont traduits par des brèches intercalées dans les Klastische Trogkofelschichten; des fragments de roches du Carbonifère moyen (étage de Moscow) sont inclus dans ces brèches. Enfin cette tendance arrive à son comble à la limite Permien inférieur/Permien moyen, lorsque la plateforme se désintègre avec formation de la Taviser Brekzie dans des effondrements local.Le cycle de sédimentation alpin débute par les Grödener Schichten — encore terrestres à l'Ouest, mais surtout marines dans les Alpes carniques et les Karawanken occidentales — et par les Bellerophonschichten marquant une mer en transgression.

---

, , (GB) , ( ) (UPK). , , , , . — , — , -, . . , . . , — — .

     

Stable isotope and fluid inclusion studies of gem-bearing granitic pegmatite-aplite dikes,San Diego Co., California

Late Cretaceous, granitic pegmatite-aplite dikes in southern California have been known for gem-quality minerals and as a commercial source of lithium. Minerals, whole-rock samples, and inclusion fluids from nine of these dikes and from associated wall rocks have been analyzed for their oxygen, hydrogen, and carbon isotope compositions to ascertain the origins and thermal histories of the dikes. Oxygen isotope geothermometry used in combination with thermometric data from primary fluid inclusions enabled the determination of the pressure regime during crystallization.Two groups of dikes are evident from their oxygen isotope compositions (¹⁸O_qtz+10.5 in Group A, and +8.5 in Group B). Prior to the end of crystallization, Group A pegmatites had already extensively exchanged oxygen with their wall rocks, while Group B dikes may represent a closer approximation to the original isotopic composition of the pegmatite melts. Oxygen isotope fractionations between minerals are similar in all dikes and indicate that the pegmatites were emplaced at temperatures of about 730 ° to 700 ° C. Supersolidus crystallization began with the basal aplite zone and ended with formation of quench aplite in the pocket zone, nearly to 565 ° C. Subsolidus formation of gem-bearing pockets took place over a relatively narrow temperature range of about 40 ° C (approximately 565–525 ° C). Nearly closed-system crystallization is indicated.Hornblende in gabbroic and noritic wall rocks (D_w.r. = –90 to –130) in the Mesa Grande district crystallized in the presence of, or exchanged hydrogen with, meteoric water (D –90) prior to the emplacement of the pegmatite dikes. Magmatic water was subsequently added to the wall rocks adjacent to the pegmatites.Groups A and B pegmatites cannot be distinguished on the basis of their hydrogen isotope compositions. A decrease in D of muscovite inward from the walls of the dikes reflects a decrease in temperature. D values of H₂O from fluid inclusions are: –50 to –73 (aplite and pegmatite zones); –62 to –75 (pocket quartz: Tourmaline Queen and Stewart dikes); and –50 ± 4 (pocket quartz from many dikes). The average ¹³C of juvenile CO₂ in fluid inclusions in Group B pegmatites is –7.9. In Group A pegmatities, ¹³C of CO₂ is more negative (–10 to –15.6), due to exchange of C with wall rocks and/or loss of ¹³C-enriched CO₂ to an exsolving vapor phase.Pressures during crystallization of the pockets were on the order of 2,100 bars, and may have increased slightly during pocket growth. A depth of formation of at least 6.8 km (sp. gr. of over burden = 3.0, and P _fiuid=P _load) is indicated, and a rate of uplift of 0.07 cm/yr. follows from available geochronologic data.     

Age and origin of the Cortlandt Complex,New York: Implications from Sm-Nd data

Sm-Nd systematics for nine whole-rock samples of hornblende norites, pyroxenites and a lamprophyre from various parts of the Cortlandt Complex were analyzed. Six of these samples from the central and eastern parts of the complex give an isochron age of 430±34 (2) Ma with an _Nd value of –2.9±0.5, and the other three samples from the western part, including the lamprophyre, define a similar age of 394±33 (2) Ma but with a distinctly different _Nd value of –1.4±0.4. The two different initial ¹⁴³Nd/¹⁴⁴Nd ratios corresponding to these -values are interpreted to reflect continental crustal contamination of the lamprophyric parental liquid prior to final emplacement and crystal fractionation to produce the different rock types of the complex. The intrusion age of 430 Ma for the complex clearly post-dates the major metamorphic event of the Taconic orogeny. The Nd-isotopic data also suggest a relationship between the Cortlandt Complex and a belt of lamprophyric dike rocks to the west, known as the Beemerville trend, which cuts across the metamorphic trends of the Taconic (Ratcliffe 1981).     

Thermodynamic properties,low-temperature heat-capacity anomalies,and single-crystal X-ray refinement of hydronium jarosite, (H<Subscript>3</Subscript>O)Fe<Subscript>3</Subscript>(SO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>6</Subscript>

Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO₄ solutions. Single-crystal XRD refinement with R₁=0.0232 for the unique observed reflections (|F_o| > 4_F) and wR₂=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, V^o=160.11(4) cm³, and fractional positions for all atoms except the H in the H₃O groups. The chemical composition of this sample is described by the formula (H₃O)_0.91Fe_2.91(SO₄)₂[(OH)_5.64(H₂O)_0.18]. The enthalpy of formation (H^o_f) is –3694.5 ± 4.6 kJ mol^–1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, -FeOOH, MgO, H₂O, and -MgSO₄. The entropy at standard temperature and pressure (S^o) is 438.9±0.7 J mol^–1 K^–1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (C_p) data between 273 and 400 K were fitted to a Maier-Kelley polynomial C_p(T in K)=280.6 + 0.6149T–3199700T^–2. The Gibbs free energy of formation is –3162.2 ± 4.6 kJ mol^–1. Speciation and activity calculations for Fe(III)–SO₄ solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the C_p data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in C_p, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H₃O)Fe₃(SO₄)₂(OH)₆ was estimated: G^o_f= –3226.4 ± 4.6 kJ mol^–1, H^o_f=–3770.2 ± 4.6 kJ mol^–1, S^o=448.2 ± 0.7 J mol^–1 K^–1, C_p (T in K)=287.2 + 0.6281T–3286000T^–2 (between 273 and 400 K).     

Phase relationships and fluid inclusions characteristics of the metamorphosed,stratiform sulfide deposit Matchless,Namibia

Silicate assemblages in the wallrocks of the metamorphosed, stratiform sulfide deposit Matchless, Damara orogen, Namibia, yield temperatures between 550 and 625 °C and pressure of 6–8 kbar. Highest density fluid inclusions, containing either pure CO₂ or CO₂-CH₄/N₂ mixtures, display isochors which extrapolate through these metamorphic peak conditions. A second group of inclusions with the same fluid compositions exhibit isochors conforming to lower pressures, i.e. 3 kbar as deduced from sphalerite barometer, at the same temperatures. From these observations, we deduce an isothermal uplift path after peak of amphibolite facies metamorphism.

---

Zusammenfassung Silikatparagenesen im Nebengestein der metamorphen, schichtgebundenen Kupferlagerstätte Matchless, Damara-Orogen, Namibia, ergeben Temperaturen zwischen 550 und 625 °C und Drucke von 6–8 kbar beim Höhepunkt der Me-tamorphose. Fluideinschlüsse hoher Dichte, die mit CO₂ oder mit CO₂+CH₄/N₂ gefüllt sind, besitzen Isochoren, die durch diese P-T-Kombination verlaufen. Eine zweite Gruppe von Fluideinschlüssen mit der gleichen Zusammensetzung entspricht niedrigeren Druckbedingungen von etwa 3 kbar, wie sie sich aus dem Zinkblende-Barometer ableiten lassen, jedoch bei unveränderter Temperatur. Daraus läßt sich auf eine isotherme Heraushebung nach dem Höhepunkt der Metamorphose schließen.

---

Résumé Le dépôt de sulfure stratiforme métamorphique de Matchless (orogenèse Damara, Namibie) sont encaissés dans des roches dont les paragenèses silicatées indiquent des températures comprises entre 550° et 625° et des pressions de 6 à 8 Kb. Des inclusions fluides, très denses, contenant soit du CO₂ pur, soit des mélanges CO₂ + CH₄/N₂ fournissent des isochores qui traversent ce champ (P, T). Les isochores d'un deuxième groupe d'inclusions de mêmes compositions donnent, pour les mêmes températures, des pressions plus basses, de l'ordre de 3 Kb, déduites du baromètre à blende. On en déduit un soulèvement isothermique de la région après le pic du métamorphisme, qui s'est produit dans les conditions du faciès des amphibolites.

---

, Matchless, , , , , , 550 65 6 8 . , ₂, ₂ + CH₄/N₂ , . , 3 , , . , , .

     

Anomalous Sm−Nd ages for the early Archean Onverwacht Group Volcanics

New Sm–Nd isotopic data for eight samples of basalt and komatiite from the Tjakastad Subgroup (lower Onverwacht Group) of the Barberton Greenstone Belt (BGB) of the Kaapvaal craton in southern Africa are reported. They give new constraints on the interpretation of Sm–Nd ages for the Subgroup and highlight the petrogenesis of Tjakastad volcanics. Although Sm–Nd isotopic data earlier reported for volcanic rocks from the Tjakastad Subgroup yielded an isochron age of 3526±48 Ma, the new results give a much younger isochron date of 3269±84 Ma. The 3526±48 Ma isochron age has been obtained in combining samples ranging in composition from felsic volcanics to ultrabasic komatiites and is thus considered suspect with regard to the pre-requisite of geochronology that all the studied rocks must have had identical initial isotopic compositions. The new isochron date of 3269±84 Ma has been obtained in combining samples solely of basic/ultrabasic composition. It might thus represent a more correct age for the eruption of the Tjakastad Subgroup volcanism. In fact, owing to the potential problem of source heterogeneity and also in the light of geochronological and geochemical arguments we show that this date also has little chance to have any strict chronological meaning. Most likely, the Tjakastad volcanics were formed 3450 Ma ago. Also most likely, their source rocks were isotopically heterogeneous and the 3530 Ma and 3270 Ma linear arrays are not true but apparent isochrons. Based on the calculated _Nd (3450) values and other geochemical arguments, we show that three possible sources might have been involved: depleted mantle, primitive mantle and older continental crust.     

Sulfur isotopes and the origin of stibnite mineralisation in New England,Australia

Stibnite mineralisation in the antimony province of New England can be divided into Central type ores (veins of stibnite + quartz ± berthierite) and Peripheral type ores of stibnite + quartz + native antimony ± berthierite. The Central stibnites have ³⁴S_CDT values of –5±2 (1) which may represent equilibrium precipitation from mantle sulfur at about 200°C. Peripheral stibnites have ³⁴S values between 0 and –25, with a large group at 0±2. They represent precipitation from a limited supply of mantle sulfur and the acquisition of sedimentary sulfur. We consider that the different ore types were produced from distinct ore solutions derived from two immiscible melts. These originated in the deep mantle, were mobilised by tectonic activity and supplied the antimony and most of the sulfur to the ores.     

Examples of structural control of hydrothermal mineralization: Fault zones in epicontinental sedimentary basins — A review

By compiling data available from progressive geoscientific research and by complementary studies it seems possible for an increasing number of mineral districts to find indications of the regional structural control of hydrothermal ore deposits. The relationship between hydrothermal mineralization, faulting or fault reactivation and related processes can, of course, best be examined at the less deformed sedimentary record of ancient epicontinental sedimentary basins. There is increasing evidence that the sediment-hosted submarine-hydrothermal Zn-Pb-Ba mineralizations such as Meggen, Rammelsberg, Howards Pass and McArthur River are spatially and genetically related to deep synsedimentarily active fault systems of intracontinental basins or stretched passive continental margins. For long periods of basin development the faults provided favorable conduits for episodically released basin-dewatering brines, which precipitated the leached metals into third-order basins situated next to these faults. There are many similarities to epigenetic mineralizations, which formed either pre-, syn- or postorogenetically from brines leaching the (meta)sediments and associated magmatites and migrating along fault zones to their near-surface trap structures.

---

Zusammenfassung Für viele Erzprovinzen bietet sich durch Kompilation der in den letzten Jahrzehnten rasch angewachsenen Zahl geowissenschaftlicher Daten und durch gezielte ergänzende Untersuchungen die Möglichkeit, Anhaltspunkte oder Belege für die regionale Strukturbindung von hydrothermalen Vererzungen zu finden. Naturgemäß lassen sich die Zusammenhänge zwischen hydrothermaler Mineralisation, Störungstektonik und begleitenden Prozessen am besten an nur gering orogen überprägten ehemaligen Sedimentbecken untersuchen. So gibt es vermehrt Belege dafür, daß die sediment-gebundenen, submarin-hydrothermalen Zn-Pb-Ba Vererzungen wie Meggen, Rammeisberg, Howards Pass, McArthur River etc. räumlich und genetisch an tiefreichende Störungssysteme gebunden sind, die während der Sedimentation in intrakontinentalen Riftbecken oder auf ausgedünnten passiven Kontinentalrändern aktiv waren. Diese Störungssysteme bündelten die salinaren Beckenentwässerungsströme und ermöglichten über lange Zeiträume ihren episodischen Aufstieg zum Meeresboden, wo die aus den Sedimenten aufgenommenen Metalle als Sulfide in Spezialbecken abgesetzt wurden. Es bestehen prinzipielle Ähnlichkeiten mit vielen epigenetischen prä-, syn- oder postorogenen Mineralisationen; auch hier migrierten die salinaren Lösungen, die ihren Metallgehalt aus den (Meta)Sedimenten und eingelagerten Magmatiten bezogen, entlang von Störungssystemen bis zu den oberflächennahen Fallenstrukturen.

---

Résumé La compilation des données géologiques qui se sont largement multipliées au cours des dernières décennies, complétée par des rechercùhes complémentaires particulières, nous offre pour beaucoup de régions la possibilité de trouver des indices ou même des preuves d'un contrôle structural des minéralisations hydrothermales. Il va de soi que les relations entre la minéralisation hydrothermale, la formation et la réactivation des failles ainsi que les processus associés sont les plus faciles à étudier dans les couches sédimentaires les moins touchées par une orogenèse postérieure. Ainsi, il existe des preuves que les gisements stratiformes et sédimentaires de Zn-Pb-Ba du type Howards Pass, McArthur River et Meggen sont liés à des systèmes de failles synsédimentaires profondes, en relation avec des bassins intracontinentaux du genre rift et avec des marges continentales passives en extension. Ces systèmes de failles synsédimentaires ont drainé des eaux salines intersticielles et ont rendu possible leur mouvement ascendant vers les fonds marins où leur contenu métallifère s'est déposé dans des bassins locaux. Des nombreuses recherches récentes mettent en évidence la ressemblance avec les minéralisations épigénétiques formées avant, pendant et après l'orogenèse; là aussi, les eaux interstitielles, après avoir reçu leur contenu métallifère des (meta)sédiments et des magmatites associées ont migré le long de systèmes de failles jusqu'aux pièges structuraux proches de la surface.

---

, , , , 280 . , , . , - , , , .: Meggen, Rammerlsberg, Howards Pass, McArthur River ., , . , , , , , . -, - - ; , // , .