Trace element behaviour during anatexis in the presence of a fluid phase

If anatexis takes place in the presence of an immiscible volatile-rich fluid phase, the behaviour of a trace element depends not only on partition coefficients D^{i ?} between mineral i and silicate liquid, but also on coefficients D^{i f} between mineral i and fluid. The limited experimental data available, as well as theoretical arguments, suggest that for common minerals these coefficients differ, in some cases (REE) notably.Theory has been developed to permit calculation of concentration variations with the fraction F of rock melted, if D-values are known. In its present state this theory neglects the influence of T, P and composition variations in liquid and fluid on the D-values: if such variations can be expressed as functions of F, appropriate modifications can be made. The theory also neglects the effects of T and P on the solubility of fluid in liquid and the resulting modifications to phase petrology, by assuming simply that a given mass of rock melts in the presence of a mass proportion v of fluid. By choosing different values of v, from 0 (dry melting) to 100 (large fluid excess), the response of the trace element concentrations can be followed, beginning with concentration c_o in the unmelted rock. Such treatment is highly idealised, but serves to indicate some limits on what can occur.     

Decoupling between the middle and upper crust during transpression-related lateral flow: Variscan evolution of the Aston gneiss dome (Pyrenees, France)

We present a structural, AMS, microstructural and kinematic study of the Aston gneiss dome (French Pyrenees), which consists of a core made up of orthogneiss and paragneiss intruded by numerous sills of Carboniferous peraluminous granite. The orthogneiss corresponds to a former Ordovician granitic laccolith. Four Variscan events have been evidenced in this gneiss dome: (i) D1 deformation observed only as relics in the orthogneisses and their country-rocks located above the sillimanite isograd, and characterized by a NS to NE–SW non coaxial stretch associated to top to the south motions (NS convergence); (ii) D2-a deformation observed in the orthogneisses and their country-rocks, mainly migmatitic paragneisses, located below the sillimanite isograd and in the peraluminous granites whatever their structural level, and characterized by an EW to N120°E stretch associated to a top to the east flat shearing (lateral flow in the hot middle crust in a transpressive regime); (iii) D2-b deformation characterized by EW-trending megafolds corresponding to the domes in the middle crust and by EW-trending tight folds with subvertical axial planes in the metasedimentary upper crust; (iv) subvertical medium-temperature mylonitic bands developed by the end of the transpression.The Aston massif is a good example of decoupling between a cold upper crust and a hotter middle crust overheated by a thermal event originated in the upper mantle. This decoupling allowed the lateral flow of the migmatitic middle crust along a direction at high angle with respect to the more or less NS-trending direction of convergence. We suggest that the HT-LP metamorphism developed before the formation of the domes during D2-a, coevally with the emplacement of numerous sills of peraluminous granite, whereas the emplacement of the large calc-alkaline plutons in the upper crust occurred by the end of D2-b. Our data invalidate the previous geodynamical models based on either early or late extensional regime to explain the development of the HT-LP metamorphism. This new interpretation of the dynamics of the Variscan crust of the Pyrenees is consistent with recent studies conducted in Archaean and Palaeoproterozoic hot continental crusts having undergone oblique convergence, and characterized by a competition between vertical thickening and lateral flow induced by the important rheological contrast between two thermally different levels.     

Rare earth element geochemistry of the Scourian complex N.W. Scotland — Evidence for the granite-granulite link

Medium-to high-pressure granulite facies complexes represent samples of lower crustal material and are, therefore, important in the study of crustal processes. New rare earth element data for the Scourian granulite facies terrain of the Precambrian Lewisian complex of N.W. Scotland indicate that:

1. Overall, the Scourian complex has a light rare earth enriched pattern with a small but distinct positive Eu anomaly;
2. While some rare earth element trends for the complex as a whole (e.g., Σ REE vs SiO₂) are similar to those observed in upper crustal cogenetic sequences, others (e.g., Eu/Eu* vs SiO₂) are reversed;
3. Compared to average upper crust, the Scourian complex is depleted in REE (except Eu) by a factor of 2 to 3.

These new data, along with previously reported major and trace element data, isotopic abundances, and trace element modelling support the hypothesis that the Scourian terrain is the residuum left after genesis and removal of granitic melts.     

Thermal interaction of middle and upper crust during gneiss dome formation: example from the Montagne Noire (French Massif Central)

This paper aims to decipher the thermal evolution of the Montagne Noire Axial Zone (MNAZ, southern French Massif Central) gneiss core and its metasedimentary cover through determination of P–T paths and temperature gradients. Migmatitic gneiss from the core of the dome record a clockwise evolution culminating at 725 ± 25 °C and 0.8 ± 0.1 GPa with partial melting, followed by a decompression path with only minor cooling to 690 ± 25° C and 0.4 ± 0.1 GPa. Field structural analyses as well as detailed petrological observations indicate that the cover sequence experienced LP‐HT metamorphism. Apparent thermal gradients within the cover were determined with garnet–biotite thermometry and Raman Spectroscopy on Carbonaceous Matter. High‐temperature apparent gradients (e.g. ～530 °C km^?1 along one transect) are explained by late brittle–ductile extensional shearing evidenced by phyllonites that post‐date peak metamorphism. In areas where normal faults are less abundant and closely spaced, gradients of ～20 to 50 °C km^?1 are calculated. These gradients can be accounted for by a combination of dome emplacement and ductile shearing (collapse of isotherms), without additional heat input. Finally, the thermal evolution of the MNAZ is typical for many gneiss domes worldwide as well as with other LP‐HT terranes in the Variscides.     

Trace and rare-earth element behaviors during alteration and mineralization in the Attepe iron deposits (Feke-Adana,southern Turkey)

Hydrothermal-metasomatic iron ores consisting mainly of siderite, ankerite and hematite are located in the Lower–Middle Cambrian limestone marbles of the Eastern Taurus Belt. The siderite, ankerite, hematite and host rock samples from the deposits have been investigated for major, trace, and rare-earth elements (REE) to evaluate the element mobility and mass transfer during fluid–rock interactions.     

Garnet Lherzolites from the Kaapvaal Craton (South Africa): Trace Element Evidence for a Metasomatic History

Kimberlites from the Kaapvaal craton have sampled numerous mantlegarnet lherzolites in addition to garnet harzburgites. Traceelement characteristics of constituent clinopyroxenes allowtwo groups of garnet lherzolites to be distinguished. Traceelement compositions of all clinopyroxenes are characterizedby enrichment in light rare earth elements (LREE) and largeion lithophile elements and by a relative depletion in Ti, Nb,Ta, and to a lesser extent Zr and Hf. However, the LREE enrichmentand the depletion in Nb and Zr (Hf) are less in the Type 1 clinopyroxenesthan in the Type 2 clinopyroxenes. Our study suggests that thetwo melts responsible for the metasomatic imprints observedin the two garnet lherzolite groups are highly alkaline maficsilicate melts. Type 1 clinopyroxenes that have trace elementsimilarities to those of PIC (Phlogopite–Ilmenite–Clinopyroxene)rocks appear to have crystallized from, or been completely equilibratedwith, the same melt related to Group I kimberlite magma. TheType 2 clinopyroxenes have trace element similarities to thoseof MARID (Mica– Amphibole–Rutile–Ilmenite–Diopside)rocks and are therefore probably linked to melt related to GroupII kimberlite magma. KEY WORDS: garnet lherzolites; Kaapvaal craton; mantle xenoliths; mantle metasomatism; trace elements     

Chemistry of rainwater in the Massif Central (France): a strontium isotope and major element study

Atmospheric aerosols (sea salt, crustal dust, and biogenic aerosols) are the primary source of dissolved species in rainwater as well as one of the sources of dissolved species in river water. Chemical weathering studies require quantification of this atmospheric input. The crustal component of atmospheric input can have various origins, both distant and local. The proportions of the various inputs (marine, distant or local) are determined in this study.Strontium isotope ratios and Ca, Na, K, Mg, Al, Cl, SO₄, NO₃ and Sr concentrations were measured in rainwater samples collected in the Massif Central (France) over a period of one year. Each sample, collected automatically, represents a monthly series of rain events. Chemical composition of the rainwater samples varied considerably and the ⁸⁷Sr/⁸⁶Sr ratios ranged between 0.709198 and 0.713143.Using Na as an indicator of marine origin, and Al for the crustal input in rain samples, the proportion of marine and crustal elements was estimated from elemental ratios. A marine origin of 4 to 100% of Cl, of 0.6 to 20% of the SO₄, of <1 to 10% of Ca, <1 to 40% of K, 4 to 100% of Mg and 1 to 44% of Sr was determined.Strontium isotopes were used to characterize the crustal sources. The ⁸⁷Sr/⁸⁶Sr ratios of the crustal sources varied considerably from 0.7092 to 0.71625 and indicate the occurrence of multiple sources for the crustal component in the analysed rainwaters.     

The behaviour of arsenic in muddy sediments of the Bay of Biscay (France)

We have studied particulate and dissolved arsenic species in sediment and porewaters at sites in the Bay of Biscay, France, ranging in depths from 150 to 2,800 m. At all stations, major redox species (oxygen, nitrate, ammonia, total and reactive iron and manganese, sulphate and sulphur) reflect early diagenetic depth sequences of redox reactions comparable to other marine environments. Vertical distributions of dissolved and particulate As species and major redox species are related to changes in redox conditions and their major carrier phases, such as Fe and Mn-oxides. Arsenic diagenesis appears strongly dependent on Fe cycling. A subsurface maximum of dissolved As and surface enrichment of particulate As correspond to dissolution and precipitation of Fe (III) phases. Except for the shallowest and most bioturbated site, flux calculations show three different vertical diffusive As fluxes: two upwards and one downwards. Phase changes of recycled As result in local accumulations of reactive As at different redox fronts. Mass-balance calculations indicate that the upward As flux toward the oxidized layer can explain the enrichment of HCl extractable particulate As in this layer. A portion of the upward diffusing As can escape the sediment and may be fixed onto settling Fe-oxides by adsorption or co-precipitation and contribute to reactive particulate As input (i.e., As is recycled across the water sediment interface).     

Evidence for millennial-scale climatic events in the sedimentary infilling of a macrotidal estuarine system,the Seine estuary (NW France)

High-resolution sedimentological and rock magnetic analyses from sediment cores collected in the Seine estuary record changes in coastal sedimentary dynamics linked to climatic variations during the late Holocene. Using AMS ¹⁴C and paleomagnetic data we present a first attempt in developing a reliable age model on macrotidal estuarine archives, with a decadal resolution. Correlations between sedimentary successions from the outer Seine estuary document the main sedimentary infilling phases of the system during the last 3000 years. Between 3000 and 1150 cal. BP sedimentary patterns reveal that sequence deposition and preservation are predominantly controlled by marine and tidal hydrodynamics while severe storm events are recorded at ca. 2700 and 1250 cal. BP in the outermost estuary. Conversely, the Medieval Warm Period (MWP; 900–1200 AD) is characterized by a drastic waning of the influence of marine hydrodynamics on sediment preservation. Pronounced episodes of Seine river floods indicate a much stronger impact of continental inputs on sedimentary patterns during this period. The onset of the Little Ice Age marks a diminishing influence of continental inputs while tidal and open marine hydrodynamics again exerted a primary control on the sedimentary evolution of the system during 1200–2003 AD. Coastal sedimentary dynamics as preserved within sedimentary successions appear to have been largely influenced by changes in storminess during the last 3000 years. We have matched the preservation of MWP Seine river floods, as revealed by sedimentological and rock magnetic proxy data, to a prolonged interval of weakened storminess in Normandy permitting the preservation of estuarine flood deposits within a context of reduced coastal erosion in northern Europe. The preservation of sedimentary successions in the Seine estuary is therefore maximal when climate conditions resembled those of the preferred low phase of the NAO on multidecadal timescales such as during 800–1200 AD (MWP). In contrast, increased removal and transport of estuarine sediments occur when winter storm activity greatly intensified over northwestern France. We report four prominent centennial-scale periods of stronger storminess, occurring with a pacing of ～1500 years, which are likely to be related to the last four Bond's Holocene cold events. Our results documenting a close link between coastal sedimentary dynamics, millennial-scale variations in Holocene climate and North Atlantic atmospheric circulation are fairly consistent with other records from Scandinavia, central Greenland and southern Europe.     

Evidence for Modal Metasomatism in the Orogenic Spinel Lherzolite Body from Caussou (Northeastern Pyrenees, France)

Metasomatic mineral-bearing and/or trace element-enriched ultramaficassemblages have been reported from very few Alpine-type massifs.The small ultramafic body from Caussou (Ari?ge, northeasternPyrenees) compared with other north Pyrenean ultramafic complexesshows distinctive features which are similar to those of modallymetasomatized mantle xenoliths found in alkali basalts. It ismainly composed of clinopyroxene-rich spinel lherzolites (cpx/opxratios 1), with subordinate titanian pargasite-rich peridotites,both greatly depleted in orthopyroxene. Moreover the Caussouperidotites differ from other Ari?ge peridotites in the presenceof ilmenite, the abundance of sulfide inclusions in pyroxenesand amphiboles, higher Al, Ca, Na, K, Ti, and lower Mg contents,and enrichment in incompatible trace elements (ITE). Such mineralogicaland geochemical features are interpreted as resulting from modalmetasomatism produced by influxes of silicate melt into theperidotites. At Caussou, the metasomatic assemblage comprisesTi-pargasite+Ti-bearing clinopyr oxene+ilmenite+Ti-phlogopite+sulphide+fluid,suggesting that K, Ti, Na, ITE (including S, H₂O CO₂ and possiblyFe and Ca, were introduced by the metasomatizing agent. Thismetasomatism was probably imposed on an ultramafic associationdominated by LREE-depleted peridotites similar to the northPyrenean spinel lherzolites. These features indicate that, underupper lithospheric mantle conditions, a mafic melt locally infiltratedlherzolites by a grain-boundary percolation process and reactedwith the original mineral assemblage. The infiltration of alkali-basalticliquids into spinel peridotite led to: (1) partial dissolutionof orthopyroxene and, locally, spinel; (2) crystallization ofclinopyroxene directly from introduced melts; and (3) re-crystallization/equilibrationof pre-existing clinopyroxene with these magmatic liquids. Inthe last stage of the metasomatism, segregation of more fractionatedsilicate liquids, coexisting with a (CO₂+H₂O) fluid phase, mayhave been responsible for the crystallization of titanian pargasite,possibly by means of hydro-fracturing mechanism. The pervasive modal metasomatism at Caussou was contemporaneouswith the segregation of amphibole-bearing dykes in the Lherz-Freychin?debodies (northeast Pyrenees) (101–103 Ma). They representtwo manifestations of the same magmatic event in the lithosphericmantle, probably related the Middle Cretaceous alkaline magmatisrnof the Pyrenees.     

Trace elements behavior in granite genesis: A case study The calc-alkaline plutonic association from the Querigut complex (Pyrénées,France)

The different granitoids of the zoned Querigut complex (Hercynian Pyrenees) are associated with a series of basic to intermediate rocks ranging from hornblende-bearing peridotites to quartz-diorites. The whole complex appears as a calc-alkaline plutonic suite typical of orogenic zones. The distribution of lanthanides and other trace elements amongst coexisting minerals indicate they are essentially held by accessory phases, particularly in granitoids. This restricts the use of those elements in the calculation of petrogenetic models for acidic plutonic rocks. Magmatic differentiation, mainly by hornblende + plagioclase fractionation, can produce the basic series. This differentiation cannot directly produce the different granitoids, which require a preponderant contribution of crustal melts. The sequence of different granitoids can be explained either by an heterogeneity in the source region, or by magmatic differentiation. The most plausible interpretation of the whole complex calls for the emplacement of a mantle-derived magma into a wet, anatectic continental crust, with interactions between basic rocks and the soproduced acidic melts.     

Evidence for an enriched mantle source for jøtunite (orthopyroxene monzodiorite) associated with the St. Urbain anorthosite, Quebec

Mafic orthopyroxene monzodiorite (jøtunite) lithologies are exposed in the St. Urbain plutonic suite as a marginal facies to quartz mangerite and massif anorthosite intrusive bodies and as dikes within a variety of host rocks. High concentrations of Ti, Fe, P, K, Ba, Nb, La, Ce, Zn, Ga, Zr and Y characterize these rocks and are distinctive of many mafic lithologies associated with anorthosite massifs worldwide. Characteristically low concentrations of Ni and Cr, in conjuction with low Mg numbers, have been used by previous investigators as evidence for either partial melting of mafic granulitic lower crust or extensive fractional crystallization of a mantle-derived magma. In an attempt to distinguish between these competing models, we note that jøtunite display many features that bear a strong resemblance to continental tholeiitic flood basalts, including chemical signatures on normalized multi-element (‘spider’) diagrams. Ratios of incompatible trace elements and patterns on rare earth and ‘spider’ diagrams collectively indicate that the jøtunite rocks were derived from an enriched, rather than depleted, mantle source. Enrichment may have occured by subduction-derived fluids or by mixture with a plume component prior to partial melting so that isotopic and trace-element compositions are decoupled. Small amounts of partial melting of mafic granulite has been advanced as an alternative model; we show, however, that the experimental data on which this model is built are not applicable. Our preferred model begins with partial melting of a trace-element enriched mantle source that fractionates olivine at high to moderate pressures. Increasing concentrations of P (and Ti) eventually caused a contraction of the olivine stability field in favor of orthopyroxene. Fractional crystallization may yield the series of rocks from anorthosite, leuconorite, oxide-apatite gabbronorite, to jøtunite. Mafic magmas emplaced into continental crust are typically attributed to incipient rifting or mantle upwelling, which are features common to many models for the genesis of anorthosite and related rocks.     

The Hospitalet gneiss dome (Pyrenees) revisited: lateral flow during Variscan transpression in the middle crust

A new structural and kinematic study of the Hospitalet dome (Pyrenees) is presented. This dome corresponds to the eastern half of an EW‐trending antiformal structure made of an orthogneissic core intruded by granitoids, partly covered by Upper Proterozoic to Lower Ordovician metapelites. Its Variscan evolution can be split into four stages: (i) development of a strong high temperature pervasive deformation associated with subhorizontal foliations and lineations, and with non‐coaxial top‐to‐the‐east kinematics; (ii) formation of a south‐verging overturned megafold; (iii) emplacement of calc‐alkaline granitoids; and (iv) formation of mylonitic bands on the southern border of the dome, with reverse dextral kinematics. The flat lying pervasive high‐T deformation is interpreted as a large lateral flow developed in a dextral transpressive regime inducing an important uncoupling between the middle and upper crusts. The next stages happened in a progressive deformation in the same transpressive regime.     

Lead isotope behaviour in a polyphased Pb-Zn ore deposit: Les Malines (Cévennes,France)

Les Malines Mine (Cévennes, France) provides an example of Pb-Zn ore bodies that underwent a polyphased evolution. On the Pb-Pb diagram, experimental points of the Les Malines ore types always plot in the central part of the field defined for the Cévennes metallogenic province. This homogeneity, the similarities with the Pb isotopic compositions of the surrounding rocks and the comparison with Pb isotopes evolution of whole rocks and minerals of the neighbouring continental lands, which emerged during Triassic and Liassic times, rule out the participation of extraneous Pb during the successive concentration stages and agree with an initial metal stock transformed and mobilized in a Pb closed system. Hercynian K-feldspars are the main source of the Pb preconcentrated during Triassic times. Most of the Cévennes Pb-Zn ore bodies could derive from this Triassic metal stock; nevertheless, local Pb isotopic anomalies suggest hydrothermal sources for some Pb-Zn ore bodies located along the Cévennes fault.     

The timing of migmatization in the northern Arabian–Nubian Shield: Evidence for a juvenile sedimentary component in collision‐related batholiths

Collision‐related granitoid batholiths, like those of the Hercynian and Himalayan orogens, are mostly fed by magma derived from metasedimentary sources. However, in the late Neoproterozoic calcalkaline (CA) batholiths of the Arabian–Nubian Shield (ANS), which constitutes the northern half of the East African orogen, any sedimentary contribution is obscured by the juvenile character of the crust and the scarcity of migmatites. Here, we use paired in situ LASS‐ICP‐MS measurements of U–Th–Pb isotope ratios and REE contents of monazite and xenotime and SHRIMP‐RG analyses of separated zircon to demonstrate direct linkage between migmatites and granites in the northernmost ANS. Our results indicate a single prolonged period of monazite growth at 640–600 Ma, in metapelites, migmatites and peraluminous granites of three metamorphic suites: Abu‐Barqa (SW Jordan), Roded (S Israel) and Taba–Nuweiba (Sinai, Egypt). The distribution of monazite dates and age zoning in single monazite grains in migmatites suggest that peak thermal conditions, involving partial melting, prevailed for c. 10 Ma, from 620 to 610 Ma. REE abundances in monazite are well correlated with age, recording garnet growth and garnet breakdown in association with the prograde and retrograde stages of the melting reactions, respectively. Xenotime dates cluster at 600–580 Ma, recording retrogression to greenschist facies conditions as garnet continued to destabilize. Phase equilibrium modelling and mineral thermobarometry yield P–T conditions of ~650–680°C and 5–7 kbar, consistent with either water‐fluxed or muscovite‐breakdown melting. The expected melt production is 8–10 vol.%, allowing a melt connectivity network to form leading to melt segregation and extraction. U–Pb ages of zircon rims from leucosomes indicate crystallization of melt at 610 ± 10 Ma, coinciding with the emplacement of a vast volume of CA granites throughout the northern ANS, which were previously considered post‐collisional. Similar monazite ages (c. 620 Ma) retrieved from the amphibolite facies Elat schist indicate that migmatites are the result of widespread regional rather than local contact metamorphism, representing the climax of the East African orogenesis.     

Trace element evolution in the Phlegrean Fields (Central Italy): fractional crystallization and selective enrichment

Trace element analysis of Phlegrean Fields volcanic products shows that they belong to a homogeneous series whose evolution is due mainly to fractional crystallization. However quantitative modelling of crystal fractionation using measured mineral/groundmass and computed bulk distribution coefficients singles out K, Sb, Cl and F as elements which are selectively enriched in the differentiation process with respect to proven hygromagmaphile (HYG) elements. This anomalous enrichment involving elements that are easily transported in a magmatic fluid phase is thought to be due to percolation of such a fluid through a stored magma body. This enrichment is strongest in products resulting from protracted differentiation (trachybasalts-latites). Isotopic data from the literature are reinterpreted in terms of this model and show good agreement. Trace elements concentrations in clinopyroxenes provide evidence that complex differentiation paths were followed to produce latitic magmas. Their origin by mixing of magmas from different parentages is not indicated by the data. However, mixing of batches of the same parentage but of different degree of evolution seems likely. Because HYG elements ratios are not modified in the evolution of the magmas, they record these ratios in the partial melting zone. For example, the Th/Ta ratio in the source areas of the magmas has apparently been quite heterogeneous.     

Fluid-rock interaction in the Mo-bearing Nebelstein greisen complex,Bohemian Massif (Austria)

Summary In the Nebelstein area, molybdenite-bearing greisens occur together with peraluminous leucogranites. In the compositional change of the granites to the greisens, there is an almost complete loss of Na, combined with a decrease in Ca, Mg, Sr, and Ti concentrations. The progressive alteration is reflected by lower homogenization temperatures and increasing salinity in aqueous fluid inclusions. The fluid regime prior to greisenization was water-dominated with low salt contents, while the early stage of the greisen development was characterized by a mixed fluid containing carbon dioxide and water. This was succeeded by a moderate saline aqueous fluid which caused the mineralization by exchange of metal ions for Na⁺(Ca²⁺, K⁺). A negative correlation between salt content in fluid inclusions and Na₂O concentrations in the bulk rocks supports this model. Mass balance calculations for this interaction yield a minimum fluid-rock ratio of approximately 2 : I. Greisenization took place at a minimum pressure of 180 MPa (1.8 kb) and in a temperature range between 200 and < 400 °C.

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Fluid-Gestein-Wechselwirkung in dem Molybdänit führenden Greisenkomplex Nebelstein, Böhmische Masse (Österreich)

Zusammenfassung Die Molybdänglanz führenden Greisengesteine des Nebelsteins sind an peraluminöse Granite gebunden. Bei der Alteration der Granite ist für den Übergang Biotitgranit zu Greisen eine weitestgehende Verarmung an Na zu beobachten, gleichzeitig nehmen auch die Gehalte an Ca, Mg, Sr und Ti ab. Die fortschreitende Greisenbildung dokumentiert sich in den wäßrigen Flüssigkeitseinschlüssen durch steigende Salinität bei sinkenden Homogenisierungstemperaturen. Die fluide Phase war vor der Greisenbildung H₂O dominiert und niedrig salinar. Der Beginn der Alterationsprozesse ist durch CO₂ und H₂O hältige Fluide gekennzeichnet. Danach folgt ein Anstieg der Salinität, der auf den Austausch von Metallchloridlösungen gegen Na⁺, K^- und Ca^2- zurückgeführt wird. Dies läßt sich durch eine negative Korrelation der Salinität in den Flüssigkeitseinschlüssen mit dem Na-Gehalt der Gesteine belegen. Daraus wurde die Volumsbeziehung der den Granit durchströmenden fluiden Phase relativ zum Gestein mit mindestens 2 : 1 abgeleitet. Die Mineralisation fand bei einem Minimaldruck von 1,8 kb in einem Temperaturbereich von 200 - < 400 °C statt.

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This paper was presented at the IGCP 291 Project Symposium Metamorphic Fluids and Mineral Deposits, ETH Zürich, March21–23,1991.     

Trace element cycling in a subterranean estuary: Part 1. Geochemistry of the permeable sediments

Subterranean estuaries are characterized by the mixing of terrestrially derived groundwater and seawater in a coastal aquifer. Subterranean estuaries, like their river water-seawater counterparts on the surface of the earth, represent a major, but less visible, hydrological and geochemical interface between the continents and the ocean. This article is the first in a two-part series on the biogeochemistry of the subterranean estuary at the head of Waquoit Bay (Cape Cod, MA, USA). The pore-water distributions of salinity, Fe and Mn establish the salt and redox framework of this subterranean estuary. The biogeochemistry of Fe, Mn, P, Ba, U and Th will be addressed from the perspective of the sediment composition. A second article will focus on the groundwater and pore-water chemistries of Fe, Mn, U and Ba.Three sediment cores were collected from the head of Waquoit Bay where the coastal aquifer consists of permeable sandy sediment. A selective dissolution method was used to measure the concentrations of P, Ba, U and Th that are associated with “amorphous (hydr)oxides of iron and manganese” and “crystalline Fe and Mn (hydr)oxides.” The deeper sections of the cores are characterized by large amounts of iron (hydr)oxides that are precipitated onto organic C-poor quartz sand from high-salinity pore waters rich in dissolved ferrous iron. Unlike Fe (hydr)oxides, which increase with depth, the Mn (hydr)oxides display midcore maxima. This type of vertical stratification is consistent with redox-controlled diagenesis in which Mn (hydr)oxides are formed at shallower depths than iron (hydr)oxides. P and Th are enriched in the deep sections of the cores, consistent with their well-documented affinity for Fe (hydr)oxides. In contrast, the downcore distribution of Ba, especially in core 3, more closely tracks the concentration of Mn (hydr)oxides. Even though Mn (hydr)oxides are 200-300 times less abundant than Fe (hydr)oxides in the cores, Mn (hydr)oxides are known to have an affinity for Ba which is many orders of magnitude greater than iron (hydr)oxides. Hence, the downcore distribution of Ba in Fe (hydr)oxide rich sediments is most probably controlled by the presence of Mn (hydr)oxides. U is enriched in the upper zones of the cores, consistent with the formation of highly reducing near-surface sediments in the intertidal zone at the head of the Bay. Hence, the recirculation of seawater through this type of subterranean estuary, coupled with the abiotic and/or biotic reduction of soluble U(VI) to insoluble U(IV), leads to the sediments acting as a oceanic net sink of U. These results highlight the importance of permeable sediments as hosts to a wide range of biogeochemical reactions, which may be impacting geochemical budgets on scales ranging from coastal aquifers to the continental shelf.