Melt viscosity, temperature and transport processes, Troodos ophiolite, Cyprus

The lava section in the Troodos ophiolite, Cyprus, is chemically stratified and divided into a shallow lava sequence with low TiO₂ content and a deeper lava sequence with high TiO₂ content. We calculate the viscosity at magmatic temperature based on major element chemistry of lavas in Cyprus Crustal Study Project (CCSP) Holes CY-1 and 1A. We find that typical shallow low-Ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high-Ti lavas. This implies that, after eruption on-axis, Troodos low-Ti lavas would have been able to flow down the same slope faster and farther than high-Ti lavas. The calculated lava viscosity increases systematically from the lava-sediment interface to the bottom of the composite Hole CY-1/1A. This suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo Troodos rise may have been responsible for the chemical stratification in the Troodos lava pile. Calculated magmatic temperature and molar Mg/(Mg+Fe), or Mg#, decrease systematically down-section, while SiO₂ content increases. Correlation of Mg# in the lavas with Mg# in the underlying, lower crustal plutonic rocks sampled by CCSP Hole CY-4 shows that the shallow lavas came from a high-temperature, lower crustal magma reservoir which is now represented by high-Mg# pyroxenite cumulates, while the deeper lavas were erupted from a lower-temperature, mid-crustal reservoir which is now represented by gabbroic cumulates with lower Mg#.     

Chilling statistics indicate an ocean-floor spreading origin for the Troodos complex,Cyprus

Different models for the generation of ophiolite complexes lead to differing predictions of the nature, extent and consistency of one way chilling (see text) of dykes in the sheeted unit of such complexes. Measurements of the degree of one way chilling were made on a number of transects of the Diabase (sheeted) unit of the Troodos complex. Statistical analysis of the results strongly favours an ocean-floor spreading model over the other models considered for the generation of the complex, with the spreading axis lying to the west of the complex in its present orientation. In addition, the analysis shows that the stratigraphically central portion of the sheeted unit must be composed entirely of dykes. This method can be used to determine the origin of other ophiolite complexes that have sheeted dyke units.     

Oxygen and hydrogen isotope studies of plutonic granitic rocks

The primary δD values of the biotites and hornblendes in granitic batholiths are remarkably constant at about ?50 to ?85, identical to the values in regional metamorphic rocks, marine sediments and greenstones, and most weathering products in temperate climates. Therefore the primary water in these igneous rocks is probably not “juvenile”, but is ultimately derived by dehydration and/or partial melting of the lower crust or subducted lithosphere. Most granitic rocks have δ¹⁸O = +7.0 to +10.0, probably indicating significant involvement of high-¹⁸O metasedimentary or altered volcanic rocks in the melting process; such an origin is demanded for many other granodiorites and tonalites that have δ¹⁸O = +10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of all batholiths, locally producing very low δ¹⁸O values (particularly in feldspars) during subsolidus exchange. Some granitic plutons in such environments also were emplaced as low-¹⁸O magmas probably formed by melting or assimilation of hydrothermally altered roof rocks. However, the water/rock ratios were typically low enough that over wide areas the only evidence for meteoric water exchange in the batholiths is given by low D/H ratios (δD as low as ?180); for example, because of latitudinal isotopic variations in meteoric waters, as one moves north through the Cordilleran batholiths of western North America an increasingly higher proportion of the granitic rocks have δD values lower than ?120. The lowering of δD values commonly correlates with re-setting of K-Ar ages, and in the Idaho batholith two broad zones (10,000 km²) can be defined where δD biotite <?100 and K-Ar “ages” have all been re-set to values less than 60 m.y., suggesting that the Ar loss was caused by the meteoric-hydrothermal circulation systems. In certain Precambrian batholiths, a much different type of very low-temperature, regional alteration by surface-derived waters took place over an extended period long after emplacement, producing “brick-red” feldspars and markedly discordant Rb-Sr isochron “ages”.     

Metal-depleted root zones of the Troodos ore-forming hydrothermal systems, Cyprus

The cupriferous pyrite deposits of Cyprus were precipitated from hydrothermal solutions derived by interaction of contemporaneous seawater with hot mafic rock at the ancient Troodos spreading centre. Here we identify the zones in which this interaction took place. The zones occur in the lower part of the sheeted dyke complex, and within them 30–50% of the rock is made up of epidosite, an epidote-quartz rock, replacing the dykes as sheets and pipes. The epidosites contain abundant fluid inclusions, which give trapping temperatures of 350–400°C or even higher, and contain water normally near seawater in salinity. Zones of epidosite are elongate parallel to the strike of the sheeted dykes, and are up to 1 km wide. The rocks throughout these zones are strongly depleted in Cu and Zn, and the metals removed are sufficient to supply the ore deposits. In fact several large ore deposits lie along strike from zones of epidosite. All of these features support the identification of the epidosites as the hydrothermal reaction zones.The location of the epidosite zones immediately above the gabbros of the plutonic complex supports the hypothesis that the heat to drive the ore-forming systems came from the underlying magma, as is also likely for modern black smoker springs.     

Oxygen isotope evidence for submarine hydrothermal alteration of the Del Puerto ophiolite, California

The oxygen isotope compositions and metamorphic mineral assemblages of hydrothermally altered rocks from the Del Puerto ophiolite and overlying volcaniclastic sedimentary rocks at the base of the Great Valley sequence indicate that their alteration occurred in a submarine hydrothermal system. Whole rock δ¹⁸O compositions decrease progressively down section (with increasing metamorphic grade): +22.4‰ (SMOW) to +13.8 for zeolite-bearing volcaniclastic sedimentary rocks overlying the ophiolite; +19.6 to +11.6 for pumpellyite-bearing metavolcanic rocks in the upper part of the ophiolite's volcanic member; +12.3 to +8.1 for epidote-bearing metavolcanic rocks in the lower part of the volcanic member; +8.5 to +5.7 for greenschist facies rocks from the ophiolite's plutonic member; +7.6 to +5.8 for amphibolite facies or unmetamorphosed rocks from the plutonic member.

Modelling of fluid-rock interaction in the Del Puerto ophiolite indicates that the observed pattern of upward enrichment in whole rock δ¹⁸O can be best explained by isotopic exchange with discharging¹⁸O-shifted seawater at fluid/rock mass ratios near 2 and temperatures below 500°C.¹⁸O-depleted plutonic rocks necessarily produced during hydrothermal circulation were later removed as a result of tectonism. Submarine weathering and later burial metamorphism at the base of the Great Valley sequence cannot by itself have produced the zonation of hydrothermal minerals and the corresponding variations in oxygen isotope compositions. The pervasive zeolite and prehnite-pumpellyite facies mineral assemblages found in the Del Puerto ophiolite may reflect its origin near an island arc rather than deep ocean spreading center.     

Oxygen and carbon isotope evidence for seawater-hydrothermal alteration of the Macquarie Island ophiolite

Rocks of the Miocene Macquarie Island ophiolite, south of New Zealand, have oxygen and carbon isotopic compositions comparable to those of seafloor rocks. Basalt glass and weathered basalts have δ¹⁸O values at 5.8–6.0‰ and 7.9–9.5‰, respectively, similar to drilled seafloor rocks including samples from the Leg 29 DSDP holes near Macquarie Island. Compared to the basalt glass, the greenschist to amphibolite facies metaintrusives are depleted in¹⁸O (δ¹⁸O=3.2–5.9‰) similar to dredged seafloor samples, whereas the metabasalts are enriched (δ¹⁸O=7.1–9.7‰). Although the gabbros are only slightly altered in thin-section they have exchanged oxygen with a hydrothermal fluid to a depth of at least 4.5 km. There is an approximate balance between¹⁸O depletion and enrichment in the exposed ophiolite section. The carbon isotopic composition of calcite in the weathered basalts (δ¹³C=1.0–2.0‰) is similar to those of drilled basalts, but the metamorphosed rocks have low δ¹³C values (?14.6 to 0.9‰).These data are compatible with two seawater circulation regimes. In the upper regime, basalts were weathered by cold seawater in a circulation system with high water/rock ratios (?1.0). Based on calcite compositions weathering temperatures were less than 20°C and the carbon was derived from a predominantly inorganic marine source. As previously suggested for the Samail ophiolite, it is postulated that the lower hydrothermal regime consisted of two coupled parts. At the deeper levels, seawater circulating at low water/rock ratios (0.2–0.3) and high temperatures (300–600°C) gave rise to¹⁸O-depleted gabbro and sheeted dikes via open system exchange reactions. During reaction the seawater underwent a shift in oxygen isotopic composition (δ¹⁸O=1.0–5.0‰) and subsequently caused¹⁸O enrichment of the overlying metabasalts. In the shallower part of the hydrothermal regime the metabasalts were altered at relatively high water/rock ratios (1.0–10.0) and temperatures in the range 200–300°C. The relatively low water/rock ratios in the hydrothermal regime are supported by the low δ¹³C values of calcite, interpreted as evidence of juvenile carbon in contrast to the inorganic marine carbon found in the weathered basalts.     

New radiolarian ages from the Troodos ophiolite and their tectonic implications

Abstract Newly obtained radiolarian biostratigraphic age combined with previous isotopic age of the Troodos ophiolite shows that the ophiolite becomes systematically younger from east to west: Turonian, early Campanian, and late Campanian. The youngest late Campanian part of the ophiolite is directly covered by the volcaniclastic sediment derived from an active island arc, whereas the older part is covered by pelagic radiolarite. These facts constitute evidence that the Troodos ophiolite was probably emplaced during the subduction of an active spreading ridge.     

The origins of rare earths in metalliferous sediments of the Troodos Massif. Cyprus

The ferromanganoan umbers which overlie the uppermost pillow lavas of the Troodos Massif, Cyprus, are strongly enriched in REE relative to normal pelagic clays. Together, the umbers, related ferruginous sediments, and the ochres which are associated with massive cupriferous sulphides located within the lavas, all show light REE enrichment. There is a marked negative Ce anomaly which is characteristic of seawater. Field and chemical data suggest that the REE were incorporated into ferruginous precipitates derived from deep leaching of tholeiitic lavas by seawater. All the metalliferous sediments can be interpreted as related events in the evolution of the Troodos ocean ridge. In contrast, REE patterns of bentonitic clays above the umbers indicate a predominantly continental derivation.     

Origin of waters responsible for serpentinization of the Izu-Ogasawara-Mariana forearc seamounts in view of hydrogen and oxygen isotope ratios

Many serpentinite seamounts occur over a region 20–120 km west of the trench axis in the Izu-Ogasawara-Mariana forearc regions. The hydrogen and oxygen isotopic compositions of serpentine from these regions indicate that there are at least two kinds of waters responsible for serpentinization: seawater and water derived from dehydration of the descending slab. Serpentine from two Mariana and two Torishima samples with microscopically ductile and sheared texture (sheared-type) have lowerδD(−63to−52‰) and slightly higherδ18O values (+6.1 to +8.2‰) than that of other nine Ogasawara samples with mesh texture (mesh-type) (δD= −43to−49‰ andδ¹⁸O= +5.8to+6.7‰). This suggests that the sheared-type serpentine with lowerδD and slightly higherδ¹⁸O values was formed within the wedge mantle by interaction with water derived from a descending slab. The sheared texture is likely to have been produced during diapiric uplift. The unaltered portion of the ultramafic bodies later interacted with seawater after emplacement at or near the seafloor, resulting in formation of the mesh-type serpentine with higherδD values.     

Neodymium and strontium isotope study of ophiolite and orogenic lherzolite petrogenesis

Neodymium isotopic analyses have been measured on nine ophiolites and four orogenic lherzolites. ε_Nd varies from +12 to +3 in the ophiolites and from +18 to +2 in the orogenic lherzolites. The majority of the analyses plot on a ε_Nd-ε_Sr correlation line as defined by Nd and Sr isotopic analyses of oceanic basalts. However, certain ophiolitic and lherzolitic samples exhibit high⁸⁷Sr/⁸⁶Sr ratios and as such lie to the right of the correlation line towards seawater compositions.From these data one can postulate several origins for ophiolites including that of mid-ocean ridges and ocean islands. If the orogenic lherzolites are interpreted as representative of the mantle occurring below active ridges a more complex model is required involving mantle heterogeneity and multi-episodic chemical fractionation starting prior to 2 Ga ago.     

Experimental hydrogen isotope studies,II. Fractionations in the systems epidote-NaCl-H2O,epidote-CaCl2-H2O and epidote-seawater,and the hydrogen isotope composition of natural epidotes

The hydrogen isotope fractionation factors between epidote and aqueous 1 M and 4 M NaCl, 1 M CaCl₂ solutions, and between epidote and seawater, have been measured over the temperature range 250–550°C over which the degree of dissociation of dissolved species varies significantly. Measured fractionations at 350°C are decreased by up to 12‰, 9‰ and 7‰ relative to pure water in seawater, 1 M CaCl₂ and 1 M NaCl respectively, while above 500°C fractionations are not measurably dependent on fluid composition. Water—solution fractionation factors are derived which are generally applicable to the correction of mineral—water hydrogen isotope fractionations for the composition of the fluid phase.The hydrogen isotope compositions of natural epidotes are interpreted in the light of experimental fractionation data for situations where temperature, δD (fluid), and, in some cases, fluid chemistry, are independently known. Epidotes from active geothermal systems have hydrogen isotope quench temperatures consistent with or close to measured well temperatures unless the measured temperature has declined substantially since epidote formation or there is uncertainty in the D/H ratio of the water associated with the epidote because of isotopic heterogeneity in the well waters. Hydrothermal and metamorphic epidotes show closure temperatures of 175–225°C and 200–250°C. There is no evidence that retrograde metamorphic fluids, if present, are isotopically different from prograde fluids.The water-solution fractionations indicate strong solute-solvent interactions between 250 and 450°C and imply that both dissociated and associated species contribute to the fractionation effects through modification of the orientations and structure of the water molecules. Solute-solvent interactions become negligible at temperatures around 550°C.     

Oxygen and hydrogen in the primitive atmosphere

From time to time there appears in the literature the assertion that photolysis of water vapor could have maintained an appreciable concentration of oxygen in the primitive (prebiological) atmosphere. The implausibility of this assertion is argued in this paper.By itself, photolysis does not provide a source of oxygen because it is usually followed by recombination of the products of photolysis. Only the escape to space (at a much smaller rate) of the hydrogen produced by photolysis of water results in a net source of oxygen. The oxidation state of the primitive atmosphere depended on the relative magnitudes of this net source of oxygen and a volcanic source of hydrogen and other reduced gases. Today the volcanic source of reduced gases is approximately equal to the oxygen source provided by photolysis followed by escape. The oxygen source depends on the mixing ratio of water vapor in the stratosphere, which ultimately determines the rate of escape of hydrogen produced from water vapor. Its magnitude may not have been very different in the past. The volcanic source of hydrogen, on the other hand, is likely to have been much larger when the earth was tectonically young. Hydrogen was therefore released to the primitive atmosphere more rapidly than oxygen, probably. Photochemical reactions with the excess hydrogen maintained oxygen mixing ratios at negligibly small levels. The hydrogen mixing ratio was determined by a balance between the volcanic source (reduced by recombination with oxygen) and escape to space.In time, either because of decline of the volcanic source of hydrogen or because of addition of a biological source of oxygen, the input of oxygen to the atmosphere rose above the input of hydrogen. The oxidation state of the atmosphere changed rapidly. Volcanic hydrogen was now consumed by photochemical reactions with excess oxygen, while the oxygen mixing ratio was determined by a balance between the source (reduced by recombination with volcanic hydrogen) and consumption in reactions with reduced material at the surface.     

Oxygen and hydrogen isotope ratios of chert from the Sixtymile Formation in Grand Canyon National Park,USA:a warm palaeoclimate,freshwater deposit

New oxygen and hydrogen isotope ratios of chert from middle, intraformational breccias, and upper breccia members of the Sixtymile Formation(SMF) in eastern Grand Canyon National Park(AZ) yield palaeoclimate estimates between 27 and 33 °C. The isotopic compositions of cherts define a domain approximately parallel to the meteoric water line when plotted on a δD–δ~(18)O diagram; these data indicate that meteoric water was involved during formation of the chert. In thin section, the absence of interlocking mega quartz(35 lm) and silicafilled fractures and veins, along with preserved micromorphological silica fabrics, suggest that the chert has not been permeated by later hydrothermal fluids. Petrographic observations in thin section such as cyclic silica precipitation phases and glaebular micromorphologic fabrics lend support to the interpretation that meteoric waters were involved during chert precipitation. The post 742 Ma SMF has been correlated with diamictite(transition) beds of the Kingston Peak Formation(CA), which in turn have been interpreted to have been deposited during the Sturtian Ice Age(~750–700 Ma). Absence of facetted and striated clasts and other diagnostic glaciogenic features in the SMF,an unconformable contact with the stratigraphically older Chuar Group, coupled with warm palaeotemperature data inferred from stable isotope values of chert, tentatively suggest that deposition of sediment in the SMF likely did not take place during the Sturtian Ice Age.     

Oxygen and hydrogen isotope ratios in tree rings: how well do models predict observed values?

We have measured annual oxygen and hydrogen isotope ratios in the α-cellulose of the latewood of oak (Quercus robur L.) growing on well-drained ground in Norfolk, UK. We compare the observed values of isotope ratios with those calculated using equations that allow for isotopic fractionation during the transfer of oxygen and hydrogen from source water taken by the tree to cellulose laid down in the cambium. The equations constitute a model in which isotopic fractionation occurs during evaporative enrichment within the leaf and during isotopic change between carbohydrates and water in the trunk during cellulose synthesis. From the relationship between isotope ratios in precipitation and α-cellulose, we deduce that the source water used by the tree comprises a constant mixture of groundwater and precipitation, chiefly from the months of May, June and July of the growth year. By selection of isotopic fractionation factors and the degree of isotope exchange within the trunk, we are able to model the observed annual values of oxygen isotope ratios of α-cellulose to a significant level (r=0.77, P<0.01). When we apply the same model to hydrogen isotope ratios, however, we find that, although we can predict the average value over the time series, we can no longer predict the year-to-year variation. We suggest that this loss of environmental signal in the hydrogen isotopes is caused by differences in the kinetic isotope effects of the biochemical reactions involved in the fixation of hydrogen in different positions of the glucose molecule. Owing to these effects, the hydrogen isotope ratios of cellulose can vary in a way not anticipated in current models and hence may induce non-climatic ‘noise’ in the hydrogen isotope time series.     

Oxygen isotope geothermometry and origin of smectites in the Atlantis II Deep,Red Sea

The sediments underlying the hot brine pool of the Atlantis II Deep, a localised area of geothermal activity in the Red Sea, comprise a diversity of facies characterised by combinations of one or more of five species assemblages, sulphide, sulphate, silicate, oxide and carbonate, each including several mineral phases. The silicate mineral assemblage is dominated by geothermal authigenic smectites. Previous studies of these smectites have reported iron-rich varieties only, nontronite in particular, and only one environment of formation. In three cores from the Southwest Basin of the Atlantis II Deep, of the present study, three smectites comprising two species have been distinguished [10,21] evidently from three different environments of formation. Two of these smectites are nontronites, one from sulphide/silicate/amorphous facies, the other from silicate/carbonate/oxide facies. The third is a montmorillonite/beidellite from sulphate/sulphide/silicate/oxide facies.The oxygen isotopic compositions of samples of the three smectites have been determined from which formation temperatures have been calculated. Six samples of the “anoxic” nontronite have formation temperatures in the range 90–140°C. A single sample of the “oxic” nontronite has a formation temperature of about 80°C. Four samples of the montmorillonite/beidellite have formation temperatures in the range 160–200°C.The formation temperature range of the two nontronites is intermediate between the temperature of the brine at or prior to discharge (up to 250°C [12]) and the temperature of the brine pool in the Deep (about 50–60°C [13,14]). The nontronite formation temperature range reflects genesis by combination of isotopically light silica supplied by the incoming brine and isotopically heavier iron oxyhydroxide settling from the upper layers of the brine pool. Evidently, the “anoxic” nontronite forms at greater depth (hotter) in the brine pool than the “oxic” nontronite resulting in a relatively greater contribution from silica but diminished contribution from iron oxyhydroxide in the former compared to the latter. The wide range of the formation temperatures for the “anoxic” nontronite is related to the different actual locations of the samples in the sulphide/silicate/amorphous facies.The formation temperature range of the montmorillonite/beidellite is approaching the estimated temperature of the brine at or prior to discharge. The montmorillonite/beidellite formation temperature range reflects genesis by combination of isotopically light silica and aluminium, both supplied by the incoming brine, at the site of an active discharge vent. The wide range of the formation temperatures for the montmorillonite/beidellite may in part reflect a possible thermal event at the brine source, likely to have occurred during deposition of the sulphate/sulphide/silicate/oxide facies and which, it appears, has contributed to the formation of this facies [10,20].     

Oxygen and carbon isotope composition from the UHP Shuanghe marbles, Dabie Mountains, China

Investigations on the oxygen and carbon isotope compositions from the ultrahigh-pressure (UHP)-metamorphosed Shuanghe marbles, that occur as a member of a UHP slab, show that the δ¹⁸ O values range from +11.1% to+20.5% SMOW, and δ¹³ C from+1.0% to+5.7% PDB, respectively. The variations in isotope compositions show a centimeter scale of homogeneity and a heterogeneity of regional scale larger than 1 meter. In contrast to the eclogite marbles from Norway, the Shuanghe marbles have inherited the carbon isotope compositions from their sedimentary precursor. The δ¹³C shows positive correlation to the content of dolomite. The depletion in¹⁸O, compared with the pmtolithic carbonate strata, might result from three possible geological processes: 1) exchanging oxygen isotope with meteoric water before the UHP metamorphism, 2) decarbonation during the UHP metamorphism, and 3) exchanging oxygen isotope with country gneiss at local scale during retrograde metamorphism. It seems that the advection of fluid in the orogenic belt was very limited during subduction and exhumation of UHP rocks. Project supported by a U. S. -China cooperative project led by Prof. Cong Bolin of the Institute of Geology. Chinese Acade-my of Sciences, and Prof. J. G. Liou of the Department of Geological and Environmental Sciences, Stanford University and by the National Natural Science Foundation of china (Grant No. 49794042). Chinese Academy of Sciences (Grant No. KZ951-A1-401r, and National Science Foundation (Grant No. EAR-95-26700).     

Oxygen isotope variations in phosphate of biogenic apatites,III. Conodonts

The method for measurement of the isotopic composition of oxygen in phosphates has been improved and adapted for analysis of small quantities of apatite, down to 10 mg. This extension enables one to analyze hand-picked conodont samples with an analytical reproducibility better than ±0.5‰ (1σ).46 samples of conodonts ranging from the Ordovician to the Pennsylvanian of North America were analysed. Some insoluble phosphatic residues, ichthyoliths and inarticulate brachiopods of the same time range were also measured. The range of the δ¹⁸O values of the analysed conodonts is between 15 and 19‰. It shows a general trend of decreasing d¹⁸O with increasing age, from an average value of about 19 in the Pennsylvanian to 17 in the Ordovician. This trend parallels that previously detected in marine phosphorites. For the time range between the Devonian and Pennsylvanian our data are in agreement with independent paleoclimatic information. Specifically, we detect maximum¹⁸O enrichment at the end of the Pennsylvanian, and minimum enrichment at the end of the Devonian. The difference between these two extremes is equivalent to about 10°C (from about 40° to 30°C), assuming constant isotopic composition of ocean water.The success of oxygen isotopic analysis of conodonts raises the possibility of their use in Paleozoic paleo-oceanography in a similar way to foraminifera in the Mesozoic-Cenozoic.     

Oxygen isotope fractionation in dissolved oxygen in the deep sea

¹⁸O variations in dissolved oxygen have been measured at five stations from the eastern equatorial Pacific, at the GEOSECS-I and -II intercalibration stations in the North Pacific and North Atlantic, and along an Antarctic-South Pacific section from MONSOON expedition. Relative to atmospheric oxygen, dissolved oxygen in the ocean is enriched in¹⁸O up to a maximum of 14‰, the extreme enrichments occurring in the oxygen-minimum region of the eastern Pacific. The vertical diffusion-advection model has been used to determine the isotopic fractionation of deep-water in-situ oxygen consumption ascribed to bacterial metabolism. The single-stage enrichment, ε, in Pacific Deep Water below 1 km is 10‰ (α = 0.99,¹⁶O consumed preferentially). The model calculations show that the isotopic data cannot be fit without the introduction of a fractionation factor, just as the dissolved oxygen data cannot be fit without an in-situ consumption parameter. The consistency of the positive sign for ε and the negative source term for O₂, observed in all deep Pacific profiles analyzed to date, provide strong evidence that vertical transport and in-situ consumption terms dominate the horizontal tracer flux terms, and indicate the presence of a significant “deep metabolism” in abyssal ocean waters.     

Geochemical and lead isotope evidence for a mid-ocean ridge type mineralization within a polymetamorphic ophiolite complex (Monte del Forno, North Italy/Switzerland)

Major, trace element and Pb isotope investigations show the presence of a mid-ocean ridge-type mineralization within the polymetamorphic Monte del Forno Unit. Detailed analysis of the lithostratigraphy of the amphibolites demonstrates a close similarity to recent oceanic crust: a dyke zone at the bottom, a hydrothermally altered zone with a stockwork-type FeCuZn mineralization and a pillow zone at the top. Effects of hydrothermal seafloor alteration are restricted to an approximately 50 m thick horizon. Sulfide mineralization is accompanied by Ca and Sr depletion and Mn and minor Na and Mg enrichments. Mineralogically the horizon distinguishes itself from the unmineralized amphibolites by the presence of chlorite and contact metamorphic magnesio-cummingtonite. The chemical imprint of the hydrothermal seafloor alteration survived a regional greenschist and an upper amphibolite facies contact metamorphism.

The MORB signature of the Pb isotopes is preserved in the central parts of the approximately 300 m thick amphibolite sequence. During the regional greenschist facies metamorphism the isotope characteristics of the amphibolites were almost completely changed at the contact to the metasediments. The contact metamorphism of the Bregaglia Intrusion produced no obvious Pb contamination even within amphibolite xenoliths in the granodiorite.