Stable lead isotopes as tracers of groundwater pollution in the water supply for a small village

High lead (Pb) concentration has been measured in the incoming water to the water supply for a small Swedish village since the 1990s. There are several sources of the contamination and the objective of this study was to identify these by analysis of Pb isotopes. Lead has four stable isotopes in nature (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb) and the relative proportions of these vary according to their geological source. The study showed that two anthropogenic sources of Pb, a glassworks deposit and a highway, had similar Pb isotope ratios and thus it was not possible to separate them. However, the very high Pb concentration in the glassworks deposit suggested that this is the main source of the very high concentrations observed occasionally in low flow conditions. The soil in the recharge area of the most important well for the water supply had elevated Pb concentrations compared with background values in soils. Moreover, the Pb ratios in this soil differed from those in the anthropogenic sources. Several sites of mineralisation or natural enrichment have been identified in outcrops about 14?km northwest of the site and several anomalies in Pb exist in the glacial till. The conclusion was that Pb originating from the soil in the recharge area generally dominates and leads to Pb concentrations in water of 1?C2???g?L^?1. However, at higher concentrations, e.g. around 10???g?L^?1, water transported in cracks and fissure from the glassworks deposit becomes more important.     

Copper stable isotopes as tracers of metal-sulphide segregation and fractional crystallisation processes on iron meteorite parent bodies

We report high precision Cu isotope data coupled with Cu concentration measurements for metal, troilite and silicate fractions separated from magmatic and non-magmatic iron meteorites, analysed for Fe isotopes (δ⁵⁷Fe; permil deviation in ⁵⁷Fe/⁵⁴Fe relative to the pure iron standard IRMM-014) in an earlier study (Williams et al., 2006). The Cu isotope compositions (δ⁶⁵Cu; permil deviation in ⁶⁵Cu/⁶³Cu relative to the pure copper standard NIST 976) of both metals (δ⁶⁵Cu_M) and sulphides (δ⁶⁵Cu_FeS) span much wider ranges (−9.30 to 0.99‰ and −8.90 to 0.63‰, respectively) than reported previously. Metal-troilite fractionation factors (Δ⁶⁵Cu_M-FeS = δ⁶⁵Cu_M − δ⁶⁵Cu_FeS) are variable, ranging from −0.07 to 5.28‰, and cannot be explained by equilibrium stable isotope fractionation coupled with either mixing or reservoir effects, i.e. differences in the relative proportions of metal and sulphide in the meteorites. Strong negative correlations exist between troilite Cu and Fe (δ⁵⁷Fe_FeS) isotope compositions and between metal-troilite Cu and Fe (Δ⁵⁷Fe_M-FeS) isotope fractionation factors, for both magmatic and non-magmatic irons, which suggests that similar processes control isotopic variations in both systems. Clear linear arrays between δ⁶⁵Cu_FeS and δ⁵⁷Fe_FeS and calculated Cu metal-sulphide partition coefficients (D_Cu = [Cu]_metal/[Cu]_FeS) are also present. A strong negative correlation exists between Δ⁵⁷Fe_M-FeS and D_Cu; a more diffuse positive array is defined by Δ⁶⁵Cu_M-FeS and D_Cu. The value of D_Cu can be used to approximate the degree of Cu concentration equilibrium as experimental studies constrain the range of D_Cu between Fe metal and FeS at equilibrium to be in the range of 0.05-0.2; D_Cu values for the magmatic and non-magmatic irons studied here range from 0.34 to 1.11 and from 0.04 to 0.87, respectively. The irons with low D_Cu values (closer to Cu concentration equilibrium) display the largest Δ⁵⁷Fe_M-FeS and the lowest Δ⁶⁵Cu_M-FeS values, whereas the converse is observed in the irons with large values D_Cu that deviate most from Cu concentration equilibrium. The magnitudes of Cu and Fe isotope fractionation between metal and FeS in the most equilibrated samples are similar: 0.25 and 0.32‰/amu, respectively. As proposed in an earlier study (Williams et al., 2006) the range in Δ⁵⁷Fe_M-FeS values can be explained by incomplete Fe isotope equilibrium between metal and sulphide during cooling, where the most rapidly-cooled samples are furthest from isotopic equilibrium and display the smallest Δ⁵⁷Fe_M-FeS and largest D_Cu values. The range in Δ⁶⁵Cu_M-FeS, however, reflects the combined effects of partial isotopic equilibrium overprinting an initial kinetic signature produced by the diffusion of Cu from metal into exsolving sulphides and the faster diffusion of the lighter isotope. In this scenario, newly-exsolved sulphides initially have low Cu contents (i.e. high D_Cu) and extremely light δ⁶⁵Cu_FeS values; with progressive equilibrium and fractional crystallisation the Cu contents of the sulphides increase as their isotopic composition becomes less extreme and closer to the metal value. The correlation between Δ⁶⁵Cu_M-FeS and Δ⁵⁷Fe_M-FeS is therefore a product of the superimposed effects of kinetic fractionation of Cu and incomplete equilibrium between metal and sulphide for both isotope systems during cooling. The correlations between Δ⁶⁵Cu_M-FeS and Δ⁵⁷Fe_M-FeS are defined by both magmatic and non-magmatic irons record fractional crystallisation and cooling of metallic melts on their respective parent bodies as sulphur and chalcophile elements become excluded from crystallised solid iron and concentrated in the residual melt. Fractional crystallisation processes at shallow levels have been implicated in the two main classes of models for the origin of the non-magmatic iron meteorites; at (i) shallow levels in impact melt models and (ii) at much deeper levels in models where the non-magmatic irons represent metallic melts that crystallised within the interior of a disrupted and re-aggregated parent body. The presence of non-magmatic irons with a range of Fe and Cu isotope compositions, some of which record near-complete isotopic equilibrium implies crystallisation at a range of cooling rates and depths, which is most consistent with cooling within the interior of a meteorite parent body. Our data therefore lend support to models where the non-magmatic irons are metallic melts that crystallised in the interior of re-aggregated, partially differentiated parent bodies.     

Tungsten isotopes as tracers of core-mantle interactions: The influence of subducted sediments

The postulated difference in W isotopic composition of the Earth’s core of ∼2 ε_W units, compared to the bulk silicate earth (BSE) has previously been used to search for evidence of core-mantle interaction (CMI) in ocean island basalts (OIB). The absence of W isotope anomalies has thus been taken as evidence that CMI does not occur. However, the addition of subducted sediment with high W to the sources of OIB could obscure a core signature. This possibility brings into question the utility of W isotopes as tracers for CMI. To accurately consider the effects of sediment addition to mantle sources of OIB with respect to W requires improved constraints on the abundances of W in subducting sediment. Here, we present high-precision W abundance data (and other HFSE) for a suite of sediments from the Banda subduction regime in East Indonesia. Subducting East Indonesian sediments have trace element concentrations that resemble those of average upper continental crust (UCC), making these sediments valuable to consider as typical of subducted sediments. Average W abundances of 2.1 ppm, corrected for carbon content coupled with current models of 0.5% core addition and 1% sediment addition to EM1 or HIMU plume, suggest that a model hybrid source should exhibit values of ε_W = −0.24 with ∼25 ppb W. Prior studies have not reported such low W isotopic compositions or high estimated W concentrations present in the sources of either Hawaiian or French Polynesian lavas, so such large additions of core material to these plume sources seems unlikely. Given these constraints, core contributions to these source, if present, can be no more than ∼0.1%.     

Natural radionuclides as tracers of coastal biogeochemical processes

This paper presents an overview of the ways that natural radionuclides have been used as tracers of biogeochemical processes occurring in the coastal ocean. The radionuclides involved include those in the uranium and thorium decay series and those produced in the atmosphere by the interaction of cosmic rays with atmospheric gases (i.e., cosmogenic radionuclides). The property of radioactivity provides a chronometer with which to measure the rates of oceanic processes, and the fundamental biogeochemical behavior of the radionuclide determines which process(es) it may be used to trace. Examples from recent research will be presented in the talk.     

Geochemistry and water dynamics: II. Trace metals and Pb–Sr isotopes as tracers of water movements and erosion processes

A dynamic scheme for water movements over a flood in a small watershed was proposed in a previous paper [Ben Othman, D., Luck, J.M., Tournoud, M.G., 1997. Geochemistry and water dynamics: application to short-time scale flood phenomena in a small Mediterranean catchment: I. Alkalis, alkali-earths and Sr isotopes. Chem. Geol. [140] 9–28] based on major, alkali–alkali earth trace elements and Sr isotopes. In the present paper, metal contents and Pb isotope data are reported for the same samples, and compared to natural and anthropogenic local sources analysed for this purpose. Water treatment plants have stable but show relatively large variations, presumably related to yet unidentified (industrial?) Pb sources. Some Cu–Zn-rich chemicals used extensively on vineyards have unradiogenic around 17.7. Local rains sampled over two years show roughly similar low values. Pb from rocks is variable and more radiogenic ( ≈18.8–22.8). Except for the first hours, trace element concentrations in the dissolved load are similar to or slightly higher than those observed over the year, and similar to other moderately anthropogenic areas [Shiller, A.M., Boyle, E.A., 1987. Variability of dissolved trace metals in the Mississippi river. Geochim. Cosmochim. Acta [51] 3273–3277] for [Pb] (0.05–0.45 nM), [Zn] (10–75 nM) and [Cd] (0.03–0.18 nM). Dissolved [U] and [Co] show simple variations correlated to carbonate, related to local natural sources. Most trace elements in the particulate load are strongly correlated. Lead isotopes in the dissolved and particulate loads show ranges over the flood, again similar to those observed over the year ( ≈17.9–18.3), implying the same sources. The very good alignments observed in and vs. diagrams, especially for the particulate, are interpreted as mixing phenomena. Generally Pb isotopic signature of the dissolved load is less radiogenic than the particulate, indicating differences in sources or proportions and absence of isotopic equilibration with respect to the time of transfer. Pb isotopes shift regularly with time away from the road runoff endmember towards more radiogenic values. The strong negative correlation between Pb isotopes in the particulate load and Sr isotopes in the dissolved load, observed for the first time in a small watershed, probably reflects the local coupling between mechanical and chemical erosion, respectively.     

Strontium isotopes as tracers of airborne fly ash from coal-fired power plants

Fly ash generated by coal-fired power plants is in part collected by filters in the emission stacks while a small portion is vented into the atmosphere. Since many of the coalfired power plants in the western United States are located in the desnrt, the ability to monitor fly ash emissions requires a chemical tracer that utilizes desert soil and plant interactions with the fly ash deposited in the desert environment. This investigation presents the results of a controlled greenhouse experiment in which a native desert plant, the brittlebush (Encelia farinosa), was grown on admixtures of desert soils and fly ash. The fly ash is strongly enriched in Sr and the brittlebush is a Sr accumulator. The data demonstrate that (1) the brittlebush isotopically equilibrates with desert soils whose fly ash components are as low as 0.25% by weight, (2) the fly ash Sr is apparently more available to the plant, than Sr derived from the soils, and (3) the difference between the⁸⁷Sr/⁸⁶Sr ratio of the fly ash (0.70807) and soils (0.71097 to 0.71117) warrants further investigations in the natural environment to determine the practicality of this method as a natural tracer of fly ash in the environment.     

Evaluation of osmium isotopes and iridium as paleoflux tracers in pelagic carbonates

New osmium (Os) isotope and platinum group element (PGE) concentration data are used in conjunction with published ³He and Th isotope data to determine the relative proportions of lithogenic, extraterrestrial and hydrogenous iridium (Ir) in a Pacific pelagic carbonate sequence from the Ocean Drilling Program (ODP) Site 806 on the Ontong Java Plateau (OJP). These calculations demonstrate that lithogenic and extraterrestrial contributions to sedimentary Ir budget are minor, while hydrogenous Ir accounts for roughly 85% of the total Ir. Application of analogous partitioning calculations to previously reported data from a North Pacific red clay sequence (LL44-GPC3) yields very similar results. Total Ir burial fluxes at Site 806 and LL44-GPC3 are also similar, 45 and 30 pg cm⁻² kyr⁻¹, respectively. Average Ir/³He and Ir/xs²³⁰Th_initial ratios calculated from the entire Site 806 data set are similar to those reported earlier for Pacific sites. In general, down-core profiles of Ir, ³He and xs²³⁰Th_initial, are not well correlated with one another. However, all three data sets show similar variance and yield sediment mass accumulation rate estimates that agree within a factor of two. While these results indicate that Ir concentration has potential as a point-paleoflux tracer in pelagic carbonates, Ir-based paleoflux estimates are likely subject to uncertainties that are similar to those associated with Co-based paleoflux estimates. Consequently, local calibration of Ir flux in space and time will be required to fully assess the potential of Ir as a point paleoflux tracer. Measured ¹⁸⁷Os/¹⁸⁸Os of the OJP sediments are systematically lower than the inferred ¹⁸⁷Os/¹⁸⁸Os of contemporaneous seawater and a clear glacial-interglacial ¹⁸⁷Os/¹⁸⁸Os variation is lacking. Mixing calculations suggest Os contributions from lithogenic sources are insufficient to explain the observed ¹⁸⁷Os/¹⁸⁸Os variations. The difference between the ¹⁸⁷Os/¹⁸⁸Os of bulk sediment and that of seawater is interpreted in terms of subtle contributions of unradiogenic Os carried by particulate extraterrestrial material. Down-core variations of ¹⁸⁷Os/¹⁸⁸Os with Pt/Ir and Os/Ir also point to contributions from extraterrestrial particles. Mixing calculations for each set of several triplicate analyses suggest that the unradiogenic Os end member cannot be characterized by primary extraterrestrial particles of chondritic composition. It is noteworthy that in efforts aimed at determining the effect of extraterrestrial contributions, ¹⁸⁷Os/¹⁸⁸Os of pelagic carbonates has greater potential compared to abundances of PGE. An attempt has been made for the first time to estimate sediment mass accumulation rates based on amount of extraterrestrial Os in the OJP samples and previously reported extraterrestrial Os flux. Throughout most of the OJP record, Os isotope-based paleoflux estimates are within a factor of two of those derived using other constant flux tracers. Meaningful flux estimates cannot be made during glacial maxima because the OJP sediments do not record the low ¹⁸⁷Os/¹⁸⁸Os reported previously. We speculate that this discrepancy may be related to focusing of extraterrestrial particles at the OJP, as has been suggested to explain down-core ³He variations.     

Lead isotopes as tracers of anthropogenic pollution in urban topsoils of Mexico City

A survey was performed to trace the main source of anthropogenic Pb pollution in Mexico City through Pb isotopic signatures (²⁰⁸Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁷Pb, and ²⁰⁸Pb/²⁰⁷Pb) from 103 urban topsoil (0–5 cm) samples. Those were collected in the metropolitan area of Mexico City and compared with isotopic compositions of leaded gasoline (LG), domestic Pb ores (DLO) and parent rock (PR). The isotope ratios (IRs) of Pb were determined by inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) and total Pb concentration analyzed by wavelength dispersive X-ray fluorescence (WDXRF). The range of Pb concentrations levels in urban topsoil samples was 15–473 mg/kg. The IR values obtained for these samples were 37.965–39.718 (²⁰⁸Pb/²⁰⁴Pb), 18.375–19.204 (²⁰⁶Pb/²⁰⁴Pb), 1.177–1.218 (²⁰⁶Pb/²⁰⁷Pb) and 2.443–2.496 (²⁰⁸Pb/²⁰⁷Pb). Analyzed topsoil samples with low Pb content (<50 mg/kg) displayed high dispersion in ²⁰⁸Pb/²⁰⁴Pb values, which are determined by different natural sources. Samples with 51–200 mg/kg Pb content, shown low dispersion that revealed the mixing between the natural Pb and anthropogenic Pb. The assessment of the IR values shown that, as Pb concentration increases, a trend toward gasoline IR data has been observed. The results obtained by this research suggest that although the use of leaded petrol had been banned in Mexico since 1997, the Pb pollution in the urban topsoils due to the historical use of Pb in petrol is still significant.     

Stable isotopes in mineralogy

Stable isotope fractionations between minerals are functions of the fundamental vibrational frequencies of the minerals and therefore bear on several topics of mineralogical interest. Isotopic compositions of the elements H, C, O, Si, and S can now be determined routinely in almost any mineral. A summary has been made of both published and new results of laboratory investigations, analyses of natural materials, and theoretical considerations which bear on the importance of temperature, pressure, chemical composition and crystal structure to the isotopic properties of minerals. It is shown that stable isotope studies can sometimes provide evidence for elucidating details of crystal structure and can be a powerful tool for use in tracing the reaction paths of mineralogical reactions.     

Archaebacterial ether lipids: Natural tracers of biogeochemical processes

Methanogen ether lipids have been quantified in sediments from a Florida swamp and the Atlantic ocean. Swamp cores containing acyclic and monocyclic isopranyl ethers are clearly differentiated from deep sea sediments which also contain bicyclic compounds. A concentration maximum near the bottom of the sulfate reducing zone in deep sea sediments may reflect a biogeochemical system in which methanogenesis and sulfate reduction are coupled by the process of methane oxidation. Lipid diagenesis is evident in the deep sea sediments. Species zonation, possibly caused by oxygen sensitivity, is detected in the relative lipid abundances in swamp sediments.     

Oxygen and nitrogen isotopes as tracers of fluid activities in serpentinites and metasediments during subduction

Summary N and O isotope systematics of a suite of high-pressure (HP) and ultrahigh-pressure (UHP) metasediments of the Schistes Lustrés nappe and metaperidotites of the Erro Tobbio Massif from the Alpine-Appennine system are compared with their unmetamorphosed or hydrothermally-altered equivalent from the same localities and from the South West Indian Ridge (SWIR). The HP and UHP rocks studied represent a sequence of pelagic sediments and altered ultramafic rocks subducted to different depths of down to 90 km along a cold geothermal gradient (8 °C/km). Unmetamorphosed and HP metasediments show the same range in δ¹⁵N values irrespective of their metamorphic grade and bulk nitrogen concentrations. Together with several other geochemical features (K, Rb and Cs contents, δD), this indicates that δ¹⁵N values were unaffected by metamorphism and N was not released during subduction. N isotope analysis of serpentinites coupled with δ¹⁸O systematics suggests the involvement of a mafic (crustal) component during partial deserpentinization of the subducted oceanic mantle at the depth locus of island arc magmatism. This does not imply large-scale fluxes as the metagabbros are spatially associated with the analyzed serpentinites. It rather indicates preservation of presubduction chemical and isotopic heterogeneities on a local scale as documented for the metasediments.     

Copper isotopes as monitors of redox processes in hydrothermal mineralization

The stable copper isotope composition of 79 samples of primary and secondary copper minerals from hydrothermal veins in the Schwarzwald mining district, South Germany, shows a wide variation in δ⁶⁵Cu ranging from −2.92 to 2.41‰. We investigated primary chalcopyrite, various kinds of fahlores and emplectite, as well as supergene native copper, malachite, azurite, cuprite, tenorite, olivenite, pseudomalachite and chrysocolla. Fresh primary Cu(I) ores have at most localities copper isotope ratios (δ⁶⁵Cu values) of 0 ± 0.5‰ despite the fact that the samples come from mineralogically different types of deposits covering an area of about 100 by 50 km and that they formed during three different mineralization events spanning the last 300 Ma. Relics of the primary ores in oxidized samples (i.e., chalcopyrite relics in an iron oxide matrix with an outer malachite coating) display low isotope ratios down to −2.92‰. Secondary Cu(I) minerals such as cuprite have high δ⁶⁵Cu values between 0.4 and 1.65‰, whereas secondary Cu(II) minerals such as malachite show a range of values between −1.55 and 2.41‰, but typically have values above +0.5‰. Within single samples, supergene oxidation of fresh chalcopyrite with a δ value of 0‰ causes significant fractionation on the scale of a centimetre between malachite (up to 1.49‰) and relict chalcopyrite (down to −2.92‰). The results show that—with only two notable exceptions—high-temperature hydrothermal processes did not lead to significant and correlatable variations in copper isotope ratios within a large mining district mineralized over a long period of time. Conversely, low-temperature redox processes seriously affect the copper isotope compositions of hydrothermal copper ores. While details of the redox processes are not yet understood, we interpret the range in compositions found in both primary Cu(I) and secondary Cu(II) minerals as a result of two competing controls on the isotope fractionation process: within-fluid control, i.e., the fractionation during the redox process among dissolved species, and fluid-solid control, i.e., fractionation during precipitation involving reactions between dissolved Cu species and minerals. Additionally, Rayleigh fractionation in a closed system may be responsible for some of the spread in isotope compositions. Our study indicates that copper isotope variations may be used to decipher details of natural redox processes and therefore may have some bearing on exploration, evaluation and exploitation of copper deposits. On the other hand, copper isotope analyses of single archeological artefacts or geological or biological objects cannot be easily used as reliable fingerprint for the source of copper, because the variation caused by redox processes within a single deposit is usually much larger than the inter-deposit variation.     

Hydrodynamic modelling of some metamorphic processes

The P–T paths for metamorphic complexes from the Precambrian shields and fold belts of different ages may result from advection, i.e. one-cycle convective processes in the lithosphere. This conclusion has been exemplified by the metamorphic evolution of several well-known complexes, for which an advective model can be successfully applied. Numerical simulations of the above processes in terms of Newtonian rheology by using a two-dimensional finite element program have been conducted.
Two representative models for intracontinental gravitational ordering initiated presumably by mantle activity are considered: (i) a thermally activated multi-layered rhythmic sequence and (ii) huge rising diapiars causing circulation, in which crustal lithologies underwent high- P metamorphism (above 10–15 kbar) and subsequent ascent toward the Earth's surface.     

Stable chlorine isotopes in Phanerozoic evaporites

Modern seawater has a uniform δ³⁷Cl value (0.0‰), with an exception in the upper current of the Bosphorus (0.4‰). Marine halite ranging in age from Cambrian to Miocene has δ³⁷Cl values of 0.0 ± 0.9‰, with most of the data in the range 0.0 ± 0.5‰. Mean δ³⁷Cl values differ measurably between basins, with no evident relationship to basin size or to age. Smaller evaporite bodies have the largest δ³⁷Cl ranges. Potash facies halite has mean δ³⁷Cl values lower than those of halite facies salt in the East Siberia and Zechstein basins. The bulk δ³⁷Cl of bedded halite preserving sedimentary textures cannot be shifted measurably after deposition under plausible natural conditions. During the Phanerozoic, a detectable change in the δ³⁷Cl values of the oceans is unlikely as a result of Cl fluxes to and from the mantle and evaporites. In halite, the values of δ³⁷Cl that cannot be explained by fractionation occurring on crystallization are best explained by the addition of non-marine Cl with δ³⁷Cl ≠ 0.0‰ to evaporite brine.     

B and Li isotopes as intrinsic tracers for injection tests in aquifer storage and recovery systems

Boron and Li isotopes have been tested as environmental tracers of treated sewage injected into a sandy aquifer (Shafdan reclamation project, Israel). During a 38 days injection test in a newly dug injection well, a conservative artificial tracer (Br⁻) was monitored together with δ¹¹B and δ⁷Li in the injectate, in the unsaturated soil zone (porous cup) and an observation well in the aquifer. In spite of B and Li concentrations in the injectate close to background values, significant shifts of the isotope signatures could be observed over the duration of the injection test. Boron isotope ratios show a breakthrough curve delayed with respect to Br⁻ breakthrough due to some reversible sorption on the aquifer material. No isotope fractionation was observed in the unsaturated or the saturated zone so that B isotopes can be considered as conservative in the investigated part of the aquifer system. Lithium isotopes are strongly fractionated, probably due to sorption processes. Lithium concentrations point to a Li sink in the system, δ⁷Li values vary strongly with a tendency of ⁷Li depletion in the liquid phase over the duration of the experiment. This is opposite to the expected preferential sorption of ⁶Li onto clay minerals. Boron isotopes reveals a valuable tracer of artificial recharge of freshwaters derived from treated sewage, both for short term tracer tests and for long-term monitoring of artificial recharge, even if in aquifers with higher clay contents, sorption-linked isotope fractionation cannot be excluded. More data are needed on Li isotope fractionation in natural groundwater systems to assess the potential of this tracer as monitoring tool.     

Permeability generation and resetting of tracers during metamorphic fluid flow: implications for advection-dispersion models

Advection-dispersion fluid flow models implicitly assume that the infiltrating fluid flows through an already fluid-saturated medium. However, whether rocks contain a fluid depends on their reaction history, and whether any initial fluid escapes. The behaviour of different rocks may be illustrated using hypothetical marble compositions. Marbles with diverse chemistries (e.g. calcite + dolomite + quartz) are relatively reactive, and will generally produce a fluid during heating. By contrast, marbles with more restricted chemistries (e.g. calcite + quartz or calcite-only) may not. If the rock is not fluid bearing when fluid infiltration commences, mineralogical reactions may produce a reaction-enhanced permeability in calcite + dolomite + quartz or calcite + quartz, but not in calcite-only marbles. The permeability production controls the pattern of mineralogical, isotopic, and geochemical resetting during fluid flow. Tracers retarded behind the mineralogical fronts will probably be reset as predicted by the advection-dispersion models; however, tracers that are expected to be reset ahead of the mineralogical fronts cannot progress beyond the permeability generating reaction. In the case of very unreactive lithologies (e.g. pure calcite marbles, cherts, and quartzites), the first reaction to affect the rocks may be a metasomatic one ahead of which there is little pervasive resetting of any tracer. Centimetre-scale layering may lead to the formation of self-perpetuating fluid channels in rocks that are not fluid saturated due to the juxtaposition of reactants. Such layered rocks may show patterns of mineralogical resetting that are not predicted by advection-dispersion models. Patterns of mineralogical and isotopic resetting in marbles from a number of terrains, for example: Chillagoe, Marulan South, Reynolds Range (Australia); Adirondack Mountains, Old Woman Mountains, Notch Peak (USA); and Stephen Cross Quarry (Canada) vary as predicted by these models. Received: 3 February 1997 / Accepted: 26 June 1997     

Stable isotopes in the ostracod shell: a preliminary study

Oxygen and carbon isotope ratios of Recent ostracods from six localities are presented. The δO¹⁸ data are consistent with precipitation of the shells in isotopic equilibrium with seawater. although additional data are necessary to confirm equilibrium precipitation. No strong correlation between carbon isotope ratios and temperature or salinity were observed for ostracods.     

Theory of metamorphic segregation and related processes

Metamorphic segregation is defined here as the formation and growth of bands or domains of different bulk compositions within an originally unbanded rock. It can result from an instability arising in some deforming rocks when diffusion transfer is significant. The nature of this instability is demonstrated separately for a differentiation associated with a crenulation (Type C) and for one without crenulation (Type L). However, the continuous gradation between the two types observed in nature is also expected from the models.A requirement for both types is that one chemical component of the rock, typically silica, diffuse more rapidly than others, e.g. phyllosilicate components, in response to stress-induced pressure gradients. In addition, Type L requires that, at least at an early stage, portions of the rock rich in this mobile component, e.g. rich in quartz, be more competent than those portions which are poor in quartz. By a process akin to the development of pressure shadows, silica diffuses toward domains which are already the richest in quartz. Alternatively, if the rock is chemically open to loss of silica, this silica preferentially originates from dissolution of quartz in the more mica-rich domains. In either case the compositional contrast between domains is increased and metamorphic segregation results. Type C differentiation is best explained if we accept the suggestion made by many petrographers that layer silicates catalyse the pressure-induced transfer of silica. This suggestion can in fact account for other features of metamorphic segregation bands.Metamorphic segregation should also proceed during the development of quartz segregations, quartz rods, slaty cleavage and mylonite banding. The stress distribution argued for Type L bands may also occur in many banded migmatites. In migmatites. however, the assemblage crystallizing in the leucosome may be derived from a circulating hydrothermal fluid as well as from diffusion transfer.     

Characteristics of metamorphic processes in Early Precambrian

Relatively high geothermal gradient and greater depths of the erosion plane may account for certain distinctions of the Lower Archean metamorphic complexes. Their granulites and amphibolites could not have been formed at depths shallower than 15-16 and 10 km, respectively, as indicated by review of the available data. The granulite fades of metamorphism is not restricted to Precambrian rocks, but is found also in younger complexes, some of which outcrop on the surface of the ground, although they were subjected to intensive autodiaphthoresis, which usually accompanied crystallization of anatectic melts within the amphibolitic range of the temperature.     

Lithium, boron and chlorine as tracers for metasomatism in high-pressure metamorphic rocks: a case study from Syros (Greece)

High-pressure metamorphic (HPM) rocks (derived from igneous protoliths) and their metasomatised rinds from the island of Syros (Greece) were analysed for their B and Cl whole-rock abundances and their H₂O content by prompt-gamma neutron-activation analysis (PGNAA) and for their Li and Be whole-rock abundances by ICP-OES. In the HPM rocks, B?/Be and Cl?/Be ratios correlate with H₂O contents and appear to be controlled by extraction of B and Cl during dehydration and prograde metamorphism. In contrast, samples of the metasomatised rinds show no such correlation. B?/Be ratios in the rinds are solely governed by the presence or absence of tourmaline, and Cl?/Be ratios vary significantly, possibly related to fluid inclusions. Li/Be ratios do not correlate with H₂O contents in the HPM rocks, which may in part be explained by a conservative behaviour of Li during dehydration. However, Li abundances exceed the vast majority of published values for Li abundances in fresh, altered, or differentiated oceanic igneous rocks and presumably result from metasomatic enrichment of Li. High Li concentrations and highly elevated Li/Be ratios in most metasomatised samples demonstrate an enrichment of Li in the Syros HP mélange during fluid infiltration. This study suggests that B and Cl abundances of HPM meta-igneous rocks can be used to trace prograde dehydration, while Li concentrations seem to be more sensitive for retrograde metasomatic processes in such lithologies.