Strontium Isotopic Constraints Research Articles

Rare earth elements and neodymium and strontium isotopic constraints on provenance switch and post-depositional alteration of fossiliferous Ediacaran and lowermost Cambrian strata from Arctic Norway

The Digermulen Peninsula in northeastern Finnmark, Arctic Norway, comprises one of the most complete Ediacaran–Cambrian transitions worldwide with a nearly continuous record of micro- and macrofossils from the interval of the diversification of complex life. Here, we report on the provenance and post-depositional alteration of argillaceous mudstones from the Digermulen Peninsula using rare earth elements and Sm–Nd and Rb–Sr isotopic systematics to provide an environmental context and better understand this important transition in Earth’s history. The studied sections comprise a mid-Ediacaran glacial–interglacial cycle, including the Nyborg Formation (ca. 590 Ma) and Mortensnes Formation (related to the ca. 580 Ma-old Gaskiers glaciation), and the Stáhpogieddi Formation (ca. 560–537 Ma), which yields Ediacara-type fossils in the Indreelva Member and contains the Ediacaran–Cambrian boundary interval in the Manndrapselva Member and basal part of the informal Lower Breidvika member (ca. 537–530 Ma). Three sample groups, (1) Nyborg and Mortensnes formations, (2) the lowermost five samples from the Indreelva Member and (3) the remaining samples from the Indreelva as well as from the Manndrapselva and Lower Breidvika members, can be distinguished, belonging to distinct depositional units. All samples have negative εNd(T) values (−6.00 to − 21.04) indicating a dominant input of terrigenous detritus with an old continental crust affinity. Significant shifts in Sm–Nd isotope values are related to changes in the sediment source, i.e. Svecofennian province vs Karelian province vs Svecofennian province plus in addition likely some juvenile (late Neoproterozoic volcanic) material, and probably reflect palaeotectonic reorganisation along the Iapetus-facing margin of Baltica. The combined Rb–Sr isotopic data of all samples yield an errorchron age of about 430 Ma reflecting the resetting of the Rb–Sr whole-rock isotope systems of the mudstones during the Scandian tectono-metamorphic event in the Gaissa Nappe Complex of Finnmark. Preservation of palaeopascichnids coincides with the sedimentation regimes of sample groups 2 and 3 while other Ediacara-type fossils, e.g. Aspidella-type and frondose forms, are limited to the sample group 3. Our results are similar to those of earlier studies from the East European Platform in suggesting oxic seafloor conditions during the late Ediacaran.

Magmatic Fluids Implicated in the Formation of Propylitic Alteration: Oxygen, Hydrogen, and Strontium Isotope Constraints from the Northparkes Porphyry Cu-Au District, New South Wales, Australia

AbstractIn porphyry ore deposit models, the propylitic alteration facies is widely interpreted to be caused by convective circulation of meteoric waters. However, recent field-based and geochemical data suggest that magmatic-derived fluids are likely to contribute to development of the propylitic assemblage. In order to test this hypothesis, we determined the oxygen and hydrogen isotope compositions of propylitic mineral separates (epidote, chlorite, and quartz), selected potassic mineral separates (quartz and magnetite), and quartz-hosted fluid inclusions from around the E48 and E26 deposits in the Northparkes porphyry Cu-Au district, New South Wales, Australia. In addition, the strontium isotope composition of epidote was determined to test for the potential contribution of seawater in the Northparkes system given the postulated island-arc setting and submarine character of some country rocks.Oxygen isotope geothermometry calculations indicate potassic alteration occurred between ~600° and 700°C in magmatic/mineralized centers, persisting to ~450°C upon lateral transition into propylitic alteration. Across the propylitic facies, temperature progressively decreased outward to <250°C. These temperature estimates and additional data from chlorite geothermometry were utilized to calculate the oxygen and hydrogen isotope composition of the fluid in equilibrium with the sampled minerals. Results show that propylitic fluids spanned a range of compositions with δ18O between 0.5 and 3.7‰ and δD between –49 and –17‰. Comparison of these results with the modeled compositions of meteoric and/or magmatic fluids during their evolution and isotopic exchange with local country rocks shows that a magmatic fluid component must exist across the propylitic halo during its formation. Strontium isotope data from propylitic epidote provide initial (based on formation at ~450 Ma) 87Sr/86Sr values in the range of 0.704099 to 0.704354, ruling out the presence of seawater as a second fluid in the system. Although we cannot exclude magmatic-meteoric mixing, especially toward the fringes of the system, our results support a model in which magmatic-derived fluid is the primary driver of propylitic alteration as it undergoes cooling and chemical equilibration during outward infiltration into country rocks. This is consistent with chemical mass transfer calculations for Northparkes and published chemical-thermodynamic models that only require a magmatic fluid for the production of propylitic assemblages. In view of this and supporting data from other deposits, we suggest that magmatic fluids are essential drivers of propylitic alteration in porphyry systems.

Carbon, oxygen and strontium isotopic constraints on fluid sources, temperatures and biogeochemical processes during the formation of seep carbonates - Secchia River site, Northern Apennines

Understanding authigenic seep carbonate formation provides clues for hydrocarbon exploration and insights into contributions to gas budgets of marine environments and the atmosphere. Seep carbonates discovered in the outcropping succession along the Secchia riverbanks (near Modena, Italy) belong to the Argille Azzurre Formation of Early Pleistocene age deposited in an upper shelf environment overlying the Miocene foredeep successions, which include hydrocarbon fields. The fluid migration from the hydrocarbon fields, up to the surface, is presently active on land and started in the marine succession during the Late Miocene.Authigenic globular carbonate concretions and carbonate chimneys are interspersed along the strata throughout the section. A comprehensive geochemical characterisation of the carbonates has been carried out to understand the processes leading to their formation. The carbonate concretions are the record of past hydrocarbon vents linked to the Miocene petroleum system of the Northern Apennines. The samples are composed of >50% microcrystalline dolomite. The δ13C signatures identify two groups in the samples according to different type of formation processes. Globular concretions have positive values that suggest an influence of CO2 associated to secondary methanogenesis due to microbial degradation of higher hydrocarbons. The analysed chimney, with negative δ13C values, is interpreted as former conduit where carbonate precipitation is promoted by Anaerobic Oxidation of Methane coupled with Sulfate Reduction. The δ18O range, coupled with 87/86Sr signatures, indicate that the contribution of deep connate water from the Miocene reservoirs is up to 23% during the formation of the globular concretions. The connate water occurrence is also documented by higher ambient temperatures. The different isotope signatures in seep carbonates result from the relative contribution of the recognised gas and water components, linked to different plumbing systems and fluid supply from a well-defined hydrocarbon field. The seep carbonate characteristics have enlightened variations in biogeochemical processes, which can be rarely quantified in ancient and present-day marine environments.

Strontium isotope constraints on solute sources and mixing dynamics in a large impoundment, South-Central USA

Strontium concentration and isotope composition ( 87Sr/ 86Sr) are important proxies for tracing solute and suspended load origins in surface water systems, but have not been widely applied to lakes. We determined these proxies, in addition to major ion compositions, in waters collected from Lake Texoma, a large crescent-shaped (concave-west) impoundment on the border of Texas and Oklahoma that marks the downstream culmination of the Upper Red River Basin (URRB), and its major influent rivers. These rivers enter the lake at its southerly and northerly arms and have distinctly different salinities and discharges, respectively: the more saline Red River (mean total dissolved solids, TDS = 2930 mg/L; 1290 mg/L weighted for discharge, Gainesville gauging station) and the fresher Washita River (TDS = 634 mg/L; 376 mg/L weighted for discharge, Dickson gauging station). These two influent river systems combine to mix in the deepest (main) part of the lake. The URRB involves a heterogeneous mix of bedrock geologies, notably high solubility Permian marine evaporites (gypsum, halite) and carbonates, and riverine solute load and composition can vary accordingly with regional precipitation. Most river and lake waters show an strong positive correlation between 87Sr/ 86Sr and 1/Sr. Red River system waters have variable 87Sr/ 86Sr values, ranging from 0.7076 to 0.7097 (n = 34), whereas the Washita River system appears to be less isotopically variable, with 87Sr/ 86Sr values ranging from 0.7082 to 0.7087 (n = 16). Lake Texoma 87Sr/ 86Sr varies spatially between 0.7087 and 0.7092, reflecting mixing of the two input rivers. Variations in Sr isotope composition, Ca/Na and Sr/Ca of river and lake waters are consistent with the majority of the solute load being derived from dissolution of Permian marine chemical sediments, dominantly halite with lesser gypsum. Waters with Sr isotope composition more radiogenic than Permian seawater reflect the contribution of Permian and Pennsylvanian siliciclastic sediments in the case of the Red River watershed, and Precambrian granite in the Washita River watershed. In the main lake water column, Sr concentrations and filtered 87Sr/ 86Sr increase substantially with depth during summer stratification (from 0.7087 to 0.7091), likely reflecting desorption of Sr from suspended solid surfaces and breakdown of oxyhydroxides below the metalimnion. The differences in 87Sr/ 86Sr between filtered and unfiltered aliquots suggest that Sr associated with the suspended load (calcite) is either less radiogenic than the dissolved load or more radiogenic than surface waters but only released to the dissolved pool under deeper reducing conditions below the metalimnion. Either possibility has important implications for Sr cycling in binary mixing models and paleolake reconstruction from Sr isotopes.

Hydrogen, oxygen, helium and strontium isotopic constraints on the formation of oilfield waters in the western Qaidam Basin, China

Surveys and exploration for oil and gas have revealed many oil-field brines in the Tertiary strata of the western Qaidam Basin, China. The source and formation of these brines are unclear. Brine samples collected from petroleum wells in the oilfield area were analyzed for their general chemical composition and for hydrogen, oxygen, strontium and helium isotopes in order to trace their origin, formation, and resource distribution. Results show that the concentrations of resources such as K, B, Sr, Br and Li are unusually enriched in oil-field brines and have excellent potential for development and utilization in future. The geology, H, O, Sr and He isotopic composition, the chemical composition, and the geophysical measurements all show that the tertiary oil-field brines predominantly originated from the deep mixture of formation water and magmatic residual fluids in the western Qaidam Basin. The tectonic geology, intense and frequent magmatic-hydrothermal activity as well as high 3He/ 4He ratio indicate that the magmatic fluids possibly originate from the upper mantle-derived magmatic fluids. Thus, according to our interpretation of the formation and evolution of the oilfield brine, this source is adequate for the volume and composition to be explored and utilized latterly.

Trace element and strontium isotope constraints on sedimentary environment of Ediacaran carbonates in southern Anhui, South China

Two units of sedimentary carbonates in association with Neoproterozoic glaciations occur in the Lantian Formation of the Ediacaran system, equivalent to the Doushantuo Formation elsewhere in South China. Trace elements and strontium isotopes in the carbonates were analyzed by stepwise dissolutions with different strengths of acid. The results show that the carbonate component dissolved in 0.5 M acetic acid (Dissolution I) is principally composed of calcite and almost free from non-carbonate contamination. Thus its element and isotope compositions can be used to unravel the geochemical feature of water from which the carbonates precipitated. The strontium concentrations and isotope ratios, REE + Y concentrations and patterns of Dissolution I from the carbonates are similar to each other between the two units. They exhibit high strontium concentrations and high 87Sr/ 86Sr ratios, depletion of LREE relative to MREE and HREE, minor positive La anomalies, subtle negative Ce, Gd and Er anomalies, and near-chondritic Y/Ho ratios. These indicate that the Lantian carbonates were precipitated from the similar nature of basin water, which is devoid of trace element composition typical of normal marine carbonates but similar to those in freshwater carbonates. The negative Ce anomalies in the carbonates of the two units suggest oxygenation of the precipitation water. Therefore, the Lantian carbonates would form in a continental marginal basin, with predominant contributions from freshwater and high sedimentary rates. Since the Doushantuo–Lantian carbonate secessions may probably correspond to the chemical sediments of post-Marinoan and post-Gaskiers, it is hypothesized that the freshwater would originate from locally deglacial meltwater with significant inputs from continental weathering.

Strontium isotope constraints on fluid flow in the sheeted dike complex of fast spreading crust: Pervasive fluid flow at Pito Deep

Fluid flow through the axial hydrothermal system at fast spreading ridges is investigated using the Sr‐isotopic composition of upper crustal samples recovered from a tectonic window at Pito Deep (NE Easter microplate). Samples from the sheeted dike complex collected away from macroscopic evidence of channelized fluid flow, such as faults and centimeter‐scale hydrothermal veins, show a range of 87Sr/86Sr from 0.7025 to 0.7030 averaging 0.70276 relative to a protolith with 87Sr/86Sr of ∼0.7024. There is no systematic variation in 87Sr/86Sr with depth in the sheeted dike complex. Comparison of these new data with the two other localities that similar data sets exist for (ODP Hole 504B and the Hess Deep tectonic window) reveals that the extent of Sr‐isotope exchange is similar in all of these locations. Models that assume that fluid‐rock reaction occurs during one‐dimensional (recharge) flow lead to significant decreases in the predicted extent of isotopic modification of the rock with depth in the crust. These model results show systematic misfits when compared with the data that can only be avoided if the fluid flow is assumed to be focused in isolated channels with very slow fluid‐rock exchange. In this scenario the fluid at the base of the crust is little modified in 87Sr/86Sr from seawater and thus unlike vent fluids. Additionally, this model predicts that some rocks should show no change from the fresh‐rock 87Sr/86Sr, but this is not observed. Alternatively, models in which fluid‐rock reaction occurs during upflow (discharge) as well as downflow, or in which fluids are recirculated within the hydrothermal system, can reproduce the observed lack of variation in 87Sr/86Sr with depth in the crust. Minimum time‐integrated fluid fluxes, calculated from mass balance, are between 1.5 and 2.6 × 106 kg m−2 for all areas studied to date. However, new evidence from both the rocks and a compilation of vent fluid compositions demonstrates that some Sr is leached from the crust. Because this leaching lowers the fluid 87Sr/86Sr without changing the rock 87Sr/86Sr, these mass balance models must underestimate the time‐integrated fluid flux. Additionally, these values do not account for fluid flow that is channelized within the crust.

Oxygen, carbon and strontium isotope constraints on the mechanisms of nappe emplacement and fluid–rock interaction along the subhorizontal Naukluft Thrust, central Namibia

The Naukluft Thrust forms the floor thrust to the Naukluft Nappe Complex, a far-travelled, nappe stack of the Pan-African Damara belt in Namibia. The thrust tectonostratigraphy comprises three dolomitic components, a calc-mylonite horizon, and a discrete brittle fault. Stable isotope data indicate that the leading edge is characterized by positive δ 13 C values, whereas the trailing edge is characterized by negative δ 13 C values. There is a significant range in the δ 18 O values, over 15‰ in different sections, with the leading edge showing a larger range than the trailing edge. δ 18 O values are characteristic of burial dolomites and secondary dolomitization is indicated by the presence of networks above and below the Naukluft Thrust zone. The large range in δ 18 O values and variations in δ 13 C vs. δ 18 O patterns are interpreted to be the result of interaction between the precursor to the Naukluft Thrust zone dolomites and fluids derived from different footwall lithologies. 87 Sr/ 86 Sr isotope ratios indicate that some fluids were derived from the basement. The data presented in this study suggest that an original carbonate-dominated horizon existed prior to thrusting and that the basal thrust of the nappe complex exploited this horizon.

Carbon, oxygen and strontium isotopic constraints on fluid flow in the Junggar Basin (NW China)

Carbon, oxygen and strontium isotopic constraints on fluid flow in the Junggar Basin (NW China)

Strontium isotope constraints on fluid flow in the upper oceanic crust at the East Pacific Rise

Strontium isotopes are useful tracers of fluid–rock interaction in marine hydrothermal systems and provide a potential way to quantify the amount of seawater that passes through these systems. We have determined the whole-rock Sr-isotopic compositions of a section of upper oceanic crust that formed at the fast-spreading East Pacific Rise, now exposed at Hess Deep. This dataset provides the first detailed comparison for the much-studied Ocean Drilling Program (ODP) drill core from Site 504B. Whole-rock and mineral Sr concentrations indicate that Sr-exchange between hydrothermal fluids and the oceanic crust is complex, being dependent on the mineralogical reactions occurring; in particular, epidote formation takes up Sr from the fluid increasing the 87Sr/ 86Sr of the bulk-rock. Calculating the fluid-flux required to shift the Sr-isotopic composition of the Hess Deep sheeted-dike complex, using the approach of Bickle and Teagle [1] [M.J. Bickle, D.A.H. Teagle, Strontium alteration in the Troodos ophiolite: implications for fluid fluxes and geochemical transport in mid-ocean ridge hydrothermal systems. Earth Planet. Sci. Lett. 113 (1992) 219–237] gives a fluid-flux similar to that determined for ODP Hole 504B. This suggests that the level of isotopic exchange observed in these two regions is probably typical for modern oceanic crust. Unfortunately, uncertainties in the modeling approach do not allow us to determine a fluid-flux that is directly comparable to fluxes calculated by other methods.

Strontium and carbon isotope constraints on carbonate-solution interactions and inter-aquifer mixing in groundwaters of the semi-arid Murray Basin, Australia

Strontium and carbon isotopes are used to study the effect of carbonate mineral dissolution, and inter-aquifer mixing on the chemical evolution of groundwater from the Murray Group Limestone Aquifer in semi-arid SE Australia. The 87Sr/ 86Sr ratio of groundwater evolves from 0.7097 at the basin margin to a less radiogenic value of 0.7084 about 250 km down-gradient, which is similar to the 87Sr/ 86Sr ratio of the aquifer carbonate matrix. The concomitant increase in δ 13C DIC, Sr/Ca and Mg/Ca ratios in the groundwater along a 250 km transect suggests that incongruent dissolution of high Mg-calcite controls the carbonate geochemistry in this aquifer. Further down-gradient, the groundwater is characterized by a relatively more radiogenic 87Sr/ 86Sr ratio caused by upward leakage and mixing with more radiogenic groundwater from the Renmark Group Sand Aquifer. A mixing model using 87Sr/ 86Sr ratio suggests that the fraction of water that contribute to the Murray Group Aquifer through upward leakage from underlying Renmark Group Aquifer ranges from 15 to 85%.

Strontium and oxygen isotopic constraints on fluid mixing, alteration and mineralization in the TAG hydrothermal deposit

Strontium- and oxygen-isotopic measurements of samples recovered from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound during Leg 158 of the Ocean Drilling Program provide important constraints on the nature of fluid–rock interactions during basalt alteration and mineralization within an active hydrothermal deposit. Fresh Mid-Ocean Ridge Basalt (MORB), with a 87 Sr/ 86 Sr of 0.7026, from the basement beneath the TAG mound was altered at both low and high temperatures by seawater and altered at high temperature by near end-member black smoker fluids. Pillow breccias occurring beneath the margins of the mound are locally recrystallized to chlorite by interaction with large volumes of conductively heated seawater (>200°C). The development of a silicified, sulfide-mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of chloritization followed by mineralization and the development of a quartz+pyrite+paragonite stockwork cut by quartz–pyrite veins. Initial alteration involved the development of chloritic alteration halos around basalt clasts by reaction with a Mg-bearing mixture of upwelling, high-temperature (>300°C), black smoker-type fluid with a minor (<10%) proportion of seawater. Continued high-temperature (>300°C) interaction between the wallrock and these Mg-bearing fluids results in the complete recrystallization of the wallrock to chlorite+quartz+pyrite. The quartz+pyrite+paragonite assemblage replaces the chloritized basalts, and developed by reaction at 250–360°C with end-member hydrothermal fluids having 87 Sr/ 86 Sr ≈0.7038, similar to present-day vent fluids. The uniformity of the 87 Sr/ 86 Sr ratios of hydrothermal assemblages throughout the mound and stockwork requires that the 87 Sr/ 86 Sr ratio of end-member hydrothermal fluids has remained relatively constant for a time period longer than that required to change the interior thermal structure and plumbing network of the mound and underlying stockwork. Precipitation of anhydrite in breccias and as late-stage veins throughout most of the mound and stockwork, down to at least 125 mbsf, records extensive entrainment of seawater into the hydrothermal deposit. 87 Sr/ 86 Sr ratios indicate that most of the anhydrite formed from ≈2:1 mixture of seawater and black smoker fluids (65%±15% seawater). Oxygen-isotopic compositions imply that anhydrite precipitated at temperatures between 147°C and 270°C and require that seawater was conductively heated to between 100°C and 180°C before mixing and precipitation occurred. Anhydrite from the TAG mound has a Sr–Ca partition coefficient K d≈0.60±0.28 (2 σ). This value is in agreement with the range of experimentally determined partition coefficients ( K d≈0.27–0.73) and is similar to those calculated for anhydrite from active black smoker chimneys from 21°N on the East Pacific Rise. The δ 18 O SO 4 of TAG anhydrite brackets the value of seawater sulfate oxygen (≈9.5‰). Dissolution of anhydrite back into the oceans during episodes of hydrothermal quiescence provides a mechanism of buffering seawater sulfate oxygen to an isotopically light composition, in addition to the precipitation and dissolution of anhydrite within the oceanic basement during hydrothermal recharge at the mid-ocean ridges.

Strontium isotope constraints on the seasonal variation of the provenance of base cations in rain water at Kawakami, Central Japan

The 87Sr/ 86Sr ratios in monthly precipitation in the forested basin at Kawakami, central Japan, varied seasonally from 0.709 to 0.711 in spring to as low as 0.7062 ± 0.0004 in autumn over nine years from 1987 to 1995. The seasonal variation can be explained in terms of the mixing of three sources of Sr: sea salt ( 87Sr/ 86Sr∼ 0.70917), soluble eolian minerals originating from deserts in continental Asia (∼ 0.711), and biogenic materials growing on soils derived from the volcanic rock substrates in the vicinity ( ∼ 0.706). It is estimated that the contribution of sea-salt Sr into the Kawakami rain Sr is relatively constant (10 ± 5%) and that more than half of the Sr is of biogenic origin throughout the year except during spring rains when 50% of the Sr is due to the dissolution of Ca-minerals from Asian dusts. The dominant contribution of eolian components to spring rains is consistent with the high concentration of 3H, which is typical of air masses from Asia.

Neodymium and strontium isotopic constraints on soil sources in Barbados, West Indies

Neodymium and strontium isotopic compositions and SmNd ratios are used to constrain the sources of silicate-rich soils developed on uplifted Pleistocene coral-reef limestones on Barbados, West Indies. The geographic and geologic setting of Barbados facilitates the application of these tracers to the evaluation of the following soil sources: (1) Pleistocene reef limestone regolith, (2) Tertiary carbonate rocks, sandstones, and mudstones that are exposed in northeastern Barbados, (3) volcanic ash erupted from the Lesser Antilles arc, (4) Saharan dust transported by trade winds, and (5) fertilizer. The soils have ϵNd values that range from −6.6 to −1.9, 87Sr86Sr values of 0.70890 to 0.71067, and SmNd ratios of 0.223–0.260. The Pleistocene limestone component is the most significant source of Sr in the soils and a negligible source of Nd. Comparison of Sm and Nd concentrations and neodymium isotopic compositions of soil samples that are weathered to varying extents indicates that Sm and Nd are relatively unfractionated and retained in the soils during weathering. ϵNd and SmNd variations in the soils, therefore, primarily reflect the compositions and proportions of the silicate sources. Mass balance calculations based on Sm‐Nd systematics require that the silicate soil components contain between 30–85% volcanic ash, with the remaining silicate fraction comprised of old, continentally-derived sediment. In contrast to Sm and Nd, Sr is mobilized and removed from the soils during weathering. Strontium from volcanic and carbonate sources is preferentially removed relative to continental silicate sources. The strontium isotopic compositions of the soils, therefore, reflect the combined effects of the degree of weathering and the compositions and proportions of the soil sources. Mass balance calculations indicate that at least 35–60% of the initial Sr in the soils has been removed by weathering. These results illustrate (1) the utility of radiogenic isotopes in identifying and quantifying soil sources and weathering processes, (2) the compositional influence of numerous sources on soils, even those developed in a relatively isolated area such as Barbados, and (3) the domination of Barbados soil Sm‐Nd systematics by nonregolith eolian components.

Evolution of the upper mantle of the Earth's Moon: Neodymium and strontium isotopic constraints from high-Ti mare basalts

Isotopic studies of mare basalts have led workers to conclude that their sources are heterogeneous on both large and small scales. Furthermore, these studies have led workers to postulate that depletion within the lunar mantle occurred early in its evolution and was a result of accumulation of mafic minerals from a LREE-enriched magma ocean. High-Ti basalts from the Apollo 11 and 17 landing sites and ilmenite basalts from Apollo 12 are secondary evidence of this extreme, early depletion event. KREEPy rocks are the complementary enriched component in the Moon. A total of fourteen high-Ti basalts have now been analyzed from the Apollo 11 landing site for neodymium and strontium isotopes. A Sm-Nd internal isochron on basalt 10058 yields an age of 3.70 ± 0.06 Ga, similar to 40Ar 39Ar ages of other Group B1 basalts. A compilation of all previously determined ages on Apollo 11 high-Ti basalts indicates four distinct phases of volcanism at 3.85 ± 0.02 Ga (Group B2), 3.71 ± 0.02 Ga (Group B3), 3.67 ± 0.02 Ga (Group B1), and 3.59 ± 0.04 Ga (Group A). Wholerock Sm-Nd isotopic data for all Apollo 11 high-Ti basalts form a linear array, which yields the age of the Moon (4.55 ± 0.30 Ga). A similar regression of all uncontaminated high-Ti basalts from the Moon (both Apollo 11 and Apollo 17) yields an age of 4.46 ± 0.17 Ga. Both arrays are interpreted as average source ages of the high-Ti basalts and are consistent with the formation of these sources by precipitation of cumulates from a magma ocean early in the history of the Moon. These new strontium and neodymium isotopic data, coupled with previously published data, are consistent with a two component model for the upper mantle of the Moon. These two-components include mafic adcumulates precipitated from a magma ocean prior to 4.4 Ga and small amounts (< 2%) of trapped, KREEPy, late-stage, magma ocean differentiates. The mafic adcumulate evolves from 4.5 Ga, with 147Sm 144Nd = 0.318 and 87Rb 86Sr = 0.005 to extremely radiogenic neodymium isotopic ratios and very unradiogenic strontium isotopic ratios. The KREEPy trapped liquid has a 147Sm 144Nd = 0.168 and 87Rb 86Sr = 0.235 and thus, evolves toward very unradiogenic neodymium and radiogenic strontium isotopic ratios. Because the KREEPy trapped liquid is enriched in both rubidium and the REEs by over an order of magnitude compared to the mafic adcumulate, trapping of even small proportions of this liquid in the adcumulate will control the radiogenic isotopic composition of the source. The apparent heterogeneity in the source regions of mare basalts could be caused by trapping of variable, yet small, proportions of this LILE-enriched liquid in the cumulate pile.

Oxygen, Carbon and Strontium Isotopic Constraints on Fluid Movement During the Acadian Metamorphism in South-Central Maine

8t80, 813C and STSr/S6Sr isotopic tracers have been used to constrain the timing, as well as the magnitude and direction of cross-layer fluid flow in Acadian metamorphic rocks in south-central Maine. The area contains pelites, calcareous greywackes and the ~100 m thick Waterville Limestone which were isoclinaUy folded and then intruded by granitoid plutons during the Devonian Acadian orogeny. Metamorphism to andalusite and sillimanite facies and granitic plutonism are thought to be closely associated. Baumgartner and Ferry (1991) and Ferry (in press) use the progress of decarbonation reactions to estimate metamorphic time-integrated fluid fluxes up to ~2.10 4 m3/m 2, with the flow uptemperature and layer parallel (lithological units strike at a high angle to isograds). This study tests predictions of this model by: 1) Determining the timing of fluid-flow from small-scale Rb-Sr isochron systematics. 2) Estimating the magnitude of the cross-layer component. Significant

Strontium isotopic constraints on the origin of ore-forming fluids of the Viburnum Trend, southeast Missouri

We have measured 87Sr 86Sr and Rb and Sr concentrations in several minerals, primarily sulfides, spanning the paragenesis of hydrothermal mineralization in the Viburnum Trend in southeast Missouri. Separate measurements were made for fluid inclusions opened by crushing or thermal decrepitation and for the solids. For comparison, measurements were also made on samples of probable local aquifers, the Bonneterre Formation and the Lamotte Sandstone. For some of the samples, concentrations of K, Ca, Cl, Na, and Mg are also reported. In several cases 87Sr 86Sr ratios are different (higher) in the solids than in the fluid inclusions. We have investigated the possibility that either type of sample gives spurious results, e.g., that the fluid inclusions are secondary or contaminated by host dolomite, or that Sr in the solids reflects a detrital rather than an authigenic source. Consideration of mass balance, overall solute chemistry, and examination of non-sulfide dissolution residue, however, suggests that both types of sample reflect primary fluid Sr composition. We thus adopt the working hypothesis that Sr isotopic composition in fluids at the time of hydrothermal mineralization was highly variable. The observed results for the Viburnum Trend do not conform well to expected trends for Mississippi Valley-type (MVT) mineralization based primarily on analyses of gangue carbonates, barite, and fluorite. Fluid inclusion Sr in some of the Viburnum Trend samples is more radiogenic than in the host dolomite but only moderately so; in other samples, notably main-stage octahedral galena, fluid inclusion Sr composition is within the range observed for the host dolomite. In contrast, Sr in some of the sulfides is very radiogenic, much more so than previously reported for MVT minerals, and is very radiogenic early in the paragenesis (pyrite, chalcopyrite, sphalerite), less radiogenic during main-stage ore deposition, and again more radiogenic in later paragenetic stages.

Neodymium and strontium isotopic constraints on the origin of the peraluminous granitoids of the South Mountain Batholith, Nova Scotia, Canada

Nd isotopic data from the South Mountain batholith show that its peraluminous granitic rocks have considerably higher ϵ t Nd ( t=372 Ma) values (−5.2 to −1.4) than the extensive flyschoid country rocks (−11.2 to −8.7). These isotopic data are combined with Sr isotopic measurements to estimate the involvement of the country rocks, either as source materials or as contaminants, in the batholith. Metasedimentary enclaves in the batholith generally have Nd and Sr isotopic compositions intermediate between those of the batholith and the country rocks, and provide evidence of contamination by the country rocks. Integrated with other geochemical data, the isotopic results show at least two possible origins for the South Mountain batholith: either this large batholith is a hybrid of mantle and crustal materials and contains a large proportion of mantle-derived Nd and Sr, or the magma is derived from deeper metasedimentary crustal material, with younger model ages or higher Sm/Nd ratios than the upper crust.

Oxygen, hydrogen, and strontium isotope constraints on the origin of granites

ABSTRACTOxygen isotope data are very useful in determining the sourcerocksof granitic magmas, particularly when used in combination with Sr, Pb, and Nd isotope studies. For example, unusually high δ18O values in magmas (δ18O&gt; +8)requirethe involvement of some precursor parent material that at some time in the past resided on or near the Earth's surface, either as sedimentary rocks or as weathered or hydrothermally altered rocks. The isotopic systematics which are preserved in the Mesozoic and Cenozoic batholiths of western North America can be explained by grand-scale mixing of three broadly defined end-members: (1) oceanic island-arc magmas derived from a “depleted” (MORB-type?) source in the upper mantle (δ18O c.+6 and87Sr/86Src.0·703); (2) a high-18O (c.+13 to +17) source with a very uniform87Sr/86Sr (c.0·708 to 0·712), derived mainly from eugeosynclinal volcanogenic sediments and (or) hydrothermally altered basalts; and (3) a much more heterogeneous source (87Sr/86Src.0·706 to 0·750, or higher) with a high δ18O (c.+9 to +15) where derived from supracrustal metasedimentary rocks and a much lower δ18O (c.+7 to +9) where derived from the lower continental crust of the craton. These end-members were successively dominant from W to E, respectively, within three elongate N–S geographic zones that can be mapped from Mexico all the way N to Idaho.18O/16O studies (together with D/H analyses) can, however, play a more important and certainly a unique role in determining the origins of the aqueousfluidsinvolved in the formation of granitic and rhyolitic magmas. Fluid-rock interaction effects are most clear-cut when low-18O, low-D meteoric waters are involved in the isotopic exchange and melting processes, but the effects of other waters such as seawater (with a relatively high δDc.0) can also be recognised. Because of these hydrothermal processes, rocks that ultimately undergo partial melting may exhibit isotopic signatures considerably different from those that they started with. We discuss three broad classes ofpotentialsource materials of such “hydrothermal-anatectic” granitic magmas, based mainly on water/rock (w/r), temperature (T), and the length of time (t) that fluid-rock interaction proceeds: (Type 1) epizonal systems with a wide variation in whole-rock δ18O and extreme18O/16O disequilibrium among coexisting minerals (e.g. quartz and feldspar); (Type 2) deeper-seated and (or) longer-lived systems, also with a wide spectrum of whole-rock δ18O, but with equilibrated18O/16O ratios among coexisting minerals; (Type 3) thoroughly homogenisedandequilibrated systems with relatively uniform δ18O in all lithologies. Low-18O magmas formed by melting of rocks altered in a Type 2 or a Type 3 meteoric-hydrothermal system are the only kinds of “hydrothermal-anatectic” granitic magmas that are readily recognisable in the geological record. Analogous effects produced by other kinds of aqueous fluids may, however, be quite common, particularly in areas of extensional tectonics and large-scale rifting. The greatly enhanced permeabilities in such fractured terranes make possible the deep convective circulation of ground waters and sedimentary pore fluids. The nature and origin of low-18O magmas in the Yellowstone volcanic field and the Seychelles Islands are briefly reviewed in light of these concepts, as is the development of high-D, peraluminous magmas in the Hercynian of the Pyrenees.

Strontium isotopic constraints on the origin of proterozoic anorthosites

Abstract The Proterozoic Giles Complex, central Australia contains an almost complete range of anorthosite types from minor or major layers in gabbronorite intrusions to large anorthosite-troctolite bodies to small orthopyroxene anorthosite massifs; each type has a distinctive Sr isotopic signature. Anorthosite-dominated masses have a regular relationship between ferromagnesian mineralogy, initial 87 Sr/ 86 Sr and anorthite contents in plagioclase: anorthosite-troctolite bodies have significant olivine, relatively low initial 87 Sr/ 86 Sr (0.7038–0.7043) and An 50–69 ; orthopyroxene-dominant anorthosites have relatively high 87 Sr/ 86 Sr (0.7045–0.7063) and An 45−60 . The pattern is found worldwide. Detailed study of one intrusion demonstrates that contamination by wall-rock granulite produces the higher 87 Sr/ 86 Sr values, anti-correlation between 87 Sr/ 86 Sr and An, and determines olivine/orthopyroxene proportions. Olivine-bearing anorthosites form from a primary aluminous tholeiite magma with plagioclase dominating the liquidus; progressive contamination of this parent magma produces a gradation to orthopyroxene anorthosites.