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Water and chemical transport from a point source within vadose zone sediments at Hanford were examined with a leak test consisting of five 3800-liter aliquots of water released at 4.5 m depth every week over a 4-week period. The third aliquot contained bromide, D2O and 87Sr. Movement of the tracers was monitored for 9 months by measuring pore water compositions of samples from boreholes drilled 2-8 m from the injection point. Graded sedimentary layers acting as natural capillary barriers caused significant lateral spreading of the leak water. D2O concentrations >50 percent of the concentration in the tracer aliquot were detected at 9-11 m depth. However, increased water contents, lower d18O values, and geophysical monitoring of moisture changes at other depths signified high concentrations of leak fluids were added where D2O concentrations were

[1] Sr, Nd, and Os isotopic measurements were made on 110 Mauna Kea lava and hyaloclastite samples from the drillcore retrieved from the second phase of the Hawaii Scientific Drilling Project (HSDP-2). The samples come from depths of 255 to 3098 meters below sea level, span an age range from 200 to about 550-600 kyr, and represent an ordered record of the lava output from Mauna Kea volcano as it drifted a distance of about 40 km over the magma-producing region of the Hawaiian hot spot. The deepest (oldest) samples represent the time when Mauna Kea was closest to the center of the melting region of the Hawaiian plume. The Sr and Os isotopic ratios in HSDP-2 lavas show only subtle isotopic shifts over the ∼400 kyr history represented by the core. Neodymium isotopes (ɛNd values) increase systematically with decreasing age from an average value of nearly +6.5 to an average value of +7.5. This small change corresponds to subtle shifts in 87Sr/86Sr and 187Os/188Os isotope ratios, with small shifts of ɛHf, a large shift in 208Pb/204Pb and 208Pb/207Pb values, and with a very large shift in He isotope ratios from R/RA values of about 7–8 to values as high as 25. When Mauna Kea was closest to the plume core, the magma source did not have primitive characteristics for Nd, Sr, Pb, Hf, and Os isotopes but did have variable amounts of “primitive” helium. The systematic shifts in Nd, Hf, Pb, and He isotopes are consistent with radial isotopic zoning within the melting region of the plume. The melting region constitutes only the innermost, highest-temperature part of the thermally anomalous plume mantle. The different ranges of values observed for each isotopic system, and comparison of Mauna Kea lavas with those of Mauna Loa, suggest that the axial region of the plume, which has a radius of ∼20 km, is a mixture of recycled subducted components and primitive lower mantle materials, recently combined during the formational stages of the plume at the base of the mantle. The proportions of recycled and primitive components are not constant, and this requires there be longitudinal (vertical) heterogeneity within the core of the plume. The remainder of the plume, outside this plume “core zone,” is less heterogeneous but distinct from upper mantle as represented by mid-ocean ridge basalt (MORB). The plume structure may provide a detailed view of mantle isotopic composition near the core-mantle boundary.

The nitrogen and oxygen isotopic compositions of nitrate in pore water extracts from unsaturated zone (UZ) core samples and groundwater samples indicate at least four potential sources of nitrate in groundwaters at the U.S. DOE Hanford Site in south-central Washington. Natural sources of nitrate identified include microbially produced nitrate from the soil column (delta15N of 4 - 8 per thousand, delta18O of -9 to 2 per thousand) and nitrate in buried caliche layers (delta15N of 0-8 per thousand, delta 18O of -6to 42 per thousand). Isotopically distinctindustrial sources of nitrate include nitric acid in low-level disposal waters (delta15N approximately per thousand, delta 18O approximately 23%o) per thousandnd co-contaminant nitrate in high-level radioactive waste from plutonium processing (6'5delta1of 8-33 % o, per thousand18delta oO -9 to 7%0). per thousandThe isotopic compositions of nitrate from 97 groundwater wells with concentrations up to 1290 mg/L NO3- have been analyzed. Stable isotope analyses from this study site, which has natural and industrial nitrate sources, provide a tool to distinguish nitrate sources in an unconfined aquiferwhere concentrations alone do not. These data indicate that the most common sources of high nitrate concentrations in groundwater at Hanford are nitric acid and natural nitrate flushed out of the UZ during disposal of low-level wastewater. Nitrate associated with high-level radioactive UZ contamination does not appear to be a major source of groundwater nitrate at this time.

The objective of this research is to develop and advance isotopic approaches for characterizing fluid flow and chemical transport through the vadose zone to groundwater. Previous research has been concentrated on developing and comparing different isotopic systems (e.g., hydrogen, oxygen and strontium isotopes) for determining fluid infiltration rates and pathways in the vadose zone (e.g., Maher et al., 2003; DePaolo et al., 2004; Singleton et al., in press). The results demonstrate the unique advantage of studies of multiple isotopic systems for distinguishing short-term versus long-term processes. The focus of our current efforts is on using the isotopic compositions of different chemical phases (e.g., uranium, nitrate) to track their movement through the vadose zone. Preliminary results indicate that this will be a powerful tool for assessing environmental risks associated with vadose zone contamination.

In the mid-1990s, a groundwater plume of uranium (U) was detected in monitoring wells in the B-BX-BY Waste Management Area at the Hanford Site in Washington. This area has been used since the late 1940s to store high-level radioactive waste and other products of U fuel-rod processing. Using multiple-collector ICP source magnetic sector mass spectrometry, high-precision uranium isotopic analyses were conducted of samples of vadose zone contamination and of groundwater. The isotope ratios 236U/238U, 234U/238U, and 238U/235U are used to distinguish contaminant sources. On the basis of the isotopic data, the source of the groundwater contamination appears to be related to a 1951 overflow event at tank BX-102 that spilled high-level U waste into the vadose zone. The U isotopic variation of the groundwater plume is a result of mixing between contaminant U from this spill and natural background U. Vadose zone U contamination at tank B-110 likely predates the recorded tank leak and can be ruled out as a significant source of groundwater contamination, based on the U isotopic composition. The locus of vadose zone contamination is displaced from the initial locus of groundwater contamination, indicating that lateral migration in the vadose zone was at least 8 times greater than vertical migration. The time evolution of the groundwater plume suggests an average U migration rate of approximately 0.7-0.8 m/day showing slight retardation relative to a groundwater flow of approximately 1 m/day.

[1] We report measurements of U-series disequilibria, Sr, Nd, Hf, and Pb isotopic compositions and major and trace element abundances in a suite of well-located, off-axis MORBs that span the East Pacific Rise (EPR) ridge crest from 9°48′–52′N and across it for ∼4 km on either side. The geological context of the samples are well constrained as they were collected by submersible in an area that has been extensively imaged by remote sensing techniques. Sr, Nd, Hf and 208Pb/206Pb isotopic compositions of the off-axis N-MORB are identical to the axial lavas from this same region, suggesting that their sources are similar and that melting processes are the dominant influence in establishing the U-Th-Ra disequilibria and trace element fractionations. A majority of off-axis samples have U-Th and Th-Ra disequilibria that are larger, and model ages that are younger, than would be predicted from their off-axis distance and the time-integrated spreading rate. There are, however, a few off-axis samples with U-Th ages that are consistent with their spreading rate ages. It is likely that these samples erupted within or close to the axial summit trough (AST) and aged at a rate proportional to the spreading rate. The anomalously young ages determined for most of the off-axis lavas suggest that volcanic construction along this region is occurring over a zone that is wider (at least 4 km) than the AST (10s to 100s of meters). The combined observational, chemical and isotopic data support a model for the 9°0′N area that includes a significant component of crustal accumulation resulting from lavas that breach the AST and flow down the flanks of the EPR ridge crest. However, these data also require a minor component of off-axis eruptions that occur on distinct pillow mounds and ridges. This suggests that MOR construction involves several volcanic and tectonic processes acting in concert to form a complex patchwork of lava ages and compositions along, and across, this fast spreading ridge crest.

[i] Sr isotope ratios were measured in the pore water, acid extracts, and sediments of a 70-m vadose zone core to obtain estimates of the long-term infiltration flux for a site in the Hanford/DOE complex in eastern Washington State. The 8 7 Sr/ 8 6 Sr values for the pore waters decrease systematically with depth, from a high value of 0.721 near the surface toward the bulk sediment average value of 0.711. Estimates of the bulk weathering rate combined with Sr isotopic data were used to constrain the long-term (century to millenial scale) natural diffuse infiltration flux for the site given both steady state and nonsteady state conditions. The models suggest that the infiltration flux for the site is 7 ′ 3 mm/yr. The method shows potential for providing long-term in situ estimates of infiltration rates for deep heterogeneous vadose zones.

[1] Variations in the isotopic content (18O/16O and D/H ratios) of water in the natural environment provide a valuable tracer of the present-day global hydrologic cycle and a record of the climate over at least 400,000 years that is preserved in glacial ice. The interpretation of observed isotopic ratios in water vapor, rain, snow, and ice depends on our understanding of the processes (mainly phase changes) that produce isotopic fractionation. Whereas equilibrium isotopic fractionation is well understood, kinetic effects, or diffusion-controlled fractionation, has a limited experimental foundation. Kinetic effects are significant during evaporation into unsaturated air and during condensation to form ice from vapor. Kinetic effects are also thought to control the deuterium excess (d = δD − 8δ18O) of precipitation. We describe experiments to observe kinetic effects associated with evaporation. Analysis of our own and previous experiments shows that surface cooling of the liquid is a crucial variable affecting fractionation from evaporating water that has not been properly considered before. Including the effects of evaporative surface cooling reconciles observed D/H fractionation with kinetic theory and removes the need to invoke an unusual size for the HDO molecule. Thus the isotopic molecular diffusivity ratios are D(H218O)/D(H216O) = 0.9691 and D(HD16O)/D(H216O) = 0.9839. Implications of this work for representation of kinetic fractionation in global circulation models and cloud physics models are briefly discussed.

[1] New He isotopic data from the HSDP pilot hole core, lava accumulation rate models, and data from the literature are used to develop a 200,000 year isotopic record for the lava erupted from the Mauna Loa volcano. This record, coupled with an analogous record from Mauna Kea from the Hawaii Scientific Drilling Project (HSDP) pilot hole project and other literature data from the GEOROC database, are used to construct a “map” of lava isotopic compositions for the island of Hawaii. The isotopic map is converted to a map of the He and Nd isotopic compositions of melts from the mantle plume, which can be compared with a published melt supply map derived from geodynamic modeling. The resulting map of the plume indicates that values of helium 3He/4He > 20 Ra are confined to the core of the plume (radius ≈ 20–25 km) and correspond to potential temperatures >1565°C, suggesting the He isotopic signal is derived from deep in the mantle. The 3He/4He map has closed contours down to 10 Ra; the contours are teardrop-shaped and elongated in the general direction of plate motion. The closed contours indicate that most of the plume He signal is lost during the early stages of melting, which is consistent with helium behaving as a strongly incompatible element (KHe ≤ 0.001). The eNd contours (and by inference the contours for Sr, Pb, Hf, and Os) do not all close on the scale of the island of Hawaii but instead partially follow material flow lines within the plume beneath the lithosphere. The plume signal for Nd extends circa 100 km in the direction of plate motion, which is consistent with the moderately incompatible behavior of Nd (KNd ≈ 0.02). Downstream from the plume core epicenter, plume Nd occurs with asthenospheric He; this could be mistaken for an additional plume component, whereas it may be only a manifestation of differing incompatibility. Data from Mauna Loa suggest the presence of a low-3He/4He plume component that has low eNd and high 87Sr/86Sr. The plume map suggests that this component may be a blob (circa 20 km scale), located between Mauna Loa and Hualalai and separated from the main plume core by a zone of more asthenosphere-like material. The HSDP data preclude a proposed model where this material represents a ring of entrained material from the lower mantle. The orientation of the elongation of contours on the plume He and Nd isotope maps (∼N45°W) does not match the modern plate motion as measured by GPS (N65°W) nor does it match the trend of the ridge axis between Maui and Loihi (N30°W). The geochemical evidence, as well as the locations and growth histories of the Hawaiian volcanoes, suggest that the plume, as well as the Pacific plate, has been moving at a velocity of several centimeters per year over the past 1 to 2 million years.

Hawaiian volcanoes are the most comprehensively studied on Earth. Nevertheless, most of the eruptive history of each one is inaccessible because it is buried by younger lava flows or is exposed only below sea level. For those parts of Hawaiian volcanoes above sea level, erosion typically exposes only a few hundred meters of buried lavas (out of a total thickness of up to 10 km or more).Available samples of submarine lavas extend the time intervals of individual volcanoes that can be studied. However, the histories of individual Hawaiian volcanoes during most of their ~1-million-year passages across the zone of melt production are largely unknown.

Significant variations in the isotopic composition of marine calcium have occurred over the last 80 million years. These variations reflect deviations in the balance between inputs of calcium to the ocean from weathering and outputs due to carbonate sedimentation, processes that are important in controlling the concentration of carbon dioxide in the atmosphere and, hence, global climate. The calcium isotopic ratio of paleo-seawater is an indicator of past changes in atmospheric carbon dioxide when coupled with determinations of paleo-pH.

The purpose of this project was to explore and refine applications of isotope ratio measurements for guiding environmental remediation strategies. The isotopic compositions of samples collected from field sites were analyzed to address both basic scientific issues and site-specific problems.

The atmospheric budget of methyl bromide (CH 3 Br), an ozone-depleting gas, is highly uncertain, because it has complex sources and sinks. Although oceans, biomass burning, and industrial production are identified as the major sources, the fraction of CH 3 Br that is contributed by each source is not well known. A mass-balance approach that exploits differences in the carbon isotopic signature (δ 13 C) of CH 3 Br sources and sinks may provide a means of reducing uncertainties in the atmospheric budget. This approach depends on the distinctiveness of industrially produced methyl bromide. Our δ 13 C measurements of industrial CH 3 Br from the three largest manufacturers worldwide yield a weighted average of −54.4‰ relative to the Peedee Belemnite standard. This result suggests that industrial CH 3 Br is isotopically distinct and that the carbon isotopic composition of atmospheric CH 3 Br may indicate what fraction of atmospheric CH 3 Br is anthropogenic.

Publisher Summary This chapter discusses the integrated stratigraphy of the middle to upper Miocene pelagic sequence of the Conero Riviera (Marche region, Italy). In the western Tethyan domain, the middle to upper Miocene is usually represented by thick flysch and post-flysch sequences. In the northern Apennines, turbiditic sandstones, conglomerates and molasses derived from the denudation of the emerging Alpine-Apennine orogenic system, and collected in various minor basins with complex topography and facies characteristics; interrupt thick sequences of pelagic carbonate rocks, which are deposited in a subsiding epeiric basin since the early Jurassic. The principal aim of this chapter is to calibrate the boundaries between the Langhian, Serravallian, Tortonian, and Messinian stages recognized on the basis of biostratigraphic criteria, by interpolating from radioisotopically dated volcaniclastic layers contained in this pelagic sequence. This leads to the compilation of an integrated stratigraphic model for the late Miocene time scale, which being derived from an apparently complete and continuous sequence, may serve as a reference for the proposal of integrated stratigraphic definitions of Serravallian/Tortonian, and Tortonian/Messinian boundaries.

Oxygen isotope records of deep-sea paleotemperature indicate that the Cenozoic is a period of dramatic climatic change. Deep-ocean temperatures were highest 55 million years ago, and have declined by 12 to 15°C to the present.1–2 The Earth’s climate appears to have changed as dramatically during the past 55 million years as it has over any similar period in the last 500 million years.

The shapes of typical Hawaiian volcanoes are simply parameterized, and a relationship is derived for the dependence of lava accumulation rates on volcano volume and volumetric growth rate. The dependence of lava accumulation rate on time is derived by estimating the eruption rate of a volcano as it traverses the Hawaiian plume, with the eruption rate determined from a specified radial dependence of magma generation in the plume and assuming that a volcano captures melt from a circular area centered on the volcano summit. The timescale of volcano growth is t = 2 R/ν_plate where R is the radius of the melting zone of the (circular) plume and νplate is the velocity of the Pacific plate. The growth progress of a volcano can be described by a dimensionless time t′ = tν_plate/2R, where t′ = 0 is chosen to be the start of volcano growth and t′ = 1 approximates the end of “shield” growth. Using a melt generation rate for the whole plume of 0.2 km^(3)/yr, a plume diameter of 50 km, and a plate velocity of 10 cm/yr, we calculate that the lifetime of a typical volcano is 1000 kyr. For a volcano that traverses the axis of the plume, the “standard” dimensions are a volume of 57,000 km^3, a summit thickness of 18 km, a summit elevation of 3.6 km, and a basal radius of 60 km. The volcano first breaches the sea surface at t′ ≈ 0.22 when it has attained only 5% of its eventual volume; 80% of the volume accumulates between t′ = 0.3 and t′ = 0.7. Typical lava accumulation rates start out over 50 m/kyr in the earliest stages of growth from the seafloor, and level out at ∼35 m/kyr from t′ ≈ 0.05 until t′ = 0.4. From t′ = 0.4 to t′ = 0.9, the submarine lava accumulation rates decrease almost linearly from 35 m/kyr to ∼0; subaerial accumulation rates are about 30% lower. The lava accumulation rate is a good indicator of volcano age. A volcano that passes over the plume at a distance 0.4R off to the side of the plume axis is predicted to have a volume of about 60% of the standard volcano, a lifetime about 8% shorter, and lava accumulation rates about 15–20% smaller. The depth-age data for Mauna Kea lavas cored by the Hawaii Scientific Drilling Project are a good fit to the model parameters used, given that Mauna Kea appears to have crossed the plume about 15–20 km off-axis. The lifetime of Mauna Kea is estimated to be 920 kyr. Mauna Loa is predicted to be at a stage corresponding to t′ ≈ 0.8, Kilauea is at t′ ≈ 0.6, and Loihi is at t′ ≈0.16. The model also allows the subsurface structure of the volcanoes (the interfaces between lavas from different volcanoes) to be modeled. Radial geochemical structure in the plume may be blurred in the lavas because the volcanoes capture magma from a sizeable cross-sectional area of the plume; this inference is qualitatively born out by available isotopic data. The model predicts that new Hawaiian volcanoes are typically initiated on the seafloor near the base of the next older volcano but generally off the older volcano's flank.

Measurements of uranium/thorium and samarium/neodymium isotopes and concentrations in a suite of Hawaiian basalts show that uranium/thorium fractionation varies systematically with samarium/neodymium fractionation and major-element composition; these correlations can be understood in terms of simple batch melting models with a garnet-bearing peridotite magma source and melt fractions of 0.25 to 6.5 percent. Midocean ridge basalts shows a systematic but much different relation between uranium/thorium fractionation and samarium/neodymium fractionation, which, although broadly consistent with melting of a garnet-bearing peridotite source, requires a more complex melting model.

Publisher Summary This chapter discusses integrated stratigraphy of the Chattian to mid-Burdigalian pelagic sequence of the Contessa valley. The pelagic sequence exposed in active quarries and along road cuts in the Contessa valley, near Gubbio in the Umbrian region of central Italy, represents a remarkably continuous stratigraphic record from the upper Jurassic to the mid Miocene. The numerous volcaniclastic layers within the Eocene to Miocene portion of this sequence present the rare opportunity to radioisotopically date magnetostratigraphic, biostratigraphic, and chemostratigraphic events. The stratigraphic interval comprising the upper Chattian, Aquitanian, and lower Burdigalian in the Contessa sequence, is represented by the upper part of the Scaglia Cinerea formation, and the overlying lower part of the Bisciaro formation. This sequence has been studied in three sections—namely, the Contessa Quarry (CQ), the Contessa Testimone (CT) and the Contessa Valderchia (CV), the locations of which are shown. The sharp contact between the Scaglia Cinerea and the Bisciaro formations is marked by the so-called “Raffaello Level”, an 18 cm thick bentonite layer, which represents a regional marker bed, easily recognizable in the field.

Publisher Summary This chapter discusses integrated stratigraphy of the upper Burdigalian-lower Langhian section at Moria. The 100 m thick Moria section is exposed along a mountain road between the villages of Palcano and Moria (Pontedazzo exit of the State Highway No. 3, near Cantiano), on the west side of Monte Petrano, in the Marche region of Italy. The chapter exposes in continuity the transition between the Bisciaro formation and the overlying Schlier formation, as well as the stratigraphic interval containing the Burdigalian/Langhian boundary. Moreover, several intercalations of volcaniclastic horizons permit direct radioisotopic age calibration of relevant biostratigraphic, magneto-stratigraphic, and chemostratigraphic events. Thus, the Moria section represents an advantageous circumstance for the comprehensive definition of the lower/upper Miocene boundary in a representative Tethyan pelagic facies. The top part of the Bisciaro Formation is exposed in the lower 20 m of the Moria section. The formation is characterized in the lower half of this interval by the greyish-beige “siliceous-calcareous-tuffitic member”, which is overlain by the “upper marly member”.

Many volcanoes are associated with subduction zones or mid-ocean ridges, but other areas of unusually high volcanism (or "hotspots") do not have such a direct connection to plate tectonic processes. DePaolo and Manga argue in their Perspective that some hotspots are caused by narrow upwelling plumes originating in the lower mantle, while others are caused by broad, hot upwellings or "superswells." Yet others are not obviously associated with either deep plumes or superswells. Direct evidence for mantle plumes will require seismic imaging at higher resolution than is currently accessible. In a related Perspective, Foulger and Natland take the opposing view that hotspot volcanism is a by-product of plate tectonics and does not require deep plumes.

Temporal and spatial variations in the Nd isotopic compositions of Tertiary caldera-forming rhyolite tuffs, and Cretaceous and Tertiary granites of the western U.S.A. are used as a basis for a model that accounts for the observed proportions of crustal versus mantle contributions to silicic magmas in terms of two parameters: the ambient crustal temperature and the rate of supply of basaltic magma from the mantle. The crustal contribution to silicic igneous rocks is measured in terms of the Neodymium Crustal Index (NCI). The relationships between crustal temperature, basalt supply and NCI are quantified using a model of a magma chamber undergoing continuous recharge, wall-rock assimilation and fractional crystallisation

The vast Wrangellia terrane of Alaska and British Columbia is an accreted oceanic plateau with Triassic strata that contain a 3- to 6-kilometers thick flood basalt, bounded above and below by marine sedimentary rocks. This enormous outpouring of basalt was preceded by rapid uplift and was followed by gradual subsidence of the plateau. The uplift and basalt eruptions occurred in less than approximately 5 million years, and were not accompanied by significant extension or rifting of the lithosphere. This sequence of events is predicted by a mantle plume initiation, or plume head, model that has recently been developed to explain continental flood volcanism. Evidence suggests that other large oceanic basalt plateaus, such as the Ontong-Java, Kerguelen, and Caribbean, were formed as the initial outbursts of the Louisville Ridge, Kerguelen, and Galapagos hot spots, respectively. Such events may play an important role in the creation and development of both oceanic and continental crust.

Measurements of marine carbonate samples indicate that during the past 2.5 million years the (87)Sr/(86)Sr ratio of seawater has increased by 14 x 10(-5). The high average rate of increase of (87)Sr/(86)Sr indicates that continental weathering rates were exceptionally high. Nonuniformity in the rate of increase suggests that weathering rates fluctuated by as much as +/-30 percent of present-day values. Some of the observed shifts in weathering rates are contemporaneous with climatic changes inferred from records of oxygen isotopes and carbonate preservation in deep sea sediments.

We present 238 U- 230 Th - 226 Ra disequilibria and cosmogenic 3 He and 36 Cl data for the Bluewater flow of the Zuni-Bandera volcanic field in western New Mexico. The 238 U- 230 Th disequilibria measured on separated groundmass phases yield an internal isochron age of 68 ka (+24/–20 ka; 2σ). This value cannot be directly compared with surface exposure ages unless erosion rates are known. The apparent (zero erosion) ages determined from both the 3 He concentration (47.5 ± 5 ka; 2σ) and the 36 Cl concentration (41.2 ± 8.8 ka; 2σ) are significantly younger than the U-Th isochron age. When minimum estimates of surface erosion based on flow morphology are considered, the 3 He concentrations indicate a minimum exposure age of 60 ka, in good agreement with the U-Th isochron age, with a minimum erosion rate of 1.7 mm/k.y. and an erosion rate as high as 5 mm/k.y. in other locations. Correcting for erosion has little effect on the model 36 Cl age and, as a result, the 36 Cl age is significantly younger than the U-Th isochron age and erosion-corrected 3 He ages; this discordance is attributed to a lack of closed-system behavior in the 36 Cl system. These new ages have local significance for the geochronology of the Zuni-Bandera volcanic field; however, their larger significance is in their applicability to dating Quaternary basalts and quantifying erosion rates.

The Hawaii Scientific Drilling Project (HSDP) sampled a 3 km section of basalt lava from the Mauna Kea volcano, covering ages of 200–600 ka. When combined with surface and dredge samples, there is a 600 ky record of the lava output from Mauna Kea as well as a 200 ky record from Mauna Loa. The isotopic stratigraphy of the core samples (Gcubed Theme) reflects the geochemical structure of the Hawaiian plume, given a model for the sampling of the plume by melting and melt transport (Bryce et al., 2005). The data show that there is radial geochemical zoning of the plume in terms of He, Pb, Nd, Sr and Hf isotopes. Data from other volcanoes indicate there is also heterogeneity along the axis of the plume, as well as asymmetry (Loa-Kea dichotomy). To first order, the radial geochemical structure of the plume represents the vertical structure at the thermal boundary layer from which the plume originates. Numerical models of plumes show that this vertical structure, which corresponds to potential temperature, is likely to be preserved during passage of the plume through the mantle. In the case of Hawaii, all of the lavas are derived from melting of mantle that originates from within 20–50 km of the base of the mantle, so the inferred radial geochemical structure maps to stratigraphy at the base of the mantle. HSDP data indicate that the high He/He anomaly (R/ Ra > 16) is restricted to the innermost core of the plume and is much larger in amplitude and smaller in diameter than the Nd, Sr or Hf anomalies. The He-3 anomaly must have an origin different from that of other isotopes; the anomalous mantle is restricted to the lowermost 10–20 km of the plume source. This observation accords with along-ridge variations of Nd, Sr, and He near Iceland. The HSDP data are consistent with two models for the configuration of the plume source. If the plume originates from the top of a dense layer separating the main mantle from the outer core, then the high-He signal must be attributed to the dense layer and distinguishes it from the main lower mantle. The dense layer is not significantly different from the rest of the lower mantle in terms of Nd, Sr, or Hf isotopes. If the plume originates directly from the CMB, then probably the He signal comes from the core. Models for core formation can accommodate this possibility. The dense layer may have distinctive He because it receives He from the outer core, or because it has retained primordial He. Hawaii and Iceland data imply that the lower mantle also has large regions with elevated He/He, but these typically have R/Ra 6 16.

Stable isotopes of oxygen and hydrogen were measured in pore fluid extracted from a sediment core located in a relatively undisturbed area in the Hanford site near the S-SX Tank Farms. Pore fluids from most of the 70-meter thick vadose zone section have ?18O values that are shifted to higher values than those for winter precipitation (and Columbia River water) by 3 to 4?. The shift of ?18O, and the ?18O-?D slope of about 4 in the deep vadose zone pore fluids is attributed to partial evaporation during residence in the upper meter of the soil section. A model relating the isotopic shift to recharge and soil properties suggests that the shift should be inversely proportional to recharge, and larger for coarser soils with lower water retention. When applied to Hanford soils and precipitation patterns, the model predicts that vadose zone pore fluids at Hanford should typically be shifted by +2? to +6? in ?18O relative to the values in wet season precipitation, even for relatively high values of annual net infiltration (up to 100 mm/yr or 60% of annual precipitation). The model has implications for groundwater as well as vadose zone ?18O. The effects of vegetation are not included,more » so only upper limit values for the net infiltration flux can be inferred from vadose zone ?18O. The shifted ?18O of natural pore fluids allows identification of the presence of subsurface water that comes from industrial discharges at the Hanford site. An example is provided by a low-?18O, high water content horizon at a depth of 44m in the core, which is interpreted as industrial water that was transported laterally above a capillary barrier as a result of a nearby, near-surface point discharge that happened within the past 50 years.« less

Hot spot volcanic chains are a fundamental feature of the Earth's crust, but their origins are still poorly understood [Okal and Batiza, 1987]. The Hawaiian-Emperor volcanic chain, which dominates the topography of the central Pacific ocean floor, is the best developed and most intensely studied of the known hot spot tracks. It continues to be one of the world's most important field laboratories for the study of igneous processes, plate movements, mantle convection, structure, geochemical evolution, and the properties of the lithosphere. Despite continued effort, fundamental questions regarding the composition, structure, and evolution of Hawaiian volcanos and their magma sources remain unanswered. This is largely due to the fact that only lavas representing the late stages in the evolution of the volcanos can be sampled at the surface. Most of the internal structure of the volcanos and evidence of their growth history and geochemical evolution are hidden from view. The most deeply eroded volcanos are exposed only to depths of a kilometer or so, whereas the volcanos rise some 5–15 km above the old ocean floor [Moore, 1987].

The provenance of fore-arc sandstones of the Upper Cretaceous Great Valley Group was investigated by using measurements of Sm-Nd and Rb-Sr isotope ratios and major and trace element concentrations. The objective was to elucidate the erosional history of the adjacent Sierra Nevada arc and its relation to magmatism and fore-arc sedimentation. Relatively detailed provenance information is obtainable because the currently exposed plutonic rocks of the arc (and, by inference, the Cretaceous volcanic rocks) have large geographic gradients in isotopic composition, mainly from west to east across the arc. The relation of ϵ Nd to ϵ Sr and to major and trace elements in Great Valley sandstones is the same as in the plutonic rocks of the Sierra Nevada; this observation confirms that the arc was the primary source of sediment and that the chemistry of the detritus was not significantly disturbed by sedimentary processes. With decreasing ϵ Nd , there is an increase in K 2 O, Rb, Pb, Ba, La, and U, and MgO, Ni, and Cr decrease. The ϵ Nd of sandstones decreases with decreasing depositional age from -0.7 to -5.0 in 97 to 73 Ma strata. The ϵ Nd and age of the sandstones closely match those of the plutonic rocks of the Sierra Nevada. This observation indicates that the location of the mean sandstone source and the active volcanic front were nearly coincident in time and space; thus, the isotopic compositions of the sandstones directly record the temporal and geographic variations in arc magmatism. The volcanic "roof" of the Sierra Nevada arc was evidently chemically and isotopically equivalent to its plutonic roots. Prearc metasedimentary rocks did not significantly contribute to the isotopic or geochemical composition of the fore-arc sandstones.

Samarium-neodymium isotope systematics provide a means of determining the age of the continental crust, where ”age“ refers to the amount of time the crustal rock material has been isolated from the convecting mantle. This age is referred to as the Sm-Nd model age or the mantle separation age. Systematic analysis of rocks from the continents can be used to determine the age distribution by mass in the continents; this is a unique record of the thermal and chemical evolution of Earth over almost 4 b.y. The establishment of mantle separation ages for crustal rock reservoirs requires precise geochronological information typically provided by U-Pb isotopic measurements of zircon, plus models for (1) Nd isotopic evolution in the mantle source of continental crust, (2) Nd isotopic evolution of crustal reservoirs subsequent to formation, (3) petrogenesis of certain types of granitic rocks in the crust, (4) the mechanisms of formation of hybrid crust (mixtures of material newly differentiated from the mantle with material derived from older crust), and (5) the evolution of the lower parts of the continental crust during continental margin magmatism and intracontinental extension. Methods are presented for treating isotopic data on continental rock materials to obtain meaningful mantle separation ages. The greatest uncertainties come from a lack of adequate information on lower crustal processes and composition. The methods are applied to the Precambrian and Mesozoic rocks of the southwestern United States to produce a mass-age distribution for the region, which represents 1% of the global continental mass. The results suggest episodic crustal growth, with short growth periods at circa 2.8, 1.8, and 0.1 Ga. About 90% of the crust was formed by 1.8 Ga, the remaining 10% was added in the Phanerozoic. The mean age of this section of continent is determined to be 1.84 Ga. Almost all of the Phanerozoic additions occur in areas where there was no preexisting continental basement; Mesozoic magmatic additions account for only about 2% of the crustal mass in the area underlain by Precambrian basement. In the region of post-Precambrian continental growth, the regions farthest from the Precambrian continental edge are composed of mostly (>85%) of material newly differentiated from the mantle. Adjacent to the Precambrian continental edge the new crust is typically composed about 50% of material newly differentiated from the mantle and 50% of old crustal material. There is no correlation between crustal thickness and the crust formation age in the area studied; the age-area and age-mass curves are nearly identical.

In this paper we present geologic mapping, K-Ar chronology, major and trace element data, mineral chemistry, and Nd, Sr, and O isotopic data for volcanic rocks of the Mount Taylor volcanic field (MTVF). The MTVF lies on the tectonic boundary between the Basin and Range province and the southeastern Colorado Plateau and is dominated by Mount Taylor, a composite volcano active from ≈ 3 to 1.5 m.y. ago. Growth of the volcano began with eruption of rhyolite, followed by quartz latite and finally latite. Basalts erupted throughout the lifetime of the volcano. The compositional variations of MTVF rocks resulted primarily from fractional crystallization of mildly alkaline hy-hawaiite magmas. Differentiation of magmas occurred in multiple, short-lived magma chambers, a result of a low magma flux from the mantle that could not support a single, long-lived chamber. Rare mixing of evolved hy-hawaiite and rhyolite produced a few intermediate magmas, primarily in the early history of the field. Mixing may have occurred when rhyolite magmas in the lower crust ascended to upper crustal levels and were injected into the bases of mafic magma chambers. Small amounts of crustal assimilation accompanied fractional crystallization and affected all the evolved MTVF rocks. Assimilation/fractional crystallization occurred primarily in the lower crust as hy-hawaiite differentiated to mugearite or latite. Early in the history of the field, evolved lower crustal magmas ascended into the upper crust, where density filtering and a reduced tensional stress field inhibited further ascent until magmas evolved to rhyolite or quartz latite. Later in the history of the field, latite magmas ascended directly from the lower crust and erupted without further significant differentiation because of increased crustal extension.

Regional patterns of Nd, Sr, and O isotopic ratios of ∼500 Ma granitic rocks are used to identify the ages and areal extents of three crustal provinces in the central Transantarctic Mountains. One of the provinces is the edge of the East Antarctic Craton, which isotopic analyses show is composed of Archean rocks thrust over Proterozoic rocks. The other two provinces compose the Beardmore microcontinent, which we deduce was allochthonous to East Antarctica and was emplaced in late Precambrian or early Paleozoic time. Evidence for a former ocean basin between the Beardmore microcontinent and East Antarctica is provided by basalt and gabbro of mid-ocean ridge character, dated by Sm-Nd at ∼760 Ma, associated with marine sediments now located at the suture. The granitic rocks formed over a westward-dipping subduction zone that was active at ∼500 Ma. The East Antarctic Craton is exposed in the Miller Range, which is a tectonic composite of reworked Archean and early Proterozoic material containing ∼500 Ma peraluminous granites with model ages (TDM) of 2.0 Ga, high δ18O (+11 to +12‰) and high initial 87Sr/86Sr (0.7324 to 0.7417). East of the Marsh Glacier the granitic rocks are metaluminous to weakly peraluminous with model ages of 1.3 to 1.8 Ga, high δ18O (+9 to +13‰) and lower 87Sr/86Sr (0.7068 to 0.7191). East of the Shackleton Glacier, gabbro, tonalite, diorite, and granodiorite have low δ18O (+6 to +7‰), low initial 87Sr/86Sr (0.7045 to 0.7059) and high ϵNd (+0.4 to +1.7). These isotopic provinces correspond to differences in age and composition of the middle and lower crust at the time of formation of the granitic magmas. The boundaries of the isotopic provinces also correspond to discontinuities in provenance, lithology, structural style, and grade of metamorphism of prebatholithic metasedimentary rocks. The isotopic data indicate that the granitic magmas were formed mostly by crustal anatexis in the areas west of the Shackleton Glacier. This is typical of early Paleozoic granitic batholiths elsewhere in the world and has led to speculation that subduction was not involved in granitic magmatism at that time of earth history. However, the granitic rocks located west of the Shackleton Glacier, by virtue of their mantle-like isotopic compositions and their association with metavolcanic rocks, appear to be subduction-related. The tectonic history deduced for the Gondwana margin, as represented in the central Transantarctic Mountains, began with deposition of sediments on an Atlantic-type rifted margin at ∼760 Ma. The Beardmore microcontinent was most likely accreted in association with folding of the clastic sedimentary rocks before middle Early Cambrian time (550 Ma). Carbonate sedimentation and volcanism along the eastern margin of the Beardmore microcontinent commenced in Cambrian time. Folding and metamorphism of all older units occurred in late Cambrian time followed by emplacement of granitic rocks at ∼500 Ma.

The K-Ca radioactive parent-daughter system provides a tool for tracing the origins of igneous rocks. It is complementary to other isotopic systems because as stoichiometric constituents of major minerals, the concentrations of K and Ca, and the K/Ca ratio in rocks, are simply related to mineralogy. In this paper we report the first high-precision calcium isotopic analyses of continental granitic rocks, island arc rocks, and mid-ocean ridge basalts. These data show that mid-ocean ridge basalts have the low 40 Ca 42 Ca ratios expected for the Earth's mantle, but that island arc rocks have slightly higher 40 Ca 42 Ca ratios indicative of crustal calcium in their magma sources. Many granitic rocks have high initial 40 Ca 42 Ca ratios, and in conjunction with independent evidence for the age of the crustal sources, these ratios provide constraints on the K/Ca ratios, and in turn on the silica contents and residual mineralogy, of the deep crustal magma sources.

High precision mass spectrometric determination of calcium isotope ratios allows the 40K → 40Ca radioactive decay to be used for dating a much broader range of geologic materials than is suggested by previous work. 40Ca42Ca is used to monitor enrichments in 40Ca and can be measured to ±0.01% (2σ) using an exponential mass discrimination correction (Russell et al., 1978) and large ion currents. The earth's mantle has such a low KCa (∼0.01) that it has retained “primordial” 40Ca42Ca = 151.016 ± 0.011 (normalized to 42Ca44Ca = 0.31221), as determined by measurements on two meteorites, pyroxene from an ultramafic nodule, metabasalt, and carbonatite. 40Ca42Ca ratios can be conveniently expressed relative to this value as ϵCa in units of 10−4. To test the method for age dating, a mineral isochron has been obtained on a sample of Pikes Peak granite, which has been shown to have concordant KAr, RbSr, and UPb ages. Plagioclase, K-feldspar, biotite, and whole rock yield an age of 1041 ± 32 m.y. (2σ) in agreement with previous age determinations (λK = 0.5543 b.y.−1, λβ−λK = 0.8952, 40K = 0.01167%). The initial 40Ca42Ca of 151.024 ± 0.016 (ϵCa = +0.5 ± 1.0), indicates that assimilation of high K/Ca crust was insufficient to affect calcium isotopes. Measurements on two-mica granite from eastern Nevada indicate that the magma sources had K/Ca ≈ 1, similar to intermediate-composition crustal rocks. These results show that the KCa system can be used as a precise geochromometer for common felsic igneous and metamorphic rocks, and may prove applicable to sedimentary rocks containing authigenic K minerals. The relatively short half-life of 40K, the non-volatile daughter, and the fact that potassium and calcium are stoichiometric constituents of many minerals, make the KCa system complementary to other dating methods, and potentially applicable to a variety of geologic problems.

The results of isotopic and trace-element-abundance analyses of Ir-enriched Cretaceous-Tertiary-boundary clay sediments from Caravaca, Spain, and of adjacent carbonate and marl layers, are presented. Acetic-acid and HCl leachates and residues were analyzed by isotope dilution to determine K, Rb, Sr, Sm, and Nd concentrations and Sr-87/Sr-86 and Nd-143/Nd-144 ratios. The stable isotope ratios delta-D, delta-(C-13), and delta-(0-18) were also determined. The results are presented in tables and graphs and compared with published data on the Caravaca sediments and on samples from other locations. The boundary clay is found to be distinguished from the adjacent layers by its isotopic ratios and to be of mainly terrestrial, lithospheric (deeper than 3-km) origin. Although submarine-weathering effects are evident and difficult to quantify, the degree of variation in Ni, Ir, Sr, and REE concentrations is considered too large to be attributed to postdepositional processes alone. These findings are seen as evidence for the ocean impact of a large single asteroid producing a worldwide blanket of ejecta, a large injection of water vapor into the atmosphere, and perhaps a gigantic tsunami, at the end of the Cretaceous period.

Equations describing trace element and isotopic evolution in a magma chamber affected simultaneously by fractional crystallization and wallrock assimilation are presented for a model where the mass assimilation rate(Ṁa) is an arbitrary fraction(r) of the fractional crystallization rate(Ṁc). The equations also apply to recharge of a crystallizing magma. Relatively simple analytical expressions are obtained for both radiogenic isotope variations (Nd, Sr, Pb) and stable isotopes (O, H) including the effects of mass-dependent fractionation. Forr = 1 a modified zone refining equation is obtained for trace element concentrations, but forr < 1 behavior is a combination of zone refining and fractional crystallization. Asr → ∞, simple binary mixing is approached. The isotopic and trace element “mixing” trends generated can be much different from binary mixing, especially forr < 1. The model provides the basis for a more general approach to the problem of wallrock assimilation, and shows that binary mixing models are insufficient to rule out crustal assimilation as a cause of some of the isotopic variations observed in igneous rocks, including cases where clustering of isotopic values occurs partway between presumed endmember values. The coupled assimilation-fractional crystallization model provides an explanation for certain trace element and isotopic properties of continental margin orogenic magmas (e.g. Sr concentration versus87Sr/86Sr) which had previously been interpreted as evidence against assimilation. So-called “pseudoisochrons” can be understood as artifacts of contamination using this model. A significant correlation exists between country rock age and low143Nd/144Nd ratios in continental igneous rocks, clearly suggestive that crustal contamination is generally important.

An analysis of the initial neodymium and hafnium isotopic compositions of modern and ancient mantle-derived rocks shows that the divergence of the mantle 143Nd/144Nd and 176Hf/177Hf ratios from the chondritic ratios over the past 2 billion years has been much less than would be expected based on the Sm/Nd and Lu/Hf ratios of the mantle source of mid-ocean ridge basalt. The most likely explanation is that rock material from the continents has been recycled semi-continuously back into the mantle over this time period. The required rate of return of continents to the mantle, calculated from a simple transport model, is about 0.35±0.15 of the continental mass per billion years (∼2.5 km³/year). This result is consistent with the hypothesis that the mass of the continents has been approximately constant since the late Archean, and implies that recycled continental material must be an important component of the upper mantle.