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The relative abundances of calcium isotopes in the mass range 40-44 were measured in primitive and differentiated meteorites and igneous rocks from Earth and Mars in search of non-mass-dependent variations that could provide clues about early solar system processes. Most bulk samples of planetary materials have calcium isotopic compositions identical with Earth's within the current resolution of about 0.01% in 40Ca/44Ca. Possible exceptions include carbonaceous chondrites, some ordinary chondrites, and two samples of calcium-aluminum-rich inclusions, which have small excesses of 40Ca. The samples with 40Ca excesses are also known to have 50Ti and 135Ba excesses and 142Nd and 144Sm deficits. Collectively these data from refractory elements suggest that the planetary embryos represented by chondrites preserve isotopic heterogeneity that reflects different nucleosynthetic sources. No late admixture from a single nucleosynthetic source can explain all observations. The results are most compatible with variable proportions of material derived from Type II supernovae. The initial calcium isotope compositions of Earth and Mars are indistinguishable and similar to the 40Ca abundance found in some chondrites and all differentiated meteorites studied. It appears that isotopic heterogeneity in calcium was still present at the completion of disk formation but was homogenized during planetary accretion.

The lunar meteorite Northwest Africa (NWA) 032 is a low-Ti basalt that has incompatible-element abundances and Th/Sm ratios characteristic of the involvement of late stage magma ocean crystallization products (urKREEP) in its petrogenesis. This sample is very fine-grained and contains terrestrial weather products. A progressive leaching procedure was therefore developed and applied to magnetic separates and whole rock fractions to obtain Rb–Sr and Sm–Nd ages. Although many of the leachates, as well as the unleached mineral and whole rock fractions contain terrestrial alteration products, selected residue fractions yield concordant Rb–Sr and Sm–Nd ages. Rubidium–Sr isotopic analyses yield an age of 2947 ± 16 Ma with an initial 87 Sr/ 86 Sr of 0.700057 ± 17. These characteristics indicate NWA 032 is derived from a source region with an 87 Rb/ 86 Sr ratio of 0.044 ± 0.001. This value is higher than all but those determined for KREEP basalts, and suggests that NWA 032 is derived from a source region that has higher incompatible-element abundances than other low-Ti basalts. Samarium–neodymium isotopic analysis yield a concordant age of 2931 ± 92 Ma and an initial e Nd of +9.71 ± 0.74 corresponding to a source region with 147 Sm/ 144 Nd ratio of 0.246 ± 0.004. The initial Nd isotopic composition stands in contrast to the initial Sr isotopic composition by requiring NWA 032 to be derived from a source with lower incompatible-element abundances than most low-Ti basalts. The source of NWA 032 is therefore unlike those of other lunar basalts. Modeling of magma ocean cumulate formation demonstrates that unlike other low-Ti basalt source regions the NWA 032 source is a mixture of olivine, pigeonite, and clinopyroxene bearing cumulates and only a small amount of urKREEP. Furthermore, unlike other mare basalt sources, the NWA 032 source does not contain appreciable quantities of plagioclase. Partial melting models demonstrate that the incompatible-element characteristics of the NWA 032 result from formation by smaller degrees of partial melting than other mare basalts. Thus, the incompatible-element geochemical signature that is observed in NWA 032 appears to reflect the combined effects of generation from an unusual plagioclase-free incompatible-element-depleted source region by very small degrees of partial melting. This study demonstrates that both the presence of urKREEP in the source region and small degrees of partial melting generate magmas with similar, but not identical, incompatible-element characteristics. In addition, it underscores the fact that there is significantly more geochemical diversity on the Moon than is represented by samples collected by the American and Soviet lunar missions.

Velika Palagruža (Pelagosa) is the largest island of the Palagruža archipelago (central Adriatic Sea, Croatia). Despite its minute size the island bears a certain geological interest being the only exposed piece of land in the central part (Mid-Adriatic ridge) of the common Adriatic foreland of the Apenninic and the Dinaridic orogenic domains. The litho-, bio-, and chemostratigraphic (strontium and sulphur isotopes) characteristics of the sedimentary units, along with tectono-structural and geomorphic characteristics of the island, are described in this paper. The oldest Žalo unit is composed of highly deformed siliciclastics containing gypsum, and carbonates of Middle Triassic (Ladinian) age. This unit represents a transitional fl uvial-to-shallow marine, occasionally evaporitic environment, typical of the Middle Triassic rifting phase of the Adriatic microplate. Soft and strongly deformed Žalo unit deposits are found along a probably still active, WNW–ESE striking, subvertical, oblique-slip fault that crosses the entire length of the island. The Žalo unit is probably in diapiric contact with the Lanterna unit, poorly defi ned as Late Triassic, and characterized by dolomite with chert and dolomite breccia, presumably deposited in a transitional platform-to-basin environment of an evolving Adriatic basin. The Lanterna unit deposits are capped by Miocene biocalcarenites of the Salamandrija unit over an almost perpendicular discordance, possibly representing an unconformity, suggesting that an early deformational phase preceded a Miocene marine transgression. Talus, landslide deposits, and humic soil make up the cover of the bedrock sedimentary succession, and they represent the ultimate phase of emersion of the island, which probably occurred during Pliocene(?) to Quaternary times. An active neotectonic regime of the central Adriatic is evidenced by present-day seismicity, while recent uplifting of the island is shown by the presence of remnants of pebbly palaeobeach deposits, marine (erosional) straths, and cyanobacterial supratidal encrustations (pelagosite) currently observed at various elevations above mean sea level.

Understanding the complex interactions between pressurized fluids and porous rock is a key prerequisite to the safe sequestration of carbon dioxide and radioactive waste

Amphitheater-headed canyons have been used as diagnostic indicators of erosion by groundwater seepage, which has important implications for landscape evolution on Earth and astrobiology on Mars. Of perhaps any canyon studied, Box Canyon, Idaho, most strongly meets the proposed morphologic criteria for groundwater sapping because it is incised into a basaltic plain with no drainage network upstream, and approximately 10 cubic meters per second of seepage emanates from its vertical headwall. However, sediment transport constraints, 4 He and 14 C dates, plunge pools, and scoured rock indicate that a megaflood (greater than 220 cubic meters per second) carved the canyon about 45,000 years ago. These results add to a growing recognition of Quaternary catastrophic flooding in the American northwest, and may imply that similar features on Mars also formed by floods rather than seepage erosion.

We measured Ca stable isotope ratios (δ 44/40 Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m −2 ) is present as sulfates (2.5 kmol m −2 ), and to a lesser extent carbonates (0.4 kmol m −2 ). In aqueous extracts of variably hydrated calcium sulfate minerals, δ 44/40 Ca E values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ 44/40 Ca in the top six horizons resembles that of sulfate-δ 44/40 Ca, but with values 0.1–0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ 44/40 Ca values observed on Earth. Linear correlation among δ 44/40 Ca, δ 34 S and δ 18 O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ 34 S values are positively correlated with sulfate-δ 18 O values ( R 2 = 0.78) and negatively correlated with sulfate δ 44/40 Ca E values ( R 2 = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ 44/40 Ca fractionation factor of −0.4‰ in CaSO 4 . The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO 4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events.

Processes of fluid flow and chemical transport through the vadose zone can be characterized through the isotopic systematics of natural soils, minerals, pore fluids, and groundwater. In this contribution, we first review our research using measured isotopic variations, due both to natural and site-related processes, of the elements H, O, N, Sr, and U, to study the interconnection between vadose zone and groundwater contamination at the Hanford Site in south-central Washington State. We follow this brief review with a presentation of new data pertaining to vadose zone and groundwater contamination in the Waste Management Area (WMA) T-TX-TY vicinity. Uranium isotopic data for the C3832 core (WMA TX) indicate the involvement of processed natural U fuel and link the observed U contamination to releases near single-shelled tank TX-104. The data also preclude contamination from an early 1970s TX-107 leak. In the case of the C4104 core (WMA T), the U isotopic data indicate a mixture of processed natural and enriched U fuels consistent with the major leak from T-106 in 1973. Uranium and strontium isotopic data for the cores also provide direct evidence for chemical interaction between high-pH waste fluid and sediment. Isotopic data (δ15N and δ18O) for groundwater nitrate contamination in multidepth samples just to the northeast of WMA T are distinct from that seen in surrounding wells and suggest tank waste (possibly from the 1973 T-106 event) as a source of very high 99Tc concentrations recently observed at the northeast corner of WMA T.

The calcium isotopic compositions (δ44Ca) of 30 high-purity nannofossil ooze and chalk and 7 pore fluid samples from ODP Site 807A (Ontong Java Plateau) are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor (αs−f) between calcite and dissolved Ca2+ and the rates of post-depositional recrystallization in deep sea carbonate ooze. The value of αs−f at equilibrium in the marine sedimentary section is 1.0000 ± 0.0001, which is significantly different from the value (0.9987 ± 0.0002) found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean. We hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for Ca isotope fractionation during calcite precipitation. We describe a steady state model that offers a unified framework for explaining Ca isotope fractionation across the observed precipitation rate range of ∼14 orders of magnitude. The model attributes Ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface. This model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation. The Ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly-deposited carbonate ooze is 30–40%/Myr, and decreases with age to about 2%/Myr in 2–3 million year old sediment. The recrystallization rates determined from Ca isotopes for Pleistocene sediments are higher than those previously inferred from pore fluid Sr concentration and are consistent with rates derived for Late Pleistocene siliciclastic sediments using uranium isotopes. Combining our results for the equilibrium fractionation factor and recrystallization rates, we evaluate the effect of diagenesis on the Ca isotopic composition of marine carbonates at Site 807A. Since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite, the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation. Accordingly, diagenesis produces a maximum shift in δ44Ca of +0.15‰ for Site 807A sediments but will have a larger impact where sedimentation rates are low, seawater circulates through the sediment pile, or there are prolonged depositional hiatuses.

Zoisite-fluid trace element partition coefficients have been determined and are used to model fluids generated during the breakdown of hydrous phases in a subducting slab. Partition coefficients were determined for Rb, Ba, Th, U, Nb, Ta, Pb, Sr, and nine rare earth elements (REE) at 750–900 °C and 2.0 GPa. Our results show that Sr and Pb are extremely compatible in zoisite relative to other high-pressure phases (DSrN100, DPbN10). The zoisite-fluid partition coefficients are combined with previously determined partition coefficients for eclogite-facies minerals (garnet, cpx, rutile, phengite) to generate reaction-specific partition coefficients as well as bulk eclogite-fluid partition coefficients. Batch equilibrium models are then used to approximate the conditions of fluid formation in the slab. Two kinds of fluids are modeled — 1) fluids resulting from specific zoisite breakdown reactions, and 2) combined fluids from a variety of continuous and discontinuous reactions that have had the opportunity to equilibrate with a residual zoisite-bearing eclogite assemblage. We find that fluids generated during individual reactions have much more extreme trace element enrichments than those that have equilibrated with the eclogitic slab as a whole. In particular, fluids generated during zoisitebreakdown reactions tend to be strongly enriched in Pb and Sr. Fluids of extreme composition from individual dehydration reactions maybepreservedasfluidinclusionsandveins inexhumedeclogites.Fluidsthathaveequilibratedwithzoisite-bearingeclogitehavea more moderate trace element enrichment pattern that is more consistent with the fluid component sampled by island arc basalts (IAB) in the zone of melting beneath the volcanic front. We propose that fluids generated in the slab, which may initially have extreme patterns of trace element enrichment and fractionation, and mayshow extreme variation on asmall spatial scale, are ultimately at least partially equilibrated with the bulkeclogite. Subsequent mixing with partial melts of the sediment layer and exchange with the mantle wedge en route to the melting region serve to further homogenize the fluids. Therefore, we expect that “metamorphic” subduction zone fluids recorded by inclusions and veins in eclogite will differ quite markedly in composition from “metasomatic” subduction zone fluids that contribute to the formation of island arc basalts, even though their origin may be the same. © 2007 Elsevier B.V. All rights reserved.

This paper presents new U–Th/He crystallization and 3 He exposure ages on olivine for a suite of basalt samples from the Snake River Plain, Idaho. The results provide the first demonstration that U–Th/He geochronology can be successfully applied to Late Quaternary basalts using measurements of olivine phenocrysts as opposed to U-rich minerals. The approach employs abrasion to remove the outer rinds of olivine crystals to avoid problems with 4 He implantation from the groundmass, and a specially designed extraction system that allows U, Th and He concentrations and isotopic compositions to be measured on the same olivine grains. We demonstrate the viability of the U–Th/He technique on basalts with ages of 373 ka and 3400 ka previously dated by Ar–Ar, and show that concurrent measurements of U–Th– 4 He ages and 3 He cosmogenic exposure ages improve the accuracy of both methods. The combinedU–Th– 4 He– 3 HetechniqueisappliedtoaseriesofbouldersandflowsfromBoxCanyonnearHagerman,Idaho,toevaluate the ages of the basalts and to use exposure ages to make inferences about the age and mode offormation of the canyon. The U–Th/He ages of the Box Canyon basalts are between 86±12 ka and 130±12 ka (1-sigma errors) and suggest at least two periods of eruption. The ages are similar to a ca. 95 ka Ar–Ar age previously assigned, and distinguish the Box Canyon basalts from nearby ca. 395 ka basalt. Exposure ages of boulders within the canyon at the mouth, head and middle of Box Canyon are 21±1, 19±3 and 48±3 ka; scoured bedrock at the canyon head has an exposure age of 45±5 ka. These data indicate that the canyon was carved before the

Mantle-derived magmas at convergent plate boundaries provide unique insights into the nature of materials subducted to and recycled from depths. Here we present a study of Os–Sr–Mg isotopes on the Oligocene–Miocene ultrapotassic rocks aimed at better understanding sediment subduction and recycling beneath southern Tibet. New isotopic data confirm that ultrapotassic rocks in southern Tibet are of mantle origin, but underwent crustal contamination as evidenced by the variably high 187Os/188Os that obviously deviates from normal mantle reservoir. Still some samples with mantle-like 187Os/188Os exhibit δ26Mg significantly lower than mantle and crustal lithologies, suggesting that the isotopically light Mg may not result from crustal contamination but retain specific fingerprint of carbonate-related metasomatism in mantle sources. Mantle carbonatite metasomatism is manifested by the inverse δ26Mg–87Sr/86Sr correlations, as well as the depletion of high field strength elements relative to rare earth elements and the enrichment of CaO in ultrapotassic rocks. The positive co-variations between δ26Mg and Hf/Sm defined by those low-187Os/188Os ultrapotassic rocks provide evidence for the potential of recycled dolomites to modify mantle Mg isotopic composition. The correlated spatial variations of δ26Mg and Hf/Sm are interpreted to reflect carbonatitic metasomatism associated with the northward subduction of the Neo-Tethyan oceanic slab and its profound influence on postcollisional ultrapotassic magmatism.

Carbon capture and storage (CCS) is likely to be a critical technology to achieve large reductions in global carbon emissions over the next century. Research on the subsurface storage of CO2 is aimed at reducing uncertainties in the efficacy of CO2 storage in sedimentary rock formations. Three key parameters that have a nanoscale basis and that contribute uncertainty to predictions of CO2 trapping are the vertical permeability kv of seals, the residual CO2 saturation Sg,r in reservoir rocks, and the reactive surface area ar of silicate minerals. This review summarizes recent progress and identifies outstanding research needs in these areas. Available data suggest that the permeability of shale and mudstone seals is heavily dependent on clay fraction and can be extremely low even in the presence of fractures. Investigations of residual CO2 trapping indicate that CO2-induced alteration in the wettability of mineral surfaces may significantly influence Sg,r. Ultimately, the rate and extent of CO2 conversion to mineral phases are uncertain due to a poor understanding of the kinetics of slow reactions between minerals and fluids. Rapidly improving characterization techniques using X-rays and neutrons, and computing capability for simulating chemical interactions, provide promise for important advances.

Current issues with carbon emissions need to be understood in terms of natural geologic processes that move carbon on the Earth. Comparison of modern emissions with the norms and extremes of natural processes emphasizes the enormity of the current challenge, and also the reason there are uncertainties about the future effects. Reaching sustainable emissions in the future can be viewed as a need to systematically reduce the carbon intensity of energy production. Achieving sustainable carbon emissions requires understanding of Earth’s natural carbon cycles. Geologic processes move carbon in large quantities between Earth reservoirs, including in and out of the deeper reaches of the planet, and regulate Earth’s surface temperature within a narrow range suitable for life for the past 3–4 billion years. There have been large changes in atmospheric CO2 in the geologic past; the largest to offset changes in the brightness of the Sun. Atmospheric CO2 has been much higher in the past, but not since humans evolved. Geologic processes act slowly, even during times in the geologic past regarded as examples of catastrophic climate change. In contrast, over the past 100 years, Earth’s carbon cycles have undergone revolutionary change as a result of a greatly accelerated transfer of carbon from geologic storage to the atmosphere. Today, about 98% of the movement of carbon out of geologic reservoirs (coal-, oil-, and gas-bearing sedimentary rocks and limestone) into the atmosphere is due to human activities; the total carbon flux is 40–50 times the geologic flux. The extremely large modern carbon flux is unprecedented in Earth history. Returning to a sustainable carbon cycle requires systematic lowering of the carbon emission intensity of energy production over the next century.

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Both chemical weathering rates and fluid flow are difficult to measure in natural systems. However, these parameters are critical for understanding the hydrochemical evolution of aquifers, predicting the fate and transport of contaminants, and for water resources/water quality considerations. 87 Sr/ 86 Sr and ( 234 U/ 238 U) activity ratios are sensitive indicators of water–rock interaction, and thus provide a means of quantifying both flow and reactivity. The 87 Sr/ 86 Sr values in ground waters are controlled by the ratio of the dissolution rate to the flow rate. Similarly, the ( 234 U/ 238 U) ratio of natural ground waters is a balance between the flow rate and the dissolution of solids, and α-recoil loss of 234 U from the solids. By coupling these two isotope systems it is possible to constrain both the long-term (ca. 100’s to 1000’s of years) flow rate and bulk dissolution rate along the flow path. Previous estimates of the ratio of the dissolution rate to the infiltration flux from Sr isotopes ( 87 Sr/ 86 Sr) are combined with a model for ( 234 U/ 238 U) to constrain the infiltration flux and dissolution rate for a 70-m deep vadose zone core from Hanford, Washington. The coupled model for both ( 234 U/ 238 U) ratios and the 87 Sr/ 86 Sr data suggests an infiltration flux of 5 ± 2 mm/yr, and bulk silicate dissolution rates between 10 −15.7 and 10 −16.5 mol/m 2 /s. The process of α-recoil enrichment, while primarily responsible for the observed variation in ( 234 U/ 238 U) of natural systems, is difficult to quantify. However, the rate of this process in natural systems affects the interpretation of most U-series data. Models for quantifying the α-recoil loss fraction based on geometric predictions, surface area constraints, and chemical methods are also presented. The agreement between the chemical and theoretical methods, such as direct measurement of ( 234 U/ 238 U) of the small grain size fraction and geometric calculations for that size fraction, is quite good.

The 87Sr/86Sr ratios and Sr concentrations in sediment and pore fluids are used to evaluate the rates of calcite recrystallization at ODP Site 807A on the Ontong Java Plateau, an 800-meter thick section of carbonate ooze and chalk. A numerical model is used to evaluate the pore fluid chemistry and Sr isotopes in an accumulating section. The deduced calcite recrystallization rate is 2% per million years (%/Myr) near the top of the section and decreases systematically in older parts of the section such that the rate is close to 0.1/age (in years). The deduced recrystallization rates have important implications for the interpretation of Ca and Mg concentration profiles in the pore fluids. The effect of calcite recrystallization on pore fluid chemistry is described by the reaction length, L, which varies by element, and depends on the concentration in pore fluid and solid. When L is small compared to the thickness of the sedimentary section, the pore fluid concentration is controlled by equilibrium or steady-state exchange with the solid phase, except within a distance L of the sediment–water interface. When L is large relative to the thickness of sediment, the pore fluid concentration is mostly controlled by the boundary conditions and diffusion. The values of L for Ca, Sr, and Mg are of order 15, 150, and 1500 meters, respectively. LSr is derived from isotopic data and modeling, and allows us to infer the values of LCa and LMg. The small value for LCa indicates that pore fluid Ca concentrations, which gradually increase down section, must be equilibrium values that are maintained by solution-precipitation exchange with calcite and do not reflect Ca sources within or below the sediment column. The pore fluid Ca measurements and measured alkalinity allow us to calculate the in situ pH in the pore fluids, which decreases from 7.6 near the sediment–water interface to 7.1 ± 0.1 at 400–800 mbsf. While the calculated pH values are in agreement with some of the values measured during ODP Leg 130, most of the measurements are artifacts. The large value for LMg indicates that the pore fluid Mg concentrations at 807A are not controlled by calcite-fluid equilibrium but instead are determined by the changing Mg concentration of seawater during deposition, modified by aqueous diffusion in the pore fluids. We use the pore fluid Mg concentration profile at Site 807A to retrieve a global record for seawater Mg over the past 35 Myr, which shows that seawater Mg has increased rapidly over the past 10 Myr, rather than gradually over the past 60 Myr. This observation suggests that the Cenozoic rise in seawater Mg is controlled by continental weathering inputs rather than by exchange with oceanic crust. The relationship determined between reaction rate and age in silicates and carbonates is strikingly similar, which suggests that reaction affinity is not the primary determinant of silicate dissolution rates in nature.

High precision uranium isotope measurements of marine clastic sediments are used to measure the transport and storage time of sediment from source to site of deposition. The approach is demonstrated on fine-grained, late Pleistocene deep-sea sediments from Ocean Drilling Program Site 984A on the Bjorn Drift in the North Atlantic. The sediments are siliciclastic with up to 30% carbonate, and dated by δ18O of benthic foraminifera. Nd and Sr isotopes indicate that provenance has oscillated between a proximal source during the last three interglacial periods – volcanic rocks from Iceland – and a distal continental source during glacial periods. An unexpected finding is that the 234U/238U ratios of the silicate portion of the sediment, isolated by leaching with hydrochloric acid, are significantly less than the secular equilibrium value and show large and systematic variations that are correlated with glacial cycles and sediment provenance. The 234U depletions are inferred to be due to α-recoil loss of 234Th, and are used to calculate “comminution ages” of the sediment — the time elapsed between the generation of the small (≤ 50 μm) sediment grains in the source areas by comminution of bedrock, and the time of deposition on the seafloor. Transport times, the difference between comminution ages and depositional ages, vary from less than 10 ky to about 300 to 400 ky for the Site 984A sediments. Long transport times may reflect prior storage in soils, on continental shelves, or elsewhere on the seafloor. Transport time may also be a measure of bottom current strength. During the most recent interglacial periods the detritus from distal continental sources is diluted with sediment from Iceland that is rapidly transported to the site of deposition. The comminution age approach could be used to date Quaternary non-marine sediments, soils, and atmospheric dust, and may be enhanced by concomitant measurement of 226Ra/230Th, 230Th/234U, and cosmogenic nuclides.

New Sr and Nd isotopic data are combined with available information on the composition and petrology of lavas and the thermal and seismic structure of the underlying crust to develop a detailed model for the deep structure and magmatic processes of the Clear Lake volcanic system in northern California. The isotopic data require a two-stage model for magmatic evolution. In stage I, basaltic magma (eNd = + 6 to + 8; 87Sr / 86Sr = 0.703 to 0.7035; SiO2 ≤ 50%) is fed from the mantle into the lower and middle crust and evolves through combined crustal assimilation and fractional crystallization to basaltic andesite (eNd = + 5 to + 0.4; 87Sr / 86Sr = 0.70328 to 0.70485; SiO2 ≈ 55% to 57%). In stage II, the basaltic andesite magmas are transported upward and are either erupted at the surface or stored in shallow magma chambers where they evolve by fractional crystallization to form dacitic and rhyolitic magmas (SiO2 ≈ 65% to 70%). High-silica rhyolites (SiO2 ≈ 75%; high 87Sr / 86Sr) show evidence that further crustal assimilation can occur where upper crustal temperatures are elevated. Calculated densities of Clear Lake lavas indicate that basalt should pond at a depth of 12–18 km where seismic data show a pronounced density boundary within the crust. Thermodynamic models of assimilation require that mid-crustal temperatures are at least 600–800 °C to allow for enough assimilation to explain the isotopic data. Both surface heat flow and thermobarometry of crustal xenoliths in andesites are consistent with these inferred high temperatures. The Clear Lake volcanic system provides an opportunity to cross-calibrate petrological, geochemical and geophysical approaches. The results confirm that magma supply, magma buoyancy, and crustal temperatures control magmatic evolution. A temporal trend of increasing eNd over the past 2 million years suggests that magma supply in the Clear Lake volcanic field has been increasing and is still high. This is consistent with high heat flow in the area and high 3He / 4He in thermal gasses. The current pause in eruptive activity could represent a period of silicic magma accumulation in the crust. Future eruptions in the Clear Lake area may therefore be silicic and larger in volume than those over the past 100 ky.

Strontium isotope ratios (87Sr/86Sr) measured in groundwater samples from 273 wells in the Pasco Basin unconfined aquifer below the US Department Of Energy Hanford Site show large and systematic variations that provide constraints on groundwater recharge, weathering rates (for saturated zone dissolution) of the aquifer host rocks, communication between unconfined and deeper confined aquifers, and vadose zone–groundwater interaction. The impact of millions of cubic meters of wastewater discharged to the vadose zone (103–105 times higher than ambient drainage) shows up strikingly on maps of groundwater 87Sr/86Sr. Extensive access through the many groundwater monitoring wells at the site allows for an unprecedented opportunity to evaluate the strontium geochemistry of a major aquifer, hosted primarily in unconsolidated sediments, and relate it to both long term properties and recent disturbances. Groundwater 87Sr/86Sr increases systematically from 0.707 to 0.712 from west to east across the Hanford Site, in the general direction of groundwater flow, as a result of addition of Sr from the weathering of aquifer sediments and from diffuse drainage through the Pasco Basin vadose zone. The lower 87Sr/86Sr groundwater reflects recharge waters that have acquired Sr from Columbia River Basalts. Based on a steady-state model of Sr reactive transport and drainage, there is an average natural drainage flux of 0–1.4 mm/yr near the western margin of the Hanford Site, and ambient drainage may be up to 30 mm/yr in the center of the site assuming an average bulk rock weathering rate of 10−7.5 g/(g/yr). The weathering rates cannot be higher than those assumed by more than a factor of 2–3, in order to match Sr isotope data.

A mathematical model is presented that describes the effects of pore fluid aqueous diffusion and reaction rate on the isotopic exchange between fluids and rocks in reactive geo-hydrological systems where flow is primarily through fractures. The model describes a simple system with parallel equidistant fractures, and chemical transport in the matrix slabs between fractures by aqueous diffusion through a stagnant pore fluid. The solid matrix exchanges isotopes with pore fluid by solution–precipitation at a rate characterized by a time constant, R (yr −1 ), which is an adjustable parameter. The effects of reaction on the isotopes of a particular element in the fracture fluid are shown to depend on the ratio of the diffusive reaction length for that element ( L ) to the fracture spacing ( b ). The reaction length depends on the solid–fluid exchange rate within the matrix, the partitioning of the element between the matrix pore fluid and the matrix solid phase, the porosity and density of the matrix, and the aqueous diffusivity. For L / b L / b relative to a standard porous flow (single porosity) model. For L / b > 1, the parallel fracture model is no different from a porous flow model. If isotopic data are available for two or more elements with different L values, it may be possible to use the model with appropriate isotopic measurements to estimate the spacing of the primary fluid-carrying fractures in natural fluid–rock systems. Examples are given using Sr and O isotopic data from mid-ocean ridge (MOR) hydrothermal vent fluids and Sr isotopes in groundwater aquifers hosted by fractured basalt. The available data for MOR systems are consistent with average fracture spacing of 1–4 m. The groundwater data suggest larger effective fracture spacing, in the range 50–500 m. In general, for fractured rock systems, the effects of fracture–matrix diffusive exchange must be considered when comparing isotopic exchange effects for different elements, as well as for estimating water age using radioactive and cosmogenic isotopes.

Pore water chemistry and 234U/238U activity ratios from fine-grained sediment cored by the Ocean Drilling Project at Site 984 in the North Atlantic were used as constraints in modeling in situ rates of plagioclase dissolution with the multicomponent reactive transport code Crunch. The reactive transport model includes a solid-solution formulation to enable the use of the 234U/238U activity ratios in the solid and fluid as a tracer of mineral dissolution. The isotopic profiles are combined with profiles of the major element chemistry (especially alkalinity and calcium) to determine whether the apparent discrepancy between laboratory and field dissolution rates still exists when a mechanistic reactive transport model is used to interpret rates in a natural system. A suite of reactions, including sulfate reduction and methane production, anaerobic methane oxidation, CaCO3 precipitation, dissolution of plagioclase, and precipitation of secondary clay minerals, along with diffusive transport and fluid and solid burial, control the pore fluid chemistry in Site 984 sediments. The surface area of plagioclase in intimate contact with the pore fluid is estimated to be 6.9 m2/g based on both grain geometry and on the depletion of 234U/238U in the sediment via α-recoil loss. Various rate laws for plagioclase dissolution are considered in the modeling, including those based on (1) a linear transition state theory (TST) model, (2) a nonlinear dependence on the undersaturation of the pore water with respect to plagioclase, and (3) the effect of inhibition by dissolved aluminum. The major element and isotopic methods predict similar dissolution rate constants if additional lowering of the pore water 234U/238U activity ratio is attributed to isotopic exchange via recrystallization of marine calcite, which makes up about 10–20% of the Site 984 sediment. The calculated dissolution rate for plagioclase corresponds to a rate constant that is about 102 to 105 times smaller than the laboratory-measured value, with the value depending primarily on the deviation from equilibrium. The reactive transport simulations demonstrate that the degree of undersaturation of the pore fluid with respect to plagioclase depends strongly on the rate of authigenic clay precipitation and the solubility of the clay minerals. The observed discrepancy is greatest for the linear TST model (105), less substantial with the Al-inhibition formulation (103), and decreases further if the clay minerals precipitate more slowly or as highly soluble precursor minerals (102). However, even several orders of magnitude variation in either the clay solubility or clay precipitation rates cannot completely account for the entire discrepancy while still matching pore water aluminum and silica data, indicating that the mineral dissolution rate conundrum must be attributed in large part to the gradual loss of reactive sites on silicate surfaces with time. The results imply that methods of mineral surface characterization that provide direct measurements of the bulk surface reactivity are necessary to accurately predict natural dissolution rates.

Reactive transport modeling is an essential tool for the analysis of coupled physical, chemical, and biological processes in Earth systems, and has additional potential to better integrate the results from focused fundamental research on Earth materials. Appropriately designed models can describe the interactions of competing processes at a range of spatial and time scales, and hence are critical for connecting the advancing capabilities for materials characterization at the atomic scale with the macroscopic behavior of complex Earth systems. Reactive transport modeling has had a significant impact on the treatment of contaminant retardation in the subsurface, the description of elemental and nutrient fluxes between major Earth reservoirs, and in the treatment of deep Earth processes such as metamorphism and magma transport. Active topics of research include the development of pore scale and hybrid, or multiple continua, models to capture the scale dependence of coupled reactive transport processes. Frontier research questions, that are only now being addressed, include the effects of chemical microenvironments, coupled thermal–mechanical–chemical processes, controls on mineral–fluid reaction rates in natural media, and scaling of reactive transport processes from the microscopic to pore to field scale.

A 20 million year marine Ca isotope record is constructed based on measurements of 36 nannofossil ooze samples from DSDP Site 590. The Ca isotopic composition (d 44 Ca, relative to our bbulk EarthQ average) of carbonate ooze samples varies between � 0.2 and � 0.9, and the inferred d 44 Ca of seawater calcium varies between +1.1 and +0.4 (modern seawater value is +0.95 on our scale). Fluctuations in the seawater Ca isotope ratio occur in 2 to 4 million year intervals and indicate that the supply and removal of Ca from the oceans are seldom in balance. The Ca isotope record is used to reconstruct past weathering fluxes and marine calcium concentrations. The inferred marine concentration of Ca fluctuates by about F20% on 2–4 million year time scales and by about F50% over the 20 million year record. These variations are presumably superimposed on the previously inferred 100 million year variations. The seawater Ca concentration was about 0.5 times present at 20 Ma and generally increased until 6 Ma (1.7 times present). Local maxima in seawater calcium concentrations occur at 6.4 and 4 Ma, preceding periods of enhanced marine productivity and high mass accumulation rates at Site 590. While the calculated calcium concentrations are sensitive to the assumed d 44 Ca value for the weathering flux, the 2–4 million year features of the derived

The effect of continental weathering on the iron isotope compositions of natural materials is investigated. Unweathered igneous rocks, pelagic clay, and dust fall within the range δ 56 Fe=0±0.3‰. Rivers with large suspended loads also have δ 56 Fe values near zero. Dilute streams have δ 56 Fe values that trend towards lower δ 56 Fe (∼−1) suggesting that dissolved riverine iron is isotopically light relative to igneous rocks. Bulk soil and soil leaches display systematically different δ 56 Fe profiles, indicating that isotopically distinct Fe pools are generated during pedogenesis. Nannofossil ooze, which contains Fe scavenged from the ocean water column, has δ 56 Fe≈0, but is consistent with seawater dissolved Fe having negative δ 56 Fe. It is inferred that continental weathering under modern oxidizing Earth surface conditions preferentially releases dissolved Fe with negative δ 56 Fe, which is transported in rivers to the ocean. A preliminary analysis of the marine Fe budget suggests that riverine Fe has a substantial role in determining the δ 56 Fe of both the modern and ancient oceans, but other inputs, particularly that from diagenesis of marine sediments, may also be important. Since the chemical pathways of Fe processing during weathering are dependent on oxidation state and biological activity, Fe isotopes may prove useful for detecting changes in these parameters in the geologic past.

We agree with George et al. that high 226 Ra/ 230 Th and correlated Ba/Th, 238 U/ 230 Th, and other elemental and isotopic ratios are the result of fluid addition to the wedge. The Feineman and DePaolo [Earth Planet. Sci. Lett. 215 (2003) 339–355] manuscript addresses more specifically the pathway followed by the fluid en route to the surface. In particular, we address whether the 226 Ra/ 230 Th data directly yield the total time available for the fluid to migrate from the point of origin in the slab to the melting region in the wedge, and ultimately to the surface with the melt. All of the processes involved in generating arc volcanism would of necessity have to take place very rapidly within the constraints proposed by George et al. The alternative is that the upward fluid movement is hindered by the fluid flow regime and solid mantle flow, allowing more time for reaction with the mantle wedge. There is evidence in the U-series data for more than one time scale, and our model helps to explain how this could come about. The extent to which grainscale 226 Ra/ 230 Th disequilibrium affects the inferred melt transport time is yet another issue, and this could be achieved either with the small-degree melt mechanism we discussed or by additional effects associated with melt and fluid migration at later stages in the melt generation process. We agree that the final melt transport event is likely to happen quickly. One of the most puzzling features of volcanic arcs is that the volcanic front emerges considerably behind the presumed location of primary water release from the subducting slab (depth to slab is ∼120 km beneath the volcanic front, as opposed to the predicted ∼80 km). This offset between the expected site of fluid release and the location of the volcanic front can be attributed to coupled transport of the fluid—in the form of hydrous minerals, such as amphibole and/or phlogopite in the wedge [J. Geophys. Res. 97 (1992) 2037–2070] or redirection of the melt due to flow and stresses in the mantle matrix as the magma slowly percolates upward [J. Geol. 95 (1987) 285–307]. In either case, the fluid and/or melt is required to ascend slowly, at least for some part of its journey, to explain the location of the volcanic front. Furthermore, 238 U/ 230 Th disequilibria in arc lavas seem to suggest a relatively long interval for fluid addition (∼30–60 ka) [Science 292 (2001) 1363–1366; Earth Planet. Sci. Lett. 179 (2000) 581–593; Geochim. Cosmochim. Acta 61 (1997) 4855–4884], which is difficult to reconcile with the 226 Ra/ 230 Th disequilibrium observed in the lavas. For these reasons, we sought to find a means whereby apparently short-lived isotopic disequilibria could be maintained during complex fluid and melt migration in the mantle wedge. Below, we address a few of the main issues presented by George et al. [Earth. Planet. Sci. Lett. this issue] in their comment.

Bulk dissolution rates for sediment from ODP Site 984A in the North Atlantic are determined using the 234U/238U activity ratios of pore water, bulk sediment, and leachates. Site 984A is one of only several sites where closely spaced pore water samples were obtained from the upper 60 meters of the core; the sedimentation rate is high (11–15 cm/ka), hence the sediments in the upper 60 meters are less than 500 ka old. The sediment is clayey silt and composed mostly of detritus derived from Iceland with a significant component of biogenic carbonate (up to 30%). The pore water 234U/238U activity ratios are higher than seawater values, in the range of 1.2 to 1.6, while the bulk sediment 234U/238U activity ratios are close to 1.0. The 234U/238U of the pore water reflects a balance between the mineral dissolution rate and the supply rate of excess 234U to the pore fluid by α-recoil injection of 234Th. The fraction of 238U decays that result in α-recoil injection of 234U to pore fluid is estimated to be 0.10 to 0.20 based on the 234U/238U of insoluble residue fractions. The calculated bulk dissolution rates, in units of g/g/yr are in the range of 4 × 10−7 to 2 × 10−6 yr−1. There is significant down-hole variability in pore water 234U/238U activity ratios (and hence dissolution rates) on a scale of ca. 10 m. The inferred bulk dissolution rate constants are 100 to 104 times slower than laboratory-determined rates, 100 times faster than rates inferred for older sediments based on Sr isotopes, and similar to weathering rates determined for terrestrial soils of similar age. The results of this study suggest that U isotopes can be used to measure in situ dissolution rates in fine-grained clastic materials. The rate estimates for sediments from ODP Site 984 confirm the strong dependence of reactivity on the age of the solid material: the bulk dissolution rate (Rd) of soils and deep-sea sediments can be approximately described by the expression Rd ≈ 0.1 Age−1 for ages spanning 1000 to 5 × 108 yr. The age of the material, which encompasses the grain size, surface area, and other chemical factors that contribute to the rate of dissolution, appears to be a much stronger determinant of dissolution rate than any single physical or chemical property of the system.

The Pb isotopic compositions of phenocrystic phases in young basaltic lavas have been investigated using the Getty-DePaolo method (Getty S. J. and DePaolo D. J. [1995] Quaternary geochronology by the U-Th-Pb method. Geochim. Cosmochim. Acta 59, 3267 3272), which allows for the resolution of small isotopic differences. Phenocryst, matrix, and whole rock analyses were made on samples from the 17 Myr-old Imnaha basalts of the Columbia River Group, a zero-age MORB from the Mid-Atlantic Ridge, and a ca. 260 kyr-old tholeiite from Mount Etna. Plagioclase feldspar phenocrysts have low-(U, Th)/Pb, and in each sample the plagioclase has significantly lower 206Pb/207Pb and 208Pb/207Pb values than whole rock, matrix, and magnetite-rich separates. The Pb isotopic contrast between plagioclase and matrix/whole rock is found in three samples with varying grain sizes (0.5 2 cm for the Imnaha basalt and MORB and

The abundances of 18 O and deuterium in the present and past hydrologic cycle have proven to be an important tool in Earth systems science. In contrast, the abundance of 17 O in precipitation has thus far been assumed to carry no additional information to that of 18 O. Here, we demonstrate, using known constraints on oxygen isotope abundances from the O 2 cycle and existing data about the natural abundance of 17 O in water, that the relationship between the discrimination against 17 O and 18 O in water may vary. This relationship, presented here as θ = ln ( 17 α)/ln ( 18 α), is found to be 0.511 ± 0.005 for kinetic transport effects and 0.526 ± 0.001 for equilibrium effects, with very low temperature sensitivity. As a result, the 17 Δ of precipitation is controlled primarily by kinetic effects during evaporation of the initial vapor and, in contrast to the deuterium excess, is independent of the temperature at the evaporation (and condensation) site. This makes 17 Δ a unique tracer that complements 18 O and deuterium, and may allow for a decoupling of changes in the temperature of the ocean, that serves as the vapor source, from changes in the relative humidity above it. In addition, the 17 Δ of ice caps is influenced by the kinetic effects in ice formation, and therefore measurement of ice 17 Δ can be used as an additional constraint for better understanding and parameterization of these effects.

Waste material buried in drums in the shallow subsurface at the Radioactive Waste Management Facility (RWMC) of the Idaho National Engineering and Environmental Laboratory (INEEL) contained significant amounts of organic compounds including lubricating oils and chlorinated solvents. CO{sub 2} concentrations in pore gas samples from monitoring wells in the vicinity of the disposal pits are 3 to 5 times higher than the concentrations in nearby background wells. The stable carbon isotope ratios ({delta}{sup 13}C values) of CO{sub 2} from the disposal pits averaged 2.4. less than CO{sub 2} from the background wells, indicating that the elevated CO{sub 2} concentrations around the pits were derived from source materials with {delta}{sup 13}C values in the range of -24{per_thousand} to -29{per_thousand}. These {delta}{sup 13}C values are typical of lubricating oils, but higher than most solvents. The radiocarbon ({sup 14}C) contents of CO{sub 2} across most of the site were significantly elevated above modern concentrations due to reactor blocks buried in a subsurface vault at the site. However, several samples collected from the high-CO{sub 2} zone on the far side of the RWMC from the reactor blocks had very low {sup 14}C contents (less than 0.13 times modern), confirming production from lubricating oils manufactured from fossil hydrocarbons. The magnitude of the CO{sub 2} anomaly observed at the site is consistent with intrinsic biodegradation rates on the order of 0.5 to 3.0 metric tons of carbon per year.

Available constraints on metamorphic reaction rates derived from the study of natural systems are similar to, or slightly lower than, the bulk strain rates measured in the same rocks. Here, we explore whether this apparent relationship is merely coincidence or due to a more fundamental mechanistic link between reaction and strain. Grain boundary migration accommodated dislocation creep (GBMDC) or grain boundary diffusion creep (GBDC) (i.e. pressure solution), both of which involve dissolution-precipitation as we define it, will occur simultaneously with mineral reactions involving dissolution-precipitation in the presence of a non-zero deviatoric stress. The exact relationships between reaction and strain are different depending on whether GBMDC or GBDC is controlling strain, but the mechanistic link exists in both cases. We present theoretical arguments which show that bulk strain by GBMDC or GBDC, which may additionally be accommodated by processes not involving dissolution-precipitation, such as dislocation glide and climb or grain boundary sliding, should in most cases be somewhat faster than the bulk reaction rates as observed. With few exceptions, for natural metamorphic systems undergoing plastic deformation, strain rates provide an upper limit for bulk reaction rates occurring simultaneously in the same rocks. The data suggest that mineral reaction rates may typically be within one order of magnitude of the strain rate.

The U–Th/He system can potentially be used for dating volcanic rocks with ages as young as a few thousand years and as old as several million years, thus providing a valuable supplement to radiocarbon and K–Ar dating. Garnet phenocrysts from the 79 AD eruption of Mt. Vesuvius were dated to evaluate the accuracy with which the necessary measurements and corrections can be made. The determined age, corrected for diffusive loss of He, alpha ejection, and initial U-series disequilibrium, is 1885±188 yr which compares well with the known age of 1923 yr. U and Th concentrations were measured by isotope dilution on different aliquots than were used for He concentration measurements. Step-wise degassing yielded an Arrhenius relationship for He diffusion in garnet with an activation energy of 91.31±5.76 kJ/mol and ln D0/a2=−2.00±0.56. The uniformity of U and Th concentrations in garnet was checked by ion microprobe analysis. The 234U/238U and 230Th/238U activity ratios were measured by MC-ICPMS. The results suggest that with proper analysis and corrections, the U–Th/He method can be used to date young volcanic minerals with useful precision and accuracy, and may therefore be valuable for dating volcanic rocks that have low K or are otherwise difficult to date accurately with Ar–Ar or radiocarbon.

Measurements of the concentrations of the 238U decay series isotopes (234U, 230Th, 226Ra) have been used to estimate the rates at which magma is generated and transported in the mantle. The usual assumption is that solid mantle minerals are in radioactive equilibrium prior to melting. However, if one or more of the nuclides in the chain is strongly concentrated by a minor mineral, and if the diffusivity of that nuclide is large enough, steady-state radioactive disequilibrium can result in the solid phase. It can be inferred from available data that radium is strongly concentrated in minor hydroxyl-bearing mantle minerals (phlogopite and amphibole) relative to Th, and Ra diffusion in clinopyroxene is fast relative to the typical grain diameter at ca. 1100°C. Consequently, we show with simple analytical models that a steady-state Ra deficiency in clinopyroxene (cpx), accompanied by a complementary steady-state Ra excess in neighboring phlogopite (phlog) or amphibole (amph), is likely to be the normal situation in hydrous mantle peridotite with average clinopyroxene grain radii of ca. 1 mm. The steady state (226Ra/230Th) (parentheses indicating activity ratio) in the hydrous mineral is limited roughly by the mass ratio with clinopyroxene (i.e. cpx/phlog or cpx/amph) and could be as high as 10–100. The exceptionally high (226Ra/230Th) of some island arc lavas could therefore be a result of preferential contribution of phlogopite or amphibole during partial melting of hydrous mantle. This effect may ease time constraints for source-to-surface melt migration at island arcs. Incipient melting of hydrous minerals from channel walls during melt transport and/or late-stage incorporation of phlogopite or amphibole into arc magmas may also contribute to generating high (226Ra/230Th). Steady-state (226Ra/230Th) disequilibrium due to diffusive loss of 226Ra from clinopyroxene is also important for melt/solid and fluid/solid partitioning, and must be incorporated into models relating isotopic disequilibrium to melt and fluid transport rates. Diffusive effects could be important for other U-series nuclides in some circumstances.

Experimental diffusion couples were used to study chemical diffusion between molten rhyolite and basalt with special emphasis on the associated fractionation of calcium and lithium isotopes. Diffusion couples were made by juxtaposing firmly packed powders of a natural basalt (SUNY MORB) and a natural rhyolite (Lake County Obsidian) and then annealing them in a piston cylinder apparatus for times ranging from 0.1 to 15.7 h, temperatures of 1350–1450°C, and pressures of 1.2–1.3 GPa. Profiles of the major elements and many trace elements were measured on the recovered quenched glasses. The diffusivities of all elements except lithium were found to be remarkably similar, while the diffusivity of lithium was two to three orders of magnitude larger than that of any of the other elements measured. Chemical diffusion of calcium from molten basalt into rhyolite was driven by a concentration ratio of ∼18 and produced a fractionation of 44Ca from 40Ca of about 6 ‰. Because of the relatively low concentration of lithium in the natural starting materials a small amount of spodumene (LiAlSi2O6) was added to the basalt in order to increase the concentration difference between basalt and rhyolite, which was expected to increase the magnitude of diffusive isotopic fractionation of lithium. The concentration ratio between Li-doped basalt and natural rhyolite was ∼15 and the resulting diffusion of lithium into the rhyolite fractionated 7Li from 6Li by about 40‰. We anticipate that several other major rock-forming elements such as magnesium, iron and potassium will also exhibit similarly larger isotopic fractionation whenever they diffuse between natural melts with sufficiently large differences in the abundance of these elements.

The relatively low rates of magma production in island arcs and continental extensional settings require that the volume of silicic magma involved in large catastrophic caldera-forming (CCF) eruptions must accumulate over periods of 105 to 106 years. We address the question of why buoyant and otherwise eruptible high-silica magma should accumulate for long times in shallow chambers rather than erupt more continuously as magma is supplied from greater depths. Our hypothesis is that the viscoelastic behavior of magma chamber wall rocks may prevent an accumulation of overpressure sufficient to generate rhyolite dikes that can propagate to the surface and cause an eruption. The critical overpressure required for eruption is based on the model of Rubin (1995a). An approximate analytical model is used to evaluate the controls on magma overpressure for a continuously or episodically replenished spherical magma chamber contained in wall rocks with a Maxwell viscoelastic rheology. The governing parameters are the long-term magma supply, the magma chamber volume, and the effective viscosity of the wall rocks. The long-term magma supply, a parameter that is not typically incorporated into dike formation models, can be constrained from observations and melt generation models. For effective wall-rock viscosities in the range 1018 to 1020 Pa s–1, dynamical regimes are identified that lead to the suppression of dikes capable of propagating to the surface. Frequent small eruptions that relieve magma chamber overpressure are favored when the chamber volume is small relative to the magma supply and when the wall rocks are cool. Magma storage, leading to conditions suitable for a CCF eruption, is favored for larger magma chambers (>102 km3) with warm wall rocks that have a low effective viscosity. Magma storage is further enhanced by regional tectonic extension, high magma crystal contents, and if the effective wall-rock viscosity is lowered by microfracturing, fluid infiltration, or metamorphic reactions. The long-term magma supply rate and chamber volume are important controls on eruption frequency for all magma chamber sizes. The model can explain certain aspects of the frequency, volume, and spatial distribution of small-volume silicic eruptions in caldera systems, and helps account for the large size of granitic plutons, their association with extensional settings and high thermal gradients, and the fact that they usually post-date associated volcanic deposits.

Interpretation of U-series disequilibria in midocean ridge basalts is highly dependent on the bulk partition coefficients for U and Th and therefore the mineralogy of the mantle source. Distinguishing between the effect of melting processes and variable source compositions on measured disequilibria (238U-230Th-226Ra and 235U-231Pa) requires measurement of the radiogenic isotopes Hf, Nd, Sr, and Pb. Here, we report measurements of 238U-230Th-226Ra and 235U-231Pa disequilibria; Hf, Nd, Sr, and Pb isotopic; and major and trace element compositions for a suite of 20 young midocean ridge basalts from the East Pacific Rise axis between 9°28′ and 9°52′N. All of the samples were collected within the axial summit trough using the submersible Alvin. The geological setting and observational data collected during sampling operations indicate that all the rocks are likely to have been erupted from 1991 to 1992 or within a few decades of that time. In these samples, 230Th excesses and 226Ra excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of 226Ra, this inverse correlation between 230Th and 226Ra excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study, 226Ra and 230Th excesses also vary with lava composition: 226Ra excesses are negatively correlated with Na8 and La/Yb and positively correlated with Mg#. Conversely, 230Th excesses are positively correlated with Na8 and La/Yb and negatively correlated with Mg#. Th/U, 230Th/232Th, and 230Th excesses are also variable and correlated to one another. 231Pa excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na8. The isotope ratios 143Nd/144Nd, 176Hf/177Hf, 87Sr/86Sr, and 208Pb/206Pb are constant within analytical uncertainty, indicating that they were derived from a common source. The source is homogeneous with respect to parent/daughter ratios Lu/Hf, Sm/Nd, Rb/Sr, and Th/U; therefore, the measured variations of Th/U, 230Th, and 226Ra excesses and major and trace element compositions in these samples are best explained by polybaric melting of a homogeneous source, not by mixing of compositionally distinct sources.

Bulk metamorphic reaction rates have been measured using Sr isotopes as tracers of reactive transport processes at a field site near Simplon Pass, Switzerland, employing the technique described in Part I of this study (Baxter and DePaolo, 2002a). The reaction rate, R, inferred, similar to the value first reported in Baxter and DePaolo (2000), is 1.4 + 1 . 1 - 0 . 4 x 10 - 7 yr - 1 (or R = 7.0 + 5 . 5 - 2 . 0 x 10 - 9 g/cm 2 /yr normalized to the geometric surface area of plagioclase) which is several orders of magnitude slower than extrapolations of laboratory data. This rate suggests that attainment of local equilibrium may require 10's of millions of years, and consequently equilibrium may not be closely approached in dynamic metamorphic systems if significant changes in P-T-X occur over shorter timescales. These data suggest that there is a problem in current extrapolations or applicability of laboratory kinetic data to natural high-temperature systems. Possible reasons for this discrepancy include differences in reactive surface area, reactant supply limited by slow diffusion of major elements, or an armoring effect by which access to grain interiors is limited. On the basis of microchemical patterns in plagioclase, we identify grain boundary migration (GBM) as the rate limiting process for accessing grain interiors, and thus for bulk reaction. Calculation of reaction quotients for six balanced mineral reactions suggests local disequilibrium for garnet with respect to its Ca content. The characteristic equilibrium lengthscales (Le) for Sr and Ca are both about 1 meter. Equilibrium based geochemical tools such as isochron geochronology or geothermobarometry are compromised within a distance Le of the lithologic contact. Bulk Sr diffusivity measured at the field site changes from 10 - 9 to 10 - 7 m /yr, suggesting that the characteristics of the intergranular transporting medium (ITM) changed significantly during the early retrograde history of the field site.

Knowledge of metamorphic reaction rates is crucial to accurately interpret rock and mineral chemistry. The local equilibrium assumption, used in geochronology, geothermobarometry, and material flux estimates, requires that local reaction rates among system phases are fast relative to local rates of P-T-X change. Natural metamorphic reaction rates are essentially unknown and difficulties and disagreements exist regarding extrapolations of existing laboratory data to natural conditions. Growing recognition of natural effects that could be attributed to slower reaction rates justifies the need for a field-based quantification of reaction rates to assess the accuracy of lab-based predictions. We describe in detail the theory and methodology of a technique for extracting bulk reaction rates directly from isotopic data derived from natural samples (Baxter and DePaolo, 2000). Reaction rates measured using this technique may be judiciously applied to isotopic exchange or net reaction kinetics in other natural systems. The technique requires collection of whole rock and garnet 87Sr/86Sr data along a sampling traverse normal to a lithologic contact where there was, prior to metamorphism, a sharp isotopic discontinuity. Garnet data provide information on syn-metamorphic conditions and the time interval for the exchange process. Forward modeling of the reactive transport process using numerical methods and the equations for diffusive reactive transport allow determination of the reaction rate and bulk Sr diffusivity that provides the best fit to the data. Measurement of reaction rates is best constrained if an isotopic step is preserved at the contact, the size of which is directly proportional to the bulk reaction rate. This contribution is intended to serve as a template for future use and development of this technique to acquire natural reaction rate data from metamorphic systems.

Forty-four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40Ar/39Ar and demonstrate the importance of bulk Ar diffusivity and system geometry—factors not usually considered in the interpretation and collection of 40Ar/39Ar age data. The resulting 40Ar/39Ar apparent ages range from 11.30 ± 0.05 Ma to 17.90 ± 0.10 Ma. The ages (and excess argon contents) are spatially and lithologically correlated. The pelite samples all yield ages clustering around ∼12 Ma, the age expected for cooling through biotite closure (∼360°C) in this region of the Alps. Ages in the amphibolite biotites are older, showing a smooth trend between 15 Ma at the contact with the pelite to 18 Ma, 34 cm from the contact. This data shows that characterization of the Ar closure age for biotite in a given system should not rest on a single sample, as otherwise irresolvable differences in age between samples within the same outcrop can exist. A generalized mechanistic model for excess argon is presented. The presence (or absence) of excess Ar depends on an intrinsic system parameter, τT, the transmissive timescale, which is the characteristic time for 40Ar to escape through the local intergranular transporting medium (ITM) to some sink for argon. To prevent buildup of geochronologically significant excess 40Ar, τT must be very short relative to the true closure age of the mineral. A FORTRAN code including radiogenic Ar production, diffusive loss of Ar from biotite, and bulk Ar diffusion through the ITM has been developed. Application of numerical modeling suggests that the time-averaged effective bulk diffusivity, DeffAr, in the biotite-amphibolite rock during early retrograde cooling is 2.2 ± 1.0 × 10−8 m2/yr (assuming steady state conditions) - the first such measurement available. Numerical modeling also provides information about the transmissivity and geologic history specific to the field site, including a drop in DeffAr at 15.5 ± 1.0 Ma. The timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack.

The timing of garnet growth during metamorphism associated with the Grampian Orogeny in the sillimanite zone of the Barrovian type-locality in Glen Clova, Scotland, was determined by Sm/Nd geochronology. Two high precision garnet-whole-rock ages were achieved by employing HF partial dissolution of garnet separates to optimize purity. Multiple garnet growth generations were identified on the basis of the geochronology and detailed textural and chemical data: an early stage, at 472.9 ± 2.9 Ma, during D 2 deformation under garnet zone conditions (c. 500–550 °C), and a later stage, at 464.8 ± 2.7 Ma, during or slightly after D 3 deformation mostly under sillimanite zone conditions (peak temperature of c. 660 °C), but possibly including some growth during kyanite zone conditions. When combined with recently published garnet ages from the kyanite and garnet zones the data suggest that peak metamorphic temperatures in at least these three of Barrow9s zones were achieved roughly contemporaneously. The difference between garnet zone and sillimanite zone peak temperature attainment is 2.8 ± 3.7 Ma. The near contemporaneity of peak temperature attainment in different metamorphic zones requires an additional source of heat beyond thermal relaxation of a variably over-thickened crust. We suggest that local igneous intrusions, with synmetamorphic ages, provided that additional heat.

In-situ recovery (ISR) of uranium (U) from sandstone-type roll-front deposits is a technology that involves the injection of solutions that consist of ground water fortified with oxygen and carbonate to promote the oxidative dissolution of U, which is pumped to recovery facilities located at the surface that capture the dissolved U and recycle the treated water. The ISR process alters the geochemical conditions in the subsurface creating conditions that are more favorable to the migration of uranium and other metals associated with the uranium deposit. There is a lack of clear understanding of the impact of ISR mining on the aquifer and host rocks of the post-mined site and the fate of residual U and other metals within the mined ore zone. We performed detailed petrographic, mineralogical, and geochemical analyses of several samples taken from about 7 m of core of the formerly the ISR-mined Smith Ranch–Highland uranium deposit in Wyoming. We show that previously mined cores contain significant residual uranium (U) present as coatings on pyrite and carbonaceous fragments. Coffinite was identified in three samples. Core samples with higher organic (> 1 wt.%) and clay (> 6–17 wt.%) contents yielded higher 234U/238U activity ratios (1.0–1.48) than those with lower organic and clay fractions. The ISR mining was inefficient in mobilizing U from the carbonaceous materials, which retained considerable U concentrations (374–11,534 ppm). This is in contrast with the deeper part of the ore zone, which was highly depleted in U and had very low 234U/238U activity ratios. This probably is due to greater contact with the lixiviant (leaching solution) during ISR mining. EXAFS analyses performed on grains with the highest U and Fe concentrations reveal that Fe is present in a reduced form as pyrite and U occurs mostly as U(IV) complexed by organic matter or as U(IV) phases of carbonate complexes. Moreover, U–O distances of ~ 2.05 A were noted, indicating the potential formation of other poorly defined U(IV/VI) species. We also noted a small contribution from U O at 1.79 A, which indicates that U is partially oxidized. There is no apparent U–S or U–Fe interaction in any of the U spectra analyzed. However, SEM analysis of thin sections prepared from the same core material reveals surficial U associated with pyrite which is probably a minor fraction of the total U present as thin coatings on the surface of pyrite. Our data show the presence of different structurally variable uranium forms associated with the mined cores. U associated with carbonaceous materials is probably from the original U mobilization that accumulated in the organic matter-rich areas under reducing conditions during shallow burial diagenesis. U associated with pyrite represents a small fraction of the total U and was likely deposited as a result of chemical reduction by pyrite. Our data suggest that areas rich in carbonaceous materials had limited exposure to the lixiviant solution, continue to be reducing, and still hold significant U resources. Because of their limited access to fluid flow, these areas might not contribute significantly to post-mining U release or attenuation. Areas with pyrite that are accessible to fluids seem to be more reactive and could act as reductants and facilitate U reduction and accumulation, limiting its migration.

Preferential flow paths are expected in many groundwater systems and must be located because they can greatly affect contaminant transport. The fundamental characteristics of radiogenic isotope ratios in chemically evolving waters make them highly effective as preferential flow path indicators. These ratios tend to be more easily interpreted than solute-concentration data because their response to water-rock interaction is less complex. We demonstrate this approach with groundwater {sup 87}Sr/{sup 86}Sr ratios in the Snake River Plain aquifer within and near the Idaho National Engineering and Environmental Laboratory. These data reveal slow-flow zones as lower {sup 87}Sr/{sup 86}Sr areas created by prolonged interaction with the host basalts and a relatively fast flowing zone as a high {sup 87}Sr/{sup 86}Sr area.

Absolute accelerator mass spectrometry (AMS) measurements require reliable standards for calibration. The Space Sciences Laboratory at the University of California, Berkeley, and the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory (LLNL) have prepared new 41Ca AMS standards. The highly enriched 41CaCO3 was obtained from Oak Ridge National Laboratory. After purification of the CaCO3, the precise 41Ca/Ca ratio in the original material was determined by surface ionization mass spectrometry. The 41Ca was sequentially diluted with natural Ca to obtain 41Ca/Ca ratios of 1.155×10−10, 9.755×10−12, 5.139×10−12, 1.095×10−12 and 5.884×10−13. A solution with a 41Ca/Ca ratio of 10−6 was also prepared for biomedical research. The solutions having 41Ca/Ca ratios between 10−10 and 10−12 were measured by AMS. Although the present blank level, 5×10−14 41Ca/Ca, was higher than anticipated, excellent linearity was obtained from 6×10−13 to 1.2×10−10 41Ca/Ca.

A one-dimensional model of meridional water vapor transport is used to evaluate the factors that control the spatial and temporal variations of oxygen (δ18O) and hydrogen (δD) isotopic ratios in global precipitation. The model extends Rayleigh descriptions of isotopes in precipitation by including (1) effects of recharge to air masses by evaporation and (2) horizontal transport by both eddy fluxes and advection. Globally, spatial variations in precipitation δ18O and δD depend on the ratio of evaporation to the product of horizontal moisture flux and horizontal temperature gradient. At low latitudes, where this ratio is large, precipitation δ18O and δD are closely tied to the isotopic ratios of oceanic evaporation. At high latitudes the ratio is small, and δ18O and δD are controlled by the ratio of advective transport to eddy transport. Transport by eddy fluxes induces less fractionation than transport by advection, resulting in a smaller gradient of isotopic ratios with temperature. The model-predicted temporal relationships between δ18O (or δD) of Antarctic precipitation and temperature do not necessarily coincide with the modern spatial relationship and depend strongly on the proximity of the precipitation site to the ocean evaporation source. Sensitivity of δ18O to temporal changes in local surface temperature is low at coastal sites and increases with distance inland. These results suggest a possible explanation of the apparent discrepancy between borehole temperature inversion estimates of glacial temperatures and temperatures inferred from the modern spatial δ18O—surface temperature relationship.

The chemical compositions, ages, and Nd and Sr isotopic compositions of Miocene basaltic volcanic rocks from the southwestern Basin and Range Province were measured. The data presented, and other data obtained from the literature show that temporal variations in the Nd and Sr isotopic compositions are typically correlated with the timing of local crustal extension. The isotopic variations cannot be accounted for by crustal contamination, and are best interpreted as indicating the involvement of subcontinental lithospheric mantle as a major source of basalt magma before and during the early stages of extension, giving way to asthenospheric sources as extension proceeds. It is inferred that the basalt isotopic and age data provide information about the amount and timing of changes of the overall lithosphere thickness during extension. If this inference is valid, the inferred lithosphere thickness vs. time histories can be compared to the amount and timing of upper crustal extension to evaluate competing models for lithospheric strain during continental extension. The depth of the lithosphere–asthenospshere boundary is inferred primarily from the isotopic data, but can be refined by accounting for differences in the depth of origin of the basalts as suggested by differences in the degree of silica saturation (nepheline normative vs. hypersthene normative). The transition between silica saturated and undersaturated magmas is dependent on many factors, but for the conditions likely to apply to the Basin and Range Miocene volcanism, it corresponds to a depth of origin of 50±10 km. The data presented show strong support for the inferred mantle geochemical stratigraphy, and the inferred modern depths to the lithosphere–asthenosphere boundary are consistent with available seismic refraction and structural data. In the highly extended regions near the eastern and western margins of the southwestern Basin and Range, temporal changes in basalt isotopic compositions indicate that the subjacent lithospheric mantle thinned contemporaneously with the upper crust, as would be predicted for pure shear. These areas are also characterized by relatively high (although still low) basalt extrusion rates. In contrast, upper crustal extension in the Death Valley area was not accompanied by significant thinning of the subjacent lithospheric mantle, which implies that the location of lithospheric thinning was offset from the location of upper crustal extension as in a simple shear model. The total amount of lithospheric thinning is generally less than would be expected from the magnitude of upper crustal extension and is more evenly distributed; this suggests that there is also a component of distributed shear.

Measurements of 238U-230Th-226Ra and 235U-231Pa disequilibria in a suite of tholeiitic-to-basanitic lavas provide estimates of porosity, solid mantle upwelling rate and melt transport times beneath Hawaii. The observation that (230Th/238U) > 1 indicates that garnet is required as a residual phase in the magma sources for all of the lavas. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Hawaiian basalts. For chromatographic porous flow, the calculated maximum porosity in the melting zone ranges from 0.3–3% for tholeiites and 0.1–1% for alkali basalts and basanites, and solid mantle upwelling rates range from 40 to 100 cm yr−1 for tholeiites and from 1 to 3 cm yr−1 for basanites. For dynamic melting, the escape or threshold porosity is 0.5–2% for tholeiites and 0.1–0.8% for alkali basalts and basanites, and solid mantle upwelling rates range from 10 to 30 cm yr−1 for tholeiites and from 0.1 to 1 cm yr−1 for basanites. Assuming a constant melt productivity, calculated total melt fractions range from 15% for the tholeiitic basalts to 3% for alkali basalts and basanites.

revious studies of calcium isotope geochemistry document the existence of natural mass-related variations in the cal- cium isotopic composition of rocks, minerals, and biological samples (1-3). The biological causes and significance of the variations observed in organisms have not, however, been sys- tematically studied. This paper presents evidence of natural intraorganismal calcium isotope fractionation and documents a systematic isotopic difference between mineralized tissue and soft tissue calcium. Measurements of this difference may be useful for studying skeletal mineral balance in vertebrates. The processes that are involved in calcium use and transport in organisms have traditionally been studied by using artificially enriched isotope tracers that are introduced and tracked (e.g., refs. 4 and 5). These processes are also likely to cause small changes in the isotopic composition of natural calcium. Such isotopic discrimination by organicymetabolic processes is well documented for carbon, nitrogen, and oxygen isotopes (6, 7). The small isotopic differences produced in natural calcium cannot be observed with the techniques commonly used to measure artificially induced enrichments in calcium isotopes, but these differences are detectable with high-precision mass spec- trometry.

The magma generation at Unzen volcano may be considered as the product of crustal material mixed with mantle magma accompanied by fractional crystallization (AFC). The magma in the Unzen volcano is estimated to consist of about 50–80% of residual magma (F) and about 30–70% assimilated crustal material (A) relative to the original magma. Concerning the 1991–1995 eruption, it is estimated that the magma formed as the result of mixing of about 50–60% crustal material and about 55–65% of residual magma. An alternative magma eruption model for the 1991–1995 eruption is proposed here. In the early stage, the isotopic characteristics of 1991 eruption are defined by AFC process in the deeper magma chamber. Later, the magma ascended through the conduit and quiescently stayed for a long time in a shallow reservoir before eruption. The minerals continuously crystallized as phenocrysts especially at the chilled top and outer margin in the shallow chamber. The crystallized phenocryst mush was reworked into the central part of the magma chamber by means of magma convection and rapid magma ascent. Therefore, the reaction between phenocrysts and melt occurs only in internal chemical disequilibrium in the magma chamber. In contrast, the isotopic compositions of the original magma shall be little influenced by the above processes throughout its eruptive history. The 1991–1995 eruptive rocks of the Unzen volcano show their characteristics in Sr and Nd isotopic values independent of their two previous eruptions. However, the isotopic values of early eruptive product could represent the original magma value. This result also supports the previous work of Chen et al. (1993) [Chen, C.H., DePaolo, D.J., Nakada, S., Shieh, Y.N., 1993. Relationship between eruption volume and neodymium isotopic composition at Unzen volcano. Nature 362, 831–834], that suggested the eNd of early or precursory eruptive products could be a qualitative indicator of the maximum size of a continuing or impending eruption.