Don Porcelli

Mass transfer of helium, neon, argon, and xenon through a steady-state upper mantle

We have examined the steady-state upper mantle model for helium, neon, argon, and xenon following the mass transfer approach presented by Kellogg and Wasserburg (1990) for helium and Porcelli and Wasserburg (1995a) for xenon. The model explains the available observational data of mantle helium, neon, argon, and xenon isotope compositions and provides specific predictions regarding the rare gas isotopic compositions of the lower mantle, subduction of rare gases, and mantle rare gas concentrations. Rare gases in the upper mantle are derived from mixing of rare gases from the lower mantle, subducted rare gases, and radiogenic nuclides produced in situ. Isotopic shifts in the closed system lower mantle are due to decay of uranium and thorium decay series nuclides, ^(40)K, ^(129)I, and ^(244)Pu over 4.5 Ga, while isotopic shifts in the steady-state upper mantle are due to decay of uranium and thorium series nuclides, and ^(40)K over a timescale of ∼1.4 Ga. The model predicts that the shift in ^(21)Ne/^(22)Ne in the upper mantle relative to that in the lower mantle is the same as that for ^4He/^3He between the mantle reservoirs. This is compatible with the available data for MORB and ocean islands. Subduction of atmospheric helium and neon is not significant. All of the ^(40)Ar in the lower mantle has been produced by ^(40)K decay in the lower mantle. In the upper mantle, ^(40)K decay further increases the radiogenic 40Ar from the lower mantle by a factor of ∼3. The calculated minimum lower mantle ^(40)Ar/^(36)Ar ratio is substantially greater than the atmospheric ratio. The inferred rare gas relative abundances of the lower mantle are different from those of the atmosphere and are consistent with possible early solar system reservoirs. Both the calculated ^3He/^(22) and ^(20)Ne/^(36)Ar ratios of the lower mantle are within the range for meteorites with ‘solar’ neon isotope compositions. The ^(130)Xe/^(36)Ar ratio of the lower mantle is greater than that of the atmosphere, and may be possibly as high as the ratio found for meteoritic “planetary” rare gases. The model treats the atmosphere as a separate reservoir with rare gas isotope compositions that are distinct from those in the mantle. If the Earth originally had uniform concentrations of rare gases as represented by those in the lower mantle, then degassing of the upper mantle would have provided only a small proportion of the nonradiogenic rare gases presently in the atmosphere. The remainder may have been derived from late-accreted material with a much higher concentration of rare gases than the lower mantle. However, the amount of radiogenic ^(129)Xe and ^(136)Xe in the atmosphere as well as the lower mantle implies a substantial loss of rare gases. It is most likely that rare gases have been lost during late accretion and/or during; the hypothesized moon-forming impact. The nonradiogenic rare gases in the atmosphere were then supplied by subsequently accreted material with nonradiogenic xenon, possibly in comets. Fractionation of atmospheric xenon isotopes relative to other early solar system components must have occurred either on the late-accreting materials or during subsequent loss from the Earth.

The behavior of U- and Th-series nuclides in groundwater

Groundwater has long been an active area of research driven by its importance both as a societal resource and as a component in the global hydrological cycle. Key issues in groundwater research include inferring rates of transport of chemical constituents, determining the ages of groundwater, and tracing water masses using chemical fingerprints. While information on the trace elements pertinent to these topics can be obtained from aquifer tests using experimentally introduced tracers, and from laboratory experiments on aquifer materials, these studies are necessarily limited in time and space. Regional studies of aquifers can focus on greater scales and time periods, but must contend with greater complexities and variations. In this regard, the isotopic systematics of the naturally occurring radionuclides in the U- and Th- decay series have been invaluable in investigating aquifer behavior of U, Th, and Ra. These nuclides are present in all groundwaters and are each represented by several isotopes with very different half-lives, so that processes occurring over a range of time-scales can be studied (Table 1⇓). Within the host aquifer minerals, the radionuclides in each decay series are generally expected to be in secular equilibrium and so have equal activities (see Bourdon et al. 2003). In contrast, these nuclides exhibit strong relative fractionations within the surrounding groundwaters that reflect contrasting behavior during release into the water and during interaction with the surrounding host aquifer rocks. Radionuclide data can be used, within the framework of models of the processes involved, to obtain quantitative assessments of radionuclide release from aquifer rocks and groundwater migration rates. The isotopic variations that are generated also have the potential for providing fingerprints for groundwaters from specific aquifer environments, and have even been explored as a means for calculating groundwater ages. View this table: Table 1. Radionuclides important in groundwater studies. The highly fractionated nature of the …

Thallium isotope variations in seawater and hydrogenetic, diagenetic, and hydrothermal ferromanganese deposits

Results are presented for the first in-depth investigation of Tl isotope variations in marine materials. The Tl isotopic measurements were conducted by multiple collector-inductively coupled plasma mass spectrometry for a comprehensive suite of hydrogenetic ferromanganese crusts, diagenetic Fe–Mn nodules, hydrothermal manganese deposits and seawater samples. The natural variability of Tl isotope compositions in these samples exceeds the analytical reproducibility (±0.05‰) by more than a factor of 40. Hydrogenetic Fe–Mn crusts have ϵ205Tl of +10 to +14, whereas seawater is characterized by values as low as −8 (ϵ205Tl represents the deviation of the 205Tl/203Tl ratio of a sample from the NIST SRM 997 Tl isotope standard in parts per 104). This ∼2‰ difference in isotope composition is thought to result from the isotope fractionation that accompanies the adsorption of Tl onto ferromanganese particles. An equilibrium fractionation factor of α∼1.0021 is calculated for this process. Ferromanganese nodules and hydrothermal manganese deposits have variable Tl isotope compositions that range between the values obtained for seawater and hydrogenetic Fe–Mn crusts. The variability in ϵ205Tl in diagenetic nodules appears to be caused by the adsorption of Tl from pore fluids, which act as a closed-system reservoir with a Tl isotope composition that is inferred to be similar to seawater. Nodules with ϵ205Tl values similar to seawater are found if the scavenging of Tl is nearly quantitative. Hydrothermal manganese deposits display a positive correlation between ϵ205Tl and Mn/Fe. This trend is thought to be due to the derivation of Tl from distinct hydrothermal sources. Deposits with low Mn/Fe ratios and low ϵ205Tl are produced by the adsorption of Tl from fluids that are sampled close to hydrothermal sources. Such fluids have low Mn/Fe ratios and relatively high temperatures, such that only minor isotope fractionation occurs during adsorption. Hydrothermal manganese deposits with high Mn/Fe and high ϵ205Tl are generated by scavenging of Tl from colder, more distal hydrothermal fluids. Under such conditions, adsorption is associated with significant isotope fractionation, and this produces deposits with higher ϵ205Tl values coupled with high Mn/Fe.

U-TH ISOTOPE SYSTEMATICS FROM THE SOREQ CAVE, ISRAEL AND CLIMATIC CORRELATIONS

Precise ^(230)Th-^(234)U ages were obtained on thirty-one growth laminae in speleothem samples which are self-consistent with the detailed layer stratigraphy. Samples with low ^(232)Th/^(238)U ratios give ages with analytical uncertainties of 40 years at 2 ky and 1000 years at 90 ky. Some growth zones with high but variable ^(232)Th/^(238)U were dated by intermal isochrons. This permits the determination of the initial ^(230)Th/^(232)Th assuming equilibrium of ^(232)Th and ^(238)U series in the source of the high ^(232)Th component. This shows initial (^(230)Th/^(232)Th) (in activity units) of from 1.3 to 2.9. The calculated atomic ratios of ^(232)Th/^(238)U for the high ^(232)Th component range from 1.08 to 2.4 which is well below the average crustal value. Speleothem materials with high ^(232)Th/^(238)U are found to exhibit clear correlations of ^(232)Th with Si, Al and Fe, while ^(238)U correlates with Sr and Ba. Analyses of Soreq cave drip waters show that the particulates in the waters have high ^(232)Th concentrations and a ^(232)Th/^(238)U ratio much lower than that found in the high ^(232)Th component in speleothems but with ^(230)Th/^(232)Th) = 1.0 to 2.4. We infer that the trapped high-Th component in speleothems is from particulate matter in water with a large concentration of adsorbed U and not simply from detrital material. The speleothems have only small234U excess The initial ^(234)U/^(238)U)0 show a range of 1.02 to 1.14 that was found to correlate with age over the past 25 ky. The youngest samples have values in the same range as the modern drip waters. There appears to be a correlation of ^(234)U/^(238)U)0 with the δ^(18)O values. There is a drop of δ^(18)O in the time interval 20 to 15 ky which then remains relatively constant to recent times. As the high δ^(18)O values have been related to rainfall and associated climatic conditions, we suggest that the ^(234)U/^(238)U in the speleothem reflects the effects of rainfall and soil weathering conditions on drip-water composition and may provide a proxy for climate change.

Nonconservative behavior of dissolved organic carbon across the Laptev and East Siberian seas

Climate change is expected to have a strong effect on the Eastern Siberian Arctic Shelf (ESAS) region, which includes 40% of the Arctic shelves and comprises the Laptev and East Siberian seas. The ...

The core as a possible source of mantle helium

The core has often been suggested as a source for He presently found in the mantle. If the core served as the long-term storage reservoir for He in ocean island basalts with distinctively high 3He/4He ratios, then the difficulties of maintaining an isolated mantle reservoir separate from the source of mid-ocean ridge basalts are removed. However, the possibilities of trapping He into the core and releasing it into the overlying mantle have not yet been systematically evaluated. Here some of the factors to be considered are discussed. Appealing to the core as a source of rare gases necessarily evokes specific conditions of terrestrial rare gas acquisition and core formation, as well as core composition characteristics. It is shown that even if partition coefficients between silicates (solid or melt) and liquid Fe are low, there may have been sufficient gas present in the mantle during core segregation to supply a substantial quantity of He and Ne to the core. Transfer from the core to the mantle by either bulk entrainment of core material or chemical interaction at the core–mantle boundary may provide a reasonable mechanism for supplying relatively unfractionated rare gases to plumes. However, this remains speculative. While this process may have considerable impact on other trace elements, further limits on the rate of such transfer are only possible once further constraints are available on the concentration of other elements in the outer core. There are presently insufficient data available to establish whether or not the core is a plausible source of mantle He with high 3He/4He ratios. Therefore, further systematic investigation of this possibility should be conducted.

PARTICLE TRANSPORT OF 234U-238U IN THE KALIX RIVER AND IN THE BALTIC SEA

The role of particles for U isotope transport was investigated in the Kalix River watershed, a particle-poor, Fe/Mn-rich river in northern Sweden, and in the Baltic Sea estuary. Particles >0.45μm are strongly enriched in U and contain 20-50% of the total riverine uranium budget and <1% of the total U in brackish waters (3-7 PSU). The particles have high δ^(234)U which is close to that of dissolved U in the associated water, indicating that U on particles is dominantly nondetrital and isotopically exchanges rapidly with the ambient dissolved U. Particles at the river mouth are dominated by nondetrital Fe-Mn oxyhydroxides. Uranium and Fe are strongly correlated, clearly demonstrating that secondary Fe-oxyhydroxide is the major carrier of U in river water. There is no evidence for significant association of U with Mn-oxyhydroxide. Apparent U distribution coefficients (K_d^(Fe)) were calculated for U between the authigenic Fe on particles and the solution. These values appear to be relatively constant throughout the year. This suggests an equilibrium between Fe in solution and authigenic Fe-oxyhydroxides on detrital particles. High values of K_d^(Fe) calculated for one summer as well as high U concentrations in brackish waters can be explained by U scavenging by biogenic phases with low authigenic Fe content.

Helium and strontium isotopes in ultramafic xenoliths

Abstract The isotopic compositions of He and Sr have been obtained for suites of ultramafic xenoliths brought to the surface by Recent volcanic eruptions in continental environments. Samples include a range of petrologic types from Lashaine, Eledoi and Pello Hill (Tanzania), Bullenmerri maar (Victoria, Australia), Puy Beaunit (Massif Central, France) and Ataq (South Yemen). Primordial mantle He was found in all samples, with whole-rock analyses yielding 3 He 4 He ratios ( R ) that show a restricted range from 6 to 10 times the atmospheric ratio ( R A ). This is within the documented range for mid-ocean ridge basalts and indicates that the He at these diverse locations, and by implication associated fluids such as CO 2 , are derived from the same mantle circulation as sampled by volcanicity at spreading ridges. Small but significant intermineral differences between coexisting olivine and clinopyroxene are due to minor radiogenic additions of He in the latter. Although the xenoliths exhibit a relatively uniform He isotopic signature, separated clinopyroxenes have 87 Sr 86 Sr ratios that vary from 0.7036 to 0.8360, and demonstrate a complete decoupling of the isotopes of He from those of the lithophile trace elements.

In search of lost planets – the paleocosmochemistry of the inner solar system

The depletion of moderately volatile elements in planetesimals and planets is generally considered to be a result of removal of hot nebula gases. This theory can be tested with Sr isotopes. The calculated initial 87Sr/86Sr of the angrite parent body (APB), eucrite parent body (EPB), the Moon and the Earth are significantly higher than the initial Sr isotopic composition of the solar system despite the volatile-depleted nature of all of these objects. Calculated time-scales required to accomplish these increases in 87Sr/86Sr with a solar Rb/Sr in a nebula environment are >2 Myr for the APB, >3 Myr for the EPB and >10 Myr for the Moon. These times are more than an order of magnitude longer than that expected for cooling the nebula in the terrestrial planet-forming region and correspond to the period during which most of the mass already should have been accreted into sizeable planetesimals and even planets. Therefore, incomplete condensation of the nebula does not provide an adequate explanation for the depletion in moderately volatile elements. The data are better explained by a protracted history of depletion via more than one mechanism, including processes completely divorced from the earliest cooling of the circumstellar disk. The Sr model ages are maximum formation ages of the APB and EPB and indicate that these are most probably secondary objects. With independent estimates of their minimum age, a time-integrated Rb/Sr can be calculated for the precursor materials from which they formed. These are consistent with accretion of the APB and EPB from objects that at one stage may have resembled carbonaceous chondrite parent bodies in terms of volatile budgets. At some late stage there were large losses of volatiles, the most likely mechanism for which is very energetic collisions between planetesimals and proto-planets that, in the case of the Asteroid Belt, have since been lost. The same applies to the Moon, which presently has Rb/Sr=0.006 even though the material from which it formed had a time-integrated Rb/Sr ratio of ∼0.07, consistent with a precursor planet (Theia) that was even less volatile element-depleted than the present Earth (Rb/Sr=0.03). The time-integrated Rb/Sr of Theia is similar to the present Rb/Sr of Mars (0.07). There is suggestive evidence of a similar time-integrated value for the proto-Earth (∼0.09). Therefore, prior to the later stages of planet formation involving giant impacts between large objects, the inner solar system may have had relatively uniform concentrations of moderately volatile elements broadly similar to those found in volatile-depleted chondrites. Correlations of the present Rb/Sr ratios in planets and planetesimals with ratios of other volatile elements to Sr can be used to infer the time-integrated composition of precursor materials. The time-integrated inferred K/U ratios of the proto-Earth, as well as Theia, were ∼20 000, so that early radioactive heat production may have been ∼40% greater than that calculated by extrapolating back from the Earth’s present K/U. Higher C and S bulk concentrations may have led to concentrations in proto-cores of 0.6–1.5% C and 4–10% S. These are significantly higher than those anticipated from the degree of volatile depletion of the present silicate Earth (∼0.12% C, ∼1.3% S). If the late history of accretion did not involve large-scale re-equilibration of silicates and metal, the present core may have inherited such high C and S concentrations. In this case, S would be the dominant light element in the present core.

The importance of colloids for the behavior of uranium isotopes in the low-salinity zone of a stable estuary

Particle-mediated removal processes of U isotopes were investigated during spring flood discharge in the low-salinity zone (LSZ, up to 3 practical salinity units [psu]) of a stable estuary. A shipboard ultrafiltration cross-flow filtration (CFF) technique was used to separate particles (>0.2 μm) and colloids (between 3000 daltons (3 kD) and 0.2 μm) from ultrafiltered water (<3 kD) containing “dissolved” species. Sediment traps were used to collect sinking material. Concentration of Fe and organic C, which are indicators of the major U carrier phases, were used to interpret the behavior of ^(234)U-^(238)U during estuarine mixing. Colloids dominated the river water transport of U, carrying ≈90% of the U. On entering the estuary, colloids accounted for the dominant fraction of U to about a salinity of 1 psu, but only a minor fraction ( 1 psu, there is a general correlation between U and salinity in all filtered fractions. The ^(234)U/^(238)U ratios in different filtered fractions and sinking particles were generally indistinguishable at each station and showed enrichment in ^(234)U, compared with secular equilibrium (δ^(234)U = 266–567). This clearly shows that all size fractions are dominated by nondetrital U. Consideration of U isotope systematics across the estuary reveals that substantial U exchange must occur involving larger particles at least to 1 psu and involving colloids at least to ≈1.5 psu. Further exchange at higher salinities may also occur, as the proportion of U on colloids decreases with increasing salinity. This may be due to decreasing colloid concentration and increasing stabilization of uranyl carbonate complexes during mixing in the estuary. The results show that although U is a soluble element that shows generally conservative mixing in estuaries, removal occurs in the very low salinity zone, and this zone represents a significant sink of U. Variation in composition and concentration of colloidal particles between different estuaries might thus be an important factor for determining the varying behavior of U between estuaries.