No abstract

The 238 U/ 235 U isotopic composition of uranium in seawater can provide important insights into the modern U budget of the oceans. Using the double spike technique and a new data reduction method, we analyzed an array of seawater samples and 41 geostandards covering a broad range of geological settings relevant to low and high temperature geochemistry. Analyses of 18 seawater samples from geographically diverse sites from the Atlantic and Pacific oceans, Mediterranean Sea, Gulf of Mexico, Persian Gulf, and English Channel, together with literature data (n = 17), yield a d 238 U value for modern seawater of À0.392 ± 0.005& relative to CRM-112a. Measurements of the uranium isotopic compositions of river water, lake water, evaporites, modern coral, shales, and various igneous rocks (n = 64), together with compilations of literature data (n = 380), allow us to estimate the uranium isotopic compositions of the various reservoirs involved in the modern oceanic uranium budget, as well as the fractionation factors associated with U incorporation into those reservoirs. Because the incorporation of U into anoxic/euxinic sediments is accompanied by large isotopic fractionation (D Anoxic/Euxinic-SW = +0.6&), the size of the anoxic/euxinic sink strongly influences the d 238 U value of seawater. Keeping all other fluxes constant, the flux of uranium in the anoxic/euxinic sink is constrained to be 7.0 ± 3.1 Mmol/yr (or 14 ± 3% of the total flux out of the ocean). This translates into an areal extent of anoxia into the modern ocean of 0.21 ± 0.09% of the total seafloor. This agrees with independent estimates and rules out a recent uranium budget estimate by Henderson and Anderson (2003). Using the mass fractions and isotopic compositions of various rock types in Earth's crust, we further calculate an average d 238 U isotopic composition for the continental crust of À0.29 ± 0.03& corresponding to a 238 U/ 235 U isotopic ratio of 137.797 ± 0.005. We discuss the implications of the variability of the 238 U/ 235 U ratio on Pb-Pb and U-Pb ages and provide analytical formulas to calculate age corrections as a function of the age and isotopic composition of the sample. The crustal ratio may be used in calculation of Pb-Pb and U-Pb ages of continental crust rocks and minerals when the U isotopic composition is unknown.

No abstract

Zr stable isotopes exhibit >5‰ fractionations at magmatic conditions, yielding novel insights into melt chemical evolution.

No abstract

No abstract

The equilibrium Fe isotopic fractionation factors of goethite and jarosite have considerable importance for interpreting Fe isotope variations in low temperature aqueous systems on Earth and possibly Mars in the context of future sample return missions. We measured the b-factors of goethite FeO(OH), potassium-jarosite KFe 3 (SO 4 ) 2 (OH) 6 , and hydronium-jarosite (H 3 O)Fe 3 (SO 4 ) 2 (OH) 6 , by Nuclear Resonant Inelastic X-ray Scattering (NRIXS, also known as Nuclear Resonance Vibrational Spectroscopy -NRVS or Nuclear Inelastic Scattering -NIS) at the Advanced Photon Source. These measurements were made on synthetic minerals enriched in 57 Fe. A new method (i.e., the general moment approach) is presented to calculate b-factors from the moments of the NRIXS spectrum S(E). The first term in the moment expansion controls iron isotopic fractionation at high temperature and corresponds to the mean force constant of the iron bonds, a quantity that is readily measured and often reported in NRIXS studies. The mean force constants of goethite, potassium-jarosite, and hydronium-jarosite are 314 ± 14, 264 ± 12, and 310 ± 14 N/m, respectively (uncertainties include statistical and systematic errors). The general moment approach gives 56 Fe/ 54 Fe b-factors of 9.7, 8.3, and 9.5& at 22°C for these minerals. The b-factor of goethite measured by NRIXS is larger than that estimated by combining results from laboratory exchange experiments and calculations based on electronic structure theory. Similar issues have been identified previously for other pairs of mineral-aqueous species, which could reflect inadequacies of approaches based on electronic structure theory to calculate absolute b-factors (differences in b-factors between aqueous species may be more accurate) or failure of laboratory experiments to measure mineral-fluid equilibrium isotopic fractionation at low temperature. We apply the force constant approach to published NRIXS data and report 1000 Â ln b for important Fe-bearing phases of geological and biochemical relevance such as myoglobin, cytochrome f, pyroxene, metal, troilite, chalcopyrite, hematite, and magnetite.

No abstract