Publications by authors named "Nicolas Dauphas"

22 Publications

  • Page 1 of 1

Imprint of chondrule formation on the K and Rb isotopic compositions of carbonaceous meteorites.

Sci Adv 2021 Dec 1;7(49):eabl3929. Epub 2021 Dec 1.

Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA.

[Figure: see text].
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http://dx.doi.org/10.1126/sciadv.abl3929DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8635422PMC
December 2021

Survival of presolar -nuclide carriers in the nebula revealed by stepwise leaching of Allende refractory inclusions.

Sci Adv 2021 Jul 9;7(28). Epub 2021 Jul 9.

The Isotoparium, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.

The Rb-Sr radiochronometer provides key insights into the timing of volatile element depletion in planetary bodies, yet the unknown nucleosynthetic origin of Sr anomalies in Ca-Al-rich inclusions (CAIs, the oldest dated solar system solids) challenges the reliability of resulting chronological interpretations. To identify the nature of these Sr anomalies, we performed step-leaching experiments on nine unmelted CAIs from Allende. In six CAIs, the chemically resistant residues (0.06 to 9.7% total CAI Sr) show extreme positive μSr (up to +80,655) and Sr variations that cannot be explained by decay of Rb. The extreme Sr but more subdued Sr anomalies are best explained by the presence of a presolar carrier enriched in the -nuclide Sr. We argue that this unidentified carrier controls the isotopic anomalies in bulk CAIs and outer solar system materials, which reinstates the chronological significance of differences in initial Sr/Sr between CAIs and volatile-depleted inner solar system materials.
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http://dx.doi.org/10.1126/sciadv.abf6222DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8270483PMC
July 2021

Titanium isotopes constrain a magmatic transition at the Hadean-Archean boundary in the Acasta Gneiss Complex.

Sci Adv 2020 Dec 9;6(50). Epub 2020 Dec 9.

Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA.

Plate subduction greatly influences the physical and chemical characteristics of Earth's surface and deep interior, yet the timing of its initiation is debated because of the paucity of exposed rocks from Earth's early history. We show that the titanium isotopic composition of orthogneisses from the Acasta Gneiss Complex spanning the Hadean to Eoarchean transition falls on two distinct magmatic differentiation trends. Hadean tonalitic gneisses show titanium isotopic compositions comparable to modern evolved tholeiitic magmas, formed by differentiation of dry parental magmas in plume settings. Younger Eoarchean granitoid gneisses have titanium isotopic compositions comparable to modern calc-alkaline magmas produced in convergent arcs. Our data therefore document a shift from tholeiitic- to calc-alkaline-style magmatism between 4.02 and 3.75 billion years (Ga) in the Slave craton.
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http://dx.doi.org/10.1126/sciadv.abc9959DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7725461PMC
December 2020

Triple iron isotope constraints on the role of ocean iron sinks in early atmospheric oxygenation.

Science 2020 10;370(6515):446-449

Department of Earth and Planetary Sciences, University of California, Riverside, CA 92521, USA.

The role that iron played in the oxygenation of Earth's surface is equivocal. Iron could have consumed molecular oxygen when Fe-oxyhydroxides formed in the oceans, or it could have promoted atmospheric oxidation by means of pyrite burial. Through high-precision iron isotopic measurements of Archean-Paleoproterozoic sediments and laboratory grown pyrites, we show that the triple iron isotopic composition of Neoarchean-Paleoproterozoic pyrites requires both extensive marine iron oxidation and sulfide-limited pyritization. Using an isotopic fractionation model informed by these data, we constrain the relative sizes of sedimentary Fe-oxyhydroxide and pyrite sinks for Neoarchean marine iron. We show that pyrite burial could have resulted in molecular oxygen export exceeding local Fe oxidation sinks, thereby contributing to early episodes of transient oxygenation of Archean surface environments.
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http://dx.doi.org/10.1126/science.aaz8821DOI Listing
October 2020

Triggered Star Formation inside the Shell of a Wolf-Rayet Bubble as the Origin of the Solar System.

Proc Int Astron Union 2018 08 13;14(S345):78-82. Epub 2020 Jan 13.

Clemson University.

A constraint on Solar System formation is the high Al/Al abundance ratio, 17 times higher than the average Galactic ratio, while the Fe/Fe value was lower than the Galactic value. This challenges the assumption that a nearby supernova was responsible for the injection of these short-lived radionuclides into the early Solar System. We suggest that the Solar System was formed by triggered star formation at the edge of a Wolf-Rayet (W-R) bubble. We discuss the details of various processes within the model using numerical simulations, and analytic and semi-analytic calculations, and conclude that it is a viable model that can explain the initial abundances of Al and Fe. We estimate that 1%-16% of all Sun-like stars could have formed in such a setting.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6425492PMC
August 2018

SciPhon: a data analysis software for nuclear resonant inelastic X-ray scattering with applications to Fe, Kr, Sn, Eu and Dy.

J Synchrotron Radiat 2018 Sep 21;25(Pt 5):1581-1599. Epub 2018 Aug 21.

Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60615, USA.

The synchrotron radiation technique of nuclear resonant inelastic X-ray scattering (NRIXS), also known as nuclear resonance vibrational spectroscopy or nuclear inelastic scattering, provides a wealth of information on the vibrational properties of solids. It has found applications in studies of lattice dynamics and elasticity, superconductivity, heme biochemistry, seismology, isotope geochemistry and many other fields. It involves probing the vibrational modes of solids by using the nuclear resonance of Mössbauer isotopes such as Fe, Kr, Sn, Eu and Dy. After data reduction, it provides the partial phonon density of states of the Mössbauer isotope that is investigated, as well as many other derived quantities such as the mean force constant of the chemical bonds and the Debye velocity. The data reduction is, however, not straightforward and involves removal of the elastic peak, normalization and Fourier-Log transformation. Furthermore, some of the quantities derived are highly sensitive to details in the baseline correction. A software package and several novel procedures to streamline and hopefully improve the reduction of the NRIXS data generated at sector 3ID of the Advanced Photon Source have been developed. The graphical user interface software is named SciPhon and runs as a Mathematica package. It is easily portable to other platforms and can be easily adapted for reducing data generated at other beamlines. Several tests and comparisons are presented that demonstrate the usefulness of this software, whose results have already been used in several publications. Here, the SciPhon software is used to reduce Kr, Sn, Eu and Dy NRIXS data, and potential implications for interpreting natural isotopic variations in those systems are discussed.
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http://dx.doi.org/10.1107/S1600577518009487DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6140397PMC
September 2018

Oxygen isotopic evidence for accretion of Earth's water before a high-energy Moon-forming giant impact.

Sci Adv 2018 03 28;4(3):eaao5928. Epub 2018 Mar 28.

Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK.

The Earth-Moon system likely formed as a result of a collision between two large planetary objects. Debate about their relative masses, the impact energy involved, and the extent of isotopic homogenization continues. We present the results of a high-precision oxygen isotope study of an extensive suite of lunar and terrestrial samples. We demonstrate that lunar rocks and terrestrial basalts show a 3 to 4 ppm (parts per million), statistically resolvable, difference in ΔO. Taking aubrite meteorites as a candidate impactor material, we show that the giant impact scenario involved nearly complete mixing between the target and impactor. Alternatively, the degree of similarity between the ΔO values of the impactor and the proto-Earth must have been significantly closer than that between Earth and aubrites. If the Earth-Moon system evolved from an initially highly vaporized and isotopically homogenized state, as indicated by recent dynamical models, then the terrestrial basalt-lunar oxygen isotope difference detected by our study may be a reflection of post-giant impact additions to Earth. On the basis of this assumption, our data indicate that post-giant impact additions to Earth could have contributed between 5 and 30% of Earth's water, depending on global water estimates. Consequently, our data indicate that the bulk of Earth's water was accreted before the giant impact and not later, as often proposed.
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http://dx.doi.org/10.1126/sciadv.aao5928DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5873841PMC
March 2018

Triggered Star Formation inside the Shell of a Wolf-Rayet Bubble as the Origin of the Solar System.

Astrophys J 2017 Dec 22;851(2). Epub 2017 Dec 22.

Department of Physics and Astronomy, Clemson University, Clemson, SC, 29634-0978, USA;

A critical constraint on solar system formation is the high Al/Al abundance ratio of 5 × 10 at the time of formation, which was about 17 times higher than the average Galactic ratio, while the Fe/Fe value was about 2 × 10, lower than the Galactic value. This challenges the assumption that a nearby supernova (SN) was responsible for the injection of these short-lived radionuclides into the early solar system. We show that this conundrum can be resolved if the solar system was formed by a triggered star formation at the edge of a Wolf-Rayet (W-R) bubble. Al is produced during the evolution of the massive star, released in the wind during the W-R phase, and condenses into dust grains that are seen around W-R stars. The dust grains survive passage through the reverse shock and the low-density shocked wind, reach the dense shell swept-up by the bubble, detach from the decelerated wind, and are injected into the shell. Some portions of this shell subsequently collapse to form the dense cores that give rise to solar-type systems. The subsequent aspherical SN does not inject appreciable amounts of Fe into the proto-solar system, thus accounting for the observed low abundance of Fe. We discuss the details of various processes within the model and conclude that it is a viable model that can explain the initial abundances of Al and Fe. We estimate that 1%-16% of all Sun-like stars could have formed in such a setting of triggered star formation in the shell of a W-R bubble.
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http://dx.doi.org/10.3847/1538-4357/aa992eDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6430574PMC
December 2017

Titanium isotopic evidence for felsic crust and plate tectonics 3.5 billion years ago.

Science 2017 09;357(6357):1271-1274

Department of Geology, University of Johannesburg, Post Office Box 524, Auckland Park, 2006, Republic of South Africa.

Earth exhibits a dichotomy in elevation and chemical composition between the continents and ocean floor. Reconstructing when this dichotomy arose is important for understanding when plate tectonics started and how the supply of nutrients to the oceans changed through time. We measured the titanium isotopic composition of shales to constrain the chemical composition of the continental crust exposed to weathering and found that shales of all ages have a uniform isotopic composition. This can only be explained if the emerged crust was predominantly felsic (silica-rich) since 3.5 billion years ago, requiring an early initiation of plate tectonics. We also observed a change in the abundance of biologically important nutrients phosphorus and nickel across the Archean-Proterozoic boundary, which might have helped trigger the rise in atmospheric oxygen.
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http://dx.doi.org/10.1126/science.aan8086DOI Listing
September 2017

Iron isotopic fractionation between silicate mantle and metallic core at high pressure.

Nat Commun 2017 02 20;8:14377. Epub 2017 Feb 20.

Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA.

The +0.1‰ elevated Fe/Fe ratio of terrestrial basalts relative to chondrites was proposed to be a fingerprint of core-mantle segregation. However, the extent of iron isotopic fractionation between molten metal and silicate under high pressure-temperature conditions is poorly known. Here we show that iron forms chemical bonds of similar strengths in basaltic glasses and iron-rich alloys, even at high pressure. From the measured mean force constants of iron bonds, we calculate an equilibrium iron isotope fractionation between silicate and iron under core formation conditions in Earth of ∼0-0.02‰, which is small relative to the +0.1‰ shift of terrestrial basalts. This result is unaffected by small amounts of nickel and candidate core-forming light elements, as the isotopic shifts associated with such alloying are small. This study suggests that the variability in iron isotopic composition in planetary objects cannot be due to core formation.
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http://dx.doi.org/10.1038/ncomms14377DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5321738PMC
February 2017

The isotopic nature of the Earth's accreting material through time.

Authors:
Nicolas Dauphas

Nature 2017 01;541(7638):521-524

Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA.

The Earth formed by accretion of Moon- to Mars-size embryos coming from various heliocentric distances. The isotopic nature of these bodies is unknown. However, taking meteorites as a guide, most models assume that the Earth must have formed from a heterogeneous assortment of embryos with distinct isotopic compositions. High-precision measurements, however, show that the Earth, the Moon and enstatite meteorites have almost indistinguishable isotopic compositions. Models have been proposed that reconcile the Earth-Moon similarity with the inferred heterogeneous nature of Earth-forming material, but these models either require specific geometries for the Moon-forming impact or can explain only one aspect of the Earth-Moon similarity (that is, O). Here I show that elements with distinct affinities for metal can be used to decipher the isotopic nature of the Earth's accreting material through time. I find that the mantle signatures of lithophile O, Ca, Ti and Nd, moderately siderophile Cr, Ni and Mo, and highly siderophile Ru record different stages of the Earth's accretion; yet all those elements point to material that was isotopically most similar to enstatite meteorites. This isotopic similarity indicates that the material accreted by the Earth always comprised a large fraction of enstatite-type impactors (about half were E-type in the first 60 per cent of the accretion and all of the impactors were E-type after that). Accordingly, the giant impactor that formed the Moon probably had an isotopic composition similar to that of the Earth, hence relaxing the constraints on models of lunar formation. Enstatite meteorites and the Earth were formed from the same isotopic reservoir but they diverged in their chemical evolution owing to subsequent fractionation by nebular and planetary processes.
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http://dx.doi.org/10.1038/nature20830DOI Listing
January 2017

Origin of uranium isotope variations in early solar nebula condensates.

Sci Adv 2016 Mar 4;2(3):e1501400. Epub 2016 Mar 4.

Origins Lab, Department of the Geophysical Sciences, and Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA.

High-temperature condensates found in meteorites display uranium isotopic variations ((235)U/(238)U), which complicate dating the solar system's formation and whose origin remains mysterious. It is possible that these variations are due to the decay of the short-lived radionuclide (247)Cm (t 1/2 = 15.6 My) into (235)U, but they could also be due to uranium kinetic isotopic fractionation during condensation. We report uranium isotope measurements of meteoritic refractory inclusions that reveal excesses of (235)U reaching ~+6% relative to average solar system composition, which can only be due to the decay of (247)Cm. This allows us to constrain the (247)Cm/(235)U ratio at solar system formation to (1.1 ± 0.3) × 10(-4). This value provides new clues on the universality of the nucleosynthetic r-process of rapid neutron capture.
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http://dx.doi.org/10.1126/sciadv.1501400DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4783122PMC
March 2016

Geochemical arguments for an Earth-like Moon-forming impactor.

Philos Trans A Math Phys Eng Sci 2014 Sep;372(2024):20130244

Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA.

Geochemical evidence suggests that the material accreted by the Earth did not change in nature during Earth's accretion, presumably because the inner protoplanetary disc had uniform isotopic composition similar to enstatite chondrites, aubrites and ungrouped achondrite NWA 5363/5400. Enstatite meteorites and the Earth were derived from the same nebular reservoir but diverged in their chemical evolutions, so no chondrite sample in meteorite collections is representative of the Earth's building blocks. The similarity in isotopic composition (Δ(17)O, ε(50)Ti and ε(54)Cr) between lunar and terrestrial rocks is explained by the fact that the Moon-forming impactor came from the same region of the disc as other Earth-forming embryos, and therefore was similar in isotopic composition to the Earth. The heavy δ(30)Si values of the silicate Earth and the Moon relative to known chondrites may be due to fractionation in the solar nebula/protoplanetary disc rather than partitioning of silicon in Earth's core. An inversion method is presented to calculate the Hf/W ratios and ε(182)W values of the proto-Earth and impactor mantles for a given Moon-forming impact scenario. The similarity in tungsten isotopic composition between lunar and terrestrial rocks is a coincidence that can be explained in a canonical giant impact scenario if an early formed embryo (two-stage model age of 10-20 Myr) collided with the proto-Earth formed over a more protracted accretion history (two-stage model age of 30-40 Myr).
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http://dx.doi.org/10.1098/rsta.2013.0244DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4128266PMC
September 2014

Geochemistry: Sulphur from heaven and hell.

Authors:
Nicolas Dauphas

Nature 2013 Sep 4;501(7466):175-6. Epub 2013 Sep 4.

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http://dx.doi.org/10.1038/nature12554DOI Listing
September 2013

Distribution coefficients of 60 elements on TODGA resin: application to Ca, Lu, Hf, U and Th isotope geochemistry.

Talanta 2010 May 22;81(3):741-53. Epub 2010 Jan 22.

Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA.

Batch equilibration experiments are conducted to measure the distribution coefficients (K(d)) of a large number of elements in nitric, nitric plus hydrofluoric, and hydrochloric acids on Eichrom TODGA extraction chromatography resin. The K(d)s are used to devise a multi-element extraction scheme for high-precision elemental and isotopic analyses of Ca, Hf, Lu, Th and U in geological materials, using high-purity lithium metaborate (LiBO(2)) flux fusion that allows rapid digestion of even the most refractory materials. The fusion melt, dissolved in nitric acid, is directly loaded to a TODGA cartridge on a vacuum chamber for elemental separation. An Ln-Spec cartridge is used in tandem with TODGA for Lu purification. The entire procedure, from flux digestion to preparation for isotopic analysis, can be completed in a day. The accuracy of the proposed technique is tested by measuring the concentrations of Ca (standard bracketing), Hf, Lu, Th and U (isotope dilution), and the isotopic composition of Hf in geostandards (USNM3529, BCR-2, BHVO-1, AGV-1 and AGV-2). All measurements are in excellent agreement with recommended literature values, demonstrating the effectiveness of the proposed analytical procedure and the versatility of TODGA resin.
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http://dx.doi.org/10.1016/j.talanta.2010.01.008DOI Listing
May 2010

Iron isotope fractionation during magmatic differentiation in Kilauea Iki lava lake.

Science 2008 Jun;320(5883):1620-2

Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA.

Magmatic differentiation helps produce the chemical and petrographic diversity of terrestrial rocks. The extent to which magmatic differentiation fractionates nonradiogenic isotopes is uncertain for some elements. We report analyses of iron isotopes in basalts from Kilauea Iki lava lake, Hawaii. The iron isotopic compositions (56Fe/54Fe) of late-stagemeltveins are 0.2 permil (per thousand) greater than values for olivine cumulates. Olivine phenocrysts are up to 1.2 per thousand lighter than those of whole rocks. These results demonstrate that iron isotopes fractionate during magmatic differentiation at both whole-rock and crystal scales. This characteristic of iron relative to the characteristics of magnesium and lithium, for which no fractionation has been found, may be related to its complex redox chemistry in magmatic systems and makes iron a potential tool for studying planetary differentiation.
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http://dx.doi.org/10.1126/science.1157166DOI Listing
June 2008

Analytical developments for high-precision measurements of W isotopes in iron meteorites.

Anal Chem 2007 Apr 8;79(8):3148-54. Epub 2007 Mar 8.

Department of the Geophysical Sciences, The University of Chicago, Illinois 60637, USA.

A procedure was developed to accurately measure the W isotopic compositions of iron meteorites with a precision of better than +/-0.1 epsilon on epsilon182W and epsilon184W (normalized to 186W/183W). Purification of W was achieved through a two-step, ion-exchange procedure. In most cases, the yield is better than 80%, and purified W solutions are clear of matrix elements and direct isobars of W. The final W solutions were analyzed using a Micromass Isoprobe multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Tests performed on mixtures of terrestrial standards and meteorite samples demonstrate that the method is accurate and that epsilon182W variations as small as approximately 0.1 epsilon can be detected. Analyses of three different aliquots of the Gibeon (IVA) iron meteorite obtained over a period of 6 months show identical epsilon182W values with a weighted mean of 3.38 +/- 0.05, consistent with literature data for IVA iron meteorites, and indicating that the metal-silicate differentiation event in its parent body was either contemporaneous with or slightly postdated (by up to approximately 2.5 My) the formation of refractory inclusions. We demonstrate our ability to measure epsilon184W accurately and precisely (within +/-0.1 epsilon), which is useful for characterizing cosmogenic and nucleosynthetic effects that may be present in iron meteorites. We also report for the first time measurements of epsilon180W, albeit with large error bars (<+/-4 epsilon, in most cases).
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http://dx.doi.org/10.1021/ac062040cDOI Listing
April 2007

High precision measurements of non-mass-dependent effects in nickel isotopes in meteoritic metal via multicollector ICPMS.

Anal Chem 2006 Dec;78(24):8477-84

Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA.

We measured the Ni isotopic composition of metal from a variety of meteorite groups to search for variations in the 60Ni abundance from the decay of the short-lived nuclide 60Fe (t(1/2) = 1.49 My) and for possible nucleosynthetic effects in the other stable isotopes of Ni. We developed a high-yield Ni separation procedure based on a combination of anion and cation exchange chromatography. Nickel isotopes were measured on a single-focusing, multicollector, inductively coupled mass spectrometer (MC-ICPMS). The external precision on the mass-bias-corrected 60Ni/58Ni ratio (+/-0.15 epsilon; 2sigma) is comparable to similar studies using double-focusing MC-ICPMS. We report the first high-precision data for 64Ni, the least abundant Ni isotope, obtained via MC-ICPMS. The external precision on the mass-bias-corrected 64Ni/58Ni ratio (+/-1.5 epsilon; 2sigma) is better than previous studies using thermal ionization mass spectrometry. No resolvable excesses relative to a terrestrial standard in the mass-bias-corrected 60Ni/58Ni ratio were detected in any meteoritic metal samples. However, resolvable deficits in this ratio were measured in the metal from several unequilibrated chondrites, implying a 60Fe/56Fe ratio of approximately 1 x 10(-6) at the time of Fe/Ni fractionation in chondritic metal. A 60Fe/56Fe ratio of (4.6 +/- 3.3) x 10(-7) is inferred at the time of Fe/Ni fractionation on the parent bodies of magmatic iron meteorites and pallasites. No clearly resolvable non-mass-dependent anomalies were detected in the other stable isotopes of Ni in the samples investigated here, indicating that the Ni isotopic composition in the early solar system was homogeneous (at least at the level of precision reported here) at the time of meteoritic metal formation.
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http://dx.doi.org/10.1021/ac061285mDOI Listing
December 2006

Mass spectrometry and natural variations of iron isotopes.

Mass Spectrom Rev 2006 Jul-Aug;25(4):515-50

Origins Laboratory, Department of the Geophysical Sciences, Enrico Fermi Institute, and Chicago Center for Cosmochemistry, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA.

Although the processes that govern iron isotope variations in nature are just beginning to be understood, multiple studies attest of the virtue of this system to solve important problems in geosciences and biology. In this article, we review recent advances in the geochemistry, cosmochemistry, and biochemistry of iron isotopes. In Section 2, we briefly address the question of the nucleosynthesis of Fe isotopes. In Section 3, we describe the different methods for purifying Fe and analyzing its isotopic composition. The methods of SIMS, RIMS, and TIMS are presented but more weight is given to measurements by MC-ICPMS. In Section 4, the isotope anomalies measured in extraterrestrial material are briefly discussed. In Section 5, we show how high temperature processes like evaporation, condensation, diffusion, reduction, and phase partitioning can affect Fe isotopic composition. In Section 6, the various low temperature processes causing Fe isotopic fractionation are presented. These involve aqueous and biologic systems.
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http://dx.doi.org/10.1002/mas.20078DOI Listing
December 2006

The U/Th production ratio and the age of the Milky Way from meteorites and Galactic halo stars.

Authors:
Nicolas Dauphas

Nature 2005 Jun;435(7046):1203-5

Origins Laboratory, Department of the Geophysical Sciences, Enrico Fermi Institute, and Chicago Center for Cosmochemistry, The University of Chicago, 5734 South Ellis Avenue, Chicago Illinois 60637, USA.

Some heavy elements (with atomic number A > 69) are produced by the 'rapid' (r)-process of nucleosynthesis, where lighter elements are bombarded with a massive flux of neutrons. Although this is characteristic of supernovae and neutron star mergers, uncertainties in where the r-process occurs persist because stellar models are too crude to allow precise quantification of this phenomenon. As a result, there are many uncertainties and assumptions in the models used to calculate the production ratios of actinides (like uranium-238 and thorium-232). Current estimates of the U/Th production ratio range from approximately 0.4 to 0.7. Here I show that the U/Th abundance ratio in meteorites can be used, in conjunction with observations of low-metallicity stars in the halo of the Milky Way, to determine the U/Th production ratio very precisely (0.57(+0.037)(-0.031). This value can be used in future studies to constrain the possible nuclear mass formulae used in r-process calculations, to help determine the source of Galactic cosmic rays, and to date circumstellar grains. I also estimate the age of the Milky Way (14.5(+2.8)(-2.2)Gyr in a way that is independent of the uncertainties associated with fluctuations in the microwave background or models of stellar evolution.
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http://dx.doi.org/10.1038/nature03645DOI Listing
June 2005

Clues from Fe isotope variations on the origin of early Archean BIFs from Greenland.

Science 2004 Dec;306(5704):2077-80

Origins Laboratory, Department of the Geophysical Sciences, and Enrico Fermi Institute, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA.

Archean rocks may provide a record of early Earth environments. However, such rocks have often been metamorphosed by high pressure and temperature, which can overprint the signatures of their original formation. Here, we show that the early Archean banded rocks from Isua, Akilia, and Innersuartuut, Greenland, are enriched in heavy iron isotopes by 0.1 to 0.5 per mil per atomic mass unit relative to igneous rocks worldwide. The observed enrichments are compatible with the transport, oxidation, and subsequent precipitation of ferrous iron emanating from hydrothermal vents and thus suggest that the original rocks were banded iron formations (BIFs). These variations therefore support a sedimentary origin for the Akilia banded rocks, which represent one of the oldest known occurrences of water-laid deposits on Earth.
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http://dx.doi.org/10.1126/science.1104639DOI Listing
December 2004

Chromatographic separation and multicollection-ICPMS analysis of iron. Investigating mass-dependent and -independent isotope effects.

Anal Chem 2004 Oct;76(19):5855-63

Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA.

A procedure was developed that allows precise determination of Fe isotopic composition. Purification of Fe was achieved by ion chromatography on AG1-X8 strongly basic anion-exchange resin. No isotopic fractionation is associated with column chemistry within 0.02 per thousand /amu at 2sigma. The isotopic composition was measured with a Micromass IsoProbe multicollection inductively coupled plasma hexapole mass spectrometer. The Fe isotopic composition of the Orgueil CI1 carbonaceous chondrite, which best approximates the solar composition, is indistinguishable from that of IRMM-014 (-0.005 +/- 0.017 per thousand /amu). The IRMM-014 reference material is therefore used for normalization of the isotopic ratios. The protocol for analyzing mass-dependent variations is validated by measuring geostandards (IF-G, DTS-2, BCR-2, AGV-2) and heavily fractionated Fe left after vacuum evaporation of molten wüstite (FeO) and solar (MgO-Al(2)O(3)-SiO(2)-CaO-FeO in chondritic proportions) compositions. It is shown that the isotopic composition of Fe during evaporation of FeO follows a Rayleigh distillation with a fractionation factor alpha equal to (m(1)/m(2)()1/2), where m(1) and m(2) are the masses of the considered isotopes. This agrees with earlier measurements and theoretical expectations. The isotopic composition of Fe left after vacuum evaporation of solar composition also follows a Rayleigh distillation but with a fractionation factor (1.013 22 +/- 0.000 67 for the (56)Fe/(54)Fe ratio) that is lower than the square root of the masses (1.018 35). The protocol for analyzing mass-independent variations is validated by measuring terrestrial rocks that are not expected to show departure from mass-dependent fractionation. After internal normalization of the (57)Fe/(54)Fe ratio, the isotopic composition of Fe can be measured accurately with precisions of 0.2epsilon and 0.5epsilon at 2sigma for (56)Fe/(54)Fe and (58)Fe/(54)Fe ratios, respectively (epsilon refers to relative variations in parts per 10 000). For (58)Fe, this precision is an order of magnitude better than what had been achieved before. The method is applied to rocks that could potentially exhibit mass-independent effects, meteorites and Archaean terrestrial samples. The isotopic composition of a 3.8-Ga-old banded iron formation from Isua (IF-G, Greenland), and quartz-pyroxene rocks from Akilia and Innersuartuut (GR91-26 and SM/GR/171770, Greenland) are normal within uncertainties. Similarly, the Orgueil (CI1), Allende (CV3.2), Eagle Station (ESPAL), Brenham (MGPAL), and Old Woman (IIAB) meteorites do not show any mass-independent effect.
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http://dx.doi.org/10.1021/ac0497095DOI Listing
October 2004
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