Extinct Isotopes Research Articles

Extinct isotope heterogeneities in the mantles of Earth and Mars: Implications for mantle stirring rates

AbstractHeterogeneities in terrestrial samples for 182W/183W and 142Nd/144Nd are only preserved in Hadean and Archean rocks while heterogeneities in 129Xe/130Xe and 136Xe/130Xe persist to very young mantle‐derived rocks. In contrast, meteorites from Mars show that the Martian mantle preserves heterogeneities in 182W/183W and 142Nd/144Nd up to the present. As a consequence of the probable “deep magma ocean” core formation process, we assume that the Earth and Mars both had a very early two‐mantle‐reservoir structure with different initial extinct nuclide isotopic compositions (different 182W/183W, 142Nd/144Nd, 129Xe/130Xe, 136Xe/130Xe ratios). Based on this assumption, we developed a simple stochastic model to trace the evolution of a mantle with two initially distinct layers for the extinct isotopes and its development into a heterogeneous mantle by convective mixing and stretching of these two layers. Using the extinct isotope system 182Hf‐182W, we find that the mantles of Earth and Mars exhibit substantially different mixing or stirring rates. This is consistent with Mars having cooled faster than the Earth due to its smaller size, resulting in less efficient mantle mixing for Mars. Moreover, the mantle stirring rate obtained for Earth using 182Hf‐182W is consistent with the mantle stirring rate of ~500 Myr constrained by the long‐lived isotope system, 87Rb‐87Sr and 147Sm‐143Nd. The apparent absence of 182W/183W isotopic heterogeneity in modern terrestrial rocks is attributed to very active mantle stirring which reduced the 182W/183W isotopic heterogeneity to a relatively small scale (~83 m for a mantle stirring rate of 500 Myr) compared to the common sampling scale of terrestrial basalts (~30 or 100 km). Our results also support the “deep magma ocean” core formation model as being applicable to both Mars and Earth.

Inheritance of solar short- and long-lived radionuclides from molecular clouds and the unexceptional nature of the solar system

Apparent excesses in early-solar 26Al, 36Cl, 41Ca, and 60Fe disappear if one accounts for ejecta from massive-star winds concentrated into dense phases of the ISM in star-forming regions. The removal of apparent excesses is evident when wind yields from Wolf–Rayet stars are included in the plot of radionuclide abundances vs. mean life. The resulting trend indicates that the solar radionuclides were inherited from parental molecular clouds with a characteristic residence time of 108 yr. This residence time is of the same order as the present-day timescale for conversion of molecular cloud material into stars. The concentrations of these extinct isotopes in the early solar system need not signify injection from unusual proximal stellar sources, but instead are well explained by normal concentrations in average star-forming clouds. The results imply that the efficiency of capture is greater for stellar winds than for supernova ejecta proximal to star-forming regions.

Thermal evolution and differentiation of planetesimals and planetary embryos

In early Solar System during the runaway growth stage of planetary formation, the distribution of planetary bodies progressively evolved from a large number of planetesimals to a smaller number of objects with a few dominant embryos. Here, we study the possible thermal and compositional evolution of these planetesimals and planetary embryos in a series of models with increasing complexities. We show that the heating stages of planetesimals by the radioactive decay of now extinct isotopes (in particular 26Al) and by impact heating can occur in two stages or simultaneously. Depending on the accretion rate, melting occurs from the center outward, in a shallow outer shell progressing inward, or in the two locations. We discuss the regime domains of these situations and show that the exponent β that controls the planetary growth rate R ˙ ∝ R β of planetesimals plays a crucial role. For a given terminal radius and accretion duration, the increase of β maintains the planetesimals very small until the end of accretion, and therefore allows radioactive heating to be radiated away before a large mass can be accreted. To melt the center of ∼500 km planetesimal during its runaway growth stage, with the value β = 2 predicted by astrophysicists, it needs to be formed within a couple of million years after condensation of the first solids. We then develop a multiphase model where the phase changes and phase separations by compaction are taken into account in 1-D spherical geometry. Our model handles simultaneously metal and silicates in both solid and liquid states. The segregation of the protocore decreases the efficiency of radiogenic heating by confining the 26Al in the outer silicate shell. Various types of planetesimals partly differentiated and sometimes differentiated in multiple metal–silicate layers can be obtained.

Trace-element composition of Fe-rich residual liquids formed by fractional crystallization: Implications for the Hadean magma ocean

New isotopic studies of 142Nd, the daughter product of the short-lived and now extinct isotope 146Sm, have revealed that the accessible part of the silicate Earth (e.g., upper mantle and crust) is more radiogenic in 142Nd/ 144Nd than that of chondritic meteorites. The positive 142Nd anomaly of the Earth’s mantle implies that the Sm/Nd ratio of the mantle was fractionated early in Earth’s history and that the complementary low 142Nd reservoir has remained isolated from the mantle since its formation. This has led to the suggestion that an early enriched reservoir, formed within Earth’s first hundred million years (the Hadean), resides permanently in the deep interior of the Earth. One hypothesis for a permanently isolated reservoir is that there may be an Fe-rich, and hence intrinsically dense, chemical boundary layer at the core-mantle boundary. The protoliths of this chemical boundary layer could have originated at upper mantle pressures during extreme fractional crystallization of a global magma ocean during the Hadean but testing this hypothesis is difficult because samples of this early enriched reservoir do not exist. This hypothesis, however, is potentially refutable. Here, we investigate a post-Archean magnetite-sulfide magma formed by extreme magmatic differentiation to test whether residual Fe-rich liquids of any kind have the necessary trace-element signatures to satisfy certain global geochemical imbalances. The magnetite-sulfide magma is found to have high Pb contents (and low U/Pb ratios), high Re/Os ratios, and anti-correlated Sm/Nd and Lu/Hf fractionations. Permanent segregation of such a magma would (1) provide a means of early Pb sequestration, resulting in the high U/Pb ratio of the bulk silicate Earth, (2) be a source of radiogenic 187Os in the source regions of plumes, and (3) provide an explanation for decoupled Hf and Nd isotopic evolution in the early Archean, which is not easily produced by silicate fractionation. However, the magnetite-sulfide magma is not highly enriched in K, and thus, at face value, this magma analog would not serve as a repository for all of the heat producing elements. Nevertheless, other Fe–O–S liquids reported elsewhere are enriched in apatite, which carries high concentrations of K, U and Th. Given some promising geochemical fractionations of the Fe-rich liquids investigated here, the notion of a Hadean Fe-rich residual liquid deserves continued consideration from additional experimental or analog studies.

Mass‐Independent Isotope Fractionation of Molybdenum and Ruthenium and the Origin of Isotopic Anomalies in Murchison

Dauphas et al.'s model for the nucleosynthetic origin of Mo and Ru anomalies in meteorites leaves the case of Murchison (CM2) unexplained. We explore the possibility that such a discrepancy is due to mass-independent effects controlled by nuclear field shift with, in particular, staggering between odd and even masses. We first demonstrate the existence of such mass-independent fractionation of Mo and Ru isotopes by chemical exchange of Mo and Ru between DC18C6 crown ether and aqueous solutions. Our results fit the nuclear field shift theory of Bigeleisen. We then review the correlation between the mean-square charge radius (which controls the nuclear field shift) and the isotopic anomalies found in an Allende CAI and in Murchison. Although Mo and Ru in the Allende CAI show a clear indication of nucleosynthetic components, the mass-independent anomalies observed in Murchison show a strong correlation with the nuclear charge distribution. We therefore argue that some isotopic anomalies observed in meteorites may be due to nuclear field shift rather than nucleosynthetic processes. Such effects are temperature dependent and may represent either genuine nebular processes or analytical artifacts. This new interpretation may help assess the existence of anomalies due to the extinct isotopes 97Tc and 98Tc.

The depletion of tungsten in the bulk silicate earth: Constraints on core formation

The depletion of the siderophile element tungsten (W) in the bulk silicate Earth (BSE), due to core formation, provides clues to the formation and early evolution of the Earth. This study significantly improves our knowledge of the abundance of W in the continental crust, an important reservoir for W in the BSE. Tungsten is a highly incompatible element, whose absolute concentrations are variable due to igneous processes. Therefore, the abundance of W is normalized to the highly incompatible lithophile element Th, to correct for igneous fractionation since the end of core formation. Similar W/Th ratios are observed in several terrestrial reservoirs, including the depleted mantle (nodules and Mid-Ocean Ridge Basalts), the old continental crust (upper continental sediments and Archean granulites), and the young continental crust (continental and oceanic arcs). The use of Th as a normalizing element is inappropriate, however, in the case of granulite xenoliths from the lowermost continental crust. These samples have higher W/Th ratios due to loss of Th (and U) compared to other incompatible elements including Ba and W. The few komatiite samples investigated have high ratios of W to the normalizing elements, possibly due to loss of the normalizing elements during alteration. The depletion of W in the BSE is determined, by using a mass balance calculation based on the W/ Th ratios of mantle and crustal reservoirs, and including the uncertainty in the initial abundance of W in the Earth, based on W abundances in chondritic meteorites. The resulting depletion of W relative to the refractory lithophile element Th is 0.06, with a range from 0.03–0.1. The depletion range for W overlaps the depletions of the compatible siderophile elements Co and Ni. This observation is consistent with the heterogeneous accretion theory for the Earth ( Newsom, 1990; O'Neill, 1991). In contrast, the observed depletions for W, Co, and Ni are not consistent with their experimental high temperature equilibrium metal-silicate partition coefficients (( Hillgren and Drake, 1994). The depletion of W also provides important constraints on the timing of core formation, based on the decay of the now extinct isotope 182Hf to the isotope 182W in the early solar system ( Harper and Jacobsen, 1994).

Extinct Radionuclide Decay Signatures and Mantle Evolution

We are developing three promising extinct radionuclide systems for studies of early Earth differentiation and geodynamics: 146Sm-142Nd (tl/2 = 103 Ma), lS2Hf2s2w (tl/2 = 9 Ma), and 244puRu,Pd (tl/2 = 80 Ma). Each of these systems has potential for advancing understanding of different aspects of Earth history. The utility of an extinct isotope system is determined by four factors. First, the cosmo/geochemical properties of the parent/ daughter element pair determine the potential advantages or disadvantages of the system for investigating different modes of planetary differentiation and geodynamic transport. Second, the half-life of the parent defines the timescale for development of isotopic differences between differentiated reservoirs at early times. Relevant 'boundary conditions' of Earth history (accretion rate, giant impacts, late additions, geodynamic mixing rates, etc.) are a third generic factor and set the stage for the geochemical behavior associated with the events and processes one seeks to understand. The final factor is the range of radiogenic isotope effects in rocks which determines the practicality of signal resolution by high precision isotopic measurements. We have focused recently on developing mass spectrometry for Nd and are now able to reproduce some isotope ratios to better than __+4 ppm (___0.04 e-units) at the 26 level for single measurements. This experience has

Fossil Alpha-Particle Recoil Tracks: A New Method of Age Determination

The discovery of a new type of fossil nuclear track in mica is reported. This track is produced by the recoil nucleus accompanying the alpha-particle decay of uranium and thorium impurities. The tracks are very short and can be seen with phase contrast microscopy. Measurement of fossil alpha-recoil track densities, coupled with determinations of the thorium and uranium contents, provides a new dating technique analogous to the previously discovered "fission-track method." The primary advantage of the alpha-recoil method is a several-thousand-fold increase in sensitivity over the fission-track technique. The alpha-recoil method should also prove useful in studying the problem of extinct isotopes in meteorites.

Spontaneous fission tracks from extinct Pu244in meteorites and the early history of the solar system

A new method is proposed for exploring the early history of the solar system, based on the search in meteoritic crystals for fossil fission tracks from the presently extinct isotope Pu244 (half-life = 7.6 × 107 years). The method employs the experimental techniques we have developed for dating terrestrial minerals; it is simple and applicable to many meteoritic minerals. If such tracks can be definitely identified, it is possible to infer values for ΔT0, the time interval between the end of nucleosynthesis and the formation of cooled meteoritic material. We first describe the method and then present experimental results giving evidence for Pu tracks in two meteorites. Alternative explanations are considered and are shown to be less likely.