Isotopic evolution of the protoplanetary disk and the building blocks of Earth and the Moon (original) (raw)
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Isotopic Evolution of the Inner Solar System Inferred from Molybdenum Isotopes in Meteorites
The Astrophysical Journal
The fundamentally different isotopic compositions of non-carbonaceous (NC) and carbonaceous (CC) meteorites reveal the presence of two distinct reservoirs in the solar protoplanetary disk that were likely separated by Jupiter. However, the extent of material exchange between these reservoirs, and how this affected the composition of the inner disk, are not known. Here we show that NC meteorites display broadly correlated isotopic variations for Mo, Ti, Cr, and Ni, indicating the addition of isotopically distinct material to the inner disk. The added material resembles bulk CC meteorites and Ca-Al-rich inclusions in terms of its enrichment in neutron-rich isotopes, but unlike the latter materials is also enriched in s-process nuclides. The comparison of the isotopic composition of NC meteorites with the accretion ages of their parent bodies reveals that the isotopic variations within the inner disk do not reflect a continuous compositional change through the addition of CC dust, indicating an efficient separation of the NC and CC reservoirs and limited exchange of material between the inner and outer disk. Instead, the isotopic variations among NC meteorites more likely record a rapidly changing composition of the disk during infall from the Sun's parental molecular cloud, where each planetesimal locks the instant composition of the disk when it forms. A corollary of this model is that late-formed planetesimals in the inner disk predominantly accreted from secondary dust that was produced by collisions among pre-existing NC planetesimals.
The great isotopic dichotomy of the early Solar System
Nature Astronomy
he Solar System formed by the gravitational collapse of a molecular cloud core, which resulted in the formation of a circumsolar disk of gas and dust (sometimes called the 'solar nebula'). This disk was ultimately transformed into a planetary system consisting of a single central star, the Sun, surrounded by four terrestrial planets in the inner Solar System, four giant planets in the outer Solar System beyond the 'snow line' , and a multitude of smaller bodies, including asteroids, moons, dwarf planets and comets. To understand how the Solar System evolved towards its present-day configuration, the events and processes occurring during the earliest stages of Solar System history must be reconstructed at a very high temporal and spatial resolution. Although astronomical observations 1 and dynamical modelling 2 provide fundamental insights into the structure and dynamics of protoplanetary disks, and the processes of planetary accretion, the study of meteorites allows the reconstruction of the Solar System's earliest history with unprecedented resolution in time and space. Recent analytical advances in the precision of isotope ratio measurements make it possible not only to date meteorites at submillion-year precision 3-5 (see Box 1) but also to identify distinct nucleosynthetic isotopic signatures. This allows genetic links between planetary materials to be determined and helps constrain the area of the disk a given meteorite originated from 6-8. Most meteorites derive from asteroids that are at present located in the main asteroid belt between Mars and Jupiter (at ~2.0-3.3 au), and have traditionally been viewed as samples from bodies that formed where they are found today. However, recently, this perspective has changed dramatically with the discovery of a fundamental genetic dichotomy observed in the nucleosynthetic isotope signatures of non-carbonaceous (NC) and carbonaceous (CC) meteorites 6,8,9. This discovery, combined with the establishment of a precise chronology for the accretion of meteorite parent bodies, has enabled the integration of meteoritic constraints into large-scale models of disk evolution and planet formation.
The rates of accretion, core formation and volatile loss in the early Solar System
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2001
Nuclides with half-lives of 10 5 -10 8 yr permit the elucidation of nebula time-scales and the rates of accretion of planetesimals. However, the 182 Hf-182 W system with a half-life of 9 ± 2 Myr also provides new and very useful constraints on the formation of the terrestrial planets. This technique allows one to address the timing of metal-silicate equilibration in objects as different as chondrites and the Earth. With improvements in sensitivity and precision, very small time differences in metal segregation in asteroids should be resolvable from measuring iron meteorites. It is already clear that the formation and differentiation of some asteroidal-sized objects was completed in less than 10 Myr. Accretion and core formation were protracted in the case of the Earth (greater than 50 Myr) relative to Mars (probably less than 20 Myr). Indeed, the Martian mantle appears to retain both chemical and isotopic heterogeneities that are residual from the process of core formation. Such early features appear to have been eliminated from the Earth's mantle presumably because of 4.5 Gyr of relatively efficient convective mixing. Tungsten isotope data provide compelling support for the 'giant impact' theory of lunar origin. The Moon is a high Hf/W object that contains a major component of chondritic W. This is consistent with a time of formation of greater than 50 Myr after the start of the Solar System. New highly precise oxygen isotope data are unable to resolve any difference between the source of components in the Earth and Moon. Therefore, the giant impact itself may have produced some of the differences in moderately volatile element budgets between these objects. This finds support in precise Sr isotopic data for early lunar samples. The data are consistent with the proto-Earth and Theia (the impactor) having Rb/Sr ratios that were not very different from that of present day Mars. Therefore, the extended history of accretion, rather than nebular phenomena, may be responsible for some of the major differences between the terrestrial planets.
Geochimica et Cosmochimica Acta, 2009
We evaluate initial ( 26 Al/ 27 Al) I , ( 53 Mn/ 55 Mn) I , and ( 182 Hf/ 180 Hf) I ratios, together with 207 Pb/ 206 Pb ages for igneous differentiated meteorites and chondrules from ordinary chondrites for consistency with radioactive decay of the parent nuclides within a common, closed isotopic system, i.e., the early solar nebula. The relative initial isotopic abundances of 26 Al, 53 Mn, and 182 Hf in differentiated meteorites and chondrules are consistent with decay from common solar system initial values, here denoted by I(Al) SS , I(Mn) SS , and I(Hf) SS, respectively. I(Mn) SS and I(Hf) SS = 9.1 ± 1.7 Â 10 À6 and 1.07 ± 0.08 Â 10 À4 , respectively, correspond to ''canonical" I(Al) SS = 5.1 Â 10 À5 . I(Hf) SS so determined is consistent with I(Hf) SS = 9.72 ± 0.44 Â 10 À5 directly determined from an internal Hf-W isochron for CAI minerals. I(Mn) SS is within error of the lowest value directly measured for CAIs. We suggest that erratically higher values measured for CAIs in carbonaceous chondrites may reflect proton irradiation of unaccreted CAIs by the early Sun after other asteroids destined for melting by 26 Al decay had already accreted. The 53 Mn incorporated within such asteroids would have been shielded from further ''local" spallogenic contributions from within the solar system. The relative initial isotopic abundances of the short-lived nuclides are less consistent with the 207 Pb/ 206 Pb ages of the corresponding materials than with one another. The best consistency of shortand long-lived chronometers is obtained for ( 182 Hf/ 180 Hf) I and the 207 Pb/ 206 Pb ages of angrites. ( 182 Hf/ 180 Hf) I decreases with decreasing 207 Pb/ 206 Pb ages at the rate expected from the 8.90 ± 0.09 Ma half-life of 182 Hf. The model solar system age thus determined is T SS,Hf-W = 4568.3 ± 0.7 Ma. ( 26 Al/ 27 Al) I and ( 53 Mn/ 55 Mn) I are less consistent with 207 Pb/ 206 Pb ages of the corresponding meteorites, but yield T SS,Mn-Cr = 4568.2 ± 0.5 Ma relative to I(Al) SS = 5.1 Â 10 À5 and a 207 Pb/ 206 Pb age of 4558.55 ± 0.15 Ma for the LEW86010 angrite. The Mn-Cr method with I(Mn) SS = 9.1 ± 1.7 Â 10 À6 is useful for dating accretion (if identified with chondrule formation), primary igneous events, and secondary mineralization on asteroid parent bodies. All of these events appear to have occurred approximately contemporaneously on different asteroid parent bodies. For I(Mn) SS = 9.1 ± 1.7 Â 10 À6 , parent body differentiation is found to extend at least to 5Mapost−TSS,i.e.,untildifferentiationoftheangriteparentbody5 Ma post-T SS , i.e., until differentiation of the angrite parent body 5Mapost−TSS,i.e.,untildifferentiationoftheangriteparentbody4563.5 Ma ago, or 4564.5Maagousingthedirectlymeasured207Pb/206PbagesoftheD′Orbigny−clanangrites.The4564.5 Ma ago using the directly measured 207 Pb/ 206 Pb ages of the D'Orbigny-clan angrites. The 4564.5Maagousingthedirectlymeasured207Pb/206PbagesoftheD′Orbigny−clanangrites.The1 Ma difference is characteristic of a remaining inconsistency for the D'Orbigny-clan between the Al-Mg and Mn-Cr chronometers on one hand, and the 207 Pb/ 206 Pb chronometer on the other. Differentiation of the IIIAB iron meteorite and ureilite parent bodies probably occurred slightly later than for the angrite parent body, and at nearly the same time as one another as shown by the Mn-Cr ages of IIIAB irons and ureilites, respectively. The latest recorded epi-0016-7037/$ -see front matter Published by Elsevier Ltd. Geochimica et Cosmochimica Acta 73 (2009) 5115-5136 sodes of secondary mineralization are for carbonates on the CI carbonaceous chondrite parent body and fayalites on the CV carbonaceous chondrite parent body, both extending to $10 Ma post-T SS . Published by Elsevier Ltd.
Solar system genealogy revealed by extinct short-lived radionuclides in meteorites
Little is known about the stellar environment and the genealogy of our solar system. Short-lived radionuclides (SLRs, mean lifetime τ shorter than 100 Myr) that were present in the solar protoplanetary disk 4.56 Gyr ago could potentially provide insight into that key aspect of our history, were their origin understood. Aims. Previous models failed to provide a reasonable explanation of the abundance of two key SLRs, 26 Al (τ 26 = 1.1 Myr) and 60 Fe (τ 60 = 3.7 Myr), at the birth of the solar system by requiring unlikely astrophysical conditions. Our aim is to propose a coherent and generic solution based on the most recent understanding of star-forming mechanisms.
Hf–W chronology of the accretion and early evolution of asteroids and terrestrial planets
Geochimica et Cosmochimica Acta, 2009
The 182 Hf-182 W systematics of meteoritic and planetary samples provide firm constraints on the chronology of the accretion and earliest evolution of asteroids and terrestrial planets and lead to the following succession and duration of events in the earliest solar system. Formation of Ca,Al-rich inclusions (CAIs) at 4568.3 ± 0.7 Ma was followed by the accretion and differentiation of the parent bodies of some magmatic iron meteorites within less than 1Myr.ChondrulesfromHchondritesformed1.7±0.7MyrafterCAIs,aboutcontemporaneouslywithchondrulesfromLandLLchondritesasshownbytheir26Al−26Mgages.Somemagmatismontheparentbodiesofangrites,eucrites,andmesosideritesstartedassoonas1 Myr. Chondrules from H chondrites formed 1.7 ± 0.7 Myr after CAIs, about contemporaneously with chondrules from L and LL chondrites as shown by their 26 Al-26 Mg ages. Some magmatism on the parent bodies of angrites, eucrites, and mesosiderites started as soon as 1Myr.ChondrulesfromHchondritesformed1.7±0.7MyrafterCAIs,aboutcontemporaneouslywithchondrulesfromLandLLchondritesasshownbytheir26Al−26Mgages.Somemagmatismontheparentbodiesofangrites,eucrites,andmesosideritesstartedassoonas3 Myr after CAI formation and may have continued until 10Myr.Asimilartimescaleisobtainedforthehigh−temperaturemetamorphicevolutionoftheHchondriteparentbody.Thermalmodelingcombinedwiththeseageconstraintsrevealsthatthedifferentthermalhistoriesofmeteoriteparentbodiesprimarilyreflecttheirinitialabundanceof26Al,whichisdeterminedbytheiraccretionage.Impact−relatedprocesseswereimportantinthesubsequentevolutionofasteroidsbutdonotappeartohaveinducedlarge−scalemelting.Forinstance,Hf−WagesforeucritemetalspostdateCAIformationby10 Myr. A similar timescale is obtained for the high-temperature metamorphic evolution of the H chondrite parent body. Thermal modeling combined with these age constraints reveals that the different thermal histories of meteorite parent bodies primarily reflect their initial abundance of 26 Al, which is determined by their accretion age. Impact-related processes were important in the subsequent evolution of asteroids but do not appear to have induced large-scale melting. For instance, Hf-W ages for eucrite metals postdate CAI formation by 10Myr.Asimilartimescaleisobtainedforthehigh−temperaturemetamorphicevolutionoftheHchondriteparentbody.Thermalmodelingcombinedwiththeseageconstraintsrevealsthatthedifferentthermalhistoriesofmeteoriteparentbodiesprimarilyreflecttheirinitialabundanceof26Al,whichisdeterminedbytheiraccretionage.Impact−relatedprocesseswereimportantinthesubsequentevolutionofasteroidsbutdonotappeartohaveinducedlarge−scalemelting.Forinstance,Hf−WagesforeucritemetalspostdateCAIformationby20 Myr and may reflect impact-triggered thermal metamorphism in the crust of the eucrite parent body. Likewise, the Hf-W systematics of some non-magmatic iron meteorites were modified by impact-related processes but the timing of this event(s) remains poorly constrained.
EVIDENCE FOR MAGNESIUM ISOTOPE HETEROGENEITY IN THE SOLAR PROTOPLANETARY DISK
With a half-life of 0.73 Myr, the 26Al-to-26Mg decay system is the most widely used short-lived chronometer for understanding the formation and earliest evolution of the solar protoplanetary disk. However, the validity of 26Al-26Mg ages of meteorites and their components relies on the critical assumption that the canonical 26Al/27Al ratio of ~5 × 10–5 recorded by the oldest dated solids, calcium-aluminium-rich inclusions (CAIs), represents the initial abundance of 26Al for the solar system as a whole. Here, we report high-precision Mg-isotope measurements of inner solar system solids, asteroids, and planets demonstrating the existence of widespread heterogeneity in the mass-independent 26Mg composition (μ26Mg*) of bulk solar system reservoirs with solar or near-solar Al/Mg ratios. This variability may represent heterogeneity in the initial abundance of 26Al across the solar protoplanetary disk at the time of CAI formation and/or Mg-isotope heterogeneity. By comparing the U-Pb and 26Al-26Mg ages of pristine solar system materials, we infer that the bulk of the μ26Mg* variability reflects heterogeneity in the initial abundance of 26Al across the solar protoplanetary disk. We conclude that the canonical value of ~5 × 10–5 represents the average initial abundance of 26Al only in the CAI-forming region, and that large-scale heterogeneity—perhaps up to 80% of the canonical value—may have existed throughout the inner solar system. If correct, our interpretation of the Mg-isotope composition of inner solar system objects precludes the use of the 26Al-26Mg system as an accurate early solar system chronometer.
Earth and Planetary Science Letters, 2011
The early evolution of the solar nebula involved substantial transport of mass, resulting in mixing and homogenization of isotopically diverse materials that were contributed to the solar system from multiple stellar nucleosynthetic sources. The efficiency of this mixing, as well as its timescale can be quantified by determining nucleosynthetic isotope variations among meteorites and terrestrial planets. Here we present Mo isotopic data for a wide range of samples, including Ca-Al-rich inclusions, chondrites and differentiated meteorites, as well as martian and terrestrial samples. Most meteorites are depleted in s-process Mo relative to the Earth, and only the IAB-IIICD irons, angrites and martian meteorites have terrestrial Mo isotopic compositions. In contrast, most Ca-Al-rich inclusions are enriched in r-process Mo, but one inclusion is characterized by a large s-process deficit. Molybdenum isotopic anomalies in the bulk meteorites correlate with those in Ru exactly as predicted from nucleosynthetic theory, but no obvious correlation is apparent between Mo and Ni anomalies. Therefore, s-process Mo and Ru seem to be hosted in the same carrier, which must be distinct from the carrier responsible for isotopic anomalies in the Fe-group elements (Ni, Cr, Ti). Furthermore, the isotopic heterogeneity in Mo (and other elements) contrasts with the isotopic homogeneity for Hf and Os, indicating that different s-process carriers once existed in the early solar nebula and that only some of these were heterogeneously distributed. The Mo isotopic anomalies of meteorites and their components decrease over time and with increasing size of the parent bodies, providing evidence for a progressive homogenization of the solar nebula. However, the carbonaceous chondrites exhibit larger Mo anomalies than expected for their age, indicating that they received a greater portion of material from the outer solar system (where homogenization was slow) than other meteorite parent bodies and terrestrial planets. Compared to the meteorites, Earth is enriched in s-process Mo and must have accreted from material distinct from the meteorites. Combined Mo and O isotopic data show that the composition of the Earth cannot be reconstructed by any known combination of meteorites, implying that meteorites may be inappropriate proxies for the isotopic composition of the bulk Earth. This is exemplified by the covariation of 92 Mo and 142 Nd anomalies in chondrites, showing that the 142 Nd deficit of chondrites compared to the accessible Earth may not unequivocally be interpreted as a signature of an early differentiation of the Earth. However, further high precision isotopic data are needed to evaluate the role of chondrites in defining the isotopic composition of the Earth.