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We present a report on the status of 107Pd (τ = 9.4
× 106 y) in the early solar system and the implications
of its presence for protoplanet evolution. Over the last two decades we
have carried out an extensive search for the evidence of presently
extinct 107Pd in meteorites. From these results we conclude
that: 1) 107Ag* (excess 107Ag) is present i...
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... The longlived radioactivities which are synthesized from O-burning, s-, and r-process start to decay as soon as they are generated. Although it is unclear how the supernova rate or neutron star merger rate has changed over time, actinide abundances for the early Solar System are roughly that expected for near-uniform production (179,180). ...
Life on Earth emerged at the interface of the geosphere, hydrosphere and atmosphere. This setting serves as our basis for how biological systems originate on rocky planets. Often overlooked, however, is the fact that the chemical nature of a rocky planet is ultimately a product of galactic chemical evolution. Elemental abundances of the major rock-forming elements can be different for different stars and planets formed at different times in galactic history. These differences mean that we cannot expect small rocky exoplanets to be just like Earth. Furthermore, age of the system dictates starting nuclide inventory from galactic chemical evolution, and past, present and future mantle and crust thermal regimes. The bulk silicate mantle composition of a rocky planet modulates the kind of atmosphere and hydrosphere it possesses. Hence, the ingredients of a rocky planet are as important for its potential to host life as proximity to the so-called habitable zone around a star where liquid water is stable at the surface. To make sense of these variables, a new trans-disciplinary approach is warranted that fuses the disciplines of Geology and Astronomy into what is here termed, Geoastronomy.
... Silver has two stable isotopes, 107 Ag (51.4%) and 109 Ag (48.6%). Natural variations of 109 Ag/ 107 Ag are of interest in cosmochemistry, planetary core formation and volatile element budgets (Chen and Wasserburg, 1996;Hauri et al., 2000;Woodland et al., 2005;Schönbächler et al., 2007;Schönbächler et al., 2008Schönbächler et al., , 2010 and as a 'stable isotope' tracer in economic geology (Hauri et al., 2000;Chugaev and Chernyshev, 2012;Tessalina, 2015;s Argapadmi et al., 2018; Silver is a ubiquitous component in natural gold (lode and detrital, Hough et al., 2009;Hough et al., 2007) and in archaeological gold artefacts (e.g., Schlosser et al., 2012), varying in abundance from tracelevels to % levels, and reaching > 30% in some gold nuggets (Hough et al., 2009). Both elements are often transported together in hydrothermal systems and surficial fluids (Saunders et al., 2014;Seward et al., 2014). ...
We describe a technique for high-precision analysis of ¹⁰⁹ Ag/ ¹⁰⁷ Ag ratios in natural and archaeological gold (1.0–120 mg/g Ag) with particular emphasis on the control of Ag yield and effects of instrumental mass bias. After dissolution of mg-sized gold samples in aqua regia and conversion to chlorides, a miniature column filled with the anion exchange resin [email protected] was used for removal of Au from the sample solutions. In a second miniature column made of the Triskem TBP resin Ag was converted from the chloride to the nitrate form. Isotopic analyses on the MC-ICP-MS employed the combination of Pd-doping and standard bracketing and considering the measurements of replicate dissolutions and chromatographic separations for SRM978a and CEZAg reference solutions, the combined analytical uncertainty (2 s) of the analytical method is better than 0.016‰. A solution prepared from several gold nuggets was characterised isotopically (CEZAg with δ ¹⁰⁹ Ag = 0.043 ± 0.015‰) for use as a silver-in‑gold reference material in Ag isotope studies of natural and processed gold. Results for gold nuggets from three continents indicate a wide variation in Ag concentrations (5 to 123 mg/g) and this is matched by an equally wide range in isotopic compositions (δ ¹⁰⁹ Ag −0.58 to +0.83‰). This is larger than the variation found so far in in native gold from primary deposits (−0.42 to +0.5‰). Although small isotopic effects to lower δ ¹⁰⁹ Ag during strong Ag loss are possible within a single grain, most analysed nuggets are internally isotopically homogeneous. This suggests preservation of the primary δ ¹⁰⁹ Ag in the supergene environment and that gold was sourced from several distinct gold deposits in the catchment. Recent studies, however, indicate that even single primary deposits may be isotopically heterogeneous implying that Ag isotope fractionation is caused by numerous deposition-dissolution cycles in hydrothermal systems. Fine-grained detrital gold from three placers along the Rhine river in Germany show only small differences in δ ¹⁰⁹ Ag (−0.005 ± 0.047 to +0.124 ± 0.007‰, despite being distributed along 480 km of river length. This may indicate thorough mixing of gold grain populations during transport. The small sample size required for Ag isotope work on gold opens the way for detailed micro-sampling approaches. These may be used to further examine the potential for within-grain isotopic variability related to fluviatile processing and can be used to correlate the composition of the placers with that of grains from primary sources.
... The amount ratio n( 107 Pd)/n( 107 Ag) is used in geochronology to date major events in the Solar System [344][345][346][347][348]353]. Although 107 Ag is naturally occurring, it is also the daughter product by beta decay of 107 Pd. ...
... Ag and 107 Pd (with a half-life of 6.5 × 10 6 years) are present in a sample of extraterrestrial Unauthenticated Download Date | 6/18/19 1:01 AMorigin, then the material would have formed sometime after107 Pd decayed. The n( 107 Pd)/n( 107 Ag) amount ratio can be measured to help determine when the 107 Pd decay process began and how much time has elapsed since the material was formed[344][345][346][347][348]. ...
The IUPAC (International Union of Pure and Applied Chemistry) Periodic Table of the Elements and Isotopes (IPTEI) was created to familiarize students, teachers, and non-professionals with the existence and importance of isotopes of the chemical elements. The IPTEI is modeled on the familiar Periodic Table of the Chemical Elements. The IPTEI is intended to hang on the walls of chemistry laboratories and classrooms. Each cell of the IPTEI provides the chemical name, symbol, atomic number, and standard atomic weight of an element. Color-coded pie charts in each element cell display the stable isotopes and the relatively long-lived radioactive isotopes having characteristic terrestrial isotopic compositions that determine the standard atomic weight of each element. The background color scheme of cells categorizes the 118 elements into four groups: (1) white indicates the element has no standard atomic weight, (2) blue indicates the element has only one isotope that is used to determine its standard atomic weight, which is given as a single value with an uncertainty, (3) yellow indicates the element has two or more isotopes that are used to determine its standard atomic weight, which is given as a single value with an uncertainty, and (4) pink indicates the element has a well-documented variation in its atomic weight, and the standard atomic weight is expressed as an interval. An element-by-element review accompanies the IPTEI and includes a chart of all known stable and radioactive isotopes for each element. Practical applications of isotopic measurements and technologies are included for the following fields: forensic science, geochronology, Earth-system sciences, environmental science, and human health sciences, including medical diagnosis and treatment.
... Moreover, samples from the volatile-depleted iron meteorite groups IVA and IVB have much higher Pd/Ag ratios than those from the less volatile-depleted groups IIAB and IIIAB. This difference is commonly attributed to the volatile-depleted nature of the IVA and IVB irons (Chen and Wasserburg, 1996;Walker et al., 2008;McCoy et al., 2011;Horan et al., 2012). ...
To constrain the timescales and processes involved in the crystallization and cooling of protoplanetary cores, we examined the Pd-Ag isotope systematics of the IVA iron meteorites Muonionalusta and Gibeon. A Pd-Ag isochron for Muonionalusta provides an initial ¹⁰⁷Pd/¹⁰⁸Pd = (2.57 ± 0.07) × 10⁻⁵. The three metal samples analyzed from Gibeon plot below the Muonionalusta isochron, but these samples also show significant effects of cosmic ray-induced neutron capture reactions, as is evident from ¹⁹⁶Pt excesses in the Gibeon samples. After correction for neutron capture effects on Ag isotopes, the Gibeon samples plot on the Muonionalusta isochron, indicating that these two IVA irons have indistinguishable initial ¹⁰⁷Pd/¹⁰⁸Pd. Collectively, the Pd-Ag data indicate cooling of the IVA core below Pd-Ag closure between 2.9 ± 0.4 Ma and 8.9 ± 0.6 Ma after CAI formation, where this age range reflects uncertainties in the initial ¹⁰⁷Pd/¹⁰⁸Pd ratios of the solar system, which in turn result from uncertainties in the Pb-Pb age of Muonionalusta. The Ag isotopic data indicate that the IVA core initially evolved with a modestly elevated Pd/Ag, but the low Ag concentrations measured for some metal samples indicate derivation from a source with much lower Ag contents and, hence, higher Pd/Ag. These contrasting observations can be reconciled if the IVA irons crystallized from an initially more Ag-rich core, followed by extraction of Fe-S melts during compaction of the nearly solidified core. Owing to its strong tendency to partition into Fe-S melts, Ag was removed from the IVA core during compaction, leading to the very low Ag concentration observed in metal samples of IVA irons. Alternatively, Ag was lost by evaporation from a still molten metallic body just prior to the onset of crystallization. The Pd-Ag isotopic data indicate that Muonionalusta cooled at >500 K/Ma through the Pd-Ag closure temperature of ∼900 K, consistent with the rapid cooling inferred from metallographic cooling rates for IVA irons. Combined, these observations are consistent with cooling of IVA irons in a metallic body with little or no silicate mantle.
... These workers find that there was a rather short time between τ P1 and τ CAI (several million years, see Kruijer et al. 2014b, their supplemental data, Table 6). In contrast, for 107 Pd we know from internal isochrons for three meteorites (Gibeon, Duchesne, Muonionalusta) that 107 Pd/ 108 Pd = 2.4 × 10 −5 (Chen & Wasserburg 1996;Horan et al. 2012) and see Matthes et al. (2015) for the most precise value for Muonionalusta. The 107 Pd/ 108 Pd ratio for these samples is the value when the diffusion process stopped between the coexisting phases in these objects. ...
We explore the possibility that the short-lived radionuclides $^{26}$Al, $^{60}$Fe, $^{107}$Pd, and $^{182}$Hf inferred to be present in the proto-solar cloud originated from $3-8 M_{\odot}$ Asymptotic Giant Branch (AGB) stars. Models of AGB stars with initial mass above 5$M_{\odot}$ are prolific producers of $^{26}$Al owing to hot bottom burning (HBB). In contrast, $^{60}$Fe, $^{107}$Pd, and $^{182}$Hf are produced by neutron captures: $^{107}$Pd and $^{182}$Hf in models $\lesssim 5 M_{\odot}$; and $^{60}$Fe in models with higher mass. We mix stellar yields from solar-metallicity AGB models into a cloud of solar mass and composition to investigate if it is possible to explain the abundances of the four radioactive nuclides at the Sun's birth using one single value of the mixing ratio between the AGB yields and the initial cloud material. We find that AGB stars that experience efficient HBB ($\geq 6 M_{\odot}$) cannot provide a solution because they produce too little $^{182}$Hf and $^{107}$Pd relative to $^{26}$Al and $^{60}$Fe. Lower-mass AGB cannot provide a solution because they produce too little $^{26}$Al relative to $^{107}$Pd and $^{182}$Hf. A self-consistent solution may be found for AGB stars with masses in-between ($4-5.5 M_{\odot}$), provided HBB is stronger than in our models and the $^{13}$C($\alpha$, n)$^{16}$O neutron source is mildly activated. If stars of M $< 5.5 M_{\odot}$ are the source of the radioactive nuclides, then some basis for their existence in proto-solar clouds needs to be explored, given that the stellar lifetimes are longer than the molecular cloud lifetimes.
... If Earth's differentiation occurred within 40 million years (approximately five half-lives) of the beginning of the Solar System, an isotopic excess of 107 Ag should exist within the core. Equally, because Ag is a moderately volatile element, whereas Pd is more refractory than Ag, large ranges in Pd/Ag have been observed in volatiledepleted iron meteorites (up to 100,000), compared with a Solar Pd/Ag of ~ 3, leading to 107 Ag/ 109 Ag ratios > 9, compared with the solar value of 1.079 (Chen and Wasserburg 1996). For the Pd-Ag isotope system, the initial 107 Pd/ 108 Pd has been determined as 5.9 ± 2.2 × 10 −5 (Schönbächler et al. 2008), with 107 Ag/ 109 Ag typically reported in parts per ten thousand notation relative to the NIST SRM978a silver standard ( Ag Ag/ Ag / Ag/ Ag -1 × 10,000 ε = ...
... The Pd-Ag isotope system can be used to define the chronology of iron meteorite formation and the timing and mechanisms of early volatile depletion, since Ag is a moderately volatile siderophile element (50% condensation temperature = 996 K). As a result, some volatile-depleted iron meteorites have Pd/Ag >10 5 , compared with a solar Pd/Ag ~3, leading to very high 107 Ag/ 109 Ag in some irons (Chen and Wasserburg 1996), and measureable differences in carbonaceous chondrites (Schönbachler et al. 2008). The resulting ages of iron meteorites defined by Pd-Ag chronology are ~9-20 Ma after Solar System formation and are longer than Hf-W isotope chronology (~1-3 Ma) (e.g., Kruijer et al. 2014). ...
The highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au) are key tracers of planetary accretion and differentiation processes due to their affinity for metal relative to silicate. Under low-pressure conditions the HSE are defined by having metal–silicate partition coefficients in excess of 104 (e.g., Kimura et al. 1974; Jones and Drake 1986; O’Neill et al. 1995; Borisov and Palme 1997; Mann et al. 2012). The HSE are geochemically distinct in that, with the exception of Au, they have elevated melting points relative to iron (1665 K), low vapour pressures, and are resistant to corrosion or oxidation. Under solar nebular conditions, Re, Os, Ir, Ru, Rh, and Pt, along with the moderately siderophile elements (MSE) Mo and W, condense as refractory-metal alloys. Palladium and Au are not as refractory and condense in solid solution with FeNi metal (Palme 2008). Assuming abundances of the HSE in materials that made up the bulk Earth were broadly similar to modern chondrite meteorites, mass balance calculations suggest that >98% of these elements reside in the metallic core (O’Neill and Palme 1998). In practical terms, the resultant low HSE abundance inventories in differentiated silicate crusts and mantles enables the use of these elements in order to effectively track metallic core formation and the subsequent additions of HSE-rich impactors to planets and asteroids (Fig. 1). In detail, the absolute and relative abundances of the HSE in planetary materials are also affected by mantle and crustal processes including melting, metasomatism, fractional crystallization, and crust-mantle remixing, as well as later impact processing, volatility of Re under oxidizing conditions, and low-temperature secondary alteration (cf., Day 2013; Gannoun et al. 2016, this volume). In the absence of metal, the HSE are chalcophile, so these elements are also affected by processes involving growth and breakdown of sulfides. Work over the last several decades has led to a large available database for understanding processes affecting the HSE for planetary bodies. This chapter summarises this progress for rocky Solar System bodies, including the Earth, Moon, Mars and some asteroids, and examines the conceptual framework for interpreting these data. The first section outlines the motivation for measuring the HSE in planetary materials. The second section briefly considers methods for measuring and interpreting HSE abundance and Os isotopic data. The third section provides an outline of natural HSE abundance variations and Os isotope compositions in planetary materials. The fourth section outlines current interpretations of the available data and outstanding issues. The final sections offer some comparative planetology, implications for terrestrial planet formation, synthesis and future directions. This chapter does not consider nucleosynthetic variations, as these are the subject of a review by Yokoyama and Walker (2016, this volume), and does not provide a detailed consideration of experimental work, which is the subject of Brenan et al. (2016, this volume). While comparisons are made with terrestrial HSE compositions, these data are considered in detail elsewhere in this volume, or in Walker et al. (1997), Shirey and Walker (1998), Carlson (2005), Walker (2009), and Day (2013).
... Furthermore, the Pd/Ag ratios of different samples from a given group of iron meteorites also vary, reflecting Pd/Ag fractionation during fractional crystallization of the metal cores (Chabot and Drake, 1997). Finally, within a magmatic iron meteorite the Pd/Ag ratio is strongly fractionated between metal and sulfide, making it possible to determine precise internal isochrons and, hence, cooling ages of individual iron meteorites (Chen and Wasserburg, 1983, 1996Horan et al., 2012). ...
... For instance, different metal samples of the IVA iron meteorite Gibeon define an isochron corresponding to an initial 107 Pd/ 108 Pd of (2.40 ± 0.05) Â 10 À5 at the time of Pd-Ag closure, the highest and most precise ratio determined so far (Chen and Wasserburg, 1990). A detailed Pd-Ag study on a large set of magmatic iron meteorites has demonstrated the presence of excess 107 Ag in almost all investigated magmatic iron meteorites, with inferred initial 107 Pd/ 108 Pd ratios of between $1.5 Â 10 À5 and $2.5 Â 10 À5 (Chen and Wasserburg, 1996). ...
... The IVB iron meteorites are some of the most strongly volatile element-depleted meteorites and, hence, are characterized by exceedingly high Pd/Ag ratios. Although most of the IVBs analyzed for Pd-Ag isotope systematics exhibit radiogenic Ag isotope compositions, they all plot below but approximately parallel to the Pd-Ag isochron of the IVA iron Gibeon (Chen and Wasserburg, 1996). This is surprising, because it would indicate a less radiogenic initial 107 Ag/ 109 Ag for the IVBs, in spite of their more volatile-depleted composition compared to IVA iron meteorites. ...
The short-lived 107Pd-107Ag system is a versatile tool for dating iron meteorites, but neutron capture reactions during cosmic ray-exposure might have modified Ag isotope compositions. These cosmic ray-induced effects would vary depending on the exposure time of a sample and its location within the parent meteoroid and, therefore, could bias the age information inferred from Pd-Ag isotope systematics. Our new combined Pd-Ag and Pt isotope data for iron meteorites in conjunction with model calculations reveal large cosmic ray-induced downward shifts of 107Ag/109Ag, which preclude the determination of Pd-Ag isochrons based on measured Ag isotope compositions. For the strongly irradiated iron meteorites Ainsworth (IIAB) and Carbo (IID) these shifts are similar to or even larger than the effects from radiogenic ingrowth resulting from 107Pd-decay. For the less strongly irradiated IIIAB iron meteorites Boxhole, Grant and Henbury, the cosmic ray-induced shifts are smaller than the radiogenic 107Ag excesses, but are nevertheless significant. We have developed a method to quantify the cosmic ray-induced Ag isotope shifts using a neutron capture model and Pt isotope compositions as the neutron dose monitor. After correction, Pd-Ag isochrons are obtained for all investigated iron meteorites, even for the most strongly irradiated samples. The Pd-Ag ages inferred from the isochrons are in good agreement with other chronological data for iron meteorites, indicating that our neutron capture model provides a reliable correction method for quantifying cosmic ray-induced shifts on measured Ag isotope compositions. The Pd-Ag ages for iron meteorites obtained in this and previous studies indicate rapid crystallization and cooling of the parental metal cores within a few Ma after core formation and solar system formation. Such rapid cooling can be attributed to either small parent body sizes or collisional erosion of the insulating silicate mantle from larger bodies. The collisions would have facilitated rapid cooling below Pd-Ag isotopic closure and so in this case the Pd-Ag ages would effectively date the time of the collisions.
... Such sulphide inclusions would have to be small, however, because none were visually observed during sample preparation. Similar observations and interpretations were previously made for IVA iron meteorites (Chen and Wasserburg, 1996;Horan et al., 2012). In line with these authors, we conclude that minor troilite inclusions are likely to form and record isotopic closures at the same time as the Fe-Ni metal and thus any mixing that has occurred does not hamper the time information obtained from the isochrons. ...
The extinct 107Pd-107Ag decay system (half-life ~6.5Ma) is a useful chronometer to constrain the thermal evolution of the IAB parent body. To this end, Pd/Ag concentrations and the Ag isotope compositions of metals separated from 6 different IAB iron meteorites were determined. The samples show ε107Ag variations between +0.1 and +15.8 with 108Pd/109Ag ratios between 38 and 200. The data can be divided into two groups based on their petrology, each defining an isochron: a graphite and troilite rich inclusion bearing group (A), with the IAB meteorites Toluca, Odessa and Canyon Diablo and a more silicate rich group (B), which includes Campo Del Cielo, Caddo County and Goose Lake. Using the initial abundance of 107Pd derived from carbonaceous chondrites, the corresponding age for the group (A) is 18.7 (+3.6/-5.0)Ma after the start of the solar system and 14.9 (+2.5/-4.9)Ma for the group (B). This suggests that the last thermal event to reach high enough temperatures to melt metal on the IAB parent body occurred within the first 15Ma of our solar system.
... Â 10 À5 [40,103] (Figure 10.9a). This [40,103,104]. (b) Pb-Tl isochron data for metal fragments from the IAB irons Canyon Diablo and Toluca [39] define an isochron, which first established that live 205 Pb was present in the early solar system. ...
IntroductionExtraterrestrial SamplesOrigin of Cosmochemical Isotopic VariationsUse of MC-ICP-MS in CosmochemistryApplications of MC-ICP-MS in CosmochemistryConclusion
AcknowledgmentsReferences
