Laboratory technology and cosmochemistry

Abstract

Recent developments in analytical instrumentation have led to revolutionary discoveries in cosmochemistry. Instrumental advances have been made along two lines: (i) increase in spatial resolution and sensitivity of detection, allowing for the study of increasingly smaller samples, and (ii) increase in the precision of isotopic analysis that allows more precise dating, the study of isotopic heterogeneity in the Solar System, and other studies. A variety of instrumental techniques are discussed, and important examples of discoveries are listed. Instrumental techniques and instruments include the ion microprobe, laser ablation gas MS, Auger EM, resonance ionization MS, accelerator MS, transmission EM, focused ion-beam microscopy, atom probe tomography, X-ray absorption near-edge structure/electron loss near-edge spectroscopy, Raman microprobe, NMR spectroscopy, and inductively coupled plasma MS.

Cosmochemistry is concerned with the study of the Solar System (SS), its formation, its history, and the processes that shaped the planets. This is mainly accomplished through the analysis of extraterrestrial samples, astronomical observations from Earth and space, and in situ measurements by robotic spacecrafts. The latter is the topic of a separate chapter; this chapter is concerned with the question of how recent advances in laboratory instrumentation have led to revolutionary discoveries in cosmochemistry.

The study of extraterrestrial samples includes the analysis of meteorites (from asteroids, Mars, and the Moon), lunar samples (returned by Apollo), micrometeorites (1), interplanetary dust particles (IDPs) (2, 3), samples returned from comets (Stardust) (4), asteroids (HAYABUSA) (5), and samples from the Sun (Genesis) (6).

Throughout the 20th century, continuous development of analytical instrumentation has led to a deep understanding of how the SS formed, what material contributed to its formation, what processes took place in its early history, and what was the time scale. An important step in the development of instrumental capabilities was the Apollo Program. Funding for instrumentation for the analysis of returned lunar samples led to analytical advances that, in turn, resulted in pathbreaking discoveries in other fields. Examples are the discovery of ¹⁶O excesses in Ca-Al–rich inclusions (CAIs) (7) and evidence for the initial presence of ²⁶Al in the early SS (ref. 8; see also ref. 9).

The last two decades have seen a plethora of instrumental developments that have led to revolutionary discoveries. The whole field of the analysis of extraterrestrial materials has grown to an extent and involves so many different analytical techniques that a detailed survey is completely out of the question in the framework of this paper. In our treatment, we will emphasize the impact of instrumentation on cosmochemistry. This will exclude many conventional techniques, such as X-ray analysis in the electron microprobe and SEM, neutron activation analysis, conventional X-ray fluorescence, and X-ray diffraction analysis, which are important to cosmochemistry but are discussed extensively elsewhere.

Recent progress in instrumental developments has mainly been made along two lines.

• i) Increase in spatial resolution and sensitivity of detection, allowing for the study of increasingly smaller samples such as IDPs, stellar condensates (presolar grains), and returned material from the Stardust and Genesis missions. Examples of instruments include the transmission EM (TEM), secondary ion MS (NanoSIMS), resonance ionization MS (RIMS), Auger probe, atom probe, and synchrotron X-ray fluorescence (μ-XRF).

• ii) Increase in the precision of isotopic analysis that allows for more precise dating, the study of isotopic heterogeneity in the SS, and other studies. An example is multicollector inductively coupled plasma MS (MC-ICP-MS).

In some cases, one can achieve both high precision at fairly high spatial resolution. Examples are ion microprobe analysis of the Al-Mg system and O-isotopic measurements of chondrules and CAIs with large-sector ion microprobes and Faraday cup detection.

The small amounts of material returned by the Stardust and Genesis missions have not only provided motivation to try new analysis and sample handling (10) techniques but have fostered collaborations and combination of a multitude of different analytical instruments. As an example, the reader is referred to a special issue of Meteoritics and Planetary Science (11) that is devoted to the analysis of material returned by the Stardust mission.

In the following, we will shortly describe various instrumental techniques and discuss important results obtained by their application.

Ion Microprobe

The ion microprobe has become the most important instrument for the isotopic analysis of small samples (12, 13). It uses SIMS. In this technique, a primary ion beam (O or Cs) of typically 15–20 keV energy is focused into a small spot onto the sample. Of the atoms sputtered by the primary beam bombardment, a certain fraction (the ionization efficiency strongly depends on the element) is emitted as ions, and these secondary ions are analyzed in a double-focusing magnetic sector mass spectrometer either by single ion counting in electron multipliers or by charge measurement in Faraday cups (FC).

One of the major applications of the ion microprobe is the isotopic analysis of presolar grains (14, 15). These grains, found in primitive meteorites, are bona fide stardust. They condensed in the expanding atmospheres of late-type stars [red giant branch (RGB) and asymptotic giant branch (AGB) stars] and in the ejecta from supernova (SN) explosions. They carry the isotopic compositions of their stellar sources and thus, give information about stellar nucleosynthesis and the isotopic evolution of the Galaxy. The ion microprobe makes it possible to measure isotopic ratios in presolar grains down to less than a micrometer in diameter.

A considerable advance in the study of small grains has been brought about by a new type of ion microprobe, the NanoSIMS (16). This instrument is characterized by a small primary beam spot (down to 50 nm for Cs), high secondary ion transmission at high mass resolution (needed for separation of atomic ions from molecular interferences), and up to seven detectors. The NanoSIMS has played an essential role in the discovery of presolar silicates (17, 18). Although presolar silicates in primitive meteorites are more abundant than other presolar grain species such as SiC, graphite, corundum, and spinel (14, 15), presolar silicates are small (typically 250–300 nm in diameter) and have to be detected in the presence of an overwhelming number of isotopically normal silicates of SS origin. This makes it necessary to measure the isotopic compositions of thousands and tens of thousands of grains, achieved by isotopic raster imaging. Fig. 1 shows O-isotopic ratio images of a 10-μm × 10-μm area covered with small grains from the Acfer 094 primitive meteorite (18). One grain is identified by its anomalous isotopic compositions (excess in ¹⁷O and deficit in ¹⁸O). Its enlarged SEM image is also shown.

Fig. 1.

Isotopic ratio images of O calculated from isotopic raster images obtained with the NanoSIMS. An enlarged SEM picture of a presolar silicate identified from the isotopic images is also shown. Modified from Nguyen and Zinner (18).

Isotopic imaging in the ion microprobe has been used for the detection of rare grain types such as SiC grains of type X, which have ²⁸Si excesses, indicating an SN origin (19). Even rarer are SiC grains with large ²⁹Si and ³⁰Si excesses, also believed to have an SN origin. Recent automatic imaging with the NanoSIMS identified such grains, and their isotopic ratios provide information on SN nucleosynthesis (20).

Isotopic raster imaging in the NanoSIMS has identified D- and ¹⁵N-rich hotspots, small areas with extremely large D and ¹⁵N excesses in organic matter from primitive meteorites (21) and in primitive IDPs (22, 23). These excesses indicate the presence of material from the presolar molecular cloud. Primitive IDPs not only have hotspots but also bulk enrichments in ¹⁵N as well as high abundances of presolar grains (silicates and SiC). The last study (23) is especially interesting, because in addition to the NanoSIMS, it applies the TEM, Raman microprobe, and X-ray absorption near-edge structure (XANES; see below) to the analysis of IDPs believed to have originated from the comet 26P/Grigg-Skjellerup.

The ion microprobe has also been important in the analysis of material returned from the comet Wild 2 by the Stardust mission. The NanoSIMS was used to identify true O-rich stardust material in impact craters on Al foil (24) and a presolar SiC grain in an aerogel capture cell (25). Ion microprobe oxygen isotopic analysis of Inti, a refractory inclusion captured by Stardust, showed a ¹⁶O-rich composition (26), confirming its similarities to CAIs. The presence of a refractory inclusion formed in the inner SS in a comet from the Kuiper belt indicates mixing of material from the inner to the outer solar nebula. Another piece of evidence confirming mixing was obtained by the ion-probe analysis of chondrule-like objects in cometary material from Stardust, which had O-isotopic compositions similar to those of chondrules from carbonaceous chondrites (27).

With large-radius ion microprobes and multidetection with FC, a precision approaching that of thermo-ionization mass spectrometry (TIMS) or inductively coupled plasma-source MS equipped with multiple collectors (MC-ICP-MS) has been achieved, whereas the primary beam diameter is still only ∼25 μm (28). Such measurements on chondrules have established that ²⁶Al was uniformly distributed in the early SS, which can be used as a fine-scale chronometer for early SS events (28, 29). A noteworthy technical advance with large-radius ion microprobes that have direct ion imaging capability, such as the Cameca ISM 1270/1280, has been the stacked CMOS-type active pixel sensor (SCAPS). This device allows one to obtain high-spatial resolution isotopic images with high sensitivity. It has not only been successfully used in the search for presolar silicates (30), but it has also led to the discovery of a reservoir extremely enriched in ¹⁷O and ¹⁸O in the early SS (31).

The MegaSIMS combines SIMS and accelerator MS (AMS). Its main design characteristic is not high lateral spatial resolution but high depth resolution and utmost sensitivity (32, 33). It was especially built for the isotopic analysis of solar wind N and O implanted into samples exposed during the Genesis mission (6). Because solar wind ions are implanted at shallow depth and low fluence, high depth resolution and sensitivity are extremely important. MegaSIMS analysis of Genesis samples has established the N- and O-isotopic compositions of the Sun (34, 35). Both have been the topic of intense debate for decades (36, 37).

Laser Ablation Gas MS

The Sun's N-isotopic composition obtained with the MegaSIMS agrees with that obtained by laser ablation gas MS (LA-GMS), where the solar wind-implanted N was excavated layer by layer by UV laser pulses (38). This technique was also used for the analysis of noble gases in Genesis samples (39). If differs from IR laser bombardment, where sample heating leads to the release of noble gases and where no depth profiles can be obtained. An example is the analysis of He and Ne in cometary matter from Stardust (40).

Auger EM

Although the electron beam used for X-ray analysis in the electron microprobe or SEM can be focused into a very small spot of only a few nanometers, the size of the volume from which X-rays are emitted is typically 1 μm in size. This is much larger than the typical size of presolar silicate grains identified by isotopic imaging in the NanoSIMS. Auger electrons, in contrast, are detected only from the top few nanometers of the sample and can be used for elemental analysis on this spatial scale (41, 42). Thus, Auger EM (AEM) is the ideal complement to the NanoSIMS for the in situ elemental analysis of submicrometer grains.

RIMS

One of the limitations of SIMS is that, generally, it cannot separate isobaric interferences (e.g., ⁹²Zr from ⁹²Mo). One of the great advantages of RIMS is that it uses a finely tuned laser beam to selectively ionize the neutral atoms of a given element sputtered from the sample by an ion beam or desorbed by a laser pulse (43, 44). These ions are then analyzed for their masses in a time of flight (TOF) mass spectrometer. The other advantage is that a large fraction (up to 50%) of the atoms released from the sample can be ionized and detected.

These features have been used to measure the Mo- and Zr-isotopic compositions of presolar SiC grains (45). Fig. 2 shows the Mo-isotopic patterns of two presolar SiC grains, identified by their C-, N-, and Si-isotopic ratios as originating from an AGB star (mainstream grain) and a supernova (type X grain) (46). The patterns of these two grains are completely different, indicating different nucleosynthetic processes (s process and neutron pulse, respectively). Another important RIMS result is the identification of the initial presence of ⁹⁹Tc in presolar SiC grains (47). The detection of Tc, which does not have any stable isotopes, in stars (48) played an important role in the history of the theory of nucleosynthesis (49).

Fig. 2.

Molybdenum isotopic patterns measured by RIMS in two different presolar SiC grains. The C-, N-, and Si-isotopic ratios of these grains are also given. Modified from Pellin et al. (46).

Microprobe Two-Step Laser MS

A variation of RIMS that is applied to the analysis of organic molecules is microprobe two-step laser MS (μL²MS). In this technique, a whole family of organic compounds desorbed from the sample with a pulsed IR laser beam is ionized resonantly by a UV laser beam. For example, UV light with a wavelength of 266 nm results in electronic excitation of the aromatic ring so that aromatic compounds can be selectively ionized in this way. The detailed masses of these compounds are determined in a TOF mass spectrometer. This techniques has led to the identification of polycyclic aromatic hydrocarbons (PAHs) and alkylated derivatives in IDPs (50) and PAHs in presolar graphite grains (51). Recently, it has resulted in the identification of N-rich aromatic compounds in cometary samples from Stardust (52), similar to what is observed in IDPs. This finding is consistent with the belief that some IDPs have a cometary origin.

AMS

In AMS, negative ions typically produced by sputtering are accelerated to energies of a few MeV and sent through a stripping foil or gas cell, which turns them into positive ions that subsequently go through a magnetic sector mass spectrometer. The outstanding characteristic of this technique is the extreme sensitivity to rare (radioactive) isotopes in the presence of high abundances of other (stable) isotopes. One of the most exciting discoveries by AMS is the detection of the radioisotope ⁶⁰Fe in deep-sea samples deposited 2.8 Myr ago (53). This finding indicates a nearby SN explosion a few million years ago.

TEM

TEM enables analysis of grain structure, elemental composition, and local bonding at scales ranging from micrometers to subnanometers. TEM images are formed from electrons transmitted through a thin sample (<200 nm) illuminated with high-energy electrons (usually 200 or 300 keV). The contrast in a given image depends on the electron beam illumination conditions (e.g., parallel or focused probe) and the selection of transmitted electrons that have undergone specific kinds of interactions with the sample (e.g., diffraction, high-angle scattering, or inelastic scattering). The most common types of TEM images in cosmochemistry studies are bright-field TEM (BF-TEM) for determination of microstructure, high-resolution TEM (HR-TEM) for imaging of lattice structure and atomic planes, selected area electron diffraction (SAED) for crystal structure determination, scanning TEM annular dark field (STEM-ADF) for atomic number contrast, and energy loss-filtered TEM (EF-TEM) for elemental mapping. In addition to imaging, TEM analysis often includes two types of spectroscopy: energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS); both provide elemental composition data, and in the latter case, they also provide information about bond configurations and oxidation states. More details of these techniques can be found in Williams and Carter (54). Applications of EELS are also discussed herein along with the related synchrotron-based XANES analysis.

The structural and chemical data obtained from TEM have fundamentally changed our understanding of the conditions of formation and alteration of SS and extrasolar materials. Studies of presolar grains provide direct constraint on circumstellar dust condensation conditions including temperature, pressure, order of phases condensed, and deviations from thermodynamic equilibrium (55–58) (Fig. 3). For SS materials, such as the Stardust comet Wild 2 samples, TEM data regarding crystallinity and the presence of subgrains inform our understanding of both nebular condensation conditions and radial transport processes (4, 59). Signatures of shock as well as thermal, aqueous, and radiation processing can all be identified. As the quality of electron optics and stability of the electron emitters in transmission electron microscopes has improved, the ability to detect ever more subtle processing signatures has also improved. Field emission sources, aberration-corrected lenses, and monochromation now permit detection of signatures as minute as single-atom impurities in some cases (60, 61).

Fig. 3.

Scanning TEM annular dark-field (STEM-ADF) image of a type-Z presolar SiC grain. The grain was first isolated from the host meteorite by physical disaggregation and then, was sectioned by focused ion-beam microscopy to form an electron transparent section (123). The STEM-ADF image reveals a crack that extends from the grain surface into the interior as well abundant subgrains. Energy dispersive X-ray spectroscopy (EDX) shows that the crack is oxidized and that the subgrains include multiple phases, some rich in Fe and Ni and others rich in Ti and V.

Focused Ion-Beam Microscopy

Focused ion-beam microscopy (FIB) addresses the need to prepare TEM samples of very heterogeneous materials (e.g., sections of chondrite matrix or rims associated with chondrules and CAIs) (62, 63) and/or isotopically anomalous or otherwise unique grains ranging in size from microns to submicrons (56, 64). The FIB is similar to an SEM with a Ga⁺ ion beam in place of the electron beam (FIB) or in combination with an electron beam (FIB-SEM) (65). The user controls the Ga⁺ beam current and raster pattern to remove sample material by sputtering and creates a site-specific electron transparent membrane. The membrane is then extracted from the sample and attached to a TEM support grid by in situ or ex situ micromanipulation for subsequent TEM, SIMS, or XANES analysis. Needle-shaped specimens for atom probe tomography can also be produced.

Atom Probe Tomography

Atom probe tomography is a time of flight MS method for atom by atom tomographic reconstruction of needle-shaped specimens (66). Atoms are evaporated from the needle tip by a combination of applied high-voltage and pulsed laser heating and are collected with a 2D position sensitive detector. Using an estimate of the radius of curvature of the needle tip (50–150 nm) and the TOF and position of a detected atom, the mass and the original location of the atom in the specimen can be determined. Recent atom probe measurements of SiC grains show promise for characterization of trace element segregation in presolar grains that could result from heterogeneous nucleation or ion implantation of circumstellar or interstellar gas (67, 68). Preparation of samples that evaporate uniformly is the major challenge in all atom probe experiments. Needles of pure metal specimens can be obtained by electrochemical etching, but most samples are now prepared by FIB techniques.

Analysis with Synchrotron Radiation

The installation of third-generation electron storage rings specifically designed for the production of synchrotron radiation resulted in new capabilities for microanalysis. In these facilities, high-intensity X-rays are produced by insertion devices (wigglers and undulators) in the path of the electron beam and after selection of a small energy range by a monochromator, can be focused down to diameters of <1 μm by microfocusing mirrors (69). This makes the high-sensitivity analysis of small samples possible. There are several analytical techniques making use of synchrotron radiation (70) that have been applied to extraterrestrial samples. Among the techniques are XRF, X-ray computed microtomography (XRCMT), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), and XANES.

XRF Analysis.

In synchrotron XRF analysis, the (usually monochromatic) finely focused X-rays are used to produce characteristic X-rays from the sample, which are analyzed for their energy, giving information on the elemental composition of the sample (71). Because of the atomic charge (Z) dependence of fluorescence, XRF is especially suited to the analysis of heavier elements. Element images can be obtained by raster scanning. This method has been successfully applied to determine the location and composition of cometary fragments along dust particle tracks in Stardust aerogel (72, 73). These studies found compositional differences between the debris along tracks and terminal particles. Cluster IDPs, analyzed in a similar way, might be an analog to cometary particles collected by Stardust (74). An impressive SXRF study is the measurement of trace element abundances in individual presolar SiC grains ∼2 μm in size, providing evidence that the short-lived isotope ⁹³Zr had condensed into the grains at the time of their formation (75).

XRCMT.

The 3D internal structure of an object can be obtained by combining many X-ray images, usually by rotating the sample (71, 76). Images of X-ray attenuation yield 3D density structure. This technique has been applied to determine the location and size of cometary particle fragments in Stardust aerogel tracks (77, 78). X-ray tomography can also be done from XRF images and thus, provides the 3D distribution of different elements. However, this analysis requires a pencil beam and is much more time intensive.

Synchrotron Fourier Transform Infrared Spectroscopy.

The use of infrared light from synchrotron light sources leads to an increase in brightness of 100–1,000 times over conventional IR sources and allows imaging with diffraction-limited beam spots. The achieved high sensitivity is instrumental for the analysis of small samples such as volatile cometary material deposited in aerogel tracks from the Stardust mission, allowing for the identification of several organic compounds such as amines and amino acids (79, 80).

XANES.

Synchrotron XANES (81) and TEM-based electron loss near-edge spectroscopy (ELNES) (82) probe the energy-level structure of atoms in a material. In XANES, the absorption of a monochromated, synchrotron X-ray source because of excitation of electrons from an inner shell is measured as a function of energy. In ELNES, a type of TEM-EELS, the intensity of inelastic scattering of the incident electron beam by the sample because of core-level excitation is measured as a function of the energy loss. Both techniques are commonly used in cosmochemistry to determine oxidation states of Fe (83–85), Ti (25, 86), and other transition metals and to fingerprint carbon bond distributions (87) (e.g., graphite vs. diamond vs. different organic molecules) to constrain formation conditions. In general, soft X-ray XANES has greater sensitivity and energy resolution (<0.1 eV) than does most ELNES (1 eV) because of the bright monochromated X-ray source. Soft X-rays are also less damaging to many materials, particularly organics, than high-energy electrons. Hard X-ray XANES can be used to characterize thicker samples with ∼1 eV energy resolution, including submicron particles captured in aerogel (85). However, the achievable spatial resolution of ELNES (subnanometer) is much greater than that of XANES (>10 nm), and 0.1 eV energy resolution can be achieved in a monochromated electron microscope (60, 88).

Raman Microprobe

Raman spectroscopy gives information on molecular bonds, and the Raman microprobe allows analysis on a submicrometer level. It has been successfully applied to the study of insoluble organic matter in primitive meteorites (89), where it was found that metamorphic processes and radiation affect the Raman signal to the identification of ultra-primitive IDPs (23) (see above), the analysis of returned Stardust samples (90), and presolar SiC grains (91). In the last study, the Raman spectrum of a supernova grains indicates crystallographic disorder, whereas grains from AGB stars have a well-crystallized cubic structure.

NMR Spectroscopy

Meteorites and IDPs contain organic material, some of which might have a presolar origin. Recently, NMR analysis has been used to determine the structure of organic macromolecules, the insoluble organic matter (IOM). In solid-state NMR spectroscopy, the resonant frequency of a spin one-half nucleus such as ¹H or ¹³C depends on its electronic environment. Thus, an NMR spectrum provides information about the bonding of these nuclei (92). The study of IOM from primitive meteorites revealed many aromatic and aliphatic functional groups as well as diamond (92, 93). It was found that the relative abundances of aromatic and aliphatic carbon as well as nanodiamonds vary with meteorite group and might provide information on the oxidative nature of aqueous fluids on the meteorites’ parent bodies.

ICP-MS

The mid-1990s saw the development of ICP-MS equipped with multiple collectors (MC-ICP-MS). ICP sources can ionize most elements and have permitted the precise isotopic measurement of elements, such as transition metals W, Hf, and Zr, which are poorly ionized by the state of the art TIMS. Because of rapid sample throughput and because the instrumental mass bias varies smoothly during an analytical session, the correction of the instrumental mass bias can be made with an element different from the one being analyzed (e.g., Cu for Zn) and by bracketing standards (94). The combination of laser ablation with MC-ISP-MS has permitted precise in situ measurements of Mg- and Si-isotopic ratios (95, 96).

During the last 15 y, MC-ICP-MS has become one of the most important techniques in isotope geochemistry and cosmochemistry laboratories. Here, we focus on high-precision MC-ICP-MS isotopic analyses that have led to breakthrough discoveries in cosmochemistry.

Isotopic Heterogeneity in the Solar Nebula.

The presence of presolar grains in primitive meteorites is evidence for incomplete homogenization of presolar material during SS formation. Although presolar grain can be analyzed individually, recent improvements in analytical methods and instrumentation, notably MC-ICP-MS, have permitted resolution of small isotopic heterogeneities for several elements in bulk meteorites (e.g., Ti and Ni) (97–99), whereas Fe and Zn have uniform isotopic compositions (100, 101). Measuring the abundance of several isotopes produced by the same process provides useful constraints on the nucleosynthetic processes responsible for the isotopic anomalies. In addition, correlated isotopic effects are used to quantify homogeneity in the SS distribution of extinct radionuclides (e.g., ⁶⁰Fe T_1/2 = 1.5 Myr) that could possibly be used as chronometers. For instance, ⁵⁸Fe and ⁶⁰Fe are synthesized together in core collapse supernovae or AGB stars by neutron capture reactions (102). Correlated isotopic anomalies of ⁵⁰Ti, ⁵⁴Cr, ⁶²Ni, and ⁶⁴Ni (97–99, 103) have been observed in meteorites and suggest some large-scale isotopic heterogeneity that has survived mixing within the solar nebula. However, the absence of ⁵⁸Fe-isotopic anomalies larger than 0.3–0.5 ε in bulk meteorites argues for the injection of ⁶⁰Fe in the early SS and its homogenization before the formation of planetary bodies (100).

Tungsten Isotopes: Age of the Metal/Silicate Differentiation on Earth and Asteroids and the Giant Impact as the Origin of the Moon.

Although W is a moderately siderophile element, Hf is a lithophile element and is strongly fractionated from W during core formation. ¹⁸²Hf/¹⁸²W chronometry can date Hf/W fractionations that occurred up to ∼60 Myr after the formation of ¹⁸²Hf (half-life = 9 Myr). Comparison of terrestrial mantle samples with chondrites has been used to evaluate the timing of terrestrial core formation (104–107). Although Lee and Halliday (105) did not find any difference in the W-isotopic composition between terrestrial samples and chondrites, later studies detected a small but resolvable ∼20 ppm excess of ¹⁸²W in the Earth relative to chondrites (106–108). These results imply that core mantle differentiation occurred probably within 30 Myr after SS formation.

Iron meteorites, thought to represent the core of asteroids, have deficits of 3–5 ε in ¹⁸²W/¹⁸⁴W ratio compared with terrestrial samples (109–112). These deficits correspond to differentiation ages of ∼1–5 Myr after SS formation. The first Pb-Pb dating of Group IVA iron meteorites confirmed this rapid differentiation of iron meteorite parent bodies (113).

The silicate fraction of the Moon has the same ¹⁸²W/¹⁸⁴W ratio as the Earth's mantle (114). The implications of this result on the time of the giant impact as the origin of the Moon depend on its Hf/W ratio. A higher than terrestrial Hf/W ratio, derived from the W/U ratio of lunar rocks (114), implies a time for the giant impact >60 Myr after SS formation. However, Yin et al. and Yin and Antognini (115, 116) argued that the Hf/W ratios of the two planetary bodies were similar, reducing the time to ∼30 Myr, as initially suggested (106, 107).

Variations of Stable Isotopes of Heavy Elements: Tracers of SS Processes.

Small stable isotopic fractionations of heavy elements have been measured by MC-ICP-MS in meteorites and lunar samples for moderately volatile elements (e.g., Zn and Cd) (117, 118) and more refractory elements (e.g., Fe and Ni) (119, 120). These isotopic variations, which are too small to be measured by any other instrument, brought additional constraints on the volatile history and the physical processes (e.g., impacts), which have modified the meteorite parent bodies and the Earth. The abundance of Zn, a moderately volatile element, in carbonaceous chondrites decreases in this order: CI > CM > CV-CO > Earth (121). Its isotopic composition decreases in the same order from isotopically heaviest in CIs to isotopically lightest in the Earth (117). The opposite pattern is expected if the Zn depletion in Zn in chondrites and Earth relative to CI chondrites is because of volatilization. The observed pattern is an argument in favor of an incomplete accretion origin for the volatile depletion in the SS (122).