Cosmochemistry, isotopic analysis

Farquhar J, Wing BA (2003) Multiple sulfur isotope analysis applications in geochemistry and cosmochemistry. Earth Planet Sci. Lett 213 1-13 Farquhar J, Johnston DT, Wing BA, Habicht KS, Canfield DE, Airieau S, Thiemens MH (2003) Multiple sulphur isotope interpretations for biosynthetic pathways implications for biological signatures in the sulphur isotope record. Geobiology 1 27-36... 

Magnetic sector ion microprobes are becoming increasingly important in isotopic analysis of extratenestrial materials as spot size decreases and the precision and accuracy of the measurements improve. The first of the commercially available ion microprobes used in cosmochemistry were the Cameca ims 3f-7f series machines, which initially became available in the mid-1980s. These multipurpose instruments are able to measure isotopic compositions of most elements of interest in cosmochemistry and can also be used to measure trace element abundances. Their main drawback is that the relatively small mass spectrometer can only be operated at mass-resolving powers below about 9000, and at this mass resolving power, the transmission of the mass spectrometer is very low. 

Other mass spectrometric techniques such as RIMS and AMS possess high isotope selectivity for extreme ultratrace and isotope analysis of, in particular, radiotoxic isotopes ( C, "Ca, Sr, Tc, Pb, U and plutonium isotopes) in the environment, in cosmochemistry, radiodating, nutrition and biomedical research. RIMS has become as an nltrasensitive and selective analytical technique for the determination of extremely low isotope abundances. In spite of the excellent analytical features of RIMS (detection limit for isotopes 10 atoms per sample) and exciting applications for the determination of extremely low abundances and isotope ratios of long-lived radionuclides, such as all plutonium isotopes (including Pu), U or " Ca, but no commercial instrument is available on the analytical market. 

Application of Multiple-Collector Inductively Coupled Plasma Mass Spectrometry to Isotopic Analysis in Cosmochemistry... 

Given the importance of isotopic analysis to cosmochemical research, it is unsurprising that improvements in mass spectrometric methods and instrumentation have played a significant role in advancing the scientific field. The advent of multiple-collector (MC) inductively coupled plasma mass spectrometry (ICP-MS) in the early 1990s [10-13] is no exception in this regard. The success of this instrumental technique, which is now routinely used in well over 100 laboratories world-wide, has had an enormous effect on cosmochemistry, with many studies published over the last two decades that showcase novel methods and applications. 

This chapter provides an overview of the impact that MC-ICP-MS has had on cosmochemical research. To this end, it encompasses (i) an introduction to the extraterrestrial samples, and in particular meteorites, which are analyzed (ii) a brief explanation of the origin and significance of the most important types of isotopic anomalies that are measured (iii) a brief introduction to the particular advantages that MC-ICP-MS provides for isotopic analysis in cosmochemistry and to common analytical procedures and (iv) an overview of important applications of MC-ICP-MS in cosmochemistry. This last section highlights selected novel findings and their scientific significance, while also discussing particular analytical procedures and potential pitfalls. 

Although chemical and isotopic analysis by remote sensing using either Earth- or satellite-based instruments play an important role in some cosmochemical studies, laboratory measurements on real samples remain the predominant means of data acquisition. In cosmochemistry, extraterrestrial materials are the most... 

In the following, we provide (i) a short overview of the particular advantages that the MC-ICP-MS technique features for isotopic analysis in cosmochemistry and (ii) a brief outline of the analytical techniques that are utilized in such measurements. 

The high ionization efficiency that characterizes plasma source mass spectrometers also has drawbacks, however. In particular, isotopic analysis by MC-ICP-MS is more susceptible than TIMS measurements to spectral interferences from isobars, molecular ions, and doubly charged species. Such interferences are furthermore much more problematic for isotope ratio than for element concentration measurements, as isotopic compositions typically need to be measured with an accuracy and precision of better than 100 ppm. In order to reduce spectral interferences to tolerable levels, isotopic analyses of complex natural samples in geo- and cosmochemistry are generally carried out following separation of the... 

There are two principal means of sample introduction for isotopic analysis by MC-ICP-MS in geo- and cosmochemistry. Most common is the technique of solution nebulization, whilst laser ablation (LA) systems have been applied for in situ isotope ratio measurements [32]. As such, MC-ICP-MS complements both the TIMS and SIMS methodologies, which are the most common alterative methods for bulk sample and in situ isotopic analysis, respectively, in cosmochemical research. 

Isotopic analysis in cosmochemistry has further profited immensely from instrumental improvements that were developed over the last decade, including new skimmer cone geometries, improved desolvation nebulizer systems, and the use of higher capacity vacuum pumps for the expansion chamber. These... 

Nonetheless, MC-ICP-MS has been used successfully for Nd isotopic analysis in cosmochemistry. A good example is the combined use of the Sm-Nd and Lu-Hf systems by Blichert-Toft et al. [70], They analyzed 18 eucrites (Table 10.1) and found an Sm— " Nd age of 4464 + 75 Ma, with three cumulative eucrites defining a better constrained isochron of 4470 + 22 Ma. This suggests that cumulative eucrites have a crystallization age that postdates the formation of the solar system by about 100 Ma and implies a fairly protracted crystallization history for the eucrite parent body, which is thought to be the asteroid 4 Vesta (Table 10.1). Combined " Nd and Nd ages of eucrites [71, 72] give a some-... 

Lead has four stable isotopes, ° Pb, Pb, Pb, and ° Pb, but three of these are radiogenic, such that there is no invariant isotope that can be used for internal correction of instrumental mass fractionation. This poses a significant analytical challenge for precise Pb isotopic analysis. This problem is most readily overcome with MC-ICP-MS, because the Pb isotope ratio measurements can utilize the ratio of added T1 for external normalization of the Pb isotope ratio data (see also Chapter 5). The simplicity of this procedure is the main reason why MC-ICP-MS Pb isotopic analysis is now commonly used in geo- and cosmochemistry. This approach needs to be applied with care, however, and it has been argued that Pb isotopic analysis that utilizes the double spike methodolc (either in conjunction with TIMS or MC-ICP-MS) is typically superior in accuracy and precision [86, 87]. 

The application of MC-ICP-MS has had a profound impact on isotopic research in cosmochemistry over the last two decades. This immense impact primarily reflects two factors. First, MC-ICP-MS instruments are comparatively affordable and straightforward to use. As a result, there are now many laboratories world-wide in which MC-ICP-MS instruments are in routine use on a daily basis. The second factor is the performance characteristics of the instrumental technique, which is both versatile and suitable for high-precision isotopic analysis. As such, MC-ICP-MS can been applied to resolve small natural isotopic variations for a wide range of metallic and metalloid elements. Furthermore, it is equally suitable for the analysis of radiogenic and nucleosynthetic isotope anomalies and also mass-dependent isotope fractionations. As such, the technique of MC-ICP-MS is ideally suited for exploring the wealth of isotopic variations that are present in extraterrestrial materials and many successful investigations, which have yielded novel and important results, have been carried out in the recent past. 

Turekian, K. K., Chan, L. H., The Marine Geochemistry of the Uranium Isotopes, 230Th and 231Pa, In Activation Analysis in Geochemistry and Cosmochemistry, Universitetsforlaget, Oslo, 311-320 (1971). 

Resonance-ionization mass-spectrometry is still in the development stage in terms of its application to cosmochemistry. The Charisma instrument, which is operated by Argonne National Laboratories, uses multiple lasers to resonantly ionize only the elements of interest, which are then analyzed with a time-of-flight mass spectrometer. The Charisma instrument has made isotopic measurements of molybdenum, zirconium, strontium, ruthenium, barium and other elements in presolar grains. These measurements are made possible by the high ionization efficiency of the RIMS technique and its ability to completely eliminate isobaric interferences. Work is now underway to build a RIMS instrument that can be operated by an individual investigator in a university laboratory. If this succeeds, RIMS will play an increasing role in analysis of extraterrestrial materials. 

Figure 1 Studies published between 1988 and 1997 reporting use of secondary ion mass spectrometry (SIMS) in geochemistry and cosmochemistry. Papers are subdivided according to theme Hvy Iso, studies of heavy isotope ratios, principally for U-Pb dating Lt Iso, studies of light stable isotope ratios (H, B, C, O, S) Hvy El, studies primarily focused on analysis of elements >40 amu (e.g., rare earth elements) Lt El, studies primarily focused on analysis of elements <40 amu (e.g., water content) Prec Met, analysis of precious metal contents (e.g., Au, Ag, Pt) Expt, analysis of experimental run products Misc, other mis-cellanous studies utilizing SIMS.

Analysis of extraterrestrial materials, and in particular meteorites, is an important focus of cosmochemical research, as such samples preserve chemical and isotopic records of early solar system conditions and processes. The first studies of meteorites, which recognized that such samples have an extraterrestrial origin, date back to the late eighteenth century [3], but modem research in cosmochemistry has a much more recent origin. This is traced back by many to the founder of contemporary geochemistry, V.M. Goldschmidt, as he produced early, but well-founded, compilations of cosmic element abundances, based on data acquired for meteorites [4, 5]. Goldschmidt s work was later continued and extended by Suess in collaboration with Urey and their study on the abundances of the elements [5] is still an important milestone in cosmochemistry. 

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