Anal. Chem. 2001, 73, 2687-2700
Geochemical and Cosmochemical Materials Michael E. Lipschutz,*,† Stephen F. Wolf,‡ John M. Hanchar,§ and F. Bartow Culp†
Department of Chemistry, BRWN/WTHR Building, Purdue University, West Lafayette, Indiana 47907-1393, CMT/205, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439-4837, and Department of Earth and Environmental Sciences, The George Washington University, 2029 G Street N.W., Washington, DC 20006 Review Contents Geostandards Sample Preparation Multitechnique Symposia and Reviews Inductively Coupled Plasma Mass Spectrometry Solution Laser Ablation Sector-Based High-Resolution and Multicollector ICPMS Solution Laser Ablation Mass Spectrometry Electron Microbeam Techniques Nuclear Techniques Miscellaneous Methods Spectroscopy Chromatography Others Literature Cited
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This review surveys the literature on the chemical analysis of terrestrial and extraterrestrial solids for the two-year period October 1998 to October 2000. We continue to include extraterrestrial materials as we did for the 1996-1998 period (A1) because analyses of these samples on Earth continue to present new challenges to the analytical geochemist. We focused upon rocks and minerals and did not include gaseous species. We included aqueous solutions in our survey only where these bore upon geologic processes, but not environmental problems. We carried out comprehensive on-line searches of the pertinent chemistry and geoscience literature to support the reviewers’ individual reading and subject expertise. We searched the following databases: GEOREF, established by the American Geological Institute in 1966, is the on-line complement of the printed Bibliography and Index of Geology. It is the most comprehensive database in the geosciences, containing over 2 million references to journal articles, books, conference proceedings, reports, and theses. The particular format used for these searches was the on-line version of the database covering 1998-2000. CAPLUS, produced by the Chemical Abstracts Service (CAS) of the American Chemical Society, is the most current version of Chemical Abstracts Online, including the entire Chemical Abstracts file from 1967 with additional new and unindexed entries. It covers †
Purdue University. Argonne National Laboratory. § The George Washington University. ‡
10.1021/ac010280g CCC: $20.00 Published on Web 04/21/2001
© 2001 American Chemical Society
the world’s chemical literature comprehensively, with specific sections on geochemistry and cosmochemistry. We used Scifinder Scholar, the new search engine developed by CAS, to search these database. In searching each database, we followed two strategies. First, we very broadly searched over the period (1998-2000) combining the concept terms for “geochemistry” and “chemical analysis”. These searches resulted in large numbers of recordssover 400 from each database. These records were scanned by the reviewers to check for their relevance to the scope of the review. Accordingly, we compiled a list of key terms for specific analytical procedures and carried out follow-up searches using these terms in the context of geochemistry. In addition, we manually searched The Analyst, Analytica Chimica Acta, Analytical Chemistry, American Mineralogist, The Canadian Mineralogist, Chemical Geology, Contributions to Mineralogy and Petrology, Critical Reviews in Analytical Chemistry, Earth and Planetary Science Letters, Geochimica et Cosmochimica Acta, Journal of the American Society for Mass Spectrometry, Journal of Chromatography, Journal of Geophysical Research E, Journal of Radioanalytical and Nuclear Chemistry, Meteoritics and Planetary Science, Nature, Science, Spectrochimica Acta B, and Talanta for bulk elemental and isotopic analysis of geochemical and cosmochemical materials during the period of interest. In this connection, we note that Geochemical Transactions, a new electronic journal published by the Royal Society of Chemistry in collaboration with the Division of Geochemistry of the American Chemical Society, was launched in March 2000. This is the latest project of the Scholarly Publishing and Academic Resources Coalition (SPARC), which encourages competition in the scholarly communications market. According to its mission statement, Geochemical Transactions intends to “provide a medium for the rapid publication of high quality research in all areas of chemistry as it relates to materials and processes occurring in the Earth’s aquasphere and geosphere”, The journal is available in electronic form only, and beginning in January 2001, any article published in it is free to all for a period of four months after its publication. Information about this journal can be found at its Web site (A2). Mindful of Editorial instructions to focus on a more limited number of exciting areas, we chose to highlight trends in trace analysis, particularly of smaller samples. In fact, this follows analytical trends in geochemical and cosmochemical research, which essentially is leading to getting ever more compositional information from ever smaller samples. In the past two years, this trend has particularly been characterized by an explosion of studies using inductively coupled plasma mass spectrometry Analytical Chemistry, Vol. 73, No. 12, June 15, 2001 2687
(ICPMS) with all of its hyphenated permutations. Other trace analysis techniques are falling by the wayside more or less rapidly. The most noteworthy of these is accelerator mass spectrometry (AMS), long considered as promising even though it suffered the drawback of requiring access to an accelerator. During the past two years, few studies employing it to quantify long-lived radionuclides have appeared. Almost none of these studies is of sufficiently general interest to be cited here. A second trend in geochemical research reported during the past two years is the application of two or more advanced techniques to characterize a terrestrial sample or suite of samples. This “consortium approach” has long been a staple in cosmochemistry, where samples are rare and usually small and where bulk host material is not generally plentiful. In geochemical studies where bulk material is usually plentiful, the major appeal of the multitechnique approach is that data sets generally prove synergistic. Even in such materials, it may be only rare grains that are of interest (for example, a single zircon that is to be dated.) Also, as techniques improve so that more and more information can be obtained from lesser quantities of sample, compositional heterogeneity becomes a potentially more serious problem. Whether this problem is serious can be determined by gathering several sorts of data, to establish whether sample heterogeneity can be affecting the conclusions of the primary study. Our search was not myopic however since we also tried to identify important studies that might not be included in the focus areas. GEOSTANDARDS A recent editorial in Geostandards Newsletter: The Journal of Geostandards and Geoanalysis (GN) carried the title “Into the Electronic Future” and discussed the rapid changes that electronic communication is bringing to the world of scientific publishing (A3). In the specific case of GN, this means the appearance of simultaneous electronic and print issues and the relocation of its Web site to the Centre de Recherches Pe´trographiques et Ge´ochimiques (CRPG, a division of CNRS, the French National Center for Scientific Research), the publisher of GN. (The new URL is http://www.crpg.cnrs-nancy.fr/Geostandards/) An example of the utility of the electronic format is the brief abstract of the results of GeoPT4 and GeoPT5, the fourth and fifth international proficiency tests for analytical geochemistry laboratories (A4). The complete text and lengthy data tables that comprise this article are only available in the Web version. It is the intention of the editors of GN that the electronic version of the journal will be maintained permanently; the larger issues of what “permanent” means, and of the future access to such information, are those facing the scientific publishing world in general. Kane and Potts (A5) reviewed the International Organization for Standardization (ISO) recommendations for certification of reference materials and have proposed ways they can be adapted to new geostandard reference materials (GRM). These authors point out that only 8 of the over 300 GRM qualify as certified reference materials (CRM) according to ISO guidelines. Jochum et al. (A6) have sought to address this last point by preparing eight new geological reference glasses for in situ microanalytical investigations and establishing the trace element concentrations to match ISO guidelines. Preliminary reference values for more than 60 elements were calculated from the analytical results with 2688
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uncertainties between 1 and 15%. One key necessity for any trace element or isotopic analysis using secondary ion mass spectrometry (SIMS) is some suitable standard(s), required to obtain precise and accurate quantitative data. Hinton (A7) investigated the degree of homogeneity of the commonly used National Institute of Standards and Technology (NIST) 600 series standard reference materials (SRM) glasses. These glasses nominally contain 500 (SRM 610, 611), 50 (SRM 612, 613), 1 (SRM 614, 615), and 0.02 (SRM 616, 617) µg/g of 61 elements. Ion probes cannot give absolute elemental concentrations but can accurately provide elemental abundance ratios between glasses of similar major element compositions. Hinton (A7) determined that although the absolute concentrations of the NIST 600 series glasses are significantly lower than the nominal values, the actual dilution factors are close to the weighed amounts. The consistency between the ratios of random samples of glasses (e.g., SRM 610/SRM 612 and SRM 611/SRM 613) supports a high degree of homogeneity at all scales. As with analytical geochemistry in general, the field of geostandards has been dominated by the “more from less” theme: the attempt to wrest more information from smaller samples. This has occasionally been accomplished by newer methodologies, such as the use of high-resolution, magnetic sector ICPMS (HR-ICPMS) to determine elements at nanogram per gram levels in rock reference materials (A8). The greater resolving power of HR-ICPMS reduces the effects of polyatomic interference and makes it especially valuable in determining elements with masses less than 80 amu, that is, Sc, which Robinson et al. (A9) claimed as being incompletely resolved spectrally from 29Si16O and 28Si16O1H if quantified by quadrupole ICPMS. They determined Sc, Y, and rare earth elements (REE) in four basaltic, one andesitic, and three ultramafic SRM without prior separation or preconcentration steps. Making use of the sensitivity of multiple collector ICPMS (MCICPMS) and employing new sample preparation and ion-exchange separation methods, Yi et al. (A10) determined Cd, In, and Te in six international GRM. The detection limits were 4.0 Ga) continental crust, a result at odds with Zr/Hf data for samples from a western Greenland site, the only early Archaean suite studied previously. This difference (E20) raises the issue of whether the early Archaean mantle was regional or global in extent. The Li isotopic compositions of open ocean seawater, nine international rock standards, and a C1 chondrite were studied to provide a benchmark for ensuring the reliability of Li isotope data between different laboratories and different techniques such as TIMS, MC-ICPMS, and ion probe (E21). Tomascak et al. (E22) described a method for the preparation of geological materials for determination of Li isotopic composition by MC-ICPMS requiring only 40 ng of Li. In another study, Tomascak et al. (E23) demonstrated that MC-ICPMS measurement of the Li isotopic composition is more rapid than using TIMS and that MC-ICPMS results are at least as accurate for such small Li amounts. They demonstrated that mass fractionation of Li does not occur at the
per mil level during basaltic liquid/crystal fractionation at >1050 °C. Neither Cu nor Zn has been widely used as a geochemical tracer because no convenient technique existed to quantify their isotopic compositions. Mare´chal et al. (E24) developed a methodology using MC-ICPMS to measure the isotopic compositions of even 20 ng of Cu or Zn. Mass fractionation was corrected for using a nonisotopic standard, i.e., via spiking with Zn for a Cu sample or Cu for a Zn sample. The technique demonstrated an internal precision of 20 µg/g and an external reproducibility of 40 µg/g (95% confidence level). Zhu et al. (E25) utilized two calibration methodologies to determine the Cu isotopic composition by MC-ICPMS, Zn spiking and sample-standard bracketing. The simultaneous multielement capability of ICPMS has made possible the precise determination of the Tl isotopic composition. Measurement of Tl isotopic composition by TIMS has been limited by the fact that Tl possesses only two naturally occurring isotopes, precluding correction for instrumental mass discrimination by measurement of an independent isotope ratio. Rehka¨mper and Halliday (E26) used the unique multielement capabilities of MCICPMS to correct for instrumental bias by spiking the sample with a Pb spike with known isotopic composition. The procedures that the authors developed were capable of a precision of 0.01-0.02% for terrestrial rocks and meteorites, a factor of g3-4 better than TIMS. The results for some terrestrial samples (ferromanganese crusts) demonstrate Tl isotopic fractionation during precipitation from seawater (cf. ref E12). Thus, Tl is the heaviest element established to be mass-fractionated by a geochemical process (E26). They also quantified the isotopic composition of Tl in the Allende CV3 chondrite. Tl is known to be easily vaporized by heating of primitive meteorites such as chondrites. Since some 205Tl would have formed from live 15-Ma 205Pb in the early Solar System, the Tl isotopic composition may, in the future, provide important information on the early evolutionary history of early objects. Nonisotopic spiking was also employed by White et al. (E27) for high-precision determination of Pb isotopic composition by MC-ICPMS using Tl to correct for mass-dependent isotopic fractionation. Fractionation coefficients for Tl and Pb proved different, but the ratio of the two coefficients was found to be constant over the course of an experimental run, allowing calculation of the Pb fractionation coefficient from that of Tl. The precision of the MC-ICPMS analyses were 2-6 times better than those of TIMS for 206Pb/204Pb and 208Pb/206Pb, respectively. Laser Ablation. There have been several papers utilizing recent developments in LA-ICPMS and LA-MC-ICPMS technology. Stirling et al. (E28) developed a method for rapid, in situ determination of U-Th isotopes at the semimicrometer scale using laser ablation sampling. This is the first study attempting to measure in situ U-series isotopes by combining a laser ablation apparatus with MC-ICPMS. This technique uses a Q-switched and frequency quadrupled 266-nm Nd:YAG laser to ablate samples containing ∼100 µg/g levels of U with a spatial resolution of 150 µm. This corresponds to 1-4 ng of 238U, ∼70-200 fg of 234U, and 20-60 fg of 230Th per analysis. Samples analyzed in this study included synthetic glass standards and naturally occurring samples of zircon and opal, which were used to assess the precision and accuracy of the LA-MC-ICPMS method. The234U/238U values were Analytical Chemistry, Vol. 73, No. 12, June 15, 2001
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indistinguishable from ICPMS solution nebulization values. With an Ar gas system, Th/U ratios are systematically lower and the apparent 238U-234U-230Th are systematically younger than the true values. Using He gas, elemental fractionation between U and Th is entirely eliminated while precision and accuracy in measurement of the U isotopes are retained. Yin et al. (E29) reported new Zr isotopic evidence for live 92Nb in the early Solar System using LA-MC-ICPMS analyses of zircon from the Chaunskij mesosiderite and rutile from the IAB iron meteorite, Zagora. The use of the laser ablation for in situ analyses of those minerals and the MC-ICPMS instrument allowed determination of 90Zr, 91Zr, 92Zr, 93Nb, 94Zr, 95Mo, 96Zr, and 97Mo. An example of a multitechnique study, is that of Griffin et al. (E30), who used a LA microprobe connected to an MC-ICPMS to analyze zircons in kimberlites from Zaire. They determined Lu and Hf isotopic compositions by LA-MC-ICPMS and a large number of trace elements by LA-ICPMS. The Lu and Hf data compare well with earlier TIMS results on the same zircons, and Griffin et al. (E30) compared their dating results with these obtained for zircons from other sources (Australia, Siberia, southern Africa), mainly by the SHRIMP ion microprobe. They concluded that the source magma for the kimberlites derived from depleted mantle or ocean island basalts that reacted with the subcontinental lithospheric mantle. MASS SPECTROMETRY Traditionally, high-precision TIMS has been used to measure isotopic ratios in geologic and cosmochemical materials. It is competitive with HR-ICPMS and MC-ICPMS in terms of sensitivity and ability to determine isotopic compositions in small samples but is much more labor- and time-intensive. It continues to be used for several isotopic systems. During 1998-2000, a number of studies have been made using MC-ICPMS and TIMS or comparing results of these two techniques, with MC-ICPMS usually emerging as the method of choice. These were reviewed earlier and cited in earlier sections (B2, E2, E4, E11, E13, E19, E21, E23, E26, E30). Identification and precise determination of Fe isotope fractionation is a prerequisite for its use as an indicator of extraterrestrial biotic activity. Beard and Johnson (F1) presented a double-spike method for high-precision (0.014% RSD) determination of Fe isotopic composition by TIMS, and they identified naturally occurring, mass-dependent Fe isotope fractionation using this technique. As part of the Alle´gre Festschrift (C2), Kurz and Geist (F2) measured He isotopic compositions (by noble gas mass spectrometry) and those of Sr, Nd, and Pb (by TIMS) in a number of lava flows from the Galapagos Archipelago. The combination of data establish links between source formation ages and sampling and mixing with asthenospheric material during oceanic intraplate volcanism. A new technique for determining 231Pa in silicate rocks by IDTIMS was developed by Bourdon et al. (F3). The method does not require 237Np for spike preparation and allows determination down to 100 fg of 231Pa with a 1-2% uncertainty at the 2σ level. This method was used to determine 231Pa-235U disequilibrium in Tonga-Kermadec island arc lavas and was able to discriminate between the time scales of fluid addition and partial melting (F4). 2694
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Liu et al. (F5) developed an isobaric interference-free TIMS method for precise determination of isotopic ratios of LREE. The method utilizes 16O-enriched O2 as a source for formation of the MO+ species. Isobaric interferences caused by O were reduced by up to 6 and 70 times for 17O and 18O, respectively. One critical aspect of analyzing smaller samples is blank minimization. Hattori et al. (F6) reported an N-TIMS method for determination of Re, Os, and Pt isotopic ratios that yields low blank levels by using Ta filaments containing lower PGE concentrations. The problem of O2 consumption by the Ta filament was solved by prebaking the filament to remove tantalum oxides prior to sample loading. The use of N-TIMS for even the smallest samples was demonstrated by Pearson et al. (F7). They reported that N-TIMS is capable of measuring Re-Os isotopic compositions of individual syngenetic sulfide inclusions from three different growth zones within a central cross section plate cut from a single Siberian diamond. Chen et al. (F8) developed chemical procedures and an N-TIMS technique to study Re-Os systematics in meteorites. A number of other long-lived chronometers exist that permit dating of objects/events in the early Solar System but these involve lithophile elements. Both Re and Os are siderophilic and a method based upon 43-Ga 187Re provides a chronometer for events involving iron meteorites and metallic phases in chondrites. N-TIMS determinations of the 187Re-187Os isotopic composition exhibit high sensitivity due to increased efficiency for production of negative ions. Chen et al. (F8) used their method to study the 187Re-187Os system in chondrites, applying it to iron meteorites to confirm that the technique yields reliable data. The chemical and N-TIMS methods developed in this study proved to be marked improvements upon earlier ones based upon SIMS and resonance ionization mass spectrometry (RIMS). This study provides a great deal of new information on PGE fractionations during formation of chondrites and iron meteorites and various phases in them. The TIMS technique can often be applied in tandem with others to obtain more information. Two research groups identified as carbonaceous chondrite-like the object that impacted with Earth 65 Ma ago, causing profound ecologic changes at the K-T boundary. Shukolyukov and Lugmair (F9) used TIMS to quantify Cr isotopic compositions in K-T boundary sediments and found high 53Cr/52Cr ratios like those in refractory inclusions from the Allende CV3 chondrite. The excess 53Cr in such material is accepted as reflecting decay of 3.7-Ma 53Mn now extinct but originally present in the early Solar System. Kyte (F10) made his identification from a 2.5-mm fossil meteorite recovered from K-T boundary sediments at the DSDP 576 oceanic drill site using EMPA and INAA. The meteoritic mineral halite (NaCl) is rarely encountered, presumably because its small crystals are easily dissolved by airborne water vapor during the meteorite’s terrestrial residence. Zolensky et al. (F11) identified e3-mm crystals of halite in the Monahans H5 chondrite regolith breccia within 48 h of its fall. The halite contained sylvite (KCl) and fluid inclusions, all of which were analyzed by a variety of techniques. When analyzed mass spectrometrically, 1 mg of halite/sylvite gave a Rb-Sr model age of 4.7 ( 0.2 Ga. Raman microprobe spectra demonstrated that the fluid inclusions were aqueous brines but unsaturated with respect to NaCl. This brine is the first sample discovered of the
actual liquid apparently responsible for aqueous alteration of early Solar System material. Halite-containing fluid inclusions were also discovered in the Zag H3-6 regolith breccia and studied by a variety of techniques (F12). Xenon, determined by RIMS in the halite, consisted essentially of pure 129Xe, presumably produced by decay of 16-Ma 129I in the early Solar System. Its I-Xe age was only 1.7 ( 0.9 Ma more recent than that of the oldest known solar system mineral, magnetite from the Murchison CM2 chondrite. Ar-Ar ages for gray matrix and a light clast were 4.25 ( 0.03 Ga but for two halite crystals were 4.03 ( 0.05 and 4.66 ( 0.08 Ga. Whitby et al. (F12) suggested that the halite formed in the early Solar System. Zag parent material acquired solar gases during its exposure as regolith for 66 Ma during the first 300 Ma of the Solar System, and during a collision 4.25 Ga ago, halite was admixed. Some halite (sylvite?) was altered by heating and/ or moisture: the younger halite with its 4.0 Ga Ar-Ar age reflects that alteration episode. Since sulfide ores are currently one of the most important sources for Au, Steele et al. (F13) compared SIMS and synchrotron X-ray fluorescence (SXRF) techniques to quantify it in sulfides from a paradigmatic Brazilian ore. The SIMS technique has imaging capability and
