Cosmogenic Isotope Variations

Variations in isotopic abundances that are caused by nuclear reactions induced by cosmic rays are most commonly utilized in cosmic ray exposure dating, but this employs isotopes that are measured by either accelerator or noble gas mass spectrometry [28, 29]. In fact, there are only a very limited number of elements that are suitable for the study of cosmogenic isotopic variations, which can be readily analyzed by either TIMS or MC-ICP-MS [28]. The most important application of these techniques are studies of the secondary neutron fluxes that are generated by (primary) cosmic rays. Such measurements aim to detect anomalies in Sm, Gd, and Cd isotopic abundances that are produced by (n,y) reactions, for example Cd(n, y) Cd. Many of these investigations were conducted by TIMS [137-139], but some cosmogenic Cd isotope variations of lunar rocks and soUs were evaluated based on MC-ICP-MS isotope ratio data that were originally acquired as part of a stable isotope study [134]. 

More important to MC-ICP-MS measurements are the minor cosmogenic isotope anomalies that can interfere with studies of nudeosynthetic, radiogenic and stable isotope effects. A prominent example are the cosmodiemical MC-ICP-MS W isotopic analyses that are carried out primarily to detect variations in the abundance of from the decay of now extinct Hf (Table 10.2). Such variations are most commonly detected by the measurement of or which are 

Given the high precision that is now attained in the isotopic analysis of various elements by MC-ICP-MS, it is important that cosmochemical studies carefully consider and evaluate possible cosmogenic effects. Without such an evaluation, it is possible that cosmogenic isotopic variations are erroneously interpreted as nucleosynthetic, radiogenic, or mass-dependent stable isotope effects. 

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. 

In particular, the technique of MC-ICP-MS has significantly furthered the routine application of a number of extinct radionuclide systems, such as Al— Mg, and ° Pd— Ag, for a wide range of extraterrestrial samples, enabled routine analysis of the long-lived chronometer, and 

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Bhandari N, Bonino G, Cirri Castagnoli G (1994) The 11-year solar cycle variation of cosmogenic isotope production rates in chondrites. Meteoritics 29 443-444... 

Authigenic Deposits. Carbon Cycle. Cenozoic Climate - Oxygen Isotope Evidence. Cenozoic Oceans - Carbon Cycle Models. Cosmogenic Isotopes. Mid-Ocean Ridge Geochemistry and Petrology. Rare Earth Elements and their Isotopes in the Ocean. River Inputs. Stable Carbon Isotope Variations in the Ocean. Uranium-Thorium Series Isotopes in Ocean Profiles. 

However, cosmogenic reactions can also produce small variations in the abundances of stable isotopes. Such anomalies can be difficult to resolve from radic enic, nudeosynthetic, and stable isotope effects and, if undiscovered, can lead to erroneous data interpretations. Targeted measurements of isotopes that are generated at particularly high levels by such cosmogenic reactions (e.g., Sm, have... 

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