APPLICATIONS OF MULTIPLE COLLECTOR-ICPMS TO COSMOCHEMISTRY, GEOCHEMISTRY, AND PALEOCEANOGRAPHY

Multiple collector-inductively coupled plasma mass spectrometry (MC-IC PMS) is a new technique for the measurement of isotopic compositions a t high precision, and is of great relevance to planetary, earth, ocean , and environmental sciences. The method combines the outstanding ioni zation efficiency of the ICP source with the superior peal; shapes ach ievable from the ion optical focal plane of a large dispersion magneti c sector mass spectrometer, utilizing simultaneous multiple collection to achieve the most precise isotopic measurements yet made for many e lements-particularly those with high first ionization potential. The a ddition of a laser facilitates studies for which spatial resolution is required. This method is still in its infancy, yet diverse applicatio ns have already led to a number of important scientific developments. Here we review some of these accomplishments and the potential for fur ther work. The Lu-Hf isotopic system, for many years considered analyt ically challenging, is now relatively straightforward and offers great promise in fields as diverse as garnet geochronology, hydrothermal fl uxes to the oceans, and crustal evolution. The age of the Earth's core , the Moon, and Mars have been measured using a new short-lived chrono meter Hf-182-W-182. Other such new chronometers will follow. High prec ision isotope dilution measurements of the Earth's inventory of many p oorly understood elements such as In, Cd, Te, and the platinum group e lements are providing tests for models for the accretion of the inner solar system. The small natural isotopic variations in elements such a s Cu and Zn, produced by mass dependent fractionation, are now measura ble at high precision with this method, and entirely new fields of sta ble isotope geochemistry can be developed. Similarly, measurements of small nucleosynthetic isotopic anomalies should be made easier for som e elements. Measurements of U and Th isotopic compositions at very hig h sensitivity and reproducibility are now possible, allowing the devel opment of higher resolution Quaternary geochronology. Finally, using l aser ablation, the first precise in situ Sr, Hf, W, and Pb isotopic me asurements have been made in natural materials, opening up a range of microanalytical isotopic studies in petrology and marine geochemistry. MC-ICPMS offers exciting times ahead in areas well beyond the bounds of geochemistry. Indeed, MC-ICPMS is likely to become the method of ch oice for many isotopic measurements because it is a more user friendly and efficient method for the acquisition of high precision data. It i s also much more versatile, permitting elements to be measured that we re previously considered intractable, and allowing the acquisition of data in situ, all with a single mass spectrometer. The limiting factor on the sensitivity is the transmission which is less than or equal to 2% for all instruments thus far designed. If it is found possible to improve the transmission still further, thermal ionization mass spectr ometry, the technique that has, thus far, provided the high precision measurements necessary for most of the vast field of radiogenic isotop e geochemistry, may be relegated to specialized applications. Copyrigh t (C) 1998 Elsevier Science Ltd.