HR: 11:05h
AN: V51K-04 INVITED     [PDF]
TI: Advantages of Secondary Ion Mass Spectrometry (SIMS) for Stable Isotope Microanalysis of Trace Light Elements
AU: * Layne, G D
EM: glayne@whoi.edu
AF: Woods Hole Oceanographic Institution, MS23 Clark114A, Woods Hole, MA 02543 United States
AB: SIMS has several general advantages for the determination of light stable isotopes occurring at trace element concentrations in natural samples. Ion microprobe SIMS instruments sputter nanogram quantities of material from a well defined, micrometer-sized analytical crater. The extremely small quantity of sample extracted allows analysis of very small objects, such as igneous melt inclusions. Sputter ionization of many light elements (e.g.; Li, B, S, Cl) is efficient enough ($\gg$1%) to allow precise determination of isotope ratios at elemental concentrations as low as 1 - 100 ppm. Primary bombardment of the sample is performed in close proximity to the initial extraction optics of the mass spectrometer, enabling very stable control of the ionization process. Consequently, instrumental mass fractionation (IMF) can be maintained at a very consistent and reproducible level. In situ SIMS microanalysis has a particular advantage for samples where the elemental concentration is less than that which would provoke chemical blank problems during preparation of the purified samples necessary for other types of mass spectrometry. B isotopes. Use of SIMS for the determination of $delta^{11}$B is simplified because compositionally diverse matrices are amenable to calibration for IMF with a single standard; usually a high silica glass containing 100s - 1000s ppm total B$^{1}$.This attribute is particularly convenient in subduction-related volcanic systems, where tephra sequences may contain a wide spectrum of major element chemistries. The combination of $\delta^{11}$B and trace element microanalyses has been particularly valuable in these same systems. For example, the inverse correlation of $\delta^{11}$B with LILE/Nb ratios in Neogene fallout tephra was used to infer the contribution of a metasomatized mantle wedge to the Izu Arc Front volcanics$^{2}$. Li isotopes. For Li, IMF is more dependent on matrix chemistry, requiring a well-determined suite of standards. IMF may also drift in response to elemental mobility under ion beam charging. Nevertheless, Li ionizes (as Li+) with extremely high efficiency under O- bombardment ($\gg$10%). With careful attention to calibration and sputtering stability, $\delta^{7}$Li may be quantitatively determined in glass or silicate mineral samples containing as little as 1 ppm total Li. Contrasting aqueous solubility behavior makes $\delta^{7}$Li a valuable complement to $\delta^{11}$B in the study of processes involving high temperature hydrothermal fluids. Cl isotopes. $\delta^{37}$Cl is readily amenable to SIMS microanalysis in a variety of matrices, due to the vigorous ionization of Cl (as Cl-) under bombardment by positive primary ion beams such as Cs+. Determination of $\delta^{37}$Cl in glassy matrices requires using a Normal Incidence Electron Gun (NEG) to compensate for sample charging. A series of well-typified standards are also essential to calibrate for IMF, which can vary up to 10 per mil relative between basaltic and rhyolitic matrices$^{3}$. However, precise determinations of $\delta^{37}$Cl are possible for glasses or silicate minerals with as little as 100 - 200 ppm total Cl. Further, certain natural fluids (e.g., seawater or submarine hydrothermal vent fluids) can be prepared by evaporation/integration with conductive binders, and then sputtered directly for SIMS analysis. In this manner, $\delta^{37}$Cl can be determined in microliter fluid aliquots, containing as little as 1 microgram of total Cl$^{4}$. \\ \noindent $^{1}$Chaussidon, M. et al, 1997, GSNL 21(1), 7-17; $^{2}$S.M.Straub and G.D.Layne, 2002, EPSL, 198, 25-39; $^{3}$A.V.Godon et al, 2001, EOS, 82(47), F1392; $^{4}$W.Bach et al, 2002, EOS, 83(47), V61B-1367.
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting