HR: 1340h
AN: B13A-0891 INVITED    [Abstracts]
TI: Advances in laser ablation MC-ICPMS isotopic analysis of rock materials
AU: * Young, E D
EM: eyoung@ess.ucla.edu
AF: University of California Los Angeles, 595 Charles E. Young Drive East, Geology Building, Los Angeles, CA 90095,
AB: Laser ablation multiple-collector inductively coupled plasma-source mass spectrometry (LA-MC-ICPMS) is a rapid method for obtaining high-precision isotope ratio measurements in geological samples. The method has been used with success for measuring isotope ratios of numerous elements, including Pb, Hf, Mg, Si, and Fe in terrestrial and extraterrestrial samples. It fills the gap between the highest precision obtainable with acid digestion together with MC-ICPMS and thermal ionization mass spectrometry (TIMS) and the maximum spatial resolution afforded by secondary ion mass spectrometry (SIMS). Matrix effects have been shown to be negligible for Pb isotopic analysis by LA-MC-ICPMS (Simon et al., 2007). Glass standards NBS 610, 612, and 614 have Pb/matrix ratios spanning two orders of magnitude. Our sample-standard bracketing laser ablation technique gives accurate and precise ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb for these glasses. The accuracy is superior to that obtained when using Tl to correct for mass fractionation. Accuracy and precision (± 0.2 ‰) for Pb in feldspars is comparable to that for double-spike TIMS. Data like these have been used to distinguish distinct sources of magmas in the Long Valley silicic magma system. LA-MC-ICPMS analyses of Mg isotope ratios in calcium-aluminum-rich inclusions (CAIs) from carbonaceous chondrite meteorites have revealed a wealth of new information about the history of these objects. A byproduct of this work has been recognition of the importance of different mass fractionation laws among three isotopes of a given element. Kinetic and equilibrium processes define distinct fractionation laws. Reservoir effects can further modify these laws. The result is that the linear coefficient β that relates the logarithms of the ratios n₂/n₁ and n₃/n₁ (n_i refers to the number of atoms of isotope i) of isotopes with masses m₃ > m₂ > m₁ is not unique. Rather, it is process dependent. In the case of Mg, this coefficient ranges from 0.521 for single-step equilibrium processes to 0.510 or even lower for kinetic processes. Rayleigh fractionation involving a kinetic process with a single-step β of 0.510 produces an effective β of 0.512. Such differences in fractionation laws can be crucial for determining excesses or deficits in isotopes relative to mass fractionation. Contrary to some assertions, Si isotope ratios can be measured with high accuracy and precision using 193 nm excimer lasers with nanosecond pulse widths (Shahar and Young, 2007). Silicon isotope ratios in CAIs measured by 193 nm LA-MC-ICPMS have been combined with Mg isotope ratios to constrain the astrophysical environments in which these oldest solar system materials formed. Accuracy of the measurements was determined using gravimetric standards of various matrix compositions. The results establish that matrix effects for Si are below detection at the ± 0.2 ‰ precision of the laser ablation technique. High mass resolving power (m/Δ m ~ 9000) is necessary to obtain accurate Si isotope ratios by laser ablation. High-precision LA-MC-ICPMS measurements of ¹⁷⁶Hf/¹⁷⁷Hf in zircons can be obtained by normalizing to ¹⁷⁹Hf/¹⁷⁷Hf assuming an exponential fractionation law and no mass-dependent Hf, Lu, or Yb stable isotope fractionation. With corrections for interfering ¹⁷⁶Lu and ¹⁷⁶Yb precision for this method can be on the order of 0.3 epsilon (0.03 ‰). The approach has been used to infer the existence of continental crust on Earth 4.4 billion years before present (Harrison et al., 2005).
DE: 1028 Composition of meteorites (3662, 6240)
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting