Chapter 2 : in Situ Oxygen Isotope Geochemistry by Ion Microprobe

Mineralogical Association of Canada Short Course 41, Toronto, May 2009, p. 19-63 INTRODUCTION High precision and accuracy with in situ analysis has been a “holy grail” for generations of stable isotope geochemists who employed a range of strategies for separation of heterogeneous samples and for interpretation of data from mixed samples. However, in situ techniques have in the past suffered from relatively poor precision and accuracy. Recent advances by ion microprobe have improved precision and accuracy, and make these time-consuming, uncertain approaches unnecessary. In situ analyses can be correlated with textures and imaging, offering the promise of new and fundamental information about patterns of isotope distribution. The importance of stable isotope geochemistry is well established in many disciplines (see Clark and Fritz 1997, Griffiths 1998, Kendall & McDonnell 1998, Criss 1999, Ambrose & Katzenberg 2000, Valley and Cole 2001, Unkovich et al. 2001, Hoefs 2004, De Groot 2004, Johnson et al. 2004, Sharp 2007). Many seminal studies are included in these reviews. However, the conventional analytical techniques employed in these studies require homogenization of bulk samples that are larger than the scale of zonation in a wide range of samples. Important information has been lost by analysis of powders. The ion microprobe’s ability to analyze isotope ratios with high precision and accuracy from micrometre-scale spots in natural samples (as well as experimental products) in situ from a microscope slide reduces sample size by factors of ten thousand to one billion, and allows isotopic data to be correlated with other geochemical information in spatial context with textures and imaging (Fig. 2-1). The accuracy of these data now approaches that of conventional techniques that require far larger samples. These advantages are revolutionizing stable isotope geochemistry, just as the electron microprobe revolutionized in situ chemical analysis and the SHRIMP (ion probe) revolutionized zircon geochronology (Ireland 1995, Hinton et al. 1995, Ireland & Williams 2003).