|Title:||Geochemical analyses of small samples: Micro–analytical techniques for the nineties and beyond|
|Authors:||C.R. Neal, J.P.Davidson, and K.D. McKeegan|
|Book Title:||National Report to the International Union of Geodesy and Geophysics|
As technology becomes more sophisticated, the number and type of tools available to the geochemist continue to grow and the amount of data collected from a single sample is dramatically increased. Micro‐analytical techniques developed over the last 20–30 years allow the geochemist to obtain an almost overwhelming wealth of data from single samples. Not only can bulk analysis be routinely conducted, but detailed major — and trace — element determinations, as well as isotopic analysis of individual components from geological, metallic, ceramic, and plastic materials are opening up a whole new avenue of research and petrogenetic interpretation. In situ measurements have become an important aspect of many geoscience applications, following the realization that much of the information relating to the origin and evolution of rock systems is retained at the scale of individual mineral grains [e.g., Reed,1989,1990, 1993]. The new generation of high resolution ion probes has furthered our ability to perform trace element and isotope analyses at the micron (µm = micron = 10−3mm) scale. Laser ablation techniques, coupled with Inductively Coupled Plasma Mass Spectrometry (ICP‐MS) analyses have opened up a parallel capacity for trace element analysis, while laser fluorination oxygen extraction is fast becoming the tool of choice for silicate oxygen isotope geochemistry. With these analytical techniques available, a detailed data base on a single sample or suite of samples can be quickly built up. For major and minor element analyses, the electron microprobe is normally used and as this has generally become a routine analytical technique and has recently been extensively reviewed by Reed , it will not be discussed here. This paper is concerned with the analysis of geologic materials on a sub‐mm scale allowing the recognition of major and trace element, as well as isotopic zonation across individual mineral grains. The primary concentration is on work conducted in laboratories in the United States, although reference to work conducted in other countries is made where deemed appropriate. Three analytical techniques will be discussed: 1) In situ microsampling for isotopic analysis; 2) Secondary Ion Mass Spectrometry (SIMS); and 3) Laser Ablation Inductively Coupled Plasma–Mass Spectrometry (LA‐ICP‐MS). The object of the paper is to give the reader a current overview of the applications of each of the three analytical techniques as well as a detailed description of the newer LA‐ICP‐MS procedure.