Microsampling technology has been utilized for several years to investigate problems of diagenesis and records of temperature fluctuations during crystal growth [e.g. Given and Lohmann, 1985]. The carbonate materials analyzed in these studies are easy to drill, and contain high abundances of carbon and oxygen---analysis for which the material is separated. This contrasts with the ppm level of Sr, Nd, and Pb tracers. As a consequence, much smaller samples can be separated for C and O isotopic analysis, and spatial resolution is superior to microsampling for Sr, Nd, and Pb isotopes. Dettman and Lohmann [1993] reported carbon and oxygen isotopic analyses of 10-20 mg samples from individual growth layers from fresh water bivalves. These data were used to infer temperature variations and mixing relationships between water reservoirs. A similar study of microlayering in mollusk shells was used to estimate bottom temperature variations in the north Atlantic [ Weidman et al., 1994]. For such applications, fine scale milling techniques were employed, controlled by computer rather than a human operator.
The utility of microsampling in igneous petrology is clear. Isotopic disequilibrium between crystals and groundmass glass has been demonstrated by many studies [e.g. Christensen and DePaolo, 1993; Ruiz and Duffield, 1994]. In situ microsampling enables us to further identify inter-crystal and intra-crystal variability, and investigate quantitatively isotopic gradients within crystals and between groundmass and xenocrysts, xenoliths or magmatic inclusions. Isotopic gradients can be used to infer crystal growth rates [as demonstrated for metamorphic garnets by Christensen et al., 1989] or to track the evolution of differentiating magma systems [e.g., Davies et al., 1994].
An elegant study from the University of Michigan laboratory
shows how isotopic variability in crystal phases from the Tuscany
rhyolites is related to precipitation from a magma evolving by
assimilation of wallrock material (i.e., open system evolution)
[ Feldstein et al., 1994]. A number of studies reported at
the recent International Conference On Geochronology,
Cosmochronology, and Isotope Geology in Berkeley, CA, also attest
to the growing interest in microanalytical approaches. Strontium
isotopic zonation within single feldspar crystals has been
identified, and used to investigate growth rates in high Rb/Sr
magmas [e.g. Heumann et al., 1994] or diffusion time scales
in xenocrysts [ Tepley et al., 1994]. Disequilibrium melting
of crustal rocks during assimilation has been identified [
Tommasini et al. 1994] via microsampling of plagioclase crystals.
The less radiogenic 
Sr/
Sr signature of the feldspars
relative to the source rock were interpreted as being consistent
with the feldspars not having undergone equilibration with the bulk
rock during melting. This observation will surely force us to
re-evaluate our assumption that contaminants are characterized by
the isotopic ratios of the bulk rock. Churikova and
Kostitsyn [1994] on the basis of core-rim 
Sr/
Sr
variations in plagioclase phenocrysts (increasing from 0.7034 to
0.7037 core to rim) have reported that the effects of
combined assimilation and fractional crystallization can be recorded
in single crystals. Baker et al. [1994] drew similar
conclusions from Nd-Sr-O isotope analysis of zoned clinopyroxene
crystals in a high MgO basalt from Yemen flood basalts. Such
observations should stimulate caution in the interpretation of bulk
rock geochemistry in the context of mantle sources and processes.
The effects of crustal differentiation can be faithfully recorded
in individual crystal phases, and would otherwise be missed
adopting a bulk chemistry approach.
It is evident that in situ micro-sampling techniques to the investigation of geological processes has the potential to provide details of geochemical signatures and processes that would have been overlooked using a bulk rock analytical approach. Expansion of this technique is expected over the next four years which will allow the petrologist to obtain an intimate picture of geological processes. Use of in situ microsampling with other micro-analytical techniques will enable the refinement of many petrogenetic models and possibly negate others.