The application of SIMS analysis to geological materials has
mushroomed over the last four years. The measurement of isotopic
ratios of the low-mass elements which undergo significant mass
fractionation during many geochemical processes, the so-called
`stable isotopes' of H, C, O, S, etc., has been a continuing
interest of ion microprobe applications in both cosmochemistry and
geochemistry for the past decade. Carbon isotopes have been
measured in numerous meteoritic samples, but so far not many
terrestrial studies have been performed. An exception is provided
by studies of diamond [ Harte and Otter, 1992] which may be
analyzed with sub-permil [permil (t%
) as opposed to
percent (%)] precision by negative SIMS since it becomes
semiconducting under Cs
bombardment. Sulfur isotopes have been
analyzed in a variety of mineral phases (e.g., chalcopyrite,
pyrite, galena, sphalerite) from different geologic environments
ranging from sulfide ores [ Macfarlane and Shimizu, 1991] to
sedimentary sulfide deposits [e.g., Eldridge et al.,
1993] to small inclusions in diamond [ Eldridge et al., 1991].
In every case, an extremely large range in 
S [usually
spanning several percent---see Faure, 1986 for definition of
delta notation] was found reflecting the complex geochemical
behavior of sulfur. These isotopic heterogeneities, which are
largely averaged out by conventional bulk analyses, are highly
diagnostic for sources of sulfur and hence processes for ore
formation, etc., and therefore we can expect further application of
ion microprobe measurements in this area.
The natural variability of oxygen isotope ratios and their
well-established utility in thermometry and as monitors of
fluid-rock interactions has made the in situ measurement
of 
O/
O at the 10 
m spatial scale an important
goal of ion microprobe technique development. Several laboratories
have approached the problems of sample charging and the control of
instrumental mass discrimination differently, and remarkable
progress has been made. Valley and Graham [1991, 1992, 1993]
have used high mass resolution (
2000 channels per atomic mass
unit) and exploited the fact that magnetite is sufficiently
conducting in order to measure 
O to a precision of 1
permil (%
). These authors also employed depth profiling
to determine very fine-scale isotopic gradients that cause a shift
in 
O of up to 
1% over a distance of 
10
m from rim to core of individual crystals. From such data
they were able to constrain possible cooling histories and isotopic
exchange with fluids in this granulite-facies marble following
regional metamorphism [ Valley and Graham, 1991]. Further
detailed analyses of 
O across a large (few mm) magnetite
grain showed evidence for fluid transport along healed cracks, and
led the authors to urge caution in modeling bulk-mineral oxygen
isotope data in terms of thermometry or diffusive processes without
knowledge of the microscale variations in 
O [ Valley
and Graham, 1993]. For the analysis of electrically insulating
samples (such as silicates), Hervig and colleagues attacked
the charging problem by developing a focused high-energy electron
gun in order to balance incoming and outgoing positive and negative
currents during an analysis, and also employed extreme energy
filtering to discriminate against molecular interferences [
Hervig, 1992]. The method was used to correlate oxygen isotopic
zoning (measured in 30 
m spots) across a hydrothermal garnet
with other indicators of changing fluid chemistry, such as fluorine
and REE content [ Jamtveit and Hervig, 1994].
The proportion of ion microprobe studies devoted to terrestrial, as opposed to meteoritic, samples has been increasing during the present quadrennium and this trend will undoubtedly continue. Many terrestrial applications concern geochronology, where the SHRIMP has been employed with great success to date thousands of zircons ranging from the oldest terrestrial minerals and rocks [ Nutman and Collerson, 1991; Liu et al., 1992; Mueller et al., 1992] to some of the youngest tectonically active regions on Earth [e.g., Zeitler et al., 1993]. Ancient materials can provide unique geochemical information regarding differentiation processes in Earth's early evolution. For example, Maas et al. [1992] concluded, based on extensive morphological, mineralogical, and elemental (including trace element) characterization, that these tiny grains of ancient zircon provide good evidence for a 'differentiated continental crust of substantial thickness' as their source. Kinny et al. [1991] used the SHRIMP to measure the Hf isotopic compositions of some of these zircons demonstrating concordance of Hf-model ages with U/Pb up to 4.2Ga.
One of the most exciting applications of SIMS has been in the
study of discrete relict interstellar dust grains in primitive
meteorites [ Anders and Zinner, 1993, and references therein].
Trace amounts of tiny grains of diamond, SiC, and graphite were
identified from chondrites by virtue of their unique chemical
properties (relative to the silicate host) and their correlations
with concentrations of various exotic (i.e., non-solar system)
noble gas components. Since the isotopic abundance patterns in
each presolar grain were fixed in specific, and potentially unique,
stellar sources, it is essential to measure isotope ratios on
individual particles. While the diamonds, with a typical grain
size of only 10's of nanometers, are far too small to permit any
isotopic measurements on individual grains, fortunately SiC and
graphite can often range up to several microns in diameter, ideal
for SIMS analysis. Only the ion microprobe has the sensitivity to
perform this task; moreover in many cases it has proven possible to
measure C, N, and Si isotopes, as well as the isotopic composition
of several minor elements on SiC and graphite particles. The
isotopic correlations have been used to distinguish groups of
grains that may derive from similar stars and these data provide
strong experimental constraints on theories of nucleosynthetic
processes in various stellar environments [e.g., Anders et
al., 1991; Amari et al., 1992; Alexander, 1993;
Hoppe et al., 1993a,b]. Correlated isotopic measurements of C and
N have also been performed on many individual graphite grains [
Amari et al., 1993]. Carbon isotopic compositions are exceedingly
anomalous, with both depletions and excesses of 
C by more than
a factor of 20 relative to the solar 
C/
C ratio. However,
nitrogen isotopic ratios are relatively close to the terrestrial
value and do not correlate with N abundance which varies by more
than a factor of 100 among graphite grains.
Zinner et al. [1991a] documented huge 
Mg excesses in
SiC and graphite particles which they interpreted as ``fossil''
anomalies due to the decay of now-extinct 
Al. The inferred
Al/
Al at the time the grains condensed ranges up to 0.06 in
graphite and 0.2 in SiC; this is much higher than the value of
5 x 10
found in many refractory oxide inclusions in
carbonaceous chondrites. The signatures of the short-lived
nuclides 
Ti and 
V may also have been found by Amari et
al. [1992] in the calcium and titanium isotopes of SiC grains.
Based on their major element isotope systematics, these grains are
considered to be possible supernova ejecta, although there are
difficulties in modeling their high 
Al abundances from a
supernova source. The Ba and Nd isotopic compositions of
aggregates of SiC particles were also demonstrated to have a
distinct ``s-process'' (supernova) signature by Zinner et al.
[1991b].
The incorporation of short-lived isotopes, especially 
Al
(half life = t
=0.7Ma), into mm to cm sized refractory
inclusions (i.e., objects that formed in the early solar nebula) in
meteorites has important implications for the evolution of this
phase of solar system formation. Disturbances of the Al-Mg system,
which affect the use of 
Al as a high-precision relative
chronometer, have been studied by Podosek et al. [1991] and
MacPherson and Davis [1993]. In addition to Mg, isotopic
anomalies in O, Ca, and Ti in refractory phases from primitive
meteorites have been investigated [e.g., Davis et al., 1991;
Ireland et al., 1991, 1992; Lundberg et al., 1994].
The role of physical processes, such as diffusion, melting, and distillation by evaporation, in determining isotopic and trace element abundance patterns in refractory inclusions and chondrules has been receiving increased attention by ion microprobe analyses of natural samples [ Simon et al., 1991; Ireland et al., 1992] and experimental run-products [e.g., Kennedy et al., 1993; Ryerson and McKeegan, 1994; Simon et al., 1994]. Sheng et al. [1992] determined self-diffusion rates for Mg in spinel and co-existing melts and were therefore able to use the existence of natural isotopic heterogeneities in spinel to constrain possible cooling rates, and hence formation mechanisms, of plagioclase-olivine inclusions. Similarly, Ryerson and McKeegan [1994] attempted to constrain thermal histories of a certain class of Ca-Al-rich inclusions (CAI) by comparing ion probe measurements of self-diffusion rates for oxygen in the minerals †nkermanite, anorthite, diopside, and spinel with known patterns of oxygen isotopic anomalies in these phases. They concluded that existing O isotopic data cannot be explained by simple gas-solid or gas-melt diffusive exchange processes, but probably require multiple thermal events including alteration and recrystallization of some CAI minerals.
Extra-terrestrial samples other than primitive meteorites have also been scrutinized by the ion microprobe, principally trace element studies. For example, Floss and Crozaz [1991, 1993] investigated rare earth element (REE) distributions in reduced meteorites. Igneous activity on the Moon has been examined by Shearer and co-workers [1990a,b] as well as Snyder et al. [1993] and Jolliff et al. [1993], with results demonstrating a complex source mixing and metasomatism for lunar magmas. Igneous meteorites thought to be derived from Mars have been measured for REE contents [ Lundberg et al., 1990] as well as hydrogen isotopes [ Watson et al., 1994]. The minor and trace element trends in pyroxenes, as well as the calculated compositions of parent melts, are consistent with formation of the shergottites by closed-system fractional crystallization [ Wadhwa et al. 1994]. High deuterium to hydrogen (D/H) ratios measured in hydrous phases of these meteorites are best explained by post crystallization isotopic exchange with crustal fluids having D/H values similar to the present Martian atmosphere (about five times the terrestrial atmospheric value) [ Watson et al., 1994]. These results have implications for the extent of hydrogen loss from the Martian atmosphere and its interaction with a fluid phase present in that planet's crust.