Micron-Scale Isotopic Zoning in Minerals: A Record of Large-Scale Geologic Processes

Abstract

Isotopic microanalysis can reveal details of mineralizing processes that would otherwise remain undiscovered, because minerals often exhibit micron-scale isotopic zonations recording the large-scale geologic events which have affected them. The SHRIMP (sensitive hi~,h resolution ion microprobe) performs in-situ 3 S/32S analyses of minerals, utilizing a sampling volume that is only 15-25 Ixm wide and 5 txm deep. A negativelycharged oxygen primary beam is used to sputter secondary sulphur ions for analysis in the mass spectrometer sector of the instrument. The precision and accuracy of SHRIMP 834S values are + 2~ (2or) (Eldridge et al., 1987, 1989). Ion microprobe analyses of S isotopic variations with the SHRIMP have documented numerous examples where individual minerals or mineral intergrowths exhibit micron-scale 834S variations that approach the known range of conventional bulk S isotopic compositions in nature. Moreover, the magnitudes and directions of micron-scale S isotopic zoning are often diagnostic of specific geochemical processes which have caused mineralization.