Research School of Earth Sciences
Further developments in the in situ analysis of sulphur isotopes using SHRIMP II
Richard A. Armstrong, Peter Holden and Ian S. Williams
For several years the successful measurement of sulphur isotope ratios on the SHRIMP has been frustrated by the lack of suitable standards and the difficulty in producing reproducible, accurate and precise data through instrumental problems and idiosyncrasies. Establishing suitable standards is a difficult and time-consuming process, as internationally available material might be uniform on the bulk scales they were measured at, but might show some variation in composition at the 20 mm scale commonly measured on the SHRIMP. For analyses of sulphides, the early work by Eldridge et al. (1988, 1989) on SHRIMP I showed that matrix effects require the standards to be matched to the composition of the unknown sulphides. We have spent some considerable time in analysing available sulphide standards (e.g. those described by Crowe and Vaughan, 1996) and have managed to overcome many instrumental problems, enabling us to report consistent δ34S/32S isotope measurements with external precisions of ~2‰ in standards in a variety of sulphides. Figure 1 shows results from two different composition pyrites (Balmat and Ruttan) run in a single session on SHRIMP II. These results are in excellent agreement with the reported values for these standards.
Eldrige et al. (1988, 1989) were also able to show that isotope variations on the SHRIMP scale can be large and not necessarily comparable to bulk analyses in some ore deposits. Our investigations of a number of ore deposits from around the world have confirmed this finding. Detailed small-scale analyses within and across various types of pyrite grains from the Witwatersrand deposit show ranges up to 19‰ from core to rim. Many of these traverses show characteristic rhythmic saw-toothed changes in composition, suggesting a repeated process of formation in these particular concentric, structured grains. Figure 2 illustrates both the structure and isotope variation across a concentric Archaean pyrite grain from the Witwatersand sequence.
The successful development of this analytical capability on SHRIMP II
will be extended to other more exotic applications, with an emphasis
on establishing a routine for the added analysis of 33S. This
currently requires modifications to the mulitcollector configuration,
but should be possible in the near future. This will extend our research
capabilities, enabling us to assess and measure complex mass-dependent
and mass-independent fractionation patterns relating to the early development
of the Earth's atmosphere, as described by Farquhar and Wing, 2005.
Crowe, D.E. , Vaughn, R.G. (1996). Characterization and use of isotopically
homogeneous standards of is situ laser microprobe analysis of 34S/32S
ratios. Amercian Mineralogist, 81: 187-193.