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Rapid in situ measurement of sulphur isotope ratios: new developments and results using the SHRIMP II

Richard A. Armstrong, Peter Holden and Ian S. Williams

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

 As the most important group of ore-forming minerals, sulphides are of enormous economic interest, and studies of sulphide isotope geochemistry provide fundamental information on the genesis of these ore deposits. Sulphur isotope studies also provide unique information on the geochemistry and evolution of mantle, crustal, hydrothermal, biogeochemical and atmospheric systems, and the processes within and between these reservoirs.  This information is gathered from the measurement of the natural variations of the stable sulphur isotopes, recording mass-dependent and mass-independent fractionation from a variety of processes.

Sulphur isotope ratio measurements are made using a number of methods, but the pioneering work by Eldridge et al. (1988, 1989) showed the advantages and insights to be gained from in situ analyses using the high spatial capabilities of the SHRIMP I. This technique allows the measurement of isotope ratios on ~20µm spots in a variety of sulphide minerals. Unfortunately this research and analytical capability ceased some years ago, but with recent technical developments on the SHRIMP II instrument at the Research School of Earth Sciences, ANU, renewed efforts have been made to further explore the possibility of redeveloping this analytical capability, but with the potential for greater precision and accuracy than previously possible.  In particular, the dual developments of a Cs+ primary ion source and a multicollector system have significantly improved sensitivity, stability and reduced the time required for measurement of sulphur isotopes, and indeed, other stable isotopes.

The accuracy of sulphur isotope measurements is critically dependant on the quality of the standards available.  Matrix effects also require the standards to be matched to the composition of the unknown sulphides. Great effort, therefore, has gone into the quest for standards with homogeneous sulphur isotope compositions, with several of the international standards currently available being assessed.  An important additional development which has improved analytical precision is the use of the larger mega-mounts developed for the SHRIMP oxygen isotope analytical protocol (see Hiess et al, 2006 RSES Annual Report).  Although we anticipate that further improvements in our analytical and sample preparation techniques will evolve into further improvements in analytical precision, we are currently obtaining precisions of the order of ± 0.3 permil or less for any analytical session. These standards have been used to accurately measure the isotope ratios in a number of ore deposits, including Au-bearing horizons of  the Witwatersrand Basin, magmatic systems of the Bushveld Complex and a number of other deposits from various environments and settings.

Future development work includes analysis of 33S in order to assess, inter alia,  the presence or absence of mass-independent fractionation in sulphides from anoxic Archaean and early Proterozoic environments  (e.g. Farquhar and Wing, 2005)  and the unraveling of complex mass-dependant and mass-independent fractionation patterns relating to early biological activity.

 

Figure 1.  SHRIMP analytical spots across pyrite from the Witwatersrand gold deposit, South Africa.  The SHRIMP spots are approximately 20 µm in diameter.

 


Eldridge, CS, Compston, W, Williams, IS, Both, RA, Walshe, JL, Ohmoto, H. (1988) Sulfur-isotope variability in sediment-hosted massive sulphide deposits as determined using the ion microprobe SHRIMP: 1. An example from the Rammelsberg orebody. Economic geology 83: 443-449.
Eldridge, CS, Compston, W, Williams, IS, Walshe, JL, (1989) Sulfur isotope analyses on the SHRIMP ion microprobe. U.S. Geological Survey Bulletin 1890: 163-174.
Farquhar, J, Wing, BA (2005) The terrestrial record of stable sulphur isotopes: a review of the implications for evolution of Earth's sulphur cycle. In:  McDonald, l, Boyce, AJ, Butler, JB, Herrington, RJ, Poyla, DA (eds): Mineral Deposits and earth Evolution, Geological Society, London, Special Publication 248: 167-177.