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Further developments in the in situ analysis of sulphur
isotopes using SHRIMP II
Richard A. Armstrong, Peter Holden and Ian S. Williams
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
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.
Sulphur isotope compositions
of two pyrite standards as measured on SHRIMP II during a single
analytical session.
A concentrically-grown
pyrite grain from the Witwatersrand gold deposit, South Africa,
showing a series of SHRIMP analytical spots across the grain. The
SHRIMP spots are approximately 20µm in diameter. Sulphur
isotope compositions were measured across the growth bands and
show a large range in values from +10‰ in the centre to ~-7‰ in
one of the bands near the margin.
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.
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.