Download Provenance microanalysis of sedimentary rocks: how many

Provenance microanalysis of sedimentary rocks: how
many analyses do we need and what does it really tell us
anyway?
SEDIMENTARY ROCKS ARE IMPORTANT CLUES TO GEOLOGICAL HISTORY
Sedimentary rocks are formed from the eroded detritus of other rocks and thus can
provide a valuable ‘snapshot’ of the geological environment at the time the detritus was
deposited. One method for understanding this snapshot is to use the age of uraniumbearing minerals in the sediment as a proxy for the age of the rock the minerals
originated, thus determining the ‘provenance’ of the sedimentary rock.
Analytical instruments such as ion microprobes and laser ablation mass spectrometers
can now analyse the ages of around hundred individual mineral grains per day depending
on the technique. Such studies have been used to provide information about a wide
variety of geological subjects such as continental reconstruction, the movement of desert
and beach sands and the evolution of sedimentary basins. Detrital analysis has also
revealed the oldest known minerals on Earth in the Jack Hills of Western Australia at 4.4
billion years old, derived from a source rock that has long since vanished.
THE PROBLEMS
Gathering detrital age data has presented two challenges:
1) How many analyses should be acquired for a sample to be representative
of the population? Is true representation achievable? Dodson et al. (1988)
calculated the limit as 59 and more recently, Vermeesch (2004) calculated the
limit as 117 in cases assuming a uniform age distribution (a gross oversimplification in geological terms although he also concludes that samples with
apriori distribution limits can have fewer analyses). In contrast, Andersen (2005)
concluded that a truly representative sample was unlikely and depreciated the
value and accuracy of subsequent comparison of distributions from acquired
samples (e.g. Sircombe and Hazelton, 2004; Vermeesch, 2005).
2) How well can the detrital ages limit the age of the sedimentary rock itself?
Based upon the principle of inclusions, the age of the sedimentary rock itself
must be younger than the youngest detrital mineral. Establishing such a limit is
often important where other information about the age of the sedimentary rock
such as fossils or cross-cutting relationships is absent. How to calculate the limit,
or even if such a limit has any mathematical validity have become hotly debated
topics. Is it the single youngest age minus a given uncertainty? Is a better
estimate the youngest cluster of ages that can be statistically resolved from the
remainder of the sample? Or, following Andersen (2005), is the issue too fraught
with geological and statistical difficulty to ever be useful?
WHY IS IT IMPORTANT?
Even with modern methods, gathering 60-120 analyses is resource intensive. At worst, if
such datasets are of limited value then resources could be re-distributed. However, the
geological clues potentially revealed by investigating sedimentary rocks will always be
very enticing and any approach that can refine statistical requirements to improve the
efficiency of analysis and the value of the interpretation would be of great benefit to the
discipline. This in turn will feed more reliable information into understanding geological
issues such as the mass transport of sediment in river and marine systems and the
development of petroleum or geothermal-bearing sedimentary basins.
Keith Sircombe, Geoscience Australia Geochronology Laboratory
REFERENCES
Andersen T., 2005. Detrital zircons as tracers provenance: limiting conditions from statistics and
numerical simulation. Chemical Geology 216, 249–270. doi:10.1016/j.chemgeo.2004.11.013
Andersen T., 2006. Reply to Comment on “Detrital zircons as tracers provenance: limiting
conditions from statistics and numerical simulation” Chemical Geology 226, 74-75
doi:10.1016/j.chemgeo.2005.10.008
Dodson M.H., Compston W., Williams I.S., Wilson J.F, 1988. A search for ancient detrital
zircons in Zimbabwean sediments, Journal of the Geol. Soc. (London) 145, 977– 983.
doi:10.1144/gsjgs.145.6.0977
Sircombe K. N. and Hazelton M. L., 2004. Comparison of detrital age distributions by kernel
functional estimation: Sedimentary Geology 171, 91-111. doi:10.1016/j.sedgeo.2004.05.012
Vermeesch P., 2004. How many grains are needed for a provenance study? Earth and Planetary
Science Letters 224, 441-451. doi:10.1016/j.epsl.2004.05.037
Vermeesch P., 2005. Statistical uncertainty associated with histograms in the Earth sciences,
Journal of Geophysical Research 110, B02211. doi:10.1029/2004JB003479
Vermeesch P., 2006. Comment on “Detrital zircons as tracers provenance: limiting conditions
from statistics and numerical simulation” Chemical Geology 226, 73.
doi:10.1016/j.chemgeo.2005.10.012
Figure 1: An example of interpreting detrital mineral ages from sedimentary rocks in
Tasmania, South Australia and central Australia (from Sircombe and Hazelton, 2004). Is
this really telling us anything worthwhile?
Keith Sircombe, Geoscience Australia Geochronology Laboratory