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Friday 9 February 2024

How reliable are the pXRF studies of igneous bluestones?

An interesting disagreement has emerged between Rob Ixer and David Nash on the reliability (or otherwise) of the pXRF studies of the igneous bluestone fragments found in the Stonehenge landscape.  The exchange of views comes in a post clearly written by Ixer and posted on Tim Daw's blog:

Anyway, in the conclusion to the recent paper by Ciborowski et al (2024) the authors stated:

"Our key message is that studies attempting to use surficial (pXRF) analysis to provenance any excavated artefact must demonstrate that weathering processes following burial did not significantly alter the primary chemical signature of the material before any meaningful provenance interpretations can be made."

"Any future attempts to provenance excavated dolerite fragments at the monument (likely derived from the in situ dressing of megaliths and/or the removal of flakes in more recent history) must consider differences in the weathering regime experienced by the buried fragments, exposed potential outcrops and standing stones. Due to its mineralogical composition, dolerite is more susceptible to chemical weathering than sarsen. Thus, one should expect differences in weathering to be much more significant between buried dolerite fragments exposed to subsoil weathering, and dolerite outcrops and megaliths exposed to differing intensities and durations of subaerial weathering."

Local and exotic sources of sarsen debitage at Stonehenge revealed by geochemical provenancing,
T. Jake R. Ciborowski, David J. Nash, Timothy Darvill, Ben Chan, Mike Parker Pearson, Rebecca Pullen, Colin Richards, Hugo Anderson-Whymark,
Journal of Archaeological Science: Reports, Volume 53, 2024, 104406, ISSN 2352-409X,

Ixer argues (anonymously, for reasons that are unclear) that their abundant studies of Stonehenge dolerites, rhyolites and other rock types ARE reliable, because they increasingly depend upon both geochemistry and petrology when characterising rock fragments.  That's fair enough, but then he adds that ".......all the pXRF analysis has been on exposed Stonehenge stones comparing to exposed Welsh rocks, so they are like for like comparisons. And the Newall boulder was further analysed to show it was part of a broken monolith from Craig Rhos‐y‐Felin...."   

We'll ignore the somewhat wild speculation about the Newall boulder being  a part of a broken Rhosyfelin monolith and concentrate on the claim that all of the studies so far have been on "exposed" surfaces.  What does the word "exposed" actually mean?   Are buried fragments in a chalky environment exposed in the same way as surfaces exposed to atmospheric weathering?  Clearly they are not, and David Nash clearly agrees with me. You cannot simply assume that you have made "like for like comparisons".   David says, in a comment on the Ixer claim:

"Interesting thoughts but I wouldn’t be quite so confident about work on dolerite orthostats. These would have had fresh faces when quarried, and then slowly weathered at Stonehenge. The surfaces of comparator outcrops in Wales would have been weathering for much longer. This means that the Preseli outcrops are likely to be more altered by weathering than the comparatively fresh orthostats. Plus the weathering environment in Wales will be different to that of Salisbury Plain.  pXRF work needs to bear these differences in mind." (as reported on Tim's blog)

I will disagree with David on the "fresh faces when quarried", since in my view there are no quarries and thus no fresh quarried faces. But there are fresh faces on rockfall debris and on some Stonehenge bluestone monoliths which have been dressed on site -- and especially on the spotted dolerite stones used in the bluestone circle. 

 But I agree with him that there are many different permutations with regard to the fragments and stone surfaces available for examination using pXRF techniques. The technology is still relatively young, and I suspect that many of the results in recent publications will be subject to re-interpretation in the future. Even if all of the bluestone fragments at Stonehenge have come from glacially transported erratics, the surfaces available for examination today will have had vastly different histories. On a single boulder one would expect to find geochemical differences between exposed (upward facing) surfaces and the surfaces buried in the ground. (This is crucial in sampling work for cosmogenic dating, for example.) Some boulders littered in a landscape will be partly covered with vegetation, and others will not. Those on a shady slope will not have the same weathering characteristics as those on a sunny south-facing slope, partly because they may, over time, have been protected by seasonal snow-cover. On broken fragments that are buried, the burial history is important, as is the nature of the sediments in which they are contained.

So I agree with David that there is no room for complacency with regard to the claimed accuracy of the provenancing work on bluestone fragments. It is quite possible that many of the claimed matches between Stonehenge fragments and Pembrokeshire locations are incorrect.  But at least the geologists are facing up to this issue of variable weathering profiles on sample surfaces, and I commend them for that.  I think that in some ways this is the most interesting aspect of the work being undertaken by Ixer, Bevins and their colleagues.

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