Lithological provenancing has featured heavily in the pages of Current Archaeology recently. In one of last month’s features, we discussed the recent evidence behind the potential origins of the Stonehenge bluestones, and this month we are examining the source of the monument’s celebrated sarsens. As we have yet to explore petrology or geochemistry within ‘Science Notes’, I thought it a good opportunity to rectify this and delve into the details of some of the techniques used for these projects.
At the core of the quest for the origins of Stonehenge’s stones are petrographical and geochemical analyses, and one of the most frequently used of these analytical tools is X-ray fluorescence (XRF). This technique beams an X-ray at a sample, which causes its atoms to emit secondary ‘fluorescent’ X-rays in response. These fluorescent X-rays reflect the sample’s elemental composition. In this way, it offers a quick method for identifying the unique elements that make up a sample. It has become particularly useful out in the field, as portable XRF (pXRF) machines, the size of hairdryers, are now readily available and produce results within a couple of minutes.
The main benefit of pXRF is that it is completely non-destructive, which makes it immensely useful in studying Stonehenge. It has recently been used on all 52 extant sarsen stones, and has previously been applied to the bluestones as well. Since it is not possible to destructively sample all the stones that make up the monument, this type of analysis provides a way of determining whether the stones have a similar composition to each other, and hence are likely to have come from the same source area. For example, using pXRF, the recent examination of the sarsens found that all but two probably came from the same location.
But pXRF cannot tell the whole story, and often more-destructive geochemical methods are needed in order to help pinpoint a stone’s specific origin. At Stonehenge, debitage (scattered fragments of stone debris) is most often used for destructive analysis, so as not to damage the stones themselves. The use of debitage, however, is problematic for geochemical sourcing studies, as it is rarely possible to tell which stone the loose material came from. A recently identified core drilled from one of the sarsens (see CA 352) circumvented this issue and allowed, for the first time, destructive sampling of a known sarsen. Three small samples from this core were crushed and analysed, using both inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectrometry (ICP-AES). While these techniques sound complicated, ultimately what they both do is break a sample down into its separate elements, which can then be detected and analysed – but at a much higher precision and with far-better detection limits than pXRF.
Once the elemental composition of a sample is known, to identify its origin it needs to be compared with samples from possible source locations. For the bluestones, this has helped pinpoint specific quarry sites in the Mynydd Preseli region of Wales (see CA 311 and 345); for the possible sarsen origins… you will have to check out our feature in this issue!
These are not the only destructive techniques available to the petrologist, though. One of the most-exciting new technologies is automated SEM-EDS (scanning electron microscopy with linked energy dispersive spectra), which was highlighted in last month’s feature on the bluestones. While the techniques mentioned above are able to separate a sample into its composite elements for analysis, SEM-EDS is able to identify the individual minerals present in a sample, as well as map and quantify them, which is dramatically more informative. SEM-EDS is also being applied to sarsen samples from the Stonehenge core, so watch this space for emerging results.
Finally, an additional mineralogical tool that has been used on the bluestones, and trialled on sarsens in Kent, is U-Pb and lead-lead zircon radiometric dating. It is based on the fact that zircon is a widespread mineral found within almost all types of rocks, and that it incorporates both uranium and thorium atoms into its crystal structure, but strongly rejects lead when forming. Therefore, as zircon contains little lead when it forms and the exact rate at which uranium decays into its series of lead isotopes is known, by determining the ratio of lead isotopes to uranium in a zircon sample, it is possible to establish its age. This is useful in provenancing the stones of Stonehenge by allowing researchers to match the range of ages – as well as the size, shape, and quality of zircon grains – between them and any potential source areas. It has been used on the bluestones to help complement the results from both ICP-AES and ICP-MS, as well as SEM-EDS.