More about the dolerite "spots"

There's an interesting new paper (online) about the spotted dolerite of Preseli and the origins of their spots. In it, Bevins et al  (2021) undertake a very detailed analysis of the content of the white or cream-coloured spots which have traditionally been used by geologists in identifying the locations from which the Stonehenge spotted dolerite bluestones might have come.

Details:  

Alteration fabrics and mineralogy as provenance indicators; the Stonehenge bluestone dolerites and their enigmatic “spots” (2021)Richard E.Bevins, Rob A.Ixer, DuncanPirrie, Matthew R.Power, Tom Cotterell , Andrew G.Tindle
Journal of Archaeological Science: Reports
Volume 36, April 2021, 102826

https://doi.org/10.1016/j.jasrep.2021.102826

ABSTRACT

Stonehenge is dominated by the large, locally derived sarsen stones which comprise the Outer Sarsen Circle and the Inner Sarsen Trilithon Circle. Lithologically they are a hard form of sandstone called silcrete. Less obvious are the smaller ‘bluestones’ which form the Outer Bluestone Circle and Inner Bluestone Horseshoe. The bluestone assemblage is composed of a variety of lithologies, including dolerite (almost all carrying small, cm scale, white to pink ‘spots’), rhyolites, volcanic tuffs and two different types of sandstone. The majority of the bluestones have been provenanced to the Mynydd Preseli area in west Wales some 200 km west of Stonehenge, and the distinctive spots in the dolerites were crucial evidence in making that link. However, despite the importance of the spots in that provenancing model, which provides evidence for one of the most remarkable feats of Neolithic time in north-west Europe, they have been little studied and are not fully characterized. One reason for this is the fact that they are difficult to study using standard optical microscopy because of the fine grain size of the component minerals. In order to gain a greater understanding of the mineralogy and origin of the spots we have used a range of alternative optical and analytical techniques, including high resolution photomicrography, ‘standard’ electron microscopy (SEM-EDS), automated scanning electron microscopy with linked energy dispersive analysis (automated SEM-EDS) and X-ray diffraction. These investigations, based primarily on samples from Carn Goedog and Carn Meini in the Mynydd Preseli, lead us to conclude that the spots result from a two-stage alteration history involving firstly saussuritization of primary igneous calcium-rich plagioclase phenocrysts/glomerocrysts. The saussuritization was probably a late-stage magmatic event, possibly occurring at ca. 400 °C, and led to the development of the secondary minerals clinozoisite, zoisite, albite and muscovite. Subsequently, the dolerites were affected by regional low-grade metamorphism at ca. 250–300 °C, at the transition from the prehnite-pumpellyite to the greenschist facies. During this metamorphism the background (non-spot) areas of the samples were altered with the development of the secondary minerals chlorite, actinolite, epidote, quartz, titanite and further albite. Interpretation of these data shows that the Carn Goedog sample has a higher percentage of secondary minerals compared to the Carn Meini sample, suggesting that the former shows a higher degree of alteration. Further analyses of dolerite samples from the Mynydd Preseli should provide insight into the potential of the extent of rock alteration in the further refinement of understanding the source or sources of the Stonehenge bluestone dolerites.

This is a "pure" geology paper, and it's a welcome addition to the literature.  Nonetheless, the authors can't resist taking the opportunity to perpetrate the bluestone transport myth greatly beloved of MPP and his team.  These words are from the Abstract:  However, despite the importance of the spots in that provenancing model, which provides evidence for one of the most remarkable feats of Neolithic time in north-west Europe, they have been little studied and are not fully characterized. That's nonsense -- the provenancing model does not provide any evidence at all for the so-called "remarkable feat" -- all it does is provide evidence relating to the provenancing of some of the bluestones.  So they paper is devalued by the authors' insistence on straying away from science and into the field of myth and fantasy.

In the text, there is a mention of the forthcoming paper by the MPP team (including Bevins and Ixer) which will no doubt flag up the wonders of Waun Mawn:

Parker Pearson, M., Pollard, J., Richards, C., Welham, K., Kinnaird, T., Shaw, D., Simmons, E., Stanford, A., Bevins, R., Ixer, R., Ruggles, C., Edinborough, K., 2021 (in press). The original Stonehenge? A dismantled stone circle in the Preseli hills of west Wales.  Antiquity.

We await it with interest, but already know (courtesty of Bluestone Brewery) what it will say!

The new work concentrated on 13 Stonehenge spotted dolerite samples and 12 Preseli spotted dolerite samples from different outcrops. The authors are at pains to point out that the white spots are not simply "feldspar crystals or aggregates" but are in fact clusters of alteration minerals including albite, epidote chlorite, clinozoisite, muscovite and clay minerals, as well as chrome spinel.  So they are very complicated, and detained analysis, using several high-tech methods, can be used for much more detailed provenancing than anything previously possible.

The authors are convinced that the Preseli dolerites have been altered, to a greater or lesser extent, by two phases of metamorphism which have had very subtle but still discernible effects.  Quote:

Calculation of an alteration index for samples from Preseli and Stonehenge may therefore provide a tool for refining the provenancing of the Stonehenge doleritic bluestones. So whilst the spots were the defining character that led to the provenancing of the doleritic bluestones to the Mynydd Preseli area, the character and extent of the low-grade metamorphic alteration present in the background areas of the dolerites might offer the potential for further refinement of source outcrops. This can only be advanced, however, by analysing more Preseli samples using the automated SEM-EDS method to get a clearer picture of the metamorphic alteration history of the Preseli dolerites and also to explore the range of variation at the hand specimen and rock outcrop scales.

This is impressive work -- although it should be stressed that most of the analytical work has been done on just two samples -- numbered PCM32 (from Carn Meini) and PCC11 (from Carn Goedog).  Much more sampling needs to be done laterally across the spotted dolerite outcrops in order to assess just how "diagnostic" the features described in this paper really are -- and of course this pure geology paper has no bearing at all on the debate about quarrying either at Carn Meini or Carn Goedog.