Abstract The Altar Stone at Stonehenge in Wiltshire, UK, is enigmatic in that it differs markedly from the other bluestones. It is a grey–green, micaceous sandstone and has been considered to be derived from the Old Red Sandstone sequences of South Wales. Previous studies, however, have been based on presumed derived fragments (debitage) that have been identified visually as coming from the Altar Stone. Portable X-ray fluorescence (pXRF) analyses were conducted on these fragments ( ex situ ) as well as on the Altar Stone ( in situ ). Light elements ( Z <37) in the Altar Stone analyses, performed after a night of heavy rain, were affected by surface and pore water that attenuate low energy X-rays, however the dry analyses of debitage fragments produced data for a full suite of elements. High Z elements, including Zr, Nb, Sr, Pb, Th and U, all occupy the same compositional space in the Altar Stone and debitage fragments, and are statistically indistinguishable, indicating the fragments are derived from the Altar Stone. Barium compares very closely between the debitage and Altar Stone, with differences being related to variable baryte distribution in the Altar Stone, limited accessibility of its surface for analysis, and probably to surface weathering. A notable feature of the Altar Stone sandstone is the presence of baryte (up to 0.8 modal%), manifest as relatively high Ba in both the debitage and the Altar Stone. These high Ba contents are in marked contrast with those in a small set of Old Red Sandstone field samples, analysed alongside the Altar Stone and debitage fragments, raising the possibility that the Altar Stone may not have been sourced from the Old Red Sandstone sequences of Wales. This high Ba ‘fingerprint’, related to the presence of baryte, may provide a rapid test using pXRF in the search for the source of the Stonehenge Altar Stone.
New U–Pb zircon ages from rhyolite samples of the Fishguard Volcanic Group, SW Wales, confirm a Middle Ordovician (Darriwilian) age for the group. One of the samples is from Craig Rhos-y-felin, which has recently been identified on petrological and geochemical grounds as the source of much of the debitage (struck flakes) at Stonehenge. Analysis of a Stonehenge rhyolite fragment yields an age comparable with that of the Craig Rhos-y-felin sample. Another Stonehenge fragment, thought to come from orthostat (standing stone) 48 and on petrographical grounds to be derived from the Fishguard Volcanic Group (but not Craig Rhos-y-felin), yields an age also consistent with a Fishguard Volcanic Group source. Supplementary material: Details of analytical methods and a table of data are available at https://doi.org/10.6084/m9.figshare.c.3518175 .
Lanthanite-(Ce) occurs as a secondary mineral in oxidized copper ore at the Britannia Mine, Snowdoni4 North Wales, U.K. It is found as colorless transparent plates {010} covered by radiating tufts of malachite and is associated with brochantite, posnjakite and chalcoalumite. The analytical formula, based on t7 oxygens, is (Ce6.rrl-ao.rrNdo.rrSmo.oGdo.orYo.oohr.ouCz.rOg.ot'7.96H2O, and the theoretical formula is (Ce,La,Nd)r(CO3)3.8H2O with Ce > La,Nd. Orthorhombic, space goup Pbnb, a:9.482(6), b : 16.938(ll), c : 8.965(3)A,Z:4, D(calc.) :2.79 E/carr for the ideal formula (Ce:La:Nd : 0.83 :0.59 :0.58), D (meas .l : 2.76 g/cmt , V : 144043, a:brc :0.559E :1 :0.5293. Strongest lines in the X-ray powder diffraction pauern are ftlA (I) (hkl)l 8.47(100X020), 4.746(65)l2W),4.462(62NW2),3.255(73N202) and 3.028(65\222\. The mineral is biaxial negative, a 1.532(2), P 1.594(2),y l.6lQ2); orientation X:b, Y:c, Z:a;2V(measJ:60(2)0, 2V(calc.)=62,no dispersion observed. The mineral is not fluorescent, and has H:2.5, a colorless streak, a vitreous luster and is sectile. In dilute mineral acids (HCl, HNO.), the mineral reacts with effervescence to yield a gel-like precipitate oflanthanide(Ill) hydroxides.
Analysis of the precision of the illite ‘crystallinity’technique shows that machine errors are <5%, while intra‐ and inter‐sample errors are variable but are up to 12% and 14%, respectively (1σ). Consideration of this error analysis shows that the isocryst approach, which involves close contouring (e.g. 0.03 Δ2°) of illite ‘crystallinity’data, has a very low degree of confidence (<0.5) and thus is not regarded as statistically valid. If contouring is to be undertaken with a high degree of confidence (>0.8) it is necessary that contours should be at intervals of 0.1 ΔΘ2°, which is equivalent to subdivision of the anchizone into upper and lower units. Where previous interpretations have relied upon an isocryst method of contouring at less than 0.1 ΔΘ2° the conclusions must be regarded as unsubstantiated. Centrifuge separation of clay fractions (based on a Stokes’law application) gives separations in which a significant, but variable, percentage of grains have long axes greater than the size calculated. For the typical <2‐μm fraction utilized, some 20% of grains lie in the 2–4‐μm range, although the proportion is not believed to have a significant effect upon ‘crystallinity’values. The formula is applicable for grain‐sizes down to 0.5 μm. Illite ‘crystallinity’values on samples prepared by an ultrasonic disaggregation method show a small increase on those prepared by ball mill crushing. The differences are minimal at the epi/anchizone level but increase to some 10% at the anchizone/diagenetic level. The effect on grade determinations is again thought to be minimal and indicates that concern over unsuitability of the ultrasonic disaggregation method is unfounded.
The Lower Palaeozoic succession of the Welsh Basin has suffered the effects of low grade metamorphism which shows features that do not accord with classical models of regional or burial metamorphism. Textural evidence demonstrates the development of greenschist facies metamorphism prior to deformation and cleavage development, while mineralogical characters indicate a low pressure facies series. In order to accommodate these features it is suggested that the area has suffered diastathermal metamorphism, in which an enhanced thermal flux develops early in the tectonic cycle in response to an extensional setting. Temperatures remained relatively high during final closure, associated with deformation of the basin sediments and consequent cleavage development. Such a model adequately resolves previously conflicting interpretations of burial- and deformation-related metamorphism.
Summary The first occurrence of pumpellyite in the Lower Ordovician volcanic rocks of North Pembrokeshire is reported and its form and chemistry are described. The significance of pumpellyite in relation to the grade of metamorphism attained during the Caledonian orogenic episode is discussed.
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