John Faithfull

The Lower Eastern Layered Series of Rhum

The Lower Eastern Layered Series (LELS) comprises approximately units 1–5 of previous workers. Remapping has revealed considerable along-strike lithological variation in the units of the LELS. It is suggested, on the basis of field and geochemical evidence, that two layers formerly regarded as ‘conformable intrusive sheets of fine-grained olivine gabbro’, may be evolved alii valite layers rather than later intrusions. Xenolith suites, in these layers and elsewhere, indicate a component derived from the roof or walls of the magma chamber. Cryptic variation is more extensive in the LELS than in other parts of Rhum: olivine forsterite content varies from 85.6 to 70, and clinopyroxene Mg×100/(Mg+Fe) varies from 88 to 74. Postcumulus effects and subsolidus re-equilibration have altered the initial compositions of the mineral phases. The data are consistent with a model of repeated replenishment by picritic magma, although the replenishing liquids may have been slightly less magnesian than those subsequently available, during the formation of the upper ELS.

Terpenoid composition and origin of amber from the Cape York Peninsula, Australia

The terpenoid composition of fossil resin from the Cape York Peninsula, Australia has been analysed by pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) to determine its origin. The pyrolysis products were dominated by cadalene-based C15 bicyclic sesquiterpenoids including some C30–C31 bicadinanes and bicadinenes typical of Class II resin derived from angiosperm plants of Dipterocarpaceae. This observation contrasts with the Araucariaceae (Agathis sp.) source previously suggested for the resin based on Fourier transform infrared (FTIR) analyses. Dipterocarpaceae are not known in Australian vegetation but grow abundantly in Southeast Asia including New Guinea, indicating that the geological origin of the amber is not the Australian mainland but could be traced to Southeast Asia.

Mid-Eocene renewal of magmatism in NW Scotland: the Loch Roag Dyke, Outer Hebrides

A monchquite dyke, in the vicinity of Loch Roag, Lewis, Outer Hebrides has an unusually enriched chemistry, and contains a unique assemblage of megacrysts and xenoliths from the lithosphere of the Hebridean craton. A 40Ar/39Ar plateau age of 45.2 ± 0.2 Ma (2σ) of a phlogopite megacryst from the dyke overlaps an earlier reported K–Ar age, and confirms that the British Palaeogene Igneous Province extended into the Eocene. Similar late low-volume melts were erupted in the Eocene and Oligocene in West and East Greenland, suggesting that such late-stage magmatic rejuvenescence is a widespread feature across the North Atlantic Igneous Province.

Trace-element abundances in the shallow lithospheric mantle of the North Atlantic Craton margin: Implications for melting and metasomatism beneath Northern Scotland

Abstract Bulk rock geochemistry and major- and trace-element compositions of clinopyroxene have been determined for three suites of peridotitic mantle xenoliths from the North Atlantic Craton (NAC) in northern Scotland, to establish the magmatic and metasomatic history of subcontinental lithospheric mantle (SCLM) below this region. Spinel lherzolites from the southernmost locality (Streap Com’laidh) have non-NAC mantle compositions, while the two northern xenolith suites (Loch Roag and Rinibar) are derived from the thinned NAC marginal keel. Clinopyroxene compositions have characteristic trace-element signatures which show both ‘primary’ and ‘metasomatic’ origins. We use Zr and Hf abundances to identify ancient cryptic refertilization in ‘primary’ clinopyroxenes. We suggest that Loch Roag and Rinibar peridotite xenoliths represent an ancient Archaean- Palaeoproterozoic SCLM with original depleted cratonic signatures which were overprinted by metasomatism around the time of intrusion of the Scourie Dyke Swarm (~2.4 Ga). This SCLM keel was preserved during Caledonian orogenesis, although some addition of material and/or metasomatism probably also occurred, as recorded by Rinibar xenoliths. Rinibar and Streap xenoliths were entrained in Permo-Carboniferous magmas and thus were isolated from the SCLM ~200 Ma before Loch Roag xenoliths (in an Eocene dyke). Crucially, despite their geographical location, lithospheric mantle peridotite samples from Loch Roag show no evidence of recent melting or refertilization during the Palaeogene opening of the Atlantic.

The mineral prospecting expeditions to the South Atlantic islands and Antarctic Peninsula region made by the Scottish geologist David Ferguson, 1912–1914

Synopsis David Ferguson’s mineral prospecting expeditions to South Georgia (1912), the Falkland Islands and the South Shetland Islands (1913–1914), on behalf of the Christian Salvesen whaling company of Leith and now largely forgotten, were early examples of commercially motivated terrestrial exploration in the South Atlantic region. Prior geological knowledge was very limited and Ferguson complemented his unsuccessful prospecting work with attempts to understand the regional geology of the areas that he visited. These interpretations were based on relatively cursory fieldwork undertaken in an arduous environment, and did not prove robust; but the well-documented specimen collections that Ferguson accumulated provided the basis for excellent and much-cited petrographical accounts by G. W. Tyrrell of Glasgow University. Ferguson had studied geology at the university and the influence of his mentor there, Professor J. W. Gregory, is apparent. In turn, Gregory utilized Ferguson’s observations in support of a subsided ‘South Atlantic continent’, opposing the ‘displacement hypothesis’ for that region formalized by Alfred Wegener from 1912 onwards. Ferguson’s field notebooks and most of his rock specimens are now held by Glasgow University (Archive Services and Hunterian Museum, respectively) but he distributed representative specimen collections widely, and these are extant in several other British museums. Specimens were also supplied to, and discussed with, William Speirs Bruce who, following the 1902–1904 Scottish National Antarctic Expedition, took a keen interest in Ferguson’s discoveries.

Xenolithic insights into the deep geology beneath the Ross of Mull

Synopsis A rich assemblage of xenoliths is present in three small Permian dykes that cut the Ross of Mull granite on the SW coast of Mull, bordering the Sound of Iona. The xenoliths, up to 80 mm in diameter, are abundant, and include a wide range of compositions, including spinel lherzolites, dunites, pyroxenites, meta-basic granulite-facies gneisses, anorthosites, anorthoclasites, quartzo-feldspathic gneisses and psammitic meta-sediments. Granitic xenoliths attributable to the Ross of Mull pluton are strikingly scarce. The inference is that the majority of the xenoliths were entrained from beneath the pluton. The xenoliths represent a wide cross-section of the lithosphere, from below the Moho to relatively high crustal levels. Cataclasis and intense shearing in some of the crustal xenoliths suggests that the dykes intersected one or more of the bounding Caledonian thrusts at mid-crustal depths.

High-precision 41K/39K measurements by MC-ICP-MS indicate terrestrial variability of δ41K

Potassium is a major component in continental crust, the fourth-most abundant cation in seawater, and a key element in biological processes. Until recently, difficulties with existing analytical techniques hindered our ability to identify natural isotopic variability of potassium isotopes in terrestrial materials. However, measurement precision has greatly improved, and a range of K isotopic compositions has now been demonstrated in natural samples. In this study, we present a new technique for high-precision measurement of K isotopic ratios using high-resolution, cold plasma multi-collector mass spectrometry. We apply this technique to demonstrate natural variability in the ratio of 41K to 39K in a diverse group of geological and biological samples, including silicate and evaporite minerals, seawater, and plant and animal tissues. The total range in 41K/39K ratios is ca. 2.6‰, with a long-term external reproducibility of 0.17‰ (2σ, N = 108). Seawater and seawater-derived evaporite minerals are systematically enriched in 41K compared to silicate minerals by ca. 0.6‰, a result consistent with recent findings. Although our average bulk-silicate Earth value (−0.54‰) is indistinguishable from previously published values, we find systematic δ41K variability in some high-temperature sample suites, particularly those with evidence for the presence of fluids. The δ41K values of biological samples span a range of ca. 1.2‰ between terrestrial mammals, plants, and marine organisms. Implications of terrestrial K isotope variability for the atomic weight of K and K-based geochronology are discussed. Our results indicate that high-precision measurements of stable K isotopes, made using commercially available mass spectrometers, can provide unique insights into the chemistry of potassium in geological and biological systems.

The isotope centenary: 4th December

On 4 December 1913, the journal Nature published a letter by Frederick Soddy, of the University of Glasgow, in which the term isotope was publicly used for the first time. Soddy had been at Glasgow since 1904 and realized the chemical identity of ‘mesothorium’ (228Ra—which Soddy separated from thorium minerals) and Marie Curie’s radium (226Ra—from uranium minerals). Soddy was awarded the Nobel Prize for Chemistry in 1921, largely for this work. A number of historical radioactive samples (including Soddy samples) survive at Glasgow, as well as some of his equipment, and these provide a rare glimpse into the birth of radiochemistry. High precision gamma-ray spectrometry has been carried out on them, and together with archival research, this provides new insights into their preparation and history. Since Soddy’s breakthrough, the ratio of known isotopes to elements has grown from 1 to nearly 27!Isotopic labelling with heavy hydrogen isotopes (D2 and T2) is widely used as a means to monitor the biological fate of a potential drug molecule and represents a particularly industry-facing example of chemoselective organometallic catalysis. Consequently, preliminary studies from our laboratory have allowed expedient access to a series of novel iridium complexes, such as 2, that are able to catalyse the ortho-deuteration of various coordinating functionalities and pharmacophores, such as ketones, amides and nitro compounds 2 (Scheme 1). As part of our latest studies, we recently reported an efficient protocol for ortho-deuteration using more readily accessible Ir(I)chloro-carbene complexes. Turning to more challenging substrate classes, the utility of bench-stable catalysts such as 5 has now evolved to deliver the first highly effective strategy for the ortho-deuteration of primary sulfonamides at room temperature (Scheme 2). Additionally, we have used experimental and computational methods in parallel to explain the origins of observed chemoselectivity in labelling multi-functional drug molecules like 7, highlighting the importance of substrate–complex interactions during complexation. The details of all such studies will be delineated in this lecture.

A Scottish ‘diamond’ specimen rediscovered and re-identified: the first Scottish occurrence of piemontite

Synopsis A possible occurrence of diamond near Ben Hope in Sutherland was described in 1901. For most of the 20th century, the specimen had been assumed lost, and thus the identification could not be verified. This specimen has recently come to light. Re-examination shows that the specimen contains no diamonds, but rather contains a very unusual assemblage of piemontite–spessartine–hematite–braunite–muscovite–K-feldspar–quartz. This is the first Scottish occurrence of true piemontite. Revisiting the original locality, no in-situ exposures were found, but this lithology forms a distinctive component of glacially transported boulders over an area to the east and south of Ben Hope. It is almost certainly derived from the local Moine, and represents a unit with an oxidized, coarse clastic protolith. If found in situ, this unit may be a useful marker bed in the otherwise homogenous psammitic rocks of the Moine in this area.