Geochronology and geochemistry of Carboniferous volcanism throughout the Midland Valley of Scotland

Overview

During the Carboniferous (359-259 Ma), Scotland lay near the equator in a position akin to central Africa, and therefore had a hot climate. The northern parts of Scotland were upland areas and the southern parts of Scotland were lower lying. Further to the north and west lay the North American continent, to which Scotland was joined, and to the south of what we know as England, lay a large ocean. During the Carboniferous, sea level rose and fell several times. There were times when southern Scotland lay underwater, covered by a shallow, warm sea. At other times this area lay above sea level.

When below sea level, the area was covered in sand and mud that was washed into the sea, leading to the formation of rocks such as sandstone, mudstone, and shale. Limestone also formed in part through the accumulation of dead sea creatures, but also through the precipitation of calcium carbonate in the tropical conditions. The shallow tropical seas of Carboniferous Scotland, much akin to the present-day Bahamas, were full of life and much of this life became preserved as fossils in the rocks, including shellfish, corals, crinoids, sharks and other fish. When above sea level, the land was covered by tropical swamps, where forests of large trees, that were very unlike modern trees, flourished. Giant-sized centipedes, dragonflies and spiders ruled the landscape along with amphibian and early reptiles, the forerunners of the dinosaurs. Over time, the remains of trees that had fallen into the swamps were buried deeply and compacted, eventually forming peat, and finally coal (Figure 1).

Volcanic activity was another important feature of the Carboniferous (Figure 2). The continent at this time was being ‘stretched’, allowing magma to well up from the mantle to form many volcanoes. The remains of these volcanoes are preserved as intrusive rocks (e.g., Figure 3) and lava flows (e.g., Figure 4) that dot the landscape of southern Scotland. These rocks comprise classical sites, such as Arthur’s Seat, Edinburgh.

Although the stratigraphic relationships of intrusive and extrusive volcanic rocks and the Carboniferous sedimentary rocks of Scotland are good, the absolute age and temporal framework of the rocks is poor. Lack of an accurate and precise temporal framework essentially leaves the Carboniferous rocks of Scotland ‘floating’ in time with reference to the Geological Time Scale. Without an absolute chronology the rates of climate change and evolution of flora and fauna preserved within the Carboniferous sediments throughout Scotland cannot be assessed.

The aim of this PhD project is to date, using high precision radio-isotope geochronology, the Carboniferous volcanic rocks located throughout the Midland Valley (Figure 2). The data will be used to construct a robust temporal framework that can place these sequences of rocks within the global Geological Time Scale and constrain the ages of fossil-bearing sediments. To push the limits of geochronology the project has support from Thermo Scientific (CASE Partner) and the student will get access to the latest state-of-the-art mass spectrometer technology. In addition, isotope geochemistry (co-supervisors Ellam and McPherson) will be used to understand the formation and evolution of the Carboniferous magmatic systems of Scotland.

Click on an image to expand

Image Captions

Figure 1: Spireslack is a stunning exposed section of an important coal-bearing sequence, unparalleled in Scotland. More than 1 km long, the 80-metre-deep canyon is cut into a gently dipping succession of Carboniferous-age strata that includes economically valuable coal, ironstone and oil shale.
Figure 2: Volcanic rocks of Carboniferous and Permian age in the Midland Valley, Scotland.
Figure 3: Dumbarton rock is the remains (basaltic plug) of a Carboniferous volcano.
Figure 4: Exposure of ca. 30 lava flows of Carboniferous age, Western Campsie Fells from Strathblane.

Methodology

The project will utilise both high precision Ar/Ar (lavas and explosive eruptions) and zircon U-Pb geochronology (intrusive rocks and explosive eruptions) to date rocks from throughout the Midland Valley (Figure 2). The rocks will be sampled from secure stratigraphic settings and the required mineral phases for geochronology be extracted by the student. The fieldwork will be extensive (approx. 12-15 weeks over 2 years) and rocks will be characterised using optical microscopy, XRF and elemental analysis. The geochronological data will provide absolute temporal markers for anchoring the Carboniferous rocks of Scotland to the global Geological Time Scale. Radiogenic isotopes (Nd, Sr and Pb, Ellam) will provide information about the magmatic evolution of the Carboniferous rocks and their parent magmas through time.

Project Timeline

Year 1

– Literature review of radioisotope geochronology (Ar/Ar and U-Pb)
– Literature review of Carboniferous geochronology, Scotland
– Review of field sites and development of sampling plan
– Fieldwork to sample rocks from secure stratigraphic settings
– Petrographic descriptions of rocks, XRF and elemental analyses
– Preparation of samples for high-precision geochronology (Ar/Ar and U-Pb)
– Preparation (acid cleaning, many cycles) clean room Teflon for U-Pb geochronology

Year 2

– Ar/Ar dating of lavas and explosive eruptions
– U-Pb geochronology of intrusive rocks and explosive eruptions
– Assessment of gaps in data and failed experiments and finalise final field sampling
– Fieldwork to collect final samples from secure stratigraphic settings
– Petrographic descriptions of rocks, XRD and elemental analyses
– Prepare materials for geochronology and radiogenic isotopes
– Presentation of data at UK conference

Year 3

– Complete geochronological work on samples collected/prepared year 2
– Complete geochronological experiments at Thermo Bremen
– Conduct radiogenic isotope analyses (Sr, Nd, Pb) on all rocks
– Begin construction of thesis and publication of data
– Presentation of data at international conference

Year 3.5

– Complete thesis for submission and publication of data

Training
& Skills

The student will be trained in a variety of geochronological and geochemical techniques, including mass spectrometry (e.g., noble gas, thermal ionisation). It is expected the student will become an emerging leader in geochemistry at the completion of this PhD. The project demands the student spend many hours working in laboratories. There will also be extensive fieldwork including mapping of igneous units and rocks descriptions throughout the Midland Valley of Scotland. The student will present data at both UK and International conferences and prepare data/manuscripts for publication. The student will also engage with the global EARTHTIME community (http://www.earthtimetestsite.com), which includes attendance at bi-annual Gordon Research Conferences dedicated to geochronology: https://www.grc.org/geochronology-conference/2021/.

The project requires a student from a geological background who has a desire to combine field observations with precise and accurate laboratory measurements (geochemistry and geochronology). Experience of field mapping, numerically literate and demonstrable ability to work with isotope data are key. The student will be immersed in the lively high precision geochronology group at the Scottish Universities Environmental Research Centre (SUERC) led by Professor Mark, which currently includes 6 PhD students, 2 post-doctoral researchers and 4 research technicians. Working with CASE partners Thermo Scientific will allow the student to get direct experience in the mass spectrometer industry.

References & further reading

U-Th-Pb Geochronology: https://www.sciencedirect.com/science/article/pii/B9780080959757003107?via%3Dihub

Ar/Ar geochronology:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article/doi/10.1130/B35560.1/587798/Interpreting-and-reporting-40Ar-39Ar

Carboniferous stratigraphy UK:
http://nora.nerc.ac.uk/id/eprint/13985/1/RR10007.pdf

Carboniferous stratigraphy Midland Valley (+other references):
http://earthwise.bgs.ac.uk/index.php/Carboniferous_of_the_Midland_Valley_of_Scotland

Geochronology of Carboniferous-Permian volcanism/magmatism Midland Valley:
https://jgs.lyellcollection.org/content/163/1/15/tab-article-info

Further Information

For further information please contact: Darren Mark (SUERC, University of Glasgow)
Darren.Mark@glasgow.ac.uk. The project student will join the Caledonian Geochronology & Geochemistry Research Group (www.scottishisotopes.co.uk) and be based fulltime at SUERC (https://www.gla.ac.uk/research/az/suerc/)

To apply for this position, use the following link www.gla.ac.uk/ScholarshipApp

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