Because of Covid-19 restrictions, lecture meetings for Session 163 (2020 -2021) will take the form of online Zoom meetings. They will still be held on the second Thursday of the month, but will start in September rather than October. The meetings will start at 7:30 pm and finish at around 9:00 pm. Because of the unusual circumstances this session, the lecture programme has not yet been finalised. Further details will be posted on the website when they become available.
Society members for whom we have email addresses will be sent an invitation a few days before each meeting. If you are a member but are not on our email list, or a non-member who would like to join a particular meeting, please email the society’s meetings secretary to request an invitation.
10th September 2020
Professor Simon Cuthbert, AGH University of Science and Technology, Krakow
The geology of Mercury
8th October 2020
Professor Matthew Thirlwall, Royal Holloway, University of London
New isotopic age determinations from the Northern Highlands and their implications
12th November 2020
Professor Bruce Levell, University of Oxford
The Argyll Group: two beauts
Mercury has been recognised as a “wanderer” of the night sky since Babylonian times, but its proximity to the Sun has made it a difficult body to study. The situation has changed dramatically since the recent MESSENGER mission, which has revealed a fascinating and distinctive member of the family of terrestrial planets.
Dubbed the “iron planet” it has a huge metallic core and a relatively thin rocky mantle. In some ways the geology and landforms of Mercury resemble Earth’s Moon with its heavily cratered surface, extensive smooth lava plains and lack of an atmosphere, but it is unusually dark – a possible vestige of a primordial crust made of graphite! Tectonic features suggest that the entire planet has shrunk by several kilometres since it originally solidified.
This presentation will explore current knowledge about this rather uncelebrated planet and consider how its curious nature might be inherited from the special environmental conditions close to the Sun during the very earliest history of the Solar System. A look ahead to the upcoming Bepi-Colombo mission will anticipate some exciting new insights about this mysterious “pink dot”.
Over the last 15 years we have carried out a large number of new age determinations on metamorphic rocks from the Moine and its basement inliers. Some of these are published and some not. They include Lu-Hf and Sm-Nd ages on garnets and Rb-Sr ages on white mica and biotite. Lu-Hf garnet ages are probably close to crystallization ages, while Sm-Nd garnet ages in most cases represent stages on a cooling trajectory. This can clearly be demonstrated on a few samples where core Sm-Nd ages are younger than rim Lu-Hf ages. White mica ages are in some cases substantially younger than Sm-Nd garnet, and in other cases agree well.
We see a long record of Proterozoic events, but there are few if any Archean ages on the basement inliers. There are almost no Silurian ages, neither from garnets, nor from white micas, implying that Scandian events did not result in substantial new garnet or mica growth. White mica ages largely cluster in the late Ordovician around 445Ma, and are clearly too old to reflect final collision.
Background reading: Bird, Anna; Cutts, Kathryn; Strachan, Rob; Thirlwall, Matthew F.; Hand, Martin. (2018): First evidence of Renlandian (c. 950–940 Ma) orogeny in mainland Scotland : Implications for the status of the Moine Supergroup and circum-North Atlantic correlations. Precambrian Research, 305, p. 283-294.
The Neoproterozoic Argyll Group (Dalradian Supergroup), is a well-exposed, 10 km thick, sequence recording a transition from glacial through shallow marine to slope and deep-water sediments and pillow lavas. It probably represents the rift-to-drift sequence of the Laurentide margin of Iapetus.
Two units are presented: the Port Askaig Formation, probably the Sturtian phase of “Snowball Earth”, and the Jura “Quartzite”, a 5 km thick cross-bedded, sandstone. These allow general points to be made about “Snowball Earth” and preservation bias in the sedimentary record respectively.