Herdman Symposium report
This year’s Herdman Symposium, organised by the University of Liverpool Geology Society, celebrated two of their Local Heroes, Profs HH Read and Wallace Pitcher and not surprisingly concerned itself with the rocks forever associated with these two giants of 20th Century geology, GRANITE.
Professor John Clemens brought us up to speed with what granites are, with examples from around the globe, all of which he seemed to have met in person. He illustrated how granites are, in reality, far from the simple coarse quartz, feldspar and mica rocks of the text books and can contain spectacular and indeed odd physical structures and mineralogies. In his view the classic Bowen’s reaction model of different minerals forming at different times as a magma cools doesn’t stand up as a model for granite formation., neither does the model of granite forming as plutons at depth and migrating upwards as diapirs. Most granites are shallow and tabular. He put forward the proposition that granites are unique to terrestrial planets with liquid water: i.e. water reacts with mafic crust, this hydrothermally altered crust is subducted and melts forming sodium rich felsic magmas which migrate upwards in the crust forming continental crust – without which we wouldn’t exist.
Professor Nick Petford took as a starting point analogies with 20th Century architectural theory: ‘FORM FOLLOWS FUNCTION’ i.e. SHAPE FOLLOWS PROCESS’. He claimed we have become culturally conditioned to think of granites as plutons from the depths, often depicted as a downward facing 3D parabola or cone shape disappearing to infinity. ‘We’ have done this by misapplying work done last century on salt diapirs and ignoring field evidence – diapir emplacement would result in extreme cylindrical sheer zones around granites, which are simply never seen. A model of a pulse or pulses from a deep seated feeder system fits the field and geophysical evidence of many granites being tabular. He concluded with a close up of the cross section on William Smith’s famous 1815 geological map where Smith assumed that granites were tabular, according to Prof Petford, because he was not influenced by inappropriate models!
Professor Jon Davidson explained how the isotopic composition of rocks can tell us where the magma came from: was it from the mantle, a crustal melt or some combination of the two. Isotopic analysis of minerals can tell us about the crystalisation history – but we need to understand and the structure and texture of the minerals – compositional zoning and inclusions and interpret the isotope results in this light. He illustrated how by just looking at the chemical analyses incorrect conclusions could be drawn.
Professor Jane Plant looked at the relationships between granites and the major uranium provinces of the world. Uranium deposits tend to occur in provinces and within a province many different types and ages of uranium deposits are typically observed spatially associated with high heat production granites – i.e. granites with high concentrations of radiogenic elements. Not all the deposits can be related to the time of emplacement of the granite but the evolution of the batholiths, even after it has magmatically cooled, can explain many different U deposits; i.e. deposits created by convection cells created by the radiogenically derived heat. She also pointed out that there are no U deposits pre-2500 Ma. Her theory is that the emplacement of these high heat production granites and hence the U deposits is linked to the formation and break-up of continents and this began at the end of the Archaean – 2500Ma. Without granites there would be no economic U deposits.
Dr Alan Bromley centred on mineralization associated with the Cornish Granites – which were economically viable from the Bronze Age through to 1999. An enormous range of minerals have been extracted, tin copper and iron being among the more significant. However by far the most significant mineral, by many factors of ten was, china clay aka Kaolinite, or as he described it, ‘chemically fixed water’. He presented field an experimental evidence showing that the significant stages in mineralisation occurred after the granite had cooled from a magma. He believes the tin deposits are derived from assimilated country rocks into which the granite was emplaced. These xenoliths result in residual metal-rich fluids within the granite (which has cooled on the outside to from a hard carapace) being high in volatile gases and liquids. These volatiles cause the pressure to increase which results in an explosive brecciation of the granite and the country rocks – releasing mineralising fluids AND creating the self propagating fractures needed to make the space for the mineralising fluids to deposit the minerals in. Radiogenic heat also drives mineralising convection currents. Indeed, even as the mines were closed and flooded, iron and calcium deposits could be seen being laid down by hydrothermal systems driven by the granite’s heat.
Professor Mike Edwards gave the last talk. Based on work on the Carnmenellis Granite near St Austell, he discussed field experiments studying how heat can be ‘mined’, i.e. how granite can be exploited to produce electricity from hydrothermal energy. This is far from simple engineering. To make it work the granite needs to be well understood. There are problems of water short-circuiting between the injection point and the abstraction point – thus reducing heat pickup. Also water can be lost in vast quantities though the very system of cracks and fissures which is need to make the water flow through the granite in the first place. Groundwater associated with granites can be saline – he believed this salinity was derived from acidic hydrolysis of feldspars and the groundwater contained meteoric or surface derived water. Given the reactivity of granite and the chemical soup it generates, aquifers in granites can sometimes not be the most wholesome of water for drinking purposes. He concluded with a case history of a Scottish lake, Loch Fleet, where he could demonstrate that artesian flows into the loch were passing through a calcium rich vein within granite bedrock. The result as that acidification from the atmosphere was less in this loch than others in the area, the calcium in the ground water in effect canceling out some of the acid in the rain.
All of the six lectures were outstanding and given by people clearly at the top of their game. They all combined good science, good field observation with excellent communication – the essence of good teachers.
Niall Clarke
**************************************