UNDERSTANDING EARLY EARTH PROCESSES-FACT AND FICTION
Seminars
Summer Semester
Prof. Peter Cawood obtained undergraduate and PhD degrees from the University of Sydney and has held academic positions in Australia, New Zealand, Canada, and the UK. He is currently ARC Laureate Fellow at Monash University and is an elected Fellow of the Australian Academy of Sciences, the Royal Society of Edinburgh, and the Geochemical Society and European Association of Geochemistry. Peter Cawood's research focuses on the origin of the Earth's continental lithosphere (crust and upper mantle) and the processes of its generation, stabilization and reworking. His major research contributions include: demonstrating that the archive of Earth history is not simply a record of the processes of crustal generation but markedly biased by the supercontinent cycle; innovative studies on the early stages of collisional orogenesis that link ophiolite generation to its emplacement; a model for the deformation and stabilization of accretionary orogens, and temporal relations to collisional orogenesis; the role and timing for initiation of plate tectonics on the early Earth; and the application of microanalytical techniques to unravel the provenance history and paleogeography of sedimentary basins and orogenic belts.
Additional information: Prof. Guochun Zhao, gzhao@hku.hk
The Earth is the only planet with active plate tectonics but for how long this has been the modus operandi of mantle-lithosphere coupling remains unresolved. There is a widespread view that plate tectonics has been in operation since the late Archean or the beginning of the Paleoproterozoic but commencement in the Hadean to early Archean has also been argued, as has a later and possible episodic start to plate tectonics. Differences reflect in part contrasting conceptual approaches to the interpretation of geological features and processes. Those favouring a very early start for plate tectonics highlight similarities in observations, proxies and models between the early and contemporary Earth, whereas those favouring a later start date tend to emphasise the differences, which is in part attributed to higher mantle potential temperatures following solidification of the magma ocean. Resolving these different interpretations is hindered by the limited geological record, which for the Hadean, is almost entirely restricted to isotopic proxies in mineral fragments from younger detrital rocks. The main rock record starts in the Eoarchean, at 3.8 Ga, with the commencement of craton formation, with a further change at the end of the Archean with cessation of tectonothermal activity in the cratons, their stabilization, subsidence, emplacement of dykes into rigid lithosphere, and widespread cratonic breakup. Although there is no universally accepted smoking gun to constrain when plate tectonics commenced, establishing when it did start is fundamental to understanding our planet’s evolution given plate tectonics role on the contemporary Earth in maintaining a habitable environment.
Irrespective of the tectonic mode in operation on the early Earth, conditions were fundamentally different from today with a faint young sun, a dominant water world that limited terrestrial silicate weathering, and predominant mafic volcanism. On Earth’s early water world, with a global ocean ~ 4.5 km deep, submarine basalt degassing resulted in CO2 and CO as the dominant phases with water and sulphur remaining dissolved in the basaltic melt. Enhanced CO2 degassing under a higher-pressure submarine dominated regime along with reverse weathering would help mitigate the effects of the decreased luminosity of the early Sun and likely lead to increased ocean acidity. Progressive continental emergence from ca. 3.8-2.5 Ga may have marked the start of widespread subaerial silicate weathering. However, the weak nature of the lithosphere, under the high mantle potential temperatures of the early Earth, limited topography along with associated weathering and material transport. The Earth’s atmospheric and ocean composition, land-water ratio, continental volume, tectonic mode, and solar flux have evolved through time, but the dynamic nature of the carbonate-silicate feedback loop has been able to maintain clement conditions throughout most of Earth’s 4.5 Ga existence.
This talk will discuss some of the evidence as well as potential problems with the “evidence” in constraining early Earth tectonic modes and potential impact on the carbon cycle.