The origin of the Earth: its early evolution
Dr Maud Boyet, CNRS University of Clermont- Ferrand, France
Isotope geochemistry provides undisputable evidence for early differentiation of the Earth. The indications come from 1) the comparison between measurements on terrestrial samples and extra-terrestrial objects that may represent the Earth’s building blocks, and 2) the study of radioactive systematics (short-lived and long-lived chronometers) on the oldest terrestrial samples. Excess in 142Nd (decay product of 146Sm) measured in terrestrial samples relative to chondrites has been explained by a very early differentiation event. However this scenario has been challenged by the discovery of variations in mass-independent stable Nd isotopic compositions in undifferentiated chondritic meteorites. On the basis of high-precision isotope measurements Dr Boyet will discuss which material is most likely to have participated in the Earth's accretion and the implications for the early stages and the long-term evolution of our planet.
The inception of plate tectonics on terrestrial planets
Professor Craig O’Neill, Macquarie University, Australia
The progression of a planet from a pre-plate tectonics, to a plate-tectonic regime, has been shown to be very sensitive to system parameters, such as thermal state and the specific rheology. Whilst generally it has been shown that cold-interior high-Rayleigh number convection (such as on the Earth today) favours plates, due to the ability of the interior stresses to couple with the lid, a given system may or may not have plate tectonics depending on its initial conditions. This has led to the idea that there is a strong-history dependence to tectonic evolution – and the details of tectonic transitions, including whether they even occur, may depend on the early history of a planet.
However, intrinsic convective stresses are not the only dynamic drivers of early planetary evolution. Early planetary geological evolution is dominated volcanic processes, and impacting. These have rarely been considered in thermal evolution models. Recent models exploring the details of plate tectonic initiation have explored the effect of strong thermal plumes or large impacts on surface tectonism, and found that these ‘primary drivers’ can cause a transition from a stagnant-lid state to an active lid, and, in some cases, over-ride the pre-configuration set by initial conditions.
The only detailed planetary record we have of the development comes from Earth, and is restricted by the limited geological record of its earliest history. Many recent estimates have suggested an origin of plate tectonics at ca. ~3.0Ga, inferring a somewhat monotonic transition from pre-plates to a somewhat modern plate tectonic regime around that time. However, both numerical modelling, and the geological record itself, suggest a strong non-linearity in the dynamics of the transition, and it has been noted that the early history of Archaean greenstone belts and TTGs may be one of failed subduction.
This talk will explore the history of subduction failure on the early Earth, and couple these with insights from numerical models of the geodynamic regime at the time.
Constraints on early Earth tectonics from convection models with damage theory
Professor Brad Foley, Penn State University, USA
A key requirement for plate tectonics is rheological weakening in the lithosphere that forms weak plate boundaries. As a result, a major issue for early Earth dynamics is whether such weakening is active and effective in a hotter Earth, and one with a potentially different heating mode (i.e. a different percentage of core versus internal heating). This topic has been addressed with pseudoplastic models of Earth’s lithospheric rheology, but not with more complex models considering grainsize evolution. Here Professor Foley shows that a hotter, internally heating dominated Earth is not a significant impediment to plate boundary formation, subduction, and surface plate mobility; this result contrasts with those from pseudoplastic models due to fundamental physical differences in the two mechanisms for plate boundary formation. A hotter Earth does lead to more episodic subduction, over a range of timescales, and a more sluggish subduction relative to flow in the mantle interior. Professor Foley will also discuss how sluggish, drip-like subduction in the Archean can potentially explain observations of crust forming processes during this time.
Mantle dynamics and crustal evolution through the Earth history
Professor Jun Korenaga, Yale University, USA
Resolving the modes of mantle convection through the Earth history, i.e., when plate tectonics started and what kind of mantle dynamics reigned before, is essential to the understanding of the evolution of the whole Earth system, because plate tectonics influences almost all aspects of modern geological processes. At the same time, it is a very challenging problem because plate tectonics continuously rejuvenates Earth’s surface on a time scale of about a hundred million years, destroying evidence for its past operation. This nature of plate tectonics forces us to look for indirect evidence preserved in the buoyant continental crust, which can survive over billions of years. In this contribution, Professor Korenaga will review a range of crustal growth models to gain some insights into the modes of mantle convection. Growth models proposed so far can be categorized into three types: crust-based, mantle-based, and the others, and the first two types are particularly important as their difference reflects the extent of crustal recycling, which can be related to subduction. By combining such inference from growth models with other geophysical, geochemical, and geological constraints, Professor Korenaga will discuss a promising working hypothesis for the evolution of mantle dynamics and its influence on surface environment.
Archean plume-lid tectonics and cratons formation
Professor Taras Gerya, ETH Zurich, Switzerland
Cratons - the oldest stabilised parts of Earth’s continents - have multi-stage history and are characterised by thick and heterogeneous (on the scale of tens to hundreds km) mantle roots with variable degree of depletion and metasomatic reworking. Several distinct proto-cratonic units of hundreds km size differing in crustal and mantle structure could be often found within large cratons. Geodynamic mechanisms of cratons formation remain debatable and combine both plate-tectonics-related and plume-related processes. Based on recent numerical experiments, this research will propose a new concept of Archean cratonisation intrinsically related to the operation of plume-lid (squishi-lid) tectonics by which proto-continental and proto-oceanic lithospheric domains spontaneously formed before the onset of global plate tectonics. In contrast to present day, hot felsic proto-continental domains had thinner, more deformable and less depleted mantle lithosphere compared to their cold mafic proto-oceanic counterparts formed by ultraslow spreading atop hot mantle upwellings. Numerical models show feasibility of short-lived deep subduction of the depleted proto-oceanic lithosphere to core-mantle boundary driven by eclogitisation of the thick mafic crust. Subsequent heating and buoyancy-driven separation of the eclogite and harzburgite triggered formation of strongly depleted harzburgite plumes. Rising and accretion of these chemically buoyant refractory plumes to the bottom of proto-continental domains created relatively small (hundreds km) proto-cratons. After the onset of global plate tectonics by plume-induced subduction initiation, assembling of smaller proto-cratonic terrains formed actual large cratons.