Department of Earth and Planetary Sciences Seminars
The Department of Earth and Planetary Sciences holds research seminars on a regular basis - please see below for the upcoming Department seminars. Staff and students are welcome to attend.
|9th of November 2018|
E7B (14SCO) 263 Tute Rm
|Trevor Ireland, Research School of Earth Sciences, Australian National University||Hayabusa2 mission at asteroid Ryugu|
The Hayabusa2 spacecraft is currently sitting above asteroid Ryugu. Hayabusa2 is a Japanese Aerospace Exploration Agency (JAXA) mission to sample Ryugu and bring a sample home. This is the second such sample-return mission run by JAXA. Hayabusa1 visited the S-class asteroid Itokawa and brought home stony fragments that can be related directly to LL type meteorites. Ryugu is a C-class asteroid and may be related to carbonaceous chondrites. These meteorites contain abundant organic compounds and potentially brought those materials into Earth as the seeds of life, and are also water-rich. Understanding the nature of the C-type asteroids is thus very important for understanding volatiles on Earth and how the Earth developed into a habitable, and inhabited, environment.
Will Earth-crossing asteroids such as these pose risks to the inhabitants of Earth? Well, they certainly have in the past. A key element in mitigating this risk is to understand what these asteroids are made of, as well as their internal structure. The best way of getting our hands on this sort of data is going there and bringing samples back.
|2nd of November 2018|
E7B (14SCO) 263 Tute Rm
|Vasileios Chatzaras, School of Geosciences, University of Sydney||Rheological structure of plate boundary strike-slip faults: the role of mantle-crust feedbacks||This talk will address how mantle-crust interactions may affect the rheology of the deep sections of lithospheric-scale, strike-slip faults, and will present microstructures that provide a potential new indicator of earthquake-related deformation at upper mantle conditions. I will present geological data from three field areas: 1) the San Andreas fault system in the US, 2) the Baja California shear zone in Mexico, and 3) the Bogota Peninsula shear zone in New Caledonia. Analysis of the upper mantle xenoliths from the San Andreas fault, and the lower crust and upper mantle xenoliths from the Baja California shear zone indicates that stress remains constant, and low, with depth. This result is not consistent with the typical lithospheric strength profiles constructed from deformation experiments. Data from the Bogota Peninsula shear zone, which is the exhumed mantle section of an oceanic transform zone, indicate spatial and temporal variations in stress across the strike-slip shear zone. These variations are caused predominantly by imposed localization, rather than intrinsic localization produced by strain weakening. Imposed localization is induced by the mechanical interaction between the upper, “brittle” part of the oceanic lithosphere, and the underlying upper mantle, during earthquake rupture in the transform fault. The results from the three study areas combined, indicate that crust and lithospheric mantle act together as an integrated system. Furthermore, allow us to build a picture of earthquake-related deformation in the upper mantle during the seismic cycle.|
|12th of October 2018|
E7B (14SCO) 263 Tute Rm
School of Earth Sciences,
University of Melbourne
|Geometric controls on slab deformation and expression in seismicity||The relationship between slab seismicity and subduction dynamics is a long-standing area of enquiry. The influential study of Isacks and Molnar (1971) emphasised modes of longitudinal deformation relating to competition between slab pull and stress transmission from the lower mantle. However, nearly 50 years of additional observations have revealed significant complexity, particularly at intermediate depths. Double seismic zones (DSZ) have been revealed in many locations, often consisting of extensional (lower) and compressional (upper) bands consistent with bending. However, disentangling the seismic signature of bending from other modes of deformation, as well as the strong influence of metamorphic fluid release, has proven difficult. This seminar presents new insights into these processes, focussing particularly on flat slab segments. Flat slabs reveal the strong control of slab geometry on the deformation rate, specifically through the spatial curvature gradient of the slab. The curvature gradient controls the bending rate as material is advected through the slab profile. In many locations, seismic moment rate as well as the polarity of the DSZ correlate with this simple geometric measure. In younger slabs, bending does not necessarily result in a DSZ, as only the upper half of the bending slab is seismogenic. That bending can explain a diverse range of slab seismicity patterns results from the strong sensitivity of viscous (inelastic) flexure to the slab geometry, also to the asymmetry provided by slab temperature.|
|5th of October 2018|
Hannes Brueckner, Lamont-Doherty Earth Observatory of Columbia University
|Slabs, mélanges, subduction channels? What’s going on in subduction zones where eclogites are generated and serpentinites collected? The view from the Guatemala Suture Zone.|
The eclogite-facies metamorphism of continental crust appears to be a relatively straightforward process of subduction into the mantle as a coherent slab followed by the exhumation of this slab towards the surface. The eclogitization of ocean crust in an oceanic subduction system is more controversial with some models favouring the coherent in and out process similar to continental collision while other models propose an incoherent churning in a “subduction channel” where individual units follow different P-T paths at different times. Our work in the Guatemala Suture Zone (GSZ), at the western end of the boundary between the North American and Caribbean plates, resulted in evidence for both processes.
The GSZ exposes a metamorphic complex comprised of a serpentinite-matrix mélange with tectonic blocks of high-pressure–low-temperature eclogites as well as blueschists, amphibolites and vein-related rocks (including jadeitites). Sm–Nd ages from eclogites range from 160 to 114 Ma suggesting a prolonged evolution under high P-T conditions. U-Pb zircon ages from jadeitites suggest a similar spread. But phengite Ar-Ar ages give consistent ages of ≈85 Ma suggesting a short-lived tectonic event at lower temperatures. Different mélange units record different P-T paths. These complex P-T-t paths indicate discreet, multiple or continuous tectonic events for each rock type rather than a simple, common subduction/exhumation/obduction history for all rock types. The spread in eclogite ages is consistent with continuous subduction along a Pacific margin lasting over at least 45 million years in a subduction channel with units undergoing complex, internal P-T loops. However this interpretation is not held by all of us. The consistent 85 Ma Ar-Ar ages suggest a more coherent history, at least during the final stages of the GSZ.
|14th of September|
|Gang Sha, |
Nanjing University of Science and Technology
|Atom probe tomography and its application in geoscience||Atom probe tomography (APT), as an emergent characterization technique, is capable of determining the chemical identity of each individual atom and generating 3D chemical maps to reveal the distribution of individual atoms. Its high spatial resolution (better than 0.2 nm in z direction) and high analytical sensitivity (as good as 10 appm) make it powerful in providing elemental composition of a specimen, 3D distribution of atoms, composition of phases, morphology and size of precipitates, and solute distribution across interfaces, at grain boundaries and along dislocations. In combination with laser pulsing controlled evaporation technique, APT is able to analyse not only conductive metals but also materials with poor conductivities such as semiconductors and geomaterials. Recently, the technique has been successfully applied to gain information of geomaterials. The new information unveiled by APT is transforming our fundamental understanding about geomaterials. In this talk, I will provide basic information about the principle of this ananlysis technique, and address major issues in sample preparation, data collection and 3D reconstruction of APT dataset. I will review recent progress in application of the technique in geomaterials, and discuss fundamental scientific understanding which we can gain with the information unveilled by APT.|
|7th of September 2018|
|Nathan Daczko||The recognition of melt pathways in the crust & A cryptic Gondwana-forming orogen located in Antarctica|
Pt 1: The recognition of melt pathways in the crust: A view from the base of a magmatic arc
The production of continental crust in magmatic arcs is an integral part of plate tectonics and involves the transfer of melt through the lower crust to mid and upper crustal levels. I summarize the different modes of melt transfer recognised in the lower crustal sections of the well-exposed Mesozoic magmatic arc of Fiordland, New Zealand, involving: (1) diffuse and channelized porous melt flow under conditions of low differential stress, (2) syntectonic, channelized porous melt flow and (3) brittle failure allowing melt transfer via dyking. Each mechanism has distinct field, microstructural and geochemical signatures that can be used to identify them. At the same time these signatures inform about the details of the processes involved. Common to all three mechanisms is the inference that the system is open and that the migrating melt is externally derived. Hence, it is likely to be in chemical disequilibrium with the host rocks through which it migrates. The chemical potential drives melt-rock reaction and the development of complex microstructures and rock textures. Analogous to aqueous fluid-rock interaction, features typical of reactive transport are common and include reaction fronts, finger structures and rapid replacement of the host assemblage by a distinct, high variance assemblage by dissolution and precipitation.
Pt 2: A cryptic Gondwana-forming orogen located in Antarctica
The most poorly exposed and least understood Gondwana-forming orogen lies largely hidden beneath ice in East Antarctica. Called the Kuunga orogen, its interpolation between scattered outcrops is speculative with differing and often contradictory trends proposed, and no consensus on the location of any sutures. While some discount a suture altogether, paleomagnetic data from Indo-Antarctica and Australo-Antarctica do require 3000–5000 km relative displacement during Ediacaran-Cambrian Gondwana amalgamation, suggesting that the Kuunga orogen sutured provinces of broadly Indian versus Australian affinity. Here I use compiled data from detrital zircons offshore of East Antarctica that fingerprint two coastal subglacial basement provinces between 60 and 130°E, one of Indian affinity with dominant ca. 980–900 Ma ages (Indo-Antarctica) and one of Australian affinity with dominant ca. 1190–1140 and ca. 1560 Ma ages (Australo-Antarctica). This offshore compilation is combined with existing and new onshore U-Pb geochronology and previous geophysical interpretations to delimit the Indo-Australo-Antarctic boundary at a prominent geophysical lineament which intersects the coast east of Mirny at ~94°E.
|31st of August 2018|
14SCO 100 Theatrette
|John Adams||Intraplate magmatism: its origins and place in global evolution, a comparative and phase equilibrium approach||Although volumetrically less significant than the volcanism produced by mid-ocean ridges and volcanic arcs, intraplate volcanism (of OIB type) is the most ubiquitous and compositionally diverse form of volcanism on Earth. In spite of this, neither its physical distribution nor compositional variation are random. Both can be related to systematic controls by phase equilibria and to generally prevalent mantle conditions and processes. When compared to more voluminous magma types, intraplate magmas record a wide range of conditions (particularly depths) for both their mantle production and subsequent evolution. Compositionally, they can be linked both to the peridotitic MORB source (via incompatible element fractionation during melting) and to recycled crustal components. These features, combined with the spatial and temporal distribution of intraplate magmatism, require a prevalent mantle that is relatively hot, as well as fertile, when compared to some estimates for the MORB source (contrasting with the dominant role sometimes suggested for thermal plumes). They also imply a mantle that is constantly balancing internal fractionation (that creates both incompatible element enriched and depleted mantle domains) with re-homogenisation during convection. The former process can be linked to the self-regulation of mantle volatile concentrations (via their influence on solidus temperatures) and is difficult to reconcile with the very high H2O concentrations that have been suggested on the basis of the deep mantle’s capacity to store H2O.|
8th of June 2018
|Juraj Farkas||Stable and Radiogenic Alkali/Alkaline Earth Metal Isotopes: Applications to Earth System Studies and Geochronology|
Alkaline earth metals, such as Mg, Ca and Sr, are major components of many geological and biological systems, and their biogeochemical cycles are closely linked to the global C cycle through the processes of silicate/carbonate weathering, marine carbonate formation, and/or seawater-basalt interactions at the mid-ocean ridges. These large-scale processes thus control the elemental and isotope budgets of alkaline earth metals in the oceans, and their past changes will be reflected in Mg, Ca, and Sr isotope records of seawater over geological time.
The stable and radiogenic isotope proxies of selected alkaline earth metals (d26Mg, d44Ca, d88Sr and 87Sr/86Sr), applied to marine carbonate archives, can be thus used to reconstruct the isotope composition of paleo-seawater over Phanerozoic and Neoproterozoic time scales, with implications for the Earth's system evolution. Specifically, stable Mg isotopes are used here to better constrain the past oceanic Mg fluxes, and plausible driving mechanism(s) behind the temporal changes in marine Mg/Ca ratios over Phanerozoic. A novel approach using both stable and radiogenic Sr isotopes (d88Sr, and 87Sr/86Sr) in carbonates will be applied to Neoproterozoic to infer past changes in Earth’s surface processes, i.e., carbonate weathering versus burial fluxes, during one of the most extreme environmental changes recorded on our planet.
Finally, new applications of in-situ dating and geochronology based on alkali/alkaline earth metal isotope systems, such as K/Ca and Rb/Sr radioactive decay pairs, will be illustrated on examples relevant to dating of sediments and low-temperature earth’s surface processes. These will include recent data on in-situ Rb/Sr dating (LA-QQQ) of glauconites and bulk shales.
28th of May 2018
|Zsannet Pinter||The compositions of melts in the incipient melting region|
The composition of mantle-derived magmas suggest a remarkable variety in the abundances of volatiles in the upper mantle. Volatile components like H2O and CO2, generally depress the melting point of mantle considerably (Green, 2015). However, we have little knowledge about these first, incipient melts. We know incipient melts exist in a large temperature range (~300°C) in the upper mantle, but the chemical compositions of these melts are poorly constrained, and therefore the effect of volatiles on the various melt proportions could change the behaviour of melt significantly (Foley et al., 2011).
Nature provides us with limited samples of primitive mantle-derived melts, which have mostly suffered fractionation or weathering processes. Therefore it is necessary to simplify the picture for studying the primitive melts. Experimental petrology provides better insights into the incipient melting regime in mantle conditions. This project consists of a systematic study to determine the chemistry of incipient melts using different starting compositions (Green, 2015) with various volatile compositions. We are considering the effects of temperature (provide T range), and pressure (provide P range) in the incipient melt regime using piston cylinder apparatus. Our results show that melt compositions progress from carbonate-rich to carbonated silicate and are characterised by strong increase in SiO2 (2.75 to 39 wt%), as TiO2, Na2O and K2O decrease with increasing temperature. However, MgO shows little change at given pressures.
The project is a collaboration between Macquarie University (Sydney) and ANU RSES experimental group.References:
Foley, S.F. (2011) A reappraisal of redox melting in the Earth’s Mantle as a function of tectonic setting and time. Journal of Petrology 52:1363-1391.
Green, D.H. (2015) Experimental petrology of peridotites, including effects of water and carbon on melting in the Earth´s upper mantle. Physics and Chemistry of Minerals 42:95-122.