Disorientation control on trace element segregation in fluid-affected low-angle boundaries in olivine

Tacchetto Tommaso1,2, Reddy Steven1,2, Saxey David2, Fougerouse Denis 1,2, Rickard William2, Clark Chris1

1School of Earth and Planetary Sciences, Curtin University, Perth, Australia, 2Geoscience Atom Probe, Curtin University, Perth, Australia

The interfaces between minerals (grain boundaries s.l.) play a critical role in controlling the rheological behaviour of rocks and the ability of fluids to penetrate them in the deep crust. However, the role of chemical segregation in controlling the behaviour of mineral interfaces remains largely unexplored. In this work, we combined electron backscattered diffraction (EBSD) and atom probe tomography (APT) to assess the relationship between deformation-related low-angle boundaries in naturally-deformed olivine and the degree of trace element segregation to those boundaries. The sample studied comes from the Bergen Arcs (Norway), where high-grade, dry, metamorphic rocks of the lower crust have been overprinted by fluid-present high-pressure metamorphism during the Caledonian tectonic subduction between 430 and 410 Ma. EBSD orientation mapping of deformed olivine is used to characterise the slip systems associated with deformation and the misorientation relationships within different parts of the microstructure. APT has then been used to systematically target grain boundaries of different misorientation angle (up to 8°). 

The analysed boundaries formed by sub-grain rotation recrystallisation associated with {100}<001> slip system developed during the fluid-catalysed metamorphism. APT data show that olivine trace elements segregated to the low-angle boundaries during this process. Boundaries with < 2° degrees show marked enrichment associated with the presence of multiple, non-parallel dislocation types. However, at increased misorientation (> 2°), the interface becomes more ordered with dislocation geometries defined by linear concentrations of trace elements, and which are consistent with the EBSD data. These boundaries show a systematic correlation of increasing trace element segregation with misorientation angle. In particular, elements are generally more enriched at higher degrees of distortion, where variations are mostly significant for Ca (from 0.07 up to 0.6 at%) and Cl (up to 0.3 at%). Elements that are segregated to the low-angle boundaries (Ca, Al, Ti, P, Mn, Fe, Na, Mg and Co) are here interpreted to be captured and accumulated by dislocations as they migrate to the sub-grain boundary interfaces. However, the exotic trace elements Cl and H, also enriched in the low-angle boundaries, likely reflect a small but significant contribution of an external fluid source during the fluid-related deformation. In particular, since the occurrence of H in olivine is strongly attributed to Ti defects, the segregation of Ti to grain boundaries is consistent with the detected enrichment of hydrogen in the low-angle interfaces.

The observed compositional segregation of trace elements to low-angle boundaries have significant implications for the deformation and transformation of olivine at mantle depth, the interpretation of geophysical data and the redistribution of elements deep in the Earth. Furthermore, the nanoscale correlation between heterogeneous distribution of elements like Ti and the diffusion of H along boundary interfaces within olivine might have the potential to yield important implication for the understanding of the hydrogen distribution in the upper mantle and its consequences for the rheological properties of mantle-rocks during deformation.


Biography

Tommaso completed his MSc in Geology at the Univesity of Padova (Italy) in 2017 and awarded in 2017 of a Temporary Research Fellowship. He is now completing his 3rd year of PhD at Curtin University focused on the investigation of metamorphic processes in the precence of fluids at the nanoscale.

Lapstone Structural Complex and uplift of the Blue Mountains, tectonic backdrop to western Sydney, Australia

Fergusson, Chris1 and Hatherly, Peter2

1School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, Australia, 2Lavender Bay, Sydney, Australia

The eastern margin of the Blue Mountains uplift is well defined by the Lapstone Structural Complex (LSC) with elevations typically of 150 to 200 m but locally up to 600 m at Kurrajong Heights. While it has been suggested that the LSC is related to normal faulting associated with passive margin development, this suggestion is inconsistent with a west-dipping reverse fault with minimal fault damage development (the Bargo Fault) we have found in the southern part of the LSC. We prefer the alternative view that the LSC consists of east-facing monoclines and reverse faults that dip both east and west. The underlying controlling structure could be a west-dipping thrust and the surface expression is due to the formation of a triangle zone. Further support for our interpretation is found in the northern part of the LSC where there are gravels (Rickabys Creek Gravel) and clay (Londonderry Clay) which are draped along the front of the monocline. Their position indicates that they were folded during development of the LSC. Thus the gravels do not occur in a series of terraces as was previously considered. Unfortunately no direct age of the Rickabys Creek Gravel and the Londonderry Clay has been determined but their unconsolidated state is consistent with a Neogene age thereby constraining formation of the LSC to the Neogene. Ongoing seismic activity associated with the LSC indicates neotectonic activity along the structure. The higher parts of the Blue Mountains are at elevations of over 1000 m and overall rock units of the Sydney Basin slope to the east at an angle of ca 1.5°. We consider that the latest phase of uplift in the Blue Mountains was associated with formation of the LSC and other structures including the Mt Tomah Monocline. Given earlier phases of uplift associated with formation of the eastern highlands it is problematic to identify the limits of this younger phase of Blue Mountains uplift particularly in the west where it apparently merges with the Central Tablelands. The enduring debate about the cause of the uplift of the Great Dividing Range of eastern Australia has become increasingly centred on the importance of mantle upwelling. We consider that the younger phase of uplift of the Blue Mountains associated with formation of the LSC was related to Australia’s setting west of the Southwest Pacific convergent margin rather than mantle upwelling.


Biography

Peter Hatherly is a retired geophysicist who has taken an interest in the structural and geomorphological evolution of the Blue Mountains. He has published relevant papers on the results of high resolution seismic reflection surveys, analysis of longitudinal stream profiles and mapping of semi-consolidated gravels above river systems.

Provenance of Late Cambrian-Ordovician sedimentary rocks in western, north-eastern Tasmania and southern Victoria: Constraints from U/Pb dating, zircon geochemistry and εHf isotope

Habib, Umer1, Meffre, Sebastien1, Kultaksayos, Sitthinon1, Berry, Ron1

1Centre of Ore-Deposit Geology and Earth Sciences, University of Tasmania, Hobart, Australia

Email address. umer.habib@utas.edu.au

The sedimentary sequences in western, north-eastern Tasmania and southern Victoria preserves an excellent record of distinctive Cambrian to Ordovician deposition environments and sediment provenance.  Here we integrate new and already published U/Pb detrital age data, field observations, zircon geochemistry, and Hf isotope data to establish the sediment source and tectonostratigraphic history of these rocks. Overall, the detrital age spans from 2.6 Ga to 0.476 Ga, which includes some major Precambrian and Cambrian age peaks. The 1.8-1.2 Ga peaks for Western Tasmania are consistent with derivation from the Tyenna region which incorporate sediments derived from granitoids in Laurentia (North America) and Baltica. A small population of detrital ages from 1.5-1.1 Ga from north-eastern Tasmania and southern Victoria is suggested to have Grenville provenance, possibly from central Australia or other parts of the Grenvillian Orogen. The 850-545 Ma detrital ages occur in all sedimentary rocks and are from the widely represented Pacific-Gondwana Phanerozoic sediments of south eastern Australia. These 850-545 Ma detrital ages are rare in the Owen Group, relative to the overlying Middle-Late Ordovician Pioneer Sandstone, implying a sharp shift in provenance in western Tasmania in the Early Ordovician. The Pioneer Sandstone is very similar to Early Ordovician sandstones from Waratah Bay in southern Victoria and Ordovician sandstone from eastern Tasmania. Also present in the samples are 480-520 Ma zircons. However, the proportions of these in the sedimentary rocks are variable. The Th/U ratios from the Cambrian zircons in western Tasmania support a proximal source from the Mt Read Volcanics. The Cambrian zircons in eastern Tasmania and southern Victoria have much lower Th/U ratios, implying a different provenance. The εHf isotope signatures and statistical analysis, along with sedimentological and paleocurrent data from previous studies, support a local derivation from Precambrian and Cambrian detrital sources for the western Tasmanian rock units, indicating deposition in a segmented basin-margin fault system. The eastern Tasmanian and southern Victorian sandstone were deposited in a basin offshore from the Tyennan-Delamerian Orogen during the Ordovician. 

Keywords. Western Tasmania, U-Pb dating, Tectonic configuration, Palaeozoic


Biography

Umer has done his Honors and masters degree in geology from Pakistan and worked for MOL from 2013-2015. He joined Codes in 2018 to persue his PhD.

Recent progress in laboratory studies of seismic wave attenuation relevant to the Earth’s upper mantle

Qu, Tongzhang1, Jackson, Ian1, David, Emmanuel C.1,2, Faul, Ulrich H.1,3

1Research School of Earth Sciences, Australian National University, Canberra, ACT, Australia, 2Now at Department of Earth Sciences, University College London, London, United Kingdom, 3Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

The Earth’s upper mantle is being imaged with increasing fidelity with the various methods of modern seismology. However, such images are of limited value without laboratory-based models for their robust interpretation in terms of the many factors (e.g., chemical composition, temperature, and partial melting) that influence the seismic properties of mantle materials. Over decades, we have developed a unique capability for measurement of seismic properties of rock cylinders by forced oscillation at seismic periods (1-1000 s) rather than the much shorter periods (ns-ms) of laboratory wave-propagation methods. Sustained application of these methods has documented grain-size sensitive viscoelastic relaxation, responsible for frequency dependence (dispersion) of the shear modulus and hence wavespeed and related strain-energy dissipation in polycrystalline olivine, as well as the influence of partial melting and dislocation density, and more recently, of oxidizing/hydrous conditions.

Here we report new measurements on olivine-orthopyroxene mixtures. For temperatures reaching 1200°C and seismic periods, the strain-energy dissipation and shear modulus dispersion are generally similar to those for essentially pure olivine, but somewhat diminished and slightly more temperature-sensitive with increasing orthopyroxene concentration from 5% to 95%. The viscoelastic behaviour is of the high-temperature-background type without any evidence of the superimposed dissipation peak reported by others for a melt-bearing specimen of otherwise similar composition. It is concluded that our olivine-based model for seismic wave dispersion and attenuation will require only modest modification to accommodate the role of orthopyroxene.

In order to further refine our methodology, we are also addressing the effect of uncertainty in the mechanical behaviour of the enclosing mild-steel jacket arising from the transition between the austenite and ferrite phases on cooling across the interval 900-700ºC. Variations of its microstructure and hence viscoelastic behaviour have the potential to mask the seismologically important onset, within this temperature range, of appreciably viscoelastic behaviour of upper-mantle materials. Accordingly, we have previously conducted a study in which a specimen of polycrystalline olivine is jacketed instead within a copper sleeve which retains its face-centred-cubic (fcc) structure throughout the range of the measurements – limited to 1050ºC maximum by the proximity of the melting point. Here we report new measurements to higher temperature (1200ºC) in which we employed austenitic (fcc) stainless steel (SS) as an alternative jacket material. Prior measurement of the mechanical properties of the SS jacket material allowed subtraction of its contribution to the torsional stiffness of the SS-jacketed specimen. The resulting dissipation spectrum for the olivine specimen consists of a monotonic background dissipation with a superimposed peak located within the 1-1000 s period range for temperatures between 900 and 1050ºC. The dissipation peak and associated shear modulus dispersion, potentially attributable to elastically accommodated grain-boundary sliding, display an Arrhenius dependence upon temperature – moving systematically to shorter periods with increasing temperature. Comparison of the results obtained for synthetic Fo90 olivine specimens enclosed within the alternative mild-steel, copper, and stainless-steel jackets is providing new insight into the nature of the seismologically important transition between the elastic and anelastic regimes.


Biography

Tongzhang Qu is currently a PhD student in Rock Physics at ANU, following undergraduate studies at Jilin and Pennsylvania State Universities.

Next-generation model of the Australian crust from synchronous and asynchronous ambient noise imaging

Chen, Yunfeng1,2, Saygin, Erdinc1

1Deep Earth Imaging, Future Science Platform, CSIRO, Perth, Australia, 2Department of Physics, University of Alberta, Edmonton, Canada

The proliferation of seismic networks in Australia has laid the groundwork for improved probing of the continental crust.  Despite ever-growing seismic instrumentation across the country, the last major effort of mapping continental-scale structures with seismic ambient noise was conducted more than a decade ago, thereby demanding a new appraisal of its crustal structure.  In this study, we develop a new crustal model of the Australian continent using a large dataset that consists of nearly 30 years (1992-2019) of continuous seismic recordings from over 2200 stations.  This unprecedented dataset is further exploited with the recently developed ambient noise imaging workflow of Chen & Saygin (2020) that integrates results from temporary seismic arrays deployed at different times. We compute two sets of noise correlation functions (NCFs) between 1) synchronous stations with the conventional ambient noise correlation (i.e., C1) and 2) asynchronous pairs with the high-order correlation technique (i.e., C2) based on correlational and convolutional types of source-receiver interferometry.  The C2 approach enables extracting 1-3 times more NCFs than available from using C1 alone, and the combined dataset leads to over 200,000 high-quality NCFs to image the crustal structures, significantly improved upon the most recent model constructed from 7500 measurements.  We invert the Rayleigh wave arrival times for group velocities between 4-40 sec using a trans-dimensional inversion.  This non-linear inversion approach adopts an adaptive parameterization and Bayesian inference to account for unbalanced data sampling and allows assessment of the model uncertainties.   The final 3D shear velocity model reveals fine-scale structure in the Australian crust.  The low velocities at shallow depths (<10 km) are in excellent agreement with the distribution of known sedimentary basins and also hint at the presence of unreported basins/sub-basins in previously poorly explored areas, for example, northern Australia.  At lower crustal depths (30-40 km), our model delineates the boundaries of major Archean blocks such as the Yilgarn and Pilbara cratons in Western Australia.  High-velocity structures also characterize the lower crust/uppermost mantle of the Phanerozoic New England Orogen near the eastern continent margin.  In conclusion, this study provides significantly improved constraints on the shear velocity structures and builds a new basis for the next-generation crustal model of the Australian continent.


Biography

Dr. Chen is a visiting scientist at the Deep Earth Imaging group of CSIRO.  He is also a postdoc researcher at the Global Seismology Group at the University of Alberta.

New insight into the Carpentaria Conductivity Anomaly from high-resolution MT data in the Cloncurry region

Simpson, J.M.1,  Brown, D.D.1, Duan, J.2 and Kyi, D.2.  

1 Geological Survey of Queensland, Brisbane, Australia, 2 Geoscience Australia, Canberra, Australia

The electrical structure of Queensland is dominated by the Carpentaria Conductivity Anomaly (CCA). It stretches hundreds of kilometres from the Gulf of Carpentaria in the north, possibly extending as far south as Birdsville. It is present in the crust and extends down into the mantle. The CCA is of great interest as it underlies significant mineral deposits in the Eastern Succession of Mount Isa such as Ernest Henry. Modelling of existing broadband and long period MT data has been used to suggest the CCA may be associated with collisional tectonics along the Gidyea Suture during the Proterozoic.

A new MT survey in the Cloncurry region offers insight into the crustal portion of this anomaly. The survey offers over 500 new MT sites at 2 km station spacing and was collected in 2020. Conductive anomalies are imaged by the new data which are associated with both the Mount Margaret and the Quamby/Fountain Rage Faults. The conductive response occurs from approximately 2 km depth and extends into the deep crust. These are both major structures that have accommodated significant crustal movements throughout the complex history of Mount Isa. Mount Margaret Fault is related to the Gidyea Suture, while different interpretations of deep crustal seismic data in the area suggests that the Quamby/Fountain Range Fault extends either into the mid-crust or is full crustal thickness structure.

The presence of conductive anomalies along such major structures suggests that they could be functioning as more localised fluid pathways from the deeper part of the conductive anomaly. The presence of conductive anomalies along both these structures, rather than just along the Gidyea Suture associated Mount Margaret Fault suggests that the CCA is not simply a suture related feature.


Biography

Janelle is a senior geophysicist with the Geological Survey of Queensland. She is passionate about the integration of geology and geophysics to produce more meaningful interpretations.

Geoelectric Structure of Tasmania from Multi-Scale Magnetotelluric Data

Ostersen, Thomas1,2, Reading, Anya2, Cracknell, Matthew2, Roach, Michael2, McNeill, Andrew3, Duffett, Mark3, Bombardieri, Daniel3, Thiel, Stephan4, Robertson, Kate4, Duan, Jingming5, Heinson, Graham6

1Solve Geosolutions, Hobart, Australia, 2University of Tasmania, Hobart, Australia, 3Mineral Resources Tasmania, Hobart, Australia, 4Geological Survey of South Australia, Adelaide, Australia, 5Geoscience Australia, Canberra, Australia, 6University of Adelaide, Adelaide, Australia

The current understanding of Tasmania’s enigmatic tectonic history has been informed by geological information observed or sampled at the Earth’s surface coupled with geophysical data sets sensitive to magnetic, density and seismic properties of the rocks forming the crust and mantle beneath. With the completion of the Tasmanian portion of the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP), new 3D and 2D geophysical models describing the electrical properties of the Tasmanian lithosphere at different spatial scales have been derived to compliment these data.

At the whole-of-state scale, 3D inverse models of the long period magnetotelluric (MT) data have illuminated the electrical structure of the mid-crustal to lithospheric mantle depths. This model images the full extent of the Tamar Conductivity Anomaly, a crustal-scale conductor extending from northern to southern Tasmania along the boundary between eastern and western Tasmanian geologic terrains.

In the west of the state, a 2D inverse model transecting the Cambrian Mount Reid Volcanics brings the electrical structure of the upper- to mid-crustal depth range in this economically important part of the state into sharper focus. The model images west-dipping conductive structures spatially coincident with major faults and associated copper mineralisation near Queenstown.

Finally, in the central east of Tasmania, a joint inversion incorporating legacy broadband MT data with newer AusLAMP MT data using the whole-of-state scale model as a priori geoelectric structure was conducted. Inversion results demonstrate a potential use case for regional scale AusLAMP models to improve higher resolution geoelectric structure modelling, with the joint inverse model imaging the Lemont geothermal field at higher resolution while simultaneously mapping geologically feasible 3D resistivity structures.


Biography

Thomas is a geophysicist and PhD candidate at the University of Tasmania studying the geoelectric structure of the Tasmanian lithosphere. He is now applying his geophysical and scientific programming skills to a consultant role with Solve Geosolutions.

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.