Wed 19th July @ noon, Rm 312.222

Abstract:

During hyper-velocity meteorite impacts, minerals may deform in unique manners, known as shock metamorphism. The two most commonly used U – Pb geochronometers, the accessory minerals zircon and monazite, respond in distinct ways to shock, including developing mechanical twins, various crystal-plastic deformation features and in the case of zircon may undergo a solid state transformation to high pressure polymorph reidite. Shock deformation can affect the U – Pb systematics of zircon and monazite to varying degrees. This presentation will focus on a coupled microstructural and geochronological study of shocked zircon and monazite grains from the 2020 Ma Vredefort Dome in South Africa, the 259 Ma Araguainha impact structure in Brazil and the nominally Proterozoic Yarrabubba impact structure in Western Australia.

Results of this study help to quantify diagnostic shock features in zircon and monazite and constrain impact-age resetting in various microstructural domains. Furthermore, these results highlight the importance of combing microstructural studies (EBSD) with in situ analyses (e.g. SHRIMP, LA-ICP-MS) of shocked zircon and monazite to best determine the timing of shock deformation. Finally, these results demonstrate the efficacy of monazite to date impact events, especially from ancient structures within terranes which have subsequently experienced complicated and protracted geologic histories or are deeply eroded.

Short bio:

Timmons recently completed his PhD at Curtin in early 2017. Since that time he has worked as a research associate in the Department of Applied Geology studying rocks from Western Australia to Norway. Prior to moving to Western Australia he completed an MSc at the University of Puerto Rico Mayagüez, which sparked his initial interest in shock deformation of accessory phases and the geologic record of impact cratering.

Wed 12th July @ noon, Rm 312.222

Abstract:

Cristobalite is a low-P, high-T polymorph of crystalline silica (SiO₂). Despite having a nominal stability field far outside the conditions that exist in crustal magma bodies, it forms and persists as metastable vapour-phase crystals in the pores of many volcanic rocks. Interest in volcanic cristobalite has been driven by: concerns that volcanic cristobalite may be a respiratory hazard, the possibility that it may exacerbate lava dome instability, and the fact that its persistence challenges ideas of equilibrium phase stability from geochemical and crystallographic perspectives.

We examine the occurrence, morphology, chemistry, and structure of vapour phase cristobalite. In an effort to unravel its formation mechanisms, significance, and possible utility, we have employed a large range of techniques, ranging from fieldwork to Atom Probe Tomography. We find that volcanic cristobalite is far more common than previously recognized. Textural (µ-cT) evidence points to local (e.g., individual pore scale) SiO₂ redistribution, whereby acidic volcanic gases corrode magmatic melt and redistribute its components (mainly SiO₂) as cristobalite. Coupled microchemical and microstructural information indicate temporal purification during the vapour deposition process, with progressively lower levels of impurities (mainly Al³⁺ and Na⁺), corresponding to progressively greater structural order.

We discuss the concept that volcanic cristobalite represents a natural proxy for the same type of epitaxial chemical vapour deposition used in thin film manufacturing processes. We argue that its link to particular volcanic degassing regimes and its high preservation potential make cristobalite a useful, durable, and underappreciated indicator of crucial magmatic processes.

Short Bio:

Ian Schipper is a lecturer in igneous processes at Victoria University of Wellington. His background is in submarine explosive volcanism and the analysis of textures in volcanic rocks. In recent years, his enthusiasm for all things volcanic have (overly?) broadened his interests to include all stages of magmatic evolution, degassing and eruption. His continual search for adventurous fieldwork has taken him from the bottom of the Pacific Ocean to the tops of the Andes.

Wed 5th July @ noon, Rm 312.222

Abstract:

In various shocked meteorites, low-pressure silica polymorph α-cristobalite is commonly found in close spatial relation with the densest known SiO₂ polymorph seifertite, which is stable above ~80 GPa. We demonstrate that under hydrostatic pressure a-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 11GPa cristobalite X-I forms—a monoclinic polymorph built out of silicon octahedra; the phase is not quenchable and back-transforms to α-cristobalite on decompression. There are no other known silica polymorphs, which transform to an octahedra-based structure at such low pressures upon compression at room temperature. Further compression in non-hydrostatic conditions of cristobalite X-I eventually leads to the formation of quenchable seifertite-like phase. Our results demonstrate that the presence of α-cristobalite in shocked meteorites or rocks does not exclude that materials experienced high pressure, nor is the presence of seifertite necessarily indicative of extremely high peak shock pressures.

Short bio:

I am currently studying rocks from the Moon! My project RESOLVE (Reconstructing the History of Lunar Volatiles) was funded by the European Commission in the H2020-MSCA-IF-2015 call.

It aims at understanding how the abundance and the distribution of volatile components, as well as their isotopic composition are influenced by the crystal structure of the host mineral in lunar samples. The volatile containing minerals – primarily phosphates – respond to pressure increase caused by the impact events by accommodating i.e. compressing their crystal structures, causing redistribution of the volatile, but also of the stable (e.g., H, Cl) and radiogenic isotopes (e.g., Pb). Therefore they not only provide a unique opportunity for precise age-determination of an impact event, but also simultaneous determination of the isotopic signature of the volatiles, a fingerprint to the source of the volatiles. This is a powerful approach for discriminating between indigenous and externally-derived volatile sources (solar wind, cosmic radiation, etc.), which is one of the remaining puzzles in studying the origin of water in the inner Solar System.

Wed 21st June @ noon, Rm 312.222

Abstract:

To develop a comprehensive understanding of geological processes a multidisciplinary approach much be utilized. To provide insights on the origin and duration models of LIPs generation, this research focus’ on three Continental Flood Basalts (CFB) located throughout the Southern Hemisphere; the ca. 511 Ma Kalkarindji CFB Province located in the northwest of Australia; the Karoo CFB Province located in southern Africa and parts of Antarctica; and the Tasmanian Dolerite portion of the Ferrar CFB Province outcropped in Antarctica and Tasmania.

An extensive geochemical database of the Kalkarindji igneous constituents display one of the most geochemically homogeneous CFBs currently known. Interpretations from the geochemical data preclude the involvement of the assimilation of crustal material during the emplacement of the Kalkarindji magmas. Instead the geochemistry indicate the enriched characteristics are native to the mantle source. Calculations utilizing the high 207Pb/204Pb and elevated 208Pb/204Pb for moderate 206Pb/204Pb conclude the enriched geochemical characteristics can be reproduced from an ancient enrichment to the sub-continental lithospheric mantle (SCLM) ca. 2.5 Ga into the source region of the Kalkarindji magmas. Using the geochemical data a model of mantle warming for ca. 700 Ma coupled with decompression melting of the anciently fertilized SCLM that was triggered during a large scale rotation of Gondwana ca. 500 Ma is presented for generation of the magmas of the Kalkarindji Province. An important constraint on this model is the ability to cause SCLM melting, which requires previous charging by fluids to provide a fertile and/or relatively hydrated SCLM.

The integration of 40Ar/39Ar geochronology with geochemical analyses of dolerite intrusions into the Western Cape province of the Karoo CFBs display the first direct evidence of a hydrated source for a CFB province. 40Ar/39Ar plagioclase geochronology indicate these Western Cape intrusions are some of the first magmatic expressions of the Karoo CFB province at ca 183.5 Ma. Precluding any evidence of a considerable amount of crustal contamination, the primary hydrated minerals (biotite and hornblende) are representative of a hydrated source. Furthermore, the geochemical and geographic location of the Western Cape intrusions with other low-Ti rocks of the Karoo LIP indicate subduction during the Kibaran Orogeny provided hydrated material to the base of the lithosphere preferentially proximal to the subduction zone. The direct evidence of H2O in the source region of tholeiitic CFBs provides evidence needed to suggest the SCLM could contribute notably in the generation of CFBs.

The advent of multi-collector noble gas mass spectrometers provides the ability to continue to push various geochronological techniques to new horizons. The first geologically meaningful 40Ar/39Ar plateau ages for terrestrial pyroxene has been accomplished on dolerites from the Kalkarindji and Ferrar CFBs. A thorough comparison of these 40Ar/39Ar pyroxene plateau ages with previously published results from a multitude of geochronological methods indicate these results represent a true geological event. Irradiation induced recoil resulting from extensive exsolution in some of the samples can be mitigated through a thorough petrographic investigation when selecting samples for 40Ar/39Ar geochronology. The ability to utilize the 40Ar/39Ar technique with the mineral pyroxene establishes unprecedented geochronological opportunities to the study of LIPs as well as metamorphic and thermochronologic applications.

Wed 7th June @ noon, Rm 312.222

Abstract:

The formation of impact craters is a highly dynamic and complex process that subjects the impacted target rocks to numerous types of deformation mechanisms. Understanding and interpreting these styles of micro-, meso- and macroscale deformation has proved itself challenging for the field of structural geology. The presentation introduces to the structural inventory found in craters of all size ranges on Earth, and looks into the structures of craters on other planetary bodies. Structural features are discussed here that are caused by i) extremely high pressures and temperatures that occur during the initial passage of the shock wave through the target rock and projectile, ii) the resulting flow field in the target that excavates and ejects rock materials, and iii) the gravitationally induced modification of the crater cavity into the final crater form. A special focus is put on the effects that low-angle impacting bodies have on crater formation.

 

Short bio:

Dr Thomas Kenkmann holds a professorship position at the University of Freiburg. He is the head of department, Chair of Geology and Structural Geology at the University of Freiburg. He is also vice dean of the Environment and Natural Resources faculty. His research interests gravitate around the structural geology of impact craters (field work, remote sensing), the experimental study of rock deformation under static, dynamic and shock loading conditions and deformation microstructures.

Wed 24th May @ noon, Rm 312.222 

Abstract:

Although the source of metals for porphyry deposit systems is still debated, large magma chambers are commonly assumed to develop underneath the porphyry deposits. Systematic investigations of mineral texture and chemistry, generally focused on plagioclase, have been widely used to describe the processes occurring in the magma chamber of volcanic systems; however, similar studies of porphyry Cu deposits have been limited due to significant high temperature hydrothermal alteration. The intrusive rocks at Baguio district, Philippine, provides an excellent natural laboratory due to: 1) the young formation ages of the porphyry (Late Miocene to Pliocene); 2) a wide range of rocks (mafic to felsic) with well-defined geochemistry and geochronology; and 3) the well-preserved reversely, normally and oscillatory zoned minerals in most rock types.

Short bio:

Mingjian Cao obtained his Ph.D degree in September 2013 at Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), and conducted two years post-doctor research at IGGCAS from September 2013 to December 2015. Since January 2016, he became a senior research fellow at IGGCAS. His research interest is mainly focus on using in situ microanalysis to constrain the detailed petrogenesis and metallogenesis of porphyry Cu deposits from Central Asian Orogenic Belts, Tibet, and Baguio district (Philippine).

Wed 3rd May @ noon, Rm 312.222 

Abstract:

Field, geochronological, isotopic and geophysical studies do not support plate tectonic models for subduction and arc accretion in Archaean granite-greenstone terrains. Geophysical data suggest Neoarchaean granite-greenstone sequences of the southern Superior Province in Canada formed during rifting of an older, composite Superior I craton and not as a series of accreted arcs and micro-continents as previously interpreted; a similar model is proposed for the Yilgarn Craton. How then do we explain regional folding, ductile shearing and faulting in Archaean granite-greenstone belts?

Venus, where there is no plate tectonics, was studied to see if regional shortening and large displacements of discrete blocks can occur without plate tectonics. Structures in basalt plains on Venus imaged by Magellan radar were previously attributed to mantle plumes in a stagnant lid or transitional convection regime. Interpretations of radar and filtered bouguer gravity data, however, provide a completely new view of our ‘sister planet’ that has significant repercussions in evaluating models for Archaean tectonics on Earth:

• Bouguer gravity lows imply the presence of granitic or other felsic rocks in craton-like plana, so Venus is more similar to Earth than previously thought.
• Regional transcurrent faults produce indentation and lateral escape structures resembling the Himalayan-Tibetan orogen and intraplate tectonics of central Australia.
• Fold interference patterns, refolded shear zones and cross-cutting rifts imply superposed deformation.

Structures on Venus are interpreted to result from mantle flow directed away from upwelling, mantle plume-related rifts interacting with the deep keels of plana, along with lithospheric drips, i.e. similar to the model proposed for Archaean tectonics. Venus shows that plate tectonics is not required to produce deformation features in Archaean terrains on Earth.

Short bio:

Since 2003, Lyal Harris is a professor of structural geology at INRS-ETE, an applied research university in Québec City, Canada (he previously taught at UWA). His research integrates enhancement and interpretation of geophysical data with field studies and analogue modelling for regional structural and tectonic syntheses. He is especially interested in the link between deep crustal and upper mantle structures and mineral deposits, and non-plate tectonic models for the Archaean Earth. Reinterpreting the tectonics of Venus and developing new Archaean tectonic models with Geological Survey of Canada (GSC) colleague Jean Bédard received an award as one of the top 10 scientific discoveries in Québec for 2014 by Quebec Science and the 2015 GSC team research award.

Wed 22nd February @ 12:00 pm, Rm 307.101:LT 

Formation of calcite veins and breccias in association with CO2 degassing is common along plate boundary faults in Turkey and crustal-scale intra-continental faults in central Australia.  In this presentation, I will present a precisely-dated late Quaternary record using U-series dating to investigate the timing of vein formation and to relate it to tectonic and/or climatic processes. In southwest Turkey, vein formation is focused during cold/dry climate periods. It is proposed that during wet/warm climate periods crustal and mantle-derived CO2 is transferred to the surface where it passively degassed from springs when meteoric water circulation is enhanced. By contrast, during phases when groundwater is limited, CO2 builds- up in sealed reservoirs and degassing is associated largely with rupture events. While regional tectonics is the ultimate driver of fault activity, climate-driven near surface hydrological changes may have played an important role in modulating CO2-rich fluid circulation. In south-central Australia, late Quaternary to Recent fault-controlled mantle-derived CO2 degassing occur in association with travertine vein deposits of which U-series ages cluster every ~3-4 ka since 26 ka. A neotectonic model is discussed in which recent stress field, heat flow and lithospheric structure in central Australia reactivated a number of steeply dipping Neoproterozoic faults, growing into a crustal/lithospheric-scale structure.

Short bio:

Tonguç Uysal is a geoscientist focussing on isotopic dating and tracing of hydrothermal/geothermal systems and thermal histories of sedimentary basins as well as active fault systems. Tonguc has recently joined CSIRO, after he had been a Marie Curie Research Scientist in Turkey and Research Fellow at the University of Queensland (UQ). Tonguc has a BSc (Hons) Degree from Istanbul Technical University, Master’s Degree from Aachen University in Germany, and PhD from UQ in 1999.

Wed 16th November @ noon, Rm 312.222

Abstract Following the breakup of Australia from Greater India in the Early Cretaceous, the North West Shelf of Australia (NWS) evolved as a passive margin. The combination of a wide continental shelf and relatively limited clastic influx resulted in the progradation of carbonate strata, and these deposits  constitute excellent archives of Australia’s past climate and sea levels. Neogene carbonates of the NWS are generally dominated by “heterozoan” carbonate assemblages (e.g., carbonate production controlled by organisms that usually strive in sub-tropical to colder oceans and/or areas associated with high nutrient input). However there is widespread evidence for the Miocene development of a coralgal “photozoan” (tropical, light-dependant) reef system. This presentation will review the sedimentological and geophysical evidence of a Miocene coralgal barrier reef system that extended over at least 2000 km, from the Exmouth sub-basin to the NE Bonaparte Basin. The high quality 3D seismic datasets that cover most of the North West Shelf allow reconstructing the stratigraphic architecture and geomorphology of this reef system at very high resolution. Biostratigraphic and Strontium isotope dating on more than 30 wells along the margin, together with field evidence (Cape Range) allow constraining for the first time the timing of reef development across all the NWS basins. The analysis shows that in general reef development initiated at ca. 18-17 Ma, at the onset of the Mid Miocene Climatic Optimum global warming. An extensive barrier reef rimmed the North West Shelf by 15-14 Ma, at a time where paleoclimate reconstructions indicate global mean surface temperatures up to 3-4° higher than today and atmospheric C02 concentrations ranging from pre-industrial levels (250 ppm) to 500 ppm (e.g., same range than those projected in the coming centuries). This barrier system may have persisted until 11-10 Ma, during a time interval marked by relatively high Sea Surface Temperatures in the Indian Ocean despite the onset of global cooling and the expansion of Antarctic ice. The slow demise of this barrier reef system after 11-10 Ma and most importantly after 7-6 Ma probably results from a complex interaction between global changes in oceanography (due to plate tectonic reorganization in greater SE Asia, including the Banda Arc) and local changes in subsidence rates and/or clastic influx as the result of collision. This Miocene “Great Barrier Reef” appears as a very useful analogue to understand long-term carbonate reef response to climate changes and coral reef development in a predicted warmer future. Biography Julien Bourget is a senior lecturer in Marine and Petroleum Geology at the School of Earth Sciences and the Associate Director of the Centre for Energy Geoscience of the University of Western Australia. Julien obtained his MSc and PhD from the University of Bordeaux (France) in 2006 and 2009, respectively. Following his PhD he held a postdoctoral fellowship at the Australian School of Petroleum (University of Adelaide) from 2010 to 2011. His research expertise is in Sequence and Seismic Stratigraphy with a primary focus on modern 3D seismic interpretation techniques and their applications for understanding the geodynamic and sedimentary evolution of the margins of the Indian Ocean (including the North West Shelf of Australia) in the Mesozoic and Cenozoic.

Wed 2nd November @ 12:00 pm, Rm 312.222

Earth was hotter in its early history, though few observations exist on the cooling rate of the bulk Earth. We demonstrate that rates of true polar wander–wholesale rotation of mantle and crust around the core–have significantly decreased over the past three billion years. As true polar wander is rate-limited by the viscosity structure of the mantle, we interpret the decay to reflect the secular cooling of Earth. The amplitude of the decay indicates that the viscosity of the lower mantle had increased by an order of magnitude, and such a viscosity requires a cooling rate of >100 K/Gyr for the lower mantle. This rate is compatible with recent petrological and geochemical estimates on the cooling rate of the upper mantle, suggesting that the mantle as a whole has experienced rapid cooling since at least Neoproterozoic time, and possibly since Archaean time.