Wed 1st April 12 – 1 pm Rm 312.222

One of the more difficult challenges involved in the disposal of radioactive waste in deep geological repositories is the very long time frames required by safety assessments. In the case of one Canadian project – Ontario Power Generation’s Deep Geological Repository (OPG DGR) project for the management of low and intermediate level radioactive reactor waste – the safety assessment time-frame is 1 million years. This type of time frame goes well beyond anything that can be considered in an experimental setting. Mathematical models that make long term estimates face challenges with considerable uncertainties over these time frames. There are also significant challenges with explaining the results of these models to the public and other stakeholders.

The safety case required for deep geological repository projects must use multiple lines of evidence to complement safety assessments that quantify the level of protection provided by a geological disposal facility. Geological time frames are particularly useful to support arguments associated with safety assessments, as geological time frames are often in excess of safety assessment time frames. For example, the OPG DGR project safety case considers a one million year period, and the sedimentary units that are proposed to host the repository have a well-studied, well-documented geological setting indicating that host sedimentary rocks were deposited and consolidated hundreds of millions of years ago, onto billion year old Canadian Shield rocks.

This talk will present some examples of research programs that focus on the safe long-term management of used nuclear fuel in a DGR, whose objectives are to build capabilities to assess the suitability of potential sites for a centralized deep geological repository, for all of Canada`s used nuclear fuel. Potential DGRs are assessed in terms of isolating and containing radioactive waste over geologically long time frames. Specific research topics focus on reducing uncertainties in models of long term repository performance and include (but aren`t limited to): modelling the results of large-scale shaft seal experiments, laboratory determinations of sealing material performance, modelling of the effects of past and future glacial events, natural tracers and natural analogues.

Wed 25th March 12 – 1 pm, Rm 312.222

Abstract

The IOCG deposit group includes temporally and spatially associated hydrothermal and magmatic end-members which are associated with magmas belonging to the shoshonitic series, e.g., Olympic Dam, Missouri, El Laco, Great Bear Magmatic Zone.  They have been interpreted to have formed from similar parental silicate magmas along contrasting petrogenetic paths involving either immiscible separation of Fe-P-O-S magma (e.g., El Laco) or Fe-Cu-S-rich aqueous fluid (e.g., Olympic Dam) [1].

A sample of unconsolidated tephra at El Laco is 86 mol% Fe2O3*, 12 mol% FePO4, 2 mol% SiO2 and minor S, REE.  Ovoid cavities in hematite and magnetite ash particles are wholly or partially occupied by vesicular masses of Fe-phosphate, silica, and trace  monazite interpreted to be finely crystallized melt with a composition nearly identical to that of the eutectic in the FePO4-Fe2O3 system at 1 atm (84 mol% FePO4, 1070 °C [2]).  We have confirmed this by heating the tephra to 1081 °C in an evacuated silica tube, to produce Fe-P-O melt surrounding magnetite and hematite crystals.  Ubiquitous occurrences of perlitic shoshonite glass form menisci partially lining cavities in oxide lapilli and bombs. Experimental equilibration of a synthetic shoshonite melt with magnetite, FePO4, H2O and CO2 at 900 °C, 1 GPa produced immisicible high-silica rhyolite and Fe-P-O melt.

We consider several hypotheses for the origins of coexisting shoshonite and Fe-P-O magma including liquid immiscibility and deep crustal melting of Fe-rich sedimentary phosphorite.  Fe-P-O-S magmas erupted and degassed violently, losing its H3PO4 and SO3 and cooling as magnetite lava. Quenched airfall deposits retain some phosphate minerals. Large hydrothermal IOCG deposits are associated with felsic differentiates of similar potassic magmas that need not have previously generated immisicible Fe-P-O magmas.

[1] Tornos F, (2011) 11th SGA Biennial Meeting Let’s Talk Ore Deposits, p. 26-28. [2] Zhang et al., (2011) J Amer Ceramic Soc 94, 1605-1610.

Wed 11th March 12 – 1 pm Rm 312.222

Ice Sheet Collapse, Sea Level Rise and Coastal Response

The response of polar ice sheets to modest increases in global temperature and the rate of future sea-level rise remains highly uncertain. One way of addressing these uncertainties is to examine past interglacials, which contain records of both changes in ice sheet mass balance and associated coastal response under a warm or slightly warmer climate system compared to present. However most of the geological record of Antarctic Ice Sheet extent during warmer interglacials has been lost due to glacial expansion and retreat cycles, An alternative and as yet untested approach for constraining ice sheet mass balance under warmer climates utilises the unique sea level fingerprint that results from a collapsing ice sheet. Principally the melting of individual ice sheets and glaciers over time scales of centuries to millennia drives perturbations in the Earth’s gravitational field, solid surface elevation and rotational state. Collectively, these effects contribute to significant geographic variation in eustatic sea level.

This talk I focuses on the last interglacial period spanning 128 to 116 ka. This period is characterised by global mean temperatures several degrees warmer than pre-industrial, and is therefore considered an analogue for near-future climate change. There is also a strengthening scientific consensus, and significantly from a near-future analogue perspective, that eustatic sea level peaked between 5 and 9 m above modern during the last interglacial, and was the result of the collapse of one or more of three significant Polar ice sheets, West Antarctic Ice Sheet, East Antarctic Ice Sheet or Greenland Ice Sheet. This finding carries significant implications for future polar ice sheet instability in the face of a relatively moderate global warming.

The overarching hypothesis is that LIG ice sheet melting history has left its discernable imprint of marine transgression along coastlines remote from the ice sheets and presents us with a unique opportunity to invert the problem.

Fri 20th  February  12 – 1 pm  Rm 312.222

Mineralised Cu-Mo-Au porphyries were discovered in belts of Cambrian andesite in the Stavely region of western Victoria in the early 1990s.  Deep weathering complicated early geochemical exploration and only a small number of diamond holes to several hundred metres depth were drilled.  These encountered sub-economic grades in mostly propylitic alteration.  New work shows they only drilled into porphyry dykes emanating up into the host andesites, rather than intersecting the actual porphyry bodies where economic grades could exist in the potassic zone.  This lack of early success, combined with no historic artisanal mining and uncertainty over the geological context meant there was little appetite to persevere.

In the mid 2000s university wholerock geochemical research suggested that the plate tectonic setting had been an Andean-type convergent margin – very attractive for copper porphyries  – rather than a region of arc-continent collision as previously preferred.  The geochemistry recognised boninites that can only form in supra-subduction zone settings.  The andesite belts themselves and some associated granodiorite intrusions also possessed a geochemical subduction fingerprint with relative depletion of HFSE.  A government funded deep crustal seismic transect across the region in the late 2000s imaged a crustal geometry consistent with the Andean-type convergent margin model.  The faulted belts of andesite exposed at surface can be traced down into what appears to be a larger arc edifice buried in the subsurface.

This greater confidence for Andean-like mineralised porphyries has triggered a new round of exploration and research.   A major breakthrough in prospectivity was taking some of the mineralised porphyry drill core and doing wholerock geochemistry for comparison against the existing academic geochemistry.  This shows the porphyries are related to the granodiorite intrusions rather than the belts of andesite as previously thought.  This boosts prospectivity in two important ways: 1 The intrusions are younger than the steep dipping fault slices of andesite so that the mineral systems are upright for easy vectoring rather than being variably titled and/or partly dissected; 2 The porphyries can occur anywhere within the entire Stavely region rather than being narrowly restricted to a few fault slices of andesite.  A government stratigraphic drill program is being planned to better define the margins of the Stavely Geological Zone.

Like many mineralised porphyry systems there is complexity for explorers.  Current mapping shows the most obvious targets are where the porphyries intrude into the andesites with zone of phyllic overprinting causing demagnetisation haloes (historic Victor prospect).  Less obvious are porphyries intruding in siliclastic sandstone outside of the andesite belts still preserve the prograde potassic cap and can be magnetic highs (historic Junction prospect).  The intrusions occur in clusters with a multiphase intrusion history, some mineralised and some barren.  The clusters seem to be concentrated in late conjugate faults cutting across the paleo-arc trend to define corridors of enhanced prospectivity.  Current explorers are collecting gravity and IP to generate good exploration targets through the deep weathering and cover, whilst they raise money for the deep drilling (up to 1 km) that will finally test the economic viability of this very poorly known province of mineralised porphyries.

Wed 3rd December

12 – 1 pm

Rm 312.222

Stephen Gallagher

Department of Earth and Atmospheric Sciences, Univer

International Ocean Discovery Program Expedition 356, August-September 2015

Reefs, Oceans, and Climate: A 5 million year history of the Indonesian Throughflow, Australian monsoon, and subsidence on the Northwest Shelf of Australia

Stephen J Gallagher, Craig Fulthorpe and Kara Bogus

Abstract

The Indonesian Throughflow (ITF) is a critical part of the global thermohaline conveyor. It plays a key role in transporting heat from the equatorial Pacific (the Indo-Pacific Warm Pool) to the Indian Ocean and exerts a major control on global climate. Because of the continued northward motion and impingement of the Australasian plate into the southeast Asian part of the Eurasian plate, the complex tectonic history of the Indonesian archipelago makes it difficult to reconstruct long-term (million year) ITF history from sites within the archipelago. The best areas to investigate ITF history are downstream in the Indian Ocean, in regions directly under the influence of the ITF. International Ocean Discovery Program Expedition 356 will drill a transect of cores over 10° latitude on the northwest shelf (NWS) of Australia to obtain a 5 m.y. record of ITF, Indo-Pacific Warm Pool, and climate evolution that has the potential to match orbital-scale deep-sea records in its resolution. Coring will reveal a detailed shallow-water history of ITF variability and its relationship to climate. It will allow us to understand the history of the Australian monsoon and its variability, a system whose genesis is thought to be related to the initiation of the East Asian monsoon. It also will lead to a better understanding of the nature and timing of the development of aridity on the Australian continent.  Paleobathymetric and stratigraphic data from the transect will also allow subsidence curves to be constructed to constrain the spatial and temporal patterns of vertical motions caused by the interaction between plate motion and convection within the Earth’s mantle, known as dynamic topography. The NWS is an ideal location to study this phenomenon because it is positioned on the fastest moving continent since the Eocene, on the edge of the degree two geoid anomaly. Accurate subsidence analyses over 10° of latitude can resolve whether northern Australia is moving with/over a time transient or long-term stationary downwelling within the mantle, thereby vastly improving our understanding of deep-Earth dynamics and their impact on surficial processes.

Biography

Stephen Gallagher is an Associate Professor at the University of Melbourne. He has had diverse research experience in Carboniferous to Recent microfossils, sedimentology and stratigraphy globally.  He uses applied stratigraphy & micropalaeontology to solve stratigraphic problems and to interpret (palaeo) environments, bathymetry and oceanography.