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Experimental Petrology Group

Annual Report 2001


Introduction

The Petrochemistry and Experimental Petrology (P&EP) Group uses a laboratory-based experimental approach combined with field observations to study the Earth, its origin, evolution and mineral wealth. The group operates a wide range of experimental apparatuses for generating the high temperatures and pressures that are needed to reproduce the natural conditions within the Earth. The equipment includes: high temperature furnaces capable of reaching 1800ºC, several of which are equipped for precise control of oxygen and sulfur fugacities by gas mixing; ten solid-media piston-cylinder devices for generating pressures to 6 GPa and temperatures in excess of 2000ºC, some of which are large-capacity devices capable of synthesising relatively large volumes of high pressure phases for detailed mineralogical studies; a multi-anvil apparatus, which can achieve pressures of 26 GPa; and, through collaboration with the Department of Geology, the Faculties, a well-equipped hydrothermal laboratory. These high-temperature, high-pressure apparatuses are complimented by an array of microbeam analytical techniques, including electron microprobe, ion microprobe, laser-ablation ICP-MS, FTIR spectroscopy and visible-UV spectroscopy. The group has just taken delivery of a new electron microprobe, a Cameca SX100, which will be operated in collaboration with a consortium of other Canberra users. It is expected that this new instrument will become operational in February 2002. As well as providing better imaging capabilities and more stable operation (hence accuracy), the new instrument will enable quantitative element mapping. It will replace our ageing Cameca Camebax, which is now entering its nineteenth year of operation, having obtained over 300,000 quantitative analyses an incontrovertible testament to the importance of the electron microprobe to the group's activities over the years.

Most of the group's activities are concentrated into five areas: 1) Origin of the Earth and core formation; 2) Phase equilibria, including melting relations, in mantle systems; 3) Phase equilibria related to crustal evolution and ore deposits; 4) Physics of melting and melt extraction; and 5) Spectroscopic and thermodynamic property measurements on minerals and silicate melts. The latter area of research is undertaken in the belief that better understanding of large-scale geological processes often requires detailed, fundamental knowledge of the behaviour of minerals at the atomic scale.

This year saw the formal retirement of Professor D. Green, who will continue with the group in an emeritus role. Much of the strength of the group derives from its Visiting Fellows, who bring an immense amount of intellectual diversity to the group's activities. This year, the group has benefitted from extended stays by Dr G. Witt-Eickschen of the University of Cologne, who used the laser-ablation ICP-MS to study mantle xenoliths of the Eifel graben; Dr A. Glikson, formerly of AGSO, whose interests centre around asteroid and comet impacts and early crustal evolution; Dr J.-P. Li, of the Chinese Academy of Sciences, Guangzhou, who is investigating the origin of the high-K magmas of the Tibetan Plateau; and Dr S. Redfern of Cambridge University, who is working on the relationships between the crystal structure of minerals and their physical and thermodynamic properties. During the year Dr Taylor resigned his Research Fellowship to concentrate on his diamond exploration activities.

The group continues to develop its use of X-ray absorption spectroscopy (XANES) at the Australian National Beamline Facility at Tsukuba, Japan, to characterise the chemical environment and oxidation states of trace elements in minerals and silicate melts. Drs Berry and O'Neill extended their previous work on Cr oxidation states in Fe-free systems by studying the effect of Fe. This necessitated developments in two directions: firstly, the XANES spectra of a series of glasses with variable Fe 2+ /Fe 3+ ratios have been studied, in order to develop XANES as a general analytical method for Fe oxidation states in silicate glasses. The calibration glasses were previously characterized by Mössbauer spectroscopy in collaboration with Dr Jayasuriya and Prof Campbell of the School of Physics, ADFA. Secondly, in-situ measurements of both Cr and Fe XANES spectra have been obtained at temperatures to 1500ºC and controlled oxygen fugacity, using a furnace designed and built by Mr M. Shelley.

This spectroscopic work on silicate melts compliments the thermodynamic and phase equilibrium studies carried out by the group. Dr H. O'Neill and co-workers have measured the solubilities of several siderophile elements (Fe, Ni, Co, Mo, and W) in silicate melts as a function of melt composition, and are now using these data to work out generalised models for the activity coefficients of different groups of trace elements in silicate melts. Mr A. Hack and Dr J. Mavrogenes are investigating Cu solubilities in supercritical fluids to high temperatures and pressures, necessitating the development of several new experimental techniques.

Dr J. Hermann and Prof D. Green continue their investigation of phase relations applicable to ultra-high-pressure metamorphic terrains. Drs J. Mavrogenes and H. O'Neill have extended their experimental work on the properties of sulfur in silicate melts by studying very oxidizing conditions, under which the sulfur dissolves as sulfate not sulfide. The effects of fO 2 , fSO 2 , melt composition and temperature have been quantified in this regime from experiments at atmospheric pressure; and these have been complimented by experiments at high pressure on the solubility of anhydrite (CaSO 4 ). Dr U. Faul has been investigating the constraints from U-series data on melt transport beneath mid-ocean ridges and ocean island basalts. Dr S. Kesson continues to develop expertise for the group in the field of powder X-ray diffraction using the Rietveld method, which technique is becoming an increasingly important part of the group's activities.

The group contains four PhD students, Messrs Hack, Xi and Sommacal, and Ms L. Glass. Earlier in the year, Dr C. Magee successfully completed and defended his thesis on the origin of carbonado. Mr Hack is studying metal solubility in high temperature fluids, using synthetic fluid inclusions analysed by the LA-ICP-MS. Ms L. Glass is writing up her study of what were formerly named the Antrim Plateau basalts of the Northern Territory and other related basalts of northern Australia; the stratigraphic part of her work has led her to suggest a new name, the Kalkarinji flood basalt province. Mr Sommacal, in conjunction with Dr M. Sambridge, is developing new computational ways to handle the thermodynamics of compositionally complex, multisite solid solutions, with the immediate aim of developing a thermodynamic model for phase relations in the upper mantle. Mr X. Liu has almost completed an experimental investigation of the effects of Cr 2 O 3 , K 2 O and H 2 O on mantle melting in the simplified model mantle system CaO-MgO-Al 2 O 3 -SiO 2 at 1.1 GPa.

Of the technical staff, Mr M. Shelley has constructed a high-temperature furnace for X-ray absorption spectroscopy in silicate melts under controlled CO/CO 2 atmosphere to 1500ºC, and has built a new sample cell for the laser-ablation ICP-MS apparatus. Mr W. Hibberson and Mr D. Scott have continued the development of the 6-7 GPa piston-cylinder apparatus, and have been extensively engaged in sample synthesis for many of the group's activities. Mr N. Ware maintains and operates the group's present aging electron microprobe and has been busy preparing the ground for the new Cameca SX100. Sadly, Mr J. Derlacki was forced to resign through ill health this year, and died shortly after. His quiet competence will be greatly missed.

Research Activities