Characterising carbonate occurrences from the Tumut-Eucumbene Tunnel, NSW, Australia

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The earth’s upper mantle is known to contain carbon, in an amazing variety of forms, including diamond, methane-rich fluids, as carbonate dissolved in silicate melts dissolved in metallic FeNi alloys, as carbonate melts and as solid, crystalline carbonate. Depending on its form, carbon can profoundly influence a variety of upper mantle petrological processes, particularly partial melting leading to magmatism and sometimes volcanism. Peridotite and pyroxenite xenoliths hosted in alkali basalts are direct samples of the earth’s upper mantle and can provide important information about these processes in the mantle lithosphere or deeper. However, carbonate preserved in these rocks is extremely rare, despite clear evidence that some samples clearly underwent earlier interaction with fugitive carbonate-bearing melts or fluids. A unique suite of fresh xenoliths from the now flooded Tumut-Eucumbene Tunnel in the Snowy Mountains Hydroelectric Scheme contains abundant and variable carbonates along with the normal spinel peridotitic and pyroxenitic silicate minerals. The aim of this project is to characterise these carbonate occurrences and develop a model to explain their formation and significance in these rocks. For example, are these primary crystalline mantle carbonates, or quenched carbonatite melts. Or did they exsolve or crystallise from silicate melts and if so where and why? Are they related to the host basanite? Do they tell us anything about the nature of carbon in the upper mantle lithosphere, from where these samples originated? This research will be an important step in current, international attempts to understand the earth’s deep carbon cycle.

Information on apparatus used in the project and/or fieldwork;
Techniques to be used will include high resolution imaging of thin sections of the xenoliths using a variety of techniques (optical petrographic microscopy, back-scattered electron images, elemental X-ray mapping, X-ray computed tomography) and equipment (Scanning Electron Microscope, electron probe microanalyser [EPMA], X-ray Computed Tomography) and quantitative microanalysis of carbonates and other mineral phases using EPMA and Laser-Ablation-ICMPS.

Other applicable information;
This suite of samples is unique - it was collected in the 60s by Professor John Lovering from a nepheline-basanite dyke in the Tumut-Eucumbene tunnel in the Snowy Mountains of southern NSW. The tunnel was subsequently flooded and any opportunity for further sampling lost.

A figure:

Calcite (CC) in an apatite-bearing (AP) spinel peridotite from western Victoria. Field of view is 3.5mm across. 




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