Carbonate eclogite in the upper mantle - its role in petrogenesis

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One effect of alteration of the basaltic ocean floor is addition of a few weight % carbonate in the form of calcite, or CaCO3 to the upper few 100 metres of the oceanic crust. There is abundant evidence from high pressure experimental studies and from investigations of high pressure and ultra-high pressure metamorphic rocks that some of this carbonate can survive the subduction process and is recycled back into the deeper mantle.

Geochemical evidence suggests that this material may then be important in some partial melting processes in the upper mantle, leading to formation of certain types of magmas (eg some alkali basalts, carbonatites, kimberlites etc). The recycling of carbonate from the earth's atmosphere, hydrosphere and crust, back into the mantle is thus likely to be a highly significant part of the planet's overall carbon cycle, with an important role to play in the chemical evolution of the earth. However, relatively little is known about the behaviour of carbonate-bearing mafic rocks at high pressure (i.e. carbonate eclogite) and the few studies that have been done have yielded contrasting results.

The aim of this project is to use high pressure experimental petrology to carefully constrain the high pressure melting and phase relations of carbonate eclogite, and to develop models for its involvement in upper mantle petrogenesis. The data produced will also enable calibration of a geobarometer that could be used to determine the barometric history of natural crustal high pressure carbonate + garnet assemblages. The student would be involved in a larger ARC funded project investigating the melting behaviour of heterogeneous upper mantle.

He/She would prepare high pressure experimental assemblies, run them in the high pressure experimental equipment at RSES and undertake microanalysis by electron microprobe and laser ablation ICPMS. There are also possibilities for field work aimed a collecting suitable samples for thermobarometry based on garnet-carbonate exchange equilibria.

The student would be closely involved in analysis and interpretation of the experimental and natural samples.

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