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Exploring the melting behaviour of the Earth's heterogeneous upper mantle.

Anja Rosenthal 1 , Gregory M. Yaxley 1 , David Green 1 and Carl Spandler 2

1 Research School of Earth Sciences, Australian National University , Canberra , ACT 0200, Australia
2 Institute of Geological Sciences, University of Bern , Bern , Switzerland

 

This project forms part of an ARC-funded project, which aims to determine solidus temperatures, phase relations and partial melt compositions during partial melting of upwelling eclogites (oceanic crust composition), residual garnet-bearing and garnet-free pyroxenites and refertilised lherzolites over the upper 150-200 km of the mantle, as functions of pressure, temperature and bulk composition. The phase relations thus determined control minor and trace element behaviour during partial melting and will be essential constraints on models of isotope evolution and mixing often used to explain the heterogeneity of mantle-derived magmas.

The primary technique for this research is high pressure experimental petrology, in conjunction with a range of cutting edge micro-analytical techniques (electronprobe microanalysis, laser-ablation ICP-MS). Techniques of high pressure and temperature experimental petrology are necessary to define the melting behaviour of postulated mantle materials as functions of pressure (P), temperature (T) and composition. High pressure experiments can be used to establish equilibrium melt and residual phase compositions. Careful experimental design can also constrain and clarify other melting models such as continuous melting and melt extraction, channel flow and melt pooling, etc.

A high-pressure experimental investigation of several eclogitic compositions (Res-2 to Res-5) is nearly complete, in which the effects of Na/Ca+Na and other compositional features on the phase relations and partial melt compositions have been delineated. The preparation of compositions and experimental techniques followed established procedures. High pressure experiments have been conducted over a temperature range from 1200 to 1500°C, and a pressure range form 3.0 to 5.0 GPa in a standard 1.27 cm piston-cylinder apparatus. Run products were polished and examined by reflected light microscopy, and by scanning electron microscopy (SEM) and electronprobe microanalysis (EPMA), using a JEOL 6400 SEM with EDS facility to analyse major element of the mineral assemblages (garnet and clinopyroxene) and glass.

A second essential component of the project is to assess whether evidence of processes of heterogeneous melting, melt migration and mantle refertilisation exists in natural mantle samples. Accessible for such investigation due to tectonic emplacement from upper mantle into the crust are peridotitic bodies of the Western Gneiss Region (WGR) of Norway . For example, at Almklovdalen, a range of relatively refractory pyroxenite-rich lithologies are surrounded by fertile garnet peridotite and dunite. These may be natural examples of high pressure interactions between partial melts of pyroxenites and enclosing peridotites.

Fieldwork was conducted in the Nordfjord-Stadlandet part of the Western Gneiss Region in Norway in June-July to sample these pyroxenites and peridotites, which The region is known for its excellent outcrops, making possible the compilation of spatial, temporal and genetic relationships of the layered peridotite masses, and determination and mapping of key textures in mm to m scale available.

Figure 1. Layers of garnet pyroxenite and peridotite at Alklovdalen, Western Gneiss Region.