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An experimental study on the fractional melting of metapelites: first results. Marcos García-Arias and Gary Stevens Centre for Crustal Petrology, Department of Earth Sciences, University of Stellenbosch, South Africa.
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An experimental study on the fractional melting of metapelites: first results Marcos García-Arias and Gary Stevens Centre for Crustal Petrology, Department of Earth Sciences, University of Stellenbosch, South Africa
Most of the granitic batholiths in the world are not homogeneous rocks but show an internal separation in bodies with different composition or textures called facies. • The Peninsular Pluton, one of the four plutons of the Cape Granite Suite, shows this differentiation. The facies range in composition from leucogranites to granodiorites. The boundaries between the facies are diffuse, pointing to a co-magmatic origin for the facies (Villaros et al., 2009). • The pluton is considered to have derived from partial melting of a metasedimentary source, the Malmesbury Group (Villaros et al., 2009; Harris and Vogeli, 2010). This is, the different facies have come from the same source, which underwent fractional melting. • Moreover, the variation in the composition is ascribed not only to different compositions of the melt formed at different temperatures but also to the entrainment of varying amounts of peritectic minerals into the melt.
This is interesting because, until recently, chemical and textural variation within a granite was considered an evidence of mixing different magmas or internal fractionation after a single magma was emplaced. • To demonstrate the feasibility of fractional melting and entrainment of peritectic phases to produce the observed variations in a single batholith, we have started an experimental study to reproduce these processes under controlled conditions. • As a starting material, we created a synthetic rock with the composition of a biotite-quartz-plagioclase schist of the Malmesbury group (Villaros et al. 2009). • The reported composition contained no water, which plays a fundamental role in determining the temperature of melting and the melting reactions. In order to determine it, we made thermodynamic calculations.
The starting material is a synthetic rock made from the gellification of a mixture of elements and oxides. The water is added as kaolinite, an hydrated mineral. • To check that the sample has the right composition, it was analyzed using the X Ray Fluorescency spectrometer (XRF) of the CAF.
The starting material is a synthetic rock made from the gellification of a mixture of elements and oxides. The water is added as kaolinite, an hydrated mineral. • To check that the sample has the right composition, it was analyzed using the X Ray Fluorescency spectrometer (XRF) of the CAF. • The water content of the sample must be tightly constrained. Because an XRF analysis does not provide water content (all volatiles are listed as LOI), we determined it by ThermoGravimetric Analysis (TGA).
The starting material is a synthetic rock made from the gellification of a mixture of elements and oxides. The water is added as kaolinite, an hydrated mineral. • To check that the sample has the right composition, it was analyzed using the X Ray Fluorescency spectrometer (XRF) of the CAF. • The water content of the sample must be tightly constrained. Because the XRF does not provide water content (volatiles are listed as LOI), we determined it by ThermoGravimetric Analysis (TGA). • The first experiment was made at 800 ºC and 8 kbar using a gold capsule to contain the sample and a non-end-loaded piston-cylinder device in the experimental petrology lab of the Centre for Crustal Petrology of the University of Stellenbosch. • After the experiment, the capsule was cut in half, polished and carbon-coated to obtain the chemical composition of the experimental products, melt (now glass) and minerals. The Scanning Electronic Microscope (SEM) of the CAF was used.
To simulate the segregation of a magma composed by melt and peritectic minerals, a new synthetic rock with the composition of most mineral phases (no melt, no peritectic phases) will be made. The next experiment will be made at 850 ºC, 50ºC beyond the first one. • This process of making a synthetic rock, starting an experiment, analyzing all phases and creating a new rock after removing a “magma” will be repeated again to make a final experiment at 900 ºC. After finishing the experimental campaign, the results will be compared to the facies of the Peninsular Pluton.
