Partial melting of two amphibolites: contrasting experimental results under fluid-absent conditions |
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Authors: | Tracy Rushmer |
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Institution: | (1) Department fuer Erdwissenschaften, ETH Zuerich, CH-8092, Switzerland |
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Abstract: | A large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase. As a consequence,
H2O-undersaturated or fluid-absent melting of amphibolites may be responsible for the formation of some granites and migmatites
produced during major orogenic events. In an attempt to determine the systematics of melting under fluid-absent conditions,
a series of piston-cylinder experiments was conducted on two natural amphibolites; one, a meta-alkali basalt (ABA) with a
total water content of ∼ wt% contained in hornblende, and the other, a meta-island-arc tholeiite (IAT) which has ∼1–1.3 wt%
water contained in hornblende, cummingtonite and biotite. The experimentally determined melting ranges of the two amphibolites
showed that the solidus temperatures, and sta temperature interval over which amphibole was stable, were controlled by the
amphibolites' different bulk compositions and their resulting metamorphic assemblages. The volume % of melt produced by melting
of the two amphibolites were compared with estimated amounts, based on Burnham's (1979) water-melt solubility model and the
fluid-absent melting model presented by Clemens and Vielzeuf (1987). The observed melt volumes were greater than estimated.
As the water content of melt largely detemines the volume % of melt produced, independent measurements of the water-content
of the glass formed during partial melting in the ABA were made by thermogravimetric analyses. The water content of the ABA
glass is ∼2 wt%, which is less than the assumed “melt-water” content (water content of the melt) used in previous modeling
of fluidabsent anatexis in mafic lithologies. As a consequence, more melt can be expected during fluid-absent partial melting
of mafic lower crust, as is observed in the experiments. A modification of the Clemens and Vielzeuf (1987) fluid-absent melting
model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented
here for pressures of 5, 8 and 10 kbar. Tectonic scenarios in which the crust is thickened (i.e. by collision) then undergoes
extension or where a previously thinned crust is later rethickened, provide enough heat so that amphibolite melting under
fluid-absent conditions can become importan and hence responsible for some melts produced during post-collisional magmatism.
The results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains. |
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