Basic phase relations of polybaric batch, percolative and critical melting |
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| Authors: | S Maaløe |
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| Institution: | (1) Geologisk Institutt, Bergen, Norway, NO |
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| Abstract: | Summary ?Partial melting of the mantle is polybaric which implies that the phase relations change during partial melting. In addition
to the pressure the composition of the melt depends on the melting mode. Various melting models have been suggested. Here
the basic phase relations of polybaric batch, percolative, and critical melting are considered, using a simple ternary system.
The percolative melts are in equilibrium with their residua, but differ somewhat in composition from those of batch melting.
Critical melting is a fractional type of melting where the residuum contain interstitial melt. The critical melts differ in
composition from batch melts. The linear trends of peridotites from ophiolites show that the extracted melts had nearly constant
compositions, and therefore were extracted within a small pressure interval. A comparison between the trends of mantle peridotite
and experimental batch melts suggests strongly that the melt extracted from the peridotites are in equilibrium with their
residua. This could suggest that either batch or percolative melting are relevant melting modes for the mantle. However, isotopic
disequilibria favor instead a critical mode of melting. This inconsistency can be avoided if the ascending melts are accumulated
within a source region and equilibrate with the residuum before the melt is extracted from the source region. The evidence
for equilibrium suggests that multisaturation of tholeiitic compositions in PT-diagrams is relevant for estimating pressure
and temperature of generation of primary tholeiitic magmas.
Received September 2, 2001; revised version accepted March 20, 2002 |
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