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The melting slopes of galena and pyrrhotite were determined to be +5 and +6°C/kb, whereas the slope of the decomposition curve of pyrite was found to be +9°C/kb. Zinc sulfide has a negative melting slope of −5°C/kb up to 40 kb, where the slope changes to a positive one of about +3°C/kb. The melting slopes of the metal sulfides indicate that, if silicate magma forms at depths below 80 kb, sulfide magma should also form. Its downward intrusion could help to explain the origin of deep focus earthquakes.
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W. E. Sharp (1969) studied this question.
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