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Pyrolite is a theoretical rock considered to be the best approximation of the composition of the upper mantle. The definition varies, but it is generally considered as being 1 part tholeiitic basalt and 3 parts dunite. If fused experimentally, this mix yields high pressure tholeiitic basaltic melts, and intermediate pressure alkaline basalts. The hypothetical pyrolite compositions are not compatible with trace element, isotopic and chondritic abundances data as well as evidence for mantle heterogeneity.