[4] The reason for this apparent contradiction is owed to the change in behavior between Re and Os in partial melt events.
[5] This is thought to evidence of subducted ancient high Re/Os basaltic crust that is being recycled back into the mantle.
This combination of radiogenic (187Os that was created by decay of 187Re) and nonradiogenic melts helps to support the theory of at least two Os-isotopic reservoirs in the mantle.
[6] The first reservoir is characterized by depletion in Re and proxies for melt fertility (such as concentrations of elements like Ca and Al).
However, in-situ 187Os / 188Os and 186Os / 188Os of modern plume related magmas show simultaneous enrichment which implies a source that is supra-chondritic in Pt/Os and Re/Os.
These observations are interpreted to support the theory that the Archean subducted crust contributed Os-isotope rich melts back into the mantle.