Changing tectonic settings through time: Indiscriminate use of geochemical discriminant diagrams

• Evidence for depleted and enriched mantle do not become widespread until after 2.2 Ga.
• Not all greenstones are ophiolites.
• Modern tectonic settings cannot be easily identified in rocks older than 2.2 Ga.
• Plate tectonics did not become firmly established on Earth until 2.2 Ga.

No one method should be used to identify ancient tectonic settings, and this is especially true for rocks older than 2.5 Ga, when Earth was changing from stagnant lid to plate tectonics. The most useful application of geochemical discriminant diagrams should be to constrain mantle source compositions, mantle heterogeneity, the effects of degree of melting, mixing of melts, and crustal contamination. It is only the convergence of evidence from greenstones that includes rock association, stratigraphic variation, sediment types and provenances, information from syntectonic granitoids, and geochemistry of all rock types that we can make progress in understanding evolving tectonic settings on planet Earth.