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Fig. 1 Pressure–temperature diagram relating diagenesis to metamorphic regimes and typical P–T gradients in Earth’s crust. The crustal geotherm of 10 C km− 1 is representative of stable cratons, whereas a value of 30 C km− 1 is typical of rifted sedimentary basins. Low-pressure (shallow)–hightemperature conditions are realized only in geothermal systems or in the vicinity of igneous contacts (so-called contact diagenesis: