Petrogenesis of siliciclastic sediments and sedimentary rocks explored in three-dimensional Al2O3-CaO*+Na2O-K2O-FeO+MgO (A-CN-K-FM) compositional space

Quantitatively determining the amount of chemical weathering within sedimentary rocks (and weathering profiles) took a major step forward with the creation of the chemical index of alteration (CIA) 40 years ago. The CIA relates the proportion of immobile aluminum to the mobile cations of calcium, sodium, and potassium and is grounded in empirical and modeled geochemical data for mineral reactions that occur during hydrolysis. However, the CIA should be applied cautiously because it is a one-dimensional value that in the most complex situations, as with clastic sedimentary rocks, homogenizes the compositional inputs of source, weathering, sorting, and diagenesis. Subsequently developed two-dimensional (2D) ternary diagrams (Al2O3-CaO*+Na2O-K2O; Al2O3-CaO*+Na2O+K2O-FeO+MgO) permitted the capacity to explore mineralogical-geochemical pathways in data sets that may separate those inputs, but interpreting the ternary diagrams may be complicated because they differentiate and group certain elements. Here we develop a three-dimensional tetrahedral diagram (Al2O3-CaO*+Na2O-K2O- FeO+MgO, A-CN-K-FM) that incorporates the same critical elements and permits the simultaneous assessment of felsic and mafic rocks and minerals on the same diagram while retaining the ability to separate plagioclase from alkali feldspar and monitor post-depositional potassium changes. Using the tetrahedral plot, we show that both the CIA value and positions on the 2D ternary diagrams can generate potentially misleading interpretations without properly budgeting the ferromagnesian components in parallel. We first show how the tetrahedron works, then use it with numerous previously published examples to identify how the competing mafic and felsic inputs shape the composition of source rocks, weathering profiles, actively transporting sediment, paleosols, and sedimentary rocks in sedimentary petrogenesis.

Automated summary

The creation of the chemical index of alteration (CIA) 40 years ago was a major advancement in quantitatively determining chemical weathering in sedimentary rocks. However, the CIA should be used cautiously as it homogenizes multiple compositional inputs. Two-dimensional ternary diagrams have been developed to explore mineralogical-geochemical pathways, but they can be complicated to interpret. A newly developed three-dimensional tetrahedral diagram allows for the assessment of both felsic and mafic rocks and minerals, while also monitoring post-depositional potassium changes.