The chemical index of alteration (CIA) and interpretation of ACNK diagrams

Abstract

The chemical index of alteration (CIA) and ACNK ternary diagrams (i.e., with apices of Al₂O₃, CaO + Na₂O, and K₂O) are among the most widely used tools for evaluation of chemical weathering patterns in soils and sediments. However, incorrect use of these tools has become prevalent in recent literature, necessitating a “corrective” discussion of best practices for the CIA proxy and ACNK diagrams, their implications for the chemical weathering intensity of transported sediments, and the recognition of metasomatic effects in deep-time formations. As shown by the diversity of weathering trends in modern soil profiles, the assumption of an “ideal weathering trend” (IWT) for paleosols that runs parallel to the A-CN axis is generally unjustified. Use of an IWT to infer the presence of excess potassium (K) in a paleosol can lead to incorrect inferences of K-metasomatism and to unwarranted “corrections” of CIA values. Protolith reconstruction from a weathering path requires that the latter form a well-defined linear array in ACNK space that can be accurately projected backward to the feldspar join; projections based on poorly defined data arrays are meaningless. In transported sediments, the interpretation of ACNK data as a weathering proxy is further complicated by factors such as hydraulic sorting, two-component mixing systems, and sediment recycling. In metasedimentary formations, metasomatic alteration commonly disrupts weathering paths in ACNK space accompanied by pronounced shifts toward more K- (or Ca-Na-)rich compositions; however, it is doubtful whether this process can operate on a linear sample array representing a well-defined weathering path and shift all samples in a coordinated manner to produce a new array with an equal degree of alignment but an entirely different slope. Although K-enrichment of Paleoproterozoic paleosols has been widely attributed to K-metasomatism, the potential role of supergene weathering linked to rising atmospheric oxygen levels in promoting pedogenic K retention through early illitization of clay minerals requires consideration. For maximum robustness, studies of weathering in paleosols and paleoformations must combine information for the mobile bases (i.e., Ca-Na-K, in the form of the CIA proxy and ACNK relationships) with compositional data for Fe-Mg, SiO₂, and resistate elements as well as petrographic data documenting mineral phase relationships and sample paragenetic histories. In summary, more nuanced use and interpretation of the CIA proxy and ACNK diagrams are necessary.