SILICICLASTIC PETROGENESIS IN 3D A-CN-K-FM TETRAHEDRAL SPACE: THE NEXT STEP IN THE EVOLUTION OF UTILIZING CHEMICAL WEATHERING INDEX PROXIES

Even with numerous examples, the chemical index of alteration (CIA) is the most widely utilized geochemical tool for interpreting chemical weathering, and by extension, paleoclimate in silicic rocks (bedrock-hosted weathering profiles, siliciclastic sediments and rocks) for the past forty years. Based on specific elemental components of source-rock compositions and reactions that occur during hydrolysis, particularly feldspars (which dominate the continental crust), the CIA relates the proportion of Al (A, immobile) to Ca, Na, and K (C-N-K, mobile). A drawback of the CIA is that as a 1D value, in many circumstances, it may homogenize multiple processes (source, weathering, sorting, and diagenesis). This problem can be addressed by plotting the A-C-N-K values on 2D ternary diagrams with A, CN, and K as apices, which has been very useful in separating weathering and diagenesis from source, particularly in rocks with minimal iron (F) and magnesian (M) minerals. A second diagram that uses A, CNK, and FM as apices permits the assessment of more mafic rocks and integration of the mafic index of alteration (MIA), but loses critical resolution of separating plagioclase from alkali feldspar by joining CN+K. Here we report the evolution of this tradition by introducing a 3D tetrahedral diagram that uses A, CN, K, and FM as apices. This plot permits the simultaneous assessment of silicic rocks regardless of composition on the same diagram and retains feldspar separation. Using the A-CN-K-FM tetrahedral plot, we show that both the 1D CIA value and data arrays on 2D ternary diagrams may generate very misleading interpretations without properly budgeting the FM components in parallel. A critical component of work in this space is that geochemical trends may be understood in the context of all common rock-forming mineral compositions and with a direct link to 1D MIA values. We will explore how the A-CN-K-FM tetrahedral space can be used to interpret and determine provenance, understand the extent that geochemistry relates to hydrolysis and oxidation in weathering profiles and paleosols, identify how sorting during sedimentary transport is capable of fractionating mineral phases that leads to non-weathering-related geochemical changes, and lastly determining the pathways of geochemical change that occur during diagenesis.