EXPLORING WEATHERING AND ALTERATION TRENDS IN A-CN-K-FM THREE-DIMENSIONAL SPACE

It is now common to determine alteration extent and trends in weathering profiles as 2D ternary projections in Al₂O₃-CaO*+Na₂O-K₂O (A-CN-K) and Al₂O₃-CaO*+Na₂O+K₂O-FeO+MgO (A-CNK-FM) compositional space. Although highly useful, the 2D projections impose limitations on complete visualization of the actual trends because major elements are either excluded (FM component in A-CN-K) or grouped together (all feldspars in A-CNK-FM). In an attempt to overcome these problems, we have developed a 3D A-CN-K-FM tetrahedral projection that permits simultaneous visualization of relationships among alkali and plagioclase feldspars, mafic minerals and their weathering products. The 3D projection combines major element data from virtually every major mineral constituent. Data examined here are from previously published works; the samples had been subjected to weathering of variable intensity and represent modern and ancient weathering profiles and modern sediments ranging from clay to gravel. In A-CN-K space, clearly defined linear trends for weathering profiles have been well documented for a wide range of initial bedrock compositions, including basalt. Inclusion of the FM apex to create a 3D plot reveals that the apparent linear nature of the weathering trends results both from exclusion of ferromagnesian components and from projection of data on to a 2D face. The projection problem is progressively exacerbated with increasing ferromagnesian content. Three-dimensional plots illustrating chemical weathering of basalt and granodiorite have curvilinear paths, whereas their 2-D plots are straight lines. The alteration pathway of a Proterozoic, K-metasomatized weathering profile developed on granite abruptly deflects towards the FM pole at a position equivalent to a premetasomatized CIA value of 82, which indicates not only K addition, but also secondary accumulation of iron oxides and/or immature clays high in the profile. This kinked pathway cannot be predicted from the 2D A-CN-K plot. Pathways representing production of sediment by physical disaggregation of soil (or rock) and subsequent hydraulic sorting demonstrate that ferromagnesian components are concentrated in the fine fraction.