OCTAHEDRAL CHEMISTRY OF CLAY MINERALS IN ALKALINE LAKES: AL, FE, AND MG DYNAMICS AND IMPLICATIONS FOR LACUSTRINE SEDIMENTS

Clay mineral phases in lake waters are generally produced by chemical weathering of silicates in soils and are deposited as lacustrine sediment; alteration may occur before, during, or after deposition. In general, chemical alteration or precipitation of new clay minerals in lakewater requires 1) elevated salinity and alkalinity; 2) abundant aqueous silica derived from hydrolytic weathering and or hydrothermal activity; 3) relatively low sedimentation rates. Mg-rich phases are particular hallmarks of saline alkaline waters, requiring high aqueous Mg/Ca ratios.

Review of published compositions of sub-micron fractions of authigenic clay minerals from East Africa, Brazil, the Green River Formation, Tecopa Basin, and other modern and ancient saline settings suggests that of the three major octahedral cations in lacustrine clays (Al, Fe, and Mg), solid solutions exist only between Al-Fe and Al-Mg. Plotted in ternary space, there is a gap between Fe- and Mg-rich endmembers, but continuous solid solutions along the other axes. In other words, direct alteration of Fe-rich clay minerals to form new Mg-rich clays either does not occur, or does not produce intermediate octahedral compositions between Fe- and Mg-rich endmembers. This interpretation assumes that geochemical data are representative of authigenic phases that may be orders of magnitude smaller than the scale of analyses. Additionally, the nature of the weathering parent material (e.g. Fe-rich andesitic glass versus Al-rich trachytic glass) is an important determinant.

This has several implications for understanding the authigenic clay records of lacustrine sedimentary successions, both on Earth and Mars. First, we hypothesize that reductive dissolution of Fe-rich clay minerals is required to attain high Al/Fe ratios in the octahedral layer. Such reduction also likely increases layer charge that encourages the process of illitization. Second, the use of indices such as the “Octahedral Cation Index” ((Al+Fe)/Mg), while providing a useful overall signal of paleosalinity, likely misses the reductive dissolution aspect of authigenic clay formation. These nuances of clay mineralogy, although requiring detailed analyses, can provide important perspectives on paleolimnology, particularly in saline and alkaline intervals lacking biotic indicators.