Discrimination of Multiple Authigenic Clay Phases: Coupled Mineralogy and Geochemistry In Pliocene Paleolake Olduvai, Tanzania

Using clay minerals as paleochemical proxies can be problematic, yet such silicates are in many cases the only primary precipitates remaining from a paleolake. Nanometer-scale analyses, such as HR-TEM/AEM, reveal “true” crystal chemistries, but in some settings these may not be representative. Additionally, bulk analyses of clay extracts, even with electron microprobe, clearly must be treated as “whole rock” analyses. Indeed, it appears that multiple authigenic phases are routinely present in the sub-micron fraction of saline lake clays.

Here we use coupled mineralogy and geochemistry to reveal the complexities of clay diagenesis in the saline, alkaline paleo-waters of Pliocene Paleolake Olduvai, in northern Tanzania. Multiple authigenic phases are distinguished and quantified by XRD and deconvolution of 060 peaks coupled with geochemical (EMP, HRTEM-AEM) and crystallographic (IR) investigation. By assigning geochemistry according to the XRD deconvolution, we identify three principal phases: dioctahedral Al-rich, dioctahedral relatively Fe-rich, and Mg-rich with a trioctahedral component. These have average octahedral structures of Al_1.30 FeO_0.57 Mg_0.13 (Al-rich), Al_0.56 Fe_0.80 Mg_0.62 (Fe-rich), and Al_0.09 Fe_0.23 Mg_2.09 (Mg-rich) per half formula unit. IR data confirm the geochemical assessments of individual phases. The Mg-rich phase either has both dioctahedral and trioctahedral domains within sheets, or a homogeneous octahedral sheet with an intermediate composition. This is, to our knowledge, the first report of such an intermediate octahedral structure in a 2:1 phyllosilicate.

All three phases occur in different areas of the paleolake; their proportions and octahedral compositions are variable. Total Mg content of “bulk” <0.1µm clays reflects the relative abundance and Mg content of the Mg-rich phase. This helps to identify periods of elevated salinity and alkalinity in the paleolake. In addition to shedding light on the crystal chemistry of authigenic 2:1 clay minerals, this demonstrates the paleolimnologic importance of discriminating multiple authigenic clay phases in lacustrine deposits.