Saline lake diagenesis as revealed by coupled mineralogy and geochemistry of multiple ultrafine clay phases: Pliocene Olduvai Gorge, Tanzania

Abstract

Mineralogical and geochemical analyses of ultrafine (<0.1m) extracts of Pliocene clays from Olduvai Gorge reveal the complexities of clay diagenesis in saline, alkaline paleo-waters. Multiple authigenic phases are routinely present; these may be distinguished and quantified by decomposition of XRD (060) peaks coupled with geochemical (microprobe, HRTEM-AEM) and crystallographic (IR) investigation. By assigning geochemistry based on XRD results, we identify three principal phases by octahedral composition: dioctahedral Al-rich, dioctahedral relatively Fe-rich, and Mg-rich with a trioctahedral component. Average octahedral formulae are Al 1.30 Fe 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 analyses support these octahedral structures. The Mg-rich phase either has coexisting dioctahedral and trioctahedral domains within individual sheets, or a homogeneous octahedral sheet of intermediate composition. This is, to our knowledge, the first report of an intermediate octahedral occupancy in a 2:1 phyllosilicate.HRTEM observations indicate both solid state octahedral and illitization reactions and dissolution-precipitation. Layer charge and interlayer cations suggest that during illitization, layer charge increase was due to interaction with Na-rich brines, but later K uptake may have been with either saline or fresher fluids with higher K/Na ratios.All three phases occur throughout the basin; relative proportions and octahedral compositions vary. Total Mg content of "bulk" <0.1m clay fractions, (a paleochemical indicator in this and other basins) is the product of both the relative abundance and the Mg content of the Mg-rich phase. In addition to shedding light on the crystal chemistry of authigenic 2:1 phyllosilicates, these results demonstrate the paleolimnologic importance of discriminating the presence of multiple authigenic clay phases in lacustrine deposits.

Constraints on the crystal-chemistry of Fe/Mg-rich smectitic clays on Mars and links to global alteration trends

“…Is that an indication that smectite clay minerals on Mars exhibit true solid solution between Mg-rich and Fe-rich end-members in this range? If so, it would be surprising because FeMg-smectites do not commonly occur in solid solution on Earth (Deocampo et al, 2009); intermediate compositions more typically imply the existence of both dioctahedral and trioctahedral domains either within individual sheets (Grauby et al, 1994) or in interstratification of sheets (mixed layering) (Cuadros, 2010). Therefore the questions addressed below are: how do the chemistry of Fe/Mg-rich smectites relate to their NIR features?…”

“…2). Lastly, the Mössbauer data were used to identify Fe 3+ in multiple 6-coordinated environments, including multiple sites in clay minerals (Drits et al, 1997), and in oxyhydroxide, and oxide contaminants, if present. With these data, we were able to avoid samples with contamination or correct for minor contamination if present.…”

“…Yet, most previous clay mineralogy research has suggested that a continuous and perfect solid solution between the dioctahedral Fe 3+ -rich smectite (nontronite) and the trioctahedral Mg 2+ -rich smectite (saponite) does not exist (e.g., Grauby et al, 1994). Instead, intermediate compositions result from intimate structural mixtures of trioctahedral and dioctahedral domains (Grauby et al, 1994;Deocampo et al, 2009;Cuadros, 2010). The Fe-Mg-smectitesystem is further complicated by the variable valence state of Fe and therefore, the relationship between Fe/Mg ratio in smectitic clay minerals and near-infrared spectral features (2.27-2.32 μm) is not well understood.…”

A Brine Evolution Model and Mineralogy of Chemical Sediments in a Volcanic Crater, Lake Kitagata, Uganda

“…This is not surprising considering Lake Kitagata brines have most likely been concentrated by evaporation to reach their high salinity, and bottom sediments contain calcite and magadiite. No Mg-bearing minerals, however, have yet been identified at Lake Kitagata, although detailed analysis of clay minerals may reveal the MgÁsilicate phase that readily forms in other saline alkaline lakes (Hay and Kyser 2001;Jones and Deocampo 2003;Hover and Ashley 2003;Deocampo 2004aDeocampo , 2005Deocampo et al 2009). Because Mg 2?…”

The influence of authigenic clay formation on the mineralogy and stable isotopic record of lacustrine carbonates

“…The MgO contents of clay aggregates range from 16 to 23 wt.% MgO (Table A2), compared to a modal percentage of 3-6.5 wt.% for typical dioctahedral smectites source clays from the Clay Minerals Society. However, electron microprobe analyses should be considered as representing "bulk" chemistry, of what is probably a mixture of clay crystallites with a range of compositions (Deocampo et al, 2009). The breadth and asymmetry of the trioctahedral 060 peak of all samples also permits the presence of authigenic clay crystallites with a range of Fig.…”