Lindsay J. McHenry

Jarosite in a Pleistocene East African saline-alkaline paleolacustrine deposit: Implications for Mars aqueous geochemistry

[1] Jarosite occurs within altered tephra from the saline‐alkaline paleolake deposits of Pliocene‐Pleistocene Olduvai Gorge, Tanzania. Zeolites (mainly phillipsite), authigenic K‐feldspar, and Mg/Fe‐smectites dominate the mineral assemblage, indicating saline‐ alkaline diagenetic conditions (pH > 9). As jarosite is ordinarily an indicator of acidic conditions on Earth and Mars, its association with such undisputed high‐pH indicators is unexpected. Of 55 altered tephra samples collected from the paleolake basin and margin deposits, eleven contained jarosite detectable by X‐ray Diffraction (XRD) (>0.15%). Mossbauer spectroscopy, Fourier Transform Infrared Reflectance (FTIR), Electron Probe Microanalysis (EPMA), X‐ray Fluorescence (XRF), and Scanning Electron Microscopy (SEM) analyses confirm the presence and nature of the jarosite. This paper documents this occurrence and presents mechanisms that could produce this unusual and contradictory mineral assemblage. We favor a mechanism by which jarosite formed recently, perhaps as modern ground and meteoric water interacted with and oxidized paleolacustrine pyrite, providing local and temporary acidic conditions. However, local groundwater (at modern springs) has a pH > 9. In recent studies of Mars, the presence of jarosite or other Fe or Mg sulfates is often used to indicate dominantly acidic conditions. Regardless, the current study shows that jarosite can form in sediments dominated by alkaline minerals and solutions. Its coexistence with Mg/Fe smectites in particular makes it relevant to recent observations of Martian paleolakes.

Hominin raw material procurement in the Oldowan-Acheulean transition at Olduvai Gorge

The lithic assemblages at the Oldowan-Acheulean transition in Bed II of Olduvai Gorge, Tanzania, represent a wide variety of raw materials reflecting both the diversity of volcanic, metamorphic, and sedimentary source materials available in the Olduvai basin and surroundings and the preferences of the tool-makers. A geochemical and petrographic systematic analysis of lava-derived archaeological stone tools, combined with textural and mineralogical characterization of quartzite, chert, and other metamorphic and sedimentary raw materials from two Middle and Upper Bed II sites, has enabled us to produce a comprehensive dataset and characterization of the rocks employed by Olduvai hominins, which is used here to establish a referential framework for future studies on Early Stone Age raw material provenancing. The use of rounded blanks for most lava-derived artifacts demonstrates that hominins were accessing lava in local stream channels. Most quartzite artifacts appear to derive from angular blocks, likely acquired at the source (predominantly Naibor Soit hill), though some do appear to be manufactured from stream-transported quartzite blanks. Raw material composition of the EF-HR assemblage indicates that Acheulean hominins selected high-quality lavas for the production of Large Cutting Tools. On the other hand, the HWK EE lithic assemblage suggests that raw material selectivity was not entirely based on rock texture, and other factors, such as blank shape and availability of natural angles suitable for flaking, played a major role in Oldowan reduction sequences.

Geochemistry and Mineralogy of Laetoli Area Tuffs: Lower Laetolil through Naibadad Beds

Tuffs from the Lower Laetolil, Upper Laetolil, Ndolanya, and Naibadad Beds of the Laetoli and Kakesio areas of Tanzania are characterized and compared to serve as correlation tools for this important paleoanthropological region. These beds contain variably-altered tuffs of original carbonatitic, nephelinitic, phonolitic, trachytic, and rhyolitic composition, tracking changes in volcanic source area and local diagenetic conditions. Previous studies (e.g., Hay 1987) show compositional differences between tuffs from different Laetoli area beds, but do not provide the tuff-specific information needed to create a high-resolution tephro-stratigraphic framework. This study details differences in mineral composition between individual tuffs (where distinguishable) within each bed. The Lower Laetolil tuffs have similar compositions but can generally be distinguished by their andradite compositions. Titanomagnetite and perovskite compositions help distinguish individual Upper Laetolil tuffs, and divide the bed compositionally into lower (UL 1–4) and upper (UL 5–8) units. Augite compositions distinguish the individual Ndolanya tuffs analyzed in this study, whereas glass compositions distinguish individual Naibadad tuffs. The variable degree of alteration observed within and between sites at Laetoli makes the comparison of mineral assemblages an unreliable means for identifying tuffs. Glass, nepheline, and melilite were likely significant components of the original Laetolil Beds tuff mineral assemblages, but are now recognized in few (if any) samples except by the presence of likely alteration products. Feldspar and biotite also show signs of degradation. A comparison of glass and phenocryst compositions between the Naibadad tuffs and the tuffs of lower Bed I at Olduvai Gorge does not support a direct correlation between the two sites. The presence of quartz and rhyolitic and trachytic glass in both does suggest a common source, likely Ngorongoro Crater.

Secondary minerals associated with Lassen fumaroles and hot springs: Implications for martian hydrothermal deposits

Abstract The active hot springs, fumaroles, and mud pots of the southwestern Lassen hydrothermal system include various alteration environments, which produce a range of hydrothermal mineral assemblages. Analysis of water, mineral precipitates, altered sediment, and rock samples collected at and near these features at Sulphur Works, Bumpass Hell, Little Hot Springs Valley, and Growler and Morgan Hot Springs reveals conditions ranging from ~100 °C acid-sulfate fumaroles (e.g., Sulphur Works and Bumpass Hell) to near-neutral hot springs (e.g., Growler and Morgan), and includes both oxidizing and reducing conditions. Resulting hydrothermal minerals include a wide variety of sulfates (dominated by Al-sulfates, but also including Fe2+, Fe3+, Ca, Mg, and mixed-cation sulfates), sulfides (pyrite and marcasite), elemental sulfur, and smectite and kaolinite clays. Most altered samples contain at least one silica phase, most commonly quartz, but also including cristobalite, tridymite, and/or amorphous silica. Quartz and other silica phases are not as abundant in the less altered rock samples, thus their abundance in some hydrothermally altered sediment samples suggests a detrital origin, or formation by hydrothermal alteration (either modern or Pleistocene); this requires a high degree of diagenetic (or epigenetic) maturation. These results support a previously identified model that the Lassen hydrothermal system involves the de-coupling of a vapor phase (which becomes acidic as it oxidizes near the surface, producing acid-sulfate fumaroles at higher elevations at Sulphur Works and Bumpass Hell) from the residual near neutral thermal waters that emerge as hot springs at lower elevations (Growler and Morgan). Because both acid-sulfate fumarole and near-neutral sinter-producing hot springs have been invoked to explain the silica-rich deposits observed by the Mars Exploration Rover Spirit near Home Plate in the Columbia Hills on Mars, Lassen can serve as a useful terrestrial analog for comparison.

From the Oldowan to the Acheulean at Olduvai Gorge, Tanzania – An introduction to the special issue

Even before setting foot at Olduvai Gorge, Louis Leakey was certain that he would find handaxes there (Leakey, 1951). Apparently, he did so on the very first day of the 1931 expedition, the first that Louis Leakey ever conducted at Olduvai. More unexpectedly, Louis Leakey also found an archaic, handaxefree industry in the lower beds of Olduvai (Leakey et al., 1931), which then he formally termed Oldowan (Leakey, 1936). In one way or another, all studies on the transition from the Oldowan to the Acheulean in East Africa and elsewhere, stem from Louis Leakey’s (e.g., 1936, 1951) initial reports at Olduvai, the first site where a stratigraphic sequence showed that handaxes emerged after a coreand-flake technology.