Neoformed Clays Research Articles

The Behavior of Rare Earth Elements during Green Clay Authigenesis on the Congo Continental Shelf

Clay mineral authigenesis at continental margins plays an important role in global marine element cycles. However, despite being increasingly used as tracers for both modern and past oceanographic conditions, the behavior of the rare earth elements (REEs) and their isotopes during marine clay authigenesis still remains poorly known. In this study, we report on a detailed geochemical investigation of glauconite from the West African continental shelf, near the mouth of the Congo River. Elemental, neodymium, and hafnium isotope analyses were conducted on both acid leachate and separated clay-size fractions of glauconite pellets, in order to investigate the behavior of REE during the formation of authigenic clays. Our data indicate that kaolinite dissolution and subsequent Fe-bearing clay authigenesis act as a net source of REEs to seawater. We show that enhanced glauconitization, as inferred from increasing Fe and K contents, is accompanied by significant decoupling of the REE toward markedly LREE-enriched shale-normalized patterns in neoformed clay separates. Using both Nd and Hf isotopes and SEM observations, we rule out any seawater influence and argue that this shift primarily reflects the progressively overwhelming presence of insoluble nanocrystals of detrital LREE-rich phosphates, which are known to occur in close association with kaolinite in tropical soils. Due to their marked insolubility in surface environments, such nanocrystals can be preserved during kaolinite dissolution and subsequently incorporated into the aggregates of authigenic green clays forming the peloids. Most strikingly, we show that the combined influence of net REE loss (due to kaolinite dissolution) and decoupling (due to subsequent entrapment of inherited LREE-bearing accessory phases into neoformed clay minerals) is accompanied by preferential release of a dissolved REE fraction characterized by seawater-like distribution patterns. These findings reinforce the emerging view that clay mineral dissolution and authigenesis at continental margins possibly play a major role in marine REE cycling.

Meteoric fluid infiltration in crustal-scale normal fault systems as indicated by δ18O and δ2H geochemistry and40Ar/39Ar dating of neoformed clays in brittle fault rocks

Both the sources and pathways of fluid circulation are key factors to understanding the evolution of low-angle normal fault (LANF) systems and the distribution of mineral deposits in the upper crust. In recent years, several reports have shown the presence of meteoric waters in mylonitic LANF systems at mid-crustal conditions. However, a mechanism for meteoric water infiltration to these mid-crustal depths is not well understood. Here we report paired δ 18 O and δ 2 H isotopic values from dated, neoformed clays in fault gouge in major detachments of the southwest United States. These isotopic values demonstrate that brittle fault rocks formed from exchange with pristine to weakly evolved meteoric waters at multiple depths along the detachment. 40 Ar/ 39 Ar dating of these same neoformed clays constrains the Pliocene ages of fault-gouge formation in the Death Valley area. The infiltration of ancient meteoric fluids to multiple depths in LANFs indicates that crustal-scale normal fault systems are highly permeable on geologic timescales and that they are conduits for efficient, coupled flow of surface fluids to depths of the brittle-plastic transition.

Soil micromorphogenesis and Early Holocene paleoclimate at the desert margin of Southern Arabia .

The Ramlat as-Sab’atayn desert margin near Ma’rib, Yemen, displays well-preserved Early Holocene paleosols that are documented by micromorphological and pedological data. The buried soils, which are represented by Ahb horizons, indicate soil formation mostly before 8.3 cal ka BP. In contrast, sandy cover sediments without signs of pedogenesis appeared between 8.3 and 6.6 cal ka BP due to increasing aridity. Characteristic micromorphological features of the cover sediments are a single grain microstructure, crystallitic b-fabric, predominant occurrence of fresh sideromelane, and remnants of microlayers. Micromorphological pedogenic features in the buried Ah horizons include a subangular blocky microstructure, undifferentiated b-fabric as a result of enrichment of organic matter and decalcification, and the predominant occurrence of completely altered sideromelane. Most of these horizons appeared to be nearly completely decalcified so that in parts a stipple speckled b-fabric and neoformed clay coatings could be detected as a result of stronger weathering and soil formation. Pedogenic data provide important information about Holocene climate fluctuations, including the amount of precipitation, which was calculated on the basis of geochemical data from buried A, AB and B horizons. The buried paleosols represent moist climate conditions with precipitation ranging from 400 to 600 mm a<sup>-1</sup>.

Pathways of volcanic glass alteration in laboratory experiments through inorganic and microbially-mediated processes

AbstractRhyolitic obsidian was reacted with natural waters to study the effect of water chemistry and biological activity on the composition and formation mechanisms of clay. Two sets of experiments (18 months, 6 years) used fresh, hypersaline water (Mg-Na-SO4-Cl- and NaCl-rich) and seawater. The 6-year experiments produced the transformation of obsidian into quartz, apparently by in situ re-crystallization (Cuadros et al., 2012). The most abundant neoformed clay was dioctahedral (typically montmorillonite), indicating chemical control by the glass (where Al > Mg). Altered glass morphology and chemistry in the 18-months experiments indicated in situ transformation to clay. Magnesium-rich (saponite) clay formed under water-chemistry control in the bulk and within biofilms with elevated Mg concentration (Cuadros et al., 2013). The contact between microbial structures and glass was very intimate. Glass transformation into quartz may be due to some characteristic of the obsidian and/or alteration conditions. Such combination needs not to be uncommon in nature and opens new possibilities of quartz origin.

Silcrete and alunite genesis in alluvial palaeosols (late Cretaceous to early Palaeocene, Duero basin, Spain)

An extensive formation of silcretes occurs in the upper half of a late Cretaceous-to-Palaeocene Siderolithic Unit consisting of conglomerates, sandstones and sandy mudstones along the western margin of the Tertiary Duero basin (Spain). The unit is arranged in fining-upward sequences that were deposited in a fluvial braided system with floodplains subject to periodic events of exposure and drying out. The original mineralogy of the Siderolithic Unit is seen in the less altered levels, where it consists of quartz, illite–white mica, kaolinite and, at the top of the unit, K-feldspar, all coming from the palaeoalteration mantles developed on the metasediments of the Variscan Basement. The unit is characterized by a marked development of alluvial palaeosols at the uppermost part of each sequence. At field scale, the pedogenic structures and features are evident from the mottled pattern, burrowing, cracking planes and root traces; at micromorphological scale, an intense development of birefringent fabrics is seen in clays. Micas and feldspars show evidence of alteration due to hydrolysis, while the more resistant quartz grains show common corrosion gulfs and have become the only surviving elements in the most silicified beds. The neoformed minerals are related to the pedodiagenetic processes developed on the floodplains. The main neoformed phase is opal (probably amorphous silica in origin that aged to opal-CT), which ranges from local cementations in pores to an almost complete replacement of the rock, specially the finest components (matrix). Kaolinite is the most abundant neoformed clay. Locally, veinlets of alunite accompany the opalized levels. All the processes described occurred in a seasonal tropical climate. During dry episodes, strong evaporation resulted in an increase in pH, favouring the hydrolysis of the original silicates. The paucity of Mg 2+ and Ca 2+ in the groundwater would have favoured the increase in pH. In ensuing wetter periods, pHs fell rapidly, resulting in the precipitation of kaolinite and also favouring the precipitation of opal and alunite. The alunite could have formed either by oxidation of the iron sulphides developed in reducing areas of the flood plains or by evaporitic concentration of sulphate in Ca 2+- and Mg 2+-free groundwaters.

Effects of chemical weathering on infrared spectra of Columbia River Basalt and spectral interpretations of martian alteration

We investigated the mineralogy of basalt weathering rinds and fresh basaltic rocks using visible/near-infrared (VNIR) ( λ = 0.4–2.5 μm) and thermal emission ( λ = 6–30 μm) spectroscopy to 1) constrain the effects of chemical weathering on rock spectra, and 2) further understand the context of infrared spectra of Mars, which may contain evidence for weathered rocks and particulates derived from them. VNIR spectra of weathered rock surfaces are generally redder and brighter than fresh surfaces. Thermal infrared spectra of weathered basalts show evidence for aluminous opal and clay minerals (or clay precursor mineraloids) in natural surfaces. Supporting VNIR observations generally do not show the same evidence for neoformed clays or silica because of their textural occurrence as thin coatings and microfracture-fill, and possibly due to poor crystallinity of the aluminosilicate weathering products in this context. Spectral trends observed at Mars, such as the detection of low to moderate (10–25%) abundances of silica and clay that are observed in the thermal infrared but not in the VNIR, are therefore consistent with trends observed for natural rock surfaces in the laboratory. The combined use of thermal infrared and VNIR suggest that vast areas of martian dark regions contain sandy–rocky basaltic materials with weathering rinds and thin coatings that could have formed in conditions of relatively low water/rock ratios.

Pedosedimentary, cultural and environmental significance of paleosols within pre-hispanic agricultural terraces in the southern Peruvian Andes

Paleosols were exposed in sections through four abandoned pre-Hispanic agricultural terraces surrounding an infilled mire basin in the southern Peruvian Andes. The two paleosols beneath the Tocotoccasa terrace represent the original ‘natural’ solum and a later soil formed after construction of the agricultural terrace, probably during the early Middle Horizon cultural period (615–695 AD). The soil at the current surface developed subsequent to the building up and reconstruction of the terrace, possibly during the late Late Intermediate period (1200–1400 AD). Micromorphology revealed an unexpected abundance of clay coatings within the upper terrace paleosol and surface terrace soil, a phenonemon attributed to the migration and/or accumulation of neoformed clay produced from the weathering of very unstable volcanic clasts, perhaps fuelled by arid/humid climatic oscillations and/or seasonal input of irrigation waters. The paleosols at Tocotoccasa could not be correlated with any degree of confidence with those beneath the other three terraces due to differences in pedosedimentary properties and uncertainties over chronological controls. Thus, it seems likely that either the terraces were (re)constructed and utilised over different cultural periods or that there is significant variation in the extent of weathering of material used for reconstruction of the terraces. Unfortunately, it cannot be ascertained from the data available whether the terraces were abandoned for any significant period of time prior to reconstruction and, if so, whether this was a regional phenomenon related to climate, social, or economic changes.

The role of porosity in the development of authigenic dolomite and palygorskite. An example in siliceous sandstones of continental origin in Western Spain

During the Tertiary, the Hercynian basement of the Iberian Peninsula was uneven due to Alpine tectonism which led to the development of a set of horsts and grabens forming its main geological structure. One of these grabens is the Ciudad Rodrigo Basin, located close to the Portuguese border. The south-western margin of this graben is filled with arkosic sandstones of continental origin that, once deposited, underwent important low-grade diagenetic processes related to the surface processes. The main diagenetic processes were hydrolysis, replacement by carbonate, and argillization to palygorskite. These three processes led to the differentiation of three main levels within the outcrops studied: (1) the Lower Level, where the original clayey sandstone outcrops, displaying fairly hydromorphic traits; (2) the Middle Level, where part of the clayey matrix has been replaced by dolomitic carbonate; and (3) the Upper Level, where palygorskite is the new neoformed clay. The change in porosity seems to have been the controlling factor, since it governs the dynamics of the solutions inside the rocks. These conditions changed from being diffusion-driven, at the start of the diagenesis, to flow-driven as porosity increased.

Alluvial Sandstone Composition and Paleoclimate, II. Authigenic Mineralogy

ABSTRACT Various empirical and theoretical arguments suggest that early formed cement in sandstone is a product of meteoric water whose chemistry is controlled by climate. The nature and distribution of early silicate cement in Gondwana, Fountain, and Cutler sandstone support this supposition. Sandstones from all three units have experienced a broadly similar diagenetic history characterized by two stages of authigenesis. Stage I was typified by neomorphic development of kaolinite, chlorite, smectite, quartz, and, additionally in the Cutler Formation, laumontite. Stage II was dominated by replacement reactions involving the production mostly of illite, iron oxide, carbonate minerals, and, mostly in the Gondwana sandstone, a second generation of quartz. Thermodynamic relations, hydrologic constraints, and radiometric dating of the late-stage illite indicate that early authigenesis took place within a few million years of deposition at burial depths measured in hundreds of meters. Consequently, the nature of the silicate cement was a function of the groundwater chemistry, as controlled by climate. During times of relative aridity, ionic concentration of the groundwater was high, and high proportions of smectite, chlorite, and, more rarely, laumontite formed. Such was the case in Petrofacies I, IV, V of the Gondwana Supergroup (for a definition of Gondwana petrofacies see part I of this study) in the Cutler Fm., and in the upper 150 m of the Fountain Fm. During times of high precipitation, pore water was dilute, which promoted authigen c formation of kaolinite and quartz. This is represented by sandstones in petrofacies II and VI of the Gondwana and by sandstones in the lower 200 m of the Fountain Fm. Illite is especially abundant as a pseudomorphic replacement of kaolinite in the older Gondwana sandstones. Illitization apparently began at burial depths of about 1,600 m at a temperature of about 75°C. Illite is not common in the Cutler Formation and is present only in subordinate amounts in the Fountain Formation from the study area. The 18O values of early clay cement in the Gondwana sandstones range systematically from base to top from 5.00 to 13.20. The systematic variation in 18O value of Gondwana clay reflects the gradual migratory drift of the Gondwana harm toward lower latitudes through time. Average 18O values for Cutler cement (13.23) and Fountain cement (19.82) are greater than the analogous values for Gondwana cements. This is consistent with the lower-latitude setting of deposition for the Cutler and Fountain relative to the latitudinal location of the Gondwana basins. However, the values are anomalously lower than what have been observed in modern-day neoformed clays in weathering profiles from low latitudes.

Isotopic Evidence for Clay Mineral Weathering and Authigenesis in Cryoboralfs

Clay mineral distribution and weathering were studied in two Cryoboralf pedons developed on texturally different tills in northeastern Alberta, Canada, to elucidate authigenic mineral formation in these soils. The crystalline clay minerals were identified by x-ray diffraction analyses, and quantified by analyses for cation exchange, surface area and elemental content. The total non-crystalline clay component was determined using selective dissolution treatments. The crystalline clay suite of the parent materials consisted of an admixture of smectites, kaolinite, dioctahedral mica, and chlorite. Beidellite and montmorillonite were both identified as components of the smectite group. Kaolinite was negatively enriched in the E horizons in all clay size separates, whereas mica and smectites were enriched in the Bt horizons as a result of lessivage. Routine chemical and mineralogical characterization suggested apparent neoformation of beidellite in the E horizons of these pedons. As the detrital crystalline clays in these soils are isotopically distinct from any neoformed clay species, the 18O content of both crystalline and poorlycrystalline clay species was used to explain the observed clay mineral variations within the sola. The 18O content of the crystalline clay separates indicated the observed inter-horizon variability in content of the individual phyllosilicate mineral species was a result of physical translocation rather than authigenesis. The interpretation of neoformation of beidellite in E horizons was not supported by the 18O data. Changes in measured 18O content of clay separates following selective dissolution provided evidence, however, of authigenic poorly crystalline components. Intra-profile variation in the 18O signature of the authigenic clays is explained in terms of evaporative effects on 18O enrichment of the soil water.