Sam Chaudhuri

Effects of experimental leaching on RbSr and KAr isotopic systems and REE contents of diagenetic illite

Leaching with 1 N HCl, 1 N NH4Cl, acetone, humic acid extracts, cation-exchange resin and NH4-EDTA have negligible effects on the RbSr and KAr systems of different size fractions (< 0.4 and 0.8–2 αm) of a 1M diagenetic illite. None of these treatments, including the 1 N HCl treatment, induced preferential removal of radiogenic 87Sr and 40Ar from mineral structures. However, the 1 N HCl treatment removed significant amounts of Sr and REE, as compared to the treatments by NH4-EDTA and cation-exchange resin. However, much of these HCl-derived elements resided in non-clay minerals such as carbonate and Fe-oxide phases. The KAr and RbSr ages of the samples were not affected by the HCl treatment any more than by the NH4-EDTA or the cation-exchange resin. Because of potential loss of solid material during elutriation of differentially treated samples, the HCl treatment is preferred over other methods allowing sufficient controls on the isotopic and mass balances between the leachate and the corresponding residue of an untreated sample. For RbSr isotopic dating purposes of clay minerals from sedimentary and metasedimentary rocks, leaching experiments give information about the ambient environment of crystallization. Analysis of both the leachate and residue under carefully controlled conditions is recommended.

Diagenetic Evolution of Clay Minerals in Oil-Bearing Neogene Sandstones and Associated Shales, Mahakam Delta Basin, Kalimantan, Indonesia

The clay fraction of the Mahakam Delta Basin (eastern Kalimantan, Indonesia) consists of mixed-layer illite/smectite, kaolinite/dickite, detrital illite, and chlorite. Kaolinite formed early as vermicular particle aggregates, and thick dickite booklets formed late in pore spaces. The dickite particles probably grew in deeper intervals of the sequence, in contact with brines acidified by decarboxylation of organic matter, during dissolution of K-feldspar and upward migration of potassium. In the Handil field on the nearshore anticline, evolution of the mixed-layer illite/smectite is characterized by a decrease in expandability with depth. In the Tunu field, located on the next offshore anticline, variable amounts of expandable layers suggest a different evolution. The conversion process of smectite to illite layers in the mixed layers depends on the lithology of the host rocks, transformation in the shales, and dissolution-precipitation in the sandstones. Sample location also played a role in the conversion process because the types of illite clays in the two locations differ when they are compared at similar degrees of evolution. Present-day temperatures and paleotemperatures, vitrinite reflectance, and illitization of the mixed-layer illite/smectite all suggest that organic matter and clay particles evolved differently in the Handil field, but similarly in the Tunu field. Integration of the analytical results obtained here and comparison with a two-dimensional numerical model suggest that hydrocarbon generation took place in the deeper synclinal zones of the sequence, and that oil migrated upward with brines, probably inducing most of the illitization in the upper sequence. Both models emphasize the role of faults in channelizing fluid flow during the final stages of evolution of the basin.

Fluctuations of Caspian Sea level: Beyond climatic variations?

The continued rising mode, since about 1978, in the level of the Caspian Sea cannot be ascribed to a change in the hydrologic balance that is governed solely by runoff, precipitation, and evaporation. Such a change fails to explain some of the chemical links (K/Cl, Rb/Cl, Ca/Cl, Sr/Cl, and K/Rb) between Kara Bogaz and Caspian Sea waters. A more realistic model must include a tectono-hydrodynamic factor that allows for subsurface addition of saline water to Caspian Sea and Kara Bogaz waters. The subsurface water component could be akin to the coastal thermal springs in the area, but must dissolve some evaporite deposits in the subsurface before addition to the waters of the inland sea. A budget calculation gives an estimate of 2% to 9% subterranean addition of saline water to the seawater. Such an addition could cause a rise in the level of the Caspian Sea from 63 to as much as 225 cm within the past 20 yr.

K-Ar DATING OF ILLITIC FRACTIONS OF ESTONIAN "BLUE CLAY" TREATED WITH ALKYLAMMONIUM CATIONS

Two clay fractions of a Cambrian claystone from Estonia, consisting essentially of illite and 20% expandable illite-smectite, (I-S) were treated with C12 and C18 alkylammonium cations for K-ex-change. Both the untreated and treated samples were dated by the K-Ar method. The treated clays lost several percent of their original K2O, with greater losses for longer-chain cations and for longer reaction time, in accordance with previously published studies. The dates of the treated clay fractions were 20–30 Ma lower than those of the untreated clays. The decrease in the dates suggests preferential opening of older, detrital clays. The K-Ar dates of the illite layers susceptible to K extraction by the various treatments were calculated by subtracting K2O and radiogenic 40Ar values of the consecutive step products, and they were plotted against the total % K2O removed, used as an indicator of the reaction progress. Extrapolation of the plot revealed a detrital (1550 Ma) and a diagenetic (380 Ma) age for the 2 illitic minerals present in the investigated shale sample. The inferred Devonian age of diagenesis of the Estonian clay corresponds to the period of massive dolomitization in the area. Both alteration processes can be related to a Devonian incursion of hot or alkaline fluids, which helps to explain the occurrence of 20% expandable I-S in claystones that have never been buried more than 1000 m.Extrapolated K-Ar ages and K2O contents of the illitic minerals, estimated from the X-ray diffraction (XRD) data, were used to model the experimental data. A good agreement was reached when dilution effects (chlorite and expanded illite) were taken into account.

K TRANSFER DURING BURIAL DIAGENESIS IN THE MAHAKAM DELTA BASIN (KALIMANTAN, INDONESIA)

Progressively buried sandstones and shales of the Mahakam Delta Basin in Indonesia were studied. Mineralogical, morphological and K-Ar isotopic data were obtained for clay, mica and feldspar minerals. The data indicate that K necessary for the illitization of illite/smectite mixed-layer minerals was supplied mainly from K-feldspar alteration within the sandstones and from mica within the shales. Most of the K-feldspar alteration for both the shale and sandstone samples were observed outside the main zone of illitization, which is restricted to the upper 2000 m of sediment. The feldspar grains were altered below this depth for both lithologies. Therefore, illitization requires an open sedimentary system. This is in contrast to the illitization model for deeply buried shales of the Gulf Coast. That system is commonly assumed to be a closed K system.

IMPACT OF TERMITE ACTIVITY ON SOIL ENVIRONMENT: A PERSPECTIVE FROM THEIR SOLUBLE CHEMICAL COMPONENTS

An investigation on varied types of termite mounds relative to the nearby soils that are not inhabited by the termites in different places of Cameroon show that the activity of the termites is increasing the contents of most major and some trace elements in the termite mounds, except for Si and sometimes Fe, Mn, Na and K. These released elements are relocated into newly formed mineral phases that are dissolved by either H2O or dilute HCl leachings. The Ca and Mn released by the termite activity testify for crystallization of Ca-Mg carbonates and phosphates as well as of Fe oxy-hydroxides and/or Mn hydroxides. Termite activity also induces an increase in the lanthanide contents, the mound materials being especially enriched in light lanthanides relative to the corresponding soils without termite activity. The shapes of the patterns support precipitation of Mn-Fe oxy-hydroxides and Ca carbonates-phosphates. The increased amounts of Eu and Ce linked to termite activity seem to relate to the occurrence of reducing agents that are released by the termites, modifying Eu+3 into Eu+2 and Ce+4 into Ce+3, favoring in turn selective incorporation of Eu+2 and Ce3+ in the new phases of the termite mounds. Another consequence of the termite activity is the precipitation of H2O and HCl extractable phases having low Sr/Ca ratios. Even if the K/Rb values of the termite mounds are typical for common soil-forming silicate minerals, their relocation by an inorganic process alone does not explain an abnormally high ratio in the H2O leachable mineral phases. It was also shown that the main source for K and Rb of the dissolved phases is not only the interlayer site of clay particles, but also nutrients immobilized in and by the termites.

The isotopic composition of strontium in Permian limestones, eastern Kansas

Abstract The strontium ( 87 86 ) ratios in ten samples of Permian limestones from eastern Kansas vary from 0.7070 to 0.7091. This range is beyond the possible analytical error of ± 0.0003 per single analysis determined from sixteen analyses of an interlaboratory standard. The possible reasons for this range may be: (1) actual fluctuations in local environments in Permian seas; (2) preferential leaching of radiogenic 87Sr from argillaceous constituents during the analytical procedures; (3) secondary enrichment of 87Sr in the limestones due to recrystallization; (4) some other process or a combination of any of the above. Comparison of total strontium and silica contents with the isotopic Sr ( 87 86 ) data indicate no direct correlation between these parameters; hence the range is most probably due to reason 1. In addition, attempts to pinpoint the isotopic composition of Sr on the marine geochron proposed by Hurley et al. (1965) have failed, and the data suggest that present methods of determining this geochron are inadequate at present.

Inter-basinal comparison of the diagenetic evolution of illite/smectite minerals in buried shales on the basis of K-Ar systematics

The K-Ar systematics of illite/smectite (I/S) mixed layers in deeply buried shales from the Gulf Coast, the North Sea and the Mahakam Delta basins have been compared to provide additional perspectives on the diagenetic evolution of these minerals. Comparison of the results suggests that illitization proceeds similarly in the 3 basins, at least for the increase in the illite-layer and K contents, despite differences in the provenance of the detrital components, the ages of deposition, the depths of burial and the tectonic history of the basins. Analysis of the trends with depth in the illite-layer and K contents of I/S-enriched size fractions of shales in the North Sea and the Mahakam Delta basins shows that these trends represent segments of the more complete trends from I/S minerals of the Gulf Coast area.The trends with depth in the radiogenic 40Ar contents and in the K-Ar ages of the I/S-rich fractions in the North Sea and Mahakam Delta basins suggest that, relative to the reference trends of the Gulf Coast area, the K-Ar system of the clay material is more dependent on the behavior of the radiogenic 40Ar than on the occurrence or non-occurrence of detrital grains in the size fractions. Recasting of the available data suggests that retention of radiogenic 40Ar by the illite-type minerals occurs in the intense illitization zone and release occurs in the deeper part of the basins. We therefore speculate that the illitization process of the I/S mixed layers of progressively buried shale-type sediments could be controlled by a transformation process integrating dissolution of detrital components in poral rock environments relatively impermeable to radiogenic 40Ar. These excesses, which might be partly or completely erased in deeper parts of the sedimentary basins, question the application of the K-Ar dating method on clay minerals extracted from shales.

Revisited Isotopic Dating Methods of Sedimentary Minerals for Stratigraphic Purpose

Cormier (1956) and Wasserburg et al. (1956) initiated many isotopic investigations of sedimentary minerals and whole rocks for stratigraphic purposes. Since this pioneering period, different approaches have been evaluated to identify the isotopic signatures of mineral components which may set reasonably narrow limits to the time of deposition of sediments. Many contradictory opinions have been expressed about the merits of these approaches and the significance of the various isotopic signatures in relation to the stratigraphic ages of the studied minerals. It is obvious from analysis of the available literature, that questionable dates were often generated without the necessary efforts to delineate clearly the origin of the analysed materials and to evaluate the potential impact of mineral impurities on the final dates.