Applied Geochemistry (v.16, #4)

Kinetics of montmorillonite dissolution in granitic solutions by F.J. Huertas; E. Caballero; C. Jiménez de Cisneros; F. Huertas; J. Linares (397-407).
Experiments measuring smectite dissolution rates in granitic solutions were carried out in a semi-batch reactor at 20, 40, and 60°C. The pH conditions of the solutions range from 7.6 to 8.5. Solid samples were confined within a dialysis membrane and introduced in the solution. The solution was renewed every 7 days and the dissolution reaction was investigated by the variation of Si concentration in the solutions. The average rates at pH∼8 were 10−14.13, 10−13.70, and 10−13.46 mol m−2 s−1, at 20, 40, and 60°C, respectively, and the activation energy for the dissolution reaction at pH ∼8 was 30.5±1.3 kJ mol−1. Comparison of the present results with other studies reveals that the montmorillonite dissolution rate depends strongly on the pH of the solution, with a minimum value at pH 8–8.5. At room temperature, the dissolution rate was found to be linearly dependent on proton (acidic conditions) or hydroxyl (basic conditions) activity in solution: Rate=10−11.39a H + 0.34   pH<8 Rate=10−12.31a OH 0.34   pH>8.5 The comprehension of the dissolution mechanism can be improved by using surface complexation theory. Correlation between speciation of surface sites and kinetic results indicated that at room temperature the dissolution rate was directly proportional to the surface concentration of >AlOH2 + and >AlO surface complexes, under acidic or alkaline conditions, respectively. Rate=10−8.0{>AlOH 2 +}  pH<8 Rate=10−8.2{>AlO }  pH>8.5 A multiple variable model is proposed to take into account simultaneously the effect of pH on dissolution rates and on activation energy. The rates estimated using the model are in good agreement with experimental dissolution rates.

Lead isotope geochemistry of the urban environment in the Pearl River Delta by Zhu Bing-Quan; Chen Yu-Wei; Peng Jian-Hua (409-417).
The study of the Pb isotopic and elemental composition of eolian dusts, aerosols and soils collected from urban areas in the Pearl River Delta, China, indicates that the atmospheric Pb pollution originates from automobile exhausts and industry. The Pb isotopes allow distinguishing between Pb from the natural background, automobile exhausts, and industrial pollution. The Pb isotopes show that the air-carried Pb pollution in Guangzhou and Foshan is a mixture of industrial Pb from the Fankou Pb-Zn giant deposit and automobile exhaust, whereas the pollution in Foshan is mainly from industry. The chemical compositions show that aerosols possess features of volcanic ash, and eolian dusts are mixtures of aluminosilicates-sulfides.

Helium and carbon isotope systematics of natural gases from Taranaki Basin, New Zealand by John R. Hulston; D.R. Hilton; I.R. Kaplan (419-436).
The chemical and isotopic compositions of gases from hydrocarbon systems of the Taranaki Basin of New Zealand (both offshore and onshore) show wide variation. The most striking difference between the western and south-eastern groups of gases is the helium content and its isotopic ratio. In the west, the Maui gas is over an order of magnitude higher in helium concentration (up to 190 μmol mol−1) and its 3He/4He ratio of 3.8 R A (where R A=the air 3He/4He ratio of 1.4×10−6) is approximately half that of upper mantle helium issuing from volcanic vents of the Taupo Volcanic Zone. In the SE, the Kupe South and most Kapuni natural gases have only a minor mantle helium input of 0.03–0.32 R A and low total helium concentrations of 10–19 μmol mol−1. The 3He/C ratio (where C represents the total carbon in the gas phase) of the samples measured including those from a recent study of on-shore Taranaki natural gases are generally high at locations where the surface heat flow is high. The 3He/CO2 ratio of the Maui gases of 5 to 18×10−9 is higher than the MORB value of 0.2 to 0.5×10−9, a feature found in other continental basins such as the Pannonian and Vienna basins and in many high helium wells in the USA. Extrapolation to zero CO2/3He and CO2/C indicates δ13C(CO2) values between −7 and −5‰ close to that of MORB CO2. The remaining CO2 would appear to be mostly organically-influenced with δ13C(CO2) c.−15‰. There is some evidence of marine carbonate CO2 in the gases from the New Plymouth field. The radiogenic 4He content (Herad) varies across the Taranaki Basin with the highest Herad/C ratios occurring in the Maui field. δ13C(CH4) becomes more enriched in 13C with increasing Herad and hydrocarbon maturity. Because 3He/4He is related to the ratio of mantle to radiogenic crustal helium and 3He/C is virtually constant in the Maui field, there is a correlation between R C/R A (where R C=air-corrected 3He/4He) and δ13C(CH4) in the Maui and New Plymouth fields, with the more negative δ13C(CH4) values corresponding to high 3He/4He ratios. A correlation between 3He/4He and δ13C(CO2) was also observed in the Maui field. In the fields adjacent to Mt Taranaki (2518 m andesitic volcano), correlations of some parameters, particularly CO2/CH4, C2H6/CH4 and δ13C(CH4), are present with increasing depth of the gas reservoir and with distance from the volcanic cone.

Solid–water partitioning of elements in Czech freshwaters by Josef Veselý; Vladimı́r Majer; Jan Kučera; Vladimı́r Havránek (437-450).
Partitioning of 41 elements between solids and water was studied by filtration and dialysis in situ in Czech freshwaters. Field-based distribution (partition) coefficients, K D, between suspended particulate matter (SPM) and filtrate (‘dissolved’ fraction) differed by 4 orders of magnitude. The highest K D values (log K D>2.0 l/g) were exhibited by Zr, Al, Ce, Pb, La, Ti, Fe, Sm, Th and Cr which are extremely insoluble in near-neutral water or generally poorly soluble (Zr,Ti). The K Ds decrease with element and DOC loading due to the relative increase of the element in the low molecular fraction. Log K D mostly increased linearly with pH within a range from 3.5 to 9. K D U decreased at pH >6 due to carbonate complexation. The colloidal fraction (>1 kDa <0.4 μm) in a reservoir with a pH of 6.8 was mainly preferred by Fe, Pb, Be, Nb, Y, Al, Ni, U and Zr. When the colloidal fraction is not differentiated from true solution, then incorrect information about partitioning may be obtained and the highest K D may decrease.

The thermal maturity of oils extracted from inclusions and the fluorescence colours of oil-bearing fluid inclusions have been measured in 36 sandstone samples from Australasian oil fields. The inclusion oils were analysed using an off-line crushing technique followed by GC–MS. A maturity assessment was made for each inclusion oil using 25 molecular maturity ratios, including a newly defined dimethyldibenzothiophene ratio (DMDR). Each inclusion oil was placed in one of 4 maturity brackets, approximately equivalent to early, mid, peak and post oil generation windows. The fluorescence colours of oil inclusions were visually-discriminated into “blue”, “white” and “yellow plus orange” and their proportions estimated using point counting techniques. Sixteen samples have >85% of oil inclusions with blue fluorescence, whilst other samples have more variable fluorescence colours. One sample has 100% of oil inclusions with yellow plus orange fluorescence. The results show that samples containing mainly blue-fluorescing oil inclusions have thermal maturities anywhere within the oil window. In particular, the molecular geochemical data strongly suggests that oil inclusions with blue fluorescence can have relatively low maturities (calculated reflectance <0.65%), contrary to the widely applied assumption that blue fluorescence colours indicate high maturities. Samples containing mainly white-fluorescing oil inclusions have maturities anywhere within the oil window and cannot be distinguished using molecular geochemical parameters from samples containing mainly blue-fluorescing oil inclusions. Though few in number, samples with mainly yellow and orange-fluorescing oil inclusions tend to have maturities in the lower half of the oil window. The data presented strongly suggest that although the relationship between API gravity and the fluorescence properties of crude oils is well established, the extension of this relationship to the use of the fluorescence colours of oil inclusions as a qualitative thermal maturity guide is not justified. Fluorescence colour depends in the first instance on chemical composition, which is controlled not only by maturity but by several other processes. For example, inclusions in samples from below current or residual oil zones in the Timor Sea contain a high proportion of yellow- and orange-fluorescing oil inclusions compared to the overlying oil zones, which are dominated by blue-fluorescing oil inclusions. This observation is interpreted to be due to water washing causing molecular and gross fractionation of oils prior to trapping. Fractionation of the gross composition of oil during the inclusion trapping process may also be a significant controlling process on the fluorescence colours of oil inclusions, due to the preferential adsorption of polar compounds onto charged mineral surfaces. A trapping control is strongly supported by synthetic oil inclusion work. Care should be taken when interpreting the charge history of samples containing oil inclusions with mixed fluorescence colour populations, such as those from the Iagifu-7x well in the Papuan Basin. It is possible that the different colour populations represent a single oil charge, with oil inclusions trapped under slightly different conditions or at slightly different grain surfaces, rather than multiple migration events.

The usefulness of stable isotopes of dissolved SO434S and δ18O) to study recharge processes and to identify areas of significant inter-aquifer mixing was evaluated in a large, semi-arid groundwater basin in south-eastern Australia (the Murray Basin). The distinct isotopic signatures in the oxidizing unconfined Murray Group Aquifer and the deeper reducing Renmark Group confined aquifer may be more sensitive than conventional chemical tracers in establishing aquifer connections. δ34S values in the unconfined Murray Group Aquifer in the south and central part of the study area decrease along the hydraulic gradient from 20.8 to 0.3‰. The concomitant increasing SO4/Cl ratios, as well as relatively low δ18OSO4 values, suggest that vertical input of biogenically derived SO4 via diffuse recharge is the predominant source of dissolved SO4 to the aquifer. Further along the hydraulic gradient towards the discharge area near the River Murray, δ34S values in the unconfined Murray Group Aquifer increase, and SO4/Cl ratios decrease, due to upward leakage of waters from the confined Renmark Group Aquifer which has a distinctly low SO4/Cl and high δ34S (14.9–56.4‰). Relatively positive δ34S and δ18OSO4 values, and low SO4/Cl in the Renmark Group Aquifer is typical of SO4 removal by bacterial reduction. The S isotope fractionation between SO4 and HS of ∼24‰ estimated for the confined aquifer is similar to the experimentally determined chemical fractionation factor for the reduction process but much lower than the equilibrium fractionation (∼70‰) even though the confined groundwater residence time is >300 Ka years. Mapping the spatial distribution of δ34S and SO4/Cl of the unconfined Murray Group Aquifer provides an indicative tool for identifying the approximate extent of mixing, however the poorly defined end-member isotopic signatures precludes quantitative estimates of mixing fractions.