Weathering Of Plagioclase Research Articles

Interstitial water profiles and sites of diagenetic reactions, Leg 35, DSDP, Bellingshausen Abyssal Plain

Discrepancies between predicted and observed interstitial water profiles for sites 322 and 323, Leg 35, Bellingshausen Abyssal Plain, were used to identify sites of reaction for further mineralogical and chemical investigations. Two major reaction sites were identified at site 323: (1) In the silicification zone between 410 and 505 m depth, where dissolution of biogenic (opaline) silica, plagioclase and a few coccoliths and the formation of opal-CT, Mg-rich smectite and K-feldspar are responsible for the observed silica, Ca 2+, Mg 2+, and K + interstitial water gradients. Dissolution of biogenic silica provided most of the silica for the porcelanites. (2) In the basalt, weathering of pyroxene and plagioclase and the formation of celadonite, smectite, calcite, and goethite are probably responsible for the observed Ca 2+, Mg 2+, and K + interstitial water gradients below the silicification zone. The chemistry of authigenic smectites reflects the composition of their precursor(s).

Etude théorique de l'altération des plagioclases. Bilans et conditions de stabilité des minéraux secondaires en fonction de la la pression partielle de CO2 et de la température (0°C à 100 °C)

Using a theoretical approach based on experimental thermodynamic data and solution-mineral equilibria, the chemical ways of plagioclase weathering are calculated. Influences of temperature, CO₂ partial pressure and initial composition are discussed. Evolutions towards podzolic or bauxitic soils and towards kaolinitic or montmorillonitic products are confirmed.

Chemical weathering and Rb-Sr whole rock ages

Weathered whole rock samples of five lithic units from Maine, Virginia, North Carolina and South Carolina were analyzed to evaluate the effect of chemical weathering on the Rb-Sr ages of the original rocks. Major element and turbidity analyses, as well as standard X-ray diffraction and petrographic studies were made on selected samples. All weathered granitic rocks are friable; several are almost saprolites. Volcanic rocks are powdery or are pitted as a result of the weathering of plagioclase phenocrysts. The weathering profiles studied all are young relative to the age of the unaltered rock. Considering the extent of weathering, however, ages of the weathered samples are remarkably close to the published ages for unaltered samples of the same units. Weathered samples have ages ranging from essentially the same as fresh samples to no more than 20% younger than the age obtained from the fresh samples. In one profile, we have attempted to quantify the extent of weathering: there is a positive correlation between our measure of the degree of weathering and per cent ‘lowering’ of the age. Weathering of plagioclase apparently has little effect on whole rock ages; ages are lowered significantly only where biotite and K-feldspar have undergone considerable alteration.

GENESIS, MINERALOGY, AND RELATED PROPERTIES OF WEST INDIAN SOILS

SummarySoils of the Maracas series from two sites in the Northern Range of Trinidad differ only in minor aspects. With the exception of cheluviation of iron and formation of goethite the soils preserve the mineralogy and to some extent the structure of the parent phyllite. Weathering of calcic and sodic plagioclases is rapid. Mica is well preserved even in surface horizons and resists physical weathering, but when reduced to clay‐size, it weathers rapidly to kaolinite throughout the solum with little accumulation of expandable minerals. Exchangeable K is very low in spite of the micaceous nature of the soil parent material. Low levels of availability of Ca, Mg, and particularly P are also characteristic of these soils. Total inorganic P is very low, the main fraction being reductant‐soluble iron phosphate; other forms of inorganic P are unimportant. Organic P, derived from the carbonaceous matter of the parent rock, increases with depth. From the results obtained, the Maracas series may be classified as a latosol. For these soils located in a humid tropical environment, the paradoxical situation exists that they do not undergo sufficient weathering to release and make available plant nutrients before the solum is carried downslope by erosional processes. As a result, they are relatively infertile.

Samples and Principles

Major, trace element concentrations and Nd, Sr isotope ratios were measured in micro-drilled samples of a 2.37 Ga-old, hand-specimen sized spheroidally weathered diabase from southern India. A sample of the un-weathered diabase dike was also analyzed. X-ray micro-CT imaging of the weathered sample shows three dominant mineral phases which are plagioclase, pyroxene, and a Fe-bearing phase (possibly hematite and/or ilmenite). This imaging documents the pervasive nature of two generations of ribbon-like, cross-cutting fractures. The older fracture is sealed while the more recent fracture is open without any in-filling. The values of the Chemical Index of Alteration (CIA) of the samples show a wide range but are less than 50. Despite being a relatively less weathered rock, we observe that concentrations of major, minor and trace elements vary significantly with the percentage relative standard deviation (%RSD) for the elements ranging from 10.2–41.8. The CIA of the samples do not show any trend with the position of the sample in the hand-specimen. Barring Ca and Li, whose concentrations decrease from the core to the rim of the sample, there is no significant spatial trend in the concentrations of the elements.Concentrations of Na, Al, and Sr increase with increasing CIA values while concentrations of Mg, Fe, and Sc decrease with increasing CIA. The strong positive correlations of Na and Al, as well as Na and Sr indicates preferential weathering of plagioclase in the diabase. Na/Ca increases while Mg/Al, Mg/Na, Mg/Ca, Fe/Al and Sc/Sr decrease with increasing CIA values and the un-weathered rock plots in the middle of these trends. Such variations are explained in terms of differential weathering of plagioclase (in samples with lower CIA than the un-weathered rock, W1-type) and pyroxene (in samples with higher CIA than the un-weathered rock, W2-type) which have varying resistance to weathering. At the hand-specimen scale, the variability in the weathering indices like CIA are controlled by differential weathering of minerals and might not accurately reflect the intensity of weathering. Chondrite-normalized La/Sm and Gd/Lu co-vary with CIA values indicating mobility of the REEs during spheroidal weathering even at the hand-specimen scale. The Eu anomaly also increases with increasing CIA values which is explained by differential weathering of pyroxene and plagioclase. We observe large percentage deviations of the Nb-normalized concentrations of elements from the un-weathered rock in specific samples but no spatial trend is observed. Overall, the variations in element concentrations can be explained by varying fluid mobility of the elements, selective weathering of the minerals in the diabase, and ambient environmental conditions.Considerable Nd and Sr isotopic variability is observed at the hand-specimen scale and is explained in terms of weathering-related fractionation of parent/daughter ratios. This elemental fractionation must have happened long time ago to allow for radiogenic decay of the long-lived isotopes of 87Rb and 147Sm. The spread (%RSD) in the initial Sr and Nd isotope compositions of the weathered samples reach a minimum value around 1.2–1.3 Ga which we interpret as the timing of the peak weathering event which led to fractionation of the parent/daughter ratios. For Nd isotopes, the average εNd (1.2 Ga) of the weathered samples coincides with the εNd (1.2 Ga) of the un-weathered rock. The timing of the weathering event coincides with the timing of the breakup of the Columbia supercontinent and follows wide-spread alkaline volcanism in the Indian subcontinent. This is the first such attempt to determine the timing of a weathering event in rocks using long-lived radioactive isotopes.