DIffusion of Eu and Gd in basalt and obsidian

Tracer diffusion coefficients of ¹⁵³Gd and ¹⁵²Eu in olivine tholeiite have been determined at temperatures between 1150 and 1440°C. The results are identical for both tracers within experimental error. Between 1440 and 1320°C the diffusion coefficients are given by D(Eu, Gd) = 0.058 exp(?40,600/ RT). Between 1320 and 1210°C, the diffusion coefficients are constant at D = (1.4 ± 0.4) × 10^?7 cm²s^?1 and between 1210 and 1150°C, the D values drop irregularly to 4 × 10^?9 cm²s^?1. The liquidus temperature (1270°C) lies within the region of constant D. Such anomalous behavior has not been encountered in previous studies of Ca, Sr, Ba and Co diffusion in basalt. To explain the constant D value near the liquidus, we speculate that the structure of the melt changes as a function of temperature in such a way that the normal temperature dependence of the diffusivity is compensated. For example, the rare earth ions may be displaced from their (high temperature) octahedral coordination sites to other sites where they are more readily dissociated and therefore become progressively more mobile. The behavior below 1210°C may be the result of relatively stable complexes or molecules in the melt or of the formation of a REE bearing crystalline phase that has so far escaped detection. Preliminary results for Eu diffusion in obsidian are D (Eu, 800°C) = 5 × 10^?13 cm² s^?1 and D (Eu, 950°C) = 1.5 × 10^?11 cm² s^?1. These data are consistent with an activation energy of 59 Kcal mole^?1. These low diffusivities indicate that the partitioning of REE in crystallizing intermediate and acidic melts may be controlled by diffusion in the melt rather than equilibrium between the crystal surface and the bulk melt.The diffusion data are applied to partial melting in the mantle, in an attempt to explain how LREE enriched tholeiites may be derived from a LREE depleted mantle source. In this model LREE diffuse from garnet bearing regions that have small melt fractions into garnet free regions that have relatively large melt fractions. REE diffusion is so slow that this process is quantitatively significant only in small partially molten bodies (diameter ～1 km or less) or in larger, but strongly flattened bodies. Internal convective motion during diapiric rise would also increase the efficiency of the process.     

Carbon isotopes, time boundaries and evolution

The carbon isotope composition of carbonate rocks in time-boundary zones is interpreted as a reflection of the variations in the total biomass at mass extinction events. Better stratigraphic control and identification of hiatuses can be gained by stable isotope stratigraphic methods than with any other type of stratigraphy. It is suggested that the use of carbon isotopes can help in solving some of the problems confronting palaeontologists and lead to a better understanding of the evolutionary significance of these events.     

Pleistocene paleoclimate of the arid region of Israel as recorded in calcite deposits along regional transverse faults and in veins

The δ¹⁸O and δ¹³C values of the calcites associated with E-W and NE-SW transverse faults in the Negev, Israel, indicate that calcite was deposited from meteoric water. A regional change in the δ¹⁸O and δ¹³C values was observed. The ¹⁸O content in the calcite increases, from the southwestern (δ¹⁸O = −17.8‰) to the northeastern (δ¹⁸O = −2.9‰) part of the region. The δ¹³C values show the opposite trend of the ¹³C content decrease: from +2‰ in the south to −10‰ in the northeast. These trends had to reflect changes in regional paleoclimate, suggesting a change in the isotopic composition of the solution from which the calcite was deposited in different periods. The variations in the δ¹⁸O values reflect shifts in the δ¹⁸O values of precipitation and are associated with a change in the source of moist air masses which came from the equatorial Atlantic in the early Pleistocene and from the Mediterranean during a later period. Variations in δ¹³C values reflect changes from humid to arid conditions. Two modes of calcite deposition are suggested: (1) precipitation of calcite minerals in the unsaturated zone following the dissolution in the soil or (2) calcite deposition that occurred as CO₂ was lost during emergence of paleogroundwater from Lower Cretaceous and Jurassic aquifers.     

Stable isotope and Sr2+/Ca2+ evidence of diagenetic dedolomitization in a schizohaline environment: Cenomanian of northern Israel

The Cenomanian—Turonian Upper Judea Group of Israel comprises shallow-marine hypersaline dolomites passing laterally into ‘basinal’ limestones and chalks. Chertbearing diagenetic dedolomites characterize the transitional zone. The restriction of dedolomites to a narrow zone, their light δ¹³C (up to ?11%), and their low Sr²⁺ concentration (<16 ppm) all suggest that the dedolomites were formed diagenetically through exposure of this zone to fresh meteoric waters. This type of partly changing environments was referred to as schizohaline by Folk and Siedlecka (1974).     

Oxygen,hydrogen and carbon isotope studies of the franciscan formation,Coast Ranges,California

Analyses of 230 Franciscan rock and mineral samples, including the San Luis Obispo ophiolite, show that metamorphism produces no change in the δ¹⁸O of the graywackes (+11 to +14), but that igneous rocks become enriched in ¹⁸O by 2–6% and the cherts depleted by 5–10%. The shales are of two types, a high-¹⁸O type (+16 to +20) associated with chert and a low-¹⁸O type isotopically and mineralogically similar to the graywackes. The vein quartz (δ = + 15 to + 20) is invariably richer in ¹⁸O than the host rock quartz and in most of the rocks the δ¹⁸O of the clastic quartz is similar to the δ¹⁸O of the whole rock. Mineral assemblages are typically not in isotopic equilibrium. Although the δ¹⁸O values are very uniform (+13 to +16). the δ¹³C of vein aragonite and calcite is widely variable (0 to ? 14), implying that a major source of the carbon is oxidized organic material. The δD values of 83 igneous and sedimentary rocks are -45 to -80, exceptions are the Fe-rich minerals howieite and deerite, which have δD = ?100. All of these samples could have equilibrated with H₂O having δD ≈ +10 to ?20 and δ¹⁸O ≈ ?3 to +8. assuming temperatures of 100–300°C. However, the serpentines (δD ≈ ?85 to ?110) and the vein minerals (δD = ?23 to ?55) are exceptions. The vein minerals are 10–20%, richer in deuterium than the adjacent wall rocks; they formed from a relatively D-rich metamorphic water, typically at lower temperatures than did their host rocks. The isotopic compositions of the other Franciscan rocks were affected by three distinct events: (1) hydrothermal alteration of the ophiolite complexes and volcanic rocks as a result of submarine igneous activity at a spreading center or in an island-arc environment; (2) low-temperature, high-pressure regional metamorphism and diagenesis; and (3) a late-stage, very low temperature (<100°C) alteration of the ultramafic bodies by meteoric ground waters, producing lizardite-chrysotile serpentine. In the first two cases, the pore fluid involved in the alteration of the Franciscan rocks was sea water. However, this water became somewhat depleted in D and enriched in ¹⁸O during blueschist metamorphism, evolving to values of δD ≈ ? 20 and δ¹⁸O ≈ + 6 to + 8 at the highest grades. Except for one graywacke sample, the meteoric waters that affected the serpentinites did not significantly change the $\text{D}\text{H}$ ratios of the OH-bearing minerals in any other Franciscan rock.The δ¹⁸O values of orogenic andesites are too low for such magmas to have formed by direct partial melting of Franciscan-type materials in a subduction zone. Andesites either form in some other fashion, or the melts must undergo thorough isotopic exchange with the upper mantle. The great Cordilleran granodiorite-tonalite batholiths, however, are much richer in ¹⁸O and may well have formed by large-scale melting or assimilation of Franciscan-type rocks. The range of δD values of Franciscantype rocks is identical to the ?50 to ?80 range shown by most igneous rocks. This suggests that ‘primary magmatic H₂O’ throughout the world may be derived mainly by partial melting of Franciscantype materials, or by dehydration of such rocks in the deeper parts of a Benioff zone.     

Changes in hydraulic conductivity of laboratory sand-clay mixtures caused by a seawater-freshwater interface

The influence of small amounts of clay minerals on the hydraulic conductivity of sandy aquifer was investigated by laboratory experiments. Admixture of up to 1.5% by weight of clay minerals to sand did not cause any measurable decrease of hydraulic conductivity for seawater. Increasing the clay fraction from 1.5% to 10% decreased hydraulic conductivity by one order of magnitude. Montmorillonite caused the strongest decrease; the effect of kaolinite and illite was only half as large. When seawater was flushed by freshwater, hydraulic conductivity of the montmorillonite-sand mixture decreased drastically. However, flushing with freshwater did not measurably affect the hydraulic conductivity of an illite-sand or kaolinite-sand mixture. The explanation for this behaviour is the capability of various types of clay to adsorb different quantities of water between their platelets which induces a gel-droplet formation process. This is governed by the chemical composition and the ionic strength of the solution.     

Isotope shifts in the Late Permian of the Delaware Basin,Texas, precisely timed by varved sediments

Closely spaced samples (285 in number) of varved sediments from the Upper Permian in Delaware Basin, Texas, have been analyzed for δ¹³C_carb, δ¹³C_org, δ¹⁸O_carb, C_org, C_carb, and calcite/dolomite. δ¹³C records a dramatic rise from ?2.8 to +5.7‰ in only 4400 years, detected in three sections across the basin, extrapolating smoothly through a 600-year interruption by a local (west side of the basin) fresh-water inflow evidenced by low δ¹⁸O. This continuity and low C_org within the basin, both indicate that the excess net deposition of C_org, necessary to generate the rise in δ¹³C, took place in the ocean external to the Delaware Basin. Correlation with similar records from the Zechstein Basin suggest that the event was world-wide, although this poses obvious difficulties for the carbon cycle. The rate of rise of δ¹³C, and its sustained high level, must imply conversions of oxidized carbon to reduced carbon that are very large depending on which reservoirs were involved.     

Immiscible silicate-carbonate liquids as evidenced from ocellar diabase dykes,southeast Sinai

Diabase dykes containing spherical patches of carbonate intrude the Tarr albitite complex of SE Sinai. The morphology of these dykes indicates a highly gas-charged magma. Petrographic evidence points toward equilibrium during cooling of immiscible carbonate ocelli and silicate matrix. Dolomite is the main component of these ocelli, which are geochemically and isotopically similar to carbonatite. However, the low total REE content, and the presence of considerable marble in the country rock, suggest a process of stoping and melting of carbonate, followed by the “in situ” development of silicate-carbonate immiscibility.