Knowledge Of Partition Coefficients Research Articles

Chemical Disequilibria, Lithospheric Thickness, and the Source of Ocean Island Basalts

AbstractWe examine REE (Rare-Earth Element) and isotopic (Sr–Hf–Nd–Pb) signatures in OIB (Ocean Island Basalts) as a function of lithospheric thickness and show that the data can be divided into thin- (<12 Ma) and thick-plate (>12 Ma) sub-sets. Comparison to geophysically constrained thermal plate models indicates that the demarcation age (∼12 Ma) corresponds to a lithospheric thickness of about 50 km. Thick-plate OIB show incompatible element and isotopic enrichments, whereas thin-plate lavas show MORB-like or slightly enriched values. We argue that enriched signatures in thick-plate OIB originate from low-degree melting at depths below the dry solidus, while depleted signatures in MORB and thin-plate OIB are indicative of higher-degree melting. We tested quantitative explanations of REE systematics using melting models for homogeneous fertile peridotite. Using experimental partition coefficients for major upper mantle minerals, our equilibrium melting models are not able to explain the data. However, using a new grain-scale disequilibrium melting model for the same homogeneous lithology the data can be explained. Disequilibrium models are able to explain the data by reducing the amount of incompatible element partitioning into low degree melts. To explore new levels of detail in disequilibrium phenomena, we employ the Monte-Carlo Potts model to characterize the textural evolution of a microstructure undergoing coarsening and phase transformation processes simultaneous with the diffusive partitioning of trace elements among solid phases and melt in decompressing mantle. We further employ inverse methods to study the thermochemical properties required for models to explain the OIB data. Both data and theory show that OIB erupted on spreading ridges contain signatures close to MORB values, although E-MORB provides the best fit. This indicates that MORB and OIB are produced by compositionally indistinguishable sources, although the isotopic data indicate that the source is heterogeneous. Also, a posteriori distributions are found for the temperature of the thermomechanical lithosphere-asthenosphere boundary (TLAB), the temperature in the source of OIB (Tp, oib) and the extent of equilibrium during melting (i.e. grain size). TLAB has been constrained to 1200–1300°C and Tp, oib is constrained to be <1400°C. However, we consider the constraints on Tp, oib as a description of all OIB to be provisional, because it is a statistical inference from the global dataset. Exceptional islands or island groups may exist, such as the classical ‘hotspots’ (Hawaii, Reunion, etc) and these islands may originate from hot sources. On the other hand, by the same statistical arguments their origins may be anomalously hydrated or enriched instead. Mean grain size in the source of OIB is about 1–5 mm, although this is also provisional due to a strong dependence on knowledge of partition coefficients, ascent rate and the melting function. We also perform an inversion in which partition coefficients were allowed to vary from their experimental values. In these inversions TLAB and Tp, oib are unchanged, but realizations close to equilibrium can be found when partition coefficients differ substantially from their experimental values. We also investigated bulk compositions in the source of OIB constrained by our inverse models. Corrections for crystallization effects provided ambiguous confirmations of previously proposed mantle compositions, with depleted mantle providing the poorest fits. We did not include isotopes in our models, but we briefly evaluate the lithospheric thickness effect on isotopes. Although REE data do not require a lithologically heterogeneous source, isotopes indicate that a minor enriched component disproportionately contributes to thick-plate OIB, but is diluted by high-degree melting in the generation of thin-plate OIB and MORB.

Effect of octanol:water partition coefficients of organophosphorus compounds on biodistribution and percutaneous toxicity

Knowledge of partition coefficient (log P) data can play a critical role in understanding the pharmacokinetic and pharmacodistributive properties of toxic organophosphorus (OP) compounds. Using a recently published gas chromatographic method, the octanol:water log P values for the compounds tabun (GA), sarin (GB), cyclosarin (GF), and O-ethyl-S-(2-diisopropylaminoethyl) methylphosphonothiolate (VX) were determined to be 0.384 +/- 0.033, 0.299 +/- 0.016, 1.038 +/- 0.055, and 0.675 +/- 0.070, respectively. Based on these data, the log P value of the fluorophosphonate fragment, common to GB, soman (GD), and GF, was determined to be -2.256 +/- 0.273. The predictive value for absorption and distribution of the determined log P values was compared to measured values. The time to onset of local fasciculations (47.3, 29.0, 8.8, 8.5, and 6.3 min, respectively) in guinea pigs exposed percutaneously to equilethal doses of GA, VX, GF, GB, or GD was used as an indicator of dermal penetration. There was a good correlation (r = 0.95) between the measured log P value and the rate of onset of local fasciculations. Assuming a direct correspondence, equilibrium tissue:blood log P may be estimated from octanol:water log P. Comparison of the estimated and directly measured tissue:blood log P revealed a correlation of 0.8 for GD in liver, muscle, and adipose tissue. Our results demonstrate the use of log P data to both predict absorption and determine the distribution of OP compounds in tissues. This facilitates further estimates of in vivo OP effects from in vitro experiments.

Prediction of interfacial transfer kinetics. I. Relative importance of diffusional resistance in aqueous and organic boundary layers in two-phase transfer cell

Interfacial transfer kinetics were determined for 3 series of solute homologues in a variety of aqueous : organic solvent systems in the two-phase transfer cell. Experimentally determined interfacial transfer kinetics were compared to those estimated from a theoretical equation derived by Byron et al. (1981). The importance of aqueous and organic diffusional resistance to solute transfer was examined. A method is described to calculate a theoretical solute and solvent dependent ratio that enables estimation of the dominant diffusional resistance for a particular solute in a given solvent system. Choice of solute and solvent system allowed the predictive theories to be tested under conditions where aqueous, organic or mixed diffusional control predominated. Successful prediction of the transfer kinetics of any homologue in a series was possible in all cases from a knowledge of partition coefficient and transfer kinetics of the parent compound, the partition coefficient of the homologue and some easily determined system variables.

A method for measuring nonelectrolyte partition coefficients between liposomes and water.

This series of papers describes methods for measuring solute partition coefficients between membrane suspensions and water, and applies these methods to the system dimyristoyl lecithin: water. Knowledge of partition coefficients is relevant to analyzing permeability coefficients, probing membrane structure, and evaluating permeation models. The theoretical section of this first paper derives equations to correct measured partition coefficients for the effects of trapped water in the precipitate, tritium exchange with lipids or membranes and nonsolvent water adjacent to membranes. The experimental section presents methods for measuring partition coefficients in liposomes of dimyristoyl lecithin, including techniques for minimizing effects of counter drift and quenching on double-label radioactive counting. These methods offer advantages in measuring membrane: water partition coefficients that are not high.

Yttrium geochemistry applied to petrogenesis utilizing calcium-yttrium relationships in minerals and rocks

A survey of Y data from all sources shows that Y behaves systematically in igneous, metamorphic and sedimentary rock series, due to its incorporation in a predictable and uniform manner in Ca minerals. Compared with average calc-alkali basalt melts, plagioclase, kaersutite, augitic clinopyroxene and calcite have low Y for their CaO contents; whereas hornblende, garnet, orthopyroxene, apatite, sphene, zircon and most K, Na rich minerals other than plagioclase have high Y for their Ca contents. In sedimentary processes, Ca/Y becomes lower in shales and sandstones, but higher in limestones than their source. In metamorphic processes Y appears to be inert. In all igneous series for which Y data has been assembled Ca/Y falls as Ca falls. These series can be classified into three categories: (i) a standard calc-alkali trend, used as a reference (ii) J-type trends which become progressively impoverished in Y as Ca falls when compared with the standard trend, and (iii) L-trends which become progressively enriched in Y as Ca falls when compared with the standard trend. Despite little knowledge of partition coefficients, the J-type trends appear to have a significant component of hornblende control in their fractionation processes, whereas plagioclase and augitic clinopyroxene dominate the L-type trend. Alternative names for these series might therefore be the standard series, the hornblendic and the pyroxenic series respectively. Modern ocean floor basalts appear to be significantly richer in Y than modern calc-alkali basalts, and these in turn may be richer in Y than their Archaean counterparts.