Transpiration by dryland oaks: studies in the south-western United States and Northern Israel

Increased understanding of transpiration by dryland oaks in the woodlands of the south-western United States and northern Israel has been obtained from studies in the two countries. Transpiration was estimated in both studies by the heat pulse velocity (HPV) method in stands of Quercus emoryi, a drought-deciduous species growing in the south-western United States, and stands supporting Q. ithaburensis, a deciduous oak, and Q. calliprinos, an evergreen oak, in northern Israel. Estimates of daily transpiration rates by individual trees and annual transpiration amounts on a stand basis indicate that depending on the species and stand structure, 45–80% of the precipitation inputs to the stands sampled are represented by transpiration component of the respective hydrologic cycles.     

Chronology and mechanism of depletion in Lewisian granulites

The pattern of Th, U and Rb depletion has been investigated in a suite of contrasting lithologies from the classic Lewisian granulite terrain at Scourie, N.W. Scotland. The study is based on lithologies ranging from ultramafic to tonalitic gneisses, together with calcsilicate and pelitic metasediments, sampled on the metre scale. Because of their close proximity, all of these lithologies have experienced identical P-T-t histories since depletion; however, their expected dehydration and melting characteristics are markedly different. Pb isotope analyses of the samples define the latest time of U/Pb fractionation as 2.665±0.026 Ga, and indicate substantial U and Th depletion relative to Pb in tonalitic and mafic lithologies, and in some metasediments. In contrast, ultramafic samples show no U and Th depletion. Sm-Nd isotope analyses of the mafic and ultramafic samples define an age of 2.707±0.052 Ga, which is interpreted as the time of igneous differentiation of the mafic-ultramafic bodies. This time is indistinguishable from the latest time of Th–Pb and U–Pb fractionation. Overall there is a straightforward relationship between the magnitude of depletion, lithology and the stability of mineral phases. Tonalitic and mafic gneisses are depleted in Th and U relative to Pb, whereas ultramafic gneisses which retain amphibole show no evidence for depletion. The observations are consistent with loss of U, Th and Rb in fluids produced by metamorphic dehydration and melting. A role for externally derived CO₂ in the depletion process is not excluded by the data but there is no evidence for it having existed. Limits are placed on the length scale of transport for several elements. U, Th and Rb are depleted regionally from mafic and tonalitic lithologies; transport distances must have been on the kilometre scale. In contrast, movements of Sm and Nd have been more restricted.     

Plate tectonic model for the oligo-miocene evolution of the western Mediterranean

This paper outlines a plate tectonic model for the Oligo-Miocene evolution of the western Mediterranean which incorporates recent data from several tectonic domains (Corsica, Sardinia, the Kabylies, Balearic promontory, Iberia, Algero-Provençal Basin and Tunisian Atlas). Following late Mesozoic anticlockwise rotation of the Iberian peninsula (including the Balearic promontory and Sardinia), late Eocene collision occurred between the Kabylies and Balearic promontory forming a NE-trending suture with NW-tectonic polarity. As a result of continued convergence between the African and European plates, a polarity flip occurred and a southward-facing trench formed south of the Kabylie—Balearic promontory suture. During late Oligocene time an E-W-trending arc and marginal basin developed behind the southward-facing trench in the area of the present-day Gulf of Lion. Opening of this basin moved the Corsica—Sardinia—Calabria—Petit Kabylie—Menorca plate southward, relative to the African plate. Early Miocene back-arc spreading in the area between the Balearic promontory and Grand Kabylie emplaced the latter in northern Algeria and formed the South Balearic Basin. Coeval with early Miocene back-arc basin development, the N-S-extension in the Gulf of Lion marginal basin changed to a more NW-SE direction causing short-lived extension in the area of the present-day Valencia trough and a 30° anticlockwise rotation of the Corsica-Sardinia-Calabria—Petit Kabylie plate away from the European plate. Early—middle Miocene deformation along the western Italian and northeastern African continental margins resulted from this rotation. During the early late Miocene (Tortonian), spreading within a sphenochasm to the southwest of Sardinia resulted in the emplacement of Petit Kabylie in northeastern Algeria.     

Axisymmetric transients in shells of revolution

The propagation of axisymmetric transients in shells of revolution subjected to impulsive boundary loads is treated in this paper. Consideration is restricted to linear, elastic, isotropic, homogeneous shells of revolution with straight line generators. The analysis is based on the concept of a wave as a carrier of discontinuities in the field variable and its derivatives. These discontinuities are determined from a set of recurrence relations which are in turn generated by the use of asymptotic series solutions to the equations of motion. A numerical superposition technique which enables the calculation of long time response is developed. Several numerical examples are presented in order to illustrate the method.     

Field determination of sodium and potassium in feldspars by ion-selective electrodes

A field-lab technique has been developed to aid in the geochemical exploration for feldspars involving the determination of the K/Na ratio of feldspar samples with recently improved ion-selective electrodes. Feldspar is dissolved in a mixture of m-benzene disulfonic acid and hydrofluoric acid. The solution is neutralized with tetramethylammonium hydroxide in the presence of CDTA. K and Na determinations are then made with ion-selective electrodes in the neutral solution. The precision and accuracy of the technique have been tested and closely approach that of the flame photometer.     

The formation of chondrules by open-system melting of nebular condensates

Experiments were conducted under canonical nebular conditions to see whether the chemical compositions of the various chondrule types can be derived from a single CI-like starting material by open-system melting and evaporation. Experimental charges, produced at 1580 °C and P_H2 of 1.31×10⁻⁵ atm over 1 to 18 hours, consisted of only two phases, porphyritic olivine crystals in glass. Sulfur, metallic-iron and alkalis were completely evaporated in the first minutes of the experiments and subsequently the main evaporating liquid oxides were FeO and SiO₂. Olivines from short runs (2-4 hours) have compositions of Fo₈₃-Fo₈₉, as in Type IIA chondrules, while longer experimental runs (12-18 hours) produce ∼Fo₉₉ olivine, similar to Type IA chondrules. The concentration of CaO in both olivine (up to 0.6 wt.%) and glass, and their Mg#, increased with increasing heating duration. Natural chondrules also show increasing CaO with decreasing S, alkalis, FeO and SiO₂. The similarities in bulk chemistry, mineralogy and textures between Type IIA and IA chondrules and the experimental charges demonstrate that these chondrules could have formed by the evaporation of CI precursors. The formation of silica-rich chondrules (IIB and IB) by evaporation requires a more pyroxene-rich precursor.Based on the FeO evaporation rates measured here, Type IIA and IA chondrules, were heated for at least ∼0.5 and ∼3.5 h, respectively, if formed at 1580 °C and P_H2 of 1.31×10⁻⁵ atm. Type II chondrules may have experienced higher cooling-rates and less evaporation than Type I.The experimental charges experienced free evaporation and exhibited heavy isotopic enrichments in silicon, as well as zero concentrations of S, Na and K, which are not observed in natural chondrules. However, experiments on potassium-rich melts at the same pressure but in closed capsules showed less evaporation of K, and less K isotopic mass fractionation, than expected as a function of decreasing cooling rate. Thus the environment in which chondrules formed is as important as the kinetic processes they experienced. If chondrule formation occurred under conditions in which evaporated gases remained in the vicinity of the residual melts, the extent of evaporation would be reduced and back reaction between the gas and the melt could contribute to the suppression of isotopic mass fractionation. Hence chondrule formation could have involved evaporative loss without Rayleigh fractionation. Volatile-rich Type II and volatile-poor Type I chondrules may have formed in domains with high and low chondrule concentrations, and high partial pressures of lithophile elements, respectively.