An exact, stratified model of a meddy

An exact model to describe submesoscale, coherent vortices in a uniformly stratified fluid is presented. The model allows for stratification of the eddy interior, so as to agree with observations. The closed set of equations governing the evolution of the eddy on the F-plane is derived. In the case that the interior isopycnal surfaces remain horizontal the stratified analogue of the ‘rodon’, a special solution of the ‘lens equations’ that govern the evolution of uniform-density, warm-core surface eddies, is obtained.     

Computation of additional components of the first-order ray approximation in isotropic media

This study shows that the use of the first-order additional components of the ray method in the seismic wave field modeling is easy and that it can bring a substantial improvement of the standard ray results obtained with the zero-order ray approximation only. For the calculation of a first-order additional component, spatial derivatives of the parameters of the medium and spatial derivatives of the zero-order ray amplitude term are necessary. The evaluation of the former derivatives is straightforward; the latter derivatives can be calculated approximately from neighboring rays by substituting the derivatives by finite differences. This allows an effective calculation of the first-order additional terms in arbitrary laterally varying layered media.The importance of the first-order additional terms is demonstrated by the study of individual higher-order terms of the ray series representing elementaryP andS elastodynamic Green functions for a homogeneous isotropic medium. The study shows clearly that the consideration of the first-order additional terms leads to a more substantial decrease of the difference between approximate and exact elementary Green functions than any other higher-order term. With this in mind, effects of the first-order additional terms on the ray synthetic seismograms for aVSP configuration are studied. It is shown that the use of the additional terms leads to such phenomena, unknown in the zero-order approximation of the ray method, like quasi-elliptical and transverse polarization of a singleP wave or longitudinal polarization of a singleS wave.     

Zoned (Cretaceous and Cenozoic) garnet and the timing of high grade metamorphism, Southern Alps, New Zealand

New (garnet Sm–Nd and Lu–Hf) and existing (Rb–Sr, ⁴⁰Ar/³⁹Ar, U–Pb and Sm–Nd) ages and data on deformational fabrics and mineral compositions show for the first time that the garnet growth and ductile deformation in the Alpine Schist belt and Southern Alps orogen, New Zealand are diachronous and partly Cenozoic in age. The dominant metamorphic isograds in the Alpine Schist formed during crustal thickening at a previously unsuspected time, at c. 86 Ma, immediately prior to the opening of the Tasman Sea at c. 84–82 Ma. Obvious changes in the textures and compositional zoning patterns of garnet are not always reliable indicators of polymetamorphism, and fabric elements can be highly diachronous. A detailed timing history for the growth of a single garnet is recorded by a Sm–Nd garnet–whole rock age of 97.8 ± 8.1 Ma for the inmost garnet core (zone 1), Lu–Hf ages of 86.2 ± 0.2 Ma and 86.3 ± 0.2 Ma for overgrowth zones 2 and 3, a step‐leach Sm–Nd age of 12 ± 37 Ma for zone 4, and growth of the garnet rim (zone 5) over the Alpine Fault mylonite foliation during the modern phase of oblique collision that began at c. 5–6 Ma. Plate convergence along the New Zealand portion of the Gondwana margin continued after c. 105 Ma, almost certainly culminating in the oblique collision of a large oceanic plateau (Hikurangi Plateau). The metamorphism of the Alpine Schist at c. 86 Ma is evidence of that hit. The mid‐ to late‐Cretaceous extension that is widespread elsewhere in the New Zealand region is attributed to upper plate extension and slab roll‐back. The effects of the collision with the Hikurangi Plateau may have contributed to the changing plate motions in the region leading up to the opening of the Tasman Sea at c. 82 Ma.     

Genesis and evolution of a curved mountain front: paleomagnetic and geological evidence from the Gran Sasso range (central Apennines, Italy)

The Gran Sasso range is a striking salient formed by two roughly rectilinear E–W and N–S limbs. In the past 90° counterclockwise (CCW) rotations from the eastern Gran Sasso were reported [Tectonophysics 215 (1992) 335], suggesting west–east increase of rotation-related northward shortening along the E–W limb. In this paper, we report on paleomagnetic data from Meso-Cenozoic sedimentary dykes and strata cropping out at Corno Grande (central part of the E–W Gran Sasso limb), the highest summit of the Apennine belt. Predominant northwestward paleomagnetic declinations (in the normal polarity state) from both sedimentary dykes and strata are observed. When compared to the expected declination values for the Adriatic foreland, our data document no thrusting-related rotation at Corno Grande. The overall paleomagnetic data set coupled with the available geological information shows that the Gran Sasso arc is in fact a composite structure, formed by an unrotated-low shortening western (E–W trending) limb and a strongly CCW rotated eastern salient. Late Messinian and post-early Pliocene shortening episodes documented along the Gran Sasso front indicate that belt building and arc formation occurred during two distinct episodes. We suggest that the southern part of a late Messinian N–S front was reactivated during early–middle Pliocene time, forming a tight range salient due to CCW rotations and differential along-front shortening rates. The formation of a northward displacing bulge in an overall NW–SE chain is likely a consequence of the collision between the Latium-Abruzzi and Apulian carbonate platforms during northeastward propagation of the Apennine wedge, inducing lateral northward extrusion of Latium-Abruzzi carbonates towards ductile basinal sediment areas.     

Isotopic anomalies of noble gases in meteorites and their origins—III. LL-chondrites

Nine LL-chondrites were studied by a selective etching technique, to characterize the noblegas components in three mineral fractions: HF-HCl-solubles (silicates, metal, troilite, etc.; comprising ～ 99% of the meteorite), chromite and carbon (～ 0.3–0.7%) and Q (a poorly characterized mineral defined by its solubility in HNO₃, comprising ～ 0.05% of the meteorite but containing most of the Ar, Kr, Xe and a neon component of ${}^{20}\text{Ne}{}^{22}\text{Ne}\text{= 10.9 ± 0.8}$). The ${}^{20}\text{Ne}{}^{36}\text{Ar}$ ratio in Q falls wi petrologic type and rising ³⁶Ar content, as expected for condensation from a cooling solar nebula, but contrary to the trend expected for metamorphic losses. Chondrites of different petrologic types therefore cannot all be derived from the same volatile-rich ancestor, but must have formed over a range of temperatures, with correspondingly different intrinsic volatile contents.The CCFXe (carbonaceous chondrite fission) component varies systematically with petrologic type. The most primitive LL3s (Krymka, Bishunpur, Chainpur) contain substantial amounts of CCFXe in chromite-carbon, enriched relative to primordial Xe as shown by high ${}^{136}\text{Xe}{}^{132}\text{Xe}$ (0.359–0.459, vs 0.310 for primordial Xe). These are accompanied by He and by Ne with ${}^{20}\text{Ne}{}^{22}\text{Ne}\text{≈ 8.0}$ and by variable amounts of a xenon component enriched in the light isotopes. The chromite in these meteorites is compositionally peculiar, containing substantial amounts of Fe(III). These meteorites, as well as Parnallee (LL3) and Hamlet (LL4) also contain CCFXe in phase Q, heavily diluted by primordial $\text{Xe}\text{(}{}^{136}\text{Xe}{}^{132}\text{Xe}\text{= 0.317–0.329)}$. On the other hand, LL5s and 6s (Olivenza, St. Séverin, Manbhoom and Dhurmsala) contain no CCFXe in either mineral. This deficiency must be intrinsic rather than caused by metamorphic loss, because Q in these meteorites still contains substantial amounts of primordial Ne.If CCFXe comes from a supernova, then its distribution in LL-chondrites requires three presolar carrier minerals of the right solubility properties, containing three different xenon components in certain combinations. These minerals must be appropriately distributed over the petrologic types, together with locally produced Q containing primordial gases, and they must be isotopically normal, in contrast to the gases they contain. On the other hand, if CCFXe comes from fission of a volatile superheavy element, then its decrease from LL3 to LL6 can be attributed to progressively less complete condensation from the solar nebula. Ad hoc assumptions must of the host phase Q, its association with ferrichromite and the origin of the associated xenon component enriched in the light isotopes.Soluble minerals in LL3s and LL4s contain a previously unobserved, solar xenon component, which, however, is not derived from the solar wind. Three types of ‘primordial’ xenon thus occur side-by-side in different minerals of the same meteorite: strongly fractionated Xe in ferrichromite and carbon, lightly fractionated Xe in phase Q, and ‘solar’ Xe in solubles. Because the first two can apparently be derived from the third by mass fractionation, it seems likely that all were trapped from the same solar nebula reservoir, but with different degrees of mass fractionation.     

Isotopic constraints on the genesis and age of autochthonous glaucony in the Oligo-Miocene Torquay Group, south-eastern Australia

The Oligo‐Miocene Torquay Group at Bird Rock in south‐eastern Australia comprises a sequence of fine‐grained skeletal carbonates and argillaceous and glauconitic sandstones, deposited in a cool‐water, mid‐shelf environment. The Bird Rock glaucony is autochthonous and consists predominantly of randomly interstratified glauconitic smectite, which constitutes bioclast infills and faecal pellet replacements. The results of Rb–Sr and oxygen isotopic analysis of samples taken from a single glauconitic horizon (the BW horizon) indicate that the glaucony developed through a series of simultaneous dissolution–crystallization reactions, which occurred during very early diagenesis in a closed or isochemical system, isolated from the ambient marine environment. The constituent ions of the glaucony were derived primarily from terrigenous clay minerals, but considerable potassium may have been sourced indirectly from sea water, through potassium enrichment of clay precursors. The pore fluids associated with glauconitization were marine derived, but progressively modified by the dissolution–crystallization of detrital clay minerals and autochthonous glaucony. Rb–Sr data for the BW horizon indicate that dating glauconies may be somewhat problematic, as co‐genetic glauconitic minerals can show a range of initial strontium compositions, which reflect the incorporation of strontium derived from mineralogical precursors and/or contemporaneous sea water. Rb–Sr isochrons indicate that the glaucony of the BW horizon formed at 23 ± 3 Ma. This age is in good agreement with both the established biostratigraphy and a ⁸⁷Sr/⁸⁶Sr age for the horizon (23 ± 1 Ma), but could only be determined using the independent age constraint and the estimate of the ⁸⁷Sr/⁸⁶Sr ratio of contemporaneous sea water provided by analysis of associated biogenic carbonate.     

Numerical simulation of idealized granular assemblies with plane elliptical particles

The paper presents the development of algorithms that have been implemented in a computer program used to simulate the performance of idealized granular systems composed of elliptical-shaped particles. The work is an extension of numerical simulation methods which have been successfully used in micromechanics research on disk-shaped or polygon-shaped particles by the authors and others. The simulation of elliptical-shaped particles offers the possibility to explore the influence of particle shape on the micromechanical behaviour of plane assemblies of particles and the stress-strain behaviour of these systems at the macro scale. Typical results of simulation runs are presented to illustrate the importance of particle shape on the macroscopic stress-strain response of plane systems.     

The helium abundance and ΔY/ΔZ in lower main-sequence stars

We use nearby K dwarf stars to measure the helium-to-metal enrichment ratio  Δ Y /Δ Z   , a diagnostic of the chemical history of the solar neighbourhood. Our sample of K dwarfs has homogeneously determined effective temperatures, bolometric luminosities and metallicities, allowing us to fit each star to the appropriate stellar isochrone and determine its helium content indirectly. We use a newly computed set of Padova isochrones which cover a wide range of helium and metal content.
Our theoretical isochrones have been checked against a congruous set of main-sequence binaries with accurately measured masses, to discuss and validate their range of applicability. We find that the stellar masses deduced from the isochrones are usually in excellent agreement with empirical measurements. Good agreement is also found with empirical mass-luminosity relations.
Despite fitting the masses of the stars very well, we find that anomalously low helium content (lower than primordial helium) is required to fit the luminosities and temperatures of the metal-poor K dwarfs, while more conventional values of the helium content are derived for the stars around solar metallicity.
We have investigated the effect of diffusion in stellar models and the assumption of local thermodynamic equilibrium (LTE) in deriving metallicities. Neither of these is able to resolve the low-helium problem alone and only marginally if the cumulated effects are included, unless we assume a mixing-length which is strongly decreasing with metallicity. Further work in stellar models is urgently needed.
The helium-to-metal enrichment ratio is found to be  Δ Y /Δ Z = 2.1 ± 0.9  around and above solar metallicity, consistent with previous studies, whereas open problems still remain at the lowest metallicities. Finally, we determine the helium content for a set of planetary host stars.     

Geochronological and isotopic constraints on Palaeoproterozoic skarn base metal mineralisation in the central Gawler Craton, South Australia

The mineralisation potential of Palaeoproterozoic strata from the central Gawler Craton, South Australia, is poorly known. This study defines the timing of Zn-rich skarn formation within Palaeoproterozoic calcsilicate and highlights this as a new mineralisation style for the Gawler Craton. Sulphides within the garnet–diopside skarn in the No. 17 Bore Prospect are predominantly in the form of sphalerite, associated with galena, minor chalcopyrite, pyrrhotite and pyrite. Sulphide is present in disseminated form and as a coarse-grained sulphide within a sericite-rich cavity-fill. Mineralisation is inferred to have formed at 1710 ± 16 Ma through a Sm–Nd isochron from garnet and diopside aliquots. A weakly mineralised and altered granite immediately below the calcsilicate skarn crystallised at 1729 ± 13 Ma (LA-ICPMS U–Pb zircon), within error of the skarn mineralisation. The skarn is interpreted to have formed through the initiation of fluid circulation as a result of high-level granite emplacement within the Palaeoproterozoic strata. Exploration for skarn Zn–Pb deposits such as the No. 17 Bore Prospect is assisted by their geophysical properties.