Grain structure of the Earth's inner core

The coarsening rate of an initial grain structure is calculated and compared to the inner core growth rate. An estimate of the present size of the grains in the centre of the core varies from 560 m to 12 km, depending on the value taken for the diffusion coefficient of iron in the core. Regardless of the hypotheses chosen, this size is homogeneous inside the inner core.     

High-pressure thermal aureoles around two Neoproterozoic synorogenic magmatic epidote-bearing granitoids,Northeastern Brazil

Unusual high-pressure inner thermal aureoles are described from the Minador and Angico Torto epidote-bearing tonalitic plutons that emplaced into greenschist-facies metasedimentary rocks of the Neoproterozoic Cachoeirinha-Salgueiro belt, northeastern Brazil. The foliated pelitic hornfelses display the mineral assemblage garnet, kyanite, staurolite, muscovite, biotite, plagioclase ± quartz. Rare fibrolite is only found very close to the contacts. Hornfelses display steep mineral lineations and steeply-dipping foliations concordant with magmatic contacts. Leucocratic veinlets containing quartz, oligoclase, garnet, kyanite, staurolite, rutile and ilmenite suggest that limited melting conditions were reached very close to magmatic contacts (T ? 650 °C, P around 8 kbar). These high-pressure hornfelses form a few meters thick, rigid envelopes around the two plutons. Contrary to known examples of kyanite-bearing hornfelses that recorded high-temperature decompression, the nearly isobaric cooling down to ca. 450 °C is constrained by 3.20–3.30 Si contents of retrogressive phengites from both inner hornfelses and ductilely-deformed tonalite at the pluton margins. Isograds and bathograds are, therefore, apparently telescoped due to HP/LT shearing, possibly caused by subsequent differential vertical movements affecting these two solidified plutons. The unusual depth of emplacement of these syn-kinematic calc-alkaline plutons is explained by a tentative geodynamic model involving a pre-620 Ma-subduction setting.ResumenLas aureolas internas que rodean dos plutones tonalíticos emplazados dentro de rocas cajas en facies esquistos verdes del Cinturón-plegado Cachoeirinha-Salgueiro al noreste de Brasil, contienen hornfelses pelíticos foliados con granate, kyanita, estaurolita, muscovita, biotita, plagioclasa ± cuarzo. Fibrolita es rara ó es encontrada solamente cerca de las zonas de contacto. Los hornfelses desarrollaron foliaciones concordantes con buzamiento fuerte cerca de los contactos magmáticos y muestran lineaciones minerales casi verticales. Venillas leucocraticas contienen cuarzo, oligoclasa, granate, kyanita, estaurolita, rutilo e ilmenita sugiriendo que el límite en las condiciones de fusión fueron cercanas a los contactos magmáticos (T ? 650 °C, P entorno 8 kbar). Estos hornfelses de alta presión forman una cobertura rígida de poco metros entorno a los dos plutones. Contrariamente a lo conocido en diferentes ejemplos de hornfelses con kyanita con registro de descompresión, la isóbara de enfriamiento para cerca de 450 °C está controlada por el contenido de Si 3.20–3.30 de fengitas retrógradas, en los hornfelses internos y en las márgenes de los plutones donde las tonalitas están deformadas ductilmente. Isógradas y batógradas son, por lo tanto, aparentemente telescopadas debido a cizalla de alta presión/baja temperatura, posiblemente causado por subsecuentes movimientos verticales diferenciales afectando estos dos plutones solidificados. El emplazamiento a una profundidad inusual de estos plutones calci-alcalinos syn-cinemáticos es explicado por un modelo geodinámico tentativo que involucra un escenario de subducción pre-620 Ma.     

First Results on it p Modes from GOLF Experiment

The GOLF experiment on the SOHO mission aims to study the internal structure of the Sun by measuring the spectrum of global oscillations in the frequency range 10^-7 to 10^-2 Hz. Here we present the results of the analysis of the first 8 months of data. Special emphasis is put into the frequency determination of the p modes, as well as the splitting in the multiplets due to rotation. For both, we show that the improvement in S/N level with respect to the ground-based networks and other experiments is essential in achieving a very low-degree frequency table with small errors ∼ 2 parts in 10^-5). On the other hand, the splitting found seems to favour a solar core which does not rotate slower than its surface. The line widths do agree with theoretical expectations and other observations.     

Variations of the low‐degree solar p‐mode frequencies for 10 years of GOLF observations

We experiment with a method of measuring the frequency of solar p modes, intended to extend the passband for the variations of the frequency spectrum as high as possible. So far this passband is limited to a fraction of μ Hz for the classical analysis based on numerical fits of a theoretical line profile to a power spectrum averaged over periods lasting at least several weeks. This limit for the present analysis can be shifted to the mHz range, corresponding to some of the “5 min” oscillations, but in this range we use a lower resolution which allows us to separate odd and even p modes. We show an example of the results for long term variations and apply this analysis to search for a modulation of the p‐mode frequency spectrum by asymptotic series of solar g modes. A faint signal is found in the analysis of 10 years of GOLF data. This very preliminary result possibly indicates the detection of a small number of g modes of degree l = 1. A tentative determination of an observational value of the parameter P₀ follows. P₀ is the scaling factor of the asymptotic series of g modes and is a key data for solar core physics. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Review of Ray Theory Applications in Modelling and Imaging of Seismic Data

Throughout the last twenty years, 3D seismic ray modelling has developed from a research tool to a more operational tool that has gained growing interest in the petroleum industry. Various areas of application have been established and new ones are under development. Many of these applications require a modelling system with flexible, robust and efficient modelling algorithms in the core. The present paper reviews the basic elements of such a system, based on the open model concept and the wavefront construction technique. In the latter, ervený's dynamic ray tracing is an intrinsic part. The modelling system can be used for generating ray attributes and synthetic seismograms for realistic 3D surveys with tens of thousands of shots and receivers. Moreover, some other types of application areas are illustrated: Production of Green's functions for prestack depth migration and hybrid modelling (combined ray and finite-difference modelling), attribute mapping and illumination analysis, both for survey planning and interpretation. Finally, the concepts of isochron rays and velocity rays related to seismic isochrons have been introduced recently, with very interesting future applications.     

Cretaceous seamounts along the continent-ocean transition of the Iberian margin: U-Pb ages and Pb-Sr-Hf isotopes

To elucidate the age and origin of seamounts in the eastern North Atlantic, 54 titanite and 10 zircon fractions were dated by the U-Pb chronometer, and initial Pb, Sr, and Hf isotope ratios were measured in feldspars and zircon, respectively. Rocks analyzed are essentially trachy-andesites and trachytes dredged during the “Tore Madeira” cruise of the Atalante in 2001. The ages reveal different pulses of alkaline magmatism occurring at 104.4 ± 1.4 (2σ) Ma and 102.8 ± 0.7 Ma on the Sponge Bob seamount, at 96.3 ± 1.0 Ma on Ashton seamount, at 92.3 ± 3.8 Ma on the Gago Coutinho seamount, at 89.3 ± 2.3 Ma and 86.5 ± 3.4 Ma on the Jo Sister volcanic complex, and at 88.3 ± 3.3 Ma, 88.2 ± 3.9, and 80.5 ± 0.9 Ma on the Tore locality. No space-time correlation is observed for alkaline volcanism in the northern section of the Tore-Madeira Rise, which occurred 20-30 m.y. after opening of the eastern North Atlantic. Initial isotope signatures are: 19.139-19.620 for ²⁰⁶Pb/²⁰⁴Pb, 15.544-15.828 for ²⁰⁷Pb/²⁰⁴Pb, 38.750-39.936 for ²⁰⁸Pb/²⁰⁴Pb, 0.70231-0.70340 for ⁸⁷Sr/⁸⁶Sr, and +6.9 to +12.9 for initial epsilon Hf. These signatures are different from Atlantic MORB, the Madeira Archipelago and the Azores, but they lie in the field of worldwide OIB. The Cretaceous seamounts therefore seem to be generated by melts from a OIB-type source that interact with continental lithospheric mantle lying formerly beneath Iberia and presently within the ocean-continent transition zone. Inheritance in zircon and high ²⁰⁷Pb of initial Pb substantiate the presence of very minor amounts of continental material in the lithospheric mantle. A long-lived thermal anomaly is the most plausible explanation for alkaline magmatism since 104 Ma and it could well be that the same anomaly is still the driving force for tertiary and quaternary alkaline magmatism in the eastern North Atlantic region. This hypothesis is agreement with the plate-tectonic position of the region since Cretaceous time, including an about 30° anti-clockwise rotation of Iberia.     

Impact of environmental factors on in situ determination of iron in seawater by flow injection analysis

A sensitive method for iron determination in seawater has been adapted on a submersible chemical analyser for in situ measurements. The technique is based on flow injection analysis (FIA) coupled with spectrophotometric detection. When direct injection of seawater was used, the detection limit was 1.6 nM, and the precision 7%, for a triplicate injection of a 4 nM standard. At low iron concentrations, on line preconcentration using a column filled with 8-hydroxyquinoline (8HQ) resin was used. The detection limit was 0.15 nM (time of preconcentration = 240 s), and the precision 6%, for a triplicate determination of a 1 nM standard, allowing the determination of Fe in most of the oceanic regimes, except the most depleted surface waters. The effect of temperature, pressure, salinity, copper, manganese, and iron speciation on the response of the analyser was investigated. The slope of the calibration curves followed a linear relation as a function of pressure (C_p = 2.8 × 10^{− 5}P + 3.4 × 10^{− 2} s nmol^{− 1}, R² = 0.997, for Θ = 13 °C) and an exponential relation as a function of temperature (C_Θ = 0.009e^0.103Θ, R² = 0.832, for P = 3 bar). No statistical difference at 95% confidence level was observed for samples of different salinities (S = 0, 20, 35). Only very high concentration of copper (1000 × [Fe]) produced a detectable interference. The chemical analyser was deployed in the coastal environment of the Bay of Brest to investigate the effect of iron speciation on the response of the analyser. Direct injection was used and seawater samples were acidified on line for 80 s. Dissolved iron (DFe, filtered seawater (0.4 μm), acidified and stored at pH 1.8) corresponded to 29 ± 4% of Fe_a (unfiltered seawater, acidified in line at pH 1.8 for 80 s). Most of Fe_a (71 ± 4%) was probably a fraction of total dissolvable iron (TDFe, unfiltered seawater, acidified and stored at pH 1.8).     

The mark III Nançay radioheliograph The Radioheliograph Group

A north-south array has been added to the Mark II Nançay Radioheliograph (Radioheliograph Group, 1977). This instrument gives at 169 MHz two one-dimensional images of the Sun in the east-west and north-south directions including measurement of the circular polarization ratio. The main performances of this instrument are high space and time resolutions, flexible on line data processing and the possibility of off-line interactive data reduction. The method of calibration is briefly described. A few observations are presented.