Circumstellar dynamics and transfer

This paper is both a review and a presentation of new models. Observation and modelization of circumstellar envelopes of early type or late type stars are now quickly evolving because of new techniques and facilities for observations, and increased power of computers. More and more complex physical phenomena involved in mass driving can now be modelized, at many different size scales. While most of models were previously based on informations derived from spectrophotometric data only or on measurements concerning objects observed with no spatial resolution, observations at much increased angular resolution can provide constraints on models of these phenomena. Theory and modelization must take this new situation into account. Two approaches are possible and effectively used. On the one hand, dynamical/physical self consistent models can be built; on the other hand, elaborate semi-empirical models including complicated distributions of matter with asymmetries (3D models) can be built and fitted for direct comparison with results of High Angular Resolution Measurements. Adding such constraints to classical constraints leads to a new insight in the physics of circumstellar matter and, through it, of stellar and interstellar evolution. Two examples have been chosen, in which new models are presented and assuming or not spherical symmetry is carefully discussed:

?Circumstellar matter around evolved stars

?Shock waves propagating in the circumstellar matter around evolved stars.

     

Paleoproterozoic (2155–1970 Ma) evolution of the Guiana Shield (Transamazonian event) in the light of new paleomagnetic data from French Guiana

We present a comprehensive paleomagnetic study on Paleoproterozoic (2173–2060 Ma) plutonic and metamorphic rocks from French Guiana, representative of the full range of the main Transamazonian tectonothermal steps. Twenty-seven groups of directions and poles were obtained from combination of 102 sites (613 samples) based on age constraint, similar lithology and/or geographical proximity. Paleomagnetic results show variations between rocks of different ages which are supposed to be characteristic of magnetizations acquired during uplift and cooling of successive plutonic pulses and metamorphic phases. This is also reinforced by positive field tests (baked contact and reversal tests). Recent U/Pb and Pb/Pb on zircon and complementary ⁴⁰Ar/³⁹Ar on amphibole and biotite allow questioning the problem of magnetic ages relative to rock formation ages. Estimated magnetic ages, based on amphibole dating as a proxy, enable us to construct a Guiana Shield apparent polar wander path for the 2155–1970 Ma period. It is also possible to present paleolatidudinal evolution and continental drift rates related to specific Transamazonian tectonic regimes.French Guiana and probably the Guiana Shield were located at the Equator from ca. 2155 to 2130 Ma during the Meso-Rhyacian D1 magmatic accretion phase, related to subduction of Eorhyacian oceanic crust. After closure of the Eorhyacian Ocean and collision of West African and Amazonian plates, the Guiana Shield moved. The first evolution towards 60° latitude, occurs after 2080 Ma, during the Neorhyacian D2a post collisional sinistral transcurrent phase. During the Late Rhyacian D2b phase, up to 2050 Ma, the Guiana Shield reaches the pole and starts to move to lower latitudes on an opposite meridian. By the Orosirian D2c phase, from ca. 2050 to 1970 Ma, the Guiana Shield reaches the Equator.Based on the amphibole ⁴⁰Ar/³⁹Ar dates, we estimate the continental drift between 12 and 16 cm/y for the Meso to Late Rhyacian period followed by a lower rate between 9 and 14 cm/y up to Orosirian time. This study highlights rock ages and magnetic ages are prerequisite to any continental reconstruction especially when it is shown continental drift is important for a 100–200 Ma time period. Our results confirm the possibility of APWP construction on Paleoproterozoic plutonic rocks but suggest improvement will rely on the combination with multidisciplinary approaches such as structural geology and multi-method radiometric dating.     

Zircon geochronology and Sm–Nd isotopic study: Further constraints for the Archean and Paleoproterozoic geodynamical evolution of the southeastern Guiana Shield, north of Amazonian Craton, Brazil

The eastern part of the Guiana Shield, northern Amazonian Craton, in South America, represents a large orogenic belt developed during the Transamazonian orogenic cycle (2.26–1.95 Ga), which consists of extensive areas of Paleoproterozoic crust and two major Archean terranes: the Imataca Block, in Venezuela, and the here defined Amapá Block, in the north of Brazil.

Pb-evaporation on zircon and Sm–Nd on whole rock dating were provided on magmatic and metamorphic units from southwestern Amapá Block, in the Jari Domain, defining its long-lived evolution, marked by several stages of crustal accretion and crustal reworking. Magmatic activity occurred mainly at the Meso-Neoarchean transition (2.80–2.79 Ga) and during the Neoarchean (2.66–2.60 Ga). The main period of crust formation occurred during a protracted episode at the end of Paleoarchean and along the whole Mesoarchean (3.26–2.83 Ga). Conversely, crustal reworking processes have dominated in Neoarchean times. During the Transamazonian orogenic cycle, the main geodynamic processes were related to reworking of older Archean crust, with minor juvenile accretion at about 2.3 Ga, during an early orogenic phase. Transamazonian magmatism consisted of syn- to late-orogenic granitic pulses, which were dated at 2.22 Ga, 2.18 Ga and 2.05–2.03 Ga. Most of the ε_Nd values and T_DM model ages (2.52–2.45 Ga) indicate an origin of the Paleoproterozoic granites by mixing of juvenile Paleoproterozoic magmas with Archean components.

The Archean Amapá Block is limited in at southwest by the Carecuru Domain, a granitoid-greenstone terrane that had a geodynamic evolution mainly during the Paleoproterozoic, related to the Transamazonian orogenic cycle. In this latter domain, a widespread calc-alkaline magmatism occurred at 2.19–2.18 Ga and at 2.15–2.14 Ga, and granitic magmatism was dated at 2.10 Ga. Crustal accretion was recognized at about 2.28 Ga, in agreement with the predominantly Rhyacian crust-forming pattern of the eastern Guiana Shield. Nevertheless, T_DM model ages (2.50–2.38 Ga), preferentially interpreted as mixed ages, and ε_Nd < 0, point to some participation of Archean components in the source of the Paleoproterozoic rocks. In addition, the Carecuru Domain contains an oval-shaped Archean granulitic nucleus, named Paru Domain. In this domain, Neoarchean magmatism at about 2.60 Ga was produced by reworking of Mesoarchean crust, as registered in the Amapá Block. Crustal accretion events and calc-alkaline magmatism are recognized at 2.32 Ga and at 2.15 Ga, respectively, as well as charnockitic magmatism at 2.07 Ga.

The lithological association and the available isotopic data registered in the Carecuru Domain suggests a geodynamic evolution model based on the development of a magmatic arc system during the Transamazonian orogenic cycle, which was accreted to the southwestern border of the Archean Amapá Block.     

Wintertime rates of net primary production and nitrate and ammonium uptake in the southern Benguela upwelling system

The elevated levels of primary productivity associated with eastern boundary currents are driven by nutrient- rich waters upwelled from depth, such that these regions are typically characterised by high rates of nitrate-fuelled phytoplankton growth. Production studies from the southern Benguela upwelling system (SBUS) tend to be biased towards the summer upwelling season, yet winter data are required to compute annual budgets and understand seasonal variability. Net primary production (NPP) and nitrate and ammonium uptake were measured concurrently at six stations in the SBUS in early winter. While euphotic zone NPP was highest at the stations nearest to the coast and declined with distance from the shore, a greater proportion was potentially exportable from open-ocean surface waters, as indicated by the higher specific nitrate uptake rates and f-ratios (ratio of nitrate uptake to total nitrogen consumption) at the stations located off the continental shelf. Near the coast, phytoplankton growth was predominantly supported by ammonium despite the high ambient nitrate concentrations. Along with ammonium concentrations as high as 3.6 µmol l^–1, this strongly suggests that nitrate uptake in the inshore SBUS, and by extension carbon drawdown, is inhibited by ammonium, at least in winter, although this has also been hypothesised for the summer.     

Dust Grains and Circumstellar Motion: The Problem of the Henand the Egg

In the last ten years, the number of models of circumstellar envelopes has exponentially increased, both concerning evolved stars and forming stars with their planetary systems. The main progress was intoduction of chemistry and coupling with dynamics. However, paradoxically, the nucleation theories did not evolve at the same rate; moreover, dynamics and nucleation are rarely analysed at the same degree of accuracy: one is of them is always mostly drawing attention. At the same time observations showed the presence of dust grains at a lot of sites where it could not be expected from usual quasi equilibrium theories. Self consistent systems are omnipresent. Thus, it is important to distinguish causes and effects, even if the phenomena are are strongly coupled. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electron microprobe U–Th–Pb monazite dating of the Transamazonian metamorphic overprint on Archean rocks from the Amapá Block, southeastern Guiana Shield, Northern Brazil

The Amapá Block, southeastern Guiana Shield, represents an Archean block involved in a large Paleoproterozoic belt, with evolution related to the Transamazonian orogenic cycle (2.26 to 1.95 Ga). High spatial resolution dating using an electron-probe microanalyzer (EPMA) was employed to obtain U–Th–Pb chemical ages in monazite of seven rock samples of the Archean basement from that tectonic block, which underwent granulite- and amphibolite-facies metamorphism. Pb–Pb zircon dating was also performed on one sample.Monazite and zircon ages demonstrate that the metamorphic overprinting of the Archean basement occurred during the Transamazonian orogenesis, and two main tectono-thermal events were recorded. The first one is revealed by monazite ages of 2096 ± 6, 2093 ± 8, 2088 ± 8, 2087 ± 3 and 2086 ± 8 Ma, and by the zircon age of 2091 ± 5 Ma, obtained in granulitic rocks. These concordant ages provided a reliable estimate of the time of the granulite-facies metamorphism in the southwest of the Amapá Block and, coupled with petro-structural data, suggest that it was contemporaneous to the development of a thrusting system associated to the collisional stage of the Transamazonian orogenesis, at about 2.10–2.08 Ga.The later event, under amphibolite-facies conditions, is recorded by monazite ages of 2056 ± 7 and 2038 ± 6 Ma, and is consistent with a post-collisional stage, marked by granite emplacement and coeval migmatization of the Archean basement along strike-slip shear zones.     

Spatial Variation of Sea-Spray Fluxes over a Mediterranean Coastal Zone Using a Sea-State Model

We first deal with sea-spray flux estimates for short fetch conditions in coastal Mediterranean areas. To this end, a sea-state dependent model for the whitecap fraction was included in three different formulations for the sea-spray source function. A comparison with the sea-spray fluxes, calculated on the basis of aerosol size distributions measured at the island of Porquerolles located south off the French Riviera, evaluates the predictions of different whitecap dependant flux formulations. Then we deal with the spatial distribution of the whitecap fraction and the sea-spray fluxes in the study area. To achieve this, a whitecap dependant flux formulation was forced by a wave numerical model that was implemented in the study area. Experimental results on wave conditions have been used to adjust the model in the Mediterranean coastal area. Numerical simulations of wave and whitecap coverage have been carried out during typical regional wind events, and they show a nonhomogeneous distribution of the sea-surface production over the northern Mediterranean as a consequence of the spatial variation of the sea state. In particular, we note the occurrence of a narrow band of high sea-surface production following the northern coast and along the east part of the Gulf of Lions.     

内蒙古西拉沐伦成矿带碾子沟钼矿床成矿流体地球化学特征

碾子沟钼矿床是内蒙古西拉沐伦钼多金属成矿带中石英脉型钼矿床的典型代表,矿体以石英大脉形式产于燕山期中粗粒黑云母二长花岗岩内,受断裂构造控制。流体包裹体研究发现包裹体均为气液两相,按照相比不同,可进一步分为WL型(5%～20%)和WV型(20%～50%)。Ⅰ阶段流体为低温(89.3～245.2℃)、中低盐度(2.07%～17.96%NaCleqv)流体;Ⅱ阶段流体具有中低温(134.4～458.8℃,峰值170℃～240℃)、中低盐度(0.53%～19.92%NaCleqv)特征;Ⅲ阶段流体为低温(134.9～202.4℃)、中低盐度(4.96%～14.97%NaCleqv)流体。流体成分均以H2O为主(96.1mol%),含少量挥发份CO2、N2、CH4、C2H6、Ar、H2S,阳离子以Na+为主,阴离子以SO42-、Cl-为主,属NaCl-H2O体系。各阶段成矿热液氢、氧同位素特征为:δ18OH2O介于-5.75‰～-1.90‰、δD介于-128.821‰～-109.234‰,说明成矿流体是岩浆热液与古大气降水混合而成。开放的断裂体系为流体混合创造了条件,流体的混合作用是造成碾子沟辉钼矿沉淀成矿的主要原因。这与斑岩型钼矿床的高盐度流体以及以沸腾为主的矿石沉淀机制具有显著区别。     

40Ar–39Ar geochronology across Archean and Paleoproterozoic terranes from southeastern Guiana Shield (north of Amazonian Craton,Brazil): Evidence for contrasting cooling histories

A ⁴⁰Ar/³⁹Ar geochronological study was performed on amphibole and biotite from some representative units of distinct tectonic domains of the southeastern Guiana Shield, north of the Amazonian Craton, the Amapá Block and the Carecuru Domain. In the Amapá Block, an Archean continental block involved in the Transamazonian orogenesis (2.26–1.95 Ga), the investigated minerals, from rocks of the Archean high-grade basement assemblage, give only Paleoproterozoic ages, indicating their complete resetting during the Transamazonian orogenic event. Amphibole ages vary from 2087 ± 3 to 2047 ± 20 Ma, and biotite ages spread mainly between 2079 ± 18 and 2033 ± 13 Ma. In the Carecuru Domain, in which the geodynamic evolution is related to Paleoproterozoic magmatic arc setting during the Transamazonian event, calc-alkaline granitoids yield amphibole age of 2074 ± 17 Ma, and biotite ages of 1928 ± 19 Ma and 1833 ± 13 Ma.These data reinforce the importance of the Transamazonian orogenic cycle in the investigated area, and indicate that the rocks were not significantly affected by post-Transamazonian events. When coupled with available U–Th–Pb monazite and Pb–Pb zircon geochronological records and petro-structural observations, the new ⁴⁰Ar/³⁹Ar data delineate contrasting cooling and exhumation histories for the tectonic domains. In the Amapá Block, the data suggest nearly vertical T–t paths that reflect fast cooling rates, which indicate tectonically controlled exhumation, related to collisional stages of the Transamazonian event, between 2.10 and 2.08 Ga. Conversely, in the Carecuru Domain, low cooling rates suggest that the arc-related granitoids underwent slow and monotonous cooling since their emplacement until reaching the biotite isotopic closure temperature.     

Sm–Nd isotopic investigation of Neoproterozoic and Cretaceous igneous rocks from southern Brazil: A study of magmatic processes

Nd-evolutionary paths for diversified igneous suites from southern Brazil are here re-evaluated using published results. We interpret the ε_Nd paths considering the secondary fractionation of ¹⁴⁷Sm/¹⁴⁴Nd due to major petrogenetic processes. The inclusion of Nd isotopes and geochemical data for Precambrian and Mesozoic basic rocks allow improving the discussion on the subcontinental lithosphere beneath southern Brazil. Late Neoproterozoic rocks, mostly granitoids, are exposed in two regions of the southern Brazilian shield, an eastern collisional belt and a western foreland. The latter included two geotectonic domains amalgamated at this time, the São Gabriel Arc (900–700 Ma), and the Taquarembó cratonic block. Magma genesis mainly involved mixture of crustal and incompatible-element-enriched mantle components, both with a long residence time. Continental segments are the Neoarchaean–Paleoproterozoic lower crust (ca. 2.55 Ga) in the western foreland, and Paleoproterozoic–Neoproterozoic recycled crust (2.1–0.8 Ga) in the collisional belt. Granitoids with a single crustal derivation are limited in the southern Brazilian Shield. Mixing processes are well-registered in the western foreland, where the re-enriched old mantle was probably mixed with a 900–700 Ma-old subducted lithosphere and a 2.55 Ga-old lower crust. The contribution of the latter increased from the early 605–580 Ma to the later 575–550 Ma Neoproterozoic events, which may be due either to crustal thickening or to delamination of the lithosphere. Magma sources were diversified in the 660–630 Ma collisional belt. Initially, they involved the mixing between two components with similar Nd isotopic ratios, a 2.1–0.8 Ga-old recycled crust and a subduction-processed old mantle. Regional heating and abundant production of granitic melts, with diversified contribution of enriched mantle components, mark the end of the collisional period, at 630–580 Ma. We can also attribute this to the delamination of the lithosphere, so that the same geodynamic process may explain the magmatism in the whole shield at the end of the Dom Feliciano Orogeny. Mesozoic rocks include flood basalts from the Cretaceous Paraná Province and sub-coeval alkalic suites. Multiple processes of metasomatism affected the lithospheric mantle, resulting in some complexity but they mainly register two enriched-mantle components, both generated during Neoarchaean–Paleoproterozoic events. One end-member has a more pronounced subduction signature. The other one probably resulted from the re-enrichment of the first component at the end of the Camboriú collisional orogeny (2.0 Ga).