Iron isotope fractionation in subduction-related high-pressure metabasites (Ile de Groix,France)

Characterisation of mass transfer during subduction is fundamental to understand the origin of compositional heterogeneities in the upper mantle. Fe isotopes were measured in high-pressure/low-temperature metabasites (blueschists, eclogites and retrograde greenschists) from the Ile de Groix (France), a Variscan high-pressure terrane, to determine if the subducted oceanic crust contributes to mantle Fe isotope heterogeneities. The metabasites have δ⁵⁶Fe values of +0.16 to +0.33‰, which are heavier than typical values of MORB and OIB, indicating that their basaltic protolith derives from a heavy-Fe mantle source. The δ⁵⁶Fe correlates well with Y/Nb and (La/Sm)_PM ratios, which commonly fractionate during magmatic processes, highlighting variations in the magmatic protolith composition. In addition, the shift of δ⁵⁶Fe by +0.06 to 0.10‰ compared to basalts may reflect hydrothermal alteration prior to subduction. The δ⁵⁶Fe decrease from blueschists (+0.19 ± 0.03 to +0.33 ± 0.01‰) to eclogites (+0.16 ± 0.02 to +0.18 ± 0.03‰) reflects small variations in the protolith composition, rather than Fe fractionation during metamorphism: newly-formed Fe-rich minerals allowed preserving bulk rock Fe compositions during metamorphic reactions and hampered any Fe isotope fractionation. Greenschists have δ⁵⁶Fe values (+0.17 ± 0.01 to +0.27 ± 0.02‰) similar to high-pressure rocks. Hence, metasomatism related to fluids derived from the subducted hydrothermally altered metabasites might only have a limited effect on mantle Fe isotope composition under subsolidus conditions, owing to the large stability of Fe-rich minerals and low mobility of Fe. Subsequent melting of the heavy-Fe metabasites at deeper levels is expected to generate mantle Fe isotope heterogeneities.     

Sedimentary evolution of an active margin during middle and upper cretaceous times: the north Peruvian margin from late aptian up to senonian

The sedimentary evolution of the North Peruvian margin during upper Cretaceous can be summarized as follows:The upper Aptian transgression involved the whole margin, and was accompanied by minor, local, synsedimentary tectonic features. It could be coeval with the individualization of the Western ensialic marginal basin. The transgression reached its maximum during middle Albian, and is associated with anoxic deposits, and with the main activity of the marginal basin.A regression followed during the upper Albian. It was materialized during lower Cenomanian by intertidal deposits, by a westward deltaic progradation, and by synsedimentary tectonic features. This regressive maximum is correlatable with the tectonic closure and folding of the western marginal basin.Then, the great upper Cretaceous transgression affected the eastern parts of the margin. Major deepening stages occured during late Middle Cenomanian and at the base of the Turonian. During Turonian, a widespread, calm carbonate shelf developed. A new major transgressive pulse occured at the Turonian-Coniacian boundary. As a result of a general uplift, the whole margin emerged diachronically between Santonian and Campanian, leading to the deposition of continental Red Beds. The latter recorded a new tectonic phase of uppermost Senonian age.During upper Cretaceous times, the sedimentary evolution of the North Peruvian margin appears to be controlled both by the eustatic sea-level changes, and by the uppermost Albian to early Cenomanian, and the upper Senonian tectonic events.

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Zusammenfassung Die sedimentäre Entwicklung des nordperuanischen Kontinentalrandes zeigt während der Ober-Kreide folgende Etappen: die Transgression des Oberaptes erstreckt sich über den gesamten Kontinentalrand und geht mit leichten synsedimentären tektonischen Diskonformitäten einher, die gleichzeitig mit der Entstehung des westlichen Randbeckens zu sehen sind. Die Transgression erreicht ihren Höhepunkt im mittleren Alb, sie ist verbunden mit anoxischen Ablagerungen und mit den Hauptbewegungen im randlichen Becken. Im Oberalb folgt eine Regression. Sie zeigt sich im unteren Cenoman durch intertidale Ablagerungen, durch nach Westen progradierende Deltas und durch synsedimentäre Tektonik. Der Höhepunkt der Regression ist verbunden mit der tektonischen Isolation und Faltung des westlichen Randbeckens.Die große Transgression während der Oberkreide beeinflußt vor allen Dingen die östlichen Teile des Randes. Wesentliche Eintiefungsphasen erscheinen im oberen Mittelcenoman und an der Basis des Turons. Während des Turons entwickelt sich ein weiter Karbonatschelf. Ein größerer Transgressionsvorstoß erscheint an der Grenze von Turon zu Coniac. Als Ergebnis einer generellen Heraushebung wird der gesamte Kontinentrand diachron zwischen Santon und Campan herausgehoben und es entstehen kontinentale Rotablagerungen. Diese Sedimente dokumentieren eine neue tektonische Phase im obersten Senon. Während der Oberkreide wird die sedimentäre Weiterentwicklung des nordperuanischen Kontinentalrandes durch globale Meeresspiegelschwankungen zusammen mit tektonischen Ereignissen (Oberapt, Oberalb-unteres Cenoman, Senon und oberstes Senon) kontrolliert.

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Résumé L'évolution sédimentaire de la marge nord-péruvienne au Crétacé supérieur peut être résumée ainsi:La transgression de l'Aptien supérieur affecte toute la marge, et s'accompagne d'une légère instabilité tectonique localisée. Celle-ci peut être contemporaine de l'individualisation du bassin marginal occidental. La transgression atteint son maximum à l'Albien moyen. Elle est contemporaine de dépôts anoxiques et de l'activité du bassin marginal.Une régression survient au cours de l'Albien supérieur. Elle se matérialise au Cénomanien inférieur par l'installation d'une plate-forme carbonatée, puis par la progradation du delta oriental et par des mouvements tectoniques synsédimentaires. Cette régression est en relation avec la fermeture du bassin marginal occidental.La grande transgression du Crétacé supérieur affecte ensuite les zones orientales de la marge, les étapes majeures d'approfondissement se situant à la fin du Cénomanien moyen et à la base du Turonien. Au Turonien, une vaste plate-forme carbonatée s'installe, et un nouvel approfondissement survient à la limite Turonien-Coniacien. A la suite d'une surrection générale, la marge émerge de façon diachrone entre le Santonien et le Campanien, donnant lieu au dépôt des Couches Rouges continentales qui enregistrent au Sénonien terminal un nouvel événement tectonique.Au Crétacé supérieur, l'evolution sédimentaire de la marge nord-péruvienne semble donc contrôlée à la fois par les variations eustatiques, et par les événements tectoniques de l'Albien terminal-Cénomanien inférieur, et du Sénonien.

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汉诺坝幔源单斜辉石巨晶氢同位素组成的离子探针分析：微尺度不均一性

运用离子探针技术测定了河北汉诺坝新生代玄武岩中3个单斜辉石巨晶的氢同位素组成，结果显示同一颗粒内部表现出微尺度的不均一性，2mm范围内δD的变化达到60‰。δD和氢含量之间不存在同步的变化，巨晶内部的化学成分均一，因此我们认为巨晶的氢同位素不均一性继续自母岩浆。母岩浆的氢同位素变化可能是去气过程中气相与熔体之间的分馏引起的。单斜辉石巨晶形成后很短的时间内即被寄主岩浆带至地表并经历了快速淬火。     

GGR Critical Review of Analytical Developments in 2003

The articles that comprise this critical review serve to draw attention to research papers published in specific fields of interest during 2003, provide critical comment on the relevance and importance of individual publications in these fields, and offer an overview of the comparative importance of advances in particular areas. In this way, these articles aim to assist experts in the field by keeping them informed of relevant recent publications, as well as providing an important resource for students or early career researchers who are embarking on studies in an area new to them. This year, five papers provide summaries of developments in bulk sample determinations employing (1) ICP-AES and ICP-MS (trace elements), (2) XRF and atomic absorption spectrometry and INAA, (3) isotope ratio measurements (TIMS, MC-ICP-MS, ICP-MS, ToF), as well as in situ measurements conducted using (4) secondary ion mass spectrometry and (5) laser ablation ICP-MS (trace element and isotope ratio determinations).     

2004–2005 Analytical Developments in Secondary Ion Mass Spectrometry

This review of the literature from 2004 and 2005 concerning secondary ion mass spectrometry (SIMS) highlights the contribution the technique has made in the fields of petrology, geochronology, cosmochemistry and material sciences. In petrology, much research was devoted to the measurement of stable isotopes and trace elements by developments in multicollection acquisition, with emphasis on low atomic mass number elements. Elements studied in particular were S (in sulfides), O (in garnets), C (in sedimentary organic matter), Cl (in glasses) and Si. Novel applications of SIMS to geochronology have included the measurement of young zircon grains by the U-Pb and U-Th decay methods. An increasing number of studies have combined U-Pb geochronology with the measurement of trace elements or stable isotopes in zircon.     

Structural control over equilibrium silicon and oxygen isotopic fractionation: A first-principles density-functional theory study

Isotopic fractionation factors for oxygen and silicon in selected silicates (quartz, enstatite, forsterite, lizardite, kaolinite) are calculated using first-principles methods based on density-functional theory. Good agreement between theory and experiment is found in the case of oxygen. In the case of silicon, agreement and differences with existing estimates of equilibrium fractionation factors are discussed. The relationship between silicon and oxygen fractionation factors, silicate polymerization degree and chemical composition is studied and compared with previous semi-empirical models.     

Momentum- and Heat-Flux Parametrization at Dome C,Antarctica: A Sensitivity Study

An extensive meteorological observational dataset at Dome C, East Antarctic Plateau, enabled estimation of the sensitivity of surface momentum and sensible heat fluxes to aerodynamic roughness length and atmospheric stability in this region. Our study reveals that (1) because of the preferential orientation of snow micro-reliefs (sastrugi), the aerodynamic roughness length \(z_{0}\) varies by more than two orders of magnitude depending on the wind direction; consequently, estimating the turbulent fluxes with a realistic but constant \(z_{0}\) of 1 mm leads to a mean friction velocity bias of \(24\,\%\) in near-neutral conditions; (2) the dependence of the ratio of the roughness length for heat \(z_{0t}\) to \(z_{0}\) on the roughness Reynolds number is shown to be in reasonable agreement with previous models; (3) the wide range of atmospheric stability at Dome C makes the flux very sensitive to the choice of the stability functions; stability function models presumed to be suitable for stable conditions were evaluated and shown to generally underestimate the dimensionless vertical temperature gradient; as these models differ increasingly with increases in the stability parameter z / L, heat flux and friction velocity relative differences reached \(100\,\%\) when \(z/L > 1\); (4) the shallowness of the stable boundary layer is responsible for significant sensitivity to the height of the observed temperature and wind data used to estimate the fluxes. Consistent flux results were obtained with atmospheric measurements at heights up to 2 m. Our sensitivity study revealed the need to include a dynamical parametrization of roughness length over Antarctica in climate models and to develop new parametrizations of the surface fluxes in very stable conditions, accounting, for instance, for the divergence in both radiative and turbulent fluxes in the first few metres of the boundary layer.     

Diachronous evolution of volcano-sedimentary basins north of the Congo craton: Insights from U–Pb ion microprobe dating of zircons from the Poli, Lom and Yaoundé Groups (Cameroon)

Ion microprobe U–Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700–665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100–950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa–Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west–central Africa.