The South African polymict eucrite Macibini

Abstract— The polymict eucrite Macibini is a fragmental breccia, predominantly composed of eucritic materials with minor proportions (maximum 2 vol%) of diogenitic material. Hence, it is intermediate between the Yamato‐74159‐type polymict eucrites, which contain negligible amounts of magnesian orthopyroxene, and the howardites. The present study provides mineralogical and bulk compositional data for the meteorite breccia and for six clasts. These clasts include both volcanic and igneous rocks and a variety of impact‐generated rocks. A broad range of degrees of postcrystallization metamorphism affected these materials before the final aggregation of the breccia. Clast A is a fragment of unequilibrated eucrite with subophitic texture. The edges of the zoned pyroxenes in this clast are composed of a host of Fe‐rich augite containing vermicules (blebs) and lamellae composed of a mixture of Fe‐rich olivine and silica. Similar features occur as fragments in lunar breccias and are attributed by some workers to the breakdown of pyroxferroite, an Fe‐rich pyroxenoid. However, textures and compositions of these augite‐olivine‐silica intergrowths in clast A suggest that, in this case, they are the result of decomposition in a series of steps of Fe‐rich subcalcic augite. Among the fragments of impact‐generated material in Macibini is clast 2, an earlier‐formed clastic breccia that was lithified before being broken apart and included in the meteorite breccia. Clast 3 is an impact‐melt breccia that is composed of rock and mineral fragments in a devitrified groundmass. Clast C is also an impact‐melt breccia that has a coarser‐grained, hornfelsic groundmass that resulted from extensive metamorphism after formation.     

Defining the mechanisms that disturb the Sm‐Nd isotopic systematics of the Martian meteorites: Examples from Dar al Gani 476 and Allan Hills 77005

Abstract— Microbeam studies of Martian meteorites Dar al Gani (DaG) 476 and Allan Hills (ALH) 77005 have been conducted to identify potential causes of disequilibrium exhibited in their Sm‐Nd isotopic systematics. Olivine and maskelynite mineral fractions on the DaG 476 isochron are displaced relative to their positions as dictated by measured mineral compositions. The olivine mineral fractions from ALH 77005 not only have a relatively low Sm/Nd ratio, but appear to contain an unradiogenic component that shifts the olivine mineral fraction off the isochron defined by the pyroxene and maskelynite mineral fractions. Trace components such as melt inclusions, impact melt, high‐Si mesostasis, and altered olivine were analyzed using scanning electron microscopy, quantitative electron microscopy, and secondary ion mass spectrometry to determine their potential for disturbing the isotopic systematics of the mineral fractions, assuming that the mineral fractions were not completely pure. Mixing models indicate that the presence of melt inclusions in the DaG 476 olivine mineral fraction lowered its Sm/Nd ratio. The maskelynite mineral fraction contains a related but more evolved mesostasis component that raised the Sm/Nd ratio of the fraction. The position of two olivine mineral fractions below the ALH 77005 isochron is interpreted to reflect small additions of impact melt with a light rare earth element enriched pattern and a non‐indigenous, unradiogenic Nd component. Furthermore, the presence of rare earth elements in olivine and maskelynite from both igneous and non‐igneous components such as melt inclusions, mesostasis, and impact melt is observed on a fine (<30 μm) scale. Despite the addition of this material, the Sm‐Nd ages are not affected. This study demonstrates that detailed mineral separation procedures as employed by modern geochronology laboratories permit reliable ages to be derived from shocked and altered samples.     

Inductive electric field of a time-dependent ring current

The inductive electric field generated by a time-dependent symmetric ring current has been investigated. The symmetric ring current was modelled by a population of protons drifting in a magnetic dipole field. The interaction of these protons with ion-cyclotron waves was assumed to be the dominant energy loss process for the ring current protons, at least under certain conditions. The calculation, with spectral densities for the ion-cyclotron waves that are based on experimental data, showed that an azimuthal inductive electric field of as much as 0.25 mV/m can be produced by this mechanism. Furthermore there is evidence that if the spectral density of the waves is substantially larger than the one adopted here, the electric field might increase to the order of 1.0 mV/m or more.     

Upper proterozoic volcanic graywackes from northwestern Hoggar (Algeria) — geology and geochemistry

Late-Proterozoic volcanic graywackes from the NW Hoggar (Algeria) have been investigated from geological and petrological points of view. Thirty-five samples have been analysed for major elements and the following trace elements: Li, Rb, Sr, Ba, Ni, Co, Cr, V, Zn, Cu.These graywackes constitutes a thick flysch-like formation deposited in a marine environment. They are composed of volcanic (mainly andesites) and plutonic detritus. The chemical analyses confirm this observation and demonstrate the very immature character of these rocks.The composition of the NW Hoggar graywackes differs chemically from other investigated graywackes in that they have a higher Ca, Na content and lower SiO₂, Ni content.Several geological observations suggest that the average Hoggar graywacke composition approaches the composition of the W Hoggar Late Precambrian upper crust. The graywackes were formed as a result of erosion of penecontemporaneous calc-alkali volcanic suite, emplaced after widespread mafic intrusions in the Upper Proterozoic shelf deposits.     

Modelling tidal current‐induced bed shear stress and palaeocirculation in an epicontinental seaway: the Bohemian Cretaceous Basin,Central Europe

Lower to Middle Turonian deposits within the Bohemian Cretaceous Basin (Central Europe) consist of coarse‐grained deltaic sandstones passing distally into fine‐grained offshore sediments. Dune‐scale cross‐beds superimposed on delta‐front clinoforms indicate a vigorous basinal palaeocirculation capable of transporting coarse‐grained sand across the entire depth range of the clinoforms (ca 35 m). Bi‐directional, alongshore‐oriented, trough cross‐set axes, silt drapes and reactivation surfaces indicate tidal activity. However, the Bohemian Cretaceous Basin at this time was over a thousand kilometres from the shelf break and separated from the open ocean by a series of small islands. The presence of tidally‐influenced deposits in a setting where co‐oscillating tides are likely to have been damped down by seabed friction and blocked by emergent land masses is problematic. The Imperial College Ocean Model, a fully hydrodynamic, unstructured mesh finite element model, is used to test the hypothesis that tidal circulation in this isolated region was capable of generating the observed grain‐size distributions, bedform types and palaeocurrent orientations. The model is first validated for the prediction of bed shear stress magnitudes and sediment transport pathways against the present‐day North European shelf seas that surround the British Isles. The model predicts a microtidal to mesotidal regime for the Bohemian Cretaceous Basin across a range of sensitivity tests with elevated tidal ranges in local embayments. Funnelling associated with straits increases tidal current velocities, generating bed shear stresses that were capable of forming the sedimentary structures observed in the field. The model also predicts instantaneous bi‐directional currents with orientations comparable with those measured in the field. Overall, the Imperial College Ocean Model predicts a vigorous tide‐driven palaeocirculation within the Bohemian Cretaceous Basin that would indisputably have influenced sediment dispersal and facies distributions. Palaeocurrent vectors and sediment transport pathways however vary markedly in the different sensitivity tests. Accurate modelling of these parameters, in this instance, requires greater palaeogeographic certainty than can be extracted from the available rock record.     

Caribbean hydrological variability during the Holocene as reconstructed from crater lakes on the island of Grenada

Contemporary precipitation patterns in the Caribbean region are spatially variable, and the small number of Holocene paleoclimatic records may not adequately capture patterns of variation in the past. The hydrological history of Grenada was inferred from paleolimnological analyses of sediment cores from two crater lakes on the island. The basins were formed by volcanic activity some time during the Last Termination, but were dry between ca. 13 000 and ca. 7200 cal. a BP. After filling, the lakes were initially very shallow, and sedimentation was interrupted by a hiatus ca. 6300–5500 cal. a BP, followed by deposition of a thick tephra in both sites. After 5500 cal. a BP, lake level shows considerable multi‐centennial variability, superimposed upon a long‐term trend of generally higher lake level after 3200 cal. a BP. The pattern of lake‐level variation in Grenada shows some similarity with other Caribbean paleoclimatic records in terms of the timing of transitions, but differs from several classic studies in the sign of inferred precipitation change. The differences among records may reflect spatially variable precipitation patterns in the past in response to the position of the Intertropical Convergence Zone and to sea surface temperature influences on the trade winds and Caribbean low‐level jet. Copyright © 2011 John Wiley & Sons, Ltd.     

Metamorphic veining and mass transfer in a chemically closed system: a case study in Alpine metabauxites (western Vanoise)

In western Vanoise (French Alps), karstic pockets of Triassic‐to‐Jurassic metabauxites embedded in carbonate rocks and containing several generations of metamorphic veins were studied. During blueschist facies metamorphism, a cumulative amount of ～13 vol% of water is inferred to have been produced locally by successive dehydration reactions, and part of this fluid remained in the bauxitic lenses during most of the metamorphic cycle. Field and geochemical evidence show that these rocks have been isolated from large‐scale fluid flow (closed‐system behaviour). It is proposed that the internally derived fluid has promoted the opening of fluid‐filled open spaces (as attested by the euhedral habits of vein minerals) and served as medium for mass transfer from rock to vein. Indeed, the vein infill is obviously the result of chemical interactions, at the millimetre‐to‐centimetre scale, between the rock minerals and the locally produced aqueous fluid. Two vein types can be distinguished based on mineralogical and textural features: (i) some veins are filled with newly formed products of either prograde (chloritoid) or retrograde (chlorite) metamorphic reactions; in this case, fluid‐filled open spaces seem to offer energetically favourable nucleation/growth sites; (ii) the second vein type is infilled with cookeite or pyrophyllite, that were present in the host rock prior to the vein formation. In this closed chemical system, the components for the vein infill minerals have been transferred from rock to vein through the fluid, in a dissolution–transport–precipitation process, possibly stress‐assisted. These different vein generations all contain Al‐rich mineral infills, suggesting that Al was a mobile element (cm scale) during metamorphism. In these HP rocks, fluid flow may have been restricted, and if so mass transfer occurred by diffusion in an almost stagnant fluid. Metamorphic veins can be seen as witnesses of fluid and mass redistribution that partly accommodate the rock deformation (lateral segregation).     

The estuarine clamRangia cuneata as a biomonitor of dioxins and furans in the Neches River,Taylor Bayou,and Fence Lake,Texas

Polychlorinated dibenzo-para-dioxins (PCDDs) and dibenzofurans (PCDFs) were measured in the tissues of the estuarine clamRangia cuneata at four sites in the Neches River that had been subjected to paper mill effluent and at two remote sites. One of the river samplings was taken before dioxin control measures and another 2 yr after the control measures began. Measurable concentrations of PCDD/Fs were present at all sites, but higher concentrations were present at the Neches River sites. Whole tissue concentrations of 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD) and all PCDD congeners for the two collections were not significantly different (p>0.05), but lipid-normalized concentrations for the two collections were significantly different (p<0.05). Highest concentrations of PCDDs at sites located 8 km and 16 km upriver from the paper mill effluent outfall indicated that the materials were transported upriver by saltwater intrusion and/or that nonpoint sources existed upriver. The presences of PCDD/Fs in clam tissues from the remote sites also indicated that other sources of these materials existed. Even under extreme physiological conditions (spent reproductive phase, low lipid contents, water temperature 10°C; ripe reproductive phase, high lipids, water temperature 32°C)Rangia cuneata was an effective biomonitor to determine the distribution of PCDD/Fs at specific sites. *** DIRECT SUPPORT *** A01BY069 00021