Kimberlitic sources of super-deep diamonds in the Juina area,Mato Grosso State,Brazil

The Juina diamond field, in the 1970–80s, was producing up to 5–6 million carats per year from rich placer deposits, but no economic primary deposits had been found in the area. In 2006–2007, Diagem Inc. discovered a group of diamondiferous kimberlitic pipes within the Chapadão Plateau (Chapadão, or Pandrea cluster), at the head of a drainage system which has produced most of the alluvial diamonds mined in the Juina area. Diamonds from placer deposits and newly discovered kimberlites are identical; they have super-deep origins from the upper-mantle and transition zone. Field observations and petrographic studies have identified crater-facies kimberlitic material at seven separate localities. Kimberlitic material is represented by tuffs, tuffisites and various epiclastic sediments containing chrome spinel, picroilmenite, manganoan ilmenite, zircon and diamond. The diamond grade varies from 0.2–1.8 ct/m³. Chrome spinel has 30–61 wt.% Cr₂O₃. Picroilmenite contains 6–14 wt.% MgO and 0.2–4 wt.% Cr₂O₃. Manganoan ilmenite has less than 3 wt.% MgO and 0.38–1.41 wt.% MnO. The ¹⁷⁶Hf/¹⁷⁷Hf ratio in kimberlitic zircons is 0.028288–0.28295 with ε_Hf = 5.9–8.3, and lies on the average kimberlite trend between depleted mantle and CHUR. The previously known barren and weakly diamondiferous kimberlites in the Juina area have ages of 79–80 Ma. In contrast, zircons from the newly discovered Chapadão kimberlites have a mean ²⁰⁶Pb/²³⁸U age of 93.6 ± 0.4 Ma, corresponding to a time of magmatic activity related to the opening of the southern part of the Atlantic Ocean. The most likely mechanism of the origin of kimberlitic magma is super-deep subduction process that initiated partial melting of zones in lower mantle with subsequent ascent of proto-kimberlitic magma.     

Luminosity distributions within rich clusters — II. Demonstration and verification via simulation

We present detailed simulations of long-exposure CCD images. The simulations are used to explore the validity of the statistical method for reconstructing the luminosity distribution of galaxies within a rich cluster, i.e., by the subtraction of field number-counts from those of a sight-line through the cluster. In particular, we use the simulations to establish the reliability of our observational data to be presented in Paper III. Based on our intended CCD field-of-view (6.5 × 6.5 arcmin²) and a 1σ detection limit of 26 mag arcsec⁻², we conclude that the luminosity distribution can be robustly determined over a wide range of absolute magnitude (−23 <  M _R  < −16) provided: (a) the cluster has an Abell richness 1.5 or greater; (b) the redshift of the cluster lies in the range 0.1 <  z  < 0.3; (c) the seeing is better than FWHM 1.25 arcsec, and (d) the photometric zero-points are accurate to within Δ m  = ± 0.12. If these conditions are not met, then the recovered luminosity distribution is unreliable. Finally, although the method clearly has limitations, within these limitations the technique represents an extremely promising probe of galaxy evolution and environmental dependences.     

Lower mantle diamonds from Rio Soriso (Juina area,Mato Grosso,Brazil)

The morphology, colour, fluorescence, cathodoluminescence, nitrogen content and aggregation state, internal structure and mineral inclusions have been studied for 69 alluvial diamonds from the Rio Soriso (Juina area, Mato Grosso State, Brazil). Nitrogen in most diamonds (53%) is fully aggregated as B centres, but there is also a large proportion of N-free stones (38%). A strong positive correlation between nitrogen and IR-active hydrogen concentrations is observed. The diamonds contain (in order of decreasing abundance) ferropericlase, CaSi-perovskite, magnetite, MgSi-perovskite, pyrrhotite, olivine, SiO₂, perovskite, tetragonal almandine-pyrope phase and some other minerals represented by single grains. The Rio Soriso diamond suite is subdivided into several subpopulations that originated in upper and lower mantle of ultramafic and mafic compositions, with the largest subgroup forming in the ultramafic lower mantle. Analysed ferropericlase grains are enriched in Fe (Mg#=0.43–0.89), which is ascribed to their origin in the lowermost mantle. The Juina kimberlites may be unique in sampling the material from depths below 1,700 km that ascended in a plume formed at the core–mantle boundary.Electronic Supplementary Material  Supplementary material is available for this article at

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Die statistische Auszählung von Punktediagrammen auf ebenen Projektionen

Zusammenfassung An einen Dichteplan werden verschiedene Forderungen bezüglich Übersichtlichkeit und Vergleichbarkeit von Gefügeregelungen gestellt; diese Forderungen sind durch eine einzige Beziehung erfüllbar:Auf 1% der Netzfläche entfallen x% Pole. Danach wird die statistische Auszählung der Punktediagramme vorgenommen. Die Alternative: flächentreue Projektion—winkeltreue Projektion wird untersucht und die flächentreue Projektion (Schmidtsches Netz) als günstiger befunden, da bei der stereographischen Projektion eine Vergleichbarkeit der Maximaflächen wegen veränderlicher, netzbedingter Größenverzerrungen (14 vom Zentrum zur Peripherie) nicht gegeben erscheint. Beim flächentreuen Netz wird dagegen die Bedingung an die Auszählfigur, daß sie von beständiger Form sei, nicht erfüllt. Der Kreis als richtigste Auszählform wird bei dieser Projektionsart peripheriewärts zu ellipsenähnlichen Ovaloiden abgeplättet. Es wird festgestellt, daß diese Verzerrungen innerhalb der Genauigkeitsgrenzen der Meßmethoden liegen und daher nicht störend sind. Die Entwicklung der Auszählmethoden mit Kreisen auf demSchmidtschen Netz wird verfolgt. Entscheidend für die Güte (Richtigkeit) eines Dichteplans sind die Größe des Zählkreises (Radius r) und des Zählpunktabstandes (a). Vom Zählkreis 1/4% bis zu 4% werden alle Größen, meist sogar in einem Diagramm verwendet. Dies ist abzulehnen, weil nur der 1%-Kreis der eingangs aufgestellten Beziehung entspricht; alle anderen bewirken eine Überhöhung (bei kleinerem) bzw. Verflachung (bei größerem Zählkreis). Verschiedene r in einem Diagramm stören die Vergleichbarkeit. (Vergleichbarkeit verschiedener Dichtepläne ist bei guter Gefügeregelung bereits ab zirka 60 Daten gegeben, die Genauigkeit — Feinheit — steigt jedoch mit wachsender Anzahl.) Der Zählpunktabstand a ist möglichst klein zu halten; damit steigt die Richtigkeit des Diagrammes. Beim absoluten Wert a=0 ist eineFläche gleicher Besetzungsdichte als der geometrische Ort aller Mittelpunkte, deren dazugehörige Zählkreise eine bestimmte Anzahl von Polen umschließen, definiert. Nach diesem Satz wird eine einfache Konstruktionsmethode beschrieben, die das Optimum an Richtigkeit darstellt und die für Spezialfragen empfehlenswert ist. Für bloße Übersichtsdiagramme genügt die schnellere Zählpunktmethode mit konstant gehaltenem 1%-Zählkreis (r=R/10) und Zählpunktabstand (eines Quadratnetzes) a=r/2.

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Concerning the clearness and comparability of a density plan (there) exist several requirements. Those requirements are only be fulfitted by the following relation:1% of the net surface carries x% poles. The counting procedure of the point-diagrams than follows accordingly. The alternative: surface-true projection vs. angle-true projection is examined. The surface-true projection (Schmidt-net) is considered to be the more applicable one, because in case of the stereographic projection a comparability of the maxima-surfaces does not exist. The size-distortions (14 from the center to the circumference) are too great. In case of the surface-true net on the other hand the counting area is not consistent and hence another basic condition not fulfitted. The circle, which is the most correct counting-frame, is in that case toward the circumference flattened to an ellipsoidal ovaloid. It then is stated that those distortions are within the limit of acurracy and therefore can be ignored. A consideration of the different counting procedures on theSchmidt-net follows. Decisive for the correctness of a density plan is the size of the counting-circle (radius r) and the point distance (a). All sizes of counting circles 1/4%–4% aer used, mostly even within one diagram. Such methods should not be applied, for only the 1% circle fulfitts the relation which is stated at the beginning. All others procedures cause exaggerations (in case of smaller counting-circles) or a flattening (in case of a larger counting-circle). Different radii (r) disturb the comparability of the diagrams. (A comparability of different density-plans in case of a well developed orientation of the fabric exists already starting with 60 poles, however the accuracy increases with an increasing number of poles.) A small point-distance (a) is suggested, this furthers the correctuess of the diagram. In case of the absolute value a=0,a surface of equal point-density is definedas the geometric location of all centers the counting-circles of which contain an equal number of poles.After this statement a simple method of construction is described. It gives the optimum of accuracy and should be used in case of special problems. For simple reconnaissance diagrams a faster counting method with a consistant 1% counting-circle (r=R/10) and consistant point-distance (of a square-net) a=r/2 is sufficiant.

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Mit 10 Textabbildungen     

The Aegir Rift: Crustal Structure of an Extinct Spreading Axis

Two seismic refraction and gravity lines were obtained along and normal to the axis of the Aegir Rift, an extinct spreading centre in the Norway Basin. Velocity-depth solutions and crustal structure models are derived from ocean-bottom records using two-dimensional ray tracing and synthetic seismogram modelling techniques. Gravity data are used to generate models consistent with the lateral variations in thickness of the layers in the crustal models. The resulting models require considerable degree of lateral inhomogeneity along and perpendicular to the rift axis. Crust within the extinct spreading centre is found to be thinner and of low P-wave velocity when compared with the crust sampled off-axis. To explain reduced velocities of the lower crust we suggest that, due to the relationship between fracturing and seismic velocity, the decreasing spreading rate leading up to extinction let the mechanically strong layer thicken, so that faulting and fracturing extended to greater depths . Low velocities are also observed in the uppermost mantle underlying the extinct spreading ridge. This zone is attributed to hydrothermal alteration of upper mantle peridotites. Furthermore, after spreading ceased 32-26 my ago, ongoing passive hydrothermal circulation was accompanied by the precipitation of alteration products in open void spaces, thereby decreasing the porosity and increasing the velocity. Consequently the typical low velocities of layer 2 found at active mid-ocean ridges have been replaced by values typical of mature oceanic crust.     

Life in the Goldilocks Zone: Perceptions of Place Disruption on the Periphery of the Bakken Shale

Community impacts from oil development in the Bakken Shale, located in the north central region of the United States, have been well reported in the press, including highly publicized accounts of social disruption. Less examined are communities on the periphery of such development, which experience related impacts unique to their location. Twenty-eight long-time residents from three Bakken periphery communities were interviewed to evaluate social and environmental impact perceptions, including perceived risks of place disruption. Findings suggest that whereas place meanings and identities were disrupted by the oil boom in Bakken core communities, they remain intact in these periphery communities. Findings further suggest periphery residents view their proximal location to the Bakken region as a social and economic “Goldilocks Zone:” close enough to benefit from spillover economic impacts, yet far enough away to avoid serious negative social impacts. This research extends understanding of place disruption and development impacts in energy periphery communities.     

Determining cooling rates from mica 40Ar/39Ar thermochronology data: Effect of cooling path shape

Tectonic models are commonly underpinned by metamorphic cooling rates derived from diffusive‐loss thermochronology data. Such cooling ages are usually linked to temperature via Dodson's closure temperature (T_C) formulation, which specifies a 1/time‐shaped cooling path (Contributions to Mineralogy and Petrology, 1973, 40, 259). Geologists, however, commonly discuss cooling rates as a linear temperature/time shape. We present the results of a series of simple finite‐difference diffusion models for Ar diffusion in muscovite and biotite that show that the difference in recorded age between 1/t and linear cooling paths increases significantly with hotter starting temperatures, slower cooling rates and smaller grain sizes. Our results show that it is essential to constrain the cooling path shape in order to make meaningful interpretations of the measured data.