Unravelling the effects of melt depletion and secondary infiltration on mantle Re-Os isotopes beneath the French Massif Central

Spinel lherzolite xenoliths from Mont Briançon, French Massif Central, retain evidence for multiple episodes of melt depletion and melt/fluid infiltration (metasomatism). Evidence for primary melt depletion is still preserved in the co-variation of bulk-rock major elements (MgO 38.7-46.1 wt.%; CaO 0.9-3.6 wt.%), and many samples yield unradiogenic bulk-rock Os isotope ratios (¹⁸⁷Os/¹⁸⁸Os = 0.11541-0.12626). However, many individual xenoliths contain interstitial glasses and melt inclusions that are not in equilibrium with the major primary minerals. Incompatible trace element mass balance calculations demonstrate that metasomatic components comprise a significant proportion of the bulk-rock budget for these elements in some rocks, ranging to as much as 25% of Nd and 40% of Sr Critically, for Re-Os geochronology, melt/fluid infiltration is accompanied by the mobilisation of sulfide. Consequently, bulk-rock isotope measurements, whether using lithophile (e.g. Rb-Sr, Sm-Nd) or siderophile (Re-Os) based isotope systems, may only yield a perturbed and/or homogenised average of these multiple events.Osmium mass balance calculations demonstrate that bulk-rock Os in peridotite is dominated by contributions from two populations of sulfide grain: (i) interstitial, metasomatic sulfide with low [Os] and radiogenic ¹⁸⁷Os/¹⁸⁸Os, and (ii) primary sulfides with high [Os] and unradiogenic ¹⁸⁷Os/¹⁸⁸Os, which have been preserved within host silicate grains and shielded from interaction with transient melts and fluid. The latter can account for >97% of bulk-rock Os and preserve geochronological information of the melt from which they originally precipitated as an immiscible liquid. The Re-depletion model ages of individual primary sulfide grains preserve evidence for melt depletion beneath the Massif Central from at least 1.8 Gyr ago despite the more recent metasomatic event(s).     

Modelling the relationship between magnetic fabric and strain in polymineralic rocks

We present a model that is applicable to the relatively frequent case of rocks in which the magnetic fabric is dominated by uniaxial paramagnetic minerals, and in which the deformation (pure shear) corresponds to the March-Fernandez model. Borradaile et al.'s procedure for the isolation of the magnetic fabric of monocrystals of anisotropic components allows us to obtain the magnetic properties equivalent to those of all the component minerals if they should be artificially aligned. In the case of real component minerals with similar shape parameters, χ (dependent on dimension ratio), these properties will correspond to those of a theoretical equivalent mineral. Therefore, finite strain and orientation tensors may be determined from magnetic fabric measurements of such polymineralic rocks.     

Observations of shocked [FeII] and H2 line profiles in orion bullet wakes

Recent near-IR imaging of the Orion molecular cloud has revealed a complex of dense bullets, visible as [FeII] emitting HH-objects at the tips of H₂ wakes, ejected explosively from the cloud core. Having resolved individual bow-shock structures for the first time in this bright source, we have observed [FeII] 1.644µm velocity profiles of selected bullets and H₂ 1-0 S(1), 2.122µm velocity profiles for a series of positions along and across the corresponding bow-shock wakes. We present observed profiles for the bullet M42 HH1 and its associated wake and compare with theoretical bow-shock models.     

Interactive 3-D computation of fault surfaces using level sets

Deformable implicit surfaces, implemented with level set methods, have demonstrated a great potential in the computational sciences for applications such as modeling, simulation, and segmentation. They allow implicit handling of complex topologies deformed by operations where large changes can occur without destroying the level set representation. The use of level set techniques for the computation and segmentation of plane-like and high positive curvature features is nontrivial. We present a technique for representing and extracting fault surfaces from 3-D seismic attribute data such that 2-manifolds can be interactively computed and segmented. We present our approach as an interactive workflow that generates highly accurate fault surfaces.     

Consolidation with threshold gradients

Evidence for threshold gradients is reviewed. The consolidation problem, with threshold gradient, is properly formulated and solved numerically. An approximate analytical solution is also developed. The influence of a threshold gradient on the time rate of settlement is examined, and it is shown that by modifying the definition of the degree of consolidation a good approximation to the threshold gradient problem can be obtained directly from the Terzaghi solution. It is also shown that threshold gradients will have no influence on odometer testing and their effect is, therefore, to reduce the primary compression below that predicted from standard tests.     

A catalogue of seismicity in Greece and adjacent areas

Summary. A new earthquake catalogue for Greece has been formed to cover the instrumental period 1901–78, in particular 605 earthquakes for the period 1917–63 inclusive are relocated using first arrival data from the International Seismological Summary. These relocations incorporate macro-seismically and other well-controlled master events into an ensuing joint epicentre determination technique. The largest annual average shift is 165 km for 78 earthquakes during the decade after 1917, decreasing to 17 km for the later years 1957–63. Magnitudes are redetermined mainly using readings from the Swedish network and Uppsala since as early as 1908. Catalogue completeness exists for magnitudes around 5.5 for at least the last 60 years, but magnitudes of about 4.7 are completely reported only during the most recent 15 years.
The new epicentral positions lead to a better delineation of the seismic zones than has previously been achieved. The majority of shallow earthquakes are contained in a belt parallel to the Hellenic Arc which extends north into Albania, in the south-east the epicentral locations give a more diffuse extension of this zone into the west coast of Turkey. Depths of intermediate earthquakes along the Hellenic Arc tend to relocate to shallower depths, in the south-west part of Crete no earthquake focus deeper than 100 km is found, although in the south-eastern section of the arc a tendency to increased depths is observed. A second zone starts at Leukas Island in the west and extends through central Greece to near Volos on the east coast, where it divides into two branches, which are less well defined, but which eventually join the seismicity of western Turkey. A third zone follows the Saronikos and Corinth gulfs.     

The mean lifetime of interstellar molecular clouds

We describe the results of a study of the mean lifetime of molecular clouds based on actual observations. Using the model of growth from cloud-cloud collision and observations of ¹³CO along the galactic equator between longitudes 27°.85 and 40°, we derive a lower limit of 1 × 10⁹ yr for the lifetime of the interstellar molecular clouds