Dielectric spectroscopy of oil-bearing rocks at 0.05–16 GHz

We have measured the dielectric spectra of sand and bentonite mixtures with oil and saline water (NaCl solution) in the frequency range from 50 MHz to 16 GHz at 22 °C. The measured spectra were compared with predictions in terms of the generalized refractive mixing dielectric model (GRMDM), and the respective error was estimated as a check of the model applicability to describe oil-bearing formations. The results may be useful in developing new methods of electromagnetic logging.     

Mineralogical and geochemical patterns of mantle xenoliths from the Jixia region (Fujian Province,southeastern China)

The paper discusses the results of mineralogical and petrographic studies of spinel lherzolite xenoliths and clinopyroxene megacrysts in basalt from the Jixia region related to the central zone of Cenozoic basaltic magmatism of southeastern China. Spinel lherzolite is predominantly composed of olivine (Fo_89.6–90.4), orthopyroxene (Mg# = 90.6–92.7), clinopyroxene (Mg# = 90.3–91.9), and chrome spinel (Cr# = 6.59–14.0). According to the geochemical characteristics, basalt of the Jixia region is similar to OIB with asthenospheric material as a source. The following equilibrium temperatures and pressures were obtained for spinel peridotite: 890–1269°C and 10.4–14.8 kbar. Mg# of olivine and Cr# of chrome spinel are close to the values in rocks of the enriched mantle. It is evident from analysis of the textural peculiarities of spinel lherzolite that basaltic melt interacted with mantle rocks at the xenolith capture stage. Based on an analysis of the P–T conditions of the formation of spinel peridotite and clinopyroxene megacrysts, we show that mantle xenoliths were captured in the course of basaltic magma intrusion at a significantly lower depth than the area of partial melting. However, capture of mantle xenoliths was preceded by low-degree partial melting at an earlier stage.     

Parental media of natural diamonds and primary mineral inclusions in them: Evidence from physicochemical experiment

A generalized diagram was constructed for the compositions of multicomponent heterogeneous parental media for diamonds of kimberlite deposits on the basis of the mantle carbonatite concept of diamond genesis. The boundary compositions on the diagram of the parental medium are defined by the components of minerals of the peridotite and eclogite parageneses, mantle carbonatites, carbon, and the components of volatile compounds of the C-O-H system and accessory phases, both soluble (chlorides, phosphates, and others) and insoluble (sulfides and others) in carbonate-silicate melts. This corresponds to the compositions of minerals, melts, and volatile components from primary inclusions in natural diamonds, as well as experimental estimations of their phase relations. Growth media for most natural diamonds are dominated by completely miscible carbonate-silicate melts with dissolved elemental carbon. The boundary compositions for diamond formation (concentration barriers of diamond nucleation) in the cases of peridotite-carbonate and eclogite-carbonate melts correspond to 30 wt % peridotite and 35 wt % eclogite; i.e., they lie in the carbonatite concentration range. Phase relations were experimentally investigated at 7 GPa for the melting of the multicomponent heterogeneous system eclogite-carbonatite-sulfide-diamond with a composition close to the parental medium under the conditions of the eclogite paragenesis. As a result, “the diagram of syngenesis” was constructed for diamond, as well as paragenetic and xenogenic mineral phases. Curves of diamond solubility in completely miscible carbonate-silicate and sulfide melts and their relationships with the boundaries of the fields of carbonate-silicate and sulfide phases were determined. This allowed us to establish the physicochemical mechanism of natural diamond formation and the P-T conditions of formation of paragenetic silicate and carbonate minerals and coexistence of xenogenic sulfide minerals and melts. Physicochemical conditions of the capture of paragenetic and xenogenic phases by growing diamonds were revealed. Based on the mantle carbonatite concept of diamond genesis and experimental data, a genetic classification of primary inclusions in natural diamond was proposed. The phase diagrams of syngenesis of diamond, paragenetic, and xenogenic phases provide a basis for the analysis of the physicochemical history of diamond formation in carbonatite magma chambers and allow us to approach the formation of such chambers in the mantle material of the Earth.     

Experimental study of the pyroxene-garnet phase transition in the Na2MgSi5O12 system at pressures of 13–20 GPa: First synthesis of sodium majorite

Doklady Earth Sciences -     

Anomalous concentrations of zinc and copper in highmoor peat bog, southeast coast of Lake Baikal

When examining the peat deposit discovered in Vydrinaya bog, South Baikal region, the authors encountered anomalous Zn and Cu concentrations for highmoors being up to 600–500 ppm on a dry matter basis in the Early Holocene beds (360–440 cm) formed 11 000–8500 years ago. It has been demonstrated that Zn and Cu are present inside the plant cells of peat moss in the form of authigenic sulfide minerals of micron size. Apart from Zn and Cu, native Ag particles (5–7 um) have been encountered in the peat of the Vydrinaya bog at a depth of 390–410 cm; these particles formed inside the organic matter of the plasma membrane of peat moss containing Ca, Al, S, and Cu. This study suggests probable patterns of the formation of zinc sulfides, copper sulfides, and native silver in peat moss. The results obtained indicate that biogenic mineral formation plays a significant role in this system, which is a very important argument in the discussion on the ore genesis, in which physicochemical processes are normally favored, while the role of living matter is quite frequently disregarded.     

Numerical modeling of the distribution of horizontal stresses in a moving continental plate

The distribution and time evolution of overlithostatic horizontal stresses in the vertical cross-sections of the model of a solid continent are studied in terms of a 2D numerical model. The continent is moving self-consistently with time-varying mantle flows. Over a period of 50 Ma, the continent, which had been initially located at a distance of about 1500 km from the downward mantle flow (subduction zone), first overlays the region of the marginal sea, then thrusts over the zone of subduction, thus making it inclined and deforming it. It is found that the horizontal tensile stresses observed at the initial stages are succeeded by compressive stresses, especially at the leading edge (up to 40 MPa). This is caused by the change of the dragging action of the subduction zone into the retarding action during the continent crossing over this zone. The dependence of the calculated stresses on the Rayleigh number and on the thickness of the model of the continent is estimated. It is obtained also that the distribution of horizontal stresses along the strike of the continent is rather distinctly indicative of the locations of upward and downward flows in the subcontinental mantle. With the considered values of the model parameters, the tensile stresses in the near-to central regions of the continent may be twice to thrice the tensile stresses at its margins. Hence, in the simplest case of a homogeneous continent, its division will occur in the zone directly above the upward subcontinental flow. Detachment from the continent of the fragments of its edge adjacent to the marginal sea implies that the thickness of the continental plate in that part and/or its strength is considerably lower than in the center of the continent.     

Towards global seismic monitoring of underground nuclear explosions using waveform cross correlation. Part II: Synthetic master events

Waveform cross correlation is an efficient tool for detection and characterization of seismic signals. For the purposes of the Comprehensive Nuclear-Test-Ban Treaty, cross correlation can globally reduce the threshold of detection by 0.3 to 0.4 magnitude units. However, the technique critically depends on the availability of master events. In Part I of this paper, we have demonstrated that in seismically active regions the best master events (grand-masters) replicated over a regular grid allow improving the efficiency of signal detection and event finding. In aseismic areas, there are two approaches to populate the global grid of master events for the International Monitoring System: the replication of grand-masters and calculation of synthetic seismograms for master-events in the global grid nodes. The efficiency of synthetic templates depends on the accuracy of shape and amplitude predictions controlled by focal depth and mechanism, source function, velocity structure and attenuation along the master/station path. Here we test three focal mechanisms (explosion, thrust fault, and actual Harvard CMT solution for one of the April 11, 2012 Sumatra aftershocks) and two velocity structures (ak135 and CRUST 2.0). Sixteen synthetic master events were distributed over a 1° × 1° grid covering the zone of aftershocks. We built five cross correlation standard event lists (XSEL) and compared detections and events with those built using the real and grand master events as well as with the Reviewed Events Bulletin of the International Data Centre. The XSELs were built using an explosion source and ak135 and the reverse fault with isotropic radiation pattern to demonstrate the performance similar to that of the real and grand masters. Here we have proved quantitatively that it is possible to cover all aseismic areas with synthetic masters without significant loss in seismic monitoring capabilities based on cross correlation.