A new type of noble metal mineralization in the Northern Caucasus

The weathering crust of the Beden ultrabasite massif (the basin of Big Laba River) is identified and studied. Anomalously high contents of noble metals (Au, Pt, Pd) are revealed in the basal horizon of the Jurassic part of the weathering crust. For this reason we suspect an existence of a belt of noble metal miner-alization in the Paleozoic ultrabasites in the Peredovoi Range of the Northern Caucasus.     

Seismicity and isostasy

Both a theoretical treatment and observed data show that the isostatic gravity field and the seismicity are closely related. The equation for harmonic loading shows that large deviations from isostatic equilibrium lead to large shear stresses in the crust. High seismic activity correlates well with large isostatic gravity anomalies and with large horizontal gradients of isostatic gravity anomalies. Studies of seismicity should consider the isostatic gravity field.     

Halite oolites and ripples in the Dead Sea, Israel

Clastic features in recent halite deposits are observed along the beaches of an artificially dammed part of the Dead Sea. These features include halite oolites (termed halolites in this paper) and ripples. Halite precipitates initially either at the brine surface or on the floor. It is suggested that moderate increase of wave agitation shifts the balance towards brine-surface crystallization, and keeps the growing halite grains in constant motion. In this way rippled structures are formed. A further increase of wave energy leads to the growth of coated halite grains. The accumulation of the various halite grains along the beach, to form soft rippled floor and oolitic beach ridge is brought about during shoreward winds. During calm periods the bulk of the halite crystallizes directly on the floor. It develops into a hard crust which assumes the morphology of the substrate, including the ripple forms.     

Detailed mineralogical-geochemical study of a large spinel Lherzolite xenolith in alkali basalt of Shavaryn Tsaram paleovolcano,Mongolia

Mineralogical-geochemical data obtained on a large xenolith of spinel lherzolite from basanites of Shavaryn Tsaram volcano (Middle Pleistocene) provide evidence that the xenolith was affected by heat and fluid that came from the basalt melt and induced geochemical heterogeneity of the xenolith. Olivine crystals from the central portion of the xenolith contain much more CaO than this mineral from the peripheral zones of the xenolith as well as in xenocrystals of the host basalts. The large xenolith was unevenly annealed by the host basalt melt, and this predetermined the heterogeneous diffusion-controlled “purification” of olivine crystals from their CaO admixture. The xenolith is heterogeneously enriched in LREE and some trace elements (Zr, Hf, Nb, Th, and U), and their distribution demonstrates unclearly pronounced concentric zoning: these elements enriched the outer portions of the xenolith. The enrichment of the xenolith in LREE and other mobile trace elements is explained by their occurrence not only as structurally bound (isomorphic) components in minerals but also in the form of nonstructural admixtures (contaminant), as minute grains of compounds hosted in the interstitial space and tiny cracks cutting mineral grains. The enrichment of the xenolith in these admixtures proceeded via their infiltration-controlled introduction with fluid from the basalt along minute cracks.     

Petrology of Santorini Volcano, Cyclades, Greece

The Pliocene to Recent lavas, dyke rocks, and cognate xenolithsof Santorini island group belong to four distinct series, eachof high-alumina basalt-andesite-dacite type. The oldest seriesincludes hornblende dacites and minor basaltic andesites. Theformer contain hornblende-rich cognate xenoliths of basalticcomposition, which consist essentially of crystals ‘floating’in residual acid liquid (glass). The chemical variation of theseries, like that of lavas of volcanic centres north-west ofSantorini, is of ‘calc-alkali’ type. The second and third series consist of a range of lavas frombasalt to rhyodacite. No hydrous mineral occurs as a stablephase. Augite is the phenocrystal pyroxene of basalts; augiteand hypersthene of andesites and dacites. The groundmass pyroxenesof basalts and most andesites are augite and pigeonite, whiledistinctive hornblende xenocryst-bearing andesites of the secondseries, and acid lavas of both second and third, carry augiteand hypersthene in the groundmass. Interstitial glass increasesin proportion from basalts to andesites, and forms a major componentof acid lavas. The second series, like the oldest, lacks absoluteiron enrichment. The third, however, shows weak iron enrichmentof andesitic relative to basaltic compositions. Of the youngest (historic) series, only the acid members (hyalodacites)have been extruded as lavas. The more basic members are representedby non-cumulate xenoliths of basaltic to andesitic compositionwhich, like those of the oldest series, consist of a mesh ofcrystals set in abundant glass. This modern series also displaysfeeble absolute iron enrichment. The compositional range of minerals other than plagioclase isvery limited in the two xenolithic series, but much greaterin the two lava series. Glass compositions are virtually constantwithin individual series. Estimates of temperatures and oxygenfugacities of Fe-Ti oxide mineral equilibration, and deductionsfrom liquid compositional trends indicate that the oldest serieswas characterized by higher f_O2, and f_H2O, and lower temperaturesthan the three younger, ‘dry’ series. Its silicaenrichment trend appears to have been controlled chiefly byfractionation of silica-poor hornblende, rather than magnetiteas in the younger series. The presence, in all series, of xenolithsof gabbroic cumulates, and the constancy of glass compositionssuggests that each series was generated by the tapping of adifferentiating highalumina basalt magma in a high level magmachamber.     

OIB-type basalts of the Irbistu River basin (Southeast Mountain Altai): Evidence for the HIMU component in the magmatic source

It has been found that basalts of the Irbistu River basin (southeast Altai Mountains) belong to the intraplate OIB type. The peculiarities of their material composition do not indicate interaction effects between the basic melt and continental crust, and signifies that there was a magmatic source in the moderately depleted mantle comparable to the HIMU domain. It is suggested that basalts of the Irbistu River basin, along with other manifestations of similar volcanites in the Altai Mountains, reflect the Neoproterozoic-Early Paleozoic stage of the North Asian plume’s activity during closure of the Paleoasian Ocean.     

The Effect of Inhomogeneous Magnetic Permeability on Magnetotelluric Responses: 1D and 2D Models

Izvestiya, Physics of the Solid Earth - Abstract—When solving direct and inverse problems of magnetotellurics, magnetic permeability throughout model medium is conventionally assumed to be...     

A multifractal model of crack coalescence in rocks

Geometric characteristics of fractal sets of cracks are investigated from the standpoint of their coalescence. A 1D computer model, in which a realistic character of forms of crack sets is achieved through the inducing of sets with the use of the multiplicative cascade procedure, is considered. The investigation is aimed at the determination of conditions of coalescence of cracks organized as a superposition of fractal subsets of a unified set of cracks. The process of crack coalescence is investigated for three different values of the coalescence criterion. The geometric characteristics of the sets of cracks that are necessary for the transition of the crack coalescence process into an avalanche-like stage are estimated.