Mixture Of Mantle Research Articles

Structure and evolution of the Himalaya–Tibet orogenic belt

The 1981 French–Chinese expedition to Tibet focused on the Lhasa block, extending earlier coverage 400 km north of the Tsangpo suture. The Lhasa block stood between 10 and 15° N latitude over most of the Upper Cretaceous and Eocene and, if Gondwanian in origin, had detached from Gondwana by early Permian. Seismic profiles reveal a complex Moho topography resulting both from multiple continental thrusting and large-scale strike-slip faulting. Subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated. This study emphasizes the role of smaller blocks in the accretion of the continental mosaic.

Trace-element and isotopic constraints on the source of magmas in the active volcano and Mariana island arcs, Western Pacific

Analytical results of the relative and absolute abundance of LIL-incompatible trace elements (K, Rb, Cs, Sr, and Ba) and isotopic compositions ( 18 O 16 O , 87 Sr 86 Sr , and 143 Nd 144 Nd ) are summarized for fresh samples from active and dormant volcanoes of the Volcano and Mariana island arcs. The presence of thickened oceanic crust ( T ∼ 15–20 km) beneath the arc indicates that while hybridization processes resulting in the modification of primitive magmas by anatectic mixing at shallow crustal levels cannot be neglected, the extent and effects of these processes on this arc's magmas are minimized. All components of the subducted plate disappear at the trench. This observation is used to reconstruct the composition of the crust in the Wadati-Benioff zone by estimating proportions of various lithologies in the crust of the subducted plate coupled with analyses from DSDP sites. Over 90% of the mass of the subducted crust consists of basaltic Layers II and III. Sediments and seamounts, containing the bulk of the incompatible elements, make up the rest. Bulk Western Pacific seafloor has 87 Sr 86 Sr ∼ 0.7032 , δ 18 O ∼ +7.2 , K/Rb ∼ 510, K/Ba ∼ 46, and K/Cs ∼ 13,500. Consideration of trace-element data and combined δ 18 O − 87 Sr 86 Sr systematics limits the participation of sediments in magmagenesis to less than 1%, in accord with the earlier results of Pb-isotopic studies. Combined 143 Nd 144 Nd − 87 Sr 86 Sr data indicate little, if any, involvement of altered basaltic seafloor in magmagenesis. Perhaps more important than mean isotopic and LIL-element ratios is the restricted range for lavas from along over 1000 km of this arc. Mixtures of mantle with either the subducted crust or derivative fluids should result in strong heterogeneities in the sources of individual volcanoes along the arc. Such heterogeneities would be due to: (1) gross variations of crustal materials supplied to the subduction zone; and (2) lesser efficiency of mixing processes accompanying induced convection between arc segments (parallel to the arc) as compared to that perpendicular to the arc. The absence of these heterogeneities indicates that either some process exists for the efficient mixing of mantle and subducted material parallel to the arc or that subducted materials play a negligible role in the generation of Mariana-Volcano arc melts. Consideration of plausible sources in the mantle indicates that (1) an unmodified MORB-like mantle cannot have generated the observed trace-element and isotopic composition of this arc's magmas, while (2) a mantle similar to that which has produced alkali-olivine basalts (AOB) of north Pacific “hot spot” chains is indistinguishable in many respects spects from the source of these arc lavas.

Rifts of the Baikal mountain region

Large and small graben comprising the Baikal rift zone are connected with an extensive Pliocene-Pleistocene uplift with amplitude 3,000–4,000 m above sea level. These grabens inherit the strike of the Early Neogene subsidences and they cannot be considered as the result of fracturing or sinking of the crust in the arch of the uplift. The long and discontinued basalt volcanism, which preceded and accompanied the rift formation, as well as active mountain formation and the character of fracture tectonics point to the leading role of stretching of the Earth's crust during the formation of this zone; the zone has no evident connection with the world rift system; moreover, it has a specific strain field in which, according to seismological data, near-horizontal stretching forces are dominant. They are directed across the major tectonic and orographie lines. A reverse strain pattern in earthquakes foci in the western region of the zone (near-horizontal compression), in the presence of the outer structure-morphological monotony of the latter, can testify to the growing influence on this region of conditions which are inherent in the neighbouring region of the central-Asiatic depressions (Mongolia) where earthquake foci have similar strains and seismic activity is much higher. Numerous signs of increase of rift depressions in width and depth, the presence of embryonal but active rift structures within the Stanovoi Mountains indicate the continuation of the development of the zone in the modern epoch. Its rift structures are only partially connected with the system of the boundary seam of the Siberian Platform. They also utilize the inner ancient faults in the Baikalides and Early Caledonides belt. Contrary to the opinions of some authors, the horizontal displacement component in the structure of the Baikal Rift has a clearly subordinate role. Among the existing interpretations of the gravimetrical data related to the Baikal zone the hypothesis suggested by Borisov(1965) should be particularly noted. The hypothesis proposes “a crust—mantle mixture” comprising the root of the general structure of the Baikal Rift, while the cause of considerable mobility of the crust in this zone can be found in highly heated upper layers of the upper mantle. This is supported by recent researches in Pribaikalie carried out by the magneto-telluric sounding.

Visible Radiation Characteristics of Incandescent Oxides

Energy radiated in the visible part of the spectrum of various oxides and their mixtures when heated to red brightness temperatures between 1400\ifmmode^\circ\else\textdegree\fi{} and 2000\ifmmode^\circ\else\textdegree\fi{}K by means of cathode-ray bombardment, and gas-air and oxy-gas flames was measured by an optical pyrometric method. The oxides investigated were urania, ceria, lanthana, neodymia, erbia, yttria, zirconia, thoria, alumina, beryllia, magnesia, and mixtures of thoria with one percent ceria (the Welsbach mantle mixture), one percent and less of urania, one percent neodymia, and one percent manganese oxide. These were either pressed or fused to insure good surface conditions. In general, linear relations were found between the logarithm of the red-blue intensity ratio and the reciprocal of the brightness temperature, and between the logarithm of the candles emitted per unit surface area and the logarithm of the brightness temperature. Different modes of heating gave different radiation curves for the same oxide. Tables are given of the candles per square centimeter and the relative blue brightness for red brightness temperatures of 1400, 1500, 1600, 1700, 1800, 1900, and 2000\ifmmode^\circ\else\textdegree\fi{}K.

Visible radiation characteristics of incandescent oxides

Energy radiated in the visible part of the spectrum of various oxides and their mixtures when heated to red brightness temperatures between 1400\ifmmode^\circ\else\textdegree\fi{} and 2000\ifmmode^\circ\else\textdegree\fi{}K by means of cathode-ray bombardment, and gas-air and oxy-gas flames was measured by an optical pyrometric method. The oxides investigated were urania, ceria, lanthana, neodymia, erbia, yttria, zirconia, thoria, alumina, beryllia, magnesia, and mixtures of thoria with one percent ceria (the Welsbach mantle mixture), one percent and less of urania, one percent neodymia, and one percent manganese oxide. These were either pressed or fused to insure good surface conditions. In general, linear relations were found between the logarithm of the red-blue intensity ratio and the reciprocal of the brightness temperature, and between the logarithm of the candles emitted per unit surface area and the logarithm of the brightness temperature. Different modes of heating gave different radiation curves for the same oxide. Tables are given of the candles per square centimeter and the relative blue brightness for red brightness temperatures of 1400, 1500, 1600, 1700, 1800, 1900, and 2000\ifmmode^\circ\else\textdegree\fi{}K.