Mantle Material (mantle + material)

Distribution by Scientific Domains

Selected Abstracts

K-Ar age determination, whole-rock and oxygen isotope geochemistry of the post-collisional Bizmi,en and Çalt, plutons, SW Erzincan, eastern Central Anatolia, Turkey

Ayten Önal
Abstract Post-collisional granitoid plutons intrude obducted Neo-Tethyan ophiolitic rocks in central and eastern Central Anatolia. The Bizmi,en and Çalt, plutons and the ophiolitic rocks that they intrude are overlain by fossiliferous and flyschoidal sedimentary rocks of the early Miocene Kemah Formation. These sedimentary rocks were deposited in basins that developed at the same time as tectonic unroofing of the plutons along E,W and NW,SE trending faults in Oligo-Miocene time. Mineral separates from the Bizmi,en and Çalt, plutons yield K-Ar ages ranging from 42 to 46,Ma, and from 40 to 49,Ma, respectively. Major, trace, and rare-earth element geochemistry as well as mineralogical and textural evidence reveals that the Bizmi,en pluton crystallized first, followed at shallower depth by the Çalt, pluton from a medium-K calcalkaline, I-type hybrid magma which was generated by magma mixing of coeval mafic and felsic magmas. Delta 18O values of both plutons fall in the field of I-type granitoids, although those of the Çalt, pluton are consistently higher than those of the Bizmi,en pluton. This is in agreement with field observations, petrographic and whole-rock geochemical data, which indicate that the Bizmi,en pluton represents relatively uncontaminated mantle material, whereas the Çalt, pluton has a significant crustal component. Structural data indicating the middle Eocene emplacement age and intrusion into already obducted ophiolitic rocks, suggest a post-collisional extensional origin. However, the pure geochemical discrimination diagrams indicate an arc origin which can be inherited either from the source material or from an upper mantle material modified by an early subduction process during the evolution of the Neo-Tethyan ocean. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Geochemical identification of projectiles in impact rocks

Roald Tagle
The identification of a projectile component in impactites can be achieved by determining certain isotopic and elemental ratios in contaminated impactites. The isotopic methods are based on Os and Cr isotopic ratios. Osmium isotopes are highly sensitive for the detection of minute amounts of extraterrestrial components of even <<0.05 wt% in impactites. However, this only holds true for target lithologies with almost no chemical signature of mantle material or young mantle-derived mafic rocks. Furthermore, this method is not currently suitable for the precise identification of the projectile type. The Cr-isotopic method requires the relatively highest projectile contamination (several wt%) in order to detect an extraterrestrial component, but may allow the identification of three different groups of extraterrestrial materials, ordinary chondrites, an enstatite chondrites, and differentiated achondrites. A significant advantage of this method is its independence of the target lithology and post-impact alteration. The use of elemental ratios, including platinum group elements (PGE: Os, Ir, Ru, Pt, Rh, Pd), in combination with Ni and Cr represents a very powerful method for the detection and identification of projectiles in terrestrial and lunar impactites. For most projectile types, this method is almost independent of the target composition, especially if PGE ratios are considered. This holds true even in cases of terrestrial target lithologies with a high component of upper mantle material. The identification of the projectile is achieved by comparison of the "projectile elemental ratio" derived from the slope of the mixing line (target-projectile) with the elemental ratio in the different types of possible projectiles (e.g., chondrites). However, this requires a set of impactite samples of various degree of projectile contamination. [source]

Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid

Edward R. D. SCOTT
To test whether impacts can excavate core iron and mix it with crustal material, we used a low-resolution, smoothed-particle hydrodynamics computer simulation. For 50,300 km diameter differentiated targets, we found that significant proportions of scrambled core material (and hence potential mesosiderite metal material) could be generated. For near-catastrophic impacts that reduce the target to 80% of its original diameter and about half of its original mass, the proportion of scrambled core material would be about 5 vol%, equivalent to ,10 vol% of mesosiderite-like material. The paucity of olivine in mesosiderites and the lack of metal-poor or troilite-rich meteorites from the mesosiderite body probably reflect biased sampling. Mesosiderites may be olivine-poor because mantle material was preferentially excluded from the metal-rich regions of the reaccreted body. Molten metal globules probably crystallized around small, cool fragments of crust hindering migration of metal to the core. If mantle fragments were much hotter and larger than crustal fragments, little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally. The rapid cooling rates of mesosiderites above 850 °C can be attributed to local thermal equilibration between hot and cold ejecta. Very slow cooling below 400 °C probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris. We infer that our model is more plausible than an earlier model that invoked an impact at ,1 km/s to mix projectile metal with target silicates. If large impacts cannot effectively strip mantles from asteroidal cores, as we infer, we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material. This may help to explain why relatively few olivine-rich (A-type) and metal-rich asteroids (M-type) are known. Some S-type asteroids may be scrambled differentiated bodies. [source]

Genesis and Mixing/Mingling of Mafic and Felsic Magmas of Back-Arc Granite: Miocene Tsushima Pluton, Southwest Japan

Ki-Cheol Shin
Abstract The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc-alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back-arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2,6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial 87Sr/86Sr ratios (0.7065,0.7085) and lower ,Nd(t) (,7.70 to ,4.35) than the MME of basaltic,dacitic composition (0.7044,0.7061 and ,0.53 to ,5.24), whereas most gray granites have intermediate chemical and Sr,Nd isotopic compositions (0.7061,0.7072 and ,3.75 to ,6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr,Nd,Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma,fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back-arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI-like composition, which plays an important role in the genesis of igneous rocks there. [source]

Seismic constraints on Earth's small-scale structure

Sebastian Rost
The small-scale structure of the Earth's interior is more difficult to access than the big picture. In the Bullerwell Lecture 2009, Sebastian Rost discusses seismic studies targeting small-scale structure at the core,mantle boundary, including areas containing partially molten mantle material and the remnants of subducted slabs, and assesses how they influence aspects of the Earth's dynamics. [source]

Meso-Cenozoic Mineralization Pattern in the Continent of China

CHEN Yuchuan
Abstract, Based on the complex structure and material resources, the complex geological setting of the Mesozoic-Cenozoic continent of China controlled four kinds of dynamic mechanisms of the continental tectonic-mineralization pattern, i.e. the dynamic mechanisms related to (1) underthrusting or collision, (2) activation of old tectonic belts or activity of new tectonic belts, (3) upwelling of mantle material and heat, and (4) interaction between the atmosphere, hydrosphere, biosphere and lithosphere. The four dynamic factors are related to and interact with each other; and the mantle-crust interaction leads to the regular time-space zonation of endogenetic deposits on a regional scale. The Meso-Cenozoic mineralization pattern in China can be outlined as the network tectono-metallogenic pattern constructed by NNE- and E-W-trending tectonics in eastern China, and multi-layer ring tectono-metallogenic pattern in the Qinghai-Tibet plateau and its northern and eastern neighbouring areas. [source]

Normal Faulting Type Earthquake Activities in the Tibetan Plateau and Its Tectonic Implication

Jiren XU
Abstract: This paper analyzes various earthquake fault types, mechanism solutions, stress field as well as other geophysical data to study the crust movement in the Tibetan plateau and its tectonic implications. The results show that a lot of normal faulting type earthquakes concentrate in the central Tibetan plateau. Many of them are nearly perfect normal fault events. The strikes of the fault planes of the normal faulting earthquakes are almost in the N-S direction based on the analyses of the equal area projection diagrams of fault plane solutions. It implies that the dislocation slip vectors of the normal faulting type events have quite great components in the E-W direction. The extension is probably an eastward extensional motion, mainly a tectonic active regime in the altitudes of the plateau. The tensional stress in the E-W or WNW-ESE direction predominates the earthquake occurrence in the normal event region of the central plateau. A number of thrust fault and strike-slip fault type earthquakes with strong compressive stress nearly in the NNE-SSW direction occurred on the edges of the plateau. The eastward extensional motion in the Tibetan plateau is attributable to the eastward movement of materials in the upper mantle based onseismo-tomographic results. The eastward extensional motion in the Tibetan plateau may be related to the eastward extrusion of hotter mantle materials beneath the east boundary of the plateau. The northward motion of the Tibetan plateau shortened in the N-S direction probably encounters strong obstructions at the western and northern margins. Extensional motions from the relaxation of the topography and/or gravitational collapse in the altitudes of the plateau occur hardly in the N-S direction. The obstruction for the plateau to move eastward is rather weak [source]

Petrogenesis of Cenozoic Potassic Volcanic Rocks in the Nangqên Basin

SUN Hongjuan
Abstract The Nangqên basin is one of the Tertiary pull-apart basins situated in the east of the Qiangtang block. Similar to the adjacent Dengqên basin and Baxoi basin, there occurred a series of potassic volcanic and sub-volcanic rocks, ranging from basic, intermediate to intermediate-acid in lithology. Based on the study of petrology, mineralogy and geochemistry, including REEs, trace elements, isotopic elements and chronology, the authors concluded that the Cenozoic potassic volcanic rocks in the Nangqên basin were formed in the post-collisional intraplate tectonic settings. The relations between the basic, intermediate and intermediate-acid rocks are neither differentiation nor evolution, but instead the geochemical variability is mainly attributable to the different partial melting degrees of the mantle sources formed at depths of 50,80 km. The sources of the potassic rocks are enriched metasomatic mantle that has experienced multiple mixing of components mainly derived from the crust. The recycling model can be described as follows: after they had subducted to the mantle wedge, the crust-derived rocks were metasomatized with the mantle materials. In view of the fact that the ratio of crust-derived rocks increases by the age of volcanism, it can be concluded that the sources of the potassic rocks moved upwards progressively with time. The underplating of small scattered magmas upwelling from the asthenosphere may have induced partial melting of the sources of the volcanic rocks in some pull-apart basins in the Hengduanshan area and the intense tectonic movements of large-scale strike-slip belts provided conduits for the ascending melts. [source]