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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Nabasmita Pradhan posted an Question
July 24, 2022 • 14:59 pm
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30 points
30 points
- IIT JAM
- Geology (GG)
34. which of the following is not a mantle reservoir ? (1) depleted mantle (3) enriched mantle (2) himu mantle (4) continental crust 35. which facies is known t
1. Which of the following is not a mantle reservoir ? (1) Depleted mantle (2) Enriched mantle (3) HIMU mantle (4) Continental crust 2. Which facies is known to occupy the maximum outcrop width during metamorphism of low pressure: (1) Albite-epidole hornfels facies (2) Pyroxene hornfels facies (3) Hornblende hornfels facies (4) Sanidinite facies 3. The temperature of melting during the formation of migmatites depends on: (1) Anorthite content of the rock (2) H20 pressure (3) Fe corntent of the rock (4) Anorthite content and H20 pressure 4. Which of the following is not a charnockite forming reaction: (1) Qtz + Kfs + Bio = Opx + L. (3) Qtz + Bio = Kfs +Opx + L (2) Qtz + Bio = Opx + L (4) Musc +Qtz = Kfs + Sill + L
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Banibrata majumder
A mantle reservoir is basically a region of the Earth’s mantle with a specific geochemical (generally isotopic) fingerprint. The classical model for mantle reservoirs is that they are broad regions of the mantle that have specific geochemical fingerprints– they are enriched in one element or depleted in another element– because of their origin and geochemical history. Because geologists cannot directly sample the Earth’s mantle, they use geochemical fingerprints of mantle sources found in rocks at the surface to infer the composition of the inaccessible mantle. (1) Depleted mantle: The depleted mantle is the part of Earth’s mantle from which basaltic melt has been extracted in one or multiple melting events at, for example, mid-ocean ridges, hot spots, or island arcs. Elements that do not fit into the crystal lattice of mantle minerals, the so-called incompatible elements, are preferentially incorporated into the melt and thus removed from the mantle, leaving the latter depleted with respect to these elements. This residual depleted mantle amounts to anywhere between 30 % and 100 % of Earth’s mantle. (2) HIMU Mantle: The parameter μ describes the 238U/204Pb ratio of an Earth reservoir. Mantle domains labelled HIMU (high μ) originally defined reservoirs with highly radiogenic Pb isotope ratios observed in basalts from a select number of ocean islands, St. Helena in the Atlantic Ocean and the Cook-Austral islands in the South Pacific Ocean. While some authors use the term HIMU in this original sense, others refer to HIMU as a widespread component in many mid-ocean ridge and ocean island basalt (MORB and OIB) sources. (3) Enriched mantle: The origin of enriched mantle (EM) sources remains an unsolved problem for constraining the composition and chemical evolution of the Earth's mantle, because a wealth of different, often mutually exclusive models has been suggested. The combined Sr, Nd, Pb isotope and trace element systematics of global ocean island basalts suggest that each EM source contains a unique enriched additive. Systematic variations between Th/Nb, K/La, Rb/La, and Ce/Pb ratios and 87Sr/86Sr ratios in all EM basalts suggest that all EM-type end-members share a common heritage from the continental crust. The observed coupling of relative Eu enrichments or deficits with 87Sr/86Sr isotope ratios further indicates that the inferred compositional differences of EM-type sources are caused by the addition of different proportions of lower and upper continental crust. Recycling of marine sediment and oceanic lithosphere in subduction zones accounts for the isotopic and chemical composition of EM sources with high 87Sr/86Sr and relatively constant 206Pb/204Pb ratios (e.g. Samoa), which have a high affinity for the upper continental crust. (4) Continental crust: Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its bulk composition is richer in aluminium silicates (Al-Si) and has a lower density compared to the oceanic crust, called sima which is richer in magnesium silicate (Mg-Si) minerals. Changes in seismic wave velocities have shown that at a certain depth (the Conrad discontinuity), there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character. Thus, continental crust is not a mantle reservoir. This right answer is option (4).