Please explain type of melting (addition of volatiles, decompression, heat trans
ID: 114033 • Letter: P
Question
Please explain type of melting (addition of volatiles, decompression, heat transfer) is responsible for mafic magmas in each of the following tectonic settings and why the resulting mag,a is mafic. a) Mid ocean ridges b) Continental drifts c) Oceanic and continental volcanic arcs d) Hot spots
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Örigin of Mafic Magmas te in Differ EXERCISE 10 ngs Name Course volatiles, decompression, heat transfer) is responsible for mafic of the following tectonic settings and why the (a) Mid-ocean ridges which type of melting (addition of s, decompressi resulting magma is mafic. Section Date: sfer) is responsible for mafic magmas in each (b) Continental rifts () Oceanic and continental volcanic arcs (d) Hot spotsExplanation / Answer
a) Mid-oceanic ridges- Decompression melting occurs here. Adiabatic ascent i.e. decompression of the peridotite in the mantle at constant temperature dominates the Mid-oceanic ridges where subduction of water does not occure and so, water does no aid in melting. Decompression causes the lowering of melting point of the rock due to increase of pressure. Consequently, new mafic magma (basalt) rises near or at the axis of the mid-oceanic ridges. The resulting magma is mafic because of the melting of ultramafic rock peridotite.
b) Continental Rifts - Decompression melting occurs here. Lithosphere is pulled apart along a linear zone in the rifts which is a result of extensional tectonism. The resulting magma is alkali basalts and bimodal volcanism. Melting occurs due to decrease in pressure without decreasing temperature. This is because during the formation of rifts, the lithosphere becomes thinner. Consequently, the asthenosphere just below the continental rifts rises to the shallower levels and undergoes decompression melting. The resulting magma is mafic because the asthenosphere rises above and undergoes decompression; the asthenosphere consists of ultamafic rocks.
c) Volcanic Arcs (Continental and Oceanic) - Melting occurs due to the addition of water. Deep melting of rock mixtures form volcanic rocks.
Oceanic Crust- Oceanic crust is basaltic and consists of hydrous minerals which may be formed due to hydrothermal alteration as a consequence of seepage of seawater through the younger oceanic crust at the mid oceanic ridges. Hydrous minerals (talc, amphibole, serpentine) undergo metamorphism at high pressure to form new minerals which have lost the bonded water. The water is added to the wedge of the overlying mantle. This is an example of metamorphic dewatering which causes liberation of water from the underlying crust. Melting point of the mantle is lowered to form melt. The resulting magma is mafic because of the hydrous minerals which causes metamorphic dewatering are mafic.
d) Hotspots - Decompression melting is the cause of melts in hotspots. The mantle rock is hot and rises to the surface; in the mean time, there is a great reduction of pressure without any reduction of temperature. This causes partial melting of nearly 20% of the mantle rock. The rock is ultramafic due to the partial melting of the ultramafic rocks.