What is Fractional Crystallization? (1 pts) How are Parental magmas related to p

Question

What is Fractional Crystallization? (1 pts) How are Parental magmas related to primitive magmas. (1 pts) Describe the 3 processes by which the mantle can melt. Describe how each of these processes result in melting. (3 pts) In what principal ways does a tholeiitic basalt differ from an alkaline basalt? (2) Describe how magma starts to move? (3) Describe how magma mixing affects a magma? Do you always end up with a homogenous magma? If not why not? (3 pts) Describe at least 3 processes by which magma is modified?

Explanation / Answer

1) Fractional crystallization is the main processes of magmatic differentiation, inwhich crystals are removed and segregated during crystallization from a melt. Composition of the magma is changed due to fraction crystallization. Fractional crystallization is known as crystal fractionation. It is one of the most important geochemical and physical processes which is operating within the crust and mantle. Melts become enriched in incompatible elements due to fraction crystallizaton. Due to removal of early formed crystals from an originally homogeneous magma by the process of differentiation such as gravity settling etc., these crystals are prevented from further reaction with the residual melt. Resulting in the precipitation of a sequence of different minerals.

2) When a rock melts, the liquid is known as a primary melt which have not undergone any differentiation process. It represents the primary composition of a magma.The example of primary melt is leucosomes of migmatites. Primary melts are derived from the mantle magmas. Parental melts is most important in absence of primary melts. A composition which is formed by fractional crystallization or differentiation of magma is known as parental melt. So. by fractional crystallization or differentiation process parental magma is formed from primary magma.

3)

The temperature rises deeper into the Earth. The typical geothermal gradient is 15 - 20°C / km. However, pressure also increases with increasing depth into the Earth, and increased pressure inhibits melting. Three basic ways that melt the rocks are decompression, addition of volatiles, and conduction.

Decompression

Decompression melting generaly occurs at mid-ocean ridges. In mid-oceanic ridge two plates move away from each other and it creates a space. The magma comes out to that place buoyantly from below. As this hot rock rises and the pressure decreases, it facilitate the rock melting.

Conduction

Another important point is melting of rock by conduction by which it may transfer heat. Hotter particles vibrate more and vibrational heat energy is transferred to those nearby particles when they come into contact with other nearby particles. When rocks melt, it rises toward Earth's surface. When it comes into contact with solid rock on its path upward, it can transfer enough heat to the surrounding rock and melting them.

Volatiles

Rocks are melted at a lower temperature in the presence of volatiles. When a water or volatiles saturated subducting plate sinks under the overriding plate, volatiles is forced out and percolates upward into the overlaying hot, dry mantle rock. Thus water or volatiles lowers the melting point of that mantle rock, and it begins to melt. This types of melting generally occurs at all subduction zones.

4)

5) Magma rises to the surface due to current activities due to the presence of volatiles or gases and water in the magma. It increases pressure to the Earth’s surface and weaken the crust for allowing magma to rise. Magmas are formed by partial melting of mantle rocks. As the rocks move upward as the water and volatile added to them and they started to melt a little bit. These melt migrate upward and coalesce to form larger volumes move upward.

6) Magma mixing effects is important. When the magma moves upward it assimilate the country rocks. Due to high temperature of magma it melts surrounding rocks. The heat can be supplied in two ways. such as 1) if the magma has superheat or has above liquidus temperature, then cooling without crystallization can provide the required heat till the liquidus temperature is reached 2) if the magma is already on liquidus temperature, it can provide the required heat through crystallization. During this time, ions exchange occured within mineral compostion exchanges. So, the primary magma composition diverts frm its original composition. So, we do not get generally original magma compositional rocks in the earth surface.

7) The three processes by which magma is modified are described in below:

liquid immiscibility: some liquid system break up into immiscible liquid fractions through fall of temperature. If there is a density contrast the two fractions can physically seperate. Liquid immiscibility may manifest in natural conditions but is restricted to a small compositional range.

Thermal diffusion: Accroding to Sorret's principle, when two parts of a solution are at different temperatures, there is a higher concentration of disolved substances in the parts of lower temperature than higher temperature by the process of diffusion. It makes layered rocks. For that cause magma are differentiated that may change the magma compositions. However, diffusibility can be increased by increase in temperature and water content of the system and efficiency of the process can be increased by convection.

Gravitational diffusion: Higher desity minerals are seperated from the rest melt and seggregated to the lower part of the magma. Gravitational diffusion involves seperation of elements in the homogeneous magma under the influence of gravity. A review of the molecular weight/molecular volume of the major oxides suggests enrichment of upper part of a dry magma column (in order of decreasing density) in K2O, Na2O, SiO2, and the bottom part with FeO, CaO, MgO, TiO2. This is consistent with the common differentiation trend where felsic rocks are developed towards the top and mafic minerals are developed towards the bottom part of the layerd complexs.

Tholeiite Alkali basalt 1. Midocean ridges, oceanic islands Island arcs, collision/subduction zones 2. Shallower melting almost 25% melting at <30 kms Deeper melting, almost 25% melting at 60 kms. 3. Greater % partial melting ( 30% melting at 60 km) Lesser % partial melting (20% melting at 60 km ) 4. contains normative hypersthene and olivine contains normative olivine and nepheline 5. No alkali feldspar Interstitial alkali feldspar or feldspathoid

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