Geology Course Work Example on Igneous Petrology

1. Sediment subduction:

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Igneous petrology is the study of igneous rocks. These rocks are formed from magma. The configuration of igneous rocks and minerals can be determined through various methods of varying simplicity, cost and complexity. One of the methods used is study of samples through naked eye or by use of a hand lens. This method is used to scale the general mineralogical composition of the rock which gives an intuition into the composition. Other methods used to define mineralogy are; petrographic microscope, X-ray diffraction and electron microprobe.

Beryllium-10[10Be] is a radioactive isotope of beryllium formed mainly by the cosmic ray spallation of oxygen. 10Be decays by beta decay to stable boron-10 with a determined energy of 556.2keV through a reaction where light elements in the air react with high energy galatics cosmic ray particles. Beryllium tends to occur in solutions and is liquefied to the earths surface through rain water. As the precipitation rapidly becomes more alkaline, 10Be drops out of solution thereby cosmogenic accumulates at the soil surface where its relatively long-half life is 1.387 million years permits a long residence time before it falloffs to 10B. Beryllium is also molded in nuclear eruptions by a reaction of fast neutrons with 13C in carbon dioxide in air and is one of the historic gauges of past action at nuclear locations.

Beryllium has an absorption of 2 to 6 parts per million [ppm] in the earths crust. Typically it is concentrated in the soils with 6 parts per million. Concentration of Beryllium in sea water is 0.2-0.6 parts per trillion. However, its concentration in stream water is more abundant with a concentration of 0.1 parts per billion.

2. The slab component:

The subducting oceanic slab is thought to be dehydrate making fluids which metasomatize the covering top mantle wedge where island-arc magma forms. Data suggest that an identical slab fluid subsidizes to all Izu volcan-oes but that the amount of the fluid decreases continuously with increasing depth of the subducting slab.

Chemical processes take place in the component of the slab. As oceanic slab sinks into the mantle at subduction zones and temperature and pressure increases, they dry out and release fluids. The effectiveness of this process this process as well as the chemical make-up of slab fluids depends on the pressure and temperature conditions experienced by the slab as they subduct.

Concrete expands marginally as the temperature go down. These changes in temperature may be as a result of environmental conditions or by cement hydration. The average value for the amount of thermal expansion is about 10 millionths per degree Celsius. Thermal expansion and contraction of concrete differs mostly with aggregate type, cementitious material content, water cement ratio, temperature range, concrete age and ambient relative humidity.

3. Sediment melts during subduction:

The contrast of primitive arc lavas with mid-ocean ridge basalts provides indication that are lavas are supplemented in water, large-ion lithophile elements [LILE] and to a slighter extent light rare earth elements. Exhaustive chemical analysis of oceanic crustal material revealed that subducted sediments are the central host of LILE in the slap [Plank and Langmuir, 1998]

In this paper they present the outcomes of an experimental study on the phase and melting relations in a perlite composition. A piston cylinder experiment was used for the whole dataset covering P-T range of 2.5-4.5 GPa and 600-1050 degrees Celsius suitable for sub-arc condition which removes any probable systematic errors involved in using two different experimental techniques .Stability and melting conduct of phengite to constrain the recycling of K2O from subducted sediments was experimented. Configurations of aqueous fluids and hydrous melts vary as a function of temperature. This data is used to propose that sediment melt provide the simplest way to explain the K2O and H2O systematics of primitive arc lavas.

The preliminary material was formed using a sol-gel method to eliminate problems of sluggish reaction of refractory minerals during experiments. Major elements and some trace elements were combined as nitrate solutions and mixed with tetra-ethyl orthosilicate and gradually dried to a gel.

4. Melting of subducted slab:

In an argument on melting of specific materials in some subducted zones, volcanic arcs are used as a sample to explain the circumstances under which they melt. The basaltic ocean crust comprises hydrous materials like amphiboles formed by hydrothermal alteration as seawater seeped through hot, fractured, young ocean crust at the mid ocean ridge. The pressure rises as the ocean sinks deeper into the mantle. The pressure becomes bigger at the depth of around 100km beneath the surface. This ample for the hydrous minerals to undertake metamorphosis. This procedure liberates water gradually seeps upward into the overlying wedge of hot mantle. This depresses the temperature causing in partial melting of ultramafic mantle rocks to yield mafic magma Igneous rocks which did not make it to the surface before forming lie beneath active volcanic arc. These volcanic arcs maybe volcanic island arcs such as Aleutians and Marianas.

Benioff zones are an additional evidence used in the argument. Motions on thrust faults produce earthquakes in and around ocean trenches indicating converging plates. The earthquakes are deeper the further they are from the trench. These earthquakes of Wadati-Benioff Zone happen close to the surface of the descending slab. These earthquakes occur to the pits of nearly 670km at some subduction zones. It should be noted that volcanic arc lies where the Wadati-Benioff Zone earthquakes are about 100km below the surface but Benioff zone earthquakes go past this down to 60km therefore the plate has not melted away.

5. Another slab component:

Stratigraphic and morph structural studies have been carried out in the Mt. Etna volcanic region to study in detail the distortion events that have affected the sedimentary successions creating the substratum of the volcano. Around this area, NNW-SSE-oriented compression resulting from the collision between the Eurasian and Nubian plates is still active. Dynamics of the Calabrian Arc which represents the inner chain of the Apennine-Maghrebian Mountain belt at the rear of the dynamic subduction of the Ionian Sea. The Calabrian Arc went through strong tectonic elevate since the middle Pleistocene time with maximum rates greater than 1.0mm per year and with effects that extended to the fore deep and foreland domains.

Giant diapers up to 2km in height have been accepted worldwide showing conical shapes at the surface [Fryer et al 1990; Fryer 2002]. The dooming of Mt. Etna could result from lower crust to the narrow levels. In agreement with the mantle-plume origin of Mt. Etna as suggested by petrological ,tomographic and seismotectonic observations, a mounting thermal anomaly would melt the lithospheric mantle instead of aggravating its volumetric expansion triggering the thinning of overlying crust dooming can automatically be excluded. Deposition of the formation ended when the entire area emerged and the current elevations of the top level of substrate and not the crucial arching .As a result of this, a mantle-related process as a cause of the surficial record the inhomogeneous vertical motions that the area under study underwent.

References:

Defant, M .and Drummond, M. (1989). Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature,vol347,pp.66-665.

Leeman .W. Morris, J. and Tera, F. (1990). The subducted component in island arc lavas; constrains from B-Be systematic. Nature. vol.334 ,pp.31-36.

Plank,T and Langmuir ,C. (1993). Tracing traces elements from sediments input to volcanic output and subduction zones. Nature vol.362,pp.739-743.