Sample History Paper on Petrological and Mineralogical Processes                                                       

Petrological and Mineralogical Processes

Introduction

History is full of information regarding rock formation and the processes that leads to their formation.  According to Wager, Brown & Wadsworth (1960), among the most common types of rocks are the igneous rocks that forms layered intrusions. These rocks have recorded the oldest one more that 4.2 and distributed in various locations and are vertically layered making them have varying compositions and textures. This can also be attribute to their petrological processes and the mineralogical processes. Consequently, Zhou eta al. (2005) described layered intrusion as large sill-like body composed of igneous rocks with sizes range of 100 km2 (39 sq mi) to over 50,000 km2. Consequently, Large layered intrusions are formed by igneous rocks mainly ultramafic to mafic in composition that flows and solidifies forming layers due to time of the magma flow and different mineral content of the lava.

 

Difference between petrological and mineralogical processes

According to Zipfel et al. (2000), Petrology is term that is used to denote the study of rocks such as igneous, among others and the associated processes that lead to their formation and transformation. Among them are the layered intrusions that are formed win this process due to successive crops of crystals due to cooling magma. It is during this process that due to heat loss through the boundaries of cooling magma makes a growing crystal to exerts gravitational stress on the liquid suspending it. The crystal will ten sinks or float at a rate guided by properties of the liquid crystal.  Also, due to increased rate of temperature loss and solids leads to increases the viscosity and increases strength of the crystal-liquid suspension. Therefore, if stress exerted by the crystal becomes greater than the yield strength of the liquid, it means that crystal will continues to move but if the rate of growth and density contrast are high, the result is rapid sinking or a floating enough to escape or become trapped in the solidifying matrix.

During this process, most of the heat is known to move from intrusions to the roof making crystallization be concentrated on the floor. As Jackson (1961) has pointed out, however, the positive pressure dependence of crystallization temperature implies that a homogeneous magma should crystallize more rapidly at its base than under its roof. Any concentration of volatile components near the top will augment this effect (Sorenson, 1969; Elsdon, 1970).  However, there is presence of dynamic processes during crystallization. Among the most common includes differential settling or flotation of crystals with varying densities and grain sizes. This affects flow of crystal-laden magma and crystal separation during convective fluid movement into the lava compartment. Other processes that contribute to these formations include magma injection into the compartment leading to magma mixing and formation of silicate liquid immiscibility in the chambers.

However, non-dynamic processes may be involved which includes rapid changes in crystallization conditions creating disruptions in the process curves. Formations of the layer may also originate form variation in nucleation proportions and from mineralogy due to rock heterogeneous properties. It is also known that majority of these processes are driven by degeneracy of energy making them self-organization processes to form modal layers.

Consequently, Dana (1869) defined mineralogy as the study of associated chemistry, crystal structure and physical properties of rock mineral constituents. Therefore, it is important to consider the petrological and mineralogical processes involved as they affect the environmental. As a demonstration of the process, it is known that dark minerals have higher density than light ones like pyroxene is ca. 30% denser than plagioclase and settles more rapidly to form a layer with dark minerals and vice versa forming contrasting layers.  Hence, according to Jerram, Cheadle & Philpotts (2003), crystal-sorting mechanism is the main cause of magma differentiation conditions, so the compositions of rocks, and the minerals they consist of, are interrogated to answer fundamental questions across a wide range of geological disciplines.

Crystal Fractionation

Fractional crystallization, or crystal fractionation, is one of the most important geochemical and physical processes operating within crust and mantle of a rocky planetary body, such as the Earth. It is important in the formation of igneous rocks because it is one of the main processes of magmatic differentiation. Crystal settling may occur in a surprisingly diverse range of regimes and may lead to intermittent deposition events even with small crystal concentration. It is caused by continuous separation of crystals and liquid as crystals solidifies. Consequently, more minerals in the crystals and the liquid causes change of liquid contents per unit of crystallization.  The liquid composition is varied by presence of major elements in crystals and liquid.

Therefore, the nature and proportions of the crystallising phases and relative concentrations in the liquid of elements that enter into solid solutions is also a major factor.  Fractional crystallisation of basaltic liquids usually involves separation of olivine, pyroxene and plagioclase. Consequently, Gravitational settling occurs which results to graded bedding. This is a result of denser crystals settling at the bottom of the magma body and become segregated from the residual melt. The densest particles will settle at the bottom of the magma chamber. The process can also be supported by Stokes Law of residual magmatic fluids.

Double Diffusive Convection

As result of this varying properties, double diffusive convection becomes apparent which then yields to rhythmic layering that cause repetition of zones of varying composition as the large layered intrusions.

 

 

Cyclic layering

During formation of large layered intrusions, cyclic layering is differentiated by remarkable regular spacing of the layers with geometric increase in spacing. In particular, layers are always seen to be parallel to the contact. The phenomenon of cyclic layering is to magma chamber recharge. In this, there is continuous depletion in compatible trace minerals form base to top representing the composition of the initial magma. The latter is maintained until a new base is attained which signifies an injection of fresh magma where mixing can occur leading to a mixture with a different composition.

 

Density Currents

As a result of varying mixtures properties of gases and liquids, motion is maintained by the force of gravity that acts in differences of density leading to modal layers.

Magmatic Segregation

This is a general term that refers to all the process responsible for more minerals getting locally concentrated during the processes of magma cooling and crystallization. As a result, the resultant rocks formed are magmatic cumulates. Therefore, despite the initial magma being homogeneous liquid, it can become a complex mixture along the path. It is also evident that magmatic cumulates mineral deposits are strictly in mafic and ultramafic igneous rocks due to silica exerting control exerted on the viscosity of a magma i.e. high silica content means more viscous a magma slowing down segregation proceed.

Bagnold effect

It is known to be the most important in determination of flowage differentiation due to is consideration of buoyancy. However, this phenomenon is absent in porphyritic rocks. For large channels, magmatic differentiation that exists between the core and the walls does not relate to the forced lava flow. However, the latter is determined by other factors like crystal settling among others.

Large layered intrusions

From the Zhou eta al (2005) description of layered intrusion, these rocks can be termed as composed of both heterogeneous and homogeneous masses of rock. Their formation is as result of two or more types of rocks with seamless connections between the rock types which gives the impression that different magmas were involved in their formation but did not mix before solidifying. In most cases, these rocks conform to the floor of the intrusion region but varies in thickness and length. It may be explained by the fact that the processes are dominant in these areas over varying period of time among other factors. Due to varying composition and mixture, the resultant layers of rocks have different texture and/or varying mineral proportions.

Layering in the Skaergaard intrusion: Source (Wager, L. R., and Deer, W. A. (1939)

Layered intrusions demonstrate a repetition of patterns in the layers giving an impression of a cycle of conditions leading to their formation. In many cases, the large layered intrusion contacts are planar with a characteristic of eroded hollows in some underlying layer having been filled by an overlying layer showing a repeated process of formation. The spreading of the magma as it flows causes it to cover a large area and as these processes’ repeats, they form a Large layered intrusion

Examples of these intrusions

Among the best examples of large layered intrusions includes the Macro-rhythmic layering in Greenland. In this case, the formation demonstrates sequences with initial layers being enriched with Fe–Ti oxides, clinopyroxene-rich gabbro and plagioclase-rich layer in the same order. A second example is the micro-rhythmic layering in the Bjerkreim in Norway. In this case, successive layers are highly enriched with orthopyroxene and oxide minerals followed by plagioclase. A third example is the very-fine scale layering in the Storgangen intrusion in Norway. The fourth example is the modally-graded layering in the magnetite layer in South Africa. The feature demonstrates continuous increase of plagioclase mode upwards. These are some of the known examples among many that exist.

  

 

Reference

Dana, J. D. (1869). A system of mineralogy.

Gibb, F. G. (1992). Convection and crystal settling in sills. Contributions to Mineralogy and Petrology109(4), 538-545.

Jerram, D. A., Cheadle, M. J., & Philpotts, A. R. (2003). Quantifying the building blocks of igneous rocks: are clustered crystal frameworks the foundation? Journal of Petrology44(11), 2033-2051.

Wager, L. R., Brown, G. M., & Wadsworth, W. J. (1960). Types of igneous cumulates. Journal of Petrology1(1), 73-85.

Zipfel, J., Scherer, P., Spettel, B., Dreibus, G., & Schultz, L. (2000). Petrology and chemistry of the new shergottite Dar al Gani 476. Meteoritics & Planetary Science35(1), 95-106.

Zhou, M. F., Robinson, P. T., Lesher, C. M., Keays, R. R., Zhang, C. J., & Malpas, J. (2005). Geochemistry, petrogenesis and metallogenesis of the Panzhihua gabbroic layered intrusion and associated Fe–Ti–V oxide deposits, Sichuan Province, SW China. Journal of Petrology46(11), 2253-2280.