MORB
Introduction
Mid-Ocean Ridge Basalts (MORB) are a type of igneous rock that forms at mid-ocean ridges, which are underwater mountain ranges created by plate tectonics. These basalts are significant in understanding the composition of the Earth's mantle and the processes involved in seafloor spreading. MORB is the most common volcanic rock on Earth, covering approximately 60% of the planet's surface. This article delves into the complexities of MORB, including its formation, composition, classification, and its role in geochemical cycles.
Formation of MORB
MORB is formed by partial melting of the upper mantle at divergent plate boundaries, where tectonic plates are moving apart. As the plates separate, magma rises to fill the gap, solidifying to form new oceanic crust. The process is driven by decompression melting, where the reduction in pressure as mantle material rises leads to melting. This phenomenon is crucial for understanding the dynamics of plate tectonics and the continuous renewal of the ocean floor.
Decompression Melting
Decompression melting occurs when mantle rocks ascend adiabatically, meaning they rise without losing heat to their surroundings. As the pressure decreases, the solidus temperature of the mantle material is reduced, causing it to melt. This process is responsible for generating the basaltic magma that forms MORB. The degree of partial melting and the composition of the mantle source influence the characteristics of the resulting basalt.
Magma Ascent and Crystallization
Once formed, the basaltic magma ascends through the lithosphere, often pooling in magma chambers beneath the mid-ocean ridge. As the magma cools, it begins to crystallize, forming minerals such as olivine, pyroxene, and plagioclase. The crystallization sequence and the cooling rate affect the texture and mineralogy of the MORB. Rapid cooling at the seafloor results in fine-grained, aphanitic textures typical of MORB.
Composition of MORB
MORB is characterized by a specific chemical composition that reflects its mantle source and the conditions of its formation. It is predominantly composed of mafic minerals, with high levels of magnesium and iron, and low silica content. The geochemical signature of MORB provides insights into mantle heterogeneity and the processes of mantle melting.
Major and Trace Elements
The major elements in MORB include silicon, oxygen, magnesium, iron, calcium, and aluminum. Trace elements such as titanium, sodium, and potassium are also present, albeit in smaller quantities. The concentration of these elements can vary, leading to the classification of MORB into different types based on their geochemical characteristics.
Isotopic Composition
Isotopic analysis of MORB, particularly of elements like strontium, neodymium, and lead, reveals information about the age and evolution of the mantle source. These isotopic ratios are used to trace mantle convection patterns and to understand the recycling of oceanic crust through subduction and mantle plumes.
Classification of MORB
MORB is classified into three main types based on its geochemical composition: Normal MORB (N-MORB), Enriched MORB (E-MORB), and Depleted MORB (D-MORB). Each type reflects different degrees of partial melting and mantle source characteristics.
Normal MORB (N-MORB)
N-MORB is the most common type and is characterized by a relatively uniform composition with low concentrations of incompatible elements. It is thought to originate from a depleted mantle source that has undergone previous melting events.
Enriched MORB (E-MORB)
E-MORB contains higher levels of incompatible elements and is believed to derive from a mantle source that has been enriched by recycled oceanic crust or mantle plumes. This enrichment results in a more diverse isotopic signature compared to N-MORB.
Depleted MORB (D-MORB)
D-MORB exhibits lower concentrations of incompatible elements and is associated with a highly depleted mantle source. It represents the residue of extensive melting processes, often linked to the extraction of continental crust.
Role in Geochemical Cycles
MORB plays a crucial role in the Earth's geochemical cycles, particularly in the cycling of elements between the mantle, crust, and oceans. The formation and alteration of MORB influence the distribution of elements and isotopes in the oceanic crust and seawater.
Hydrothermal Alteration
As MORB interacts with seawater at mid-ocean ridges, it undergoes hydrothermal alteration, a process that alters its mineralogy and chemistry. This interaction leads to the formation of hydrothermal vents, which are important sites for the exchange of heat and elements between the ocean and the crust.
Carbon Cycle
MORB is involved in the global carbon cycle through the subduction of oceanic crust. Carbonate minerals formed during hydrothermal alteration can be subducted into the mantle, where they contribute to the deep carbon cycle. This process influences the long-term carbon balance of the Earth.