Everything about Metasomatism totally explained
Metasomatism is the chemical alteration of a
rock by
hydrothermal and other fluids. It is also known as "alteration".
Metasomatism can occur via the action of hydrothermal fluids from an
igneous or
metamorphic source.
In the
igneous environment, metasomatism creates
skarns,
greisen, and may affect
hornfels in the contact metamorphic aureole adjacent to an
intrusive rock mass.
In the metamorphic environment, metasomatism is created by
mass transfer from a volume of
metamorphic rock at higher
stress and
temperature into a zone with lower stress and temperature, with metamorphic hydrothermal solutions acting as a
solvent. This can be envisaged as the metamorphic rocks within the deep
crust losing fluids and dissolved
mineral components as hydrous minerals break down, with this fluid percolating up into the shallow levels of the crust to chemically change and alter these rocks.
This mechanism implies that metasomatism is open system behaviour, which is different from classical
metamorphism which is the in-situ mineralogical change of a rock without appreciable change in the chemistry of the rock. Because metamorphism usually requires
water in order to facilitate metamorphic reactions, metasomatism and metamorphism nearly always occur together.
Further, because metasomatism is a mass transfer process, it isn't restricted to the rocks which are changed by addition of chemical
elements and minerals or hydrous
compounds. In all cases, to produce a metasomatic rock some other rock is also metasomatised, if only by
dehydration reactions with minimal chemical change. This is best illustrated by
gold ore deposits which are the product of focused concentration of fluids derived from many
cubic kilometres of dehydrated
crust into thin, often highly metasomatised and altered
shear zones and
lodes. The source region is often largely chemically unaffected compared to the highly hydrated, altered shear zones, but both must have undergone complementary metasomatism.
Metasomatism is more complicated in the
Earth's mantle, because the composition of
peridotite at high temperatures can be changed by infiltration of
carbonate and
silicate melts and by
carbon dioxide-rich and water-rich fluids, as discussed by Luth (2003). Metasomatism is thought to be particularly important in changing the composition of mantle peridotite below
island arcs as water is driven out of
ocean lithosphere during
subduction. Metasomatism has also been considered critical for enriching source regions of some
silica-undersaturated magmas.
Carbonatite melts are often considered to have been responsible for enrichment of mantle peridotite in
incompatible elements.
Types of metasomatites
Metasomatic rocks can be extremely varied. Often, metasomatised rocks are pervasively but weakly
altered, such that the only evidence of alteration is bleaching, change in colour or change in the crystallinity of micaceous minerals.
In such cases, characterising alteration often requires microscope investigation of the mineral assemblage of the rocks to characterise the minerals, any additional mineral growth, changes in protolith minerals, and so on.
In some cases, geochemical evidence can be found of metasomatic alteration processes. This is usually in the form of mobile, soluble elements such as
barium,
strontium,
rubidium,
calcium and some
rare earth elements. However, to characterise the alteration properly, it's necessary to compare altered with unaltered samples.
When the process becomes extremely advanced, typical metasomatites can include:
Effects of metasomatism in mantle peridotite can be either modal or cryptic. In cryptic metasomatism, mineral compositions are changed, or introduced elements are concentrated on grain boundaries and the peridotite mineralogy appears unchanged. In modal metasomatism, new minerals are formed.
Cryptic metasomatism may be caused as rising or percolating melts interact with surrounding peridotite, and compositions of both melts and peridotite are changed. At high mantle temperatures, solid-state
diffusion can also be effective in changing rock compositions over tens of
centimeters adjacent to melt conduits: gradients in mineral composition adjacent to
pyroxenite dikes may preserve evidence of the process.
Modal metasomatism may result in formation of
amphibole and
phlogopite, and the presence of these minerals in peridotite
xenoliths has been considered strong evidence of metasomatic processes in the mantle. Formation of minerals less common in peridotite, such as
dolomite,
calcite,
ilmenite,
rutile, and
armalcolite, is also attributed to melt or fluid metasomatism.
Alteration assemblages
Investigation of altered rocks in hydrothermal ore deposits has highlghted several ubiquitous types of
alteration assemblages which create distinct groups of metasomatic alteration effects, textures and mineral assemblages.
Propylitic Alteration is caused by iron and sulfur-bearing hydrothermal fluids, and typically results in epidote-chlorite-pyrite alteration, often with hematite and magnetite facies
Albite-epidote alteration is caused by silica-bearing fluids rich in sodium and calcium, and typically results in weak albite-silica-epidote
Potassic Alteration, typical of lode gold deposits, results in production of micaceous, potassic minerals such as biotite in iron-rich rocks, muscovite mica or sericite in felsic rocks, and orthoclase (adularia) alteration, often quite pervasive and producing distinct salmon-pink alteration vein selvages.
Rarer types of hydrothermal fluids may include highly carbonic fluids, resulting in advanced carbonation reactions of the host rock typical of calc-silicates, and silica-hematite fluids resulting in production of jasperoids, manto ore deposits and pervasive zones of silicification, typically in dolomite formations.
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