Primitive mantle
Encyclopedia
The primitive mantle is, in geochemistry
, a hypothetical reservoir with the composition of the Earth's crust and mantle taken together.
The currently accepted scientific hypothesis is that the Earth formed by accretion
of material with a chondritic
composition. Still during the accretionary phase planetary differentiation
started, giving rise to the Earth's core, where heavy metallic siderophile
elements accumulated. Around it was a (in this stage) undifferentiated mantle, the primitive mantle. Further differentiation would take place later, creating the different chemical reservoirs of crust and mantle, with incompatible element
s accumulating in the crust.
Today differentiation still continues in the upper mantle. Reservoirs depleted in lithophile
elements are called depleted, "fresh" undifferentiated parts of the mantle are called enriched or primitive. The last name is confusing but derives from the fact that such reservoirs are comparable in composition to the primitive mantle. Volcanic rock
s from hotspot areas
often have a primitive composition. Because the magma
at hotspots is supposed to have been taken to the surface from the deepest regions of the mantle by mantle plume
s, geochemists assume there must be a relatively closed reservoir of very primitive composition somewhere in the lower mantle. One of the hypotheses is this is the so called D"-layer at the core-mantle boundary.
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
, a hypothetical reservoir with the composition of the Earth's crust and mantle taken together.
The currently accepted scientific hypothesis is that the Earth formed by accretion
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
of material with a chondritic
Chondrite
Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids...
composition. Still during the accretionary phase planetary differentiation
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...
started, giving rise to the Earth's core, where heavy metallic siderophile
Siderophile
Siderophile means "iron-loving". This can refer to:* Siderophilic bacteria, bacteria that require or are facilitated by free iron* Siderophile elements, chemical elements such as iridium or gold that tend to bond with metallic iron, as described by the Goldschmidt classification* Siderophilia,...
elements accumulated. Around it was a (in this stage) undifferentiated mantle, the primitive mantle. Further differentiation would take place later, creating the different chemical reservoirs of crust and mantle, with incompatible element
Incompatible element
An incompatible element is a term used in petrology and geochemistry to describe an element that is unsuitable in size and/or charge to the cation sites of the minerals, and is defined by the partition coefficient between rock-forming minerals and melt being much smaller than 1..During the...
s accumulating in the crust.
Today differentiation still continues in the upper mantle. Reservoirs depleted in lithophile
Lithophile
Lithophiles are micro-organisms that can live within the pore interstices of sedimentary and even igneous rocks to depths of several kilometers....
elements are called depleted, "fresh" undifferentiated parts of the mantle are called enriched or primitive. The last name is confusing but derives from the fact that such reservoirs are comparable in composition to the primitive mantle. Volcanic rock
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...
s from hotspot areas
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
often have a primitive composition. Because the magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
at hotspots is supposed to have been taken to the surface from the deepest regions of the mantle by mantle plume
Mantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
s, geochemists assume there must be a relatively closed reservoir of very primitive composition somewhere in the lower mantle. One of the hypotheses is this is the so called D"-layer at the core-mantle boundary.