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The Mantle and the Origin of basalt
Plate tectonic theory postulates that most geological
features of the Earth can be explained on the assumption: 1)
that the Earth is a self regulating heat engine fuelled by
radiogenic heat; 2) that the Earth is insulated by a
relatively rigid outer layer through which heat is
transferred by conduction; 3) that the radiogenic heat
generated in the internal part of the Earth is transferred
by convection to the outer conducting layer; and 4) that the
thermally driven vertical convective motion is converted
into horizontal motion of the Earth's rigid surface layer
composed of a set of 'plates'.
The average temperature of the Earth is kept relatively
constant by balancing the rate of radiogenic heat
generation, the efficiency of the convective system, and the
downward growth (thickening) of its outer insulating layer.
If there were no convection, the Earth would continuously
heat up. The thermal and dynamic interaction of the various
plates forming the Earth's surface accounts for most
observed geological phenomena: volcanoes, plutonic rocks,
deep sea trenches, mid-ocean ridges, mountain systems,
continental rifts, sedimentary basins, rock deformation,
continental drift, continental growth, element
fractionation, etc., etc.
Mantle rocks rocks bear the imprint of four processes:
1) The imposition of a flow fabric during convective flow,
most easily perceived in the preferred shape orientation of
minerals and mineral aggregates.
2) Metamorphic segregation of pyroxenes and spinels to form
layered lherzolite and harzburgite.
3) Metamorphic transformation of spinel to garnet or
plagioclase.
4) Partial melting of lherzolite to form a basalt melt and
harzburgite/dunite residue as a result of isothermal
decompression related to mantle convection.
Partial melting is thought to change 'undepleted' or
'fertile' mantle material to depleted mantle typical of the
upper mantle, which is consequently known as the 'depleted'
layer. The basalt forming equation is therefore depicted as:
fertile mantle = depleted mantle + basalt
The fertile mantle is composed of ultramafic rock
called lherzolite made up of the minerals olivine
(FeMg)2SiO4, orthopyroxene (FeMg)2Si2O6, clinopyroxene
Ca(FeMg)Si2O6, Tschermak's molecule CaAl2SiO6-MgAl2SiO6,
jadeite NaAlSi2O6, and spinel MgAl2O4-FeCr2O4 or garnet
(FeMg)3Al2Si3O12. Lherzolite contains spinel at moderate
pressure, garnet at high pressure, and plagioclase at low
pressure. Tschermak's molecule and jadeite do not exist as
mineral phases but occur dissolved in orthopyroxene and
clinopyroxene.
How is basalt produced in the mantle?
Basalt is made up of the minerals clinopyroxene and
plagioclase(anorthite+albite).
When lherzolite is heated to its melting temperature
basalt is formed by the melting of clinopyroxene,
Tschermak`s molecule, jadeite and spinel or garnet:
clinopyroxene -> (clinopyroxene)liquid
jadeite + orthopyroxene -> (albite)liquid +
(olivine)residue
Tschermak's molecule + orthopyroxene -> (anorthite)liquid
+ (olivine)residue
MgAl2O4-FeCr2O4spinel + clinopyroxene + orthopyroxene ->
(anorthite)liquid + (FeCr2O4 chromite + 2 olivine)residue
The change of aluminous spinel to liquid and chromite
is known as incongruent melting. The residue after melting
is enriched in olivine and chromite, and is composed of
olivine, orthopyroxene, and chromite (harzburgite).
If lherzolite is heated to even higher temperatures,
the amount of orthopyroxene entering the melt phase will
increase. If all the orthopyroxene melts, the residue will
be composed of olivine and chromite and is known as
dunite.Dunite is the host for many economic deposits of
chromite.
The basalt melt phase will separate from the
harzburgite/dunite residue and will eventually reach the
surface of the Earth via the magma chamber beneath mid-
oceanic ridges. The olivine rich depleted mantle
(hartzburgite/dunite) will flow first vertically and then
laterally, forming the base on which the overlying basalt is
passively carried in piggy-back fashion. Because the
dunite/harzburgite is flowing in the solid state it will
take on a deformation fabric, and will appear as a
metamorphic 'tectonite`.
The order in which minerals crystallize from oceanic
basalt is: olivine + chromite - plagioclase - clinopyroxene
- orthopyroxene.