will partially melt. These two processes are: (1) the spreading and opening of the
crust-lithosphere at ridge axes, which could facilitate the upwelling of passive
mantle material which will then melt due to decompression, and/or (2) the
dynamic flow model which involves a forceful uplifting of the mantle during
partial melting as a result of localized heat conduction. This will require a region
of lower viscosity in the mantle due to thermal, compositional and density changes
caused by melting. The localized or focused mantle upwelling is associated with
thermal pulses generated by the circulation of hot mantle material in the
asthenosphere, when it enters the lithosphere. Such localized, thermal pulses were
detected by Bonatti et al. (2003) during a study of a 20 million-year-old record of
exposed lithosphere underneath the Mid-Atlantic Ridge.
Melt Impregnation of Peridotites
The heterogeneous nature and the structural and compositional diversities of
exposed oceanic crust are a reflection of the asthenospheric circulation interacting
with the rocks that make up the rigid lithosphere. The asthenosphere-lithosphere
interactions are suggested by the exposed mantle residues derived from the partial
melting of mantle material, which was impregnated with basaltic liquids that did
not reach the surface but which had previously solidified within the lithosphere.
When a melt rises after the partial melting of mantle peridotite, it will travel
through the lithosphere’s openings, breaks, faults and fissures to create new oceanic crust. Evidence for melt impregnation dispersed within the peridotite and
other formations (i.e. dunites, gabbros) forming the lithosphere is observed by the
presence of small millimeter-sized dykelets and larger veins (centimeter to meter
size) of solidified melt in the form of minerals. An increase in partial melting will
depend on a region’s spreading mechanism as well as on mantle plume upwelling,
which is able to bring up hot material during lithospheric decompression.
The composition of an impregnated melt does not necessarily reflect the
composition of the original parental mantle source. Indeed, chemical reactions
could have taken place between the impregnated liquid and the surrounding formation during the liquid’s ascent as well as during crystal-liquid fractionation,
which could alter the original mineral assemblages. The study of impregnated
melts that have solidified within the lithosphere can give us an indication about the
way magma arrives at the surface. It can also give us an idea about the extent and
the amount of melt circulating within the lithosphere. This impregnation phenomenon takes place during an intermediate stage between when a melt has been
extracted from the mantle until the moment of its extrusion on the surface during a
volcanic event.
98
4 Sea Floor Rocks
crust-lithosphere at ridge axes, which could facilitate the upwelling of passive
mantle material which will then melt due to decompression, and/or (2) the
dynamic flow model which involves a forceful uplifting of the mantle during
partial melting as a result of localized heat conduction. This will require a region
of lower viscosity in the mantle due to thermal, compositional and density changes
caused by melting. The localized or focused mantle upwelling is associated with
thermal pulses generated by the circulation of hot mantle material in the
asthenosphere, when it enters the lithosphere. Such localized, thermal pulses were
detected by Bonatti et al. (2003) during a study of a 20 million-year-old record of
exposed lithosphere underneath the Mid-Atlantic Ridge.
Melt Impregnation of Peridotites
The heterogeneous nature and the structural and compositional diversities of
exposed oceanic crust are a reflection of the asthenospheric circulation interacting
with the rocks that make up the rigid lithosphere. The asthenosphere-lithosphere
interactions are suggested by the exposed mantle residues derived from the partial
melting of mantle material, which was impregnated with basaltic liquids that did
not reach the surface but which had previously solidified within the lithosphere.
When a melt rises after the partial melting of mantle peridotite, it will travel
through the lithosphere’s openings, breaks, faults and fissures to create new oceanic crust. Evidence for melt impregnation dispersed within the peridotite and
other formations (i.e. dunites, gabbros) forming the lithosphere is observed by the
presence of small millimeter-sized dykelets and larger veins (centimeter to meter
size) of solidified melt in the form of minerals. An increase in partial melting will
depend on a region’s spreading mechanism as well as on mantle plume upwelling,
which is able to bring up hot material during lithospheric decompression.
The composition of an impregnated melt does not necessarily reflect the
composition of the original parental mantle source. Indeed, chemical reactions
could have taken place between the impregnated liquid and the surrounding formation during the liquid’s ascent as well as during crystal-liquid fractionation,
which could alter the original mineral assemblages. The study of impregnated
melts that have solidified within the lithosphere can give us an indication about the
way magma arrives at the surface. It can also give us an idea about the extent and
the amount of melt circulating within the lithosphere. This impregnation phenomenon takes place during an intermediate stage between when a melt has been
extracted from the mantle until the moment of its extrusion on the surface during a
volcanic event.
98
4 Sea Floor Rocks