orthopyroxene. Chromites, which are dark colored minerals of iron, magnesium
and aluminum oxydes, are disseminated throughout the dunite cumulates or form
independent units.
Peridotites
Peridotites are green colored rocks that have the closest composition to some
meteorites (achondrites) coming from space, probably due to the disintegration of
older planets. Peridotites have the same composition as the most abundant compounds on Earth and consist of silicates of magnesium, iron and calcium, forming
the minerals of olivine, pyroxene, spinel and garnet. Peridotites probably make up
about 50 % of the Earth’s mantle and they are found exposed both on land and
under the sea.
The first extensive dredging operation, to collect peridotites on a spreading
ridge was carried out in the summers of 1947 and 1948 with the RV ATLANTIS
under the leadership of Maurice Ewing. Several hundred pounds of rocks from
various parts of the Atlantis fracture zone on the MAR at 500–2500 fathoms were
collected and later studied by Professor Shand from Columbia University (Shand
1949). The importance of these dredged rocks became obvious when scientists
noticed the presence of some deep-seated peridotite, which was thought to form
the Earth’s mantle and was believed to be the parent melt as well as a residue of
the basalt that covers the major part of the modern sea floor. Peridotite, found in
the lower crust and upper part of the Earth’s lithosphere, was formed from a
combination of melting and crystallization processes during the ascent of solidified
minerals (mainly olivine) and melt beneath spreading ridge segments.
Two types of peridotite are found on the ocean floor: lherzolites and harzburgites, which take their names from the locations in Europe where they were first
studied (See glossary). The harzburgites are considered to be the residue left after
the partial melting of a mantle peridotite. They remained after the melt separated
from the mantle and circulated upward before giving rise to basaltic volcanism.
Residual harzburgite is quite close in composition to the original melt prior to
melting and consists essentially of olivine (Fo 85-95 ), orthopyroxene, spinel and
variable amounts of clinopyroxene (diopside-augite). During partial melting,
clinopyroxene (with the lowest melting temperature) is the first mineral to melt
and rise towards the upper part of the lithosphere where it could supply a magmatic reservoir. Unaltered peridotite, called lherzolite, consists essentially of
olivine, which is a dense silicate containing iron and magnesium, plus orthopyroxene (a similar mineral but less dense) and clinopyroxene containing aluminum
and calcium. The minor mineral constituents of all peridotite rocks are spinel and/
or garnet, which consist of chromium, magnesium, aluminum and iron oxide.
The importance of peridotite as a main constituent for the source of basaltic
volcanism was revealed during experimental laboratory work on the melting of a
synthetic peridotite, equivalent to an eclogite, a granular rock composed essentially
90
4 Sea Floor Rocks
and aluminum oxydes, are disseminated throughout the dunite cumulates or form
independent units.
Peridotites
Peridotites are green colored rocks that have the closest composition to some
meteorites (achondrites) coming from space, probably due to the disintegration of
older planets. Peridotites have the same composition as the most abundant compounds on Earth and consist of silicates of magnesium, iron and calcium, forming
the minerals of olivine, pyroxene, spinel and garnet. Peridotites probably make up
about 50 % of the Earth’s mantle and they are found exposed both on land and
under the sea.
The first extensive dredging operation, to collect peridotites on a spreading
ridge was carried out in the summers of 1947 and 1948 with the RV ATLANTIS
under the leadership of Maurice Ewing. Several hundred pounds of rocks from
various parts of the Atlantis fracture zone on the MAR at 500–2500 fathoms were
collected and later studied by Professor Shand from Columbia University (Shand
1949). The importance of these dredged rocks became obvious when scientists
noticed the presence of some deep-seated peridotite, which was thought to form
the Earth’s mantle and was believed to be the parent melt as well as a residue of
the basalt that covers the major part of the modern sea floor. Peridotite, found in
the lower crust and upper part of the Earth’s lithosphere, was formed from a
combination of melting and crystallization processes during the ascent of solidified
minerals (mainly olivine) and melt beneath spreading ridge segments.
Two types of peridotite are found on the ocean floor: lherzolites and harzburgites, which take their names from the locations in Europe where they were first
studied (See glossary). The harzburgites are considered to be the residue left after
the partial melting of a mantle peridotite. They remained after the melt separated
from the mantle and circulated upward before giving rise to basaltic volcanism.
Residual harzburgite is quite close in composition to the original melt prior to
melting and consists essentially of olivine (Fo 85-95 ), orthopyroxene, spinel and
variable amounts of clinopyroxene (diopside-augite). During partial melting,
clinopyroxene (with the lowest melting temperature) is the first mineral to melt
and rise towards the upper part of the lithosphere where it could supply a magmatic reservoir. Unaltered peridotite, called lherzolite, consists essentially of
olivine, which is a dense silicate containing iron and magnesium, plus orthopyroxene (a similar mineral but less dense) and clinopyroxene containing aluminum
and calcium. The minor mineral constituents of all peridotite rocks are spinel and/
or garnet, which consist of chromium, magnesium, aluminum and iron oxide.
The importance of peridotite as a main constituent for the source of basaltic
volcanism was revealed during experimental laboratory work on the melting of a
synthetic peridotite, equivalent to an eclogite, a granular rock composed essentially
90
4 Sea Floor Rocks