Harzburgite
Encyclopedia
The ultramafic igneous rock, harzburgite, is a variety of peridotite
consisting mostly of the two mineral
s, olivine
and low-calcium
(Ca) pyroxene
(enstatite
); it is named for occurrences in the Harz Mountains of Germany. It commonly contains a few percent chromium
-rich spinel
as an accessory mineral. Garnet
-bearing harzburgite is much less common, found most commonly as xenolith
s in kimberlite
.
Harzburgite typically forms by the extraction of partial melts
from the more pyroxene-rich peridotite called lherzolite
. The molten magma
extracted from harzburgite may then erupt on the surface as basalt
. If partial melting of the harzburgite continues, all of the pyroxene may be extracted from it to form magma, leaving behind the pyroxene-poor peridotite called dunite
. Harzburgite may also form by the accumulation of olivine and low-Ca pyroxene in large magma chamber
s of basalt deep in continental crust
(layered intrusions).
, which are thought to represent oceanic crust
and the underlying oceanic mantle exposed during collision with continental crust
. Examples of ophiolites with extensive harzburgite include the Troodos ophiolite in Cyprus
, the Semail ophiolite in Oman
, the Coast Range ophiolite of California
, and the Bay of Islands
ophiolite in Newfoundland. The so-called "abyssal peridotites" dredged from the seafloor near fracture zone
s (where oceanic lithosphere
may be exposed at the surface) are typically lherzolites, not harzburgites. Lherzolites may also form by the extraction of magma from more pyroxene-rich peridotite, but are less depleted than harzburgites. This is consistent with the observation that many ophiolites probably form in the fore-arc region of island arc
s, not at mid-oceanic ridges.
Harzburgite may also be found in some Alpine peridotite
massifs that consist mostly of lherzolite. Alpine or orogenic lherzolite
s represent subcontinental mantle
lithosphere (the upper mantle below continental crust) exposed during the plate tectonic collision of continental plates. Examples of orogenic lherzolite massif
s with harzburgite include the Lherz massif in France (Pyrenees Mountains), the Lanzo massif in northern Italy, and the Horoman massif in Japan.
Garnet
-bearing harzburgite is found as a xenolith
in some kimberlite
pipes, which are found almost exclusively in ancient continental craton
s of Archean
or Paleoproterozoic
age. The mantle lithosphere under these cratons is particularly thick (up to 200 km or more) and cool. Garnet harzburgites are less depleted in the basalt
component than most ophiolite harzburgites. Garnet harzburgite xenoliths from kimberlites in South Africa have been particularly well-characterized.
Cumulate
harzburgite is found in some large layered igneous intrusions. At the Earth's surface, basaltic magmas typically crystallize
the minerals: olivine, plagioclase
, and augite
(a high-Ca pyroxene); low-Ca pyroxenes can only co-exist with olivine at low pressure in magma that is high in both MgO and SiO2 (boninite
s). At pressures greater than 5 kilobars (0.5 GPa, or 5000x atmospheric pressure
), olivine and low-Ca pyroxene (enstatite
or bronzite
) may crystallize together from normal basalt magmas to form harzburgite. These conditions were common in some layered mafic
intrusions, most of which are Proterozoic
in age, which formed from enormous sill
-like intrusions of basalt into lower continental crust. The classic example of a Proterozoic layered intrusion with cumulate harzburgite is the Stillwater igneous complex
of Montana, U.S.A. It is also found in the Bushveld Igneous Complex
, a large layered intrusion in South Africa.
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
consisting mostly of the two mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s, olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
and low-calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
(Ca) pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
(enstatite
Enstatite
Enstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite . The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks...
); it is named for occurrences in the Harz Mountains of Germany. It commonly contains a few percent chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
-rich spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...
as an accessory mineral. Garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
-bearing harzburgite is much less common, found most commonly as xenolith
Xenolith
A xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...
s in kimberlite
Kimberlite
Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the Big Hole....
.
Harzburgite typically forms by the extraction of partial melts
Partial melting
Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the solid....
from the more pyroxene-rich peridotite called lherzolite
Lherzolite
Lherzolite is a type of ultramafic igneous rock. It is a coarse grained rock consisting of 40 to 90% olivine along with significant orthopyroxene and lesser calcic chromium rich clinopyroxene. Minor minerals include chromium and aluminium spinels and garnets. Plagioclase can occur in lherzolites...
. The molten 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...
extracted from harzburgite may then erupt on the surface as basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
. If partial melting of the harzburgite continues, all of the pyroxene may be extracted from it to form magma, leaving behind the pyroxene-poor peridotite called dunite
Dunite
Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group...
. Harzburgite may also form by the accumulation of olivine and low-Ca pyroxene in large magma chamber
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
s of basalt deep in continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
(layered intrusions).
Occurrence
Harzburgite is the most commonly found variety of peridotite in ophiolitesOphiolites
An ophiolite is a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks...
, which are thought to represent oceanic crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
and the underlying oceanic mantle exposed during collision with continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
. Examples of ophiolites with extensive harzburgite include the Troodos ophiolite in Cyprus
Cyprus
Cyprus , officially the Republic of Cyprus , is a Eurasian island country, member of the European Union, in the Eastern Mediterranean, east of Greece, south of Turkey, west of Syria and north of Egypt. It is the third largest island in the Mediterranean Sea.The earliest known human activity on the...
, the Semail ophiolite in Oman
Oman
Oman , officially called the Sultanate of Oman , is an Arab state in southwest Asia on the southeast coast of the Arabian Peninsula. It is bordered by the United Arab Emirates to the northwest, Saudi Arabia to the west, and Yemen to the southwest. The coast is formed by the Arabian Sea on the...
, the Coast Range ophiolite of California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, and the Bay of Islands
Bay of Islands, Newfoundland and Labrador
The Bay of Islands is an extensive inlet located on the west coast of the island of Newfoundland, in Canada. The Way Office was established on July 1, 1883. The first Waymaster was Thomas Carter.It is also a sub-basin of the Gulf of St...
ophiolite in Newfoundland. The so-called "abyssal peridotites" dredged from the seafloor near fracture zone
Fracture zone
A fracture zone is a linear oceanic feature--often hundreds, even thousands of kilometers long--resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics. Lithospheric plates on either side of an active transform fault move in opposite directions;...
s (where oceanic lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
may be exposed at the surface) are typically lherzolites, not harzburgites. Lherzolites may also form by the extraction of magma from more pyroxene-rich peridotite, but are less depleted than harzburgites. This is consistent with the observation that many ophiolites probably form in the fore-arc region of island arc
Island arc
An island arc is a type of archipelago composed of a chain of volcanoes which alignment is arc-shaped, and which are situated parallel and close to a boundary between two converging tectonic plates....
s, not at mid-oceanic ridges.
Harzburgite may also be found in some Alpine peridotite
Lherzolite
Lherzolite is a type of ultramafic igneous rock. It is a coarse grained rock consisting of 40 to 90% olivine along with significant orthopyroxene and lesser calcic chromium rich clinopyroxene. Minor minerals include chromium and aluminium spinels and garnets. Plagioclase can occur in lherzolites...
massifs that consist mostly of lherzolite. Alpine or orogenic lherzolite
Lherzolite
Lherzolite is a type of ultramafic igneous rock. It is a coarse grained rock consisting of 40 to 90% olivine along with significant orthopyroxene and lesser calcic chromium rich clinopyroxene. Minor minerals include chromium and aluminium spinels and garnets. Plagioclase can occur in lherzolites...
s represent subcontinental mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
lithosphere (the upper mantle below continental crust) exposed during the plate tectonic collision of continental plates. Examples of orogenic lherzolite massif
Massif
In geology, a massif is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole...
s with harzburgite include the Lherz massif in France (Pyrenees Mountains), the Lanzo massif in northern Italy, and the Horoman massif in Japan.
Garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
-bearing harzburgite is found as a xenolith
Xenolith
A xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...
in some kimberlite
Kimberlite
Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the Big Hole....
pipes, which are found almost exclusively in ancient continental craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
s of Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
or Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
age. The mantle lithosphere under these cratons is particularly thick (up to 200 km or more) and cool. Garnet harzburgites are less depleted in the basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
component than most ophiolite harzburgites. Garnet harzburgite xenoliths from kimberlites in South Africa have been particularly well-characterized.
Cumulate
Cumulate rock
Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks.-Formation:Cumulate rocks are the...
harzburgite is found in some large layered igneous intrusions. At the Earth's surface, basaltic magmas typically crystallize
Fractional crystallization (geology)
Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the...
the minerals: olivine, plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
, and augite
Augite
Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...
(a high-Ca pyroxene); low-Ca pyroxenes can only co-exist with olivine at low pressure in magma that is high in both MgO and SiO2 (boninite
Boninite
Boninite is a mafic extrusive rock high in both magnesium and silica, formed in fore-arc environments, typically during the early stages of subduction. The rock is named for its occurrence in the Izu-Bonin arc south of Japan...
s). At pressures greater than 5 kilobars (0.5 GPa, or 5000x atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
), olivine and low-Ca pyroxene (enstatite
Enstatite
Enstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite . The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks...
or bronzite
Bronzite
Bronzite is a member of the orthopyroxene group of minerals, belonging with enstatite and hypersthene to the orthorhombic series of the group...
) may crystallize together from normal basalt magmas to form harzburgite. These conditions were common in some layered mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
intrusions, most of which are Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
in age, which formed from enormous sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
-like intrusions of basalt into lower continental crust. The classic example of a Proterozoic layered intrusion with cumulate harzburgite is the Stillwater igneous complex
Stillwater igneous complex
The Stillwater igneous complex is a large layered intrusion located in southern Montana in Stillwater, Sweet Grass and Park Counties. The complex is exposed across 30 miles of the north flank of the Beartooth Mountain Range. The complex has extensive reserves of chromium ore and has a history of...
of Montana, U.S.A. It is also found in the Bushveld Igneous Complex
Bushveld igneous complex
The Bushveld Igneous Complex is a large layered igneous intrusion within the Earth's crust which has been tilted and eroded and now outcrops around what appears to be the edge of a great geological basin, the Transvaal Basin. Located in South Africa, the BIC contains some of the richest ore...
, a large layered intrusion in South Africa.