Lamproites are
ultrapotassicUltrapotassic igneous rocks are a class of rare, volumetrically minor generally ultramafic or mafic silica-depleted igneous rocks.Ultrapotassic rocks are defined by molar K2O/Na2O >3 in much of the scientific literature. In other papers written as recently as 2005, they are...
mantle-derived
volcanic rocksIgneous rock is one of the three main rock types . Igneous rock is formed by magma being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive rocks or on the surface as extrusive rocks...
. They have low
CaOCalcium oxide , commonly known as burnt lime, lime or quicklime, is a widely used chemical compound. It is a white, caustic and alkaline crystalline solid at room temperature. As a commercial product, lime often also contains magnesium oxide, silicon oxide and smaller amounts of aluminium oxide and...
,
Al2O3Aluminium oxide is an amphoteric oxide of aluminium with the chemical formula
23. It is also commonly referred to as alumina, corundum, sapphire, ruby or aloxite in the mining, ceramic and materials science communities. It is produced by the Bayer process from bauxite...
,
Na2OSodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses. Treatment with water affords sodium hydroxide.The alkali metal oxides M2O crystallise in the antifluorite structure...
, high K
2O/Al
2O
3, a relatively high
MgOMagnesium oxide, or magnesia, is a white solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of . It is formed by an ionic bond between one magnesium and one oxygen atom. Magnesium oxide is hygroscopic in nature and care must be taken to...
content and extreme enrichment in incompatible elements.
Lamproites are geographically widespread yet are volumetrically insignificant. Unlike
kimberliteKimberlite 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....
s which are found exclusively in Archaean
cratonA 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, lamproites are found in terrains of varying age, ranging from Archaean in Western Australia, to Palaeozoic and Mesozoic in southern Spain.
Lamproites are
ultrapotassicUltrapotassic igneous rocks are a class of rare, volumetrically minor generally ultramafic or mafic silica-depleted igneous rocks.Ultrapotassic rocks are defined by molar K2O/Na2O >3 in much of the scientific literature. In other papers written as recently as 2005, they are...
mantle-derived
volcanic rocksIgneous rock is one of the three main rock types . Igneous rock is formed by magma being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive rocks or on the surface as extrusive rocks...
. They have low
CaOCalcium oxide , commonly known as burnt lime, lime or quicklime, is a widely used chemical compound. It is a white, caustic and alkaline crystalline solid at room temperature. As a commercial product, lime often also contains magnesium oxide, silicon oxide and smaller amounts of aluminium oxide and...
,
Al2O3Aluminium oxide is an amphoteric oxide of aluminium with the chemical formula
23. It is also commonly referred to as alumina, corundum, sapphire, ruby or aloxite in the mining, ceramic and materials science communities. It is produced by the Bayer process from bauxite...
,
Na2OSodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses. Treatment with water affords sodium hydroxide.The alkali metal oxides M2O crystallise in the antifluorite structure...
, high K
2O/Al
2O
3, a relatively high
MgOMagnesium oxide, or magnesia, is a white solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of . It is formed by an ionic bond between one magnesium and one oxygen atom. Magnesium oxide is hygroscopic in nature and care must be taken to...
content and extreme enrichment in incompatible elements.
Lamproites are geographically widespread yet are volumetrically insignificant. Unlike
kimberliteKimberlite 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....
s which are found exclusively in Archaean
cratonA 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, lamproites are found in terrains of varying age, ranging from Archaean in Western Australia, to Palaeozoic and Mesozoic in southern Spain. They are also widely varied in age, from
ProterozoicThe 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." The Proterozoic Eon extended from 2500 Ma to 542.0 ± 1.0 Ma , and is the most recent part of the old, informally named ‘Precambrian’...
to
PleistoceneThe Pleistocene is the epoch from 2.588 million to 12 000 years BP covering the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
, the youngest known example being 56,000 ± 5,000 years old.
Lamproite volcanology is varied, with both
diatremeA diatreme is a breccia-filled volcanic pipe that was formed by a gaseous explosion. Diatremes often breach the surface and produce a tuff cone, a filled relatively shallow crater known as a maar, or other volcanic pipes...
styles and
cinder coneAccording to the , Cinder Cone is the proper name of 1 cinder cone in Canada and 7 cinder cones in the United States:In Canada: Cinder Cone In the United States:...
or cone edifices known.
Petrology
Lamproites form from partially melted mantle at depths exceeding 150 km. The molten material is forced to the surface in
volcanic pipeVolcanic pipes are subterranean geological structures formed by the violent, supersonic eruption of deep-origin volcanoes. They are considered to be a type of diatreme. Volcanic pipes are composed of a deep, narrow cone of solidified magma , and are usually largely composed of one of two...
s, bringing with it
xenolithA 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 and diamonds from the
harzburgiticThe ultramafic igneous rock, harzburgite, is a variety of peridotite consisting mostly of the two minerals, olivine and low-calcium pyroxene ; it is named for occurrences in the Harz Mountains of Germany. It commonly contains a few percent chromium-rich spinel as an accessory mineral...
peridotiteA 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...
or
eclogiteEclogite is a coarse-grained mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...
mantle regions where diamond formation is stabilized.
Recent research and lead-lead
isotope geochemistryIsotope geochemistry is an aspect of geology based upon study of the relative and absolute concentrations of the elements and their isotopes in the Earth. Variations in the abundance of these isotopes, typically measured with an isotope ratio mass spectrometer or an accelerator mass spectrometer,...
has revealed that the source of lamproites may be transition zone melts of subducted
lithosphereThe lithosphere is the rigid outermost shell of a rocky planet.- Earth's lithosphere :...
which has become trapped at the base of the lithospheric mantle. This observation also reconciles the depth of melting with the peculiar geochemistry, which is most easily explained by melting of already felsic material under deep mantle conditions.
Mineralogy
The mineralogy of lamproites is controlled by their peculiar
geochemistryThe 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 and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and their...
, with a predominance of rare silica-deficient mineral species and rare, mantle-derived minerals predominating.
Minerals typical of lamproites include:
forsteriticForsterite is the magnesium rich end-member of the olivine solid solution series. Forsterite crystallizes in the orthorhombic system with cell parameters a 4.75 Å , b 10.20 Å and c 5.98 Å .Forsterite is associated with igneous and metamorphic rocks and has also been found in meteorites...
olivine; high
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
leuciteLeucite is a rock-forming mineral composed of potassium and aluminium tectosilicate K[AlSi2O6]. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in 1821, they are not optically isotropic, and are therefore pseudo-cubic...
;
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the “space age metal”, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color.Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other...
-rich
aluminiumAluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances....
-poor
phlogopitePhlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg
3AlSi
3O
102, or...
;
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
- and titanium-rich richterite; low aluminium
diopsideDiopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the...
; and iron-rich
sanidineSanidine is the high temperature form of potassium feldspar 4O8. Sanidine most typically occurs in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower...
. A variety of rare trace minerals occur. The rocks are high in potassium with 6 to 8%
potassium oxidePotassium oxide is a compound of potassium and oxygen. This pale yellow solid, the simplest oxide of potassium, is a rarely encountered, highly reactive species...
. High
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, 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 odourless, tasteless, and malleable...
and
nickelNickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. It is one of the four ferromagnetic elements at about room temperature, other three being iron, cobalt and gadolinium...
content is typical. The rocks commonly are altered to
talcTalc is a mineral composed of hydrated magnesium silicate with the chemical formula H
2Mg
34 or Mg
3Si
4O
102. In loose form, it is the widely used substance known as talcum powder...
with
carbonateIn chemistry, a carbonate is a salt or ester of carbonic acid, characterized by the presence of the carbonate ion, or a carbonate functional group O=C2....
or
serpentineThe serpentine group describes a group of common rock-forming hydrous magnesium iron phyllosilicate minerals; they may contain minor amounts of other elements including chromium, manganese, cobalt and nickel. In mineralogy and gemology, serpentine may refer to any of 20 varieties belonging to the...
,
chloriteThe chlorite ion is ClO2−. A chlorite is a compound that contains this group,with chlorine in oxidation state +3...
, and
magnetiteMagnetite is a ferrimagnetic mineral with chemical formula Fe
3O
4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name ferrous-ferric oxide...
.
ZeoliteZeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
s and
quartzQuartz is the second most abundant mineral in the Earth's continental crust . It is made up of a continuous framework of SiO
4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO
2.There are many different varieties of...
may also occur.
Lamproites are characterized by the presence of widely varying amounts (5-90 vol.%) of the following primary phases (Mitchell & Bergman, 1991):
- titanian (2-10 wt% TiO2), aluminum-poor (5-12 wt% Al2O3) phenocrystic phlogopite
- titanian (5-10 wt% TiO2) groundmass poikilitic
Poikilitic texture refers to crystals, typically phenocrysts, in an igneous rock which contain small grains of other minerals. The texture is most easily observed in petrographic thin sections....
"tetraferriphlogopite"
- titanian (3-5 wt% TiO2) potassium (4-6 wt% K2O) richterite
- forsteritic olivine
- aluminum-poor (2O3), sodium-poor (2O) diopside
- nonstoichiometric iron-rich (1-4 wt% Fe2O3) leucite, and
- iron-rich sanidine (typically 1-5 wt% Fe2O3)).
The presence of all the above phases is not required in order to classify a rock as a lamproite. Any one mineral may be dominant, and this, together with the two or three other major minerals present, suffices to determine the petrographic name.
The presence of the following minerals precludes a rock from being classified as a lamproite: primary plagioclase, melilite, monticellite, kalsilite, nepheline, Na-rich alkali feldspar, sodalite, nosean, hauyne, melanite, schorlomite or kimzeyite.
Geochemistry
Lamproites conform to the following chemical characteristics:
- molar K2O/Na2O > 3, i.e., ultrapotassic
Ultrapotassic igneous rocks are a class of rare, volumetrically minor generally ultramafic or mafic silica-depleted igneous rocks.Ultrapotassic rocks are defined by molar K2O/Na2O >3 in much of the scientific literature. In other papers written as recently as 2005, they are...
- molar K2O/Al2O3> 6.8 and commonly > 1
- molar K2O + Na2O/ Al2O3 typically > 1 i.e., peralkaline
- typically <10 wt% each of FeO and CaO, TiO2 1-7 wt%, >2000 and commonly >5000 ppm Ba, >500 ppm Zr, >1000 ppm Sr and >200 ppm La.
Economic importance
The economic significance of lamproite became known with the 1979 discovery of the
Argyle diamond pipeThe Argyle diamond mine is a diamond mine located in the Kimberley region of Western Australia. The Argyle mine is the largest diamond producer in the world by volume, although due to the low proportion of gem-quality diamonds, is not the leader by value...
in
Western AustraliaWestern Australia is a state of Australia, occupying the entire western third of the Australian continent. Australia's largest state and the second largest subnational entity in the world, it has 2.2 million inhabitants , 85% of whom live in the south-west corner of the state.The state's capital...
. This discovery led to the intense study and re-evaluation of other known lamproite occurrences worldwide; previously only
kimberliteKimberlite 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 were considered economically viable sources of
diamondIn mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is the second most stable form of carbon, after graphite; however, the conversion rate from diamond to graphite is...
s.
The Argyle diamond mine remains the only economically viable source of lamproite diamonds. This deposit differs markedly by having a high content of diamonds but low quality of most of stones. Research at Argyle diamond have shown that most of stones are of E-type, they originate from
eclogiteEclogite is a coarse-grained mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...
source rocks and were formed under high temperature ~. The Argyle diamond mine is the main source of rare pink diamonds.
OlivineThe mineral olivine is a magnesium iron silicate with the formula
2SiO
4...
lamproite pyroclastic rocks and
dikesA dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
are sometimes hosts for
diamondIn mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is the second most stable form of carbon, after graphite; however, the conversion rate from diamond to graphite is...
s. The diamonds occur as xenocrysts that have been carried to the surface or to shallow depths by the lamproite
diapirA diapir is a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks...
ic
intrusionAn intrusion is any formation of intrusive igneous rock; rock formed from magma that cools and solidifies within the crust of a planet. In contrast, an extrusion consists of extrusive rock; rock formed above the surface of the crust....
s.
The diamonds of Crater of Diamonds State Park near
Murfreesboro, ArkansasMurfreesboro is a city in Pike County, Arkansas, United States. The population was 1,764 at the 2000 census. The city is the county seat of Pike County....
are found in a lamproite host.
Nomenclature
Lamproites, as a group, were known by a variety of localised names due to the fact their mineralogy is quite variable and because of their rarity often few examples of the following lamproite variants were known. Modern terminology classes all as lamproites but modifies this term with the mineral abundances as per the standard IUGS rules.
| Historic |
Modern |
| Wyomingite |
diopside-leucite-phlogopite lamproite |
| Orendite |
diopside-sanidine-phiogopite lamproite |
| Madupite |
diopside madupitic lamproite |
| Cedricite |
diopside-leucite lamproite |
| Mamilite |
leucite-richterite lamproite |
| Wolgidite |
diopside-leucite-richterite madupitic lamproite |
| Fitzroyite |
leucite-phlogopite lamproite |
| Verite |
hyalo-olivine-diopside-phlogopite lamproit |
| Jumillite |
olivine-diopside-richterite madupitic lamproite |
| Fortunite |
hyalo-enstatite-phlogopite lamproite |
| Cancalite |
enstatite-sanidine-phlogopite lamproite |
External links