Lamproites are geographically widespread yet are volumetrically insignificant. Unlike
kimberlites which are found exclusively in Archaean cratons, lamproites are found in crust of varying age, ranging from Archaean examples in Western Australia, to Palaeozoic in southern Spain. They are also widely varied in age, from Proterozoicto Holocene, the youngest known example being 56,000 +/- 5,000 years old.
Lamproite volcanology is varied, with both
diatremestyles and cinder coneor cone edifices known.
Lamproites form from partially melted mantle at depths exceeding 150 km. The molten material is forced to the surface in
volcanic pipes, bringing with it xenoliths and diamonds from the harzburgitic peridotiteor eclogitemantle regions where diamond formation is stabilized.
Recent research and lead-lead
isotope geochemistryhas revealed that the source of lamproites may be transition zone melts of subducted lithospherewhich 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.
The mineralogy of lamproites is controlled by their peculiar
geochemistry, with a predominance of rare silica-deficient mineral species and rare, mantle-derived minerals predominating.
Minerals typical of lamproites include:
forsteritic olivine; high
iron leucite; titanium-rich aluminium-poor phlogopite; potassium- and titanium-rich richterite; low aluminium diopside; and iron-rich sanidine. A variety of rare trace minerals occur. The rocks are high in potassium with 6 to 8% potassium oxide. High chromiumand nickelcontent is typical. The rocks commonly are altered to talcwith carbonateor serpentine, chlorite, and magnetite. Zeolites and quartzmay 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
* 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.
Lamproites conform to the following chemical characteristics:
* molar K2O/Na2O > 3, i.e., ultrapotassic
* 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.
The economic significance of lamproite became known with the
1979discovery of the Argyle diamond pipe in Western Australia. This discovery led to the intense study and re-evaluation of other known lamproite occurrences worldwide; previously only kimberlitepipes were considered economically viable sources of diamonds.
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
eclogitesource rocks and were formed under high temperature ~1400 °C. The Argyle diamond mine is the main source of rare pink diamonds. Olivinelamproite pyroclasticrocks and dikes are sometimes hosts for diamonds. The diamonds occur as xenocrysts that have been carried to the surface or to shallow depths by the lamproite diapiric intrusions.
The diamonds of
Crater of Diamonds State Parknear Murfreesboro, Arkansasare found in a lamproite host.
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.
"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
Related rock types
Ultrapotassic igneous rocks
* [http://www.em.gov.bc.ca/Mining/Geolsurv/MetallicMinerals/MineralDepositProfiles/profiles/n03.htm LAMPROITE-HOSTED DIAMONDS] retrieved June 7, 2005
* [http://www.microscopy.fsu.edu/primer/techniques/polarized/gallery/pages/lamproitesmall.html Microscopic image of lamproite] retrieved June 7, 2005
* [http://volcano.und.nodak.edu/vwdocs/volc_images/australia/argyle/argyle.html Argyle pipe] retrieved June 7, 2005
* [http://www.geol.lsu.edu/henry/Geology3041/lectures/02IgneousClassify/IUGS-IgneousClassFlowChart.htm Igenous rock classification flowchart]
* Mitchell, R.H. & Bergman, S.C., 1991. "Petrology of Lamproites". Plenum Press, New York
* Murphy D.T., Collerson K.D., Kamber B.S., 2002. "Lamproites from Gaussberg, Antarctica: Possible Transition Zone Melts of Archaean Subducted Sediments", Jounal of Petrology, "'43", pp981-1001
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