Lamproite is anultrapotassicmantle-derivedvolcanic orsubvolcanic rock. It has lowCaO,Al2O3,Na2O, high K2O/Al2O3, a relatively highMgO content and extreme enrichment inincompatible elements.
Lamproites are geographically widespread yet are volumetrically insignificant. Unlikekimberlites, which are found exclusively inArchaeancratons, lamproites are found in terrains of varying age, ranging from Archaean in Western Australia, to Palaeozoic and Mesozoic in southern Spain. They also vary widely in age, fromProterozoic toPleistocene, the youngest known example fromGaussberg in Antarctica being 56,000 ± 5,000 years old.
Lamproite volcanology is varied, with bothdiatreme styles andcinder cone or cone edifices known.
Lamproites form from partially meltedmantle at depths exceeding 150 km. The molten material is forced to the surface involcanic pipes, bringing with itxenoliths anddiamonds from theharzburgiticperidotite oreclogite mantle regions where diamond formation is stabilized.
Recent research, for example on the lamproites atGaussberg inAntarctica, and lead-leadisotope geochemistry have revealed that the source of lamproites may be transition zone melts ofsubductedlithosphere which has become trapped at the base of thelithospheric mantle.[2] This observation also reconciles the depth of melting with the peculiar geochemistry, which is most easily explained by melting of alreadyfelsic material under deep mantle conditions.
The mineralogy of lamproites is controlled by their peculiargeochemistry, with a predominance of raresilica-deficient mineral species and rare, mantle-derived minerals predominating.
Minerals typical of lamproites include:forsteritic olivine; highironleucite;titanium-richaluminium-poorphlogopite;potassium- and titanium-richrichterite; low aluminiumdiopside; and iron-richsanidine. A variety of rare trace minerals occur. The rocks are high in potassium with 6 to 8%potassium oxide. Highchromium andnickel content is typical. The rocks commonly are altered totalc withcarbonate orserpentine,chlorite, andmagnetite.Zeolites andquartz may also occur.
Lamproites are characterized by the presence of widely varying amounts (5-90 vol.%) of the following primary phases (Mitchell & Bergman, 1991):[3]
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 thepetrographic name.
The presence of the following minerals precludes a rock from being classified as a lamproite: primaryplagioclase,melilite,monticellite,kalsilite,nepheline, Na-richalkali feldspar,sodalite,nosean,hauyne,melanite,schorlomite orkimzeyite.
Lamproites conform to the following chemical characteristics:
The economic significance of lamproite became known with the discovery ofEllendale E4 and E9 lamproite pipes and better known 1979 discovery of theArgyle diamond pipe inWestern Australia. This discovery led to the intense study and re-evaluation of other known lamproite occurrences worldwide; previously onlykimberlite pipes were considered economically viable sources ofdiamonds.
TheArgyle 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 stones. Research at Argyle diamond have shown that most stones are ofE-type; they originate fromeclogite source rocks and were formed under high temperature ~1,400 °C (2,600 °F). The Argyle diamond mine is the main source of rarepink diamonds.
Olivine lamproitepyroclastic rocks anddikes are sometimes hosts fordiamonds. The diamonds occur asxenocrysts that have been carried to the surface or to shallow depths by the lamproitediapiricintrusions.
The diamonds ofCrater of Diamonds State Park nearMurfreesboro, Arkansas are found in a lamproite host.
Lamproites, as a group, were known by a variety of localised names because 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 standardIUGS rules.
| Historic | Modern |
|---|---|
| Wyomingite | diopside-leucite-phlogopite lamproite |
| Orendite | diopside-sanidine-phlogopite 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 |
