Lamproit

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Photo of a Lamproit handpiece

Lamproites are porphyry dark gray rocks that belong to Group I of the ultrapotassic rocks . Their place of origin is the upper mantle . In terms of their chemical composition, they have low contents of Al 2 O 3 , CaO and Na 2 O , their MgO contents are relatively high. They have a high K 2 O / Al 2 O 3 ratio and extremely enriched incompatible elements . Several subspecies can be distinguished on the basis of their mineralogical composition.

etymology

Lamproit is derived from the Greek λαμπρός, bright, shiny '. This property relates to the reflectivity of the mica mineral phlogopite, which is characteristic of these rocks .

Occur

Lamproites form volcanic deposits ( ash cones , pyroclastites ) or, at shallow depths, solidified sub- volcanic volcanic vents , diatrems , tunnels and tunnels . Closely related rocks are kimberlite , orangite and lamprophyr .

Lamproites usually weather to talc and carbonates or serpentine , chlorite and magnetite . Also, quartz and zeolites can decomposes in.

Occurrence

Lamproites are spatially widespread, but their volume is practically insignificant. In contrast to kimberlites, which practically only occur on cratons of the Archean , lamproites can be detected from the Archean in all time stages ( Proterozoic , Paleozoic , Mesozoic and Tertiary ). The youngest known occurrence is from the Pleistocene and is 56,000 ± 5,000 years old.

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Circle Mesa in the southeast of the Lamproitic Leucite Hills : 6 eroded ash cones made of Wyomingite tuff

Mineralogical composition

According to Mitchell and Bergman (1991), the presence of the following mineral phases as main constituents is used to classify lamproites, which can, however, be subject to large fluctuations in their respective volume percentages (5 to 90 percent by volume):

  • Characteristic for Lamproite is a mostly clear proportion of titanium-rich (2 to 10 percent by weight TiO 2 ) and low -aluminum (5 to 12 percent by weight Al 2 O 3 ) phlogopite phe- nocrystals.
  • titanium-containing (5 to 10 percent by weight TiO 2 ) poikilitic tetraferriphlogopite occurs in the base mass
  • titanium-containing (3 to 5 percent by weight TiO 2 ) potassium- containing ( 4 to 6 percent by weight K 2 O) Richterite
  • Forsterite-rich olivine (Fo 87-94 )
  • low- aluminum (<1 percent by weight Al 2 O 3 ) and low-sodium (<1 percent by weight Na 2 O) diopside
  • non-stoichiometric, iron-rich leucite (1 to 4 percent by weight Fe 2 O 3 )
  • iron-rich sanidine (usually 1 to 5 percent by weight Fe 2 O 3 ).

However, the mineralogical composition varies within very wide limits, so that each of these minerals can modally predominate and others can be absent. Not all mineral phases have to be present to classify a rock as Lamproit. Each of the listed minerals can predominate and in combination with one or two other components this is completely sufficient for a correct petrographic name. Example: Leucite-Richterite-Lamproit has Richterite as the main component and Leucite as the secondary component.

In addition to apatite , chromite (Mg), enstatite , ilmenite , magnetite and titanite, trace minerals such as priderite , wadeit , perovskite , thorite , chayesite and zircon are found in smaller quantities . Very rarely are Shcherbakovit , armalcolite , Jeppeit , Perrierit - Chevkinit . In some lamproites there is also diamond , which can be viewed as a mantle xeno crystal.

Apart from phlogopite, the lamproite minerals have only a minimal chemical range of variation. However, their chemical composition is characteristic of the respective occurrences. Strangely, lamproite mica is quite poor in Ba, although the Ba content is very high.

Analcime (very often, replaces leucite and sanidine), carbonate , chlorite , zeolites and barite act as secondary minerals . Olivines are mostly pseudomorphosed by serpentine , iddingsite , carbonate or quartz. Fresh leucite is also rarely found - it is replaced pseudomorphically by sanidine, analcime, quartz, zeolite or carbonate.

The following minerals are incompatible with lamproites: primarily formed plagioclase , melilite , monticellite , kalsilite , nepheline , sodium-rich alkali feldspar , aluminum-rich augite , sodalite , nosean , hauyn , melanite , schorlomite and kimzeyite . All these minerals are characteristic of group II and group III of the ultrapotassic rocks.

The mineralogy of lamproites is dominated by their peculiar geochemical composition - with a predominance of rare silicon-undersaturated and rare minerals from the earth's mantle.

Difference to kimberlites

The differences compared to kimberlites can be summarized as follows:

  • Lamproite contain glass
  • Richterite rich in potassium occurs in the base mass
  • Lamproite mica is enriched in Ti, Fe and Na, but depleted in Al
  • Diopsids in the lamproic matrix are richer in titanium
  • Calcite is almost completely absent from lamproites

However, olivine-rich lamproites show similarities to the kimberlites of Group II.

Geochemical composition

criteria

Lamproites are subject to the following chemical criteria:

  • their molecular quotient K 2 O / Na 2 O> 3, they are thus ultrapotassic
  • their molecular quotient K 2 O / Al 2 O 3 > 0.8, mostly even> 1
  • their molecular quotient (K 2 O + Na 2 O) / Al 2 O 3 is typically> 1 and they are therefore per-alkaline.

The following concentrations are characteristic of Lamproite:

  • FeO and CaO: <10 percent by weight
  • TiO 2 : 1 to 7 percent by weight
  • Ba:> 2000 ppm and usually> 5000 ppm
  • Sr:> 1000 ppm
  • Zr:> 500 ppm
  • La:> 200 ppm.

Main elements

The analysis tables given are intended to clarify the geochemical composition of lamproites:

Oxide
wt.%		 Average
Lamproit		 Sisco-
Lamproit		 Orciatico-
orendite		 Montecatini-
Orendit		 Gaussberg Leucite Hills Smoky butte
SiO 2 53.30 56.74 56.90 58.50 52.20 52.70 53.50
TiO 2 3.00 2.27 1.42 1.37 3.50 2.40 5.60
Al 2 O 3 9.10 10.84 11.25 12.61 10.10 10.80 9.80
Fe 2 O 3 2.00 3.25 0.81
FeO 6.30 2.42 2.92 2.84 6.10 5.10 5.40
MnO 0.10 0.06 0.08 0.10 0.09 0.09 0.12
MgO 12.10 6.63 8.29 7.15 8.20 8.40 7.40
CaO 5.80 3.12 4.40 3.74 4.70 6.70 6.40
Na 2 O 1.40 1.02 1.36 1.20 1.70 1.30 1.50
K 2 O 7.20 10.73 7.68 7.91 11.90 10.40 7.40
P 2 O 5 1.30 0.67 0.70 0.92 1.50 1.50 1.70
LOI 2.70 2.09 3.16 2.43
Mg # 0.79 0.82 0.72 0.73 0.77 0.73
K / Na 3.38 6.90 3.72 4.33 4.61 5.26 3.25
K / Al 0.86 1.07 0.74 0.68 1.28 1.04 0.82
(Na + K) / Al 1.11 1.22 0.94 0.84 1.56 1.24 1.07

In the main elements, lamproites consist of around 45-55 (up to a maximum of 61) percent by weight of silicon dioxide and are therefore classified as mafic to intermediate . In addition, they have a very high potassium content of 6 to 8, occasionally up to 12 percent by weight of K 2 O ; they are therefore ultrapotassic and therefore belong to the group of alkaline rocks . It is characterized by a very high ratio of potassium to sodium . The iron , calcium and titanium levels can be significant.

Trace elements

Trace elements
ppm		 Average
Lamproit		 Sisco-
Lamproit		 Orciatico-
orendite		 Montecatini-
Orendit		 Gaussberg Leucite Hills Smoky butte
Cr 580 340 500 380 310 460 501
Ni 420 230 280 140 230 253 344
Zn 80 90 100
Rb 272 318 612 792 300 253 102
Sr 1530 640 577 421 1830 2840 3160
Zr 922 1040 749 491 1000 1440 1660
Ba 5120 1460 1400 1370 5550 6600 9810
Ce 400 367 352 206 420 427 774
Nd 207 146 193 133 150 166 304
Sm 24 19.1 26.9 23.5 19th 21st 36
Hf 39 32.1 21.4 13.4 64
Th 46 37.9 119 112 30th 6.5

High concentrations of chromium and nickel are characteristic of trace elements .

Isotope ratios

Neodymium-strontium isotope diagram with the position of various lamproites

The following initial ratios were determined for the radioisotopes of Sr, Nd and Pb:

Isotopes Sisco Orciatico Montecatini Leucite Hills Smoky butte
87 Sr / 86 Sr 0.71227 0.7160 0.71580 0.70563-0.70591 0.70587-0.70633
143 Nd / 144 Nd 0.512149 0.51210 0.51209 0.51188-0.51208 0.51128-0.51143
206 Pb / 204 Pb 18.786 18.697 18,624-18,670 17,273-17,583 16,025-16,146
207 Pb / 204 Pb 15,692 15,698 15,638-15,642 15,482-15,504 15,190-15,218
208 Pb / 204 Pb 39.181 39,062 38.947-38.965 37.280-37.523 36,195-36,680

In the isotope diagram 143 Nd / 144 Nd versus 87 Sr / 86 Sr, lamproites are located in the enriched quadrant, but show a very widely scattered distribution. In general, two trends can be identified: a steep trend depleted in 87 Sr / 86 Sr and a flat trend towards 87 Sr / 86 Sr-rich crustal components. The Orendite and Madupite of the Leucite Hills and the Lamproite of the Smoky Butte are on the steep trend, with Smoky Butte being extremely depleted in ϵ Nd and lead isotopes. The lamproites of Tuscany, Southeast Spain and Western Australia (West Kimberley) follow the flat trend. The Lamproit from Gaußberg in Antarctica occupies a middle position.

New names

Lamproite had a large number of historical rock names, which mostly referred to their respective type locality , but were of minimal informative value petrologically. These historical rock names have now been replaced by new names by the IUGS , which take into account the scheme of Mitchell and Bergman (1991) and take into account the actual mineral inventory:

Historical rock name Renaming
Wyomingite Diopside-leucite-phlogopite-lamproite
Orendit Diopside-sanidine-phlogopite-lamproite
Madupit Diopside Madupit Lamproit
Cedricite Diopside leucite lamproite
Mamilit Leucite-Richterite-Lamproit
Volgidite Diopside-Leucite-Richterite-Madupite-Lamproit
Fitzroyit Leucite-phlogopite-lamproite
Verit Hyalo-olivine-diopside-phlogopite-lamproit
Jumillit Olivine-Diopside-Richterite-Madupite-Lamproit
Fortunit Hyalo-enstatite-phlogopite-lamproite
Cancalit Enstatite-sanidine-phlogopite-lamproite

There are thus two basic types of lamproites: phlogopite lamproite and madupite lamproite . Madupit lamproites (or Madupitic lamproites) contain phlogopite in the matrix.

Emergence

Lamproites arise in the upper mantle as partial melts, whereby the depth of formation can be less than 150 kilometers. The melt rises to the surface in volcanic tubes. During its ascent, it can carry away diamonds and rock inclusions from the surrounding peridotitic mantle rock (mostly Harzburgite , but also eclogite , which stabilizes diamond formation).

The complex mineralogical and chemical compositions of lamproites compared to the ordinary igneous rocks, which can be easily classified in the IUGS system, can be explained by the very variable metasomatoses in their melting zone, by different depths and dimensions of the partial melting and by a highly advanced gastric differentiation.

Recent research results, in particular lead isotope geochemistry, indicate that lamproites may represent melts from the transition zone to the subducted lithosphere , which was wedged under the lithospheric mantle. This assumption brings the rather large melting depth and the peculiar geochemical composition of lamproites into harmony, ie the melting of rocks with still Felsic compositions but under the conditions of the deeper mantle.

Economic importance

Diamond from a Cretaceous Lamproit, Crater of Diamonds State Park , Arkansas

Diamond-bearing lamproites are an important source of diamond production. The economic importance of Lamproites was evident in 1979 with the discovery of the Argyle diamond mine in Kimberley , Western Australia . This led to a new investigation and reclassification of known lamproite deposits worldwide. Previously, only kimberlites were considered to be the economic starting point for diamond production.

As of now, the Argyle Diamond Mine is the only economical Lamproit-based diamond mine. Their diamond content is very high, but the majority of the stones found are of poor quality. Most stones belong to the E-type and originally come from eclogites. They were formed at very high temperatures around 1400 ° C. Pink stones are also very rare.

Diamonds are also occasionally found in pyroclastic deposits of lamproitic composition as well as in lamproitic dike rocks. The diamonds are present in them as foreign crystals (xenocrystals) and were carried by the lamproit intrusions to just below or directly to the surface.

Other occurrences of diamond-bearing lamproites include Prairie Creek in the Crater of Diamonds State Park near Murfreesboro in Arkansas , Majhgawan in India and Bobi-Segeula in the Ivory Coast .

References

The lamproic volcano Cabezo Negro de Zeneta near Murcia

literature

  • RH Mitchell (Ed.) 1996: Undersaturated alkaline rocks. Mineralogy, petrogenesis and economic potential. Mineralogical Association of Canada, Nepean 1996, ISBN 0-921294-24-7 ( Mineralogical Association of Canada - Short course series 24).

Individual evidence

  1. ↑ D. Velde, O. Medenbach, C. Wagner, W. Schreyer: Chayesite, K (Mg, Fe2 +) 4Fe3 + [Si12O30]: A new rock-forming silicate mineral of the osumilite group from the Moon Canyon (Utah) lamproite . In: American Mineralogist . tape 74 , 1989, pp. 1368–1373 ( minsocam.org [PDF; 489 kB ]).
  2. ↑ E. Alietti, MF Brigatti, p Capredi AND L. Poppi: The roedderite-chayesite series from Spanish lamproites: crystal-chemical characterization . In: Mineralogical Magazine . tape 58 , December 1994, p. 655–662 ( rruff.info [PDF; 556 kB ]).
  3. ^ Scott-Smith, B. and Skinner, EMW: Diamondiferous lamproites . In: Journal of Geology . tape 92 , 1984, pp. 433-438 .
  4. ^ Bergman, SC: Lamproites and other potassium-rich igneous rocks: a review of their occurrence, mineralogy and geochemistry . In: Fitton, JG and Upton, BGJ, Alkaline igneous rocks (eds.): Geol. Soc. Sp. Publ. Volume 30 , 1987, pp. 103-189 .
  5. ↑ Mitchell, RH and Bergman, SC: Petrology of Lamproites . Plenum Press, New York 1991, ISBN 0-306-43556-X .
  6. ^ Civetta, L. et al .: Eastwards migration of the Tuscan anatectic magmatism due to anticlockwise rotation of the Apennines . In: Nature . tape 276 . London 1978, p. 604-606 .