Lherzolite

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Spinel -lherzolite in the form of a so-called “olivine bomb”, a xenolite dragged from the upper mantle . The basalt surrounding it can be seen as a dark material on the upper edge of the handpiece. Site: Dreiser Weiher , Vulkaneifel .

Lherzolite is a relatively common, ultramafic , plutonic peridotite rock of deep green to black-green color. Lherzolites form a large part of the lithospheric mantle and the asthenosphere .

Etymology and type locality

The type locality

The lherzolite was scientifically described for the first time by Jean-Claude Delamétherie in 1795 . He had named the rock after its type locality , the Étang de Lers (also Lac de Lhers or in the old spelling: Étang de Lherz ) located near Massat ( Ariège department ) in the Pyrenees . The place name Lherz (or Lers or Ers) is derived from the Altokzitan orer or ers , the participle of the verb erzer - to build up, to erect. This is likely to mean the vulgar Latin ergere . What is obviously meant is the rocky slope that builds up steeply behind the lake.

definition

Brownish, weathered lherzolite in the type locality (ballpoint pen for size comparison)
Close-up of a fresh fracture surface of a spinel lherzolite from the type locality. The blackish spinel crystallites are clearly visible.

Lherzolites consist mainly of the minerals olivine (40–90% by volume) with at least 5% ortho- and clinopyroxene each . In the QAPF diagram , they fall into field 16 of the Ultramafitolite .

Mineral inventory

Besides the three main components

  • Olivine
  • Clinopyroxene
  • Orthopyroxene

The following phases occur depending on the prevailing pressure and temperature conditions :

  • Plagioclase (up to 30 kilometers deep)
  • Spinel (Al-Spinel, Cr-Spinel and Chromite - up to 55, maximum up to 70 kilometers depth)

and below 70 to 300 kilometers depth

These three minerals also determine the lherzolite subtypes plagioclase-lherzolite , spinel-lherzolite and garnet-lherzolite . The spinel-lherzolite facies can in turn be divided into two sub-types, namely the deeper ariegite sub-facies and the shallower Seiland sub-facies . All lherzolite types in their classic sequence are exposed in the Ronda peridotite .

The following can also occur:

The modal mineral inventory is illustrated using a lherzolite from the Ivrea zone :

  • Olivine: 51.7 percent by volume
  • Orthopyroxene: 32.0% by volume
  • Clinopyroxene: 13.9% by volume
  • Spinel: 2.4% by volume

Weathering and Alteration

Lherzolites weather on the earth's surface with reddish-orange-brown to ocher-yellow hues. Are formed here carbonates , quartz , different iron hydroxides or hematite . In the hydrothermal area (up to 400 ° C) the rocks serpentinate under the influence of carbon dioxide- containing waters, whereby magnesium is removed when water is absorbed. At the same time, the new formation of serpentine minerals leads to an increase in volume .

structure

Four types of structure can be distinguished in lherzolites :

  • protogranular structure (20%)
  • porphyroclastic structure (55%)
  • uniform grain structure (20%)
  • poikiloblastic structure (5%)

Protogranular structures are created by recrystallization , which probably takes place during partial melting (with the creation of basaltic magmas ) in the earth's mantle. The original structure is completely replaced. Protogranular structures consist of large, almost undeformed olivine grains with curvilinear grain boundaries and an indistinct spatial preferred direction. Spinels and clinopyroxenes form rounded inclusions in orthopyroxes. According to Green and Radcliffe (1972), the recrystallization may have occurred syn- or post-kinematically. Thereafter, a continued recovery process , a ( engl. Recovery ), recognizable by the small number of dislocations (engl. Dislocations ) or sub grain boundaries (engl. Subgrain boundaries ).

With continued plastic flow, this structure changes into a porphyroclastic structure . This characteristic structure of coarse-grained, very heavily stressed porphyroclasts and equally large, small-grained and practically undeformed neoblasts occurs most frequently in alpinotype peridotites . The neoblasts form a distinct foliation , the orientation of which deviates from the very clear porphyroclast orientation. Spinel usually occurs interstitially between the olivine grains, but can also be in contact with the pyroxenes. Relatively high-temperature deformations are indicated internally by intracrystalline sliding processes - the porphyroclasts have clear undersized grain boundaries with edge dislocations - as well as by syn- and post-tectonic recrystallization (neoblast formation). The flow processes take place here under shear .

The even-grain structure is created by complete recrystallization from the porphyroclastic structure. It is comparable to the structure of granulites in the lower crust and is likely to have arisen from very intensive flow processes. The very small grains appearing in polygonal aggregates are characteristic. Porphyroclasts can still be present in traces, but are usually completely absent. Even-grain structures can be divided into two sub-types - one with an irregular mosaic texture and one with a well- regulated tabular texture of the olivine grains. The time of recrystallization is not clear, but the tabular texture indicates a synkinematic origin followed by static heating. The recrystallization mechanism is assumed to be subgrain rotation .

Poikiloblastic structures are rare in lherzolites. In them, large pyroxe crystals enclose many small olivines. They possibly point to metasomatic processes.

Chemical composition

To illustrate the chemical composition of lherzolites, the following examples are selected (preceded by a global average value based on 179 analyzes, followed by average values ​​for spinel lherzolites and garnet lherzolites, then a chromite lherzolite, the spinel lherzolite from Baldissero from the Ivrea Zone ( Italy ), the spinel-lherzolite from Ronda (3 analyzes), the spinel-lherzolite fragments from Witim , ( Siberia - 7 analyzes), the garnet-lherzolite from Alpe Arami (6 analyzes) and finally the normalization of the average values):

Chemical composition of lherzolites in% by weight
oxide average Spinel Lherzolite Garnet Lherzolite Chromite Lherzolite Ivrea zone Ronda Vitim Alpe Arami CIPW standard percent
SiO 2 45.43 44.2 45.89 45.31 46.00 44.65 44.64 44.45 Q
TiO 2 0.45 0.13 0.09 0.11 0.09 0.33 0.14 0.15 C.
Al 2 O 3 4.39 2.05 1.57 0.43 3.2 5.30 2.83 1.50 Or 1.50
Fe 2 O 3 5.15 3.5 8.23 dead From 4.66
FeO 7.44 8.29 dead 6.91 dead 6.52 5.5 8.74 dead 8.34 dead On 7.99
MnO 0.17 0.13 0.11 0.09 0.16 0.13 0.13 0.12 Tuesday 13.54
MgO 30.31 42.21 43.46 46.03 37.5 34.75 40.68 39.84 Hy 21.48
CaO 5.68 1.92 1.16 0.56 3.2 4.62 2.18 2.18 Oil 36.31
Na 2 O 0.59 0.27 0.16 0.13 0.18 0.48 0.22 0.49 Mt 7.00
K 2 O 0.27 0.06 0.12 0.17 0.01 0.02 0.05 Il 0.79
P 2 O 5 0.12 0.03 0.04 0.04 0.04 0.01 Ap 0.26
H 2 O /
loss on ignition		 0.71 0.49 0.61 0.66 1.53 2.61
Mg # 0.841 0.918 0.933 0.939 0.902 0.876 0.897 0.914

Lherzolites are primarily olivine-normative, quartz- undersaturated rocks. They are also hypersthenic and diopside normative. Their SiO 2 content is in a relatively narrow range from 44 to 46 percent by weight. The MgO values, on the other hand, are much wider spread with a range of variation of 30 to 46 percent by weight.

The chemical composition of the lherzolite is particularly influenced by the melting processes within the shell . These lead to the depletion of the peridotite in so-called incompatible elements , which due to the charge or the radius of their ions do not fit into the crystal structure and therefore preferentially melt into the melt. So lherzolite z. B. partially melted by pressure relief in rift zones , whereby part of the mineral inventory is removed from the rock and thus a conversion into the heavily depleted Harzburgite takes place.

According to current theory, however, the lherzolite of the uppermost mantle is itself a depleted rock, the missing components of which had migrated to form the continental crust . It is assumed that reservoirs of the original pyrolytic rock (so-called primitive mantle ) still exist in the lower earth mantle .

Lherzolite in thin section

Lherzolite in thin section

Unevenly grained lherzolite of medium grain size (diameter 1 - 3 millimeters). The main components olivine, clinopyroxene, orthopyroxene and spinel are clearly visible in the thin section. The olivines differ from the pyroxenes in their clearer and more irregular cracks. Olivine and pyroxene are in a perfect state of equilibrium , recognizable by the triple points at which the crystals meet at an angle of 120 °. Spinel (dark brown, isotropic under crossed polarizers ) occurs interstitially.

Ditto under crossed polarizers

Occurrence

Lherzolites generally arise in the upper mantle and are stable up to a depth of about 300 kilometers (from this depth the transition to high pressure phases takes place).

The stability field of the quite rare plagioclase lherzolites extends to a depth of around 30 kilometers, which corresponds to a pressure of around 0.1 gigapascals and a temperature of 1300 ° C. Among the plagioclase lherzolites are the most common spinel lherzolites, which can be found up to 70 kilometers deep. Their maximum PT conditions are 0.2 GPa and 1450 ° C. Finally, below a depth of 70 kilometers, the rarer garnet lherzolites follow.

This mantle sequence, which is characteristic of our planet, is also assumed for the other terrestrial planets ( Mercury , Venus and Mars ) as well as the moon , whereby slight differences are to be expected depending on the respective mantle composition. So instead of the garnet lherzolite, Mars probably has a garnet wehrlite . Mercury (and probably also Mars) has the layer structure of spinel-bearing plagioclase weirlite => spinel-lherzolite => spinel-garnet-weirlite due to the lack of orthopyroxene.

In Ophiolithfolgen to lherzolite found in the footwall , harzburgites are here but most often. Lherzolites form part of alpine peridotite massifs. Occasionally they also appear on the fracture zones of the mid-ocean ridges . As xenolites , they can usually be found in the chimney fillings of the kimberlites . They can also be found as bombs in the ejecta of alkali basalt volcanoes .

Basaltic magmas are mainly formed by partial melting of lherzolites.

Extraterrestrial origin

Lherzolites or lherzolite shergottites have been discovered in several meteorites from the Antarctic . Their place of origin is attributed to the planet Mars . Including the meteorites of the Allan Hills A 77005, the Lewis Cliffs LEW 88516 and the Grove Mountains GRV 99027, as well as two of the Yamato Mountains YA 1075 and Y-793605. They contain 40 to 60 vol .-% olivine, poikilitic pigeonite and , to a lesser extent , plagioclase, chromite and titanomagnetite .

References

In alpine peridotites ( alpine orogenesis context):

In peridotites of Variscan Orogeny :

In flood basalts :

In rift zones (rift association):

In ophiolites (subduction context):

In volcanic bombs and xenolites:

See also

literature

Web links

Commons : Lherzolit  - collection of images, videos and audio files

Individual evidence

  1. ↑ Obata, M .: The Ronda peridotite - garnet-lherzolite, spinel-lherzolite, and plagioclase-lherzolite facies and the PT trajectories of a high-temperature mantle intrusion . In: Journal of Petrology . tape 21 , 1980, pp. 533-572 .
  2. ^ Lensch, G .: The ultra-mafitites of the Ivrea zone . In: Ann. Univ. Saraviensis . tape 9 , 1971, p. 6-146 .
  3. ↑ a b c Mercier, JC. and Nicolas, A .: Textures and Fabrics of Upper-Mantle Peridotites as Illustrated by Xenoliths from Basalts . In: Journal of Petrology . tape 16 , no. 2 , 1974, p. 454-487 .
  4. ↑ Green, HW and Radcliffe, SV: Deformation processes in the upper mantle . In: Heard, HC u. a .: Flow and Fracture of Rocks (Ed.): Geophysical Monograph Series . tape 16 , 1972, p. 139-156 .
  5. ↑ a b Poirier, J.-P. and Nicolas, A .: Deformation induced recrystallization due to progressive misorientation of subgrains, with special reference to mantle peridotites . In: Journal of Geology . tape 83 , 1975, pp. 707-720 .
  6. ↑ Best, MG and Christiansen, EH: Igneous Petrology . Blackwell Science, 2001, ISBN 0-86542-541-8 , pp. 458 .
  7. ↑ Maaløe, S. and Aoki, K .: The major element composition of the upper mantle estimated from the composition of lherzolites . In: Contrib. Mineral. Petrol. tape 63 , 1977, pp. 161-173 .
  8. ↑ Bodinier, J.-L. u. a .: Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain) . In: Journal of Petrology . tape 49 , no. 5 , 2008, p. 999-1025 , doi : 10.1093 / petrology / egn014 .
  9. ↑ Ernst, W. G. and Piccardo, G. B .: Petro Genesis of some Ligurian peridotite - I. Mineral and bulk-rock chemistry . In: Geochimica et Cosmochimica Acta . tape 43 , 1979, pp. 219-237 .
  10. ^ Morgan, JW and Anders, E .: Chemical composition of Earth, Venus, and Mercury . In: Proc. Nati. Acad. Sci. USA . tape 77 , no. 12 , 1980, pp. 6973-6977 .
  11. ↑ Goodrich, CA: Olivine-phyric martian basalts: a new type of shergottite . In: Meteoritics & Planetary Science . tape 37 , 2002, p. B31-B34 .
  12. ^ Evans, BW and Trommsdorf, V .: Petrogenesis of garnet lherzolite, Cima di Gagnone, Lepontine Alps . In: Earth and Planetary Science Letters . tape 40 , no. 3 , 1978, p. 333-348 .
  13. ↑ Lorand, J.-P., Schmidt, G., Palme, H. and Kratz, K.-L .: Highly siderophile element geochemistry of the Earth's mantle: new data for the Lanzo (Italy) and Ronda (Spain) orogenic peridotite bodies . In: Lithos . tape 53 , 2000, pp. 149-164 .
  14. ↑ Peters, T. and Stettler, A .: Time, physico-chemical conditions, mode of emplacement and geologic setting of the Totalp peridotite in the eastern Swiss Alps . In: Switzerland. Mineral. Petrogr. Mitt. Band 67 , 1987, pp. 285-294 .
  15. ^ Reisberg, LC, Allegre, CJ and Luck, J.-M .: The Re-Os systematics of the Ronda ultramafic complex of southern Spain . In: Earth Planet. Sci. Lett. tape 105 , 1991, pp. 196-213 .
  16. ↑ Becker, H .: Geochemistry of garnet peridotite massifs from lower Austria and the composition of deep lithosphere beneath a Paleozoic convergent plate margin . In: Chem. Geol. Volume 134 , 1996, pp. 49-65 .
  17. ↑ Girod, M., Dautria, JM and de Giovanni, R .: A First Insight into the Constitution of the Upper Mantle Under the Hoggar Area (Southern Algeria): The Lherzolite Xenoliths in the Alkali Basalts . In: Contrib. Mineral. Petrol. tape 77 , 1981, pp. 66-73 .
  18. ↑ Meisel, T., Walker, RJ, Irving, AJ and Lorand, J.-P .: Osmium isotopic compositions of mantle xenoliths: a global perspective. In: Geochim. Cosmochim. Acta . tape 65 , 2001, p. 1311-1323 .
  19. ↑ Varne, R .: On the Origin of Spinel Lherzolite Inclusions in Basaltic Rocks from Tasmania and Elsewhere . In: Journal of Petrology . tape 18 , no. 1 , 1977, pp. 1-23 , doi : 10.1093 / petrology / 18.1.1 .
  20. ^ Schmidt, G. and Snow, J .: Os isotopes in mantle xenoliths from the Eifel volcanic field and the Vogelsberg (Germany): age constraints on the lithospheric mantle . In: Contrib. Mineral. Petrol. tape 143 , 2002, p. 694-705 , doi : 10.1007 / s00410-002-0372-7 .
  21. ↑ Nkouandou, OF and Temdjim, R .: Petrology of spinel lherzolite xenoliths and host basaltic lava from Ngao Voglar volcano, Adamawa Massif (Cameroon Volcanic Line, West Africa): equilibrium conditions and mantle characteristics . In: Journal of Geosciences . tape 56 , 2011, p. 375-387 , doi : 10.3190 / jgeosci.108 .
  22. ^ Morgan, JW: Ultramafic xenoliths: clues to Earth's late accretionary history . In: J. Geophys. Res. 91 (B12), 1986, pp. 12375-12387 .
  23. ↑ Luffi, P. u. a .: Lithospheric mantle duplex beneath the central Mojave Desert revealed by xenoliths from Dish Hill, California . In: Journal of Geophysical Research . 114, B03202, 2009, p. 1-36 , doi : 10.1029 / 2008JB005906 .
  24. ↑ Rudnick, RL, Gao, S., Ling, W.-L., Liu, Y.-S. and McDonough, WF: Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton . In: Lithos . tape 77 , 2004, pp. 609-637 .