Lherzolite

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

Etymology and type locality

The lherzolite in 1795 first described by Jean -Claude Delamétherie scientifically. He had the rock after its type locality, the at Massat (department Ariège) in the Pyrenees located Etang de Lers (including Lac de Lhers or in the old spelling: Etang de Lherz ) named. The place name Lherz (or moth or Ers ) is ore or from the first Altokzitanischen, the participle of the verb erzer - build, build - derived. Behind it probably the vulgar Latin ergere should hide. What is meant is obviously the steeply behind the lake constructive cliff.

Definition

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

Mineral constituents

Apart from the three main components

  • Olivine
  • Clinopyroxene
  • Orthopyroxene

Are in addition as a function of the prevailing pressure and temperature conditions, the following phases:

  • Plagioclase (up to 30 km depth)
  • Spinel ( Al- spinel, Cr - spinel and chromite - to 55, maximum to 70 km depth)

And below 70 to 3wq < 00 km depth

  • Garnet ( pyrope ).

These three minerals also determine the lherzolite - plagioclase lherzolite subtypes, spinel lherzolite and garnet lherzolite -. The spinel lherzolite facies can be divided into two subtypes turn again, that in the deeper Ariegit subfacies and in the shallower Seiland subfacies. All Lherzolithtypen in their classic series are open in Ronda peridotite.

In addition, you may experience:

  • Amphibole ( pargasite - can be 5 to 10 vol % reach )
  • Mica biotite
  • Phlogopite ( occasionally up to 10 vol %)

The modal mineral composition is illustrated by a Lherzoliths from the Ivrea zone:

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

Weathering and alteration

Lherzolites weather at the surface with reddish- orange-brown to ocher yellow tones. It formed here carbonates, quartz, various iron hydroxides or hematite. In hydrothermal range (up to 400 ° C) serpentinisieren the rocks under the effect of water containing carbon dioxide, wherein magnesium is removed when water is absorbed. At the same time it is due to the formation of Serpentinminerale to an increase in volume.

Structure

In Lherzolithen four structural types can be distinguished:

  • Protogranulare structure (20%)
  • Porphyroklastische structure (55%)
  • Equal -grained structure (20%)
  • Poikiloblastische structure (5%)

Protogranulare structure caused by recrystallization, which is likely during partial melting ( basaltic magmas to produce ) in the mantle on. The original initial structure will be replaced completely. Protogranulare structure consisting of large, nearly undeformed Olivinkörnern with curvilinear grain boundaries and indistinct trained, spatial preferred direction. Spinels and clinopyroxene form rounded inclusions in orthopyroxene. The recrystallization is likely to be, according to Green and Radcliffe (1972 ) takes place syn or postkinematisch. Then began a recovery process ( engl. recovery), recognizable by the small number of dislocations (English dislocations ) or sub- grain boundaries (English subgrain boundaries ).

With continued plastic flow this structure turns into a porphyroklastisches structure. In alpinotype peridotites of this characteristic structure type occurs in coarse-grained, very heavily used porphyroclasts and equally large, small grain and virtually undeformed neoblasts most often. The neoblasts form a distinct foliation whose orientation differs from the very significant Porphyroklastenausrichtung. Spinel commonly occurs interstitially in between the Olivinkörnern, but can also be in contact with the pyroxenes. Relatively high temperature- deformation microstructure are displayed internally by intracrystalline sliding processes - so, the porphyroclasts distinct sub- grain boundaries with dislocations ( engl. edge dislocations ) on - and by syn -and post- tectonic recrystallization ( Neoblastenbildung ). The flow processes take place here under shear.

The equal -grained structure is formed by complete recrystallization of the porphyroklastischen structure. It is similar to the structure of granulites of the lower crust and is likely to be caused by very intensive flow processes. Characteristic are the very small, occurring in polygonal aggregates grains. Porphyroclasts may still be present in trace amounts, but are usually absent completely. DC -grained structure can be divided into two subtypes - an irregular mosaic texture and with well respected regelter Tabulartextur the olivine grains. The timing of recrystallization is not unique, but the Tabulartextur indicates a synkinematischen origin followed by static heating as Rekristallisationsmechanismus is undersize rotation (English subgrain rotation) accepted.

Poikiloblastische structure are rare in Lherzolithen. In these large pyroxene crystals enclose many small olivine. You might suggest to metasomatic processes.

Chemical composition

To illustrate the chemical composition of Lherzolithen following examples are singled out ( prefixed with a global average 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 Lherzolithauswürflinge of Vitim, ( Siberia - 7 analyzes), the garnet lherzolite of Alpe Arami (6 analyzes ), and finally the normalization of the averages):

Lherzolites are primarily olivine - normative, quartz - undersaturated rocks. Moreover, they are hypersthene and diopside - normative. Your SiO2 content moves within a relatively narrow range of 44 to 46 weight percent. The MgO values ​​, however, are substantially spread further comprising a variation from 30 to 46 weight percent.

The chemical composition of Lherzoliths is particularly influenced by internal coat melting processes. This lead to the depletion of the peridotite at so-called incompatible elements which do not fit due to the charge or the radius of its ions in the crystal structure and therefore able to form preferentially in melts. So lherzolite is at rift zones partially melted by pressure relief, for example, whereby a portion of the mineral content is removed from the rock and thus takes place a transformation in the heavily depleted harzburgite.

According to current theory is the uppermost mantle lherzolite but already own a depleted rock whose missing components had migrated to the structure of the continental crust. It is believed that in the lower mantle reservoir nor the original pyrolytic rocks ( so-called primitive mantle ) exist.

Lherzolite in thin section

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

Occurrence

Lherzolites generally occur in the upper mantle and are up to a depth of about 300 km stable ( Below this depth, the transition to high-pressure phase is done ).

The stability field of the quite rare plagioclase lherzolites extends to a depth of about 30 kilometers, equivalent to a pressure of about 0.1 Giga Pascal and a temperature of 1300 ° C. Under the plagioclase Lherzolithen are the most represented in spinel lherzolites, which are found up to 70 km depth. Your maximum PT conditions are 0.2 GPa and 1450 ° C. Below 70 km depth finally follow the rarer garnet lherzolites.

This characteristic feature of our planet mantle sequence is also assumed for the other terrestrial planets (Mercury, Venus and Mars ), and the moon, which, depending on the respective surface composition slight differences should be expected. Thus, the Mars instead of the garnet Lherzoliths has probably a garnet wehrlite. Mercury (and probably Mars) has because of Orthopyroxenmangel the management structure spinellführender plagioclase wehrlite => spinel lherzolite => spinel -garnet wehrlite.

In Ophiolithfolgen Lherzolite be found in the footwall, harzburgites are here but most frequently. Lherzolite form part of Alpine Peridotitmassive. Occasionally, they also occur at the fracture zones of mid-ocean ridges to days. As xenoliths are mostly to be found in the Schlotfüllingen the kimberlites. Furthermore, they find themselves as bombs in the ejecta of alkali basalt volcanoes.

Basaltic magmas are predominantly the result of partial melting of Lherzolithen.

Extra Terrestrial origin

In several meteorites from Antarctica lherzolites or lherzolithische shergottites were discovered. Your origin is attributed to the planet Mars. Among the meteorite Allan Hills 77005 A, the Lewis Cliffs LEW 88516 and the Grove Mountains GRV 99027, as well as two of the Yamato Mountains YA 1075 and Y- 793,605th They contain 40 to 60 vol % olivine, pigeonite poikilitischen and subordinate plagioclase, chromite and titanomagnetite.

Localities

In alpine peridotites ( Alpidic orogenic context):

  • Albania - Tropoja - ophiolite
  • France - The Pyrenees Western Pyrenees - Turon de Técouère
  • Central Pyrenees - Tuc d' Esse, Moncaup
  • Eastern Pyrenees - Etang de Lers (type locality), Fontête Rouge, Freychinède, Shem, Porteteny -Pic Couder, Bestiac, Caussou, Pic de Geral
  • Ivrea zone ( Baldissero, Balmuccia, Finero )
  • Lepontine Alps ( unterpenninische Adula ceiling) - Alpe Arami, Cima di Gagnone and Monte Duria (all Granatlherzolith )
  • Liguria - Voltri group ( Erro Tobbio unit) and Monte Aiona, Montenegro, as well Suvero in the East Ligurian Riviera
  • Sesia - Lanzo Zone (Central and Northern Lanzo massif )
  • Lower Engadine window ( Nauders )
  • Styria ( Kapfenberg )

In the Variscan orogeny peridotites:

  • England - Lizard complex
  • Italy: Tonale ceiling, Ultimo Zone - Hochwart, Samer mountain pasture
  • Austria - Lower Austria - Moldanubicum, Gföhl Blanket

In flood basalts:

In grave rift zones ( Riftassoziation ):

  • Egypt - Zabargad
  • Russia - Siberia - Baikal Rift ( Vitim volcanic field)
  • People's Republic of China - Jilin Province ( Wangqing volcanic field )

In ophiolites ( Subduktionskontext ):

  • Tibet - Luobusha - ophiolite
  • United States California - Klamath Mountains ( Trinity ophiolite ) - Silurium / Lower Devonian

In volcanic bombs and xenoliths:

  • Algeria - Hoggar ( Adrar n'Ajjer, Atakor, Eggéré, Tahalra )
  • Australia New South Wales
  • Queensland - Mount Quincan
  • Tasmania
  • Victoria
  • Vogelsberg
  • Vulkan Eifel - Dreiser Weiher
  • Alaska - Nunivak Iceland
  • Arizona - Cochise Crater, San Carlos
  • Hawaii - Salt Lake Crater
  • California - Mojave Desert ( Cima Volcanic Field, Dish Hill )
  • Navajo Volcanic Field
  • New Mexico - Kilbourne Hole, Puerco Necks
  • Rhode Iceland - Lamprophyrgang of Ashaway
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