Etymology and 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.
Besides the three main components
- 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:
- Amphibole ( pargasite - can reach 5 to 10% by volume)
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 .
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.
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):
|oxide||average||Spinel Lherzolite||Garnet Lherzolite||Chromite Lherzolite||Ivrea zone||Ronda||Vitim||Alpe Arami||CIPW standard||percent|
|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|
|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
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
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.
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.
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 .
In alpine peridotites ( alpine orogenesis context):
- Albania - Tropoja Ophiolite
- France - Pyrenees
- Ivrea zone ( Baldissero , Balmuccia , Finero )
- Lepontine Alps (Lower Pennine Adula Nappe ) - Alpe Arami , Cima di Gagnone and Monte Duria (all pomegranate heartzolite)
- Liguria - Voltri group ( Erro-Tobbio unit ) and Monte Aiona , Monte Nero and Suvero in Eastern Liguria
- Sesia-Lanzo Zone (Central and Northern Lanzo Massif )
- Morocco - Beni Bousera peridotite
- Switzerland - Totalp peridotite
- Spain - Betic Cordillera - Ronda Peridotite
In peridotites of Variscan Orogeny :
- England - Lizard Complex
- Italy: Tonale ceiling , Ulten zone - Hochwart , Samerberg Alm
- Austria - Lower Austria - Moldanubikum , Gföhl ceiling
In flood basalts :
In rift zones (rift association):
- Egypt - Zabargad
- Russia - Siberia - Baikal Rift ( Vitim volcanic field )
- People's Republic of China - Jilin Province ( Wangqing Volcanic Field )
In ophiolites (subduction context):
- Tibet - Luobusha Ophiolite
- United States
In volcanic bombs and xenolites:
- Algeria - Hoggar ( Adrar n'Ajjer , Atakor , Eggéré , Tahalra )
- Brazil - Fernando de Noronha
- Italy - Sardinia ( Pozzomaggiore )
- Cameroon - Adamawa Massif ( Ngao Voglar Volcano )
- Comoros - Grande Comore
- Lesotho - Matsoku
- Mexico - Baja California ( San Quintin )
- Scotland - Caithness ( Duncansby Head )
- South Africa - Kimberley
- Tanzania - Labait
- United States
- People's Republic of China - North China Kraton
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