Wehrlite

Wehrlite is a relatively rare, ultramafic and ultrabasic plutonic ( igneous rock ), which is counted among the peridotites.

• 6.1 Extra Terrestrial origin

Etymology

Wehrlite was named in 1838 by Franz von Hess Peter after Alois Wehrle which the rock was first investigated mineralogically.

Definition

Wehrlitic contain modal between 40 and 90 volume percent olivine. Their Klinopyroxengehalt varies between 5 and 60, but is usually 30 to 50 volume percent. Orthopyroxene occurs subordinate and a maximum of 5 percent by volume.

Mineral constituents

In addition to the essential olivine wehrlitic usually lead spinel ( chromium spinel ) and clinopyroxene ( diopside ), with spinel crystallizes before clinopyroxene with olivine as primary Kumulusphase (but not all wehrlitic contain spinel). Wehrlitic are essentially two-component rocks. Accessory you may also have:

• Orthopyroxene
• Oxides ( chromite, magnetite as an inclusion in pyroxene )
• Phlogopite
• Amphibole (usually pargasite and hornblende, sometimes Edenit )
• Plagioclase (rare)

From wehrlitischen sills titanomagnetite and titanite are known. Sporadically also carbonates, apatite and glass can be encountered.

As an example of the modal mineral composition Wehrlitknollen from Morocco (7 analyzes) may serve:

• Olivine: 62.5 volume percent
• Clinopyroxene: 24.5 volume percent
• Amphibole: 10 percent by volume
• Spinel: 3 volume percent
• Orthopyroxene; track

Serpentinization

Wehrlitic are very often serpentinized before such as in the Bay-of- Islands ophiolite in Newfoundland. The conversion reaction detected mainly olivine, clinopyroxene remains largely unchanged. Be formed anew case by means of water supply the minerals chrysotile, lizardite, brucite and accessory magnetite, which simultaneously increases the volume of rock by a third. The simplified reaction equation is:

• Forsterite water = > serpentine brucite

Depending on the iron content of olivine in addition magnetite is formed.

Structure

Originally formed structure in defense of Israel are porphyroklastisch ( with olivine porphyroclasts ). Olivine later, however, be partially replaced by neukristallisierte clinopyroxene, amphibole and spinel. The newly established structure are relatively coarse - grained and equal and are called matrix units ( engl.: matrix assemblage ) refers.

Wehrlitgefüge rarely exhibit clear signs of deformation microstructures, they testify to the contrary of purely static recrystallization and intense grain boundary migration (English grain boundary migration ). This gives rise to characteristic Polygonalgefüge with grain boundary angle about 120 °. The preferred spatial orientation (English preferred crystal orientation or CPO) of olivine and Klinopyroxenkristalle is therefore usually also weak. Consequently, the seismic anisotropy is low.

Chemical composition

The chemical composition of Defense of Israel is based on the following examples from Alaska ( a total of 5 analyzes), Madagascar (average of 9 analyzes), the Comoros (5 analyzes ) and the Massif Central französisischen illustrates:

Occurrence

Wehrlitic occur at a depth of 25 to 35 kilometers or slightly above the MOHO transition zone in the transition region from the upper mantle to the lower crust (English crust -mantle transition zone, transition zone or MTZ MOHO ). For in association formed in Subduktionskontext with oceanic crust wehrlitic is because of their water content (they are amphibole - bearing) but pulled a much smaller depth of just 6 to 18 km into consideration.

Wehrlitic are usually found in ophiolites associated with island arcs of subduction zones located below. For example, believed that the basis of island arcs of thick Kumulatlagen of amphibole - defense of Israel is formed. Some of these amphibole wehrlitic may also metasomatic origin. Also in connection with continental flood basalts occur wehrlitic. Volcanic Wehrliteinschlüsse in the form of bombs and xenoliths can be ejected.

Iron-rich wehrlitic are spread worldwide, magnesium - emphasized ( with Fo > 90), however, are less frequent.

Alkaline wehrlitic have formed sills in the mountains of Oman, which broke into a Triassic continental margin sequence and were then run over in the Upper Cretaceous of Samail - ophiolite.

For the mantle of Mars a garnet wehrlite leader is assumed.

Extra Terrestrial origin

Some meteorites could be classified as wehrlite ( for example, NWA 4797 ).

Occurrence

In the oceanic area wehrlitic are usually ply formed (as Wehrlitbänder ) and walk along the mantle -crust transition zone ( MTZ ) with Duniten a close association a. A little above the petrological MOHO they form intrusive bodies in the layered gabbros of the lower crust. Because of creep in the lower crust, the intrusive Wehrlitkörper can finally be boudiniert and sheared ( they are then as Boudin, shear lenses and Phakoide ago).

In the continental area Wehrlitbomben and xenoliths sometimes be supported by explosive volcanism to days. The more complex structured continental wehrlitic provide an insight into the often multi-stage evolution of the respective continental lower crust, characterized by repeated melting processes or metasomatic effects.

In the aforementioned warehouse aisles in Oman should again be pointed at this point.

Formation by Wehrlitisierung

In general, the development of military of Israel on the basis of high-temperature, magmatic melts or hot solutions is explained that, starting traverse from the upper asthenosphere, the porous and possibly delaminated, lower lithosphere and then impose upon within refractory Mantelperidotiten, they transpire part, and thus impregnate metasomatic.

This process is generally referred to as Wehrlitisierung. A characteristic feature of this process is the reactive dissolution of orthopyroxene in Mantelperidotiten under formation of secondary olivine and clinopyroxene. Harzburgites and lherzolites are converted into wehrlitic. As a simple example, the following response was given, which was initiated by a carbonatite (dolomite ) melt or hot solution:

Orthopyroxene dolomite => olivine clinopyroxene carbon dioxide

Falls below the reaction proceeds further, it comes under absorption of Klinopyroxens finally to complete Dunitisierung the mantle rock:

Clinopyroxene dolomite => olivine calcite carbon dioxide

The rise of the melting or hot solutions can be done in various dimensions:

• As channeled pore flow in Nanno, the micrometer to sub-millimeter
• By lithospheric veins in millimeters, centimeters and meters range
• Under formation of a broad-based Perkolationsfront in the kilometer range.

After the Perkolationsfront wanders through harzburgites and lherzolites, so-called Dunitkanäle remain from extremely refractory nature. These channels are lined with military of Israel, whose thermochemical erosion, however, is much lower than that of the dunites.

Since the penetrating melt or hot liquids can cover a broad chemical spectrum ( alkaline, of silica - undersaturated to carbonate melts and carbon dioxide - containing aqueous silicate solutions ), it also possess resulting wehrlitic therefore quite variable composition. Now even multi-stage metasomatic effects are used to explain for iron-rich wehrlitic. For magnesium-rich wehrlitic sodium leading carbonatites were blamed, which had risen from very deep mantle regions and had acted on Spinellperidotite.

In addition, also other Imprägnationsprozesse for the formation of magnesium-rich defense of Israel be targeted:

Impregnation

• By highly differentiated, aqueous, carbon dioxide - rich silicate melts, as it were, represent the final product after extensive flow through the pore spaces in the surrounding Mantelperidotit ( mantle wedge above subduction zones ).
• By carbonate - rich fluids that had separated themselves due to their immiscibility of residual gas containing silicate melts.
• By silicate melts with low ASIO2 that caused by their depletion of natural pyroxene crystallizing from clinopyroxene plus olivine aggregates.

A special case is the secondary development of wehrlite from dunite ( thus reversing the Dunitisierung ) which is indexed by poikiloblastische Klinopyroxenkristalle enclosing corroded Olivine. The Neukristallisation of clinopyroxene was here at the expense of the consumed olivine crystals of the pre-existent Dunits. Also speak relict Dunitinseln and lenses in wehrlite and its virtual lack of CPO For this interpretation. The trigger for this transformation must have been basaltic magma that flowed through the dunite under relatively low pressure and recrystallized - seen at the interstitial nature of the clinopyroxene, their high Aliv / Alvi ratio and its very high TiO2 content.

Localities

In alpine peridotites ( orogenic context):

• Italy - Lanzo peridotite
• Pakistan - Magma Phonetic arc of Kohistan ( Kohistan complex)

Generally in ultramafic rocks:

• Russia - Urals

In ophiolites:

• Albania - Mirdita Ophiolite
• Canada - Newfoundland - Bay-of- Islands Ophiolite
• Oman - Samail - ophiolite

In Subduktionskontext under island arcs:

• Japan Hokkaido - Horoman - peridotite
• Megata volcano ( Ichino - Megata ) on the Oka Peninsula ( amphibole wehrlite ) - Quaternary

• Alaska - Blashke Islands, Dall Iceland, Iceland Duke, Kane Peak, Red Bluff Bay, Union Bay - Lower Cretaceous ( Albian )
• California - Klamath Mountains ( Trinity ophiolite ) - Silurium / Lower Devonian

In intraplate area:

• Australia - Western Victoria ( amphibole wehrlite )

In sills:

• Oman - in continental margin sediments beneath the Samail - Ophioliths

Flood basalt context:

• Madagascar - Antampombato - Ambatovy complex - Upper Cretaceous ( Turonian / Coniacian )

In volcanic bombs and xenoliths:

• Brazil - Fernando de Noronha
• Germany Alkali basalts of the Hessian Depression
• Urach volcanic area - Miocene ( Tortonian )
• Vulkan Eifel - Baarley, Dreiser Weiher, Gees, Rockeskyller head - Quaternary

• Rapa
• Tahiti