Wehrlit

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

etymology

Wehrlit was named in 1838 by Franz von Kobell after Alois Wehrle , who first examined the rock mineralogically .

definition

Wehrlit is predominantly a mixture of olivine and clinopyroxene.

Wehrlite modal contain between 40 and 90 percent olivine by volume . Their clinopyroxene content varies between 5 and 60, but is usually around 30 to 50 percent by volume. Orthopyroxene occurs subordinately and is a maximum of 5 percent by volume.

Mineral inventory

In addition to the essential olivine, Wehrlites usually have spinel (chromium spinel) and clinopyroxene ( diopside ), whereby spinel crystallizes out before clinopyroxene together with olivine as the primary cumulus phase (but not all Wehrlites contain spinel). Wehrlites are essentially two-component rocks. The following can be added as an accessory:

From wehrlitischen sills are Titanomagnetit and titanite known. Carbonates , apatite and glass can also be found sporadically .

Wehrlite tubers from Morocco (7 analyzes) serve as an example of the modal mineral inventory :

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

Serpentinization

Wehrlite are very often serpentinized , for example in the Bay of Islands ophiolite in Newfoundland . The conversion reaction mainly includes olivine, clinopyroxene remains largely unchanged. The minerals chrysotile , lizardite , brucite and accessory magnetite are newly formed by adding water , which at the same time increases the rock volume by a third. The simplified reaction equation is:

Depending on the iron content of the olivine, magnetite is also formed.

structure

The structures originally formed in Wehrliten are porphyroclastic (with olivine porphyroclasts). However, olivine can later be partially replaced by recrystallized clinopyroxenes, amphiboles and spinels. The newly formed structure are relatively coarse and grainy equal and are used as matrix aggregates (engl .: matrix assemblage hereinafter).

Wehrlitgefüge rarely have clear signs of deformation microstructures on, but they testify to purely static recrystallization and intensive grain boundary migration (Engl. Grain boundary migration ). This creates characteristic polygonal structures with a grain boundary angle of 120 °. The preferred spatial orientation ( crystal preferred orientation or CPO ) of the olivine and clinopyroxe crystals is therefore usually only weak. Consequently, the seismic anisotropy is also low.

Chemical composition

The chemical composition of Wehrliten is illustrated by the following examples from Alaska (a total of 5 analyzes), Madagascar (average of 9 analyzes), the Comoros (5 analyzes) and the French Massif Central :

Chemical composition of Wehrliten in% by weight
oxide Blashke Islands (3) Kane Peak Union Bay Madagascar (9) Grande Comore Ray Pic
SiO 2 39.67 42.40 39.00 42.22 42.74 42.47
TiO 2 0.08 0.41 0.09 0.04
Al 2 O 3 0.41 0.44 0.16 1.70 1.36 1.48
Fe 2 O 3 11.51 dead 2.28 (10.89 dead) 4.84 (11.01 dead) 14.17 dead 8.81 dead
FeO 8.61 6.17 8.68 dead
MnO 0.10 0.19 0.20 0.22 0.17 0.13
MgO 44.40 38.05 41.70 32.74 43.38 43.22
CaO 0.99 5.63 2.34 5.83 2.21 3.09
Na 2 O 0.18 0.04 0.32 0.31
K 2 O 0.01 0.05 0.03
P 2 O 5 0.01 0.01 0.04
H 2 O or loss on ignition 9.54 1.69 5.37 2.67 1.01 0.23
Mg # 0.96 0.90 0.95 0.84 0.90 0.91

Occurrence

Wehr Lite arising 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 ( engl. Crust-mantle transition zone , MOHO transition zone or MTZ ). For weirlites formed in the context of subduction in association with oceanic crust , a much smaller depth of only 6 to 18 kilometers is considered because of their water content (they are amphibole- bearing).

Wehrlites are mostly found in ophiolites , which are associated with subduction zones below island arcs . It is assumed, for example, that the base of island arches is formed from thick layers of amphibole defense. Some of these amphibole weirlites may also be of metasomatic origin. Wehrlite also occur in connection with continental flood basalts . Wehrlit inclusions in the form of bombs and xenolites can be ejected volcanically .

Iron-rich Wehrlites are widespread worldwide, but magnesium- emphasized (with Fo > 90 ) are less common.

Alkaline weirlites formed camp tunnels in the mountains of Oman , which penetrated a Triassic continental margin and were then run over by the Samail ophiolite in the Upper Cretaceous .

For the mantle of Mars one is garnet leading wehrlite accepted.

Extraterrestrial origin

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

Occur

In the oceanic area, weirlites are mostly layered (as weirlit bands) and are closely associated with dunites along the mantle-crust transition zone (MTZ). Somewhat above the petrological MOHO, they form intrusive bodies in the layered gabbros of the lower crust. Due to creeping movements in the lower crust, the intrusive Wehrlit bodies can finally be boudinized and sheared (they are then in the form of boudins , shear lenses and phacoids ).

In the continental area, military lit bombs and xenolites are occasionally brought to light by explosive volcanism. The much more complex continental Wehrlite provide an insight into the often multi-stage evolution of the respective continental lower crust, characterized by repeated melting processes or metasomatic influences.

At this point, we would like to point out the already mentioned camps in Oman.

Formation through military litigation

In general, the formation of weirlites is explained on the basis of high-temperature, igneous melts or hot solutions , which, starting from the upper asthenosphere, cross the porous and possibly delaminated lower lithosphere and then penetrate within refractory mantle peridotites, partially seeping through them and thereby metasomatically impregnating them.

This process is commonly referred to as military litigation . This process is characterized by the reactive dissolution of orthopyroxes in mantle peridotites with the formation of secondary olivine and clinopyroxene. Harzburgite and Lherzolite are converted into Wehrlite. As a simple example, the following reaction is given, which was initiated by a carbonatitic ( dolomitic ) melt or hot solution:

Orthopyroxene + dolomite => olivine + clinopyroxene + carbon dioxide

If the reaction continues, the clinopyroxene is absorbed and the mantle rock is completely dunitized :

Clinopyroxene + dolomite => olivine + calcite + carbon dioxide

The rise of the melts or hot solutions can take place in different dimensions:

  • as a channeled pore flow in the nanometer, micrometer to submillimeter range
  • through lithospheric veins in the millimeter, centimeter and meter range
  • with the formation of a broad percolation front in the kilometer range.

After the percolation front has passed through Harzburgite and Lherzolite , so-called dunite channels of an extremely refractory nature remain . These channels are bordered by Wehrlites, but their thermochemical erosion is much less than that of the Dunite.

Since the penetrating melts or hot liquids can cover a broad chemical spectrum (from alkaline, undersaturated with silicic acid to carbonate melts as well as aqueous, silicate solutions containing carbon dioxide ), the resulting Wehrlite therefore also have a very variable composition. For iron-rich Wehrlite even multi-level metasomatic effects are now used to explain. Sodium- bearing carbonatites were held responsible for magnesium-rich Wehrlites , which had risen from very deep mantle areas and had an effect on spinel peridotites.

In addition, other impregnation processes for the formation of magnesium-rich Wehrlites are also targeted:

impregnation

  • through highly differentiated, aqueous, carbon dioxide-rich silicate melts , which, so to speak, represent the end product after extensive flow through the pore spaces in the surrounding mantle peridotite (mantle wedge above subduction zones).
  • by carbonate- rich liquids, which, due to their immiscibility, had separated from residual, gaseous, silicate melts.
  • through silicate melts with low a SiO2 , which caused the crystallization of clinopyroxene-plus-olivine aggregates by consuming orthopyroxene.

A special case is the secondary formation of Wehrlite from dunite (thus the reversal of dunitization), which is indicated by poikiloblastic clinopyroxene crystals that enclose corroded olivine. The recrystallization of clinopyroxene took place at the expense of the consumed olivine crystals of the pre-existing dunite. Relict dunite islands and lenses in the Wehrlit as well as its almost missing CPO also speak for this interpretation. The trigger for this transformation was probably basaltic magma , which flowed through the dunite under relatively low pressure and recrystallized - this is evident from the interstitial nature of the clinopyroxenes, their high Al IV / Al VI ratio and their very high TiO 2 content.

References

In alpine peridotites ( orogenesis context):

Generally in ultramafites:

In ophiolites:

In the subduction context under island arcs:

In the intraplate area:

In warehouse aisles:

  • Oman - in continental marginal sediments below the Samail ophiolite

Flood basalt context :

In volcanic bombs and xenolites:

See also

Individual evidence

  1. ^ Department of Mineralogy and Petrography . uni-miskolc.hu. Retrieved January 12, 2013.
  2. ^ Kobell, F. von: Grundzüge der Mineralogie . Schrag, Nuremberg 1838, p. 348 .
  3. ^ Komor, SC et al.: Serpentinization of cumulate ultramafic rocks from the North Arm Mountain massif of the Bay of Islands ophiolite . In: Geochimica et Cosmochimica Acta . tape 49 , no. 11 , 1985, pp. 2331-2338 .
  4. ↑ a b Himmelberg, GR and Loney, RA: Characteristics and Petrogenesis of Alaskan-Type Ultramafic-Mafic Intrusions, Southeastern Alaska . In: US Geological Survey professional paper: 1564 . 1995.
  5. ↑ Médard, E. et al .: Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts . In: Journal of Petrology . 47 Number 3, 2006, p. 481-504 , doi : 10.1093 / petrology / egi083 .
  6. ↑ Ionov, DA, Chanefo, I. and Bodinier, J.-L .: Origin of Fe-rich lherzolites and wehrlites from Tok, SE Siberia by reactive melt percolation in refractory mantle peridotites . In: Contributions to Mineralogy and Petrology . tape 150 , 2005, pp. 335-353 .
  7. ↑ Lippard, SJ: Petrology of alkali wehrlite sills in the Oman Mountains . In: Mineralogical Magazine . tape 48 , 1984, pp. 13-20 .
  8. ^ Morgan, JW and Anders, E .: Chemical composition of Mars . In: Geochimica et Cosmochimica Acta . tape 43 , no. 10 , 1979, p. 1601-1610 .
  9. ↑ NWA 4797 (PDF; 603 kB) curator.jsc.nasa.gov. Retrieved January 12, 2013.
  10. ↑ Coltorti, M., Bonadiman, C., Hinton, RW, Siena, F. and Upton, BGJ: Carbonatite metasomatism of the oceanic upper mantle: evidence from clinopyroxenes and glasses in ultramafic xenoliths of Grande Comore, Indian Ocean . In: Journal of Petrology . tape 40 , 1999, pp. 133-165 .
  11. ↑ Xu, Y. and a .: K-rich glass-bearing wehrlite xenoliths from Yitong, Northeastern China: petrological and chemical evidence for mantle metasomatism . In: Contributions to Mineralogy and Petrology . tape 125 , 1996, pp. 406-420 .
  12. ↑ Yaxley, GM, Crawford, AJ and Green, DH: Evidence for carbonatite metasomatism in spinel peridotite xenoliths from western Victoria, Australia . In: Earth and Planetary Science Letters . tape 107 , 1991, pp. 305-317 .
  13. ↑ Laurora, A. et al .: Metasomatism and melting in carbonated peridotite xenoliths from mantle wedge: The Gobernador Gregores case (Southern Patagonia) . In: Journal of Petrology . tape 42 , 2001, p. 69-87 .
  14. ↑ Zanetti, A., Mazzucchelli, M., Rivalenti, G. and Vannucci, R .: The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism . In: Contributions to Mineralogy and Petrology . tape 134 , 1999, pp. 107-122 .
  15. ↑ Zinngrebe, E. and Foley, SF: Metasomatism in mantle xenoliths from Gees, West Eifel, Germany: evidence from genesis of calcalkaline glasses and metasomatic Ca-enrichment . In: Contributions to Mineralogy and Petrology . tape 122 , 1995, pp. 79-96 .
  16. ↑ Tommasi, A. et al .: Seismic anisotropy and compositionally induced velocity anomalies in the lithosphere above mantle plumes: a petrological and microstructural study of mantle xenoliths from French Polynesia . In: Earth and Planetary Science Letters . tape 227 , 2004, pp. 539-556 .
  17. ↑ Piccardo, GB, Zanetti, A. and Müntener, O .: Melt / peridotite interaction in the Southern Lanzo peridotite: field, textural and geochemical evidence . In: Lithos . tape 94 , 2007, p. 181-209 .
  18. ↑ Clénet, H. et al .: Thick sections of layered ultramafic cumulates in the Oman ophiolite revealed by an airborne hyperspectral survey: Petrogenesis and relationship to mantle diapirism . In: Lithos . tape 114 , 2010, pp. 265-281 .
  19. ↑ Takazawa, E. et al .: Geochemical evidence for melt migration and reaction in the upper mantle . In: Nature . tape 359 , 1992, pp. 55-58 .
  20. ^ Aoki, KI: Petrology of mafic inclusions from Ichino-Megata, Japan . In: Contributions to Mineralogy and Petrology . tape 30 , 1971, p. 314-331 .
  21. ^ Richard, M .: Géologie et petrologie d'un jalon de l'arc Taïwan - Luzon: l'île de Batan (Philippines). Doctoral thesis at the Université de Bretagne Occidentale . Brest 1986, p. 351 .
  22. ^ Stremmel, K .: Geology and Petrology of the Mafic Plutons in the Trinity Ophiolite, California (USA). Inaugural dissertation at the University of Cologne . 2012, p. 412 .
  23. ^ O'Reilly, SY and Griffin, WL: Mantle metasomatism beneath western Victoria, Australia: I. Metasomatic processes in Cr-diopside lherzolites . In: Geochimica et Cosmochimica Acta . tape 52 , 1988, pp. 433-447 .
  24. ↑ Melluso, L. et al .: Geochronology and Petrogenesis of the Cretaceous Antampombato-Ambatovy Complex and Associated Dyke Swarm, Madagascar . In: Journal of Petrology . 46 Number 10, 2005, p. 1963-1996 , doi : 10.1093 / petrology / egi044 .
  25. ↑ Zangana, NA et al .: Geochemical variation in peridotite xenoliths and their constituent clinopyroxenes from Ray Pic, French Massif Central: implications for the composition of the shallow lithospheric mantle . In: Chemical Geology . tape 153 , 1999, pp. 11-35 .
  26. ↑ Raffone, N. et al .: Metasomatism in the Lithospheric Mantle beneath Middle Atlas (Morocco) and the Origin of Fe- and Mg-rich Wehrlites . In: Journal of Petrology . 50 Number 2, 2009, p. 197-249 , doi : 10.1093 / petrology / egn069 .