Dunite

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Dunite tuber

Dunite is a relatively rare, extremely impoverished ultramafic mantle rock from the group of peridotites .

Etymology and type locality

Dunit was named by the Austrian geologist Ferdinand von Hochstetter in 1864 after the type locality Dun Mountain near Nelson in New Zealand . Hochstetter wrote: "Only on Dun Mountain itself is there a very striking rock for which a special name seems justified, and which I therefore call Dunite." The mountain range of Dun Mountain is part of an ultramafic belt caused by the Alpine Fault is offset by 600 km to the right .

Mineral inventory

Triangle diagram of olivine-orthopyroxene-clinopyroxene with the dunite field highlighted in green

By definition, the mostly coarse-grained (grain size 0.3 - 38 mm) phaneritic dunites consist of at least 90 percent by volume of the island silicate olivine , in contrast to the other peridotites, which have only 40 to 90 percent by volume of olivine. Fine-grained variants are also known, however. Due to their predominance of olivine, dunites can be regarded as almost monomineral rocks. The olivine has a forsterite content that is generally between 89 and 93% and mostly exceeds 90% (Fo 90 and higher). Minimum values ​​can go down to 82%. Restitische dunites are magnesium -rich and run Fo 93-95 , Kumulatdunite however, accumulate iron in their Olivine have thus Fo 87-90 .

Other characteristic secondary components are ortho- and clinopyroxene (often chromium diopside ) with 5 to 1 percent by volume and the oxides chromite , ilmenite and rutile . Orthopyroxene and clinopyroxene have quite high magnesium numbers Mg # of 0.9 to 0.93. Occasionally there are also interstitial plagioclase ( labradorite ), the garnet chromium pyrope and spinel (chromium spinel). Chromium spinel can occur interstitially or as poikilitic inclusions and in rare cases can reach up to 4% modally. Forsterite crystals up to 15 cm in size were found in some dunites .

In dunite bulbs there is sometimes interstitial glass , which was apparently injected from the surrounding volcanic rock. Traces of water can also be present in dunites, mostly in the form of amphibole ( pargasite ), phlogopite or as chlorite , tremolite or talc . Gas inclusions in olivines can also be observed, which is mostly carbon dioxide .

Due to its mineral content , dunite has a grass-green color, and due to the intensive weathering of olivine with subsequent limonitization in the air, the rock takes on an ocher-yellow to red-brownish color.

The dunite of the Great Dyke in Zimbabwe is an example of the modal mineral inventory : it contains 93 percent by volume olivine, 3 percent by volume orthopyroxene, 3 percent by volume of chromite and chromium spinel and 1 percent by volume of iron-titanium ores.

structure

Dunites show porphyroclastic and granular structures . The porphyroclastic, relatively coarse-grained (grain size 0.5 - 13 mm) olivines are surrounded in the former by much finer-grained (grain size 0.025 - 1 mm), newly formed olivine neoblasts and occasional oxide residues. The porphyroclasts are mostly penetrated by fissure cracks, but the neoblasts are fresh and unchanged. With the exception of orthopyroxes, all other accessories have crystallized as amoeboids either at olivine grain boundaries or at structural breakpoints . The grainy structures are uniform and have a mosaic texture . Very often tectonically heavily stressed structures are found. The olivine porphyroclasts then show fractures and deformation lamellae .

Serpentinization

The serpentinization can proceed well along the cracks in the olivine porphyroclasts . Neoblasts are not spared from it either, they are attacked from their grain boundaries. Within the newly formed serpentine (often antigorite ), sulfide minerals and magnetite are formed at the same time .

Chemical composition

To illustrate the average chemical composition of dunites based on 93 analyzes, the composition of the Great Dyke, an amphibole dunite from Zabargad, and the normalization of the average values :

Chemical composition of dunites in% by weight
oxide average Great dyke Zabargad CIPW standard percent
SiO 2 41.04 39.12 41.99 Q 0
TiO 2 0.10 0.06 C. 0.80
Al 2 O 3 1.95 1.77 1.53 Or 0.47
Fe 2 O 3 3.85 0.22 From 1.69
FeO 10.05 10.21 8.50 On 1.17
MnO 0.76 0.16 Hy 14.48
MgO 40.66 45.88 41.92 Oil 67.38
CaO 1.08 0.96 3.39 Mt 5.20
Na 2 O 0.21 0.09 0.27 Il 0.18
K 2 O 0.09 0.16 Ap 0.47
P 2 O 5 0.21 0.68
Mg # 0.87 0.91 0.90

Dunites are therefore extremely olivine-normative, quartz- undersaturated rocks. Moreover, they are hypersthene - and slightly corundum - normative.

Your magnesium number Mg # with values ​​around 0.9 is very high. The chromium number Cr # is very variable and can range from 0.2 to 0.95. Ophiolite dunites have a Cr # of 0.2 to 0.9, whereas cumulate dunites, for example from Hawaii, have a much narrower range of 0.4 to 0.6. Cumulative units of the Magmatic Greater Provinces (e.g. Karoo) record 0.65 to 0.95. The highest values, however, estimate xenolite bulbs from mantle units with 0.95 to almost 1.

The values ​​of NiO are generally very high and vary between 0.15 and 0.40 percent by weight. In this case too, the jacket units again achieve the highest values ​​(around 0.4% by weight).

Alteration

When ascending to shallow crustal areas, dunites undergo a retrograde metamorphosis ; near the surface their mineral inventory is thermodynamically unstable and the rocks serpentinize . Once they reach the surface of the earth, their mafic minerals weather very quickly.

Occurrence

Together with other peridotites, dunite is a typical rock of the Earth's upper mantle and is stable up to a depth of 400 kilometers. Dunite bulbs, which in the course of volcanic activity transported to the earth's surface as xenolites and z. B. as volcanic bombs (also known as olivine bombs ) are found in Germany in the Vulkaneifel and in the Kaiserstuhl . Host rocks here are especially members of the alkali basalt - basanite - nephelinite series, whose magmas are very rich in dissolved gases .

Dunite rocks are also associated with autopsied material in the area of subduction zones (for example in the central Troodos Mountains in Cyprus ). They are also present in accretion wedges (e.g. lower levels of the quarries near Kraubath an der Mur , Styria). Dunites are also found in alpine peridotite massifs, which were incorporated into the orogen as part of mantle chips during the continental collision.

Occur

Dunites are usually found in layers or layers (accumulation layers) and are associated with other mantle peridotites and ultramafites, usually Harzburgite and Lherzolite , but associations with Wehrlite , websterite , chromitite , gabbro , basalt and serpentinite melange are also known. Dunite can also form ducts , for example in Harzburgite, which are usually folded . In the event of further tectonic stress, the rock is sheared and is then present as shear lenses ( phacoids ) in the host rock.

Emergence

Volcanic bomb made of black basanite with an enclosed dunite fragment

There are several explanations for the formation of dunites:

  • They can be seen as restitic rocks , which were left behind in the upper mantle when basaltic melts were secreted . The partial melting rate was very high here. An example of this are basal dunites in ophiolite sequences. These residual dunites are recognizable by their very high magnesium number, chromium-rich olivine, chromium-rich spinel and magnesium-rich pyroxene. They are very poor in calcium and aluminum-rich phases.

With kimberlites to light a series of sponsored xenoliths show Granatlherzolith over Granatharzburgit and harzburgite towards dunites. An increasing depletion of the elements Al, Ca, Ti, Na and K as well as a steady increase in the magnesium number and the chromium number can be observed.

  • Another formation possibility is the action of silicate melts on lherzolites and Harzburgites of the upper mantle, whereby orthopyroxenes of the parent rock are leached and consequently residual rock gradually enriched in olivine remains.
  • The cumulative secretion in basaltic or picritic magma chambers can also be used as a model for the formation of dunites. Newly formed olivine crystals sink gravitationally to the bottom of the magma chamber. There arise such powerful layers of Kumulatduniten in so-called layered intrusions ( engl. Intrusion layered , with which Kumulatlagen of) wehrlite , Olivinpyroxenit , harzburgite and even chromitite are associated. Cumulative dunites occur in association with wehrlite, pyroxenite and gabbro in ophiolites.

This includes the quite iron-rich cumulative dunites, which, after metasomatic change in the starting mantle rocks in question, are found under igneous large provinces such as B. formed the Karoo of the Kaapvaal Kraton .

Partial melting and accumulation of deposits often lead to a succession of dunite at the base with resin burgite on top and finally orthopyroxenite. In contrast to residualdunites, cumulativedunites can show olivines and pyroxenes that are lower in magnesium and spinels that are lower in chromium than the resin burgites and orthopyroxenites that follow. The cryptic storage in the Bushveld and Stillwater complex may serve as an example.

  • A somewhat different variant sees dunite as a secretion product of picrites or tholeiites in volcanic magma chambers. As an example serves Ko'olau - shield volcano on O'ahu has promoted in its central region Dunitbomben of relatively small depth (Bottom, oceanic crust or top mantle).

use

Due to their local accumulation of heavy metals , especially the platinum group and platinum metals , and their increased concentrations of chromite and magnetite, dunites are valuable raw material carriers for chromium , gold , iridium , nickel , osmium , palladium , platinum , rhodium and ruthenium .

References

At spreading centers :

For transform faults and associated fracture zones :

In alpine peridotites ( alpine orogenesis context):

In layered intrusions:

In ophiolites (subduction context):

Rift Association :

As inclusions in volcanic bombs:

In ultramafites in general:

literature

  • Gregor Markl: Minerals and Rock. Qualities, education, investigation. S. 78. Elsevier, Munich 2004. ISBN 3-8274-1495-4

Web links

Individual evidence

  1. ^ F. Hochstetter: Geology of New Zealand. Contributions to the geology of the provinces of Auckland and Nelson . Vienna 1864, p. 218.
  2. ^ FJ Loewinson-Lessing, EA Struve: Petrografitscheski Slowar . Moskwa 1937, pp. 122-123.
  3. ↑ Sen, G. and Presnall, DC: Petrogenesis of Dunite Xenoliths from Koolau Volcano, Oahu, Hawaii: Implications for Hawaiian Volcanism . In: Journal of Petrology . 27, Part 1, 1986, pp. 197-217 .
  4. ↑ Gurney, JJ and Harte, B .: Chemical variations in upper mantle nodules from southern Africa kimberlites . In: Philosophical transactions of the Royal Society of London . A297, 1980, p. 273-293 .
  5. ^ Worst, BG: Differentiation and structure of the Great Dyke of Rhodesia . In: Transact. geol. Soc. South Africa . tape 61 , 1958, pp. 283-354 .
  6. ^ Schmidt, G. et al .: Are highly siderophile elements PGE, Re and Au fractionated in the upper mantle of the earth? New results on peridotites from Zabargad . In: Chemical Geology . tape 163 , 2000, pp. 167-188 .
  7. ↑ Best, MG and Christiansen, EH: Igneous Petrology . Blackwell Science, 2001, ISBN 0-86542-541-8 .
  8. ^ Wilson, M .: Igneous Petrogenesis . Chapman & Hall, 1989, ISBN 0-412-53310-3 .
  9. ↑ Rehfeldt, T. et al .: Fe-rich Dunite Xenoliths from South African Kimberlites: Cumulates from Karoo Flood Basalts . In: Journal of Petrology . 48, Number 7, 2007, pp. 1387-1409 .
  10. ↑ Eales, HV and Cawthorn, RG: The Bushveld Complex . In: Cawthorn, RG (Ed.): Layered Intrusions . Elsevier Science, Amsterdam 1996, pp. 181-229 .
  11. ^ Jackson, ED: Primary Textures and Mineral Associations in the Ultramafic Zone of the Stillwater Complex . In: US Geological Survey Professional Paper # 358 . 1961.
  12. ↑ Suhr, G .: Upper mantle peridotites in the Bay of Islands Ophiolite, Newfoundland: formation during the final stages of a spreading center? In: Tectonophysics . tape 206 , 1992, pp. 31-53 .
  13. ↑ Miller, RB and Mogk, DW: Ultramafic rocks of a fracture-zone ophiolite, north Cascades, Washington . In: Tectonophysics . tape 142 , 1987, pp. 261-289 , doi : 10.1016 / 0040-1951 (87) 90127-2 .
  14. ^ Kruhl, HJ and Voll, G .: Deformation and metamorphism of the western Finero Complex . In: Mem. Sci. Geol. Band 33 , 1979, pp. 85-109 .
  15. ^ Sarwar, G .: Tectonic setting of the Bela Ophiolites, southern Pakistan . In: Tectonophysics . tape 207 , 1992, pp. 359-381 .
  16. ↑ Dubois-Côté, V. et al .: Petrological and geochemical evidence for the origin of the Yarlung Zangbo ophiolites, southern Tibet . In: Chemical Geology . tape 214 , 2005, pp. 265–286 , doi : 10.1016 / j.chemgeo.2004.10.004 .
  17. ↑ Nasir, S .: The lithosphere beneath the northwestern part of the Arabian plate (Jordan): evidence from xenoliths & geophysics . In: Tectonophysics . tape 201 , 1992, pp. 357-370 .
  18. ^ McGuire, AV: The mantle beneath the Red Sea margin: xenoliths from western Saudi Arabia . In: Tectonophysics . tape 150 , 1988, pp. 101-119 .
  19. ↑ Helz, RT: Diverse olivine populations in lavas of the 1959 eruption of Kilauea volcano, Hawaii . In: EOS Trans. Am. geophys Un. tape 64 , 1983, pp. 900 .