Harzburgite

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Coarse-grained Harzburgite, Madagascar

Harzburgite is the name of an ultramafic , plutonic peridotite rock.

etymology

Harzburgite was first described petrographically by Karl Heinrich Rosenbusch in 1887 and named by him after the type locality in the Radau valley near Bad Harzburg in the Harz Mountains . The rock forms part of the Harzburg gabbro and is found as an accumulation in the lying area of this massif.

definition

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

Harzburgite mainly consists of olivine , which can be between 40 and a maximum of 90 percent by volume. Orthopyroxene is present at 5 to 60 percent by volume. Clinopyroxene is subordinate and reaches a maximum of 5 percent by volume.

Mineral inventory

In the fresh, unweathered state, the rock, which is deep dark green to black green, can contain, in addition to the defining minerals olivine, orthopyroxene and clinopyroxene, relatively small amounts of plagioclase , spinel , phlogopite , amphibole ( hornblende - pargasite ), as well as accessory apatite and ore minerals .

As an example of the modal mineral inventory, the Harzburgite of the type locality is given, which contains 43 volume percent olivine, 56.5 volume percent orthopyroxene, 0.5 volume percent plagioclase and traces of ore and apatite. Clinopyroxene is absent.

structure

In addition to the typical mantle rock structure (described in detail under Lherzolite ), the Harzburgite of the type locality shows, for example, a cumulative structure , whereby olivine is present as poikilitic inclusions in plagioclase and pyroxene ( poikilitic structure ).

Chemical composition

The chemical composition of Harzburgites is illustrated by the following examples: a global average value based on 206 analyzes, followed by Harzburgite of the type locality (2 analyzes) and the extremely depleted, Greenlandic Wiedemann Harzburgite. The CIPW norm of the average is also given.

Chemical composition of Harzburgites in% by weight
oxide average Type locality Greenland CIPW standard percent
SiO 2 43.73 39.25 42.63 Q
TiO 2 0.28 0.19 0.01 C.
Al 2 O 3 2.57 4.62 0.82 Or 0.83
Fe 2 O 3 6.00 4.13 From 2.60
FeO 7.09 6.33 6.65 dead On 4.17
MnO 0.16 0.17 0.19 Tuesday 6.93
MgO 36.34 33.79 48.43 Hy 21.13
CaO 3.18 3.18 0.19 Oil 46.22
Na 2 O 0.34 0.26 0.03 Mt 7.94
K 2 O 0.15 0.10 Il 0.50
P 2 O 5 0.14 0.03 Ap 0.30
H 2 O /
loss on ignition		 6.67 1.05
Mg # 0.859 0.857 0.941

Harzburgites are predominantly quartz - undersaturated, olivine and hypersthene - normative rocks.

Occurrence

In addition to their occurrence along oceanic spreading centers , Harzburgites are found in stratified intrusions in association with dunite , norite , gabbro , gabbronorite , diorite and quartz diorite . Harzburgites appear as volcanic bombs and xenolites under the ejecta of volcanoes , but they can also be carried along in gangue rocks ( lamprophyren etc.) and in kimberlites .

Emergence

Harzburgites are mainly formed in oceanic rift systems as a result of the formation of tholeiitic basalt melts .

The starting material for this is Lherzolite rock from the Upper Earth's mantle , which is subject to a strong pressure reduction when ascending in the spreading or rift zones . This leads to a partial melting out of up to 20% of the original lherzolite, whereby primarily certain mineral components (such as clinopyroxene, plagioclase ) from the starting material go into the tholeiitic melt. While the melt continues to rise and finally solidifies to form a new oceanic crust , the remaining parts of the mixture remain as a residual or residual layer ( restite ) in the hanging wall of the lherzolite.

The residual material harzburgite is as impoverished or depleted rock ( English depleted classified) because it very clearly incompatible elements such as aluminum and calcium lost. Due to their ionic radius or their number of electrical charges, these elements do not fit into the crystal lattice of the mantle rock and therefore preferentially go into the melt. Magnesium and iron in particular remain in the residual material , as well as small amounts of chromium , nickel and cobalt , which can, however, reach locally degradable concentrations.

References

In layered intrusions:

  • Germany - Harzburger Gabbro (type locality)

At oceanic spreading centers:

For transform faults and associated fracture zones :

In the subduction context (supra-subduction zone mantle wedge):

Generally in ophiolites:

As bombs and xenolites:

Individual evidence

  1. ↑ Rosenbusch, H .: Microscopic Physiography of Minerals and Rocks. Vol. II. Massive rocks . Swiss beard, Stuttgart. Second edition 1887, p. 877 .
  2. ↑ Best, MG and Christiansen, EH: Igneous Petrology . Blackwell Science, 2001, ISBN 0-86542-541-8 , pp. 458 .
  3. ↑ Sano, S. et al .: Petrological, geochemical and isotopical constraints on the origin of the Harzburg intrusion, Germany . In: Journal of Petrology . tape 43 , no. 8 , 2002, p. 1529-1549 .
  4. ↑ Bernstein, S. et al .: Depleted spinel harzburgite xenoliths in Tertiary dykes from East Greenland: Restites from high degree melting . In: Earth and Planetary Science Letters . tape 154 , 1998, pp. 221-235 .
  5. ↑ Suhr, G .: Upper mantle peridotites in the Bay of Islands Ophiolite, Newfoundland: formation during the final stages of a speading center? In: Tectonophysics . tape 206 , 1992, pp. 31-53 .
  6. ↑ Marchesi, C. et al .: Petrogenesis of highly depleted peridotites and gabbroic rocks from the Mayarí-Baracoa Ophiolitic Belt (eastern Cuba) . In: Contrib. Mineral. Petrol. tape 151 , 2005, pp. 717-736 , doi : 10.1007 / s00410-006-0089-0 .
  7. ↑ 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 .
  8. ↑ 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 .
  9. ↑ Shallo, M .: Geological evolution of the Albanian ophiolites and their platform periphery . In: Geologische Rundschau . tape 81, 3 , 1992, pp. 681-694 .
  10. ↑ Nasir, S .: The lithosphere beneath the northwestern part of the Arabian plate (Jordan): evidence from xenoliths & geophysics . In: Geophysics . tape 201 , 1992, pp. 357-370 .

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