Hawaiit
Hawaiite is a basalt-like volcanic rock . It is the sodium- containing variety of trachy basalt and contains olivine . Its normative plagioclase is either oligoclase or andesine . Hawaiian magmas are weakly alkaline , nepheline normative and undersaturated in SiO 2 .
Etymology and history
The name Hawaiit is derived from the Hawaiian islands. The rock was first described and named by Joseph Paxson Iddings in 1913 . Its type locality is the Hawaiian Islands.
classification
Hawaiite is chemically defined in the TAS field S1. As a sodium-rich trachy basalt, it fulfills the condition Na 2 O - 2 ≥ K 2 O. Its weight fraction of SiO 2 is between 45 and 52%, the weight fraction of the sum of Na 2 O and K 2 O between 5 and 7.3%. To further differentiate it from other volcanic rocks, the MgO content can be used, which in Hawaiite is between 4 and 8 percent by weight.
composition
Hawaiites carry 40–70 percent by volume of mafic minerals, usually augite ( clinopyroxene ) and olivine . Another main part of the mixture is plagioclase , mostly andesine or labradorite . Foids can also be present.
Mineral inventory
Phenocrystals in Hawaiites are common:
The mesostasis contains:
- Augit
- Plagioclase ( rich in anorthite )
- Magnetite
- Ilmenite
- Titanomagnetite
- Apatite
- Alkali feldspar
- Amphibole
- Olivine ( rich in magnesium )
The basic mass can be partially glassy.
Chemical composition
The Hawaiite from the Hocheifel may serve as an example of the chemical composition (Rappoldsley, Breidscheid near Adenau ):
| oxide | Weight percent | CIPW standard | percent |
|---|---|---|---|
| SiO 2 | 47.0 | Q | |
| TiO 2 | 2.5 | Or | 9.08 |
| Al 2 O 3 | 15.0 | From | 26.94 |
| Fe 2 O 3 | 4.2 | On | 19.86 |
| FeO | 5.7 | Tuesday | 19.50 |
| MnO | 0.18 | Hy | |
| MgO | 6.4 | Mt | 6.23 |
| CaO | 10.3 | Il | 4.86 |
| Na 2 O | 3.8 | Ap | 1.50 |
| K 2 O | 1.5 | Oil | 7.66 |
| P 2 O 5 | 0.62 | No | 3.21 |
| CO 2 | 0.5 | Cc | 1.16 |
| H 2 O + | 2.0 | ||
| Mg # | 0.66 |
Quartz and hypersthene are absent as standard minerals; instead, nepheline occurs.
Outward appearance
Hawaiites are fine-grain volcanic rocks with an average grain size of less than 0.25 mm. They are mostly from light gray to dark gray in color. Their number of colors varies between 40 and 70.
Origin and Association
Hawaiian magmas are formed by fractional crystallization of olivine and clinopyroxene from alkali basalts . They emerge at temperatures between 1080 ° C (on Etna) and 1250 ° C (Auvergne) as relatively thin and sometimes bubble-rich lavas on the surface of the earth. So far, relatively shallow magma chambers have been assumed as the origin of the Hawaiians. H. low printing conditions. This contradicts an occurrence in New South Wales , which has inclusions of lherzolite and pyroxene foreign crystals and thus suggests the Upper Earth mantle as the place of origin, similar to basalts.
Hawaiites belong to the following sodium-stressed magma associations:
- Ankaramit -Alkalibasalt-Hawaiite differentiation series
- Alkali Basalt Hawaiit- Mugearit - Benmoreit - Trachyt (- rhyolite ) -Differentiationsserie
Occurrence and localities
As a member of the basalt group, Hawaiites are very common and can be found in most tectonic contexts:
- In the oceanic area:
- Within oceanic islands together with OIB's . Examples are the Azores , the Canary Islands , the Comoros , Kerguelen and the Marquesas Islands
- In oceanic hotspots like Galapagos , Hawaii (type locality), Iceland and Réunion
- In seamounts and guyots , for example the Hawaii-Emperor chain
- In underwater ridges, for example the Keli Ridge ( Line Islands ) in the Pacific
- In convergence zones - southwestern Honshu Bend and the Northland Volcanic Zone in New Zealand
- In the continental area:
- In continental hotspots - Eifel , French Massif Central , Cameroon Line
- Within volcanic series of rifts - Great African Rift Valley in Ethiopia
- In continental flood basalts (CFB's), for example the Ferrar Supergroup in Antarctica
- In intraplate volcanism (plateau veins of Patagonia ) and
- In complex volcanic contexts - Etna and Hybrid Plateau .
Hawaiite sites in Europe are Germany (Hocheifel and Vogelsberg ), France ( Morvan and Chaîne des Puys ), Iceland ( Heimaey ), Italy ( Etna , Pantelleria and Vulcano ) and Scotland ( Skye and Mull ). In the Atlantic, it can also be found on Ascension , Madeira , Gough Island and Tristan da Cunha . The Hawaiian Islands as a type locality lead Hawaiite on Hawaii at Mauna Kea , Hualālai and Kohala as well as on Molokaʻi . Hawaiian lavas were also found in America in the southwest Basin and Range Province ( Nevada ) and with the basalts of the eastern Snake River . Mexico and the Austral Islands ( Tubuai ) in the Pacific region should also be mentioned .
Footnotes
- ↑ WIMMENAUER, W. (1985): Petrography of igneous and metamorphic rocks. Ferdinand Enke publishing house. ISBN 3-432-94671-6
- ↑ JFG WILKINSON & RA BINNS, Hawaiite of High Pressure Origin from North-eastern New South Wales, Nature 222, 553-555 (10 May 1969). doi : 10.1038 / 222553a0
- ↑ Hawaiit , data sheet of the GEOROC database in Mainz ( CSV file , must be saved locally before opening)
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- Roger Walter Le Maitre (Ed.): Igneous Rocks. A Classification and Glossary of Terms . Cambridge University Press, Cambridge 2002, ISBN 0-521-61948-3 .