Oceanic island basalt

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The oceanic island basalt is an important type of basalt that can be found on offshore islands in the plate interior . Its formation is usually attributed to hotspots in the Earth's mantle .

introduction

Lava flow from OIB at Piton de la Fournaise on Reunion, eruption in 2004.

Oceanic island basalts, English oceanic island basalts or OIB for short , are always found on oceanic crust and build up huge volcanic islands such as Hawaii , but also an astonishing number of seamounts and guyots . Their total share in oceanic volcanism is estimated at around 10%. The chemical composition of the OIB is variable and ranges from sub-alkaline tholeiites to alkali basalts within a single group of islands , with calcareous magmas typically never appearing (the latter are limited to plate edges). In hotspot islands, most of the volcanic structure is built up using tholeiitic OIBs during the initial shield stage . After the erosive stage , violent eruptions can occur, in which case alkali basalts and other more highly differentiated volcanic rocks with a high alkali content are extracted.

The plate tectonics provides although good explanations for the existence violenter volcanic eruptions at plate edges (z. B. at mid-ocean ridges or subduction zones ). It also shows why geological processes produce basalts with different geochemical properties at plate boundaries. Basal eruptions in the plate interior are not taken into account in plate tectonic models. Therefore, from 1963 on, models were developed that explain intraplate volcanism by means of hotspots or magma ascent in the form of mantle diapirs (plumes) from deep geological sources. These models answer the question of why OIBs are geochemically more enriched than MORBs , which are much shallower and have their starting point in the asthenosphere . However, whether the mantle plumes and hotspots postulated in the sixties and seventies actually have a physical existence is still hotly debated.

Springs in the earth's mantle

So far, various sources of oceanic island basalts have been identified in the reservoirs of the earth's mantle. Their magmas differ in the isotope ratios of radioactive elements that they have inherited from the parent rocks. In particular, the combined analyzes of the isotope ratios of 143 Nd / 144 Nd, 87 Sr / 86 Sr, 206 Pb / 204 Pb, 207 Pb / 204 Pb, 208 Pb / 204 Pb and recently also 187 Os / 188 Os and 187 Re / 188 Os made it possible to define very specific magma components in the earth's mantle. These can be divided into enriched and depleted source components.

Among the enriched components are:

Acting as depleted components are:

Component EMI , English enriched mantle I (enriched mantle I), it might concern mantle material that was contaminated by subducted pelagic sediments. However, it is also conceivable that EMI represents subcontinental lithosphere , which in turn has been contaminated with subducted pelagic sediment material. The component EMII , English enriched mantle II (enriched earth mantle II), is mantle material that was probably mixed with recycled terrigenous sediments. The component HIMU , English high mu (high my - where μ expresses the ratio 238 U / 204 Pb), probably comes from subducted oceanic crust, which could not homogenize with the surrounding mantle. The homogenization did not take place because the oceanic crust was present as gigantic "indigestible megaliths" which accumulated at the mantle transition zone at a depth of 660 kilometers or even deeper at the core-mantle boundary .

The component PREMA , English prevalent mantle (predominant mantle), represents either a depleted mixed component from all other mantle sources or a depleted reservoir formed very early in the earth's history. The component DMM , English depleted MORB mantle (depleted MORB mantle), arose due to the segregation of the earth's crust at the beginning of the earth's history and now lies beneath the mid-ocean ridges . Together with the HIMU component, it is responsible for the development of MORB. The component FOZO , English focal zone (focus zone ), is related to mantle plumes. In terms of its composition, it lies between the DMM and HIMU components. It owes its name to a tetrahedral projection of isotope values ​​that appear to fan out from the focus zone. The FOZO component is also characterized by its high helium-3 content and is associated with very deep mantle plumes. It represents either plume material rising directly from the core-mantle boundary itself or a layer of mantle material that is dragged up the face of the rising plumes.

Petrology

As already mentioned at the beginning, oceanic island basalts have a very variable geochemical composition, which is expressed in three differentiation series:

  • a saturated sub-alkaline tholeiite series picrite - basalt - icelandite - rhyodacite - commendite / pantellerite
  • an undersaturated, mildly alkaline series (such as on Ascension or the Azores) alkali basalt - mugearite - hawaiite - benmoreit - trachyte
  • an undersaturated, strongly alkaline series (such as on Tristan da Cunha) basanite - tephrite - nepheline phonolite - nephelinitic melilithite

The absence of calcareous magmas is of great importance.

Main elements

In the main elements , OIBs show a significant enrichment in TiO 2 , K 2 O and P 2 O 5 compared to MORB , but a lower Al 2 O 3 concentration. This reflects a different source region or different residual mineralogies during the partial smelting process.

The following table illustrates the differences between MORB and various OIBs:

Oxide
wt.%		 MORB OIB tholeiite
Mauna Loa		 OIB basalt
Tristan da Cunha		 OIB alkali basalt
Hualalai
SiO 2 48.77 51.63 42.43 46.37
TiO 2 1.15 1.94 4.11 2.40
Al 2 O 3 15.90 13.12 14.15 14.18
Fe 2 O 3 1.33 2.14 5.84 4.09
FeO 8.62 8.48 8.48 8.91
MnO 0.17 0.17 0.17 0.19
MgO 9.67 8.53 6.71 9.47
CaO 11.16 9.97 11.91 10.33
Na 2 O 2.43 2.21 2.77 2.85
K 2 O 0.08 0.33 2.04 0.93
P 2 O 5 0.09 0.22 0.58 0.28

Trace elements

Among the trace elements , cations with low valence ( LILE or LFS) such as cesium , rubidium , potassium , barium , lead and strontium accumulate in oceanic island basalts compared to MORB, with the most pronounced enrichment being achieved in alkali basalts. Their respective concentration depends on the source composition, residual mineralogy, degree of melting and subsequent fractional crystallization. Large cations with high valence such as the HFS elements thorium , uranium , cerium , zirconium , hafnium , niobium , tantalum and titanium also preferentially accumulate in OIB.

Transition metals

The transition metals nickel and chromium are depleted in OIB alkali basalts compared to MORB and also OIB tholeiites. This may indicate significant fractional crystallization occurring under high pressures.

Rare earth

In the case of rare earths , oceanic island basalts have an internal enrichment of their LREE compared to the HREE. Compared to normal MORB (N-MORB), even OIB tholeiites are still very rich in LREE. This shows that, unlike MORB, their mantle sources are not depleted. This difference becomes even clearer with the OIB alkali basalts and can be explained by a lower partial melting rate.

Note: Plume-MORB (P-MORB) has similarities with OIB.

Table with some rare earths and trace elements:

Rare earth
trace elements
ppm		 MORB OIB tholeiite
Mauna Loa		 OIB basalt
Tristan da Cunha		 OIB alkali basalt
Hualalai
La 2.10 7.58 18.8
Ce 21.0 43.0
Sm 2.74 4.40 5.35
Eu 1.06 1.60 1.76
Yb 3.20 1.98 1.88
Rb 0.56 4.9 110 22nd
Sr 88.7 273 700 500
Ba 4.2 75 700 300

Radioisotopes

Nd-Sr isotope diagram with the positions of the depleted shell components DMM, MORB and OIB. Oceanic island basalts are clearly separated from the basalts of the mid-ocean ridges.

Radioisotope diagrams illustrate the segregation of MORB and OIB, with MORB only occupying a very limited field in the vicinity of the DMM component. This can be clearly seen in the 143 Nd / 144 Nd- 87 Sr / 86 Sr diagrams. The field occupied by OIB is much more extensive and extends to lower εNd and higher εSr values. Only island arc basalts (IAB, English Iceland arc basalt ) are still extensive in their isotopic compositions; they enclose the OIB values ​​and also tend towards even higher εSr.

As soon as the lead isotopes are taken into account, a simple binary mixture model between a DMM and an EM component can no longer be maintained. If, for example, 143 Nd / 144 Nd or 87 Sr / 86 Sr is applied against 206 Pb / 204 Pb, four jacket components (see above) can be kept apart.

Occurrence

Oceanic island basalts are found in all oceans. Examples are:

Literature on the Plume Controversy

  • John Tuzo Wilson: A possible origin of the Hawaiian islands . In: Canadian Journal of Physics . tape 41 , 1963, pp. 493-571 .
  • John Tuzo Wilson: Mantle plumes and plate motion . In: Tectonophysics . tape 19 , 1973, p. 149-164 .
  • Morgan, WJ: Convection plumes in the lower mantle . In: Nature . tape 230 , 171, pp. 42-43 .
  • Morgan, WJ: Plate motions and deep mantle convection . In: Geol. Soc. At the. Mem. 1972, p. 7-22 .
  • Morgan, WJ: Hotspot tracks and the early rifting of the Atlantic . In: Tectonophysics . tape 94 , 1983, pp. 123-139 .
  • Zindler, A., Staudigel, H. and Batiza, R .: Isotope and trace element geochemistry of young Pacific seamounts: implications for the scale of upper mantle heterogeneity . In: Earth and Planetary Science Letters . tape 70 , 1984, pp. 175-195 .
  • Zindler, A. and Hart, S .: Chemical geodynamics . In: Am. Rev. Earth Planet. Sci. tape 14 , 1986, pp. 493-571 .

Individual evidence

  1. ^ Wilson, Marjorie: Igneous Petrogenesis - a global tectonic approach . Chapman & Hall, London 1989, ISBN 0-412-53310-3 .
  2. ↑ Niu, Y., Wilson, M., Humphreys, ER and O'Hara, MJ: The origin of intra-plate ocean island basalts (OIB): the lid effect and its geodynamic implications . In: Journal of Petrology . tape 52 (7-8):, 2004, pp. 1443-1468 , doi : 10.1093 / petrology / egr030 .
  3. ↑ Dickin, Alan P .: Radiogenic Isotope Geology (2nd ed.) . Cambridge University Press, 2005, pp. 161-162 .