Ocean island basalt
Ocean island basalts are found on islands and volcanoes located on oceanic crust. The chemical composition of these basalts can vary from tholeiite to alkali basalt within the same island group, but it is never calc-alkaline. During the shield volcano stage of many hotspot islands, tholeiitic OIBs build up most of the volcano's structure. Following the post-erosional stage this is usually accompanied by violent eruptions of alkali basalt and other more evolved volcanic rocks with high alkali content.
The theory of plate tectonics provides excellent explanations for both the existence of volcanic eruptions at plate boundaries—such as ocean ridges and subduction zones—and why geological processes produce basalts of different geochemical properties at these plate boundaries. The existence of basaltic eruptions in the interior of tectonic plates, however, is not directly related to plate tectonism and models involving magma upwelling from deep mantle sources (such as mantle plumes and hotspots) have been developed to explain this intra-plate volcanism. These models explain why OIBs contain more geochemically enriched basalt than mid-ocean ridge basalts (MORBs) that originate from much closer to the Earth's surface in the asthenosphere. The existence of mantle plumes and hotspots has, however, been hotly debated since they were first proposed in the 1960s and 1970s.
There are various sources identified for ocean island basalt magma in Earth’s mantle. These mantle sources are inferred from differences in radiogenic isotope ratios that magmas inherit from their source rock. Sources have been defined from a combined analysis of strontium (Sr), neodymium (Nd) and lead (Pb) isotopes. The sources as defined by radiogenic isotopes are:
|EMI||Enriched Mantle I||Probably mantle contaminated with material derived from subducted pelagic sediments. An alternative explanation is that this source derives from the sub-continental lithosphere which could also be contaminated by subducted pelagic sediments.|
|EMII||Enriched Mantle II||Likely mantle contaminated with material derived from the recycling[A] of terrigenous sediments from the continental crust into the mantle.|
|HIMU||High U/Pb ratio||Likely derived from subducted oceanic crust that has not been homogenized with the rest of the mantle. The lack of homogenization could be indebted to the accumulation of subducted oceanic crust in large-scale “megaliths” at the 670 km seismic discontinuity or near the core–mantle boundary.|
|PREMA||Prevalent Mantle||Possible formed by mixing of all the other mantle sources or a source formed early in Earth’s history.|
|FOZO||Focus Zone||A source associated with mantle plumes. It is of intermediate composition between DMM and HIMU. The name Focus Zone derives from the apparent fanning out of compositions from this zone when displaying isotope composition data on tetrahedron chart. FOZO contains high contents of Helium-3. The FOZO source is associated with deep mantle plumes. FOZO has been proposed to be either the plume material that rises from the core–mantle boundary or material that becomes attached to the plume as a sheet as the plume it rises from the core–mantle boundary.|
- Subduction, subduction erosion etc.
- Niu et al. 2011, Introduction, pp. 1443–1444
- Dickin 2005, p. 157
- Dickin 2005, pp. 161–162
- Dickin 2005, p. 151
- Dickin 2005, p. 164
- Niu, Y.; Wilson, M.; Humphreys, E. R.; O'Hara, M. J. (2011). "The origin of intra-plate ocean island basalts (OIB): the lid effect and its geodynamic implications" (PDF). Journal of Petrology. 52 (7-8): 1443–1468. doi:10.1093/petrology/egr030. Retrieved 15 January 2017.
- Dickin, Alan P. (2005) . Radiogenic Isotope Geology (2nd ed.). Cambridge University Press.
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