Ocean island basalt (OIB) is a volcanic rock, usually basaltic in composition, erupted in oceans away from tectonic plate boundaries. Although ocean island basaltic magma is mainly erupted as basalt lava, the basaltic magma is sometimes modified by igneous differentiation to produce a range of other volcanic rock types, for example, rhyolite in Iceland, and phonolite and trachyte at the intraplate volcano Fernando de Noronha. Unlike mid-ocean ridge basalts (MORBs), which erupt at spreading centers (divergent plate boundaries), and volcanic arc lavas, which erupt at subduction zones (convergent plate boundaries), ocean island basalts are the result of intraplate volcanism. However, some ocean island basalt locations coincide with plate boundaries like Iceland, which sits on top of a mid-ocean ridge, and Samoa, which is located near a subduction zone.
In the ocean basins, ocean island basalts form seamounts, and in some cases, enough material is erupted that the rock protrudes from the ocean and forms an island, like at Hawaii, Samoa, and Iceland. Over time, however, thermal subsidence and mass loss via subaerial erosion causes islands to become completely submarine seamounts or guyots. Many ocean island basalts erupt at volcanic hotspots, which are thought to be the surface expressions of melting of thermally buoyant, rising conduits of hot rock in the Earth's mantle, called mantle plumes. Some such hotspot volcanic chains are believed to have started with the formation of large igneous provinces. Mantle plume conduits may drift slowly, but Earth's tectonic plates drift more rapidly relative to mantle plumes. As a result, the relative motion of Earth's tectonic plates over mantle plumes produces age-progressive chains of volcanic islands and seamounts with the youngest, active volcanoes located above the axis of the mantle plume while older, inactive volcanoes are located progressively farther away from the plume conduit (see Figure 1). Hotspot chains can record tens of millions of years of continuous volcanic history; for example, the oldest seamounts in the Hawaiian–Emperor seamount chain are over 80 million years old.
Not all ocean island basalts are the product of mantle plumes. There are thousands of seamounts that are not clearly associated with upwelling mantle plumes, and there are chains of seamounts that are not age progressive. Seamounts that are not clearly linked to a mantle plume indicate that regional mantle composition and tectonic activity may also play important roles in producing intraplate volcanism.
Mantle sources There are various sources identified for ocean island basalt magma in Earth's mantle but the main component is ancient recycled basaltic oceanic crust which has inherited the trace element and isotopic signatures of a subduction zone dehydration process, with enrichment in high field strength elements. 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 but it is now possible to classify usefully and more conveniently on high field strength trace elements alone such as barium (Ba), caesium (Ce), rubidium (Rb), niobium (Nb) and terbium (Tb is chosen as proportion about constant in all OIB). : NameFull NameSourceTrace element composition Enriched sources EM1 (EMI) Enriched Mantle 1Dickin 2005, p. 157 Significant contamination with continental crust, usually lower crust. 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. Moderate Nb, Ce, Ba, La, Nb. Enrichment of Ba EM2 (EMII) Enriched Mantle 2 Significant contamination with continental crust, usually upper crust. Likely mantle contaminated with material derived from the recycling of sediments from the continental crust into the mantle.Dickin 2005, pp. 161–162 Less enriched in Nb, Ba, La, and Ce compared to EMI or HIMU but relative enrichment Ba c.f. HIMU or DMM HIMU High U/Pb ratio Negligible crustal contamination. 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.Dickin 2005, p. 151 Low Ba, high Ce and La Depleted sources PREMA Prevalent Mantle Possible formed by mixing of all the other mantle sources or a source formed early in Earth's history. DMM Depleted MORB Mantle Has mid-oceanic ridge basalt (MORB) characteristics with low 87Sr/86Sr and high 143Nd/144Nd and εHf as compared to MORB Low Ba, high to moderate Ce/Rb (i.e. low Rb) and low La 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.Dickin 2005, p. 164
Isotope geochemistry The geochemistry of ocean island basalts is useful for studying the chemical and physical structure of Earth's mantle. Some mantle plumes that feed hotspot volcanism lavas are thought to originate as deep as the core–mantle boundary (~2900 km deep). The composition of the ocean island basalts at hotspots provides a window into the composition of mantle domains in the plume conduit that melted to yield the basalts, thus providing clues as to how and when different reservoirs in the mantle formed.
Early conceptual models for the geochemical structure of the mantle argued that the mantle was split into two reservoirs: the upper mantle and the lower mantle. The upper mantle was thought to be geochemically depleted due to melt extraction which formed Earth's continents. The lower mantle was thought to be homogenous and “primitive”. (Primitive, in this case, refers to silicate material that represents the building blocks of the planet that has not been modified by melt extraction, or mixed with subducted materials, since Earth's accretion and core formation.) Seismic tomography showed subducted slabs passing through the upper mantle and entering the lower mantle, which indicates that the lower mantle cannot be isolated. Additionally, the isotopic heterogeneity observed in plume-derived ocean island basalts argues against a homogenous lower mantle. Heavy, radiogenic isotopes are a particularly useful tool for studying the composition of mantle sources because isotopic ratios are not sensitive to mantle melting. According tradition subclassification used Sr-Nd-Pb-Hf-He isotopic ratios. This means that the heavy radiogenic isotopic ratio of a melt, which upwells and becomes a volcanic rock on the surface of the Earth, reflects the isotopic ratio of the mantle source at the time of melting. The best studied heavy radiogenic isotope systems in ocean island basalts are 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 176Hf/177Hf and, more recently, 187Os/188Os. In each of these systems, a radioactive parent isotope with a long half-life (i.e., longer than 704 million years) decays to a “radiogenic” daughter isotope. Changes in the parent/daughter ratio by, for example, mantle melting, result in changes in the radiogenic isotopic ratios. Thus, these radiogenic isotopic systems are sensitive to the timing, and degree, of parent/daughter the changed (or fractionated) parent daughter ratio, which then informs the process(es) responsible for generating observed radiogenic isotopic heterogeneity in ocean island basalts. In mantle geochemistry, any composition with relatively low 87Sr/86Sr, and high 143Nd/144Nd and 176Hf/177Hf, is a referred to as “geochemically depleted”. High 87Sr/86Sr, and low 143Nd/144Nd and 176Hf/177Hf, is referred to as “geochemically enriched”. Relatively low isotopic ratios of Pb in mantle-derived rocks are described as unradiogenic; relatively high ratios are described as radiogenic.
These isotopic systems have provided evidence for a heterogenous lower mantle. There are several distinct “mantle domains” or endmembers that appear in the ocean island basalt record. When plotted in multi-isotope space, ocean island basalts tend to form arrays trending from a central composition out to an endmember with an extreme composition. The depleted mantle, or DM, is one endmember, and is defined by low 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, and high 143Nd/144Nd and 176Hf/177Hf. The DM is therefore geochemically depleted, and relatively unradiogenic. Mid-ocean ridges passively sample the upper mantle and MORBs are typically geochemically depleted, and therefore it is widely accepted that the upper mantle is composed mostly of depleted mantle. Thus, the term depleted MORB mantle (DMM) is often used to describe the upper mantle that sources mid-ocean ridge volcanism. Ocean island basalts also sample geochemically depleted mantle domains. In fact, most ocean island basalts are geochemically depleted, and <10% of ocean island basalts have lavas that extend to geochemically enriched (i.e., 143Nd/144Nd lower than the Earth's building blocks) compositions.
There are two geochemically enriched domains, named enriched mantle 1 (EM1), and enriched mantle 2 (EM2). Though broadly similar, there are some important distinctions between EM1 and EM2. EM1 has unradiogenic 206Pb/204Pb, moderately high 87Sr/86Sr, and extends to lower 143Nd/144Nd and 176Hf/177Hf than EM2. Pitcairn, Kerguelen-Heard, and Tristan-Gough are the type localities of EM1. EM2 is defined by higher 87Sr/86Sr than EM1, and higher 143Nd/144Nd and 176Hf/177Hf at a given 87Sr/86Sr value, and intermediate 206Pb/204Pb. Samoa and Society are the archetypal EM2 localities.
Another distinct mantle domain is the HIMU mantle. In isotope geochemistry, the Greek letter μ (or mu) is used to describe the 238U/204Pb, such that ‘high μ’ (abbreviated HIMU) describes a high 238U/204Pb ratio. Over time, as 238U decays to 206Pb, HIMU Earth materials develop particularly radiogenic (high) 206Pb/204Pb. If an Earth material has elevated 238U/204Pb (HIMU), then it will also have elevated 235U/204Pb, and therefore will produce radiogenic Pb compositions for both the 206Pb/204Pb and 207Pb/204Pb isotopic systems (238U decays 206Pb, 235U decays to 207Pb). Similarly, Earth materials with high U/Pb also tend to have high Th/Pb, and thus evolve to have high 208Pb/204Pb (232Th decays to 208Pb). Ocean island basalts with highly radiogenic 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb are the products of HIMU mantle domains. St. Helena, and several islands in the Cook-Austral volcanic lineament (e.g., Mangaia) are the type localities for HIMU ocean island basalts.