Ocean island basalt
[1] 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.
Over time, however, thermal subsidence and mass loss via subaerial erosion causes islands to become completely submarine seamounts or guyots.
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.
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 IOB).
[6] [A] : The geochemistry of ocean island basalts is useful for studying the chemical and physical structure of Earth's mantle.
The upper mantle was thought to be geochemically depleted due to melt extraction which formed Earth's continents.
[11] Additionally, the isotopic heterogeneity observed in plume-derived ocean island basalts argues against a homogenous lower mantle.
The final mantle domain discussed here is the common composition that ocean island basalts trend toward in radiogenic isotopic multi-space.
PREMA, or “Prevalent Mantle” was the first term coined by Zindler and Hart (1986) to describe the most common composition sampled by ocean island basalts.
[14] Hart et al. (1992) later named the location of the intersection of ocean island basalt compositions in radiogenic isotopic multi-space as the “Focus Zone”, or FOZO.
[18] For example, the EM2 hotspots of Samoa and Society are both thought to have a mantle source that contains recycled upper continental crust,[19] an idea that is supported by stable isotope observations, including δ18O and δ7Li.
Thus, hotspots that are categorized as “EM1”, “EM2”, “HIMU”, or “FOZO”, may each sample physically distinct, but compositionally similar, portions of the mantle.
Isotopic systems help to deconvolve the geologic processes that contributed to, and in some cases the timing of, the formation of these mantle domains.
[20] MIF of sulfur isotopes is a phenomenon that occurred in Earth's atmosphere only before the Great Oxidation Event ~2.3 Ga.
Thus, in plume-derived lavas, high 3He/4He is an “ancient” geochemical signature that indicates the existence of a well-preserved helium reservoir in the deep mantle.
The timing of the formation of this reservoir is constrained by observed anomalies of 129Xe/130Xe in ocean islands basalts, because 129Xe was only produced by decay of 129I during the first ~100 My of Earth's history.
