<P> High - alumina basalts have aluminium contents of 17--19 wt% Al O; boninites have magnesium contents of up to 15 percent MgO . Rare feldspathoid - rich mafic rocks, akin to alkali basalts, may have Na O + K O contents of 12% or more . </P> <P> The abundances of the lanthanide or rare - earth elements (REE) can be a useful diagnostic tool to help explain the history of mineral crystallisation as the melt cooled . In particular, the relative abundance of europium compared to the other REE is often markedly higher or lower, and called the europium anomaly . It arises because Eu can substitute for Ca in plagioclase feldspar, unlike any of the other lanthanides, which tend to only form 3 + cations . </P> <P> Mid-ocean ridge basalts (MORB) and their intrusive equivalents, gabbros, are the characteristic igneous rocks formed at mid-ocean ridges . They are tholeiitic basalts particularly low in total alkalis and in incompatible trace elements, and they have relatively flat rare earth element (REE) patterns normalized to mantle or chondrite values . In contrast, alkali basalts have normalized patterns highly enriched in the light REE, and with greater abundances of the REE and of other incompatible elements . Because MORB basalt is considered a key to understanding plate tectonics, its compositions have been much studied . Although MORB compositions are distinctive relative to average compositions of basalts erupted in other environments, they are not uniform . For instance, compositions change with position along the Mid-Atlantic ridge, and the compositions also define different ranges in different ocean basins . Mid-ocean ridge basalts have been subdivided into varieties such as normal (NMORB) and those slightly more enriched in incompatible elements (EMORB). </P> <P> Isotope ratios of elements such as strontium, neodymium, lead, hafnium, and osmium in basalts have been much studied to learn about the evolution of the Earth's mantle . Isotopic ratios of noble gases, such as He / He, are also of great value: for instance, ratios for basalts range from 6 to 10 for mid-ocean ridge tholeiitic basalt (normalized to atmospheric values), but to 15--24 and more for ocean - island basalts thought to be derived from mantle plumes . </P>

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