t %, ▁those ▁of ▁M g O ▁commonly ▁in ▁the ▁range ▁ 5 ▁to ▁ 1 2 ▁w t %. ▁ ▁High - al um ina ▁bas al ts ▁have ▁al umin ium ▁contents ▁of ▁ 1 7 – 1 9 ▁w t % ▁Al 2 O 3 ; ▁bon init es ▁have ▁mag nes ium ▁( M g O ) ▁contents ▁of ▁up ▁to ▁ 1 5 ▁percent . ▁R are ▁f eld sp ath oid - rich ▁ma fic ▁rocks , ▁a kin ▁to ▁alk ali ▁bas al ts , ▁may ▁have ▁Na 2 O ▁+ ▁K 2 O ▁contents ▁of ▁ 1 2 % ▁or ▁more . ▁ ▁The ▁abund ances ▁of ▁the ▁l anth an ide ▁or ▁rare - ear th ▁elements ▁( REE ) ▁can ▁be ▁a ▁useful ▁di agnostic ▁tool ▁to ▁help ▁explain ▁the ▁history ▁of ▁min eral ▁cry st all isation ▁as ▁the ▁m elt ▁co o led . ▁In ▁particular , ▁the ▁relative ▁abund ance ▁of ▁europ ium ▁compared ▁to ▁the ▁other ▁RE E ▁is ▁often ▁marked ly ▁higher ▁or ▁lower , ▁and ▁called ▁the ▁europ ium ▁anom aly . ▁It ▁ar ises ▁because ▁Eu 2 + ▁can ▁substitute ▁for ▁Ca 2 + ▁in ▁pl agi oc l ase ▁f eld s par , ▁unlike ▁any ▁of ▁the ▁other ▁l anth an ides , ▁which ▁tend ▁to ▁only ▁form ▁ 3 + ▁c ations . ▁ ▁Mid - o cean ▁r idge ▁bas al ts ▁( M OR B ) ▁and ▁their ▁intr us ive