▁equival ents , ▁g abb ros , ▁are ▁the ▁characteristic ▁ igne ous ▁rocks ▁formed ▁at ▁mid - o cean ▁rid ges . ▁They ▁are ▁th ole i it ic ▁bas al ts ▁particularly ▁low ▁in ▁total ▁alk alis ▁and ▁in ▁in compatible ▁trace ▁elements , ▁and ▁they ▁have ▁relatively ▁flat ▁rare - ear th ▁element ▁( REE ) ▁patterns ▁normal ized ▁to ▁mant le ▁or ▁ch ond rite ▁values . ▁In ▁contrast , ▁alk ali ▁bas al ts ▁have ▁normal ized ▁patterns ▁highly ▁en rich ed ▁in ▁the ▁light ▁RE E , ▁and ▁with ▁greater ▁abund ances ▁of ▁the ▁RE E ▁and ▁of ▁other ▁in compatible ▁elements . ▁Because ▁M OR B ▁bas alt ▁is ▁considered ▁a ▁key ▁to ▁understanding ▁plate ▁t ect on ics , ▁its ▁compos itions ▁have ▁been ▁much ▁studied . ▁Although ▁M OR B ▁compos itions ▁are ▁distinct ive ▁relative ▁to ▁average ▁compos itions ▁of ▁bas al ts ▁er upt ed ▁in ▁other ▁environments , ▁they ▁are ▁not ▁uniform . ▁For ▁instance , ▁compos itions ▁change ▁with ▁position ▁along ▁the ▁Mid - At l antic ▁R idge , ▁and ▁the ▁compos itions ▁also ▁define ▁different ▁ranges ▁in ▁different ▁ocean ▁bas ins . ▁Mid - o cean ▁r idge ▁bas al ts ▁have ▁been ▁sub div ided ▁into ▁vari eties ▁such ▁as ▁normal ▁( N M OR B ) ▁and ▁those ▁slightly ▁more ▁en rich ed ▁in ▁in compatible ▁elements ▁( EM OR B ). ▁ ▁Is ot ope ▁r ati os ▁of ▁elements ▁such ▁as ▁str ont ium