) ▁and ▁low ▁r ati os ▁of ▁Re / Os ▁in ▁the ▁lower ▁mant le . ▁In ▁this ▁regard , ▁the ▁Re – Os ▁system ▁to ▁study ▁the ▁ge och em ical ▁evolution ▁of ▁mant le ▁rocks ▁and ▁in ▁defining ▁the ▁chron ology ▁of ▁mant le ▁different iation ▁is ▁extremely ▁helpful . ▁ ▁Per id ot ite ▁x en ol ith s ▁which ▁are ▁thought ▁to ▁sample ▁the ▁upper ▁mant le ▁sometimes ▁contain ▁sup ra - ch ond rit ic ▁Os - is ot op ic ▁r ati os . ▁This ▁is ▁thought ▁to ▁evidence ▁of ▁sub duct ed ▁ancient ▁high ▁Re / Os ▁bas alt ic ▁cr ust ▁that ▁is ▁being ▁rec yc led ▁back ▁into ▁the ▁mant le . ▁This ▁combination ▁of ▁radi ogen ic ▁( 1 8 7 Os ▁that ▁was ▁created ▁by ▁decay ▁of ▁ 1 8 7 Re ) ▁and ▁non rad i ogen ic ▁mel ts ▁helps ▁to ▁support ▁the ▁theory ▁of ▁at ▁least ▁two ▁Os - is ot op ic ▁res er voir s ▁in ▁the ▁mant le . ▁The ▁volume ▁of ▁both ▁these ▁res er voir s ▁is ▁thought ▁to ▁be ▁around ▁ 5 - 1 0 % ▁of ▁the ▁whole ▁mant le . ▁The ▁first ▁res er voir ▁is ▁character ized ▁by ▁de ple tion ▁in ▁Re ▁and ▁pro x ies ▁for ▁m elt ▁fert ility ▁( such ▁as ▁concentr ations ▁of ▁elements ▁like ▁Ca ▁and ▁Al ). ▁The ▁second ▁res er voir ▁is ▁ch ond rit ic ▁in ▁composition . ▁ ▁Direct ▁measurement