▁of ▁the ▁age ▁of ▁contin ental ▁cr ust ▁through ▁Re - Os ▁d ating ▁is ▁difficult . ▁In fil tr ation ▁of ▁x en ol ith s ▁by ▁their ▁commonly ▁Re - rich ▁mag ma ▁al ters ▁the ▁true ▁element al ▁Re / Os ▁r ati os . ▁Instead , ▁determ ining ▁model ▁ages ▁can ▁be ▁done ▁in ▁two ▁ways : ▁" Re ▁de ple tion " ▁model ▁ages ▁or ▁the ▁" mel ting ▁age " ▁model . ▁The ▁former ▁finds ▁the ▁minimum ▁age ▁of ▁the ▁extra ction ▁event ▁assuming ▁the ▁element al ▁Re / Os ▁ratio ▁equals ▁ 0 ▁( kom ati ite ▁resid ues ▁have ▁Re / Os ▁of ▁ 0 , ▁so ▁this ▁is ▁assuming ▁a ▁x en ol ith ▁was ▁extracted ▁from ▁a ▁near - kom ati ite ▁m elt ). ▁The ▁latter ▁gives ▁the ▁age ▁of ▁the ▁mel ting ▁event ▁infer red ▁from ▁the ▁point ▁when ▁a ▁m elt ▁proxy ▁like ▁Al 2 O 3 ▁is ▁equal ▁to ▁ 0 ▁( anci ent ▁sub cont inental ▁l ith os phere ▁has ▁weight ▁percent ages ▁of ▁Ca O ▁and ▁Al 2 O 3 ▁r anging ▁from ▁ 0 ▁to ▁ 2 %). ▁ ▁P t – Re – Os ▁system at ics ▁ ▁The ▁radio active ▁decay ▁of ▁ 1 9 0 P t ▁to ▁ 1 8 6 Os ▁has ▁a ▁half - life ▁of ▁ 6 . 5 ( 3 ) × 1 0 1 1 ▁years ▁( which ▁is ▁longer ▁than ▁the ▁age ▁of ▁the ▁universe