The meaning of epsilon values

Epsilon values vary with time and the following approximation may be used to calculate epsilon (f) from epsilon (today)  

Time (r)- epsilon evoludon diagram for 3.8 Ga metakomadites from Labrador (adapted from CoUerson et aL, 1991). Komadites were produced from depleted mantle at 3.8 Ga with individual samples ranging in composidon from E = H-0.2 to +2.3, and with variable Sm/Nd rados q 0.S9). Individual samples have evolved to present-day epsilon values 

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