THE MEANING OF SM-ND APPARENT AGES FROM K IMBERLITE-DERIVED, COARSE-GRAINED LOW-TEMPERATURE GARNET PERIDOTITES FROM YAKUTIA

Garnet harzburgites from Yakutian kimberlite pipes are the only known mantle xenoliths which show positive Sm-Nd (isochron) slopes between c linopyroxene (Cpx) and garnet. We present new data for sample M-502-72 from Mir pipe which has the youngest (836 Ma) and the sample UV-126-9 3 from Udachnaya which has the oldest (1528 Ma) garnet-Cpx ages from t hese localities. These ages greatly exceed the age of the host kimberl ites (350 Ma). Compared with data from literature these samples have t he most unradiogenic Nd from both localities (Cpx M-502-72: 0.51308, g arnet: 0.51510; Cpx UV-126-93: 0.51062, garnet: 0.51093). The sample F rom Mir pipe (garnet of M-502-72) are strongly contaminated with Nd (8 7.5% addition) from the host kimberlite but other samples were unaffec ted. Beginning at 2038 Ma, we suggest that the closure (cooling) of th e Sm-Nd system went on for 360 Ma. During this time some harzburgites remained open and were depleted in LREE. All harzburgites cooled below the closure temperature by 1680 Ma. The rocks remained undisturbed fo r about 1330 Ma. Shortly before the ruption of the kimberlites the min erals began to re-equilibrate possibly due to elevated temperatures. A t Udachnaya re-equilibration was more extensive leading to younger ave rage ages than found for Mir pipe.