Influence Of Mantle Plumes On The Lithospheric Mantle

The effect of the cratonic lithospheric mantle by mantle plumes has been a long-term concern in solid earth science.To understand this effect,it is necessary to study the changes in the lithospheric mantle of the craton before and after mantle plume activity.This thesis choosed the peridotite xenoliths from Udachnaya(357 Ma)and Obnazhennaya(152 Ma)kimberlite pipes before and after the mantle plume activity(251 Ma)in the Siberian craton as the research object.The Sr-Nd-Hf isotopic composition of minerals in peridotite xenoliths from the Obnazhennaya kimberlite was analyzed using solution method.Additionally,the geochemical characteristics of mantle xenoliths in two kimberlite pipes before and after mantle plume activity were systematically analyzed and compared,including major and trace elements,Re-Os isotopes,and PGE patterns.The geochemical impact of Siberian mantle plume activity on the lithospheric mantle of the craton was discussed,and the metasomatism process of the activity of the Siberian mantle plume on the cratonic lithospheric mantle was revealed,which would provide new evidence for the metasomatism transformation of the mantle plume on the cratonic lithosphere.The comparison suggested that mantle peridotite xenoliths before and after mantle plume activity underwent partial melting and metasomatic transformation.The peridotite xenoliths from Udachnaya kimberlites represent the characteristics of the lithospheric mantle prior to mantle plume activity,and their clinopyroxenes have a wide range of SrNd-Hf isotopic compositions.The clinopyroxenes in harzburgite xenoliths have depleted Sr-Nd-Hf isotopic compositions,combined with the presence of unradiogenic Os isotopic compositions and lower[Pd/Ir]N characteristics of these samples,indicating that they experienced melting extracted ancient depleted cratonic mantle.The depleted Sr isotope and enriched Nd isotope of clinopyroxene in lherzolite xenoliths suggest that the lithospheric mantle in the Udachnaya area has been metasomatised.The peridotite xenoliths from the Obnazhennaya kimberlites represent the characteristics of the lithospheric mantle after the activity of the mantle plume.Clinopyroxenes and garnets in these samples have a depleted Sr isotope composition(87Sr/86Sr=0.702801-0.704087),and enriched Nd isotopes(εNd(t)=-0.1--11.2)and Hf isotopes(εHf(t)=-9.0--2.6)characteristics.Garnet has a more primitive Nd-Hf isotope composition,the value of εNd(t)ranges from+40.8 to+74.1,the value of εHf(t)ranges from+66.5 to+112.1.Through comparing the characteristics and binary mixing curves of mantle xenoliths in Udachnaya and Obnazhennaya kimberlites,the Sr-Nd-Hf isotopic composition of mantle xenoliths in Udachnaya kimberlites is more inclined to the characteristics of Udachnaya kimberlites,while the Sr-Nd-Hf isotopic composition of mantle xenoliths in Obnazhennaya kimberlites is plotted between kimberlites and mantle plume overflow basalts,but more inclined to the isotopic characteristics of overflow basalt.Therefore,the study suggests that mantle plume activity has an impact on the lithosphere of the Siberian craton,and change the isotopic composition of its lithospheric mantle.