Figure 4

Oxygen and Hf isotopes measured in dated zircons in co-registered spots demonstrating significant within-sample heterogeneity suggesting that magmas are not tapping a single homogenous reservoir and represent about equal mix of local crustal and mantle proportions. (A,B) Temporal variations, (C) correlation of O and Hf isotopes in co-registered spots. Fields are shown for zircon from neighboring volcanic centers: 1: Chegem and Zayukovo, 2: Tyrnyauz and 1.9 Ma ignimbrites in the base of Elbrus (data from30,35) Mixing line showing percentage of crustal contribution is drawn between crustal (δ18O = + 9‰, εHf = − 10, 4 ppm Hf) and mantle (δ18O = + 6‰, εHf = + 10, 2 ppm Hf ) end-members estimated as realistic for Caucasus27,30,35.