Ancient and modern subduction zone contributions to the mantle sources of lavas from the Lassen region of California inferred from Lu-Hf isotopic systematics

Abstract

Hafnium isotopic compositions have been determined on a suite of calc-alkaline and high-alumina-olivine tholeiitic lavas from the Lassen region of California and are used, in conjunction with previously published mineralogical, geochemical, and isotopic data, to constrain their petrogenesis. Positive correlation between ε Hf values and geochemical indices of the modern subduction component indicates that the isotopic compositions of the calc-alkaline lavas record addition of radiogenic Hf from the subducted slab. However, the addition of the modern subduction component increases the ε Hf values of most calc-alkaline lavas by <0·5 units over estimates of non-subduction enriched peridotites of the mantle wedge. The Lu–Hf isotopic systematics of the Lassen lavas suggest that the calc-alkaline magmas have equilibrated with garnet at some point in their history, whereas the tholeiitic magmas have not. These observations require the two lava types to be derived from different sources. The isotopic variability of the Lassen lavas cannot be produced by mixing mantle sources inferred to be present in the eastern–central Pacific and western USA with a modern subduction component. Instead, the isotopic variability is consistent with mixing of a depleted mantle source, a more fertile mantle source enriched by an ancient subduction component, and a modern subduction component.