Nd-Hf isotopic systematics of the arc mantle and their implication for continental crust growth

Abstract

The 143Nd/144Nd and 176Hf/177Hf values of depleted mid-ocean ridge basalt (MORB) mantle have long been used to calculate the Nd and Hf model ages and constrain the growth rate of the continental crust. However, because continental crust forms principally at subduction zones, the appropriate 143Nd/144Nd and 176Hf/177Hf for use in model age calculations are those of the mantle wedge that underlies the juvenile magmatic arcs. In this study, we compile the Nd and Hf isotopic compositions of the modern island arc volcanic rocks. The average 143Nd/144Nd value obtained for the arc mantle is 0.512959 (eNd = +6.3), with upper and lower 95% confidence uncertainties of 0.513115 (eNd = +9.3) and 0.512613 (eNd = -0.5), respectively. The average obtained for 176Hf/177Hf is 0.283144 (eHf = +13.2) with upper and lower limits of 0.283272 (eHf = +17.7) and 0.282927 (eHf = +5.5), respectively, at the 95% confidence level. The Nd and Hf isotopic compositions of the modern island arc volcanic rocks can be divided into three populations, the high, transitional and low e arc mantle populations, that reflect a combination of mantle heterogeneity and additions of different slab-derived components to the mantle wedge. The high e arc mantle (HAM) population (eNd +5.7 and eHf +10) were metasomatized by the transfer of unradiogenic Nd and Hf from pelagic sediments, and Fe-Mn crusts and nodules, to the mantle wedge, the HAM by fluid dominate process and the TAM by a combination of sediment melts and fluids. In contrast, melts derived from subducted sediments, particularly zircon-rich turbidites, play a more important role in the low e arc mantle population (LAM, eNd < +5.7 and eHf < +10). The average of both Nd and Hf isotopic data for the arc mantle, together with the 95% confidence limits, are used to define the arc mantle growth curves and their uncertainties. We suggest that these arc mantle growth curves should be used to calculate Nd and Hf model ages and constrain the growth rate of the continental crust.