Tracking Magmatic Processes through Zr/Hf Ratios in Rocks and Hf and Ti Zoning in Zircons: An Example from the Spirit Mountain Batholith, Nevada

Abstract

Zirconium and Hf are nearly identical geochemically, and therefore most of the crust maintains near chondritic Zr/Hf ratios of ~35-40. By contrast, many high-silica rhyolites and granites have anomalously low Zr/Hf (15-30). As zircon is the primary reservoir for both Zr and Hf and preferentially incorporates Zr, crystallization of zircon controls Zr/Hf, imprinting low Zr/Hf on coexisting melt. Thus, low Zr/Hf is a unique fingerprint of effective magmatic fractionation in the crust. Age and compositional zonation in zircons provides a record of the thermal and compositional histories of magmatic systems. High Hf (low Zr/Hf) in zircon zones demonstrates growth from fractionated melt, and Ti provides an estimate of temperature of crystallization (T(TiZ)) (Watson and Harrison, 2005). Whole-rock Zr/Hf and zircon zonation in the Spirit Mountain batholith, Nevada, documents repeated fractionation and thermal fluctuations. Ratios of Zr/Hf are ~30-40 for cumulates and 18-30 for high-SiO2 granites. In zircons, Hf (and U) are inversely correlated with Ti, and concentrations indicate large fluctuations in melt composition and T(TiZ) (>100º C) for individual zircons. Such variations are consistent with field relations and ion probe zircon geochronology that indicate a million year history of repeated replenishment, fractionation, and extraction of melt from crystal mush to form the low Zr/Hf high-SiO2 zone.