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Tracking magmatic processes through Zr/Hf ratios in rocks and Hf and Ti zoning in zircons: An example from the Spirit Mountain batholith, Nevada

¹ Earth and Environmental Sciences, Vanderbilt University, SC 5717 Science & Engineering Bldg., Stevenson Center Dr., Nashville, TN, USA
² US Geological Survey, USGS-Stanford Micro-isotopic Analytical Center, Stanford University, Green Building, 367 Panama St., Stanford, CA, USA
³ Department of Mineralogy and Petrology, Campus Fuentenueva, University of Granada, 18002 Granada, Spain

^* E-mail: lily.e.lowery{at}vanderbilt.edu

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 themselves provide 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, document 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 (>100°C) for individual zircons. Such variations are consistent with field relations and ion-probe zircon geochronology that indicate a >1 million year history of repeated replenishment, fractionation, and extraction of melt from crystal mush to form the low Zr/Hf high-SiO₂ zone.

KEYWORDS: zircon, hafnium, zirconium, high-silica granite, Ti-in-zircon thermometry, fractionation

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