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 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 (TTiZ) (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 ∼307–40 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 TTiZ (>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.

Zircon texture and chemical composition as a guide to magmatic processes and mixing in a granitic environment and coeval volcanic system

“…Timms et al 2006;Reddy et al 2009). The chemical composition of zircon may be used to elaborate many magmatic processes, including crystal fractionation (Wark and Miller 1993;Thomas et al 2002;Pettke et al 2005;Lowery Claiborne et al 2006), interaction with hydrothermal fluids (Thomas et al 2002;Hoskin 2005) and/ or magma mixing (Wang et al 2002;Belousova et al 2006), as well as being a potential source indicator (Belousova et al 2002). The zircon crystallization history may provide unique insights into magma petrogenesis, provided that the magma system is reasonably well characterised geochemically and isotopically.…”

“…Lowenstern et al 2000;Wang et al 2002;Miller and Wooden 2004;Kemp et al 2007;Lowery Claiborne et al 2006;Yang et al 2007;Miller et al 2007). Zircon is an extremely robust mineral that can crystallize over a wide range of magmatic temperatures, essentially depending on the bulk composition of the host magma (and associated source rock; Watson and Harrison 1983;Harrison and Watson 1983).…”

Accessory mineral U–Th–Pb ages and 40Ar/39Ar eruption chronology, and their bearing on rhyolitic magma evolution in the Pleistocene Coso volcanic field, California

“…In general, unpolished surface analyses (''up'' in Table 4) yield higher U/Th ratios that vary from *2.4 to *4.3 (in one case up to 12.6), indicating increasing U/Th during crystallization. Rimward enrichment of incompatible elements is consistent with magmatic zircon growth (e.g., Claiborne et al 2006). The Pleistocene and Mesozoic zircons are zoned in backscatter-electron imaging as well as cathodoluminescence images (Fig.…”

Oxygen isotope and trace element geochemistry of zircons from porphyry copper system: Implications for Late Triassic metallogenesis within the Yidun Terrane, southeastern Tibetan Plateau

“…The negligible Eu anomalies present within the PDL zircons are consistent with hydrous parental melts, as evidenced by the presence of hydrous phenocryst phases such as amphibole and biotite within bulk rock samples, a factor that probably suppressed the early crystallization of plagioclase (Blatter and Carmichael, 1998;Moore and Carmichael, 1998;Müntener et al, 2001) and resulted in weak or absent Eu anomalies. Hafnium can be used to monitor magmatic differentiation, as higher Hf concentrations are indicative of zircons generated from more evolved melts from which the early crystallized zircons have already been removed (e.g., Claiborne et al, 2006Claiborne et al, , 2010Castiñeiras et al, 2011;du Bray et al, 2011). Comparing hafnium concentrations with Eu anomalies in zircons from the same sample can therefore be used as an index of plagioclase fractionation (Castiñeiras et al, 2011;du Bray et al, 2011).…”

“…Magmatic zircon textures and compositions (e.g., REE, U, Hf, Yb, and Y) are sensitive indicators of magma compositions and the tectonic setting of magmatism (e.g., Wilde et al, 2001;Belousova et al, 2006;Claiborne et al, 2006;Grimes et al, 2007;Carley et al, 2014;Grimes et al, 2015). The PDL zircons have chondrite-normalized REE Sr/ 86 Sr = 0.715068 (Zhang, 2008).…”