Artigo Revisado por pares

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

2006; Cambridge University Press; Volume: 70; Issue: 5 Linguagem: Inglês

10.1180/0026461067050348

ISSN

1471-8022

Autores

Lily L. Claiborne, Calvin F. Miller, B. A. Walker, J. L. Wooden, F. K. Mazdab, Ф. Беа,

Tópico(s)

High-pressure geophysics and materials

Resumo

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 ( 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 ∼307–40 for cumulates and 18–30 for high-SiO 2 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-SiO 2 zone.

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