MELT INCLUSIONS IN ZIRCON

Abstract

Silicate melt inclusions (MI) are small samples of melt that are trapped during crystal growth at magmatic pressures and temperatures. The MI represent a sample of the melt that was isolated from the bulk melt during host crystallization. Thus, MI preserve the composition of the melt that was present during crystal growth and record the P-T growth conditions. As such, MI provide a valuable tool for constraining the magmatic history of igneous systems. Melt inclusions may be composed of a single-phase glass or they may contain multiple phases (vapor bubbles ± crystals) that nucleated from the melt within the inclusion during cooling, or were produced by devitrification of the glass following trapping. Heating and homogenization techniques applied to multiphase MI produce a glass suitable for microanalysis, and may also provide information regarding the temperature of MI and crystal formation (Roedder 1984). Many workers have used MI in major rock-forming minerals to constrain igneous processes, such as crystal fractionation and magma degassing (e.g., Anderson et al. 1989, Hervig and Dunbar 1992, Sobolev and Shimizu 1993, Sobolev and Danyushevsky 1994, Lu 1991), but there have been far fewer studies of MI in accessory minerals (e.g., Chupin et al. 1998, Spandler et al. 2000, Sokolov 2002, Thomas et al. 2002). Melt inclusions in zircons of igneous origin are common, and have been identified in a wide range of igneous rock types, including quartz diorite and rhyolite, and in detrital zircon grains from heavy mineral sands (Fig. 1⇓). Figure 1. Transmitted light photographs of crystalline and glass melt inclusions (MI) in zircons. The zircons commonly contain solid inclusions of apatite (labeled “ap”), feldspar (labeled “fsp”) and less commonly, oxide minerals. The zircons in the photographs are from: the 56 Ma quartz dioritic Quottoon Igneous Complex in …