Trapped liquid from a nepheline syenite: a re-evaluation of Na-, Zr-, F-rich interstitial glass in a xenolith from Tenerife, Canary Islands

Interstitial glass and pyroxene in a nepheline syenite described by Wolff (1987) have been more precisely and accurately re-analysed using a cryogenic microprobe technique. The glass compositions, with Na₂O = 13.4?17.0 wt.% and (Na+K)/Al = 1.81?2.26 at SiO₂ = 53?56.5 wt.% (on a 100% normalised basis) are more extreme than any known erupted magma. ZrO₂ contents of the glasses are 1.4%–1.8% by weight, while F ranges between 1% and 1.4%. The high Zr content is due to the failure of a Zr-silicate phase to precipitate, despite predicted zircon saturation of the liquids above their probable quench temperatures. Partitioning of Zr between pyroxene and liquid is highly variable and apparently controlled by sector zoning; Zr is preferentially incorporated into aegirine {001} growth sectors. The glasses demonstrate the extent to which peralkaline phonolitic liquids can be fractionated in closed magma systems.