Zirconium-rich nanophases in an experimental phonolitic melt

Abstract

Many experimental studies have addressed the crystallisation of zircon, not least because zircon is of paramount importance as a geochronometer. Today, it is well established at what temperature, melt composition, and Zr concentration a silicate melt reaches zircon saturation. It remains unclear though what course a Zr bearing melt takes before zircon appears as a macroscopic phase. Does zircon nucleate directly from the melt, or does it use during nucleation and growth Zr-rich nanoparticles as fundamental building blocks? The question is relevant because Zr 4+ is a high field strength (HFS) cation that should tend to polymerise in silicate melts to (ZrO 2 ) n clusters. To fill that gap of knowledge, we performed experiments with a Zr-enriched phonolitic melt at 1200 (outside zircon stability) and 900° C (inside zircon stability). We investigated the glasses with Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) for Zr-rich nanoparticles, at a spatial resolution not achieved by previous experiments involving zircon. A wide range of nanoparticles is identified, including baddeleyite, zircon, zirconium titanate ZrTiO 4 , and rutile. The diverse range of nanoparticles is probably owed to the fact that local-scale gradients in silica (aSiO 2 ) among silicate melt pools prevailed despite the high run temperatures. The smallest and presumably earliest phases are (ZrO 2 ) n nanoparticles trapped by liquidus corundum. They reach diameters around 2 nm or ~ 200 unit cells if they are crystalline. The results support the assumption that Zr 4+ dissolves in silicate melts as ZrO 2 monomers, then quickly polymerises to (ZrO 2 ) n clusters. The same may be valid for other HFS cations. Possible applications to HFS element depletions in arc basalts are discussed. At 900° C inside zircon stability, many examples are noted where zircon grows by peritectic reaction of baddeleyite nanoparticles with SiO 2 of the melt although zircon nanoparticles also exist. Larger baddeleyite grains around 50 nm seem to grow by aggregation of smaller (ZrO 2 ) n nanoparticles. That zircon also grows by particle attachment cannot be confirmed. Zircon can also crystallise directly from the melt when in melt pools the aSiO 2 and Zr contents were high enough to stabilise zircon.

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