LA-ICP-MS and TEM constraints on the magmatic and post-magmatic processes recorded by the zircon-xenotime intergrowth in pegmatite (Piława Górna, Góry Sowie Block, SW Poland)

Fabian Tramm,Richard Wirth,Bartosz Budzyń,Jiří Sláma,Anja Schreiber

doi:10.1016/j.lithos.2021.106480

Abstract

A strongly altered zircon-xenotime intergrowth in pegmatite from the Piława Górna (the Góry Sowie Block, SW Poland) was studied with transmission electron microscopy (TEM) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Distinct domains demonstrate features related to fluid-induced coupled dissolution-reprecipitation processes resulting in microscale to nanoscale porosity in the zircon and in xenotime, and diffusion-reaction processes in the metamict core of the zircon. The inclusions of secondary phases include fluorapatite, unknown layered Fe-silicate, fibrous Fe-oxides, solid solutions of coffinite-thorite and uraninite-thorianite, uraninite and secondary U-rich zircon. The fluid had high concentrations of Ca and Fe, transporting these nonformula elements through the zircon and xenotime structure while mobilizing U, Th and Pb even in robust xenotime, thus forming small inclusions in nanopores. Nanoinclusions of galena in the zircon represent major Pb mobilization that may cause disturbance during UPb geochronological analysis. The trace element composition of zircon matches patterns typical of altered zircon. The differentiation of geochemical characteristics between the metamict core and the altered rim of zircon implies different alteration mechanisms, with a stronger role of fluids in the alteration of the rim. Nanoscale observations complement trace element microanalysis and serve as an important and necessary tool to further understand the alterations of geochronometers in the presence of alkaline-rich fluid, especially in the complex environments of pegmatites.

Highlights

Zircon (ZrSiO4) is unambiguously the most frequently used mineral in the geochronology of igneous and metamorphic processes, and in provenance studies (Harley and Kelly, 2007; Nemchin et al, 2013)
The variation in light rare earth elements (LREE) grad ually decreases with decreasing ionic radius, from La to Eu
transmission electron microscopy (TEM) investigations of the zircon-xenotime intergrowth demon strated characteristic and complex features of different alteration pro cesses, such as: (i) Diffusion-reaction processes in the metamict core of the zircon generally promoting nanoporosity and partial repair of the in ternal structure of the zircon core, albeit showing limited element transport locally and accumulating aggregates of larger microinclusions. (ii) Coupled dissolution-precipitation processes resulting in major porosity and element mobilization of U, Th and Pb outward, whereas nonformula elements such as Ca and Fe were transported inward in the fluids

Summary

Introduction

Zircon (ZrSiO4) is unambiguously the most frequently used mineral in the geochronology of igneous and metamorphic processes, and in provenance studies (Harley and Kelly, 2007; Nemchin et al, 2013). Metamictization occurs in zircon through α decay of U and Th (or Pu), resulting in the destruction of the crystal lattice due to the recoil of the heavy daughter isotope when injecting a lighter alpha particle into the structure, leading to local displacement of atoms (Ewing et al, 2003; Geisler et al, 2007). The number of these recoil events destroying the zircon structure depends on the concentration of alpha emitters, their half-life and the duration of this process. The accumulation of crystal structure damage from irradiation is opposed by annealing effects, repairing the distorted crystal structure (Nasdala et al, 2001)

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Discussion

Conclusion