INSIGHTS INTO THE ANALYSIS OF MICROMETRE-THIN RIMS USING LA-ICP-MS: DEPTH-PROFILE AND GRAIN MAPPING OF UNPOLISHED CRYSTALS

English Abstract:

Zircon, in the presence of alkaline fluids, can undergo coupled dissolution-reprecipitation. This fluid mediated alteration manifests itself as µm-scale rims on the outer surfaces and within existing imperfections of zircon crystals. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was examined as a tool to interrogate the lateral and depth variation of isotopic (U-Pb) and elemental (REE) distributions within the rims of unpolished zircons from an Archean metasediment. Our LA-ICP-MS U-Pb depth profile technique was able to identify micron-thin (<3 μm), concordant, isotopically distinct mineral domains (rims) characterized by a ca. 100 m.y. younger ²⁰⁷Pb/²⁰⁶Pb age associated with 2σ age-uncertainties as low ~0.2%, as well as elevated U content relative to the zircon interior (up to an order of magnitude). Our calculated penetration rate suggests that each laser pulse excavates depths of ~0.06 μm. Ages resolved through the LA-ICP-MS U-Pb depth profile method overlap, within 2σ uncertainties, the ²⁰⁷Pb/²⁰⁶Pb ages measured using SIMS U-Pb depth-profiling on the same population of zircon. The rims are further evinced by the presence of a relative enrichment (>3 orders of magnitude) in REE measured using an independent LA-ICP-MS depth-profiling technique on the same zircon. The enrichment of REE was used as an indicator of rim material. LA-ICP-MS techniques were developed to generate trace element concentration maps, with a lateral spatial resolution of 7 µm, of the outermost 3 µm of unpolished zircon grains. These maps demonstrate the heterogeneous nature of the crystallization of rim material, commonly demonstrating preferred growth along fractures or within isolated domains on the grain surface. The LA-ICP-MS techniques are capable of quickly identifying, and chemically and isotopically characterising zircon rims, which are an indication of low temperature, yet geologically significant, fluid events that may otherwise remain unidentified.