CONSTRAINING RUBY PROVENANCE USING SR - PB ISOTOPES AND TRACE ELEMENTS AND DATING RUBY CRYSTALLISATION USING PB-PB GEOCHRONOLOGY: A NEW APPROACH

Rubies are amongst the most valuable gemstones. The commercial value of a ruby depends on its size, its colour and its clarity. For ethical reasons its geographic origin is also a critical factor. The gem industry has long been interested in establishing accurate methods to identify the provenance of rubies. Trace element analysis can potentially provide an elemental fingerprint of rubies and improve our understanding of the source characteristics of ruby-forming fluids and crystallization environments. However, because most elements in ruby are present at ultra-trace levels, fully quantitative data remain sparse. In addition, elemental geochemistry is not efficient at distinguishing between rubies from different locations that share similar geological settings. Using a novel off-line laser ablation technique we analyzed rubies from two different ruby deposits with similar geological characteristics; Namahaca (Mozambique) and Aappaluttoq (SW Greenland). We obtain the first radiogenic isotope compositions of ruby for use in provenance determination and we obtain the first ever radiometric date of ruby (re?)crystaliisation. We also produce the most extensive elemental dataset so far for in-situ LA-ICP-MS.

We obtained quantitative trace element data (> LOQ @ 10σ bg) for 30 elements. Rubies from both localities show very similar trace element characteristics, however, the Aappaluttoq rubies show greater variance and higher median concentrations for numerous elements. The broader spectrum of quantitative trace element data makes it possible to use a wider range of elements to discriminate between rubies of different localities and/or deposit type.

Sr isotopic compositions for the Namahaca rubies are less variable (⁸⁷Sr/⁸⁶Sr 0.7089 - 0.7291, n=9) than the very wide range and more radiogenic values of Aappaluttoq rubies (0.7247 to 0.934, n=8). This clearly delineates two distinct populations and reflects the contrasting age of the host rocks. Multiple Pb isotopes mirror the greater variability of Aappaluttoq rubies. The results define the first ever Pb-Pb isochron age for ruby crystallization, proving their crystallisation, or re-crystallisation and re-setting of the Pb-Pb system in the Neoarchean.