Impurities and defects in, and isotope compositions of, gemstones

• Author / Creator

  Krebs, Mandy Y

• Different laser ablation and mass spectrometry techniques and FTIR spectroscopy have been used to investigate elemental impurities, defects, and stable and radiogenic isotope compositions in natural diamond and ruby, with the aim of expanding previous research into diamond and ruby genesis, aiding exploration efforts and developing techniques to “fingerprint” a gemstone’s origin. First I use FTIR and SIMS techniques to evaluate the basis for using microdiamonds as grade predictors for diamond deposits. Predictions of the macrodiamond grade of newly discovered kimberlites are commonly obtained using size frequency distributions of microdiamonds, based on the implicit assumption of a genetic relationship between all diamonds across the entire size range. Despite the success of this approach, such a genetic relationship between microdiamonds and macrodiamonds has not been placed on a sound scientific footing. In contrast to previous comparative studies on microdiamonds and macrodiamonds from single deposits, here all diamonds analyzed – from the Misery deposit, Northwest Territories, Canada - originate from the same microdiamond bulk sample (558 diamonds; ranging in size from 0.212 to 3.35 mm). The diamonds were analyzed for their carbon isotopic compositions and nitrogen aggregation and concentration characteristics, and based on this dataset statistical comparisons were conducted across the size range to assess cogenesis. Overall, a general geochemical similarity of diamonds from across the different size fractions was established, reinforcing the use of size-frequency analysis to predict diamond grade in kimberlite diamond deposits. A novel off-line laser sampling technique has been applied to analyzing high purity diamonds. Quantitative trace element data from high-purity gem diamonds from the Victor Mine, Ontario, as well as near-gem diamonds from peridotite and eclogite xenoliths from the Finsch and Newlands mines, South Africa, acquired using an “offline” laser ablation method show that we see the same spectrum of fluids in gem diamonds that was previously documented in fibrous diamonds, strengthening the link between the parental fluids of both gem and fibrous diamonds. Differences in trace element characteristics are evident between Victor diamonds containing silicate inclusions compared with Victor diamonds containing sulphide inclusions. The sulphide-bearing diamonds show lower levels of inter-element fractionation and more widely varying siderophile element concentrations - indicating that the silicate and sulphide-bearing diamonds likely formed by gradations of the same processes, via melt-rock reaction or from a subtly different fluid source. Trace element characteristics of rubies from Greenland, Mozambique, Myanmar and Vietnam obtained using LA-ICPMS, TOF-LA-ICPMS and offline ablation followed by solution ICPMS show that, with the exception of a small number of elements that can substitute for Al3+ in the crystal structure (e.g. Ti, Fe), trace elements in ruby are present in mineral inclusions or along fractures. Thus, the absolute trace element abundances in rubies are primarily a function of impurity density. TOF-LA-ICPMS, applied to rubies for the first time, effectively maps trace element spatial variation in these gems and clearly demonstrates a mineral inclusion host in some rubies for elements such as Sr, Rb, U and Th. Using the same offline laser ablation technique that was applied to the diamonds, followed by thermal ionization mass spectrometry (TIMS) we were able to measure radiogenic isotope compositions in ruby for the first time. Pb-Pb isochron ages for the two subpopulations in the Aappaluttoq suite (2558 ± 68 Ma for a low-V group and 3059 ± 380 Ma for the high-V group) are the first ever direct ages determined on a ruby suite, independent of associated minerals and - if reflecting ruby crystallisation or re-crystallisation - prove either their crystallisation or re-crystallisation and re-setting of the U-Pb system in the Neoarchean in SW Greenland. We have demonstrated, using rubies from four different localities and two different deposit types, that Ni and Pb, in conjunction with established discriminator elements such as Ti and V show great potential for the use in geographical origin determination. A triangular plot of Ni-V-Pb makes it possible to distinguish between both the different localities and the different deposit types for all analysed rubies in this study. Measured 87Sr/86Sr and Pb isotope ratios show pronounced differences for rubies from different localities and deposit types, suggesting that radiogenic isotopes potentially offer a powerful means of provenance discrimination even for geologically similar deposits.

• Subjects / Keywords

  • Diamond
  • Ruby
  • Trace Elements
  • LA-ICPMS
  • Radiogenic Isotopes

• Graduation date

  Spring 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3CF9JN5Q

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.