COUNTRY-OF-ORIGIN DETERMINATION OF MODERN GEM PERIDOTS FROM LA-ICPMS TRACE-ELEMENT CHEMISTRY AND LINEAR DISCRIMINANT ANALYSIS (LDA)

Gem peridots in the modern market come mainly from four geographic areas: China, Pakistan, Myanmar (Burma) and the southwestern US. Even though peridot is not a highly valued gemstone; it was used in some ancient jewelry and studying country of origin of modern peridots can provide useful tools for studying the ancient artefacts.

The main goal of this paper is to describe the numerical analysis procedure and to illustrate its value for gem origin determination. We used a well established statistical method - linear discriminant analysis (LDA) to study the modern peridots and found using the following elements: Li, V, Mn, Fe, Ni, Co, Zn, Na, Al, K, Ti, Cr, Ag, and Au in LDA can provide very good separation of the four geographic sources.

The trace element concentrations were obtained using a Laser ablation-inductively coupled plasma mass spectrometer (LA-ICPMS) system, which is a Thermo X-series II quadrupole ICPMS coupled with a New Wave Research UP-213 laser ablation unit. The laser conditions were set to a 55 micrometer spot in diameter, laser is running at 7 Hz repetition rate with a fluence around 10 J/cm². The ICPMS was running at 1300 W forward power and was tuned to a condition that maximizes beryllium counts while keeping ThO/Th ratio less than 2%. NIST SRM 610 and 612 as standards are used in a typical run, three spots on each standards were run before the actual sample and three additional spots on the NIST 610 were added after two samples were run. At the end of the run, three spots on the NIST 610 are collected to correct the instrument shifts. All analysis use ²⁸Si as internal standard for the peridot spots.

The results demonstrate that a combination of LA-ICPMS trace element chemistry data and LDA can provide a basis for determining the origin of a gem peridot better than 83 % accuracy rate. Additionally, utilizing plots of various discriminant functions can provide even better separation. The same methodology can be applied to study country of origin of other gemstones, such as Paraiba tourmalines and pearls; as well as to study the ancient jewelries.