BORON & LITHIUM ISOTOPIC PROFILES IN MULTI-COLOURED PARAIBA TOURMALINES FROM BRAZIL

Copper-bearing tourmaline exhibits a wide range of colors and is the most sought after of the gem tourmalines, matching values which are usually reserved for precious minerals such as emeralds and rubies. The name “Paraiba” tourmaline was initially derived for Cu- and Mn- bearing tourmalines displaying neon blue-green coloration from the da Batalha Mine in Paraíba state, Brazil, where the gem was discovered. However since its first appearance on the market additional sources of Cu-bearing “Paraiba-type” tourmaline have been located in a neighboring Brazilian state as well as in Nigeria and Mozambique. Limited success has been achieved for distinguishing the highest quality “Paraiba-type” material from the authentic Paraiba tourmalines using chemical analysis. A recent study by Peretti et al. (2009) investigated variations in chemical concentrations in Cu-bearing color-zoned tourmalines. Using EMPA element distribution maps in conjunction with chemical profiles by LA-ICP-MS, they showed that “Paraiba-type” tourmalines are chemically complex and zoned and therefore trace element analysis alone cannot be used to distinguish between sources. However, the study by Shabaga et al. (2010) showed that the B and Li isotopic compositions of “Paraiba-type” tourmalines can be used to distinguish between tourmalines from Brazil, Nigeria and Mozambique, and provide information on the source of B and Li for these gem minerals. δ¹¹B values of tourmalines from Brazil (-42.4‰ to -32.9‰), Nigeria (-30.5‰ to -22.7‰) and Mozambique (-20.8‰ to -19.1‰) were found to be the main discriminator and are consistent with a non-marine evaporite source. Here we use secondary ion mass spectrometry (SIMS) to measure the stable isotopes of B and Li in zoned tourmalines with a range of colors (neon blue to violet-pink) from Paraiba, Brazil. The main objective of this study is to determine whether there is a correlation between zones within a single grain of tourmaline and their B and Li isotopic compositions. Data from this study could provide information on the fluid evolution associated with zoned tourmaline and further test the utility of using B and Li isotopes as provenance indictors of Paraiba tourmalines. Refs: Peretti, et al. (2009) Contributions to Gemology, 9:1-77; Shabaga, et al. (2010) Mineralogical Magazine, 74:241-255.