Application of Laser-Induced Breakdown Spectroscopy and Principal Component Analysis for Compositional Zoning of Liddicoatite Tourmaline from Anjanaboniona

Tourmalines from the Anjanabonoina pegmatite field, Central Madagascar, exhibit some of the most complex multi-color zoning patterns known. These tourmalines are also rare because of their unusual Ca- and Li-rich liddicoatite composition. Liddicoatite specimens crystallize in miarolitic pockets in pegmatites, which periodically break open and seal. Multivariate analysis of Laser-Induced Breakdown Spectroscopy (LIBS) spectra of an Anjanabonoina liddicoatite specimen allows evaluation of simultaneous changes in all elements during crystallization. LIBS is an optical emission technique in which photons emitted from a cooling laser plasma are diffracted and recorded as a spectrum. All elements present in the sample at concentrations above their inherent detection limits are represented by peaks in the spectrum. Principal Component Analysis of 123 LIBS spectra acquired in a core-to-rim traverse reveals six major compositional zones that suggest four stages of crystallization, the last three of which begin with the opening of the pocket and mixing of pegmatitic fluids with those from the metasedimentary host rocks.