SEM/EDS-Assisted LAM-ICPMS Analyses of Tourmaline of Tourmalinites Hosted in Serpentinites of the Paso Del Dragón Complex, Northeastern Uruguay

Gianna M. Garda, Sandra Andrade, Elena Peel-Canabal
2016 Microscopy and Microanalysis  
The Cerro La Tuna (NE Uruguay) is composed of deformed and metamorphosed mafic-ultramafic rocks of Neoproterozoic age. Serpentinites predominate and are associated with magnesian schists, banded micaceous schists and quartzites, and amphibolitic schists. Tourmalinite occurs as boudins in the serpentinites and is composed of radiating fans of acicular, cm-long tourmaline and chlorite [1]. Tourmaline has successfully been applied in the interpretation of geologic processes. Thanks to certain
more » ... nks to certain characteristics, tourmaline can acquire a chemical signature from the rock in which it develops and can retain that signature through geologic time [2]. As a contribution to the understanding of the still debatable origin of Paso del Dragón Complex mafic-ultramafic rocks [3, 4], we started a SEM/EDSassisted LAM-ICPMS study of the Cerro La Tuna tourmaline, aiming at its chemical characterization. LAM-ICPMS study requires a 80 micron-thick polished thin section, which was coated with carbon for the preceding SEM/EDS session. A scanning electron microscope LEO 4401 coupled with an Oxford Inca EDS system was used for the acquisition of back-scattered electron (BSE) images and semiquantitative EDS analyses of major elements in tourmaline and identification of accessory minerals (very bright phases in Fig. 1 ). The carbon coating was removed from the thin section and the spots in tourmaline analyzed by EDS were mapped, in order to perform the LA-ICPMS analyses as close as possible to the EDS analyses. The mass spectrometer Thermo Scientific iCAP Q used in this study is coupled with a New Wave Research UP-213 laser ablation system. Laser ablation of internal spots in tourmaline (e.g. P2-c4 and P2-4b in Fig. 1 ) was performed with a 65-micron laser-spot diameter, 85% laser power, 5-micron depth and 10 Hz repetition rate. Analyses along tourmaline rims (e.g. P2-r4 in Fig. 1) were performed in raster mode, adopting 70% laser power, 2 micron/sec scanning speed, 5-micron depth, and 25-micron spot size. Laser-sample interaction time was 60 sec, preceded by a 60-sec analysis of blank. Glitter 4.4.2 software aided on-line correction of instrumental drift, fractionation and data reduction. All element concentrations were normalized using Mg as an internal standard. As revealed by BSE images and EDS analyses, darker gray areas in tourmaline (Fig. 1) correspond to higher Mg contents and the MgO content used was 11.5 wt.%; for brighter gray areas, the MgO content used was 10.15 wt.% (EPMA in [1]). Standard NIST-610 was used for calibration and BHVO-2 for quality control. The advantage of monitoring LAM-ICPMS analyses is the possibility of correction of the data from interferences caused by accessory minerals, such as monazite, Y and Ti oxides (Fig. 1) . Figure 2 shows the compositional variations in brighter and darker areas in tourmaline. Similar variations to graph (a) were obtained for Al2O3 and SiO2 (EPMA in [1]) and Mn; similar to graph (b), FeO (EPMA in [1]) and Zn, and to some extent: Li, V and Sr. Chondrite-normalized REE concentrations 446
doi:10.1017/s1431927616003081 fatcat:tpkzltcfzba45kkuanjpd43vtm