SEM-EBSD Analysis of Broad Ion Beam Polished Rock Thin Sections - The MFAL Protocol

摘要

A protocol is presented to perform Broad Ion Beam Polishing (BIBP) of rock thin sections to obtain a surface that can yield high quality crystallographic preferred orientation data using SEM-EBSD. The modus operandi, referred to as Mamtani's Fabric Analysis Lab (MFAL) protocol, involves flat milling of a rotating rock thin section using a triple ion beam milling system (Leica EM TIC 3X) in which tilt angle of sample (degree) with reference to ion beam (Argon), acceleration voltage (kV) and source current (mA) can be controlled along with lateral and rotary movements. The protocol has been developed after >90 hours of BIBP experiments on various rock samples under different experimental settings (tilt angle/kV/mA) followed by SEM-EBSD analysis. Each experiment was performed at medium rotation speed (7 rpm) with a lateral movement of upto +/- 7 mm on rectangular rock thin sections (long dimension <25 mm). The best surface for SEM-EBSD analysis of rock thin sections is obtained in two steps of BIBP. In step-1, cleaning of thin section is done for 5 minutes (1.5 degrees/4 kV/1.5 mA). In step-2, polishing is done for 30 minutes (1.5 degrees/4 kV/3 mA). BIBP following MFAL protocol yields very good SEM-EBSD data from hard and soft mineral phases in rocks that enable (a) determination of mineral slip systems, (b) kinematics and (c) EBSD data-based mapping of grain boundaries of soft mineral phases (like hornblende) for petrofabric analysis in a polymineralic rock, which has always remained a challenge from mechanically polished samples. It is concluded that the MFAL protocol saves more than 5 hours of sample preparation time over other sample preparation methods for SEM-EBSD studies.