Preliminary results of a rock-magnetic study of obsidians from central Mexico

Abstract

Rock-magnetic parameters of obsidian samples from 18 localities of the Trans-Mexican volcanic belt indicate significant differences in relative contents of iron-titanium oxides and grain sizes of up to five orders of magnitude, particularly in low-field magnetic susceptibility, NRM intensity and IRM intensity. Obsidians from 13 deposits show susceptibility values between 40 and 1500 10-6 SI units, NRM intensities between 40 and 7130 mA/m, and IRM intensities between 1370 and 23900 mA/m. Samples from five localities show lower values, with susceptibilities between 10 and 20 10-6 SI, NRM intensities between 0.1 and 26 mA/m, and IRM intensities between 4 and 500 mA/m. Log-log plots of combinations of these magnetic parameters allow characterization of obsidians from different volcanic sources. However, further studies are required to permit a distinction of given deposits within the two major groups.