Experimental Melting of Obsidian by Laser Pulses with Implications for the Analysis of the Composition of Impact Glasses

摘要

The processes of melting and evaporation during impact cratering are examined in model experiments on the laser melting of obsidian of a composition close to that of the targets of many terrestrial impact craters.The experiments were-conducted using a laser at a temperature of approx 4000 K in the focusing region of the laser beam and pulses approx 10~(-3) s in duration.The process of melting and the splashing of the melt produced numerous glass spherules ranging from 0.5 to 20 mum in diameter,whose major-element composition was determined on a microprobe.The statistical treatment of the analytical data demonstrates that much of the glass underwent deep chemical transformations due to the selective evaporation of Na,Fe,and Si.Compared with pristine obsidian,the concentrations of low-volatile Al,Ca,and Ti increased in some glass fragments by factors of nearly three.Our data led us to propose a method for identifying high-temperature glasses in impact craters.The application of the method is demonstrated by the example of impact glasses from the Logoiski crater.