Spectro-chemical study of moldavites from Ries impact structure (Germany) using LIBS

Abstract

The present study primarily involves the application of LIBS (Laser Induced Breakdown Spectroscopy) technique for the characterization of moldavite, a shocked silica-rich glass observed in parts of Central Europe. This impact ejecta formed due to the collision of a km-sized chondritic meteorite about 14.81 ± 0.2 Ma ago in parts of Southern Germany forming twin impact structures (Ries and Steinheim). The physical and chemical properties of these glasses have been extensively studied to ascertain the chronology of impact event, nature of source material and the type of impactor (meteorite). In this study, for the first time two representative moldavite samples are analyzed using LIBS technique followed by routine EPMA (Electron Probe Microanalysis). A recent study has demonstrated similarities between ejecta deposits of Bakhuysen crater, Mars and Ries impact structure. Since moldavites are unique impactites associated with Ries structure, this study further extends the possibility of discovering such shocked glasses in the Red Planet which has vast stretches of unconsolidated sediments similar to target lithology of Ries. The LIBS spectra of moldavite samples are recorded in the spectral range between 200 nm and 800 nm wavelength using a Nd:YAG laser source. The spectra show the presence of atomic lines of different elements (like, Sodium, Magnesium, Aluminum, Silicon, Potassium, Calcium, Chromium, Titanium, Manganese, Iron, Cobalt, Nickel and Barium) as well as lighter elements (Hydrogen, Carbon, Nitrogen and Oxygen). The relative abundance of large ion elements over the smaller ions is in agreement with previous studies obtained by conventional analytical methods. The LIBS spectra show the unequivocal presence of various siderophile elements (Chromium, Iron, Cobalt and Nickel) except Iridium, a geochemical fingerprint of the impactor, whose presence in this study remains inconclusive although Ni/Cr > 1 similar to the chondritic impactor. The LIBS and EPMA data of moldavite generated during the present study compliment very well and also are in agreement with previous reports. The ease of in-situ rapid elemental detection of various siderophile and light elements in impactites using LIBS technique shall be extremely useful in planetary exploration, especially for future rover missions.