RAMAN SPECTROSCOPIC STUDY OF SHOCK-METAMORPHOSED ROCK FRAGMENTS RECOVERED FROM THE AUSTRALASIAN MICROTEKTITE LAYER

The Australasian microtektite layer has been found throughout the Indian Ocean, western equatorial Pacific, and the Philippine, Sulu, and Celebes Seas. Abundant microtektites and unmelted impact ejecta, compared with other Australasian microtektite-bearing sites, were found in cores from several sites in the South China Sea. At these sites, shock-metamorphosed rock fragments were found in this microtektite layer for the first time. Previous X-ray diffraction (XRD) studies indicate that these rock fragments are composed primarily of coesite, quartz, a mica phase (illite?), and, occasionally contain traces of stishovite. Previously determined major oxide compositions suggest the presence of K-feldspar and plagioclase and the following trace minerals: garnet, rutile, ilmenite, zircon, titanite, barite, apatite, Al₂SiO₅, iron oxide phases, and possibly calcite and dolomite. We studied a few of the shocked-rock fragments using Raman spectroscopy. Most grains in the rock fragments are highly fluorescent, possibly due to shock metamorphism. Many grains appear to be glassy and may be diaplectic since they contain no obvious vesicles. Raman spectroscopy confirmed the presence of coesite and a phyllosilicate, in agreement with XRD data. The presence of orthoclase, plagioclase, calcite, dolomite, titanite, ilmenite, and apatite was also confirmed. An iron-rich phase was identified as magnetite. Rutile was confirmed, but it was found that some of the rutile grains had been converted mostly or completely to the high-pressure TiO₂ polymorph with an αPbO₂ structure (TiO₂II). This is only the third known occurrence of TiO₂II associated with an impact event. Two grains originally identified as zircon based on their composition, produced Raman spectra that do not match those of zircon, reidite (a high-pressure polymorph of zircon), or baddeleyite plus silica, and may, therefore, be an unknown high-pressure polymorph of zircon. The strongest bands for this phase (or phases) occur at: 731, 388.6, 778, 447.6, and 512.5 cm-1. The mineral assemblage in the rock fragments supports a sedimentary origin for the rock fragments and the discovery of the high-pressure polymorph of rutile (TiO₂II), in addition to coesite and stishovite, supports the conclusion that the rock fragments are impact ejecta.