AN ANALYSIS OF SUBMARINE ASH DEPOSITS NEAR THE CENTRAL BASIN FAULT, PHILIPPINE SEA

Volcanic ash layers were recovered in several deep sea sediment drill cores taken near the Central Basin Fault of the Philippine Sea on the JAMSTEC (Japanese Science and Technology Center) Expedition entitled Investigation of Ryukyu Trench and Central Basin Fault (CBF), Philippine Sea Steps-III (KR99-10).The study reported here is a preliminary examination of those ash samples. The purpose of the study is to characterize the ash particles and layers, and to identify their possible origin. Samples were rinsed and dried and then separated by particle size and magnetic susceptibility. Individual particles were mounted on slides and examined using an incident light binocular microscope, polarized light microscope, and scanning electron microscope (SEM). With the SEM, glass and mineral particles were imaged with secondary and backscattered electrons and analyzed chemically using energy dispersive analysis. The minerals feldspar, pyroxene and hornblende were identified, together with volcanic glass fragments and some microfossils. The glass is easily recognized by the presence of round or elongated vesicles. Two groups of glass compositions were found, one andesitic and one rhyolitic. The Philippine Sea is surrounded by volcanic arcs, including the Philippine, Ryukyu, Japanese, and Izu-Bonin-Mariana arcs. The composition of the glasses compared to volcanics from these locations will be presented.