VOLCANIC ASH PARTICLES AS CARRIERS OF REMANENT MAGNETIZATION IN DEEP-SEA SEDIMENTS FROM THE KERGUELEN PLATEAU

Carbonate sediments from the Kerguelen Plateau (ODP Leg 120) of Eocene to Pliocene age were investigated with rock magnetic, petrographic an d geochemical methods to determine the carriers of remanent magnetizat ion. Magnetic methods showed that the major magnetic minerals were tit anomagnetites slightly larger than single domain particles. Submicrome tre to micrometre-size grains of titanomagnetite were identified as in clusions in volcanic glass particles or as crystals in lithic clasts. Volcanic fallout ash particles formed the major fraction of the magnet ic extract from each sediment sample. Three groups of volcanic ashes w ere identified: trachytic ashes, basaltic ashes with sideromelane and tachylite shards, and palagonitic ashes. These three groups could be e qually well defined based on their magnetic hysteresis properties and alternating field demagnetization curves. The highest coercivities of all samples were found for the tachylite, due to the submicrometre-siz e titanomagnetite inclusions in the matrix. Trachytic ashes had interm ediate magnetic properties between the single-domain-type tachylites a nd the palagonitic (altered) basaltic ashes with low coercivities. Sam ples which contained mixtures of these different volcanic ashes could be distinguished from the three types of ashes based on their magnetic characteristics. There was neither evidence of biogenic magnetofossil s in the transmission electron micrographs nor did we find magnetic pa rticles derived from continental Antarctica, The presence of dispersed volcanic fallout ashes between visible ash layers suggests continuous explosive volcanic activity on the Kerguelen Plateau in the South Ind ian Ocean since the early Eocene. The continuous fallout of volcanic a sh from explosive volcanism on the Kerguelen Archipelago is the source of the magnetic particles and thus responsible for the magnetostratig raphy of the nannofossil oozes drilled during Leg 120.