Chemistry of Pliocene-Pleistocene pumice from the Izu-Bonin Arc

Thick pumice deposits were found in the cored sequences of forearc, arc, and backarc sites of Leg 126 in the Izu-Bonin Arc. These deposits, composed of fragmental rhyolite pumice with the chemical composition of low-alkali tholeiites, are products of arc volcanism. Pumice deposits constitute more than half of the thickness of the sediment fill of the Sumisu Rift, a backarc rift of the Izu-Bonin Arc. They comprise five thick pumiceous beds separated by thin hemipelagic units; as such, they record four major episodes or pulses of explosive, rhyolitic volcanism during the last 0.15 Ma, separated by quiescent intervals that each lasted about 30-60 k.y. The thick pumiceous beds were deposited in the rift mainly by sediment gravity flows during and immediately after the eruption of arc volcanos, which were probably submarine. Initiation of rifting was also preceded in the Pliocene by submarine rhyolitic volcanism, as seen in samples from the top of the eastern rift flank. Thick pumice beds correlative with those in the backarc also occur in the forearc basin to the east.

(Table 1) Sulphur concentration and isotopic composition of ODP Leg 126 igneous rocks

Sulfur isotope ratios have been determined in 19 selected igneous rocks from Leg 126. The d34S of the analyzed rocks ranges from -0.1 â to +19.60 â. The overall variation in sulfur isotope composition of the rocks is caused by varying degrees of seawater alteration. Most of the samples are altered by seawater and only five of them are considered to have maintained their magmatic sulfur isotope composition. These samples are all from the backarc sites and have d34S values varying from +0.2 â to +1.6 â, of which the high d34S values suggest that the earliest magmas in the rift are more arc-like in their sulfur isotope composition than the later magmas. The d34S values from the forearc sites are similar to or heavier than the sulfur isotope composition of the present arc.

(Table 1) Remanence properties of samples from ODP Core 126-792A-7H

A combination of high sedimentation rates and high concentrations of magnetic grains in cores from Ocean Drilling Program Leg 126 resulted in the recovery of detailed direction and intensity records of the Brunhes/Matuyama geomagnetic polarity reversal. Virtual geomagnetic poles (VGPs) computed from azimuthally oriented samples taken from the cores of Hole 792A in the western Izu-Bonin forearc basin reveal that the geomagnetic pole persisted at moderate to high southern latitudes for several thousand years before a rapid migration to northern latitudes. Alternating-field demagnetization behavior, as well as NRM, NRM/ARM, and NRM/IRM intensities for samples from this same interval, and the NRM/IRM intensities derived from unoriented core samples from Holes 790C and 791B, drilled in the ~100-km distant Sumisu Rift, all suggest that the dipole field oscillated widely in intensity before the reversal. The fast polarity change occurred at the low point of an ~1100-yr field intensity cycle. This "reversal cycle" immediately followed earlier intensity cycles whose peaks rivaled or surpassed the normalized intensities of discrete samples from well above and below the reversal interval; furthermore, the troughs indicate a much diminished dipole field at their nadir.