Dehn, Jonathan (1992) Volcanic Evolution of the Eastern Indian Ocean: Broken Ridge and Ninetyeast Ridge, ODP Leg 121. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, 149 pp.

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Abstract

A tephrochronology of the past 5 Ma is constructed based on ash layers contained in Neogene sediments drilled at ODP Leg 121 Site 758 on the northern Ninetyeast Ridge. More than two hundred tephra layers occur in the uppermost 80 m of cores, ranging in thickness from a few millimeters to 34 cm. Seventeen tephra layers, at least one cm thick, were sampled and analyzed for major elements. Ages for the ash layers are estimated from the paleomagnetic and 8180 chronostratigraphies and range from 75,000 ka
to more than 5 Ma. The ash layers comprise about 1. 7% by volume of the sediments recovered in the first 72 meters. Median grain size of the ashes is about 75 μm with a maximum of 150 μm. The ash consists of rhyolitic bubble junction and pumice glass shards. Bubble wall shards dominate in the ashes (70%-90%), with blocky and platy shards of even proportions (10%-30%).
The layers are always less than 2% crystals by volume, with plagioclase and alkali feldspar present in nearly every layer. Biotite occurs only in the thickest layers. Major element compositions of the glass shards reflect fractionation trends mainly by feldspar crystallization. Three chemical groups of glass
compositions suggest several distinct magmatic systems. Dating by 8180 and paleomagnetic reversals suggests that major marine ash-layer-producing eruptions (marine tephra layers > 1 cm in thickness) occur roughly every ca. 414,000 years correlating well with land-based stratigraphic correlations and dates of Pleistocene Sumatran tuffs (375,000
yr average eruptive interval). Residence times of the magmatic systems defined by geochemical trends are 1.6 Ma, 2.5 Ma, and 1.4 Ma. The
longest time interval starts with the least differentiated magma. The Sunda Arc, specifically Sumatra, is inferred to be the source region for the ashes. Four of the five youngest ash layers recovered correlate in time
and in major element chemistry to ashes observed on land at the Toba caldera.

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