Drift pumice in the Central Indian Ocean Basin: Geochemical evidence

Abstract

Abundant white to light grey coloured pumice without ferromanganese oxide coating occurs within the Quaternary sediments of the Central Indian Ocean Basin. Two distinct groups of pumice are identified from their geochemical composition, which allow one to define two different origins linked to two separate eruptions. One group of pumice is a dacitic type characterized by high Fe, Ti, Mg, Al and Ca with comparatively low contents of Si, rare earth elements (Σ REE-69 ppm), Rb, Sr, U, Th, Ba, V, Nb, Sc, Mo and Co which strongly suggest an origin from the 1883 Krakatau eruption. The other group is rhyolitic and is characterized by low contents of Fe, Ti, Mg and Ca and high Si , ΣREE content (121 ppm), Rb, Sr, U, Th, Ba, V, Nb, Mo, Co, and Sc and correlates well with the composition of the Youngest Toba Tuff (YTT) eruption of ~ 74 ka from Northern Sumatra and is being reported for the first time. Therefore, correlation of the pumice to the 1883 Krakatau and YTT eruptions indicates that the pumice drifted to the Central Indian Ocean Basin and eventually sank when it became waterlogged. However, physical properties such as density, specific gravity, porosity and degree of saturation required for sinking of pumice for both 1883 Krakatau and YTT are almost similar.

The origin of ferro-manganese oxide coated pumice from the Central Indian Ocean Basin

“…These pumices are mostly rounded to sub-rounded, similar to the earlier observations of Pattan et al (2008) which indicates their long transportation and abrasion. Iyer and Karisiddaiah (1988) studied 48 pumices and more recently, Kalangutkar et al (2011) analysed about 400 pumice clasts from CIOB and found that majority of them are equant and oblate Zingg shapes.…”

“…The REE content of these pumice grains is very similar to each other and as well as that of the uncoated pumice from the CIOB (121 ppm, Pattan et al, 2008), These P REE contents and profiles are similar to material from the Toba Caldera; to glass shards of YTT origin from different locations such as the CIOB, Peninsular India, South China Sea, Arabian Sea; and to Layers A, C and E from ODP-758 (Westgate et al, 1998;Song et al, 2000;Pattan et al, 2001Pattan et al, , 2002). An ash layer was described from ODP 758 at w8.2 Ma (Late Miocene) which also has similar REE concentrations ( P REE 123 ppm) to the coated pumices, and this has been speculated to be of Toba origin (Padmakumari and Ahmad, 2004).…”

“…This rules out a correlation with any of the <0.8 Ma, voluminous Toba rhyolitic eruptions. The fully, partially and uncoated pumice analysed by Pattan et al (2008), in general, have nearly similar major, trace and REE compositions, and this implies that they were sourced from either a single large event or multiple events from a compositionally similar source. A scenario involving multiple events allows for the accretion of different thicknesses of FeeMn oxide coating over different periods of time.…”

“…Uncoated pumice clasts from the CIOB have already been attributed to the w74 ka YTT eruption (Pattan et al, 2008) even though these are depleted in K 2 O compared to proximal materials (CIOB pumice 2.59%; YTT caldera pumice from Sumatra 4.97%). Alkali exchange (Na/K exchange) is a common phenomenon which can affect volcanic glass after deposition.…”

“…In the CIOB, the thickness of FeeMn oxide on pumice clasts reaches up to a maximum of w10 mm, suggesting the accretion could have occurred over w10 Ma because 1 mm of FeeMn oxide accretion takes w1 Ma. Several sources for the pumice recovered from the CIOB have been suggested, including in-situ submarine silicic volcanic activity (Hedervari, 1982;Iyer and Sudhakar, 1993); the 1883 e Krakatau eruption (Mudholkar and Fujii, 1995;Pattan et al, 2008); Indonesian Arc Volcanism (Iyer and Karisiddaiah, 1988;Martin-Barajas and Lallier-Vergès, 1993) and the Youngest Toba Tuff (YTT) eruption (northern Sumatra) at w74 ka (Pattan et al, 2008). Pumice from the 1883 e Krakatau eruption drifted to most of the Indian Ocean coasts (Frick and Kent, 1984) while pumice from the YTT is known to have drifted into the CIOB (Pattan et al, 2008).…”