HR: 17:00h
AN: V32H-05    [PDF]
TI: Barren Island Volcano (NE Indian Ocean): Island-Arc High-Alumina Basalts to Andesites Caused by Troctolite Disaggregation and Plagioclase Accumulation
AU: * Luhr, J F
EM: luhr@volcano.si.edu
AF: Department of Mineral Sciences Smithsonian Institution, P.O. Box 37012 NHB-119, Washington, DC 20013-7012 United States
AU: Haldar, D
EM: haldar2115@yahoo.co.uk
AF: 4/3K/2, Ho-Chi-Minh Sarani, Shakuntala Park, Kolkata, 700 061 India
AB: Barren Island (BI) is a subduction-related volcanic island lying in the northeastern Indian Ocean, about 750 km north of the northern tip of Sumatra along the same subduction zone that carries the Indo-Australian Plate northeastward beneath the southeast corner of the Eurasian Plate. The island has a diameter of only 3 km and rises to 355 m above sea level. Three eruptive episodes are known in historical time. A cinder cone formed near the center of the island in 1787, within remnants of a pre-historical caldera 2 km in diameter and open to the west. Eruptions continued intermittently until 1832. Two other episodes occurred more recently, during March to October 1991 and December 1994 to May 1995. Each eruption included strombolian and fire-fountain activity at the central and flank vents of the cinder cone and the westward flow of block lavas through the caldera breach until they cascaded into the sea. This investigation is based on 28 samples collected from Barren Island by researchers from the Geological Survey of India during expeditions to the island prompted by the eruptions of 1991 and 1994-1995. The samples include 18 lavas, 5 scoriae, and 5 bulk ashes that can be divided into 4 age groups: pre-1787 (n=6), 1787-1832 (n=4), 1991 (n=8), and 1994-1995 (n=10). Whole-rock compositions range from 50.7 to 59.8 wt.$%$ SiO$_{2}$, and thus span the range from basalt to andesite. All samples contain phenocrysts and microphenocrysts of olivine (plus spinel), plagioclase, and clinopyroxene. A notable textural feature in many samples from all age groups is the presence of abundant (to 40 vol.$%$), large (to 4 mm) phenocrysts of plagioclase. These have clear tabular cores of homogeneous highly calcic composition at An$_{90-95}$, surrounded by normally zoned mantles, 10-150 microns thick, that range down to An$_{48}$. Plagioclase phenocryst abundances vary with whole-rock SiO$_{2}$ (negatively) and Al$_{2}$O$_{3}$ (positively) contents. All samples with $>$20 vol.$%$ plagioclase phenocrysts have $>$19 wt.$%$ Al$_{2}$O$_{3}$ and $<$53 wt.$%$ SiO$_{2}$. Samples rich in plagioclase phenocrysts also have slight positive Eu anomalies. All evidence points to generation of these high-Al basalts by accumulation of calcic plagioclase during dissagregation of troctolitic xenoliths, which are found within historical lavas and scoriae. We suppose that a magmatic mush or plutonic mass of troctolite at depth beneath Barren Island is the source of this contamination. These high-Al basalts, at least, are not magmatic compositions. Six Barren Island samples were also analyzed for Sr, Nd, and Pb isotopic compositions, which show a crude trend toward more enriched values with time: overall ranges are $^{87}$Sr/$^{86}$Sr = 0.7038-0.7041, $\epsilon$Nd = 4.1-6.8, $^{206}$Pb/$^{204}$Pb = 18.20-18.29. Trapped glass inclusions in olivine phenocrysts were analyzed for volatile contents by Fourier-transform infrared spectroscopy and electron microprobe. The highest values for total H$_{2}$O are 2.6 and 3.1 wt.$%$ from two 1991 inclusions. Carbon species were not detected, as is typical for subduction-zone glasses. Sulfur contents ranged 0.08-0.20 wt.$%$ SO$_{3}$, and Cl contents ranged 0.14-0.21 wt.$%$, reflecting the elevated Cl values that characterize subduction-related glasses worldwide, in contrast with those from mid-ocean ridges or hot spots.
DE: 3640 Igneous petrology
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting