T018-0018
Heterogeneity of subducting Indian oceanic plate and its implication to Sunda-arc magmas geochemical systematics

Wednesday, 9 December 2020
Poster
Esti Handini, Universitas Gadjah Mada, Sleman, DI Yogyakarta Yogyak, Indonesia, Toshiaki Hasenaka, Kumamoto University, Center for Water Cycle, Marine Environment and Disaster Management, Kumamoto, Japan, Takashi Sano, National Museum of Nature and Science, Department of Geology and Paleontology, Ibaraki, Japan, Tomoyuki Shibata, Hiroshima University, Graduate School of Advanced Science and Engineering, Higashi-Hiroshima, Japan, Nicholas Barber, University of Cambridge, Department of Earth Sciences, Cambridge, United Kingdom and Nugroho Imam Setiawan, Gadjah Mada University, Geological Engineering Department, Sleman, Indonesia
Abstract:
We report major and trace element of Indian ocean basalts from four IODP drilling sites. From oldest (Middle Jurassic) to youngest (Early Oligocene) they are site 261, 211, 254, and 283. Site 261 basalts are located in the Argo Basin. Site 211 basalts are part of Wharton Basin, located in smaller sub-basin to the west of Christmas Island-Roo Rise, and to the north of Cocos (Keeling) Rise. Site 254 basalts are from the southern end of Ninety East Ridge. The youngest basalts from site 283 are located in the Tasman Basin. Site 261 and 211 basalts are located in the south of Java and Flores islands, where they are believed to represent the composition of the Quaternary subducting slab that defines the long Sunda-Banda arc system.

Our result indicates that basalts from the four sites may have been generated from different type of mantle sources. Site 261 basalts have typical NMORB signatures, while those from site 211, mostly tephrite basanites, show typical OIB geochemical characteristics. Site 211’s composition resembles the alkaline volcanics from Christmas Island to the SW. The two younger basalts from site 254 and 283 mostly show EMORB characteristics but are slightly elevated in Cs, Rb, and Pb, which might be caused by low temperature hydrothermal alteration, and depleted in Ba and Sr. Site 211 basalts show the typical OIB signatures with higher (La/Sm)N and lower (Zr/Nb)N than basalts from the three other sites.

These findings lead us to make several critical observations: (1) Indian Ocean basalts are generated from different mantle sources varying from NMORB to EMORB, and (2) basalts from site 211 may have been generated from intraplate volcanism, which at the time of formation would have formed in thin oceanic crust. Subduction of depleted 261 basalts and enriched 211 alkaline-basalts would have uniquely affected geochemistry of arc basalts along Sunda-Banda arc. A simple trace element mixing model shows that subduction of 211-type enriched alkaline basalts or some parts of the Roo Rise may have been responsible for the elevated Ba (and some other LILEs) in magmas from the middle section of Sunda-arc. These findings will help to better understand the cycling of trace elements, particularly from the slabs, in subductions zones.

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