[S-CG62_2AM2] Geofluids and dynamics in subduction zones

[SCG62-10] Evaluating slab-fluid contribution into inhomogeneous mantle source: geochemical variation of Central and East Java arc

*Esti HANDINI¹, Toshiaki HASENAKA¹, Haryo edi WIBOWO², Tomoyuki SHIBATA³, Yasushi MORI⁴, Agung HARIJOKO² (1.Graduate School of Science and Technology, Kumamoto University, 2.Geological Engineering, Gadjah Mada University, 3.Institute for Geothermal Sciences, Kyoto University, 4.Kitakyushu Museum of Natural History and Human History)

Keywords:Sunda arc, slab-derived fluid, across- and along-arc variations, trace elements

The spatial distribution of the volcanoes in Central and East sections of Java arc denotes the widest and the narrowest of Java Island. Central Java section corresponds to the largest range and depth of Wadati-Benioff Zone along the island (180-360 km), whereas East Java section shows the narrowest range (190-220 km). However, both sections equally show wide geochemical variation with the function of slab-depth. Both also mark the appearance of the rear-arc alkaline suites in a different slab depth (360 km for Central Java, and 220 km for East Java). Geochemical datasets of basalt to basaltic andesite (further screened on Zr/Nb basis) from these sections were compiled to evaluate the contributions of slab-derived fluid to the mantle sources, and to assess the possible mantle sources of these magmas. We group the lavas of the Central and East Java into two series: (1) the volcanic front series (VF), calc-alkaline suites of frontal- and middle-arc region volcanoes of Central and East Java, and (2) the rear-arc series (RA) consists of alkaline suites from Central and East Java (Muria, and Ringgit-Beser and Lurus, respectively). The VF series consistently shows typical island arc geochemistry, with strong LILE enrichment (Sr, Ba, Pb, and Rb) relative to HFSE. The RA series, mainly Muria, indicate stronger enrichment of LILE than other volcanoes closer to the trench. Ringgit-Beser and Lurus, the rear-arc lavas of East Java, behave differently in LILE enrichment. Ringgit-Beser lavas shows stronger LILE enrichment than that of lavas from Lurus, within the same enrichment range of Muria lavas. In the other hand, Lurus lavas are showing obvious HFSE depletion compared to OIB. The decreasing trend of LILE/HFSE and LILE/LREE (e.g. Ba/Nb, Ba/La, Pb/Ce, Pb/Nb) is observed across both Central and East Java sections. These ratios become lower toward the rear-arc of both sections, and the lowest in the rear-arc of Central Java. In various normalized plots (such as Nb vs. Ba/Nb), the VF series are plotted within the range of typical island arc basalts (IAB). Muria lavas, the rear-arc alkaline suite of the Central Java, resemble OIB and other non-arc type alkaline rock characteristics, but with positive indications of being island arc, such as negative Nb and Ti anomalies. Ringgit-Beser and Lurus alkaline lavas of East Java, however, are associated with other arc-type alkaline rock characteristics, with stronger signature of island arc than Muria. Our analyzed samples show that lavas from East Java are closer in compositions to primitive magmas compared to Central Java's. The thicker overriding crust beneath Central Java than East Java possibly acts as the magma retainer that allows extensive fractionation. Across-arc variation of slab-derived fluid in both sections are observed as shown by decreasing LILE/HFSE and LILE/LREE toward rear-arc, suggesting the decreasing amount of slab-fluid added to the great slab-depth. The slab-fluid added to the volcanic front of East Java is slightly higher than that of Central Java, which may be controlled by the narrow range of slab dehydration area in the former that allows more fluid to concentrate. The low ratio of these trace elements in the rear-arc of both sections suggests that these parts have also been affected by dehydration of subducted slab. The stronger slab-fluid contributions in the rear-arc alkaline lavas of East Java than that of Central Java may reflect the role of shallower slab depth. Different mantle characteristics between the rear-arc of Central and East Java may reflect several possibilities: (1) the inhomogeneous mantle plume (E-type/EMI) beneath both sections, or (2) stronger EMI-type mantle contribution to Central Java than to East Java, or (3) the combination of both.