Sr‐Nd‐Pb isotope systematics of the Banda Arc, Indonesia: Combined subduction and assimilation of continental material

Abstract

We present Sr, Nd, and Pb isotope results and SiO 2, Rb, Sr, Sm, Nd, U, Th, and Pb data for six active volcanoes and one extinct volcanic island distributed over the whole length of the Banda Arc. Rock types range from low-K tholeiitic in the NE to high-K calc-alkaline in the SW. The volcanoes in the NE have "normal" arc signatures (SVSrf16Sr=0.7045-0.7055, 143Nd/144Nd=0.51273-0.51291, and 2ø6pbflø4pb=l 8.66-18.75), whereas those in the SW have extreme values (87Srf16Sr=0.7065-0.7083, 143Nd/144Nd=0.51252-0.51267, and 2ø6pb/2ø4pb= 19.28-19.43). Serua, situated in the central part, is the most anomalous volcano with regard to its Sr and Nd isotopic composition (87Srff6Sr=0.7075-0.7095 and 143Nd/144Nd=0.51240-0.51260) but not with regard to Pb isotopes (2ø6pbflø4pb=19.02-19.08). The inactive island of Romang in the SW overlaps the Serua trends. The volcanoes display variable within-suite ranges in 87Sr/86Sr and 143Nd/144Nd. Large ranges (e.g., at Nila) are consistent with assimilation (10-20%) of carbonate-bearing sediments from the arc crust. Despite the evidence for assimilation, it cannot explain all of the Sr-Nd isotopic trends found, and Banda Arc magmas must have already obtained a "continental" signature at depth before they reached the arc crust. Within-suite trends of Pb isotopes are virtually absent. We found an extreme range in the volcanics along the arc which coincide with a similar trend in sediments in front of the arc and consider this as strong evidence for the contribution of subducted continent-derived material to magma sources. Bulk addition of 0.1-2% of local sediment in the NE Banda Arc, and of 1-3% in the SW Banda Arc, to an Indian Ocean mid-ocean ridge basalt (I-MORB) source can explain the isotopic trends; both Serua and Romang require > 5% sediment. The Pb isotopes (e.g., 2ø7pb/2ø4pb -2ø8pb/2ø4pb) also suggest changes in the mantle end-member from I-MORB to oceanic island basalt (OIB) source type. The latter becomes more conspicuous toward the SW and has the high 2øspbflø4pb characteristic of Indian Ocean (Dupal) OIBs. We hypothesize that mixing of magmas in the mantle wedge and/or in the arc crust was an important mechanism by which mantle and subducted end-members were incorporated in the final products. continental crustal lithosphere, (2) large quantities of Precambrian continental material (reworked into sediments and in the form of Australian crustal lithosphere) are available in front of the trench, and (3) there are marked geochemical variations along the arc that allow us to test inferred sources and processes.In this paper we discuss new Sr, Nd, and Pb isotopic data and selected trace element results for volcanoes along the full length of the arc, our aim being to assess the relative importance of subducted and assimilated material. We present evidence for along-arc changes in the nature and contributions of mantle sources and of the subducted components, using the isotopic signatures of sediments on the adjacent Australian shelf. Finally, we discuss mixing mechanisms by which sub...

Oxygen- and Sr-Isotope Systematics of the Banda Arc Volcanics (Indonesia): Evidence for Subduction of Continental Material

“…The northern Banda Arc volcanoes (Banda Archipelago and Manuk) can be explained by 0.5-1% addition of sediment to their source, whereas the southern Banda Arc volcanics (Nila, Teon, Damar and Romang) require some 2-5%. This is in agreement with the models based on Pb-Nd isotopes (Vroon et al, 1993).…”

“…The Banda Arc has been widely cited in the literature as an example of an island-arc where magma genesis is influenced by large amounts of subducted continental sediments which are derived from the Australian craton (Whitford and Jezek, 1979;Vroon et al, 1993). This was first suggested by an combined Sr-and oxygen isotope study of Magaritz et al (1978).…”

Oxygen isotope systematics of the Banda Arc: low δ18O despite involvement of subducted continental material in magma genesis

“…2; cf. Van Bergen et al, 1989;Vroon et al, 1993): (1) low-K Banda Archipelago, and (2) medium-K Serua and Manuk, both with phenocryst assemblages of olivine, clinopyroxene, orthopyroxene, plagioclase and Fe-Ti oxides, and (3) high-K Nila, Teon and Damar, containing, in addition to these phases, amphibole and biotite. The extinct volcanic islands of Ambon and Romang are intermediate between the two latter groups.…”

“…Within-suite trends establish the extent to which arc-crust assimilation plays a role in individual centers. The results will be discussed, in combination with Sr-Nd-Pb isotope and trace-element data obtained on the same volcanic and sediment samples (Vroon, 1992;Vroon et al, 1993;Vroon et al, 1995), to distinguish mechanisms by which subducted continental material is transferred to magma sources in the sub-arc mantle.…”

“…The Banda Arc is an extreme example of the latter group, as it represents an arc system where sediment supply is strongly enhanced by the approach of a passive continental margin. Large contributions from subducted continental material (terrigenous sediments and/or continental crust) to magma sources have been revealed by trace-element and isotopic studies (Whitford et al, 1977;Whitford and Jezek, 1979;Gill and Williams, 1990;Hilton et al, 1989;1992;Vroon, 1992;Vroon et al, 1993;, whereas the influence of subducted (altered) oceanic crust is thought to be of minor importance.…”