M.J. van Bergen

Strontium, neodymium, and lead isotopic and trace-element signatures of the East indonesian sediments: provenance and implications for banda arc magma genesis

We present new trace-element and Sr-Nd-Pb isotope data for 127 surface sediments and five sediments from DSDP Site 262, distributed along and across the arc-continent collision region of the Banda Arc, East Indonesia. The results are used to evaluate the role of subducted continental material (SCM) in the genesis of the Banda Arc magmas and to assess the extent to which geochemical and isotopic signatures of SCM are controlled by sediment provenance. In the surface sediments lead and neodymium isotope ratios are variable: 206Pb/204Pb = 18.65–19.57; 143Nd/144Nd = 0.51230–0.51190, with an increase in lead isotope ratios and a decrease in the 143Nd/144Nd ratio from northeast to southwest along the Banda Arc. DSDP Site 262 sediments, farthest to the west in the Timor Trough, overlap with the surface sediments and have 206Pb/204Pb = 18.89–19.23 and 143Nd/144Nd = 0.51200–0.51220. In contrast, the trace-element ratios and REE patterns of the sediments do not show systematic along-arc variations and largely overlap with estimated values for Upper Continental Crust, Post Archean Australian Shale (PAAS), and ODP Site 765 sediments from the Argo Abyssal Plain. From the combined isotopic and trace-element ratios in the terrigenous fraction of the sediments four major provenance areas can be distinguished: North New Guinea + Seram, South New Guinea, Timor, and northern Australia. The lead isotopic variations in the shelf and wedge sediments along the Banda Arc are parallel to similar variations in the volcanics; this is considered to be strong evidence for the incorporation of subducted continental material in the arc magmas. The trace-element characteristics of both the volcanics and the sediments are also consistent with the involvement of sediments in the Banda Arc magma genesis. The hinterland of the sediments is responsible for isotopic signatures created in the Banda Arc mantle through recent subduction. This suggests that some of the mantle heterogeneities that are inferred from oceanic basalts can be explained by differences in the provenance of (ancient) subducted sediment.

Mapping magma sources in the east Sunda-Banda arcs, Indonesia: Constraints from helium isotopes

Abstract We report new helium isotope analyses of olivine and clinopyroxene separates from recent lavas for eleven volcanoes from Flores in the east Sunda arc through the inactive segment between the arcs to Banda Island at the extreme of the contiguous Banda arc. In the east Sunda arc, 3 He 4 He ratios (R) vary between 4.5 RA ( R A = air 3 He 4 He ) for the leucitic Batu Tara volcano to a remarkable low of pure radiogenic helium (0.0075 RA) for Werung at the southern tip of Lomblen Island. Lavas from the inactive zone, which represents the locus of collision of the Australian continent with the arc, have a narrower range in R R A —from 3.9 for Kisu in the straits of Pantar to 1.0 for Romang Island. Our one locality (Banda Island) in the Banda arc gives the highest R R A ratio (3.1) observed along this arc to date. The results are consistent with the involvement of crustal material in magma genesis throughout the east Sunda/Banda arcs, as far west as Iya in central Flores. We combine these helium isotope results with published and on-going strontium isotope studies, and show that the source of the helium in the crustal component is unlikely to be terrigenous sediments derived from the Australian continent; rather, degassing of Australian continental crust appears to be the dominant process controlling the helium budget. The He-Sr isotope systematics also provide a framework to account for the areal pattern of 3 He 4 He in this part of the arcs: the radiogenic crustal component is diluted with mantle helium both in a down-dip direction and with increasing lateral distance from the collision zone. These factors result in an excellent first-order relationship between the 3 He 4 He ratio, degree of He Sr enrichment (relative to the postulated mantle endmember), and alkalinity of the erupted lavas. Such a relationship has a direct bearing on models of the tectonic evolution of the collision zone, and on the observation that helium isotopes are decoupled from strontium and other geochemical signatures along the Banda arc.

Sulfur and chalcophile elements in subduction zones: constraints from a laser ablation ICP-MS study of melt inclusions from Galunggung Volcano, Indonesia

Abstract Mafic melt inclusions hosted in olivine phenocrysts (Fo89–78) in high-Mg basalts of Galunggung volcano (Java, Indonesia) were analyzed in situ by laser ablation ICP-MS to determine concentrations of chalcophile and associated trace elements. Our results indicate that sulfur in the mantle beneath Galunggung is significantly enriched relative to MORB source mantle, suggesting large-scale fluxing of sulfur into the mantle wedge during slab dehydration. Melt-inclusion compositions range from strongly undersaturated to transitional basaltic and are characterized by a wide range of sulfur contents (350–2900 ppm). Chalcophile element concentrations are not affected by exsolution of immiscible sulfide liquids and generally fall within the range of whole-rock samples from other arcs. We infer that primary Galunggung melts contain approximately 290 ppm Ni, 60 ppm Co, 190 ppm Cu, and 3 ppm Pb. Patterns of refractory trace elements point to ∼15% melting of a MORB-source mantle below Galunggung, which was enriched in LILE and LREE by slab-derived fluids before melting. Based on this value, we use melting model calculations to investigate whether slab-derived contributions to a MORB-source mantle are required to balance the budgets of sulfur and associated chalcophile elements, assuming that all Cu and S originally resided in sulfides and adopting a Cu concentration of 28 ppm in the presubduction mantle, similar to that in MORB-type mantle. Modeling results predict at least 256 to 465 ppm S in the magma source of Galunggung, which is up to twice the amount commonly assumed for MORB sources. A slab-derived origin of the excess sulfur is consistent with 34S enrichments that commonly characterize arc magmas and gases. Although modeling suggests that elevated copper concentrations relative to MORB can be the consequence of the higher solubility of sulfur in Galunggung melts, the results do not rule out that the subarc mantle was enriched by slab-derived Cu. Lead must have been added, which is readily explained by slab-derived fluids. Other chalcophile elements (Co, Ni) are largely controlled by the nature of the presubduction mantle.

U-series, SrNdPb isotope and trace-element systematics across an active island arc-continent collision zone: Implications for element transfer at the slab-wedge interface

We present U-series, SrNdz.sbnd;Pb isotope and trace-element results of a regional study of geochemical systematics across an island arc-continent collision zone in the East Sunda Arc of Indonesia. Samples from four active volcanoes exhibit a striking compositional range from low-K tholeiitic to ultrapotassic, but all are characterised by high (0.7053–0.7067), radiogenic lead isotope ratios ( = 18.99–19.15), low (0.66–0.85), and low (0.51255–0.51272), except for high (>0.51286) at the volcanic front. Low ratios are also found in terrigenous sediments in front of the arc, which, in combination with Srz.sbnd;Ndz.sbnd;Pb isotopic constraints, indicates that subducted continental material contributes to magma sources in this arc sector. The volcanoes close to the trench show a large excess of 238U over . 230Th (up to 80%) and of 226Ra over 230Th (up to 800%). In addition, they are enriched in elements thought to be mobile in hydrous fluids during slab-wedge transfer, such as Ba, Pb, and Sr. In contrast, Uz.sbnd;Thz.sbnd;Ra systematics are close to equilibrium in the volcanoes behind the front. Abundance patterns of incompatible trace elements in these rocks are similar to those of the terrigenous sediments, so that, in comparison with the arc-front lavas, they possess low Ba/La, Ba/Th, La/Th, Pb/Ce, and Zr/Nb. Higher concentration levels and less interelement fractionation form conspicuous differences with the front volcanics. Our combined isotopic and trace element data are consistent with three-component mixing whereby a slab-derived hydrous fluid and a siliceous melt are both added to the sub-arc mantle source. The hydrous fluid largely controls the input in the shallow part of the subduction zone, whereas the siliceous melt dominates the flux at deeper levels. Sedimentary material is considered to be the primary source of both. The large U-Th-Ra disequilibria at the front suggests that element transfer is a currently active process associated with present-day subduction of continental material.

Sulfur isotope systematics of basaltic lavas from Indonesia: implications for the sulfur cycle in subduction zones

Abstract We report sulfur isotope compositions of basaltic and basaltic andesite lavas from selected volcanoes in the Indonesian arc system covering the spectrum from low-K tholeiitic to high-K calc–alkaline compositions. The results of 25 samples from seven volcanoes, which are associated with different subduction regimes, show a range in δ34S values of +2.0–+7.8‰ (VCDT) with an average of +4.7±1.4‰ (1σ). Averages and within-suite variations of two larger sets of samples from Batur and Soputan volcanoes (+4.2±1.3‰ with n=9 and +5.7±1.4‰ with n=7, respectively) are comparable to those of the entire sample set. Sulfur concentrations are low (mostly between 2 and 74 ppm, average=19 ppm) and do not show correlations with sulfur isotope composition and whole-rock chemistry, or systematic changes with time in any of the lava suites. From model calculations we infer that basaltic magmas will undergo sulfur isotope fractionation during degassing, most commonly towards lower δ34S values, but that the extent is limited at P–T conditions and oxidation states of interest. Hence, δ34S signatures of basaltic lavas will generally be within a few permil from primary magmatic values, even in cases of extensive sulfur loss. Consequently, magmas in the Indonesian arc system originate from mantle sources that are enriched in 34S relative to MORB and OIB sources and are likely to have δ34S values of about +5–+7‰. The enrichment in 34S is considered to reflect addition of slab-derived material, presumably from sediments rather than altered oceanic crust, with fluids being the most likely transport medium. Absence of correlation between δ34S values of Indonesian basalts and chemical proxies for source components or processes at the slab–wedge interface suggests that sulfur isotopes are relatively insensitive to variations in subduction setting and dynamics. This is supported by the modest range in δ34S of the Indonesian volcanoes studied despite significant variations in the nature and amount of subducted material, and by the similarity with average 34S enrichments in other oceanic arc systems.

Oxygen isotope systematics of the Banda Arc: Low delta O-18 despite involvement of subducted continental material in magma genesis

Abstract This study reports new laser fluorination oxygen isotope data for 60 volcanic rocks and 15 sediments distributed over the whole length of the Banda Arc, eastern Indonesia. The melt oxygen isotope values (δ18Omelt) were calculated from phenocryst δ18O data using theoretical and empirical mineral-melt fractionation factors. The δ18Omelt of individual volcanic centers within the arc varies between 5.57 and 6.54‰, except for Serua (δ18Omelt = 6.13–7.48‰) and Ambon (δ18Omelt = 8.12–8.38‰). These δ18Omelt values are up to 2‰ lower than new and previously published oxygen isotope data obtained on whole-rock powders by conventional methods. We attribute this discrepancy to post-emplacement low-temperature alteration and/or to a systematic deviation of the bulk analysis. Sediment δ18Owr (calculated from the δ18O carbonate and silica fractions, both measured conventionally) range between 12.9 and 24.2‰. The low δ18Omelt values (excluding Serua and Ambon) overlap with the mantle range, and are in agreement with simple two-component source-mixing models that predict 1–5% addition of subducted continental material to a depleted MORB-type source in the sub-arc mantle. This percentage is consistent with previous models based on Sr–Nd–Pb–Th–He–Hf isotope data. However, correlations between incompatible trace-element ratios and oxygen isotope systematics requires involvement of partial melts derived from subducted continental material as the major slab component rather than bulk addition. The contribution of hydrous fluids, from both subducted altered oceanic crust and continental material is probably of minor importance. Magma-mantle wedge interaction models could account for the observed low δ18O signatures, but predicted effects are difficult to distinguish from models without mantle-wedge interaction. Assimilation of arc-crust material is thought to be important for the high δ18Omelt values of Serua and Ambon. AFC modelling suggests up to 20% and 80% assimilation at Serua and Ambon, respectively. Inclusions of meta-sedimentary material and whole-rock Sr–Nd isotopes point to assimilation processes at Nila, but this probably had little effect on the δ18O of phenocrysts, which record original source values. According to radiogenic isotope data, magma sources in the Banda Arc are the most heavily influenced by fluxes of subducted continental material among currently active oceanic island arcs. Hence, the results of this study suggest that high δ18O (>6.5‰) in arc lavas are difficult to reconcile with addition of subducted components to magma sources, but must reflect assimilation of arc crustal material.

Chemical and isotopic compositions of volcanic gases from the east Sunda and Banda arcs, Indonesia

The easternmost Sunda Arc and the Banda Arc represent a continent-arc collision zone where magma genesis is influenced by subducted continent-derived material. Chemical and isotopic studies of volcanic gas samples from this environment provide information on the sources of volatiles in arc magmas. These volcanic gases, some of which last equilibrated at magmatic temperatures, are characterized by anomalous low {sup 3}He/{sup 4}He values, but by common arc values of C/S {approx} 2-4, {delta}{sup 13}C {approx} {minus}3{per thousand}, and {delta}{sup 34}S{sub tot} {approx} +5{per thousand}. Abundant helium and high He/Ar ratios are consistent with the subduction of terrigenous components in local sediments (or slivers of continental crust). Although individual concentration and isotope ratios of volatile components may be explicable by complex fractionation in the recycling process, the combined data are in agreement with an important role of subducted sedimentary source components. Comparison of the authors results with volcanic gas data from other arcs indicates that the carbon and sulfur signals in arc gases are relatively insensitive to the amount and nature of sediment on the subsiding plate. Hence, a contribution to arc volcanic gases from subducted altered oceanic crust cannot be excluded.

Geochemical and tectonic relationships in the east Indonesian arc-continent collision region: Implications for the subduction of the Australian passive margin

Abstract Van Bergen, M.J., Vroon, P.Z. and Hoogewerff, J.A., 1993. Geochemical and tectonic relationships in the east Indonesian arc-continent collision region: implications for the subduction of the Australian passive margin. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near The Earths Surface. Tectonophysics, 223: 97–116. Variations in the isotopic signatures of volcanics along the East Sunda Banda Arc reflect changes in the nature and amount of sedimentary material supplied by the northeast Indian Ocean floor and the adjacent Australian passive continental margin, which form the two major domains of the Indian Ocean plate that approach the arc system. A compilation of isotopic data for 200–500-km-long arc sectors shows that the trend in magmatic signatures follows distinct subduction/collision stages reached by the corresponding oceanic and continental-margin sections entering the trench system. Maximum amounts of magma source contamination are inferred for volcanics near an extinct sector north of Timor, where the Australian continent started to collide with the arc first. Pb-Nd isotopic source mixing models point to contamination by sediments with variations in composition, similar to observed along-arc changes in sediments entering the trench. The results indicate an increasing contribution of subducted continental material in the direction of the collision region. Mass-balance calculations, considering the magmatic output and minimum input of subducted continental material required to generate the composition of the volcanic arc in the collision region, are difficult to reconcile with subduction of ocean-floor sediments alone. Thicknesses of sediments presently covering oceanic crust near the margin are close to calculated thicknesses of the sediments fluxed into the trench and magmatically returned to the arc crust, but cannot account for the additional volumes of material accreted on the overriding plate in the same period of time. It is inferred that leading portions of the Australian continental margin have reached magma generation zones in the easternmost Sunda arc and western Banda arc, which implies subduction to depths greater than 100 km.

The origin of the potassic rock suite from Batu Tara volcano (East Sunda Arc, Indonesia)

Abstract atu Tara is an active potassic volcano in the eastern Sunda arc. Its leucite-bearing rock suite can be subdivided into two groups, one less evolved with Th 20 ppm. 87Sr/86Sr, δ18O and trace-element systematics in the less evolved group suggests that existence of parental magmas with different mantle origins. The mantle below Batu Tara is most likely heterogeneous and several source components are involved in magma genesis. Trace element and isotopic compositions of Batu Tara and adjacent volcanoes are consistent with the involvement of a subducted sedimentary/crustal component as well as MORB and OIB mantle, the latter with geochemical characteristics comparable to the mantle underlying Muriah (Java). Melt extraction from this complex mixture is envisioned as a two-stage process: partial melts of the crust-contaminated MORB mantle mix in the mantle wedge with partial melts of OIB domains. Different mixtures of these two melts provide the parental magmas that enter the volcanic plumbing system, where crystallization, hybridization and refilling processes occur. The calcalkaline volcanoes in the arc segment show stronger signatures for a subducted crustal component than Batu Tara, which displays a greater influence from the OIB mantle source. The potassium enrichment can therefore be attributed to contributions both from the enriched mantle and from subducted crustal material. Mantle-type δ18O values of the Batu Tara magmas indicate that the mantle wedge below potassic orogenic volcanoes is not necessarily strongly enriched in 18O.

Subducted upper and lower continental crust contributes to magmatism in the collision sector of the Sunda-Banda arc, Indonesia

Pb isotopes in igneous rocks from the Banda-Sunda arc show extreme along-arc variations, which correspond to major lithologic changes in crustal components entering the subduction system. An increase in 2 0 6 Pb/ 2 0 4 Pb ratios toward the zone of collision with the Australian continent reflects input of subducted upper-crustal material; maximum values coincide with anomalously radiogenic 3 He/ 4 He ratios that have been earlier attributed to the involvement of the continental margin. The collision zone is further characterized by 2 0 8 Pb/ 2 0 4 Pb ratios that are higher for a given 2 0 7 Pb/ 2 0 4 Pb value than observed in the noncollisional sectors, and in the central part of the collision zone, the 2 0 6 Pb/ 2 0 4 Pb ratios are lower than the most radiogenic values in the adjacent areas. We propose that these Pb isotope signatures reflect input of subducted lower crust, mobilized as a result of slabwindow formation during arc-continent collision. Variations in Pb isotopes in the collision zone are solely determined by variations in the nature and proportions of different subducted components. The Pb isotope arrays in the noncollision area may be dominated by slab components as well and could reflect mixing between subducted oceanic crust and entrained sediments, rather than between subarc mantle and subducted sediments. Our new interpretation of the Indonesian Pb isotope data does not call for involvement of ocean-island basalt (OIB)-type mantle or Australian subcontinental lithospheric mantle, as has been suggested previously.