Indonesian Arc Research Articles

Geochemistry and zircon U-Pb geochronology of diabase, gabbros, anorthosites and ultramafic rocks from the South Andaman Island ophiolite suite on the South-Eastern margin of the Indian plate

The South Andaman Island is located on the southeastern fringe of the Indian plate. Tectonically, the islands are associated with the Burma-Andaman-Java subduction zone of the Indonesian arc system. Several Mesozoic ophiolitic exposures are preserved in the Burma-Andaman-Java subduction zone. This study reports new petrological, geochemical, and zircon U-Pb geochronological data for the diabases, gabbros, anorthosites, and harzburgites of the ophiolite suite exposed in the South Andaman Island. The diabase is composed of plagioclase and clinopyroxene exhibiting ophitic textures. Anorthosite mainly contains plagioclase (> 90 vol%), pyroxenes and Fe-oxides. Gabbros have plagioclases and clinopyroxenes occurring as the major minerals. Harzburgites consist of olivine, orthopyroxene, clinopyroxene and spinel. Geochemical investigation reveals that these rocks were formed through crystal fractionation and accumulation processes from partial melt generated from a spinel lherzolite source. Gabbro, diabase, and anorthosites are formed by ∼1–5% melting of the, whereas depleted harzburgites were generated by slightly higher degrees of partial melting (∼10–20%) of the spinel lherzolite source. Trace element modelling reveals the influence of mid-oceanic ridge basalt (MORB)-melt interaction and subduction-related mantle alterations.The anorthosite and gabbro samples yield zircon 206Pb / 238U weighted mean ages of 93.2 ± 1.0 Ma and 442.5 ± 4.0 Ma, 383.9 ± 3.3 Ma respectively. However, the harzburgite sample records multiple age population including Paleo-Mesoproterozoic grains ranging in age from 1216 to 1931 Ma as well as Neoproterozoic and Phanerozoic grains of 607 Ma, 594 Ma, 237 Ma, 105 Ma, and 85 Ma suggesting a long-lived supra subduction zone in the Burma-Andaman-Java subduction zone where the South Andaman ophiolite suite was formed, evolved, and was emplaced.

Quaternary deposition and erosion in the northeastern Sunda Strait: An interplay between sea level, tectonics, and magmatic activity

The northeastern Sunda Strait is a narrow strait separating Java and Sumatra islands. Currently, it forms a seaway between the Java Sea and the Indian Ocean. The geological setting of the region is extremely dynamic, but how the Plio-Pleistocene interplay between sea level oscillations, magmatism, and tectonics, which lead to the current setting, has not been completely understood. We analysed an important set of legacy shallow seismic data from this area to decipher these intricate relationships. Our results indicate that the tectonic extension partly dismantled the Indonesian arc since the Middle Miocene. However, volcanic products formed a barrier between the Sunda Shelf and the Indian Ocean during the Late Pliocene to the Middle Pleistocene. Marine flooding started during the Middle Pleistocene but bypassed the barrier by flooding the NW edge of Java Island. During the Late Pleistocene, high amplitudes and longer periods of the glacial-interglacial cycles ultimately connected the Java Sea with the Indian Ocean. Still, it was only during the Holocene that important erosion made this seaway efficient in transporting seawater between the two reservoirs.

Monitoring of Indonesian volcanoes with the IS06 infrasound array

Detecting and notifying ongoing volcanic explosive eruptions is crucial in supporting the Volcanic Ash Advisory Centre (VAAC). However, local monitoring systems are missing at many active volcanoes, but long range infrasound monitoring might provide useful information if able to detect and notify volcanic explosive events. Indeed, many studies have already highlighted the utility and the potential of long-range infrasound monitoring for this aim, but still open questions remain concerning its actual efficiency and reliability. In this study we investigate the potential of the IS06 array (Cocos Island, Australia) of the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) to remotely detect volcanic explosive eruptions in the Indonesian Arc between 2012 and 2019, when 11 volcanoes, positioned at a distance between 1000 and 2000 km from the array, erupted with an energy spanning from mild explosions to VEI (Volcanic Explosivity Index) 4 eruptions. For each volcano, using infrasonic data recorded at a single array and accounting for realistic infrasound propagation conditions, we calculate a range corrected Infrasound Parameter (IP) and propose two additional empirical thresholds on signal strength and persistency. The IP is used eventually to define an alert whenever an established threshold is exceeded and the corresponding reliability estimated. Results show that the range corrected IP is highly reliable for events VEI = 3 or greater under favorable propagation conditions, but smaller scale short-lasting explosive eruptions still remain usually undetected. Unresolved ambiguity remains due to short spacing among volcanoes with respect to the array. For regional scale monitoring purposes, this can be solved only considering volcanic sectors rather than single volcanic edifices that, despite preventing unambiguous notification of a given volcano, might allow to increase the attention of the VAAC over a specific area.

Role of fractional crystallization and partial melting in the generation of intermediate and acidic magmas of the Andaman Island Ophiolite suite of India

The Mesozoic‐age Andaman Island Ophiolite located in the southeastern fringe of the Indian plate margin is one of the well‐preserved supra‐subduction‐zone oceanic lithospheric slices. It is tectonically associated with the Indonesian arc system and is an extension of the Phanerozoic Indonesian‐Myanmar orogenic belt. Thus, it offers a unique opportunity to study oceanic crust formation and specify magma evolution processes in the Tethyan subduction system. The volcanics of the Andaman Islands ophiolite display a wide compositional variation. Rock types vary from basic (basalts) to intermediate (basaltic‐trachyandesites, basaltic andesites) to acidic differentiates like dacites and rhyolites. This contribution presents the petrochemistry of the intermediate rock types—basaltic‐trachyandesites, trachyandesites and acidic—dacites and rhyodacites of Andaman Island Ophiolite (India). This study examines the role of fractional crystallization and partial melting in the generation of intermediate and acidic magmas of the Andaman Ophiolite suite. Plagioclases and pyroxenes are the major mineral constituents of these rocks on petrographic observation. Plagioclases of the intermediate rocks are of albite composition Ab98‐99. The acidic rocks show compositional variation from albite to oligoclase (Ab83–99). Analysed pyroxenes of intermediate and acidic rocks are augite (Wo46–49En34–41Fs13–17). The studied calc‐alkaline rocks show variable abundance of Large‐Ion Lithophile Elements and display strong positive Pb and negative Ti anomalies. The chondrite‐normalized Rare Earth Elements display an enriched pattern with varying degrees of enrichment. Clinopyroxene thermobarometry reveals that the intermediate magma and acidic magma becomes saturated with clinopyroxene at 1,062–1,133°C and 1,016–1,079°C, respectively, at 2 kbar and H2O = 2 wt%. Petrochemical findings and model calculations using PELE are consistent with a derivation of intermediate rocks from the basaltic melt by fractional crystallization. In contrast, acidic rocks are the product of partial melting of hydrothermally altered oceanic crust. The association of basaltic and intermediate rocks suggests a relatively stable magma chamber to generate differentiated intermediate magmas by fractional crystallization.

Element flux to the environment of the passively degassing crater lake-hosting Kawah Ijen volcano, Indonesia, and implications for estimates of the global volcanic flux

Abstract Volcanoes play an important role in the global cycling of elements by providing a pathway from the deep Earth to its surface. Here, we have constrained the flux to the environment for most elements of the periodic table for the passively degassing, crater lake-hosting Kawah Ijen volcano in the Indonesian arc. Our results indicate that emissions of Kawah Ijen are dominated by acid water outflow, especially for the ligands (Cl, F, Br), with active fumaroles contributing significant (semi)metals (e.g. Se, As, Sb, Hg), as well as C and S. The compositional signature of emissions from Kawah Ijen is similar to that of major volcanic emitters such as Etna, but element fluxes are smaller. This result provides the prerequisite foundation for extrapolating a small set of measured volcanic gas emissions to a global volcanic flux estimate. However, the aqueous flux (i.e. seepage of volcanic hydrothermal fluids and volcano-influenced groundwater) is at least as important in terms of element release, and the consideration of the gaseous flux alone thus represents a significant underestimate of the impact of volcanoes on their environment and the contribution of volcanic hydrothermal systems to global element cycling. Supplementary material: The full datasets of water and fumarole gas chemical analyses are available at https://doi.org/10.6084/m9.figshare.c.2134359

First measurement of the volcanic gas output from Anak Krakatau, Indonesia

Anak Krakatau is the active cone that has built up in the caldera of Krakatau volcano after the 1883 cataclysmic eruption, in the Sunda Strait. Initially submarine, this new cone definitely emerged from the sea in 1930 and since then has progressively grown up through both explosive and effusive eruptions (~ one eruption every 3years). Here we report on the first quantification of volcanic gas output from Anak Krakatau, based on airborn UV measurements of the SO2 flux in 2014, and then discuss its implication in terms of magma degassing budget since 1930. We find that during non-eruptive activity Anak Krakatau passively emits 190±40 tons per day of SO2, which is comparable to the emission rate during lava dome extrusion at Merapi, central Java, but substantially more than those measured on few other Indonesian volcanoes (Tangkubanparahu, Slamet, Bromo and Papandayan). Anak Krakatau thus appears to be an important persistent emitter of volcanic volatiles in the Indonesian arc, even though this very active region still remains weakly documented on that aspect. Combining with available data for the composition of its high-temperature (~700°C) crater gases, Anak Krakatau may release annually 0.07Tg of SO2, 3Tg of H2O, and 0.13moles of 3He. Using published data for the sulfur content of its feeding magma, we estimate that about 1.3km3 of magma may have been degassed during its subaerial growth over the past 85years. The subaerial cone represents only 14% of this volume. Thus, a substantial fraction (1.1km3) of the degassed magma did not extrude and may have accumulated in the plumbing system. This inference is consistent with geophysical and petrologic evidence of the presence of dense magma bodies in the shallow crustal basement of Krakatau volcano.

Tephra records from abyssal sediments off western Sumatra in recent 135 ka: evidence from Core IR-GC1

Three volcanic ash layers were identified in a deep-sea Core IR-GC1 from the north-eastern Indian Ocean, adjacent to western Indonesian arc. They were dominated by glass shards with minor mineral crystals, such as plagioclase, biotite, and hornblende. According to the morphology and major element compositions of the representative glass shards, combined with the 18O-based age, it is suggested that ash Layer A is correlated to the youngest Toba tuff (YTT), Layer B is supposed to be associated with a new eruption of Toba caldera in an age of 98 to 100 ka. Ash Layer C is different the geochemistry characteristics than those of Layer A and Layer B, suggesting that Layer C was not originated from Toba but registered another volcanic eruption event.

Planktonic Foram Dates from the Indonesian Arc: Marine 14C Reservoir Ages and a Mythical AD 535 Eruption of Krakatau

The Indonesian Arc represents the subduction of the Indian-Australian plate beneath Asia. It has been the scene of catastrophic tectonic activity, including the recent 2004 M=9.1 Aceh earthquake and resulting Indian Ocean tsunami. We have dated planktonic forams associated with historic tephras (Tambora, 1815 and Krakatau, 1883) in marine sediment cores to determine radiocarbon reservoir ages for 2 locations along the arc. Our best estimates for 19th century regional reservoir corrections (ΔR) are +90 ± 40 yr for surface-dwelling species and +220 ± 40 yr for mixed planktic assemblages containing some upper thermocline species, but scatter in the data suggests that past surface reservoir ages may have varied by about ±100 yr. We used the results of this study to investigate a proposed very large AD 535 eruption at or near Krakatau. We find no evidence for ash from such an eruption, and although this is negative evidence, we consider it sufficiently strong to rule out any possibility that one took place.

Planktic Foram Dates from the Indonesian Arc: Marine 14C Reservoir Ages and a Mythical AD 535 Volcanic Eruption

The Indonesian Arc represents the subduction of the Indian-Australian plate beneath Asia. It has been the scene of catastrophic tectonic activity, including the recent 2004 M=9.1 Aceh earthquake and resulting Indian Ocean tsunami. We have dated planktonic forams associated with historic tephras (Tambora, 1815 and Krakatau, 1883) in marine sediment cores to determine radiocarbon reservoir ages for 2 locations along the arc. Our best estimates for 19th century regional reservoir corrections (ΔR) are +90 ± 40 yr for surface-dwelling species and +220 ± 40 yr for mixed planktic assemblages containing some upper thermocline species, but scatter in the data suggests that past surface reservoir ages may have varied by about ±100 yr. We used the results of this study to investigate a proposed very large AD 535 eruption at or near Krakatau. We find no evidence for ash from such an eruption, and although this is negative evidence, we consider it sufficiently strong to rule out any possibility that one took place.

Cenozoic stratigraphy of Taiwan: Window into rifting, stratigraphy and paleoceanography of South China Sea

Shallow marine sequences of the northern South China Sea (SCS) are uplifted and exposed by plate convergence in the Taiwan mountain belt. These deposits provide detailed geological information about the rifting event, stratigraphy, sedimentology, paleoclimate and paleoceanography of the shallow SCS to compare with what are recorded in the ODP 1148 deep-sea core. Seismic surveys and marine micropalentological studies show that Eocene sequences in the offshore Taiwan Strait and onland Taiwan mountain belt are all deposited in rifting basins and are covered unconformably by the Late Oligocene-Neogene post-rifting strata. Between syn-rifting and post-rifting sequences, there is a regional break-up unconformity throughout the island. Early Oligocene and Late Eocene strata are missing along the break-up unconformity equivalent to the T7 unconformity in the Pearl River Mouth Basin off south China. This may suggest that the SCS oceanic crust could have initiated between 33 and 39 Ma. Neither obvious stratigraphic gap nor slumping features are found in the Oligocene-Miocene transition interval of Taiwan. This observation highly contrasts with what has been documented from the ODP 1148 deep-sea core. This suggests that the stratigraphic gap and slumping features could only be recorded in the SCS deep sea region, but not in the shallow shelf near Taiwan. Compared to the Middle Miocene paleoceanographic re-organization events in the SCS deep sea, the geological history of the Taiwan shallow sequence shows changes of in sedimentation and faunal composition. Due to the Antarctic glacial expansion at ∼14 Ma, Middle to late Miocene strata of the Western Foothills show progressive regression sedimentations associated with a decrease of benthic foraminiferal abundance and a sharp faunal turnover event. Many Early-Middle Miocene endemic benthic foraminifers were extinct in 14-13 Ma and new benthic foraminifers of the Kuroshio Current fauna appeared from 10.2 Ma, comparable with new occurrence of Modern benthic foraminifers at 9 Ma in the Java Sea area. This reveals that the Western Boundary Kuroshio Current in the North Pacific could initiate from 10-9 Ma due to closures of the Indo-Pacific seaways by convergent tectonics between the Australian Continent and the Indonesian Arc in 12-8 Ma. Subduction of the SCS oceanic lithosphere since the Middle Miocene resulted in formation of the Hengchun Ridge accretionary prism and the North Luzon Arc. Occurrence of these two bathymetric highs (−2400 m) since the Middle Miocene and closures of the inter-arc passages in the North Luzon arc in the last 3.5 Ma would control the water exchanges between the West Pacific and the deep SCS. Accordingly, the tectonic evolution in the Central Range-Hengchun Peninsula accretionary prism and the arc-forearc Coastal Range not only control directly the route for water exchanges between the West Pacific and the SCS, but also indirectly shows a great influence on the geochemistry of deep SCS waters. The latter is best shown by much negative carbon isotope values of benthic foraminifers in the ODP 1148 deep-sea core than the West Pacific records in the last 14 Ma.

Detrital and xenocrystic zircon ages from Neoproterozoic to Palaeozoic arc terranes of Mongolia: Significance for the origin of crustal fragments in the Central Asian Orogenic Belt

The Central Asian Orogenic Belt contains many Precambrian crustal fragments whose origin is unknown, and previous speculations suggested these to be derived from either Siberia, Tarim or northern Gondwana. We present an age pattern for detrital and xenocrystic zircons from Neoproterozoic to Palaeozoic arc and microcontinental terranes in Mongolia and compare this with patterns for Precambrian rocks in southern Siberia, the North China craton, the Tarim craton and northeastern Gondwana in order to define the most likely source region for the Mongolian zircons. Our data were obtained by SHRIMP II, LA-ICP-MS and single zircon evaporation and predominantly represent arc-related low-grade volcanic rocks and clastic sediments but also accretionary wedges and ophiolitic environments. The Mongolian pattern is dominated by zircons in the age range ca. 350–600 and 700–1020 Ma as well as minor peaks between ca. 1240 and 2570 Ma. The youngest group reflects cannibalistic reworking of the Palaeozoic arc terranes, whereas the Neoproterozoic to late Mesoproterozoic peak reflects both reworking of the arc terranes as well as Neoproterozoic rifting and a Grenville-age crust-formation event. The 700–1020 Ma peak does not exist in the age spectra of the Siberian and North China cratons and thus effectively rules out these basement blocks as potential source areas for the Mongolian zircons. The best agreement is with the Tarim craton where a major Grenville-age orogenic event and early Neoproterozoic rifting have been identified. The age spectra also do not entirely exclude northeastern Gondwana as a source for the Mongolian zircons, but here the Neoproterozoic age peak is related to the Pan-African orogeny, and a minor Grenville-age peak may reflect a controversial orogenic event in NW India. Our Mongolian detrital and xenocrystic age spectrum suggests that the Tarim craton was the main source, and we favour a tectonic scenario similar to the present southwestern Pacific where fragments of Australia are rifted off and become incorporated into the Indonesian arc and microcontinent amalgamation that will evolve into a future orogenic belt.

Major ash eruptions of Barren Island volcano (Andaman Sea) during the past 72 kyr: clues from a sediment core record

Barren Island (Andaman Sea) is the northernmost active volcano of the Indonesian Arc. To construct the eruptive history of this little studied volcano, we measured 14C dates of inorganic carbon in sediment beds, and Sr and Nd isotopic ratios of seven discrete ash layers, in a marine sediment core collected from 32 km southeast of the volcano. The study reveals that the volcano had seven major ash eruptions at ~70, 69, 61, 24, 19, 15, and 10 ka. The ash layers erupted from 70 ka through 19 ka have highly uniform Nd isotopic composition, and since the ~15 ka eruption to the present the isotopic composition has been highly variable. Between ~24 ka and ~10 ka, the volcano had large ash eruptions spaced at 4,500 year intervals. Isotopically correlating the precaldera lavas and ash exposed on the volcano to the uppermost ash layer in the core, we infer that the caldera of Barren Island volcano is younger than 10 ka.

Hydrogen-isotope systematics in degassing basaltic magma and application to Indonesian arc basalts

We have developed a predictive model for hydrogen-isotope shifts during degassing of basaltic–andesitic magma that takes the variation of water species with water content and temperature into account. OH⁎ (water dissolved as hydroxyl) is the dominant dissolved water species for the range of naturally observed magmatic water contents (0–8 wt.%) and temperatures (>800 °C), and increases in importance with higher temperatures and lower water contents. We used our hydrogen fractionation model and published experimental data to derive values for hydrogen-isotope fractionation factors between melt and H 2O vapor of 10 3 ln α OH⁎ = 34.9 + 16.7 × 10 6/ T 2 for OH⁎ and 10 3 ln α H 2O ⁎ = − 30.3 + 16.7 × 10 6/ T 2 for H 2O⁎ (water dissolved as molecular water). A value of zero for the latter, as was assumed in some recent and earlier studies, is incompatible with our model results. As a consequence of the progressively increasing OH⁎/H 2O⁎ ratio in the melt during degassing, the bulk hydrogen fractionation factor also increases continuously. In contrast, the influence of temperature on the bulk fractionation factor ΔD vapor–melt is relatively small. We applied the hydrogen fractionation model to thirty subaerial lavas of basaltic composition from eight volcanoes located in the Indonesian arc system. All samples are extensively degassed, as average contents of residual water in the melt are only ∼ 0.15 wt.%. Whole-rock δD values range from − 109‰ to − 57‰ VSMOW (average = − 89‰). Oxygen isotope compositions of the same samples range from + 5.3‰ to + 6.7‰ VSMOW and are close to MORB values, indicating that crustal contamination or post-eruptive weathering was insignificant. Hydrogen isotopes fractionated markedly during degassing, and our data agree best with dominantly closed-system exsolution of magmatic water during magma ascent and storage, followed by syn-eruptive open-system degassing from depths of less than a few hundred meters. Magmatic water was derived from a mantle source with a typical D-enriched arc isotopic signature. Within-suite systematics suggest variations in initial δD and δ 18O values for some volcanic centers.

Volcanology and eruptive styles of Barren Island: an active mafic stratovolcano in the Andaman Sea, NE Indian Ocean

Barren Island (India) is a relatively little studied, little known active volcano in the Andaman Sea, and the northernmost active volcano of the great Indonesian arc. The volcano is built of prehistoric (possibly late Pleistocene) lava flows (dominantly basalt and basaltic andesite, with minor andesite) intercalated with volcaniclastic deposits (tuff breccias, and ash beds deposited by pyroclastic falls and surges), which are exposed along a roughly circular caldera wall. There are indications of a complete phreatomagmatic tephra ring around the exposed base of the volcano. A polygenetic cinder cone has existed at the centre of the caldera and produced basalt-basaltic andesite aa and blocky aa lava flows, as well as tephra, during historic eruptions (1787–1832) and three recent eruptions (1991, 1994–95, 2005–06). The recent aa flows include a toothpaste aa flow, with tilted and overturned crustal slabs carried atop an aa core, as well as locally developed tumuli-like elliptical uplifts having corrugated crusts. Based on various evidence we infer that it belongs to either the 1991 or the 1994–95 eruptions. The volcano has recently (2008) begun yet another eruption, so far only of tephra. We make significantly different interpretations of several features of the volcano than previous workers. This study of the volcanology and eruptive styles of the Barren Island volcano lays the ground for detailed geochemical-isotopic and petrogenetic work, and provides clues to what the volcano can be expected to do in the future.

Tectonic evolution of forearc nappes of the active Banda arc-continent collision: Origin, age, metamorphic history and structure of the Lolotoi Complex, East Timor

An integrated multidisciplinary investigation of the Lolotoi Complex of East Timor (Timor Leste) indicates that it is part of the Banda forearc that was metamorphosed and rapidly exhumed during the Eocene and accreted to the NW Australian continental margin during Late Miocene to present arc–continent collision. Greenschist, graphitic phyllite, quartz–mica schist, amphibolite and pelitic schist dominate metamorphic rock types. Mineral, whole rock, and trace element geochemical analyses of metabasites indicate protolith compositions consistent with tholeiitic basalt and basaltic andesite with mixed MORB and oceanic arc affinities. Metapelite schist is mostly composed of metasedimentary units derived from mafic to intermediate rocks with oceanic to continental volcanic arc provenance. Thermobarometric calculations show peak metamorphic conditions of 530 °C to 680 °C for garnet–biotite pairs and amphibole, and peak pressures of 5 to 10 kbar for garnet–aluminosilicate–quartz–plagioclase assemblages. Peak metamorphism occurred at 45.36 ± 0.63 Ma, as indicated by Lu–Hf analyses of garnet. Detrital zircon grains have a U/Pb age distribution with spikes at 663, 120 and 87 Ma, which is typical of detrital zircon ages throughout the Great Indonesian Arc of Asia, but is distinct from Australian affinity units. These data indicate deposition and later metamorphism occurred after 87 Ma. Structural analyses of the metamorphic rocks and their sedimentary and volcanic cover units reveals 5–6 deformational phases of alternating shortening and extension. There is little to no evidence of strike-slip deformation. Phases 1–4 are inferred as pre-Oligocene from age determinations. Phases 5 and 6 are most likely related to latest Miocene to Pliocene nappe emplacement and Pliocene to present collisional deformation. Kinematic indicators show mostly top to the SE directed shortening and top to the south and SE extension. Structural mapping indicates that the Lolotoi Complex and some of its cover units are in thrust contact with underlying Gondwana Sequence rocks. Asian affinity volcanic and sedimentary cover units are found mostly in normal fault contact with metamorphic rocks. These data indicate that the Lolotoi Complex of Timor Leste is correlative with the Mutis Complex of West Timor and both form part of the Banda Terrane, which is composed mostly of dispersed fragments of the eastern Great Indonesian Arc. The study demonstrates the complex nature and deformational history of forearc basement.

A Nd isotopic study of southern sourced waters and Indonesian Throughflow at intermediate depths in the Cenozoic Indian Ocean

We present Nd isotopic data for fossil fish teeth recovered from the past 40 m.y. at ODP Site 757, currently located at 1650 m water depth on the Ninetyeast Ridge in the Indian Ocean. Although Site 757 sits in a region strongly influenced by weathering inputs from the Himalayas and volcanic inputs from the Indonesian arc, the pattern of Nd isotopic variations does not appear to respond to these potential sources of Nd. Instead, secular variations correlate to changes in the composition of intermediate to deep water masses bathing the site and circulation patterns in the Indian Ocean. From ∼40 to 10 Ma, ɛNd values and the pattern of change at Site 757 closely match those of ODP Site 1090, a deep water site in the Atlantic sector of the Southern Ocean. Comparison to data from several Fe‐Mn crusts in the Indian Ocean suggests that intermediate to deep water flow paths were similar to the modern distribution of Circumpolar Deep Water. At approximately 10 Ma, Nd isotopic values increase in a step function by 2 ɛNd units, suggesting that plate motions had carried Site 757 into a region influenced by Indonesian Throughflow. Estimates of the vertical and horizontal position of this site at 10 Ma imply that Indonesian Throughflow extended as far south as ∼20°S and to a depth of ∼1500 m. From 10 to 0 Ma, Nd isotopic variations at Site 757 appear to record variations in Indonesian Throughflow. From 10 to 5.5 Ma, values at Site 757 overlap with those from crusts located in the southwest Pacific, indicating extensive flow through the Indonesian Seaway. From 5.5 to 3.4 Ma, ɛNd values become less radiogenic at Site 757 and more radiogenic in the southwest Pacific, suggesting increasing closure of the seaway and concomitant rerouting of equatorial Pacific waters. Beginning at 3.4 Ma, ɛNd values become more radiogenic again at Site 757, which may be attributed to enhanced opening of the seaway or to a change in the source of Throughflow waters from a southern to a northern Pacific region.

Analysis of long-distance earthquake tremors and base shear demand for buildings in Singapore

The seismic faults which cause long distance earthquakes for Singapore have been analysed and the maximum possible earthquake in each of these faults have been identified. A seismic hazard predictive model developed from geophysical principles, known as the component attenuation model (CAM), has been used to estimate the bedrock motion in Singapore. CAM has been validated by comparison of the estimated ground motion parameters with historical attenuation data. Earthquakes considered in the study originated from the Indonesian Arc and the Burmese Arc in the south-western part of the Eurasian plate spanning between 90°E and 105°E, and between −5°S and 25°N. An earthquake of 7.6 in Richter Magnitude, from a 400 km distance has been shown to be critical for buildings up to 25 storeys. The elastic base shear demand corresponding to this bedrock motion when accounted for amplification by soft soil is found to be below 10% of the weight of the building.

Sulfur and chlorine degassing from primitive arc magmas: temporal changes during the 1982–1983 eruptions of Galunggung (West Java, Indonesia)

The 1982–1983 eruptions of Galunggung represent a nine-month period of intermittent volcanic activity with significant changes in explosivity and emission of volatiles. Eruptions started with Vulcanian explosions but changed gradually to Strombolian activity. Compositions of juvenile material changed from basaltic andesite to high-Mg basalt, which are among the most primitive rock types known in the Indonesian arc system. Although bulk compositions suggest a single evolution trend, we infer from the compositions of melt inclusions in olivine phenocrysts that the magmas represent derivatives of a complex spectrum of primary melts. Primitive inclusions in olivine phenocrysts from magma erupted during the Strombolian phase contain up to ∼2000ppm sulfur, but concentrations decrease rapidly with increasing SiO2 down to matrix glass values (50–100ppm). ‘Vulcanian’ inclusions appear to be degassed before eruption (∼200ppm S). Chlorine concentrations increase from 750 to 2200ppm in Strombolian, and from 800 to 1500 in Vulcanian magmas, whereas matrix glass contains about 1000 ppm in both cases. Ash leachates show two cycles of decreasing S/Cl ratios: from 9.7 to 5.6 at the start of the activity, and from 12.2 to 2.0 after four months. As the second cycle follows upon increased seismic activity at shallow depth, it probably reflects degassing of fresh sulfur-rich magma arriving in the shallow Galunggung reservoir. In contrast to the degassed state of Vulcanian magma, the significant amounts of adsorbed sulfur on the ashes point to an excess source of sulfur, which was most likely derived from intruding Strombolian magma. Hence, the observed sulfur flux of 2Mt is not in accordance with a petrologic estimate of 0.09Mt. Using a published value of 550Mt of erupted material about 0.34km3 fresh undegassed magma is needed to account for the observed sulfur flux. This is close to the erupted volume of Vulcanian magma (0.26km3), which presumably was replaced completely by Strombolian magma during the eruption. Using the petrologic method, we calculate a total release of 0.3Mt chlorine, which agrees well with an output of 0.47Mt estimated independently from S/Cl ratios of the ash leachates and TOMS sulfur yields. Ash leachates show that about 35% of the sulfur and 30% of the chlorine was scavenged from the eruption plumes. Our results suggest that sulfur and chlorine were largely decoupled during degassing, which resulted in considerable variations in S/Cl ratios during the Galunggung eruptions. We infer that sulfur degassing reflects the arrival of fresh magma at shallow depth, whereas chlorine is largely derived from simultaneously erupted material. As a consequence, the petrologic estimates are more consistent with observed emissions for chlorine than for sulfur.

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

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.

Metal emissions from Kilauea, and a suggested revision of the estimated worldwide metal output by quiescent degassing of volcanoes

Measurements of a large suite of metals (Pb, Cd, Cu, Zn and several others) and sulfur at Kilauea volcano over an extended period of time has yielded a detailed record of the atmospheric injection of ordinarily-rare metals from this quiescently degassing volcano, representative of an important type. We have combined the Kilauea data with data of recent studies by others (emissions from volcanoes in the Indonesian arc; the large Laki eruption of two centuries ago; Etna; estimates of total volcanic emissions of sulfur) to form the basis for a new working estimate of the rate of worldwide injection of metals to the atmosphere by volcanoes. The new estimate is that volcanoes inject a substantially smaller mass of ordinarily-rare metals into the atmosphere than was stated in a widely cited previous estimate [J.O. Nriagu, A global assessment of natural sources of atmospheric trace metals, Nature 338 (1989) 47–49]. Our estimate, which is an upper limit, is an annual injection mass of about 10,000 tons of the metals considered, versus the earlier estimate of about 23,000 tons. Also, the proportions of the metals are substantially different in our new estimate.