P.Z. Vroon

Geochemical trends across an arc-continent collision zone : Magma sources and slab-wedge transfer processes below the Pantar Strait volcanoes, Indonesia

Abstract Four volcanoes in the Pantar Strait, the westernmost part of the extinct sector of the east Sunda arc, show remarkable across-arc variation in elemental abundances (K2O: 1.2 to 4.3%), trace element ratios (Pb/Ce: 0.4 to 0.18; Ce/Yb: 20 to 55) and isotope ratios (143Nd/144Nd: 0.51263 to 0.51245; 87Sr/86Sr: 0.7053 to 0.7068; 206Pb/204Pb: 19.29 to 19.15). Pb isotopes are decoupled from Sr and Nd isotopes, with the frontal volcanoes showing the higher Nd and Pb and lower Sr isotopic ratios. The isotopic and trace element ratios of the volcanic samples are best explained by modification of a MORB-type source (with Indian Ocean island basalt–type Pb isotopic characteristics) by a fluid and a partial melt of subducted continental material (SCM). The frontal volcano contains the highest proportion of the fluid component, with a small contribution of partial melt. The source of the rear-arc volcano is strongly influenced by a partial melt of SCM that had undergone a previous dehydration event, by which it lost most of its fluid-mobile elements such as Pb. The SCM partial melt was in equilibrium with both rutile and garnet, whereas mantle melting took place in the presence of residual mica. The relatively large across-arc increase in incompatible elements can be explained by a combination of increasing addition of SCM partial melt, changing mantle wedge fertility and smaller degrees of partial melting toward the rear of the arc. Comparison with a more westerly across-arc transect shows that the relatively low 143Nd/144Nd ratios of the frontal volcano, and the decoupling of Pb from Sr and Nd isotopes are unique to the Pantar Strait volcanoes. This is likely to reflect magma generation in a collisional environment, where the leading edge of the Australian continent, rather than subducted sediment, contributes to the magma source.

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

We present Sr, Nd, and Pb isotope results and SiO2, 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 (87Sr/86Sr = 0.7045–0.7055, 143Nd/144Nd = 0.51273–0.51291, and 206Pb/204Pb = 18.66–18.75), whereas those in the SW have extreme values (87Sr/86Sr = 0.7065–0.7083, 143Nd/144Nd = 0.51252–0.51267, and 206Pb/204Pb = 19.28–19.43). Serua, situated in the central part, is the most anomalous volcano with regard to its Sr and Nd isotopic composition (87Sr/86Sr = 0.7075–0.7095 and 143Nd/144Nd = 0.51240–0.51260) but not with regard to Pb isotopes (206Pb/204Pb = 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., 207Pb/204Pb - 208Pb/204Pb) 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 208Pb/ 204Pb 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.

Dual role of seawater and hydrothermal fluids in Early Archean chert formation: Evidence from silicon isotopes

The great variety and abundance of chert deposits in Archean terrains constitute one of the most unusual features that mark Earth9s early geological history. Proposed explanations for their origin largely relying on field observations, trace element patterns, or oxygen isotope signatures have not yielded an encompassing model. Here we document silicon isotope systematics in cherts from 3.5–3.0 Ga units in the Pilbara Craton (Western Australia) as evidence of their formation by several distinct processes in Early Archean near-surface environments. Our δ 30 Si results, in combination with geochemical and mineralogical signatures and field relations, point to three end-member sources of silica derivation. One chert type is inferred to have originated through massive transformation of precursor material by silica added from sea water. At least 2‰ differences in δ 30 Si between the two other types, produced by direct chemical precipitation on the seafloor or in conduits, discriminate seawater from hydrothermal fluid as a source of silica. A virtually continuous Si isotope trend in cherts from this group is consistent with interaction between silica-carrying fluids at submarine vent systems.

Ocean circulation and iceberg discharge in the glacial North Atlantic: Inferences from unmixing of sediment size distributions

Variability in iceberg discharge and deep-ocean circulation in the North Atlantic during the last glacial period is inferred from the grain-size distribution and trace elemental composition of terrigenous sediments in a deep-sea core taken on Reykjanes Ridge, south of Iceland. End-member modeling of the grain-size distributions is used to unmix the signals of varying bottom-current speed and iceberg discharge. The size distribution within the silt fraction appears to be influenced by both factors. Based on Th-Sc-La relationship, we established that during the ice-rafted detritus events, continental material of likely Greenlandic origin increased to 87%, and that bottom-current–derived material contains to 40% mid-oceanic ridge fines, probably of Icelandic origin. Our results have important implications for the use of silt grain size as an indicator for paleocurrent speed in the glacial North Atlantic. We show that reconstructions of variations in bottom-current speed based on the raw grain-size data are opposite to inferences from the unmixed record. The latter indicates that deep-water convection decreased during periods of enhanced iceberg discharge, which is in general agreement with paleoceanographic reconstructions of the North Atlantic.

Determination of silicon isotope ratios in silicate materials by high-resolution MC-ICP-MS using a sodium hydroxide sample digestion method

Silicon isotope ratios (28Si, 29Si and 30Si) can be measured with high precision by multi-collector inductively coupled plasma mass spectrometers (MC-ICP-MS). However, the problematic extraction of silicon from geological materials has been a major disadvantage in previous silicon isotope studies with conventional gas source mass spectrometry, whereas available silicon isotope results obtained by MC-ICP-MS techniques have been mainly restricted to waters and high purity silica. We show here that high yields of silicon (>97%) can be achieved from samples ranging from pure silica to basaltic compositions (45–52 wt.% SiO2) via a three-step digestion and purification procedure. Silicon isotope measurements, performed with a Finnigan Neptune MC-ICP-MS used in medium-resolution mode (resolving power: 2500), indicate that polyatomic interferences can be resolved and that both δ29Si and δ30Si can be determined with high accuracy and precision on interference-free peak plateaux in the mass spectrum. Instrumental blanks (20–65 mV) were reduced to acceptable values with a Cetac Aridus desolvating device fitted with a sapphire injector in the torch. Sensitivity in medium-resolution mode is in the range of ∼6 V per μg g−1 for 28Si. δ29Si and δ30Si have been determined for silicon isotope standards IRMM-018 (δ30Si = −1.75‰), IRMM-018-76 (δ30Si = −1.42‰), Diatomite (δ30Si = 1.34‰) and Big Batch (δ30Si = −10.52‰), for USGS standards BHVO-2 (δ30Si = −0.09‰) and AGV-2 (δ30Si = −0.01‰), and for Aldrich pure silica powder (δ30Si = −0.32‰). Precision on δ30Si is 0.18–0.41‰ (2 s.d.). Our combined procedure for sample preparation followed by high-resolution MC-ICP-MS analysis facilitates straightforward and safe measurement of silicon isotope ratios in silicate materials.

An improved chromatographic separation technique of Nd with application to NdO+ isotope analysis

Abstract Isotopic analysis by Thermal Ionization Mass Spectrometry (TIMS) requires efficient chemical separation of the element of interest. This statement is particularly true for NdO + measurements during which isobaric interferences must be minimized. Here, we present a new chromatographic procedure that efficiently separates Nd from other Light Rare-Earth Elements (LREE) and is ideal for NdO + measurements. After separation of the LREE fraction from the bulk matrix, Nd is chemically separated using a High Pressure Liquid Chromatographic (HPLC) technique, with Aminex resin (Bio-Rad©) and a CH 3 OH–HAc–HNO 3 eluent. This simple and low blank (10–15 pg) separation technique efficiently reduces isobaric interferences so that 140 CeO + / 144 NdO + and 141 PrO + / 144 NdO + ratios are generally below recommended values, ensuring no significant contribution from isobaric interferences. The NdO + analytical technique also requires accurate determination of the oxygen isotope composition in TIMS since it is used to recalculate the Nd isotope composition of the samples. The results of a series of measurements performed to determine the oxygen isotope ratios ( 18 O/ 16 O=0.002040 and 17 O/ 16 O=0.000386) were used to constrain the parameters influencing the measurements. Loading technique and running conditions greatly affect measured oxygen isotope ratios. Guidelines to help accurately determine the oxygen isotope ratios are also presented. Combining the Aminex chromatographic separation with NdO + measurements and accurately determined oxygen isotope ratios allows high precision measurements. A wide range of low abundance samples can be analyzed to a precision similar to that obtained on larger samples with conventional Nd + technique (±0.000020, 2 σ ).

Sulfur-induced offsets in MC-ICP-MS silicon-isotope measurements

Sample preparation methods for MC-ICP-MS silicon-isotope measurements often involve a cation-exchange purification step. A previous study has argued that this would suffice for geological materials, as the occasional enrichment of anionic species would not compromise silicon-isotope analysis. Here we report significant offsets in MC-ICP-MS silicon-isotope measurements induced by the presence of sulfur. We show that offsets in δ30Si become significant above SO4/Si ratios (wt.) of 0.02, reaching up to ca. +1.4‰ at SO4/Si ratios above ∼0.2. This finding is particularly relevant for studies of sulfur-rich waters and silicified rocks where alteration was accompanied by sulfur-enrichments. We propose an additional purification step to remove sulfur from solid sample material.

Strontium and lead isotope ratios in human hair: investigating a potential tool for determining recent human geographical movements

Three cleaning techniques that remove external contamination of human hair are assessed to investigate the potential use of Sr and Pb isotope composition of hair for human provenancing. These techniques are; (i) a centrifugation technique using diiodomethane where hair and soil particles are separated by density difference; (ii) a leaching technique of the hair surface using 2 M HNO3 acid and (iii) cleaning the hair with chloroform, methanol and ultra pure water. These techniques are validated and are successfully applied to demonstrate the capability of modern and archaeological human scalp hair and modern human facial hair to record Sr and Pb isotope variations related to geographic location. In this study, Sr isotope ratios analysed in modern human scalp hair from a female vegetarian and non-smoker register marked isotopic change on a monthly timescale when an individual moves to locations with contrasting Sr isotope compositions. Pb isotope ratios do not show significant changes after moving locations, possibly due to comparable Pb isotope ratios in the two environments. In contrast, Pb isotopes ratios analysed in facial hair from a male omnivore and smoker record isotopic changes within a two week period when moving between locations with significant differences in environmental Pb isotope compositions. Further research is needed to determine the exact rate of change in isotopic composition in scalp and facial hair in humans with different diets and life styles that move from geographical locations with isotopic contrasting composition.

Pb and Nd isotope constraints on the provenance of tectonically dispersed continental fragments in east Indonesia

Abstract Microcontinental fragments, tectonically dispersed at the triple junction of the converging SE Asian, Indo-Australian and Pacific plates, are a widespread phenomenon in the east Indonesian region. This paper investigates the potential of Pb-Nd isotope characteristics of igneous and (meta) sedimentary rocks from these fragments as indicators of their provenance. From the correspondence of isotopic signatures, it appears possible to identify provenance areas of four selected microcontinents, which were emplaced in different settings and regions of east Indonesia since late Mesozoic times. According to their isotopic affinities, the blocks of Ambon-Seram, Bacan, the Banda Ridges and Sumba originally formed part of southern New Guinea, north Australia, ‘Pacific’ New Guinea and Sundaland respectively. These conclusions are in line with geological constraints on the palaeopositions of the fragments. The results thus indicate that isotopic signatures are potentially a powerful tool in reconstructions of the tectonic history of east Indonesia.

Pb isotope analysis of ng size samples by TIMS equipped with a 1013 Ω resistor using a 207Pb–204Pb double spike

The use of the double spike technique to correct for instrumental mass fractionation has yielded high precision results for lead isotope measurements by thermal ionisation mass spectrometry (TIMS), but the applicability to ng size Pb samples is hampered by the small size of the 204Pb ion beam in the natural isotope composition analysis. To overcome this limitation, we successfully demonstrate the application of a 1013 Ω resistor in the Faraday cup amplifier feedback loop used for the collection of 204Pb in combination with a newly produced 207Pb–204Pb double spike to correct for instrumental mass fractionation. The use of a 1013 Ω resistor for the collection of the small 204Pb ion beam leads to a tenfold improvement in the signal-to-noise ratio, but necessitates an external gain correction using a secondary standard and careful monitoring of the ion beam stability. SRM 981 aliquots of 5 ng display unparalleled reproducibility of 90–125 ppm (2 SD) and are in excellent agreement with recommended values: 206Pb/204Pb = 16.9404 ± 0.0016, 207Pb/204Pb = 15.4977 ± 0.0019 and 206Pb/204Pb = 36.7193 ± 0.0042 (2 SD, n = 22). Comparable high quality data have been obtained for 5 ng aliquots of geological reference materials AGV-1 and BCR-1 (e.g.206Pb/204Pb = 18.9399 ± 0.0011 and 18.8208 ± 0.0011 respectively), indicating that analytical blank contribution is negligible for sample sizes down to 5 ng. With further reduction in, or precise correction for, analytical blanks, the combination of the double spike technique with high-resistance amplifiers has the potential to produce highly accurate and precise (<200 ppm) Pb isotope data for samples below the ng level. This methodology will open a range of research possibilities presently impeded by the low amount of Pb available for analysis.