Andreas Stracke

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

FOZO, HIMU, and the rest of the mantle zoo

Author Posting.

Domains of depleted mantle: New evidence from hafnium and neodymium isotopes

Isotope systematics of basalts provide information on the distribution of mantle components and the length scale of mantle heterogeneity. To obtain this information, high data and sampling density are crucial. We present hafnium and neodymium isotope data on more than 400 oceanic volcanics. Over length scales of several hundred to over one thousand kilometers hafnium and neodymium isotopes of mid-ocean ridge basalts are correlated and form an array of parallel trends on a global scale. On a larger scale these domains differ in the amount of highly depleted mantle material with radiogenic hafnium and neodymium isotope ratios. Compared to the Atlantic and Indian Ocean basins the asthenosphere of the Pacific basin seems to have a more uniform and a less radiogenic Hf isotopic composition for a given Nd isotopic composition. The parallel arrays of mid-ocean ridge basalts provide strong constraints on the makeup of the MORB mantle and are evidence for the presence of a highly depleted and highly radiogenic neodymium and hafnium component. This component, because of its highly depleted character, is unrecognized in the strontium-neodymium-lead isotope systems alone. Alternatively, the parallel arrays can have an ancient origin of systematic variations in the degree of depletion. Each array then represents the variations in this fossil melting regime. Individual ocean island basalt suites display different slopes in hafnium-neodymium isotope space, which are also best explained by varying amounts of highly residual mantle rather than isotopic differences in enriched mantle components as previously invoked. The ocean island basalt arrays diverge at the depleted end and project to radiogenic compositions that are similar to those of the asthenosphere through which they travel. This is strong evidence that the plume material interacts with its surrounding mantle as it ascends. The isotopic compositions of the ocean island and ridge basalts suggest that their systematics are influenced by a heretofore unrecognized depleted component.

Insights into the dynamics of mantle plumes from uranium-series geochemistry

The long-standing paradigm that hotspot volcanoes such as Hawaii or Iceland represent the surface expression of mantle plumes—hot, buoyant upwelling regions beneath the Earth’s lithosphere—has recently been the focus of controversy. Whether mantle plumes exist or not is pivotal for our understanding of the thermal, dynamic and compositional evolution of the Earth’s mantle. Here we show that uranium-series disequilibria measured in hotspot lavas indicate that hotspots are indeed associated with hot and buoyant upwellings and that weaker (low buoyancy flux) hotspots such as Iceland and the Azores are characterized by lower excess temperatures than stronger hotspots such as Hawaii. This direct link between buoyancy flux and mantle temperature is evidence for the existence of mantle plumes.

Rifting-related volcanism in an oceanic post-collisional setting: the Tabar–Lihir–Tanga–Feni (TLTF) island chain, Papua New Guinea

Abstract The Tabar–Lihir–Tanga–Feni (TLTF) volcanic island chain occurs in a zone of lithospheric extension superimposed on a post-collisonal tectonic setting along the Pacific and Indo-Australian plates northeast of Papua New Guinea. We present geochemical and Sr, Nd, and Pb isotope data for volcanic rocks from these islands and three recently discovered seamounts located at Lihir island. Major element data document an alkalic affinity of the sample suite and trachybasalts as the predominant rock type. Negative Nb-anomalies in extended trace element patterns, enrichment of the light rare earth elements, and Ce/Pb ratios of about 4 are typical of the values in calc alkaline island arc volcanics and support an origin from subduction-modified mantle. 87 Sr / 86 Sr ratios of 0.7037 to 0.7044 and e Nd values of +5.6 to +6.8 indicate that the upper mantle evolved with a time-integrated depletion in LREE, however, not as severe as that recorded in basalts from the East Pacific Rise. Variable 87 Sr / 86 Sr ratios at less variable 143 Nd / 144 Nd ratios suggest that 87 Sr / 86 Sr ratios of the melts were modified by secondary processes, such as assimilation of seawater Sr from crustal rocks. The Pb isotope ratios are uniform, moderately radiogenic ( 206 Pb / 204 Pb ca. 18.7 to 18.8), and similar to those reported for the active Mariana arc. Elevated 207 Pb / 204 Pb ratios relative to Pacific MORB suggest melting of small amounts of subducted sediments (ca. 1–2 wt.%). An important control of subducted sediment on the chemistry of the melts can also be inferred from the ratios of highly incompatible trace elements (e.g., Th, U, Pb, La, and Nb). Additional mantle enrichment by subduction derived fluids is reflected in high values of highly incompatible trace element ratios between fluid mobile (e.g., Ba) and fluid immobile elements (e.g., Th, Nb). The results of this study document that the chemical composition of igneous rocks from post-collisional tectonic settings are strongly influenced by previous plate tectonics. This conclusion implies that the information conveyed by tectonic discrimination diagrams for these rocks must be interpreted with care.

A rapid and efficient ion-exchange chromatography for Lu–Hf, Sm–Nd, and Rb–Sr geochronology and the routine isotope analysis of sub-ng amounts of Hf by MC-ICP-MS

The development and improvement of MC-ICP-MS instruments have fueled the growth of Lu–Hf geochronology over the last two decades, but some limitations remain. Here, we present improvements in chemical separation and mass spectrometry that allow accurate and precise measurements of 176Hf/177Hf and 176Lu/177Hf in high-Lu/Hf samples (e.g., garnet and apatite), as well as for samples containing sub-nanogram quantities of Hf. When such samples are spiked, correcting for the isobaric interference of 176Lu on 176Hf is not always possible if the separation of Lu and Hf is insufficient. To improve the purification of Hf, the high field strength elements (HFSE, including Hf) are first separated from the rare earth elements (REE, including Lu) on a first-stage cation column modified after Patchett and Tatsumoto (Contrib. Mineral. Petrol., 1980, 75, 263–267). Hafnium is further purified on an Ln-Spec column adapted from the procedures of Munker et al. (Geochem., Geophys., Geosyst., 2001, DOI: 10.1029/2001gc000183) and Wimpenny et al. (Anal. Chem., 2013, 85, 11258–11264) typically resulting in Lu/Hf < 0.0001, Zr/Hf < 1, and Ti/Hf < 0.1. In addition, Sm–Nd and Rb–Sr separations can easily be added to the described two-stage ion-exchange procedure for Lu–Hf. The isotopic compositions are measured on a Thermo Scientific Neptune Plus MC-ICP-MS equipped with three 1012 Ω resistors. Multiple 176Hf/177Hf measurements of international reference rocks yield a precision of 5–20 ppm for solutions containing 40 ppb of Hf, and 50–180 ppm for 1 ppb solutions (=0.5 ng sample Hf 0.5 in ml). The routine analysis of sub-ng amounts of Hf will facilitate Lu–Hf dating of low-concentration samples.

Effects of simple acid leaching of crushed and powdered geological materials on high-precision Pb isotope analyses

We present new results of simple acid leaching experiments on the Pb isotope composition of USGS standard reference material powders and on ocean island basalt whole rock splits and powders. Rock samples were leached with cold 6 N HCl in an ultrasonic bath, then on a hot plate, and washed with ultrapure H2O before sample digestion in HF-HNO3 and chromatographic purification of Pb. Lead isotope analyses were measured by Tl-doped MC-ICPMS. Intrasession and intersession analytical reproducibilities of repeated analyses of both synthetic Pb solutions and Pb from single digests of chemically processed natural samples were generally better than 100 ppm (2 SD). The comparison of leached and unleached samples shows that leaching consistently removes variable amounts of contaminants that differ in Pb isotopic composition for different starting materials. For repeated digests of a single sample, analyses of leached samples reproduce better than those of unleached ones, confirming that leaching effectively removes most of the heterogeneously distributed extraneous Pb. Nevertheless, the external reproducibility of leached samples is still up to an order of magnitude worse than that of Pb solution standards (∼100 ppm). More complex leaching methods employed by earlier studies yield Pb isotope ratios within error of those produced by our method and at similar levels of reproducibility, demonstrating that our simple leaching method is as effective as more complex leaching techniques. Therefore, any Pb isotope heterogeneity among multiple leached digests of samples in excess of the external reproducibility is attributed to inherent isotopic heterogeneity of the sample. The external precision of ∼100 ppm (2 SD) achieved for Pb isotope ratio determination by Tl-doped MC-ICPMS is thus sufficient for most rocks. The full advantage of the most precise Pb isotope analytical methods is only realized in cases where the natural isotopic heterogeneity among samples in a studied suite is substantially below 100 ppm.

In situ analysis of 230Th–232Th–238U ratios in titanite by fs-LA-MC-ICPMS

The potential of femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICPMS) for in situ analysis of U–Th disequilibria in titanite was investigated. The aim of the study was to resolve spatial variations in (230Th/238U) ratios (where parentheses denote activity) in titanite from slowly cooled magma bodies. An in-house titanite glass (TG2), determined to be in secular equilibrium by solution mode MC-ICPMS (i.e. (230Th/238U) = 1), was used to correct for U–Th elemental fractionation by sample standard bracketing. The effect of instrument operating conditions on the accuracy and reproducibility of (230Th/238U), (232Th/238U) and (230Th/232Th) ratios was studied by analyses of titanite minerals with known composition and a secondary titanite glass standard. The (230Th/232Th) data were found to be accurate and reproducible, independent of the instrument setting used, suggesting that corrections made for SEM-Faraday gain and abundance sensitivity were appropriate. However, plasma conditions, laser ablation mode, laser energy and wavelength, and titanite material properties were all found to variably influence the U–Th elemental fractionation and compromise the accuracy of the (230Th/238U) data to different extents. Hot plasma conditions significantly reduce the fractionation between U and Th. A drift in elemental fractionation was observed during single spot analyses using NIR laser ablation and results in errors of up to 29% on the (230Th/238U) data. The magnitude of the drift in the elemental fractionation was different for different laser wavelengths and energies. Ablation using the UV single spot mode was significantly less affected by variable elemental fractionation compared to NIR spot analyses, but precision was limited by lower sample uptake. Scanning mode analyses were not compromised by temporal variation of the U–Th intensity ratios but the degree of elemental fractionation was variable between analyses of different materials (e.g. glass versus minerals). This observation suggests material-dependent differences in U–Th fractionation even for near identical titanite compositions. Analyses of the secondary titanite glass standard TG1 bracketed by TG2 yield the most reproducible and accurate (230Th/238U) data, indicating more adequate correction for elemental fractionation when the calibration standard is matched in terms of material composition and structure.

Application of Isotope Dilution in Geochemistry

Isotope dilution (ID) is a highly accurate and precise technique for measuring element concentrations in a wide array of samples in the natural sciences. During ID, the element concentration is directly determined from the masses of the sample and an added element with nonnatural isotope composition, the so-called spike, as well as the measured isotope ratios of the sample, spike, and sample–spike mixture. ID is therefore a primary method of analysis, is directly traceable to SI units, and considered superior to methods that rely on calibration against external reference materials. ID is, however, not applicable to some mono-isotopic elements. Another disadvantage is its destructive nature: It requires dissolution of the sample for homogenization with the spike, often preventing analysis of other elements on the same sample aliquot. With the so-called double-spike techniques, ID can also be used to distinguish between the mass fractionation that occurs before the spike–sample equilibration from that which occurs afterward. This allows the removal of fractionation effects induced during analysis to reveal the stable isotope fractionation inherent in the sample.

Bourdon et al. reply

Replying to: D. L. Anderson & J. H. Natland 450, doi: 10.1038/nature06376 (2007)Anderson and Natland’scomment does not question our results regarding the velocity structure of mantle upwellings based on uranium-series. We stated clearly that our results do not provide direct measurements of the depth from which hotspot volcanism starts; we have not made unjustified claims by saying we have identified deep mantle plumes. However, our results do shed light on the still incompletely understood causes of hotspot volcanism.