Zhaochu Hu

Improved in situ Hf isotope ratio analysis of zircon using newly designed X skimmer cone and jet sample cone in combination with the addition of nitrogen by laser ablation multiple collector ICP-MS

The effect of three different cone combinations on the performance of laser ablation MC-ICP-MS (Neptune plus) for the in situ Hf isotope analysis of zircon were investigated. The signal sensitivities of Hf, Yb and Lu were improved by a factor of 1.4 and 2.5, respectively, with using the X skimmer cone + standard sampler cone and the X skimmer cone + Jet sample cone compared to the standard arrangement (H skimmer cone + standard sample cone). However, when using the high-sensitivity Jet sample cone, the instrumental mass fractionation for hafnium displayed a large non-linear component that could not be corrected using the normal mass fractionation laws. The magnitude of this non-linear mass fractionation was strongly related to the central gas flow rate. The in situ Hf isotope analysis of zircon standards 91500 and Mud Tank using the Jet cone displayed large deviations (410–470 ppm) at the optimum central gas flow rate for Hf, which seriously deteriorated the performance of the Jet cone. The addition of 4 ml min−1 nitrogen to the central gas flow in laser ablation MC-ICP-MS was found to not only increase the sensitivity of Hf by a factor of 2.1, but also suppress this non-linear mass fractionation. The determined Yb/Hf and Lu/Hf ratios at their corresponding optimum makeup gas flow rates for Hf intensity were found to be reduced by factors of 2 and 1.3 in the presence of nitrogen, respectively, which would benefit the accurate in situ determination of Hf isotopes in high-content Yb and Lu samples. Compared to the standard arrangement, the corresponding precision (2σ) of 176Hf/177Hf for single spot analysis of zircon standard 91500 was improved from 224 ppm to 50 ppm by using the newly designed X-skimmer cone and Jet sample cone in combination with the nitrogen addition technique. The determined 176Hf/177Hf ratios are in excellent agreement with published values in five reference zircon standards (91500, GJ-1, Mud Tank, Penglai and Plesovice). Our first Hf isotopic results from zircon standard M257 (0.281544 ± 0.000018; 2SD, n = 151) showed that it was fairly homogeneous in Hf isotopes. These results clearly demonstrate that the present analytical method has the potential to become an important tool for the pursuit of high-quality in situ Hf isotope data for zircons.

Signal enhancement in laser ablation ICP-MS by addition of nitrogen in the central channel gas

The effects of adding nitrogen to the central gas flow (Ar + He) of an Ar plasma in laser ablation inductively coupled plasma mass spectrometry are presented. The optimum central gas flow rate was found to be negatively correlated with the N2 gas flow rate. The addition of 5–10 ml min−1nitrogen to the central channel gas in LA-ICP-MS increases the sensitivity for most of the 65 investigated elements by a factor of 2–3. The degree of enhancement depends, to some extent, on the 1st ionization energy. Another important advantage of N2 mixed gas plasma for LA-ICP-MS is that the oxide ratios (ThO+/Th+) are significantly reduced (one order of magnitude). The hydride ratio (ArH+/Ar+) is also reduced up to a factor of 3, whereas the doubly charged ion ratio (Ca2+/Ca+) is increased. The background signals at masses 29, 31, 42, 51, 52 and 55 are significantly increased due to the nitrogen based polyatomic interferences. Compared to the spatial profiles of the ion distributions in the normal mode (without nitrogen), the addition of 5 ml min−1nitrogen leads to significant wider axial profiles and more uniform distribution of ions with different physical and chemical properties. Our results also show that the makeup gas flow (central channel gas) rate has a significant effect on the ion distribution of elements with different physical and chemical properties. A very consistent increase of argon signal by the addition of nitrogen (5 ml min−1) corroborates better energy transfer effect of nitrogen in the plasma.

Age and growth of the Archean Kongling terrain, South China, with emphasis on 3.3 ga granitoid gneisses

The North China craton and the Yangtze craton (South China) both contain Archean rocks in eastern China. Unlike the North China craton, where Archean rocks are widespread, in the Yangtze craton the exposed Archean rocks are only known in the Kongling terrain (360 km2). Zircon U-Pb ages and Lu-Hf isotopic compositions of three granodioritic-trondhjemitic gneisses and three metasedimentary rocks from the Kongling terrain were analyzed by LA-ICP-MS and LA-MC-ICP-MS. Igneous zircons in one trondhjemitic gneiss in the north of the Kongling terrain have an age of 3302±7 (1σ) Ma. Evidence from cathodoluminescence imaging, variations in Th/U and degree of U-Pb age discordance suggest that apparently younger zircons in the same population are variably disturbed 3302 Ma grains. Thus, this trondhjemitic gneiss is the oldest known rock in South China and predates the earlier reported ∼2900 Ma granitoid magmatism by 400 Ma. Zircon cores from one granodioritic gneiss in the north of the Kongling terrain also give a concordant age group at 3200 to 3300 Ma. Regardless as inherited or not, these cores crystallized from a magma indistinguishable in age with the trondhjemite. Concordant U-Pb ages for igneous zircons in one granodioritic gneiss in the south of the Kongling terrain yielded a weighted average 206Pb/207Pb age of 2981±13 Ma (2σ, MSWD=9.7, n=21). The zircon age and initial Hf isotopic compositions are similar to those of widespread granitoid gneisses from the north of the Kongling terrain (2903-2947 Ma), and indicate that the south and north of the Kongling terrain are correlative. The results also reinforce that magmatism of the whole Kongling terrain mainly occurred at 2900 Ma. Available Hf isotopic data from the Kongling terrain show that juvenile crustal additions occurred mainly between 3150 and 3800 Ma with a significant peak at 3300 to 3500 Ma. The ∼3300 Ma zircons from the trondhjemitic gneiss have Hf crust formation ages of 3450 to 3730 Ma, some of which have nearly chondritic εHf (t). The whole-rock depleted mantle Nd model age of this rock is 3400 Ma, close to its age of magmatism and consistent with the Hf model age. Its εNd value at 3300 Ma is nearly chondritic (1.26). These lines of evidence suggest that the 3300 Ma trondhjemite represent juvenile crust additions to the pre-existing continental crust.

A local aerosol extraction strategy for the determination of the aerosol composition in laser ablation inductively coupled plasma mass spectrometry

Aerosol transport efficiency in UV-ns LA-ICP-MS of less than 100% requires a representative aerosol composition for precise and accurate quantitative analysis. Therefore, aerosol expansion related changes in the composition of aerosols generated using a 193 nm excimer laser were studied within the ablation cell using an in-cell aerosol extraction strategy. The gas flow pattern within the ablation cell in the proposed local aerosol extraction was modelled using computational fluid dynamics techniques. Compared to commonly applied ablation cell geometry, the peak height of a single laser shot was increased by a factor of 13.5, the signal width was reduced by a factor of 12 and the washout time of the sample cell was consequently shortened to approx. 2 s, thereby almost eliminating processes of aerosol recirculation within the cell. The selective extraction of aerosol from different positions of the expanding laser plume was realized by subsequently changing the sampling distance between the ablation site and the gas outlet nozzle tip. The results show a similar distribution of siderophile elements (P, Cr, Mn, Fe, Co, Ni, Ga, Ge, Mo, W, Au), chalcophile elements (Cu, Zn, As, Se, Rh, Ag, Cd, In, Sn, Sb, Te, Pt, Tl, Pb, Bi) and some of lithophile elements (Li, B, Na, Mg, Si, K, V, Rb, Ba, U) within the expanding plume. In contrast, lithophile elements Be, Al, Ca, Sc, Y, Zr, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta and Th were significantly depleted towards increasing sampling distances from the ablation site. Furthermore, the plume composition depends strongly on the ambient gas used within the ablation cell. Evaluating various physical properties of individual elements it becomes clear that no single parameter dependence exists. However, elements with oxide melting points higher than 1500 °C tend to be depleted towards cooler regions of the expanding aerosol. The proposed local aerosol extraction strategy is suitable for the identification of position dependent and, therefore, an indirect indicator for particle size dependent elemental composition of 193 nm laser generated aerosols under He atmosphere, which could not be studied using particle separation devices. In contrast to the ablation in helium, the changes of the aerosol composition in argon were less variable amongst different elements when sampling at different distances from the ablation crater. However, Zn and Cd intensities increased when sampling further away from the ablation crater. Results indicate that aerosol expansion within the ablation in UV-ns laser ablation can be a significant source of non-stoichiometric sampling, especially induced by aerosol deposition on the sample surface.

Contrasting matrix induced elemental fractionation in NIST SRM and rock glasses during laser ablation ICP-MS analysis at high spatial resolution

The greatest strength of the LA-ICP-MS technique is its application to microsampling in which extremely small pits are obtained. The results of this study highlight some significant different laser-induced fractionations between widely used external reference materials NIST SRM 610–614 and natural silicate reference materials (e.g., USGS reference glasses (GSE-1G, GSD-1G), MPI-DING glasses, USGS basalt glasses and zircon reference material GJ-1) at high spatial resolution analysis. For the sample matrices and analytical conditions used in this study, the laser-induced elemental fractionations for 63 selected isotopes are negligible at the spot sizes of 160–44 µm. However, the laser-induced elemental fractionations of Li, Na, Si, K, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Cs and U (with respect to Ca) increase significantly with decreasing spot sizes from 44 µm to 32 µm, 24 µm and 16 µm in these natural silicate reference materials. Unlike in these sample matrices, laser-induced elemental fractionations of these elements in NIST SRM 610–614 are unique in that they are almost not affected by the change of spot sizes from 44 to 32 to 24 µm, with only slight increase at the spot sizes of 16 µm. The much less significant laser-induced elemental fractionation in NIST SRM 61X in comparison with other natural silicate materials makes them not ideal as external reference materials at high spatial resolution analysis. Alternatively, this NIST SRM 61X-specific matrix effect for Li, Na, K, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Cs and U can be minimized by using Si for internal standardization. U and Pb in zircon GJ-1 are exceptions, which are zircon-specific.

Origin and evolution of the Bainaimiao arc belt: Implications for crustal growth in the southern Central Asian orogenic belt

Recent results show that evolution of the huge Central Asian orogenic belt can be explained in terms of southwest Pacific–style accretion of arcs and microcontinents. A better understanding of the origin and evolution of arcs and microcontinents will enhance our knowledge on the evolution of the Central Asian orogenic belt. As one of the most important early Paleozoic arc systems south to the Solonker suture zone, the origin and evolution of the Bainaimiao arc belt are still not well constrained. New zircon U-Pb geochronological and geochemical results on magmatic rocks in the Bainaimiao arc belt indicate that the arc was active from 0.52 Ga to 0.42 Ga and can extend to east Siping in NE China. Zircon U-Pb geochronological results of metasedimentary rocks in the Bainaimiao arc belt indicate that they are early Paleozoic in age, not Precambrian as previously regarded. Detrital zircon analysis of metasedimentary rocks and Sr-Nd-Hf geochemical results of magmatic rocks indicate the existence of some Proterozoic basement rocks beneath the Bainaimiao arc, and it was built upon a Precambrian microcontinent that has a tectonic affinity to the Tarim or Yangtze cratons. The Bainaimiao arc is an ensialic island arc characterized by different evolution history and basement compositions from the northern North China craton. It was separated by a wide ocean from the northern North China craton during the Cambrian–Ordovician period. Successive northward subduction resulted in contraction of the ocean and final accretion of the Bainaimiao arc to the northern North China craton during the Late Silurian–earliest Devonian by arc-continent collision. Arc-continent collision could be an important mechanism for continental crustal growth and formation of the huge Central Asian orogenic belt.

Total rock dissolution using ammonium bifluoride (NH4HF2) in screw-top Teflon vials: a new development in open-vessel digestion.

Complete sample digestion is a prerequisite for achieving reproducible and accurate analytical results for geological samples. Open-vessel acid digestions successfully dissolve mafic samples, but this method cannot achieve complete dissolution of felsic samples, because of the presence of refractory minerals such as zircon. In this study, an efficient and simplified digestion technique using the solid compound NH(4)HF(2) in a screw-top vial has been developed for multielement analysis of different types of rock samples. NH(4)HF(2) has a higher boiling point (239.5 °C) than conventional acids such as HF, HNO(3) and HCl, which allows for an elevated digestion temperature in open vessels, enabling the decomposition of refractory phases. Similar to HF, HNO(3) and HCl, ultrapure NH(4)HF(2) can be produced using a conventional PFA sub-boiling (heating and cooling) purification system. A digestion time of 2-3 h for 200 mg NH(4)HF(2) in a Savillex Teflon vial at 230 °C is sufficient to digest 50 mg of the felsic rock GSP-2, which is ~6 times faster than using conventional closed-vessel acid digestion at 190 °C (high-pressure PTFE digestion bomb). The price of a Savillex Teflon vial is far less than the price of a high-pressure PTFE digestion bomb (consisting of a PTFE inner vessel and an outer stainless steel pressure jacket). Moreover, the NH(4)HF(2)-open-vessel acid digestion is not hampered by the formation of insoluble fluorides. We have successfully applied the NH(4)HF(2)-open-vessel acid digestion to the digestion of a series of international geological reference materials, including mafic to felsic igneous rocks and shales. This method provides an effective, simple, economical, and comparatively safe dissolution method that combines the advantages of both the open- and closed-vessel digestion methods.

Suppression of interferences for direct determination of arsenic in geological samples by inductively coupled plasma mass spectrometry

We have developed a method for direct determination of arsenic in geological samples using ICP-MS by reduction of interferences, without preconcentration, separation and use of the hydride generation technique. Concentrations of HNO3 have a significant effect on the arsenic signal. This type of interference cannot be corrected by internal standards (Rh and In) because the signal suppression due to HNO3 is apparently dependent on the first ionization potential of elements. Addition of 4% (v/v) ethanol to 1–10% (v/v) HNO3 was found to be an excellent method for reducing this type of matrix effect from 30–40% to less than 5% for high first ionization potential elements 75As (9.81 eV), 82Se (9.75 eV), and 126Te (9.01 eV). Direct determination of arsenic in geological samples by ICP-MS is often complicated by the presence of Nd2+, Sm2+and Eu2+ interferences, in addition to the well known interference of ArCl+, and the high first ionization potential of As (9.81 eV) also results in relatively low analytical sensitivity in ICP-MS. It is shown that both problems can be overcome by a combination of a 4% ethanol matrix modifier with nebulizer gas flow rate adjustment. For example, the interference from doubly charged ions of a rare earth element (Ce2+) is reduced by a factor of 30 with the addition of 4% ethanol at a nebulizer gas flow rate of 1.00 l min−1 and rf power of 1350 W, while the signal intensity of As is similar in both solutions. A nebulizer gas flow rate of 0.94 l min−1, an rf power of 1350 W and 4% ethanol modifier were chosen in practical sample analysis. Under these conditions, the interference of doubly charged ions of the rare earth element (Ce2+) was reduced by a factor of 6.5 and the signal intensity of As was improved by a factor of 3 relative to that in 3% (v/v) HNO3 solution at the corresponding optimum nebulizer gas flow rate of 0.98 l min−1 and rf power of 1350 W. The arsenic equivalent concentration caused by ArCl+ interference was reduced by a factor of 10 under our given experimental conditions in the presence of 4% (v/v) ethanol. The developed method was applied to the direct determination of arsenic in a series of international geological reference materials. Most of the results were found to be in reasonable agreement with the reported values in the literature, particularly for those having recommended values. This simple method shows a great potential for the direct determination of arsenic in geological and environmental samples.

First direct evidence of sedimentary carbonate recycling in subduction-related xenoliths

Carbon in rocks and its rate of exchange with the exosphere is the least understood part of the carbon cycle. The amount of carbonate subducted as sediments and ocean crust is poorly known, but essential to mass balance the cycle. We describe carbonatite melt pockets in mantle peridotite xenoliths from Dalihu (northern China), which provide firsthand evidence for the recycling of carbonate sediments within the subduction system. These pockets retain the low trace element contents and δ18OSMOW = 21.1 ± 0.3 of argillaceous carbonate sediments, representing wholesale melting of carbonates instead of filtered recycling of carbon by redox freezing and melting. They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions. Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement. The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions.

Paleo-Asian oceanic slab under the North China craton revealed by carbonatites derived from subducted limestones

National Science Foundation of China [41530211, 41125013, 90914007]; Ministry of Science and Technology of China [2013CB429806]; State Administration of Foreign Expert Affairs of China [B07039]; Specialized Research Fund for the Doctoral Program of Higher Education [20130145110001]; Ministry of Science and Technology Special Funds of the State Key Laboratory of Geological Processes and Mineral Resources (China University of Geosciences)