Keqing Zong

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.

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.

Partial melting of deeply subducted eclogite from the Sulu orogen in China

We report partial melting of an ultrahigh pressure eclogite in the Mesozoic Sulu orogen, China. Eclogitic migmatite shows successive stages of initial intragranular and grain boundary melt droplets, which grow into a three-dimensional interconnected intergranular network, then segregate and accumulate in pressure shadow areas and then merge to form melt channels and dikes that transport magma to higher in the lithosphere. Here we show, using zircon U–Pb dating and petrological analyses, that partial melting occurred at 228–219 Myr ago, shortly after peak metamorphism at 230 Myr ago. The melts and residues are complimentarily enriched and depleted in light rare earth element (LREE) compared with the original rock. Partial melting of deeply subducted eclogite is an important process in determining the rheological structure and mechanical behaviour of subducted lithosphere and its rapid exhumation, controlling the flow of deep lithospheric material, and for generation of melts from the upper mantle, potentially contributing to arc magmatism and growth of continental crust.

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)

Accurate determination of lithium isotope ratios by MC-ICP-MS without strict matrix-matching by using a novel washing method

The accurate determination of Li isotopic ratios by MC-ICP-MS has traditionally been hampered by the high background and severe memory effect of Li. In this study, a novel method to efficiently reduce the high background and memory effect of Li was developed. It was found that the Li background can be significantly reduced by a factor of 15 to 70 by using a 5% NaCl rinse solution. In addition, the “mismatching effects” reported previously, which are caused by different acid and Li concentrations between the sample and standard, were eliminated once the Li background was efficiently reduced, suggesting that the crux of the two types of matrix effects is actually the high Li instrumental background and memory effect. Applying the background reduction technique, a method without strict matrix-matching was developed for the accurate and precise determination of Li isotopic ratios. The proposed method was validated by the analysis of eight reference materials with satisfactory results, even when the Li concentration in the samples was not matched with that of standards. The external precision of this method is better than ±0.25‰ (2SD) for δ7Li, suitable for the identification of small fractionation of Li isotopes occurring in geological processes. With the proposed method, there is no need to match the Li and acid concentration of the sample and standard, which significantly reduced the sample preparation time and increased the sample throughput.

Neoarchaean crustal growth by combined arc–plume action: evidence from the Kadiri Greenstone Belt, eastern Dharwar craton, India

Abstract Field and geochemical studies combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating set important constraints on the timing and petrogenesis of volcanic rocks of the Neoarchaean Kadiri greenstone belt and the mechanism of crust formation in the eastern Dharwar craton (EDC). The volcanic rocks are divided into three suites: tholeiitic basalts, calc-alkaline high-Mg# andesites and dominant dacites–rhyolites. The basalts (pillowed in places) show flat rare earth element (REE) and primordial mantle-normalized trace element patterns, but have minor negative Nb and Ta anomalies. They are interpreted as mantle plume-related oceanic plateau basalts whose source contained minor continental crustal input. The andesites are characterized by high Mg# (0.66–0.52), Cr and Ni, with depletion of high-field strength elements (HFSE) and enrichment of light REE (LREE) and large-ion lithophile elements (LILE). They were probably derived from a metasomatized mantle wedge overlying a subducted slab in a continental margin subduction zone. The dacites–rhyolites are silicic rocks (SiO2=61–72 wt%) with low Cr and Ni, K2O/Na2O mostly 0.5–1.1, highly fractionated REE patterns, enrichments of LILE and distinctly negative HFSE anomalies. One rhyolite sample yielded a zircon U–Pb age of 2353±32 Ma. This suite is similar to potassic adakites and is explained as the product of deep melting of thickened crust in the arc with a significant older crustal component. Collision between a continental margin arc with an oceanic plateau followed by slab break-off, upwelling of hot asthenosphere and extensive crustal reworking in a sustained compressional regime is proposed for the geodynamic evolution of the area. This is in corroboration with the scenario of EDC as a Neoarchaean hot orogen as suggested recently by some workers. Supplementary material: Details of whole-rock major and trace element determination, Nd isotope analysis and zircon U–Pb dating and trace element analysis, the geographical coordinates of the samples and the values of the international rock standards analysed are available at http://www.geolsoc.org.uk/SUP18660

In-situ U-Pb dating of uraninite by fs-LA-ICP-MS

In this study, the Pb/U fractionation between zircon and uraninite during femtosecond Laser Ablation Inductively Coupled Plasma Mass Spectrometry (fs-LA-ICP-MS) analysis was studied in detail. The results show significant Pb/U fractionation between zircon and uraninite during fs-LA-ICP-MS analysis that when calibrated against the zircon standard M257, the obtained U-Pb age of the Chinese national uraninite standard GBW04420 is 17% older than the recommended value. Thus, the accurate in-situ U-Pb dating of uraninite by LA-ICP-MS requires matrix-matched external standards for calibration. Uraninite in thin sections of two U-mineralized leucogranite from the Gaudeanmus in Namibia was analyzed by a fs-LA-ICP-MS equipped with a Signal Smooth Device (SSD), using laser spot and frequency of 10 µm and 1 Hz, respectively. When calibrated using GBW04420 as the external standard, two samples give weighted mean 206Pb/238U ages of 504±3 Ma (2σ, n=21) and 503±3 Ma (2σ, n=22), and only one of two samples yields a concordia U-Pb age of 507±1 Ma (2σ, n=21). These results are consistent with ID-TIMS U-Pb ages of 509±1 and 508±12 Ma and are also indistinguishable from zircon U-Pb upper intercept ages of 506±33 Ma (2σ, n=29) and 501±51 Ma (2σ, n=29). The present study shows that in-situ U-Pb dating of uraninite can deliver more reliable formation ages of the deposit than dating coeval high-U zircon because the latter commonly suffer significant Pb loss after formation. Our results confirm that GBW04420 is an ideal matrix matching standard for in-situ U-Pb dating of uraninite.

Accurate determination of sulfur isotopes (δ33S and δ34S) in sulfides and elemental sulfur by femtosecond laser ablation MC-ICP-MS with non-matrix matched calibration

The isotopic composition of sulfur is a vital tracer used in the Earth and planetary sciences. In this study, the laser- and ICP-induced isotopic fractionation in S-rich minerals (sulfides and elemental S) with different matrices was investigated by using 257 nm femtosecond (fs) and 193 nm ArF excimer nanosecond (ns) laser ablation systems coupled to a Neptune Plus MC-ICP-MS. Compared to ns-LA-MC-ICP-MS, higher sensitivity (1.4–2.4 times) under similar instrumental conditions and better precision (∼1.6-fold) under the same signal intensity condition were achieved by fs-LA-MC-ICP-MS. In addition, a fs-laser provides less fluence and matrix dependent S isotopic fractionation, and more stable transient isotopic ratios compared to a ns-laser. Better results acquired by fs-LA-MC-ICP-MS were attributed to the smaller size of particles and less thermal effect produced by using the fs-laser, which were evidenced by the morphologies of the ablation craters and ejected aerosol particles of P-S-1 (the pressed powder pellet of IAEA-S-1) and PPP-1 (a pyrite single crystal from the Sukhoi Log deposit). The ICP-induced isotopic fractionation (matrix effect) was still found in fs-LA-MC-ICP-MS under the maximum sensitivity conditions. However, a significant reduction of the matrix effect was obtained under robust plasma conditions at a lower makeup gas flow rate (0.52–0.54 l min−1) relative to the maximum sensitivity condition (0.6 l min−1) for S isotope analysis. This could be ascribed to the particles that not only pass into the higher temperature ICP for a longer residence time at a lower makeup gas flow rate that resulted in more efficient vaporization of the particles, but also experience a more robust plasma induced by adding 4–6 ml min−1 N2 into the plasma. Furthermore, under the robust conditions, the results of six reference materials with different matrices obtained by fs-LA-MC-ICP-MS with non-matrix matched calibration with a spot size of 20–44 μm showed excellent agreement with the reference values (the accuracy of 0.01–0.15‰ for δ34S and 0.11–0.45‰ for δ33S and the precision of 0.16–0.40‰ (2 s) for δ34S and 0.35–0.78‰ (2 s) for δ33S) and the mass-dependent fractionation line, validating the applicability of the proposed approach for providing high-quality in situ isotope data (δ33S and δ34S) of sulfides and elemental sulfur at high spatial resolution using non-matrix matched analysis.

Comparison of signal intensities and elemental fractionation in 257 nm femtosecond LA-ICP-MS using He and Ar as carrier gases

Signal intensities and elemental fractionation under different ambient gas conditions (He, Ar and a pre-mixed He–Ar mixture) were investigated using 257 nm femtosecond (fs)-LA-ICP-MS. The experiments show that the change of ablation carrier gases from argon to helium increases the sensitivities of refractory elements (e.g. rare earth elements) in NIST SRM 610 by a factor of 1.05–1.20, whereas, the signal intensities of volatile elements (e.g. Tl, Cd, Se, Sn, Te, Zn, Pb, Bi, Ge, Ga, Sb, Ag, and Cu) were increased by a factor of 1.5–3.0. The much less deposited aerosol particles around the ablation craters produced in helium when compared to argon may partly account for this phenomenon. Our results also suggest that volatile elements may be enriched in small aerosol particles produced by femtosecond laser ablation whose transport efficiencies are increased when using He instead of Ar. The calculated elemental fractionation indices (with respect to Ca) of volatile elements (B, Cu, Zn, Ga, Ge, As, Cd, Mo, Ag, Sn, Sb, Pb and Bi) rapidly increased to 1.1–1.2 with increasing the spot size from 24 μm to 60 μm when using helium as the carrier gas. In contrast, they remain nearly constant at spot sizes of 24–60 μm when using argon as the carrier gas. However, the indices of volatile elements (Co, Ni, Cu, Zn, Ga, Ge, As, Ag, Cd, Sn, Sb, Pb, and Bi) are less than 1 for all the spot sizes investigated. The signal intensities of all elements obtained in the pre-mixed He–Ar mixture is similar to the sensitivities acquired in pure helium. Meanwhile, the calculated elemental fractionation indices of all elements generally remain constant and closer to 1 than those obtained in pure He or Ar at different spot sizes. The technique of using the pre-mixed He–Ar mixture as the carrier gas has been successfully used for the determination of major and trace elements in USGS and MPI-DING glasses by using fs-LA-ICP-MS.