Laser Ablation Multi-collector Inductively Coupled Plasma Mass Spectrometry Research Articles

In-situ determination of Nd isotope ratios in apatite by LA-MC-ICPMS: Challenges and limitations

In-situ measurement of SmNd isotopes in rare-earth-element bearing minerals has been successfully used in the recent years to access the history of old Earth materials (e.g. Hammerli et al., 2019; Fisher et al., 2020). However, the analytical protocol of Laser Ablation-Multi Collector-Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS) for SmNd measurements strongly depends on the instruments, the analytical parameters and the applied corrections. In this paper, we intend to summarize some of the difficulties to set up this technique. We present new results by evaluating the influence of laser parameters (spot size, fluence, frequency and He/N2 gas flows) on 143Nd/144Nd, 145Nd/144Nd and 147Sm/144Nd ratios. We also report results on tests of cone geometry and interference and mass discrimination corrections on both the accuracy and precision of Nd isotopic analyzes. We report new SmNd measurements performed by TIMS and LA-MC-ICPMS on Durango reference apatite (Mexico) and apatite crystals from 2 carbonatites (Fogo, Cape Verde and Phalaborwa, South Africa). We conclude from these measurements that the laser parameters have no influence on the 143Nd/144Nd and 145Nd/144Nd isotopic ratios. The value of the 147Sm/144Nd ratio, however, is correlated to the size of the spot (Fisher et al., 2020). More importantly, we show that 147Sm/144Nd ratio measurements can vary when the He/N2 gas flows are changed even punctually (e.g. sample exchange) during an analytical session and decrease systematically during the day, which we relate to a systematic instrumental drift. We also conclude that our Durango crystal is homogeneous for Nd isotopic ratios, and slightly heterogeneous for Sm/Nd at the level of ±1–1.5% allowing accurate measurements when used as external standard. We retrieve expected isotopic ratios for both recent Fogo (4 Ma old) and ancient Phalaborwa (2060 Ma old) samples and we achieve precisions in the order of 125–150 ppm (1.3–1.5 εNd-units) for the 143Nd/144Nd ratio for a laser spot of 40 μm, which can be considered as a reasonable size for apatite crystals in most geological samples. Finally, we present raster measurements allowing to improve the precision by a factor of ~2 with 70 ppm (0.7 εNd-units).

Shocked titanite records Chicxulub hydrothermal alteration and impact age

Hydrothermal activity is a common phenomenon in the wake of impact events, yet identifying and dating impact hydrothermal systems can be challenging. This study provides the first detailed assessment of the effects of shock microstructures and impact-related alteration on the U-Pb systematics and trace elements of titanite (CaTiSiO5), focusing on shocked granite target rocks from the peak ring of the Chicxulub impact structure, Mexico. A > 1 mm long, shock-twinned titanite grain preserves a dense network of irregular microcracks, some of which exploit shock twin interfaces. Secondary microcrystalline anatase and pyrite are heterogeneously distributed along some microcracks. In situ laser ablation multi-collector inductively-coupled plasma mass spectrometry (LA-MC-ICPMS) analysis reveals a mixture of three end-member Pb components. The Pb components are: 1) common Pb, consistent with the Pb isotopic signature of adjacent alkali feldspar; 2) radiogenic Pb accumulated since magmatic crystallization; and 3) a secondary, younger Pb signature due to impact-related complete radiogenic Pb loss. The youngest derived ages define a regression from common Pb that intersects Concordia at 67 ± 4 Ma, in agreement with the established age of 66.04 ± 0.05 Ma for the Chicxulub impact event. Contour maps of LA-MC-ICPMS data reveal that the young ages are spatially restricted to microstructurally-complex domains that correlate with significant depletion in trace elements (REE-Y-Zr-Nb-Mo-Sn-Th) and reduction in magnitude of the Eu/Eu* anomaly. Mapping by time-of-flight secondary ion mass spectrometry (ToF-SIMS) show that patterns of localised element depletion in titanite are spatially related to microcracks, which are enriched in Al. The spatial correlation of ages and trace element abundance is consistent with localised removal of Pb and other trace elements from a pervasive network of fast fluid pathways in fractured domains via a fluid-mediated element transport process associated with the impact event. Here we interpret the 67 ± 4 Ma U-Pb age to represent hydrothermal Pb-loss in the Chicxulub peak ring in the wake of the impact event. These results highlight the potential of our analytical approach using titanite geochronology and geochemistry for dating post-impact hydrothermal activity in impact structures elsewhere.

Multistage pyrites in the Nibao disseminated gold deposit, southwestern Guizhou Province, China: Insights into the origin of Au from textures, in situ trace elements, and sulfur isotope analyses

Nibao is a unique thrust fault-controlled and strata-bound disseminated gold deposit in southwestern Guizhou Province, China. In Nibao, pyrite is the major sulfide mineral and Au is structurally bound (Au+) within the pyrite lattice. In this study, we conducted detailed analyses of the pyrite chemistry and S isotope composition in Nibao using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), respectively. Through petrographic and pyrite chemical studies, four pyrite generations (Py1–Py4) were distinguished: framboidal pyrite (Py1, pre-ore sedimentary stage), clean pyrite overgrowing framboidal pyrite (Py2, pre-ore diagenetic stage), “spongy” pyrite (Py3, early ore stage), and overgrowth of narrow pyrite rims surrounding Py2/Py3 and disseminated pyrite associated with arsenopyrite (Py4, main ore stage). Among these, Py2 and Py4 are the most abundant.The trace element content in Py2 is characterized by a wide range of As, Cu, Sb, and Pb concentrations (~2480–58100 ppm, ~55.4–1610 ppm, ~29.1–232 ppm, and ~24.1–376 ppm, respectively), while Py4 has the highest Au, As, Cu, and Se contents (~70 ppm, ~4200, ~1630 ppm, and ~38.3 ppm, respectively). The δ34S values of pre-ore pyrites measured by LA-MC-ICP-MS in this study and the available data in the literature range broadly from −53.3 to 114.8‰, indicating that they were most likely generated by bacterial reduction from marine sulfate during sedimentation/diagenesis. Meanwhile, the δ34S values of ore pyrites have relative narrow δ34S values, mostly varying from −5 to 5‰, and indicating that the S was derived either from the average of sedimentary rocks or from a magmatic source.Since igneous rocks are scarce in the region and the exposed (~77–99 Ma) are clearly younger than the mineralization age of the Nibao gold deposit (~141 Ma), a magmatic source is unlikely in Nibao. All pyrites in this study show a positive correlation (R2 = 0.71) between Co and Ni, and the Co/Ni and Zn/Ni ratios of main stage Py4 are close to or within the range defined for a sedimentary–diagenetic origin, suggesting a sedimentary source is more likely in Nibao.

Evolution and origins of pyrite in sandstone-type uranium deposits, northern Ordos Basin, north-central China, based on micromorphological and compositional analysis

Pyrite is an abundant mineral throughout the uranium-bearing Middle Jurassic Zhiluo Formation, northern Ordos Basin. Three different morphologies of pyrite are identified: framboidal, euhedral and cement. Based on textural and geochemical results from optical microscopy, secondary electron (SE) and back scattered electron (BSE) imaging, energy-dispersive spectroscopy (EDS), electron probe microanalysis (EPMA), and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS), three evolutionary models are proposed as follows: aggregation of framboids to form polyframboids as a result of the bacterial sulfate reduction (BSR) processes, the formation of cement pyrite under the addition of thermal fluids with numerous framboids and microcrystals inside, and euhedral pyrite evolving from framboids. The analytical data show that there is a minor increase in major and trace element contents in transformed pyrite, and each pyrite morphology has a distinct sulfur isotopic composition (−9.9‰ to −8.0‰ for framboids, −19.1‰ to −14.2‰ for polyframboids, +18.7‰ to +20.3‰ for euhedral pyrite, and +15.7‰ to +21.2‰ for cement pyrite). This indicates that the sulfur was derived from more than one source, compared with the original framboids. The association between uranium and different types of pyrite indicates that different processes (e.g., BSR and thermogenic sulfate reduction (TSR)) were involved in the precipitation of uranium mineralization and are important to the formation of the precipitation of uranium in the Zhiluo Formation, northern Ordos Basin.

The Geochronology and Geochemistry of Zircon as Evidence for the Reconcentration of REE in the Triassic Period in the Chungju Area, South Korea

The Chungju rare-earth element (REE) deposit is located in the central part of the Okcheon Metamorphic Belt (OMB) in the Southern Korean Peninsula and research on REE mineralization in the Gyemyeongsan Formation has been continuous since the first report in 1989. The genesis of the REE mineralization that occurred in the Gyemyeongsan Formation has been reported by previous researchers; theories include the fractional crystallization of alkali magma, magmatic hydrothermal alteration, and recurrent mineralization during metamorphism. In the Gyemyeongsan Formation, we discovered an allanite-rich vein that displays the paragenetic relationship of quartz, allanite, and zircon, and we investigated the chemistry and chronology of zircon obtained from this vein. We analyzed the zircon’s chemistry with an electron probe X-ray micro analyzer (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The grain size of the zircon is as large as 50 µm and has an inherited core (up to 15 µm) and micrometer-sized sector zoning (up to several micrometers in size). In a previous study, the zircon ages were not obtained because the grain size was too small to analyze. In this study, we analyzed the zircon with laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) for dating purposes. The REE patterns and occurrence of zircon in the quartz–allanite vein match well with previous reported recrystallized zircon, while the behavior of the trace elements shows differences with magmatic and hydrothermal zircon. The 206Pb/238U ages obtained from the zircon in the quartz–allanite vein are from 240.1 ± 2.9 to 257.1 ± 3.5 Ma and this age is included in the tectonic evolution period of the study area. Therefore, we suggest that the quartz–allanite veins in the Gyemyeongsan Formation were formed during the late Permian to early Triassic metamorphic period and the zircon was recrystallized at that time. The Triassic age is the first reported age with zircon dating in the Gyemyeongsan Formation and will be an important data-point for the study of the tectonic evolution of the OMB.

Spatially-resolved uranium isotopic analysis of contaminated scrap metal using laser ablation multi-collector ICP-MS.

Laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) was applied to the detailed investigation of the uranium (U) isotopic composition (234U, 235U, 236U, and 238U) of five contaminated scrap metal samples found within the European Union. Pressed pellets of the two certified U isotopic reference materials CRM U-020 and CRM U-030 were included in the measurement protocol for mass bias correction, calculation of the ion counter gains and for quality assurance. Since the investigated samples had low U content (0.15–14.3 wt%) compared to typically analysed pure U compounds (>60 wt%), the applied experimental parameters had to be adjusted. Spatially-resolved U isotopic information was obtained by line scan analysis of each sample. While other analytical techniques used typically in nuclear forensic investigations, such as γ-spectrometry and thermal ionisation mass spectrometry (TIMS) yielded average U isotopic compositions of the entire sample, LA-MC-ICP-MS provided substantial added value, highlighting the inhomogeneous distribution of U isotopes within various scrap metal samples. Analysis of individual particles via secondary ion mass spectrometry (SIMS) confirmed the large range of 235U enrichment levels in heterogeneous scrap metal samples. Four out of five scrap metal samples contained 236U (∼0.05–∼0.11 wt%), indicating the presence of reprocessed U. Taken together, LA-MC-ICP-MS analysis provided fast and accurate spatially-resolved U isotopic information without consuming or altering the scrap metal samples, a key feature for nuclear forensics investigations.

‘Blind time’ – current limitations on laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) for ultra-transient signal isotope ratio analysis and application to individual sub-micron sized uranium particles

Gaps in acquisition between integration times for MC-ICP-MS can introduce isotope ratio bias with a mixed detector array.

Unmasking continental natal homing in goliath catfish from the upper Amazon

Abstract Amazonian goliath catfishes are widespread in the Amazon Basin. Recently, otolith 87Sr:86Sr analyses using laser ablation–multi‐collector–inductively coupled plasma mass spectrometry (LA‐MC‐ICPMS) revealed a >8,000 km trans‐Amazonian natal homing in Brachyplatystoma rousseauxii among fish caught and hatched in the largest Amazon River tributary, the upper Madeira basin. Although also suspected for fish in the upper Amazon, homing could not be demonstrated owing to less distinct environmental 87Sr:86Sr gradients along the Amazon mainstem. Using scanning X‐ray fluorescence microscopy (SXFM), a separate study provided evidence that Se:Ca and Sr:Ca are useful markers for identifying migration into Andean headwaters and the estuarine environment. We analysed otoliths of known 87Sr:86Sr profiles using SXFM mapping to test if Sr:Ca and Se:Ca patterns could demonstrate natal homing for three fish caught in the upper Amazon, using as reference two individuals that were natal homers and two forced residents (hatched after the construction of hydroelectric dams on the Madeira River) from the upper Madeira River. As hypothesised, although the Sr isotope profiles of the upper Amazon individuals were uninformative, two of them presented similar alternating mirror patterns of Sr:Ca and Se:Ca to those of the upper Madeira natal homers, indicating migrations out of the Andean region and into the estuary area. Both were therefore natal homers from the upper Amazon. The third individual from the upper Amazon presented similar Sr:Ca and Se:Ca patterns to those of the upper Madeira residents, suggesting it was a natural resident from the upper Amazon. By combining the results of 87Sr:86Sr analyses (LA‐MC‐ICPMS) and Sr:Ca and Se:Ca mappings (SXFM) that are completely independent of one another, we demonstrated that B. rousseauxii also performs natal homing in the upper Amazon. Our results indicate that the life cycle of B. rousseauxii is more complex than previous literature hypothesised, with the existence of partial migration, even in absence of physical barriers. Quantifying the relative importance of these different life‐history strategies will have important implications for fisheries management. Our results also lay the groundwork for conservation efforts in the context of hydropower development in the Amazon Basin and set testable hypotheses of the potential impacts of the Madeira River dams.

LA-MC-ICP-MS study of boron isotopes in individual planktonic foraminifera: A novel approach to obtain seasonal variability patterns

Boron isotope (δ11B) analysis using bulk foraminifera samples is a widely used method to reconstruct paleo sea water pH conditions. Although, these analyses exhibit high analytical precision, short term information is lost due to the pooling of tests with distinct and diverse boron isotope signatures resulting in average values for the time interval encompassed in the sample. Here we present and assess the analysis of δ11B of individual foraminifera by means of Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) to obtain seasonal variability patterns and to test the limits of precision of LA-MC-ICP-MS on the planktonic foraminifera Orbulina universa. The results show that relative seasonal differences (of ∼11‰) can be captured from either uncleaned or cleaned individual O. universa tests with an average precision of ±2.9‰ (2 SE). The δ11B variability among foraminifera representing the same season is on average 7.4‰ (2 SD) irrespective of cleaning state. With our approach, analyses on oxidatively cleaned O. universa do not require the use of a matrix matched standard to obtain B isotope values in the range of those expected for solution multi-specimen analyses. Our results are useful for considering the potential spread caused by foraminifera vital effects and for obtaining information of seasonal ranges of pH and possible bias related to seasonality hidden within conventional solution based δ11B analyses.

LA-MC-ICP-MS U-Pb dating of low-U garnets reveals multiple episodes of skarn formation in the volcanic-hosted iron mineralization system, Awulale belt, Central Asia

Abstract Volcanic-hosted iron deposits of the eastern Awulale metallogenetic belt in Central Asia possess a reserve of over 1.2 billion tons of iron ores and constitute one of the most important basements for high-grade iron resources in China. Skarns are widespread in these deposits and closely associated with iron mineralization. The ages of these skarns are unclear, and their genesis remains debated, preventing further investigation into their metallogenic processes. We focused on garnets in nine ore-bearing skarns from three large-scale iron deposits (Chagangnuoer, Dunde, and Beizhan) in the eastern Awulale belt. U-Pb dating was conducted on these garnets using our in-house reference material, the Taochong garnet (TC-13, Pb-Pb isochron age: 126.2 ± 2.3 Ma, initial 207Pb/206Pb ratio: 0.845 ± 0.022). Laser-ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) was employed in the garnet U-Pb dating, and high-precision U-Pb ages (0.3%–1.6%) were obtained, highlighting the advantages of LA-MC-ICP-MS in dating low-U minerals. The garnet U-Pb ages of the nine skarn samples fall into three groups, i.e., 329.0 ± 5.1–326 ± 3.3 Ma (two samples), 316.3 ± 2.9–311.2 ± 2.4 Ma (six samples), and 295.6 ± 1.0 Ma (one sample), implying three episodes of skarn alteration in the volcanic-hosted iron mineralization system. The first and second episodes of skarns formed as a result of contact metasomatism between coeval volcanic rocks and limestone, and they have economically important iron mineralization. The third was likely caused by a local postcollision granitic intrusion, but its metallogenic potential deserves further assessment.

Further Characterization of the RW-1 Monazite: A New Working Reference Material for Oxygen and Neodymium Isotopic Microanalysis

The oxygen (O) and neodymium (Nd) isotopic composition of monazite provides an ideal tracer of metamorphism and hydrothermal activity. Calibration of the matrix effect and monitoring of the external precision of monazite O–Nd isotopes with microbeam techniques, such as secondary ion mass spectrometry (SIMS) and laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICPMS), require well-characterized natural monazite standards for precise microbeam measurements. However, the limited number of standards available is impeding the application of monazite O–Nd isotopes. Here, we report on the RW-1 monazite as a potential new working reference material for microbeam analysis of O–Nd isotopes. Microbeam measurements by electron probe microanalysis (EPMA), SIMS, and LA-MC-ICPMS at 10–24 µm scales have confirmed that it is homogeneous in both elemental and O–Nd isotopic compositions. SIMS measurements yield δ18O values consistent, within errors, with those obtained by laser fluorination techniques. Precise analyses of Nd isotope by thermal ionization mass spectrometry (TIMS) are consistent with mean results of LA-MC-ICPMS analyses. We recommend δ18O = 6.30‰ ± 0.16‰ (2SD) and 143Nd/144Nd = 0.512282 ± 0.000011 (2SD) as being the reference values for the RW-1 monazite.

Bands of Zircon, Allanite and Magnetite in Paleozoic Alkali Granite in the Chungju Unit, South Korea, and Origin of REE Mineralizations

High-grade Zr–Nb–Y–rare earth element (REE) mineralization occurs as zircon–allanite–magnetite bands in layered Paleozoic alkali rocks which intruded the Gyemyeongsan Formation of the Chungju unit, South Korea. The mineralization period and genesis have been controversial. We investigated the petrological and mineralogical properties of the newly discovered zircon–allanite–magnetite bands and the geochronological properties of zircon within the bands in the alkali granite. We analyzed the zircon with laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). The repeated quartz–feldspar-rich layers in the alkali granite show grain-sized grading textures and equilibrium igneous textures. Magnetite and allanite grains in these layers varied in size and exhibited isolated, aggregated, and coalesced textures. In addition, the settling texture of zircon grains onto the other minerals was observed. These observations could reasonably be explained by the process of gravitational accumulation during the solidification of magma. The 206Pb/238U ages obtained from zircon from the zircon–allanite–magnetite-rich layer and the alkali aplite were 331.1 ± 1.5 Ma and 334.5 ± 8.9 Ma, respectively. Therefore, we suggest that the Zr–Y–Nb–REE mineralization developed in the alkali rocks and the Gyemyeongsan Formation in the Chungju unit were formed by fractional crystallization of alkali magma and hydrothermal fluids which evolved from alkali magma fractional crystallization, respectively. The correlation between alkaline granite and REE mineralization found in this study could be used as a tool for REE exploration in other regions where the permeable geological unit is intruded by the alkali granite.

Magmatic-Hydrothermal Gold Mineralization at the Lone Tree Mine, Battle Mountain District, Nevada

Abstract The Lone Tree deposit is located in the northern Battle Mountain mining district, Nevada. Prior to mine closure in 2006, Santa Fe Pacific Gold and Newmont produced 4.2 Moz of gold at an average grade of 2.06 g/t at Lone Tree, primarily from the N-S– to NNW-SSE–striking Wayne zone. The ore is located between the Roberts Mountain and Golconda thrusts in siliciclastic rocks of the Ordovician Valmy Formation and in the Pennsylvanian-Permian Battle Mountain and Edna Mountain Formations, and above the Golconda thrust in siliciclastic and carbonate rocks of the Mississippian to Permian Havallah sequence. Ore is also hosted by rhyolitic dikes that were emplaced at 40.95 ± 0.06 Ma based on zircon U-Pb chemical abrasion-thermal ionization mass spectrometry. The gold is associated with sericitic and argillic alteration of the siliciclastic rocks and dikes and with decarbonatization and Fe carbonate alteration of the carbonate-bearing units, as well as in Fe-As sulfide and finegrained quartz alteration of all rock types. Oxidation affects 30 to 45% of the deposit, penetrating into the stratigraphy along numerous steeply dipping north-south, east-west, and north-northeast–south-southwest structures. Gold is positively correlated with Ag, As, Hg, and Sb. The highest Au grades occur in quartz-sulfide ore hosted in siliciclastic and carbonate sedimentary rocks and rhyolitic intrusions. In this ore style, fine-grained quartz and sericite are intergrown with disseminated sulfide minerals (quartz-sericite-pyrite alteration), constituting cores of weakly mineralized pyrite or marcasite, which are surrounded by fuzzy arsenopyrite rims that contain up to ~2,000 ppm Au. Low gold grades occur in late-stage banded pyrite breccias consisting of a finely zoned Au-poor pyrite matrix surrounding jigsaw-fit clasts of quartz-, illite-, barite-, and adularia-altered siliciclastic rock. The timing of main-stage mineralization is bracketed between the emplacement of the dikes and an adularia 40Ar/39Ar age of 40.14 ± 0.74 Ma. Sericite intergrown with arsenopyrite-rimmed pyrite in phenocrysts of the rhyolite dikes gave δ18O values of 1.6 to 9.5‰ and δD values of –105 to –145‰. For temperatures of 300 ± 100°C, the calculated fluid isotopic compositions are consistent with felsic magmatic water and minor modifications by mixing with meteoric water and exchange with wall rocks. In the silica-sulfide ore, in situ isotopic laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) analyses of pyrite cores yielded δ34S values ranging from 3.4 to 7.7‰, with average values of 5.6‰ in the felsic dikes, 4.5‰ in the siliciclastic rocks, and 5.3‰ in the carbonate rocks. These values match conventional pyrite δ34S data reported for Eocene porphyry systems elsewhere in the district. Nanoscale secondary ion mass spectrometry analyses show that gold and associated trace elements occur in submicron-scale zones within arsenopyrite rims on pyrite. The average δ34S values of the arsenopyrite rims are 5.3 to 6.5‰ heavier than the pyrite cores, indicating cooling and an increasing H2S/SO2 ratio. The highest grades resulted from episodic pulses of a gold-rich fluid that was partly derived from, or exchanged with, the sedimentary host rocks. In situ LA-MC-ICP-MS δ34S values for the late-stage banded pyrite breccia become progressively lighter from veinlet margin to center, reaching a low of –32‰. These veinlets indicate a shift from main-stage quartz-sericite-pyrite and intermediate argillic alteration to more neutral pH and oxidizing conditions during late-stage mineralization, indicating either increasing interaction between the fluid and sedimentary sulfur sources in the host-rock package or bacterial sulfate reduction and supergene sulfide precipitation.

Early Paleozoic zircon ages of the Higher Himalayan Gneisses of the Everest region and their Pan-African/Proto-Himalayan orogenic signature

The U-Pb analysis of zircons from two independent leucosome bodies belonging to the paragneiss of the Higher Himalayan Crystallines Sequence (HHCS) in the Everest region of eastern Nepal Himalaya was carried out using laser ablation-multi collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) at the University of Arizona, Tucson, USA. The analysis of zircons from sample 07EVT3 forms a discordia with upper and lower intercepts at 478±25 and 21.5±4.1 Ma with concordant ages of 488.5±9.2 and 20.9±0.9 Ma for cores and rims, respectively. Similarly, the analysis of zircons from sample U1206 forms a discordia with upper and lower intercepts at 515±20 and 34.8±2.7 Ma, and provides concordant ages of 463.9±10.9 and 24.6±0.6 Ma for cores and rims, respectively. No inherited zircon grains with older ages were found indicating that almost all these zircons must have formed along with the leucotomies during the ca. 500 Ma metamorphism of the protoliths. The high U/Th ratio, i.e. average 11.0 for zircons from sample 07EVT3 and 253.6 for sample U1206 also support a metamorphic origin of the zircons. The occurrences of zircons in the ca. 500Ma leucosomes in the HHCS strongly points that rocks in the Himalayan area had undergone to a high-grade metamorphism during the late Pan-African time. We call this metamorphism as the Proto himalayan metamorphism. More studies along this line will help to better understand and constrain the Pan-African orogenic history of the Proto-Himalayan Orogen within the Peri-Gondwana Orogenicterrains.

The effect of matrix interferences on in situ boron isotope analysis by laser ablation multi-collector inductively coupled plasma mass spectrometry.

Boron isotope analysis of marine carbonates by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) offers the potential for rapid sample throughput, and the means to examine micron-scale variations in the δ11 B signatures of fossil skeletons and shells/tests of marine organisms. Existing studies demonstrate an acceptable level of reproducibility is achievable, but also typically show a level of accuracy outside the limits required by most applications. Here we investigate matrix interference effects as a cause of inaccuracy and imprecision. Analyses were performed on a standard format Thermo Scientific Neptune Plus MC-ICP mass spectrometer coupled to a New Wave Research 193 nm ArF laser ablation system. The effects of matrix interference on δ11 B analysis were investigated through analyses of a set of reference materials with differing B/Ca ratios. Three approaches to correct for matrix-induced effects were trialled: (1) use of matrix-matched standards, (2) utilisation of the relationship between δ11 B inaccuracy and11 B/43 Ca, 11 B/40 ArCa4+ or 11 B/Cainterference from three reference materials with known δ11 B values and varying B/Ca ratios, and (3) direct characterisation of the (sloping) interference itself. Matrix interference from scattered Ca ions on 10 B can impede both the accuracy and the reproducibility of δ11 B analysis by LA-MC-ICP-MS. Based on analyses of two in-house reference materials, deep sea coral PS69/3181 and inorganic calcite UWC-1, we find approach 2, following the 11 B/Cainterference relationship, gives the best mean accuracies (within 0.4‰ of solution values) and external reproducibilities (± 0.5‰ 2 SD for PS69/3181). This approach has been applied to analyses of an annual growth cycle of a Siderastrea siderea coral and eight Cibicidoides wuellerstorfi benthic foraminifera. Both coral and foraminifera data match solution MC-ICP-MS analyses within reported uncertainties. LA-MC-ICP-MS can produce accurate and precise δ11 B data to a 0.5‰ (2σ) level on <0.3 ng B after correction for Ca interference effects.

Petrology, geochemistry and zircon U–Pb–Lu–Hf isotopes of Paleoproterozoic granite gneiss from Bomdila in the western Arunachal Himalaya, NE India

Abstract Felsic magmatic bodies are exposed widely in the Bomdila region of the western Arunachal Himalaya, NE India. The litho-units of this region are primarily composed of two-mica (muscovite–biotite (ms–bt)) granite gneiss, referred to herein as the Bomdila granite gneiss (BGGn), and the metasediments of the Bomdila Group forming an integral part of the NE Lesser Himalayan thrust sheet. Phase petrology, whole-rock elemental geochemistry and zircon U–Pb–Lu–Hf isotopes of the BGGn have been investigated in order to decipher the origin and timing of felsic magmatism and its implications for understanding the pre-Himalayan tectonic environment. Modally, the BGGn can be classified into monzogranite, syenogranite and quartz-rich granitoids. The composition of muscovite (Ti = 0.03–0.07, average Na = 0.07 and Al IV = 2.50–2.90 apfu), biotite (FeO t /MgO = 3.1–4.6, 2Al⇌3Fe and Mg⇌Fe substitutions, and the presence of siderophyllite) and tourmaline (Fe/Fe + Mg = 0.56–0.96, Ca &lt; 0.17 apfu) implies their primary magmatic nature crystallized typically in a peraluminous (S-type) felsic parental melt. This is further supported by the presence of ms–bt, whole-rock molar Al 2 O 3 /CaO + Na 2 O + K 2 O (A/CNK = 1.03–1.64) and normative corundum. Whole-rock multi-cationic parameters indicate a syn-collisional tectonic environment. However, the content of Rb (average 300 ppm) and high-field strength elements (HFSEs) in the BGGn indicates syn- to post-collisional tectonic settings. The BGGn parental melt was most likely to have been generated by dehydration melting of metasedimentary sources at middle–upper crustal depths. Geochemical modelling constrains the evolution of the parental melt of the BGGn by a moderate degree of fractional differentiation ( F = 0.45) involving a biotite–plagioclase–K-feldspar–muscovite–titanite–apatite (bt–pl–Kfs–ms–ttn–ap) assemblage. Laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS)-analysed zircons from the BGGn yield a weighted mean 207 Pb/ 206 Pb age of 1752 ± 23 Ma as the crystallization age for the zircons in the BGGn melt, which falls well within the period of magmatism formed during the Columbia supercontinent accretionary orogen. The observed negative ε Hf (t) values (−1.67 to −7.99) and three-stage Hf-model ages (2818, 2586–2424 and 2393–2250 Ma) of zircons, strongly point to the involvement of ancient continental crust and source heterogeneity (Neoarchean–Paleoproterozoic continental crust) in the generation of the BGGn melt. The reworked ancient crustal components would have once been part of the northern Indian lithosphere, as indicated by the observed 207 Pb/ 206 Pb concordant ages (2436, 2136, 2013 Ma) of the inherited zircons.

Recycling of Paleo-oceanic crust: Geochemical evidence from Early Paleozoic mafic igneous rocks in the Tongbai orogen, Central China

Crustal subduction generally develops from oceanic to continental subduction in collisional orogens, with the disappearance of previously subducted paleo-oceanic crust. However, it is not straightforward to trace the fate of subducted paleo-oceanic crust due to modification by subsequent continental collision. In this study, we present a combined study of secondary ion mass spectrometry (SIMS) zircon UPb ages and O isotopes, laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) zircon LuHf isotopes, whole-rock major-trace elements and SrNd isotopes of Early Paleozoic mafic igneous rocks in the Tongbai orogen, central China. These geochemical data are interpreted and modelled in terms of crust-mantle interaction in a subduction channel, in order to account for geochemical transfer from the subducting crust to the mantle wedge. The results provide insight not only into paleo-oceanic crust recycling but also into the nature of orogenic lithospheric mantle. SIMS zircon UPb dating of the mafic igneous rocks yields Early Paleozoic ages of 436 ± 4 to 449 ± 4 Ma for magma crystallization. These mafic rocks have arc-type trace element distribution patterns, with enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) but depletion in high field strength elements (HFSE) relative to rare earth elements (REE) and LILE. They have depleted whole-rock SrNd isotopic compositions, with initial 87Sr/86Sr ratios of 0.7037 to 0.7047 and positive εNd(t) values of 2.7 to 5.0. The mafic igneous rocks also display positive zircon εHf(t) values of 8.9 to 14.8 and variable δ18O values of 4.4 ± 0.3 to 5.9 ± 0.3‰. These geochemical features suggest derivation from fertile mantle sources enriched in LILE and LREE but depleted in radiogenic isotopes. The mantle sources are suggested to be part of the orogenic lithospheric mantle, which was generated by metasomatic reaction of the overlying mantle wedge with previously subducted Erlangping oceanic crust-derived fluids. Some samples have high Ba/Th, Th/Nb and (La/Yb)N ratios, suggesting that the metasomatic agents include both basaltic oceanic crust-derived aqueous solutions and sediment-derived hydrous felsic melts. Model calculations confirm that the geochemical composition of the Tongbai mafic igneous rocks can be explained by reaction of depleted mid-oceanic ridge basalt (MORB) mantle peridotite with 1–10% of subducted Erlangping oceanic crust-derived aqueous solutions plus 0.05–0.1% of sediment-derived hydrous melts. This melt-peridotite reaction would generate a pyroxenite-rich lithology in the mantle source. During the tectonic transition in the Ordovician to Silurian, partial melting of such mantle sources gave rise to mafic magmatism due to back-arc extension. Thus, the Early Paleozoic mafic igneous rocks in the Tongbai orogen record the recycling of earlier subducted paleo-oceanic crust and the nature of the orogenic lithospheric mantle in the collisional orogen.

Magmatic Origin for Sediment-Hosted Au Deposits, Guizhou Province, China: In Situ Chemistry and Sulfur Isotope Composition of Pyrites, Shuiyindong and Jinfeng Deposits

Abstract The southwest Guizhou Province, China, contains numerous sediment-hosted Au deposits with Au reserves greater than 700 tonnes. To date, the source of ore fluids that formed the Guizhou sediment-hosted Au deposits is controversial, hampering the formulation of genetic models. In this study, we selected the Shuiyindong and Jinfeng Au deposits, the largest strata-bound and fault-controlled deposits in Guizhou, respectively, for detailed research on pyrite chemistry and S isotope composition using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS), respectively. Petrography and pyrite chemistry studies distinguished five generations of pyrite. Among these, pre-ore pyrite 2 and ore pyrite are the most abundant types in the deposits. Pre-ore pyrite 2 is anhedral to euhedral and with ~2,639 ppm As and wider ranges of Cu, Sb, and Pb (&lt;~22–4,837 ppm, &lt;~6 to 532 ppm, and &lt;~4 to 1,344 ppm, respectively). Gold in pre-ore pyrite 2 is below the detection limit of LA-ICP-MS (~2 ppm). Pre-ore pyrite 2 is interpreted to have a sedimentary (syngenetic or diagenetic) origin. Ore pyrite commonly rims the four identified pre-ore pyrites or occurs as individual, anhedral to euhedral crystals. Ore pyrite is enriched in Au (~641 ppm), As (~9,147 ppm), Cu (~1,043 ppm), Sb (~188 ppm), Hg (~43 ppm), and Tl (~22 ppm) in both deposits. Ore pyrite formed mainly by sulfidation of Fe in Fe-bearing host rocks, mainly Fe dolomite, and As, Cu, Sb, Hg, and Tl, also in ore fluids, were incorporated into ore pyrite. In situ δ34S isotope ratios for pre-ore pyrite 2 and ore pyrite were measured by LA-MC-ICP-MS. Pre-ore pyrite 2 from Shuiyindong and Jinfeng deposits resulted in δ34S values ranging from −0.8 to +3.4‰ and from 5.1 to 10.5‰, respectively. Analyses of ore pyrite from the Shuiyindong have δ34S values that vary from −3.3 to +2.5‰, with a median of 0.7‰; analyses of ore pyrite from the Jinfeng range from 8.9 to 11.2‰, with a median at 10.3‰. Available bulk and in situ δ34S data in the literature for pre-ore pyrites 1 and 2 and ore-related sulfide minerals including ore pyrite, arsenopyrite, and late ore-stage stibnite, realgar, orpiment, and cinnabar from several Guizhou sediment-hosted Au deposits were compiled for comparison. Pre-ore-stage pyrites from Guizhou sediment-hosted Au deposits have a broad range of δ34S values, from −33.8 to + 17.9‰ (including in situ and available bulk δ34S data). Ore-related sulfide minerals in all Guizhou sediment-hosted Au deposits, except Jinfeng, have very similar δ34S values, and most data plot between ~−5 and +5‰. In the Jinfeng deposit, the ore-related sulfide minerals exhibit δ34S values ranging from 1.9 to 18.1‰, with most data plotting between 6 and 12‰. The broad range of S isotope compositions for the sedimentary pyrites (pre-ore pyrites 1 and 2) indicate that S in these pre-ore pyrites was most likely generated by bacterial reduction from marine sulfate. The narrow range of δ34S values (~−5–+5‰) for ore-related sulfide minerals in all Guizhou sediment-hosted Au deposits, excepting the Jinfeng deposit, suggests that the deposits may have formed in response to a single widespread metallogenic event. As the S isotope fractionation between hydrothermal fluids and sulfide minerals in a sulfide-dominated system is small (&lt;2‰) at ~250°C, the initial ore fluids that formed the Guizhou sediment-hosted Au deposits would have had δ34S values similar to the ore-related sulfide minerals, between ~−5 and +5‰. At Jinfeng, initial ore fluids may have mixed with local fluids with heavier δ34S, possibly basin brine (δ34Sbasin brine &gt;18‰), resulting in elevated δ34S values of ore-related sulfide minerals and especially late ore-stage sulfide minerals. Although few igneous rocks are exposed in the mining area around these deposits, there is evidence of magmatic activity ~20 km away. Furthermore, gravity and magnetic geophysical investigations indicate the presence of a pluton ~5 km below the surface of the Shuiyindong district. Based on in situ S isotope results and recent data indicating proximal intrusions, we interpret a deep magmatic S source for the ore fluids that formed the Guizhou sediment-hosted Au deposits. However, as the age for Au mineralization of Guizhou sediment-hosted Au deposits is still debated, the mineralization-magma connection remains hypothetical. Identifying an ore fluid source and time frame for Guizhou Au mineralization continues to be a critically important research goal for this district.

Geochronology of granitoids from Psunj and Papuk Mts., Croatia

AbstractDetailed cathodoluminescence (CL) and back scattered electron (BSE) imaging of zircon crystals, coupled with in-situ U-Pb zircon dating by Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) were used to develop new insights into the evolution of a monzogranite from Omanovac Quarry (Psunj Mt.), and of monzogranites from Šandrovac Quarry and Kišeljevac Creek (Papuk Mt.), both located in the Slavonian Mountains (Croatia).U-Pb isotopic data yielded a concordia age of 380 ± 4 Ma for the monzogranite from Omanovac Quarry, and 382 ± 2 Ma and 383 ± 5 Ma for monzogranites from Šandrovac Quarry and Kišeljevac Creek. The results suggest Late Devonian magmatic activity.

Intra‐ and inter‐tooth variation in strontium isotope ratios from prehistoric seals by laser ablation multi‐collector inductively coupled plasma mass spectrometry

RationaleStrontium isotope ratios (87Sr/86Sr) in modern‐day marine environments are considered to be homogeneous (~0.7092). However, in the Baltic Sea, the Sr ratios are controlled by mixing seawater and continental drainage from major rivers discharging into the Baltic. This pilot study explores if variations in Sr can be detected in marine mammals from archaeological sites in the Baltic Sea.Methods 87Sr/86Sr ratios were measured in tooth enamel from three seal species by laser ablation multi‐collector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS). The method enables micro‐sampling of solid materials. This is the first time that the method has been applied to marine samples from archaeological collections.ResultsThe analyses showed inter‐tooth 87Sr/86Sr variation suggesting that different ratios can be detected in different regions of the Baltic Sea. Furthermore, the intra‐tooth variation suggests possible different geographic origin or seasonal movement of seals within different regions in the Baltic Sea through their lifetime.ConclusionsThe method was successfully applied to archaeological marine samples showing that: (1) the 87Sr/86Sr ratio in marine environments is not uniform, (2) 87Sr/86Sr differences might reflect differences in ecology and life history of different seal species, and (3) archaeological mobility studies based on 87Sr/86Sr ratios in humans should therefore be evaluated together with diet reconstruction.