The role of recycled oceanic crust in magmatism and metallogeny: Os-Sr-Nd isotopes, U-Pb geochronology and geochemistry of picritic dykes in the Panzhihua giant Fe-Ti oxide deposit, central Emeishan large igneous province, SW China
Tong Hou1,Zhaochong Zhang()1,John Encarnacion2,M. Santosh1,3,Yali Sun4, He Huang1
1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, 100083, China
2.Department of Earth and Atmospheric Sciences, Saint Louis University, 3642 Lindell Boulevard, St. Louis, MO 63108, USA
3.Division of Interdisciplinary Science, Kochi University, Kochi 780-8520, Japan
4.Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 5100640, China
Sample preparation and analytical methods
All our sampleswere collected from the middle portion of the dyke tominimize any contamination or alteration. After screening under the microscope, relatively fresh samples were selected and weathered surfaces of the samples were removed. To avoid potential Re contamination from steel and tungsten carbide crushing equipments, all whole-rock powders were prepared using a ceramic jaw crusher and agate mill. These were thoroughly cleaned between samples by crushing multiple aliquots of clean quartz and pre-contaminated with an aliquot of the sample.
Zircon U–Pb dating and trace element composition
One of the representative samples of the picritic dyke (sample ZZC-1) was selected for zircon separation and U-Pb dating by laser-ablation-inductively coupledplasma mass spectrometry (LA-ICP-MS) U-Pb dating. The zircons were separated using the conventional heavy liquid and magnetic techniques. Hand-picked grains were mounted in epoxyblocks, polished to obtain an even surface, and cleanedin an acid bath before LA-ICP-MS analysis.Prior to U-Pb isotopic analyses, the internal morphology of the grains was examined using cathodoluminescence (CL).
Zircon U–Pb dating was performed on single zircons using a ThermoFinniganElement2 multi-collector ICP-MS with a New Wave UP213 laserablation system housed at the National Research Center for Geoanalysis, Beijing, China. Helium was used as the carrier gas to the enhance transport efficiency of the ablated material. The helium carrier gas inside the ablation cell was mixed with argon as a makeup gas before entering the ICP to maintain stable and optimum excitation conditions. A spot size of 30 μmwith a10 Hz repetition rate and a laser power of 16-17 J/cm2were used for all analyses. All U–Th–Pb isotope measurements were performed using zircon 91500 as an external standard and Plesovicezircon was used as the reference material.Isotopic ratios were calculatedusing GLITER 4.0 (van Achterbergh et al. 2001)while common lead correction was carried out using theEXCEL program ComPbCorr# 151 (Andersen2002).
The rare earth element (REE) analyses of the zircons were conducted by LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as description by Liu et al. (2008). Laser sampling was performed using a GeoLas 2005. An Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. Helium was applied as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. Nitrogen was added into the central gas flow (Ar+He) of the Ar plasma to decrease the detection limit and improve precision (Hu et al.2008). Each analysis incorporated a background acquisition of approximately 20-30 s (gas blank) followed by 50 s data acquisition from the sample. The Agilent Chemstation was utilized for the acquisition of each individual analysis. Element contents were calibrated against multiple-reference materials (BCR-2G, BIR-1G and GSE-1G) without applying internal standardization (Liu et al. 2008). The preferred values of element concentrations for the USGS reference glasses are from the GeoReM database ( Off-line selection and integration of background and analyte signals, and time-drift correction and quantitative calibration were performed by ICPMSDataCal (Liu et al. 2008; Liu et al.2010).
Bulk chemistry
Major and trace elements were acquired on fused glass discs using a wavelength dispersive X-ray fluorescence (XRF) spectrometer at the Center of Modern Analysis, Nanjing University, China. The analytical uncertainties are less than 1%, estimated from repeated analyses of two standards (andesite GSR-2 and basalt GSR-3). Loss on ignition was determined gravimetrically after heating the samples at 980 °C for 30 min. Trace elements were determined by solution ICP-MS. Rock powders (~40 mg) were dissolved in distilled HF+HClO4 in 15 ml Savillex Teflon screw-cap breakers. Precision for most elements was typically better than 5% RSD (relative standard deviation), and the measured values for Zr, Hf and Nb were within 10% of the certified values. The detailed sample preparations, instrument operating conditions and calibration procedures follow those established by Qi and Grégoire (2000). Two standards (granite GSR-1, basalt GSR-3) were used to monitor the analytical quality.
Rb-Sr and Sm-Nd isotopes
Rb-Sr and Sm-Nd isotopic compositions were obtained using a VG354 multi-collector mass spectrometer at the Center of Modern Analysis, Nanjing University, China. The samples (~100 mg) were weighed, and spiked before dissolution with mixed isotopic tracers. After washing with 2 mol/L HCl (in a supersonic bath for 2 h at room temperature) and Milli Q water, samples were powdered to less than 200 mesh in a clean lab, and dissolved overnight using HF and HNO3, evaporated to dryness, then followed by oven dissolution in fresh HF and HNO3 for 7 days at 160 °C. Separation of Rb and Sr were carried out with a cation-exchange column (packed with AG50Wx8). Sm and Nd were further purified using a second cation-exchange column (packed with AG50Wx12). Total procedural blanks were ~200 pg for Sr and ~50 pg for Nd. The mass fractionation corrections for Sr and Nd isotopic ratios were based on 88Sr/86Sr of 8.37521 and 146Nd/144Nd of 0.7219, respectively. During our analyses, measured 87Sr/86Sr ratios for standard NBS987 were 0.710242±0.000012, and measured 143Nd/144Nd ratios for the JMC standard were 0.511124±0.000010 (in 2σ uncertainty for 18 analyses).
Re-Os isotopes
Re-Os isotopic and Os concentration analyses were performed at the Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, ChineseAcademy of Sciences, Guangzhou, China. The analytical procedure is described in detail by Xu et al. (2007). Approximately 2 g of finely powdered sample (spiked with 190Os and 185Re), 2 ml of concentrated HCl, and 6ml of concentrated HNO3 were frozen in a Carius tube, and sealed and heated to 240 °C for 24 h (Shirey and Walker, 1995). Upon opening the Carius tube, carbon tetrachloride was added to the acid mixture, and Os was extracted from the aqueous phase (Cohen and Waters, 1996; Pearson and Woodland, 2000). After cooling, the contents of the tube were poured into a PFA vessel. CCl4 extraction adding micro-distillation technique was used to separate and purify Os. Subsequently, extraction method by N-benzoyl-N-phenylhydroxyl-amine (BPHA) chloroform solution was used to separate Re from Mo, W, and Fe (Shinotsuka and Suzuki 2007). Final purification of the Os was achieved by microdistillation. Rhenium was separated and purified by anion exchange using AG1X8 resin (100–200 mesh).
The concentration of Os was analyzed by isotope dilution (ID) ICP-MS. During the measurement, the measured isotopes of 190Os and 192Os have isobaric interferences with 190Pt and 192Pt, respectively, but the isobaric interferences were totally removed because we used water solutions of Os prepared by distillation of Os as volatile OsO4. For Os isotopic measurement, isotope abundances in OsO3− and ReO4− were measured by N-TIMS method using a Triton TIMS and equipped with an oxygen gas leak valve and an ion counting multiplier. Osmium was loaded onto a high-purity Pt filament (99.999%, 1×0.025 mm, H. Cross Co. Ltd) that had already been heated in air for more than 3 min. The Re and Os isotope compositions were measured using a static multiple Faraday collector and a pulse counting electron multiplier, respectively. Instrumental mass fractionation of Os was corrected by normalizing the measured 192Os/188Os ratio to 3.08271 (Nier, 1937). Rhenium isotopes were measured using a total evaporation technique (Suzuki et al., 2004). Oxygen subtraction for Re and Os used the ratios 17O/16O=0.00037 and 18O/16O=0.002047 (Nier, 1950). Both Re and Os were corrected for blanks. Sample PZH1003 was analyzed twice to assess the reproducibility of the analytical procedure. Total blank levels were 7.1±0.3 and 2.0±0.4 pg for Re and Os (2 SD, n=13), respectively, and the blank 187Os/188Os ratio was 0.298±0.028 (2 SD, n=13). The contributions of the blanks to measured Os and Re contents were minor (<1%).
References:
Andersen T (2002) Correction of common Pb in U–Pb analyses that do not report 204Pb. Chem Geol 192:59-79
Cohen A, Waters G (1996) Separation of osmium from geological materials by solvent extraction for analysis by thermal ionization mass spectrometry. Anal ChimActa 332:269–275
Hu Z, Gao S, Liu Y, Hu S, Chen H, Yuan H (2008) Signal enhancement in laser ablation ICP-MS by addition of nitrogen in the central channel gas. J Anal Atom Spectrom 23:1093-1101
Liu Y, Gao S, Hu Z, Gao C, Zong K, Wang D (2010) Continental and oceanic crust recycling-induced melt-peridotite interactions in the Trans-North China Orogen: U-Pb dating, Hf isotopes and trace elements in zircons of mantle xenoliths. J Petrol 51:537-571
Liu Y, Hu Z, Gao S, Günther D, Xu J, Gao C, Chen H (2008) In situ analysis of major and trace elements of anhydrous minerals by LA-ICP-MS without applying an internal standard. Chem Geol 257:34-43
Nier A O (1937) The isotopic constitution of osmium. Phys Rev 52: 885–892
Nier A O (1950) A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon and potassium. Phys Rev 77: 789–793
Pearson D, Woodland S (2000) Solvent extraction/anion exchange separation and determination of PGEs (Os, Ir, Pt, Pd, Ru) and Re–Os isotopes in geological samples by isotope dilution ICP-MS. Chem Geol 165:57–107
Qi L, Grégoire DC (2000) Determination of trace elements in twenty-six Chinese geochemistry reference materials by inductively coupled plasmamass spectrometry. GeostandardNewslett 24:51-63
Shinotsuka K, Suzuki K (2007) Simultaneous determination of platinum group elements and rhenium in rock samples using isotope dilution inductively coupled plasmamass spectrometry after cation exchange separation followed by solvent extraction. Anal ChimActa 603:129-139
Shirey S, Walker R (1995) Carius tube digestion for low-blank rhenium–osmium analysis. Anal Chem 67:2136–2141
van Achterbergh E, Ryan C, Jackson S, Griffin W (2001) In: Sylvester P (eds) Appendix 3 Data Reduction Software for LAICP–MS in “Laser-Ablation-ICPMS in the Earth Sciences”, 29. Mineralogical Association of Canada, Short Course Series 29:239-243
Xu J, Suzuki K, Xu Y, Mei H, Li J (2007) Os, Pb, and Nd isotope geochemistry of the Permian Emeishan continental flood basalts: insights into the source of a large igneous province. GeochimCosmochimActa 71:2104–2119