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 samples were collected from the middle portion of the dyke to minimize 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 coupled plasma 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 epoxy blocks, polished to obtain an even
surface, and cleaned in 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 Thermo Finnigan Element 2
multi-collector ICP-MS with a New Wave UP213 laser ablation 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 μm with a 10 Hz repetition rate and a laser power of 16-17 J/cm2 were
used for all analyses. All U–Th–Pb isotope measurements were performed using zircon 91500 as an
external standard and Plesovice zircon was used as the reference material. Isotopic ratios were
calculated using GLITER 4.0 (van Achterbergh et al. 2001) while common lead correction was carried
out using the EXCEL program ComPbCorr# 151 (Andersen 2002).
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
(http://georem.mpch-mainz.gwdg.de/). 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 HNO 3, 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
87
Sr/86Sr ratios for standard NBS987 were 0.710242±0.000012, and measured
143
Nd/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, Chinese Academy 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
190
Os 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
190
Os and
192
Os have isobaric interferences with
190
Pt and
192
Pt, 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
192
Os/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
17
O/16O=0.00037 and
18
O/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
187
Os/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%).
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