RESULTADOS E DISCUSSÃO
Os resultados obtidos neste trabalho e a discussão dos mesmos serão apresentados em formato de artigo científico, conforme segue abaixo.
BB zircon – A new Sri Lankan reference material for U-Pb and Hf isotopic laser ablation ICP-MS analysis
Maristella M. Santosa,b, Cristiano Lanaa, Ricardo Scholza, Ian Buickc, Sandra L. Kamod, Axel Gerdese, Daniel J. Condonf, Fernando Corfug, Eric Tohverh, Craig D. Storeyi,j, Miguel A.S.
Baseik, Klaus Krambrockl, Cristiano Fantinil
Abstract: The increasing demand for U-Pb and Hf measurements by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has resulted in a constant need to develop well- characterized primary and secondary (control) natural reference materials. We describe a new potential zircon standard (BB zircon) for LA-ICP-MS U/Pb geochronology and Hf isotope geochemistry that was collected from a secondary placer deposit in the Sri Lankan Highland Complex. We have characterized a number of grains that come from a batch containing 300 grams of 10 mm-long, gem- quality zircon crystals. Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and LA-ICP- MS techniques were conducted in various laboratories and show that nearly all zircon samples have a concordant U-Pb age with a weighted mean 206Pb/238U date of 562.58 ± 0.26 Ma (ID-TIMS, 2,
including tracer calibration uncertainty). LA-ICP-MS spot analyses show that seventeen individual gem-quality crystals have uniform U-Pb isotope compositions. However, for one of the zircon fragments, the ID-TIMS age was 556.25 ± 0.70 Ma, suggesting that the ID-TIMS U-Pb dating of all individual zircons is needed. The average U content varies between 270 and 452 ppm. The Th/U ratio for all crystals analysed averages between 0.245 and 0.485. The BB zircons are compositionally
a Departamento de Geologia, Universidade Federal de Ouro Preto. b Instituto Federal de Minas Gerais.
c Centre for Crustal Petrology, Dept. of Earth Sciences, Stellenbosch University. d Jack Satterly Geochronology Laboratory, Department of Geology, University of Toronto. e Institute of Geosciences, Johann Wolfgang Goethe University.
f NERC Isotope Geosciences Laboratory. g Department of Geosciences, University of Oslo.
h Tectonics Special Research Center, University of Western Australia. i Department of Earth Sciences, University of Bristol.
j Department of Mineralogy, The Natural History Museum.
k Centro de Pesquisas Geocronológicas – CPGeo/IGc – Universidade de São Paulo. l Departamento de Física, Universidade Federal de Minas Gerais.
homogeneous, as shown by measured trace elements concentrations (LA-ICP-MS) and by the absence of any internal textures in cathodoluminescence images. Hf isotopic composition of all BB zircons analysed (1.29 - 1.51 wt.% Hf) is homogenous within and between the grains with a mean 176Hf/177Hf
value of 0.281674 ± 0.000018 (2 S.D.). The calculated alpha fluence of 0.79 x 1018 g-1 corresponds to
a fine zircon structure, and is within the trend of previously studied, untreated zircon samples from Sri Lanka, which enables us to conclude that the zircon has not been annealed since it crystallized. The relatively high U and Pb concentrations of the BB zircon, together with its homogeneity of trace element contents, age and Hf isotopes make it an ideal calibration and reference material for LA-ICP- MS analyses. Comparisons with other Sri Lankan zircons suggest a metamorphic genesis for the BB zircon.
Keywords: BB zircon, Sri Lankan reference material, U-Pb geochronology, Hf isotopic system, Rare
Earth Elements, LA-ICP-MS
4.1 – INTRODUCTION
Of the available accessory phases used for U/Pb geochronology, zircon has the highest utility because of its occurrence in a wide range of sedimentary, igneous and metamorphic rock types, its robustness to weathering, its low common Pb contents and low diffusivity for Pb (e.g., Finch & Hanchar 2003, Orihashi et al. 2003, Yuan et al. 2008, Xia et al. 2011). Moreover, the zircon Hf isotope composition can be used to track crustal evolution in magmatic rocks and isotopic fingerprint detrital zircon populations; its oxygen isotope composition can be used as a petrogenetic tracer (e.g., King et al. 1998, Kinny & Maas 2003, Valley 2003, Harrison et al. 2005, Hawkesworth & Kemp 2006) and in appropriately buffered mineral assemblages, its Ti content can be used as a geothermometer (e.g., Ferry & Watson 2007, Page et al. 2007, Tailby et al. 2011). Individual zircon crystals commonly preserve multiple compositional and (U-Th-Pb, Hf) isotopic domains, even through magmatic and high-temperature metamorphic cycles, thus generally requiring high-spatial resolution analytical approaches that involve SIMS (Secondary Ionisation Mass spectrometry) or LA- ICP-MS (Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry).
The increasing widespread use of instruments capable of high-spatial resolution U/Pb geochronology and isotopic tracing has placed an increased pressure on the development of reference materials for U/Pb geochronology and isotope geochemistry in general, and for zircon in particular. This problem is exacerbated for LA-ICP-MS zircon dating because: a) all LA-ICP-MS analyses are destructive in nature; and b) the high sample throughput
compared to SIMS - based approaches means that large numbers of ablations of reference materials may be undertaken in a given analytical session e.g. 40-50 ablations in working day is common; c) most LA-ICP-MS laboratories run one or more secondary zircon standards in analytical runs, for the purposes of quality control; and d) zircon has also been used, with variable degrees of success, as the standard for non-matrix matched dating of other accessory phases, for which U/Pb reference materials are difficult to obtain, do not exist (e.g. xenotime, cassiterite, columbite-tantalite, scheelite, perovskite; Gulson & Jones 1992, Batumike et al. 2008, Dewaele et al. 2011, ZhiChao et al. 2011). The development of well-characterized natural zircon reference materials for in-situ analysis via LA-ICP-MS, in particular, is therefore essential.
There are a number of natural zircons used as reference materials for geochronology and isotope geochemistry, including 91500, Mud Tank, GJ-1, M257, Temora and Plešovice (e.g., Wiedenbeck et al. 1995, Black et al. 2003a; Jackson et al. 2004; Woodhead & Hergt 2005, Nasdala et al. 2008, Sláma et al. 2008). These have been widely used over the last two decades but some most are now too limited in quantity to meet international demand. This study presents isotopic data for natural zircon material that appears to be a suitable reference material for U-Pb dating and Hf isotopic measurements by LA-ICP-MS. Our analyses focused on the determination of reliable values of U-Pb age, isotopic ratios and U and Hf concentrations, and the study of internal homogeneity/heterogeneity of the samples, in order to check if the randomly selected fragments have reliable and representative values in analytical terms. This zircon meets most of the requirements for a reference material and can be obtained upon request to the Department of Geology at the Federal University of Ouro Preto, Brazil.
4.2 – GEOLOGICAL BACKGROUND AND SAMPLE DESCRIPTION
The samples for standard zircon development come from a secondary placer deposit of the Ratnapura gemstone field (Dissanayake & Rupasinghe 1993), located in the southwestern region of the Sri Lankan Highland Complex (Kröner et al. 1994b). The Highland Complex is composed of mafic and quartz-feldspathic granulites, charnockitic rocks, marble and quartzite, all metamorphosed to granulite facies (Kröner et al. 1994b, Dissanayake et al. 2000). Extensive U-Pb isotopic studies and the application of techniques such as sensitive high mass resolution ion microprobe (SHRIMP) analysis, have contributed to establish a
geochronological framework for high-grade rocks of Sri Lanka (e.g., Hölzl et al. 1994, Kröner et al. 1994a, Nasdala et al. 2004, Sajeev et al. 2010), yielding ages of sedimentation and granulite facies metamorphism in the Highland Complex of respectively 2.0 Ga and 610- 550 Ma.
About 300 g of dark-purple zircon megacrysts (comprising some eighty grains), hereafter referred as to Blue Berry (BB) zircons, were acquired for the present study. The individual gems of the BB zircon are dark purple translucent and often larger than 10 mm in length (Fig. 4.1a). No significant fractures and inclusions were observed under a binocular microscope. Most megacrysts are homogeneous, or show broad weak zoning under cathodoluminescence imaging (CL – Fig. 4.1b).
Figure 4.1: Images of the clear translucent brown fragments of BB zircon: (a) typical crystal shapes of the BB zircons; (b) cathodoluminescence image; (c) transmitted light image. The points and lines marked in the figures refer to the LA-ICP-MS analyses.
4.3 – ANALYTICAL METHODS
Previous studies have determined key requirements for minerals such as zircon to be considered as reference material for U-Pb and Hf isotopes analyses by LA-ICP-MS (e.g., Sláma et al. 2008). First, the mineral must be dated with high precision and accuracy by independent methods. It must be homogeneous in both U-Pb age and Hf isotopic composition, presenting moderate U (tens to hundreds ppm) and Hf (a few percent contents), low common Pb and low Lu/Hf and Yb/Hf ratios. The crystals should have an adequate size for repeated analyses by laser ablation (grains several millimeters to centimeters in diameter) and should be found in large quantities for distribution to the scientific community.
To ensure complete characterization of the zircon investigated here, chemical and isotopic analyses were conducted using a number of different techniques in several
laboratories: ID-TIMS: Jack Satterly Geochronology Laboratory (JSGL; Canada), NERC Isotope Geosciences Laboratory (NIGL; UK), University of Oslo (Norway); LA-ICP-MS: J.W. Goethe University of Frankfurt am Main (JWG; Germany), Federal University of Ouro Preto (UFOP; Brazil), University of Portsmouth (UK), University of São Paulo (USP; Brazil); Cathodoluminescence imaging: USP and JWG; Raman spectroscopy: Federal University of Minas Gerais (UFMG; Brazil); X-ray powder diffraction: UFOP. Where possible, the measurements were reproduced by similar techniques in different laboratories.
4.3.1 – Trace element concentration measurements
Trace element contents (Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu, Hf, Pb, Th and U) measurements in the BB zircon were performed to characterise the chemical composition of the individual gems and to see whether they are chemically different . The analyses were acquired by an Agilent 7700x quadrupole ICP-MS coupled to a 213 Nd-YAG laser (New Wave Research UP-213) at Federal University of Ouro Preto. The laser was fired at a repetition rate of 10 Hz, using 6 J/cm2 laser energy and a linear raster of 55 μm. Each analysis consisted of 20 s background data (gas blank) followed by 50 s of laser ablation signal. Helium was used as the carrier gas. Synthetic silicate glass NIST-612 was used to calibrate the trace element concentration data and stoichiometric Si (SiO2 = 32.78 wt%) was used as the internal standard. Time-resolved signal data were processed using the Glitter software package (van Achterbergh et al. 2001).
4.3.2 – Raman spectroscopy and X-ray powder diffraction
The physical properties of the zircon were investigated via cathodoluminescence imaging (CL), Raman spectroscopy and X-ray powder diffraction. These analyses were used to constrain possible effects of radiation damage (metamictization; e.g., Holland & Gottfried 1955, Woodhead et al. 1991, Murakami et al. 1991). The Raman spectroscopy analyses were performed at the Department of Physics at UFMG. Raman measurements were carried out on a Dilor XY spectrometer equipped with a liquid N2-cooled CCD detector. The samples were focused on an optical microscope by a 50 x (N.A. = 0.55) objective and excited with an argon-ion laser at a wavelength of = 514.5 nm. Measurements were carried out on nine different grains to check for homogeneity of samples.
Raman bands were fitted assuming Lorentzian–Gaussian band profiles. Real band FWHM (full width at half band maximum) values were calculated by correcting measured FWHMs according to
𝑏 = 𝑏𝑠 × √1 − 2 (𝑏𝑠 𝑠)
2
(Irmer 1985), where 𝑏 is the real (i.e. corrected) FWHM, 𝑏𝑠 is the measured FWHM, and 𝑠 is the spectral resolution of the Raman system.
The X-ray powder diffraction analyses were performed at the X-ray diffraction laboratory, UFOP, using an Empyrean diffractometer, Panalytical type. The analyses were carried out under the following conditions: Cu Kα ( = 1.5418 Å) radiation at 45 kV and 40 mA, receiving slit of 0.2 mm, 0.02° 2θ steps, 40 seconds per step, range of 2 to 70° 2θ. Unit-cell parameters were determined by Rietveld refinement, using the High Score Plus software.
4.3.3 – U-Pb geochronology
4.3.3.1 – ID-TIMS U-Pb dating (JSGL, NIGL, University of Oslo)
Small fragments of the gem-quality zircons BB9, BB12 and BB17 were rinsed in ultrapure H2O, immersed in 30% HNO3, ultrasonically cleaned for an hour, and placed on a hotplate at 80 °C for an hour. The HNO3 was removed and the grains were again rinsed in ultrapure H2O, before being loaded into 300 μl Teflon FEP microcapsules and spiked with the EARTHTIME mixed 205Pb-233U-235U tracer (ET535). Zircon was dissolved in Parr vessels in 120 μl of 29 M HF with a trace of 30% HNO3 at 210 °C for 48 h, dried to fluorides, and then re-dissolved in 6 M HCl at 180 °C overnight. U and Pb were separated using an HCl-based anion-exchange chromatographic procedure (Krogh 1973). Pb and U were loaded together on a single Re filament in a silica-gel/phosphoric acid mixture (Gerstenberger & Haase 1997) before measurement by TIMS at the respective institutions.
For TIMS analyses at JSGL, measurements were performed on a VG354 mass spectrometer. All common Pb was assigned to procedural Pb blank. Uranium was measured in static mode or by using the axial Faraday or axial Daly collector in pulse counting mode. Dead time of the measuring system for Pb and U was 22.8 ns and 20.8 ns, respectively. The mass discrimination correction for the Daly detector was constant at 0.05%/a.m.u. Amplifier
gains and Daly characteristics were monitored using the NIST SRM 982 Pb reference material. Thermal mass discrimination corrections were 0.10%/a.m.u.
Measurements at NIGL were performed on a Thermo Triton TIMS. Two Pb analyses were measured on a MasCom SEM detector and corrected for 0.16 ± 0.04%/a.m.u. mass fractionation. The rest of the Pb analyses were done in a multidynamic Faraday-SEM mode, peak hopping mass 204 and 205 in the SEM, which corrects for the SEM gain in real time. These data were corrected for mass fractionation of 0.12 ± 0.04%/a.m.u. Linearity and dead time correction on the SEM were monitored using repeated analyses of NBS982 and U500. Uranium was measured in static Faraday mode on 1011 Ω resistors. U was run as the oxide and corrected for isobaric interferences with an 18O/16O of 0.00205. U mass fractionation was calculated in real time using the ET535 tracer solution. U-Pb dates and uncertainties were calculated using the algorithms of Schmitz & Schoene (2007) and a 235U/205Pb ratio for ET535 of 100.18 ± 0.05. The 206Pb/238U ratios and dates were corrected for initial 230Th disequilibrium using a Th/U[magma] of 4 ± 1 applying the algorithms of Crowley et al. (2007), resulting in an increase in the 206Pb/238U dates of ~100 kyr and uncertainties in calculated Th/U for zircons of ~0.002. Common Pb in the analyses was attributed to blank and subtracted based on the isotopic composition and associated uncertainties analyzed over time. Because of the radiogenic character of these samples, the reduced data are insensitive to reasonable variations in the composition of this correction.
At Oslo, measurements were performed on a MAT 262 mass spectrometer either on Faraday cups in static mode, or by peak-jumping in an ion-counting secondary electron multiplier. The secondary electron multiplier data were corrected for a non-linear bias using an exponential equation whose parameters were adjusted based on concurrent measurements of the NBS982 Pb standard. In addition, all the data were corrected for 0.1%/a.m.u. fractionation using reproducibility factors of ± 0.05%/a.m.u. for Faraday data and ± 0.1%/a.m.u. for secondary electron multiplier data. Bulk reproducibility for zircon was tested by measuring aliquots of samples split after dissolution but before chemistry, and also by analyzing separate fragments of zircon standards 91500 (Wiedenbeck et al. 1995) and GJ (provided by W. L. Griffin). The zircon analyses were corrected for a Pb blank of 2 pg and 0.1 pg U, but in some instances it was evident that the actual blank must have been higher than that and allowance has been made for that during the calculation of the data. The residual initial common Pb was subtracted using compositions calculated with the Stacey & Kramers
(1975) model for the age of the sample. The data were reduced with ROMAGE 5.1, a program originally written by T. E. Krogh and expanded by L. Heaman.
Data plotting and age calculations were performed with the program Isoplot (Ludwig 2012).
4.3.3.2 – Laser ablation Q-ICP-MS U-Pb dating (UFOP and University of
Portsmouth)
At UFOP, isotopic analysis of more than 20 zircon grains were obtained via laser ablation Quadrupole (Q)-ICP-MS followed the technique described in Takenaka (2014). An Agilent 7700x quadrupole ICP-MS coupled to a 213 Nd-YAG laser (New Wave Research UP-213) was used to measure Pb/U and Pb isotopic ratios in grains. The laser was set up to produce energy density of ca. 8 J/cm2 at a repetition rate of 10 Hz, producing spots of 25 μm in the sample. Helium was used as carrier gas and, after this output, together with particles in suspension, argon is added to the system. As primary reference material, we used the GJ-1 zircon (608.5 ± 0.4 Ma; Jackson et al. 2004) and for quality control, we used, as secondary standard, the Plešovice zircon (337.1 ± 0.4 Ma; Sláma et al. 2008). The results are within error of recommended TIMS ages. Sixty analyses of GJ1 zircon gave a Concordia age of 607.8 ± 1.9 Ma (mean 206Pb/238U age = 607.8 ± 1.8 Ma; mean 207Pb/235U age = 608.1 ± 2.1 Ma). Twenty analyses of Plešovice zircon gave a Concordia age of 338.4 ± 1.4 Ma (mean 206Pb/238U age = 338.2±1.1 Ma; mean 207Pb/235U age = 339.5±1.3 Ma). The background data were acquired for 20 s followed by 40 s of laser ablation signal. The relevant isotope rations (207Pb/206Pb, 208Pb/206Pb, 208Pb/232Th, 206Pb/238U and 207Pb/235U, where 235U is calculated from 238U counts by abundant natural reason 235U = 238U/137.88) were processed using the Glitter software package and plotting and age calculations were done using the Excel-based Isoplot program (Ludwig 2012). The data reduction included the correction of fractionation problems and of errors in the mass counting.
At the University of Portsmouth, U-Pb ages of BB9 zircon were measured by LA-Q- ICP-MS after Jeffries et al. (2003), using an Agilent 7500cs coupled to a New Wave Research UP-213 Nd-YAG laser. Isotope ratios were calculated using a modified version of Lam Tool (Košler et al. 2008), normalized to either 91500 or Plešovice through sample-standard bracketing. The amount of 204Pb in these analyses was below the detection limit, and no common Pb correction was undertaken.
4.3.3.3 – Laser ablation SF-ICP-MS U-Pb dating (JWG, UFOP)
Seventeen different grains of the BB zircon were analyzed at Federal University of Ouro Preto and J. W. Goethe University of Frankfurt am Main, using a Thermo-Finnigan Element 2 sector field ICP-MS coupled to a CETAC213 ultraviolet laser system, at UFOP, and a Resonetics M50 193 nm Excimer laser system, at JWG. Laser spot size of 20 μm was used. The typical depth of the ablation crater was 15 - 20 μm. Data were acquired in peak jumping mode during 20 s background measurement followed by 20 s sample ablation. Signal was tuned for maximum sensitivity for Pb and U while keeping oxide production well below 1%. Raw data were corrected for background signal, common Pb, laser-induced elemental fractionation, instrumental mass discrimination, and time-dependent elemental fractionation of Pb/U using an in-house MS Excel spreadsheet program. The common Pb correction was based on the Pb composition model (Stacey & Kramers 1975). Laser-induced elemental fractionation and instrumental mass discrimination were corrected by normalization to the reference zircon GJ-1 (Jackson et al. 2004), which was analyzed during the analytical session under exactly the same conditions as the samples. Prior to this normalization, the drift in elemental fractionation was corrected by applying a linear regression through all measured ratios, excluding the outliers (N ± 2 S.D.), and using the intercept with the y-axis as the initial ratio. The total offset of the measured drift-corrected 206Pb/238U ratio from the “true” ID- TIMS value of the analyzed GJ-1 grain was typically around 1 - 3%. Reported uncertainties (2σ) were propagated by quadratic addition of the external reproducibility (2 S.D.) obtained from the zircon reference material GJ-1 during the individual analytical session.
At UFOP, three secondary standards were used before and during runs: Plešovice zircon (337 ± 1 Ma; Sláma et al. 2008), M127 zircon (524.35 ± 0.92 Ma; Klötzli et al. 2009) and 91500 zircon (1065.4 ±0.6 Ma; Wiedenbeck et al. 1995). The results are within error of recommended TIMS ages. Sixty-two analyses of Plešovice zircon gave a Concordia age of 338.39 ± 0.72 Ma (mean 206Pb/238U age = 338.36 ± 0.69; mean 207Pb/235U age = 338.46 ± 0.78 Ma). Thirty-four analyses of M127 zircon gave a Concordia age of 526.7 ± 1.1 Ma (mean 206Pb/238U age = 526.9 ± 1.5; mean 207Pb/235U age = 525.7 ± 1.2 Ma). Twenty-three analyses of 91500 zircon gave a Concordia age of 1060.4 ± 3.4 Ma (mean 206Pb/238U age = 1059.5 ± 4.2; mean 207Pb/235U age = 1061.1 ± 3.7 Ma).
4.3.3.4 – Laser ablation MC-ICP-MS U-Pb dating (USP)
U-Pb analyses by LA-MC-ICP-MS were carried out using a Thermo-Finnigan Neptune multicollector ICP-MS coupled to a Photon-Machines 193 nm laser system at the Geochronology Research Center of the University of São Paulo. The mount containing zircons were cleaned in a HNO3 solution (3%) and in ultraclean water bath. The ablation was