High spatial resolution analysis of Pb and U isotopes for geochronology by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

EARTH SCIENCES

High spatial resolution analysis of Pb and U isotopes for geochronology by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

Bernhard Bühn; Márcio M. Pimentel; Massimo Matteini; Elton L. Dantas

Instituto de Geociências, Universidade de Brasília, Campus Universitário Darcy Ribeiro, Asa Norte, 70910-900 Brasília, DF, Brasil

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ABSTRACT

Age determinations using the ²³⁵U and ²³⁸U radioactive decay series to the daughter isotopes ²⁰⁷Pb and ²⁰⁶Pb, respectively, using the mineral zircon (ZrSiO4), are widely used to decipher geological processes. A new method developed in the last couple of years, the laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), overcomes previous laborious sample preparation, and yields isotopic ratios and age data with a high spatial resolution of ten of microns. The present study describes the analytical set-up and data reduction process as presently applied at the Laboratory for Geochronology of the University of Brasília. It explores the precision and accuracy of the method by cross-analysing three international zircon standards. We arrive at a precision of 1.9 to 3.7% (2σ SD) and an accuracy of 0.6 to 3.8% (2σ SD) for and U isotopic ratios of the standards. We also apply the method to two natural zircon samples, which have previously been dated by other analytical methods. A comparison of the results show a good conformity of the age data,being whitin the error limits. The data demonstrate the great analytical potential of the method for rapid, precise and accurate U-Pb isotopic analyses on the micron scale.

Key words: geochronology, U-Pb isotopes, LA-MC-ICP-MS, zircon, age dating.

RESUMO

A determinação de idades através do decaimento dos isótopos radioativos ²³⁵U e ²³⁸U para os isótopos radiogênicos ²⁰⁷Pb e ²⁰⁶Pb, respectivamente, utilizando o mineral zircão (ZrSiO₄), é amplamente aplicada para decifrar processos geológicos. Um novo método tem sido desenvolvido nos últimos anos, a ablação a laser multi-coletor espectrometria de massas com plasma indutivamente acoplado (LA-MC-ICP-MS), superando o laborioso trabalho anteriormente necessário em outros métodos, na preparação de amostras, e permite obtenção de razões isotópicas com alta resolução espacial de micrômetros. O presente estudo descreve os procedimentos analíticos e os métodos usados na redução de dados que estão sendo aplicados no Laboratório de Geocronologia da Universidade de Brasília. Explora-se a precisão e exatidão do método através da análise de três padrões internacionais de zircão. Observa-se uma precisão entre 1,9 a 3,7% (2σ desvio padrão) e uma exatidão de 0,6 a 3,8% (2σ desvio padrão) para as razões isotópicas de Pb e U dos padrões. Também foram obtidas idades pelo método LA-ICP-MS de duas amostras de zircões naturais, que já foram datadas anteriormente por outros métodos analíticos. A comparação dos resultados mostra uma boa conformidade das idades obtidas, dentro dos limites de erro.Os dados demonstram o grande potencial do método analítico para análises isotópicas rápidas, precisas e exatas de U-Pb, numa escala de micrômetros.

Palavras-chave: geocronologia, isótopos U-Pb, LA-MC-ICP-MS, zircão, datação.

INTRODUCTION

Absolute age determinations in the geosciences serve for a variety of applications including geotectonic studies, sedimentation ages and sediment provenance, as well as dating of igneous and metamorphic rocks. They use the decay of a radioactive isotope in a natural mineral with a half-life favourable for the expected age of the material or process investigated. Many of these applications are conducted in materials of considerable age (millions to billions of years), which suggests the use of the radioactive uranium isotopes ²³⁵U and ²³⁸U with half-lifes of 7.04 ×10⁸ and 4.47 ×10⁹ years, and the radiogenic daughter isotopes ²⁰⁷Pb and ²⁰⁶Pb, respectively. Analysis of the parent and daughter isotopes allows the determination of the crystallization age of the mineral. The mineral analysed should show certain physicochemically favourable characteristics, such as high resistance to chemical exchange and recrystallization, and reasonably high U and radiogenic Pb contents for analysis, amongst others. These are met by the abundantly occurring mineral zircon (ZrSiO₄) which is, therefore, the most traditional and widely used mineral for age determinations of various geological applications (e.g. Garbe-Schönberg and Arpe 1997, Kosler et al. 2002).

Apart from the improvement in precision, accuracy and mass resolution on the detector side of the instrumental set-up, a new method of material sampling is gaining more and more importance. Instead of the introduction of the sample in aqueous solution, a laser microprobe for sampling of solid material on the micron scale is connected to an ionization cell, which ionizes the sampled material in an inductively coupled plasma (ICP) and supplies single-charged ions to a magnetic sector or quadrupole mass spectrometer. The technique is referred to as "laser ablation inductively coupled mass spectrometry" (LA-ICP-MS). It avoids the laborious sample cleaning, dissolution and element separation in an ultra-clean laboratory environment required by the traditional isotope dilution thermal ionization mass spectrometry(ID-TIMS). Since the pioneering work by Feng et al. (1993) and Hirata and Nesbitt (1995), significant analytical progress of the LA-ICP-MS method was achieved (see Vanhaecke and Moens 1999, Günther et al. 2000, Horn et al. 2000, Stirling et al. 2000, Simonetti et al. 2006, Paquette and Tiepolo 2007, Simon et al. 2007).

In this study, advances of this particular dating method, the multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) coupled to a laser microprobe, which has recently been set up at the Geochronology Laboratory of the University of Brasilia, are reported. We present results of three international zircon standards cross-calibrated one against the other, and two natural zircon samples which have previously been analysed by other analytical methods. The data demonstrate the capacity of the method in terms of rapid, precise and accurate U and Pb isotope analyses with high spatial resolution.

RATIONALE

For the case of two radioactive isotopes of a single element, as for the decay systems of ²³⁸U and ²³⁵U, the ratios between the stable decay products and the radioactive parent isotopes (²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U) are interrelated. Given closed-system conditions, they plot on a curve which extends from today towards the formation of the Earth and the solar system at approximately 4.53 Ga ago (Fig. 1). The ratios have to concord one to the other following the relationship:

and define the "Concordia curve", with λ being the decay constant of the respective radioactive isotope. Another age information can be obtained by the ²⁰⁷Pb/²⁰⁶Pb age, based on the relationship:

With the constant present-day ²³⁸U/²³⁵U ratio of 137.88, this age information considers only the daughter isotopes. Hence, the more accurate and valid age information is obtained from the ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U isotope ratios.

There are two fundamental problems with this method. Firstly, the material analysed may contain initial common Pb of non-radiogenic origin. The non-radiogenic Pb fraction displaces the isotope analyses towards values below the Concordia curve (Fig. 1). The problem of this "common Pb" content can be resolved by correction procedures using crustal ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb compositions. These Pb isotope ratios, however, change with time in terrestrial materials, due to the radiogenic growth of ²⁰⁶Pb and ²⁰⁷Pb from the decay of ²³⁸U and ²³⁵U, respectively. The respective, time-dependant isotope ratio used for correction may be calculated from the average crustal Pb isotopic composition, or may directly be analysed in cogenetic minerals which contain predominantly common, and little radiogenic Pb.

In order to avoid these corrections, it is always more effective to analyse minerals which, due to their crystal lattice characteristics, contain sufficient U to analyse the parent and the daughter isotope but, on the other hand, do not accommodate common Pb in the structure, because of the mismatch between ionic charge and effective ionic radius (Shannon 1976). These are, for example, zircon (ZrSiO₄) or monazite (Ce,La,Nd,Th)PO₄, which generally contain sufficient U to be analysed, ranging from 1 to > 10000 ppm for zircon and 282 to > 50000 ppm for monazite (Heaman and Parish 1991). The common Pb content of the two minerals is generally (but not always) very low.

Secondly, the system must have been closed to the gain or loss of the parent and daughter isotopes. In most cases, we deal with a loss of radiogenic lead which can be easily mobilized by geological processes because, as in the case of zircon, radiogenic Pb²⁺ does not fit the Zr⁴⁺ ion characteristics (Mezger and Krogstad 1997). The result is that the isotopic analyses plot on a "Discordia line" which intersects the Concordia curve twice(Fig. 1): the upper intercept generally represents the time of crystallization of the mineral; the lower intercept may, in cases, be interpreted as the geological event when the crystal lost part of his radiogenic lead. Mineral standards for isotopic analyses by LA-ICP-MS or ion microprobe should meet these two requirements, i.e. they should be concordant, and void of common lead.

METHOLOGICAL PROGRESS OF Pb-U ISOTOPIC ANALYSIS

Conventional Pb-U analyses for absolute age determination of geological materials commonly use the technique of ID-TIMS (Isotope Dilution Thermal Ionization Mass Spectrometry). Sample preparation is laborious (e.g. Nebel-Jacobsen et al. 2005, Amelin and Davis 2006). The minerals have to be separated from the rock matrix, cleaned and sometimes abraded if the outer margins show alteration features (Mattinson 2005). The minerals are dissolved, and the Pb and U isotopes separated by resin columns in an ultra-clean laboratory to allow sub-nanogram Pb isotopic analysis. The solutions are deposited on rhenium filaments for thermal ionization and introduction of the ionized species into a magnetic sector mass spectrometer.

This techniques has been successfully applied for decades and is still commonly used for U-Pb age determinations and other isotopic geochemical applications worldwide. Low-blank laboratories normally achieve a very high precision of the age determinations (Parrish and Noble 2003, Krymsky et al. 2007). However, shortcomings of the technique are the need to separate the minerals from the matrix, the lack of spatial resolution of the analyses, and the laborious wet chemical preparation under ultra-clean laboratory conditions. On the other hand, the quality of the analyses is excellent, reaching precisions down to < 0.1% for the Pb/U isotopic ratios (e.g., Mattinson 2005).

Another method which overcomes the need for wet chemical preparation is the Sensitive High Resolution Ion Micro Probe (SHRIMP). It focuses a high-energy ion beam (usually ) onto an area of usually < 30µm for in situ isotopic analysis of minerals such as zircon. The method reaches a precision and accuracy of about 1.0% (e.g., Cocherie et al. 2005). The SIMS method (Secondary Ion Mass Spectrometry), on the other hand, employs a primary ion beam and analyses the ejected secondary ions. It focuses an O^- ion beam of typically > 10µm in diameter on a solid sample surface, and is primarily applied to quantify major and trace element contents (for example, Pettke et al. 2004).

Many recent analytical methods, instead, use an "Inductively coupled plasma" ionization method (ICP) of the sample, generating a fine aerosol which is ionized in an Ar plasma (Horn et al. 2000). With Ar having a high first ionization potential of 15.759 V, the plasma ionizes all other elements to single-charged ions except for He, F and Ne. The method was developed in the early 1980s for introduction of dissolved samples. Likewise, the ICP array may be connected to a laser system which ablates a solid sample whose vaporized material is then transported by a He gas flow into the ICP unit. This method overcomes the need of ultra-clean laboratory conditions and allows the selective sampling of solid material on the micrometer scale. On the detector side, the recent technology came up with multi-collector (MC) arrangements which allow the simultaneous detection of the relative abundance of various isotopes (see Simonetti et al. 2005). The conventional Faraday cup detectors (normally 9 in a line) may be equipped with additional multichannel ion counters(MICs) for high-sensitivity isotope analyses. The equipment with additional MICs is only a spatial problem within the Faraday cup arrangement and, generally,6-8 MICs may easily be accommodated, allowing the simultaneous analysis of about 15 isotopes.

The shortcomings of the laser ablation multi-collector inductively coupled plasma mass spectrometry(LA-MC-ICP-MS) relative to SHRIMP or conventional TIMS analyses have been its lower precision. The laser ablation process is not as stable as the SHRIMP method, or as the introduction of the sample in solution as in the case of TIMS. Firstly, this is due to the fragmentation and vaporization process of the solid material ablated. The laser beam may ablate parts of the mineral which are slightly fractured, altered or otherwise physically weakened or in homogenized by previous geological processes, including the destruction of the mineral lattice by the proper radioactive radiation. The result is that the ablation and vaporization process of the material may vary in efficiency and completeness and, thus, the material transport to the plasma may not be continuous. Secondly, the carrier gas flux, normally ultrapure He, has an appreciable effect on the stability of the gas flow. Hence, the laser settings, such as laser intensity, laser frequency and beam diameter, together with the gas flow parameters, are fundamental for a reasonably stable signal reaching the detectors.

With the more recent developments, however, the analytical uncertainties are getting closer to those of SHRIMP (see Willigers et al. 2002). Bruguier et al. (2001) arrived at 6%, Horn et al. (2000) at 2%, Chang et al. (2006) at 4%, and Gerdes and Zeh (2006) at3-4% precision or accuracy for U-Pb isotope ratios of natural materials. These values depend very much on the nature of the material analysed, such as its composition, presence of fractures, inclusions and heterogeneities. The LA-ICP-MS analyses may be even more precise for synthetic glass standards which are chemically and isotopically homogeneous (Bernal et al. 2005). Often, the largest uncertainty is given by the analysis of the standards (Chang et al. 2006) which are generally natural minerals including all the potential pitfalls. With a precision of that magnitude, however, with the control of the spatial resolution of the material analysed, and with the relatively easy preparation of the samples, the laser ablation MC-ICP mass spectrometry certainly is the method of choice for the coming years for isotopic analysis of solid materials in the geo- and environmental sciences (Willigers et al. 2002, Košler and Sylvester 2003).

SET-UP OF THE LASER ABLATION MC-ICP-MS U-Pb METHOD

The analytical equipment used in this study was installed at the "Laboratório de estudos geocronológicos, geodinâmicos e ambientais" of the Geosciences Institute of the University of Brasília, Brasília, Brazil, by initiative of Petróleo Brasileiro S/A - PETROBRAS and the "Ministério de Minas e Energia". The laboratory is part of the "Rede de Estudos Geodinâmicos e Ambientais - GEOCHRONOS". It was inaugurated in December 2006 and consists, apart from other installations and instrumentation, of a Thermo Finnigan Neptune multi-collector inductively coupled plasma mass spectrometer. Attached to this is a New Wave 213µm Nd-YAG solid state laser which provides the ablated material to the mass spectrometer.

SAMPLE PREPARATION

Sample preparation is straight forward. The minerals of interest, in this case zircon, are separated from the rock matrix by crushing the rock, concentrating the heavy mineral fraction by panning, separating the zircon grains by a Frantz magnetic separator, and final hand-picking of individual zircon grains. In situ analysis directly on a rock slab or thin section is also possible. However,it was found that the procedure of pre-concentration largely facilitates the measurements and localization of the zircons, so that it is worthwhile to be done.

The zircon grains are then located under the binocular one by one on a glass wafer covered with a double-sided tape in order to secure the grains. A 9 mm diameter plastic ring is placed on the tape around the zircons, and filled with two-compound epoxy. After 1-2 days of drying, the mounts are removed from the tape. Polishing of the mounts is done with 3µm followed by 1 µm diamond paste in order to expose unaltered and clean zircon surfaces. It is advisable to locate the grains on existing flat crystal surfaces (if present) to avoid excessive polishing. The mounts are then taken to an ultrasonic bath with 3% HNO₃, and then cleaned with distilled water. A Nylon sample holder which takes three mounts was especially constructed. The three sites may be occupied with one standard and two samples, or with two standards and one sample.

THE LASER SET-UP

The sample chamber is flushed with He gas of analytical quality, which samples the ablated and vaporized material to be transported via Tygon tubes into the ICP. Before entering the laser sample chamber, the He passes through a glass tube filled with gold-coated quartz crystals (gold trap) to remove the largest part of mercury (Hg) whose isotope ²⁰⁴Hg interferes with the ²⁰⁴Pb isotope analysed to apply the common Pb correction. The He gas flux is commonly 0.35-0.45 L/min.

The laser is run at a frequency of 7-10 Hz and an energy of 30-36%, resulting in a laser energy of ca. 0.5 to 1.2 J/cm². A small laser spot size of say 30µm applied to a single spot with these laser conditions would not produce enough material for a reasonable signal, but at least 40-50µm would be required, with the signal intensity deteriorating along duration of the analysis because of the deepening of the laser pit. This suggested the use of a raster scheme of the laser which samples only the surface of the zircon. The raster is composed of two parallel and a connecting line with a 20µm spacing. With a 30µm beam diameter applied here, the sampled area of the zircon is a square of about 70µm on each side. With a speed of 1µm/sec, the propagating laser spot takes about 1 min to pass the raster, which is enough time to collect the signal. This setting also avoids the prominent down-hole fractionation of isotopes which would require an additional correction, and a significantly higher laser energy to drill the zircon. Before signal collection, a pre-ablation run along the raster is applied to clean the mineral surface. The running conditions of this cleaning run are: 40mm spot size, frequency 10 Hz and 25% laser energy.

THE ICP-MS AND MULTI-COLLECTOR SET-UP

Before entering the plasma, the He gas coming from the laser with the ablated material, is combined with Ar gas of analytical quality which sustains the plasma. The detector arrangement consists of a central Faraday cup and 4 Faraday cups each on the high side and low side of the centre cup (Fig. 2). For high-sensitivity analyses of the Pb and U isotopes, the mass spectrometer is equipped with 6 ion counters (ICs, or multichannel ion counters MICs). Four of them are fixed on the outermost Faraday cup of the low side (cup L4), one on the third cup of the low side (cup L3), and one on the outermost Faraday cup on the high side (cup H4). The latter is used only for the analysis of ²³⁸U in materials with extremely low U content. For the present application, we analysed ²³⁸U on the Faraday cup H4. The other isotopes are collected on the low side. The four ICs on the cup L4 are fixed at defined distances referring to: one mass unit betweenL4 and IC5, one mass unit between IC5 and IC4, two mass units between IC4 and IC3, and two mass units between IC3 and IC2. The signals collected on the four ICs are: ²⁰⁷Pb on IC5, ²⁰⁶Pb on IC4, ²⁰⁴Hg/²⁰⁴Pb on IC3, and ²⁰²Hg on IC2. Because of the always present contamination of the He gas with Hg, a correction for the isobaric interference between ²⁰⁴Hg and ²⁰⁴Pb has to be applied, using the signal of ²⁰²Hg collected on IC2.

DATA COLLECTION

With this collector set-up, five isotopes are analysed simultaneously. Before analysis, the Faraday cups are calibrated for baseline and gain, and the ICs for their yield. To ensure the optimal operating voltage and the same sensitivity for the ICs, a signal of about 300,000 counts per second (cps) is successively directed into the4 ICs using different analysing lines. We use a solution of ²⁰⁶Pb for this cross-calibration. Based on the signal response for each IC, the operating voltage of each IC is adjusted in order to ensure measuring conditions with the same sensitivity for all ICs used.

For standard and sample analysis, the signals are collected in a single block with 40 cycles of 1.049 sec. each. The laser is started and, when the signal at the detectors has reached its maximum, the 40 cycles are taken. A typical analysis therefore takes about 1 min and is illustrated in Figure 3. The isotopes ²⁰²Hg, ²⁰⁴(Pb+Hg), ²⁰⁶Pb and ²⁰⁷Pb are read in counts per second (cps), and ²³⁵U in mV, which is transferred into cps by multiplication by a factor of 62,500. We apply a standard-sample bracketing technique, analysing a blank, a standard, and three samples, blank, standard, and so on. The method accounts for the drift of the instrument, and eliminates the need to correct for the mass bias of individual isotopes.

DATA REDUCTION

The isotope data of the 40 analysed cycles are transferred into an in-house Excel spreadsheet for data reduction. The isotope ratios of the 40 cycles are evaluated on a 2σ standard error rejection basis. The routine then calculates the error correlation value ρ (rho) for the ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U data. The isobaric contribution of ²⁰⁴Hg is subtracted from the joint ²⁰⁴Pb/²⁰⁴Hg peak by considering the analyzed ²⁰²Hg/²⁰⁴Hg ratio. The spreadsheet allows to chose between three ways to determine the ²⁰²Hg/²⁰⁴Hg value which is applied for correction: (i) from the mean of the two bracketing blanks, (ii) from the mean of the two bracketing (common Pb-free) standards, or (iii) from the mean of the four. Normally, the difference between the three determinations is minor. We consider this more favourable than applying the natural ²⁰²Hg/²⁰⁴Hg ratio of 4.346, which would require an absolutely precise cross-calibration of IC2 and IC3. Because ²³⁵U cannot be analysed with this type of cup configuration, the natural ratio ²³⁸U/²³⁵U of 137.88 is used to calculate the ²³⁵U intensity.

The correction for instrumental drift and mass bias is done by considering the two standards analysed before and after the samples, and the two blank analyses run before and after the samples, respectively, after applying a blank correction for all isotopes analysed.The normalization of the isotopic ratios of the samples (R_spl) follows the formula of Albarède et al. (2004):

for which R_std is the true isotopic ratio of the standard (²⁰⁷Pb/²⁰⁶Pb or ²⁰⁶Pb/²³⁸U ratios, respectively), r_spl is the analysed isotopic ratio of the sample, and r_std1 and r_std2 are the analysed isotopic ratios of the two standards before and after the samples along the sequence. The variable θ (theta) is a time factor (Albarède et al. 2004) or, in other words, gives respective weight to the two standard measurements. It depends on the location of the samples within the analysing sequence, and takes values between 0 and 1. For example, three successive analyses run between two standards and two blanks suggest θ = 0.25, 0.50 and 0.75 for samples 1, 2 and 3, respectively. The term ( × ) is a linear interpolation of the logarithms of the isotopic ratios, which accounts for the generally exponential instrumental drift. The same algorithm is applied to correct for the drift in the blank analyses. An alternative would be to inject a diluted thallium (Tl) solution to determine the mass bias of the Pb isotopes (e.g., Horn et al. 2000) which, however, does not allow the correction of U mass bias. The standard-sample bracketing approach applied here, therefore, certainly bears the least error for that analytical method, as it does not require intensive mass bias corrections.

The corrected ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb ratios so obtained are transferred into absolute age information using the software of Ludwig (2003). The software was also used to plot the Concordia diagrams. The errors of the ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb ratios were propagated by quadratic addition of the external reproducibility and the within-run precision, following equation (4):

with SD being the sample standart deviation σ:

where N is the number of analyses, x_i is the value of each individual analyses x_i to x_N, and x_m is the arithmetic mean of all analyses. Whereas the SD is independent of the number of analyses, the standard error SE:

decreases with the number of analyses N taken. The external reproducibility (2 ) is represented by the standard deviation obtained by repeated analysis of standard zircon GJ-1 which were run during the analytical session (generally ~ 0.8% for ²⁰⁷Pb/²⁰⁶Pb, and ~ 1% for ²⁰⁶Pb/²³⁸U). The within-run precision (2 ) is the standard error calculated for each analysis. The resulting errors are 2σ standard deviations.

Zircon U-Pb isotopic analyses may require a correction for the common lead contribution to the total ²⁰⁶Pb and ²⁰⁷Pb signals. There are a couple of methods to do this in the literature (see Andersen 2002, Horstwood et al. 2003). For minerals with a low common Pb content as, for example, zircon, we use a correction based on the amount of non-radiogenic ²⁰⁴Pb. The counts per second of ²⁰⁴Pb (including a correction for ²⁰⁴Hg) are used to deduce the amount of non-radiogenic, common ²⁰⁶Pb and ²⁰⁷Pb, given a common Pb composition following the model of Stacey and Kramers (1975). With the fraction f_c of common ²⁰⁶Pb and ²⁰⁷Pb so obtained:

where Pb_c is the value (in counts per second) of common Pb, Pb_t the total Pb, and Pb_r the radiogenic Pb, with Pb being ²⁰⁶Pb or ²⁰⁷Pb, respectively, we use the relationships:

and

of Andersen (2002) to arrive at common Pb-corrected ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U isotopic ratios. The necessity for a correction is judged from the ²⁰⁴Pb signal intensity. Values smaller than ~ 25 cps (blank-corrected), resulting in ²⁰⁶Pb/²⁰⁴Pb ratios well beyond 3000, suggest an insignificant common Pb content which would not recognizably effect the calculated ages. All zircons analysed in this study met this condition, and therefore did not require a correction.

STANDARDS AND SAMPLES

Three international zircon standards were analysed, using each one as an external standard and correcting the remaining two against it. This yields a set of 6 isotope ratio and age data which are compared with the certified values of the standards (see Table I). The standards are: (i) GJ-1 zircon standard provided by the ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), Australia. This is a gem-quality zircon crystal of about 0.7 cm in diameter which was broken and prepared for analysis. Its reference age after Jackson et al. (2004) is: ²⁰⁷Pb/²⁰⁶Pb age = 608.6 ±1.1 Ma, ²⁰⁶Pb/²³⁸U age = 600.4 ± 1.8 Ma, ²⁰⁷Pb/²³⁵U age = 602.1 ±3.0 Ma; (ii) Temora 2, provided by Lance Black, Geoscience Australia, Australia, which comes from the Middledale gabbroic diorite in eastern Australia; the zircon crystals have a size of about 300-500 µm and, prior to sample preparation, the concentrate was hand-picked to obtain a high-purity zircon concentrate. Its reference age after Black et al. (2004) is: ²⁰⁷Pb/²⁰⁶Pb age = 419.1 ±9.1 Ma, ²⁰⁶Pb/²³⁸U age = 416.2 ±1.1 Ma, ²⁰⁷Pb/²³⁵U age = 416.7 ±1.7 Ma;(iii) FC1 zircon concentrate provided by Richard Armstrong, Australian National University, Canberra, Australia; the crystals have sizes of approximately 50-100mm, and were hand-picked prior to sample preparation; they were extracted from the Duluth Complex in the U.S.A. Its reference age (Paces and Miller 1993) is: ²⁰⁷Pb/²⁰⁶Pb age = 1098.9 Ma, ²⁰⁶Pb/²³⁸U age = 1099.9 Ma, ²⁰⁷Pb/²³⁵U age = 1100.0 Ma.

For an additional test of the method, two zircon samples from our own laboratory which had previously been dated by other analytical methods were also investigated here. One is a relatively young zircon concentrate (sample HD66) from a hornblende-bearing orthogneiss of the Neoproterozoic Goiás magmatic arc which has previously been dated at 630 ±5 Ma (2σ) by U-Pb ID-TIMS in our laboratory (Laux et al. 2005). Its average ²⁰⁷Pb/²⁰⁶Pb TIMS age is 633.3 ± 5.7 Ma. The other sample (EC69A) has much older zircon crystals and went through a more complex geological history. They were extracted from a migmatized hornblende- and biotite-bearing tonalite from an Archean nucleus of the São José do Campestre Massif in the Borborema Province, NE Brazil. The zircons yielded an upper intercept age of 3065 ±12 Ma determined by SHRIMP analysis (Dantas et al. unpublished data).

RESULTS

Between 13 and 23 analyses were run for the standards GJ1, Temora 2 and FC1. Figure 4 shows the ages obtained for each standard normalized against one another in a Concordia diagram, and Table I summarizes the data obtained. The concordance between the values obtained and the referenced standard data of the ²⁰⁷Pb/²⁰⁶Pb,²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U age data is well satisfactory. We obtained precisions of 1.9 to 3.7% (2σ standard deviation) for the ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U age data, depending on the isotope system and on the particular standard. The accuracy was calculated as the percentage of deviation between the analysed Concordia age and the referenced ²⁰⁷Pb/²⁰⁶Pb age. The Concordia age, following Ludwig (2003) is, "the most-probable age for a data-point (or weighted-mean data-point) on a Concordia diagram, where the true location of the data point is assumed to fall precisely on the Concordia curve". Because not all referenced standards provide the error correlation coefficient to calculate that age,their referenced ²⁰⁷Pb/²⁰⁶Pb age was used. The accuracy so obtained varies between 0.6 and 3.8% (2σ standard deviation).

The majority of the 23 analyses obtained for the natural sample HD66 (Table II) were taken as multiple analyses on different localities of individual grains.They are all concordant (Fig. 5), yielding a Concordia age of 627.6 ±1.3 Ma, which is identical within error to the TIMS age of 630 ±5 Ma. None of the zircons required a common Pb correction. The ²⁰⁴Hg-corrected ²⁰⁴Pb signal was 30 cps at most, yielding calculated ²⁰⁶Pb/²⁰⁴Pb ratios between 3000 and 50000. The weighted average of 24 ²⁰⁷Pb/²⁰⁶Pb ages resulted in a ²⁰⁷Pb/²⁰⁶Pb age of 628.7 ±6.0 Ma, compared with the TIMS ²⁰⁷Pb/²⁰⁶Pb age of 633.3 ±5.7 Ma.

Sample EC69A shows a more complex zircon population, as seen from 24 SHRIMP (Table III) and 35LA-ICP-MS analyses (Table IV). Most of the LA-ICP-MS analyses were taken as multiple shots on individual zircon grains. None of the zircons required a common Pb correction. The ICP-MS analyses display various groups of zircons plotting on supposedly different Discordia lines (Fig. 6). The youngest upper intercept age, defined by four discordant and three concordant analytical points, yields the age of 3045 ±24 Ma,which agrees within error with the SHRIMP age of3065 ±12 Ma. However, various older zircon populations were encountered in the sample. One is defined by 17 discordant zircon analyses which yield an upper intercept age of 3305 ±51 Ma. A minor population composed of three discordant analyses gives an upper intercept age of 3625 ±38 Ma. The two latter populations may relate to 3.4-3.5 Ga ages reported from the same area by Dantas et al. (2004). Another group of five highly discordant zircon analyses with extremely low²⁰⁶Pb/²³⁸U ratios may represent still another, even older geological event. Three analyses on grain 14 (see Table IV, but not included in Fig. 6) yielded one concordant and two nearly concordant data with ages between 540 and 570 Ma. They represent another event of resetting of the isotope system during the Neoproterozoic/Cambrian Brasiliano cycle. Therefore, the Discordias of all older zircon populations were anchored to 558 Ma, which is the mean of these three analyses.

DISCUSSION AND CONCLUSIONS

A comparison of the LA-ICP-MS results of the natural sample HD66 with TIMS analyses shows a fine conformity even within the limits of error, yielding 627.6 ±1.3 Ma for LA-ICP-MS and 630 ±5 Ma for the TIMS analyses. Moreover, the LA-ICP-MS analyses indicate that there was no Pb loss in the zircons (no discordant zircons), in contrast to three out of four TIMS analyses (Fig. 5a). It is most probable, that weathered or otherwise isotopically disturbed surfaces of the zircons are responsible for the discordant U-Pb TIMS data,since the zircons were not abraded before digestion to remove possible alterations or disturbances. The LA-ICP-MS method avoids this by polishing of the zircon grains before analysis, and by a laser pre-ablation which cleans the uppermost part of the mineral surface.

The LA-ICP-MS analyses of the natural sample EC69A show a complex history of zircon growth and lead loss. Three concordant zircon analyses, together with 4 other analyses, define an upper intercept age of 3045 ± 24 Ma (Fig. 6) which agrees, within error, with the 3065 ± 12 Ma SHRIMP age. There are other discordant zircon analyses which, although being discordant, indicate a complex history of crystallization and partial resetting. In that particular case, the internal structure of the zircons would have to be imaged by cathodo luminescence or back-scattered electron techniques prior to analysis, for ensuring an unequivocal interpretation of the various age populations. The analyses presented here show, nevertheless, that it is possible to detect an even complex geological history from a set of only 10-15 zircon grains, by high spatial resolution LA-ICP-MS analysis.

The LA-MC-ICP-MS method presented here is designed for rapid analysis of large sample sets, which often are needed in sediment provenance or other regional geological studies. It overcomes extensive sample preparation and careful wet chemical isotope separation, which may always be a source of sample contamination. Apart from that, however, it serve as well as a precise and accurate geochronological tool. With a precision of 1.9 to 3.7% (2s SD) and an accuracy of 0.6 to 3.8% (2σ SD) for the ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U isotopic analyses, the LA-ICP-MS method is a highly competitive method for rapid but nonetheless high-quality analyses of geological materials.

ACKNOWLEDGMENTS

The LA-ICP-MS facility was installed at the Universidade de Brasília with funds provided by PETROBRAS and the Ministério de Minas e Energia do Brasil, which are gratefully acknowledged. Thanks go to Lance Black, Elena Belousova and Richard Armstrong for providing the zircon standards. We also thank the three anonymous journal referees for valuable and clarifying comments and suggestions.

Manuscript received on March 13, 2008; accepted for publication on July 15, 2008; contributed by MARCIO M. PIMENTEL^*

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*

Member Academia Brasileira de Ciências

Publication Dates

• Publication in this collection
  20 Feb 2009
• Date of issue
  Mar 2009

History

• Accepted
  15 July 2008
• Received
  13 Mar 2008