r ' " " , - -

ELSEVIER Chemical Geology 133 (1996) 145-156

CHEMICAL GEOLOGY

I ~ ¢ C / / J O / ~

ISOTOPE GEOSCIENCE

Microanalysis of minerals by an Excimer UV-LA-ICP-MS system

F. Bea a p. Montero b A. Stroh c j. Baasner c

a Dept. Mineralogla y Petrologla, Univ. Granada, Campus Fuentenueva, E-18002 Granada, Spain b Centro de lnstrumentaci6n Ciendfica, Univ. Granada, Campus Fuentenueva, E-18002 Granada, Spain

c Bodenseewerk Perkin-Elmer GmbH, P.O. Box 101761, D-88647 Uberlingen, Germany

Received 18 January 1996; accepted 30 May 1996

Abstract

This paper is a performance evaluation of a prototype laser-ablation microanalytical system composed of an UV Excimer laser and a high-sensitivity inductively coupled plasma mass spectrometer (XLA-ICP-MS). The laser was optimized for trace-element microanalysis of silicate minerals, and the effects of different parameters (laser power, focus, carder gas flow, etc.) on the performance characteristics were studied. The crater size and shape produced by the XLA system were compared to a solid state IR LA system (Nd:YAG) and proved to be superior. Crater sizes achievable in thin sections vary from 1000 to 10 Ixm, although the best analytical results for this prototype were achieved at crater diameters of ~ 40-60 I~m and depths of ~ 30-40 p,m. Detection limits vary from ~ 1 ppb for 1000-1xm craters to ~ 1000 ppb for 10-1~m craters. The precision obtained for the measurements depends on both crater size and concentration in the ablated mineral. Typical RSD for five replicate analyses of the above-mentioned minerals were +4% and ___ 10% for concentrations > 10 and 1-10 ppm, respectively. Comparison of XLA with solution nebulization analysis shows an excellent agreement for most elements. The XLA source, in contrast with other laser sources currently available, produced no noticeable chemical fractionation during ablation for all elements except radiogenic 2°Tpb and, to a lesser extent, 2°6pb. This effect is a serious limitation for using LA-ICP-MS for in situ lead geochronology.

Keywords: Microanalysis; Minerals; Trace elements; Laser ablation; ICP-MS

1. Introduction

Laser ablation connected to inductively coupled plasma mass spectrometry ( L A - I C P - M S ) was first introduced as a method for avoiding sample solution (Arrowsmith, 1987), and was then quickly adopted for bulk-rock analysis of geological samples (Jarvis and Williams, 1993; Perkins et al., 1993) [but see

criticism in Morrison et al. (1995)]. Geoscientists soon recognized the potential of L A - I C P - M S for trace-element and isotope microanalysis of single mineral grains (e.g., Jackson et al., 1992; Feng et al., 1993; Jenner et al., 1993; Bea et al., 1994a,b; Fryer et al., 1995; Jarvis et al., 1995; Jeffries et al., 1995; Ludden et al., 1995), a task for which the high sensitivity, scarcity of interferences, and inherent

0009-2541/96/$15.00 Copyright © 1996 Elsevier Science B.V. All fights reserved. PH S0009-2541 (96)00073-3

146 F. Bea et al. / Chemical Geology 133 (1996) 145-156

rapidity of this technique could eventually make it a unique tool for geochemical research capable, in principle, of rivaling the more expensive ion probe.

The first commercially available laser ablation systems were based on infrared (IR) lasers, mostly of Nd:YAG type operated at their fundamental wave- length (1064 nm). IR lasers, however, produce an ablation where melting, boiling, and evaporation ef- fects predominate over direct photon-solid interac- tion. This causes significant chemical fractionation during ablation, especially for the most volatile ele- ments, and makes it essential that the composition of the standards closely match that of samples, obvi- ously not easy for crystalline samples. In addition, the heating of fluid inclusions within the ablation zone causes microexplosions (Fig. 1), which makes it very difficult to control the size of the ablated region and increases signal instability.

The realisation that many of the problems with IR lasers were related to unwanted thermal effects oc- curring during the laser-solid interaction led to a search for lasers with shorter wavelengths, especially in the ultraviolet (UV) range, which are expected to produce ablation solely due to photon-solid interac- tion, with no thermal effects. Nowadays, two major types of UV lasers are available: frequency quadru- pled Nd:YAG lasers operated at 266 nm, and Ex- cimer gas lasers which, depending on the gas compo-

sition, have various fundamental UV wavelengths. The first type still have many of the thermal effects of Nd:YAG lasers operated at 1064 nm, whereas the second type appear to be almost free of such effects and, apparently, could represent an almost ideal tech- nique for LA- ICP-MS.

This paper presents a study of the analytical capability and applications for mineral microanalysis of a new prototype of a system comprising an Ex- cimer laser operated at 308 nm and a high-sensitivity ICP-MS spectrometer (XLA-ICP-MS). The objec- tives are as follows:

to study how laser parameters affect crater mor- phology and the analytical performance of the system;

• to determine ablation characteristics of rock-for- ming minerals;

• to determine the sensitivity and precision of the method at different concentration levels in miner- als with distinct ablation features;

• to determine the accuracy of the method by com- paring results of laser ablation with conventional dissolution ICP-MS analyses of the same miner- als. Based on these data, the applicability of the

method to trace-element and isotope analysis of min- erals are discussed.

2. Experimental

2.1. Instrument

Fig. 1. SEM image of a crater produced by firing an IR laser at a rock thin section. Note the great width and irregular surface of the external zone.

2.1.1. The laser probe

An Excimer laser (Sopra SEL 510) was used for ablation. The laser head was modified so that it could be installed in a Perkin-Elmer Laser Sampler 320. The laser is operated at 308 nm with the cavity containing a premixed gas mixture consisting of 5000 ppm Xe, 1700 ppm HC1, and 300 ppm H e in neon. The diameter of the laser beam may be contin- uously adjusted from 2 mm to ~ 10 i~m by focusing it with an optical lens and using a diaphragm as beam restrictor. The laser repetition rate may be adjusted between 1 and 5 Hz with a pulse duration of 30 ns, The beam energy is ~ 80 rnJ at a repetition rate of 1 Hz with a pulse-to-pulse stability of ___ 5%.

F. Bea et al. / Chemical Geology 133 (1996) 145-156 147

At 80% energy, beana divergence is less than 0.6 * 0.6 mrad. The focused laser beam is directed via a semi-transparent mirror to the sampling cell. The cell is flushed with an adjustable argon stream to trans- port the ablated material into the ICP-MS system. The position of the :sampling cell with respect to the laser beam is adjustod with stepper motors in x - y - z directions, to ablate different locations on the sample and to focus the laser beam for optimum ablation.

2.1.2. The ICP-MS spectrometer The inductively coupled plasma mass spectrome-

ter used for this work was an Perkin-Elmer Sciex ELAN 6000. A detailed description of the key fea- tures of this instrument can be found in Denoyer (1995).

Data acquisition was performed using multichan- nel analysis with only one channel per mass unit, in order to increase the analytical speed and reduce the time of laser firing with no loss of sensitivity. Oper- ating conditions on the spectrometer were: forward power, l l00 W; plasma gas flow, 15 l rain-l; auxiliary gas flow, 0.8 1 ra in-1 ; purge gas flow, 0.40 l rain-l; sampler, platinum; skimmer, platinum; in- terface pressure, 2-.3 Tort; quadrupole pressure, 1.5 • 10 -5 Torr; data acquisition, peak hopping; dwell time, 50 ms; 20 sweeps.

Data reduction was performed with the To- talQuant If1 © software implemented in the ELAN 6000. This program reads a full mass spectrum, with the spectral data presented as a series of integrated intensities for each mass measured. It then creates a preliminary estimalle of intensity for each element based on measured intensities and an interpretation of isotopic natural abundances. Initial estimates of intensity are also made for polyatomic species, with intensity values linfited to given fractions of a con- stituent element, q~ese fractions are contained in tables of polyatomic species-to-parent ion ratios. By formulating a set of heuristic principles, intensity assignments are made first for multi-isotopic ele- ments and their at;sociated polyatomic and doubly charged species, artd then for monoisotopic elements and their associated polyatomic and doubly charged species. Intensity a:~signments are made in prioritized fashion to avoid errors between elements having overlapping isotopes. The assignment order is dy-

namically rearranged if conflicts arise during the calculation process.

Once the intensity assignment process is com- pleted, TotalQuant III© performs an analysis of con- centrations for the elements present in the sample. A table of relative response factors (ion intensity values for given concentrations of analyte) is used to con- vert intensity data into concentration values. The response factors for all the elements can be adjusted by calibrating the system using an external standard with just a few elements covering the mass range. Therefore, we used NBS-612 glass as external stan- dard - - which contains SiO 2 (72.0 wt%), CaO (12.0 wt%), Na20 (14.0 wt%), AI203 (2 wt%), Ba (41 ppm), B (32 ppm), Ce (39 ppm), Co (35.5 ppm), Cu (37.7 ppm), Dy (35 ppm), Er (36 ppm), Eu (36 ppm), Gd (39 ppm), Au (5 ppm), Fe (51 ppm), La (36 ppm), Pb (38.57 ppm), Mn (39.6 ppm), Nd (36 ppm), Ni (38.8 ppm), Rb (31.4 ppm), Sm (39 ppm), Ag (22 ppm), Sr (78.4 ppm) T1 (15.7 ppm), Th (37.79 ppm), Ti (50.1 ppm), U (37.38 ppm), Yb (42 ppm) - - hut calibrating only with the elements certified in that standard. The blank was determined by running an analytical measurement without abla- tion. Concentration values were later refined using silicon as internal standard, previously determined by electron microprobe on the same minerals.

2.2. Samples and sample preparation

Samples were prepared as polished thin sections with a thickness of ~ 80 Cm. Thinner sections are not recommended, since they tend to break upon laser impact and minerals could easily be pierced by the laser. Shot points were carefully selected under the petrographic microscope to avoid inclusions, cracks, etc., which can produce spurious results.

To check analytical performance and optimize the system, we used fairly homogeneous large specimens of four minerals with different ablation character- istics: (1) a biotite from a miaskite pegmatite (Miass, the Urals); (2) a cordierite from a peraluminous granite pegmatite (Barraco, Iberia); (3) a plagioclase feldspar from a low-pressure granulite (Gredos, Iberia); and (4) an astrophyllite from a nepheline syenite (Khivine, Kola).

Chips of these four mineral specimens as well as mineral concentrates extracted from some of the rock

148 F. Bea et al. / Chemical Geology 133 (1996) 145-156

samples were also analyzed by solution-nebulization ICP-MS after HNO 3 + HF digestion of 0. I000 g of sample powder in a Teflon-lined vessel for 150 min at ~ 180°C and ~ 10 bar, evaporation to dryness, and subsequent dissolution in 100 ml of 4 vol% HNO 3. Measurements were carried out in triplicate with a Perkin-Elmer Sciex ELAN 5000 spectrometer using Rh and Re as internal standards.

3. Results

3.1. Optimization of laser parameters

3.1.1. Craters and crater morphology Laser craters comprise an internal zone - - where

laser-matter interaction was stronger - - encircled by a peripheral halo mostly caused by thermal ef- fects from the laser impacts. The relative extension of each zone depends on the laser wavelength (see SEM images in Figs. 1 and 2) and has a strong influence on signal stability. IR lasers produce a wide ( ~ 400 t~m) peripheral zone with a broken surface (Fig. 1), indicating that microexplosions were produced during ablation, probably due to fluid in- clusions. The peripheral zone of UV laser craters, in contrast, is much smaller (Fig. 2), and its surface appears to be made of glassy material, with no signs of microexplosions. Since chemical fractionation re-

Fig. 2. SEM image of a crater produced by firing an UV laser at a rock thin section. Note that the width of the crater external zone is much smaller than one produced by the IR laser, despite the diameter of the internal zone being approximately the same.

Fig. 3. SEM image of an UV-laser crater on the same material and laser conditions as the one shown in Fig. 2, except that the laser beam diameter was reduced to the minimum. Note how the diameter of the internal zone is reduced to ~ 10 Ixm, but the width of the external zone is approximately the same as above.

lated to thermal effects is more likely to happen in the peripheral zone than in the internal zone, the smaller the peripheral zone, the better. With this in mind, and leaving other considerations aside, simply looking at the differences between the IR and UV laser craters shown in Figs. 1 and 2 it is easy to realize the superiority of UV lasers.

When the diameter of the laser beam is reduced with a diaphragm to improve the spatial resolution, the internal zone is proportionally reduced, but the width of the peripheral zone remains almost the same. Fig. 3 shows a crater produced on the same material - - a K-feldspar crystal - - and with the same laser parameters as the crater in Fig. 2, except that the diaphragm aperture was reduced to the mini- mum. Despite the diameter of the internal zone being reduced by an order of magnitude of 5, the width of the peripheral halo is nearly the same. The width of the peripheral zone in the UV laser seems to be related to the quality of the optical mirror which, if not highly reflecting in the UV range, tends to produce some diffraction effects resulting in a wider defocused beam coaxial to the laser beam. Current investigation with new mirrors specifically designed to fit the requirements of the UV laser system indi- cates that this effect can be eliminated almost com- pletely.

F. Bea et al. / Chemical Geology 133 (1996) 145-156 149

3.1.2. Crater diameter and sensitivity The price to be paid for improving spatial resolu-

tion through reducing the diameter of the laser beam is a corresponding decrease in sensitivity. Fig. 4 shows the variation of detection limits (defined as the concentration equivalent to a signal twice that of the standard background deviation) for 93Nb as a function of the crater diameter in the Miass biotite (Nb ~ 202 ppm). It should be noted that decreasing the crater size from 1000 to 100 ixm increases the detection limit by one order of magnitude, from 1 to 10 ppb. However, decreasing the crater size from 100 to 10 I~m causes a two-order-of-magnitude in- crease in the detection limit, from 10 to 1000 ppb. The behaviour of other elements, from Li to U, is very much like that of Nb.

1 g=

.<

I L I I I 2 3 5

Ne:Xe:CIH pressure (bars)

Fig. 5. Sensitivity as a function of the Ne-Xe-CIH gas pressure in the SOPRA laser. Note the plateau between 3 and 3.8 bar.

3.1.3. Laser powet, sensitivity, and fractionation during ablation

Laser power in gas lasers depends on the gas pressure in the laser cavity. On the other hand, once the cavity is filled with gas at a given pressure, the laser power depends on the number of laser firings and time. With the system working continuously at a repetition rate of 2 Hz, we observed that the laser power remains constant for ~ 4 hr ( ~ 30,000 single laser shots) and then decreases quickly, it being necessary to purge the laser cavity and fill it with new gas to achieve flae original power. It is therefore necessary to study how the decline in laser power affects analytical performance.

1000 ,~ 100!

E

Z

1 10 100 1000

Crater diameter (It)

Fig. 4. Plot of detection limits for Nb in the Miass biotite against the crater internal diam~;ter. Detection limits were calculated as the concentration equiwtlent to a signal twice the plasma torch background standard deviation.

The first observation is that the laser power af- fects essentially crater depth, but has little influence on crater diameter. Fig. 5 shows the variation of sensitivity for a typical analyte as a function of decreasing gas pressure. Note the evident presence of a small plateau between 3 and 3.8 bar, where sensi- tivity stays almost constant. We therefore fixed the pressure of the Ne-Xe-C1H gas at 3.8 bar, which produced craters of 30-60 ixm in depth, depending on the mineral being ablated. Dark, aluminium-rich minerals generally result in shallower craters than white, aluminium-poor minerals (see below). Higher gas pressures give better sensitivity, but the risk of piercing the mineral being ablated and getting some contribution from below (either another mineral or the glass support) is too high.

If variations in laser power influence all analytes in the same way, the effect may be easily corrected using an internal standard (see discussion above). Otherwise, it will produce matrix effects extremely difficult to correct. To check whether these effects occur, we did several replicates of the Miass biotite with gas pressure from 6 to 1 bar at intervals of 0.5 bar. Binary plots of the apparent concentration of three elements with very different volatility proper- ties situated on the low, middle, and high side of the mass range, Li, Nb, and Pb respectively (Fig. 6), show excellent linear correlation, clearly indicating that there is no significant chemical or mass fraction- ation related to variations in laser power.

150 F. Bea et al. / Chemical Geology 133 (1996) 145-156

3.1.4. Laser focus Sample position with respect to the laser focus

strongly influences sensitivity and analytical perfor- mance. Best results are obtained when the distance between the sample surface and the focusing lens is equal to the focal length. As this distance increases, the laser reaches the sample progressively defocused, thus increasing the crater diameter and decreasing the crater depth, with significant los of sensitivity. As this distance decreases, the laser focuses within the sample, thus producing higher sensitivities due to the increase of the amount of material being ablated as a consequence of increasing crater depth. This may be a way to obtain more sensitivity, but care should be taken not to pierce thinner specimens.

° ~

0.32 0.36 0.40 0.44 0.48 0.52

carrier gas flow (I/min)

Fig. 7. Sensitivity of different elements as a function of carrier gas flow. Note how light elements give maximum sensitivity at higher flows.

3.2. Carrier gas flow

This parameter affects the way analyses are trans- ported to the plasma torch as well as the relative position of the plasma with respect to the mass

400

3o0 g ~

200

0~

0.6

-'~ 0.5

~ 0.4

~ 0.3

eL 0.2

,~ 0.1

O.C

A

I I

100 200

Nb (app.conc. in ppm) , / B •

loo

300

i

200 300 Nb (app.conc. in ppm)

Fig. 6. Li and Ph vs. Nb apparent concentration plots for the Miass biotite obtained by firing with different gas pressures, i.e. laser power. The excellent linear correlation between elements with such different masses indicates that chemical fractionation is negligible.

spectrometer interface, and has a dramatic influence on analytical performance. Fig. 7 shows how the maximum sensitivity for lighter elements occurs at higher gas flow than for heavier elements. A com- promise value of ~ 0.40 1/min gives good results in most cases, but the parameter may be optimized for each specific situation. Obviously, the carrier gas flow should be kept constant throughout an analyti- cal run by using a mass-flow controller.

3.3. Recommended procedure

The best analytical performance was obtained when the Sopra Ne-Xe-HCI gas laser was set at 3.8 bar gas pressure with a repetition rate of 2 Hz, and the laser beam diameter was set to produce craters at the end of ablation with an external diameter of ~ 80 ~m (internal diameter ~ 60 Ixm) and 40-60 I~m in depth, depending on the ablated mineral, with a carrier gas flow of 0.40 1/min. Data were acquired for 50 s, starting acquisition 10 s after ablation was initiated.

4. D i s c u s s i o n

4.1. Ablatability of minerals

The behaviour of minerals with respect to ablation varies greatly. To compare different minerals, it is

F. Bea et al. / Chemical Geology 133 (1996) 145-156 151

useful to define an "ablatability index", expressed as the ratio between the apparent concentration of a major element, usually silicon, as measured by LA - ICP-MS analysis and the concentration of the same element determined by electron microprobe. Obvi- ously, this is a relat!tve scale which depends on the standard used for calibrating the LA-ICP-MS sys- tem. Ablation values greater than one, equal to one, and lower than one mean respectively better, equal, and worse ablatability than the standard material. The ablatability indLexes relative to the NBS-612 glass for common minerals calculated from a large data set of ~ 1500 ~nalyses are shown in Table 1. It seems that carbonates ablate better than phosphates, which ablate better than silicates. Among silicates, the easiest species to ablate are those rich in alkali and alkaline-earth elements but poor in AI and Fe - Mg.

element contained in the mineral under analysis, which may be accurately analyzed by another method. The procedure used for this study was to analyze with the electron microprobe those points selected for LA-ICP-MS analyses before being fired, and then use some of these data for internal standardization. The best choice as internal standard is silicon, because it is abundant enough in most minerals, varies between narrow limits for a given mineral, and is reasonably free of interferences in ICP-MS analysis. However, since quartz from the plasma torch produces somewhat high-silicon blanks, it is not advisable to use this element as internal standard when the silica concentration in the mineral being analyzed is < 20 wt%.

4.3. Precision

4.2. Internal standard

It is clear from the above that throughout an analytical run the amount of sample ablated in each laser firing is not constant, but depends mainly on the nature of the rrdneral, sample surface features, laser power, etc. Therefore, at the present state-of- the-art in ICP-MS analysis, the only way to get quantitative analyses is by using an internal standard. Since it is not possible to spike solid minerals with any other substance, the only choice is to use an

Table 1 Ablatability index (A.I., referred common minerals

to the NBS-612 glass) of some

Mineral A.I. Mineral A.I.

Carbonates 2.1-3.6 K-feldspar 0.92 Spodumene 1.61 Plagioclase 0.90 Nepheline 1.46 Al-poor biotite 0.83 Apatite 1.34 Garnet 0.79 Aegirine 1.21 Al-rich biotite 0.76 Riebeckite 1.19 Cordierite 0.73 Glaucophane 1.17 Muscovite 0.60 Omphacite 1.14 Titanite 0.58 Monazite 1.07 Olivine 0.63 Clinopyroxene 1.05 Zircon 0.59 Paragonite 0.99 Orthopyroxene 0.52 Amphibole 0.98 Tourmaline 0.52 Epidote 0.97 AI2SiO 5 polymorphs 0.29

For calculating the precision of LA-ICP-MS analysis we analysed five points within a small area inside each of the four minerals selected for this purpose (see above); the resulting statistics are shown in Table 2. It should be taken into account that the variance reflected by these values represents the sum of the variance associated to LA-ICP-MS analysis plus the heterogeneity inherent to natural minerals due to zoning, existence of domains, micro-inclu- sions, etc. Although the values in Table 2 are not easy to interpret, it seems that those elements struc- turally bound to minerals typically have coefficients of variation (CV = 100. SD/mean) better than 4% and 10% for concentrations of > 10 and 1-10 ppm, respectively. For example, U and Th in the Khivine astrophyllite give CV of 6.4% and 17.5% at concen- tration levels of 8.6 and 0.28 ppm, respectively. Regarding REE, coefficients of variation for 11 and 46 ppm of La are 7.6% and 4.2% respectively. For 4 ppm Eu, CV are ~ 4-7%, and for 0.5 ppm Dy and Yb, CV are ,-, 10%. In the case of astrophyllite crystals, the CV for Ce, Nd, and Sm seems to be abnormally high, probably due to the effects of crystal heterogeneity. Fig. 8 gives a graphic appraisal of precision for REE by showing chondritic patterns for each replicate in the Gredos plagioclase and Khivine astrophyllite.

It should be kept in mind that these values were obtained for crater diameters of ~ 60 ixm. When the

152 F. Bea et al. / Chemical Geology 133 (1996) 145-156

crater size is reduced, the CV increases, not only because the number of ions per second decreases, but also due to the increasing contribution of ions from the external relative to the internal zone of craters. An approximate idea of the intensity of this effect may be grasped from the fact that at 10-txm craters, the coefficient of variation for Ga in the Gredos plagioclase is 62. 1% (CV - 1.2% for 60-t~m craters), whereas 60-1xm craters for Sm - - which produces approximately the same amount of ions/second as Ga at 10-1Lm craters - - gives a CV of 14.8%.

It is worth mentioning, however, that preliminary results of current research at Uberlingen aimed at improving the optics of the laser system, especially by eliminating the above-mentioned diffraction ef- fects in the semi-transparent mirror, indicate that detection limits for very small craters may be signifi- cantly reduced in the future.

4.4. Accuracy

The accuracy of the Excimer laser ablation (XLA) ICP-MS analyses was checked by comparison with solution nebulization (SN) ICP-MS analysis of: (1) chips of the same minerals which were used for replicate studies; and (2) concentrates of minerals obtained by magnetic and density separation.

In the first case, the homogeneity of the material was carefully controlled by optical examination be- fore choosing the chip to be analyzed. It was possi- ble to get enough clean and homogeneous material for SN analysis for the Miass biotite, Gredos plagio- clase, and Barraco cordierite, but not for Kola astro- phyllite, due to the abundance of apatite inclusions. Results from both methods are shown in Table 3. For Miass biotite, LA data match SN data almost per- fectly for all elements except Sc, Cu, Mo, Zr, Y, U,

Table 2

Precis ion o f L A - I C P - M S analysis ca lcula ted f rom five replicates o f some minerals

Biotite Cordieri te Plagioclase Astrophyll i te

m sd cv m sd cv m sd cv m sd cv

Li 145 4.22 2.9 814 8.72 1.1 5.52 4 .46 80.8 71.1 4.38

Rb 582 13.6 2.3 0.17 0 .16 93.6 0.73 0.73 100 7 .64 0.87

Cs 19.6 0 . 3 7 1.9 12.9 1.10 8.6 0.75 0.79 100 0.14 0.09

Sr 10.8 1.50 13.9 - - - 551 26.3 4.8 3.27 a 0.09

B a 1025 42.2 4.1 - - - 74.0 7.82 10.6 18.7 a 0 .64

V 428 7.31 1.7 . . . . . . 138 2.20

Co 20.3 0.48 2.4 9.01 0.77 8.6 - - - 6 .12 1.01

Ni 328 14.7 4.5 91.9 11.5 12.6 - - - 39.3 1.73

Zn 335 17.7 5.3 62.2 5.71 9.2 - - - 11.0 1.14

G a 72.3 2.35 3.3 54.3 3.73 6.9 39.9 0.49 1.2 57.3 2.09

Y . . . . . . 0.55 0.05 9.9 0.57 0.11

N b 202 5.67 2.8 . . . . . . 12.9 0 .64

Ta 4.19 0.11 2.6 . . . . . . 961 10.0

Tl 1.71 0 .24 14.1 . . . . . . 150 7.19

Z r . . . . . . . . . 235 9.11

U . . . . . . 0 .08 0,11 137 8.63 0.55

Th . . . . . . 0.03 0,03 100 0.28 0.05

L a . . . . . . 11.0 0.84 7.6 45.6 1.93

Ce . . . . . . 17.8 1.34 7.5 15.7 2.59

Nd . . . . . . 5.05 0 .36 7.2 1.06 0.28

S m . . . . . . 0.51 0.75 14.8 0.19 0 .04

Eu . . . . . . 3.88 0.24 6.3 4.71 0.21

Dy . . . . . . 0 .36 0.07 19.9 0 .16 0.01

Yb . . . . . . 0 .14 0.01 8.4 0.62 0.07

6.2

11.4

69.2

2.7

3.4

1.6

16.6

4.4

10.3

3.64

18.9

5.0

1.0

4.8

3.9

6.4

17.5

4.2

16.5

26.5

18.6

4 .4

9.7

11.3

Values are given for those elements which are s t ructural ly bound to the mineral lattices and do not reside within inclusions.

mean average, s tandard deviat ion and coeff ic ient o f variation, respectively.

a Results in %.

m, sd and cv

F. Bea et al. / Chemical Geology 133 (1996) 145-156 153

Th, and REE, whose values are systematically higher in SN analysis, due to the effect of txm-sized inclu- sions of magnetite, .sulfides, zircon, and monazite, revealed by SEM. For plagioclase, XLA and SN analyses are also very close, although SN values tend to be slightly higher, especially for Li and Rb. The worst is cordierite, whose SN analyses give much higher values in Li and Rb, Sr, Cr, Zn, Zr, Hf, Y, U, Th, and REE than XLA analysis, due to the effect of mica, sulfide, ilmenite, zircon, and apatite inclusions.

To illustrate the effect of inclusions, we also compared XLA data with SN analyses of amphibole concentrates from tyro Ural rocks: Tagil diorite, and Verkhisetsk granodiorite. The amount of accessory minerals in Tagil diorite is very low, and under SEM only REE-poor apalite and very rare zircon were detected. In contrasl~, Verkhisetsk granodiorite has abundant titanite, allanite, apatite, monazite, zircon, and huttonite, frequently as small grains included within amphibole (Bea, 1996). One would therefore expect the amphibole concentrate from Tagil diorite to be freer of accessory inclusions than the one from

Verkhisetsk granodiorite. Accordingly, XLA and SN analysis of Tagil amphiboles are in good agreement for most elements; only Cr, Zr, and REE are slightly higher in SN analysis. The picture for Verkhisetsk amphiboles is, however, quite different. Only ele- ments such as Li, Rb, Sr, Ba, Ga, and Sn agree closely, whereas Sc, V, Cr, Cu, Zn, Y, Pb, U, Th, and REE are much higher in mineral concentrates (see chondritic patterns in Fig. 8).

4.5. Isotope analysis

One exciting field of application for LA-ICP-MS may be "in situ" isotopic analysis of minerals, especially for U-Th-Pb systems where the only isobaric interferences may come from 2°4Hg, which was never detected in such minerals as zircon, mon- azite, sphene, etc. Feng et al. (1993) and Ludden et al. (1995) have applied an IR laser probe to lead geochronology in zircons and concluded that attain- able precision is better than + 1%. Hirata and Nes- bitt (1995), using a frequency quadrupled Nd:YAG

1000

100

10 ] (,~ 1.0

0.10

0.01 10000

lOOO

o m

lOO @

10

I A I i ; i i i i i i i i i i i

plagioclase

".<._ ~ , l l l l l l l l t l l t l l

I l l l l l l l l l l l l

C Ta~l amphibo~

LA ICP-M$ "

1 I i i i i i i i i i i i i q I

La Pr Pm Eu Tb Ho Tm Lu Ce Nd Sm Gd Dy Er Yb

1000

~ 100

~ 1.0

o.10

i i i i i i i i i i i i i i i

B astrophyllite

0 . 0 1 i i i i i i i i i i i i i i i ,0000} b' i','rih;;,,;k'.;.k/,go'l; 1000r'X"

i1001 ~ o ' ~ " ~ ~ _ ICP-MS

I

~ i i i t i i : I I I I i i i i ~ i

La Pr Pm Eu Tb Ho Tm Lu Ce Nd Sm Gd Dy Er Yb

Fig. 8. A and B. REE chandritic patterns of each replicate obtained on the Gredos plagioclase and the Miass astrophyllite. C and D. REE chondritic pattern of amphibole mineral concentrates analysed by solution nebulization, and LA analysis of the same minerals in thin section. Note that for the Tagil diorite, which is remarkably free of REE-rich accessories, the two patterns are very close, whereas for Verkhisetsk granodiorite, rich in REE-rich accessories, the profiles are completely different. This discrepancy illustrates the effect of accessories when analysing REE in mineral concentrates.

154 F. Bea et al. // Chemical Geology 133 (1996) 145-156

UV laser (266 nm), were able to obtain accurate results on zircons with crater sizes as small as 10-15 ~m. Unfortunately, our own experience in this field is clearly disappointing. First, the common lead cor- rection needs very accurate determinations of iso- topic ratios as low as 2 ° 4 p b / / 2 ° 6 p b ~,~ 0.0001-- 0.00001. Such precision is almost impossible to achieve in the ICP-MS system due to: (a) memory effects, especially if minerals with high common

lead contents were previously ablated; and (b) the low mass resolution, even using a magnetic sector instead of a quadrupole, related to the wide energy distribution of accelerated ions, caused by the ele- vated number of ion collisions within the sampled plasma. Second, radiogenic lead is the only element appreciably fractionated during ablation with a UV laser. Hirata and Nesbitt (1995) described a serious fractionation effect in the 2°6pb//238U ratio because

Table 3 Comparison between trace composition of some minerals analyzed by LA- (a colums) and SN- (b colums) ICP-MS analysis (see text)

Biotite Plagioclase Cordierite Amphibole 1 Amphibole 2

a b a b a b a b a b

Li 145 138 5.52 7.62 814 957 1.76 2.00 10.5 Rb 582 600 0.73 2.11 0.17 3.98 0.44 0.28 1.34 Cs 19.6 19.9 0.75 0.93 12.9 14.1 0.01 0.00 0.00 Be 0,54 1.30 b.d.l, b.d.l 8.87 9.17 0.30 0.40 0.62 Sr 10,8 18.5 551 572 1.31 15.5 402 379 31,8 Ba 1025 926 74.0 81.1 3.98 5.89 14.8 18.0 32,4 Sc 0,29 3.66 b.d.1 b.d.1 b.d.1 0.21 80.1 55.2 58.2 V 428 459 b.d.1 b.d.1 b.d.l 1.39 320 314 217 Cr 1,38 14.6 b.d.1 b.d.1 154 451 273 418 101 Co 20,3 19.3 b.d.l b.d.1 9.01 9.37 42.0 64.7 35,3 Ni 3.28 4.50 b.d.1 b.d.1 91.9 110 34.5 35.7 69,3 Cu 2.09 24.1 b.d.1 b.d.1 0.68 1.14 17.2 25.5 0.96 Zn 335 315 b.d.1 b.d.1 66.3 199 52.0 45.4 111 Ga 72.3 70.4 39.9 40.1 54.3 51.9 14.8 12.2 27,2 Y 0,24 0.82 0.55 0.75 0.04 0.21 9.71 31.8 17.3 Nb 202 212 b.d.1 b.d.1 0.01 0.13 0.22 0.82 5.37 Ta 4.19 3.35 b.d.l b.d.1 b.d.1 b.d.l 0.01 0.00 0.36 Zr 1.56 4.51 b.d.1 b.d.l b.d.l 29.8 5.29 45.0 8.91 Hf 0.15 0.18 b.d.1 b.d.l b.d.l 0.61 0.00 1.70 1.76 Mo b.d.1 0.57 b.d.l b.d.1 b.d.1 b.d.1 0.00 0.10 0.16 Sn 11.9 10.7 b.d.1 b.d.1 b.d.l b.d.l 0.00 0.00 2.47 Pb b.d.l 0.89 b.d.1 b.d.1 b.d.l b.d.l 0.05 7.00 0.24 U b.d.1 0.28 0.08 0.18 b.d.1 0.36 0.14 0.10 0.39 Th b.d.1 1.33 0.03 0.06 b.d.1 0.22 0.00 1.90 0.01 La 0.53 1.80 11.0 12.3 b.d.l 0.14 3.84 5.93 7.14 Ce 0.11 1.25 17.8 19.4 b.d.l 0.17 17.9 20.3 37.3 Pr 0.13 0.65 1.82 2.01 b.d.l 0.02 4.07 4.50 6.95 Nd 0.41 1.61 5.05 5.49 b.d.1 0.08 19.2 20.6 33.6 Sm b.d.l 0.30 0.51 0.61 b.d.1 0.02 5.67 5.94 10.3 Eu 0.04 0.17 3.88 4.03 b.d.1 0.00 1.69 1.85 1.56 Gd b.d.l 0.14 0.36 0.37 b.d.l 0.03 5.54 6.04 9.36 Tb b.d.1 0.02 0.05 0.08 b.d.1 0.01 0.91 1.04 1.29 Dy b.d.l 0.12 0.36 0.39 b.d.l 0.06 5.29 6.46 7.25 Ho b.d.l 0.03 0.06 0.08 b.d.l 0.02 1.05 1.25 1.41 Er b.d.l 0.09 0.15 0.17 b.d.l 0.05 2.83 3.25 3.59 Tm b.d.1 0.01 0.02 0.03 b.d.1 0.09 0.40 0.47 0.51 Yb b.d.l 0.05 0.14 0.15 b.d.1 0.04 2.59 2.83 3.16 Lu b.d.l 0.01 0.02 0.03 b.d.1 0.01 0.42 0.43 0.58

12.20 1.81 0.00 0.80

39.1 30.0

187 656 201

66.2 123 26.7

232 29.8 60.2

6.14 0.51

148 4.50 0.60 2.71

31.3 4.00

78.1 251 451

47.6 160 23.9

4.43 19.1 2.20

11.2 2.17 5.69 0.77 5.08 0.82

b.d.l. = below detection limit,

F. Bea et al. / Chemical Geology 133 (1996) 145-156 155

2 .o "d

1 O " ' - L 7. ~. ~2- ~ . . O (:~06pb/238U) x 10

Number of ablated spots

Fig. 9. Decrease of 2°Tpb/235U and 2°6pb/238U isotopic ratios in five replicate shots in ~L single crystal of titanite.

of non-constant focusing of the laser, but they were able to minimize it by constantly refocussing the beam during ablation. We did not find the same effect with the Exciimer laser. However, we noted that the 2°7pb//235U ratio strongly depends on whether the crystal being ablated had previously been fired with the laser. Fig. 9 shows the variation of 2°6pb/238U and :!°7pb/235U ratios in five differ- ent replicate analyses on a single unzoned crystal of titanite. It is obvious that both ratios decrease as the crystal underwent successive ablations, but the 2°7pb/235U decreases much faster than 2°6pb/238U.

This effect is surely related to the fact that, within the mineral lattice, radiogenic lead is placed in sites damaged by the radiation issued by the decaying U or Th isotope. Therefore, being loosely bonded to the structure, the heating of the crystal by successive laser impacts can "distil" radiogenic lead from a volume larger than the crater, so that after several ablation procedures the remaining crystal becomes selectively impoverished in radiogenic lead. Since this effect should be more intense for isotopes with a higher decay energy, 235U, it is logical to expect that its daughter, 2°7pb, is lost more easily, exactly as is shown in Fig. 9.

with small external zones from which "b ad " contri- butions may be expected. UV lasers may be focused to produce craters with an internal diameter of 10 Ixm. However, the contribution from the external zone makes it very difficult to get reliable results in minerals with diameters smaller than 30-40 Ixm. For craters 10 ixm in internal diameter, detection limits are ",, 1 ppm and coefficients of variation for 100 ppm analyte (obtained by replicate shots) are ~ 50%. For craters with internal diameters of 60 p,m, detec- tion limits are ~ 10 ppb and coefficients of variation for analyte concentrations of > 10 ppm are in the 2-4% range.

The ablatability of minerals depends on their composition, it being highest in Na-K-Ca-rich but A1-Fe-Mg-poor species. With the same laser condi- tions, easy ablatable minerals produce narrower but deeper craters than difficult-to-ablate minerals. The crater width is mostly controlled by the diameter of the laser beam, but the crater depth depends primar- ily on laser power, which in its turn depends on the pressure of the Ne-Xe-C1H mixture in the laser gas cavity and the number of laser firings after replenish- ing the cavity with new gas.

The sensitivity of the instrument critically de- pends on the carder gas-flow, and because analytes are affected differently, this parameter should be fixed at a compromise value and kept constant throughout the analytical session.

UV lasers do not produce measurable chemical fractionation during ablation other than for radio- genic lead, thus allowing a wide range of materials to be analyzed with a single standard. Solution nebu- lization and laser ablation analysis of the same mate- rials are in excellent agreement when the mineral to be analyzed is homogeneous and free of inclusions, which reveals that the accuracy of the LA system is high enough for most trace-element geochemical ap- plications.

5. Summary and conclusions

UV laser ablation inductively coupled plasma mass spectrometry i:~, at the present state-of-the-art, a powerful and reliable technique for analysing trace elements at sub-ppm levels in minerals, but can hardly be used for accurate isotopic work. Compared with IR lasers, UV lasers produce cleaner craters,

Acknowledgements

We are indebted to two anonymous referees for their constructive revisions, which greatly con- tributed to improve the original manuscript. The help of C. Laurin to improve the English is also gratefully

156 F. Bea et al. / Chemical Geology 133 (1996) 145-156

acknowledged. This research was financially sup- ported by the CICYT grants AMB-0535 and AMB- 1432.

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