High spatial resolution 40Ar39Ar investigations using an ultra-violet laser probe extraction technique

GeochimicaetCosmochimicaActa,Vol.58,No. 16,pp.3519-3525. 1994 Copyright 0 1994ElsevierScienceLtd Printedin the USA. Allrights reserved OOl6-7037/94 $6.00+ .OO

Pergamon

0016-7037(94)00146-4

SCIENTIFIC

COMMENT

High spatial resolution 40Ar/39Ar investigations using an ultra-violet laser probe extraction technique S. P. KELLEY, N. 0. ARNAUD, and S. P. TURNER Department of Earth Sciences, Open University, Milton Keynes, Buckinghamshire MK7 6AA. UK (Received

November

20,

1993; accepted

in revised.form

March

17,

1994)

Abstract-Ultra-violet laser extraction offers a significant new approach to 40Ar/39Ar dating. The technique surmounts two important drawbacks to the existing visible and near IR laser probe 40Ar-39Ar techniques; the difficulty in analysing minerals such as quartz, plagioclase, and K-feldspar which are transparent at these wavelengths, and the spatial resolution (-50 pm) limited by argon loss due to beam reflection/ refraction into the surrounding minerals. Three experiments demonstrate the advantages of UV laser ablation using a quadrupled Nd-YAG laser (h = 266 nm) in applications to geological problems. ( 1) A traverse across a biotite/quartz boundary demonstrates that UV laser ablation extracts negligible amounts of argon from beyond the visible laser pit, even in quartz. (2) Analyses of a large plagioclase phenocryst from the Paran volcanic field, Brazil, differentiated pristine and altered areas. Analysis of the feldspar grain margin caused no detectable argon loss from groundmass only 20 pm distant. (3) A depth profile in K-feldspar produced by repeatedly rastering the UV laser beam across an area achieves yet higher spatial resolution. Argon diffusion profiles around 20 pm deep, were measured with a spatial resolution of 2-3 grn in a sample of gem quality K-feldspar, thermally treated at 700°C and 2 kbar argon pressure. INTRODUCIION

described illustrating the technique; a traverse across a quartz/ biotite boundary, a high resolution analysis of a plagioclase phenocryst, and a depth profile into K-feldspar.

40Ar/39Ar laser microprobe analyses using focused visible and near IR lasers have achieved spatial resolutions of around 50-100 Frn in minerals that are strong absorbers in the visible/ near IR region, such as biotite ( ONSTOTT et al., 199 1 ), hornblende ( LEE et al., 1990; KELLEY and TURNER, 199 1 ), phlogopite (PHILLIPS and ONSTOTT, 1988), and phengite (SCAILLET et al., 1990). However, several important minerals including K-feldspar, plagioclase, and quartz are very poor absorbers at visible and infra-red laser wavelengths. In such minerals, beam reflection and refraction spreads power hundreds of microns from the focused laser spot, heating inclusions, imperfections, and adjacent grains, making results very difficult to interpret ( GIRARD and ONSTOTT, 1991; BURGESS et al., 1992). Ultra-violet laser ablation offers a range of new extraction techniques, using conventional gas handling and mass spectrometry by taking advantage of the fact that UV laser light is absorbed strongly by most silicates including plagioclase, K-feldspar, and quartz. Most UV laser sources such as quadrupled Nd-YAG and Eximer lasers are Q-switched pulsed lasers with repetition rates from a few hertz to a few kilohertz, delivering power to the sample surface in a controllable manner. Such systems enable the analyst to extract argon from complex shapes machined out of the samples without causing significant heating of the surrounding material, and depth profiles measured with a spatial resolution of 2-3 pm. The aim of the present study is to demonstrate some of the ways in which UV laser ablation may be applied to geological problems not accessible to other laser techniques. After a brief discussion of UV laser ablation, three examples will be

UV LASER ABLATION The advantages of UV laser ablation for geochemical analysis (CHENNERY and COOK, 1993; GEERTSEN et al., 1994; JENNER et al., 1994) and stable isotope analysis (FRANCHI et al., 1993) are only now being explored, though the effect of UV laser light upon organic material has been extensively studied (SRINIVASAN, 1986). The difference between laser/ solid interaction for UV lasers and visible/near IR lasers, reflects increasing photon energies which approach bond dissociation energies for UV wavelengths ( 1064 nm photon energy = 1.2 eV, 266 nm photon energy = 4.7 eV). Unlike near IR lasers, absorption of UV laser energy is not dependant on the transition element contents of the target (JACKSON et al., 1992). The precise mechanism or combination of mechanisms responsible for ablation of geologically relevant material is not well known, and material is probably ablated by a variable combination of photothermal and photochemical processes including thermal vaporisation, electronic excitation (excitation to repulsive or weakly bonded states), droplet expulsion, exfoliation, and perhaps others (see review in DARKE and TYSON, 1993). Sample ablation using a quadrupled Nd-YAG laser (X = 266 nm) is a slow process in comparison with sample melting by visible and near IR lasers. The average laser power during UV laser ablation is around 0.01 W (1 mJ/pulse at 10 Hz), over two orders of magnitude less than typical powers used during visible/near IR laser extraction. Sample ablation 3519

S. P. Kelley. N. 0. Amaud,

3520

and S. P. Turner

03

W

FIG. 1. SEM photographs of the effects of UV laser ablation. Scales are shown on the individual photographs. (a) Three joined, UV laser pits in plagioclase showing progressively greater ablation times. The lowermost pit passed entirely through the sample. a distance of 200 pm. (b) Close-up of la. showing overlap of melt blankets on the laser pit walls and spheres of molten glass on the surface. (c) A 200 pm square . 22-24 pm deep pit in K-feldspar, produced by rastering the laser beam. Eleven analyses were obtained by progressively deepening this pit. Ejccta in the pit and on the surface surrounding the pit appear rounded rather than angular, implying either melting or condensation from a plasma. (d) Parallel trenches ablated in a biotite grain. The ejecta reach up to 200 pm from the trench. The discharge of particles and plasma formed during laser ablation of successive trenches rework earlier deposits leading to the radial streaking effects. (e) Two traverses across a biotite/quartz boundary (drawn black line on the photograph. biotite is on the left). The top traverse, performed using the pulsed UV laser without changing power. exhibits little change in pit size from the biotite to the quartz, though the pits in quartz were associated with fracturing probably due to decrepitation of fluid inclusions. The lower traverse performed using a continuous IR laser without changing power. shows large melt pits in the biotite but no visible effects within the quartr.

3521

40Ar/39Ar dating with an ultra-violet laser takes place because the high power density of the laser pulses is typically absorbed in the top few microns of the sample ( 10 ns pulses were focused in this experiment to a laser spot around 10 pm diameter - 1.3 X 10” W mm2). The resulting laser pits in materials such as plagioclase show evidence of

melting (Fig. la,b) and individual particles are subspherical and have the appearance of molten blebs (Fig. 1b). Larger pits produced by rastering the laser beam are generally blanketed internally by ejecta (Fig. 1c). The progressive ablation or “machining” is insensitive to compositional changes or cracks in the sample, though such weaknesses can contribute to mechanical failure and exfoliation as seen in the removal of a flake adjacent to the crack in the upper left-hand corner of the upper laser pit in Fig. la. Ablated material may travel several hundred microns from the site of ablation (Fig. Id). Subsequent ablation causes exfoliation of earlier ejecta blankets, producing radial streaking effects (Fig. Id) though the amount of argon extracted equates closely with the measured size ofthe laser pits. Finally, the laser power required to ablate different minerals varies very little in comparison with the variation associated with melting by visible and near IR lasers. A traverse from biotite, typically the most absorbent (least transparent) mineral to visible and near IR lasers into quartz, typically the least absorbent (most transparent), without varying the laser power, shows little variation in pit size using the UV laser (Fig. 1e). However, note that some of the UV laser pits in quartz resulted in fractu~ng at the top of the pit (the pits are several tens of microns deep). A similar traverse using a continuous IR laser ( Nd-YAG, 8 W Continuous power for 10 ms) causes melting in the biotite but has no effect upon the quartz (Fig. le). EXPERIMENTAL

~~~THODS

A sample of the Strontian granite in northern Scotland and a sample of the Chapeco rhyolite from the Parana continental flood basalt field were prepared as thin slabs ( 100-150 pm thick) polished using 0.3 pm A1203. A large phenocryst in the rhyolite sample exhibits some growth zoning and bands of melt inclusions. Minor alteration was centred around cracks ~net~ting the phenoctyst. A mirror image of the phen~~st was polished for electron probe analysis for direct comparison with the laser argon results. The phenocryst was uncoloured and transparent apart from some slight brown staining along the cracks. As part of a laboratory study of argon diffusion in K-feldspar (in coIlaboration with D. S. Draper and M. R. Carroll, University of Bristol, UK), a single polished slice of clear, colourless, gem quality K-feldspar approximately 1 mm X 3 mm X 5 mm from Madagascar (supplied by I. Villa, University of Bern. Switzerland), was heated in an argon atmosphere of 2 kbars at 700°C for 14 days. The sample remained clear and the polished surface remained unaltered, though it broke into several pieces probably during the initial stages of compression. The data presented below represent a pilot study run on one ofthe broken surfaces before irradiation, in order to determine the feasibility of detecting short diffusion profiles. J values of 0.01095 c 0.~05 and 0.0105 t 0.~05 were calculated for the irradiated samples based on analysis of the hornblende standard Mmhbl (520.4 Ma, SAMSON and ALEXANDER,1987). Corrections were made for blanks, mass spectrometer and reactor interferences (cf. HAWKESWORTH et al., 1992 ). The samples were loaded into a standard laser port with a UVgrade fused-silica window which has a high transmission coefficient for light at 266 nm. An additional sapphire window, 1mm thick was placed between the laser port window and the samples to prevent ablated material coating the laser port window. A Spcctron Laser

Systems SL401 with two temperature and angle controlled KD*P crystals was used as the source of UV light. The laser produces pulsed light with a wavelength of 1064 nm (IR) which is frequency doubIed bv the first KD * P crvstal to a wavelength of 532 nm ( VISt and then d&bled again by the-second crystal to awavelength oi266 nrn (UV). The wavelengths were separated using a Pellin Brocca prism, and the resulting ultra-violet laser pulses have energies of up to 20 mJ per pulse, and a pulse length of 10 ns at a repetition rate of 10 Hz. The beam was directed using high reflectance, oxide coated mirrors, into a customised Leica Metallux 3 microscope. Inside the microscope, the beam was re-directed through an ultra-violet refracting objective lens, and focused at the sample surface. Spot sizes of less than 10 pm are achieved with the present system, and although this is not particularly small (CHENNERYand COOK, 1993), sample size is determined by the current analytical limits of the mass spectrometer, not by the laser spot size. APPLICATIONS OF UV LASER ABLATION TO QAr/39Ar ANALYSIS

Analysis of a Quartz/Biotite

Boundary

Analysis of a biotite/quartz boundary is particularly pertinent since biotite is the strongest absorber at the wavelengths of visible and near IR lasers, whereas quartz transmits a high proportion of the power. Attempts to perform laser 40Ar/ 39Ar profiles across such boundaries using visible or near IR lasers, produce results such as those in Fig. le. In fact the first noticeable effect of laser impact in the quartz is that light refracted within the quartz causes melting in the adjacent biotite, several hundred microns from the spot. A biotite/quartz boundary from the Strontian Granite, Scotland, was examined by machining narrow, parallel trenches, crossing the boundary. Initially a 500 pm long, 40 pm wide trench was ablated in the quartz between 40 and 80 pm from the biotite grain boundary, to the full depth of the section ( - 100 (urn). Parallel 20 pm wide trenches were then machined approaching and crossing the biotite grain boundary. The first and second analyses (Table 1, Fig. 2) reflect only quartz. Reactor induced 38Ar from chlorine (probably in fluid inclusions) and trace amounts of 39Ar (from potassium) yield 39Ar/3sAr ratios of 3.5 i: 2.8 and 1.1 + 0.4, respectively (Fig. 2a). The third analysis exhibits a large increase in the amount of 39Ar released (Table 1) and increase in the 39Ar/38Ar ratio to 47 f 16, reflecting the ablation of the biotite grain margin (observed as a change in the colour of the ablated material). The biotite did not lose significant argon prior to this point, despite the fact that each of the quartz analyses consisted of 300 s of continuous laser ablation. Further parallel trenches in the biotite yielded slowly rising 39Ar/38Ar ratios, possibly reflecting chlorine zonation in the biotite (ONSTOTT et al,,

Tabie 1. IFalch

4%d39Ax3%&?4r3%f3Qk (x loo)

W0

3QAr

(x 100)

~Asv

O-40 40-60 6CMiI

100.7 503.5 35.5

28.7 89.5 2.1

0.0 106.7 1.1

0.09 0.09 2.00

80-100 100125 12&140

26.4 25.6 24.9

2.1 2.0 1.7

0.4 0.1 0.1

5.29 13.25 15.20

100.7 188.1 32.3 25.4 25.2 24.6

MO-160

25.1

1.7

0.0

4.03

25.1

39Ar am0nn% x lo-lzaa3

STP.

i

Age@@

f

-AZ 52.1

1330 490

M.4 5.3 1.9 0.7 0.6 23

2010 430 540 80 440 30 436 11 427 10 435 35

S. P. Kelley. N. 0. Arnaud. and S. P. Turner

3522

Distance (microns) (b)

2500 2000

3

1500

0

40

80

120

160

Distance (microns) FIG. 2. Results of a UV laser traverse across a quartz/biotite boundary. (a) The K/Cl ratio, monitored by the 39Ar/38Ar ratio changed suddenly at the boundary as the biotite was encountered. There was no evidence of argon release from the biotite prior to that point. (b) Age traverse corresponding to (a). Note the ages in the biotite are within errors of the intrusion age for the granite, a slightly older age in the boundary sample reflects excess argon which becomes more obvious in the analyses of uncontaminated quartz.

199 1) (Fig. 2a). There was no detectable age variation in the biotite (mean age = 432 f 7 Ma), though the analysis which straddled the boundary yielded a slightly older age of 540 + 80 Ma reflecting excess argon (Fig. 2b), the large errors reflect low 39Ar since most of the sample extracted was quartz. The quartz analyses also yielded anomalously old ages ( 1330 + 490 Ma and 2010 + 430 Ma), reflecting the presence of excess argon. Analysis of a Plagioclase Phenocryst Separated plagioclase grains are now commonly stepheated using a defocused laser (e.g., RENNE et al., 1992), although as noted by PRINGLE et al. (1992), there are problems associated with the gaussian beam intensity profile and uneven absorptions of the laser energy. Plagioclase phenotrysts can also be analysed “in situ” with an IR laser, although the poor absorption of the plagioclase and relatively strong absorption of inclusions and groundmass, limits such analysis to phenocrysts several hundred microns across. All UV laser ablation analyses were extracted from a 3 mm2 zone, close to the margin of the phenocryst. Two types of site were selected for analysis, pristine transparent colourless plagioclase, and altered/cracked or cloudy plagioclase (Table 2). The shapes of the areas analysed varied from trenches 1 mm X 100 Nrn and 100 pm deep to areas 200 pm X 250 pm and about 200 pm deep, extracted by rastering the laser beam. The amounts of 39Ar released from the plagioclase equate closely with the visible size of the holes measured using the microscope XY stage, confirming the observations from SEM analysis that the plagioclase was melting during ablation. Two adjacent analyses separated by

less than 50 pm were undertaken, one at the margin of the plagioclase, the other in the adjacent groundmass (Table 2). No evidence was found for cross contamination of either “Ar or 39Ar. In addition to the plagioclase, areas of groundmass were analysed using both the UV laser and IR laser, for comparison (Table 2). IR analyses were performed using 2-4 100 pm diameter pits, UV analyses were extracted by rastering the beam over a 250 pm X 250 pm area to ablate pits around 100 pm deep. The groundmass points extracted by both IR and UV laser systems fall on the same isochron (Fig. 3c), though the IR points generally contained larger atmospheric components and show greater variation than the UV analyses. This may be due to the much larger power input from the IR laser. The isochron yields an age of 13 1.8 + 1.4 Ma with an atmospheric intercept of 296 + 7 and an MSWD of 0.5 (removing one point which falls slightly off the line). Thus the argon isotope ratios of samples extracted using UV ablation do not differ from those extracted by IR laser melting. Five analyses of transparent colourless plagioclase using the UV laser, yielded a narrow range of low atmospheric contents (9 to 28%) and consequently fell close to the 39Ar/ 40Ar axis of the correlation plot (Fig. 3a). Analyses including altered and cracked plagioclase yielded a wide range of atmospheric argon contents (35 to 78%) (Fig. 3b). In addition to the dichotomy in the atmospheric contents, the two types of sites yielded differing Ca/ K ratios (Table 2). The pristine plagioclase yielded a narrow Ca/K range from 8.9 to 10.2 (the electron microprobe data for the same outer zones of the phenocryst indicated a Ca/K ratio variation from 7.6 to 10.6). Conversely, the Ca/K ratio of the altered plagioclase varied from 3.0 to 8.6. Clearly some of the areas sampled were not pure plagioclase but contained a higher potassium phase, possibly K-feldspar or clays which are common products found in plagioclase during basalt weathering ( BANF’IELD et al., 1991). The pristine plagioclase data clustered close to the 39Ar/ 40Ar axis and did not yield an isochron (Fig. 3a). However, the weighted mean age of 13 1.7 + 5.4 Ma agreed closely with a least squares fit of the groundmass data. Both ages are identical within errors to ages from other samples in the south east of the ParanP CFB outcrop ( HAWKESWORTH et al., 1992;

clear pl 1 clear p12 clear p13* clear d 4 clear p1 5 altp11 altp12 altp13 alto14 &Pl5 ir gndmss iI gndmss ir gndmss ir gndmss iI gndmss U” gndmss “V

g”dmss*

YYgndmss uv nndmss

8.43 8.40 8.05 7.68 7.36 29.37 18.12 12.68 12.50 10.02 16.06 12.13 9.35 9.17 9.13 0.94 8.90 8.86 8.57

(x loo) 0.22 0.19 0.19 0.23 0.29 0.10 0.12 0.18 0.13 0.17 0.15 0.16 0.19 0.17 0.18 0.16 0.17 0.18 0.17

5.57 4.85 5.21 4.98 4.83 4.68 3.24 3.46 2.11 1.62 0.80 0.23 0.23 0.20 0.21 0.20 0.18 0.14 0.13

b loo) 0.79 0.40 0.52 0.20 0.22 7.80 3.71 2.06 1.71 1.18 3.09 1.62 0.79 0.76 0.75 0.69 0.72 0.64 0.63

8.39 14.18 14.32 4.41 7.73 10.49 10.00 6.83 11.86 19.10 91.85 25.23 86.16 94.46 198.25 49.06 40.44 30.66 54.83

10.22 8.91 9.55 9.14 0.86 8.59 5.95 6.35 3.07 2.98 1.46 0.42 0.42 0.37 0.39 0.37 0.33 0.25 0.24

39~r amounts x 1~lZnn3 STP. *~dyses tithin>~~ muo,at the plagioch the grmJ@dmass.

39Ar 6.11 7.23 6.52 7.08 6.12 6.31 7.15 6.58 7.45 6.52 6.92 7.33 7.00 6.91 6.93 6.89 6.76 6.96 6.72 ti

(h4i) 117 137 124 135 128 121 136 126 141 124 132 139 133 132 132 131 129 133 128

and within

13 8 12 28 14 30 17 12 7 4 3 8 2 2 1 3 5 4 2

40Ar/39Ar dating with an ultra-violet

3523

laser

(a)

0

20

15

10

5

25

FIG. 4. Argon concentration (ppm) vs. depth (microns) profile into K-feldspar. Note the anomalously high argon concentration in the first analysis, due either to surface adsorption or open defects at the surface. Diffusive gain profiles for atmospheric argon are similar in depth to diffusive loss profiles for radiogenic argon.

0

0.04

0.08

0.12

0.16

0.12

0.16

3gArlrn!

0,0

0.04

0.08

3?4d %f FIG. 3. Correlation diagrams of UV and IR analyses ofa plagioclase phenocryst. (a) Clear plagioclase yields closely bunched analyses which in radiogenic argon which fall close to the 39Ar/40Ar axis. Although the data do not yield an isochron, assuming an atmospheric endmember (asterisk symbol) yields an age of I3 1.7 f 5.4 Ma. (b) Altered plagioclase yields a range of much higher atmospheric contents. (c) Analyses of groundmass using both UV (X = 266 nm) and IR (X = 1064 nm) lasers. Both sets of analyses fall on the same isochron but the IR analyses spread to higher atmospheric contents.

RENNE

TabLe 3.

The large errors assigned to ages from the plagioclase data are due to the small sample size, (typically around 10 pg) used in order to ensure pure transparent plagioclase, and the low potassium content of the plagioclase. The altered plagioclasc yielded an isochron age of 123.2 f 6.7 Ma with a 40Ar/36Ar intercept of 3 12 + 21 and an MSWD of 1.2 (Fig. 3b), significantly younger than the groundmass age at the 1B level. Analysis

rastering the beam over areas up to 200 pm square, sufficient argon can be obtained for a precise isotopic measurement. Repeatedly rastering the beam over the same area results in a square depression with a flat base (Fig. lc) and a concentration/depth profile such as that in Fig. 4. Beam rastering was achieved in the present experiment using a programmable motorised XY stage, and provides a new tool to study argon diffusion profiles. The high transmission characteristics of Kfeldspar preclude similar techniques using visible and near IR lasers. The feldspar sample, a clear gem quality K-feldspar from Madagascar heated for 14 days at 700°C and 2 kbar using argon as the pressure medium. No visible breakdown or alteration were seen and the argon concentration gradient in the K-feldspar was assumed to be a diffusion profile. The UV laser was rastered over one of the K-feldspar surfaces exposed to the argon atmosphere during the experiment, producing a 200 pm X 200 hrn depression (Fig. 1c). The incident power was less than 1 mJ per pulse and the laser spot moved over the sample surface at 20 pm per s. The process was repeated eleven times over the same area, measuring the gas released after each pass. The concentrations presented here (Table 3) were obtained using the final depth of the laser pit measured using stereoscopic SEM images since the sample was unirradiated. The bottom of the pit (Fig. lc) varied between 22

et al.,

1992).

of an Argon Diffusion

Profile in K-Feldspar

Laser ablation has been used to produce concentration/ depth profiles in real time for trace elements in garnet with a spatial resolution approaching 1 pm (JACKSON et al., 1992). Laser ablation pits lo-20 pm in diameter and 1 rrn deep produce insufficient argon for isotopic measurement, but by

Analvsis N6 1

2 3 4 5 6 7 8 9 10 11

4oAr

zv

(00 1 10.5:6 3.117 2.234 1.573 1.250 1.036 0.819 0.766 0.599 0.536 0.500

36‘4r (RP

0.023 0.008 0.009 0.008 0.007 0.006 0.006 0.008 0.006 0.006 0.007

**

1

0.03:2 0.0094 0.0060 0.0037 0.0023 0.0013 0.0006 0.0006 0.0001 0.0001 0.0002

O.OOB7 0.0003 0.0002 0.0002 0.0002 0.0002 0.0002 0.0801 O.OtlOl 0.0001 0.0002

* Errors shown do not include those introduced by SBM measurement of the pit depth ( f 10%).

S. P. Kelley, N. 0. Amaud. and S. P. Turner

3524

and 24 pm below the original surface after eleven analyses. Absolute concentrations were calculated using the total volume of the pit and analyses of depth profiles in unheated samples. The first analysis including the surface exposed to argon during the heating run exhibited anomalously high atmospheric high argon concentrations, corresponding to around 10 ppm (Fig. 4)) similar concentrations were measured in the surface analyses of other depth profiles from the same surface. This may be caused by defects open to the surface or may simply be a surface absorption effect. The second and subsequent analyses decreased smoothly from 3.1 ppm to around 0.5 ppm, 20 pm below the surface (Table 3). The shape of the curve corresponds closely with error function curves predicted for one dimensional diffusion (CRANK, 1975). The concentration of radiogenic argon in each analysis was calculated by assuming that the argon introduced during heating had an atmospheric ratio (a fact validated by analyses of similarly treated modern glass). The concentration of radiogenic 40Ar increased from 0.15 ppm in the first analysis, to a value of around 0.5 ppm 20 pm below the surface (Table 3, Fig. 4). The value of 0.5 ppm corresponds well with the known potassium content and age of the sample (440 f 4 Ma, 20 errors). Other rastered laser pits of varying sizes machined into the same K-feldspar surface confirmed the depth of the diffusion profiles. The heating experiment thus resulted in transport of argon across the mineral/gas interface, since both 40Ar and 36Ar exhibited similar concentration profiles, and radiogenic 40Ar was simultaneously lost from the mineral. In fact, dynamic argon exchange at the mineral/gas interface produced an isotope exchange effect reflected in the apparent loss of radiogenic 40Ar. The point to note is that both transport and isotope exchange took place over similar distance scales, implying similar activation energies for loss of radiogenic argon and gain of atmospheric argon. The results indicate argon diffusion at comparable, though significantly slower rates than predicted by FOLL\ND ( 1974). The difference in diffusion rates may reflect the unusually high iron content of the Madagascar feldspar and more detailed work on this aspect of the results will be attempted on irradiated aliquots of this sample. CONCLUSIONS

1) Ultra-violet

laser ablation using a pulsed Nd-YAG laser with a wavelength of 266 nm for 40Ar/39Ar analysis is effective in extracting samples from not only those minerals associated with “in situ” laser 40Ar/39Ar analysis but also transparent minerals such as plagioclase, quartz, and K-feldspar.

2) The rate of ablation of different minerals is similar and relatively insensitive to cracks and imperfections, allowing crucial investigations of mineral boundaries. In addition, the lack of heating beyond the visible pit walls and low average power of the laser allows long irradiance times and pristine crystal lattice can be machined selectively out of less pristine areas even in clear minerals such as plagioclase. 3) Depth profiles can be measured using a beam raster technique with a spatial resolution of better than 3 pm. A

depth profile of K-feldspar heated in an argon atmosphere resulted in diffusion profiles 15-20 pm deep. Apparently simultaneous gain of introduced atmospheric argon and loss of resident radiogenic argon were measured over similar distance scales, though the process is actually a combination of isotope exchange and bulk transport across the mineral/gas interface. Acknowledgments-The authors would like to thank T. C. Onstott, R. J. Fleck, and T. K. Kyser for their constructive and helpful comments on this manuscript and A. E. Fallick and G. B. Dalrymple for commenting on an earlier version of the manuscript. During this work, SPT was funded under NERC grant No. GR3/8339 and NOA under EC grant No. GT920643. The Ar-Ar laboratory and UV laser were funded by the Open University. SEM work was undertaken with the help of Naomi Williams at the Open University. Editorial handling: G. Faure REFERENCES BANFIELDJ. F.,

JONESB. F., and VEBLEN D. R. (1991) An AEMTEM study of weathering and diagenesis, Albert Lake, Oregon: I. Weathering reactions in the volcanics. Geochim. Cosmochim. Acta 55,278 l-2793. BURGESSR., KELLEYS. P., PARSONSI., WALKER F. D. L., and WORDENR. H. ( 1992) 40Ar-39Aranalysis of perthite microtextures and fluid inclusions in alkali feldspars from the Klokken syenite, South Greenland. Earlh Planet. Sci. Left. 109, 147-167. CHENNERYS. and COOKJ. M. ( 1993) Determination of rare earth elements in single mineral grains by laser ablation microprobeInductively coupled plasma mass spectrometry-Preliminary study. J. Anal. Atmos. Spectrom. 8, 299-303. CRANKJ. ( 1975) The Mathematics of Dijiision. Clarendon Press. DARKE S. A. and TYSON J. F. ( 1993) Interaction of laser radiation with solid materials and its significance to Analytical Spectrometry. J. Anal. Atmos. Spectrom. 8, 145-299. FOLAND K. A. ( 1974) 40Ardiffusion in homogeneous orthoclase and an interpretation of Ar diffusion in K-feldspars. Geochim. Cosmochim. Acta 38, 15 I - 166. FRANCHII. A., KELLEYS. P., PILLINGERC. T., and MILLEDGEJ. H. ( 1993) Analysis of carbon and nitrogen in specific locations in diamond using a focussed laser heating technique. Program with ahslracts of the Diamond Conference I I-14th July, Bristol Vniversity, UK, 14.1-14.3. GEERTSENC., BRIANDA., CHARTIERF., LACOURJ-L., MAUCHIEN P., SJOSTROMS., and MERMETJ-M. ( 1994) Comparison between infrared and ultraviolet laser ablation at atmospheric pressureImplications for solid sampling inductively coupled plasma spectrometry. J. Anal. Atmos. Spectrom. 9, 17-22. GIRARD J-P. and ONSTO~TT. C. ( 1991) Application of 40Ar/‘9Ar laser-probe and step-heating techniques to the dating of diagenetic K-feldspar overgrowths. Geochim. Cosmochim. Acfa 55, 37773793. HAWKESWORTHC. J., GALLAGHERK., KELLEYS., MANTOVANI M., PEATE D. W., REGELOUSM., and ROGERSN. W. ( 1992) Paranl magmatism and the opening of the South Atlantic. In Magmatism and the Causes of Conlinental Breakup (ed. B. C. ST~REY et al.); Geol. Sot. Lo&. Spec. Publ. 68, 221-240. JACKSON S. E., LONGERICHH. P.. DUNNINGG. R.. and FRYER B. J. ( 1992) The application of laser-ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) to in situ traceelement determinations in minerals. Canadian Mineral. 30, 10491064. JENNERJ. A., FOLEYS. F., JACKSONS. E., GREEN T. H., FRYER B. J., and LONGERICHH. P. (1994) Determination of partition coefficients for trace elements in high pressure-temperature experimental run products by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS). Geochim. Cosmochim. Acta 57, 5099-5 103.

40Ar/39Ar dating with an ultra-violet laser S. P. and TURNER G. ( 199 I ) Laser probe 40Ar-39Armeasurements of loss profiles within individual hornblende grains from the Giants Range Granite, northern Minnesota, USA. Earth Planet.

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