Quantification of Single Fluid Inclusions by Combining Synchrotron

Anal. Chem. 2004, 76, 3988-3994

Quantification of Single Fluid Inclusions by Combining Synchrotron Radiation-Induced µ-X-ray Fluorescence and Transmission J. Cauzid,*,†,‡ P. Philippot,† A. Somogyi,‡ A. Simionovici,‡,§ and P. Bleuet‡

Laboratoire de Ge´ osciences Marines, Institut de Physique du Globe de Paris, Case 89, 4 place Jussieu, 75252 Paris Cedex 05, France, European Synchrotron Radiation Facility, 6 rue Jules Horowitz, BP 220, F-38043 Grenoble Cedex, France, and EÄ cole Normale Supe´ rieure, 46 alle´ e d’Italie, 69007 Lyon, France

Fluid inclusions represent the only direct samples of ancient fluids in many crustal rocks; precise knowledge of their chemical composition provides crucial information to model paleofluid-rock interactions and hydrothermal transport processes. Owing to its nondestructive character, micrometer-scale spatial resolution, and high sensitivity, synchrotron radiation-induced µ-X-ray fluorescence has received great interest for the in situ multielement analysis of individual fluid inclusions. Major uncertainties associated with the quantitative analysis of single fluid inclusions arise from the inclusion depth and the volume of fluid sampled by the incident beam. While the depth can be extracted directly from the fluorescence spectrum, its volume remains a major source of uncertainty. The present study performed on natural and synthetic inclusions shows that the inclusion thickness can be accurately evaluated from transmission line scans. Experimental data matched numerical simulations based on an elliptical inclusion geometry. However, for one nonelliptical inclusion, the experimental data were confirmed using a computed absorption tomography reconstruction. Good agreement between the imaging and scanning techniques implies that the latter provides reliable fluid thickness values independent of the shape of the inclusion. Taking into consideration the incident angle, the incident beam energy, the inclusion fluid salinity, and the transmission measurement stability resulted in errors of 0.3-2 µm on calculated fluid inclusion thicknesses. Fluids can be transported for large distances in the lithosphere and can successively equilibrate with various types of magmatic, metamorphic, or sedimentary rocks at different pressure and temperature conditions. Material that is leached from one location and deposited elsewhere, at different pressures and temperatures, might result, for example, in an economic ore deposit or a reservoir bearing oil. Reconstructing past fluid-rock interactions in crustal rocks is therefore central for understanding the geological history of a specific zone. Fluid inclusions are consid* Corresponding author. Fax: + 33 4 76 88 99 69. E-mail: cauzid@ ipgp.jussieu.fr. † Institut de Physique du Globe de Paris. ‡ European Synchrotron Radiation Facility. § EÄ cole Normale Supe´rieure.

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ered to be representative of the fluid present during either the growth of minerals or the later healing of fluid-filled cracks, and as such, they represent the only direct samples of ancient fluids in many crustal rocks and provide crucial information for studies of fluid-rock interactions.1 Many natural minerals contain several generations of fluid inclusions, each one being representative of a distinct hydrothermal event.2 Therefore, there is general recognition of the uncertainties inherent in bulk analytical approaches such as crush-leach, which results in homogenizing several fluid populations. For more than one decade, synchrotron radiation X-ray fluorescence analysis (SR-XRF) has been recognized as a powerful technique for the analysis of single fluid inclusions.3-16 Its growing importance is mainly due to its high sensitivity, nondestructive character, high spatial resolution, and in situ multielement analytical capability. (1) Goldstein, H. R.; James Reynolds, J. T. T. Systematics of Fluid Inclusions in Diagenetic Minerals, SPEM Short Course No 31, SPEM, 1994. (2) Roedder, E. Rev. Mineral. 1984, 12, 644p. (3) Frantz, J. D.; Mao, H. K.; Zhang, Y. G.; Wu, Y.; Thompson, A. C.; Underwood, J. H.; Giauque, R. D.; Jones, K. W.; Rivers, M. L. Chem. Geol. 1988, 69, 235-244. (4) Mavrogenes, J. A.; Bodnar, R. J.; Anderson, A. J.; Bajt, S.; Sutton, S. R.; Rivers, M. L. Geochim. Cosmochim. Acta 1995, 59, 3987-3995. (5) Philippot, P.; Me´nez, B.; Chevallier, P.; Gibert, F.; Legrand, F.; Populus, P. Chem. Geol. 1998, 144, 121-136. (6) Me´nez, B. Les microsondes photon et proton applique´es a` l’analyse ponctuelle d’inclusions fluides: un outil pour reconstituer l’e´volution des pale´osyste`mes hydrothermaux. Ph.D. Thesis, ParisVII University, 1999; p 208. (7) Philippot, P.; Me´nez, B.; Simionovici, A.; Chabiron, A.; Cuney, M.; Snigirev, A.; Snigireva, I. Terra Nova 2000, 12, 84-89. (8) Philippot, P.; Me´nez, B.; Drakopoulos, M.; Simionovici, A.; Snigirev, A.; Snigireva, I. Chem. Geol. 2001, 173, 151-158. (9) Me´nez, B.; Philippot, P.; Bonnin-Mosbah, M.; Simionovici, A.; Gibert, F. Geochim. Cosmochim. Acta 2002, 66, 561-576. (10) Cline, J. S.; Vanko, D. A. Magmatically generated saline brines related to molybdenum at Questa, New Mexico, USA. In Magmas, Fluids and Ore Deposits; Thompson, J. H. F., Ed.; Short Course Series 23; Mineralogical Association of Canada, 1995; pp 153-174. (11) Vanko, D.; Bonnin-Mosbah, M.; Philippot, P.; Roedder, E.; Sutton, S. Chem. Geol. 2001, 173, 227-238. (12) Rankin, A. H.; Ramsey, M. H.; Coles, B.; Van Langevelde, F.; Thomas, C. R. Geochim. Cosmochim. Acta 1992, 56, 67-69. (13) Vanko, D. A.; Sutton, S. R.; Rivers, M. L.; Bodnar, R. J. Chem. Geol. 1993, 109, 125-134. (14) Bodnar, R. J. Fluid inclusion evidence for a magmatic source for metals in porphyry copper deposits. In Magmas, Fluids and Ore Deposits, Thompson, J. H. F., Ed.; Short Course Series 23; Mineralogical Association of Canada, 1995; pp 139-152. 10.1021/ac035533f CCC: $27.50

© 2004 American Chemical Society Published on Web 06/11/2004

Figure 1. Schematic view of optics hutch and experimental hutch 1 at ID22, ESRF.

The quantitative analysis of individual fluid inclusions is associated with two main uncertainties, the fluid inclusion depth (d) and the volume of analyzed fluid. Progress toward quantitative determination of the fluid inclusion depth has utilized the KR/ Kβ ratio of relatively light elements such as Ca.5 This in turn allowed correcting self-absorption in the host mineral directly from the fluorescence X-ray spectrum and optimizing calibration procedures using different kinds of external standards.9 Considering a quasi-parallel X-ray beam, the problem of estimating the volume of fluid sampled by the incident beam can be linked with good approximation to the thickness of the measured fluid layer (h). Until now, this value has been estimated optically, assuming that the thickness of the fluid layer is equal to the maximum distance between the two inclusion walls measured perpendicular to the sample surface. However, considering that most fluid inclusions display irregular geometries and that SR-XRF measurements are performed at 30°-60° from the sample surface, the optical method is imprecise. This in turn has critical influence on the calculated elemental concentrations. The aim of this paper is to show how precise information on the fluid inclusion thickness and hence volume of analyzed fluid can be obtained using transmission measurements. MATERIALS AND METHODS Sample Preparation. Three types of fluid inclusions were analyzed in this study. The first type is synthetic aqueous fluid inclusions (5-60 µm) containing 2000 ppm Ca and 2000 ppm Zn (sample c6sol2). The second is natural aqueous brine from the Brusson gold deposit, western Italian Alps (sample LD886; courtesy of L. Diamond and B. Yardley). The sample contains a unique population of fluid inclusions of large dimension (100 µm) that has been analyzed for its bulk chemistry17 and for testing the KR/Kβ ratio technique.8 The third sample contains different fluid inclusion populations of small dimension (10-20 µm) that are typical of the size encountered in average fluid inclusionbearing crustal rocks. The sample studied is from the North Pole (15) Vanko, D. A.; Mavrogenes, J. A. Synchrotron-source X-ray fluorescence microprobe: Analysis of fluid inclusions. In Applications of Microanalytical Techniques to Understanding Mineralization Processes; McKibben, M. A., Shanks, W. C., Ridley, W. I., Eds.; Society of Ecomonic Geologists: Littleton, CO, 1998; Vol. 6, pp 251-263. (16) Bu ¨ hn, B.; Rankin, A. H. Geochim. Cosmochim. Acta 1999, 63, 3781-3797. (17) Yardley, B. W. D.; Banks, D. A.; Bottrell, S. H.; Diamond, L. W. Mineral. Mag. 1993, 57, 407-422.

hydrothermal site in the Pilbara, Western Australia (Pi02-39k). All samples are doubly polished thin sections of quartz of different thicknesses (C6sol2 290 µm, LD886 160 µm, and Pi02-39k 60 µm). Prior to each experiment, samples were washed in acetone in an ultrasonic bath to remove dust, resin, or polishing compounds. Samples were then glued onto the top of pure silica capillaries before mounting them onto goniometer heads. X-ray Fluorescence and Transmission. The microfluorescence measurements were performed at the µ-fluorescence, µ-imaging, and µ-diffraction beamline (ID22, Insertion Device number 22) of the European Synchrotron Radiation Facility (ESRF). Figure 1 shows a schematic view of the beamline. A double-crystal fixed-exit monochromator was used to create the monochromatic X-ray beam of 12- (C6sol2 and LD886) or 14-keV (Pi02-39k) energy. The beam was focused with a KB mirror to a 2 × 4 µm2 spot. The spot size and focal plane was established by the so-called knife-edge technique using a thin gold cross. The intensity of the focused beam was 2.5 × 1011 photons/s. An optical microscope of 70 magnification was used to align the fluid inclusion into the focused beam. The sample position was controlled with a four-axis (x, y, z, R) sample stage with an accuracy of 0.1 µm (y, z). The angle of incidence, R, was 58° during the scanning and spot XRF measurements. The X-ray fluorescence spectra were recorded with a Gresham Si(Li) detector with 140eV resolution/Mn KR placed at 90° to the incident beam in the polarization plane to reduce scattering. The sample and the semiconductor detector were enclosed in a helium chamber having 80-µm Kapton windows in the path of the X-ray beam and 25-µm Mylar windows for visible light. The incident and transmitted intensities were monitored by two silicon PIN diodes positioned before and after the helium chamber. X-ray fluorescence spectra were collected at the fluid inclusion for 600 s real time. Transmission scanning measurements were performed with 5-10 s per point acquisition time and 2-10-µm step sizes. Computed Tomography. The tomography acquisitions were made in the second hutch of ID22 using a monochromatic full beam of 1.5 mm fwhm. The axis of rotation was chosen to be horizontal. The sample was rotated perpendicular to the beam (Figure 2) while taking an absorption image at each rotation angle using a high-speed FReLoN (Fast Read Low Noise, ESRF) CCD camera.18 Each image consists of up to 2048 × 2048 pixels with 0.7 × 0.7 µm2 effective pixel size. To obtain a spatial resolution of Analytical Chemistry, Vol. 76, No. 14, July 15, 2004

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Figure 2. Schematic view of experimental hutch 2 at ID22, ESRF.

∼1 µm in the tomographic reconstruction, 625 projections were taken over 180°. The tomographic reconstruction was performed with the standard filtered back-projection method.19 THEORETICAL BASIS The relationship (as detailed elsewhere6) between the intensity of the measured characteristic X-ray line (i) of element Z, N(Ei,Z), and its concentration in the fluid inclusion, C, if secondary and higher order effects are neglected, is

e-((µm /sinR)+(µm /sinβ))dFm Ω N(Ei,Z) ) I(E0) (Ei)CσZEi0 ‚ 4π µEf i µEf 0 + sin R sin R sin β E0

(

Ei

)

(1 - e-((µf

E0/sinR)+(µ Ei/sinβ)hF ) f f

) (1)

where E0 is the energy of the incoming beam, Ei is the energy of the detected characteristic X-ray line (i) of element Z, I(E0) is the intensity of the incoming beam (photons/s), Ω/4π is the solid angle of detection (sr), (Ei) is the detection efficiency, σZEi0 is the fluorescence cross section of X-ray line (i) of element Zi at the energy of the incoming beam, µEf 0, µEf i are the absorption coefficients of the inclusion fluid f at energies E0 and Ei, respectively, µEm0, µEmi are the absorption coefficients of the host mineral m at energies E0 and Ei, respectively, R is the incident angle to sample surface, β is the detection angle to sample surface, Ff, Fm are the fluid and mineral mass densities, respectively, d is the depth of the inclusion inside the mineral, and h is the thickness of fluid layer analyzed normal to the sample surface. During the calculation, the analyzed microscopic matrix and fluid volumes are assumed to be homogeneous. Figure 3 shows a typical geometry illustrating how the path length of the fluorescent beam through the host quartz (fluid inclusion depth, d) and fluid inclusion (fluid inclusion thickness, h) can be markedly different from the fluid inclusion depth (dopt) and fluid inclusion thickness (hopt) determined using an optical microscope. Using the dopt and hopt values will result in overestimating the analyzed volume of the inclusion fluid, hence in underestimating the elemental concentrations. The optical method is thus introducing a systematic error in the calculation, even if its statistical error is satisfactory ((1 µm). To obtain precise (18) Bravin, A.; Fiedler, S.; Coan, P.; Labiche, J.-C.; Ponchut, C.; Peterzol, A.; Thomlinson, W. Nucl. Instrum. Methods Phys. Res., Sect. A 2003, 510, 3540. (19) Natterer, F.; Wubbeling, F. Mathematical Methods in Image Reconstruction; Society of Industrial and Applied Mathematics; Philadelphia, 2001.

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Figure 3. Typical cross section through a nonspherical fluid inclusion. The optical values of dopt (optical depth of the inclusion) and hopt (optical thickness of the inclusion) are based on the maximum distance between two points of the inclusion, measured perpendicular to the sample front surface. The values needed for accurate concentration calculations are d and h, which take into account the analyzed volume of the fluid inclusion.

quantitative results, a knowledge of the d and h values is required. As discussed above, the KR/Kβ ratio technique provides an effective means of estimating the fluid inclusion depth.5 With regard to the fluid inclusion thickness, the ratio between the transmitted intensities measured above the inclusion (Iinc) and the mineral host (background intensity, Ibkg) should be proportional to the analyzed thickness of the inclusion fluid. The transmitted intensity above the inclusion is of the form,

Iinc ) I0e-(µm /sinR)(t-h)Fme-(µf /sinR)hFf 0

0

(2)

where t is the total thickness of the sample, whereas the background intensity is

Ibkg ) I0e-(µm /sinR)tFm 0

(3)

Dividing eq 2 by eq 3 gives the relationship between h and Iinc/ Ibkg.

h)

( )

Iinc sin R ln 0 Ibkg µm Fm - µf Ff 0

(4)

The relative error made on the calculated h values was checked using the following error propagation formula:

∆h )

x(

∂h ‚∆R ∂R

) ( 2

+

∂h ‚∆Abs ∂Abs

) ( 2

)

∂h ∆R ∂RI I

+

2

(5)

where

Abs ) µm0Fm - µf0Ff RI ) Iinc/Ibkg Applying this formula to eq 5 gives

∆h )

x(

cosR ln(RI)∆R Abs

) ( 2

+

) (

sin R ln(RI)∆Abs (Abs)2

2

+

)

Figure 4. Simulated background and transmitted intensities through a 100-µm-thick quartz sample, containing an elliptical inclusion of 20 × 40 µm2. The incoming beam is at 58° from the sample surface. The different lines correspond to calculations made using different salinities of the liquid phase and water vapor: 26 wt % NaCl (black line), 20 wt % NaCl (black dashes), 10 wt % NaCl (gray line), and liquid H2O (gray dashes), and H2O vapor (gray dots), background signal (black dots).

sin R ∆RI 2 Abs RI (6)

where

∆Abs ) x(µm0∆Fm)2 + (Fm∆µm0)2 + (µf0∆Ff)2 + (Ff‚∆µf0)2 ∆RI )

x( ) ( ∆Iinc Ibkg

2

+

)

Iinc∆Ibkg Ibkg

2

2

To investigate if the Iinc/Ibkg transmitted intensity ratio can be used to estimate precisely the fluid inclusion thickness, hence the volume of irradiated fluid, a three-step protocol was developed. The numerical simulation of the transmitted signal emerging from a fluid inclusion of known geometry (elliptical) was first performed in order to evaluate the variability of the Iinc/Ibkg ratio as a function of the inclusion and host mineral thickness. Transmission scans were then performed on fluid inclusions of different sizes and geometries, and the results were compared to the numerical simulations. Finally, computed tomography was performed on a single fluid inclusion for which the experimentally determined transmitted line scans could not be fitted numerically using a simple elliptical fluid inclusion geometry. RESULTS The transmitted signal through a quartz chip containing an elliptical fluid inclusion has been calculated as a function of the lateral beam position above the inclusion. Figure 4 shows the modeled transmitted intensity through a 100-µm-thick quartz wafer, containing an elliptical inclusion of 20 × 40 µm2. The transmitted intensity emerging from the inclusion is 1-2% higher than the background intensity. Note that, in the case of an inclusion several tens of micrometers thick, the salinity of the liquid phase can significantly affect the transmission signal. Figure 5 shows two transmission line scans performed above a single fluid inclusion of large dimension (20 × 70 µm2, sample

Figure 5. Feasibility tests of transmission scans through samples LD886 (a) and c6sol2 (b). In both samples, the dashed line corresponds to the transmission scan performed through an inclusion free part and the plain line corresponds to a scan performed across the inclusion (LD886) or the inclusion-rich part of the sample (c6sol2); scans are located on the corresponding photographs.

LD886) and above a fluid inclusion plane composed of several inclusions sizing from a few micrometers to a few tens of micrometers (sample c6sol2). In both cases, the inclusions can be clearly visualized owing to a marked increase of the transmitted intensity emerging from above the inclusions (plain lines) compared to the background-transmitted intensity (dashed lines). Figure 6 shows the transmission scans obtained on a single inclusion of samples c6sol2 and LD886 together with the corresponding simulation of the transmitted signal calculated using a model elliptical inclusion geometry. Figure 6a shows a good agreement between the measured and calculated transmitted profiles indicating that the inclusion can be well approximated with an elliptical shape. By contrast, in Figure 6b, the distribution Analytical Chemistry, Vol. 76, No. 14, July 15, 2004

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Figure 6. Experimental data and simulation of the signal based on an elliptical fluid inclusion model for two fluid inclusion geometries: (a) c6sol2, (b) LD886, and (c) calculated values of h obtained using the measured transmission profile shown in (a) and (b).

Figure 7. Comparison between tomography reconstruction and calculated path length through the LD886 inclusion. Calculated fluid thicknesses at each point of analysis are reported on this reconstructed cross section of the sample at the transmission scan position (thin black lines). This image is a reconstruction of the sample in the horizontal plane.

of the calculated signal arising from the model inclusion is significantly different from the measured transmitted profile, thus indicating that the model elliptical geometry is inappropriate. Fluid inclusion thicknesses were calculated using the transmitted intensity for both inclusions (Figure 6c). To investigate the source of errors associated with the fluid inclusion geometry of sample LD886, absorption tomography of the whole sample was performed. Figure 7 is a reconstructed absorption tomography slice showing the internal geometry of the sample. It is a cross-cut of the sample at the position of the transmission scan performed on the same sample. Both are displayed as a plain black line on the photograph in Figure 5a. The investigated slice was chosen in a way to avoid the gas bubble. The hatched pattern is the scaled 3992

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incident beam path length calculated from the corresponding h values at each step of the transmission line scan above the inclusion (from Figure 6c, bottom). Note the excellent agreement between the tomography reconstruction and calculated fluid inclusion thickness. This indicates that the transmitted intensity ratio can provide reliable information on the fluid inclusion thickness independent of the fluid inclusion geometry. To constrain the accuracy of the transmitted intensity technique, a time scan of the transmitted beam intensity emerging from a fixed spot of sample pi02-39k was performed (Figure 8a). The variability of the transmission signal with time is ∼0.01% of the incoming intensity. Considering the experimental conditions used for sample pi02-39k, this beam intensity fluctuation corresponds to an error of less than 0.7 µm for the fluid inclusion thickness. Figure 8b shows that, for a typical fluid inclusion size of 20 µm, the transmission intensity fluctuation is ∼0.2% of the transmitted signal, which is significantly larger than the statistical fluctuation. Figure 8c plots calculated h values as a function of optically determined h values. For four inclusions of sample pi0239k out of the five analyzed, the “optical” h value is significantly greater than the “calculated” h value, corresponding to the overestimation of the analyzed fluid layer thickness by the optical method. In all cases, note that the uncertainty on calculated h values is lower than the one associated with optical h values. DISCUSSION Transmission measurements (Figure 5) showed that 10-µmsized fluid inclusions can be detected in 100-300-µm-thick quartz samples. Numerical simulations predicted that the difference between the fluid inclusion and background-transmitted intensities should be between a fraction of a percent and a few percent of the incoming intensity, depending on the inclusion size and beam

Figure 8. Tests on sample pi02-39k. (a) Time variation of the ratio Iinc/Ibkg at a single point. This variation reflects the limits of detection of the PIN diodes and will be one of the major limiting factors on calculated h accuracy. (b) Typical transmission scan performed through a 20-µm large inclusion hosted in quartz, reflecting the lower absorption of fluid compared to that of silica. (c) Optical vs calculated h values for five fluid inclusions hosted in sample pi02-39k.

energy. This is in good agreement with the measured transmitted line scans obtained on samples c6sol2 (Figure 6a), LD886 (Figure 6b), and pi02-39k (Figure 8b), which show a transmitted peak of ∼1% of the incoming intensity. Simulations also predicted that calculated h values should be significantly influenced by the assumed fluid salinity (Figure 4). Sample c6sol2 contains inclusions having a salinity of 0.4 wt % equivalent NaCl. The calculated transmission profile using an elliptical geometry yielded an experimental curve located between a 1 wt % NaCl solution and water vapor. This is in good agreement with the composition of the fluid inclusion, which contains a low-salinity liquid water phase and a gas bubble (Figure 6a). This shows that a gas phase will increase the transmitted signal and lead to an overestimation of the fluid thickness if the density and absorption coefficient of the liquid phase are used for the calculation. With the experimental conditions used for this sample, a 30-µm-thick inclusion hosted in a 100-µm-thick quartz would be detected as 31 µm thick if a 10-µm gas bubble is present in the inclusion and as a 34-µm-thick inclusion if it is a gas-only inclusion. In the latter case, however, the gas is visible and should be considered as the internal medium of the fluid inclusion and its density and absorption coefficient used instead. The opposite case is having a daughter crystal or a trapped solid inside the inclusion, which would decrease the transmitted signal. Yet, the transmitted signal linked with the fluorescence data can help detecting analysis of solid phases, which cannot be considered as representative of the liquid phase and should therefore be discarded. With regards to sample LD886, calculated transmission signal using an elliptical fluid inclusion geometry yielded inclusion thickness values significantly different from the ones obtained using measured transmission line scans (Figure 6b). The absorption tomography reconstruction showed

that the inclusion thicknesses deduced from the transmission scans are appropriate (Figure 7), thus establishing that the height of the inclusion at the spot of fluorescence analysis can be determined from a line scan or transmission measurement. This also implies that tomography, which permits identifying detailed geometrical features, can be used as a complementary tool when very precise concentrations are required or when a unique, precious, or rare sample is to be analyzed. The time variation of the Iinc/Ibkg transmitted intensity ratio can be used for evaluating the detection limits of calculated h values. As shown in Figure 8a, variation of the transmitted intensity obtained on sample pi02-39k was (0.01% of the incoming intensity, which led to an error of (0.3-(0.85 µm on the calculations of h in pi02-39k inclusions (Figure 8c) and (0.4-(2 µm on the calculations of h in the different measurement points of the inclusion hosted in LD886. Thus, for inclusions smaller than 15 µm in thickness (sample pi02-39k), the error made on calculated h is significantly lower than the one derived from optical measurements (