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Synchrotron FTIR microscopy of synthetic and natural CO2–H2O fluid inclusions
Vibrational Spectroscopy 75 (2014) 136–148
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Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Synchrotron FTIR microscopy of synthetic and natural CO2 –H2 O fluid inclusions夽 Michél K. Nieuwoudt a,∗ , Mark P. Simpson b , Mark Tobin c , Ljiljana Puskar c a b c
School of Chemical Sciences, University of Auckland, 23 Symonds St., Auckland 1142, New Zealand GNS Science, Wairakei Research Centre, 114 Karetoto Road, RD4, Taupo 3377, New Zealand Australian Synchrotron, 800 Blackburn Rd., Clayton 3168, VIC, Australia
a r t i c l e
i n f o
Article history: Received 25 March 2014 Received in revised form 3 August 2014 Accepted 3 August 2014 Available online 26 August 2014 Keywords: FTIR spectroscopy Synchrotron Fluid inclusions CO2 dissolved in H2 O Spectral images
a b s t r a c t Fluid inclusions provide a record of the physical and chemical composition of fluids that flow through the Earth’s crust. Here, we describe a novel use of synchrotron Fourier Transform Infrared (FTIR) microscopy to map three parameters in individual fluid inclusions: (1) very low concentrations of CO2 , (2) variations of the state of CO2 and (3) variations in water structure. The intensity of the synchrotron light source allows compositional mapping of individual fluid inclusions, which provides high-resolution images of the distribution of different CO2 and H2 O species within individual inclusions. High resolution spectra of the CO2 , recorded here for the first time in fluid inclusions, show the rotational lines of CO2 gas. The results reveal predicted as well as unexpected distributions of CO2 gas in the inclusions. © 2014 Elsevier B.V. All rights reserved.
Introduction Epithermal Au–Ag deposits are an important source of gold and silver, and the fluid inclusions trapped in veins are the only remains of the fluids present during the formation of these ore deposits. Microthermometric (heating and freezing) measurements from individual fluid inclusions provide valuable information on the temperature and apparent salinity (due to salt and/or dissolved CO2 ) of these inclusions, and element concentrations can be determined from laser ablation inductively coupled plasma mass spectrometry (LA-ICP–MS). Raman microscopy is a powerful method for analysis of a number of gases including CO2 as the excitation laser beam can be focused to micron dimensions and the use of confocal systems enables analysis within transparent matrices. Besides gases, this technique has also been used to analyse hydrocarbons and minerals in fluid inclusions; the latter either precipitated from solution or accidentally trapped [1–6]. For most epithermal deposits, however, the amount of CO2 within the inclusions, and its contribution to the apparent salinity is
夽 Paper presented at the 7th International Workshop on Infrared Microscopy and Spectroscopy with Accelerator-Based Sources (WIRMS), Melbourne, Australia, 10–13th November 2014. ∗ Corresponding author. Tel.: +64 99238875. E-mail addresses: [email protected] (M.K. Nieuwoudt), [email protected] (M.P. Simpson), [email protected] (M. Tobin), [email protected] (L. Puskar). http://dx.doi.org/10.1016/j.vibspec.2014.08.003 0924-2031/© 2014 Elsevier B.V. All rights reserved.
poorly known, and generally cannot be determined from Raman microscopy because the CO2 concentrations are typically lower than the detection limit of this technique [7]. Determining the contribution of CO2 is significant because it has implication for pressure and formation depth calculations, as well as impacts determining the absolute concentration of elements from LA-ICP–MS analyses which require an external standard that is based on the final ice melting temperature (Tm ). The Tm values can be due to either aqueous salts, dissolved CO2 or a combination of the two [17]. In this study, we used synchrotron FTIR microscopy in order to investigate the feasibility of this technique to detect very low concentrations of CO2 in fluid inclusions in epithermal veins from the Hauraki Goldfield, New Zealand and to provide semi-quantitative information on the amount of contained CO2 . In addition, spectral image maps of individual fluid inclusions were used to measure variations in the distribution of CO2 and H2 O compositions of the inclusions. Synchrotron FTIR microspectroscopy is an effective technique for examining the H2 O, CO2 and methane contents of fluid inclusions [8–10]. Due to the high brightness of synchrotron light, the aperture (size) of the incident or detected infrared (IR) beam can be reduced to values as small as 3 m with minimal loss of signal, which enables analyses of individual fluid inclusions and the areas inside them. The appearance and frequency of the 3 asymmetric stretch of CO2 differs distinctly depending on whether the CO2 occurs as a gas, liquid, or gas dissolved in a liquid [8,11–14]. For the solid and liquid phases of CO2 and for liquids or solids containing dissolved
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of dissolved CO2 is 3.5 wt.%, which would contribute −1.5 ◦ C to Tm [17]. Overall, there is little variation between primary, pseudosecondary and secondary inclusions based on the microthermometry, although some differences in chemical composition and concentrations are clearly evident from LA-ICP–MS analyses of individual fluid inclusions [7,18]. Material and methods
Fig. 1. Simplified map of the Hauraki Goldfield showing the location of the Karangahake, Waiorongomai and Tokatea deposits. Inset shows the location of the Hauraki Goldfield within the North Island of New Zealand [15].
CO2 gas, the 3 occurs as a single band ranging between 2336 and 2345 cm−1 [8,13]. At the spectral resolution of 4 cm−1 , which is generally used for solids and liquids, CO2 in the gas phase occurs as two overlapping but resolved bands centred at 2349 cm−1 , with the Pbranch around 2338 cm−1 and the R-branch at 2361 cm−1 . At higher resolutions (0.5 cm−1 and up) the rotational lines of the P- and Rbranches can be clearly seen. Therefore, in this study, spectra were also recorded at higher resolution (0.5 cm−1 ) in order to investigate the presence and variation of CO2 in the gas phase in the different fluid inclusions. Fluid inclusions analysed are from the Karangahake, Waiorongomai and Tokatea deposits located in the Hauraki Goldfield, New Zealand (Fig. 1), a 200 km long by 40 km wide metallogenic belt that contains the greatest concentration of precious metal deposits in New Zealand [15]. The Karangahake deposit (0.9 M oz. Au) was the third largest Au producer in the goldfield, whereas the main veins at both Tokatea and Waiorongomai are essentially barren with localised mineralisation restricted to splay veins. Fluid inclusions from all three deposits have overlapping ranges in homogenisation temperatures (Th ) and apparent salinities; the latter is determined from final ice melting temperatures (Tm ) and is reported as weight percent NaCl equivalent [16], but can be due to included CO2 [17]. Inclusions in quartz veins from the Karangahake deposit have a Th range of 198◦ to 287 ◦ C and less than 3.9 wt.% NaCl equivalent [7] and those from Waiorongomai a Th range of 195◦ to 298 ◦ C and salinities of less than 3.5 wt.% NaCl equivalent. Fluid inclusions in amethyst from the Tokatea deposit have a Th range of 175◦ to 280 ◦ C and apparent salinities of less than 2.3 wt.% NaCl equivalent [18]. No clathrate has been identified in any of the fluid inclusions from the three deposits and therefore the maximum concentration
Fluid inclusions measured from the Karangahake, Waiorongomai and Tokatea deposits occur in doubly polished plates of quartz and amethyst that are 150–200 m thick and were washed with acetone prior to analyses. Cavities occupied by the fluids range in size from 10 to 50 m and at room temperature consist of a liquid (75 to 80 vol%) and a vapour. This is a bubble that was created due to the contraction of the contained fluid on cooling. In the following, we use the term bubble to refer to the vapour component of the fluid inclusion. Measurements were made on carefully defined fluid inclusion assemblages (FIAs) identified prior to microthermometric measurements; these are a group of inclusions that were trapped simultaneously in a single event, such as an individual growth zone or a healed fracture [19,20]. Identification of FIAs were confirmed by narrow Th ranges and these inclusions are the focus of the FTIR study. FTIR spectra were collected in transmission mode with a Bruker Hyperion 2000 microscope coupled to a Bruker Vertex V80v FTIR spectrometer at the IR beamline of the Australian synchrotron in Melbourne. A narrow-band mercury cadmium telluride (MCT) detector and ×36 reflecting objective and condenser optics of 0.5 numerical aperture were used. Spectra were taken with microscope knife edge apertures set to 5 m, and spectral resolutions of 4 cm−1 and 0.5 cm−1 referred to hereafter as low and high resolution, respectively. The number of scans averaged for the low and high resolution spectra were 64 and 256, respectively, and the data acquisition rate was 40 kHz, equivalent to a mirror velocity of 1.28 cm/s. Bruker OPUS 6.5 spectroscopic software was used to collect the data and OPUS 7 to process the spectra. The microscope sample stage was enclosed in a Perspex chamber with continuous N2 purging and the chamber was purged for 10 min before analyses to eliminate atmospheric CO2 . The background spectrum of the surrounding quartz matrix was recorded before and after every spectrum, to remove the effects of any CO2 dissolved in the quartz matrix and to assess any changes in atmospheric CO2 due to ineffective purging. The spectral maps of individual fluid inclusions were recorded at step sizes of 2.5 m. The distribution and relative amounts of gaseous CO2 and aqueous CO2 , as well as variation in H-bonding were determined from chemical images of these maps, generated by integrating absorbance peak intensities of relevant functional groups using OPUS 7 spectroscopic software. A reference spectrum was also obtained of pure, de-ionized water by placing a drop of Milli-Q de-ionized water on a BaF2 disk and recording the spectrum in transmission mode at 4 cm−1 resolution using a Continuum FTIR microscope and Thermo Fisher Nicolet 8700 FTIR spectrometer. In addition, spectra were recorded for synthetic CO2 –H2 O inclusions in quartz with known concentrations of CO2 as references. These standards are routinely used for calibration of the microthermometric fluid inclusion stage and were provided by Professor Bob Bodnar, Virginia Tech. The total CO2 concentrations are 10, 25 and 50 mol% CO2 , with the latter two containing liquid CO2 . Results and discussion CO2 as free gas and dissolved in water Fig. 2 shows a representative example of both the low resolution (red spectrum), and high resolution (blue spectrum) asymmetric
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Fig. 2. (a) Low (red) and high (blue) resolution spectra of the 3 stretching mode of CO2 in the gas phase for a fluid inclusion in amethyst from the Tokatea deposit. (b) Low (red) and high (blue) resolution spectra of the 3 stretching mode of CO2 dissolved in water. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
stretch modes of CO2 (3 ) for the gas and liquid phase in a fluid inclusion. Fig. 2a shows the gas phase mode and Fig. 2b the same mode for CO2 dissolved in the water phase, where it occurs as a single band at 2346 cm−1 due to loss of rotational freedom. This spectrum also shows a shoulder at 2362 cm−1 . The rotational lines in the high resolution spectrum of the water phase are due to some atmospheric CO2 which may have leaked in during a lag in the N2 purging. FTIR spectra of individual inclusions FTIR spectra for the bubble and liquid phases have been collected for numerous carefully selected FIAs from the Karangahake and Waiorongomai deposits; Figs. 3 and 4 show representative spectra for each of the two deposits. FTIR analyses of FIAs hosted in quartz from the Karangahake deposit also detected CO2 in some inclusions with CO2 present as a gas phase in both the water and bubble phases (Fig. 3). The similarity of the intensities of the rotational lines in the region 2 of the water phase and the bubble (region 0), and no visible single band for CO2 dissolved in water indicate very low concentrations of CO2 in this inclusion, and/or possible atmospheric contamination. FTIR analyses of individual FIAs hosted in quartz from the Waiorongomai deposit showed that some inclusions contained CO2 whereas in many others it was not detectable. In Fig. 4, we show an inclusion in which the spectra of both the bubble and water phases show a single band representing CO2 dissolved in the water phase, suggesting significant concentrations of CO2 in the inclusion; the spectrum of the water phase also appearing in the bubble region because the bubble is surrounded by water. The rotational lines in the high resolution spectrum of the liquid phase can only be explained by some lags in the N2 purging that allowed leakage of atmospheric CO2 . Leakage of atmospheric gas into the sample chamber was confirmed by performing a Fourier deconvolution on some of the spectra to remove the interference fringing in the spectrum originating from the quartz inner surfaces. This revealed weak, sharp bands characteristic of atmospheric water vapour (Fig. 5). This leakage occurred intermittently. Comparison of the O-H stretch mode (3 ) of water between 3100 and 3700 cm−1 for the fluid inclusions from the two different deposits in Figs. 3 and 4 reveal variations in shape in the bubble and liquid phases within the same inclusions. This band is characteristically broad as a result of the large number of different
extents of hydrogen bonding (H-bonding) experienced by the water molecules and the shape and maximum frequency are known to be strongly influenced by the presence of dissolved species [21,22]. The H bond coordination numbers range from 0 to 4, resulting in the simultaneous occurrence of monomeric, dimeric and several polymeric species in rapid equilibrium, each with distinct, but very close vibrational frequencies. In some FTIR studies, the different populations have been described by five or six species with resolution of the band into six components [22,23] at around: (i) 3626 cm−1 (free OH), (ii) 3557 cm−1 (zero or one H bond), (iii) 3436 cm−1 (2 coordinated H2 O), (iv) 3251 cm−1 (3-coordinated H2 O), (v) 3154 cm−1 (3 or 4 coordinated H2 O) and (vi) 3062 cm−1 (4 coordinated H2 O). In other studies, however [21], the different populations have been described in only three components: (1) low energy component around 3250 cm−1 reflecting water molecules connected to three or four other molecules, i.e., those molecules building up a supermolecular connective network, (2) a component around 3450 cm−1 representing two-to-three coordinated water molecules, and (3) a higher energy component around 3590 cm−1 representing “free” water molecules which form either zero or only one H-bond to neighbouring water molecules. Dissolved gases and ionic salts in the water will change the Hbonding and also the relative proportions of these species, resulting in changes in the overall band shape and frequency [21,22,24]. In order to better characterise the variation of these different species in the bubble and water phases, we resolved the OH stretch mode between 3100 and 3700 cm−1 into six components and compared them with similarly resolved components for spectra also recorded in transmission mode of pure Milli-Q water and of synthetic CO2 –H2 O fluid inclusions in quartz whose CO2 concentrations and pressures were known: 10 mol% CO2 (at ∼60 bar), 25 mol% (65 bar) and 50 mol% CO2 (∼73 bar). At room temperature the 10 mol% inclusion consists of two phases: water and CO2 vapour phase (Fig. 6(a)), the 25 mol% inclusions contains three phases: liquid water, liquid CO2 and CO2 vapour (Fig. 6(b)) and the 50 mol% inclusion contains two phases: liquid water and liquid CO2 (Fig. 6(c)) and at these pressures the solubility of CO2 in the water in these three inclusions would be about 0.32, 1.38 and 1.56 molal, respectively. Although three components adequately fitted the OH mode of pure water, six were needed to adequately fit the band for the synthetic CO2 –H2 O standard inclusions. Band resolution of the OH stretch mode was performed using the Peak resolution function of the OMNIC spectroscopic software
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Fig. 3. Low resolution spectra of bubble (0) and liquid phases (1, 2) in an inclusion from sample AU62232, Karangahake deposit. Insets show expanded, high resolution of the CO2 3 band around 2340 cm−1 .
Fig. 4. Low resolution spectra of the bubble (0) and liquid phase (1) in an inclusion from sample AU62868, Waiorongomai deposit. Insets show expanded, high resolution of the CO2 3 band around 2340 cm−1 .
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Fig. 5. Comparison of spectra of atmospheric H2 O and CO2 , the bubble region and the quartz matrix adjacent to the fluid inclusion, after Fourier deconvolution to remove interference fringes. The presence of atmospheric CO2 and H2 O vapour bands in the quartz matrix and bubble region indicate leakage of atmospheric gas into the N2 purge chamber. The H2 O vapour bands are shown more clearly in the expanded spectra in the inset. Table 1 Component bands for OH stretch mode of standards, pure water and natural inclusions fitted by band resolution. Band Component
Coordination value [22]
Pure H2 O (cm−1 )
0.32 m CO2 (cm−1 )
1.32 m CO2 (cm−1 )
1.56 m CO2 (cm−1 )
Kar. bubble phase (cm−1 )
Kar. H2 O phase (cm−1 )
Waior. bubble Phase (cm−1 )
Waior. H2 O phase (cm−1 )
i ii iii iv v vi
“Free” 0 and 1 2 3 3 and 4 4
3649 3577 3440 3235 3333 3078
3646 3592 3483 3352 3229 3124
3754 3605 3478 3371 3225 3037
3710 3647 3519 3380 3249 3156
3669 3527 3374 3220 3090 2911
3746 3607 3489 3369 3208 3074
3718 3614 3498 3368 3202 3070
3771 3587 3477 3348 3220 3078
which fits components of combined Gaussian and Lorentzian algorithms to the band using least squares regression. The resolved species in pure water are given in Fig. 7(a) and those in the three standards are given in Fig. 7(b)–(d). The fitted component frequencies are given in Table 1 and a plot of the relative areas of three differently coordinated H2 O species as described in [21] (obtained by combining (i + ii) to represent 0–1 coordinated H2 O, (iii + iv) to represent 2–3 coordinated and (v + vi)) representing 3–4 coordinated H2 O molecules) is given for each of the above standards as a function of concentration of dissolved CO2 in Fig. 8. This plot shows different trends for the differently coordinated H2 O molecules with increasing dissolved CO2 . There appears to be an increase in the relative amounts of low-coordination H2 O molecules (less networking
or H-bonding) with increasing concentration of dissolved CO2 , and a concomitant decrease in the more highly coordinated (3–4) H2 O molecules, or more networked species. The less networked species (represented by band components i and ii), with lower coordination of 0 to 1 increase with increasing CO2 concentrations when compared with pure water. Accordingly the presence of dissolved CO2 appears to increase the less networked water species, which are poorly connected to their environment compared with pure water, thus contributing to structure breaking of the water, or decreasing the level of H-bonding and shifting the band maximum to higher wavenumbers. Similar increases in the higher energy maxima with increasing mol% CO2 were observed in the OH stretch mode of spectra recorded at 300 ◦ C of synthetic CO2 –H2 O inclusions
Fig. 6. A 10 mol% CO2 inclusion (∼60 bar) showing two phases of CO2 vapour in the bubble and surrounding H2 O liquid (B) 25 mol% CO2 inclusion (∼65 bar) with three phases of CO2 vapour in the bubble surrounded by CO2 liquid and H2 O liquid, 5 (C) 50 mol% CO2 inclusion (∼73 bar) with two phases: CO2 liquid surrounded by H2 O liquid.
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Fig. 7. a O H stretching mode of pure H2 O, (b) 3 mode for H2 O phase of 10 mol% CO2 inclusion, (c) 3 mode for H2 O phase of 25 mol% CO2 inclusion, (d) 3 mode for H2 O phase of 50 mol% CO2 inclusion (e) 3 mode for spectrum of bubble phase of Karangahake inclusion (AU62232), (f) 3 mode for H2 O phase (area 2) of Karangahake inclusion. (g) O H stretching mode in spectrum of bubble phase of Waiorongomai inclusion 2 (sample AU62868), (H) O H stretching mode in H2 O phase of Waiorongomai inclusion 2. Fitted components: (i) 3649 cm−1 (free OH, navy blue), (ii) 3577 cm−1 (zero or one H bond, pink), (iii) 3440 cm−1 (2-coordinated H2 O, cyan), (iv) 3333 cm−1 (3-coordinated H2 O, green), (v) 3235 cm−1 (3- or 4-coordinated H2 O, purple) and (vi) 3078 cm−1 (4-coordinated H2 O, blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. Plot showing trends in relative amounts of differently-coordinated groups of H2 O molecules with increasing concentration of dissolved CO2 .
between 12.5 and 65 mol% CO2 [25]. This suggests that CO2 dissolved in water contributes to a decrease in the H-bonding, or structure breaking, of the water, even at very low concentrations, and that this effect increases with increasing amounts of dissolved CO2 . In addition, there also appear to be slight shifts to higher frequencies in some of the fitted components with increasing CO2 concentration, as shown in the band components given in Table 1. Compared to pure water, the bubble region of the Karangahake inclusion shows a red shift of the band maximum, to about 3200 cm−1 from about 3400 cm−1 for pure water (Fig. 7(a) and (e)). This appears to be mainly the result of increased relative intensities for the 3 and 4-coordinated species, or more highly networked species (component bands v and vi), which suggests that the water molecules around the bubble region exhibit increased H-bonding. In addition, there appear to be slight shifts of some of the band components to lower frequencies. A detailed study of the effect of dissolved salts on the relative intensities of the band components and shape of the O H stretch mode in D2 O [24] has shown that the presence of dissolved salts such as alkali fluorides can also contribute to such shifts. It was found that while dissolution of chloride and iodide salts results in shifts of the band maximum to higher frequencies, showing structure breaking character, dissolved fluoride salts shift the OH band to lower frequencies, showing structure making character [24]. However, the shifts were significant only for water:salt mole ratios of less than 25:1. For both inclusions shown in Figs. 3 and 4, with their corresponding resolved H2 O bands shown in Fig. 7(e) and (g), respectively, the red shifts in band maxima due to increased H-bonding appear to occur in the bubble region only, which would not be expected to contain dissolved salts. A more likely explanation for the observed shifts of the maxima to lower frequencies for our inclusions may be the occurrence of increased networking of the water molecules near the water–gas interface (Fig. 9). 2-D IR spectral images 2-D spectral images for four fluid inclusions from the Karangahake and Waiorongomai deposits are presented below and show the variation of the occurrence of CO2 gas (2361 cm−1 ), CO2 dissolved in water (2380–2520 cm−1 ) and H2 O (2520–2330 cm−1 ) in the inclusions. The 2-D spectral images of two different fluid inclusions mapped from the Waiorongomai deposit are shown in Figs. 10 and 11. Both inclusions are from the same FIAs and have essentially identical Tm values of −1.0 and −1.1 ◦ C that correspond to apparent salinities of 1.7 and 1.9 wt.% equiv. NaCl. The spectral image for the first
Fig. 9. Plot showing the variation in relative amounts of differently-coordinated groups of H2 O molecules in the bubble and liquid regions of the Karangahake and Waiorongomai inclusions, compared with those for pure water.
inclusion shows that the greatest concentration of CO2 gas is within the bubble (Fig. 10(c)) and the highest concentrations of CO2 dissolved in water (Fig. 10(d) coincide with the maximum amount of water (Fig. 10(b) which indicates the deepest part of the inclusion. The spectral images for the second inclusion shown in Fig. 11 also show that the vapour bubble contains CO2 gas, however, there is additional CO2 gas in two large areas of the inclusions where CO2 gas was not expected; in part of the water phase but also in the quartz matrix (Fig. 11d). CO2 gas was also seen in the water phase in the high resolution spectra in Fig. 4a. These either reflect atmospheric CO2 leaking at two different periods while these two areas were being mapped, because of the long collection times needed for the spectral mapping, or movement of the bubble phase during the measurement. The presence of CO2 in the inclusion is confirmed by the single band for dissolved CO2 . The spectra in the lower part of Fig. 11 show shifts in the maximum to lower frequencies for the 3 mode of H2 O around the bubble region, which indicates increased networking of the water around the bubble. Although both inclusions are from the same FIA and essentially have the same Tm values, the greater intensity of the CO2 in the bubble phase in the first inclusion (Fig. 10) suggests it has a higher CO2 concentration, shown also by the greater intensity of the single peak due to CO2 dissolved in the water phase. This would imply that they have different concentrations of dissolved salts. The 2-D spectral images of two fluid inclusions mapped from the Karangahake deposit are shown in Figs. 12 and 13. The two fluid inclusions are from separate FIAs and have very different Tm values of −3.3◦ and −0.2 ◦ C that correspond to apparent salinities of 5.4 and 0.4 wt.% equiv. NaCl, respectively. Spectral images for first inclusion show that CO2 gas is present in both the vapour and dissolved in the H2 O phase (Fig. 12). CO2 is concentrated within the bubble but is also enriched in two regions away from the bubble; the latter may be caused by movement of the bubble. The greatest intensity for CO2 dissolved in water overlaps with the region of greatest H2 O intensity and coincides with the thickest part of the inclusion. Although the amount of CO2 present cannot be quantified, the maximum amount that can possibly be present is 3.5 wt.% CO2 : this amount is constrained by the absence of any clathrate seen on freezing and would contribute −1.5 ◦ C to the Tm value [17]. Accordingly this inclusion must have at least 1.7 wt.% equiv. NaCl in addition to the CO2 . In contrast, the 2-D spectral image for the second Karangahake inclusion (Fig. 13) reveals that CO2 gas is absent from where the bubble phase should be. Fig. 13(b) shows CO2 gas concentrated at the lower left side of the inclusion, coinciding with the quartz matrix and part of the liquid phase. The CO2 gas can only be atmospheric and due to inadequate purging during collection of
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Fig. 10. A Image of Waiorongomai inclusion 1. (B) Mapping of O H stretch mode, with integration area shown in (C). (D) Mapping of CO2 R branch for gas phase (2381–2353 cm−1 ) (E) Mapping of dissolved CO2 in water, occurring as a single peak (2354–2333 cm−1 ). Integration for (D) and (E) maps is shown in (F).
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Fig. 11. A Image of a fluid inclusion from the Waiorongomai deposit (AU62868) inclusion 2 (B) Mapping of H2 O mode (3700–3350 cm−1 ) with corresponding integration areas shown in (C). (D) Mapping of CO2 R branch for gas phase (2381–2353 cm−1 ) with integration areas shown in (F). (E) Mapping of CO2 dissolved in water, occurring as a single peak over the area (2350–2338 cm−1 ) shown by integration region in Fig (G).
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Fig. 12. A Image for Karangahake (AU62232) inclusion 1. (B) Map of total H2 O mode with integrated region covering the range 3722–3013 cm−1 shown in (C). (D) Map of CO2 gas (R branch at 2385–2353 cm−1 and (E) map of CO2 gas dissolved in water (2352–2316 cm−1 ), both integrated regions shown in (F).
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Fig. 13. A Image of Karangahake (AU62232) inclusion 2 (B) Map of CO2 gas with integration area covering both P and R branches over 2384–2305 cm−1 as shown in (C)). (D) Map of higher energy H2 O mode represented by integrated peak areas 3708–3398 cm−1 shown in (F). (E) Map of lower energy H2 O mode represented by integrated peak areas 3312–2978 cm−1 in (F).
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CO2 ; this correlates with its low Tm value of −0.2 ◦ C. The Tm of the Karangahake 1 inclusion is −3.3 ◦ C and while the spectral maps in Fig. 12 show that CO2 is present, the comparable intensities with Waiorongomai inclusion 1 suggests that it must have higher dissolved salt concentrations. The two Karangahake inclusions were from the same FIA but show very different Tm values and also different levels of trapped CO2 . Conclusions
Fig. 14. Comparison of spectra of the liquid phase of the Wairongomai inclusion 1 (AU62868) with that of a standard synthetic inclusion containing 10 mol% CO2 . The single band due to dissolved CO2 is indicated by arrows in each spectrum.
the set of points over this region, as can be seen by CO2 gas bands in spectra of the quartz matrix shown in Fig. 13(c). This is also confirmed by the absence of any single band for CO2 dissolved in H2 O in the spectra of the inclusion. The spectra in Fig. 13(f) show two distinct groups for the H2 O band with different shapes, with maxima occurring around 3620 cm−1 and 3180 cm−1 . The H2 O mode has therefore been plotted in two maps (Fig. 13(d) and (e)). The spectra with maxima shifted to higher wavenumbers in Fig. 13(d) coincide with the water phase. The maxima at the lower frequency side of the H2 O band coincide with the bubble region in Fig. 13(e) and this suggests that water molecules around the bubble region have increased H-bonding. This may be due to increased networking of the water molecules at the vapour/liquid interface of the bubble. The spectral images show that this inclusion has no detectable CO2 and accordingly in this case the Tm value, though dilute, must be mainly due to dissolved salts. The thickest portions of the inclusions were revealed from the intensity of H2 O band. The variation in thickness of the inclusions implies that it would not be possible to obtain quantitative measurements of the concentration of CO2 in the inclusions from absorbance measurements using Beer’s law (A = ∈bC), where A is the absorbance, ∈ the absorption coefficient, b the path length (inclusion depth) and C the concentration of CO2 . While it is not possible to quantify the levels of CO2 in the inclusions from the spectra, a comparison of the relative intensities of the dissolved CO2 single band and the H2 O band in the 10 mol% CO2 standard inclusion and the Wairongomai inclusion 1 (Fig. 14) does show that they are very low. The detection of such low CO2 levels in these inclusions, not possible with Raman microscopy, is enabled by the increased sensitivity afforded by the high intensity of the synchrotron infrared source. The presence of CO2 , albeit at such low levels, has implications for interpretation of the measured Tm values, to which both dissolved salts and CO2 contribute. The spectral maps show that the vapour bubble in both the Waiorongomai deposit inclusions and the first inclusion in the Karangahake deposit contain CO2 gas, and that the H2 O phase also contains dissolved CO2 , however, the amounts of CO2 are very low. The Tm values of the Waiorongomai inclusions 1 and 2 are −1.0 ◦ C and −1.1 ◦ C, respectively. The spectral images and intensities of the single CO2 band in Fig. 10 show more CO2 in the Wairongomai 1 inclusion, suggesting that inclusion 2 has relatively more dissolved salts. In contrast, the spectral map for the Karangahake 2 inclusion in Fig. 13 shows no detectable
Very low concentrations of CO2 have been detected in individual fluid inclusions in epithermal quartz veins from the Waiorongomai and Karangahake deposits by FTIR microscopy using the synchrotron IR beam. Previous laser Raman analyses of inclusions from this deposit could not detect CO2 because the concentrations were below the level of detection. Semi-quantitative information about the CO2 gas in the bubble and dissolved in the liquid phases in the inclusions was obtained from observations of the 3 band around 2400 cm−1 , and by comparison with the 3 band of H2 O around 3400 cm−1 . In addition, shifts in the maximum of the H2 O band indicated changes in the network structure, or H-bonding of the water in the inclusions that is presumably related to dissolved CO2 . Comparison of the H2 O 3 band of pure water with those of standard CO2 –H2 O inclusions with increasing amounts of dissolved CO2 in the H2 O phase showed that the dissolved CO2 shifts the band maximum to higher frequencies because of increases in free and one- or zero-coordinated species in the water. Similar, but smaller, shifts were observed in the water phases for some of the inclusions which contained CO2 gas in the bubble phase, indicating the presence of some dissolved CO2 ; and supported by the presence of a single CO2 band at 2344 cm−1 . By contrast, the H2 O 3 band in the bubble phases of the inclusions showed a shift to lower frequencies, suggesting increased H-bonding in the region around the bubble. This could also result from structure-making dissolved species such as NaF, KF, LiF and MgCl2 , however, the presence of fluoride salts are unlikely and concentrations of dissolved salts in these inclusions estimated from the Tm measurements are too low to cause the shifts observed. Instead, we propose that the shift is due to increased networking of the water molecules at the bubble/water interface, but further studies are needed to confirm this. Spectral image maps of individual fluid inclusions showed variations in the distribution of CO2 and H2 O compositions within the inclusions. Variations in CO2 content between inclusions were also detected, with all but one inclusion clearly showing CO2 in the bubble. Mapping the distribution of CO2 in the bubble phase of Wairongomai inclusion 2 was hampered by intermittent leakage of atmospheric CO2 during the point by point mapping. Overall the CO2 distributions shown in the mapping images correlate with the measured Tm values, indicating some contribution to these from CO2 , although the CO2 levels were well below 10 mol%. In addition, shifts were observed in the maximum of the O H stretch mode around the bubble regions, suggesting that H-bonding of the water molecules occurs in the bubble and liquid regions. Further work is under way to quantify the very low levels of CO2 in inclusions detected here, which are too low to be detected by Raman microscopy. This has significant implications for indirectly determining pressure and the formation depth of various systems in deposits in the Earth’s crust where fluid inclusions can be studied. Acknowledgements This research was undertaken on the Infrared Microspectroscopy beam line at the Australian Synchrotron, Victoria, Australia. We thank Professor J.L. Mauk for his support of this work and valuable advice, and New Talisman Gold Mines Limited for their
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continuing support of our research, for their assistance with sample collection, and for permission to publish. We thank Professor Bob Bodnar of Virginia Tech for the synthetic CO2 –H2 O fluid inclusions and for helpful discussions. We also thank the New Zealand Ministry of Business, Innovation and Employment for financial support. References [1] E.A. Burke, Lithos 55 (2001) 139–159. [2] R.C. Burruss, in: I. Samson, A. Anderson, D. Marshall (Eds.), Fluid Inclusions, Analysis and Interpretation, Mineralogical Association of Canada Short Course Series, Vol. 32, Mineralogical Association of Canada, 2003, pp. 279–290. [3] H.M. Lamadrid, W.M. Lamb, M. Santosh, R.J. Bodnar, Gondwana Res. 26 (1) (2014) 301–310. [4] M.L. Frezzotti, S. Ferrando, F. Tecce, D. Castelli, Earth Planet. Sci. Lett. 351–352 (2012) 70–83. [5] B. Wopenka, J.D. Pasteris, J. Freeman, Geochim. Cosmochim. Acta 54 (1990) 519–533. [6] J.D. Pasteris, C.D. Kuen, R.J. Bodnar, Econ. Geol. 81 (1986) 915–930. [7] M.P. Simpson, J.L. Mauk, R.J. Bodnar, AusIMM New Zealand Branch Annual Conference, New Zealand, 2010. [8] N. Guilhaumau, P. Dumas, Oil Gas Sci. Technol.—Rev. IFP 60 (5) (2005) 763–779.
[9] N. Guilhaumou, V. Sautter, P. Dumas, Chem. Geol. 223 (1–3) (2005) 82–92. [10] N. Guilhaumou, P. Dumas, G.L. Carr, G.P. Williams, Appl. Spectrosc. 52 (8) (1998) 1029–1034. [11] H. Yamada, W.B. Person, J. Chem. Phys. 41 (8) (1964) 2478–2487. [12] G. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules, D. van Nostrand Company Inc., Princeton, NJ, 1957. [13] O. Barres, J. Dubessy, M. Pagel, Appl. Spectrosc. 41 (6) (1987) 1000–1008. [14] J.P. Blitz, C.R. Yonker, R.D. Smith, J. Phys. Chem. 93 (18) (1989) 6661–6665. [15] A.B. Christie, M.P. Simpson, R.L. Braithwaite, J.L. Mauk, S.F. Simmons, Econ. Geol. 102 (2007) 785–816. [16] R.J. Bodnar, Geochim. Cosmochim. Acta 57 (1992) 683–684. [17] J.W. Hedenquist, R.W. Henley, Econ. Geol. 80 (1985) 1379–1406. [18] M.P. Simpson, J.L. Mauk, R.J. Bodnar, 11th Pan-American Current Research on Fluid Inclusions Conference (PACROFI-XI), University of Windsor, Windsor, Canada, 2012. [19] R.H. Goldstein, T.J. Reynolds, Short Course 31, Society for Sedimentary Geology (SEPM), Tulsa, OK, 1994, pp. 199. [20] R.H. Goldstein, Lithos 55 (2001) 159–193. [21] J.B. Brubach, A. Mermet, A. Filabozzi, A. Gerschel, P. Roy, J. Chem. Phys. 122 (18) (2005) 184509(1)–184509(7). [22] D.A. Schmidt, K. Miki, J. Phys. Chem. A 111 (2007) 10119–10122. [23] T. Yokono, S. Shimokawa, M. Yokono, H. Hattori, Water 1 (2009) 29–34. [24] Z.S. Nickolov, J.D. Miller, J. Colloid Interface Sci. 287 (2) (2005) 572–580. [25] R.J. Linnen, H. Kepler, S.M. Sterner, Can. Mineral. 42 (2004) 1275–1282.
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Synchrotron FTIR microscopy of synthetic and natural CO2 –H2 O fluid inclusions夽 Michél K. Nieuwoudt a,∗ , Mark P. Simpson b , Mark Tobin c , Ljiljana Puskar c a b c
School of Chemical Sciences, University of Auckland, 23 Symonds St., Auckland 1142, New Zealand GNS Science, Wairakei Research Centre, 114 Karetoto Road, RD4, Taupo 3377, New Zealand Australian Synchrotron, 800 Blackburn Rd., Clayton 3168, VIC, Australia
a r t i c l e
i n f o
Article history: Received 25 March 2014 Received in revised form 3 August 2014 Accepted 3 August 2014 Available online 26 August 2014 Keywords: FTIR spectroscopy Synchrotron Fluid inclusions CO2 dissolved in H2 O Spectral images
a b s t r a c t Fluid inclusions provide a record of the physical and chemical composition of fluids that flow through the Earth’s crust. Here, we describe a novel use of synchrotron Fourier Transform Infrared (FTIR) microscopy to map three parameters in individual fluid inclusions: (1) very low concentrations of CO2 , (2) variations of the state of CO2 and (3) variations in water structure. The intensity of the synchrotron light source allows compositional mapping of individual fluid inclusions, which provides high-resolution images of the distribution of different CO2 and H2 O species within individual inclusions. High resolution spectra of the CO2 , recorded here for the first time in fluid inclusions, show the rotational lines of CO2 gas. The results reveal predicted as well as unexpected distributions of CO2 gas in the inclusions. © 2014 Elsevier B.V. All rights reserved.
Introduction Epithermal Au–Ag deposits are an important source of gold and silver, and the fluid inclusions trapped in veins are the only remains of the fluids present during the formation of these ore deposits. Microthermometric (heating and freezing) measurements from individual fluid inclusions provide valuable information on the temperature and apparent salinity (due to salt and/or dissolved CO2 ) of these inclusions, and element concentrations can be determined from laser ablation inductively coupled plasma mass spectrometry (LA-ICP–MS). Raman microscopy is a powerful method for analysis of a number of gases including CO2 as the excitation laser beam can be focused to micron dimensions and the use of confocal systems enables analysis within transparent matrices. Besides gases, this technique has also been used to analyse hydrocarbons and minerals in fluid inclusions; the latter either precipitated from solution or accidentally trapped [1–6]. For most epithermal deposits, however, the amount of CO2 within the inclusions, and its contribution to the apparent salinity is
夽 Paper presented at the 7th International Workshop on Infrared Microscopy and Spectroscopy with Accelerator-Based Sources (WIRMS), Melbourne, Australia, 10–13th November 2014. ∗ Corresponding author. Tel.: +64 99238875. E-mail addresses: [email protected] (M.K. Nieuwoudt), [email protected] (M.P. Simpson), [email protected] (M. Tobin), [email protected] (L. Puskar). http://dx.doi.org/10.1016/j.vibspec.2014.08.003 0924-2031/© 2014 Elsevier B.V. All rights reserved.
poorly known, and generally cannot be determined from Raman microscopy because the CO2 concentrations are typically lower than the detection limit of this technique [7]. Determining the contribution of CO2 is significant because it has implication for pressure and formation depth calculations, as well as impacts determining the absolute concentration of elements from LA-ICP–MS analyses which require an external standard that is based on the final ice melting temperature (Tm ). The Tm values can be due to either aqueous salts, dissolved CO2 or a combination of the two [17]. In this study, we used synchrotron FTIR microscopy in order to investigate the feasibility of this technique to detect very low concentrations of CO2 in fluid inclusions in epithermal veins from the Hauraki Goldfield, New Zealand and to provide semi-quantitative information on the amount of contained CO2 . In addition, spectral image maps of individual fluid inclusions were used to measure variations in the distribution of CO2 and H2 O compositions of the inclusions. Synchrotron FTIR microspectroscopy is an effective technique for examining the H2 O, CO2 and methane contents of fluid inclusions [8–10]. Due to the high brightness of synchrotron light, the aperture (size) of the incident or detected infrared (IR) beam can be reduced to values as small as 3 m with minimal loss of signal, which enables analyses of individual fluid inclusions and the areas inside them. The appearance and frequency of the 3 asymmetric stretch of CO2 differs distinctly depending on whether the CO2 occurs as a gas, liquid, or gas dissolved in a liquid [8,11–14]. For the solid and liquid phases of CO2 and for liquids or solids containing dissolved
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of dissolved CO2 is 3.5 wt.%, which would contribute −1.5 ◦ C to Tm [17]. Overall, there is little variation between primary, pseudosecondary and secondary inclusions based on the microthermometry, although some differences in chemical composition and concentrations are clearly evident from LA-ICP–MS analyses of individual fluid inclusions [7,18]. Material and methods
Fig. 1. Simplified map of the Hauraki Goldfield showing the location of the Karangahake, Waiorongomai and Tokatea deposits. Inset shows the location of the Hauraki Goldfield within the North Island of New Zealand [15].
CO2 gas, the 3 occurs as a single band ranging between 2336 and 2345 cm−1 [8,13]. At the spectral resolution of 4 cm−1 , which is generally used for solids and liquids, CO2 in the gas phase occurs as two overlapping but resolved bands centred at 2349 cm−1 , with the Pbranch around 2338 cm−1 and the R-branch at 2361 cm−1 . At higher resolutions (0.5 cm−1 and up) the rotational lines of the P- and Rbranches can be clearly seen. Therefore, in this study, spectra were also recorded at higher resolution (0.5 cm−1 ) in order to investigate the presence and variation of CO2 in the gas phase in the different fluid inclusions. Fluid inclusions analysed are from the Karangahake, Waiorongomai and Tokatea deposits located in the Hauraki Goldfield, New Zealand (Fig. 1), a 200 km long by 40 km wide metallogenic belt that contains the greatest concentration of precious metal deposits in New Zealand [15]. The Karangahake deposit (0.9 M oz. Au) was the third largest Au producer in the goldfield, whereas the main veins at both Tokatea and Waiorongomai are essentially barren with localised mineralisation restricted to splay veins. Fluid inclusions from all three deposits have overlapping ranges in homogenisation temperatures (Th ) and apparent salinities; the latter is determined from final ice melting temperatures (Tm ) and is reported as weight percent NaCl equivalent [16], but can be due to included CO2 [17]. Inclusions in quartz veins from the Karangahake deposit have a Th range of 198◦ to 287 ◦ C and less than 3.9 wt.% NaCl equivalent [7] and those from Waiorongomai a Th range of 195◦ to 298 ◦ C and salinities of less than 3.5 wt.% NaCl equivalent. Fluid inclusions in amethyst from the Tokatea deposit have a Th range of 175◦ to 280 ◦ C and apparent salinities of less than 2.3 wt.% NaCl equivalent [18]. No clathrate has been identified in any of the fluid inclusions from the three deposits and therefore the maximum concentration
Fluid inclusions measured from the Karangahake, Waiorongomai and Tokatea deposits occur in doubly polished plates of quartz and amethyst that are 150–200 m thick and were washed with acetone prior to analyses. Cavities occupied by the fluids range in size from 10 to 50 m and at room temperature consist of a liquid (75 to 80 vol%) and a vapour. This is a bubble that was created due to the contraction of the contained fluid on cooling. In the following, we use the term bubble to refer to the vapour component of the fluid inclusion. Measurements were made on carefully defined fluid inclusion assemblages (FIAs) identified prior to microthermometric measurements; these are a group of inclusions that were trapped simultaneously in a single event, such as an individual growth zone or a healed fracture [19,20]. Identification of FIAs were confirmed by narrow Th ranges and these inclusions are the focus of the FTIR study. FTIR spectra were collected in transmission mode with a Bruker Hyperion 2000 microscope coupled to a Bruker Vertex V80v FTIR spectrometer at the IR beamline of the Australian synchrotron in Melbourne. A narrow-band mercury cadmium telluride (MCT) detector and ×36 reflecting objective and condenser optics of 0.5 numerical aperture were used. Spectra were taken with microscope knife edge apertures set to 5 m, and spectral resolutions of 4 cm−1 and 0.5 cm−1 referred to hereafter as low and high resolution, respectively. The number of scans averaged for the low and high resolution spectra were 64 and 256, respectively, and the data acquisition rate was 40 kHz, equivalent to a mirror velocity of 1.28 cm/s. Bruker OPUS 6.5 spectroscopic software was used to collect the data and OPUS 7 to process the spectra. The microscope sample stage was enclosed in a Perspex chamber with continuous N2 purging and the chamber was purged for 10 min before analyses to eliminate atmospheric CO2 . The background spectrum of the surrounding quartz matrix was recorded before and after every spectrum, to remove the effects of any CO2 dissolved in the quartz matrix and to assess any changes in atmospheric CO2 due to ineffective purging. The spectral maps of individual fluid inclusions were recorded at step sizes of 2.5 m. The distribution and relative amounts of gaseous CO2 and aqueous CO2 , as well as variation in H-bonding were determined from chemical images of these maps, generated by integrating absorbance peak intensities of relevant functional groups using OPUS 7 spectroscopic software. A reference spectrum was also obtained of pure, de-ionized water by placing a drop of Milli-Q de-ionized water on a BaF2 disk and recording the spectrum in transmission mode at 4 cm−1 resolution using a Continuum FTIR microscope and Thermo Fisher Nicolet 8700 FTIR spectrometer. In addition, spectra were recorded for synthetic CO2 –H2 O inclusions in quartz with known concentrations of CO2 as references. These standards are routinely used for calibration of the microthermometric fluid inclusion stage and were provided by Professor Bob Bodnar, Virginia Tech. The total CO2 concentrations are 10, 25 and 50 mol% CO2 , with the latter two containing liquid CO2 . Results and discussion CO2 as free gas and dissolved in water Fig. 2 shows a representative example of both the low resolution (red spectrum), and high resolution (blue spectrum) asymmetric
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Fig. 2. (a) Low (red) and high (blue) resolution spectra of the 3 stretching mode of CO2 in the gas phase for a fluid inclusion in amethyst from the Tokatea deposit. (b) Low (red) and high (blue) resolution spectra of the 3 stretching mode of CO2 dissolved in water. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
stretch modes of CO2 (3 ) for the gas and liquid phase in a fluid inclusion. Fig. 2a shows the gas phase mode and Fig. 2b the same mode for CO2 dissolved in the water phase, where it occurs as a single band at 2346 cm−1 due to loss of rotational freedom. This spectrum also shows a shoulder at 2362 cm−1 . The rotational lines in the high resolution spectrum of the water phase are due to some atmospheric CO2 which may have leaked in during a lag in the N2 purging. FTIR spectra of individual inclusions FTIR spectra for the bubble and liquid phases have been collected for numerous carefully selected FIAs from the Karangahake and Waiorongomai deposits; Figs. 3 and 4 show representative spectra for each of the two deposits. FTIR analyses of FIAs hosted in quartz from the Karangahake deposit also detected CO2 in some inclusions with CO2 present as a gas phase in both the water and bubble phases (Fig. 3). The similarity of the intensities of the rotational lines in the region 2 of the water phase and the bubble (region 0), and no visible single band for CO2 dissolved in water indicate very low concentrations of CO2 in this inclusion, and/or possible atmospheric contamination. FTIR analyses of individual FIAs hosted in quartz from the Waiorongomai deposit showed that some inclusions contained CO2 whereas in many others it was not detectable. In Fig. 4, we show an inclusion in which the spectra of both the bubble and water phases show a single band representing CO2 dissolved in the water phase, suggesting significant concentrations of CO2 in the inclusion; the spectrum of the water phase also appearing in the bubble region because the bubble is surrounded by water. The rotational lines in the high resolution spectrum of the liquid phase can only be explained by some lags in the N2 purging that allowed leakage of atmospheric CO2 . Leakage of atmospheric gas into the sample chamber was confirmed by performing a Fourier deconvolution on some of the spectra to remove the interference fringing in the spectrum originating from the quartz inner surfaces. This revealed weak, sharp bands characteristic of atmospheric water vapour (Fig. 5). This leakage occurred intermittently. Comparison of the O-H stretch mode (3 ) of water between 3100 and 3700 cm−1 for the fluid inclusions from the two different deposits in Figs. 3 and 4 reveal variations in shape in the bubble and liquid phases within the same inclusions. This band is characteristically broad as a result of the large number of different
extents of hydrogen bonding (H-bonding) experienced by the water molecules and the shape and maximum frequency are known to be strongly influenced by the presence of dissolved species [21,22]. The H bond coordination numbers range from 0 to 4, resulting in the simultaneous occurrence of monomeric, dimeric and several polymeric species in rapid equilibrium, each with distinct, but very close vibrational frequencies. In some FTIR studies, the different populations have been described by five or six species with resolution of the band into six components [22,23] at around: (i) 3626 cm−1 (free OH), (ii) 3557 cm−1 (zero or one H bond), (iii) 3436 cm−1 (2 coordinated H2 O), (iv) 3251 cm−1 (3-coordinated H2 O), (v) 3154 cm−1 (3 or 4 coordinated H2 O) and (vi) 3062 cm−1 (4 coordinated H2 O). In other studies, however [21], the different populations have been described in only three components: (1) low energy component around 3250 cm−1 reflecting water molecules connected to three or four other molecules, i.e., those molecules building up a supermolecular connective network, (2) a component around 3450 cm−1 representing two-to-three coordinated water molecules, and (3) a higher energy component around 3590 cm−1 representing “free” water molecules which form either zero or only one H-bond to neighbouring water molecules. Dissolved gases and ionic salts in the water will change the Hbonding and also the relative proportions of these species, resulting in changes in the overall band shape and frequency [21,22,24]. In order to better characterise the variation of these different species in the bubble and water phases, we resolved the OH stretch mode between 3100 and 3700 cm−1 into six components and compared them with similarly resolved components for spectra also recorded in transmission mode of pure Milli-Q water and of synthetic CO2 –H2 O fluid inclusions in quartz whose CO2 concentrations and pressures were known: 10 mol% CO2 (at ∼60 bar), 25 mol% (65 bar) and 50 mol% CO2 (∼73 bar). At room temperature the 10 mol% inclusion consists of two phases: water and CO2 vapour phase (Fig. 6(a)), the 25 mol% inclusions contains three phases: liquid water, liquid CO2 and CO2 vapour (Fig. 6(b)) and the 50 mol% inclusion contains two phases: liquid water and liquid CO2 (Fig. 6(c)) and at these pressures the solubility of CO2 in the water in these three inclusions would be about 0.32, 1.38 and 1.56 molal, respectively. Although three components adequately fitted the OH mode of pure water, six were needed to adequately fit the band for the synthetic CO2 –H2 O standard inclusions. Band resolution of the OH stretch mode was performed using the Peak resolution function of the OMNIC spectroscopic software
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Fig. 3. Low resolution spectra of bubble (0) and liquid phases (1, 2) in an inclusion from sample AU62232, Karangahake deposit. Insets show expanded, high resolution of the CO2 3 band around 2340 cm−1 .
Fig. 4. Low resolution spectra of the bubble (0) and liquid phase (1) in an inclusion from sample AU62868, Waiorongomai deposit. Insets show expanded, high resolution of the CO2 3 band around 2340 cm−1 .
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Fig. 5. Comparison of spectra of atmospheric H2 O and CO2 , the bubble region and the quartz matrix adjacent to the fluid inclusion, after Fourier deconvolution to remove interference fringes. The presence of atmospheric CO2 and H2 O vapour bands in the quartz matrix and bubble region indicate leakage of atmospheric gas into the N2 purge chamber. The H2 O vapour bands are shown more clearly in the expanded spectra in the inset. Table 1 Component bands for OH stretch mode of standards, pure water and natural inclusions fitted by band resolution. Band Component
Coordination value [22]
Pure H2 O (cm−1 )
0.32 m CO2 (cm−1 )
1.32 m CO2 (cm−1 )
1.56 m CO2 (cm−1 )
Kar. bubble phase (cm−1 )
Kar. H2 O phase (cm−1 )
Waior. bubble Phase (cm−1 )
Waior. H2 O phase (cm−1 )
i ii iii iv v vi
“Free” 0 and 1 2 3 3 and 4 4
3649 3577 3440 3235 3333 3078
3646 3592 3483 3352 3229 3124
3754 3605 3478 3371 3225 3037
3710 3647 3519 3380 3249 3156
3669 3527 3374 3220 3090 2911
3746 3607 3489 3369 3208 3074
3718 3614 3498 3368 3202 3070
3771 3587 3477 3348 3220 3078
which fits components of combined Gaussian and Lorentzian algorithms to the band using least squares regression. The resolved species in pure water are given in Fig. 7(a) and those in the three standards are given in Fig. 7(b)–(d). The fitted component frequencies are given in Table 1 and a plot of the relative areas of three differently coordinated H2 O species as described in [21] (obtained by combining (i + ii) to represent 0–1 coordinated H2 O, (iii + iv) to represent 2–3 coordinated and (v + vi)) representing 3–4 coordinated H2 O molecules) is given for each of the above standards as a function of concentration of dissolved CO2 in Fig. 8. This plot shows different trends for the differently coordinated H2 O molecules with increasing dissolved CO2 . There appears to be an increase in the relative amounts of low-coordination H2 O molecules (less networking
or H-bonding) with increasing concentration of dissolved CO2 , and a concomitant decrease in the more highly coordinated (3–4) H2 O molecules, or more networked species. The less networked species (represented by band components i and ii), with lower coordination of 0 to 1 increase with increasing CO2 concentrations when compared with pure water. Accordingly the presence of dissolved CO2 appears to increase the less networked water species, which are poorly connected to their environment compared with pure water, thus contributing to structure breaking of the water, or decreasing the level of H-bonding and shifting the band maximum to higher wavenumbers. Similar increases in the higher energy maxima with increasing mol% CO2 were observed in the OH stretch mode of spectra recorded at 300 ◦ C of synthetic CO2 –H2 O inclusions
Fig. 6. A 10 mol% CO2 inclusion (∼60 bar) showing two phases of CO2 vapour in the bubble and surrounding H2 O liquid (B) 25 mol% CO2 inclusion (∼65 bar) with three phases of CO2 vapour in the bubble surrounded by CO2 liquid and H2 O liquid, 5 (C) 50 mol% CO2 inclusion (∼73 bar) with two phases: CO2 liquid surrounded by H2 O liquid.
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Fig. 7. a O H stretching mode of pure H2 O, (b) 3 mode for H2 O phase of 10 mol% CO2 inclusion, (c) 3 mode for H2 O phase of 25 mol% CO2 inclusion, (d) 3 mode for H2 O phase of 50 mol% CO2 inclusion (e) 3 mode for spectrum of bubble phase of Karangahake inclusion (AU62232), (f) 3 mode for H2 O phase (area 2) of Karangahake inclusion. (g) O H stretching mode in spectrum of bubble phase of Waiorongomai inclusion 2 (sample AU62868), (H) O H stretching mode in H2 O phase of Waiorongomai inclusion 2. Fitted components: (i) 3649 cm−1 (free OH, navy blue), (ii) 3577 cm−1 (zero or one H bond, pink), (iii) 3440 cm−1 (2-coordinated H2 O, cyan), (iv) 3333 cm−1 (3-coordinated H2 O, green), (v) 3235 cm−1 (3- or 4-coordinated H2 O, purple) and (vi) 3078 cm−1 (4-coordinated H2 O, blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 8. Plot showing trends in relative amounts of differently-coordinated groups of H2 O molecules with increasing concentration of dissolved CO2 .
between 12.5 and 65 mol% CO2 [25]. This suggests that CO2 dissolved in water contributes to a decrease in the H-bonding, or structure breaking, of the water, even at very low concentrations, and that this effect increases with increasing amounts of dissolved CO2 . In addition, there also appear to be slight shifts to higher frequencies in some of the fitted components with increasing CO2 concentration, as shown in the band components given in Table 1. Compared to pure water, the bubble region of the Karangahake inclusion shows a red shift of the band maximum, to about 3200 cm−1 from about 3400 cm−1 for pure water (Fig. 7(a) and (e)). This appears to be mainly the result of increased relative intensities for the 3 and 4-coordinated species, or more highly networked species (component bands v and vi), which suggests that the water molecules around the bubble region exhibit increased H-bonding. In addition, there appear to be slight shifts of some of the band components to lower frequencies. A detailed study of the effect of dissolved salts on the relative intensities of the band components and shape of the O H stretch mode in D2 O [24] has shown that the presence of dissolved salts such as alkali fluorides can also contribute to such shifts. It was found that while dissolution of chloride and iodide salts results in shifts of the band maximum to higher frequencies, showing structure breaking character, dissolved fluoride salts shift the OH band to lower frequencies, showing structure making character [24]. However, the shifts were significant only for water:salt mole ratios of less than 25:1. For both inclusions shown in Figs. 3 and 4, with their corresponding resolved H2 O bands shown in Fig. 7(e) and (g), respectively, the red shifts in band maxima due to increased H-bonding appear to occur in the bubble region only, which would not be expected to contain dissolved salts. A more likely explanation for the observed shifts of the maxima to lower frequencies for our inclusions may be the occurrence of increased networking of the water molecules near the water–gas interface (Fig. 9). 2-D IR spectral images 2-D spectral images for four fluid inclusions from the Karangahake and Waiorongomai deposits are presented below and show the variation of the occurrence of CO2 gas (2361 cm−1 ), CO2 dissolved in water (2380–2520 cm−1 ) and H2 O (2520–2330 cm−1 ) in the inclusions. The 2-D spectral images of two different fluid inclusions mapped from the Waiorongomai deposit are shown in Figs. 10 and 11. Both inclusions are from the same FIAs and have essentially identical Tm values of −1.0 and −1.1 ◦ C that correspond to apparent salinities of 1.7 and 1.9 wt.% equiv. NaCl. The spectral image for the first
Fig. 9. Plot showing the variation in relative amounts of differently-coordinated groups of H2 O molecules in the bubble and liquid regions of the Karangahake and Waiorongomai inclusions, compared with those for pure water.
inclusion shows that the greatest concentration of CO2 gas is within the bubble (Fig. 10(c)) and the highest concentrations of CO2 dissolved in water (Fig. 10(d) coincide with the maximum amount of water (Fig. 10(b) which indicates the deepest part of the inclusion. The spectral images for the second inclusion shown in Fig. 11 also show that the vapour bubble contains CO2 gas, however, there is additional CO2 gas in two large areas of the inclusions where CO2 gas was not expected; in part of the water phase but also in the quartz matrix (Fig. 11d). CO2 gas was also seen in the water phase in the high resolution spectra in Fig. 4a. These either reflect atmospheric CO2 leaking at two different periods while these two areas were being mapped, because of the long collection times needed for the spectral mapping, or movement of the bubble phase during the measurement. The presence of CO2 in the inclusion is confirmed by the single band for dissolved CO2 . The spectra in the lower part of Fig. 11 show shifts in the maximum to lower frequencies for the 3 mode of H2 O around the bubble region, which indicates increased networking of the water around the bubble. Although both inclusions are from the same FIA and essentially have the same Tm values, the greater intensity of the CO2 in the bubble phase in the first inclusion (Fig. 10) suggests it has a higher CO2 concentration, shown also by the greater intensity of the single peak due to CO2 dissolved in the water phase. This would imply that they have different concentrations of dissolved salts. The 2-D spectral images of two fluid inclusions mapped from the Karangahake deposit are shown in Figs. 12 and 13. The two fluid inclusions are from separate FIAs and have very different Tm values of −3.3◦ and −0.2 ◦ C that correspond to apparent salinities of 5.4 and 0.4 wt.% equiv. NaCl, respectively. Spectral images for first inclusion show that CO2 gas is present in both the vapour and dissolved in the H2 O phase (Fig. 12). CO2 is concentrated within the bubble but is also enriched in two regions away from the bubble; the latter may be caused by movement of the bubble. The greatest intensity for CO2 dissolved in water overlaps with the region of greatest H2 O intensity and coincides with the thickest part of the inclusion. Although the amount of CO2 present cannot be quantified, the maximum amount that can possibly be present is 3.5 wt.% CO2 : this amount is constrained by the absence of any clathrate seen on freezing and would contribute −1.5 ◦ C to the Tm value [17]. Accordingly this inclusion must have at least 1.7 wt.% equiv. NaCl in addition to the CO2 . In contrast, the 2-D spectral image for the second Karangahake inclusion (Fig. 13) reveals that CO2 gas is absent from where the bubble phase should be. Fig. 13(b) shows CO2 gas concentrated at the lower left side of the inclusion, coinciding with the quartz matrix and part of the liquid phase. The CO2 gas can only be atmospheric and due to inadequate purging during collection of
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Fig. 10. A Image of Waiorongomai inclusion 1. (B) Mapping of O H stretch mode, with integration area shown in (C). (D) Mapping of CO2 R branch for gas phase (2381–2353 cm−1 ) (E) Mapping of dissolved CO2 in water, occurring as a single peak (2354–2333 cm−1 ). Integration for (D) and (E) maps is shown in (F).
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Fig. 11. A Image of a fluid inclusion from the Waiorongomai deposit (AU62868) inclusion 2 (B) Mapping of H2 O mode (3700–3350 cm−1 ) with corresponding integration areas shown in (C). (D) Mapping of CO2 R branch for gas phase (2381–2353 cm−1 ) with integration areas shown in (F). (E) Mapping of CO2 dissolved in water, occurring as a single peak over the area (2350–2338 cm−1 ) shown by integration region in Fig (G).
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Fig. 12. A Image for Karangahake (AU62232) inclusion 1. (B) Map of total H2 O mode with integrated region covering the range 3722–3013 cm−1 shown in (C). (D) Map of CO2 gas (R branch at 2385–2353 cm−1 and (E) map of CO2 gas dissolved in water (2352–2316 cm−1 ), both integrated regions shown in (F).
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Fig. 13. A Image of Karangahake (AU62232) inclusion 2 (B) Map of CO2 gas with integration area covering both P and R branches over 2384–2305 cm−1 as shown in (C)). (D) Map of higher energy H2 O mode represented by integrated peak areas 3708–3398 cm−1 shown in (F). (E) Map of lower energy H2 O mode represented by integrated peak areas 3312–2978 cm−1 in (F).
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CO2 ; this correlates with its low Tm value of −0.2 ◦ C. The Tm of the Karangahake 1 inclusion is −3.3 ◦ C and while the spectral maps in Fig. 12 show that CO2 is present, the comparable intensities with Waiorongomai inclusion 1 suggests that it must have higher dissolved salt concentrations. The two Karangahake inclusions were from the same FIA but show very different Tm values and also different levels of trapped CO2 . Conclusions
Fig. 14. Comparison of spectra of the liquid phase of the Wairongomai inclusion 1 (AU62868) with that of a standard synthetic inclusion containing 10 mol% CO2 . The single band due to dissolved CO2 is indicated by arrows in each spectrum.
the set of points over this region, as can be seen by CO2 gas bands in spectra of the quartz matrix shown in Fig. 13(c). This is also confirmed by the absence of any single band for CO2 dissolved in H2 O in the spectra of the inclusion. The spectra in Fig. 13(f) show two distinct groups for the H2 O band with different shapes, with maxima occurring around 3620 cm−1 and 3180 cm−1 . The H2 O mode has therefore been plotted in two maps (Fig. 13(d) and (e)). The spectra with maxima shifted to higher wavenumbers in Fig. 13(d) coincide with the water phase. The maxima at the lower frequency side of the H2 O band coincide with the bubble region in Fig. 13(e) and this suggests that water molecules around the bubble region have increased H-bonding. This may be due to increased networking of the water molecules at the vapour/liquid interface of the bubble. The spectral images show that this inclusion has no detectable CO2 and accordingly in this case the Tm value, though dilute, must be mainly due to dissolved salts. The thickest portions of the inclusions were revealed from the intensity of H2 O band. The variation in thickness of the inclusions implies that it would not be possible to obtain quantitative measurements of the concentration of CO2 in the inclusions from absorbance measurements using Beer’s law (A = ∈bC), where A is the absorbance, ∈ the absorption coefficient, b the path length (inclusion depth) and C the concentration of CO2 . While it is not possible to quantify the levels of CO2 in the inclusions from the spectra, a comparison of the relative intensities of the dissolved CO2 single band and the H2 O band in the 10 mol% CO2 standard inclusion and the Wairongomai inclusion 1 (Fig. 14) does show that they are very low. The detection of such low CO2 levels in these inclusions, not possible with Raman microscopy, is enabled by the increased sensitivity afforded by the high intensity of the synchrotron infrared source. The presence of CO2 , albeit at such low levels, has implications for interpretation of the measured Tm values, to which both dissolved salts and CO2 contribute. The spectral maps show that the vapour bubble in both the Waiorongomai deposit inclusions and the first inclusion in the Karangahake deposit contain CO2 gas, and that the H2 O phase also contains dissolved CO2 , however, the amounts of CO2 are very low. The Tm values of the Waiorongomai inclusions 1 and 2 are −1.0 ◦ C and −1.1 ◦ C, respectively. The spectral images and intensities of the single CO2 band in Fig. 10 show more CO2 in the Wairongomai 1 inclusion, suggesting that inclusion 2 has relatively more dissolved salts. In contrast, the spectral map for the Karangahake 2 inclusion in Fig. 13 shows no detectable
Very low concentrations of CO2 have been detected in individual fluid inclusions in epithermal quartz veins from the Waiorongomai and Karangahake deposits by FTIR microscopy using the synchrotron IR beam. Previous laser Raman analyses of inclusions from this deposit could not detect CO2 because the concentrations were below the level of detection. Semi-quantitative information about the CO2 gas in the bubble and dissolved in the liquid phases in the inclusions was obtained from observations of the 3 band around 2400 cm−1 , and by comparison with the 3 band of H2 O around 3400 cm−1 . In addition, shifts in the maximum of the H2 O band indicated changes in the network structure, or H-bonding of the water in the inclusions that is presumably related to dissolved CO2 . Comparison of the H2 O 3 band of pure water with those of standard CO2 –H2 O inclusions with increasing amounts of dissolved CO2 in the H2 O phase showed that the dissolved CO2 shifts the band maximum to higher frequencies because of increases in free and one- or zero-coordinated species in the water. Similar, but smaller, shifts were observed in the water phases for some of the inclusions which contained CO2 gas in the bubble phase, indicating the presence of some dissolved CO2 ; and supported by the presence of a single CO2 band at 2344 cm−1 . By contrast, the H2 O 3 band in the bubble phases of the inclusions showed a shift to lower frequencies, suggesting increased H-bonding in the region around the bubble. This could also result from structure-making dissolved species such as NaF, KF, LiF and MgCl2 , however, the presence of fluoride salts are unlikely and concentrations of dissolved salts in these inclusions estimated from the Tm measurements are too low to cause the shifts observed. Instead, we propose that the shift is due to increased networking of the water molecules at the bubble/water interface, but further studies are needed to confirm this. Spectral image maps of individual fluid inclusions showed variations in the distribution of CO2 and H2 O compositions within the inclusions. Variations in CO2 content between inclusions were also detected, with all but one inclusion clearly showing CO2 in the bubble. Mapping the distribution of CO2 in the bubble phase of Wairongomai inclusion 2 was hampered by intermittent leakage of atmospheric CO2 during the point by point mapping. Overall the CO2 distributions shown in the mapping images correlate with the measured Tm values, indicating some contribution to these from CO2 , although the CO2 levels were well below 10 mol%. In addition, shifts were observed in the maximum of the O H stretch mode around the bubble regions, suggesting that H-bonding of the water molecules occurs in the bubble and liquid regions. Further work is under way to quantify the very low levels of CO2 in inclusions detected here, which are too low to be detected by Raman microscopy. This has significant implications for indirectly determining pressure and the formation depth of various systems in deposits in the Earth’s crust where fluid inclusions can be studied. Acknowledgements This research was undertaken on the Infrared Microspectroscopy beam line at the Australian Synchrotron, Victoria, Australia. We thank Professor J.L. Mauk for his support of this work and valuable advice, and New Talisman Gold Mines Limited for their
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continuing support of our research, for their assistance with sample collection, and for permission to publish. We thank Professor Bob Bodnar of Virginia Tech for the synthetic CO2 –H2 O fluid inclusions and for helpful discussions. We also thank the New Zealand Ministry of Business, Innovation and Employment for financial support. References [1] E.A. Burke, Lithos 55 (2001) 139–159. [2] R.C. Burruss, in: I. Samson, A. Anderson, D. Marshall (Eds.), Fluid Inclusions, Analysis and Interpretation, Mineralogical Association of Canada Short Course Series, Vol. 32, Mineralogical Association of Canada, 2003, pp. 279–290. [3] H.M. Lamadrid, W.M. Lamb, M. Santosh, R.J. Bodnar, Gondwana Res. 26 (1) (2014) 301–310. [4] M.L. Frezzotti, S. Ferrando, F. Tecce, D. Castelli, Earth Planet. Sci. Lett. 351–352 (2012) 70–83. [5] B. Wopenka, J.D. Pasteris, J. Freeman, Geochim. Cosmochim. Acta 54 (1990) 519–533. [6] J.D. Pasteris, C.D. Kuen, R.J. Bodnar, Econ. Geol. 81 (1986) 915–930. [7] M.P. Simpson, J.L. Mauk, R.J. Bodnar, AusIMM New Zealand Branch Annual Conference, New Zealand, 2010. [8] N. Guilhaumau, P. Dumas, Oil Gas Sci. Technol.—Rev. IFP 60 (5) (2005) 763–779.
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