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Extraction of gypsum hydration water for oxygen isotopic analysis by the guanidine hydrochloride reaction method
Chemical Geology 217 (2005) 89 – 96 www.elsevier.com/locate/chemgeo
Extraction of gypsum hydration water for oxygen isotopic analysis by the guanidine hydrochloride reaction method Elisabet Playa`a,*, Clemente Reciob, John Mitchellc a
Department of Geoquı´mica, Petrologia i Prospeccio´ Geolo`gica, Facultat de Geologia, Universitat de Barcelona, Martı´ Franque´s, s/n. 08028 Barcelona, Spain b Department of Geology and Stable Isotope Laboratory, Universidad de Salamanca, Salamanca, Spain c University of Newcastle, Newcastle upon Tyne, England, United Kingdom Received 17 May 2004; received in revised form 18 November 2004; accepted 16 December 2004
Abstract The release and collection of gypsum hydration water without isotopic fractionation, or contamination by adsorbed moisture, is complicated by the fact that gypsum starts to lose part of its water at low temperatures, which prevents heating the sample as a cleaning strategy. Additionally, results indicate that even at low vacuum conditions (from 102 to 103 mb) hydration water of gypsum can be lost, given time enough, particularly if associated with moderately raised temperature. In this report we describe an analytical procedure suitable for the complete release and collection of gypsum hydration water, at the same time that external water is efficiently excluded. The critical step is the elimination of external moisture without the removal of any hydration water. To minimize water losses to the walls of the vacuum line, a low-volume, transfer system (TS) has been designed and built. The procedure requires only about 20–25 mg of gypsum sample (4–5 Al of water) for the oxygen isotopic analysis by the guanidine hydrochloride reaction method, to an overall reproducibility of F0.5x or better. D 2004 Elsevier B.V. All rights reserved. Keywords: Hydration water; Gypsum; Oxygen isotope analysis; Guanidine hydrochloride; Water extraction
1. Introduction The depositional conditions and diagenetic changes of gypsum evaporites can frequently be identified by petrographic means, and the isotopic analysis of gypsum hydration water can provide * Corresponding author. Tel.: +34 93 402 1401; fax: +34 93 402 1340. E-mail address: [email protected] (E. Playa`). 0009-2541/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2004.12.005
additional evidence of the fluids involved in primary precipitation and diagenesis. However, little work is available on the release and isotopic analysis of gypsum hydration water (though see, for example, Fontes, 1965; Sofer, 1978; Pierre, 1982; Halas and Krouse, 1982; see Table 1). Likewise, there is a great lack of information regarding isotope composition of hydration water of other minerals of potential interest. The release of gypsum hydration water is a twostage process. First gypsum transforms to bassanite,
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Table 1 Main works on the topic of gypsum hydration water, with the laboratory procedures (time and temperature) used to remove moisture and hydration water of gypsum Author(s)
Gonfiantini and Fontes (1963) Fontes (1965) Fontes et al. (1973) Sofer (1978) Pierre and Fontes (1978) Longinelli (1979/1980)
Moisture elimination
Hydration water extraction
Stage 1
Stage 2
Not described
400 8C, unknown period of time 40 min at 400 8C
15–20 min at room temperature Not described 15–20 min at room temperature Not described Not described
Not described 45 min at 450 C
Not described 400 8C, unknown period of time Pierre and Ortlieb (1980) Not described Not described Pierre (1982) Not described Not described Halas and Krouse (1982) 30 min at Tb30 8C 400 8C, unknown period of time Bath et al. (1987) Not described 250 8C, unknown period of time Yonge and Krouse (1987) Not described Not described Khademi et al. (1997) 30 min at Tb45 8C 30 min at 450 8C Vacuum conditions used at stage 1 not specified by the respective authors.
and subsequently to anhydrite (reactions (1) and (2), respectively). 2CaSO4 d2H2 OY2CaSO4 d1=2H2 O þ 3H2 O
ð1Þ
2CaSO4 d1=2H2 OY2CaSO4 þ H2 O
ð2Þ
dry environment, gypsum slowly starts to transform to bassanite at 70 8C. In saturated solution and under high pressure (N350 bar), gypsum and the hemihydrate (bassanite) can coexist to temperatures higher than 100 8C (McConnell et al., 1987). In experiments from 60 to 120 8C, and heating rates from 0.1 8C/min to 5 8C/min, De las Cuevas (1992) observed that the dehydration temperature depends on heating rate. Mees and Stoops (2003) performed heating experiments using gypsum crystals and observed that dehydration of gypsum to bassanite (without advancing to anhydrite formation) started at 80 8C. The influence of vacuum conditions on the dehydration of gypsum has not otherwise been described in detail and most of the studies are mainly based in untested suppositions. The aim of the present study was therefore to identify the most convenient analytical procedure to ensure complete dehydration of gypsum and quantitative collection of released hydration water for oxygen isotopic analysis, while ensuring that nonhydration components were effectively excluded. The method was optimized to small quantities of gypsum samples (20 mg). Hydrogen isotopic analysis (given the large mass difference between H and D, and the associated large fractionations) poses additional difficulties, and is currently the subject of on-going research. Moreover, the experience we have gained from the study of gypsum has encouraged us to extend our investigations to other hydrated minerals, such as zeolites and various magnesian sulphates.
2. Materials and procedure The process is mainly controlled by water activity, temperature and pressure, and all of these factors may vary widely in the geological environment. In aqueous systems at atmospheric pressure, gypsum starts to lose its hydration water at 42 8C (Braitsch, 1971; Deer et al., 1983), but it can remain as a metastable hydrated mineral up to, at least, 93 8C (Holser, 1979). In saline systems, the dehydration temperature decreases as water activity diminishes (Hardie, 1967). All the studies quoted above were carried out under specified conditions in bwetQ environments. However, stable isotopic analysis of hydration water is carried out under bdryQ (vacuum) conditions which effectively exclude adsorbed water and atmospheric moisture. Deer et al. (1983) pointed out that, at atmospheric pressure in a
2.1. The extraction of the hydration water of gypsum It is obviously essential to exclude bnon-hydrationQ water (moisture, adsorbed water) from the analysis, while at the same time, early loss of hydration water must be avoided. As a general rule the process of water extraction is divided into two stages (Table 1): Stage 1 Sample pre-treatment for a bshortQ period of time and at low (room) temperature, in order to eliminate non-hydration water from gypsum. This component of water (moisture, absorbed water) is eliminated by pumping, but the type of vacuum pump and vacuum
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conditions achieved are not specified in the literature. Stage 2 Extraction of the hydration water of gypsum, at a higher temperature and for a longer period of time, again under unspecified vacuum conditions.
Table 3 Analytical procedures tested to eliminate moisture and collect gypsum hydration water Procedure
1
Different time and temperature conditions were tested for gypsum pre-treatment (stage 1) to ensure that bnon-hydrationQ water was eliminated without loss of hydration water (Table 2, tests A to D). Based
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2 3 4
Moisture elimination
Hydration water extraction
Stage 1
Stage 2
20 min at 45 8C, HV 10 min at 45 8C, HV 10 min at room temperature, HV 2 min at room temperature, HV
30 min at 400 8C, vacuum 45 min at 450 8C, vacuum 45 min at 450 8C, vacuum 45 min at 450 8C, vacuum
Table 2 Different time–temperature settings tested to check the mobility of gypsum hydration water under vacuum conditions
HV: high vacuum (better than 103 mb).
Test
Laboratory conditions
A
50 8C, 30 min, LV
0.1
B
130 8C, 30 min, LV
20.1
C
Room temperature, LV 30 min 2h 6h 24 h 3 days
on our findings, we adopted procedures 1 to 4 (see Table 3) and examined their respective effect on the oxygen isotopic composition measured on the stage 2 water released from four gypsum samples (Table 4). The main difference between procedures was in stage 1 (Table 3) as we sought a complete separation of bnon-hydrationQ water; the extraction of hydration water (stage 2) largely followed the general procedures described in previous literature. In order to extract, separate and collect the hydration water of gypsum we developed a so-called
D
Weight loss (%)a
0 0 0 0 0.3
4 days
1
7 days
4.2
8 days
4.9
10 days
6
13 days
7.1
Room temperature, HV 15 min 2h 1 day
0 0 1.8
2 days
4.6
5 days
13.8
Type of extracted water Moisture (+some hydration water?) Moisture+hydration water Moisture Moisture Moisture Moisture Moisture+some hydration water Moisture+some hydration water Moisture+some hydration water Moisture+some hydration water Moisture+some hydration water Moisture+some hydration water Moisture Moisture Moisture+some hydration water Moisture+some hydration water Moisture+some hydration water
LV: low vacuum (between 102 and 103 mb); HV: high vacuum (better than 103 mb). Two to three fragments (1–2 mm in size each one) of pure crystalline gypsum samples are used in all the tests. a Percentage of weight lost with respect to the original whole gypsum sample, which is assumed to correspond to the weight of water extracted from the gypsum.
Table 4 Amount of water extracted (wt.%) by the different procedures (1 to 4, as defined in Table 3) and its d 18OV-smow obtained by the GuHCl– water reaction method d 18Ov-smow (x)
Procedure
Sample
Hydration water (%)
1 2 3 4
CY-23
11.6 13.4 18.3 20.6
6.6 4.7 2.4 2.4
1 3 4
MOR-49
14.0 20.0 19.6
6.8 4.2 4.0
1 2
MRA-17
17.7 19.3
3.0 2.7
1 3
RS-17
19.4 19.3
0.4 0.2
Stoichiometric gypsum should contain 20.9% H2O. Hydration water measured as the weight percent loss with respect to the original bulk sample.
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btransfer systemQ (TS), composed of 3 parts (Fig. 1): (i) a 6 mm O.D. quartz tube which contains the sample; (ii) a 12 mm O.D. quartz tube, used as a cold trap; and (iii) a 6 mm O.D. borosilicate (Pyrexk) glass tube, where the water–guanidine reaction takes place. The remainder of the TS was constructed from 6 mm O.D., 1 mm wall stainless steel pipe using Swagelokk couplings and fittings, and Nuprok valves to isolate it from the vacuum pumping system and to isolate individual sections. The glass parts were connected to the stainless steel section by means of CAJONk O-ring unions. The main advantages of the TS are its small overall dimensions (4060 cm, approximately) and internal volume (tubes of 3 and 4 mm I.D.), the absence of plastic valves and a robust construction (metal and glass). Its construction
allowed the TS to be fully degassed with a heat gun and/or hand torch (but not exceeding 200 8C on the guanidine hydrochloride tube), and the facility to keep it heated during the period of water release. In addition, a flexible electric heating tape was wrapped around the whole length of the piping in order to keep it warm, thus preventing cold spots where water could condense. The overall volume (including valves and fittings) is below 25 ml. The dimensions adopted were dictated by the available tubular furnaces and dewars. There is no reason, however, why the internal volume could not be significantly reduced if required. Thus, within the TS, mineral dehydration and water transfer could be fully controlled and the drawbacks of large glass vacuum lines avoided. Water was quantitatively collected, as the system ensured that there was no adsorption to the glass walls or cold plastic valves. 2.2. The guanidine hydrochloride reaction method
Fig. 1. The transfer system (TS). (A) Sample tube: quartz, 6 mm O.D.3.5 mm I.D.200 mm long. (B) Water trap: quartz, 12 mm O.D.9 mm I.D.150 mm long. (C) Reaction tube: borosilicate glass (Pyrexk), 6 mm O.D.3.5 mm I.D.200 mm long. (a) Sample; (b) guanidine hydrochloride; (c) electric heater; (d) stainless steel line, 6 mm O.D.4 mm I.D.400 mm overall length; (e) NUPROk vacuum valves.
To perform oxygen isotope analyses on the extracted hydration water we used the guanidine hydrochloride–water reaction (GuHCl) method (Dugan et al., 1985; Viglino et al., 1985) rather than CO2–H2O equilibration (Dostrovsky and Klein, 1952; Epstein and Mayeda, 1953). Although the GuHCl method was efficiently adapted by Yang et al. (1996) to study fluid inclusions in halite and brines, it had not been tested in gypsum before. Despite the fact that a complete analysis using the GuHCl method requires about 2 working days per sample (though 4–6 samples can conveniently be processed in parallel), involving several discrete tasks using hazardous laboratory reagents (GuHCl, phosphoric acid), it offers the significant advantage that only a small sample is required (20 to 25 mg of gypsum, to yield 4–5 Al of water), in contrast with the 5–10 g required for the CO2–H2O equilibration (Bath et al., 1987; Khademi et al., 1997). Since the sample is maintained under vacuum throughout the procedure, the chance of contamination by atmospheric water is minimal. The analyses were performed by weighing 20 to 25 mg of gypsum sample and 70 mg of guanidine hydrochloride, respectively, into tubes A and C of the TS (Fig. 1). After evacuating the whole line and degassing it using a heat gun, the sample was allowed to outgas at low vacuum. When the pressure was below 103 mb, it was pumped for a further 2–10 min at room
E. Playa` et al. / Chemical Geology 217 (2005) 89–96
temperature to high vacuum (see Section 3.1), and then it was isolated from the pumping line. The GuHCl was simultaneously purified by melting under vacuum at 215 8C for 5 min, in order to eliminate adsorbed water. Dehydration of the gypsum was begun by heating the sample to 450 8C using a tube furnace, and the released water was continuously collected in the liquid nitrogen-cooled quartz trap B for 45 min. Any CO2 mixed with the water released by the mineral was separated by replacing the liquid nitrogen (LN2) with a higher temperature trap (solid CO2 plus acetone slush). Non-condensible gases were pumped away to vacuum, and the water was transferred from trap B to the GuHCl tube C, by immersing its lowermost part in LN2. Trap B is required to ensure that only pure water is transferred to the GuHCl tube C. While still at LN2 temperature, the bottom part of tube C is sealed by means of an oxygas torch and removed from the TS. From here on, the general procedure of Dugan et al. (1985) and Viglino et al. (1985) was followed. To extract the CO2 for isotopic analysis, the sealed glass tubes were cleaned with alcohol, scored at an appropriate height to ease rupturing and put into a cracker assembly containing 1 ml 103% H3PO4. The cracker was attached to the vacuum line and pumped overnight. The next morning, the sample tube was broken and the cracker put into a furnace at 80 8C for 1 h, so that the ammonium carbamate carrying the oxygen of interest reacts with the acid to release CO2. The CO2 thus produced was isolated and purified for mass spectrometric analysis in a vacuum line, using an acetone/dry ice slush trap and finally collected using LN2. The CO2 volume was measured manometrically, and analysed using a dual inlet VG ISOGAS SIRA series II mass spectrometer. The results have been normalised to the SLAP-SMOW scale, and are reported in the conventional d 18O (x) notation relative to V-SMOW (Vienna Standard Mean Ocean Water, Craig, 1961; Coplen et al., 1996).
3. Results and discussion 3.1. Effect of vacuum on the release of hydration water of gypsum In an attempt to gain information about the release of hydration water under reduced pressure, 100 mg
93
aliquots of a very pure, crystalline selenitic gypsum (sample CY-23) were subjected to various conditions of temperature, heating duration and vacuum (Table 2, tests A to D). After each test, weight loss of each aliquot was determined in order to assess the degree of loss of hydration water. This gravimetric approach was possible because CY-23 was 100% pure gypsum (determined by XRD and thin section petrography), and its geochemical characteristics (Sr content, and O, S and Sr isotopic compositions) were totally consistent with its marine origin (Playa` , 1998). For procedures A, B and C, so-called low vacuum (LV; between 102 and 103 mb), was achieved using a dual-stage rotatory vane pump, and was measured using Piranik gauges. Below 102 mb, these measurements can be confirmed by using Penningk gauges. Procedure D was carried out under high vacuum (HV; better than 103 mb) conditions, using an oil diffusion pump. The results of the experiments show that even at LV, hydration water of gypsum can be removed, given enough time, and that increased temperature, particularly above 50 8C, accelerates its removal (Table 2). The results clearly indicate that time, temperature and vacuum parameters are all critical to the process of gypsum dehydration. 3.2. Identification of temperature and time conditions for hydration water extraction The d 18OV-smow of hydration water extracted from several gypsum samples by the different protocols listed in Table 3 (procedures 1 to 4) was analysed as described in Section 2.2. The amount of water recovered (expressed as the equivalent weight percent loss with respect to the original bulk sample) shows that the yield achieved in each extraction is proceduredependent (Table 4). The amount of hydration water collected in stage 2 increased (from almost 12% to nearly 21%) from procedure 1 to 4, as the stage 1 pumping time and temperature decreased. The lowest yield was obtained when the sample was outgassed to HV for 20 min at 45 8C (procedure 1). This also yielded the heaviest d 18OV-smow values. When stage 1 was carried out at room temperature (procedures 3 and 4), however, the yields at stage 2 were higher and the isotopic values lighter (Table 4). The most plausible explanation is that procedures 1
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and 2 caused a partial extraction of hydration water during stage-1 outgassing. The first water to be released (adsorbed atmospheric moisture) would be expected to be isotopically lighter than hydration water in the mineral, with progressively heavier values emerging as dehydration progressed. If incomplete dehydration were occurring at stage 2, the d 18OV-smow would become heavier with increasing yields. However, Table 4 indicates that on approaching the theoretically expected yields, d 18Ov-smow% became lighter, which supports our suggestion that prolonged degassing, even at low temperature, results in loss of hydration water. The results therefore indicate that only limited pumping (approximately two min, and certainly not more than 10 min) to HV at room temperature should be adopted for eliminating adsorbed water in order to ensure that no hydration water is removed at stage 1 (procedures 3 and 4, Table 3). However, pumping to LV can be used for periods up to 24 h at room temperature, but not more than a couple of days (Table 2) without detectable loss of hydration water. Hydration water extraction and collection for isotopic analysis was generally achieved by heating to 450 8C for 45 min under vacuum. The X-ray diffractions of the final solid residues after water extractions indicate that gypsum samples were totally transformed to anhydrite in all cases; thus, complete hydration water extractions and collections were ensured. 3.3. Results on laboratory standards and field samples Application of the GuHCl method to the analysis of the hydration water of minerals is hampered by the fact that there is currently no international standard or reference material of similar characteristics to gypsum samples. We have, however, sought to establish that no fractionation occurred during transfer of water through the different parts of the TS. To this end, we have performed repeated analyses of around 4 to 5 Al of reference waters (V-SMOW, GISP and SLAP). The sample size was chosen to be approximately equivalent to the amount of water that would be released from 20 to 25 mg of gypsum. The reference waters were loaded into the TS in sealed capillaries, and then
were transferred into the GuHCl tubes following the same steps as were used for the solid mineral samples (with the obvious difference that no stage 1 dehydration step could be reproduced). The results obtained for d 18OV-smow values of these reference waters were 0.10F0.29 (n=12; range 0.47x to +0.57x) for V-SMOW; 25.16F0.41 (n=3; range 24.91x to 25.73x) for GISP and 55.18F0.69 (n=11; range 56.16x to 53.80x) for SLAP. The weighted mean of individual analyses of the three standard waters indicates an overall reproducibility of F0.5x. The analytical procedures have been applied to gypsum samples collected from different Spanish evaporitic basins of Tertiary age. The hydration water was extracted by the optimal procedures 3 and 4 defined in Table 3, pumping to high vacuum for 2–10 min at room temperature and extracting hydration water at 450 8C for 45 min. The oxygen isotopic compositions of the hydration water released were reproducible to better than F0.2 x (Table 5). These values all lie within the range of published results from other Tertiary gypsum deposits (Table 6) around the Mediterranean Basin. The range of published values can be due to several factors that make comparisons among different basins an extremely delicate task: primary gypsum lithofacies, origin of the brines (marine, or continental-related), and diagenetic history (calcium sulphate cycle, late diagenetic recrystallisation) all should be considered. Although some selenite results are very positive (up to +9x; Pierre, 1982; Longinelli, 1979/1980), lower values are reported also. Samples CY-23 and MRA-45 probably reflect late partial recrystallisation in non-
Table 5 Oxygen isotopic composition of hydration water of some selected gypsum samples Sample CY-23
Procedure d 18Ov-smow (x) Lithofacies
4 4 3 MRA-45 4 4 MOR-49 4 4 3
+2.4 +2.4 +2.4 +3.9 +3.7 +4.2 +4.0 +4.2
Basin
primary selenite
Sorbas (SE Spain)
primary selenite macrolenticular interstitial (diagenetic)
Sorbas (SE Spain) Calatayud (NE Spain)
Procedures 3 and 4 as defined in Table 3.
E. Playa` et al. / Chemical Geology 217 (2005) 89–96 Table 6 Range of published d 18 v-smow of hydration water of different Tertiary gypsum samples (both primary and secondary varieties) Reference
Analytical method
Gypsum petrology
d 18 v-smow (x)
Fontes et al. (1973)a Pierre and Fontes (1978)a
CO2–H2O equilibration CO2–H2O equilibration
Secondary gypsum Selenites, balatino Secondary gypsum (alabastrine) Selenites, gypsarenites, balatino Secondary gypsum Selenites, balatino Secondary gypsum Secondary gypsum Primary gypsum Secondary gypsum (alabastrine)
+3.9/+4.1
Longinelli (1979/1980)a
Pierre (1982)a
Halas and Krouse (1982) Playa` et al. (2000)
CO2–H2O equilibration
CO2–H2O equilibration
CO2–H2O equilibration GuHCl reaction
+6.4/+9.3 +2.3/+5.4
+1/+9
3/+1.5 3/+9
95
during the procedure due to its small size and robust construction. The whole TS may also be warmed throughout its operation, thus avoiding cold spots that could lead to fractionation. By performing the oxygen isotopic analysis on CO2 obtained via the well-established guanidine hydrochloride reaction method, a vacuum can be maintained during the entire sample treatment, thus eliminating contamination by moisture, and only small quantities of gypsum sample are required (20– 25 mg, yielding 4–5 Al of water) to achieve consistent yields and d 18O values. The process described should be readily applicable to many other hydrated mineral phases with minor modifications as appropriate.
7/+2
Acknowledgements 11b/+0.2 +6.1/0.2 +0.3
a Original values in the reference are corrected for the isotopic fractionation effect of gypsum (+4x). b Gypsum sample with organic matter.
marine waters. This is, however, beyond the scope of this work.
The present work was supported by the Spanish Projects DGICYT PB94-0882 and BTE 2001-3201 and the bGrup Consolidat de Recerca Geologia Sedimenta`riaQ 2001/SGR/75 (Catalan Government). The isotopic analysis and laboratory experiments were carried out in the Servicio General de Ana´lisis de Iso´topos Estables at the Universidad de Salamanca. The authors are indebted to Dr. J.J. Pueyo for helpul comments that have greatly improved the manuscript. [LW]
References 4. Conclusions The critical procedure in determining correct isotopic ratios of hydration water in gypsum and other hydrated minerals is the elimination of adsorbed moisture without the removal of any hydration water, since this would invariably result in isotopic fractionation of the water collected and analysed. In the case of gypsum, the optimum conditions for isolating the hydration water are to evacuate the sample at low vacuum, followed by 2–10 min at high vacuum (better than 103 mb) or not more than a couple of days at low vacuum (102–103 mb) at room temperature. Hydration water can then be fully extracted by heating to 450 8C for 45 min. The Transfer System (TS) avoids possible loss of released hydration water
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Extraction of gypsum hydration water for oxygen isotopic analysis by the guanidine hydrochloride reaction method Elisabet Playa`a,*, Clemente Reciob, John Mitchellc a
Department of Geoquı´mica, Petrologia i Prospeccio´ Geolo`gica, Facultat de Geologia, Universitat de Barcelona, Martı´ Franque´s, s/n. 08028 Barcelona, Spain b Department of Geology and Stable Isotope Laboratory, Universidad de Salamanca, Salamanca, Spain c University of Newcastle, Newcastle upon Tyne, England, United Kingdom Received 17 May 2004; received in revised form 18 November 2004; accepted 16 December 2004
Abstract The release and collection of gypsum hydration water without isotopic fractionation, or contamination by adsorbed moisture, is complicated by the fact that gypsum starts to lose part of its water at low temperatures, which prevents heating the sample as a cleaning strategy. Additionally, results indicate that even at low vacuum conditions (from 102 to 103 mb) hydration water of gypsum can be lost, given time enough, particularly if associated with moderately raised temperature. In this report we describe an analytical procedure suitable for the complete release and collection of gypsum hydration water, at the same time that external water is efficiently excluded. The critical step is the elimination of external moisture without the removal of any hydration water. To minimize water losses to the walls of the vacuum line, a low-volume, transfer system (TS) has been designed and built. The procedure requires only about 20–25 mg of gypsum sample (4–5 Al of water) for the oxygen isotopic analysis by the guanidine hydrochloride reaction method, to an overall reproducibility of F0.5x or better. D 2004 Elsevier B.V. All rights reserved. Keywords: Hydration water; Gypsum; Oxygen isotope analysis; Guanidine hydrochloride; Water extraction
1. Introduction The depositional conditions and diagenetic changes of gypsum evaporites can frequently be identified by petrographic means, and the isotopic analysis of gypsum hydration water can provide * Corresponding author. Tel.: +34 93 402 1401; fax: +34 93 402 1340. E-mail address: [email protected] (E. Playa`). 0009-2541/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2004.12.005
additional evidence of the fluids involved in primary precipitation and diagenesis. However, little work is available on the release and isotopic analysis of gypsum hydration water (though see, for example, Fontes, 1965; Sofer, 1978; Pierre, 1982; Halas and Krouse, 1982; see Table 1). Likewise, there is a great lack of information regarding isotope composition of hydration water of other minerals of potential interest. The release of gypsum hydration water is a twostage process. First gypsum transforms to bassanite,
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Table 1 Main works on the topic of gypsum hydration water, with the laboratory procedures (time and temperature) used to remove moisture and hydration water of gypsum Author(s)
Gonfiantini and Fontes (1963) Fontes (1965) Fontes et al. (1973) Sofer (1978) Pierre and Fontes (1978) Longinelli (1979/1980)
Moisture elimination
Hydration water extraction
Stage 1
Stage 2
Not described
400 8C, unknown period of time 40 min at 400 8C
15–20 min at room temperature Not described 15–20 min at room temperature Not described Not described
Not described 45 min at 450 C
Not described 400 8C, unknown period of time Pierre and Ortlieb (1980) Not described Not described Pierre (1982) Not described Not described Halas and Krouse (1982) 30 min at Tb30 8C 400 8C, unknown period of time Bath et al. (1987) Not described 250 8C, unknown period of time Yonge and Krouse (1987) Not described Not described Khademi et al. (1997) 30 min at Tb45 8C 30 min at 450 8C Vacuum conditions used at stage 1 not specified by the respective authors.
and subsequently to anhydrite (reactions (1) and (2), respectively). 2CaSO4 d2H2 OY2CaSO4 d1=2H2 O þ 3H2 O
ð1Þ
2CaSO4 d1=2H2 OY2CaSO4 þ H2 O
ð2Þ
dry environment, gypsum slowly starts to transform to bassanite at 70 8C. In saturated solution and under high pressure (N350 bar), gypsum and the hemihydrate (bassanite) can coexist to temperatures higher than 100 8C (McConnell et al., 1987). In experiments from 60 to 120 8C, and heating rates from 0.1 8C/min to 5 8C/min, De las Cuevas (1992) observed that the dehydration temperature depends on heating rate. Mees and Stoops (2003) performed heating experiments using gypsum crystals and observed that dehydration of gypsum to bassanite (without advancing to anhydrite formation) started at 80 8C. The influence of vacuum conditions on the dehydration of gypsum has not otherwise been described in detail and most of the studies are mainly based in untested suppositions. The aim of the present study was therefore to identify the most convenient analytical procedure to ensure complete dehydration of gypsum and quantitative collection of released hydration water for oxygen isotopic analysis, while ensuring that nonhydration components were effectively excluded. The method was optimized to small quantities of gypsum samples (20 mg). Hydrogen isotopic analysis (given the large mass difference between H and D, and the associated large fractionations) poses additional difficulties, and is currently the subject of on-going research. Moreover, the experience we have gained from the study of gypsum has encouraged us to extend our investigations to other hydrated minerals, such as zeolites and various magnesian sulphates.
2. Materials and procedure The process is mainly controlled by water activity, temperature and pressure, and all of these factors may vary widely in the geological environment. In aqueous systems at atmospheric pressure, gypsum starts to lose its hydration water at 42 8C (Braitsch, 1971; Deer et al., 1983), but it can remain as a metastable hydrated mineral up to, at least, 93 8C (Holser, 1979). In saline systems, the dehydration temperature decreases as water activity diminishes (Hardie, 1967). All the studies quoted above were carried out under specified conditions in bwetQ environments. However, stable isotopic analysis of hydration water is carried out under bdryQ (vacuum) conditions which effectively exclude adsorbed water and atmospheric moisture. Deer et al. (1983) pointed out that, at atmospheric pressure in a
2.1. The extraction of the hydration water of gypsum It is obviously essential to exclude bnon-hydrationQ water (moisture, adsorbed water) from the analysis, while at the same time, early loss of hydration water must be avoided. As a general rule the process of water extraction is divided into two stages (Table 1): Stage 1 Sample pre-treatment for a bshortQ period of time and at low (room) temperature, in order to eliminate non-hydration water from gypsum. This component of water (moisture, absorbed water) is eliminated by pumping, but the type of vacuum pump and vacuum
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conditions achieved are not specified in the literature. Stage 2 Extraction of the hydration water of gypsum, at a higher temperature and for a longer period of time, again under unspecified vacuum conditions.
Table 3 Analytical procedures tested to eliminate moisture and collect gypsum hydration water Procedure
1
Different time and temperature conditions were tested for gypsum pre-treatment (stage 1) to ensure that bnon-hydrationQ water was eliminated without loss of hydration water (Table 2, tests A to D). Based
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2 3 4
Moisture elimination
Hydration water extraction
Stage 1
Stage 2
20 min at 45 8C, HV 10 min at 45 8C, HV 10 min at room temperature, HV 2 min at room temperature, HV
30 min at 400 8C, vacuum 45 min at 450 8C, vacuum 45 min at 450 8C, vacuum 45 min at 450 8C, vacuum
Table 2 Different time–temperature settings tested to check the mobility of gypsum hydration water under vacuum conditions
HV: high vacuum (better than 103 mb).
Test
Laboratory conditions
A
50 8C, 30 min, LV
0.1
B
130 8C, 30 min, LV
20.1
C
Room temperature, LV 30 min 2h 6h 24 h 3 days
on our findings, we adopted procedures 1 to 4 (see Table 3) and examined their respective effect on the oxygen isotopic composition measured on the stage 2 water released from four gypsum samples (Table 4). The main difference between procedures was in stage 1 (Table 3) as we sought a complete separation of bnon-hydrationQ water; the extraction of hydration water (stage 2) largely followed the general procedures described in previous literature. In order to extract, separate and collect the hydration water of gypsum we developed a so-called
D
Weight loss (%)a
0 0 0 0 0.3
4 days
1
7 days
4.2
8 days
4.9
10 days
6
13 days
7.1
Room temperature, HV 15 min 2h 1 day
0 0 1.8
2 days
4.6
5 days
13.8
Type of extracted water Moisture (+some hydration water?) Moisture+hydration water Moisture Moisture Moisture Moisture Moisture+some hydration water Moisture+some hydration water Moisture+some hydration water Moisture+some hydration water Moisture+some hydration water Moisture+some hydration water Moisture Moisture Moisture+some hydration water Moisture+some hydration water Moisture+some hydration water
LV: low vacuum (between 102 and 103 mb); HV: high vacuum (better than 103 mb). Two to three fragments (1–2 mm in size each one) of pure crystalline gypsum samples are used in all the tests. a Percentage of weight lost with respect to the original whole gypsum sample, which is assumed to correspond to the weight of water extracted from the gypsum.
Table 4 Amount of water extracted (wt.%) by the different procedures (1 to 4, as defined in Table 3) and its d 18OV-smow obtained by the GuHCl– water reaction method d 18Ov-smow (x)
Procedure
Sample
Hydration water (%)
1 2 3 4
CY-23
11.6 13.4 18.3 20.6
6.6 4.7 2.4 2.4
1 3 4
MOR-49
14.0 20.0 19.6
6.8 4.2 4.0
1 2
MRA-17
17.7 19.3
3.0 2.7
1 3
RS-17
19.4 19.3
0.4 0.2
Stoichiometric gypsum should contain 20.9% H2O. Hydration water measured as the weight percent loss with respect to the original bulk sample.
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btransfer systemQ (TS), composed of 3 parts (Fig. 1): (i) a 6 mm O.D. quartz tube which contains the sample; (ii) a 12 mm O.D. quartz tube, used as a cold trap; and (iii) a 6 mm O.D. borosilicate (Pyrexk) glass tube, where the water–guanidine reaction takes place. The remainder of the TS was constructed from 6 mm O.D., 1 mm wall stainless steel pipe using Swagelokk couplings and fittings, and Nuprok valves to isolate it from the vacuum pumping system and to isolate individual sections. The glass parts were connected to the stainless steel section by means of CAJONk O-ring unions. The main advantages of the TS are its small overall dimensions (4060 cm, approximately) and internal volume (tubes of 3 and 4 mm I.D.), the absence of plastic valves and a robust construction (metal and glass). Its construction
allowed the TS to be fully degassed with a heat gun and/or hand torch (but not exceeding 200 8C on the guanidine hydrochloride tube), and the facility to keep it heated during the period of water release. In addition, a flexible electric heating tape was wrapped around the whole length of the piping in order to keep it warm, thus preventing cold spots where water could condense. The overall volume (including valves and fittings) is below 25 ml. The dimensions adopted were dictated by the available tubular furnaces and dewars. There is no reason, however, why the internal volume could not be significantly reduced if required. Thus, within the TS, mineral dehydration and water transfer could be fully controlled and the drawbacks of large glass vacuum lines avoided. Water was quantitatively collected, as the system ensured that there was no adsorption to the glass walls or cold plastic valves. 2.2. The guanidine hydrochloride reaction method
Fig. 1. The transfer system (TS). (A) Sample tube: quartz, 6 mm O.D.3.5 mm I.D.200 mm long. (B) Water trap: quartz, 12 mm O.D.9 mm I.D.150 mm long. (C) Reaction tube: borosilicate glass (Pyrexk), 6 mm O.D.3.5 mm I.D.200 mm long. (a) Sample; (b) guanidine hydrochloride; (c) electric heater; (d) stainless steel line, 6 mm O.D.4 mm I.D.400 mm overall length; (e) NUPROk vacuum valves.
To perform oxygen isotope analyses on the extracted hydration water we used the guanidine hydrochloride–water reaction (GuHCl) method (Dugan et al., 1985; Viglino et al., 1985) rather than CO2–H2O equilibration (Dostrovsky and Klein, 1952; Epstein and Mayeda, 1953). Although the GuHCl method was efficiently adapted by Yang et al. (1996) to study fluid inclusions in halite and brines, it had not been tested in gypsum before. Despite the fact that a complete analysis using the GuHCl method requires about 2 working days per sample (though 4–6 samples can conveniently be processed in parallel), involving several discrete tasks using hazardous laboratory reagents (GuHCl, phosphoric acid), it offers the significant advantage that only a small sample is required (20 to 25 mg of gypsum, to yield 4–5 Al of water), in contrast with the 5–10 g required for the CO2–H2O equilibration (Bath et al., 1987; Khademi et al., 1997). Since the sample is maintained under vacuum throughout the procedure, the chance of contamination by atmospheric water is minimal. The analyses were performed by weighing 20 to 25 mg of gypsum sample and 70 mg of guanidine hydrochloride, respectively, into tubes A and C of the TS (Fig. 1). After evacuating the whole line and degassing it using a heat gun, the sample was allowed to outgas at low vacuum. When the pressure was below 103 mb, it was pumped for a further 2–10 min at room
E. Playa` et al. / Chemical Geology 217 (2005) 89–96
temperature to high vacuum (see Section 3.1), and then it was isolated from the pumping line. The GuHCl was simultaneously purified by melting under vacuum at 215 8C for 5 min, in order to eliminate adsorbed water. Dehydration of the gypsum was begun by heating the sample to 450 8C using a tube furnace, and the released water was continuously collected in the liquid nitrogen-cooled quartz trap B for 45 min. Any CO2 mixed with the water released by the mineral was separated by replacing the liquid nitrogen (LN2) with a higher temperature trap (solid CO2 plus acetone slush). Non-condensible gases were pumped away to vacuum, and the water was transferred from trap B to the GuHCl tube C, by immersing its lowermost part in LN2. Trap B is required to ensure that only pure water is transferred to the GuHCl tube C. While still at LN2 temperature, the bottom part of tube C is sealed by means of an oxygas torch and removed from the TS. From here on, the general procedure of Dugan et al. (1985) and Viglino et al. (1985) was followed. To extract the CO2 for isotopic analysis, the sealed glass tubes were cleaned with alcohol, scored at an appropriate height to ease rupturing and put into a cracker assembly containing 1 ml 103% H3PO4. The cracker was attached to the vacuum line and pumped overnight. The next morning, the sample tube was broken and the cracker put into a furnace at 80 8C for 1 h, so that the ammonium carbamate carrying the oxygen of interest reacts with the acid to release CO2. The CO2 thus produced was isolated and purified for mass spectrometric analysis in a vacuum line, using an acetone/dry ice slush trap and finally collected using LN2. The CO2 volume was measured manometrically, and analysed using a dual inlet VG ISOGAS SIRA series II mass spectrometer. The results have been normalised to the SLAP-SMOW scale, and are reported in the conventional d 18O (x) notation relative to V-SMOW (Vienna Standard Mean Ocean Water, Craig, 1961; Coplen et al., 1996).
3. Results and discussion 3.1. Effect of vacuum on the release of hydration water of gypsum In an attempt to gain information about the release of hydration water under reduced pressure, 100 mg
93
aliquots of a very pure, crystalline selenitic gypsum (sample CY-23) were subjected to various conditions of temperature, heating duration and vacuum (Table 2, tests A to D). After each test, weight loss of each aliquot was determined in order to assess the degree of loss of hydration water. This gravimetric approach was possible because CY-23 was 100% pure gypsum (determined by XRD and thin section petrography), and its geochemical characteristics (Sr content, and O, S and Sr isotopic compositions) were totally consistent with its marine origin (Playa` , 1998). For procedures A, B and C, so-called low vacuum (LV; between 102 and 103 mb), was achieved using a dual-stage rotatory vane pump, and was measured using Piranik gauges. Below 102 mb, these measurements can be confirmed by using Penningk gauges. Procedure D was carried out under high vacuum (HV; better than 103 mb) conditions, using an oil diffusion pump. The results of the experiments show that even at LV, hydration water of gypsum can be removed, given enough time, and that increased temperature, particularly above 50 8C, accelerates its removal (Table 2). The results clearly indicate that time, temperature and vacuum parameters are all critical to the process of gypsum dehydration. 3.2. Identification of temperature and time conditions for hydration water extraction The d 18OV-smow of hydration water extracted from several gypsum samples by the different protocols listed in Table 3 (procedures 1 to 4) was analysed as described in Section 2.2. The amount of water recovered (expressed as the equivalent weight percent loss with respect to the original bulk sample) shows that the yield achieved in each extraction is proceduredependent (Table 4). The amount of hydration water collected in stage 2 increased (from almost 12% to nearly 21%) from procedure 1 to 4, as the stage 1 pumping time and temperature decreased. The lowest yield was obtained when the sample was outgassed to HV for 20 min at 45 8C (procedure 1). This also yielded the heaviest d 18OV-smow values. When stage 1 was carried out at room temperature (procedures 3 and 4), however, the yields at stage 2 were higher and the isotopic values lighter (Table 4). The most plausible explanation is that procedures 1
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and 2 caused a partial extraction of hydration water during stage-1 outgassing. The first water to be released (adsorbed atmospheric moisture) would be expected to be isotopically lighter than hydration water in the mineral, with progressively heavier values emerging as dehydration progressed. If incomplete dehydration were occurring at stage 2, the d 18OV-smow would become heavier with increasing yields. However, Table 4 indicates that on approaching the theoretically expected yields, d 18Ov-smow% became lighter, which supports our suggestion that prolonged degassing, even at low temperature, results in loss of hydration water. The results therefore indicate that only limited pumping (approximately two min, and certainly not more than 10 min) to HV at room temperature should be adopted for eliminating adsorbed water in order to ensure that no hydration water is removed at stage 1 (procedures 3 and 4, Table 3). However, pumping to LV can be used for periods up to 24 h at room temperature, but not more than a couple of days (Table 2) without detectable loss of hydration water. Hydration water extraction and collection for isotopic analysis was generally achieved by heating to 450 8C for 45 min under vacuum. The X-ray diffractions of the final solid residues after water extractions indicate that gypsum samples were totally transformed to anhydrite in all cases; thus, complete hydration water extractions and collections were ensured. 3.3. Results on laboratory standards and field samples Application of the GuHCl method to the analysis of the hydration water of minerals is hampered by the fact that there is currently no international standard or reference material of similar characteristics to gypsum samples. We have, however, sought to establish that no fractionation occurred during transfer of water through the different parts of the TS. To this end, we have performed repeated analyses of around 4 to 5 Al of reference waters (V-SMOW, GISP and SLAP). The sample size was chosen to be approximately equivalent to the amount of water that would be released from 20 to 25 mg of gypsum. The reference waters were loaded into the TS in sealed capillaries, and then
were transferred into the GuHCl tubes following the same steps as were used for the solid mineral samples (with the obvious difference that no stage 1 dehydration step could be reproduced). The results obtained for d 18OV-smow values of these reference waters were 0.10F0.29 (n=12; range 0.47x to +0.57x) for V-SMOW; 25.16F0.41 (n=3; range 24.91x to 25.73x) for GISP and 55.18F0.69 (n=11; range 56.16x to 53.80x) for SLAP. The weighted mean of individual analyses of the three standard waters indicates an overall reproducibility of F0.5x. The analytical procedures have been applied to gypsum samples collected from different Spanish evaporitic basins of Tertiary age. The hydration water was extracted by the optimal procedures 3 and 4 defined in Table 3, pumping to high vacuum for 2–10 min at room temperature and extracting hydration water at 450 8C for 45 min. The oxygen isotopic compositions of the hydration water released were reproducible to better than F0.2 x (Table 5). These values all lie within the range of published results from other Tertiary gypsum deposits (Table 6) around the Mediterranean Basin. The range of published values can be due to several factors that make comparisons among different basins an extremely delicate task: primary gypsum lithofacies, origin of the brines (marine, or continental-related), and diagenetic history (calcium sulphate cycle, late diagenetic recrystallisation) all should be considered. Although some selenite results are very positive (up to +9x; Pierre, 1982; Longinelli, 1979/1980), lower values are reported also. Samples CY-23 and MRA-45 probably reflect late partial recrystallisation in non-
Table 5 Oxygen isotopic composition of hydration water of some selected gypsum samples Sample CY-23
Procedure d 18Ov-smow (x) Lithofacies
4 4 3 MRA-45 4 4 MOR-49 4 4 3
+2.4 +2.4 +2.4 +3.9 +3.7 +4.2 +4.0 +4.2
Basin
primary selenite
Sorbas (SE Spain)
primary selenite macrolenticular interstitial (diagenetic)
Sorbas (SE Spain) Calatayud (NE Spain)
Procedures 3 and 4 as defined in Table 3.
E. Playa` et al. / Chemical Geology 217 (2005) 89–96 Table 6 Range of published d 18 v-smow of hydration water of different Tertiary gypsum samples (both primary and secondary varieties) Reference
Analytical method
Gypsum petrology
d 18 v-smow (x)
Fontes et al. (1973)a Pierre and Fontes (1978)a
CO2–H2O equilibration CO2–H2O equilibration
Secondary gypsum Selenites, balatino Secondary gypsum (alabastrine) Selenites, gypsarenites, balatino Secondary gypsum Selenites, balatino Secondary gypsum Secondary gypsum Primary gypsum Secondary gypsum (alabastrine)
+3.9/+4.1
Longinelli (1979/1980)a
Pierre (1982)a
Halas and Krouse (1982) Playa` et al. (2000)
CO2–H2O equilibration
CO2–H2O equilibration
CO2–H2O equilibration GuHCl reaction
+6.4/+9.3 +2.3/+5.4
+1/+9
3/+1.5 3/+9
95
during the procedure due to its small size and robust construction. The whole TS may also be warmed throughout its operation, thus avoiding cold spots that could lead to fractionation. By performing the oxygen isotopic analysis on CO2 obtained via the well-established guanidine hydrochloride reaction method, a vacuum can be maintained during the entire sample treatment, thus eliminating contamination by moisture, and only small quantities of gypsum sample are required (20– 25 mg, yielding 4–5 Al of water) to achieve consistent yields and d 18O values. The process described should be readily applicable to many other hydrated mineral phases with minor modifications as appropriate.
7/+2
Acknowledgements 11b/+0.2 +6.1/0.2 +0.3
a Original values in the reference are corrected for the isotopic fractionation effect of gypsum (+4x). b Gypsum sample with organic matter.
marine waters. This is, however, beyond the scope of this work.
The present work was supported by the Spanish Projects DGICYT PB94-0882 and BTE 2001-3201 and the bGrup Consolidat de Recerca Geologia Sedimenta`riaQ 2001/SGR/75 (Catalan Government). The isotopic analysis and laboratory experiments were carried out in the Servicio General de Ana´lisis de Iso´topos Estables at the Universidad de Salamanca. The authors are indebted to Dr. J.J. Pueyo for helpul comments that have greatly improved the manuscript. [LW]
References 4. Conclusions The critical procedure in determining correct isotopic ratios of hydration water in gypsum and other hydrated minerals is the elimination of adsorbed moisture without the removal of any hydration water, since this would invariably result in isotopic fractionation of the water collected and analysed. In the case of gypsum, the optimum conditions for isolating the hydration water are to evacuate the sample at low vacuum, followed by 2–10 min at high vacuum (better than 103 mb) or not more than a couple of days at low vacuum (102–103 mb) at room temperature. Hydration water can then be fully extracted by heating to 450 8C for 45 min. The Transfer System (TS) avoids possible loss of released hydration water
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