Eur. J. Mineral. 1998, 10, 1097-1108

This paper was presented at the ECROFI XIV Symposium in Nancy, France (July 1997)

Quantitative solute analysis of single fluid inclusions in halite by LA-ICP-MS and cryo-SEM-EDS:

complementary microbeam techniques

THOMAS J. SHEPHERD1, CARLOS AYORA2, DIΟNI I. CENDON2 , SIMON R. CHENERY1

and ALAIN MOISSETTE3

'British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK 2 Institut de Ciencies de la Terra, CSIC, Marti I Franques, E-08028 Barcelona, Spain

3CREGU, BP 23, F-54501 Vandoeuvre les Nancy, France

Abstract: An inter laboratory study was carried out to assess the complementariness of laser ablation-induc-tively coupled plasma-mass spectrometry (LA-ICP-MS) and cryogenic SEM-energy dispersive analysis (cryo-SEM-EDS) for the quantitative solute analysis of single inclusions in halite. The halite samples were selected from diverse ancient and recent evaporite environments to give a wide range of solute compositions, and were characterized by abundant, homogeneous populations of large (1—100 µm diameter) primary brine inclusions. Homogeneity of the inclusions was important because both techniques are destructive and had to be carried out on different individual inclusions. Cryogenic SEM-EDS was chosen because it is one of the few, well established fluid inclusion techniques to provide absolute concentrations for the major solutes (Na, K, Mg, Ca, Cl, SO4), and could be used to convert the relative element concentration ratios determined by LA-ICP-MS (K, Mg, Ca, Cl, Br, Sr, Li, B) into true concentration units. Normalization was achieved by referring to chlorine.

The results confirm that LA-ICP-MS and cryo-SEM-EDS are perfectly complementary and yield values for Mg and K that agree within \a standard deviation. Calcium concentrations were generally too low to be measured by cryo-SEM-EDS, but values determined by LA-ICP-MS are consistent with the hypothesis that the brines are saturated with respect to anhydrite. Overall analytical precision (relative standard deviation) for the major solutes is 2-3 times better for cryo-SEM-EDS than for LA-ICP-MS (5-15% vs. 10-35% respectively). Internal consistency of results was checked by charge mass balance estimates and halite satu­ration indices. Though the study utilized halite-saturated inclusions, cryo-SEM-EDS and LA-ICP-MS can be applied equally to lower salinity fluids and, with care, to inclusions that contain daughter minerals.

Key-words: quantitative chemical analysis, single fluid inclusions, laser ablation ICP-MS, cryogenic SEM-EDS, halite.

Introduction

Recent developments in instrumental methods of analysis, in particular microbeam techniques, have presented fluid inclusion researchers with an un­paralleled opportunity to investigate the solute chemistry of individual fluid inclusions. No longer is the small size of an inclusion an overrid­ing constraint since the majority of these tech­niques (synchrotron X-ray fluorescence (SXRF),

proton-induced X-ray excitation (PIXE), second­ary ion mass spectrometry (SIMS), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), cryogenic SEM-energy dispersive spectroscopy (cryo-SEM-EDS) and micro-laser Raman spectroscopy (LRS)) have beam sizes less than a few micrometres. The limiting problems are now (1) the heterogeneous (solid-liquid-va­pour) nature of inclusions, (2) their sub-surface lo­cation within the sample, (3) the ability to

0935-1221/98/0010-1097 $ 3.00 © 1998 E. Schweizerbart'sche Verlagsbuchhandlung. D-70176 Stuttgart

1098 T.J. Shepherd et al.

measure trace elements {e.g., metals) at low solute concentrations, and (4) whether destructive ana­lysis is acceptable. Of these four, the first and sec­ond probably impose the greatest constraint. This is especially true in the case of SXRF and PIXE where data reduction procedures are designed for surface or thin layer samples. Modelling the SXRF and PIXE excitation of heterogeneous sub­surface inclusions is extremely complex and re­quires a gross simplification of inclusion geom­etry to quantify the spectra. For LA-ICP-MS and cryo-SEM-EDS, the two main destructive tech­niques used for single inclusion analysis, the depth of an inclusion beneath the surface is less critical (see below). In the case of LA-ICP-MS the greatest uncertainties are the efficiency of fluid re­lease during laser ablation, transfer of vapourised material to the plasma without chemical fraction-ation, and calibration. Such uncertainties do not apply to cryo-SEM-EDS but unlike the other tech­niques referred to above, one cannot pre-select the inclusion for analysis (see below).

Nevertheless, despite these difficulties an in­creasing number of papers report quantitative ana­lyses for the solute chemistry of individual aqueous fluid inclusions by one or more micro-beam techniques (Diamond et al., 1990; Ramsey et al, 1992; Vanko et al., 1993; Ghazi et al., 1996). This marks an important watershed in in­clusion research and heralds the possibility of carrying out quantitative geofluid analysis. Appli­cations are too numerous to mention but key areas of future research include 1) direct measurement of mineral-fluid trace element distribution coef­ficients, 2) influence of diagenetic fluids on the cementation of hydrocarbon reservoirs, 3) fluid fluxes associated with subduction and thrust tec­tonics, 4) secular variations in ocean water chem­istry, and 5) the metal-carrying capacity of hydro-thermal fluids. However, to exploit these new techniques it must first be proven that the analyses are fit for purpose. Until now, inclusion analyses have been treated with some degree of scepticism. This is not entirely unjustified since many of the older published inclusion analyses (pre-1975) do not stand up to inspection and are at best qualita­tive. To date, the most reliable solute analyses refer either to exceptional large inclusions (> 100 µm) or milligram quantities of hand-picked, care­fully selected fluid inclusion-rich monomineralic samples that have been analysed using crush-leach techniques. To demonstrate the complete­

ness of analysis, analyses are often recalculated to show cation-anion charge balances. Values close to charge neutrality indicate a high degree of con­fidence in the bulk inclusion analysis. If analyses for single inclusions obtained by microbeam tech­niques are to be regarded with the same degree of confidence they must be accompanied by similar figures of merit (precision, accuracy, etc.). Work in this direction has already begun but there re­mains the need to establish the extent of agree­ment between different microbeam techniques (Rankin et al., 1992). At the moment, no one tech­nique provides a complete anion-cation analysis of individual <50 µm inclusions. It is important therefore that in pooling data, any systematic technique-specific biases that may influence the quality of the analysis are clearly recognized.

Research objectives

This paper presents the results of an interlabora-tory study to assess the use of two complementary microbeam techniques, LA-ICP-MS (Shepherd & Chenery, 1995; Moissette et al., 1996) and cryo-SEM-EDS (Ayora & Fontarnau, 1990; GarcRa-Veigas, 1993; Ayora et al, 1994a and b), to obtain quantitative data for the solute chemistry of fluid inclusions in halite. Since the latter is one of the few well established single inclusion techniques that yields absolute element concentrations, it af­forded an excellent benchmark for normalizing and interpreting the relative element concentration ratios obtained by LA-ICP-MS. Brief descriptions of each technique are given below. Only a limited number of elements were selected for analysis and were chosen principally to satisfy the strengths of cryo-SEM-EDS (Na, K, Mg, Ca, S and Cl), whilst at the same time testing the extended trace ele­ment capability of LA-ICP-MS (Li, B, Br, Sr). Halite was chosen as the host mineral because it provided the ideal matrix for cryo-SEM-EDS anal­ysis and because it typically contains growth zones with large populations of primary brine in­clusions. The inclusions were considered suffi­ciently abundant and chemically homogeneous to compare the capabilities of both techniques even though different inclusions were analysed. The use of halite also has other advantages. Under most evaporitic conditions, NaCl-saturated brines are saturated with respect to anhydrite which allows Ca concentrations to be independently cal­culated from solubility data.

Quantitative solute analysis of single fluid inclusions 1099

Samples selected for analysis

To present a wide range of brine compositions, halite samples were selected from a number of well documented, contrasting, ancient and recent evaporite environments (Table 1).

BI-425 and BI-389 are from the Biurrun bore­hole (Navarra, Spain), located in the Upper Eocene evaporite sequence of the South Pyrenean fore­land basin. This sequence, approximately 100 m in thickness, contains four units (Rosell, 1983): a) basal anhydrite unit, b) lower halite unit, c) potash unit (sylvite and carnallite), and d) upper halite unit. Sample BI-425 belongs to the lower halite unit and was precipitated in an evolved marine evaporitic environment prior to potash-salt pre­cipitation. Sample BI-389 comes from the upper halite unit and was precipitated from continental waters which had recycled previous evaporites.

L-212 and L-147 are from borehole S-4 lo­cated in the Upper Miocene (Messinian) evaporite sequence of the Lorca Basin (SE Spain). This se­quence, 250 m in thickness (Garcia-Veigas et ai, 1990), contains a lower halite unit sub-divided into Lower and Upper sections and an overlying gypsum unit. Sample L-212 is from the Lower section of that unit and corresponds to a shallow non-potash marine environment. Sample L-147 is located in the transition zone between the Lower and Upper sections and shows evidence of recy­cling of previous evaporites by continental waters.

A3 and A8/3 are from evaporites of the Upper Miocene (Messinian) Gessoso Solifera Formation (Zone C) of the Central Sicilian Basin (Italy). Samples were collected at the Realmonte Mine at a depth of 128 m below sea level. Sample A3 is a sec­ondary halite precipitate within the mine workings.

SR-825 is from a Neogene halite bed exposed at the margin of the Salar del Rincon (Argentina). This halite was precipitated within a closed basin, continental environment.

DS1B is from the floor of the northern basin of the Dead Sea and represents a recent halite pre­cipitate.

DSW provided a comparative sample of Dead Sea water collected at a depth of 150 m.

Except for DSW, the samples were sliced under paraffin and, zones of chevron and hopper halite (Shearman, 1970) removed, and prepared as 1 mm thick polished wafers. Potential variability of inclusions within the same sample was min­imized by preparing duplicate 'mirror image' slices. Samples that could not be prepared in du-

Table 1. Halite and related brine samples selected for analysis.

Sample Material Locality

no.

other details

BI-389 Halite Biurrun, Spain Upper Eocene halite. Borehole sample

BI-425 Halite Biurrun, Spain Upper Eocene halite.

Borehole sample

L-212 Halite Lorca, Spain Upper Miocene halite.

Borehole sample

L-147 Halite Lorca, Spain Upper Miocene halite.

Borehole sample

A8/3 Halite Realmonte Upper Miocene Mine, (Messinian) halite.

Sicily, Italy In situ sample

SR-825* Halite Salar del Neogene halite. Rincon, In situ sample

Argentina

DS1B Halite Dead Sea, Recent halite from Israel the floor of the Dead

Sea. In situ sample

DSW Brine Dead Sea, Dead Sea water collect-Israel ed at a depth of 150 m

* samples not prepared as duplicate wafers

plicate are identified in Table 1. The transparency of the wafers was perfectly adequate to locate in­clusion-rich growth bands and to identify individ­ual inclusions within the chevron halite. Typically the inclusions had a negative cuboid habit and ranged from 100 to < l µ m in diameter (edge dimension). All were liquid dominant (i.e. one phase liquid inclusions). Where a vapour bubble was observed the inclusion was normally adjacent to a fracture or cleavage plane and had probably leaked during sample preparation. These were ex­cluded from analysis. No daughter minerals were evident, although the presence of sub-micrometre solid phases could not be absolutely discounted.

Methodologies

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)

Instrumentation and calibration strategy

Comprehensive accounts of the technique have al­ready been published (Shepherd & Chenery, 1995; Ghazi et ai, 1996; Moissette et al., 1996; Gunther

l loo T.J. Shepherd et al.

et al., 1997) and only specific aspects are high­lighted here. Laser ablation ICP-MS is based upon a well established analytical technique that has been modified to allow the determination of the major, minor and trace element constituents of in­dividual brine inclusions >10 µm in size. The technique utilizes a laser to drill into selected in­clusions and vapourise the contents for ICP-MS analysis. The micrometre sampling capability of the BGS laser microprobe is achieved using a Spectron Nd.YAG laser coupled to a Leitz Aris-tomet microscope with Cassegrain objective lenses. This configuration allows excellent optical resolution of the inclusions before, during and after ablation. The laser is operated in the Q-switched mode with frequency quadrupling to produce a 266 nm beam in the UV. At this wave­length most minerals absorb strongly, ensuring controlled and reproducible ablation. Samples are contained within a Perspex ablation cell (working volume 25 cm3), the inner profile of which is aerodynamically shaped to minimize the residence time of ablated material; thus improving the sig­nal/background ratio of the transient peak. After selecting the inclusion, the laser beam is focused onto the surface of the sample to a diffraction-limited point (<1 µm) and allowed to drill down to the inclusion. Depending upon the laser condi­tions, inclusions within 5 µm of the surface are opened almost immediately while those at a depth of 50 µm may take up to 100 s before break­through is achieved. Operating parameters are given in Table 2. [N.B. Voltages to the laser oscil­lator and amplifier flash tubes were varied accord­ing to the diameter (typically 10 µm at the sur­face) and depth of the ablation crater, and rate of ablation required.]

The ICP-MS instrument was operated in a dual gas flow mode whereby a nebulized solution was introduced into the plasma simultaneously with the laser-ablated material (Chenery & Cook, 1993; Querol & Chenery, 1995; Chenery et al., 1996). Using a simple tuning solution, this per­mits rapid optimization of the system for inclusion analysis (Shepherd & Chenery, 1995). To enhance the light element response, gas flow and ion lens voltages were adjusted to maximize the 59Co+ sig­nal whilst at the same time minimizing the l4(Ce+/15fX2eO+ ratio to reduce potential oxide in­terferences. During analysis of the inclusions a 0.1 % HNO3 blank solution was run continuously through the nebulizer. Gas flow rates and opera­ting conditions for the ICP-mass spectrometer are

Table 2. Operating conditions for laser ablation micro-probe.

Laser Wavelength Maximum energy

Mode Pulse Length Laser repetition rate Microscope Laser focusing objective

Spectron SL803 Nd:YAG 266 nm (quadrupled from 1064 nm) 70 mJ (energy changed with size of crater desired) Fully Q-switched, TEMπo 10 ns 10 Hz Leitz Aristomet Ealing Electro-Optics 36X reflecting lens coated for 266 nm

Table 3. Operating conditions for ICP-mass spectro­meter.

Spectrometer Forward power Coolant gas flow rate Auxiliary gas flow rate Ablation cell gas flow rate Nebulizer gas flow rate Nebulizer Total Injector gas flow rate Data acquisition mode Points per peak Dwell time per peak Integration Mode Integration time for all isotopes per time slice

VG PlasmaQuad 2+ 1100W 13 L min~' 0.8 L min-1

0.6 L min-1

0.4 L min-1

de Galaan V-groove 1 L min-1

Peak jumping 3 5 ms Time resolved analysis 1-2 s

shown in Table 3. The r.f. power applied to the ICP was 1100 W rather than 1350 W as for con­ventional solution analysis, since this provided better signal/background ratios for many first row transition elements (Querol & Chenery, 1995 ).

Data were acquired in a time-resolved fashion using peak-jumping mode for a limited number of isotopes to maximize the signal/background ratio and minimize any isotope discrimination from the transient nature of the signal (Table 3). The acqui­sition of data in a time-resolved form has several advantages: subtraction of matrix blanks immedi­ately prior to opening the inclusion; observation of any contamination from the matrix; integration of the signal from the inclusion only while it was significantly above background (the larger the in­clusion volume, the longer the transient signal); observation of any time-dependent fractionation of the signal for different elements.

Quantitative solute analysis of single fluid inclusions 1101

60

"E 40

-Ω (5 30

c o α CO 2 20 o

I p~v"5Γ

1 ! M g E 1

1

Δ R

Γl Li L • 1 1 1

1 1 1 1 1 1 1 r* —

If I T 1

I"-"-*-'"" 1

400

3

TO

200 &

100

50

2

0 20 40 60 80 100 120 140 160 180 200 Time (s)

Fig. I. Time-resolved LA-ICP-MS traces for Cl and Mg for the analysis of a 50 µm diameter brine inclusion in halite at a depth of 30 µm below the sample surface. Time segment A (0-50 s) refers to the instrumental background before ablation; segment B (50-140 s) the halite background during laser ablation but before inclusion rupture; segment C (140-170 s) the signal due to laser vapourisation of the inclusion contents. Cl and Mg responses are in arbitrary concentration units.

Due to the composition of the matrix and the need to normalize to the Cl concentration of the fluid, particular care was taken to monitor any contribution of Cl from the matrix to the fluid sig­nal. Fig. 1 shows the time-resolved traces for Cl and Mg for a 50 µm diameter inclusion at a depth of 30 µm. Since Mg is absent from the matrix, the strong Mg signal corresponds to the instance of brine vapourisation. Prior to inclusion rupture the trace may be divided into two segments. Time in­terval A (1-50 s) corresponds to the instrumental background before ablation, whereas time interval B (51-140 s) corresponds to the matrix back­ground during ablation. It is evident that the two segments for Mg are statistically indistinguish­able, thus confirming the absence of Mg in the solid halite. By comparison, the matrix back­ground for Cl is slightly higher than the in­strumental background during the same ablation period. However, the difference is very small and several orders of magnitude less than the fluid sig­nal (time interval C, 141-165 s). Thus by integrat­ing the Cl signal over the same time interval as for

Mg, a value can be derived for the fluid phase. Where the fluid signal is low (i.e. a small inclu­sion) a minor background correction can be ap­plied. A simple calculation indicates that a 50 µm cuboid inclusion (125000 µm3) containing 200 000 ppm Cl contains approximately 1.5 ×102 times more Cl than an equivalent 5µm cube (125µm3) of solid halite (p 2.16 g cm-3). Because the laser is continuously ablating the salt, it is considered that 125 µm3 represents the maximum amount of halite likely to contribute to the fluid signal at the mo­ment of inclusion rupture.

Calibration was carried out using the micro-well-solution method described by Moissette et al. (1996). By ablating solutions of known con­centration and inclusions of unknown composi­tion under identical conditions, sensitivity factors were calculated for each element. This allowed count ratios (counts per sec) to be converted into relative concentration ratios. To allow for variable amounts of material in the inclusion (i.e. inclusion size) and variable amounts of solution ablated from the micro-wells, an internal standard element

I 102 TJ. Shepherd et ah

was used to calculate absolute concentration ratios from the relative concentration ratios. In this study the data were ratioed to Cl (35C1+ isotope). Though the determination of chlorine by ICP-MS is difficult due to poor ionization, the naturally oc­curring high level of chloride in these brines gives rise to an almost ideal signal level. Using the average absolute concentrations of Cl as deter­mined by cryo-SEM-EDS, it was possible to con­vert the LA-ICP-MS concentration ratios to true concentration units for all elements (major, minor and trace).

Detection limits are controlled by the mass of analyte extracted from the inclusion and by polyatomic interferences generated in the ICP source, and generally decrease with increasing atomic number. Smaller inclusions (<l0µm) can be analysed but with considerable reduction in the number of elements measured due to much lower signal/background ratios. Any analytical uncer­tainty due to chemical heterogeneity of the inclu­sions was considered negligible.

Cryogenic SEM-energy dispersive analysis

Instrumentation and calibration strategy

Cryogenic SEM-EDS is a modification of conven­tional Scanning Electron Microscopy-Energy Dis­persive Analysis in which the sample is held at cryogenic temperatures during EDS analysis. The technique was pioneered by Ayora, Fontarnau and

Garcia-Veigas (Ayora & Fontarnau, 1990; Garcfa-Veigas, 1993; Ayora et ah, 1994a and b) and in­volves placing a cleavage fragment of halite and droplets of standard aqueous solutions onto a sample holder, freezing them in liquid nitrogen and then quickly transferring the frozen assembly into the SEM chamber which is held at -170°C. Whilst still frozen, the halite is broken and the fractured surface coated with aluminium. Using this procedure, inclusions as small as 15 µm diameter can be analysed quantitatively for Na, K, Mg, Ca, Cl and SO4 (as S). For inclusions greater than 15 µm there is no X-ray interference between the host mineral and the frozen inclusion. Operat­ing parameters are given in Table 4. Analytical precision varies for each element and for each analytical run, but for this study was consistently <10%. Detection limits for Ca and K were typ­ically 0.01 mol/kg (400 ppm), SO4 0.05 mol/kg (4800 ppm), Mg 0.31 mol/kg (7500 ppm) and Na from 0.54 to 0.97 mol/kg (14900 ppm), depend­ing upon the Mg content of the inclusion. Internal consistency of results was checked against the cat-ion/anion charge balance ([Σcation - Σanion]/ [(Σcation + Σanion)/2]) and the calculated halite saturation index (log[aNa × ac]/Kha]ite]). For details see Ayora et ah (1994a). Analyses with charge im­balances <l±0.20l and a halite-saturation index lower than l±0.15l were considered to be within the range of analytical error. The technique how­ever is unsuitable for the measurement of trace elements, and for most evaporite brines the con-

Table 4. Operating conditions for cryo-SEM-EDS.

Approximate sample size Standard droplets

Cryogenic chamber (CC) : CT 1500 Oxford Instruments Ltd

CC temperature and pressure Thickness of sample Al coating N2 slush temperature -160°C and 10_i torr

Microscope : Scanning Electron Microscope JSM-840 Jeol

Accelerating voltage Beam current intensity Working temperature and pressure Working distance

X-ray detector : Solid state Si(Li) detector AN 10000 Link Analytical Ltd

Detector window Counting time for each analysis Resolution

10 x 8 × 1 mm 4

-160°Cand 10-2torr 50-100 nm <-200°C

15 KV 1.5 nA

-170to-190°Cand 10"6torr 39 mm

8µm beryllium 200 s 166 eV (Mn peak width half height)

Quantitative solute analysis of single fluid inclusions 1103

centrations of B, Br, Li and Sr (using the X-ray de­tectors installed at the Serveis Cientific-Tècnics, Universidad de Barcelona) are below detection.

Results

Analytical data for the brine inclusions are sum­marized in Tables 5 and 6. Table 5 shows the aver­age solute concentrations (in ppm), standard devi­ation ( lc) and number of individual inclusions (n) analysed by LA-ICP-MS and cryo-SEM-EDS. Table 5 summarises the mean solute values (mol/kg) obtained by cryo-SEM-EDS. Conven­tionally, cryo-SEM-EDS analyses are expressed as mol/kg H2O. To allow comparison with LA-ICP-MS, these were transformed into ppm concentra­tion units using the density of the brine as calcu­lated from the Mg content of the inclusion. For this transformation an empirical relationship was used where

p(kg/l) = cMg(kg/l)× 1.2+ 1.1828 (McCaffrey et ciL, 1987).

Although the numerical model of Monnin (1989) yields more accurate density measure­ments for Na-Ca-Cl-SO4 brines, the McCaffrey method is better suited to marine, Mg-rich samples. Any differences noted, however, are in the second decimal place and one to two orders of magnitude below the standard deviation of the chemical analyses. All cryo-SEM-EDS analyses are within the accepted charge balance error range of l±0.20l. Though strictly qualitative, equivalent charge balances for the LA-ICP-MS data based on a sulphate-free brine also satisfy the same criteria.

As described above, cryo-SEM-EDS is re­stricted to the measurement of Na, Mg, K, Ca, Cl and S (SO4). Unfortunately, with the exception of sample DS1B, Ca concentrations were close to or below the limit of detection (0.01 mol/Kg H2O). In Table 4, the Ca values enclosed by brackets were derived from numerical calculations (Ayora et ai, 1994b) which equilibrated the brine with anhydrite, a common accessory mineral throug­hout the halite sequences studied. For LA-ICP-MS, no values are reported for Na and S. This is because polyatomic interferences at m/e 32 and 34 (32S and 34S) preclude the measurement of sul­phur, whilst for sodium the poor signal/back­ground ratio adversely affects the acquisition of reliable measurements.

Discussion

Due to differences in the detection limits for each technique and limitations imposed by the nature of the halite and size of inclusion amenable to analysis, not all elements were analysed by both methods. It is not possible therefore to make a full two-way comparison between LA-ICP-MS and cryo-SEM-EDS. Nevertheless important conclu­sions can be drawn (Table 5). Ideally, to compare both techniques, an independent estimate of the chlorine concentration is required. However, for the following reasons, this proved impracticable. Microthermometric measurements of ice melting or halite daughter mineral dissolution as used by Heinrich et al. (1992) for aqueous inclusions in quartz are not applicable to halite. On freezing brine inclusions in halite, the brine reacts with the host to form hydrohalite (NaCl • 2H2O) which melts incongruently at + 0.1 °C. Thus hydrohalite melting temperatures do not provide a measure of salinity (i.e., NaCl equivs.) and cannot be used to estimate the chlorine concentration. Similarly, it is not possible to estimate salinity from known halite saturation curves. The Na content of evap-orite brines is extremely dependent on other ion concentrations, and especially on the activity coefficient of divalent cations such as Mg (Cen-don et al, 1998). Thus Cl and Na estimates derived from halite saturation at room temperature in the system NaCl-H2O can lead to serious analy­tical errors when applied to chemically complex brines. In view of these constraints, the cryo-SEM-EDS chlorine estimates were considered to offer the best measure of normalization.

Comparison of LA-ICP-MS and cryo-SEM-EDS

The first conclusion to be drawn from the analyses given in Table 5 is that the absolute and estimated concentrations of Mg and K by LA-ICP-MS and cryo-SEM-EDS are in good agreement and over­lap at the 1 α level. Agreement is especially close for those samples where duplicate slices were pre­pared (L-147, L-212, BI-425, DS1B). Values for calcium tend to show greater variability. This is not entirely unexpected since the values are either close to or below the detection limits for LA-ICP-MS and cryo-SEM-EDS. Only for the Dead Sea halite sample (DS1B) was the Ca concentration high enough to be measured by cryo-SEM-EDS.

1104 TJ. Shepherd et al.

Table 5. Comparison of mean analyses (ppm) for single brine inclusions in halite by LA-ICP-MS and cryo-SEM-EDS. All LA-ICP-MS element concentrations have been calculated by normalization to the corresponding cryo-SEM-EDS chlorine concentrations. Calcium values in brackets have been estimated assuming equilibration of the brine with anhydrite (see text for details).

Sample

A8/3

ICP n=8 EDS n=7

L-147

ICP n=ll

EDS 2

n=7

L-212

ICP n=10

EDS 2

n=6

BI-389

ICP n=13

EDS n=7

BI-425

ICP n=10

EDS 2

n=7

SR-825

ICP n=13

EDS 5

n=9

DS1B

ICP n=9 EDS 2

n=9

Li

4 ±0.9

20 ±5

14 ±3

201 ±33

48 ±41

268 ±66

22 ±3

D S W water*

ICP n=18

20 ±10

Dead Sea water**

17

B

57 ±21

288 ±61

243 ±66

381 ±55

747 ±331

251 ±87

69 ±10

40 ±10

Na

112931

±1052

38287

±2919

51699

±4274

40480

±350

52602

±4540

120157

±2471

33664

±1058

30253

Mg

10878

±2085

10027

±632

66640

±11580

58088

±5611

54510

±20437

50935

±2825

54432

±2920

56514

±7441

43502

±3953

2521

±405

3679

±212

51687

±8114

45604

±3012

30610

±9080

36145

S0 4

12632

±842

17389

±2217

28629

±8930

6720

±1920

11973

±1116

9273

±879

<900

Cl

(179645)

179645

±3113

(222677)

222677

±23730

(199456)

199456

±4531

(236430)

236430

±5280

(213666)

213666

±13589

(173869)

173869

±9062

(213928)

213928

±4899

(179625)

179625

K

4478

±571

3087

±343

11008

±3981

10617

±902

8099

±2868

9887

±2726

2667

±525

4301

±1560

25935

±4322

22627

±2725

5145

±889

6069

±149

6925

±1239

7912

±360

4970

±1210

6487

Ca

619 ±157

(702)

±175

447 ±32

(363)

±46

342 ±192

(217)

±93

(508)

±40

931 ±376

(357)

±47

392 ±112

(1051)

±9

21480

±2508

19504

±120

14240

±2890

13602

Br

408 ±58

3340

±1638

1927

±1127

32 ±25

2587

±848

45

3501

±1108

4420

±1120

4303

Sr

39 ±6

17 ±7

9 ±10

22 ±7

24 ±19

9 ±3

407 ±51

300 ±70

269

K/Mg

0.41

±0.07

0.31

±0.03

0.17

±0.03

0.18

±0.02

0.15

±0.08

0.19

±0.05

0.08

±0.03

0.46

±0.081

0.52

±0.08

2.04

±0.270

1.66

±0.13

0.14

±0.02

0.17

±0.01

0.16

±0.03

0.18

Ca/Mg

0.06

±0.01

0.07

±0.02

0.007

±0.001

0.006

±0.001

0.006

±0.002

0.004

±0.002

0.009

±0.001

0.018

±0.006

0.008

±0.001

0.16

±0.03

0.29

±0.03

0.40

±0.05

0.43

±0.01

0.47

±0.002

0.38

* Data normalized to mean chlorine concentration of Dead Sea water (Nissenbaum, 1977) ** Dead Sea water at a depth of 310 m (Nissenbaum, 1977) All LA-ICP-MS element concentrations have been calculated by normalization to the corresponding cryo-SEM-EDS chlorine concentrations. Calcium values in brackets have been estimated assuming equilibration of the brine with anhydrite (see text for details).

Quantitative solute analysis of single fluid inclusions 1105

Table 6. Mean solute values (mol/kg) determined by cryo-SEM-EDS for single brine inclusions in halite.

Sample

A8/3

L-147

L-212

BI-389

BI-425

SR-825

DS1B

Na

5.61

1.84

2.49

1.95

2.57

5.99

1.60

Mg

0.47

2.64

2.32

2.47

2.01

0.17

2.05

so4

0.15

0.20

0.33

0.08

0.14

0.11

Cl

5.79

6.94

6.23

7.37

6.77

5.62

6.59

K

0.09

0.30

0.28

0.12

0.65

0.18

0.22

Ca

0.02

b.d

b.d

b.d

b.d

0.03

0.53

Σcations

6.68

7.44

7.42

7.04

7.26

6.57

6.98

Σanions

6.09

7.34

6.89

7.52

7.05

5.84

6.61

CB

0.09

0.01

0.07

-0.07

0.03

0.12

0.05

ISHUI

0.005

0.008

0.002

0.041

0.001

-0.035

-0.147

p g/cm3

1.19

1.25

1.24

1.25

1.24

1.19

1.24

CB = charge balance ([Σcations - Σanions/(Σcations + Σanions)/2]). ISHal = saturation index for halite, p = density of brine. b.d refers to Ca values that are close to or below the detection limit (0.01 mol/kg).

For DS1B the mean Ca values by LA-ICP-MS and cryo-SEM-EDS overlap at the 1 α level. It is satisfying to note the high degree of correspond­ence between the theoretical (numerically mod­elled) and measured Ca concentrations suggesting that Ca is indeed controlled by anhydrite satura­tion.

In keeping with previous studies, the relative standard deviations (RSD) for K and Mg indicate that the overall analytical precision for cryo-SEM-EDS (Mg 7%, K 14%) is better than LA-ICP-MS (Mg 20%, K 22%) by a factor of 2 to 3. Also shown in Table 5 are the mean LA-ICP-MS values for Dead Sea water (sample DSW). These have been normalized to the mean chlorine value for Dead Sea water (179625 ppm) and it can be seen that there is very close agreement (within 10-20%) between the normalized solute concentra­tions estimated for DSW and the absolute concen­trations measured by Nissenbaum (1977). Since the estimated element concentrations for DS1B are systematically higher than both water samples, it is thought that the Cl value of 179 625 ppm for DSW is an underestimate of the present day halite-saturated bottom waters referred to by Anati & Stiller (1991). Further discussion of this topic is outside the scope of this paper.

As a further illustration of comparability be­tween LA-ICP-MS and cryo-SEM-EDS, Fig. 2 and 3 show the correspondence for simple ele­ment concentration ratios. This approach cancels

0.40-,

0.00 0.10 0.20 0.30 0.40 Ca/Mg (cryo-SEM-EDS)

0.50

Fig. 2. Comparison of measured Ca/Mg ratios for single fluid inclusions in halite by cryo-SEM-EDS and LA-ICP-MS. Values refer to sample means. The solid triangle is a measured data point using both methods; the open triangles refer to Ca/Mg ratios where the values for Ca have been obtained by (a) direct LA-ICP-MS and (b) estimated assuming saturation of the brine with respect to anhydrite (see text for details). The dashed curve is the ideal 1:1 relationship between the two methods.

out any errors due to the measurement of chlorine. For Ca/Mg and K/Mg, the regressions (r = 0.96 and 0.99, respectively) are highly significant indi-

1106 T.J. Shepherd et al.

l

PH

y i

<

1

2.00-

1.50-

_

1.00-

0.50-

0.00-

r = 0 . 9 9 /

//

Λ s/

^ ' 1 ' 1 ' Γ

/ / / Theoretical

K/Mg

i | i |

0.00 0.40 0.8 1.20 1.60 2.00 K/Mg(cryoSEM-EDS)

Fig. 3. Comparison of measured K/Mg ratios for single fluid inclusions in halite by cryo-SEM-EDS and LA-ICP-MS. Values refer to sample means. The dashed curve is the ideal 1:1 relationship between the two methods.

eating excellent agreement between the two tech­niques. With the exception of sample SR-825, for which the anhydrite-saturated Ca estimate is al­most twice that of the measured LA-ICP-MS value, the slope of the regression line is very close to 1. Thus for Ca and Mg, no systematic technique bias is evident. For K/Mg, the slope of the re­gression is slightly higher suggesting a slight overestimate by LA-ICP-MS. However, to be stat­istically proven one would need to analyse addi­tional samples with high K/Mg ratios. The ac­curacy of LA-ICP-MS is further substantiated by the good agreement between analyses obtained for DSW (Table 5, Dead Sea water) and the analyses for deep Dead Sea water published by Nissen-baum (1977). This is first ICP-MS analysis of its kind obtained by direct laser ablation of a brine and the results strongly justify further investiga­tion of this methodology as a means of analysing high TDS solutions (>50000 µg ml1) . In conven­tional ICP-MS analysis, the nebulization of solu­tions containing more than 5000 µg ml"1 is not ad­visable and causes serious degradation of analyte precision and sensitivity (Jarvis et al., 1992). However, new simultaneous analogue/pulse counting detectors may allow the analysis of high TDS solutions, assuming of course that problems of sampler cone blockage can be overcome.

Conclusions

The study clearly shows that LA-ICP-MS and cryogenic SEM-EDS are perfectly complementary and well suited for the analysis of single brine in­clusions in halite. Values for Mg and K are in very good statistical agreement (±1 α s.d.) and data for Ca by LA-ICP-MS are consistent with theoretical estimates of Ca in the brines. The ability to con­vert relative concentrations ratios determined by LA-ICP-MS into absolute element concentrations is especially valuable, and allows easy quantifica­tion of important process-sensitive trace elements such as Li, B, Cs, Rb, Sr and Br. Furthermore, the ability to measure Cl/Br ratios by LA-ICP-MS of­fers an exciting potential for investigating the origins of salinity in many types of basinal brine (Kesler et aL9 1995).

At present, both techniques have difficulty in measuring low concentrations of Ca (<400 ppm). This is seen as a temporary problem for LA-ICP-MS since new developments in instrument design should overcome or reduce the polyatomic inter­ferences which complicate the analysis of Ca and preclude the detection of sulphur. With continued improvements in detector design for the light ele­ments, detection limits for Li, B and Br by cryo-SEM-EDS can also be expected to decrease. Cryo-SEM-EDS is not restricted to the analysis of brine inclusions and analyses have been obtained for other types of inclusion (aqueous!variants daughter minerals) in other mineral matrices (Ayora & Fontarnau, 1990; Garcfa-Veigas, 1993; Tritlla & Cardellach, 1997). Its principal limita­tion however is that it does not allow pre-selection of inclusions and, for inclusion-poor samples or mineral with poor cleavage, much patience and ef­fort is required. Finally, for LA-ICP-MS there re­mains the question of converting relative element concentrations into absolute concentrations. Ulti­mately it is hoped to calibrate and quantify the ICP signals as illustrated by Ghazi et al (1996). For the moment we are restricted to various kinds of external normalization. Where we have low or high salinity inclusions in quartz, one can use microthermometric salinity measurements to esti­mate Cl concentrations. Although this method has an uncertainty of ±10-20% it is easy to apply and is compatible with the overall level of analytical precision for LA-ICP-MS. In general, this is more than adequate for many trace element investiga­tions. Notwithstanding these limitations, the pres­ent study demonstrates that cryo-SEM-EDS and

Quantitative solute analysis of single fluid inclusions 1107

LA-ICP-MS are capable of providing high quality, quantitative data for a large range of cation and anion species in single inclusions, and sets a benchmark against which other techniques may be assessed. It is hoped to extend this exercise to other microbeam techniques (PIXE and synchro­tron XRF) to provide fluid inclusion researchers with a practical guide for selecting the best 'fit for purpose' single inclusion technique based on pre­cision, accuracy and limits of detection. A subsi­diary but nevertheless important conclusion to be drawn from the study is that inclusions in halite provide a working alternative to synthetic fluid in­clusions for calibration and development.

Acknowledgments: The authors would like to thank their respective Institutes for encouraging the study and to the British Council and Ministe-rio de Educacion y Ciencia for providing essential funding (Acciones Integradas Grant no. 1898) for exchange visits between Spain and the UK. We also wish thank Prof. A. Starinsky for providing samples of halite and sea water from the Dead Sea. The authors gratefully acknowledge Dr. C. Ramboz (University of Orleans, France) and Dr. A. Audetat (ETH Zhrich, Switzerland) for their constructive comments on the paper which did much to improve its value and presentation. Pub­lication for TJS and SRC is by permission of the Director, British Geological Survey (NERC).

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Received 10 October 1997 Modified version received 20 March 1998 Accepted 14 May 1998