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Mass spectrometer measurements in the thermal areas of New Zealand
Geochimicaet CosmochimicaActa, 1962, Vol. 26, pp. 399 to 410. Pergamon Press Ltd. Printed in Northern Ireland
Mass spectrometer measurements in the thermal areas of New Zealand Part
2. Carbon isotopic ratios
J. R. HTJLSTONand W. J. MCCABE Institute
of Nuclear Sciences, Department of Scientific and Industrial Lower Hutt, New Zealand
Research,
Abstract-The variation of the (.F/C12 ratio of gas and water samples from pools, bores and fumaroles of the North Island of New Zealand have been studied. In general the isotopic composition is in the range KY = 0 to -7%, w.r.t. the PDB Belemnite Standard. Calculations of isotopic equilibrium temperatures from the &Y values of methane and carbon dioxide have been made and indicate temperatures of 350” and 440” C on Big Donald Mound, White Island; of approximately 250’ C from bores in t*he Wairakei area and 280” C from Champagne Pool at Waiotapu. These latter results are in good agreement with measured bore temperatures at depth. INTRODUCTION
As the art of Mass Spectrometry advanced it became possible to measure isotopic ratios more accurately, and this has led to the study of the natural variations in these ratios. One of the first surveys of the stable carbon isotopes was made by NIER and GULBRANSEN(1939) who grouped most of their samples into the classifications : Limestones, Igneous carbons, and Plant forms. They found, with an accuracy of about O-5 per cent, that compared to the Limestone, the Igneous carbon was depleted in Cl3 by O-5 to 1.5 per cent, and the Plant carbon was depleted by 2-3 per cent. With the development of double collection (NIER 1947) and rapid sample switching techniques (MCKINNEY et al., 1950), the accuracy of measurement has been increased considerably, and present day results are usually expressed in terms of parts per thousand difference (%,) with accuracies around O-l%,. RANKAMA (1954) has summarised the result of surveys done by a number of workers up to that date. The work on C?3/CF ratios in thermal areas appears, however, to be rather scant. CRAIG (1953) has measured as part of his general survey the ratios of a number of carbon dioxide and methane samples from Yellowstone National Park, U.S.A. The Cl3 content of carbon dioxide samples varied by up to 5x0, while the methane samples were depleted in Cl3 relative to carbon dioxide by approximately SO%,. CRAIU considered it likely that this difference between methane and carbon dioxide was due to isotopic equilibrium being established deep underground and that these gases were brought to the surface in a time much shorter than that necessary for equilibrium to be re-established at the lower temperatures. Since the fractionation between CO, and CH, is a function of temperature, it should thus be possible to estimate the temperatures underground. In this project it was decided to make a survey of U3/C12 ratios in the larger thermal areas of New Zealand. As part of this work it was desired to assess the usefulness and validity of the use of methane-carbon dioxide isotopic measurements in the estimation of underground temperatures for geothermal steam development in New Zealand. 399
400
J.
R. HULSTONand \V. ,J. MC&BE
SAMPLE (‘oLLEcTr0~
AND PREPARATION
Part I of this paper described the collection and preparation of carbon dioxide samples for mass spectrometer isotopic analysis. The residual gas samples were further processed as it is necessary to convert the methane present to carbon dioxide for isotopic carbon analysis. The conversion of methane to carbon dioxide was carried out in an evacuated system consisting of a Toepler pump to handle and cycle the gases, a vacuum gauge, a copper oxide furnace to oxidise the methane, a dry ice trap to remove water and a liquid oxygen trap to condense the carbon dioxide. The copper oxide furnace was run at 700°C which was the maximum temperature that could be used without producing appreciable quantities of oxygen from the decomposition of cupric oxide. It was found necessary to pass the gases over the copper oxide up to thirty times to obtain a better than 95 per cent Isotopic analysis of less completely conversion of methane to carbon dioxide. oxidised samples indicated that fractionation occurred, so that complete combustion of samples is desirable. This is particularly difficult where the concentration of methane in the residual gases is low. Samples of t’he gas remaining after the combustion of the methane and of the carbon dioxide produced were taken for mass spectrometer analysis. M.sss SPECTROMETERAXALYSIS The mass spectrometer used is a 60’ Nier type using double collection and gas switching methods, and its use for the measurement of sulphur isotopic ratios has previously been described (HULSTO~; and SHILTOS, 196X). For the measurement of C13/CY2ratios it is necessary to move the collectors closer together in order that, masses 4.5 and 44 may be collected simultaneously. All results are given as the enrichment, in C l3 of the sample compared with a standard and are expressed in parts per thousand (%,), i.e.,
sample -- (C113/(!12) standard
Modification of the equations given by Craig (1957) for the calculation from the masses 44, 45 and 46 ratios gives BC13 =~:1.068 6 (G/44)
-~ 0.034 B (46/44)
(1) of ?A””
(2)
where 6 (45/44) and 6 (46/44) are defined by equations analogous to equation (1). New Zealand Te Kuiti limestone was used as the working standard for t’his work, but for ease of comparison with the results of other workers, the results given in this paper have been converted to BC13values relative to CRAIG’S PDB standard. The Te Kuiti limestone standard has been measured by CRAIG (1957) and found to be -1.67%, with respect to the PDB standard and -0*61~& with respect to N.B.S. Solenhofen limestone reference sample No. PO. As a check on the complete process of preparation and measurement, t,his latter comparison has been repeated in this laboratory, and the value of -O..i%, obtained, with a standard deviation in measurement of 0.1 5x0. This is considered satisfa,ctory. The standard deviation of the results given in Table 2 is approximately 0.3%,.
Mass spectrometer measurements in the thermal areas of New Zealand
401
CARBON ISOTOPIC EQUILIBRIA Carbon dioxide-Bicarbonate system In order to attempt to understand the isotopic relationships obtained between gas and water phases it is necessary to understand the fractionation which occurs when carbon containing molecules are shared between the two phases. If the two phases are in equilibrium it is possible to calculate the fractionation by statistical mechanics. A table of isotopic equilibrium constants at different temperatures has been given by CRAIG (1953), and the COsCH, and CO,CO,” values are given in Table 1. These latter values approximate those expected for the CO,-HCO,exchange. Table 1. Isotope exchange equilibria for CO,, CO,= and CH, (after CRAIG, 1953)
T “K
cPo,= ~ c120,=
Cl30
IL Cl20
0
00
273.1 298.1 400 500 600
1.016 I.012 1.004 0.9994 0.9975
2
0 0.935 0.943 0.963 0.975
0.981
It will be seen from Table 1 that at 20% the isotopic equilibrium between CO, and HCO,- ions is such that 6CY3 of the HCO,- is approximately la%,, positive w.r.t. the CO, but that this reduces to zero at about 220°C and becomes negative If a reaction is a long way from equilibrium, the fractionaabove this temperature. tion is produced by the kinetic isotope effect, and a totally different 6 value may be obtained. For example, in the absorption of CO, from air by a tray of sodium hydroxide at room temperatures, the 6Ci3 value of the CO,= is initially approximately 15x,, negative w.r.t. the CO,, i.e., in the opposite direction to that for equilibrium. If a partial equilibrium has been obtained, then the results may be anywhere between these two conditions. It should be noted here that under equilibrium conditions there is no significant fractionation between CO, above a solution and CO, dissolved in the solution. Thus the fractionation between gas and water phases of a bore or pool depends on the state of isotopic equilibrium, the temperature and the ratio of bicarbonate to dissolved CO, in the water phase. Although there are other ions present in these waters, it is sufficiently accurate for this work to calculate the (bicarbonate + carbonate) to (dissolved CO,) ratio from the pH. The state of equilibrium can be determined if the rates of the reactions involved are known. Information concerning rates at high temperatures is difficult to find and the values used here are obtained by extrapolation from the results of HIMMELBAU and BABB (1958). Methane-Carbon dioxide equilibria Previous workers (e.g., CRAIG, 1953) in this field have considered the isotopic equilibrium of methane and carbon dioxide in geothermal discharges. If equilibrium existed, it would be possible to estimate underground temperatures by measurement of the relative isotopic compositions of the CH, and CO,.
402
J. Ii.HULHTON and W. J. MCCABE
The overall mechanism
usually suggested is the reaction
The isotopic equilibrium constants given by UREV (1947) and CJUIG (1953) (see Table 1) have been used in the calculation of the temperatures deduced later in this paper. No experimental measurements of the CH, equilibrium constants are known to the authors. J)XSCt-SSION
OP &EXTI,TS
The results of (!r3/C’12ratio measurements on carbon dioxide in the gas phase (cK’~~~~), botal carbon dioxide in the water phase (dCnls) and on methane in the gas phase (methane 6 (P) are given in Table 2. From these values and the relative quantities of each as given in Table 1 of Part 1 (HIXSTOX and MC(‘ABE 1962) the
Fig. 1. &I?~, w.r.t. PDB, of carbon from various; Kern Zealand sources. Closed circles show approximate average 6C’3 values for whole discharge. Open circles show W3 for methane only.
average value (&I’,‘“) has been calculated for the total discharge and is given in Table 2. Most of the BCr13 (or cYC~>~) and methane SP values have been plotted inFig. 1. It will be seen that while the 6C’13 values of carbon dioxide in the gas phase (which forms t,he bulk of the carbon discharged) vary from + 2 to --9x0. the results For example, the &? for certain areas are nearly constant within themselves. values for Waiora Valley, Wairakei, range form -0.5 to -.-l..5,%o, while the six: The pools sampled at Waiotapu have 6CL3 x7itiues in the range -- 5.8 to -6*9%,. methane samples contain much less C’13, and the relationship of these results to underground temperatures is discussed below.
Bores. The 6C13 values for CO, in t’he gas phase are very constant (-3-0 -3.9%,) while the d03 values for the water phase sre slightly less so (-4-O
to to
_____.__:
Whaktltane area Awakeri Springs Itoborna Soda Springs Onepu Small fool Onopu Bore Onepu Large Pool
Whale Island pools Boiling Pool Beach Pool 13each Pool nr Cliff
712A 612 Katattthi
712
Sorth Island fumarolcs
White Island fumarolos Big Donald (a) Big Donald (b) Big Donald (c) Seven Dwarfs
Wairakei bores Bore 11 18 30 44 203 13
_.
Location
R470/6 1 R472/t3 / R473/1 X473/2 / R473/3
I
R477/6
’ R477/1 ! R477/4
1
It43311 R433/2 i R62 i R125
R507/2 it93 R507/6
/ R.507/3
!
! It400 j R402 R403 / R404 / R405 R401
/
.3.g
-4.9
-54 -4-7
-1*2 -3.6
-5.3 -6.8 _._6.3
-4-4 -5 -6.6
-9.0 -2.6
-9.1
-3.9
!:
-.5.3
-3.0 -3.5
-1 -3.1 -4.7 -4.5
-40 -5.7 -4.9
I
1 -3.4 j -3.6 ’ -3.3
__
-39.0
- 29.7
-- 28.5 -27.9
- 16.1 -- 22.4 -23.3
- 27.0 -27.3 -26.8 __27.3
’
1
I
j : /
____ __I ---
-12 -2.7
-
-3.9 -4.3
-1 -3.1 -5.4
I
,
;
!
i j j
i 130
250
j
,I
II
’
I
_ _..--
/
I
I
I
235 250
440 345 250
245
-3.6 -4-2
260
250 245 250
_
-3.5 -
-3-7
__..
tx,
I / : Carbon dioxide ’ Methane Average ! BCa’Y K!p dc!‘3 I &p (%Q) (X0) (Zo)
j__.__l~_-..-___
/ xs so.
)
Table 2. C13/C?aRatios of carbon dioxide and methane
140
255
250 265
I 945
330 365
240
190
260 200
240
/ 1
235 260 260 220 190
--I--i 203
54 50 61
453*
8.6 5.6
6.8 6.9
7.6 7.7 7.7
_I_
I
-_
404
J. R. WTJLSTONand W. J. MCCABE
Mass spectrometer
-1
measurements
in the thermal areas of New Zealand
405
406
J. K. HULSTONand W. a. MCCABIS
-9*1%,). If isotope equilibrium exists, then the difference in BC’L3between gas and water would indicate equilibrium temperatures ranging from 240” C! for Korc 1 I through 290°C for Bore 30 to 4SO”C for Bore 13. (ieothermograph readings (Table 2) show however, maximum temperatures of approximately 200”( ‘, :!60”(’ (‘onsideration of the time of travel of and 190°C respectively for these bores. water in the bores, and reaction rates, show that) it would be unlikely that the equilibrium existing at the bottom of a bore would hc maintained at t,he top. It’ t’akes approximately PO-30 seconds for a water molecule to travel t,he length of a bore. The reaction HCIO, -+ (‘0, + OH-- at 160”(’ has a half-time of approximately 0.5 sec. A possible explanation of the more negativt, ijC13 value for the liquid phase may be that all free (‘0, is swept from the water phase by flashing steam. Due to the partial pressure of (!O, in t,he gas phase, rc-solution of CO, occurs to a certain extent. Due to t,he k’mctic isotope eff&t (i120, will dissolve more rapidly, thus decreasing the BC’13value of the water and bv isotopic exchange the 6C13value of the remaining HCO:, --will also decrease. ( ‘alculat ions of solubilitg using the data of ELLIS and FPFE (1957) show t,hat at, 1fiO”f approsimately t /&mole of CO, per mole of water phase is present as free ( ‘0,. excluding .Hore 13 which gives 0.1 ,u M/M. Table 1 of Part I shows this to be approximatcl,y IO per cent of the total CO, in the water phase. Isotopic measurements of free (‘0, and H(Y):,- in four bore water samples at room temperatures gave resu1t.s agreeing with theoretical isotopic equilibrium calculations for CO, and (‘0,~ (UKEB 1947) and assuming that C’O, and HCO,mm have a similar equilibrium. PzcmnroZe.s. The results at both White Island and Jlrairakei are similar to those of the bores, but the spread is greater suggesting a similar origin but with more The 8c”” value for Iietatahi is disturbing influences in the case of the fumaroles. more negative than that, for the bores, but, is still within the spread found in the pools. Pools. While the C13/C’12ratios of most of the areas arc constant within themselves, others, particularly Geyser Valley, are very variable and probably are the result of a number of different effects, such as dilution with groundwater CO, having a large negative Xl3 value, fractionation by gas loss before entering the pool and fractionation between gas and water in the outflow from the pool. Dilution with groundwater CO, will only have a significant effect where t,he total CO, output is low. Because of the possibility of loss of gas prior to entering the pool, the quantity of groundwater CO, is best estimated from the quantity of argon present. Fractionation due to loss of gas would be significant, only in pools of higher pH (e.g., Pool 39, Geyser Valley), where if equilibrium conditions exist, CO, depleted in Cl3 will be lost and the remaining solution will have a more positive 6C13 value. Measurements of KY3 for both gas and water phases have heen made on several pools. These measurements give, in general, smaller fractionations than would be expected from the pH and temperature measurements if a state of isotopic equilibrium existed. This is demonstrated in Table 3 which compares measured and calculated differences in SCl3 between gas and water phases. The calculated difference is obtained from the relationship
Xass spectrometer rn~~s~~rnents in the thermal areas of New Zealand
fKY3
water -
KY3 gas
a “+” b , where: = --
= (HCO,-)/(CO,)
=I=antilog (pH -
407
6-4)
and z = (SC&os- - BC$). It will be seen in Table 3 that the Awakeri Springs calculated value is very different from the measured value. This seems to indicate that either there is a kinetic isotope effect due to CO, being absorbed in the water, or else there is mixing of two water supplies just under the surface, and equilibrium is not re-established. It will be seen that, if it is desired to know the 6Cr3 value for the whole discharge, and if a significant fraction of the COz is in the water phase, it is not possible to predict the KY of one from the other, and hence both phases must be collected and measured. Methane-Carbon
dioxide system
On the assumption that isotopic equilibrium exists for this system, temperatures have been calculated for all samples where KY3 values have been measured for both CH, and CO,, and these are recorded in Table 2. Reference to Table 1 will show that a standard deviation of O-3%,, in both the CH, and CO, KY3 values corresponds to a standard deviation of approximately 8°C for temperatures in the region of 25O’C. For comparison the surface temperatures of pools have been recorded, and in the case of the Wairakei bores temperatures at the bottom of the bores, as supplied by New Zealand ministry of Works, have been recorded. It will be seen from Table 2 that all the known high temperature areas, Wairakei, Waiotapu, Whale Island and White Island, have given high isotopic equilibrium temperatures. Table 3. Carbon dioxide fractionation between gas and water phases in pools and bores
-
Temp.
Location
I
(“C)
Rotoma Soda Champagne Pool 55 Pool 48 Bore 186 Bore 203 Bore 13 Bore 30 Awakeri Pool 39 /
50 75 93 61 84 220 190 260 54 94
i
[GC&0,_ - SC&] calculated
+9 17 +6 18 +6.5 0
j
+1 -1 ts.5 +6
(at
:F
C)
[a&13 -
fxp]
,calculated measured
I _-
5.6 5.7 6.8 5.9 6.7 6.8 6.9 7.7 8.6 8.8
$-I.2 +1.2 $- 1.3 +2.0 +4 0 +0.6 -0.9 18.5 +6
i
+ 1.0 +0,3 +o‘@ +3*3 1-112 -1.8 -5.2 -1.6 -7.8 +0.6
-
Although K$,$3 has not been measured, it would appear that Tiger Pool at Ngawha also gives a high equilibrium temperature. This agreement provides some evidence that isotopic equilibrium does exist. This is strengthened by the much more negative KY3 values obtained for methane If the gases from these samples from colder areas, e.g., HelensviIle and Lyttelton. two areas are in isotopio equilibrium, then they indicate temperatures of about
408
J.R. HULSTON and V’. J. MCCABE
50°C which would seem qnite reasonable. Even if equilibrium does not exist in these samples, the large variations in the methane 6CX3values in Table 2 would at least support the existence of isotopic equilibrium in the samples from the hotter areas. ‘Ihe evidence for the existence of chemical equilibrium discussed in Part 1 of this paper is also a strong indication that isotopic equilibrium may also exist. The temperatures obtained from chemical equilibrium calculations in Part I have been included in Table 2 in order that comparisons may be made with methane isotopic temperatures and with those measured directly at the bottom of the bores. As would be expected from the errors involved in the measurements for chemical equilibrium, the best correlation is between the measured values and the methane isotopic value, but it is interesting to note that there is a great,er correlation between the chemical equilibriun~ and the isotopic temperatures than between the chemical equilibrium and t,he measured temperatures. This is pres~~n~abl~~because the isotopic and chemical equilibria are attained at a point some distance from the bore. It would therefore appear that the water and gases have been at a temperature of 220-260°C for some time, and thus do not come direct,ly from higher temperature spots. Another method of checking for isotopic equi~i~riul~~ is to measnro She natural radioactive 04 content of the CH, and CO, gases. This has been attempted by FERGUSSON (pers. comm.), but so far no conclusive results have been obtained. It is hoped that the Ngawha area (North Auckland, New Zealand) may yield a suitable sample. Even the Cl4 method may be subject to error if small quantit#ies of modern groundwater carbon dioxide entered the gas stream just below the surface. This would increase the CL* activity of the carbon dioxide but would not aReot the (‘la activity of the methane because of the absence of suficient time to re-establish isotopic equilibrium between the CO, and CH,. This type of situation is thought to exist at Awakeri. Present evidence thus appears to support the existence of isotopic equilibrium, and it, would appear that this technique can give valuable infornlation on temperatures of underground water in new areas. Mean isotopic compositior6 cmd origin qf the Cmh?L ‘The aver&ge 8C13 value (BC3,13)is included in Table 2, in order that the origin of the carbon may be considered. It will be seen that, the averages for t,he thermal samples range form -3+5 to -~ 4*47&,for Wairakei bores and fl~~naroles through -6 to -7x0 for Waiotapu to - 12x, for Awakeri. Pull information is not available on samples from Big Donald E’umarole (a) and some of the Wairakei pools, but the results in most cases suggest that the average 6C1,13values are around -3 to -3%0, (CRAIG{1953) obtained values of - 1.1 to --6*4x, for carbon dioxide from pools in Yellow&one Park, U.S.A. Since the relative quantities of C!O, in the gas and water phases and of methane are not given by CIRAIG,it is not possible to compare these results directly, but it will be seen that they are in the same range as the CO, gas from the Wairakei pools in Table 2. A possible source of the relatively large quantities of carbon dioxide from these
Mass spectrometer
measurements
in the thermal
areas of New
Zealand
409
thermal samples is the heating of carbonate rocks at depth. CRAIG (1953) reports marine limestones with 6C13 values of from +2*4x0 to --3*3x,, and dolomites with 6C13 values of +2*7x, to -2*3%,. The 6C,13 values in Table 2 in general contain less Cl3 than these carbonate rocks. It is possible that the main source in the thermal areas is similar to that of the carbonate rocks of lowest Cl3 isotopic composition, i.e. --3x,, but that this is being mixed with a proportion of organic carbon dioxide of 6C13 s -27x, from the deep recirculating groundwater. For example, the concentration of 23 1~M/M of CO, in the water phase (HULSTON and MCCABE, 1962: Table 1) at H2W13 = -21*9%, (see Table 2) measured for the sample of artesian water from Hutt Park (near Wellington) (R489/3) may result from a mixing of approximately 5 y M/M of carbon dioxide at SC13 = -3x0 and 18 ,u M/M of organic COz at El3 = -27x,. The CO, (organic)/Argon ratio of IS/O*29 = 60 obtained for this sample may be used as an indication of groundwater CO, to be expected in the thermal areas. This could explain the 6Cr13 value of -12x0 obtained for Awakeri, but this ratio would not give sufficient groundwater carbon to account for the value of -6*5x,, obtained for Champagne Pool at Waiotapu. In this latter case the non groundwater portion of the CO, must have a 6CX3value of approximately -6x,, which is significantly less than that expected from marine carbonate rocks. The above hypothesis thus explains some but not all the lower 6C13 values. The remaining variations in 6C13 must be attributed to variations in the isotopic ‘composition of the carbon at its main source. This does not rule out the possibility of carbonate rocks as the main source of the carbon, as the carbonates in some areas (e.g. Waiotapu) could have been formed in freshwater systems which contained organically derived carbon dioxide. This was suggested by CRAIG (1953) to explain the 6C13 values around -9%,, obtained for some freshwater carbonates. However, the possibility of juvenile carbon, of as yet undetermined isotopic composition, entering the system and effecting the Xl3 value should also be considered. CoNCLUsIoNs
Carbon dioxide isotope measurements on bores, fumaroles and pools show that the isotopic composition of CO, appears to be most uniform in bores and least uniform in pools. The variation in pool results is probably due to variable groundwater contamination. The differences between gas and water phases from bores and pools imply that isotopic equilibrium is not attained. In this type of measurement it is essential to measure all products in all phases in order to estimate the isotopic composition of the whole discharge. It does appear from the measurements made in this work that the carbon in geothermal gases in New Zealand comes from a common origin which has a &Y3 value close to that of carbonate rocks and also similar to that found by CRAIG (1953) for CO, from the Yellowstone Thermal Area. The temperatures obtained by measuring the 6Cia of CO, and CH, and assuming them to be in equilibrium in the reaction CO, + 4H, $ CH, + 2H,O agree reasonably well with temperatures obtained by direct measurement. As the reaction rate for this reaction is very slow, the equilibrium system must have been in existence for several years.
410
J. R. HULSTON and W. J. MCCABE REFEREENCES
CBAIG H. (1953) The geochemistry of t,he stable oarbon isotopes. 3, 53-92.
Gee&m.
et Casmoehirn.
Actn
CRAIG H. (1957) Isotopic standards for carbon and oxygen and correction factors for massspectrometric analysis of carbon dioxide. Geochim. et Cvsmochim. Acta 12, 133-149. ELLISA. J. and FYFE W. S. (1957) Hydrothermal Chemisky. RF?>.Pur. i2@. Chem. 7, 261-316. FERGUSSONG. J-. Personal Communication. HIMMELBLAUD. M. and BABB A. L. (1958) Kinetic Studies of Carbonation Reactions using Radioactive Tracers. ,i. Amer. In.& Chem. E:ngnr. 4, 143-152. HULSTONJ. R. and MCCABE W. J. (1962) Mass Spwtromet,er Measurement,s in the Thermal Areas of New Zealand. Part I-Carbon dioxide and residual gas analyses. Geoclzirri. et C~~oc~~rn. Acta 26, 353-398. HULSTONJ. R. and SHILTONB. W. (1958) Sulphur Isotopic \‘ariations in Nature. Part 4-- ~~e~~ernent of Sulphur Isotopic Rat,io by Mass Spectrometry. S.Z. J. 55. 1,91-102. MCKINNEY C. R., MCCREEJ. M., EPSTEIN S., ALLEN W. A., UREY H. C. (1950) Improvements in Mass Spectrometers for the Measurement, of Small Differences in Isotope Abundance Ratios. Rev. Sci. Innstrum. 21, 724-730. NrER A. 0. (1947) A Mass Spectrometer for Isotope and (:as Analysis. Ho?+.&.% Inetnur!. 18, 39&411. NIER A. 0. and GUI;BR_&NSEN E. A, (1939) Variations in t,he relati\-e abundance of the Carbon Isotopes. J. Amer. Chem. Sot. 61,697-698. RANKAMAK. (1954) IsoEope Geology. 535 pp. Pergamon Press, London. THOMPSONG. E. K. Personal Communication. UREY H. C. (1947) The Thermodynamic Properties of Psotopic Subatanccs. ,T. (:l~em. Sot., 562-581.
Mass spectrometer measurements in the thermal areas of New Zealand Part
2. Carbon isotopic ratios
J. R. HTJLSTONand W. J. MCCABE Institute
of Nuclear Sciences, Department of Scientific and Industrial Lower Hutt, New Zealand
Research,
Abstract-The variation of the (.F/C12 ratio of gas and water samples from pools, bores and fumaroles of the North Island of New Zealand have been studied. In general the isotopic composition is in the range KY = 0 to -7%, w.r.t. the PDB Belemnite Standard. Calculations of isotopic equilibrium temperatures from the &Y values of methane and carbon dioxide have been made and indicate temperatures of 350” and 440” C on Big Donald Mound, White Island; of approximately 250’ C from bores in t*he Wairakei area and 280” C from Champagne Pool at Waiotapu. These latter results are in good agreement with measured bore temperatures at depth. INTRODUCTION
As the art of Mass Spectrometry advanced it became possible to measure isotopic ratios more accurately, and this has led to the study of the natural variations in these ratios. One of the first surveys of the stable carbon isotopes was made by NIER and GULBRANSEN(1939) who grouped most of their samples into the classifications : Limestones, Igneous carbons, and Plant forms. They found, with an accuracy of about O-5 per cent, that compared to the Limestone, the Igneous carbon was depleted in Cl3 by O-5 to 1.5 per cent, and the Plant carbon was depleted by 2-3 per cent. With the development of double collection (NIER 1947) and rapid sample switching techniques (MCKINNEY et al., 1950), the accuracy of measurement has been increased considerably, and present day results are usually expressed in terms of parts per thousand difference (%,) with accuracies around O-l%,. RANKAMA (1954) has summarised the result of surveys done by a number of workers up to that date. The work on C?3/CF ratios in thermal areas appears, however, to be rather scant. CRAIG (1953) has measured as part of his general survey the ratios of a number of carbon dioxide and methane samples from Yellowstone National Park, U.S.A. The Cl3 content of carbon dioxide samples varied by up to 5x0, while the methane samples were depleted in Cl3 relative to carbon dioxide by approximately SO%,. CRAIU considered it likely that this difference between methane and carbon dioxide was due to isotopic equilibrium being established deep underground and that these gases were brought to the surface in a time much shorter than that necessary for equilibrium to be re-established at the lower temperatures. Since the fractionation between CO, and CH, is a function of temperature, it should thus be possible to estimate the temperatures underground. In this project it was decided to make a survey of U3/C12 ratios in the larger thermal areas of New Zealand. As part of this work it was desired to assess the usefulness and validity of the use of methane-carbon dioxide isotopic measurements in the estimation of underground temperatures for geothermal steam development in New Zealand. 399
400
J.
R. HULSTONand \V. ,J. MC&BE
SAMPLE (‘oLLEcTr0~
AND PREPARATION
Part I of this paper described the collection and preparation of carbon dioxide samples for mass spectrometer isotopic analysis. The residual gas samples were further processed as it is necessary to convert the methane present to carbon dioxide for isotopic carbon analysis. The conversion of methane to carbon dioxide was carried out in an evacuated system consisting of a Toepler pump to handle and cycle the gases, a vacuum gauge, a copper oxide furnace to oxidise the methane, a dry ice trap to remove water and a liquid oxygen trap to condense the carbon dioxide. The copper oxide furnace was run at 700°C which was the maximum temperature that could be used without producing appreciable quantities of oxygen from the decomposition of cupric oxide. It was found necessary to pass the gases over the copper oxide up to thirty times to obtain a better than 95 per cent Isotopic analysis of less completely conversion of methane to carbon dioxide. oxidised samples indicated that fractionation occurred, so that complete combustion of samples is desirable. This is particularly difficult where the concentration of methane in the residual gases is low. Samples of t’he gas remaining after the combustion of the methane and of the carbon dioxide produced were taken for mass spectrometer analysis. M.sss SPECTROMETERAXALYSIS The mass spectrometer used is a 60’ Nier type using double collection and gas switching methods, and its use for the measurement of sulphur isotopic ratios has previously been described (HULSTO~; and SHILTOS, 196X). For the measurement of C13/CY2ratios it is necessary to move the collectors closer together in order that, masses 4.5 and 44 may be collected simultaneously. All results are given as the enrichment, in C l3 of the sample compared with a standard and are expressed in parts per thousand (%,), i.e.,
sample -- (C113/(!12) standard
Modification of the equations given by Craig (1957) for the calculation from the masses 44, 45 and 46 ratios gives BC13 =~:1.068 6 (G/44)
-~ 0.034 B (46/44)
(1) of ?A””
(2)
where 6 (45/44) and 6 (46/44) are defined by equations analogous to equation (1). New Zealand Te Kuiti limestone was used as the working standard for t’his work, but for ease of comparison with the results of other workers, the results given in this paper have been converted to BC13values relative to CRAIG’S PDB standard. The Te Kuiti limestone standard has been measured by CRAIG (1957) and found to be -1.67%, with respect to the PDB standard and -0*61~& with respect to N.B.S. Solenhofen limestone reference sample No. PO. As a check on the complete process of preparation and measurement, t,his latter comparison has been repeated in this laboratory, and the value of -O..i%, obtained, with a standard deviation in measurement of 0.1 5x0. This is considered satisfa,ctory. The standard deviation of the results given in Table 2 is approximately 0.3%,.
Mass spectrometer measurements in the thermal areas of New Zealand
401
CARBON ISOTOPIC EQUILIBRIA Carbon dioxide-Bicarbonate system In order to attempt to understand the isotopic relationships obtained between gas and water phases it is necessary to understand the fractionation which occurs when carbon containing molecules are shared between the two phases. If the two phases are in equilibrium it is possible to calculate the fractionation by statistical mechanics. A table of isotopic equilibrium constants at different temperatures has been given by CRAIG (1953), and the COsCH, and CO,CO,” values are given in Table 1. These latter values approximate those expected for the CO,-HCO,exchange. Table 1. Isotope exchange equilibria for CO,, CO,= and CH, (after CRAIG, 1953)
T “K
cPo,= ~ c120,=
Cl30
IL Cl20
0
00
273.1 298.1 400 500 600
1.016 I.012 1.004 0.9994 0.9975
2
0 0.935 0.943 0.963 0.975
0.981
It will be seen from Table 1 that at 20% the isotopic equilibrium between CO, and HCO,- ions is such that 6CY3 of the HCO,- is approximately la%,, positive w.r.t. the CO, but that this reduces to zero at about 220°C and becomes negative If a reaction is a long way from equilibrium, the fractionaabove this temperature. tion is produced by the kinetic isotope effect, and a totally different 6 value may be obtained. For example, in the absorption of CO, from air by a tray of sodium hydroxide at room temperatures, the 6Ci3 value of the CO,= is initially approximately 15x,, negative w.r.t. the CO,, i.e., in the opposite direction to that for equilibrium. If a partial equilibrium has been obtained, then the results may be anywhere between these two conditions. It should be noted here that under equilibrium conditions there is no significant fractionation between CO, above a solution and CO, dissolved in the solution. Thus the fractionation between gas and water phases of a bore or pool depends on the state of isotopic equilibrium, the temperature and the ratio of bicarbonate to dissolved CO, in the water phase. Although there are other ions present in these waters, it is sufficiently accurate for this work to calculate the (bicarbonate + carbonate) to (dissolved CO,) ratio from the pH. The state of equilibrium can be determined if the rates of the reactions involved are known. Information concerning rates at high temperatures is difficult to find and the values used here are obtained by extrapolation from the results of HIMMELBAU and BABB (1958). Methane-Carbon dioxide equilibria Previous workers (e.g., CRAIG, 1953) in this field have considered the isotopic equilibrium of methane and carbon dioxide in geothermal discharges. If equilibrium existed, it would be possible to estimate underground temperatures by measurement of the relative isotopic compositions of the CH, and CO,.
402
J. Ii.HULHTON and W. J. MCCABE
The overall mechanism
usually suggested is the reaction
The isotopic equilibrium constants given by UREV (1947) and CJUIG (1953) (see Table 1) have been used in the calculation of the temperatures deduced later in this paper. No experimental measurements of the CH, equilibrium constants are known to the authors. J)XSCt-SSION
OP &EXTI,TS
The results of (!r3/C’12ratio measurements on carbon dioxide in the gas phase (cK’~~~~), botal carbon dioxide in the water phase (dCnls) and on methane in the gas phase (methane 6 (P) are given in Table 2. From these values and the relative quantities of each as given in Table 1 of Part 1 (HIXSTOX and MC(‘ABE 1962) the
Fig. 1. &I?~, w.r.t. PDB, of carbon from various; Kern Zealand sources. Closed circles show approximate average 6C’3 values for whole discharge. Open circles show W3 for methane only.
average value (&I’,‘“) has been calculated for the total discharge and is given in Table 2. Most of the BCr13 (or cYC~>~) and methane SP values have been plotted inFig. 1. It will be seen that while the 6C’13 values of carbon dioxide in the gas phase (which forms t,he bulk of the carbon discharged) vary from + 2 to --9x0. the results For example, the &? for certain areas are nearly constant within themselves. values for Waiora Valley, Wairakei, range form -0.5 to -.-l..5,%o, while the six: The pools sampled at Waiotapu have 6CL3 x7itiues in the range -- 5.8 to -6*9%,. methane samples contain much less C’13, and the relationship of these results to underground temperatures is discussed below.
Bores. The 6C13 values for CO, in t’he gas phase are very constant (-3-0 -3.9%,) while the d03 values for the water phase sre slightly less so (-4-O
to to
_____.__:
Whaktltane area Awakeri Springs Itoborna Soda Springs Onepu Small fool Onopu Bore Onepu Large Pool
Whale Island pools Boiling Pool Beach Pool 13each Pool nr Cliff
712A 612 Katattthi
712
Sorth Island fumarolcs
White Island fumarolos Big Donald (a) Big Donald (b) Big Donald (c) Seven Dwarfs
Wairakei bores Bore 11 18 30 44 203 13
_.
Location
R470/6 1 R472/t3 / R473/1 X473/2 / R473/3
I
R477/6
’ R477/1 ! R477/4
1
It43311 R433/2 i R62 i R125
R507/2 it93 R507/6
/ R.507/3
!
! It400 j R402 R403 / R404 / R405 R401
/
.3.g
-4.9
-54 -4-7
-1*2 -3.6
-5.3 -6.8 _._6.3
-4-4 -5 -6.6
-9.0 -2.6
-9.1
-3.9
!:
-.5.3
-3.0 -3.5
-1 -3.1 -4.7 -4.5
-40 -5.7 -4.9
I
1 -3.4 j -3.6 ’ -3.3
__
-39.0
- 29.7
-- 28.5 -27.9
- 16.1 -- 22.4 -23.3
- 27.0 -27.3 -26.8 __27.3
’
1
I
j : /
____ __I ---
-12 -2.7
-
-3.9 -4.3
-1 -3.1 -5.4
I
,
;
!
i j j
i 130
250
j
,I
II
’
I
_ _..--
/
I
I
I
235 250
440 345 250
245
-3.6 -4-2
260
250 245 250
_
-3.5 -
-3-7
__..
tx,
I / : Carbon dioxide ’ Methane Average ! BCa’Y K!p dc!‘3 I &p (%Q) (X0) (Zo)
j__.__l~_-..-___
/ xs so.
)
Table 2. C13/C?aRatios of carbon dioxide and methane
140
255
250 265
I 945
330 365
240
190
260 200
240
/ 1
235 260 260 220 190
--I--i 203
54 50 61
453*
8.6 5.6
6.8 6.9
7.6 7.7 7.7
_I_
I
-_
404
J. R. WTJLSTONand W. J. MCCABE
Mass spectrometer
-1
measurements
in the thermal areas of New Zealand
405
406
J. K. HULSTONand W. a. MCCABIS
-9*1%,). If isotope equilibrium exists, then the difference in BC’L3between gas and water would indicate equilibrium temperatures ranging from 240” C! for Korc 1 I through 290°C for Bore 30 to 4SO”C for Bore 13. (ieothermograph readings (Table 2) show however, maximum temperatures of approximately 200”( ‘, :!60”(’ (‘onsideration of the time of travel of and 190°C respectively for these bores. water in the bores, and reaction rates, show that) it would be unlikely that the equilibrium existing at the bottom of a bore would hc maintained at t,he top. It’ t’akes approximately PO-30 seconds for a water molecule to travel t,he length of a bore. The reaction HCIO, -+ (‘0, + OH-- at 160”(’ has a half-time of approximately 0.5 sec. A possible explanation of the more negativt, ijC13 value for the liquid phase may be that all free (‘0, is swept from the water phase by flashing steam. Due to the partial pressure of (!O, in t,he gas phase, rc-solution of CO, occurs to a certain extent. Due to t,he k’mctic isotope eff&t (i120, will dissolve more rapidly, thus decreasing the BC’13value of the water and bv isotopic exchange the 6C13value of the remaining HCO:, --will also decrease. ( ‘alculat ions of solubilitg using the data of ELLIS and FPFE (1957) show t,hat at, 1fiO”f approsimately t /&mole of CO, per mole of water phase is present as free ( ‘0,. excluding .Hore 13 which gives 0.1 ,u M/M. Table 1 of Part I shows this to be approximatcl,y IO per cent of the total CO, in the water phase. Isotopic measurements of free (‘0, and H(Y):,- in four bore water samples at room temperatures gave resu1t.s agreeing with theoretical isotopic equilibrium calculations for CO, and (‘0,~ (UKEB 1947) and assuming that C’O, and HCO,mm have a similar equilibrium. PzcmnroZe.s. The results at both White Island and Jlrairakei are similar to those of the bores, but the spread is greater suggesting a similar origin but with more The 8c”” value for Iietatahi is disturbing influences in the case of the fumaroles. more negative than that, for the bores, but, is still within the spread found in the pools. Pools. While the C13/C’12ratios of most of the areas arc constant within themselves, others, particularly Geyser Valley, are very variable and probably are the result of a number of different effects, such as dilution with groundwater CO, having a large negative Xl3 value, fractionation by gas loss before entering the pool and fractionation between gas and water in the outflow from the pool. Dilution with groundwater CO, will only have a significant effect where t,he total CO, output is low. Because of the possibility of loss of gas prior to entering the pool, the quantity of groundwater CO, is best estimated from the quantity of argon present. Fractionation due to loss of gas would be significant, only in pools of higher pH (e.g., Pool 39, Geyser Valley), where if equilibrium conditions exist, CO, depleted in Cl3 will be lost and the remaining solution will have a more positive 6C13 value. Measurements of KY3 for both gas and water phases have heen made on several pools. These measurements give, in general, smaller fractionations than would be expected from the pH and temperature measurements if a state of isotopic equilibrium existed. This is demonstrated in Table 3 which compares measured and calculated differences in SCl3 between gas and water phases. The calculated difference is obtained from the relationship
Xass spectrometer rn~~s~~rnents in the thermal areas of New Zealand
fKY3
water -
KY3 gas
a “+” b , where: = --
= (HCO,-)/(CO,)
=I=antilog (pH -
407
6-4)
and z = (SC&os- - BC$). It will be seen in Table 3 that the Awakeri Springs calculated value is very different from the measured value. This seems to indicate that either there is a kinetic isotope effect due to CO, being absorbed in the water, or else there is mixing of two water supplies just under the surface, and equilibrium is not re-established. It will be seen that, if it is desired to know the 6Cr3 value for the whole discharge, and if a significant fraction of the COz is in the water phase, it is not possible to predict the KY of one from the other, and hence both phases must be collected and measured. Methane-Carbon
dioxide system
On the assumption that isotopic equilibrium exists for this system, temperatures have been calculated for all samples where KY3 values have been measured for both CH, and CO,, and these are recorded in Table 2. Reference to Table 1 will show that a standard deviation of O-3%,, in both the CH, and CO, KY3 values corresponds to a standard deviation of approximately 8°C for temperatures in the region of 25O’C. For comparison the surface temperatures of pools have been recorded, and in the case of the Wairakei bores temperatures at the bottom of the bores, as supplied by New Zealand ministry of Works, have been recorded. It will be seen from Table 2 that all the known high temperature areas, Wairakei, Waiotapu, Whale Island and White Island, have given high isotopic equilibrium temperatures. Table 3. Carbon dioxide fractionation between gas and water phases in pools and bores
-
Temp.
Location
I
(“C)
Rotoma Soda Champagne Pool 55 Pool 48 Bore 186 Bore 203 Bore 13 Bore 30 Awakeri Pool 39 /
50 75 93 61 84 220 190 260 54 94
i
[GC&0,_ - SC&] calculated
+9 17 +6 18 +6.5 0
j
+1 -1 ts.5 +6
(at
:F
C)
[a&13 -
fxp]
,calculated measured
I _-
5.6 5.7 6.8 5.9 6.7 6.8 6.9 7.7 8.6 8.8
$-I.2 +1.2 $- 1.3 +2.0 +4 0 +0.6 -0.9 18.5 +6
i
+ 1.0 +0,3 +o‘@ +3*3 1-112 -1.8 -5.2 -1.6 -7.8 +0.6
-
Although K$,$3 has not been measured, it would appear that Tiger Pool at Ngawha also gives a high equilibrium temperature. This agreement provides some evidence that isotopic equilibrium does exist. This is strengthened by the much more negative KY3 values obtained for methane If the gases from these samples from colder areas, e.g., HelensviIle and Lyttelton. two areas are in isotopio equilibrium, then they indicate temperatures of about
408
J.R. HULSTON and V’. J. MCCABE
50°C which would seem qnite reasonable. Even if equilibrium does not exist in these samples, the large variations in the methane 6CX3values in Table 2 would at least support the existence of isotopic equilibrium in the samples from the hotter areas. ‘Ihe evidence for the existence of chemical equilibrium discussed in Part 1 of this paper is also a strong indication that isotopic equilibrium may also exist. The temperatures obtained from chemical equilibrium calculations in Part I have been included in Table 2 in order that comparisons may be made with methane isotopic temperatures and with those measured directly at the bottom of the bores. As would be expected from the errors involved in the measurements for chemical equilibrium, the best correlation is between the measured values and the methane isotopic value, but it is interesting to note that there is a great,er correlation between the chemical equilibriun~ and the isotopic temperatures than between the chemical equilibrium and t,he measured temperatures. This is pres~~n~abl~~because the isotopic and chemical equilibria are attained at a point some distance from the bore. It would therefore appear that the water and gases have been at a temperature of 220-260°C for some time, and thus do not come direct,ly from higher temperature spots. Another method of checking for isotopic equi~i~riul~~ is to measnro She natural radioactive 04 content of the CH, and CO, gases. This has been attempted by FERGUSSON (pers. comm.), but so far no conclusive results have been obtained. It is hoped that the Ngawha area (North Auckland, New Zealand) may yield a suitable sample. Even the Cl4 method may be subject to error if small quantit#ies of modern groundwater carbon dioxide entered the gas stream just below the surface. This would increase the CL* activity of the carbon dioxide but would not aReot the (‘la activity of the methane because of the absence of suficient time to re-establish isotopic equilibrium between the CO, and CH,. This type of situation is thought to exist at Awakeri. Present evidence thus appears to support the existence of isotopic equilibrium, and it, would appear that this technique can give valuable infornlation on temperatures of underground water in new areas. Mean isotopic compositior6 cmd origin qf the Cmh?L ‘The aver&ge 8C13 value (BC3,13)is included in Table 2, in order that the origin of the carbon may be considered. It will be seen that, the averages for t,he thermal samples range form -3+5 to -~ 4*47&,for Wairakei bores and fl~~naroles through -6 to -7x0 for Waiotapu to - 12x, for Awakeri. Pull information is not available on samples from Big Donald E’umarole (a) and some of the Wairakei pools, but the results in most cases suggest that the average 6C1,13values are around -3 to -3%0, (CRAIG{1953) obtained values of - 1.1 to --6*4x, for carbon dioxide from pools in Yellow&one Park, U.S.A. Since the relative quantities of C!O, in the gas and water phases and of methane are not given by CIRAIG,it is not possible to compare these results directly, but it will be seen that they are in the same range as the CO, gas from the Wairakei pools in Table 2. A possible source of the relatively large quantities of carbon dioxide from these
Mass spectrometer
measurements
in the thermal
areas of New
Zealand
409
thermal samples is the heating of carbonate rocks at depth. CRAIG (1953) reports marine limestones with 6C13 values of from +2*4x0 to --3*3x,, and dolomites with 6C13 values of +2*7x, to -2*3%,. The 6C,13 values in Table 2 in general contain less Cl3 than these carbonate rocks. It is possible that the main source in the thermal areas is similar to that of the carbonate rocks of lowest Cl3 isotopic composition, i.e. --3x,, but that this is being mixed with a proportion of organic carbon dioxide of 6C13 s -27x, from the deep recirculating groundwater. For example, the concentration of 23 1~M/M of CO, in the water phase (HULSTON and MCCABE, 1962: Table 1) at H2W13 = -21*9%, (see Table 2) measured for the sample of artesian water from Hutt Park (near Wellington) (R489/3) may result from a mixing of approximately 5 y M/M of carbon dioxide at SC13 = -3x0 and 18 ,u M/M of organic COz at El3 = -27x,. The CO, (organic)/Argon ratio of IS/O*29 = 60 obtained for this sample may be used as an indication of groundwater CO, to be expected in the thermal areas. This could explain the 6Cr13 value of -12x0 obtained for Awakeri, but this ratio would not give sufficient groundwater carbon to account for the value of -6*5x,, obtained for Champagne Pool at Waiotapu. In this latter case the non groundwater portion of the CO, must have a 6CX3value of approximately -6x,, which is significantly less than that expected from marine carbonate rocks. The above hypothesis thus explains some but not all the lower 6C13 values. The remaining variations in 6C13 must be attributed to variations in the isotopic ‘composition of the carbon at its main source. This does not rule out the possibility of carbonate rocks as the main source of the carbon, as the carbonates in some areas (e.g. Waiotapu) could have been formed in freshwater systems which contained organically derived carbon dioxide. This was suggested by CRAIG (1953) to explain the 6C13 values around -9%,, obtained for some freshwater carbonates. However, the possibility of juvenile carbon, of as yet undetermined isotopic composition, entering the system and effecting the Xl3 value should also be considered. CoNCLUsIoNs
Carbon dioxide isotope measurements on bores, fumaroles and pools show that the isotopic composition of CO, appears to be most uniform in bores and least uniform in pools. The variation in pool results is probably due to variable groundwater contamination. The differences between gas and water phases from bores and pools imply that isotopic equilibrium is not attained. In this type of measurement it is essential to measure all products in all phases in order to estimate the isotopic composition of the whole discharge. It does appear from the measurements made in this work that the carbon in geothermal gases in New Zealand comes from a common origin which has a &Y3 value close to that of carbonate rocks and also similar to that found by CRAIG (1953) for CO, from the Yellowstone Thermal Area. The temperatures obtained by measuring the 6Cia of CO, and CH, and assuming them to be in equilibrium in the reaction CO, + 4H, $ CH, + 2H,O agree reasonably well with temperatures obtained by direct measurement. As the reaction rate for this reaction is very slow, the equilibrium system must have been in existence for several years.
410
J. R. HULSTON and W. J. MCCABE REFEREENCES
CBAIG H. (1953) The geochemistry of t,he stable oarbon isotopes. 3, 53-92.
Gee&m.
et Casmoehirn.
Actn
CRAIG H. (1957) Isotopic standards for carbon and oxygen and correction factors for massspectrometric analysis of carbon dioxide. Geochim. et Cvsmochim. Acta 12, 133-149. ELLISA. J. and FYFE W. S. (1957) Hydrothermal Chemisky. RF?>.Pur. i2@. Chem. 7, 261-316. FERGUSSONG. J-. Personal Communication. HIMMELBLAUD. M. and BABB A. L. (1958) Kinetic Studies of Carbonation Reactions using Radioactive Tracers. ,i. Amer. In.& Chem. E:ngnr. 4, 143-152. HULSTONJ. R. and MCCABE W. J. (1962) Mass Spwtromet,er Measurement,s in the Thermal Areas of New Zealand. Part I-Carbon dioxide and residual gas analyses. Geoclzirri. et C~~oc~~rn. Acta 26, 353-398. HULSTONJ. R. and SHILTONB. W. (1958) Sulphur Isotopic \‘ariations in Nature. Part 4-- ~~e~~ernent of Sulphur Isotopic Rat,io by Mass Spectrometry. S.Z. J. 55. 1,91-102. MCKINNEY C. R., MCCREEJ. M., EPSTEIN S., ALLEN W. A., UREY H. C. (1950) Improvements in Mass Spectrometers for the Measurement, of Small Differences in Isotope Abundance Ratios. Rev. Sci. Innstrum. 21, 724-730. NrER A. 0. (1947) A Mass Spectrometer for Isotope and (:as Analysis. Ho?+.&.% Inetnur!. 18, 39&411. NIER A. 0. and GUI;BR_&NSEN E. A, (1939) Variations in t,he relati\-e abundance of the Carbon Isotopes. J. Amer. Chem. Sot. 61,697-698. RANKAMAK. (1954) IsoEope Geology. 535 pp. Pergamon Press, London. THOMPSONG. E. K. Personal Communication. UREY H. C. (1947) The Thermodynamic Properties of Psotopic Subatanccs. ,T. (:l~em. Sot., 562-581.