Geochimica et Cwmochimka
Acts, 1999, Vol. 33, pp. 997 to 199% Pergamon Prew
Printed in Northern Ireland
NOTE!3
Carbon-13fractionation between aragonite and calcite MI~EAEL RWINSON Department of Chemistry, The University of Chicago, Chicago, Illinois 60637 and ROBERT N. CLAYTON Enrico Fermi Institute, and Departments of Chemistry and Geophysioal S&noes The University of Chicago, Chicago, Illinois 60037 (Received30 August 1968; acoe~~ted in revisedform 15 April 1989) &&a&-Ar8gonite and caloite were precipitated from bicarbonate solutions by slow removal of oarbon dioxide at 26°C. Carbon isotope fractionation faotors (expressed as 1000 Ina) are: oaloite-bicarbonate ion 0.9 f 0.2; aragonite-bicarbonate ion 2.7 f 0.2; aragonite-oalcite 1.8 f 0.2. Theoretical oalculation of the aragonite-oalcite fractionation gives 0.9.
TARUTANI et al. (1969) have shown that aragonite concentrates O’s relative to calcite at 26°C by a small factor. This fraotionation is in reasonably good agreement with the result of a calculation based upon an extension of the Urey-Bigeleisen-Mayer treatment. As a further test of the applicationof the theory to solid phases, we have measuredthe carbon isotope fractionation between the same two phases. There are several published sets of vibration81 frequenoies for the oarbonate ion in calcite and aragonite (HEXTER, 1968; MILLERet al., 1960; HUANQ and Tuna, 1960; ADLER and KERR, 1963; WEIR and LIPPINCOTT, 1961; SCHROEDER et al., 1962; Ross and GOLDSMITH,1964). Preliminary calculations with some of these frequencies yielded the surprising prediction that the equilibrium carbon isotope fractionation factor between aragonite and calcite at 25” might be as large as 1.00’7. It appeared therefore that an experimental determination of this factor would permit selection of the best set of vibrational frequenoies. It wae neoessary to make one sign&cant modification of the experimental procedure used by T~LRUTANI et al. (1969) in precipitation of aragonite and calcite for oxygen isotope measurements: that is to provide in the solution a carbon reservoirwhich is large relative to the amount of carbonate and oarbon dioxide to be removed. This ~8s accomplished by adding sodium bicarbonate in about lOO-fold excess over the calcium carbonate. The conoentration of bicarbonate ion was varied by a factor of 1.4 to give a variation by a factor of 2 in calcium concentration in the starting solutions. Aragonite was produoed by adding magnesium chloride to the solution. Both calcite and aragoniteprecipitationswere carriedout in solutionsmade up to ionic strength of 0.5 by addition of sodium chloridein order to maintain a nearly constant ionio environment during the course of the precipitation of calcium carbonate and the removal of carbon dioxide.
EXPERIMENTAL PROCEDURE One liter solutions were made up with compositionsgiven in Table 1, and 250 mg of reagent grade calcium carbonate was added to each. Carbon dioxide gas was bubbled through the 7
997
Sates
99x
solutions for four to five hours, until the solution was saturated with CO, gas. Each solution was then allowed to equilibrate overnight, filtered to remove the excess CaCO,, and again allowed
Table 1. Compositions of starting solutions (concentrations in millimoles/l) Run number -______
---.
Cl c2 c3 Al A2 A3
N&l .--
._~ _.~___
328 328 420 300 300 390
NaHCO,
M&q
_ ~~_
12; 1*7 I HO 127 127 80
9 9 10
Ionic
strength
___~.___ 0.455 0.455 0.500 0.466 0.455 0.500
__
to stand overnight to assure isotopio equilibrium among the dissolved oarbon species. About 500 ml of each solution was placed in a gas washing bottle in a water bath at 25°C and ?i, was bubbled through the solution at the rate of 2-5 calmin. Precipitations were run in pairs (one aaloite and one aragonite) with the same nitrogen flow rate and reaction time. At the beginning and end of precipitation, the pH of the solution was determined, and the concentration and isotopic composition of total dissolved carbon were measured. To determine the oarbon-13 abundance, a 2 ml aliquot was placed into a bulb with a sidearm containing 5 ml of 100 % I+,PO,. The bulb was attached to a vacuum system but not pumped on. The aliquot in the bulb was frozen with liquid nitrogen and the air pumped out of the system. The solution was then melted, reacted with the aoid, and frozen with a dry ice bath. The CO, was extracted and collectted; the solution was melted, refrozen, and the CO, again extraoted. Isotopic composition of the CO, was determined on a double-collecting rnw spectrometer. After from two to four days, the precipitated CaCO, from each solution was collected, filtered, washed with water and acetone, and dried. The crystals were examiaed under a microscope to determine their identity as aragonite or calcite by morphology and refractive index. The identification was verified by X-ray diffraction. A portion of the solid oarbonate was reacted with 100 ‘A qPO4 and the resulting CO, ~a.3 analyzed mass speatromatriaally. EXPERIMENTAL
RESULTS
The results of the experimental measurements are given in Table 2. Carbon isotope aompositions are in the 6 notation, relative to the Chicago PDB standard, and have been corrected for mass spectrometer valve mixing, background, mass-44 tail and for 01’ content (CRAIU, 1957). Some uncertainty is involved in evaluating the isotopic composition of the bicarbonate ion in solution. Its oomposition changes during the experiment aa a result of (a) an increase of SCzs for the total oarbon in solution due to loss of isotopically lighter CO,, and (b) a change in proportionsof COs2-, HCOs- and CO, in solution due to increaseof pH. The abundances of the carbon-containingspecies in solution were calculated from the measured values of pH and total carbon content, using the values 6.36 and IO.33 for the first and second pK’s for carbonic acid. Activity aoeffioientsfor HCO,- and COs2- were taken from GARRELS et aE. (1961). The equilibrium constant for carbon isotope fractionation between bioarbonate ion and aqueous CO, was taken as 1.0082 (VOGEL, 1961). The constant for carbonate ion-bicarbonate ion was calculated from the values given by THODE etal. (1905) for bicarbonate ion-CO, gas and carbonate ion-CO, gas. For this latter fractionation, they provided a lower limit of 1.0133, and a maximum observed value of 1.0166. We have calculated our data using both of these factors. This introduces a systematio unoertainty of about O*lyWin the estimates of fractionation of soIid carbonaterelative to bicarbonateion, but the effect cancelsout in the estimation of the aragonitecalcite fractionation. For each experiment, a mean value of the bicarbonate isotopic composition was found by averaging the initial and &al values. Since the change in bicarbonate isotopic composition is small, no significant uncertainty is introduced by this process. The amount of change is also variable from one experiment to another, but there appears to be no systematic relationship 1000 lna to the isotopic fractionation factors. The resultingfractionations at 25”C, expressedEUY are: aragonite-caloite 1.8 f 0.2; oak&e-bicarbonate ion 0,9 + 0.2; aragonite-bicarbonate
136 128 90.5
138 129 90.6
63 66 96
63 66 96
(W
Time
101 107 68.5
104 114 70
(b) Using Kco,~-co~
= 1.0166.
(a) Using ~~~~~~~~~ = 1.0133.
Cl c2 c3 Mean Al A2 A3 Mt%lJl
RuIl No.
Total diss. carbon (mmole/l) Initial Filld Find 8.78 8.00 8*22 8.40 8.30 8.38
Initial
6.69 6.96 6.60
6.67 6.90 6.67
PH
-31.24 -31.57 -30.91
-31.03 -30.92 -31.08
Initial
(a)
6HCO,-
-31.42 -30.84 -30.66
-31.46 -31.02 -30.93
Find
Table 2. Results of precipitation
-31.24 -31.67 -30*91
-31.03 - 30.92 -31.08
Initial
(b)
dHCO,-
experiments
-31.66 -31.02 -30.89
-31.95 -31.11 -31.08
Final
0.92 0.86 0.72 0.83 2.82 2.77 2.42 2.67
- 30.36 -30.12 -30.30 -28.60 -28.62 -28.43
(a)
SC&O,
1.18 0.92 0.80 0.97 2.93 2.86 2.66 2.78
(b)
1000 ln a CeCOS-HCOa
1000
Xotes
ion 27 f 0.2. measurements, equilibrium.
The error assigned to these fraction&ions is the average analytical error of the and does not include any systematic errors, such as that due to departure from 1hcuss1ox
Calculations of isotopic partition function ratios for aragonite and calcite were made using an extension of the UREY (1947) and BIOELEISEN-MAYER (1947) theory as described by O'NEIL et al. (1969). The same approximations were used in the calculations reported by C’PiEIL el ILL. (1969) and TAXJTANI et al. (1969), i.e. internal vibrational degrees of freedom of the carbonate ion were treated separately, using the fundamental frequencies as determined by infrared spectroscopy, and lattice modes were treated by a single Debye function with 0 chosen to fit the low-temperature hea.t capacity data, after subtraction of the vibrational contribution to the heat capacity. Since these B’s are identical for calcite and aragonite (400”K), the difference between their partition function ratios arises entirely from differences in the internal vibration frequencies of the carbonate ion. In the case of aragonite, the lower site-symmetry of the carbonate ion leads to a removal of the degeneracy of rs and Ye, resulting in a splitting of the i.r. absorption peaks. In our calculations, we used an average value of the observed frequencies, and applied a single value of the isotopic shift factor, as determined by means of a HEATH and LINNETT (1948) force-field for a symmetrical carbonate ion. These approximations probably introduce a smaller error than t,hat associated with assigning a frequency to the broad and intense vs band (see below). BOTTINGA (1968) found that the frequency data of SCI~ROEDER et al. (1962) gave the best agreement with experiment for calcite-water and calcite-CO, isotope fractionations. The frequencies used in the present work are given in Table 3. The symmetric stretching frequency v1 does not contribute to carbon isotope fractionation. The frequencies are taken from SCHROEDER Table
3. Vibrational
frequencies
and calculated _
VI (cm-‘) % % V4 In (Q13/Q1*) internal* In (Q’8/Q’e) lattice In (Q’8/Q12) total
partition
function
ratios at 25°C
Calcite
Aragonitx
1070 881 1460 712 0.18308 0.00216 0.18523
1066 866 1475 708 0+18400 0.00216 0.16615
* Reduced partition function ratios as defined by UREY (1947).
et al. with the exception of vs for aragonite, which was taken from since no value was given by SCHROEDER et al. This is the same The TARUTANI et al. for calculation of oxygen isotope fractionations. function ratios are given in Table 3. The calculated aragonite-calcite 0.9 at 25°C.
ADLER and KERR (1963). set of frequencies used by resulting isotopic partition
fractionation:
1000 lnu =
CONCLUSIONS The slow precipitation of caloite or aragonite from bicarbonate solutions appears to give an equilibrium isotope fractionation for carbon as well as for oxygen. There is no proof that isotopic equilibrium was attained in the experiments, but the insensitivity of the measured fractionation to variations in solution concentration and precipitation rate make such an assumption plausible. The possibility cannot be ruled out, however, that the factors which lead to the precipitation of metastsble aragonite may also bring about a non-equilibrium isotopic distribution. Such an effect might occur in nature as well. According to our experiments, Cl3 is enriched in aragonite relative to calcite by l*S%, and in calcite relative to bicarbonate ion by O-9%,. VOGEL (1961) gives experimental determinations of carbon isotope fraction&ions of oarbon dioxide gas relative to bicarbonate ion and relative to calcium carbonate (presumably calcite). These results yield a c&it+bicarbonate fractionation
Notes
1001
Combination of BAERTEICEI’S (1957) determination of a carbon of 1000 lna = 12 f 0.8 (22%). dioxide+al&e faotor of l*OlO with a CO,-HCOsfaotor of f-077 (Voorr~, 1961) yields a value of 1000 Ina = 2.3. All of these values are compatible with the observations of CRAIG (1964) that average limestones have 6C1s about 27&,greater than average oceanic bicarbonate. KEITH et al, (1964) observed differences in 6018 and KY* between fibrous aragonite in external ligaments and shell carbonate (calcite or aragonite) in several marine and fresh water peleoypods. The mean differences were enrichments in the aragonite of 05% for oxygen and 1.0% for carbon. These differences are similar to those seen in inorgsnio preuipitation of aragonite and calcite at 25”, i.e. O*Sy&,for oxygen (TARUTA~Iet at., 1969) and 13yW for carbon (this work). Although the natural pelecypod data are not simple aragonite-calcite differences the coincidence of the results suggests a dominant control by equilibrium factors. The agreement of the experimental aragonite-calcite fractionation with the theoretical value is fair if the vibrational frequencies of SCHROEDER et al. (1962) are used, The serious
disagreementfound if other publishedfrequenciesare used arisesprincipallyfrom the rs frequency of both calcite and aragonite. In all the other referencescited here, this frequency for oalcite is given as about 1432 cm-r, whereas SCHROEDER et al. and Ross and GOLDSMITB. (1964) give a value of 1460 cm-i. In room temperature spectra, this absorption is intense and very broad. The liquid helium temperature measurements of SCHROEDER et aE. allowed a more accurate determinationof the fundamental frequency. This single change of frequency produces a change of 3*7x,, in the calculated partition function ratio at 25°C. This example shows that great caution must be used in the application of theoreticallycalculated isotopic fractionationfactors. Ac~w~ag~~-~. RUBINSONwas a participant in the National Science Foundation Undergraduate Reseamh Program. The researchwas also supported by NSF Grant Number GA-614. REVERENCES ADLER H. H. and KERR P. F. (1963) Infrared absorption frequency trends for anhydrous normal carbonates. Amer. Mineral. 42, 124-137. BAERTSCHIP. (1957) Messuug und Deutung Relativer Ha~gkei~va~ationen von Or8 und C!lsin K~bo~tges~~en und ~~eralien. S&we&r.~~~~~. P&ogr. &f&t. 37,73-162. BI~ELEISENJ. and &vx~ M. G. (1947) Calculations of equilibrium oonstants for isotopic exohange reactions. J. Chem. Phy8. 16, 261-267. BOTTINGA Y. (1968) Calculationof fractionationfactors for carbon and oxygen isotopic exchange in the system calcitecarbon dioxide-water. J. Phye. Chem. 78, 800-808. CRAIGH. (1954) C-13 in plants and the relation between C-13 and C-14 in nature. J. #eoE. 62, 115-14s. CRAIU H. (1957) Isotopic standards for carbon and oxygen and correction factors for mass speotrometric analysis of carbon dioxide. &oeh&m. Cosmoehim. Acta 12, 133-149.
GARRELSR. M., THOMSON M. E. and SIE~J~RR. (1961) Control of carbonate solubility by carbonate complexes. Amer. J. Sci. 259, 24-46. HEATR D. F. and LINNETT J. W. (1948) Molecular force fields-Part III-The vibration frequencies of some planar XY, molecules. Tram. Faraday Sot. 44, 8733878. HEXTERR. M. (1958) High resolution, ~mperature-dependent spectra of calcite. Spctrochim. Acta 10, 281-290. HUAN~ C. K. and KERR P. F. (1960) Infrared study of carbonate minerals. Amer. Mineral. 45, 311-324.
KEITHM. L., ANDERSONG. M. and EICHLERR. (1964) Carbon and oxygen isotopic composition of mollusk shells from marine and fresh-water environments. Geochim. Cosmochim. Acta 22,1757-1786. MILLERF. A., CARLSONG. L., BENTLEYF. F. and JONES,W. H. (1960), Infrared spectra of inorganic ions in the cesium bromide region (70~300 cm-r). S~~t~~h~rn. Aeta 16,135-235. O’NEIL J. R., CLAYTONR. N. and MAYEDAT. K. (1969) Oxygen isotope fractionation in divalent metal carbonates. In preparation. Ross S. D. and GOLDSMITH J. (1964) Factors affecting the infrared spectra of planar anions with Dgh symmetry--I. Carbonates of the main group and first row transition elements. Spectrochim. Acta 20, 781-784.
1002
Notes
R. A., WEIR C. E. and LIPPINCOTT E. R. (1962) Lattice frequencies and rotational barriers for inorganic carbonates and nitrates from low temperature infrared spectrosoopy. J. Rea. U.S. Nat. Bur. Stand. @A, 407-434. I!.~R~TANI T., CLAYTON R. N. and MAYEDA T. K. (1969) The effect of polymorphism and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Qeochim. Coamochim. Acta. 33, 987-996. THODE H. G., SHIMA M., REES C. E. and KRISWAMURTY K. V. (1965) Carbon-13 isotope effects in systems containing carbon dioxide, bicarbonate, carbonate, and metal ions. Can. J. Chem. 48, 682-595. UREY H. C. (1947) The thermodynamic properties of isotopic substances. J. Chem. Sot. (Lonthn~ 562-581. VOUEL J. C. (1961) Isotope separation factors of carbon in the equilibrium system CO,-HCO, .COs2-. In Summer Course on N&ear Geology, Varenna, pp. 216-221. Comitato Nazionale per L’Energia Nucleare, Pisa, Italy. WEIR C. E. and LIPPINCOTT E. R. (1961) Infrared studies of aragonite, calcite and vaterite type structures in the borates. carbonates and nitrates. J. Res. U.S. ANat. RUT. Stand. %5A, 173-184. SCHROEDEB
Geochimica et Cosmochimica Acts, 1969, Vol. 33, pp. 1002 to 1006. Pergamon Preen. Printed in Northern Irelaod
Rubidium and strontium determinations by X-ray fluorescence spectrometry and isotope dilution below the part per million level B. W.
CHAPPEU of Geology, Australian National University, Canberra, A.C.T. Australia and W. COMPSTON, P. A. ARRIENS and M. J. VERWO~ Department of Geophysics and Geoohemistry, Australian National University, Canberra, A.C.T., Australia
Department
(Received
20 January 1969;
accepted
in revised form
17 March 1969)
Al&r&--The rubidium and strontium contents of dunite DTS-1 and peridotite PCC-1 and of rubidium in a standard pyroxene Px-1 have been measured by X-ray fluorescence speotrometry and by isotope dilution. Both techniques are useful in determining low concentrations of these elements. THIS paper arose from a study of the rubidium and strontium contents of several standard rock and mineral samples, with the aim of comparing results obtained by X-ray fluorescence spectrometry and isotope dilution. Because of the low contents of rubidium and strontium in the standards DTS-1 (dun&e) and PCC-1 (peridotite) (FLANAGAN, 1967, 1969) and of rubidium in the pyroxene Px-1 (GOLDICH et al., 1967), it seemed that an attempt to measure these elements by X-ray speotrometry would provide information on the practioal detection limits obtainable with this technique and illustrate the problems associated with the measurement of trace elements present in concentrations near or below one part per million. In addition, a comparison co~llrl be made with results obtained by isotope dilution. ISOTOPE DII;UTION MEASUREMENTS Isotopic analyses were made on a modified Metropolitan-Viokers MS-2 mass spectrometer having magnetic peak switohing and digital output as described by CO~~~PSTON et al. (1965).