Measurement of O18O16 ratios of total oxygen of carbonates

Gewhimica et Cosmochimica Acta1985.Vol.2%pp.lS_lito 1353.Pergmnon PressLtd. Prhbed iuNorthern

Measurement of O"/Ofs ratiosof

total

oxygen

Ireland

of

carbonates*

TALESHWARSHARMA~and ROBERTN. CLAYTON l’ho Enrico Fcrlni Jnetitut,e for Nuclear Studies and Drpartmcnt of C’hemist,ry,University of C’hicago,C’hicago (Receiced2 July 1964) Abstract--The commonly used phosphoric acid procedure for the determination of oxygen isotope abundancrs in carbonates involves a lsrgc kinetic isotope effect. The value of the fractionation fwtor, 2, defined as: (G1”/Wc!o, a = (o’~/O%Brbonate has been mearurccl for srwral alkalino earth and transition metal carbonates. The value of 11100lna ranges from 9.9 t,o 11.2. The previously used assumpt,ionthat calcitcrand dolomite have the same value of TVleads to an overestimate of t,hc?clolomito-calcite oxygen isotope fractionation by 0.8 per mil.

THE most commonly used method for the extraction of cerbon dioxide from carbonates for isotopic analysis of oxygen and carbon involves the decomposition of the carbonates with loo:/, phosphoric acid at 25°C (MCCREA, 1950). Many carbonates react to completion in a few hours, but, according to the stoichiometry of the reaction, yield only two-thirds of the total oxygen of the carbonate as carbon dioxide. There is a fract,ion&ion of oxygen isotopes in the reaction, HOthat in order to determine the 018/01S ratio in the carbonate sample by this method, it is necessary to know the isotopic fractionation factor Q, defined as: (01”/01%W2 O1= ol*/O%lrboni,te Two analytical procedures have beon described for the quantitative extraction of oxygen from a variety of compounds. One met.hod, based on reduction with carbon at high temperatures has been described by BAERTSCHIand SCXWANDER (1962), SCH~ANDER(1953), CLAYTONand EPSTEIN(1958), ‘~T~KOGRADOV et cc.l.(1958) and DONTSOVA(1959). The second procedure, in which oxygen is liberated by reaction with fluorine or halogen fluorides has been described by BAERTSCXIand SILVERMAN(1951), HOEKSTRAand KATZ (1955), SHEE’Tet u.2.(1956, 1957), TUIXXE (1960), TAYLORand EPSTEIN(1963), CLAYTONand MAPEDA(1963) and SHAKMAand CLAYTON(1964). The first of these two procedures was used by CLAYTON and EPSTEIN (1958) to determine a value of a = 1.010 for manganese carbonate (rhodocrocite). The second procedure, according to the following reaction: GCaCO, + 4BrF3 -+ GCaF, + 3Br, -!- BCO, + 30,, * This work was supported in part by a grant from the National Science Foumdation NSF-GP-2019. t Present a&lross: Department of C!hemistry, Indian In&itute of Technology, Kanpur. U.P., India. 1317

was used by CLASTON(1961) to determine thefractionationfactor for the phosph~~ric~ acid reaction with calcite. A value of cc -.=l-00999 was reported. In all previously published w-ork in which oxygen isotope measurcment!s m-c' reported for more than one carbonate mineral (for example, CLAYTON and EPSTEIN (1958), EX+EL eta,!. (1958), JAMESand CLAYTON(1962), FRIEDMANand HALLL(1 !U), LOVERIXG, MCCARTHY and FRIEUM~S (1963), EPSTEIN, GRAF and DEGENS (1964) and DEGENSand EPSTEIN(1964)) it has been assumed that the isotopic fractionation

in the phosphoric acid reaction is the same from one carbonate to another. Yrelinrinary experiments by Dr. James R,. O’Neil in this laboratory indicated that this assumption might not be valid for dolomite, and led to this investigation covering several carbonates of divalent cations. Included also in this study were the anhydrous carbonates of cobalt, nickel, and magnesium which show negligible reaction with phosphoric acid, and for which no procedure for isotopic analysis has ber>nreport,ed. Three variations of the halogen fluoride oxidation procedure ha\-e been used in the present work: 1. The carbonate sample was thermally decomposed to give CO,. The resulting oxide was treated with the bromine pentafluoride reagent to give oxygen. 2. The carbonate sample was reacted with bromine pentafluoride at, 12.W to give CO, and O,, according to an equation such as: BrF, $- CaCO, --f CaF, + BrF, + CO, + 40, 3. The carbonate sample was reacted with bromine pentafluoride at 700°C to give oxygen, according to an equation such as: 3BrF, + CaCO, --+ CaF, + 3BrF, + CF, + %O, ANALYTICAL YROCEUURES 1. Thermal decomposition methods. Samples of about 20 mg were decomposed in one of two ways: (a) by induction heating in a platinum crucible inside an evacuated water-cooled glass vessel, with CO, condensed in an adjacent, cqld-trep; (b) by heating in a closed nickel tube of volume 60 cm3 in an electrical resistance furnace. The first method is suitable in cases where the oxide product can be handled in air without hydration or exchange, such as COO, NiO, CdO, MnO. The second method is necessary for carbonates of the alkaline earths. The oxides were then reacted in nickel tubes with bromine pentafluoride in the procedure described by CLAYTONand MAYEDA (1963) and SHUU and CLAYTON (1964). 2. In the low temperature reaction of BrF,, the reaction was carried out in nickel tubes at 125°C; CO s and 0 Bwere extracted through a cold trap at the melt&g point of ethanol ( - 112“C), which removed the excess reagent and other bior&ne and fluorine coinpounds. CO, was separated from oxygen using a liquid nitrogen trap; 0, was converted to CO, which was then mixed wifh the separated GO,. 3. The high temperature bromine pentafluoride reaction was carried out. at 700°C. Oxygen was extracted through a liquid nitrogen trap. Miist of the CF, formed in the reaction was trapped at this temperature. 0, was converted to GO, and the remaining CF, present was removed by first freezing the GO,-CF, mixture

hleasurementof 01*/016 ratios of tote1 oxygen of carbon&es

I S‘S9

in a trap at liquid nitrogen temperat,ure, then warming the trap to - 160% with a bat,h of liquid-solid isopentane, for one minute. The CF, was pumped away. 4. The phosphoric acid reaction was carried out in the conventional manner The reactions of carbonates of calcium, strontium, barium (MCCREA, 1950). and lead were complete in one day; carbonates of zinc, cadmium, and manganese, as well as dolomite. required two or three weeks. The isotopic compositions of oxygen and carbon in the carbon dioxide produced in these slow reactions have been found not to vary beyond experimental error during the course of the reaction, so that complete reaction is not necessary for the determination of the isotopic compositions. Mass specbrometer analyses were made on a six-inch double collecting isotope ratio spect,rometer designed after NIER (1947) and MCKINNEY et al. (1950). The isotopic data are reported in t,he P,terminology defined as:

The data are reported relative to the standard oxygen SMOW (CRAIG, 1961). The measured values are corrected for spectrometer valve mixing, and for Cl3 variations according to the equations given by CRAW (1957). The carbonate samples used in this work were as follows: synthebic*

~~CO3 CaMg(CO,),

natural dolomite

CaCO,(calcite)

natural calcite

CaCO, (aragonite)

laboratory

reagent

SrCO,

laboratory

reagent

BaCO 3

F. Wickman laboratory

MnCO,

synthetic *

standard reagent

coca,

synthetic*

NiCO,

synthetic*

CdCO,

laboratory

reagent

PbCO,

laboratory

reagent

* Prepared under CO, pressure in cold-seal bombs by Mr. David A. Northrop. RESULTS AFD DISWSSION The yields of oxygen and carbon dioxide from the various reactions are shown in Table 1. Oxygen isotope analyses are reported in Table 2. 1. Thermal decomposition method: Quantibative yields of carbon dioxide and oxygen were obtained. There is a fairly large kinetic isotope effect in the decomposition, resulting in a concentration of 01* in the carbon dioxide phase. The fractionation factor decreases with increasing temperature as illustrated in the case of dolomite in Table 3. The observation made by MCCREA (1950) that the thermal

0.08

:- 0.06

14.76 f

1502

+ 0,

2.89 f 1.80 f

0.06

8.04

5.91 & 0.03

3.56

ZnCO,

CdCO,

PbCO,

0.08

0.06

0.06

3.99 & 0.07

-f 0.07

+ Yields reported in ~moles/mg. I Thermal decomposition of carbonate and BrFF, reaction with oxide. II Low temperature BrF, reaction with carbonate. III High temperature BrF, reaction with caxbonate. IV Phosphoric acid reaot.ion wit,h carbonate.

* 0.10

f

4.35

f

8.78 f

NiCO,

4.40

0.06

8.70

cocos

& 0.06

8.77 f

M&O,

7.20 _I 0.12 4.30 5 0.05

_k 0.07

co,

B&O, 0.06

5.33

10.76 -_c0.15

:& 0.00

0,

10.07 * 0.07

(fwagonite)

C&O,

588

5 0.02

1181

CO,

II

SrCOs

(wlcite)

_ ..__..

C.&O,

C~g(COs),

W%

Sample

I __-

&- 0.13

0s

0.10

If 0.05

f

f

12.37

0.07

0.10

7.97 j_- 0.02

12.00 * 0.10

f

12.46

7.15 :1; 0.10

10.00

15.02

15.94 + 0.15

17.78

-__-

III

IV

4.20

5.80 3.74

3.40 :! 0.08

7.98

1.87

2.90

3.99

8.42 4.21

8.41

O-08

5.39 i

7.42 I, 0.12

n.r.

n.r.

2.63 s.70

8-35 2 0.10

4.33

3.39 .566

5.00

10.00 5.01 & 0.04

0.04 0.08

RU

10.88

5.00

5-93

11.86 10.00

0,

CO,

Calculated from formula

6.78

6.34 f

9.86 f

9.61 $1 0.16

10.59 & 0.09

n.r.

CO, --.._------

Table 1. Oxygen and carbon dioxide yields from various reactions*

-

Measurement

of W/O16

2351

rabies of total oxygen of carbonat,es

Table. 2. Oxygen isotolw analyses of carbonates Sample

MgCO,

I

-.

IT

--

JII

_--___

C’aMg(C’O,),

11.54

11.51

+ 0.06 (7,

CaC!O, (calcite)

18.20

.:- 0.05 (4)

C!aCO, (aragonik)

15.82

+ 0.06 (2)

WY),

14.57

BaCO,

14.30 22.18

MnC’O,

.~

2.59 -k 0.05 (3)

2.71 * 0.06 (3)

-; 0.10 (10,

047 (lo)

17.i6

;-

.! 0.09 (4)

14.49

1. ow7 (5)

-I-.0.04 (4)

13.!13 $ 0.02 (5)

+ 0.19 (3)

C’OC’O,

7.90 1: 0.12 (3)

5.70

t 0~05 (3)

XC’O,

6.49 I_ 0.03 (3)

6.43 Ii.77 . .

t WO:!(3) ’ 04l.l (3)

16.11

t 0.04 (3)

ZnCO,

15.80

C’dc’O,

16.29 f

Yb(‘0,

* 0.07 (3) 0.04 (3)

9.27 -1 0.10 (3)

r

Thermal decomposition and Brl?, reaction with oxide. Low temperature HrF, reaction wit,h carbonate. 11L High temperature BrF, reaction wit,h carbonate. ( ) Number of analyses. .tMean deviation.

I1

Table 3. Jsot,opic fractionation ‘I’omprrat~uro “C

(

on dccomposit,ion of dolomite bOe

WO,

519 (3)

17.60

_L 0.02

532 (3)

17.24

1. -_ n.O!I

-0.66

.k 0.12

0.18 4: 0~03

bl%Wb0”EitP 11.5”

-I_ 0.03

11.56 f

0.06

1000 lna 6.00 5.62

570 (1)

16.56

1.1-C

11.42

.WIH

600 (1)

16.07

2.00

11.38

4.64

) number of measurcmcnt~s.

decomposition of calcite does not give CO, of reproducible isotopic composition However, when the may be due t,o temperature differences in the decompositions. W/Ola ratio for the total carbonate is determined as the weighted mean of the isotopic compositions of the CO, and the metal oxide, t,he result is found to be well reproduced. and independent of t,he decomposition temperature. 3. Low temperature BrF, method: Of the carbonates studied here, only t.hose of calcium. strontium and barium react with bromine pentafluoride at a low enough temperature (r~lO0”C) that the further react,ion of carbon dioxide by the reaction: CO, + 3BrF,

--t CF, + SBrF,

10,

occurs to a negligible extent. As was observed bp CLAYTON (1961) there is an isot80pic fractionation in this reaction. 018 being concentrated in CO, relative to 0, by about, 30 per mil. The oxygen and carbon dioxide yields are in good agreement with stoichiometric requirements, and the Ol~/Ws ratios of the total oxygen are reproducible. Oxygen yields were generally good, with 3. High temperature BrF, reaction: Reproducibility of t,hc isotopic somewhat low values for CdCO, and dolomite.

analyses was good. Comparison of these results with those obtaineci bg tht: Iit-st two methods indicates a small systematic deviation, with lower CP,KP ratios in thth oxygen from the high t.smperature reaot,ion.* The isotopic fraotionatiou facturx for the phosphoric acid reaction with difierrtut. carbonates are shown in Table 4.

nd_h Sample .---_C&O, (calcite) C&O8

(ctragonita)

4. hotopic fractionation factor (x WO, H,PO, - -_ %+4B

26.14

WO,

Gc

1000 Ina

18.20Q

1~01008

LO.02

15-82’

1~01016

10.11

srco, BaCO, Dolomite

25.02

14.57’”

1~01030

10.25

25.23

14.30”

1.01078

10.72

C&g(CO,), ZnCO,

22.55

1163b

1*01090

10&I

27.05

15.77b

1~01111

11,o.i

CdCO,

l&20*

1.01125

MnCO,

27.59 32.35

1.00995

PbCO,

19.32

22.W 9.27c

11.19 9.90

X*00996

9.91

5 Low temperature BrFF‘,react.ion. b Mean of thermal + BrF, and BrF, high temperature ma&ions. c Thermal + BrFF,reactions. CONCLUSIONS

There are significant differences in the isotopic fractionation factqra associa.t+d with the phosphoric acid liberation of CO, from various carbonates. This must be taken into account in oomparing the isotopio compositions of different carbonates when the phosphoric acid procedure is used. The difference is of particular importance in the case of calcite and dolomite, since many calcite-dolomite mineral p@s from rocks have been enalyzed. The result of assuming the acid- fractio&ion factors for both calcite and dolomite to be 1.Olod leads to d values for doIomi&e which are too large by O-8 per mil. Thus the dolomite-calcite isotope fiaotion: ations previously reported in the literature are too large by this amount. The thermal decomposition + BrF, procedure described can be used to give oxygen and carbon isotope analyses for carbonates which do not show appreciable reaction with phosphoric acid, such as lUgCOS, CoCO,, NiCO,. * A possible explanation for this effect is the production of small smounti of COF,, either in the reaction of carbonates or carbon dioxide with bromine pentafiuoride,or in the conversion of oxygen to carbon-dioxide over hot carbon (some CF, is present at this stage). This impurity would produce an ion COW in the m.sss qxatrometxx, which has mama47 and is c@ected wi&h the mass 44 ion beam on the large CoIleotorof the double-co&et&g spectrometer, making the apparent 01*/C@’ ratio too small. The effect is worst in those cases in whioh the carbonate reacts at low temp+,ures to j#ve CO,, which then subsequently r&&a with BrP, to give 0, and CF,. An experiment was o&ed out in which carbon dioxide samples of known 018/016 ratio were treated with BrF, at 799” in the niokel rea&ion tubes. The resulting oxygen was then pmoesmd in the-me-way as the product of the oarbonatareaoticms; The overall recovery of CO, on four runs ranged from 98 to 99 per oent. The product showed an apparent decrease in @*/Ol” ratio of O-5 per mil.

Measurrment of 018/01eratios of total oxygen of carbonat,es

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