Stable carbon isotopes in selected granitic, mafic, and ultramafic igneous rocks

inNorthern Ireland Ckochimica et Cosmochimica Act.%, IQ’&Vol.87.pp.2609to2521. Pergamon Press.Printed

Stable carbon isotopes in selected granitic, mafic,

andUMUIU~~ igneon~ d~3*

Shell Development

A. N. FUEX Company, Belleire Research Center, P.O. BOX 481, Houston, Texas 77001, U.S.A. and

DONALD R. BAKER Department of Geology, Rice University, Houston, Texas 77001, U.S.A. (Received 13

Jwe

19’73; accs@ed ir, revised form 3 July 1973)

A~~ci-~on~rbon~~ (comb~ion) and c~rbon&~ (acid decomposi~o~) carbon w~1(3 separately enslyzed in 18 grmitic rocks from &group of related Tertiary intrusionsnear Crested Butte, Colorado, and 14 mafic and ultramafic rocks from various localities in the western United Statttae. Among the granites, carbonate carbon ranges from nil to 0376 per cent with &Y3-values from -5.6 to -Q*Oy&, (vs PDB); noncarbormte carbon varties from 32-360 ppm with 6C’3values from - 197 to -266%,. The m&c and ultram&c rooks have carbonate carbon contents ranging from 53 ppm to about 2 per cent with GC13-valuesfrom -i_2.9 to - 10*3y&,;noncarbonate carbon vszies from 26 to 150 ppm with ~13-v&lues of -22.2 to -27-l& For these samples, carbonate carbon ranges from 12.0 to 29.4x0 heavier than coexisting noncarbonate carbon. This consistent difference between 6Cis of carbon&e and noncarbonate carbon may be an isotopic fractionation effect. Because the specific indigenous form of noncarbonate (combustion) carbon is in doubt, conclusive interpretations regarding isotopic equilibration and fractionation cannot be made. These results have be&g on the assessment of the isotopic composition of mrantle carbon and consequently are germane to the question of the origin (source) and history of crust&l carbon. If mantle carbon is isotopically similar to noncarbonate (combustion) carbon, i.e. then a simple mantle degassing source for orustal carbon SC13-vrtlues from -19.7 to -27*1& is improbable. Such a result would indicate en additional source of crustal carbon such as from a primitive atmosphere or extra-terrestrial accretion.

VERY LITTLE previous work has been done on the extraction and isotopio characterization of carbon in igneous rocks. Since the early work by CRAIG (1953), in. which he analyzed the noncarbonate carbon in eleven igneous rocks, no carbon isotope analyses were available until those published recently by several Russian authors (LEBEDEV and PETERSIL’YE, 1964; VINO~RADOV et a&, 1965; VINO~RADOV and KEOPOTOVA, 1968). This is somewhat s~prising when one considers the general significance and implications that data on igneous carbon may have to a number of important geological problems. For example, data on carbon in igneous rocks have a bearing on the hypothesis of mantle degassing (RUBEY, 1951) vs other primary sources, such as a primitive atmosphere and weathering of igneous rocks, of crustal (lithosphere, hydrosphere, atmosphere and biosphere) carbon. In addition, the dist~butio~ and state of carbon within the mantle may have considerable influence on mantle properties * Based on a thesis submitted by the senior author in partial fulfilment of the requirements for the Ph.D. degree, Department of Geology, Rice University, Rouston, Texas. 2509

2510

A, N. FIJEXand DONUDR, BAKES

and processes, Specifically, carbon, e.g. CO,, along with other volatiles may influence partial melting processes (WYLLIE and TUTTLE, 1959). If sufficiently concentrated, it may be an important flux in particular regions, a8, for example, in the low-velocity zone in the upper mantle. In addition, it aeems probable that volatile forms of carbon have played a significant role in the origin of some special rock types such as kimberlitea and related ultramafics, alkalic ultrabasics, and carbonatites. Finally, as recently pointed out by BRETT (1971) and discussed by RINUWOOD(1971), the state of mantle carbon, specifically its chemical oxidation state, has considerable bearing on the question of core-mantle equilibrium and acceptable models for an accretion origin of the Earth. I~ormation and data on carbon in igneous rocks and related materials provides a direct ob~~~~~~l approach which may heIp set some boundary conditions or limits on these and other important problems, and may in some instances provide specific answers to significant questions. It was for this reason that we initiated the research reported here. METHODS Carbon extraction Great care was taken with 8ll samples to avoid contamination. Field samples were wrapped in ahuninum foil; weathered rook surfaoes were removed; crushed samples were treated with 15 per Gent HaOs for 18 hr (to remove easily oxidizsble surficial Org8& m&tter), and grinding ~8s done on 8 ceramic buckboard and in a muhite mortar. All extractions were made on very finely powered samples. Two methods of c8rbon extraction were used: (If vacuum acid decomposition for removal of aoidsoluble forms of carbon, e.g. Galcite and other carbonates; and (2) vacuum combustion of acid-treated sample portions for the removal of reduced forms of carbon and other undesignated forms which might be released during the high-temperature oxidation procedure. Carbon obtained by these two procedures is referred to 8s carbonate and noncarbonate carbon, respeotively. Carbonate carbon was extracted in the form of CO, by reacting the powdered sample in either HCl (2.4 N) or HsPO, (100 per cent). Reaction tubes were designed so that the sample and a&d could be held in separate compartments until 8 suitable vacuum was schieved in the tube. The resulting gas was purified by passing it through 8 dry-ice cooled U-tube and collected in a liquid nitrogen cooled trap. Similar isotopic results are obtained with either HCI or H&Q, extractions, although the phosphoric 8cid extraction generally resuhed in higher oon~ntratio~. This is explained by a partial retention of CO, in the frozen hydrochloric 8oid (the HCI-sample mixture had to be frozen with a dry-ice slurry during reoovery of the CO, from the reaction vessel, whereas H,PO, has a suffloiently low vapor pressure to make freezing unnecessary). At least two, 8nd sometimes three Or more, thaw-freeze-eXtr8Gt cycles were necessary to 8ssure 8 95 per cent recovery when using HCI. In only two of eleven comparisons were the differences in SCis-values significant (i.e. greater than 0.2%). In both oases, HCl extraction yielded 8 heavier carbon by about 05%. A preferential retention of the lighter Cl* in the frozen HCI seems to be indicated, but the effect is negligible when the concentration of carbonate carbon exceeds several hundred parts per million. The two acids tested for carbonate decomposition each have special advantages and disadvantages. Hydrochloric acid reacts more rapidly than phosphoric and, as is well known, will dissolve less soluble c8rbonates such as dolomite, siderite, and magnesite more readily than phosphoric acid. Phosphoric acid is easier to handle in 8 vacuum (no freezing necessary), and the CO, formed can be transferred away fmm it in just 8 few minutes. However, the acid presents 8 problem when large samples (5-8 g) are reacted, beesuse it is diffiouh to wet a huge amount of

Stable

carbon isotopes

in selected

granitio,

mafia and ultramafio

igneous rocks

2511

Another advantage of using HsPO, is that meaningful 601*sample with the viscous HsPO,. values may be determined on the same gas sample, whereas when HCl is used, exchange between the oxygen in the aqueous hydrochloric acid and the CO, product gives rise to meaningless 601*-values (see MCCREA, 1950). Noncarbonate carbon was extracted in a combustion apparatus very similar to that used by CEUIU (1953). Finely powdered samples were combueted in a fused silica tube, half-filled with CuO and silver wire, at a nominal temperature of 900%. Samples were generally combusted for 2) hr in an atmosphere of pure oxygen (6-8 cm Hg pressure) circulated with an automatic Toepler pump. Separate traps were provided for the removal of water and CO, during the combustion run. Combustion boats of fused silica (4 in. x 6 in. i.d.) allowed as much as lo-13 g of rock powder to be combusted in a single run. For samples with very low noncarbonate carbon concentrations (i.e. less than about 3040 ppm C) the CO, from two or three runs was combined to obtain enough gas for a 6Ci3 determination. The blank for the acid decomposition experiment was much less than 1 ymol carbon. The procedural-blank (i.e. duplicating the normal complete combustion procedure without sample) blank for the combustion apparatus was approximately 2 prnol carbon. The acid decomposition is negligible in all cases; however, the combustion blank can account for as much as 10 per cent of the CO, obtained from samples with less than about 50 ppm carbon. Carbon concentration values were determined by measuring the amount of CO, formed in All of the CO, amounts were determined on the extraction process with a mercury manometer. a manometer having a volume of only about 7 cm3, so that amounts of CO, as small as 10 pmol could be easily measured. The overall precision of carbon concentration values is estimated from repeated analyses to be 5 10 per cent of the measured concentration.

Isotope ratio determination The isotope values reported here were determined on a Nier-McKinney type, six-inch radius, The gas sample consists of purified CO*, which is ad60” deflection, ratio mass spectrometer. mitted into the source alternately with a working standard CO, gas for precise ratio determinations. The working standard CO, has a &Y3-value of +4*7yW compared to the common All values of 6Ci3 reported here are relative to the PDB standard. Following the PDB standard. procedures outlined by CRAW (1957), a correction to the GC13-values for the Oi’ contribution to the mass 45 beam was applied. The isotopic reproducibility of the mass spectrometer is about -&O-l%,, for carbon and &0.2& for oxygen. Overall reproducibility of SC13-values obtained with the acid decomposition method is better than -+0.2’& for samples with greater than about 200 ppm C, but slightly less precise for samples with smaller carbonate carbon concentrations. The precision of &isvalues extracted by the combustion method is better than *0.5x,.

Limitations

and problema

Because some of the samples analyzed contained ss much as two orders of magnitude more carbonate carbon than noncarbonate carbon, a test was performed to assess the possibility and effect of trace amounts of carbonate remaining in the HCl-treated samples used in combustion experiments. An additional portion of a sample (initially found to contain 1900 ppm carbonate carbon and 64 ppm noncarbonate carbon) wss prepared for combustion by the usual treatment The portion was then ground for an (i.e. grinding to a fine powder, followed by HCI treatment). additional hour in an automatic mortar, and then extracted with HP,O, in the usual manner. This sample yielded an additional 3.7 ppm carbonate carbon. Assuming this carbonate carbon had a &Y3-value of -7*O%, (an average for igneous carbonate carbon) a corrected value for the 6C13 of the noncarbonate carbon would be -23.3x,, as compared to -22.4x,, the value actually determined using the routine experimental procedures. For most samples, the error in GCi3-values of noncarbonate carbon due to incomplete removal of carbonate carbon is estimated to be less than 1x0. For samples with less than 50 ppm noncarbonate carbon, the error may be as high as 1-2y&,. In all cases, the true value, i.e. corrected value, will be isotopically lighter than the measured value because carbonate carbon is consistently heavier than noncarbonate carbon.

2612

A. N. Fmrx snd DONALD R. BAKER

Some preliminary experiments indiccbted th8t recovery of aombnstion carbon was in some c18sespoor. Recomb&km of s8mpies yielded insignificant amounts of addition81 c8rbon. However, some samples which were thoroughly reground before recombustion yielded 8ddition81 carbon. The results indicate that, for some samples, recovery of combustion cerbon may be low by 8 factor of two or more. Because the isotopic values of carbon recovered by additional grinding and recombustion 8re similar to values obtained by routine treatment, poor recoveries of combustion cctrbon probably c8use only 8 small error in the BC1$-valueof the combustion carbon determinations. Thus, recovery problems sre not believed to osuse 8ny serious limitation to the interpret8tion of results presented in this paper. Supplemental recovery after regrinding msy shed light on the state of combustion carbon in igneous rocks. For ex8mple, it indicated th8t the oarbon may be occluded 8s minute ges or solid p8rticle inclusions within the minerals, or possibly substituting for Sip+ in ail&&es, and hence protected from expulsion and oxidation during the combustion procedure.

SAMPLES The samples analyzed in this study included 18 gr8nitic rocks from 8 group of related Tertiary intrusions ne8r Crested Butte, Colorado, and 14 mafic and ultrem8fio rocks from various localities in the western United States. The granitic rocks 8re from the West Elk laccolithic cluster in west-central Colorado ( GODWIN and C~ASKILL, 1964; GASKILL et al., 1967). The laccoliths are aligned on both sides of 8 central dike sw8rm along 8 north-northe&-trending fracture zone at least 25 miles long. Small stocks, sill, and dikes 8re associ8ted with the 18ooolithic cluster at the northern end of the fracture zone. The gr8nitic rocks intrude early Tertiary (Ohio Creek and Was&oh Formations), Upper Cretacteous (Mesa Verde and Mancos Form8tions) 8s well 8s older Mesozoic and Psleozoio sediment8ry rooks. According to GODWIN snd G~~KELL (1964), %he time of emplacement of the West Elk intrusions is clearly post-Paleocene and most likely post-early Eocene”. More recent studies utilizing K-Ar dating (~s~oN~, 1969; OBRA~O~IOHef caE.,1969) have defmed sever81 different episodes of igneous activity in the 8re8, extending through late Miocene or early Pliocene. OBRADOVICH et aE. (1969) dated 8 sample from the Paradise stock, which is located only &bout two miles north of our sample lactations,at 29 my. Most of the samples from the Crested Butte are8 ceme from the northern third of the Oh-BeJoyful quadrangle (GASKILL et al., 1967). The following individual intrusions were sampled: (1) the August8 stock; (2) the Elkton sill; (3) the Gothic Mountain laccolith; (4) sever81 small dikes, sills and laccoliths; and (5) 8 very large dike (part of the central dike swarm) which crosses U.S. Highway 135 west of the town of Crested Butte. Petrographically, most of the samples are quartz monzonite and quartz monzonite porphyry. In addition to quartz, pl8gioclase and aikali feldspar (orthoclsse), biotite and hornblende 8re common minor constituents. Some of the rocks 8re un8ltered; however, in many s8mples the feldspar is partially serioitized and in 8 few cs8es may be repl8ced by c8lcite. In some s8mples oalcite appears prim8ry occurring 8s smell interstitial gr8ins. These features 8re noted for the individual ssmples in T8ble 1. The mefict and ultr~~u samples were obtdned from the following locations: (1) an exposure of 8 o8mptonite dike on U.S. Highw8y 93 about 8 miIes south of Boulder Dam in Arizona; (2) Vuloan’s Throne volcano in Grand Canyon National Monument, Arizona; (3) Knippa Querry, near Uvalde, Texes; (4) Peridot Cove, near Globe, Arizona; (5) Blaok Point Mesa, ne8r Flagstaff, Arizona; (6) Sunset Crater, neaP Flagstaff, Arizona; and (7) the Stillwater Complex, Montana. The c8mptonite dike was s8mpled at 8 rosd cut exposure, The dike is 4 feet thick and has chilled msrgins about 11 in. wide. It intrudes poorly consolidated alluvial fan gr8vels, which are of post-ectrly-Pliocene age (D-ON et al., 1967). However, the dike itself w8s dsted at 3.7 my by means of K-Ar methods (DAMON et al., 1967). They found 8 large excess of Ar40 in the chilled m8rgin a;nd smaller amounts in large kaorsutite and plagioclase phenocrysts from the central part of the dike. Our samples were taken along 8 profile from the margin of the dike toward its center, 8nd from 8n associated b8s8lt flow about 200 yards north of the s8mpled outcrop. In 8ddition to k8ersutite 8nd pl&~ocl~e, olivine also occurs as phenocrysts. In 811of the c-ptonite s8mples, the olivine is dtered to serpentine. Calcite occurs in all of the camptonite samples

Stable carbon isotopes in selected granitic, m&e snd ~tr&rn~fic igneous rocka

2513

Table 1. Grsnitic rocks from Crested Butte, Colorado

Sample CB-101 Quartz monzonite

dike CB-102 Da&s porphyry dike CB-IO3A Quartz mouzonita porphyry sill CB-103B Quartz monzonittt porphyry sill CB-105 Quartz- monzonite porphyry, Elkton sill CB-106A Quartz monzonits porphyry, Gothic Mt. lsocolith CB-106~ Quartz monzonite porphyry, Gothic Mt. Iaoeolith CB-108 Qusrtz monzonito porphyry dike CB-1IOA Biotits granodiorits, Augusta stock CB-110~ Biotite grmodiorite, Augusta stock CB-1i1B porphyry. Augusta stock CB-112 Quartz monzonite, Augusta stock CB-113 Quartz monzonite, Augusta stack CB-114 Quartz monzonite, Augusta stook CB-116 Biotite granodiorito, Augusta stock CB-116~ Granodiorita porphyry sill CB-IIBC Granodiorite Porphyry sill CB-117B Quartz monzonita porphyry dike

Notes

Carbonate carbon Cono. (PPm)

Noncarbonate carbon Cone. &‘* (PPm) (%v)

60

-9.0

+‘7*6

42

-24.1

Highly sericitized: interstitial calcite

7600

-7.7

-1.8

67

-23.1

HighIy ssricitizsd; interstitial cfdoite

7280

-7.1

-1.5

79

-19.7

Highly sarieitizsd: interstitial calcite

7160

-7.6

-2.2

85

-19.7

Moderate s&cite and calcite alteration; interstitial calcite; vacuoles Slightly sericitized; calcite alteration

4900

-8.6

15.8

360

- 24.5

4200

-5.7

+11-a

111

-25.0

Slightly sericitiaed; calcite alteration and interstitial grains

3300

-6.6

+9*5

91

-24.2

48

-8.6

+12-o

36

- 22+7

-

-

64

-24.0

-

-

66

-23.8

Highly scrioitised; vacoles

No alteration

No alteration

No alteration

Moderate sericite and calcite altaration; *ems vacuoles Very slightly sericitized, interstitial cakzite grains; so** vaouolcs Slightly sericitized:

<1 11

1900

-7.7

-+-4-o

64

--22.4

680

-8.1

+5.3

34

-21.6

88

-8.9

+11‘s

85

-23.9

330

-9.0

+10-o

32

-22.8

20

-1.1

$14.3

49

-23.8

Highly sericitized

22

-

-

76

-24.8

Highly serioitized

300

- 8.9

+10.0

192

-26.6

-7.7

j-5.4

43

-24.0

VaCUOleS

Slightly serioitieed; vacuoles Tars calcite grain No alteration

Stight calcite a&oretion

i200

2514

A. N. Fo~x and DONALD R. BA.KER

and the related basalt as disseminatedgrains, veinlets, and along vesicle walls. The dike was of special interest because of its content of excess argon, suggestingthat not all of its primary gases (e.g. C0.J were released during emplacement. A sample of fresh, olivine basalt taken from the base of Vulcan’s Throne volcano and a sample of a large peridotite nodule (a hamburg&e with about 1 per cent deep red spinel) from the basalt were analyzed. None of the minerals in either the basalt or nodule displayed alteration. No carbonate minerals were observed. DAMONet al. (1967) obtained an estimated K-Ar age of 10,000 years for the basalt (whole rock) and an apparent age of 114 my for the peridotite nodules. They concluded that the nodules contain excess argon; hence, the nodules might also be expected to have retained primary gases. The Knippa Quarry site is located just west of Knippa, Texas, a few hundred yards south of U.S. Highway 90. The Knippa Quarry site consists of nephelineolivine basalt with up to B-in. olivine inclusions or nodules. Samples of the basalt and nodules were analyzed separately to provide a comparison of the carbon in the basalt with that in its enclosed olivine. The olivine nodules are very pure and unaltered. Similarly, the mineralsof the basalt are fresh and unaltered. CARTER (1965) has concluded that the nodules have formed from fractional crystallization of a mafic (picritic?) magma. The Peridot Cove sampling site is about 24 miles southwest of San Carlos in the extreme west-central part of the San Carlos quadrangle, Arizona. A peridotite nodule (harzburgitewith about 3 per cent deep red spinel) was taken from a scoriaceousbasalt layer underlying the cinder cone (Peridot Hill). The minerals of the nodule showed no sign of alteration. The peridotite nodules at this locality were of special interest because CARTER(1966) has suggested that they may have been derived from the upper mantle. The Black Point Mesa sample site is located just west of the Little Colorado River and about 6 miles east of U.S. Highway 89 near Flagstaff, Arizona. Samples of the Black Point basalt (an olivine basalt with abundant plagioolasephenocrysts), which forms the mesa, were analyzed. The plagioclase is all very fresh, but the olivine is usually slightly altered around the grain borders. Calcite grains (0.05-0.1 mm) are distributed in the groundmassand occur on the walls of some cavities (vesicles). According to COOLEY(1962), the Black Point basalt is late Pliocene (9). Using the K-Ar method, DAMONet al. (1967) obtained anomalous ages of 2.4 and 9.3 my on the whole rock and plagioclase phenocrysts, respectively. As with the camptonite dike samples and Vulcan Throne nodules, the Black Point Mesa basalt probably contains excess argon and might also contain primary CO, in its plagioclase phenocrysts. At the Sunset Crater site, samples of the Bonita lava flow were taken. The lava flow is located just east of Sunset Crater, an extinct volcano, in Sunset Crater National Monument, Arizona. The scoriaceousbasalt of this lava flow was chosen for analysis primarily because it is a recent and very fresh appearing flow. The rock is aphanitic with fresh plagioclase laths and subhedral olivine. Only two samples from the Stillwater Complex were studied. These included a pyroxenite (bronzitite) and a chromitite, both from the ultrama& zone (IFEss, 1960). The pyroxenite is coarse-grainedand composed principally of orthopyroxenewith clinopyroxene,calcic plagioclase, and about 5 per cent olivine. The minerals are all fresh and unaltered. The chromitite sample includes about 25 per cent interstitial ortho- and clinopyroxenes. The data on the mafic and ultrama& rock samples are summarized in Table 2. The granitic suite of rocks may representa sampling of igneous carbon derived from crustal materials, whereas the mafic and ultramafic group of samples may in some manner be representative of the subcrustal, i.e. mantle carbon. Although this is a highly simplifiedview of these samples, the objective of examining igneous meterials which might be expected to contain carbon from crustal and mantle sources is worthy. RESULTS Branitic rocb

The data on the granitic rocks are summarized varies from nil to O-76 per cent and noncarbonate

in Table 1. Carbonate carbon from 32 to 360 ppm. Median

Stablecarbon isotopesin selectedgranitic,mai%and ultramaflcigneousrocks

2515

Table 2. Data on m&o and ultramafio rocks

Sample

CamptoniteDike

C-l Interior of dike

c-2

Notes Olivine altered to @orpentine end limonite: caloitio Olivine sitared to BeFpentine; c&&c

chilled margin of dike Olivine altered to 8~. c-3 pentine; celoitio Outer chilled margin of dike c-5 No alteration Amphibole phenooryata interior of dike Olivine altered to scrC-7 &salt aseocisted with pentine; calcitio dike Vulcan’s l’hrhroncr No alteration v-3 Peridotite nodule V-5A No alter&ion Olivine base& Iinippa Quawg K-l No sltexation Olivine nodule K-4 No alteration Nepheline olivine besalt Perdot Cove No alteration PC-3 Peridot&e nodule Bzuck Point Mew Olivine alightly altered; BPM-6 c&&e Olivine baas& Sunset Crater SC-1 No 8brStiOn Olivine basalt S~illwoter Complex STL-2 Very slight. alteration Pyroxenite STL-4 No rdtertstion Chromitita Intwior

2%

+1*tJ

-

130

-26.4

I-2%

+1-4

+20-a

460

-18.7

1.2%

+2*9

+23*8

88

-26.5

-

49

-27.0

-

87

-24.9

-

40

-24.9

j-23-9

67

- 24.2

-

64

- 23.8

-

-

68

-23.2

-8.9

-

26

-27.I

+0-a

-

160

-22.2

89

-7.9

-

68

-24.4

180

-6.7

-

81

-24.9

-

-

67

-26.6

+PProw

9*23x

+2*7

117

63 620

-

-10.3

* Camptonite dike samplea carbonate carbon concentration in weight yc.

values for the I4 samples are 450 ppm and 65 ppm for carbonate and noncarbonate, respectively. There is a wide gap in carbon isotope vahes for the two types of carbon. Carbonate carbon is richer in Cx5 with &Y-values ranging from about -5.6 to -9.0 per cent vs PDB. Nonoarbonate carbon is enriched in Cl” with 8C!13-valuesranging from - 19.7 to -26*6yW. As will be shown, this difference of from 10 to 20x, between carbonate and noncarbonate carbon is consistently displayed by all of the samples anaIyzed in this work. The type of carbonate in these is probably calcite, because it readily decomposes in H,PO, and decomposition with HCI never gave significantly more CO,. There seems to be a rough correlation between the amount of carbonate in a sample and the degree of alteration of the feldspar. The samples with most of their feldspar extensively altered have the greatest concentration of carbonate and those

2516

A. N. Fuzx and

DONALD

R. BAKER

with entirely unaltered feldspar have the smallest concentrations of carbonate. This may be the result of the carbonate being deposited from the same solutions that alter the feldspar. The noncarbonate carbon concentrations are unrelated to the degree of feldspar alteration. Mafic and ultramaJic rocks

The analytical results on this group of samples are summarized in Table 2. For the camptonite dike and related basalt, carbonate carbon varies from 0.23 to about 2 per cent, whereas noncarbonate carbon ranges from 49 to 130 ppm (the high value of 460 ppm displayed by C-2 is probably in error due to incomplete removal of carbonate carbon before combustion). Again there is a wide gap in GCWvalues with carbonate carbon as much as 29x,,heavier than coexisting noncarbonate carbon. An interesting observation with regard to the &Y-values for the carbonate carbon in the camptonite dike is that the chilled margin (C-3) and the associated basalt flow (C-7) contain carbon with essentially the same isotopic composition (about +2*8x,), whereas samples from the interior of the dike (C-l and C-2) contain carbon with a 6C13 of about +1*5%,,. The difference (l-3%,), although small, is certainly real. The chilled margin and the basalt probably contain carbon which is representative of the original carbon in the magma; whereas, the interior of the dike has a carbon composition characteristic of the CO, in a late-stage enrichment in volatiles. Thus the carbon data indicate that there might be a process of fractionation during the crystallization of the magma, which causes an enrichment in the lighter isotope in the residual magma or volatile phase. The carbonate carbon in the basalts and peridotite nodules from Vulcan’s Throne, Knippa Quarry, and Peridot Cove ranges from nil to 117 ppm and noncarbonate carbon from 26 to 68 ppm. Compared to associated carbonate carbon, noncarbonate carbon is again enriched in Cl2 with BCla-valuesranging from -23.2 to -27*1%,. The Vulcan’s Throne and Knippa Quarry samples provide a oomparison between the combustion carbon in nodules and their enclosing basalt. In both cases, the carbon concentration is slightly higher in the basal@ but the differences are probably not significant. Similarly, within analytical error, the &Y-values of each pair are virtually identical. It seems reasonable to conclude that the carbon of the peridotite nodules and enclosing basalt either had a common source or that the carbon was homogenized throughout the basalt-nodule system. In the Black Point Mesa, Sunset Crater, and Stillwater samples, noncarbonate carbon is again isotopically much lighter than the associated carbonate carbon. Concentration values are low. The variation in 6CYafor noncarbonate carbon between samples is relatively small. As would be expected, the &Y-values of the chromitite and pyroxenite from the Stillwater ultramafic zone are practically identical. DISCUSSION The 6CWresults on all of the samples reported in this study are summarized in Fig. 1. The median values for both noncarbonate and carbonate carbon for both the granitic and ma& sample suites are shown.

Stable carbon isotopes in selected granitic, mafic and ultramafic igneous

SC” %. -30

-25

-20

NONCARB.

<

IGNEOUS

8

VOLCANIC

ULTRAMAFIC ,

FROM

OTHER

I

1

SOURCES

CARBONATITES , (4.5.6.71

2,‘)

CH4

DIAMONDS I I (1.2.8) VOLCANIC CO2

____--

1

(1.9)

CRUSTAL

%o

I C

ROCKS

,1.9.10) -25

+5

ROCKS

ROCKS (I

0 I

I

C

DATA

,

-5 CARB.

MAFIC

2517

PDB

I

GRANITIC 1

r

vs -10

I

I

I

I

-15

rocks

0 %o

CARBON -7%*

da

SED. ORGANIC (“27%)

I

MANTLE (?I

C

SED.

C CqRF3N;;ES

C

Pig. 1. Summary of K?3-values for granitic, mefio and ultramafic samples analyzed in this study with analogous data from other SOUW~S (see references listed below) for igneous rocks, carbonatites, diamonds and volcanic gases. Median values shown by tick-mark below the bar. Also shown is an evaluation of the isotopic composition of mantle carbon ( -7o/, vs PDB) based on estimates of the isotopic composition and relative amounts of the main crustal carbon reservoirs, and on the assumption of mantle degassing as the exclusive source of crustal carbon. [Data sources: (1) CRAIQ, 1953; (2) VINOURADOVand KROPOTOVA,1968; (3) VINOGRADOVet cd., 1965; (4) BAERTSCHI,1957; (5) VINO~RADOVet al., 1967; (6) ECKERMANN et al., 1952; (7) TAYLORet al., 1967; (8) WICEMAN, 1956; (9) HULSTONand MCCABE, 1962; (10) DWROVA and NESMELOVA,1968.1 Perhaps

the most

striking

overall

aspect of the results is their consistency.

For both groups of samples noncarbonate -20

to

-27x,.

Carbonate

carbon

carbon ranges in GCPvalues

values

are

more

The granitic rocks display about +3 to -lo%,. carbon distribution with a median value of about -8x,. and variable of samples.

distribution

of &Y-value

As a consequence

variable, a

from

very tight carbonate There is clearly no wide

either within or between

of the consistency

from about

ranging

the two groups

in GCP-values, the isotopic dif-

ference between noncarbonate and carbonate carbon is also consistent. Carbonate carbon is isotopically always heavier than coexisting noncarbonate carbon. For the granitic samples, carbonate carbon ranges from 12.0 to 19.3%,, heavier than noncarbonate carbon. The suite of mafic samples is more variable in this regard, with carbonate carbon ranging from 13.9 to 29*%x, heavier than coexisting noncarbonate 11

carbon.

These internal consistencies suggest that the carbon in this

2518

A. N. FUEX

and DONALD

R. BAKEX

collection of igneous rocks may in part be related through a common source and/or have experienced a common isotopic history, e.g. fractionation and eq~libration under magmatic conditions. Our work offers no direct evidence that carbonate and noncarbonate carbon phases formed as a coexisting mixture under conditions of chemical equilibrium. Nevertheless, it is instructive from an interpretative viewpoint to consider the analytical data on the assumption of isotopic exchange equilibrium. Specifically, the consistent differences between the carbonate and noncarbonate carbon may be the result of isotopic fractionation. The work of BOTTINGA(1969) is very pertinent to this consideration. He has computed fractionation factors for a range of temperature and a variety of carbon species including diamond, graphite, calcite, CO,, and CH,. The application of his results to natural igneous systems and specifically the results presented here is severely hampered because the precise molecular form of the noncarbonate carbon is not known. Assuming that it is CO, which has equilibrated with calcite (carbonate carbon), BOTTIN~A’Sdata (1969, p, 55, Table 4) on the CO,-calcite equilibrium indicate a low equilibration temperature, i.e. near 25°C or less. Equilibration between CO, and calcite at magmatic temperatures, e.g. 700°C, would yield a CO, isotopically slightly heavier than coexisting calcite, which is contrary to our observations. On the other hand, Bottinga’s calculation of isotopic fractionation between calcite and CH, indicate methane will be lo-20x, lighter than coexisting calcite in a temperature range of about 300-600°C. Thus if the noncarbonate carbon in our igneous rock samples is in reduced form e.g. CH,, it may have developed an isotopically light character by equilibration with calcite at submagmatic temperatures, or perhaps during the cooling stages of these rocks. The problem is complex and no satisfactory conclusions can be reached until the carbon-bearing molecular species and solid phases which coexisted at the time and under the equilibration conditions are precisely identified. For comparison, Fig. 1 also graphically displays the range of KY-values for carbon from other similar or related materials. Although the range is greater, noncarbonate carbon from other igneous rocks is similar to the values for the samples discussed here. comparison with reduced carbon gases (methane) from volcanic sources which are also isotopically light suggests that noncarbonate carbon extracted by combustion from igneous rocks may also be in a reduced form. Oxidized carbon (CO,) from volcanic sources as shown in Fig. 1, is isotopically heavier than volcanic methanes. This suggests that carbonate carbon (calcite) in igneous rocks may hav6 precipitated from an isotopically heavy CO, component of the voliatile fraction. Perhaps the &Y3-results reported here are reflecting the establ~hment of isotopic equilibrium between oxidized carbon,e.g. CO,,andreduced forms of carbon,e.g.methane or graphite, during some stage of the magmatic history. As discussed above, this possibility is strongly supported by the calculated fractionation factors of BOTTIN~A (1969, p. 55) which consistently indicate that oxidized forms of carbon should be isotopically heavier than reduced forms. These results have bearing on an assessment of the isotopic composition of pristine ( 1) mantle carbon. It has often been assumed (WICKMAN,1956) that the isotopic composition of diamonds and carbonatites might coincide with that of mantle carbon. The results on igneous carbon indicate that the problem is more

Stable carbon isotopas in selected granitic, mafic and ultramafic igneous

rocks

2819

complex. If, for example, the isotopically light noncarbonate carbon observed in igneous rocks constitutes a rather large proportion of mantle carbon, then the isotopic composition of mantle carbon would be significantly lighter than that of carbonatites or diamonds. Clearly, the question of the isotopic composition of mantle carbon hinges around a consideration of the relative proportion and isotopic composition of the possible forms of carbon. To assume that diamond is the principal form of mantle carbon may be erroneous and is clearly at odds with the common observation of CO, fluid inclusions (Ro~DD~~, 1965) in olivine nodules which are often considered mantle derived. For the rocks examined here, depending mostly upon the relative amounts of carbonate and noncarbonate carbon, the isotopic composition of ‘total’ carbon varies from near -5~0 to -15*0%,. Because of the likelihood of volatile loss of carbon from igneous materials during their emplacement, it is difficult to recognize reliable &Y3-values for the initial total carbon content. Thus, total carbon &Y3-values of igneous materials, even when thought to be primary, may not be readily useful for evaluating the isotopic composition of pristine mantle carbon. These considerations are germane to the question of the origin and history of crustal carbon. According to RUBEY’Scalculations (1951), carbon contributed from weathering of primary igneous rocks yielded a small proportion of crustal carbon. Carbon inventory estimates show that sedimentary carbonate rocks contain about 73 per cent of the total crustal carbon and that the 27 per cent balance is principally in the form of organic matter, reduced carbon, in sedimentary rocks. The amount of carbon in all forms in the atmosphere, hydrosphere and biosphere is less than 0.2 per cent of the total crustal carbon inventory. The average isotopic composition of the two main carbon reservoirs is relatively well-known, carbonate rocks have a 6C13of near O%, and reduced carbon, approximately --25%,. Using the percentage figures cited above, the KP-value of crustal carbon calculates to be about -7x0 (Fig. 1). CRAIG(1953), who made a similar calculation, arrived at a value between -7*O%, and -15.6x,, using two extreme estimates for the proportion of limestone fo shale. If, as Rubey theorized, degassing of the mantle was the primary source of crustal carbon, then the GtY3-value of mantle carbon should be about -7%,. This corresponds with the value of diamonds and carbonatites, and hence supports the idea that they are characteristic of pristine carbon. If, as discussed above, isotopically light noncarbonate carbon is a significant proportion of mantle carbon, then the isotopic composition of average carbon degassed from the mantle during geologic time might be considerably lighter than -7x,. This consequence would pose several questions and problems. For example, it would indicate the possibility of significant error in the estimates of the relative size of the two main crustal carbon reservoirs or in our estimates of their average isotopic compositions. More importantly, a discrepancy between the KY3-value of mantle carbon calculated from crustal carbon abundance and that estimated by direct observation on carbon from ‘mantle-derived’ (igneous) sources might seriously challange Rubey’s degassing hypothesis. Specifically, such a result would indicate an additional significant source of crustal carbon such as from a primitive atmosphere or perhaps extra terrestrial accretion. These considerations are complicated by the possibility that some mantle carbon may be recycled via incorporation of crustal and supracrustal

2520

A, N. FUEX and DONALD

R. BAKER

carbon into the upper mantle along subduction zones. Thus, the mantle may contain both pristine and secondary (recycled) carbon, and the distinction between the two may be d,ifEcult and perhaps impossible to make. If recycled carbon constitutes a significant amount of mantle carbon, then geochemical computations, as made above, are indeed tenuous. Acknowledgement+-We sre grateful to Shell Development Company, Houston, Texas, for providing mass spectrometerequipment time, and to R. M[.LLOYDand H. DAY of Shell Development Company for helpful instrumental suggestionsand assistance. Mr. E. Hs.~XMNN aided in the con&u&ion of much of the equipment used in this investigation. We are grateful to JOHNA. SWEENEYfor assistancein the field. A~~ow~edgement is made to the donors of The Petroleum Research Fund, administeredby the American Chemical Society, for partial support of this research. In addition, funds furnished by The Robert A. Weloh Foundation (Grant No. C-249) provided partial support for this research and are gratefully acknowledged. REFERENCES R. L. (1969) K-Ar dating of the laccolithio centers in the Colorado Plateau and vicinity. Bull. &ol. Sot. Amm. 80, 2081-2086 BAERTSCHI P. ( 1957) Messung und Deutung relativer Haufigkeitsvariationenvon 0- 18 and C- 13 in Karbonatgesteinen und -mineralen. Schweiz. Mineral. Pekogr. Mitt. 57, 73-162. BOTT~N~AY. (1969) Calculated fractionation factors for carbon and hydrogen isotope exchange in the system calcite-carbon dioxid~~aphi~methane-hydrogen-water vapor. #~ch~m. ~o~~h~rn. Acta 8,49-64. Bnsmr R. (1971) The earth’s core: speed&ion on its chemical equi~brium with the mantle. ~ch~rn. ~o~~h~rn. A~ta 35,203-211. CASTERJ. L. (1965) The origin of olivine bombs and related inclusionsin basalts. Ph.D. Thesis, Rice University, Houston. COOLEYM. E. (1962) Geomorphology and the age of volcanic rocks in northeastern Arizona. Ariz. Gwl. Sot. Digest V, Tucson. CRAIGH. (1953) The geochemistry of the stable carbon isotopes. CTeochim. Cosmochim. Acta 3, 53-92. CRAIGH. (1957) Isotopio standards for carbon and oxygen correction factors for mass speotrometric analysis of carbon dioxide. &ochim. Cos?nochim.Acta %j, 133-149. DAMONP. E., LAUUELINA. W. and PERCIOUSJ. K. (1967) Problem of excess argon-40 in volcanic rocks. In Radioactive Bat&g and Methods of Low-Level Coqmting, pp. 463-481. (Proceedings Series) International Atomic Energy Agency, Vienna. DUBROVAN. V. and Nr%rnznovAZ. N. (1968) The isotopic composition of carbon in natural methane. i&o&em. list. 5,872-876. EC~N~MANNH. VON,UBISCH H. VON and WICKXANF. E. (1952) A preliminaryinvestigationinto the isotopic composition of carbon from some alkaline intrusions. Gwchim. Cbmochim, Acta 2, 207-210. GASKILLD. L., GODU~IN L. H. and MUTSCELER F. E. (1967) Geologic map of the Oh-be-Joyful quadrangle, Gunnieon county, Colorado. U.S. &ol. Sure. Map GQ-578. GODWIN L. H. and GASKILLD. L. (1964) Post-Paleooene West Elk laccolithic cluster, westcentral Colorado. U.S. Gwl. Sure. Prof. Paper 501-C, C66-C68. HESS H. H. (1960) The Stillwater Igneous Complex, Montana. cfwl. Sot. Amer. Mem. f$i),230 pp. HULSTONJ. R. and MoCtirr W. J. (1962) Mass speotrometermeasurementsin the thermal areas of New Zealand: Part II. Carbon isotope ratios. Beochim.coamochim. Acta 26, 399-410. LEBEDEVV. S. and PETERSLL’YE I. A. (1964) On the isotopic composition of the carbon of the hydrocarbon gas and bitumens of the igneous rocks of the Kola Peninsula. Ilokt. Akad. Naak SSSR 158,1102--1104. MC&IXA J. M. (1950) On the isotopic chemistry of carbonates and a paleotemperature scale. J. Chem. Phya, 18, 849-857. ARMSTRONG

Stable carbon isotopes in selected granitic, maI% and ultram&c

igneous rooks

2521

OBRADOVICH J. D., MUTSCHLER F. E. and BRYANTB. (1969) Potassium-argon ages bearing on the igneous and teotonic history of the Elk Mountains and vicinity, Colorado: a preliminary report. Bull. Qeol. Sot. Amer. 99, 1749-1766. RINUWOODA. E. (1971) Core-mantle equilibrium: comments on a paper by R. Brett. Geochim. Coamochim. Acta 35, 223-230. ROEDDERE. ( 1965) Liquid CO, inclusionsin olivine-bearingnodulesand phenocrystsfrom basalts. Amer. Mineral. 50, 1746-1782. RWEY W. W. (1951) Geologic history of sea water: an attempt to state the problem. Bull. Gwl. Sot. Amer. 92, 1111-l 148. TAYLOR H. P., FRECISENJ. and DEUENSE. T. (1967) Oxygen and carbon isotope studies of carbonatites from the Lascher See District, West Germany and the AlnB District, Sweden. Gwchim. Cosmochim. Acta 31, 407-430. VINO~RADOV A. P. and KROPOTOVA 0. I. (1968) The isotopic fractionationof carbon in geological processes. Int. Beol. Rev. 10,497-506. VINOQRADOV A. P., KROPOTOVA 0. I. and USTINOVV. I. (1965) Possible sources of the carbon of natural diamonds according to Cla/C13data. Gwchim. Cosmochim. Actu 2, 495-503. VINOURADOV A. P., KROP~TOVA0. I., EPSETEYNYE. M. and GRINENKOV. A. (1967) Isotopic composition of carbon in calcites representativeof different temperaturestages of carbonatite formation and the problem of genesis of carbons&es. Geo&em. Int. 4, 431-441. WICKMANF. P. (1956) The cycle of carbon and the stable carbon isotopes. Cfeochim.Cosmochim. A&a 9, 136-163. WYLLIE P. J. and TUTTLE0. F. (1959) Effect of carbon dioxide on the melting of granite and feldspars. Amer. J. Sci. 257, 648-655.