Carbon isotopes in submarine basalts

Earth and Planetary Science Letters, 70 (1984) 196-206 Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands

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Carbon isotopes in submarine basalts D.P. Mattey, R.H. Carr, I.P. Wright and C.T. Pillinger Planetary Sciences Unit, Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA (U.K.)

Received December 12, 1983 Revised version accepted June 24, 1984

High-sensitivity stepped extraction reveals two isotopically distinct forms of carbon in submarine basalt glasses: an isotopically light carbon component released by combustion from 200 to 600 o C and an isotopically heavy CO 2 liberated from vesicles (magmatic carbon) from 600 to 1200 o C. The 813CpDa of the low release temperature carbon varies from - 24 to - 30%0 and is believed to be surficial organic contamination. A survey of various types of oceanic glasses demonstrates that the 813C of magmatic CO 2 varies from - 4 . 2 to -7.5700 in mid-ocean ridge basalt (MORB), from - 2 . 8 to -6.7%0 in glasses from Hawaii and Explorer Seamount and from - 7 . 7 to -16.3%0 in glasses from the Scotia Sea and Mariana Trough. Magmatic CO 2 in back-arc basin basalts (BABB) is on average 5%0 lighter than equivalent CO 2 in M O R B and can be explained by the mixing in the source regions for BABB m a g m a s of juvenile (MORB-like) C O 2 with an organic carbon component from subducted pelagic sediments. It is inferred that significant a m o u n t s of pelagic carbonate carbon (813C = 0%0) must be recycled into the mantle.

1. Introduction

Studies of the stable isotopes of carbon have shown that a wide range in 813C * of over 40%0 exists in igneous rocks and mantle-derived materials. Although the isotopic composition of carbon in carbonatites and kimberlites appears to be confined to a narrow interval from - 2 to -8%o [1,2], diamonds possess a significantly greater spread in 813C which extends from - 3 5 to +2%o [3-6]. However, the vast majority of diamonds also have 813C values in the - 2 to -10%o range with a mode at -5%o [3-6] and consequently, a value of this order has become accepted as typical of the mantle. Early measurements of the isotopic composition of carbon in basic igneous rocks [3,7] showed that this carbon was isotopically lighter than typical " m a n t l e " values, and from further

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studies [8-10] developed the idea that significant fractionation of carbon isotopes was possible during igneous processes. More recent research in isotope geochemistry has demonstrated that carbon with an isotopic composition similar to that of " m a n t l e carbon" (0 to - 10%o) is preserved as CO 2 within vesicles in submarine glasses [8-11] together with a second component (not necessarily gaseous) which is isotopically much lighter. Two models have been put forward to explain this duality: (1) Des Marais et al. [11], who employed a stepped combustion technique to extract carbon from the glasses, considered the isotopically light carbon, which was released at low combustion temperatures, to be surficial organic contamination, whereas (2) Pineau and Javoy [10] attach considerable petrological significance to isotopitally light carbon suggesting it to be the residue after partial outgassing from the magma. Although several authors [7-10,12] favour a model in which fractionation during igneous processes can produce the isotopic variability observed in basic rocks from a restricted range of

197 mantle values, experiment has shown only a difference of 4.5%0 between oxidised and reduced carbon at l l 0 0 ° C in basaltic melts [9]. With respect to diamond genesis, recent studies of internal isotopic zoning [13,14] reveal a similar degree of isotopic variability ( < 5%o). In order to understand the significance of coexisting light and heavy carbon in basaltic glasses, we have employed stepped combustion and pyrolysis extraction techniques to examine, in detail, the release characteristics and isotopic variability of carbon from a selection of basaltic glasses. To gain some insight into how crustal recycling can contribute to the variability of upper mantle carbon reservoirs, we have measured the isotopic composition of carbon contained in submarine basalt glasses from mid-ocean ridges, ocean islands, back-arc basins and island arcs.

2. Sample localities and experimental methods Sixteen well characterised basalt glasses were selected for this study and relevant background details are given elsewhere [15-20]. Glass chips (100-200 mg) were washed in 0.3N HC1, distilled water and then cleaned ultrasonically in a 1 : 1 mixture of methanol and toluene. After drying at 110 ° C for 24 hours to remove the organic solvents, each aliquot was weighed and loaded into a prebaked quartz tube. The sample was degassed at 25 o C overnight at a pressure of 10-5 Torr. In one instance, carbon isotope compositions were measured on CO 2 at ultra-high sensitivity by a triple collector mass spectrometer operated in the static mode [21]. This new technique permits isotopic measurements to be determined on ca. 10 ng of carbon, thus allowing study of the detailed release characteristics and isotopic variation in samples of only 1-3 mg weight. An overview of the carbon geochemistry was obtained using a stepped heating technique [22] and a conventional dynamic stable isotope mass spectrometer (V.G. Micromass 602E). This instrument requires a minimum of 3 ~g of carbon for isotopic measurement and for typical basaltic glasses at least 100 mg of sample is necessary for stepped extraction at 200 ° C temperature intervals. A typical run was as follows: the glass

chips were first combusted in pure oxygen ( - 500 Tort) at 400 o C for 1 hour and then pyrolysed for 1 hour in 2 0 0 ° C steps to 1200°C. Essentially all carbon is released after pyrolysing for 1 hour at 1200 °C; no further carbon is liberated by subsequent pyrolysis or combustion. Carbon dioxide was separated cryogenically from SO 2 and H 2 0 , any CO converted to CO 2 over cupric oxide at 4 5 0 ° C and isotopic measurements were made using the Micromass 602E mass spectrometer. Carbon blanks were negligible and 13C/12C ratios are reported relative to PDB to a precision of +0.1%o. The overall precision for the smallest sample using the ultra-high sensitivity method is +5%o, however, the relative error within an individual stepped extraction is considerably less [21]. As the proportion of CO 2 trapped in vesicles relative to other carbon species will be lower in smaller sized glass chips, the yields obtained by the two methods cannot be directly compared.

3. Results The ultra-high sensitivity static mass spectrometric technique was used to characterise the detailed carbon isotopic variation in Mariana Trough glass (MV1549). The abundance and isotopic variation of carbon released by combustion in 50 o C steps for this sample is shown in Fig. 1. The carbon release profile shows a major component that reaches a maximum at 350 ° C and two smaller emissions at 800 and 1000°C. The isotopic composition of the carbon, however, is broadly bimodal, featuring a marked change at 800 o C. Over 70% of the total carbon in the sample is released below this temperature and has 813C values which vary from - 2 5 to -30%o with the exception of a sharp spike at 5 5 0 ° C which reaches -20%o. Carbon released from 800 to 1000°C is significantly heavier and of more constant isotopic composition, varying only from - 1 1 to -9%0. The total carbon content of MV1549 was 416 p p m and the bulk isotopic composition was -21.3%o. The above results are consistent with a two-component model for the carbon in the Mariana Trough glass with one species isotopically light and the other

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heavy. At the narrow temperature interval where the two components overlap (550 o C) intermediate isotopic compositions are possible. Based on the above conclusion, a lower-resolution study (200 o C steps) has been performed employing conventional sensitivity level mass spectrometry. All the CO2-richbasaltic glasses surveyed in the investigation confirm the two-component model approach. A summary of the results of stepped combustion and pyrolysis in 200 °C temperature steps of the fifteen glasses is given in Table 1. The isotopic composition and release profiles of the CO 2 liberated by stepped heating fall into three general categories which incorporate (1) mid-ocean ridge basalts (MORB) and ocean island basalts (OIB), (2) back-arc basin basalt (BABB) glasses from the Scotia Sea and Mariana Trough, and (3) the glasses from the Mariana Arc. Fig. 2 shows the isotopic variation at each extraction temperature with cumulative amount of carbon released for representative samples. The glass samples from the Juan de Fuca Ridge,

the Mid-Atlantic Ridge at 45 o N, Kilauea Rift and Explorer Seamount all have strongly bimodal release profiles consisting of a minor component of isotopically light CO2 at approximately 4 0 0 ° C and a major emission of CO2, relatively enriched in 13C, at temperatures near the melting point of the glass. Up to 41 ppm are liberated by combustion below 4 0 0 ° C with 813C values of - 2 4 . 4 to - 28.5%o. Less than 5 ppm of carbon, however, are released either by pyrolysis or combustion during the 400-600 o C step. By the 800 ° C step, the onset on the major component is encountered and this is marked by a sharp jump in isotopic composition (Fig. 2), just as previously seen in the high-resolution experiment (Fig. 1). Between 55 and 167 ppm of carbon as CO 2 are evolved by pyrolysis during the three steps from 800 to 1200 o C. Concurrently with major amounts of H20, SO 2 and lesser quantities of non-condensible gases (N2, H2, CH 4 and noble gases) are observed. The CO z above 800 o C has 813C values which vary between - 2 . 8 and - 7.5%o. Individual samples show irregular fluctuations of up to 4%o in 813C with increasing temperature. The samples from back arc basins again show evidence for a bimodal release of CO 2 but these materials differ from the M O R B / O I B glasses in two important respects. Firstly, a greater propor-

200 tion of the total carbon in these samples, between 34 and 75 ppm, is liberated below 400 o C, except in one case where less than 9 ppm carbon constituted the 4 0 0 - 6 0 0 ° C combustion step, Secondly, whilst the isotopic composition of the combustion-produced CO 2 released below 400 o C from the BABB glasses was essentially the same as that evolved from the M O R B / O I B glasses ( - 2 4 . 6 to -29.9%0, cf. - 2 4 . 4 to -28.59'o0) the CO 2 from pyrolysis above 800°C is systematically lighter ( - 7 . 6 to -16.3%o) than the corresponding fraction from the M O R B / O I B glasses ( - 2 . 8 to -7.5%0). Irregular fluctuations of up to 7.5%0 are observed in individual 'samples across the three high-temperature pyrolysis steps. The release profiles of the glass from the Mariana Arc are very different from either M O R B / O I B or BABB samples (Fig. 2). Firstly, arc glasses are relatively depleted in carbon with the bulk of the total carbon, up to 71 ppm, in fractions below 400°C. Only between 3 and 26 ppm carbon are released above 800 o C. Secondly, no jump in isotopic composition is observed between the low- and high-temperature carbon. The carbon released at all temperatures has 813C values ( - 2 4 . 5 to -29.9%o) which are indistinguishable from those observed for low-temperature carbon in M O R B / O I B and BABB glasses.

4. Discussion

Coexisting light and heavy carbon was first identified by Pineau et al. [8] in unusually volatile-rich basalts ("popping rocks") from the Mid-Atlantic Ridge. The CO 2 liberated from vesicles by crushing was found to have 813C values of -7.6%0 ___0.5%0, whereas in volatile carbon released from the residue by fusion was isotopically lighter with 813C values of - 1 2 to -13.7%0. Stepped heating of a popping rock showed that relatively heavy CO2 was released at higher temperatures. Prior to the Pineau et al. [8] study, all carbon isotopic measurements on basalts had given light values of between - 2 5 to -30%o [3,7]. The co-existence of light carbon with isotopicaUy heavy CO 2 (similar to the composition of other forms of mantle carbon such as diamonds, kimberhtes and

carbonatites), is problematical in view of the high temperatures of formation of basaltic magmas. In a recent paper, Pineau and Javoy [10] investigated, in more detail, the relationship between light and heavy carbon in basaltic glasses from the MidAtlantic Ridge and the East Pacific Rise. By crushing 1-10 g of glass, 3-222 ppm of carbon as CO 2, with 813C values of between - 0 . 7 to -8.0%o, was liberated. Fusion of the residues after crushing revealed a further 100 to 201 ppm of carbon having ~13C values in the range - 8.6 to - 20.6%0. Pineau and Javoy [10] did not think that the spread observed on the fused residues was a result of the mixing of isotopically unequilibrated carbon species because such an effect ought to have been obliterated by isotopic exchange at magmatic temperatures. Instead they proposed that the light carbon derived from an outgassing mechanism in association with fractionation between CO 2 and reduced carbon dissolved in tholeiitic melts. However, since the data did not appear to fit either an equilibrium or a pure Rayleigh distillation fractionation model (see [10]), a two-stage outgassing model was required. The first step initiated under near equilibrium conditions when the magma is not yet connected within the rising mantle material and is followed by the distillation of CO 2 when bubbles ascend rapidly within a connected liquid. Assuming that the dissolved light carbon was the residue after the degassing process outlined above, Pineau and Javoy [10] and Javoy et al. [12] calculated that mid-ocean ridge magmas initially must have contained large quantities of carbon (200-10,000 ppm) which are outgassed into the atmosphere and hydrosphere. The stepped heating experiments conducted in our study confirm the co-existence of light and heavy carbon in all glasses from mid-ocean ridges and extends the observation to include material from ocean islands and back-arc basins. However, our interpretation of that part of the data concerning light carbon differs significantly from that preferred by Pineau and Javoy [10] as is described below. 4.1 lsotopically light carbon in ocean basalts

Our high-resolution study of the Mariana Trough glass (Fig. 1) shows that virtually all the

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isotopically light carbon is liberated by 500 o C. In the lower-resolution experiments, this limit for removing light carbon by combustion is 400°C. Only minor quantities of CO 2, except in the case of A56.245, are produced if combustion is extended to 600 o C. Very little CO 2 could be released by pyrolysis at 400 °C, suggesting that the light carbon needs to be oxidised to become volatile. If the 400 o C combustion step was omitted, considerably more CO 2 was observed at 600°C. Such properties are incompatible with the light carbon phase being directly dissolved in the glass, although, it is possible that such carbon may have subsequently exsolved. Nevertheless, we doubt whether the light carbon is wholly residual carbon left over after distillation processes as proposed by Pineau and Javoy [10] or Javoy et al. [12]. The results of our experiments suggest the light carbon component resides on surfaces of the sample freely exposed to oxygen, and most likely was introduced as an organic contaminant after fracturing. The ultra-sonic precleaning of glass chips in a toluenemethanol mixture removed a variable but generally small proportion of the isotopically light carbon therefore the contamination must be polymeric in nature; indeed its combustion temper-

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ature is remarkably similar to that of kerogen from oil shales [23]. In support of the above argument, we note that the isotopic composition of CO 2 released by 400 and 600 °C combustions from all the samples studied is remarkably uniform, with 813C values averaging -27.67oo+1.8 (Fig. 4). Considering the diversity of the sample localities, the depths of eruption of the glasses and their different chemistries, it seems highly improbable that any carbon residues from an outgassing event would have such a consistent composition. Contamination, be it introduced in the laboratory or from the sedimentary environment where samples were collected, is probably much more likely to have a common isotopic composition. Even lunar samples and meteorites, which have been carefully stored and protected, have components cornbusting up to 400 °C with 813C values in the range -25 to -30%o, demonstrating that organic contamination is a serious problem for all samples having a carbon content less than 2000 p p m [22]. Ocean basalt glass most certainly falls into this category and is particularly vulnerable to contamination in its deep-sea location and by subsequent collection and handling. The abundances and isotopic composition of

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202 carbon released from basalt glasses by stepped heating (this study), crushing [10] and bulk fusion [10,27] experiments are compared in Fig. 3. We note that for MORB (and OIB) samples, the isotopic composition of carbon released as CO 2 above 600 °C by stepped pyrolysis is identical to that of CO 2 released by crushing (see also Fig. 4). Fig 3 also clearly demonstrates that bulk fusion of samples releases a minimum of 100 ppm of carbon which is mostly of intermediate isotopic composition ( - 1 0 to -25%o). We suggest that the abundance and isotopic composition of carbon released from a sample which has not been pre-combusted at 400-500 °C may contain significant quantities of light contaminant carbon. An example mixing curve between magmatic and contaminant carbon is shown in Fig. 3. The addition of up to 100 ppm of a contaminant with a ~13C of -27.5%o to glass samples containing variable abundances of indigenous carbon with a ~13C of -67~ would cover most of the observed isotopic variation shown by the bulk fusion data in Fig. 3. The presence of organic contamination rather than fractionation processes associated with multi-stage degassing models [10], provides a plausible alternative explanation of the origin of light reduced carbon in igneous samples. An important outcome of the multi-stage degassing model proposed by Pineau and Javoy [10] to explain the isotopically light carbon in MORB samples is that the original carbon concentration in the magma must have been ca. 2000-10,000 ppm. Such estimates have been questioned by Walker [25] on the grounds of mass balance and are at variance with measurements of carbon abundance in glass inclusions within olivine phenocrysts [26]. Furthermore, loss of 97-99% of the original carbon cannot be reconciled with the high concentrations of He preserved in submarine glasses [27]. If, however, as suggested by Des Marais et al. [11] and ourselves, the light carbon in basalts is organic contamination and not the residue of an outgassing process, then the above estimates may be inappropriate. 4.2 Magmatic carbon in M O R B , OIB and B A B B

The isotopicaUy heavy carbon dioxide which is released by pyrolysis between 600 and 1200°C

along with other volatile species such as SO2, H 2 0 and N 2 is believed to be a magmatic component. In the high-resolution experiment, this uniformly heavy component is seen in successive bursts at 800 and 1000°C which is consistent with the gas being liberated from decrepitating micro-vesicles (Fig. 1). The jump in isotopic composition to -20%o observed at 550 °C (Fig. 1) might thus be a minor burst coming off at a time when organic contamination was still being combusted to produce gas of mixed origin. The low-resolution study demonstrates that isotopically heavy CO 2 is liberated by pyrolysis above 600 °C from all samples studied, with the exception of those from the Mariana Arc. The latter samples, owing to their shallow depth of eruption (1100 m [17]) may have mostly degassed. The isotopic composition of carbon dioxide liberated from above 600 °C from individual samples can vary. Whilst a maximum variation of 7.6%o is noted, gas released at successive temperature steps generally only varies by less than 37c¢ and, in two cases, by less than 1%o. Reasons for these fluctuations are not yet well understood but some possible explanations include: (1) the presence in some samples of a second component of different ~13C released at similar temperatures may explain these variations; the most likely candidates would be dissolved gas [8-10] or elemental carbon adhering to vesicle walls [24] which produces CO 2 by reaction with associated silicates at high temperatures (2) small fluctuations in isotopic composition could be the result of kinetic fractionations of 13C from 12C caused by the diffusion of gas out of fluid inclusions during progressive softening and fusion of glass (3) it is possible that the delayed release of contaminant organics to some extent overlaps with the magmatic carbon, and (4) whilst carbon blanks are negligible when measured on empty quartz tubes (both for the conventional and high-sensitivity methods), reaction of molten basalt glass with the walls of the sample vessel might increase blank levels significantly at high temperatures in the case of small samples. Whilst we expect to resolve between some of these explanations in future studies by high-res-

203

olution measurements on well characterised sampies, we consider that the integral isotopic composition for fractions of CO 2 observed between 600 and 1200°C to represent the true bulk value for trapped magmatic gas. Integrated isotopic compositions of carbon from each glass are compared in Fig. 4. The composition of magmatic CO 2 in OIB appears to be similar to that of MORB magmatic CO 2. However, magmatic CO 2 in glasses from back-arc basins such as the Mariana Trough and the Scotia Sea with integrated 3t3c values of between - 9 . 8 and

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-13.2%o, are isotopically distinctly different from MORB glasses ( ~ 1 3 C = - 3 . 5 to -7.47~). It is very important to point out that the spread of data for basalts from the two environments overlap by only 0.5%0, even if the 313C values for the individual extraction steps (Table 1) and the data of Pineau and Javoy [10] are considered. Thus, we report that on average, magmatic carbon in BAB lavas is approximately 5%o lighter than that from MORB and OIB. BABB and MORB magmas are chemically very similar and several authors [28-31] have demonstrated that it is extremely difficult to establish absolute criteria to discriminate between the two types. Whereas Sr-Nd-Pb isotope systematics [20,32-37] and the presence of 1°Be [38] indicate a clear sedimentary signal in arc volcanics, a contribution to erupted basalts from pelagic sediments subducted deeper into the mantle to the source region of marginal basins is harder to confirm. As many current models of global evolution invoke the recycling of crustal materials [20,39-41], unambiguous evidence in favour of this process is of some importance. However, as He isotope evidence [43] implicates a mantle source for at least part of the volatile content of island arc basalts, it seems unlikely that the 8~3C values measured in BABB could be derived entirely from subducted carbon without any input from a deep-seated source. The lighter carbon isotopic signatures associated with BABB magmas, as compared to MORB, are best explained within a model where CO 2 of mantle origin and juvenile composition (cf. MORB) is mixed with carbon derived from the descending slab. Assuming that indigenous CO 2 in basaltic crust is almost entirely lost during magma degassing, crystallisation and hydrothermal alteration, the principal reservoirs of carbon in the oceanic lithosphere exist as (1) minor secondary carbonates in weathered and hydrothermally altered basalts, having 813C values of 0 to +27~ [44,45], or (2) pelagic carbonates with 3a3C - 07~ [45,47], authigenic carbonates and kerogens with 3a3C of - 2 0 to -307~ [44,47]. However, it is the subduction of pelagic clays (in which pelagic carbonates and minor kerogenous material combine to form the dominant crustal reservoir of carbon), rather than altered basaltic crust, which

204 provides the most likely pathway for the recycling of carbon into the mantle. Kerogenous material, with 813C between - 2 0 and -30%0 [42,45-47] will be more volatile than carbonate and might be expected to be the first carbon phase thermally degraded during subduction. Assuming typical 813C values for MORB magmatic CO 2 to be within the range - 3 . 5 to -7.4%0, then the observed shift of - 5900 to lighter values in BABB magmas would be consistent with a contribution of carbon from sedimentary organics. If the flux from the deep mantle remains the same at the BABB locations, then in order to achieve the lightest 813C values observed in the high temperature fractions of Scotia Sea samples (-16%0), approximately 1 : 1 mixing of sedimentary organic carbon with juvenile carbon would be required. This mixing ratio is an upper limit but is nevertheless consistent with a recent observation that the volatile content of lavas from the Scotia Sea [19] and Mariana Trough [17] are enriched in both carbon dioxide and water, by factors of up to 2 and 10 respectively, compared to MORB. Thermal cracking of kerogen, however, leads to low-molecular-weight hydrocarbons substantially enriched in the light isotope of carbon (813C ca. -50%0) [48], which, provided oxidation occurred in the magma, could account for the decrease in 813C observed for BABB samples at a very much lower level of sediment contribution. A good test for such a process might be the presence of isotopically light methane in BABB glasses; preliminary results (L.P. Carr, unpublished data. 1983) certainly suggest the light hydrocarbon is more abundant in Mariana trough samples than typical MORB glasses. Since there is no evidence yet for BABB volatiles being derived from degassing of subducted marine or hydrothermal carbonate, large amounts of carbon with 813C values close to 0%0 may be recycled deep into the Earth. Such a process would also provide a return pathway for Sr into the mantle and may explain anomalously low 875r//86Sr ratios in island arc magmas [49]. The fate of such carbon and its effect on the overall isotopic composition of the upper mantle is at present unknown, however, the recycling of crustal carbon provides an important mechanism by which isotopically distinct carbon reservoirs may develop in the mantle.

Acknowledgements We are grateful to D.A. Meunow and R.K. O'Nions for supplying splits of characterised glass provided to them b y P, Barker, A.D. Saunders, J. Tarney and J.G. Moore. We thank Mark Javoy for a constructive criticism of the manuscript. Financial support to the Planetary Sciences Unit from the Science and Engineering Research Council is gratefully acknowledged.

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