Carbon isotopes in xenoliths from the Hualalai Volcano, Hawaii, and the generation of isotopic variability

Geochimica d Cosmochimica Copyright Q 1990

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0016~7037/90/$3.00

Vol. 54, pp. 211-227

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Carbon isotopes in xenoliths from the Hualalai Volcano, Hawaii, and the generation of isotopic variability F. PINEAU’ and E. A. MATHEZ~ ‘Iaboratoire de Geochimie des Isotopes Stables, Universitk de Paris 7 and Institut de Physique du Globe, 2 Place Jussieu, 75251 Paris Cedex 05, France *Department of Mineral Sciences, American Museum of Natural History, New York, NY 10024, USA (Received April 2 1, 1989; accepted in revised form October 30, 1989)

Abstract-The isotopic composition of carbon has been determined in a suite of xenoliths from lava of the 1800- 180 1 Kaupulehu eruption of Hualalai Volcano, Hawaii. Several lithologies are represented in the suite, including websterite, dunite, wehrlite, pyroxenite, and gabbro. In addition, there are composite xenoliths in which contacts between lithologies are preserved. Most of the xenoliths represent deformed cumulates. The contact relations in the composite samples indicate that the lithologies originated from the same source region, which, based on pressures determined from fluid inclusions, is estimated to be at a depth of =20 km, or near the crust-mantle boundary. Samples were heated in steps from 200 to 1475’C to obtain separation of the different carbonaceous phases, and the isotopic composition of carbon released at each step was determined. Grossular glass was found to be a suitable flux to fuse refractory samples. Upon heating, carbon exhibits the typical bimodal evolution behavior observed in other studies of xenoliths and basalts. Carbon extracted from all samples at temperatures below 900°C is characterized by a 613C of about -25%0 vs PDB and is thought to be composed dominantly of graphitic and organic material, which is known to be present on virtually all cracks. The 613Cof the carbon fraction extracted at 1200°C and above from wehrlite and dunite is in the range - 1.5%0to -5.2%0, whereas that extracted from websterite ranges from -22%0 to -26%0. Similarly, in one composite sample, the compositions of dunite and websterite were found to be -2.4%0 and -7.0%0, respectively. The large difference can be associated with specific petrographic features unique to each lithology. In wehrlite and dunite, carbon exists mostly as CO&h inclusions in arrays representing partially annealed microcracks. The websterite xenoliths contain megascopic zones of large, irregularly-shaped inclusions. The zones traverse entire thinsections and are interpreted to represent fractures annealed at depth. Most of the carbon is believed to exist in the inclusion-rich zones and to consist of carbonaceous material precipitated from fluid. The observations and isotopic results demonstrate that isotopic variability can be generated by multistage fractionation processes such as degassing of COZ from magma and precipitation of C02-rich fluids to form graphitic compounds. Such processes operated over regions the scales of which were determined by style and intensity of deformation and by lithology. ascribed to localized fractionation processes (PINEAUand JAVOY, 1983; JAVOYet al., 1986). If the mantle is characterized by large-scale carbon isotopic heterogeneities, then this raises the question of how they developed. One possibility is that biogenic organic carbon was subducted and mixed to varying extents with a homogeneous primordial carbon reservoir. This process was invoked to account for isotopically light diamonds (MILLEDGEet al., 1983) and back arc basin basalts (MATTEY et al., 1984). Although some of these basalts possess helium and hydrogen isotopic ratios consistent with contamination by a small proportion of ocean crust (POREDA, 1985) for diamonds there is no independent evidence for the involvement of organic carbon in their formation. Thus, DEINESet al. (1987) suggested that the isotopic variability in diamonds was inherited from Earth accretionary processes, a possibility permitted by the variability of isotopic composition of meteorites. Yet a third possibility is that isotopic heterogeneities are primarily due to a combination of isotopic fractionation between carbonaceous phases and distillation mechanisms. The combined processes were first proposed by JAVOY( 1972) to explain extreme 13C enrichment of a certain kimberlite. A general fractionationdistillation mechanism can obviously produce almost any

INTRODUCTION THE UTILITY OF CARBONisotopes in deducing mantle evolution depends on knowledge of carbon distribution and abundance and identifying the manner and extent that various processes fractionate the isotopes. This is particularly so because mantle samples display a large range in carbon isotopic composition. For example, although 613Cvalues of most diamonds fall in the range -5L to -8%0, some exhibit more extreme values from +3%0 to -35%0 (e.g., DEINES, 1980; DEINESet al., 1984; GALIMOV, 1984). It has been argued that these variations indicate the presence of distinct, large reservoirs of different isotopic composition. Alternatively, the variation may be due to fractionation processes operating on a local scale. This is supported by the fact that 613Cvariations of up to 4k have been reported for individual diamonds (SWART et al., 1983; JAVOYet al., 1984; BOYD et al., 1987). A similar problem is posed by the variability in carbon isotopic composition of basalts. While compositional differences among mid-ocean ridge, back arc basin, and hotspot basalts may reflect similar differences over large regions of the mantle (EXLEY et al., 1986), the isotopic composition ranges of the three basalt types overlap each other and therefore may be 217

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F. Pineau and E. A. Mathez

isotopic composition, provided the fractionation between phases is in the right direction and the distillation proceeds to an extreme or there are multiple distillation events. However, it remains to be demonstrated that such processes contributed to the isotopic variability of submarine basalts, diamonds, or other mantle-derived materials. The points of this investigation are to develop the means of interpreting carbon isotope signatures of peridotite and gabbro xenoliths from alkah basalts, to associate the isotopic characteristics with specific petrographic features in the rocks, and to use such data to better understand the sources of carbon isotopic variability in the mantle. The study of xenoliths is complicated by several factors. The carbon in them appears to have multiple sources (MATHEZ, 1987), and isotopically distinct fractions of the total carbon are extracted at different temperatures and by different techniques (WATANABEet al., 1983). Indeed, stepped heating procedures have revealed extremely complex abundance and isotopic release patterns (MATTEY et al., 1985; NADEAU et al., 1990). Therefore, it is necessary to develop further the procedures for extracting useful information from xenoliths. A suite of xenoliths from the Hualalai Volcano, Hawaii, was chosen as the focus of this study. These rocks are highly deformed and enriched in carbon compared to most other peridotite xenoliths (MATHEZ, 1987) and thus were judged most likely to exhibit isotopic variability. Carbon isotopes in Hualalai xenoliths have been determined by WATANABEet al. (1983), who concluded that the underlying lithosphere is isotopically heterogeneous. Some geologic control is provided by the fact that several different lithologies are present and in contact with each other in composite samples. Also, because the rocks originated from depths of ~20 km (see below), they can be used to qualitatively represent processes of defo~ation and the chemical equilibria involving carbon that are characteristic of deeper levels of the upper mantle, despite the fact that the lithologies differ from upper mantle ones. SAMPLES The 1800- 1801 KaupuIehu flow of Hualalai Volcano is we&known for its abundant xenoliths (RICHTERand MURATA, 196 1). The flow is an alkali basalt, and the geologic setting of it and its xenolith beds are described by B~HRSON and CLAGUE(1988). According to a count of more than 3000 xenoliths by JACKSONet al. (198 l), 60% are metamorphosed dunite and wehrlite in which deformation and recrystallization have completely obliterated original textures; the remainder are partiahy deformed and recrystallized cumulates of dunite, wehdite, webstetite, troctolite, clinopyroxene gabbro, olivine-clinopyroxene gabbro, and anorthosite. The genetic relations among these various rock types are not well-established. The lithologies included in this study are described below. Samples with designations such as 63KAP9 are from E. D. Jackson’s collections and are housed at the U.S. National Museum. The remainder are from the collections of the American Museum of Natural History. Websterite (samples HL86-I,

63KAP-9 and 65KAP-12)

Rocks in which orthopyroxene is present as a cumulate phase are rare, making up only about 0.6% of the xenolith suite (JACKSON et al., 1981). BOHRSON and CLAGUE (1988) argued that these cumulates crystallized from tholeiitic magma, which in order for orthopyroxene to be on the liquidus requires pressures in the range 4.5 to 9 kb. Samples 63KAP-9 and 65KAP-12 are also included in their study.

The three samples used in this investigation are composed of about 70% cumulus orthopyroxene (0.5-Z mm diameter) poikilitically enclosed by clinopyroxene. Other primary minerah are Cr-spinet and non~umulus plagioclase, both of which exist in trace amounts. Clinopyroxene exhibits extensive exsolution of orthopyroxene, which is myrmekitically intergrown with the host crystal rather than along rational crystallographic planes (see JACKSONand WRIGHT, 1970, Plate 2). The texture probably developed as a result of thermal metamorphism when the xenoliths were entrained in the magma followed by quenching after eruption (rather than from slow cooling). Temperatures of pyroxene unmixing have been estimated to range from 1045 to 1090°C (BOHRSONand CLAGUE,1988). Some grain boundaries, particularly where they are coincident with inclusion-rich zones, are highly irregular and exhibit evidence of partial melting, being serated and occupied by bubbles, films of glass, and unidentified opaque minerals (see below). The rocks are also deformed. Clinopyroxene oikocrysts were originally larger but are now composed of subgrains separated by planar extinction continuities. All minerals contain planar tluid and mineral inclusion arrays indicative of annealed fractures. With respect to carbon, the most distinctive feature of the websterites is the presence of quasi-planar megascopic mineral/fluid inclusion-rich zones which transect entire thinsections (Fig. la). The zones were once through-going fractures but are now composed of one or mom sets of relatively large inclusions (=70-400 pm diameter) and families of smaller (~20 pm diameter) inclusion arrays (Fig. lb). The fatter are typically orthogonal to the trend of the main set (Fig. la), and thus the two inclusion sets must be genetically related. Most of the smaller inclusions are fluid-filled spherical to necked bubbles; the large ones usually have irregular shapes and may be empty or partially filled with opaque material (Fig. lb). Most of this material probably consists of sulfides and oxides. However, some of it is extremely opaque and lacks color, even at the edges of grains, and thus may be graphitic. The petrographic observations suggest that most of the carbon in the websterites is present as solid material in the megascopic inclusionrich zones rather than as fluid in microscopic fluid inclusion arrays throughout the rock. The former are interpreted to have been originally fractures which were partially annealed prior to incorporation of the xenohth in magma. Had such fractures formed during quenching and thus remained unannealed, the xenoliths would simply have broken apart. Prominent inclusion-rich zones containing high proportions of opaque material are generally absent in other xenoliths, so the proportion of graphite to fluid is probably higher in the websterites. Dunite (samples HL78-4, HL79-I, 651yAP-16 and UNTIL)

65-115-40,

~lSN~ll~77O/I

7,

All the dunites of this study are metamorphic. Although isotopic data were collected for only two samples (HL78-4 and ULTRAS), others were examined petrographically to understand the variability in deformation, fluid inclusion density, or other factors which might inguence isotopic composition. The dunites contain two generations of olivine. Primary olivine, which is present in plates up to 5 mm in diameter, is composed of numero~ subgrains separated by extinction discontinuities and is relatively rich in secondary fluid inclusions. The primary grains are surrounded by granular aggregates of smaller (co.5 mm in diameter), polygonal, secondary olivine, which contain few fluid inclusions or optical discontinuities. The overall textures vary from porphyroclastic to granular mosaic, depending on the proportions of secondary olivine. Chromite modes vary from trace to 5%. Samples HL79-1 and 65-11 S-40 also contain small amounts of clinopyroxene. Hualalai dunites have been studied in detail by GREEN and RADCLrm (1975) and KIRBY and GREEN(1980), and photomicrographs illustrating some of the textural and deformation features may be found in these references. In them it is argued that the metamorphic textures developed by dynamic and syntectonic recrystallization and deformation of an initially coarse-gmined dunite, which is represented by the primary olivine. R~~~~i~tion involved formation of the dislocation structures in primary olivine and its progressive reorganization into finer-grained, secondary olivine. Fluid inclusions were

Isotope composition of carbon in xenoliths from Hualalai, Hawaii

219

across) is relatively inclusion-rich and composed of subgrains; secondary ohvine appears as small (10.3 mm) polygonal crystals in which fluid inclusions and subgrains are mostly absent. One of the most obvious characteristics of these rocks is that the clinopyroxene porphyroblasts are extremely turbid in comparison to surrounding olivine because of their high inclusion content (MATHEZ et al., 1984, Fig. 3). The porphyroblasts contain two distinct families of fluid inclusions (Fig. 3). One is of large (50-200 pm diameter), spherical or negative crystal-shaped inclusions disposed in arrays which are parallel or subparallel to each other and clustered in planar zones. These zones can be seen to traverse several individual porphyroclasts within a single thinsection. However, they are not contiguous through the secondary olivine between porphyroclasts. This and the inclusion-rich character of pyroxene compared to olivine are probably due to the fact that olivine behaves less brittly than pyroxene at high pressure and therefore continued to recrystallize after the formation of the inclusions. A second family of fluid inclusions are small (~20 Frn diameter) and exhibit the range of spherical to necked shapes characteristic of inclusions formed during the annealing process (WANAMAKERand EVANS, 1985). These are disposed in discontinuous arrays parallel or subparallel to the major clinopyroxene cleavage direction { 110) (Fig. 3), and thus their orientations depend on individual porphyroclast orientations, which are random. With respect to the carbon isotopes, the important characteristics of the wehrlites are that nearly all inclusions contain fluid and that the amount of opaque material associated with them is mu:h smaller than in the websterites. The ratio of CO2 to solid carbons is clearly higher in wehrlite than websterite. Pyroxenite (sample HL78-5)

FIG. I. Photomicrograph of websterite HL86-1. (a) Megascopic, through-going, inclusion-rich zones traversing orthopyroxene oikotrysts (light grains) poikilitically enclosed by clinopyroxene (dark), crossed nicols. Note the arrays of small inclusions orthogonal to the larger set. The clinopyroxene contains myrmekitic exsolutions of orthopyroxene. The photomicrograph is 820 pm across. (b) Magnified view of the two sets of inclusions in orthopyroxene, plane light. Note the irregular shape of the large inclusions and that they contain opaque material. The photomicrograph is 4 10 pm across.

The xenolith is a granular olivine pyroxenite composed of clinopyroxene (85%), tabular crystals 0.5-2 mm long of orthopyroxene (10%) and equant 0.3-0.8 mm diameter crystals of olivine (5%) in the intersticies. It is unlike the other samples and appears to belong to the clinopyroxenite suite of BOHRSONand CLACUE(1988). Extensive recrystallization along the grain boundaries has obscured the original texture. The grain boundaries are now occupied by a finegrained, interlocking mass of pyroxene, oxide and possibly other material. Mineral inclusions and cracks are preserved in the center of clinopyroxene crystals but are generally absent from their rims. The rock is also transected by numerous inclusion-rich zones. Composite rocks (Samples 71KAP-1, 66KAP-7 and HL78-2)

Sample 7 1KAP- 1 is a websterite-dunite composite. Both lithologies closely resemble those of the samples described above. The Webster&e

swept through the crystals by the recrystallization fronts and are now concentrated on grain boundaries of secondary olivine (Fig. 2). The principal textural variation among samples is the proportion of primary to secondary olivine, which reflects the extent of recrystallization. However, no relationship has been found between the proportions of fluid inclusions and secondary olivine in the rocks. In particular, about 30% of the olivine in ULTRAS is secondary, compared to about 70% of that in HL78-4. Yet the isotopic compositions and abundances of carbon in these two samples are similar (Table 1). The origin of the metamorphic dunites is poorly understood. Most evidence indicates that they originated as cumulates (see below). Wehrlite (samples HL78-3 and 63kTAP-5)

The metamorphosed wehrlites possess a distinctive porphyroclastic texture. They are bimineralic rocks in which porphyroblasts of clinopyroxene, which make up 30-35% of the modes and are up to 6 mm in diameter, are enclosed in a matrix of olivine. As in the dunites, two generations of olivine are present: Primary olivine (OS-l.5 mm

FIG. 2. Photomicrograph of fluid inclusions concentrated at grain boundaries ofsecondary olivine in dunite 65KAP-16, crossed nicols. The photomicrograph is 410 pm across.

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F. Pineau and E. A. Mathez

contains megascopic, through-going, inclusion-rich zones in which opaque mat&al is common, but in the dunite such zones are absent. This suggests that the websterite crystallized first; however, the op posite is also possible if such zones originally in the dunite were obliterated by recrystagization. Unfo~unately, the contact between the two Ii~ol~~ offers no clues to this question. Sample HL78-2 is a wehrlitedunite composite cut by a 2 cm wide dikelet of fine-g&red, olivincbearing gabbro, and sample 66KAP7 is a metamorphosed dunite + wehrlite. Again, the lithologies in these composite samples are identical to those described above. The isotopic composition of sample 66KAP-7 has not been determined.

From their textures, the Webster&es are clearly cumulates. The evidence available from other studies suggests that the Hualalai dunites and wehrlites also originated in that way, although how the various cumulate lithologies became juxtaposed is not clear. Dunite xenoliths

similar to those at H&alai exist in the Honolulu Volcanics and are thought to be metamorphosed cumulates formed from tholeiitic magma (SEN, 1983). In contrast, rare earth element contents and strontium isotope ratios of clinopyroxenes separated from Hualalai xenohths are consistent with their accumulation from the alkali bar&s that feed the volcano (CHEN et al., 1985), and helium and argon isotopic ratios of Hualalai dunite differ from those of any Hawaiian tholeiite or alkali basalt (KYSER and RISON, 1982). The discrepancies among these different sets of data are not resolved. Whatever were the parent magmas of the xenolith suite, the reservoirs in which these rocks formed are thought to have been at depths of 15 to 25 km (SW, 1983; CLAGUE, 1987). Few data have been reported on the densities of fluid inclusions in the xenoiiths. The maximum CO* density reported by ROEDDER (1965) and MURCK et al. (1978) for speeificaily Hualalai rocks is 0.62 s/cm’, which corresponds to a fluid trapping pressure of 2.8 kb at 1000°C. In order to further define the depth of origin, densities and compositions of fluids were determined in wehrhte HL78-3 and

Isotope composition of carbon in xenoliths from Hualalai, Hawaii

221

WEHRLITE

HL78-3

PPm c

al3c

0

! -10

-20

-30

-40

800 1200

FIG. 3. Photomicrograph of clinopyroxene in wehrlite HL78-3 containing two generations of fluid inclusions, plane light. The arrays of smaller inclusions are oriented approximately parallel to { 1 lo}. The photomicrograph is 4 10 pm across.

1450 1475 fl

200

0

150

-10

100

-20

50

-30

-40

0 flf2n

600

websterite 63KAP-9. In both rocks, freezing points of -56.9 to -585°C were observed, indicating the fluids are >95% C02. (The freezing point of pure CO2 is -56.6”C). Homogenization temperatures of most inclusions fall in the range - 1.2 to +2.5”C, corresponding to fluid densities of 0.92 to 0.94 g/cm’ (assuming pure CO*) and a maximum trapping pressure of ~6.3 kb at 1000°C or e5.4 kb at 800°C. Bearing in mind the problems inherent in using fluid inclusions to estimate pressures (e.g., ROEDDER,1983), the data indicate that the xenoliths were deformed (and probably formed) in the mantle at or near the moho, which is at a depth of 19 km below sea level at Hualalai (MOORE, 1987). Two geologic constraints are provided by the composite xenoliths. First, the exposed contacts between wehrlite and dunite (sample 66KAP-7) websterite and dunite (sample 7 IKAP-I), and wehrlite, dunite, and gabbro (sample HL78-2) demonstrate that the constituent lithologies exist at the same depth in contact with each other. Second, the rocks represent a time span such that some were deformed and recrystallized before others had even formed. A distinct contrast exists between inclusions in the wehrlites and websterites. In the former, there are large inclusions arranged in prominent, through-going zones and small inclusions in arrays the orientations of which are crystallographically controlled by host crystals. Both inclusion types contain fluid. The websterites possess megascopic inclusion-rich zones in which most inclusions are empty or partially filled with solid material, some of which is believed to be carbonaceous. In the dunites, inclusions typically contain fluid. EXPERIMENTAL

‘UT) FIG. 4. Carbon abundances and isotopic compositions extracted during heating of wehrlite HL78-3. (a) Flux of grossular glass was added after the second 1475” heating cycle in order to fuse the sample (exp. 4, Table I). (b) Clinopyroxene separate (exp. 6).

WEBSTERITE ppm C

65KAP-12 al3c

TECHNIQUES

During crushing, samples were frequently passed through a 1.4 mm sieve to maximize the proportion of coarse material and thus the preservation of CO2 inclusions. For most samples 1.5 to 2.2 g of material were analyzed. Carbon was extracted by a stepped-heating procedure whereby samples were heated at progressively higher temperatures from 200 to 1475°C under low oxygen pressure (1 to 12 mm), the point being to obtain separation of the different carbonaceous phases (e.g., Figs. 4-7). Several experiments to temperatures of 1600°C were also conducted. The extraction procedures are described in detail by NADEAU et al. (1990). Steps up to 900°C were of one hour duration, and those at higher temperature were for a half-hour. Extractions were repeated at the same temperature when CO* was found to be abundant. Some experiments were performed on pure samples and others on samples attacked with phosphoric acid. In addition, a procedure was developed to mse samples by addition of a flux in order to insure

0

-40 600 900 1200

1400

‘W’C) FIG. 5. Stepped-heating experiment on websterite 65KAP-12 illustrating the effect of oxygen pressure on extraction efficiency of carbon from melt and the light isotopic compositions of the carbon fractions extracted at high-temperature. (a) O2 pressure was maintained at 8 torr for the first two fusions and then decreased to 0.1 torr for the second two (see text) (exp. 2 1, Table 1). (b) Experiment after attack with phosphoric acid (exp. 24).

F. Pineau and E. A. Mathez

222

calibrated manometer in which there are several calibrated volumes,

DUNITE ULTRAS PPmc

a13c

1"

the smallest one corresponding to 0.025 cm3. Isotopic analyses were performed on VG 602 or Finnigan MAT mass spectrometers,both of which give analytical precisions of kO.03 for samples > 0.3 pmole and 20.06 for smaller ones. The precision is better by a factor of 10 than analyses of two different aliquots of the same sample each treated in the same way. The results are expressed in the conventional unit of 6 relative to PDB. EXPE~ME~

‘K’C) FIG. 6. Stepped-heating experiments on dunite ULTRA 5 (a) with fusion by addition of flux (exp. 19, Table I) and (b) without fusion (exp. 20).

complete carbon extraction. Most traditional fluxes are inappropriate for extraction under vacuum conditions because they are hydrous and thus contribute matter to the evolved gas. It was found that addition of a glass of grossular composition, the liquidus of which is about 1320°C, to dunite and other xenoliths lowered melting temperatures to below 142O”C, or within the range of the maximum temperature attainable by the induction furnace. In addition to being anhydmus, grossular glass was chosen because it possesses a single oxidation state, is of low viscosity, and is easily made in large quantities. Mixtures of 30% dunite, the most refractory composition, and 70% grossular glass were observed to undergo complete fusion at 145O’C in less than one-half hour. This was verified by thinsection examination of one of the fused samples, in which only quenchproduced spine1 crystals were observed. Less refractory materials melted completely in shorter times, typically about 10 minutes, and required smaller proportions of flux. To be sure of the purity of the flux-bearing blank, the flux was loaded alone in the crucible (Ft + 10% Rb) and slowly heated under high vacuum to the melting temperature of 1320°C and then to 1475°C for two hours. Oxvaen __ was introduced and the flux reheated to confirm that additional carbon was not extracted. In this way blanks equivalent to those without fluxes were established. The blanks with flux yielded x0.1 #mole carbon per step for steps up to 900°C and a mean of 0.26 ~fi0.03 rmoles per 30 minute fusion at 1475’C. The cont~bution of 0.1 gmole from the blank is equivalent to 1.2 ppm carbon in a 1 g sample, so the blank is usuallv insignificant. A tvnical vield for the blank is given in Table 1, and ai1the sample data are blank-corrected. The s”E of the blank was -27.5 + 0.02%~ After the blank was established, the sample was added and the stepped-heating procedure conducted in the normal way after outgassing at 200°C under hip-vacuum for a minimum of 2 hours. In one experiment (described below) the flux was added to the sample after it had been heated to 1475’C. Heat was provided by an induction furnace calibrated in temperature for all the experimental conditions (with and without oxygen). Oxygen was generated by heating CuO at 850°C and removed by cooling it to 450°C. The gaseous species evolved during oxidative heating were condensed by liquid nitrogen at the end of the heating cycle and as oxygen pressure was decreased. The separation of CO2 from other trapped species, which consist mostly of H,O, was done using a cold methanol trap. The yield of CO2 was measured in a

AND RESULTS

Most samples yielded the characteristic bimodal carbon abundance and isotopic release patterns observed in other investigations of xenoliths (e.g., WATANABEet al., 1983) and submarine basalts (e.g., PINEAU et al., 1976). The data are presented in Table I. During the first heating step (200 to 6oO*C), I6 to 5 1%of the total carbon was released. This fraction was always found to be isotopically light, in the range -22%0 to -26%. Some of this carbon must have originated by adsorption from the atmosphere (e.g., PINEAU and JAVOY, 1983). However, the contribution of adsorbed carbon was minimized by minimizing sample surface area, and there is usually so much carbon released during this step that most of it must represent carbonaceous particles and films on the surfaces of fractures and microcracks. Carbonaceous material in xenoliths was described by MATHEZ (1987), who proposed that some of it originated by condensation of volcanic gas on new crack surfaces formed during cooling of the xenoliths at the surface. Even if some of the low-tem~rature carbon was originally indigeneous, i.e., present in the xenolith in its source, it is not possible to separate it from carbon that was introduced and, therefore, it is not examined further. Low-temperature release patterns of spine1 lherzolite xenoliths have been in-

GABBRO-WEHRLITE

HL78-2 ai3c

ppmc

600

900

80

-10

600

900

1200

1400

TW) FIG. 7. ~~~-h~ti~ experiments on composite xenolith HL782. (a) Wehdite (exp. 30, Table 1) and (b) gabbro (exp. 3 1). The gabbro was completely molten at 14OO*C.

Isotope composition of carbon in xenoliths from Hualalai, Hawaii vestigated in detail by NADEAU et al. (1990), who showed that several specific fractions may be resolved. At 9OO”C, only 1 to 6% of the total carbon was released and has a 613C in the range -10 to -28.5%0. Most of this probably represents a mixture of matter that would have been extracted at 600°C but was trapped when fractures partially annealed and of CO2 from a few inclusions that decrepitated. Previous experience has shown that carbonate break-down occurs between 600 and 800°C and is isotopically heavy (NADEAU et al., 1990). Carbonate has not been observed in thinsection in any of the Hualalai rocks, and the relatively low 613C of the carbon extracted at this step confirms its absence. Carbon released at higher temperatures is usually isotopically heavier and presumably represents both fluid and solid material trapped in fluid inclusions, microcracks in crystal interiors or other isolated locations. Of note is the fact that the high-temperature carbon in the wehrlites possesses an unusually high 613C, whereas that in the websterites is unusually low (see Table 1 and discussion below; cf. Figs. 4 and 5). The high-temperature gas-release behavior was found to be complicated by several problems. Among them are the difficulties of extracting all carbon and of distinguishing carbon fractions of different isotopic compositions. Therefore, several different types of experiments were conducted.

Melting experiments The importance of melting is illustrated by an experiment on wehrlite HL78-3 (exp. 4 in Table 1 and Fig. 4a). Before addition of the grossular flux, one-hour extractions at 800, 1200, 1450, and 1475’C produced a total yield of 89 ppm carbon. After addition of the flux, the charge was reheated to 900°C to extract carbon adsorbed on surfaces of the sample, flux, crucible, and furnace interior. A subsequent fusion cycle yielded an additional 92 ppm carbon (corrected for the flux contribution). The isotopic composition of carbon extracted during melting (613C = -1.50/w) was 2%0 heavier than that extracted during the first 145O’C heating cycle before melting. This implies that CO2 remained trapped in fluid inclusions until the sample was melted, probably for the combined reasons that the crystals partially annealed during heating and that COZ inclusions were small enough so that they did not burst. The extent to which melting liberated additional carbon varied from sample to sample. For several of the dunites, the carbon yields with fusion were essentially identical to those without fusion. Again, the differences in behavior probably reflect the particular size or shape of fluid inclusions, the extent to which microcracks were able to anneal or other characteristics specific to each sample. The extraction of carbon from the melt is complicated by the facts that the process is sluggish and dependent on the 02 pressure. The complexities are illustrated in Fig. 5a (exp. 2 1 in Table 1). AFter extractions performed at 600 and 9OO”C, websterite 65KAP- 12 was fused twice at 1475 “C for 10 minutes under an oxygen pressure of about 8 torr, yielding first 17 and then 6 ppm carbon. A third 10 minute fusion under an oxygen pressure of 0.1 torr yielded an additional 25 ppm

223

carbon, and an additional 40 minute fusion under low oxygen pressure produced another 30 ppm. This general behavior was observed repeatedly (exp. 21,25,26, and 28 in Table 1). The rate of carbon degassing from the melt is clearly dependent on oxygen pressure even at low pressure, the extraction efficiency being enhanced by lower oxygen pressure. Regardless of the particular conditions or sample compositions, extraction of all the carbon from the melt always required more than 30 minutes at elevated temperature. Why degassing is so sluggish is unclear. One carbon-rich sample was fused for 10 minutes, sectioned, and examined optically. The glass, which presumably retains a substantial fraction of its original carbon, appears as swirls of various shades of pale green and contains some quench-produced microlites, but bubbles or opaque particles were not observed. If such features exist in the glass, they are submicroscopic. However, in other samples large bubbles were observed to form at the melt surface, and it should be expected that when such growing bubbles burst will depend on the pressure over them-i.e., on the 02 pressure. On the other hand, if the rate of carbon release is controlled by the diffusivity of carbon in the melt, then it is not obvious why diffusivity is sensitive to oxygen pressure. Another curious feature of the fusions is that repeated extractions always yielded progressively lighter carbon (e.g., exps. 2 1 and 22 in Table 1). This extraction behavior cannot be an artifact of mixing between sample and blank carbon because the latter is insignificant in the important steps of our experiments. The systematic variation probably represents a fractionation related to the outgassing, but because neither the specific phases nor reactions involved have been identified it is not possible to speculate further on its cause. Abundance measurements One consequence of the release behavior of carbon from melt is that some of the analyses of carbon-rich xenoliths of MATHEZ et al. (1984) are in error. Their analyses involved extraction of carbon from samples as CO2 by fusion with a lithium-tetraborate flux under a continuous flow of oxygen. This was followed by coulometric-alkalimetric titration, which continued until the COZ content of the carrier gas decreased to an unmeasurable concentration. Typically, this occurred within about three minutes from the beginning of fusion. Some samples were subjected to multiple fusions by this technique, but no additional CO* was extracted. Reanalysis of several samples studied by MATHEZ et al. ( 1984) yielded systematically higher concentrations than determined in the initial study (Table 2). The largest differences are for samples rich in carbonaceous material (the webster-

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F. Pineau and E. A. Mathez

ites), and the smallest are for those containing fluid inclusions (the wehrlites). The new analyses also confirm the existence of relatively large compositional heterogeneities among gramsize aliquots. For typical spine1 lherzolites and other samples containing less than several tens of ppm carbon, the two techniques are in accord, and therefore most of the data of Table 1 of MATHEZ et al. (1984) are correct. Combustion without melting A comparison was made of carbon extracted by fusion at a few heating steps and that extracted by heating at many steps but without fusion. In one of the fusion experiments (exp. I9 in Table l), dunite ULTRA5 (+flux) was first heated to 900°C and then subjected to three fusion cycles (Fig. 6a). Presumably, the carbon liberated by the first fusion cycle was dominated by CO2 since it is relatively enriched in 13C. The stepped-heating sequence without melting (exp. 20 in Table 1 and Fig. 6b) involved eight heating cycles. The total amounts of carbon extracted by the two procedures are essentially identical. However, the eight-cycle experiment revealed the presence of carbon having a significantly higher 613Cthan that extracted by the three cycle fusion experiment. This is interpreted to mean that carbon released during the fusion experiment represents a mixture of heavy CO2 and light carbonaceous material originally on cracks but trapped as crystals annealed during the initial heating. This interpretation is supported by the fact that the 613C of the carbon fraction released by the second heating cycle at each temperature is lower than that released during the first cycle at each temperature. This behavior was observed repeatedly from one sample to another. It is also possible that each heating cycle sampled inclusion populations having different sizes or densities and that this gave rise to some of the variability. This possibility exists because different size inclusions exist in these rocks, as noted above. In any case, the important point is that the “resolution” of isotopic measurements is enhanced by a large number of heating steps. The wehrlites As noted above, fusion of wehrlite HL78-3 led to the recovery of 92 ppm carbon having a 613Cof - 1.5%0(Fig. 4a). The wehrlites were noted to contain numerous fluid inclusions. Therefore, the heavy carbon is interpreted to represent COZ. Several experiments were also conducted on clinopyroxene and olivine separates (Fig. 4b) because of the inclusion-rich nature of the former. The mean carbon contents of two clinopyroxene separates (exp. 5 and 6 in Table 1) and the bulk wehrlite (exps. 1,2, and 4) are 3 14 and 192 ppm, respectively, and an olivine separate yielded 88 ppm (exp. 7). Since clinopyroxene makes up about 40% of the rock, most of the carbon is obviously contained in it. The isotopic compositions and release patterns of the whole-rock and clinopyroxene are the same. The websterites The websterites are much different isotopically than any other xenoliths from Hualalai or elsewhere in that their high-

temperature carbon is extremely light, the 6°C being in the range -22%~ to -26%0 (Fig. 5a, exp. 21, 22, 25, 26, and 28 in Table 1). The high-temperature fraction is interpreted to be composed of solid carbons trapped in interior locations of crystals isolated from their exteriors. Also, different aliquots yielded significantly different carbon contents, indicating significant bulk heterogeneities. In order to determine if a heavier carbon fraction exists in these rocks, they were treated in two different ways, both of which were designed to remove as much solid carbon as possible. First, a sample was crushed to ~400 pm grain-size and washed with a 1: 1 mixture of dichloromethane and methanol in an ultrasonic bath to dissolve organic compounds and remove graphite particles (exp. 23 in Table 1). Second, several samples were treated with phosphoric acid to leach surfaces and material in cracks into which the acid was able to penetrate (exps. 24, 27, and 29 in Table 1). Two features are evident from comparison of the experiments on leached (Fig. 5b) and unleached aliquots. First, the effect of both leaching procedures is to dramatically decrease the amount of carbon released at all temperatures. This suggests that the high-temperature carbon is dominantly in locations that, although not exposed by heating to 900”, is exposed by leaching and crushing. Second, the high-temperature carbon released from the treated samples is isotopically heavier than that released from untreated samples at equivalent temperatures and is interpreted to represent a mixture of isotopically heavy CO* and light solid material. This implies that the amount of COZ in the websterite is only about 10 ppm. The heterogeneity and effects of acid attack on isotopic composition are consistent with the presence of most of the carbon in a solid form in the prommant inclusion-rich zones, which, as described above, are unique to the websterites. When the samples are crushed, they likely fracture along such zones. During acid attack, the close proximity of most of the inclusions to the exposed surfaces and the probably higher crystal dislocation densities in these regions result in their being more readily leached. The small quantities of heavy carbon released from leached samples are interpreted to originate from smaller fluid inclusions unrelated to the prominant inclusion-rich zones. Gabbro in composite nodule HL78-2 As described above, sample HL78-2 consists of gabbro in contact with dunite and wehrlite. The gabbro shows a distinctive stepped-heating profile (Fig. 7) in that 95 ppm carbon (613C = -24.5%0) was extracted at 600°C which is significantly more than the 50 ppm typically extracted from other lithologies. The isotopic difference is interpreted to reflect different geologic histories. The gabbro is less deformed than the surrounding rocks and thus was probably emplaced subsequent to the deformation of the wehrlite and dunite. The grain boundaries in the gabbro appear dark and may contain carbonaceous matter. If the gabbroic magma was saturated with CO* at a pressure of 5 kb, extrapolation of the lowpressure solubility curve of STOLPERand HOLLOWAY(1988) indicates that the melt would have contained approximately 580 ppm dissolved carbon. Some of this carbon may have precipitated from fluid exsolved during crystallization. This

Isotope composition of carbon in xenoliths from Hualalai, Hawaii may account for the magnitude and isotopic composition of the low-temperature fraction. The remaining carbon was extracted in two well-defined peaks at 1200 and 14OO”C, at which temperature the gabbro was completely melted. The 613C of the carbon released at fusion (-8.9%0) may be the memory of the carbon originally dissolved in the melt and trapped by growing crystals. Websterite and dunite of composite nodule 71kYAP-I The carbon extracted at high temperatures from the websterite (exp. 32 and 33 in Table 1) is considerably lighter than that extracted from the dunite (exp. 34 in Table 1), even though the absolute carbon contents of both lithologies are low. This demonstrates that significant isotopic heterogeneity on the scale of centimeters can be preserved. GENERATION OF ISOTOPIC HETEROGENEITY The isotopic data together with the petrographic observations and contact relations among lithologies indicate that large and local isotopic variations exist in the mantle. The generation of small-scale variability is not an expected result of mixing isotopically light, biogenic carbon from the crust with a heavier primordial mantle reservoir because the mixing processes should yield heterogeneities on a much larger scale. Also, there is no obvious mechanism by which surlicial material could have contaminated the mantle beneath Hawaii. The scale of the isotopic heterogeneity suggests that it was generated by in situ processes. Several senarios could be devised to account for the distinctly different isotopic compositions of the websterites compared to the wehrlites and dunites. The isotopic fractionation between CO2 and graphite is such that the latter is relatively enriched in “C (e.g., BOTTINGA, 1969). Precipitation of graphite from CO2 proceeds via the reaction path 2C02 + 2C0 + O2 and 2C0 + C + CO*, which when combined gives the mass balance between CO2 and graphite as coz = c + 02.

225

that the mass of the fluid is sufficiently large compared to that of the graphite so that the compositions of both phases remain constant as precipitation proceeds, e.g., the system remains open to COz . In this circumstance graphite of - 15%0 is produced. Suppose that this graphite is trapped in the rocks and that during a subsequent event the rock is reheated to 700°C under an_fQ such that 80% ofthe graphite is vaporized to C02, again according to mass balance (1). As illustrated in Fig. 8 (case A), the compositions of the residual graphite and evolved CO2 would now be -22%0 and -14%0, respectively. Were most of the latter lost, rocks having isotopic characteristics of the websterites would have been generated. Consider the wehrlites and other xenoliths containing CO2 with 613Cof -1.5 to -5%0. The fractionation of carbon between magma and CO2 is such that the latter is enriched in 13Cby about 4.5%0(JAVOYet al., 1978). Thus, magma having an initial 613Cof -7%0 would initially generate CO2 of -2.5%0 by degassing. The 613C of CO2 emanating from Hawaiian fumaroles falls in the range of -3 to -4%0 (GERLACH and THOMAS, 1986; FRIEDMANet al., 1987), which is consistent with magma having a S13Cof -7 to -9%0, depending on the extent of outgassing. Starting with CO2 of 613C of -2.5%0 trapped in the rock, precipitation of 12% of the fluid to graphite at 700°C in a closed system would have resulted in graphite with 6r3C of -9.5%0 and CO2 of - 1.5%0(case B in Fig. 8). CONCLUSION The point of the above discussion is that multistage processes involving condensation and vaporization reactions between graphitic compounds and COz-rich fluid can generate a wide range of isotopic compositions. These simple examples were chosen to illustrate that the extent to which phase separation occurs in a closed or open system may in part determine the extent to which isotopic variability develops. There

(1)

This emphasizes the fact that the relative stabilities of fluid and graphite depend on oxygen fugacity (f0,) as well as temperature and pressure. At low to moderate pressures, a mafic or ultramafic assemblage is expected to cool down a T-fOz path approximately parallel to one of the synthetic oxygen buffers such as Ni-NiO (e.g., MATHEZ, 1990). For a system that cools in this manner, graphite will eventually become stable. It should be remembered that the solid carbons in nature are probably highly disordered and chemically complex and thus only approximated by graphite. Consider the precipitation of graphite from a C-O-H fluid containing 5% total hydrogen at 700°C and at a total pressure of 5 kb flowing through newly-formed fractures in cooler rock. Under these conditions, the 6r3C of the graphite would be about 8%0lighter than that of the CO2 (BOTTINGA, 1969) and the maximumf02 would be about 1.5 log units below that of Ni-NiO. Assume that the 613C of the CO2 is -7%0, that precipitation proceeds at a constant temperature and

0.0

0.2

0.4

0.6

0.8

1.0

C/(C+COZ) FIG. 8. The differencein 613Cbetween graphite and CO2as a function of the proportion of phases present during CO,-graphite nrecipitation or vaporization in a closed system at 700°C. In case A, vaporization by 80%of graphite having an initial 613Cof - 15 results in formation of CO* and graphite of :22 and - 14, respectively. In case B, CO2 of 6°C of -2.5 precipitates by 12% to produce CO1 and graphite of - 1.5 and -9.5, respectively.

F. Pineau and E. A. Mathez

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are undoubtably other senarios that could be devised to generate the observed variations. In fact, it is likely that the Hualalai rocks, and probably most other mantle and deep crustal xenoliths, experienced multiple deformation and recrystallization events. Therefore, caution must be exercised when using samples in which these processes are recorded to deduce large-scale characteristics of the mantle. This raises an important question, namely, do the carbon isotopic compositions of diamonds, basal&, and other mantle materials really reflect large-scale heterogeneities in the mantle, as is implicit in some theories of how heterogeneity developed, or do they reflect small-scale processes operating over small regions represented by individual or few samples? Of all elements, carbon presents a complex problem of interpretation because it may exist in both condensed (graphite, diamond, carbonate, silicate melt) and volatile phases. Therefore, its distribution and isotopic composition are extremely sensitive both to intensive thermodynamic parameters and to processes, such as deformation, that affect fluid permeability and porosity. Despite this, the vast majority of materials originating from the mantle possess 613Cvalues of -7 + 1%. This implies that processes that operate over relatively large regions, such as the generation of magma, tend to homogenize variations that exist on smaller scales.

Acknowledgments-We thank A. J. Irving and the U.S. National Museum for providing samples and D. Mattey, P. Deines, D. Clague, G. Bussod, and M. Javoy for critical reviews. The paper was improved significantly as a result of their efforts. This study was begun during a stay by EAM at the Laboratoire de Geochimie des Isotopes Stable, Universite de Paris 7 and Institut de Physique du Globe. Support from these institutions, from the CNRS (France) and from NSF grant EAR8409834 is gratefully acknowledged.

Editorial handling: J. D. Macdougall

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