Carbonaceous matter in peridotites and basalts studied by XPS, SALI, and LEED

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Carbonaceous

matter in peridotites and basalts studied by XPS, SALI, and LEED

TRACY N. TINGLE,‘,’ EDMONDA. MATHEZ,~and MICHAELF. HOCHELLAJR.‘,~ ‘Department of Geology, Stanford University, Stanford, CA 94305, USA ‘Molecular Physics Laboratory, SRI International, Menlo Park, CA 94025, USA 3Department of Mineral Sciences, American Museum of Natural History, New York, NY 10024, USA “Center for Materials Research, Stanford Unive~ity, Stanford, CA 94305. USA (Receiwd h4u.v 2 I, 1990; accepted in

revisedjimz h4mch 4. 199 1f

Abstract-Carbonaceous matter in peridotite xenoliths and basalt from the Hualalai Volcano, in a basalt glass collected directly from an active lava lake on the east rift of Kilauea, in garnet and diopside megacrysts from the Jagersfontein kimberlite, and in gabbros from the Stillwater and Bushveld Complexes has been studied by X-ray photoelectron spectroscopy (XPS), thermal-desorption surface analysis by laser ionization (SALI), and low-energy electron diffraction (LEED). The basalt and two of the four xenoliths from Hualalai and both Jagersfontein megacrysts yielded trace quantities (5 10 nanomoles) of organic compounds on heating to 700°C. Organics were not detected in the rocks from the layered intrusions, and neither carbonaceous matter nor organics were detected in the glass from the lava lake. Where detected, organics appear to be associated with carbonaceous films on microcrack surfaces. Carbonaceous matter exists as films less than a few nm thick and particles up to 20 pm across, both of which contain elements expected to be present in significant quantities in magmatic vapors, namely Si, alkalis, halogens, N, and transition metals. LEED studies suggest that the carbonaceous films are amorphous. The data suggest two possible mechanisms for the formation of the organics. One is that they are a product of abiotic heterogeneous catalysis of volcanic gas on new, chemically active mineral surfaces formed by fracturing during cooling. Alternatively, organics may have been assimilated into the volcanic gases prior to eruption and then deposited on cracks formed during eruption and cooling. in any case, there is no evidence to suggest that the organics represent laboratory or field biogenic contamination. INTRODUCTION

CRACK SURFACESIN BASALTS and many xenoliths display a peculiar bluish-pu~le coloration that probably goes unnoticed by many investi~tors. MATHEZ and DELANEY( 198 1) noted that many of the crack surfaces and vesicles in fresh submarine basalt glasses, in glass inclusions in phenocrysts in basalts, and in peridotite xenoliths in basalts displayed this peculiar iridescence. Utilizing C X-ray mapping, they found C to be ubiquitous in these samples and concluded that C was deposited on the various surfaces from magmatic vapor sometime during eruption and cooling of the basalts. This ill-defined C, or carbonaceous matter as it is commonly referred to, was suggested to be graphite, carbynes, or perhaps organic material. Later, MATHEZ (1987) showed that the carbonaceous matter in peridotite xenoliths exists as both particles (generally less than 20-30 ym across) and very thin films (no more than a few tens of nm thick). Carbonaceous particles were found to contain Si, alkalis, and halogens, which led Mathez to suggest that the particles were graphite intercalation compounds. X-ray photoelectron spectroscopic (XPS) studies of the carbonaceous films showed that the films consist of a complex mixture of species having C-C and/or C-H bonds and various C-O bond linkages, and that other elements, such as Al, Si, 0, N, Na, Cu, and Ni are sometimes associated with the films (TINGLE et al., 1990; MATHEZ, 1987). MATHEZ (1987) interpreted the C-H and C-O bond linkages as further evidence that organic compounds are associated with the carbonaceous films, which he speculated were formed by Fischer-Tropsch-like reactions of volcanic gas on new (and

thus chemically active) mineral surfaces formed by fracture during cooling. More recently, TINGLE et al. (1990) utilized very sensitive the~al~de~~tion surface analysis by laser ionization (SALI) to confirm that organic compounds are present in carbonaceous films on crack surfaces in olivine single crystals from mantle xenoliths at Hualalai Volcano, Hawaii, and San Car10s Arizona. One of the principal findings of their study was that the organic compounds were not derived from laboratory or instrument contamination or from field contamination after cooling of the host basalt. They inferred, as did MATHEZ and DELANEY(198 1) and MATHEZ (1987) that the carbonaceous films in the olivine single crystals were deposited by volcanic gases on fractures formed during eruption and cooling. As for the thermally desorbed organic material, TINGLE etal.(1990)concludedthat either organic compounds formed abiotically by catalysis of volcanic gases on active crack surfaces, as envisioned by MATHEZ (19X7), or the organics were assimilated into the volcanic gas from a crustal biogenic source prior to eruption and then deposited on newly formed crack surfaces. The present study addresses several of the important remaining questions about the origin of organic material and carbonaceous films on crack surfaces. (1) What is the structure of the C films; are they graphite, carbynes, macromolecular organic material, or amorphous C? The question is addressed by performing low-energy electron diffraction (LEED) of carbonaceous films in San Carlos olivine. (2) Are organicbearing carbonaceous films unique to olivine crystals or is any silicate mineral or glass surface suitable for formation of such films? Among the new samples studied here by XPS 1345

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SALI are microcrystalline and glassy basalts and pyroxene-rich xenoliths from Hawaii; in addition, garnet and diopside megacrysts from a South African kimberlite diatreme were analyzed. XPS is used to determine the presence of carbonaceous films, their thickness and chemistry, and the speciation of C. SAL1 is used to determine if organic compounds are present in the films. (3) Are organic-bearing carbonaceous films formed only in volcanic environments or do they form in plutonic environments where the rocks cool more slowly and the source of C is a supercritical fluid and not volcanic gas? This question is addressed by analyzing gabbros of the Stillwater and Bushveld layered intrusions where evolution of reduced C-bearing fluids previously has been documented (e.g., MATHEZ et al., 1989, and refs. therein). (4) What is the isotopic composition of C in films on crack surfaces and is this C related to the isotopically light low-temperature C observed in basalts and mantle xenoliths by stepped-heating methods (e.g., MATTEY, 1988)? To address this question, we present here the preliminary stepped-heating C isotopic analysis of carbonaceous films in San Carlos olivine. (5) Lastly, the possibility that organic compounds can be produced abiotically on the Earth’s surface has important geologic and biologic ramifications. Is a further elucidation of the origin of the organic material possible? For these reasons, additional study of carbonaceous matter in basalts and mantle xenoliths is warranted. and

SAMPLES

AND METHODOLOGY

The Hualalai rocks include websterites 65KAP-12, 63KAP-9, and HL86-1 (which contained a l-2 cm thick rind of vesicular basalt) and wehrlite HL78-3. Detailed petrographic descriptions of these samples and data on their C abundances and isotopic compositions have been presented by PINEAU and MATHEZ (1990). In addition, olivine porphyroblasts from Hualalai dunites HU-I and HU-9 of KIRBY and GREEN (I 980) were analyzed by XPS (analyses of the latter by SAL1 were reported by TINGLE et al., 1990). In all of these rocks, carbonaceous matter is present on microcrack surfaces and fluid inclusion ~~~~~(TINGLE et al., 1990; MATHEZ, 1987). In addition, the olivine porphyroblasts contain large (up to 100 pm diameter) CO*-rich inclusions. Carbon in the websterites is dominantly in megascopic inclusion-rich zones and appears to consist mainly of solid carbonaceous material rather than fluid, whereas that in the wehrlites is dominantly COZ in inclusions (PINEAU and MATHEZ, 1990). The east rift Kilauea basalt was recovered from the active lava pond near Puu Kahaualea in March 1990 with the help of Frank Trusdell (University of Hawaii). The sample was obtained by throwing a chain into the molten lava. Melt adhered to the chain, and upon cooling. chunks several cm across were sealed in polypropylene bags until they were analyzed. The Stillwater samples include a troctolite (ST8 I- 1. I), two norites (ST8 l-3.4 and ST88- 16), and a plagioclase-orthopyroxene-olivine pegmatoid (ST81-l.9), all of which are from the platinum-bearing Howland (or J-M) Reef in the Stillwater River valley, Montana. A mineralized orthopyroxene-plagiocclase-chromite pegmatoid from the Merensky Reef of the Bushveld Complex, South Africa (B5A), was also analyzed. Samples B5A and ST88-16 were collected underground in the mines. The mineralized rocks contain sulfides and small amounts ofC (MATHEZ et al., 1989: BALLHAUS and STLIMPFL, 1985). Garnet and diopside megacrysts from the Jagersfontein kimberlite, South Africa, were obtained from the MacGregor collection at the University of California, Davis. Each crystal was 2-3 cm across and the small fragments taken from the interior of these samples showed no visible alteration. Carbon abundances ofa few tens of ppmw were reported by MATTEY et al. (1989b) in other diopside megacrysts from South African kimberlite. The presence of carbonaceous matter on

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cleavage and crack surfaces was verified by C X-ray mapping utilizing an electron microprobe. Tools for handling the specimens were cleaned with successive rinses of acetone and methanol to remove organic contamination. Large crystals were fractured repeatedly (in air) to obtain interior portions having none of the original external surface. Samples were cleaved to produce nearly flat-sided fragments. These were attached to a resistively heated Ta heating stage with fine Ta or Ni wire for the thermal-desorption SAL1 experiment or to a sample stub with In foil for XPS. For both techniques, samples were then introduced to 10-9-10~‘o torr vacuum within several minutes after fracture and analysis began immediately. All exposed surfaces of any rock or mineral are potentially sites where field or laboratory organic contamination may occur. TINGLE et al. (1990)minimized this problem by using large single crystals of olivine where it was possible to remove pre-existing surfaces by cleavage and fracture. For some xenoliths in this study. the grain size did not permit us to sample individual crystals and remove all pre-existing surfaces and grain boundaries: for those samples, polycrystalline fragments were recovered from interior portions of the xenoliths away from weathered or sawcut surfaces. Organics were not detected in many of the polycrystalline samples regardless of whether they were collected at the surface or from underground mines. In those polycrystalline samples that do contain organics, the character and quantity of organic material resembles that on crack surfaces in large single crystals, and the mass spectra are unlike those anticipated from pyrolyzed biogenic material (TINGLE et al., 1990; PERIERA and ROSTAD, 1983). Several fragments of every sample were analyzed to ensure that the potential heterogeneous distribution of organics (TINGLE et al.. 1990) and carbonaceous matter did not affect our results. The thermaldesorption SAL1 mass spectra presented here are representative of the 2-5 spectra collected for each sample. Mineral separates were analyzed in coarser-grained samples, but in no case were organics observed to be present in one phase and not in another. SAL1 was performed using thermal desorplion and non-resonant single-photon ionization of the thermally desorbed neutral species. The sample, attached to a heating stage. was positioned 3-4 mm below the ion extraction cone of the time-of-flight mass spectrometer in an ion-pumped ultrahigh vacuum (UHV) chamber (BECKER and GILLEN, 1984; SCHOHLE et al., 1988). Photoionization was accomplished by passing a beam of focused vacuum ultraviolet (VUV) light 1 mm above and parallel to the sample surface. Coherent 118 nm (10.5 eV) VUV light was obtained by frequency tripling the third harmonic of a pulsed (10 Hz) Nd:YAG laser in 160 torr Ar and 16 torr Xe. This photoionization scheme is ideally suited for organic compounds because most have ionization potentials 5 10.5 eV (SCHOHLE et al., 1988; PALLIX et al.. 1989). Samples were heated from room temperature to 700-800°C in 50°C increments or linearlq at 30”C/min. Time-of flight spectra ofthe photoions analyzed during 1000 pulses of the laser were acquired continuously, each representing a 50°C heating interval. Peaks were mass-analyzed from 10 to 500 amu, with mass resolution of -750 (i.e., unit mass resolution at m/z = 750). Temperature-integrated mass spectra were computed by averaging the mass spectra from each temperature interval. It should be noted here that the sensitivity of this particular instrument has been improved by a factor of 3-5 since the data reported bq TINGLE et al. (1990) were collected. This was accomplished by replacing the I” LiF lens at the exit window of the tripling cell with a 2” MgFz lens, which has a higher transmittance for I I8 nm light and allows for better separation of the 118 nm beam from the 355 nm pump beam (SCHUHLE et al.. 1988). The SAL1 technique is described in more detail by TINGLE et al. (199 I). XPS was performed on a VG ESCALAB Mk II using non-monochromatic Al X-rays. Survey scans were collected to determine the near-surface composition of the samples, and narrow scans of the C,, energy region were collected to determine the atomic environment of C. The area of the crystal surface analyzed was roughly 9 mm*. All measurements were made in the lo-” torr range. Low-energy electron diffraction (LEED) was used to determine the structural state of surfaces analyzed by XPS. The LEED instrument. a VG model 640-2 RVL reverse view system, is mounted on the same VG ESCALAB UHV chamber in which the XPS mea-

Carbonaceous matter in mantle-derived rocks

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surements were made. Therefore, it was possible to perform LEED and XPS on the same surface without any additional exposure to atmosphere. The electron beam diameter was 0.5 mm and beam energies ranged between 50 and 200 eV. As discussed in more detail by HOCHELLAet al. (1989), low-energy electrons travel such short distances in solids that if coherent diffraction occurs, it takes place in only the top few layers of atoms. Thus, LEED is capable of determining the degree of atomic ordering of surfaces and of thin surface films, such as the carbonaceous films which are the focus of this study. LEED analysis of San Carlos olivine (010) surfaces is discussed below and in HOCHELLA( 1990). RESULTS Atomic Structure of Carbonaceous Films in San Carlos Olivine Carbonaceous films on crack surfaces in San Carlos olivine are typically 3-4 nm thick and continuous or semi-continuous over areas of several mm2 (TINGLE et al., 1990). The degree of atomic order in these films was investigated by LEED. Of every ten fragments larger than a few mm produced by fracture from these samples, about three to four will contain at least one iridescent surface, although most of these surfaces are too irregular to analyze by LEED. After obtaining a suitable fragment with a flat {0 10) surface and the peculiar iridescence, we were unable to obtain diffraction patterns of any kind at a variety of analytical conditions. The same surface was analyzed by XPS, and a very intense C,, peak indicative of a carbonaceous film was observed along with N, Al, and excess 0 as has been observed previously. Our inability to obtain a LEED pattern on the iridescent surface cannot be attributed to sample charging, an amorphous layer of adventitious contamination, or malfunction of the LEED apparatus, because LEED patterns of pristine {010) olivine surfaces were readily obtained; Fig. 1 shows a LEED pattern obtained at 117 eV. The orthorhombic pattern and calculated cell dimensions are those expected for olivine of this particular composition (Fos9). Cell dimensions calculated from Fig. 1 are a = 0.475 nm and c = 0.603 nm (? l%), in good agreement with a = 0.4762 nm and c = 0.5994 nm determined by singlecrystal X-ray diffraction for Fog0 olivine (BIRLE et al., 1968). After LEED, this same surface was analyzed by XPS and only a very small C,, peak was observed, typical of adventitious C contamination due to momentary exposure to air or to residual gases in the UHV chamber (HOCHELLA, 1988, and refs. therein). The most reasonable interpretation of these data is that the carbonaceous film on the iridescent crack surface is amorphous. Carbonaceous Films and Organic Compounds in Xenolitbs and Basal& from Hawaii XPS survey spectra for (0 10) surfaces of two olivine porphyroblasts in dunites from Hualalai are shown in Fig. 2. In both crystals, the magnitude of the C,, photoelectron peak is greater than can be attributed to adventitious C contamination. High resolution scans (not shown) of the C,, energy region are similar to those reported previously (TINGLE et al., 1990; MATHEZ, 1987) and showed a prominent peak at 285 eV with subsidiary peaks at higher binding energies. Ionsputter depth profiling experiments indicate that the surfaces

FIG. 1. LEED diffraction pattern (at 117 eV) obtained on a virgin {0 IO} surface of San Carlos olivine that did not exhibit iridescence. Subsequent XPS analysis of this surface showed only C due to momentary exposure to air. An iridescent {010) crack surface in a San Carlos crystal was examined by LEED, but no diffraction patterns of any kind could be obtained at a variety of instrumental conditions. Subsequent XPS of the iridescent surface showed an intense C,, signal indicative of the presence of a thin carbonaceous film. We interpret the absence of any diffraction from the C film to indicate that the film is amorphous.

are at least partially covered by thin carbonaceous films (<34 nm thick). Sodium, aluminum, copper, and nickel were also observed in addition to the major constituents of olivine, in agreement with XPS spectra reported previously (TINGLE et al., 1990; MATHEZ, 1987). The change in intensity of the Cu and Ni signals with sputtering suggests that these elements are present in particles (or perhaps discontinuous films) estimated to be smaller than 20 nm in diameter. Both crystal surfaces displayed the peculiar iridescence which has been noted previously (MATHEZ and DELANEY, 1981; MATHEZ, 1987; TINGLE et al., 1990). In fact, in this and the previous work (TINGLE et al., 1990), every surface (with one exception) which displayed iridescence was found to contain a carbonaceous film. The coloration of the one sample without a carbonaceous film was cobalt-blue and distinct from the typical iridescence, and its origin is unknown. Thermal-desorption SAL1 mass spectra for the Hualalai xenoliths are shown in Fig. 3. Websterites 63KAP-9 and HL86- 1 (and its basalt crust) yielded organics, whereas none were evolved from websterite 65KAP-12 or wehrlite HL783. The quantity of organic material analyzed in 63KAP-9 and HL86- 1 is estimated to be I IO nanomoles, based on the sensitivities determined by TINGLE et al. (1990, 199 1). The organic species in all samples were desorbed between 350 and 650°C. The mass peaks observed in the organic-bearing samples are similar to those previously observed from olivine single crystals from San Carlos and Hualalai. Masses (m/z) 42, 56, 70, 84, and 98 may represent an alkene series; masses 78 and 92 probably correspond to benzene and toluene; mass

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XPS Survey Spectra Volcano Olivine Porphyroblast with Basalt Rind

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alone. The survey spectrum reveals no other elements except those in the host diopside. The analyzed surface displayed faint discontinuous iridescence consistent with our interpretation that this particular surface is also partially covered by a very thin carbonaceous film. Garnet and diopside megacrysts from Jagersfontein yielded weak organic signals (Fig. 5) similar to those observed from Hualalai olivine (TINGLE et al., 1990). The organics were evolved between 350 and 600°C and consisted of masses 42, 56,70, 78,84,92, 106, and 120. The intensity of the C,, and organic signals is consistent with observations by C X-ray microscopy that carbonaceous matter on cracks is more scarce in the megacrysts than in Hualalai or San Carlos samples. Organic Compounds in Rocks from Layered Igneous Intrusions

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No organic signals above background were observed from the Stillwater and Bushveld rocks in either polycrystalline fragments or fragments from separated minerals (Fig. 6). Some spectra showed mass 64 (Sz), consistent with the presence of sulfides.

FIG. 2. XPS survey spectra of {0 10) surfaces of large olivine porphyroblasts in dunite xenoliths from the 1801 flow of Hualalai Volcano, Hawaii. Both surfaces analyzed displayed moderate iridescence; the surface represented by the upper spectra included both ohvine crystal and a small amount of basalt rind (<25’%). Auger and photoelectron lines for prominent peaks are marked. In addition to peaks corresponding to the major constituents of olivine, C, Na, Al. Cu, and Ni were observed. Ion-sputter depth profiling indicates that the C. Na, and Al exist in a thin film on the surface as has been previously observed in San Carlos olivine (TINGLEet al., 1990); similarly, depth profiling suggests that the Cu and Ni are present either as discontinuous films or as particles smaller than 20 nm.

64 is due to SZ, probably from sulfides or S in cracks; and

masses 23 and 39 may represent Na and K, although the latter may also be a hydrocarbon fragment. One spectrum (63KAP-9, Fig. 3) showed several odd mass peaks at m/z = 43,55,69,83, and 95, which previously have been observed in San Carlos olivine (TINGLE et al., 1990). Three separate fragments of the basalt crust on HL86-1 were analyzed. Organics were detected in each, and the mass spectra are similar to those of the xenoliths. The surface of one sample was visibly weathered (i.e., reddish- to yellowishbrown and covered with a very thin layer of fine-grained clays). The weathered sample was not found to be enriched in organics compared to fresh samples. Seven fragments of the Puu Kahaualea basalt were analyzed: none of these yielded organics in excess of the procedural blanks. In some samples, mass 64 (S,) was observed. The absence of organics is consistent with C X-ray microscopy, indicating that neither carbonaceous films nor particles exist in these samples. Carbonaceous Films and Organic Compounds in Jagersfontein Megacrysts

XPS spectra of a { 110) surface of a Jagersfontein diopside megacryst are shown in Fig. 4. As noted previously, the C,, peak is more intense and its detailed shape is more complex than one would expect from adventitous C contamination

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FIG. 3. Temperature-integrated thermal-desorption SALI mass spectra of small polycrystalline fragments (a few mg) of websterite and wehrlite xenoliths from the A.D. 180 1 flow of Hualalai Volcano, Hawaii. The procedural blank is shown for comparison.

Carbonaceous matter in mantle-derived

C was released at 300-400°C. Neither sample showed appreciable releases of C above 800°C. The small amount of C in excess of the system blank analyzed in the olivine sample without iridescence is interpreted as adventitous C contamination (MATTEY and TINGLE, in prep.), as has been documented here and previously (TINGLE et al., 1990; HOCHELLA, 1990). Similarly, the C released from the olivine sample with iridescent cracks contains three components: (1) system blank, (2) adventitious C contamination, and (3) carbonaceous films. If we consider only the low-temperature (
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FIG. 4. XPS survey and C,, narrow scan spectra of a

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of a large diopside megacryst from the Jagersfontein kimberlite, South Africa. The survey spectra reveals a small C,, peak in addition to the components of diopside. The intensity of the C,, peak is greater than one would expect from adventitious C contamination due to momentary exposure to air, and the detailed shape of the C,, peak indicates that C exists in more than one structural state, unlike that observed for typical adventitious C contamination. The C peak is interpreted as evidence of a very thin or partially continuous carbonaceous film on the analyzed crack surface. (The binding energy scales have not been corrected for charge shifting.)

Our LEED results suggest that the carbonaceous films are amorphous, and hence they cannot be strictly macromolecular organic material, graphite, or carbynes. Carbynes (HAYATSU and ANDERS, 198 1; HAYATSU et al., 1980) and graphite

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Preliminary C Isotopic San Carlos Olivine

Analysis

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Films in

Approximately 30 olivine crystals were cleaved and fractured to reveal iridescent surfaces (carbonaceous films). A composite sample of fragments with exposed iridescent crack surfaces was prepared, Fragments displaying what were judged to be virgin crack surfaces comprised a second sample. MATTEY and TINGLE (in prep.) performed stepped-heating C isotopic analysis of the two composite samples each weighing = 160 mg. Combustion took place in 500 mbar O2 at 100°C intervals lasting 30 min up to 1200°C. The samples were handled only with precleaned tools. In order to avoid removing or modifying the carbonaceous material, the samples were not crushed further or subjected to any of the usual precleaning treatments (e.g., MATTEY et al., 1989a; EXLEY et al., 1987) used to minimize surface contamination. The sample crucible and combustion chamber were baked-out for several hours at high temperature before these measurements were made. Prior to analyzing either of the olivine samples, the abundance and isotopic composition of the background C in the system was evaluated by analyzing the blank sample crucible. The system blank yielded 4.7 ppm total C with an average 613C = -26%0. Next, the olivine without iridescence was analyzed. The pristine olivine sample yielded 5.6 ppm C with 613C = -26%0 with a well-defined release at 500-700°C. Finally, the olivine with iridescent cracks yielded 10.2 ppm C with 613C = -29%0; 40% of that

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FIG. 5. Temperature-integrated thermal-desorption SAL1 mass spectra of interior portions of garnet and diopside megacrysts from the Jagersfontein kimberlite, South Africa. These data were collected with the older tripling cell. Although the intensity of the organic signals are weak, they are consistently and reproducibly greater than background levels. The intensity of these signals is also consistent with our observations by C X-ray microscopy that carbonaceous material on cracks is less abundant in the megacrysts than in xenoliths from alkali basalts.

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FIG. 6. Representative temperature-integrated thermal-desorption SAL1mass spectra of mineral separate and polycrystalline fragments of Stillwater and Bushveld troctoIites and norites collected from horizons mineralized with F’t-groupelements. Blank strip heater is shown for comparison. None of the samples analyzed yielded organics in excess of the procedural blanks. Some samples showed mass 64 corresponding to Sz from thermal decomposition of sulfide minerals.

intercalation compounds (MATHEZ, t 987) remain intriguing possibilities for the carbonaceous particles. One can imagine a number of possible biogenic sources for the organics associated with carbonaceous films, so we must proceed to consider these. First, laboratory (including instrument) contamination has been excluded by careful analytical procedures and by frequent analysis of blanks (see also, TINGLE et al., 1990). Second is the possibility that lavas and xenoliths have been contaminated after eruption and cooling. TINGLE et al. (1990) showed that cracks containing carbonaceous films in San Carlos olivine were not permeated by organic solutions at ambient conditions for periods up to several months. More impo~antly, a post-eruptive contamination should be pervasive, and yet it has been observed that organics are totally absent from some samples of xenoliths containing numerous cracks from the same locality (xenolith beds in the AD 180 1 Hualalai flow) as samples with organics. A post-eruptive biogenic contamination, introduced via aqueous surface fluids or otherwise, does not appear to be plausible. The most likely agent for introducing organic matter is the volcanic gas present during eruption and cooling at the surface. Third is the possibility that the originaf volcanic gas contained organic com~unds from biomatter consumed by lava (e.g., GRAEBERet al., 1979), and these compounds were then deposited on mineral surfaces during cooling. Unsubstituted polycyclic aromatic hydrocarbons which are typical of pyrolyzed vegetation (PEREIRA and ROSTAD, 1983) have not been detected in any samples. Our detection limit for these compounds is = 1 picomole (TINGLE et al., 1991), and the total amount of organic material extracted during thermal desorption is estimated to be I-10 nanomoles (TINGLE et al., 1990). Yet a fourth possibility is that biogenic material was assimilated into the volcanic gas prior to eruption. Some recent examples of this include volcanic gases of the Green Tuff

region of Japan (SAKATA et al., 1989), hydrothe~~ fluids of the Guaymas Basin (SIMONEIT et al., 19SS), and steam from geothermal fields of the western US (DESMARAISet al., 198 1). The C isotopic composition of light hydrocarbons in these systems is typically much less than -30%0 613C and shows the characteristic decrease in 613C with increasing C number typical of thermal degradation of biomatter (e.g., DESMARAISet al., 198 I). In addition, biomolecules have been identified in the hydrothermal fluids at Guaymas Basin and in the volcanic gases of the Green Tuff region. If organic material was present in the volcanic gases at Hualalai during the AD 180 1 eruption, it is di~cult to explain the presence of organics in some samples and the absence of organics in other samples, all collected from an area less than several square meters. The weight of the present data suggests to us that the organics were produced abiogenically, although a biogenic origin cannot be dismissed at this time. The chemical environment of most basaltic eruptions is much too oxidizing for graphite to be stable, and this raises the obvious question of why reduced carbonaceous material (amorphous C and organics) exists in them at all. Lavas and especially xenoliths experience extensive cracking during quenching at the Earth’s surface. It is reasonable to assert that these new crack surfaces must be chemically active and therefore represent good substrates on which permeating volcanic gas may react. Whether or not complex carbonaceous compounds actually attain megascopic (i.e., thermodynamic) stability in cooling lava, they may be stabilized in the local environment of the new crystal surface. Phase stabilization by surface energy is a well-known phenomenon, a recent example being the formation of microdiamonds at low pressure (e.g., BADZ~AGet al., 1990). Similarly, reduced carbonaceous matter may be stabilized on crack surfaces in basalts and mantle xenoliths. Ifone accepts this hypothesis, then the absence of organics in some Hualalai xenohths must be due to the presence ofsome chemical species (or phase) in those particular rocks that inhibits their formation or cont~butes to their destruction; our studies to date provide no indication of what that species might be. No organics were detected by SALI and no C films or particles were detected by C X-ray microscopy in the glass collected from the molten lava pond on the Kilauea east rift. Either the specific conditions necessary for the formation of carbonaceous material on crack surfaces did not exist in the sample before it was removed from the lava lake or fresh basalt glass surfaces are not good reaction substrates. Experiments with mineral and glass surfaces to test their catalytic properties for production of organic compounds, as suggested by TINGLE et al. (1990) will probably provide the most meaningful test of the abiotic synthesis hypothesis. The organics detected by SAL1 in megacrysts from kimberlite are interpreted to be associated with the carbonaceous material identified on crack surfaces by XPS and not with graphite and serpentine alteration (PASTERIS,198 1) because optical examination revealed no evidence of alteration in the analyzed fragments. Our tentative conclusion is that crack surfaces in diopside and garnet are suitable substrates for formation of carbonaceous material and organic compounds. However, the possibility that post-emplacement crustal fluids permeated these samples and deposited carbonaceous matter on crack and cleavage surfaces cannot be ruled out.

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Carbonaceous matter in mantle-derived rocks The lack of organic compounds in rocks from the layered intrusions is curious. The Stillwater and Bushveld complexes locally contain megascopic masses of graphite (BALLHAUS and STUMPFL, 1985; VOLBORTHand HOUSLEY, 1984); unidentified C compounds on microcrack and grain boundary surfaces also have been observed in these rocks (MATHEZ et al., 1989). In the case of the Stillwater, most olivine-bearing rocks are partially serpentinized, and small amounts of highly reduced fluid containing methane and possibly higher hydrocarbons may be produced during serpentinization (FROST, 1985). In fact, methane has been released during mining activities in the Bushveld complex, even where serpentinization is less intense (BUNTIN et al., 1985). During slow cooling, the Stillwater and Bushveld rocks experienced continuous chemical and textural reequilibration, and therefore it is likely that the compositions of the C-O-H fluids and solid carbonaceous compounds in them were primarily controlled by the massively dominant crystalline assemblages. Apparently, the specific conditions were such that only graphite (and possibly amorphous C) and low molecular weight hydrocarbons were preserved. The latter components being gaseous must be restricted to fluid inclusions in minerals (e.g., KONNERUPMADSENet al., 1988) and other small voids. These are probably inhomogeneously distributed throughout the rocks and simply may not have been represented in our samples. The 613C of carbonaceous films in San Carlos olivine is close to -32%0 (MATTEY and TINGLE, in prep.). Measuring the abundance and isotopic composition of carbonaceous matter in basalts and mantle xenoliths is complicated by adventitious C contamination and instrument background C. Due to these complications, the presence of an indigenous low-temperature C component in any of the previous steppedheating analyses of C in basalts and mantle xenoliths (see MATTEY, 1988, and refs. therein) cannot be established with any confidence without additional studies. However, it is clear from the new data presented here that an indigenous lowtemperature C component does exist in some samples. If present in amounts similar to those analyzed here in the San Carlos olivines (=4 ppm), carbonaceous films would account for less than 5% of the total C in most of these rocks. The inferred 613Cof -32%0 for carbonaceous films is consistent with either derivation from thermally degraded biomatter or the abiotic origin advocated here (TINGLE et al., 1990). On the one hand, hydrocarbons in volcanic gases and hydrothermal fluids approach -30%0 (DESMARAIS et al., 1981; TINGLE et al., 1990). On the other hand, very large carbon isotopic fractionations are expected to occur with synthesis of hydrocarbons by Fischer-Tropsch-like processes. Extrapolating the 613Cfractionation between carbonate C and organic C observed by HAYATSUand ANDERS(198 1) in their study of abiotic synthesis and origin of organic matter in meteorites to the formation of carbonaceous films implies that carbonaceous films form at temperatures of a few hundred degrees centigrade, a rather realistic estimate in light of the large uncertainties associated with the extrapolation. An additional uncertainty is that the proportion of organic C relative to amorphous (and presumably elemental) C is poorly known, and the C isotopic composition of these two components may not be equal. Previously, TINGLE et al. ( 1990) assumed that all the thermally desorbed C was organic and thus estimated the organic C to be lo-50% of the total

C in carbonaceous films. It is conceivable that the carbonaceous films may be a mixture of C from catalysis of volcanic gases on crack surfaces (amorphous C) and C assimilated into the volcanic gas from a crustal biogenic source and deposited on crack surfaces (organic C). CONCLUSIONS Carbonaceous matter is common in basalts and mantle xenoliths (including xenocrysts in kimberlite) where it exists as particles and films on crack surfaces, grain boundaries, and the walls of fluid inclusions. Carbonaceous films are amorphous and typically less than a few nm thick; their chemistry is characterized by the presence of Si, Al, alkalis, halogens, and/or transition metals, all of which were probably present in the volcanic gas. Organic compounds are associated with carbonaceous matter in some, but not all, samples. Where found, the organics represent neither laboratory nor field contamination. We believe that the present data favor an abiogenic origin for the organics whereby Fischer-Tropschlike reactions involving volcanic gases and fresh fractures (formed during eruption and cooling) produced carbonaceous films and their associated organics, as hypothesized by MATHEZ (1987). However, deposition of organic compounds having a crustal biogenic origin directly from volcanic gas cannot be ruled out. The association of organics with carbonaceous films on cracks appears to be a phenomenon restricted to erupted materials; samples from the Stillwater and Bushveld layered intrusions do not contain organics despite the fact that they contain graphite and evidence for the presence of reduced C-bearing fluids during crystallization. However, not all erupted materials contain carbonaceous material on cracks and vesicle or fluid inclusion walls, suggesting that the physical conditions required for the formation of carbonaceous films and organics do not exist in all volcanic eruptions. These observations are all consistent with the catalytic hypothesis. Acknowledgments-This research has been supported by the Gas Research Institute under GRI Research Grant 5087-260-1626 (to MFH and TNT) and the National Science Foundation under NSF EAR89-03645 (to EAM). The authors would like to thank H. W. Green II for the loan of Hualalai specimens HU- I and HU-9, I. D. MacGregor, A. A. Finnerty, and R. Schaal for help in obtaining the Jagersfontein megacrysts, and F. Trusdell for help in sampling the lava pond on the east rift of Kilauea. P. Schiffman and M. Giaramita assisted with the C X-ray microscopy performed at the University of California, Davis. Discussions with C. H. Becker, R. Malhotra, and P. Westcott have helped us to develop the ideas expressed in this paper. Technical assistance from L. Jusinski, B. Olsen, and S. Young are also gratefully acknowledged. We are especially grateful to D. P. Mattey for collaborating with us on a study ofthe isotopic composition of C in carbonaceous films. Thought-provoking reviews by B. E. Taylor, J. D. Pasteris, and D. J. DesMarais were particularly helpful to us in clarifying our ideas and presentation of results; their time and effort in formulating constructive criticism were appreciated very much. Editorial handling: B. E. Taylor

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