Organic compounds on crack surfaces in olivine from San Carlos, Arizona and Hualalai Volcano, Hawaii

0016-7037/90~3.00+ .CKl

Geochimica et Cosmochimica Acfa Vol. 54, pp. 417-485 Copyright 0 1990 Pergamon Press in U.S.A.

plc.Printed

LETTER

Organic compounds on crack surfaces in olivine from San Carlos, Arizona, and Hualalai Volcano, Hawaii TRACY N. TINGLE,‘**MICHAELF. HOCHELLAJR., *s3CHRISTOPHERH. BECKER,’and RIPUDAMANMALHOTRA~ ‘Molecular Physics Laboratory, SRI International, Menlo Park, CA 94025, USA 2Department of Geology, Stanford University, Stanford, CA 94305, USA %enter for Materials Research, Stanford University, Stanford, CA 94305, USA 4Chemistry Laboratory, SRI International, Menlo Park, CA 94025, USA (Received September 27,

1989; accepted in revised form December 20, 1989)

Abstract-Organic compounds associated with thin carbonaceous films on crack surfaces have been detected by thermal-desorption photoionization mass spectrometry in large single crystals of olivine from San Carlos, Arizona, and Hualalai Volcano, Hawaii. Alkalis, silicon, aluminum, and halogens are also present in the 3-4 nm thick carbonaceous films. The organics probably were not derived from the upper mantle or lower crust or from environmental biogenic contamination after eruption and cooling. It is likely that the carbonaceous films and organics were deposited or formed on crack surfaces created during eruption and cooling of the host alkali basal&. Whether the organics were produced abiotically by FischerTropsch-like reactions involving volcanic gases and fresh-fractured surfaces where reduced carbon was deposited, or whether the organics represent biogenic material that was assimilated into the magmatic system prior to or during magma ascent, cannot be ascertained at this time due to their low abundance. INTRODUCI’ION

spectroscopy (XPS) have revealed thin (a few nm thick) carbonaceous films and particles on crack surfaces in diopside and olivine; Na, K, Al, Si, Cl, and Ca are present (see also, GREEN, 1979) in addition to reduced C whose structural environment is characterized by C-C and/or C-H bonding (MATHEZ, 1987; this study). Based on these XPS data, MATHEZ( 1987) speculated that the carbonaceous films consist of organic compounds and the particles consist of graphite intercalation compounds containing alkalis, silica, and halogens. The discovery of organic compounds on crack surfaces in olivine is unexpected because, as will be shown later, there were no obvious sources of contamination. Recently, it has been shown that many volcanic gases and hydrothermal fluids contain hydrocarbons (predominantly methane, propane, butane, and ethane have been analyzed), and among these there are examples of biogenic and abiogenic derivation ( GRAEBERet al., 1979; WELHAN,1987; SIMONEITet al., 1988; KONNERUP-MADSEN,1989; SAKATAet al., 1989). Sensitive detection of the trace quantities of organics in the crystals analyzed here ( c 10 I4 organic molecules/mg olivine) was achieved using thermal-desorption SALI (surface analysis by laser ionization), a new technique that combines single- or multiphoton ionization of desorbed neutral atoms and molecules with analysis by time-of-flight mass spectrometry. Such low concentrations of organic molecules are difficult, if not impossible, to analyze by the principal methods used to discriminate the origin of hydrocarbons, namely molecular C isotopes ( DESMARAIS et al., 198 1) and structural identification of specific compounds. Although much new information about the character of the ubiquitous carbonaceous matter in mantle xenoliths is reported here, it is not possible

IT IS GENERALLYTHOUGHT that carbon in the Earth was derived from chondrites in which C exists as elemental C, organic compounds, and trace amounts of carbonates. The extent of modification of these primordial forms of C during accretion and differentiation of the Earth is poorly known. Approximately 7 X 10 *’ mol C are present now at the surface (sediments, biosphere, atmosphere, and oceans) as a direct consequence of degassing of the Earth (JAVOY et al., 1982). Each year, an estimated 10” mol C or more are degassed from the Earth’s mantle where an estimated 10z3 mol C remains (JAVOYet al., 1982; MARTYand JAMBON,1987). How magmatic and tectonic processes have influenced the modern distribution and concentration of C in the Earth remains a controversial subject, and, clearly, a detailed knowledge of the Earth’s C geochemistry is important for understanding the history of the Earth. The results reported here are only a small piece of the C geochemical puzzle, but a necessary one for a complete description of C evolution in the Earth. Organic compounds have been detected in large olivine single crystals from San Carlos, Arizona, and Hualalai Volcano, Hawaii, by thermal-desorption photoionization mass spectrometry. It is shown that these organics are not laboratory or environmental contamination. Rather, these organics are correlated with the carbonaceous material that exists on nearly all crack surfaces and grain boundaries in mantle-derived xenoliths and basal& Previously, MATHEZ( 1987; MATHEZ and DELANEY, 198 1) proposed that this reduced carbonaceous material was deposited by volatiles degassed from the host magma during eruption and cooling at the Earth’s surface. X-ray microscopy (Fig. 1) and photoelectron 471

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Foci. 1. Corresponding scanning electron and C X-ray micrographs of (a. b) a large fluid Inclusion m olivine fronHU-1 (KIRBY and GREEN, 1980). a dunite xenolith collected from the A 11 1x01 basalt flow at Hualalai Volcano Hawaii, and (c, d) a large single crystal of olivine from San Carlos, Arizona. The concentrations of white dots in (h I and (d) correspond to locations where C Kru X-rays were detected. In (b) the white area corresponds to a carbonaceou\ particle in the fluid inclusion. X-ray energy-dispersive analysis did not detect Ca or Mg. and so it likely is not :I carbonate. The white areas shown in (d) correspond to carbonaceous material (films) on the cxposcd surface or on cracks obliquely intersecting the exposed surface

at present sociated

to unambiguously

determine

the origin of the as-

organics. METHODS

Large ( l-2 cm) single crystals of olivine were obtained from San Carlos, Arizona (FREYand PRINZ, 1978)) and the 180 1 Kaupulehu flow of Hualalai Volcano, Hawaii (KIRBYand GREEN. 1980). One batch of the San Carlos olivine crystals had been tumble-polished and contains crystals with no optically visible solid or fluid inclusions or cracks. A second batch of crystals contains cracks and CO2 fluid inclusions. The Hualalai crystals ( porphyroblasts removed from dunite xenoliths) contain numerous fluid CO* inclusions and microcracks (ROEDDER, 1965; KIRBY and GREEN, 1980). Tools for handling the specimens were cleaned with successive rinses of acetone and finally methanol to remove organic contamination. Initially, the external surfaces of the crystals were cleaned similarly. Later, this step was determined to be unnecessary because the crystals were always fractured repeatedly (in air) to obtain interior portions with no vestige of the original external surface. Fracture along the (010) cleavage plane commonly produced nearly flat-sided fragments which

were attached to a heating stage (resistively heated ‘Ta strip) with tine Ta or Ni wire for the thermal desorption experiment. or to a sample stub with In or Pt foil for X-ray photoelectron spectroscopy. For both techniques, samples were then introduced to ~10 -’ Torr vacuum within several minutes after fracture and analysis began immediately. Single-photon ionization of the thermally desorbed neutral species was accomplished by passing a beam of focused vacuum ultraviolet (VUV) light 1 mm above and parallel to the sample surface, which itself 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; SCH~HLE et al., 1988 ). Coherent 1 I8 nm (10.5 eV) VUV light was obtained by frequency tripling the third harmonic of a pulsed ( IO 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 2 10.5 eV ( SCHWLE et al., 1988). Samples were heated from room temperature to 7OO’C in 50°C increments. At each temperature, a spectrum of the photoions analyzed during 1000 pulses of the laser was acquired. Peaks were mass-analyzed from IO to 500 amu, with mass resolution of =750. Temperature-integrated mass spectra were computed by averaging the mass spectra from each temperature in-

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Organic compounds on crack surfaces in olivine terval. The sensitivity and general nonselectivity of the ionization technique has been demonstrated previously with stimulated desorption of organic compounds (SCHOHLE et al., 1988) and bulk polymers ( PALLIXet al., 1989)) and with thermal desorption of organic compounds in the Murchison and Allende carbonaceous chondrites (TINGLEand BECKER,unpubl. data). Although the technique is extremely sensitive (detection limits for most organic compounds are in the picomole range; TINGLEand BECKER,unpubl. data), it is difficult to quantify the organic abundance from the thermal-desorption mass spectra. Lack of knowledge regarding photoionization cross-sections, volatility of the various organic compounds, dimensions of the analyzed surfaces, and sample heterogeneity precludes anything more than the semi-quantitative estimate presented later in the discussion. X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB Mk I1 using non-monochromatic Al X-rays. Survey scans were collected to determine the near-surface chemistry 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 mm2. RESULTS First, it is essential to show that fracture in air and the UHV atmosphere do not introduce significant organic contaminants to our samples. To demonstrate this, a crack- and inclusion-free single crystal from the San Carlos tumble-polished stones was analyzed; a typical spectrum of the heating stage with no sample is shown for comparison (Fig. 2a). The peak at mass 17 corresponds to residual ammonia (NH!) in the vacuum chamber. The smaller peaks observed from

the crack-free crystal probably correspond to incidental contamination due to fracture in air or to low-level contamination in the sample introduction system (note that HZO, COZ, and CH4 are not detected because their ionizations potentials are greater than 10.5 eV). The blank sample heater was routinely analyzed after one or two organic-bearing samples to ensure that the system background was kept to a minimum and no cross-contamination of samples occurred. The XPS survey spectrum of a similar crack- and inclusionfree olivine crystal from San Carlos is shown in Fig. 3a. As expected, 0, Mg, Si, and Fe photoelectron peaks are seen, along with 0 and Mg Auger peaks, X-ray satellites, and energy loss features. The only other feature in the spectrum is a very weak C photoelectron line that is commonly seen on silicate surfaces that have been exposed to air, due to the sorption of CO1 and other C-containing gas molecules ( HOCHELLA et al., 1986; MATHEZ, 1987). This contamination, which also includes oxygen and water and is probably only one or two monolayers thick, is consistent with the small amount of organic contamination observed by thermal desorption (Fig. 2a). XPS spectra of surfaces from samples fractured to expose pre-existing cracks in large single crystals are markedly different than spectra from crack-free crystals which have been fractured to expose a virgin surface. In a typical XPS survey scan of a crack surface in San Carlos olivine (Fig. 3b), the 0, Mg, Si, and Fe lines are still present (Mg, Si, and Fe are

Ib)

(cl

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0

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loo

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(a)

FIG. 2. Temperature-integrated thermal-desorption mass spectra; (a) blank heater (lower) and tumble-polished olivine from San Carlos with no cracks or inclusions; (b) interior fragment cleaved from San Carlos olivine with microcracks; (c) another cleavage fragment from same olivine crystal sampled in (b); (d) fragments of San Carlos olivine containing only healed cracks. Prominent mass peaks analyzed are indicated. Mass assignments are discussed in the text; (e) interior portions from two separate Hualalai olivine porphyroblasts.

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FIG. 3. X-ray p~ot~~~tr~~ spectra oftin Cark)s &vine surfaces: fa) (010) surface in tumb~-plushy crystal with no cracks or inclusions; (b) iridescent crack surface (010) with carbonaceous film: (c)same crystal surface after I min of 1 keV Ar ion sputtering. Photoelectron and Auger lines for the constituent peaks are indicated.

reduced relative to 0), but the C line is two orders of magnitude more intense than that observed from the crack-free specimen (Fig. 3a). En addition, weak N, Na, and Al fines are present in Fig. 3b. Figure 3c shows the same surface afier 1 min of 1 keV Ar ion sputtering. The C peak is considerably reduced relative to the major olivine constituent peaks and the N peak is gone; the Al and Na lines remain essentially unchanged, From these results, we can infer that this crack s&ace is covered with a continuous (or ~mi-continuous) thin film, a few nanometers thick, with C as its principal

constituent (note that H cannot be detected with XPS). The film also contains 0 and smaller amounts of N, Na, and Al, similar to the chemistry of carbonaceous particles on crack surfaces anaiyzed by MA~~HEZ( 1987 ). XPS survey spectra of Hualalai olivine are similar to spectra of San cd0s olivine, except that Cu and Ni are also present in the carbonaceous films. In another experiment, the XPS survey spectrum of a cfeavage fragment 0fSan Carjos &vine indicated the presence of a carbonaceous film. The crystal was heated in UHV (in

Organic compounds on crack surfaces in olivine the XPS instrument without additional exposure to air) to 500°C for 4 h and then reanalyzed. The intensity of the Cl, photoelectron line was reduced by 50-60%. The intensity of the remaining C peak was still significantly greater than the C signal resulting from adventitious surface contamination. Not all crack surfaces from crystals with cracks yield an intense C photoelectron signal indicative of the presence of a carbonaceous film. It is obvious when a large crystal is fractured that many virgin crack surfaces are formed as we attempt to expose pre-existing cracks (those formed during eruption and cooling). Experience has shown that iridescent crack surfaces always yield intense C signals, whereas crack surfaces lacking iridescence generally do not. Although it is tempting to attribute the iridescence to the presence of the carbonaceous film, it was noted that the iridescence persisted after heating to 500°C in UHV and soaking in organic solvents and weak acids. These XPS results complement MATHEZ’S (1987) XPS study, in which he documents the ubiquity of carbonaceous matter in mantle xenoliths from several localities. MATHEZ ( 1987) reported narrow scans of the C,, energy region for carbonaceous films on olivine and diopside crack surfaces, which revealed a prominent peak at 285 eV, and smaller peaks at higher binding energies convoluted with the main peak. Mathez interpreted the smaller peaks at binding energies above 285 eV to represent various C-O bond linkages; the main peak at 285 eV is characteristic of C-C and C-H bonding. On the basis of these data, Mathez suggested that organic compounds existed in the carbonaceous films. Narrow scans ofthe Cr, region in the San Carlos and Hualalai olivine completed by us are similar to those reported by MATHEZ( 1987). Until now, more specific information about the forms of C present has been lacking. Our purpose in analyzing crack surfaces in olivine with thermal-desorption photoionization mass spectrometry was to test specifically for the presence of organic compounds. A fragment from the interior portion of a San Carlos crystal with cracks produced the temperature-integrated mass spectrum in Fig. 2b; mass (m/z) 212 desorbed from 50-200°C; masses 105, 106, and 122 desorbed from 250-400°C; and the majority of the organic signal, evolved above 400°C was represented by masses 42, 56, 70, 78, 84, 92, 98, and 108. Also, the intensity of mass 17 increased dramatically above background levels in the vacuum chamber above 400°C. A second cleavage fragment from the same crystal produced the mass spectrum in Fig. 2c. No low-temperature species were observed and the prominent masses analyzed above 35O’C include 32, 43, 55, 60, 70, 73, 85, 98, 114, and 126 (Fig. 2~). Such heterogeneity in a single crystal has been observed for other San Carlos crystals; these two spectra are judged to be representative of San Carlos olivine. Interior portions of two separate Hualalai olivine crystals produced the mass spectra in Fig. 2e. The intensities of the organic signals from the Hualalai crystals are consistently a factor of two less than those of the San Carlos samples, yet still in excess of our blanks (Fig. 2a). Peaks at masses 42, 56, 70, 84, and 98 were evolved above 300°C. Most of the organics detected in the San Carlos and Hualalai crystals were low molecular weight species desorbed above 300°C. In general, low molecular weight compounds

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are relatively volatile and would probably desorb from crack surfaces at room temperature in UHV. Most likely, the analyzed species are pyrolysis products of higher molecular weight compounds or macromolecular organic material. Alternatively, if the analyzed species correspond to the actual species that exist in the carbonaceous films, then the relatively high temperature of desorption implies that opening of cracks during heating, or diffusion out of the film, controls the desorption process. Regardless, the thermal-desorption mass spectra do not contain detailed information about the nature and origin of the organic compounds on crack surfaces in these crystals. Any mass assignments must be regarded as tentative because our technique does not allow us to determine uniquely the atomic composition or structure of the thermally desorbed species. Nevertheless, some possible assignments for the analyzed peaks are: mass 17 is assigned to ammonia; masses 42, 56, 70, 84, 98, and 112 (Fig. 2b, e) may correspond to an alkene series (butene, propene, etc.); masses 78 and 92 (Fig. 2b, e) may be assigned to the aromatic compounds benzene and toluene; masses 94 and 108 (Fig. 2b) may represent phenol and cresol ( or benzyl alcohol ) . Masses 106 and 122 (Fig. 2b) may represent benzaldehyde and benzoic acid; masses 43 and 55 (Fig. 2c) may represent alkyl fragments (butyl and propyl). Mass 39 (Fig. 2e) may represent IS, and mass 64 (Fig. 2d) is probably Sz, presumably derived from sulfide inclusions. The absence of mass 256 corresponding to Ss suggests that the S is not present in elemental form. San Carlos olivine contains two distinct generations of cracks; the first generation is produced in the lower crust by stress in the presence of CO1 ( ROVETTAet al., 1986). These early cracks are recognizable now as semi-circular planar arrays of fluid inclusions produced by diffusional crack healing ( WANAMAKERand EVANS, 1985). The second generation is probably formed during or after eruption at the Earth’s surface due to thermal stresses produced by cooling ( MATHEZ, 1987 ) . The presence or absence of organics on healed cracks, because they have trapped an early evolved magmatic fluid, provides an important constraint on the origin of organics in these crystals. By careful sampling and fracturing, we obtained two San Carlos crystals (from the tumble-polished stones) which contained only healed cracks. In one sample, the healed crack had formed on (0 10) and it was readily exposed by cleaving. In the second sample, the healed crack was oblique to, but intersected, one of the surfaces formed during fracturing. We failed to detect organic compounds in both samples (Fig. 2d). The absence of organics is consistent with our observations by X-ray microscopy that healed cracks do not contain carbonaceous films or particles. Having demonstrated that organics are associated only with the later-formed cracks, it is necessary to determine if these cracks, produced during eruption ( MATHEZ, 1987), remain “open” after cooling. To test the potential permeability of the later-formed cracks to environmental biogenic contamination, crystals from San Carlos containing these cracks were soaked in a solution of acetone with dissolved anthracene and diphenyl-anthracene. The same procedures were employed to sample interior portions of the doped crystals as previously described. Crystals analyzed after doping for up to 6 months did not yield masses 58, 178, or 330 correspond-

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ing to our dopant solution, and, significantly, the organic mass spectra depicted in Fig. 2b and c were reproduced. We interpret this result to mean that the later-gene~tion cracks are now sealed and probably have remained sealed since cooling. Similarly, crack-bearing San Carlos crystals were heated in a hydrothermal vessel at 450°C for 4 days in benzene saturated with coronene. The autogenous pressure inside the vessel was x 150 bars. After exposing interior crack surfaces in the heat-treated crystals by fracture, it was noted that many cracks displayed an intense dark discoloration. XPS survey spectra showed that these crack surfaces were covered with carbonaceous films a few nm thick. Narrow scans revealed a single C,, peak centered at 285 eV. Benzene and coronene were detected in the carbonaceous films by SALI. Thus, it appears that the later-formed cracks are permeable to organicbearing fluids and gases at temperatures of a few hundred “C. DISCUSSION Organic ~m~unds have been detected in ohvine crystals with cracks on which carbonaceous films are known to exist. The carbonaceous films are a few nm thick and contain alkalis, silica, and halogens in addition to reduced C species, Much of the C in these films can be volatilized on heating to 500°C. Analyses of crack- and inclusion-free olivine crystals indicate that our analytical procedures do not introduce significant organic contaminants. The cracks containing carbonaceous films and organic compounds are not permeable to a room-temperature organic solution, and so it seems unlikely that the organics we have detected were derived from an environmental (biogenic) source after cooling. Furthermore, organic com~unds have not been detected on healed cracks, and so we can establish that the organics were introduced sometime during eruption and cooling, probably synchronous with the deposition of the carbonaceous films. The organics and carbonaceous films an crack surfaces in olivine probably were not produced by C dissolved in the crystals that segregated to and reacted on the crack surfaces, as proposed by FREUND et al. ( 1980 f . The high-tem~rature solubility of C in olivine at low and high pressures is probably less than 30 ppmw C or else the diffusivity is less than IO-” m */s (TINGLE et al., 1988>, in agreement with the findings of MATHEZet al. ( 1984,1987) and TSONG et al. ( 1985 ) that mantle-derived olivine crystals, such as those from San Caries, contain less than 25 ppmw dissolved C. These three independent investigations contradict the claims of Freund and others ( FREUND et al., 1980; OBERHEUS~Ret al., 1983) that atomic C is both soluble (> 100 ppmw C) and highly mobile (> 10c8 m’/s) in olivine. The most reasonable explanation of the data of Freund and his coworkers is that the highly mobile C species analyzed by them is due to adventitious C contamination (TSONG et al., 1985 ). The most obvious medium for introducing organics along cracks would appear to be volcanic gas. Of course, vOlCaniC gases are composed predominantly of Hz0 and CO2 with only minor amounts of reduced C species, like CO and CH4. Do reactions occur in the C-D-H gas phase or on the crack surfaces to produce hydrocarbons, as envisioned by MATHEZ

( I987 )? Alternatively, is biogenic material somehow incorporated into the volcanic gas prior to or during eruption? Several examples of abiogenic and biogenic hydrocarbons in magmatic systems have been documented recently, Hydrocarbons (methane. propane, butane, ethane) in hydrothermal fluids emanating from the East Pacific Rise ( WELHAN, 1987) and hydrocarbons in fluid inclusions and “bitumen” disseminated in alkaline intrusions at Ilimaussaq, Greenland. and the Kola Penisula and Khibiny, USSR f KONN~RUP-MADSEN, 1989 ) , are examples of abiogenic derivation. Examples of hydrocarbons in magmatic systems of biogenic origin include volcanic gases in the Green Tuff region of Japan ( SAKATA et al.. 1989), hydrothermal fluids in the Guaymas Basin ( SIMONEITet al., I988 ), steam from geothermal areas. the Geysers, Yellowstone, and Cerro Prieto (DESMA~IS et al., i 98 t ), and hydro~r~ns in volcanic gases collected from basalt flows that have overrun vegetation (GRAEBERet al., 1979). The distinction between biogenic and abiogenic derivation in these studies is based on the identification ofbiomolecules and/or molecular C isotope data. DESMARAISet al. ( 198 I ) demonstrated with the latter technique that hydrocarbons produced by thermal degradation of organic matter are characterized by increasing 6°C with increasing C number, whereas abiotically produced hydrocarbons are characterized by decreasing 613Cwith increasing C number. Also. C isotope values below - 20%0(relative to PDB) are generally indicative ofbiogeni~ de~vation, although this criterion is by no means rigorous. MUENOW( 1973 ) reported hydrocarbons associated with gases trapped in Pele’s tears. Although he seems to conclude that they were abiotically produced, his interpretation probably could be verified by molecular C isotope data. It seems unlikely that the organics in the Hualalai crystals were derived from combusted vegetation (e.g., GRAEBERet al., 1979) for the following reasons. First, although the A.D. 1801 eruption almost certainly overran vegetation, the xenoliths were extruded only after a large volume of lava had effused. Field evidence suggests that the xenoliths were transported downslope in a lava tube in the flow and therefore were never exposed to surface biogenic matter (BOHRSON and CLAGUE, 1988). Second, the particular dunite xenoliths from which our crystals were taken exhibit abundant evidence of volatile degassing (KIRBY and GREEN, 1980): many of these xenoliths are enclosed by a highly vesiculated rind of basalt. Hence, volatiles were moving away from. and not towards, these xenoliths. Third, oombusted biogenic matter consists largely of unsubstituted polycyclic aromatic compounds (PEREIRA and ROS~AD, 1983), which we have not detected in any of our samples. The greatest hinderance to determining the origin of organics on crack surfaces in the San Carlos and Hualalai olivine is their low abundance, which precludes identification of possible biomolecules or me~u~ment of molectdar C isotope ratios by the conventional methods. As pointed out earlier, a quantitative analysis of the thermal-desorption mass spectra is not possible, but it is possible to place some broad constraints on the abundance of organics. Analysis of the Murchison meteorite by thermal-desorption SAL1 for mass 178 corresponding to phenanthrene/anth~cene, the concentration of which has been determined to be 5 ppmw (HAHN et

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Organic compounds on crack surfaces in olivine al., 1988; PERINGand PONNAPERUMA, 197 1 ), indicates that

the sensitivity is w 10 picomoles per ion detected (TINGLE and BECKER, unpubl. data). From this we Calculate that 10 Is organic molecules were desorbed from the olivine crystals. The amount of C analyzed below 600°C in stepped-heating C isotope measurements of mantle xenoliths is lo-50 ppmw C ( NADEAUet al., 1990; PINEAUand MATHEZ, 1990), and for the moment it is taken as an estimate of the abundance of crack-situated C. (It has been argued previously ( MATHEZ, 1987; NADEAUet al., 1990) that the low-temperature C may be due to reduced C on cracks and grain boundaries, but the reader should keep in mind that its origin is quite controversial.) Olivine crystal fragments weighing 15 mg with lo50 ppmw C would contain 10 I6 C atoms. Using a mean thickness of 3 nm deduced from the XPS sputtering experiment and assuming the crack-situated C films are composed of closest-packed atomic C, one calculates that 10 I6 C atoms would correspond to a film 10 mm * (for perspective, the surface area of a cleavage fragment weighing 15 mg is approximately 20-40 mm*). It follows that roughly 10% of the amount of C assumed to exist on crack surfaces is organic. Clearly, this is a crude analysis with an order-of-magnitude uncertainty. Nevertheless, it agrees with the results of the XPS heating experiment, which showed that 50% of the total C in one carbonaceous film was volatilized on heating to 5OO”C, and so it appears that as much as lo-50% of the C on the crack surfaces may be organic. Unfortunately, the concentration of individual molecular species is insufficient to be characterized structurally or isotopically. Stepped-heating C isotope measurements of basalts and mantle xenoliths have shown that the C liberated below 600°C has a relatively uniform 6 13C = -26% ( DESMARAIS and MOORE, 1984; PINEAUand MATHEZ, 1990; NADEAUet al., 1990). For the purpose of distinguishing whether the crack-situated organics are abiogenic or biogenic, it is assumed now that ail the low-temperature C is organic, which yields an estimate of 613C = -26% for the organic C. The thermal desorption behavior of organics on crack surfaces mimics

that of this isotopically light, low-temperature C, suggesting that the two have a common origin. However, the steppedheating measurements were conducted under an oxygen partial pressure of = 10m3bars in the presence of a strong oxidizing agent, usually CuO (e.g., PINEAUand MATHEZ, 1990), whereas the thermaldesorption mass spectra were obtained in UHV. However, even if the 6 13C of organics on olivine crack surfaces is approximated by that of the low-temperature C, their assumed composition does not uniquely determine their origin (Fig. 4). It has been pointed out by DESMARAIS( 1986) that 613C = -26% overlaps the 613C of bulk sedimentary organic matter, and this has been taken as evidence that the low-temperature C is some form of biogenic contamination. However, if we are correct in deducing that the organics must have been deposited on the crack surfaces above 25”C, and possibly as high as several hundred “C, and we assume that the organics were derived from a biogenic source, then it seems likely that the organic compounds in the volcanic gas have undergone thermal degradation. It follows that their C isotopic composition would be lighter than that of the parents, which in fact has been observed in the Green Tuff region ( SAKATAet al., 1989) and Guaymas Basin ( SIMONEITet al., 1988); the hydrocarbons in the volcanic gases and hydrothermal fluids are much lighter than the organic matter from which they presumably were derived (Fig. 4). Of course, if the molecular weights of the parent material and the cracksituated organics are similar, then the isotopic shift would be relatively small. Similarly, it can be argued that if the organics (low-temperature C) are abiogenic, then the parent C should be heavier. The 613Cof the low-temperature C is substantially lighter than any hydrocarbons from documented abiogenic sources, yet it is consistent with abiotic synthesis of the organics from magmatic C (6°C = -5 to -8%; JAVOYet al., 1982). In summary, organic compounds are associated with carbonaceous films on crack surfaces in olivine from mantle xenoliths in alkali basalts; they are apparently deposited by

BIOGENIC q n q

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INTRUSIONS

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-60 FIG. 4. Histogram of 6 ‘% of hydrocarbons in volcanic gases,fluid inclusions, and hydrothermal fluids, and 6 “C of low-temperature C in basalts and mantle xenoliths. Data compiled from the following sources: DESMARAIS et al. (1981), DE~MARAIS and Mooa~ (1984), WELHAN (1987), SIMONEIT et al. (1988), KONNERUP-MADSEN et al. (1988), SAKATAet al. ( 1989), PINEAIJand MATHEZ( 1990), NADEAUet al. ( 1990). The parent biogenic matter for the Guaymas Basin and Green Tuff region hydrocarbons has a’% = -20 to -30 (Simoneit et al., 1988; Sakata et al., 1989). Even if the b’“Cof the low-temperature C in basalts and mantle xenoliths approximates that of the organics on crack surfaces in olivine, the assumed 6°C does not uniquely determine their source.

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volcanic gases on crack surfaces formed during eruption and cooling. Our results confirm MATHEZ’S ( 1987) conjecture that such carbonaceous films contain organic compounds, but whether or not they are synthesized from volcanic gas on fresh-fractured olivine surfaces by Fischer-Trop~h-like reactions as he hypothesized remains an open question. At this point, we regard the possibilities of abiogenic and biogenic derivation as equally likely. Additional studies will be required to distinguish the origin of organics on crack surfaces in mantle xenoliths. However, the simple fact that organics have been detected on olivine crack surfaces raises a number of interesting questions. First. it would be desirable to determine if indeed olivine and diopside surfaces are suitable catalysts for the synthesis of organic compounds or other reduced C species, like carbynes (HAYATSIJ and ANDERS, 198 1: HAYATSU et al.. 1980). Fresh olivine surfaces could be exposed to various C-O-H gases at high temperature followed by analysis with XPS and thermaldesorption photoionization mass spectrometry. Second, there is abundant evidence now for assimilation of biogenic material into magmatic systems, and some organic species are apparently capable of surviving magmatic and hydrothermal conditions. It would be desirable to know what the chemistry and structure of such high-temperature organic complexes might be. Third, the structure of these ubiquitous carbonaceous films remains unknown: are they graphite. amorphous c’, carbynes, macromolecular organic material. or some combination of these? ~~~~owi(~dg~ents-Constructive reviews by E. Mathez, D. Mogk, and D. DesMarais were greatly appreciated. This research was supported by the Gas Research Institute under Grant 5087-260-1626. H. W. Green II provided the specimens from H&alai and the tumblepolished peridote crystals from San Carlos; F. Freund provided the crack-bearing San Carlos olivine crystals. The present study was an outgrowth of an earlier study initiated by F. Freund with CHB and RM to study organics in olivine crystals. The authors would like to thank the following people for sharing ideas which Led to improvements in the manuscript: S. Brassell, D. Clague, H. Green. E. Mathez, J. Vajo, and P. Westcott. P. Schiffman assisted with the C X-ray microscopy which was performed in the ~pa~ment of Geology, University of California, Davis. Technical assistance from G. St. John, B. Olsen. P. Wurz. and J. Pallix was greatly appreciated. Editoriul handling: G. Fame

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