C. T. Pillinger

C and N isotopic composition and the infrared absorption spectra of coated diamonds: evidence for the regional uniformity of CO2H2O rich fluids in lithospheric mantle

Abstract The δ 13 C and δ 15 N values, nitrogen abundances and nitrogen aggregation states of coated diamonds from Botswana, Angola, Sierra Leone and Siberia have been determined. A single cubic micro-diamond from the Northern Territory, Australia was also analysed. The (cubic) coats of the diamonds and the cubic diamond from Australia had a restricted range in isotope composition: δ 13 C= −7.2 to −4.1‰ , δ 15 N= −8.7 to −1.7‰ . In contrast, the cores of the coated diamonds were found to be highly variable: δ 13 C= −21.1‰ to −1.9‰ , and δ 15 N= −2.8‰ to +12.1‰ (with the majority being positive). All of the coats gave the type IaA absorption spectra, together with others due to micro-inclusions dominated by H 2 O and CO 2 , whereas the cores contained nitrogen that was more highly aggregated. The results suggest that coated diamonds were formed following the influx of CO 2 H 2 O rich fluids into diamond-bearing lithosphere. Pre-existing diamonds acted as seeds for renewed growth and these are now the cores of the diamonds. These cores may be very variable in terms of morphology, isotopic composition, age, nitrogen aggregation state and crystallinity depending on the particular history of the source regions and the conditions of diamond growth. It is believed that the influx of volatiles and the growth of the coats was linked to kimberlite magmatism. The δ 13 C and δ 15 N results for the coats indicate that, in terms of C and N isotope composition, the source of CO 2 H 2 O rich fluids is globally quite homogeneous. It is probably located beneath continental lithosphere and, in addition, may have characteristics similar to the source of the ocean island basalts.

Carbon isotopic fractionation between CO2 vapour, silicate and carbonate melts: an experimental study to 30 kbar

The carbon isotopic fractionation between CO2 vapour and sodamelilite (NaCaAlSi2O7) melt over a range of pressures and temperatures has been investigated using solid-media piston-cylinder high pressure apparatus. Ag2C2O4 was the source of CO2 and experimental oxygen fugacity was buffered at hematite-magnetite by the double capsule technique. The abundance and isotopic composition of carbon dissolved in sodamelilite (SM) glass were determined by stepped heating and the δ13C of coexisting vapour was determined directly by capsule piercing. CO2 solubility in SM displays a complex behavior with temperature. At pressures up to 10 kbars CO2 dissolves in SM to form carbonate ion complexes and the solubility data suggest slight negative temperature dependence. Above 20 kbars CO2 reacts with SM to form immiscible Na-rich silicate and Ca-rich carbonate melts and CO2 solubility in Na-enriched silicate melt rises with increasing temperature above the liquidus. Measured values for carbon isotopic fractionation between CO2 vapour and carbonate ions dissoived in sodamelilite melt at 1200°–1400° C and 5–30 kbars average 2.4±0.2‰, favouring13C enrichment in CO2 vapour. The results are maxima and are independent of pressure and temperature. Similar values of ≈2‰ are obtained for the carbon isotopic fractionation between CO2 vapour and carbonate melts at 1300°–1400° C and 20–30 kbars.

The carbon and oxygen isotopic composition of meteoritic carbonates

The 13C12C and 18O16O isotopic ratios of carbonates from carbonaceous and ordinary chondrites have been measured on CO2 released by the action of H3PO4 on whole-rock samples. Carbonates from CI, CM and CR carbonaceous chondrites exhibit a range in δ18O of ca. 15%. (+20.5%. to +35.1%. relative to SMOW). Limited data from CO2-water equilibration experiments suggest that meteoritic carbonates do not possess grossly anomalous 17O isotopic compositions; therefore, they are truly enriched in 13C, with δ13C between +23.7%. and +80.7%. relative to PDB. Large internal variations in δ13C and δ18O were found in individual meteorites and suggest that two or more isotopically distinct carbonates of different origin may be present. The abundance, δ13C and δ18O of carbonate in CM2 chondrites may be related to the extent of aqueous alteration of the meteorites. Carbonates in CI and CR chondrites have a median δ13C ca. +50 to +60%., whereas δ13C of CM meteorites lie in the range +40 to +50%., although exceptions exist in both sets of samples. CV3 and CO3 carbonaceous chondrites and unequilibrated ordinary chondrites release small amounts of CO2 on acid treatment, which might be from carbonate dissolution, but which is not enriched in 13C, exhibiting δ13C values ca. 0 ± 10%. The exception to this is Bishunpur, with δ13C ca. −23.5%.. The difference in δ13C of the CI, CM and CR vs. CV, CO and ordinary chondrite carbonates may be a result of the progressive enrichment in 13C of percolating fluids, brought about by increasing solubilization of “exotic” 13C-enriched grains.

Chassigny and the nakhlites: Carbon-bearing components and their relationship to martian environmental conditions

Abstract The carbon and nitrogen inventories of Chassigny and the nakhlites have been investigated by low-resolution (100°C temperature increment) stepped combustion; in addition, the contents and isotopic compositions of carbonate minerals have been assessed by the use of an acid-dissolution technique. Low-temperature carbon (i.e., that which combusts below 500°C) appears to be from two different sources: one component is present in variable quantities (300–700 ppm) and has δ 13 C > −26%. , consistent with identification as terrestrial organic contamination; the other is isotopically light with δ 13 C ca. −36%., which is unusual for normal sources of terrestrial contamination. Curiously, the latter material is present in each sample in similar concentrations ( 45 ± 15 ppm ). Since this level of carbon is too high to be ascribed to a system blank, it is considered that this component is probably indigenous to the meteorites. The low temperature of release of the isotopically light carbonaceous material is testimony to its highly labile nature; it is evidently organic material of some description. The accompanying low-temperature nitrogen also appears to be distributed between two components (one with δ 15 N of ca. −6.5 to −2.4%.; the other with δ 15 N > +4%. ) but it is not yet possible to establish relationships between the nitrogen and carbon components released below 500°C. The meteorites investigated were found to contain 2.5–30 ppm carbon as carbonate, with δ 13 C between −5 and +11%. and δ 18 O between +23 and +29%.. Variation in δ 13 C and δ 18 O of the carbonates indicates either a change in conditions during formation of the carbonate minerals or that there may be two distinct carbon sources. Magmatic species, which are released on combustion at temperatures above 700°C, apparently have an isotopically light carbon isotopic composition (between −30 and −20%.), and are associated with light nitrogen ( δ 15 N ). Superimposed on the magmatic species is a release of spallogenically produced isotopically heavy nitrogen, seen most clearly in Chassigny. There is no evidence in any of the samples analysed for trapped martian atmospheric gases.

Carbon, oxygen and nitrogen isotopic compositions of possible martian weathering products in EETA 79001

Abstract Carbon and oxygen isotopic analyses of carbonate-rich “white-druse” material isolated from a contact between lithologies A and C of the EETA 79001 shergottite indicate that this component is probably extraterrestrial in origin, and not simply an Antarctic weathering product. The δ13C(+6.8%.) and δ18O (+21.1 %.) values of the calcium carbonate (predominantly calcite) are unlike δ 13 C δ 18 O combinations of most common terrestrial carbonates (whether biogenic or abiogenic). However, δ13C and δ18O of the carbonates in EETA 79001 are very similar to the corresponding values for carbonates analysed from Nakhla (a meteorite closely allied to shergottites). It seems apparent that the carbonate minerals in these SNC meteorites were formed on the meteorites parent body (Mars?). Isotopic analysis of nitrogen liberated during combustion of a carbonate-rich fraction of EETA 79001 does not give any clear evidence for the 15N-enriched component isolated from EETA 79001 lithology C glass and postulated as trapped martian atmosphere. However, substantial amounts of nitrogen were present; the origin of these components is unclear but the low temperature of release poses some constraints.

Isotopic anomalies of Ne, Xe, and C in meteorites. I - Separation of carriers by density and chemical resistance

Abstract In an attempt to characterize the carriers of presolar noble gases, 19—mainly non-colloidal—separates from the Murray and Murchison C2 chondrites were isotopically analyzed for Ne, Xe, and in several cases for C and N. Although most samples were strongly depleted in Xe-HL and enriched in Ne-E(H), Ne-E(L), and Xe-S, the separation methods failed to isolate the noble-gas carriers in pure form. Nonetheless, several new properties of the carriers were established. The carriers of Ne-E(H) and Xe-S are resistant to HCl, HF, boiling HClO4, and CrO3-H2SO4, and thus must be either diamond or some resistant carbide or oxide (SiC, hibonite, etc.), but not spinel, graphite, or amorphous carbon. These two noble-gas components seem to be associated with two heavy carbon components ( δC 13 ⋍ +1000% . and +1400%.), combusting at ~ 900 and ~ 1200°C, but cannot yet be assigned to either. The carrier of Ne-E(L), Cα, is slowly oxidized by Cr2O−7 and HNO3, and rapidly by boiling HC1O4. It may be some form of amorphous carbon, of δC 13 ⋍+340% . A new carbon component, Cθ, was found as 0.2–2 μm inclusions in Murchison spinel. It is amorphous, has δC13 = −50%, and contains little or no noble gas, though the surrounding spinel contains a component of Ne 20 Ne 22 ⋍ 10.7 ± 0.2 , possibly Ne-C from solar flares. The Cθ grains require a gas phase of C/O ≥ 1 for their formation, well above the solar ratio of 0.6, and presumably the occluding spinel, too, formed from such a gas. Both Cθ and spinel are enriched by ~4% in C12 and O16, respectively. This is of interest in connection with the suggestion by Schramm and Olive (1982) that much of the C12, O16, and Ne20 in the solar system was contributed by a massive, nearby supernova. A new heavy nitrogen component (δN15 ≥ +252%.) has been found. It has an abundance of ~ 1 ppm in the bulk meteorite, combusts at 450–500°C, and may be associated with isotopically normal carbon or with Cα.

Isotopic composition of CO2 and dissolved carbon species in basalt glass

The isotopic composition of coexisting fluid and dissolved carbon species in MORB glass have been studied using a combination of replicate stepped heating experiments, vacuum crushing and grain-size analysis. Stepped heating carbon release profiles demonstrate the presence of isotopically light carbon released by combustion below 600{degree}C (LTC) and isotopically heavier carbon released above 600{degree}C (HTC). Up to 95% of the LTC component, which mostly resides on the surface of the glass, can be removed by extraction in dichloromethane; combustion at 400 and 600{degree}C ensures its complete removal prior to extraction of HTC species above 600{degree}C. Assuming that HTC consists of fluid (i.e. CO{sub 2} trapped within vesicles) and dissolved (i.e. carbonate ion complexes) species, limits on the net difference between the isotopic composition of coexisting fluid and dissolved carbon have been estimated from stepped heating data. The apparent magnitude of carbon isotopic fractionation between CO{sub 2} vapor and basaltic melt is greater than 1{per thousand} but significantly less than 4.5{per thousand} and localized fractionation processes associated with magma degassing are associated with {Delta}{sub CO{sub 2}}{sub -basalt melt} values of {approx}2{per thousand}.

ALH 85085: Nitrogen isotope analysis of a highly unusual primitive chondrite

ALH 85085 is an unusual, fine-grained primitive chondrite which does not fit into any of the known meteorite classes. It shows many chemical similarities to the equally unusual meteorites, Bencubbin, Weatherford and Renazzo. Nitrogen isotope analysis shows that ALH 85085, like Bencubbin, Weatherford and Renazzo, contains highly 15 N-enriched nitrogen. Indeed, ALH 85085 has the highest δ 15 N measured in any whole-rock meteorite, totalling to +858‰, but reaching +1497‰ in the 850–875°C temperature increment. However, the distribution of isotopically heavy nitrogen in ALH 85085 is different from that observed for Bencubbin, thus precluding a direct relationship between these meteorites. Isotopically heavy nitrogen occurs in ALH 85085 in two sites: carbonaceous material (δ 15 N ca. +860‰; δ 13 C ca. +1‰; C/N ca. 65–75) and in fine-grained Fe Ni metal or opaque chondritic matrix clasts (δ 15 N ca. +990‰). A third, very 15 N-enriched nitrogen carrier, with δ 15 N⩾ 1500‰ might also be located in the matrix. It is probable that there is only a single component with elevated δ 15 N(> 1500‰), and that it occurs in at least three discrete sites, mixed with varying amounts of nitrogen having more isotopically “normal” δ 15 N(0 ± 50‰). The identity of the hosts of the isotopically heavy nitrogen have not been fully determined, but it is becoming clear that an increasing number of brecciated meteorites, some of which do not seem to be related in other ways, show evidence of having sampled a reservoir rich in 15 N. This may be a result of the introduction of heavy nitrogen during brecciation, its δ 15 N values and distribution throughout the meteorites brought about by mixing with variable amounts of more isotopically normal nitrogen and subsequent vaporization and re-condensation processes.

Recondite interstellar carbon components in the Allende meteorite revealed by preparative precombustion

Prolonged combustion at low temperatures (390-450{degree}C and 450-510{degree}C) removes large quantities of isotopically normal carbon from an acid (HF/HCl) residue of the Allende meteorite to reveal two isotopically heavy carbon components. One of them, called C{sub {aleph}} (carbon aleph) because of its possible relationship to C{sub {alpha}} of the host of Ne-E(L), has {delta}{sup 13}C of +345{per thousand} and a combustion temperature of 700-750{degree}C. It is believed that the presence of C{sub {aleph}} is masked in the bulk acid residue because of a tailing effect in the combustion of less stable forms of carbon. The second heavy carbon component burns over the temperature regime 900-1000{degree}C and has a {delta}{sup 13}C of > 527{per thousand}. This component called C{sub {kappa}} (carbon kappa) is distinguishable from the well known types of heavy carbon called C{sub {beta}} and C{sub {epsilon}}, the hosts of s-Xe and NeE(H), respectively, and now recognized as forms of SiC. C{sub {kappa}}, however, could also be a form of very poorly crystalline SiC; it appears to be released from the Allende acid residue because the precombustion slowly degrades a secondary hot mineral, which is possibly a variety of spinel. Likewise, a component of light carbon, possibly C{sub {theta}}, poorlymorexa0» crystalline graphite, is also liberated by the precombustion reaction after between 52 and 108 h of treatment. The discovery of C{sub {aleph}}, C{sub {kappa}}, and C{sub {theta}}, together with C{sub {lambda}}, a component very rich in {sup 12}C confirms that primitive meteorites are a prolific source of interstellar grains. Preparative precombustion demonstrates that the legacy to be found in Allende is far more complicated than at first envisaged from the noble gas record of this meteorite.«xa0less

Acfer 182: search for the location of15N-enriched nitrogen in an unusual chondrite

Abstract Acfer 182, an unusual chondrite recovered from the Sahara in 1990, is highly enriched in 15 N (bulk δ 15 N ca.+600‰; δ 15 N max = +1584‰ at 900°C), reinforcing conclusions that it is related to the very unusual chondrite ALH 85085. Stepped combustion of whole-rock Acfer 182 releases 15 N over a wide temperature range, excluding a single carrier phase for the 15 N-enriched nitrogen. The highest relative abundance of 15 N is found in phase “N C ”, so far unidentified mineralogically, with a C/N of ca. 8, which releases its nitrogen on combustion of the whole rock at 850–950°C. Attempts to isolate N C by physical means proved unsuccessful. Hence a chemical treatment was tried, with preparation of an HF/HCl-resistant residue. Approximately two-thirds of the original amount of nitrogen in the sample was lost on dissolution. Almost all of component N A , the carbonaceous component, was removed, without significant change in δ 15 N( +950‰) or C/N ratio (75). N B , originally thought to be nitrogen in metal, was unchanged in abundance, δ 15 N( +500‰) and C/N ratio (ca. 20), indicating that it could not be present in metal, unless as an insoluble nitride; it might even be an isotopically light entity mixing with residual N A and N C . This last component now combusted at a lower temperature than in the whole rock, and has a lower δ 15 N value ( +1274‰). A fourth, minor component N D is revealed on combustion of the residue, which burns above 675°C, with δ 15 N ca.+ 1000‰ and C/N ratio ca. 50. The 15 N-enriched nitrogen in almost all constituents of Acfer 182 is interpreted as emanating from a single source, i.e. interstellar grains, which were incorporated within the Acfer 182 parent body during aggregation. Vaporisation, condensation and mixing of components caused redistribution of 15 N-enriched nitrogen throughout the parent, with nitrogen either trapped or implanted into silicates and metal.