Release of bound aromatic hydrocarbons from late Archean and Mesoproterozoic kerogens via hydropyrolysis

Hydrogen-lean kerogens (atomic H/C<0.4) isolated from the 2.5-billion-year-old (Ga) Mt. McRae Shale, Hamersley Group, at Tom Price, Western Australia, were studied via hydropyrolysis, a continuous-flow technique that degrades organic matter in a stream of high-pressure hydrogen assisted by a dispersed Mo catalyst. The hydropyrolysates yielded predominantly phenanthrene and pyrene, and higher polyaromatic hydrocarbons and alkylated homologues were generated in low relative concentrations. Saturated hydrocarbons were not detected. The molecular and carbon isotopic compositions of the hydropyrolysates are very similar to aromatic hydrocarbons obtained by solvent extraction of the host rocks. Because molecular structures covalently attached to kerogen are unaffected by contamination, this indicates that both the bound and extractable aromatic fractions are syngenetic with the host rocks. Therefore, the results of the hydropyrolysis experiments provide compelling evidence for preserved bitumen of Archean age. The very high proportion of nonalkylated polyaromatic hydrocarbons in the hydropyrolysates is consistent with hydrothermal dehydrogenation of the kerogen, and a marked concentration difference of pyrene in rock extracts and hydropyrolysates might be explained by hydrothermal redistribution of the bitumen. The kerogen and bitumen composition is therefore consistent with models suggesting a hydrothermal origin for the giant iron ore deposits at Mt. Tom Price. Comparison of the Archean samples with hydropyrolysates from immature Mesoproterozoic kerogens from the Roper Group, McArthur Basin, Northern Territory, and with pyrolysis experiments on Proterozoic kerogens in the literature suggests that Precambrian kerogens are frequently highly aromatic and lipid-poor regardless of their degree of thermal preservation.     

A reconstruction of Archean biological diversity based on molecular fossils from the 2.78 to 2.45 billion-year-old Mount Bruce Supergroup, Hamersley Basin, Western Australia

Bitumens extracted from 2.7 to 2.5 billion-year-old (Ga) shales of the Fortescue and Hamersley Groups in the Pilbara Craton, Western Australia, contain traces of molecular fossils. Based on a combination of molecular characteristics typical of many Precambrian bitumens, their consistently and unusually high thermal maturities, and their widespread distribution throughout the Hamersley Basin, the bitumens can be characterized as ‘probably of Archean age’. Accepting this interpretation, the biomarkers open a new window on Archean biodiversity. The presence of hopanes in the Archean rocks confirms the antiquity of the domain Bacteria, and high relative concentrations of 2α-methylhopanes indicate that cyanobacteria were important primary producers. Oxygenic photosynthesis therefore evolved > 2.7 Ga ago, and well before independent evidence suggests significant levels of oxygen accumulated in the atmosphere. Moreover, the abundance of cyanobacterial biomarkers in shales interbedded with oxide-facies banded iron formations (BIF) indicates that although some Archean BIF might have been formed by abiotic photochemical processes or anoxygenic phototrophic bacteria, those in the Hamersley Group formed as a direct consequence of biological oxygen production. Biomarkers of the 3β-methylhopane series suggest that microaerophilic heterotrophic bacteria, probably methanotrophs or methylotrophs, were active in late Archean environments. The presence of steranes in a wide range of structures with relative abundances like those from late Paleoproterozoic to Phanerozoic sediments is convincing evidence for the existence of eukaryotes in the late Archean, 900 Ma before visible fossil evidence indicates that the lineage arose. Sterol biosynthesis in extant eukaryotes requires molecular oxygen. The presence of steranes together with biomarkers of oxygenic photosynthetic cyanobacteria suggests that the concentration of dissolved oxygen in some regions of the upper water column was equivalent to at least ∼1% of the present atmospheric level (PAL) and may have been sufficient to support aerobic respiration.     

Aliphatic hydrocarbons in the bitumens of the Puertollano oil shales

The aliphatic fraction of the bitumens from 3 bands of the Puertollano oil shale field have been studied by gas chromatography and gas chromatography/mass spectrometry. Previous work published over the last years reached different conclusions about the origin of the organic matter in the Puertollano oil shales. A reappraisal of some of the biomarkers was then needed for a better understanding of the aliphatic hydrocarbons assemblage. The identified compounds, mainly n-alkanes in the medium to high boiling point region, acyclic isoprenoids and hopane-related compounds point towards a predominant algal/bacterial input. No unequivocally higher plant derived biomarkers have been detected in significant amounts, suggesting that the bulk of the organic matter derive from authocthonous organisms living in the basin. The biomarker maturity ratios indicate that the 3 Puertollano oil shales have similar maturities and are close to the onset of oil generation, assessing a slightly higher maturity than that determined by petrographic techniques.     

Molecular composition and indigenity of organic matter in Late Neoproterozoic sedimentary rocks from the Yangtze region, South China

1 Introduction As a linkage between the biosphere and the geosphere, organic geochemistry, especially molecular markers, has become a powerful tool for investigating important geological events and the evolutionary history of ancient life on Earth (Kvenvo…     

The characteristics of the biomarkers and δ13C of n-alkanes released from thermally altered solid bitumens at various maturities by catalytic hydropyrolysis

Solid bitumen occurs extensively in the paleo-reservoirs of marine sequences in southern China. The fluids in these paleo-reservoirs have usually experienced severe secondary alteration such as biodegradation and/or thermal maturation. The concentrations of extractable organic matter (EOM) in the resulting solid bitumens are too low to satisfy the amount required for instrumental analysis such as GC–MS and GC–IRMS. It is also difficult to get enough biomarkers and n-alkanes by dry pyrolysis or hydrous pyrolysis directly because such solid bitumens are hydrogen poor due to high maturities. Catalytic hydropyrolysis (HyPy) can release much more EOM from solid bitumen at mature to highly over-mature stages than Soxhlet extraction, dry pyrolysis and hydrous pyrolysis. However, whether the biomarkers in hydropyrolysates can be used for bitumen-source or bitumen–bitumen correlations has been questionable. In this study, a soft biodegraded solid bitumen sample of low maturity was thermally altered to various maturities in a closed system. HyPy was then employed to release bound biomarkers and n-alkanes. Our results show that the geochemical parameters for source and maturity based on biomarkers released from these thermally altered bitumen residues by HyPy are insensitive to the degree of thermal alteration. Furthermore, the maturity parameters are indicative of lower maturity than bitumen maturation products at a corresponding temperature. This suggests that biomarker source and maturity parameters, based on the products of HyPy, remain valid for bitumens which have suffered both biodegradation and severe thermal maturation. The distributions of δ¹³C of n-alkanes in hydropyrolysates are also insensitive to the temperature used for bitumen artificial maturation. Hence, the δ¹³C values of n-alkanes in hydropyrolysates may also provide useful information in bitumen–bitumen correlation for paleo-reservoir solid bitumens.     

Generation characteristics of organic matter and distribution of biomarkers in bitumens of the Riphean,Vendian, and Cambrian source rocks of the Siberian Platform

The distribution of biomarkers in bitumens of source rocks of different ages in the southern Siberian Platform is studied to elucidate the correlation between oils and their sources. It is shown that the bitumens of Riphean, Vendian, and Lower-Middle Cambrian source rocks are different in some parameters reflecting the type of primary biogenic material, first of all, in the distribution of n-alkanes and acyclic isoprenoids and in the portions of steranes and triterpanes among polycyclic naphthenes. The revealed difference in the composition and distribution of biomarkers in bitumens of source rocks of different ages can be used to identify the sources of hydrocarbons in discovered pools.     

Laboratory methods for evaluating migrated high molecular weight hydrocarbons in marine sediments at naturally occurring oil seeps

A laboratory study has been conducted to determine the best methods for the detection of C₁₀–C₄₀ hydrocarbons at naturally occurring oil seeps in marine sediments. The results indicate that a commercially available method using n-C₆ to extract sediments and gas chromatography–flame ionization detection (GC–FID) to screen the resulting extract is effective at recognizing the presence of migrated hydrocarbons at concentrations from 50 to 5000 ppm. When non-biodegraded, the amount of oil charge is effectively tracked by the sum of n-alkanes in the gas chromatogram. However, once the charge oil becomes biodegraded, with the loss of n-alkanes and isoprenoids, the amount of oil is tracked by the quantification of the unresolved complex mixture (UCM). Gas chromatography–mass spectrometry (GC–MS) was also found to be very effective for the recognition of petroleum related hydrocarbons and results indicate that GC–MS would be a very effective tool for screening samples at concentrations below 50 ppm oil charge.     

Biomarker characterisation and hydrous pyrolysis of bitumens from Tertiary volcanics, queen Charlotte islands, British Columbia, Canada

A number of bitumen samples have been recovered from vugs and fractures in outcropping Tertiary basalts of the Queen Charlotte Islands off the west coast of British Columbia, Canada. n-Alkanes and acyclic isoprenoids are not present and the polycyclic biomarker distribution of the four samples analyzed by GC and GC-MS has been altered. One sample contains no remaining recognizable biomarkers. Two bitumens were also obtained from Lower Jurassic potential source rocks, including one from the Sandilands Formation, which was considered previously to be the most likely source of the Tertiary bitumens. Although these two bitumens were also taken from outcrops, they are considerably less biodegraded. The Sandilands Formation bitumen contains 28, 30-bisnorhopanes and since these compounds were not detected in the Tertiary bitumens, there is no evidence from the initial results for a Sandilands Formation contribution to the hydrocarbons in the Tertiary basalts. The presence of 18α(H)-oleanane in the saturate fraction of two of the Tertiary bitumens from widely separated locations indicates that they are at least partially sourced from Tertiary organic matter.Because the distribution of biomarkers in some samples has been severely affected by biodegradation, the asphaltenes of the bitumens were hydrously-pyrolysed and the saturate fractions of the resulting pyrolysates analysed for possible additional information on the origin of the bitumens. The pyrolysates from the more degraded samples contain compounds not detected in the saturate fractions of the original bitumen and show some of the expected characteristics of the original non-degraded bitumen. However, the compounds most useful for correlation are not present in the pyrolysates apparently due to their non-incorporation into the kerogen macromolecule. Our results suggest that hydrous-pyrolysis of asphaltenes is of limited use in the correlation of biodegraded samples and in determining their origins.     

Genesis modelling for the Hamersley BIF-hosted iron ores of Western Australia: a critical review

Enrichment iron ore of the Hamersley Province, currently estimated at a resource of over 40 billion tonnes (Gt), mainly consists of BIF (banded iron-formation)-hosted bedded iron deposits (BID) and channel iron deposits (CID), with only minor detrital iron deposits (DID). The Hamersley BID comprises two major ore types: the dominant supergene martite–goethite (M-G) ores (Mesozoic–Paleocene) and the premium martite–microplaty hematite ores (M-mplH; ca 2.0 Ga) with their various subtypes. The supergene M-G ores are not common outside Australia, whereas the M-mplH ores are the principal worldwide resource. There are two current dominant genetic models for the Hamersley BID. In the earlier 1980–1985 model, supergene M-G ores formed in the Paleoproterozoic well below normal atmospheric access, driven by seasonal oxidising electrochemical reactions in the vadose zone of the parent BIF (cathode) linked through conducting magnetite horizons to the deep reacting zone (anode). Proterozoic regional metamorphism/diagenesis at ～80–100°C of these M-G ores formed mplH from the matrix goethite in the local hydrothermal environment of its own exhaled water to produce M-mplH ores with residual goethite. Following general exposure by erosion in the Cretaceous–Paleocene when a major second phase of M-G ores formed, ground water leaching of residual goethite from the metamorphosed Proterozoic ores resulted in the mainly goethite-free M-mplH ores of Mt Whaleback and Mt Tom Price. Residual goethite is common in the Paraburdoo M-mplH-goethite ores where erratic remnants of Paleoproterozoic cover indicate more recent exposure.

Deep unweathered BIF alteration residuals in two small areas of the Mt Tom Price M-mplH deposits have been used since 1999 for new hypogene–supergene modelling of the M-mplH ores. These models involve a major Paleoproterozoic hydrothermal stage in which alkaline solutions from the underlying Wittenoom Formation dolomite traversed the Southern Batter Fault to leach matrix silica from the BIF, adding siderite and apatite to produce a magnetite–siderite–apatite ‘protore.’ A later heated meteoric solution stage oxidised siderite to mplH + ankerite and magnetite to martite. Weathering finally removed residual carbonates and apatite leaving the high-grade porous M-mplH ore. Further concepts for the Mt Tom Price North and the Southern Ridge Deposits involving acid solutions followed, but these have been modified to return essentially to the earlier hypogene–supergene model. Textural data from erratic ‘metasomatic BIF’ zones associated with the above deposits are unlike those of the typical martite–microplaty hematite ore bodies. The destiny of the massive volumes of dissolved silica gangue and the absence of massive silica aureoles has not been explained. Petrographic and other evidence indicate the Mt Tom Price metasomatism is a localised post-ore phenomenon. Exothermic oxidation reactions in the associated pyrite-rich black shales during post-ore removal by groundwater of remnant goethite in the ores may have resulted in this very localised and erratic hydrothermal alteration of BIF and its immediately associated pre-existing ore.     

Hydrothermal origin for the 2 billion year old Mount Tom Price giant iron ore deposit, Hamersley Province, Western Australia

Giant iron-ore deposits, such as those in the Hamersley Province of northwestern Australia, may contain more than a billion tonnes of almost pure iron oxides and are the world's major source of iron. It is generally accepted that these deposits result from supergene oxidation of host banded iron formation (BIF), accompanied by leaching of silicate and carbonate minerals. New textural evidence however, shows that formation of iron ore at one of those deposits, Mount Tom Price, involved initial high temperature crystallisation of magnetite-siderite-iron silicate assemblages. This was followed by development of hematite- and ferroan dolomite-bearing assemblages with subsequent oxidation of magnetite, leaching of carbonates and silicates and crystallisation of further hematite. Preliminary fluid inclusion studies indicate both low and high salinity aqueous fluids as well as complex salt-rich inclusions with the range of fluid types most likely reflecting interaction of hydrothermal brines with descending meteoric fluids. Initial hematite crystallisation occurred at about 250 °C and high fluid pressures and continued as temperatures decreased. Although the largely hydrothermal origin for mineralisation at Mount Tom Price is in conflict with previously proposed supergene models, it remains consistent with interpretations that the biosphere contained significant oxygen at the time of mineralisation. Received: 16 February 1999 / Accepted: 14 May 1999     

Bitumens associated with lead,zinc and flnorite ore minerals in North Derbyshire,England

A bitumen deposit in north Derbyshire, England, is described and studied using the techniques of gas chromatography, infra-red spectrophotometry and elemental analysis. The bitumens are associated with lead-zinc-fluorite ore minerals concentrated along an unconformable contact between the Carboniferous Limestone and the overlying shales. Three varieties of bitumen are distinguished and are compared with the dispersed bitumens in both the Carboniferous Limestone and the Edale Shales, with crude oils believed to be derived from these shales, and with inclusions in the hydrothermal mineral fiuorite, associated with the limestone-shale contact. One of the varieties recognized, a brittle brown solid, contains aliphatic hydrocarbons very similar to those of the shales and to those of the crude oils likely derived from these shales. The other two are a viscous oil and a brittle, black solid, which geologic evidence suggests were originally derived from the limestone. They contain a very complex mixture of aliphatic hydrocarbons quite different from those in the limestone, however. Aliphatic hydrocarbons isolated from fluorite, which is hydrothermally formed, in nearby massive deposits are almost identical to those of the limestone and shale. This observation indicates that alteration of the aliphatic hydrocarbons in the sediment by hydrothermal fluids is an unlikely explanation for the origin of the complex mixtures of hydrocarbons in the viscous oil and brittle, black solid. On the basis of indirect evidence of high nitrogen content, presence of unsaturated hydrocarbons, and suitability of the environment for bacterial growth, it is suggested that selective bacterial alteration of the aliphatic hydrocarbons is the main process responsible for their present composition. A geologic history for the deposit is postulated that involves a two-phase introduction of bitumens. Each phase is suggested to have begun as a pulse of warm saline fluids migrating along the shale-limestone unconformity passed through the topographic high at Windy Knoll. Microbial oxidation of the bitumens may have taken place during the deposition process or, more likely, as a recent secondary oxidation process.     

Millimeter-scale concentration gradients of hydrocarbons in Archean shales: Live-oil escape or fingerprint of contamination?

Archean shales from the Pilbara in Western Australia contain biomarkers that have been interpreted as evidence for the existence of cyanobacteria and eukaryotes 2.7 billion years (Ga) ago, with far reaching implications for the evolution of Earth’s early biosphere. To re-evaluate the provenance of the biomarkers, this study determined the spatial distribution of hydrocarbons in the original drill core material. Rock samples were cut into millimeter-thick slices, and the molecular content of each slice was analyzed. In core from the Hamersley Group (∼2.5 Ga), C_<13 alkanes had gradually increasing concentrations from the surfaces to the center of the rock while the abundance of steranes, hopanes and C₁₅₊ alkanes decreased with distance from the outer surfaces. In samples from the Fortescue Group (∼2.7 Ga), hydrocarbons were overwhelmingly concentrated on rock surfaces.Two mechanisms are proposed that may have caused the inhomogeneous distribution: diffusion of petroleum products into the rock (contamination model), and leaching of indigenous hydrocarbons out of host shales driven by pressure release after drilling (‘live-oil’ effect). To test these models, the hydrocarbon distributions in the Archean shales are compared with artificially contaminated rocks as well as younger mudstones where leaching of live-oil had been observed.The results show that chromatographic phenomena associated with live-oil escape and contaminant diffusion have strong effects on molecular ratios and maturity parameters, potentially with broad implications for oil-source rock correlation studies and paleoenvironmental interpretations.For the Archean shales, the live-oil effect is consistent with some of the observed patterns, but only the contamination model fully explains the complex chromatographic fingerprints. Therefore, the biomarkers in the Pilbara samples have an anthropogenic origin, and previous conclusions about the origin of eukaryotes and oxygenic photosynthesis based on these samples are not valid.However, the study also identified indigenous molecules. The spatial distribution of particular aromatic hydrocarbons suggests they are syngenetic. Although devoid of biological information, these aromatics now represent the oldest known clearly-indigenous terrestrial liquid hydrocarbons.     

Correlation between hydrogen isotope ratios of lipid biomarkers and sediment maturity

We investigated the influence of thermal maturity on the hydrogen isotope ratios of sedimentary hydrocarbons to prove that the isotope ratio of hydrocarbons mirrors paleoclimate signatures. δD values from n-alkanes and acyclic isoprenoids of two sediment sections (Kupferschiefer [KS], 258 Ma, and Posidonienschiefer [PS], 184 Ma) with different maturation history were investigated. Both covered thermal maturity from 0.48 to 1.3 R_c (vitrinite reflectance and reflectance calculated from MPI1). Sediment burial up to 4500 m caused thermal maturation of organic matter in the KS horizon from the Early Zechstein basin of Poland, whereas contact metamorphic thermal maturation originated in the Early Toarcian PS (Posidonienschiefer) of the North German Vlotho Massif. The δD values of the extracted n-alkanes positively correlate with thermal maturity in the KS (y = 56‰ × MPI1[x] − 160‰ [VSMOW]) and in the PS (y = 104‰ × MPI1[x] − 200‰ [VSMOW]). The δD values of isoprenoids (i.e., pristane, phytane) were even more enriched with increasing maturity (y = 179‰ × MPI1[x] − 341‰ [VSMOW] in the KS; y = 300‰ × MPI1[x] − 415‰ [VSMOW] in PS).These results explain why isotope ratios of n-alkanes and isoprenoids in mature sediments are generally enriched in D and do not have the expected isotopic difference between n-alkanes and isoprenoids of ∼190‰. Moreover, the correlation between sediment maturity parameters and δD values suggests that after correction the δD values of n-alkanes can be used to reconstruct climate and environment in the geological past.     

Assessment of petroleum biodegradation using stable hydrogen isotopes of individual saturated hydrocarbon and polycyclic aromatic hydrocarbon distributions in oils from the Upper Indus Basin,Pakistan

The stable hydrogen isotopic compositions (δD) of selected aliphatic hydrocarbons (n-alkanes and isoprenoids) in eight crude oils of similar source and thermal maturity from the Upper Indus Basin (Pakistan) were measured. The oils are derived from a source rock deposited in a shallow marine environment. The low level of biodegradation under natural reservoir conditions was established on the basis of biomarker and aromatic hydrocarbon distributions. A plot of pristane/n-C₁₇ alkane (Pr/n-C₁₇) and/or phytane/n-C₁₈ alkane (Ph/n-C₁₈) ratios against American Petroleum Institute (API) gravity shows an inverse correlation. High Pr/n-C₁₇ and Ph/n-C₁₈ values and low API gravity values in some of the oils are consistent with relatively low levels of biodegradation. For the same oils, δD values for the n-alkanes relative to the isoprenoids are enriched in deuterium (D). The data are consistent with the removal of D-depleted low molecular weight (LMW) n-alkanes (C₁₄–C₂₂) from the oils. The δD values of isoprenoids do not change with progressive biodegradation and are similar for all the samples. The average D enrichment for n-alkanes with respect to the isoprenoids is found to be as much as 35‰ for the most biodegraded sample. For example, the moderately biodegraded oils show an unresolved complex mixture (UCM), loss of LMW n-alkanes (<C₁₅) and moderate changes in the alkyl naphthalene distributions. The relative susceptibility of alkyl naphthalenes at low levels of biodegradation is discussed. The alkyl naphthalene biodegradation ratios were determined to assess the effect of biodegradation. The dimethyl, trimethyl and tetramethyl naphthalene biodegradation ratios show significant differences with increasing extent of biodegradation.     

Iron in Precambrian rocks: implications for the global oxygen budget of the ancient Earth

Banded iron formations (BIF) are prominent in sediments older than 2 Ga. However, little is known about the absolute abundance of BIF in Archean and Early Proterozoic sediments, and the source of the Fe is still somewhat uncertain. Also unknown is the role that Fe may have played in the maintenance of low oxygen pressures in the Archean and Early Proterozoic atmosphere. An analysis of the chemical composition of Precambrian rocks provides some insight into the role of Fe in Precambrian geochemical cycles. The Fe content of igneous rocks is well correlated with their Ti content. Plots of Fe vs. Ti in Precambrian sandstones and graywackes fall very close to the igneous rock trend. Plots of Fe vs. Ti in Precambrian shales also follow this trend but show a definite scatter toward an excess of Fe. Phanerozoic shales and sandstones lie essentially on the igneous rock trend and show surprisingly little scatter. Mn/Ti relations show a stronger indication of Precambrian Mn loss, perhaps due to weathering under a less oxidizing early atmosphere. These data show that Fe was neither substantially added to nor significantly redistributed in Archean and early Proterozoic sediments. Enough hydrothermal Fe was added to these sediments to increase the average Fe content of shales by at most a factor of 2. This enrichment would probably not have greatly affected the near-surface redox cycle or atmospheric oxygen levels. Continued redistribution of Fe and mixing with weathered igneous rocks during the recycling of Precambrian sediments account for the excellent correlation of Fe with Ti in Phanerozoic shales and for the similarity between their Fe/Ti ratio and that of igneous rocks.     

Generation and expulsion of oils from Permian coals of the Sydney Basin,Australia

Organic geochemical and petrological assessment of coals/coaly shales and fine grained sediments, coupled with organic geochemical analyses of oil samples, all from Permo–Triassic sections of the Southern Sydney Basin (Australia), have enabled identification of the source for the widely distributed oil shows and oil seeps in this region. The Permian coals have higher hydrogen indices, higher liptinite contents, and much higher total organic matter extract yields than the fine grained sediments. A variety of source specific parameters obtained from n-alkanes, regular isoprenoids, terpanes, steranes and diasteranes indicate that the oil shows and seeps were generated and expelled predominantly from higher plant derived organic matter deposited in oxic environments. The source and maturity related biomarkers and aromatic hydrocarbon distributions of the oils are similar to those of the coals. The oil-coal relationship also is demonstrated by similarities in the carbon isotopic composition of the total oils, coal extracts, and their individual n-alkanes. Extracts from the Permo–Triassic fine grained sediments, on the other hand, have organic geochemical signatures indicative of mixed terrestrial and prokaryotic organic matter deposited in suboxic environments, which are significantly different from both the oils and coal extracts. The molecular signatures indicating the presence of prokaryotic organic matter in some of the coal extracts and oils may be due to thin sections of possibly calcareous lithologies interbedded within the coal measures. The genetic relationship between the oils and coals provides new evidence for the generation and expulsion of oils from the Permian coals and raises the possibility for commercial oil accumulations in the Permian and Early Triassic sandstones, potentially in the deeper offshore part of the Sydney Basin.     

Carbon geochemistry of serpentinites in the Lost City Hydrothermal System (30°N, MAR)

The carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered at the Lost City Hydrothermal Field (LCHF) and drilled at IODP Hole 1309D at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) was examined to characterize carbon sources and speciation in oceanic basement rocks affected by long-lived hydrothermal alteration. Our study presents new data on the geochemistry of organic carbon in the oceanic lithosphere and provides constraints on the fate of dissolved organic carbon in seawater during serpentinization. The basement rocks of the Atlantis Massif are characterized by total carbon (TC) contents of 59 ppm to 1.6 wt% and δ¹³C_TC values ranging from −28.7‰ to +2.3‰. In contrast, total organic carbon (TOC) concentrations and isotopic compositions are relatively constant (δ¹³C_TOC: −28.9‰ to −21.5‰) and variations in δ¹³C_TC reflect mixing of organic carbon with carbonates of marine origin. Saturated hydrocarbons extracted from serpentinites beneath the LCHF consist of n-alkanes ranging from C₁₅ to C₃₀. Longer-chain hydrocarbons (up to C₄₀) are observed in olivine-rich samples from the central dome (IODP Hole 1309D). Occurrences of isoprenoids (pristane, phytane and squalane), polycyclic compounds (hopanes and steranes) and higher relative abundances of n-C₁₆ to n-C₂₀ alkanes in the serpentinites of the southern wall suggest a marine organic input. The vent fluids are characterized by high concentrations of methane and hydrogen, with a putative abiotic origin of hydrocarbons; however, evidence for an inorganic source of n-alkanes in the basement rocks remains equivocal. We propose that high seawater fluxes in the southern part of the Atlantis Massif likely favor the transport and incorporation of marine dissolved organic carbon and overprints possible abiotic geochemical signatures. The presence of pristane, phytane and squalane biomarkers in olivine-rich samples associated with local faults at the central dome implies fracture-controlled seawater circulation deep into the gabbroic core of the massif. Thus, our study indicates that hydrocarbons account for an important proportion of the total carbon stored in the Atlantis Massif basement and suggests that serpentinites may represent an important—as yet unidentified—reservoir for dissolved organic carbon (DOC) from seawater.     

Preservation of hydrocarbons and biomarkers in oil trapped inside fluid inclusions for >2 billion years

Oil-bearing fluid inclusions occur in a ca. 2.45 Ga fluvial metaconglomerate of the Matinenda Formation at Elliot Lake, Canada. The oil, most likely derived from the conformably overlying deltaic McKim Formation, was trapped in quartz and feldspar during diagenesis and early metamorphism of the host rock, probably before ca. 2.2 Ga. Molecular geochemical analyses of the oil reveal a wide range of compounds, including CH₄, CO₂, n-alkanes, isoprenoids, monomethylalkanes, aromatic hydrocarbons, low molecular weight cyclic hydrocarbons, and trace amounts of complex multi-ring biomarkers. Maturity ratios show that the oil was generated in the oil window, with no evidence of extensive thermal cracking. This is remarkable, given that the oils were exposed to upper prehnite-pumpellyite facies metamorphism (280-350 °C) either during migration or after entrapment. The fluid inclusions are closed systems, with high fluid pressures, and contain no clays or other minerals or metals that might catalyse oil-to-gas cracking. These three attributes may all contribute to the thermal stability of the included oil and enable survival of biomarkers and molecular ratios over billions of years. The biomarker geochemistry of the oil in the Matinenda Formation fluid inclusions enables inferences about the organisms that contributed to the organic matter deposited in the Palaeoproterozoic source rocks from which the analysed oil was generated and expelled. The presence of biomarkers produced by cyanobacteria and eukaryotes that are derived from and trapped in rocks deposited before ca. 2.2 Ga is consistent with an earlier evolution of oxygenic photosynthesis and suggests that some aquatic settings had become sufficiently oxygenated for sterol biosynthesis by this time. The extraction of biomarker molecules from Palaeoproterozoic oil-bearing fluid inclusions thus establishes a new method, using low detection limits and system blank levels, to trace evolution through Earth’s early history that avoids the potential contamination problems affecting shale-hosted hydrocarbons.     

澳大利亚西部哈默斯利铁成矿省BIF富铁矿的成矿特征与控矿因素

澳大利亚西部哈默斯利铁成矿省含有世界级高品位的赤铁矿体。主要铁矿床包括芒特维尔贝克、汤姆普莱斯山、帕拉伯杜等,它们均产于元古宙早期布罗克曼BIF型含铁建造中。高品住铁矿体的空间分布明显受到元古宙区域隆起和拉张环境下形成的古老正断层系统的控制。该成矿省高品位铁矿层的形成可分为3个阶段：第1阶段为深层阶段,该阶段硅从含铁建造中淋滤出来,留下薄层状富含铁氧化物、碳酸盐岩、硅酸镁和磷灰石的残余物;第2阶段为深部大气水氧化阶段,该阶段含铁建造的磁铁矿-菱镁矿组合被氧化为赤铁矿-铁白云石,并以发育假象赤铁矿为特征;第3阶段为浅层风化作用。通过对成矿特征和成矿模式的总结,认为成矿时代、断层、褶皱等构造特征及流体和表生风化作用是富铁矿床形成的主要控矿因素。     

Geophysical differences in the lithosphere between phanerozoic and precambrian Australia

Cannikin atomic bomb recordings indicate that there are differences in travel-times from the Aleutian Islands test site to Phanerozoic and Precambrian provinces in Australia of up to 1.1 s. Explosion seismic studies in central and southeastern Australia enable travel-time corrections for crustal and upper mantle structure to be made to recordings of such teleseismic events. Structure in the upper 60 km can account for, at most, about 0.2 s of the residual difference, but attempts to constrain the remaining residual time to the region above the Lehmann discontinuity at about 200 km depth are difficult to reconcile with explosion seismic models. Regional differences in seismic velocity structure between Phanerozoic and Precambrian Australia therefore appear to exist at depths greater than 200 km.Electrical conductivities within the mantle have been investigated using two methods. Long-period electromagnetic depth sounding using magnetometer arrays demonstrates that conductivities increase at about 200 km under Phanerozoic Australia but not until about 500 km depth under Precambrian Australia. Shorter period magnetotelluric measurements can only resolve shallower structures; these too indicate a similar trend but with sub-crustal conductivities increasing at less than 100 km under Phanerozoic Australia. Magma at these depths and shallower may be the source for Cainozoic volcanism in eastern Australia. Under Precambrian central and northern Australia magnetotelluric investigations indicate that pronounced conductivity increases do not occur until depths of 150–200 km are reached.Oceanic magnetic observations indicate that the Australian lithospheric plate as a whole is separating from Antarctica at a rate of about 7 cm/yr. The seismic and conductivity structures under the continental region of this plate indicate that lateral inhomogeneities possibly extend to depths as great as 500 km and are probably caused by the passage of eastern Australia over a hot spot. Hawaiian studies indicate that hot spots are not local features but result from large scale disturbances in the mantle. Conductivity increases commencing in the depth range 100–250 km may give an indication of uppermost zones within which the Palaeozoic lithospherc has been substantially modified resulting in elevated surface heat flow, volcanism and seismic travel-time anomalies.