B53B-0986 1340h
Astrobiology Drilling Program of the NASA Astrobiology Institute
Access to unweathered and uncontaminated samples of the least altered, oldest, sedimentary rocks is essential for understanding the early history of life on Earth and the environments in which it may have existed. For this reason, the NASA Astrobiology Institute (NAI) has embarked on two international programs, a series of Field Workshops aimed at making the most important surface samples available to investigators, and the Astrobiology Drilling Program (ADP), which serves to provide access to fresh subsurface samples when the scientific objectives require them. The Astrobiology Drilling Program commenced in Western Australia in 2003 with the initiation of its first project, the Archean Biosphere Drilling Project (ABDP). Funding for the ABDP came mainly from the Japanese Government through Kagoshima University and from NASA through the NAI Team at Pennsylvania State University, but significant technical and logistic support was provided by the Geological of Western Australia and, to a lesser extent, by the University of Western Australia. Six diamond drill cores totalling 1.4 km were obtained from astrobiologically important successions in the 3.3-3.5 Ga-old Pilbara Craton of northern Western Australia. Drilling in 2004 also occurred in Western Australia. The Deep Time Drilling Project (DTDP), a spin-off from the NAI's Mission to Early Earth Focus Group, completed one long hole, aimed mainly at fossil biomolecules (biomarkers) and other geochemical indicators of early life. The DTDP and the ABDP also jointly drilled two other important holes 2004, one through the oldest known erosion surface (and possible soil profile). The other intersected well-preserved middle Archean sediments. These efforts parallel other drilling initiatives within the wider astrobiological community that are taking place in Western Australia, South Africa, Spain, and arctic Canada. The ADP is managed by the NAI through a Steering Committee appointed by the NAI Director. Samples of cores obtained through ADP projects are available to the whole community, following a one year embargo, upon application to project PIs and the ADP Steering Committee.
http://nai.nasa.gov/ADP
B53B-0987 1340h
The Pilbara: one Billion Years of the Early Evolution of Earth's Surface Environments and Life
The Pilbara contains the most complete sequence of sedimentary and volcanic rocks dating from 3.5 to 2.4 Ga. Because many of these rocks have experienced only low-grade metamorphism it is our best available natural laboratory for studying the origins and early evolution of life on Earth (and other planets) and the environments it inhabited. Indeed discoveries of the oldest possible microfossils, stromatolites and molecular fossils, as well as key mineral, geochemical and isotopic evidence of surface environments and the biosphere have all been reported from Pilbara rocks. Unfortunately complex geology and deep weathering (since the Mesozoic) makes interpretation of some of this record ambiguous leading to heated debates over evidence for life and environmental conditions. It is not surprising then that the first stages of the Astrobiology Drilling Program have been drilled in the Pilbara to obtain samples of sedimentary rocks unaffected by modern weathering. The drill cores potentially provide the best evidence yet of when and how life evolved on Earth, the nature of the environments it inhabited and a template for evaluating possible evidence of life from Mars and other planets. This talk will briefly outline the evolution of the Pilbara as one of the Earth's first continents and the interpreted environmental settings of the range of sites being drilled by the Archean Biosphere and Deep Time Drilling Programs. These include 3.52 to 2.5 Ga submarine and lacustrine black shales, 3.45 Ga deep- and shallow-marine banded cherts, 2.72 Ga stromatolite reefs, an interpreted 2.76 Ga paleosol, and banded iron formations.
B53B-0988 1340h
Paleomagnetism of the early Archean Marble Bar Chert Member, Pilbara craton, Western Australia: Implication for Archean geomagnetic reversals and Paleogeography
We conducted Paleomagnetic study on the early Archean rocks in Pilbara Craton, Western Australia. The Pilbara craton is the one of the best exposed and complete record during Archean evolution of the Earth. Archean Biosphere Drilling Project (ABDP) had drilled a continuous 250 m long oriented core from the Towers Formation in north-eastern part of the Pilbara craton. This drilling was essential to obtain the fresh samples from below the influence of intense weathering. Lithofacies of the core are divided into basalt/dolerite and various colored chert named the Marble Bar Chert Member. The Marble Bar Chert Member, dated between 3463 to 3454 Ma (Thorpe et al., 1992), is a world famous early Archean unit with little metamorphism. Initial magnetic measurements of the whole core revealed that the black or red layers in the Marble Bar Chert Member display strong Natural Remanence Magnetization. Petrographic observations revealed that the black and red layers are preserving the primary structure of sedimentation. Hematite, magnetite, and siderite were separately precipitated as primary minerals. Rock magnetic analysis indicates an existence of the large amount of magnetite associated with minor amount of hematite. Through the thermal demagnetization, we had found that the remanent magnetizations define two components: (1) a mid-temperature (<$450\deg$C) component; (2) a high-temperature component (450 to $575\deg$C) with stable decay going to origin. This results indicate that the main magnetic carrier of remanence likely to be magnetite, which is consistent with petrologic and rock magnetic analysis. Characteristics Remanent Magnetization directions of the 2nd component are put on nearly opposite side of the hemisphere. The polarity transition from normal to reverse recorded in continuous stratigraphic section. Petrologic, rock magnetic, and paleomagnetic analysis suggested that the Marble Bar Chert Member is probably preserving the primary magnetization. This result may suggest that the geomagnetic field has reversed at 3.46 Ga, which is the oldest observed geomagnetic reversal. The previous oldest geomagnetic reversal was reported from the Kaap Valley pluton of South Africa at 3.2 Ga (Layer et al., 1996). Our result is at least 260 m.y. older than that reported geomagnetic reversal. We also present a new paleomagnetic pole at 3.5 Ga and argue the APWP and paleogeography before 1.7 Ga. In contrast to the paleomagnetic poles from McElhinny and Senanayake (1980), our paleomagnetic pole is located in southern hemisphere, near the other poles about 2.7 Ga. Therefore, two possibilities should be considered: (1) our paleomagnetic pole is over print, which might be recorded about 2.7 Ga; (2) our paleomagnetic pole is primary, and recording the geomagnetic filed at 3.5 Ga.
B53B-0989 1340h
Evidence for Early Life in $\sim$3.5 Billion-Year-Old Pillow Lavas
Recently discovered biosignatures in the formerly glassy rims of $\sim$3.5 billion-year-old pillow lavas from the Barberton Greenstone Belt (BGB) in South Africa suggest they were colonized by microbes early in Earth's history. These subaqueous volcanic rocks represent a new geological setting in the search for early life on Earth. This is not entirely surprising since microbial alteration of basaltic glass in pillow lavas and volcaniclastic rocks has been well documented from recent oceanic crust and well-preserved ophiolites. The BGB magmatic sequence contains exceptionally well-preserved mafic to ultramafic pillow lavas, sheet flows, and intrusions interpreted to represent 3.48 to 3.22 billion-year-old oceanic crust and island arc assemblages. We observed micron-sized tubular structures mineralized by titanite in the formerly glassy rims of the BGB pillow lavas. Based on their similarity to textures observed in recent glassy pillow basalts we interpret these structures to represent ancient traces of microbial activity formed during biogenic etching of the originally glassy pillow rims as microbes colonized the glass surface. Petrographic observations coupled with overlapping metamorphic and magmatic dates indicate this process occurred soon after eruption of the pillow lavas. Subsequent greenschist facies seafloor hydrothermal alteration caused the structures to be mineralized by titanite; a process also observed in ophiolitic pillow lavas of much younger age. X-ray mapping reveals the presence of carbon along the margins of the tubular structures interpreted as residual organic material. Disseminated carbonates within the microbially-altered BGB pillow rims have low carbon isotope values consistent with microbial oxidation of organic matter. In contrast, disseminated carbonate in the crystalline pillow interiors have carbon isotope values bracketed between Archean marine carbonate and mantle carbon dioxide. It remains to be seen how deep into the Archean oceanic crust these microbes penetrated. In modern oceanic crust their highest activity occurs near 300m subsurface at temperatures around 70 degrees Celsius. If, as some suggest, the Archean ocean was relatively hot, then the depth distribution of the microborings should be biased to pillows shallower in the crust than in the modern seafloor. Based on the observed petrographic and geochemical features we propose the glassy rims of the BGB pillow lavas hosted microbial life that left behind biomarkers $\sim$3.5 billion years ago. Remnants of Archean oceanic crust may thus be one of the most promising places to search for vestiges of early life on Earth.
B53B-0990 1340h
A Possibility For Biological Origin Of Iron Oxides In The Marble Bar Chert, Pilbara Craton, Western Australia
The Marble Bar Chert contains abundant hematite-rich bands, and has been interpreted that the hematite was produced by modern weathering. On the other hand, other geologists have emphasized the oxidation of aqueous ferrous ions by iron bacteria. To understand the precipitation of the iron minerals in Marble Bar Chert, we have examined the mineral assemblages, textures and chemical compositions in fresh core which obtains by Archean Biosphere Drilling Project (ABDP). The ABDP core of the Marble Bar Chert is composed of organic-rich black chert, silicified white chert, and rhythmically banded chert consisted of white, red and black band, in order of sedimentation. Only the rhythmically banded chert has high magnetic susceptibility, which is associated with the existence of numerous hematite and magnetite grains. The colour from red to black is correlated with the proportion of hematite and magnetite. Euhedral grains of pyrite of 1 to 50 micrometer in diameter are observed to be layered in bands as well as to be precipitated in veins. This texture suggests that the pyrite might be of syngenetic. Red bands in the rhythmically banded chert contain numerous hematite grains, which are accumulated as spherical grains of about 100 nm in diameter to euhedral grains up to 500 nm in quartz with the size of 10-30 micrometer in diameter. The quartz with hematite grains shows botryoidal texture. Black bands also contain numerous euhedral magnetite grains of about 1 micrometer in diameter, and the occurrence is similar to that of red bands. These botryoidal textures resemble that of modern bacterial mat of benthic coccoid cyanobacteria. Both bands contain euhedral grains of siderite of micron size, in the botryoidal quartz. The carbon isotopic ratios of siderite are from _E to +2 per mill, suggesting siderite was produced by the reaction between dissolved ferrous ions and bicarbonate ions in seawater. SEM images show bacteria-like cells connecting with and enveloping hematite grains in the botryoidal quartz. The intensity of cathodoluminescence from the botryoidal quartz is heterogeneous, and its two-dimensional image shows bacteria-like texture. The carbon X-ray photoelectron spectra acquired from banded cherts contain a peak at about 285 eV, suggesting C-C and C-H bonding. Furthermore, the Raman spectrum from the black band chert exhibits the peaks at about 1300cm$^{-1}$ and 1600cm$^{-1}$, derived from organic carbon. The scanning image of Raman spectrum reveals the heterogeneous distribution of organic carbon. The occurrences of mineral and organic carbon suggest that the rhythmically banded cherts have not been affected by hydrothermal alteration, metamorphism or modern weathering, but suggest that the original texture has remained unchanged. The occurrences suggest further that the precipitation of iron oxides was related with biological activity.
B53B-0991 1340h
Iron Isotope Analyses for the Origin of Fe-Bearing Minerals in the 3.46 Ga Marble Bar Chert, Pilbara Craton, Western Australia, Recovered by Archean Biosphere Drilling Project (ABDP)
In 2003, "Archean Biosphere Drilling Project" recovered 3.46 billion-years-old modern-weathering free banded chert of the Marble Bar Chert Member of the Towers Formation of the Warrawoona Group in the East Pilbara Granite-Greenstone Terrane, Western Australia. The chert samples contain variable abundance of Fe-bearing minerals such as hematite, magnetite, siderite and pyrite. Hematite bands of mm or sub-mm order in thickness are composed of micro-crystals of hematite and cut by several generations of quartz veins. Pyrite crystallization is found both in the quartz veins and the hematite bands. These observations suggest that the hematite bands, probably syn-depositional, were formed earlier than the pyrite crystals. Iron isotope analyses were performed on individual Fe-bearing minerals separated by micro-drilling of the chert samples. Such data, when combined with published data for Fe isotope fractionation between minerals and fluids in natural environments and laboratory experimental system, are expected to give information on (1) the Fe isotope compositions of the fluid or precursor Fe-bearing minerals from which the Fe-bearing minerals were formed, (2) the degree of involvement of microbiological processes such as bacterial Fe reduction / oxidation in the mineralization of Fe-bearing minerals, and (3) redox state of the Early Archean deep marine environments. If hematite mineralization was primary and precipitation of Fe-(oxy)hydroxide as its precursor occurred during sedimentation by oxidizing dissolved ferrous Fe by oxygen, or if microbes were involved in the formation of Fe-bearing minerals, then far-reaching implications will emerge for the redox evolution of the atmosphere and oceans and the evolution of biosphere in the Early Archean.
B53B-0992 1340h
Sulfide Mineralization In The Marble Bar Greenstone Belt Around Mount Edger Batholith, Pilbara Craton, Western Australia
Pilbara Craton is one of the most important regions in the world to understand the evolution of early Earth, because the geological history is well preserved through the metamorphism of low grade. Pilbara Craton, Western Australia, consists of two different tectonic components formed 3.6 and 2.8 Ga; an older Archean granite occupying the east Pilbara and greenstone belt. In the east Pilbara, the most conspicuous structures are broad domal granitoid complex separated by narrow synformal greenstone belts, and a model of continuous lithostratigraphy in the greenstones in which the dominant structures were produced by multi-stage granitoid diapirism. The Marble Bar greenstone belt is distributed around the Mount Edgar of granitoid pluton, and numerous hydrothermal gold veins are distributed in the greenstone near the boundary of pluton. Also base-metal veins and volcanogenic sedimentary type deposits are located in the same area. In this study, we examined the hydrothermal mineralization observed in the core samples of the Marble Bar greenstone belt, drilled at the Salgash area by the Archean Biosphere Drilling Project (ABDP). The Salgash drill hole is composed of tuff breccias with numerous fragments of black shale of 100 m in thickness, alternation of sandstone and shale of 40 m in thickness, basaltic lava and tuff of 30 m in thickness, and shale and sandstone of 110m in thickness with some sills of basalt and ultramafic rock. The rocks had been metamorphosed, and the grade is near the boundary between green schist and amphibolite facies. Low REE content (43 to 88 ppm), low La/Yb ratio (6.3 to 14.3), and high Eu/Eu* ratio (0.9 to 1.3) of the volcanic rocks are ordinal as the basaltic rock in Archean greenstone belts. On the other hand, these rocks show extremely high values of Cr (1500ppm), Ni (700ppm), Co (70 ppm), and Zn (600 ppm). The C isotopic ratios of carbonate in the volcanic rocks are around -3.8 permil. The clastic sediment sandstone and black shale show similar features, although the absolute values are slightly different. These data suggest that the clastic sediments had been delivered mainly from the volcanic rocks. The C isotopic ratios of organic carbon in the black shale are from -26 to -30 permil, which implies the activity of bacteria. Horizontal veins are often recognized in the clastic sediments, and are composed of quartz, calcite, sphalerite, pyrite, arsenopyrite, pyrrhotite and chalcopyrite. The C isotopic ratios of vein carbonates are from -4.6 to -5.2 permil. The S isotopic ratios of sulfide minerals are concentrated from -1.5 to +0.9 permil. The quartz and calcite veins which are 10 cm in maximal diameter run through the sediment rock. The homogenization temperature of fluid inclusion ranges from 150 to 250 degree with the average of about 200 degree centigrade which concludes well with metamorphic grade, suggesting the origin of metamorphism. The fluid is composed of H$_{2}$O, CO$_{2}$, CH$_{4}$ and C, those constituents seem to reflect the characteristic environments of early Earth, which differ from the accretional material of Phanerozoic, suggesting different tectonic circumstance.
B53B-0993 1340h
3.2-Ga Dixon Island Formation VS. 3.5-Ga Marble Bar Chert: Stratigraphic correlation of the volcano-hydrothermal sequences in the Pilbara, Australia.
The 3.2-Ga Dixon Island Formation in the coastal Pilbara terrane and the 3.5 Ga Marble Bar Chert in the Warrawoona Group in Pilbara Craton of western Australia, are well preserved an Archean hydrothermal stratigraphic sequence containing organic black chert and Fe rich iron chert or BIF. We did detail mapping (1/500 and 1/100 scales) to recognize previous ocean floor environments. Especially the Dixon Island Formation is exposed excellent preservation along the coast (7 km long) which is only location of the greenstone belt in the Pilbara. The stratigraphy of these sequences preserved quite resembles. They form volcanics (rhyolite tuff, pillow basalt), highly altered zone with hydrothermal black chert vein, black chert, varicolored (black and white) chert and red chert or BIF from bottom to top. Many black-chert vein swarms imply intensive low-temperature hydrothermal activity during deposition of black chert above the basement volcanics. These resemble stratigraphy, which is called Black chert-BIF (BCB) sequence, indicate the one of the standard sedimentary sequence of Archean oceanic hydrothermal environments. Metamorphic grade and structural deformation is different. The Dixon Island Formation is situated less than prehnite-pumpellyite facies with D2 left-lateral strike-slip deformation (Kiyokawa et al., 2002). It contains many previous sedimentary structures. On the other hand, the Marble Bar Chert is affected NNW compressional deformation and lower greenschist facies metamorphic grade and most carbonaceous materials are decomposed. In detail, absence of detrital sediments of continental origin in the Dixon Island formation implies that this sedimentary facies represents a hydrothermal environment at about 500~2000 m in paleo-depth. Microbial material has been preserved well in the black chert bed, which is composed of massive black chert and laminated black chert. The massive black chert has carbonaceous peloids (0.3 mm~2 mm in diameter) similar to those in the black chert veins. The massive black chert of the Dixon Island Formation contains wriggle-, rod- and dendrite-shaped bacterial-shape material. The black chert of the Marble Bar Chert, however, preserved more deformed black carbonate materials and poor rod-shape matarials. Geochemical data of the Dixon Island Formation as follows: total organic carbon (TOC) in the black chert and black chert veins varies within 0.05 ~ 0.16% (average 0.1%) and the carbon isotope (delta 13C) values of these rocks are -35~ -27 per mil (average 30 per mill). Sulfur isotope (delta 34S and delta 33S) values of the pyrite in black chert rocks are -1~ -9.9 per mil and <ETH>1.3~5.6 per mil. This evidence suggests that the carbonaceous grains and bacteria-shaped material in the black cherts in the Dixon Island Formation are biogenic and formed close to a hydrothermal vent system. Based on the field observations and geochemical evidences suggest that the Dixon Island Formation and Marble Bar Chert are quite resemble sedimentary environments on the ocean floor with biogenic microbial colony near hydrothermal vents in the Archean. The black carbon materials in the Marble Bar Chert may be decomposed by diagenesis and metamorphism.
B53B-0994 1340h
Carbonaceous Matter in the Black Cherts from the Dixon Island Formation, Western Pilbara, Australia (3.2 Ga)
The Dixon Island Formation is one of the best locations that preserve an Archean sea-floor hydrothermal system. The black chert vein is a characteristic for the Archean sea-floor hydrothermal system, and it contains three types of carbonaceous particles that may be derived from Archean microbes (Kiyokawa et al., 2001). In this investigation, we tried to characterize the carbonaceous matter in the black cherts. After washing the samples with acetone, they were powdered and extracted sonifically with chloroform : methanol = 1 : 2. The extracts were analyzed by GC-MS, as TMS derivatives prepared with BSTFA. The residues were treated with HF/HCl (10M-1M) to remove silicates. After centrifuging and freeze-drying, black powders were obtained. One sample of them was saponified with 0.5 M KOH-MeOH and extracted with ether (basic-neutral fraction). The water layer was acidified (pH2.0) and extracted with ether in the same manner (acidic fraction). A portion of the black powders were subjected to elemental analyses. Gas chromatograms of the chloroform : methanol -extracts show various organic compounds including straight-chain hydrocarbons, however it remains uncertain whether they are indigenous or not. Compounds were rarely found in the ether fractions from saponified sample, suggesting that the carbonaceous macromolecular matter in the black chert has already lost ester bonds and that now it is on the way to graphite. This is consistent with the following results of the elemental analyses. H/C ratios of the residues after HF/HCl treatment range from 0.40 to 0.52, indicating the early metagenesis stage of which temperature is around 175 - 200 $^{o}$C for oil source rocks. However, because the black cherts are much older than typical oil source rocks, the same maturity may be reached at much lower temperature.
B53B-0995 1340h
Carbon Isotope Excursions of Archean Organic Matter (~3.0-2.5Ga) from Four Drilling Cores at the Hamersley Basin, Western Australia
Archean biological activities have played an important role for the evolution of the earth's surface environment. In particular, the carbon isotopic composition of Archean organic matter has been used to infer types and predominance of the biological activities. As a part of Archean Biosphere Drilling Project (ABDP), we determined carbon isotopic compositions of bulk organic matter in Archean shales (~3.0-2.5Ga) from four drilling cores at the Hamersley Basin, Western Australia. Organic carbon from Mosquito Formation (~2.9-3.2Ga) has ~-31% (relative to PDB) with much less variation through a ~130m core section, suggesting normal photosynthesis by algae and/or cyanobacteria using rubisco. At the ~2.8Ga black shale interbedded in Mount Roe basalt, organic matter is extremely 13C-depleted (~-50%) through ~20m, implying abundant methane assimilation. Through a ~140m core section in the Jeerinah Formation (~2.7Ga), carbon isotopic composition is highly variable between -44 and -35% with high organic carbon contents (~8 wt%). Then at the Mount McRae Shale (~2.5Ga), organic carbon is gradually enriched in 13C, varying from -37 and -32% through a ~25m core section. The carbon isotope excursion of this study indicates that methane assimilation suddenly occurred in the Hamersley Basin at ~2.8Ga, followed by gradually recovering to normal photosynthesis around ~2.5Ga.
B53B-0996 1340h
Microbial Ecosystem In The Oldest Freshwater Lake Revealed From A Drill Core Of The 2.76 Ga Hardey Formation, Pilbara District, Western Australia
The Hardey Formation in the Pilbara district of Western Australia, is the oldest known (2.76 Ga) lacustrine deposit, was one of eight targets of the Archean Biosphere Drilling Project (ABDP). Rocks in the drilling area have been subjected to very low-grade metamorphism (zeolites facies). The recovered drill core (~145 m in depth) is divisible into two depth zones: (1) the upper zone (<~80 m), comprising mostly coare-grained, gray, arkosic sandstones, and (2) the lower zone, comprising alternating layers of finely laminated, black shales and calcareous sandstones. We selected 26 samples from the lower zone at 2.5-m intervals, and conducted petrographical, mineralogical, and geochemical investigations (e.g., chemical mapping of major and trace element (including REEs); organic and carbonate C, and S contents; C isotope). Bedding planes are well preserved, comprising alternating layers of organic-C rich clay and organic-C poor carbonate (10-20 ?m). Presence of cross-lamination suggests a low energy depositional environment, probably a shallow lake. Major and trace element contents of all samples are quite homogeneous. Organic C- and pyrite S contents fall in narrow ranges, from 0.2 to 0.7wt% (a mean value = 0.5wt% and s.d. = 0.1wt%) and from 0.01 to 0.1wt% (a mean value = 0.04 wt%; s.d. = 0.02 wt%), respectively. Compared to average Archean marine shales (organic C = 0.45 wt%; S = 0.1 wt%), the Hardy shales are comparable in organic C but are much depleted in pyrite S contents. The low S/C ratios of the Hardy shales concur with a freshwater depositional interpretation. The C isotope values for the organic C vary from -36.6 to -27.6% (a mean value = -32.7%; s.d. = 1.8%). There is no stratigraphic trend, but theycorrelate well with the lithology: -31.9 to -27.6% (a mean value = -30.4%) for disseminated-grains and seams of organic C in calcareous sandstone; and -36.6 to 32.6% (a mean value = -33.4%) for finely-laminated black shales. The heavier values may represent microbial communities in shallow water, while the lighter values represent microbial communities in deeper water. The carbon isotope systematics in the Hardy Formation are essentially identical to those of the 2.75 Ga marine shales at Kidd Creek, Ontario, Canada. These data suggest that the fresh-water ecosystems were already as diversed as marine ecosystems at 2.76 Ga ago.
B53B-0997 1340h
Significance of nickel-rich sulfides in drilled core samples of the 2.7 Ga Mt. Roe sedimentary rocks: TEM and magnetic studies
The Mt. Roe Basalt is considered to be a 2.7 Ga flood basalt, widely occurring in the northern Pilbara area of Western Australia. Sedimentary rocks interbedded in the basaltic flows have been recognized by previous investigators. However, their sedimentation environments are poorly understood. Sedimentary sections of the Mt. Roe Basalt were directly drilled during the course of this study, and here we report on unknown and unique characteristics of organic carbon rich sediments in these core samples. Six basaltic lava flows of 3 to 10 m in thickness are recognized around the drilled site, about 10 km southeast from Whim Creek. Two sedimentary sections of ca. 10 m thickness are found on specific lava flows. These sedimentary rocks are rich in clastic components, and contain cross laminations and ripple marks, suggesting rapid change in the paleo-current of shallow ocean water. These sedimentary rocks and basalts were drilled and ca. 300 m successive core sections were recovered. Surprisingly organic carbon- and sulfide-rich shale was found in the two sedimentary sections. To identify the sulfide mineral assemblages, chemical composition and crystal structure analyses were carried out by using a high-resolution transmission electron microscope (TEM) with analytical electron microscopy. Sulfides occur as complex mineral assemblages: pyrrhotite, pyrite, pentlandite, chalcopyrite, and sphalerite. These were not recognized in the heavily weathered outcrop samples. Pyrrhotite was a dominant mineral in the assemblage. Notable features of these sulfides was the development of crystal defects in Fe-sulfides (po and py), and their abnormally high nickel concentrations (po: $\sim$3 wt.%; py: $\sim$1 wt.%). These crystal habits are most likely diagenetic features rather than a later low-grade metamorphic signature. Magnetic intensities of the basalts and sedimentary rocks were also measured in the drilled core samples, varying from 1 x 10$^{-3}$ to 1 x 10$^{2}$ A/m. It is remarkable that the magnetic intensities of the sulfide-rich sedimentary rocks in the lower sedimentary section are higher than those of fresh basalt samples. Such magnetism is preserved in the Fe-sulfide minerals and crystal habits and magnetic intensities are highly correlated. Because of the high probability for a diagenetic origin of the examined sulfides, the 2.7 Ga geomagnetic field was probably recorded and preserved in the Fe-sulfide minerals. This is the first attempt to indicate that Archean diagenetic sulfides are useful to tool to reconstruct the ancient geomagnetic field.
B53B-0998 1340h
Role Of Hydrothermal System At Shallower Depth In 2.77 Ga Alteration Of Mt. Roe Basalt, Pilbara, Western Australia
The sericite-chlorite alteration zone of the 2.77 Ga Mt. Roe basalt near Whim Creek, Pilbara, Western Australia, has been attracting attention if it is paleo-weathering profile (paleosol) formed under anoxic atmosphere or if it is hydrothermally altered zone by reduced fluid. It is also interesting in the standpoint of biological activity at late-Archean era, as it has been reported that the black veins in alteration zone and the sediment above it are characterized by the organic carbons with d13C values of methanogen and methanotroph. A fresh and consecutive core of Mt Roe basalt including alteration zones, which was drilled by ABDP (Archean Biosphere Drilling Project) in July 2003, gives new and more detailed insight into this concern. The core is ca. 300 m long, and is composed of amygdaloidal/massive basaltic lavas and tuffs with clastic sediments interbedded. About seventy selected samples were studied mineralogically using microscope, XRD, XGT and EPMA, and were analyzed chemically for major and trace elements using XRF and ICP-mass. The results show that (1) the core is composed of at least three lava/tuff units with different Ti/Zr and Zr/Hf ratios which are separated by thick sedimentary units, (2) the sedimentary units are comprised of sandstone, siltstone and shale, all of which are black-colored and rich in organic carbon, (3) the strongly sericitized horizons of several meters were recognized just below these sedimentary units, (4) thin sericitized layers are interbetted in amygdaloidal basalt and tuff horizons, the major part of which is associated with the fine-grained, black and organic carbon-rich quartz vein, (5) in some parts of sericitized zone, sulfide, Zn-rich and Ti-REE veins were observed, (6) all of the sericitized zone is characterizes by the depletion of Fe, Mg, Ca, Na and P and the enrichment of K, (7) several other kinds of veins, such as quartz, calcite, and chlorite, distribute across the basalt horizon, (8) organic carbons in black vein show d13C values of -40 to -50 per mil (PDB), and (9) methane was the only gas detected from the fluid inclusions in quartz vein. These observations suggest the followings, (1) there might be at least two stages of alteration; (2) the first one could be responsible for the alteration of plagioclase to calcite and mafic minerals to chlorite, for the calcite and chlorite veins, and for filling up amygdules; (3) the second one might relates to submarine hydrothermal activity at shallower depth, (4) the hydrothermal fluid might circulate through depositing sediment and top of erupted basalt/tuff, (5) methanogene could inhabitate in hydrothermal vein producing methane and made the fluid strongly reducing, and (6) methanotroph could inhabitate around seafloor. As the black hydrothermal vein was not observed in uppermost massive lava unit, the timing of this alteration would have been essentially concurrent with the volcanic activity. This is concordant with the previously reported SHRIMP age of around 2.8 Ga for euhedral apatite minerals in hydrothermal vein.
B53B-0999 1340h
Biomarker Record From the Tisdale Group (2707 - 2705 Ma) and Porcupine Group (2685 - 2673 Ma) of the Abitibi Subprovidence, Timmins, Ontario, Canada
Saturated/unsaturated and aromatic hydrocarbon fractions extracted from greenshist facies metasedimentary greywackes, siltstones, and shales of the Tisdale (2707-2705 Ma) and Porcupine Group (2685-2673 Ma) of the Abitibi Subprovince, Ontario Canada were analyzed for biomarkers. Core and hand samples were obtained from three mines run by Porcupine Joint Ventures and the core library at the Ministry of Northern Development and Mines, Timmins, Ontario. Tisdale Group sedimentary facies were previously interpreted as laminated massive sulfide and/or interflow sediments interstratified by subaqueous komatitic-tholeiitic flows. Porcupine Group turbidite facies were interstratified by massive, brecciated alkaline lava flows and reworked volcaniclastic facies. All Porcupine Group samples contained primary depositional fabric. Biomarker maturity calculations indicate the sediments are mature and either reached or surpassed oil generation. All samples contained hopanes C$_{27}$ 22,29,30-trisnorhopane-II (Ts & Tm), C$_{29}$ $\alpha$$\beta$-30-norhopane, C$_{29}$ 18$\alpha$-30-norneohopane, C$_{30 }$ $\alpha$$\beta$ hopane, C$_{30}$ $\beta$$\alpha$- hopane, and C$_{31}$-C$_{35}$ (22S & R) homohopanes. Most samples contained C$_{27}$-C$_{29}$ (20S & R) $\beta$$\alpha$ diacholestane, C$_{27}$-C$_{29}$ $\alpha$$\beta$$\beta$ cholestane (20S & R), C$_{27}$-C$_{29}$ $\alpha$$\alpha$$\alpha$ cholestane (20S & R), and 4-methyl steroids. Tricyclic terpanes ranging from C$_{19}$-C$_{29}$, but were not present in all samples. Biomarkers from the Tisdale Group samples are similar to those found in modern hydrothermal systems and include unresolved complex mixtures, abundant branched alkanes with quaternary carbon atoms, alkyl-cyclohexanes (C$_{16}$-C$_{29}$ with strong odd over even dominance), alkyl-cyclopentanes (C$_{16}$-C$_{29}$) containing only even carbon number homologs), and C$_{37}$-C$_{40}$ acyclic and cyclic archeal isoprenoids. Biomarkers from the Porcupine Group samples include C$_{16}$-C$_{35}$, branched alkanes, C$_{16}$-C$_{29}$ alkyl-cyclohexanes and alkyl-cyclopentanes (with no carbon number preference). Aromatic fraction for this group were generally low and included mono, di, and trimethyl naphthalenes, phenanthrenes, and aryl-isoprenoids. Several of these samples also contained abundant C$_{37}$-C$_{40}$ acyclic and cyclic archeal isoprenoids. The diversity of biomarkers within these two stratigraphic groups suggests that the hydrocarbons are syngenetic with their host rock and represent a change in depositional environments from that of a sulfidic hydrothermal vent system to one later overlain by continental shelf sediments.
B53B-1000 1340h
Variations in Organic $\delta ^{13}$C and Radiolarian Population Dynamics in the Silurian Cape Phillips Formation, Cornwallis Island, Canadian Arctic
Samples of graptolitic calcareous shale from the Silurian Cape Phillips Formation, located on Cornwallis Island, Canadian Arctic were collected for geochemical and paleontological study. Radiolarians were recovered and identified from 16 sample horizons from a 21 meter section. This section encompasses most of the \textit{perneri} Graptolite Zone. Calcareous shale from 16 horizons was analyzed for $\delta ^{13}$C$_{organic}$ and $\delta ^{13}$C$_{carbonate}$. At the base of the section there is a baseline shift in $\delta ^{13}$C$_{organic}$ of -2.5\permil\ over 4.5 meters. The $\delta ^{13}$C$_{organic}$ curve then stabilizes at about 6 meters until the top of the section with the exception of two excursions: a -1\permil\ excursion at 11 m; and a -2\permil\ excursion at 20m. $\delta ^{13}$C$_{carbonate}$ shows a similar baseline shift and prominent excursions versus section height. However, the $\delta ^{13}$C$_{carbonate}$ excursions occur approximately 1m higher in the section than the $\delta ^{13}$C$_{organic}$ excursions. Radiolarians recovered from this section exhibit changes in relative abundance that correlate closely to the $\delta ^{13}$C$_{organic}$ 11m and 20m excursions. Inaniguttidae, Palaeoscenidiidae, and a new species, \textit{taxon 4} are observed to increase in relative abundance at the negative $\delta ^{13}$C$_{organic}$ excursions. In contrast, a second new species, \textit{taxon 6}, was observed to decrease in relative abundance at the negative excursions. The increase in palaeoscenidiids is most conspicuous with numbers exceeding 50{%} of the population at the 11.5 m horizon. The coincidence of fluctuations in the relative abundances of the radiolarian species and shifts in the values of $\delta ^{13}$C$_{organic}$ and $\delta ^{13}$C$_{carbonate}$ suggest an environmental control. Modern radiolarians exhibit similar fluctuations in relation to changes in eutrophication, surface circulation patterns, or watermass structure. Likewise, changes in productivity or surface water characteristics may be factors that drive the relative abundance fluctuations in the Cape Phillips Formation.