Dixon Island Formation Research Articles

Tourmaline in a Mesoarchean pelagic hydrothermal system: Implications for the habitat of early life

The RNA World hypothesis requires the synthesis of RNA to allow the emergence of life on Earth. Hydrothermal systems have been proposed as potential candidates for constructing complex biomolecules. However, in order to successfully form RNA, it is necessary to stabilize ribose, a RNA carbohydrate component. Borate has been found to stabilize ribose. Therefore, boron rich hydrothermal systems are important environments concerning the origin of life on Earth.The 3.2-Ga Dixon Island Formation of the West Pilbara Superterrane, Western Australia, is a volcano-sedimentary sequence. The Formation represents a Mesoarchean pelagic hydrothermal system, which formed adjacent to an immature island arc. Fine-grained tourmaline, in addition to biogenic carbonaceous matter and spherulitic and tubular bacteriomorphs, are found in black chert. A boron-rich environment was responsible for the formation of these deposits. To explore the implications of such a boron enriched environment on microbial activity, modes of occurrence and chemical compositions of the tourmaline were examined.The tourmaline is schorl or dravite of the alkali tourmaline group and the boron isotope compositions range in δ11B from -7.3 to +2.6‰. The tourmaline occurs in microcrystalline quartz matrix of black chert veins that cross cut a volcanic unit and also in a bedded black chert, which overlays the volcanic unit. The volcanic unit contains highly altered zones with hydrothermal veins. The associated lithologic and stratigraphic features suggest that the black chert veins were the conduits for upward moving hydrothermal fluids, which reached the sea floor. Subsequently, the volcanic unit was covered by organic matter-rich cherty sediments that in part were fed, and/or altered, by the hydrothermal fluids.These results suggest that the origin of boron enrichment to form Dixon Island tourmaline is not the associated sedimentary mineral assemblage, which includes diagenetic clay, low-grade metamorphic mica, and organic matter. Instead, the tourmaline was directly precipitated from hydrothermal fluid, enriched in boron. Furthermore, the hydrothermal fluids had already concentrated the boron, in the Mesoarchean pelagic system, prior to the apex of organic matter production and microbial activity. Our findings support a hypothesis that the boron-enriched hydrothermal environment aided the survival and evolution of early life.

An ocean-floor carbonaceous sedimentary sequence in the 3.2-Ga Dixon Island Formation, coastal Pilbara terrane, Western Australia

The Dixon Island Formation of the Pilbara terrane, Western Australia, extends from Cleaverville Beach to the Dixon Island coast, and is the only example worldwide of a coastal outcrop of a 3.2–3.1Ga low-grade greenstone belt. The Dixon Island Formation was situated in an immature island arc setting and comprises siliceous, carbonaceous deep-water sediments that contain evidence for hydrothermal and microbial activity. The extensive outcrop along the coastline makes it possible to examine in detail the characteristics of Mesoarchean sedimentation in a hydrothermal environment. This study focuses on a continuously exposed carbonaceous, black chert succession in the central part of the northern coastline outcrop on Dixon Island. At this site, a 20-m-thick, carbonaceous, black chert sequence conformably overlies basement rocks of highly altered komatiite–rhyolite tuffs. The black chert sequence formed well-bedded black chert with carbonaceous peloidal matter and fragmented grains, and the sequence is homogeneous and finely laminated. In this sequence, evidence of low-temperature hydrothermal fluid, sediments and alteration stractures are well preserved in the lowermost section, indicating that high levels of hydrothermal activity occurred on the ocean floor during deposition. In particular, swarms of black chert veins provide evidence of post-volcanic hydrothermal activity that released organic matter and silica to the ocean. The carbonaceous peloidal textures and δ13Corg values of sediments located just above the basement, which hosts the vein swarms, suggest that the veins were the conduits for hydrothermal fluid which contained organic-rich silica material and that flowed onto the seafloor to form the homogeneous carbonaceous cherts along with hydrothermal-related sediment. The δ13Corg values of organic matter in the black cherts range from −42‰ to −22‰ (average=−31.9‰; n=313). Lighter δ13C values (−35‰ to −42‰) characterize carbonaceous laminated black chert located ∼5m above the basement, where biogenic structures (e.g. biomat bed, microfossil structures) are found. The lighter δ13C values might be related to methanotrophic micro-organic activity within the sediments during hydrothermal activity. In summary, we reconstructed the sedimentary environment upon a Mesoarchean hydrothermal ocean floor that was a site of microbial activity and local methanogenesis.

Distribution and trend of black chert veins in 3.2Ga Dixon Island Formation

Distribution and trend of black chert veins in 3.2Ga Dixon Island Formation

Sea floor surface sediment in Archean: 3.2Ga Dixon Island Formation in Pilbara Australia

Sea floor surface sediment in Archean: 3.2Ga Dixon Island Formation in Pilbara Australia

Field occurrence and lithology of hydrothermal vein in 3.2Ga Dixon Island Formation, Western Australia

Field occurrence and lithology of hydrothermal vein in 3.2Ga Dixon Island Formation, Western Australia

Lateral variations in the lithology and organic chemistry of a black shale sequence on the Mesoarchean seafloor affected by hydrothermal processes: The Dixon Island Formation of the coastal Pilbara Terrane, Western Australia

AbstractThe Dixon Island Formation of the coastal Pilbara Terrane, Western Australia is a 3.2 Ga volcanic–sedimentary sequence influenced by syndepositional hydrothermal activity formed in an island‐arc setting. We documented lateral variations in stratigraphy, hydrothermal alteration, and biological activity recorded in the sedimentary rocks (over several kilometers), with the aim of identifying areas of biological activity and related small‐scale structures. The Dixon Island Formation comprises volcaniclastics, black chert, and iron‐rich chert within seven tectonic blocks. Based on detailed geological mapping, stratigraphic columns, carbon isotope composition, and organic carbon (Corg) content, we found lateral (>5 km) variations in stratigraphy and carbon isotope compositions in a black chert sequence above the Mesoarchean seafloor with hydrothermal activity. Two felsic tuff layers are used as stratigraphic marker beds within a black chert sequence, which was deposited on altered volcanic rocks. The black chert sequence in each tectonic block is 10–20 m thick. Thickness variations reflect topographical undulations in the paleo‐ocean floor due to faulting. Early‐stage normal faults indicate extensional conditions after hydrothermal activity. Black chert beds in the topographically subsided area contain higher Corg contents (about 0.4 wt%) than in areas around the depression (<0.1 wt%). Carbon isotope compositions for the black chert vary from −40 to −25‰, which are similar to values obtained for a black chert vein within the komatiite–rhyolite tuff sequence (underlying the black chert sequence). Those for other rock types in the Dixon Island Formation are −33 to −15‰. Results indicate that deformation occurred soon after the final stages of hydrothermal activity. After this early‐stage deformation, organic‐rich sediments were deposited over an area several kilometers across. The organic‐rich sediments indicate stagnant anoxic conditions that resulted in the deposition of siliceous and organic matter from hydrothermal vein systems. When hydrothermal activity terminated, normal faulting occurred and organic matter was deposited from the sea surface and silica from the seafloor.

Isotopic fractionation of nitrogen and carbon in Paleoarchean cherts from Pilbara craton, Western Australia: Origin of 15N-depleted nitrogen

Nitrogen and carbon isotopic compositions, together with mineralogy and trace element geochemistry, were studied in a few kerogen-rich Paleoarchean cherts, a barite and a dolomitic stromatolite belonging to the eastern (Dixon Island Formation) and western (Dresser and Strelley Pool Chert Formations; North Pole Dome and Marble Bar) terranes of Pilbara Craton, Western Australia. The aim of the study was to search for 15N-depleted isotopic signatures, often found in kerogens of this period, and explain the origin of these anomalies. Trace elements suggest silica precipitation by hydrothermal fluids as the main process of chert formation with a contamination from volcanoclastic detritus. This is supported by the occurrence of hydrothermal-derived minerals in the studied samples indicating precipitation temperatures up to 350 °C. Only a dolomitic stromatolite from Strelley Pool shows a superchondritic Y/Ho ratio of 72 and a positive Eu/Eu * anomaly of 1.8, characteristic of chemical precipitates from the Archean seawater. The bulk δ 13C vs. δ 15N values measured in the cherts show a roughly positive co-variation, except for one sample from the North Pole (PI-85-00). The progressive enrichment in 15N and 13C from a pristine source having δ 13C ⩽ −36‰ and δ 15N ⩽ −4‰ is correlated with a progressive depletion in N content and to variations in Ba/La and Co/As ratios. These trends have been interpreted as a progressive hydrothermal alteration of the cherts by metamorphic fluids. Isotopic exchange at 350 °C between NH 4 + (rock) and N 2(fluid) may explain the isotopic and elemental composition of N in the studied cherts. However, we need to assume isotopic exchange at 350 °C between carbonate C and graphite to explain the large 13C enrichment recorded. Only sample PI-85-00 shows a large N loss (90%) with a positive δ 15N value (+11‰), while C (up to 120 ppm and δ 13C −38‰) seems to be unaffected. This pattern has been interpreted as the result of devolatilization and alteration (oxidation) of graphite by low-temperature fluids. The 15N- 13C-depleted pristine source has δ 15N values from −7‰ to −4‰ and 40Ar/ 36Ar ratios from 30,000 to 60,000, compatible with an inorganic mantle N source, although the elemental abundance ratios N/C and 40Ar/C are not exactly the same with the mantle source. The component alternatively could be explained by elemental fractionation from metabolic activity of chemolithoautotrophs and methanogens at the proximity to the hydrothermal vents. However, ambiguities between mantle vs organic sources of N subsist and need further experimental work to be fully elucidated.

DXCL掘削報告2：オーストラリア・ピルバラ海岸グリーンストーン帯における32億年前のデキソンアイランド層・DXサイトの例

DXCL掘削報告2：オーストラリア・ピルバラ海岸グリーンストーン帯における32億年前のデキソンアイランド層・DXサイトの例

Middle Archean volcano-hydrothermal sequence: Bacterial microfossil-bearing 3.2 Ga Dixon Island Formation, coastal Pilbara terrane, Australia

The 3.2 Ga Dixon Island Formation in the Cleaverville Group of the coastal Pilbara terrane, Australia, is one of the most complete and best-preserved examples of middle Archean oceanic stratigraphy and contains possible microbial material. Field observations and geochemical evidence suggest that this formation contains a low-temperature hydrothermal vent system with a biogenic microbial colony from the Archean ocean. The Dixon Island Formation is ∼350 m thick and consists of the Rhyolite Tuff, Black Chert, and Varicolored Chert Members, in ascending order. The Rhyolite Tuff Member contains many vein swarms, such as quartz and black chert veins, and highly altered rhyolite tuff layers, which are identified as an underground bypass zone for circulating hydrothermal fluid. Many black chert vein swarms in the Rhyolite Tuff Member imply intensive low-temperature hydrothermal activity during deposition of the Black Chert Member, which is 10–15 m thick. The Black Chert Member is composed of massive black chert, laminated black chert, dark-greenish siliceous shale and tuffaceous laminated chert, which are mainly composed of very fine quartz. Abundant pseudomorphs of silica after aragonite, barite, and gypsum, and a distinctly continuous, stromatolite-like biomat layer (10–20 cm thick), are preserved within the laminated black chert bed. The stromatolite-like biomat bed is formed of fine iron or iron-coated quartz pisolite within fine-grained silica. The absence of detrital sediment of continental origin and the many vein injections imply that this sedimentary facies represents a pelagic hydrothermal environment at ∼500–2000 m paleodepth, and may have been on the slope of an immature island arc. Microbial material has been preserved well in the lower part of Black Chert Member. The massive black chert has carbonaceous peloids (0.3–2.0 mm in diameter), which are similar to those in the black chert veins. The massive black chert contains spiral-, rod, and dendrite-shaped bacterial material. The total organic carbon (TOC) value of massive black chert in the lower part of the Black Chert Member is higher (TOC = 0.15–0.45%) than that of the overlying laminated chert section (TOC = 0.02–0.15%) and the black chert vein (TOC = 0.1–0.13%), and the carbon isotope (δ 13 C) values of this lithology (−33‰ to ∼−27‰) are also lighter than for the black chert veins (−29‰ to ∼−26‰) and the laminated black chert in the upper part of the Black Chert Member and the Varicolored Chert Member (−27‰ to ∼−13‰). This evidence suggests that the carbonaceous grains and bacteria-shaped material in the lower part of the Black Chert Member are of bio-genic origin and were formed close to a low-temperature hydrothermal vent system. The microbial colony may have been rapidly fossilized by silicification related to hydrothermal activity. Laminated black chert in the upper part of the Black Chert and the Varicolored Chert Members may have formed by cyanobacterial sedimentation from the ocean surface.

The Dixon Island Formation of the Western Pilbara Coastal Greenstone Terrane

Hydrothermal activity is considered to be responsible for ubiquitous silicification of Archean supracrustal rocks. Here, we show well-preserved evidence of hydrothermal silicification with in the Dixon Island Formation (3.2Ga) of Pilbara Craton, Australia. The formation is a part of immature oceanic island arc origin Cleaverville Group consists of three cycles of volcano?sedimentary sequence (Kiyokawa and Taira, in press). Each sequence consists of thick volcanics (basalt, rhyolite), black shale, ferruginous chert and BIF to the top. Especially, the Dixon Island Formation, which is the best exposed sequence, is composed of the Rhyolite Tuff, Black Chert and Varicolored Chert Members. The Rhyolite Tuff Member contains the multiple black chert veins which is feeders of the hydrothermal vent. These veins probably produced the chemogenic sediments of the upper members.