Birger Rasmussen - Profile on Academia.edu (original) (raw)

Papers by Birger Rasmussen

Geology, 2005

Petroleum generation largely occurs through the thermal decomposition of organic matter. The pres... more Petroleum generation largely occurs through the thermal decomposition of organic matter. The presence of oil-bearing fluid inclusions and pyrobitumen in Archean rocks suggests that similar processes operated as early as ca. 3.25 Ga. However, direct evidence of petroleum generation from potential source rocks is lacking, and an abiogenic origin has been proposed for some Archean carbonaceous residues. Pilbara craton ca. 3.2 Ga and ca. 2.63 Ga black shales were found to contain abundant kerogenous streaks and laminae, as well as bitumen nodules (comprising a radioactive mineral core surrounded by a carbonaceous rim) and pyrobitumen (formerly petroleum) globules, films, and aggregates. The bitumen nodules formed around detrital radioactive grains via polymerization of fluid hydrocarbons generated within the shale and represent diagnostic indicators of oil generation in ancient shales. The bitumen globules, films, and masses are preserved within authigenic pyrite and demonstrate that a separate hydrocarbon phase had developed in the shale matrix during burial, providing compelling evidence for in situ petroleum generation and expulsion. The abundance of bitumen nodules and residual pyrobitumen in black shales across the Pilbara craton suggests that hydrocarbon generation from kerogenous shales was a common phenomenon during the Middle to Late Archean. The petroleum was generated from organic matter that accumulated in marine environments, most probably comprising the remains of photosynthetic and chemosynthetic organisms, pointing to a sizeable biomass as early as 3.2 Ga.

Contributions to Mineralogy and Petrology, Jul 29, 2005

Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimen... more Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimentary rocks worldwide, ranging in age from mid-Archaean to Palaeozoic. The outgrowths comprise minute (typically <3 lm) crystals that form an irregular fringe on detrital zircon grains, and in a few cases, on diagenetic xenotime outgrowths. Textural relationships indicate that while zircon growth postdates diagenetic xenotime precipitation, it precedes or is synchronous with metamorphic xenotime formation. Unlike xenotime, zircon outgrowths are absent in unmetamorphosed sedimentary rocks, and only appear in prehnite-pumpellyite facies rocks, suggesting that zircon growth commences at temperatures of $250°C. The greater abundance of zircon outgrowths in shales than in to other sedimentary rocks may relate to higher halogen concentrations, which have been linked to enhanced zirconium mobility in hydrothermal systems. The growth of zircon in metasedimentary rocks indicates that zirconium was transported in aqueous fluids, possibly as fluorine complexes, during very low-grade metamorphism.

In Situ SHRIMP U-Pb Geochronology and Petrography of Mafic Dykes in the Yilgarn Craton, Western Australia, and Vestfold Hills and Bunger Hills, East Antarctica

AGUFM, Dec 1, 2015

THE IMPACT OF EARLY-DIAGENETlC ALUMINOPHOSPHATE PRECIPITATION ON THE OCEANIC PHOSPHORUS BUDGET

Marine Authigenesis: From Global to Microbial, 2000

Petrology and Stratigraphy of Subsurface Sandstones Near the Permo-Triassic Boundary, Northern Perth Basin, Western Australia

Diagenesis of Reservoir Sandstones from Selected Western Australian Sedimentary Basins, with Emphasis on Bitumen-rimmed Radioactive Heavy Minerals

Geology, 2005

Petroleum generation largely occurs through the thermal decomposition of organic matter. The pres... more Petroleum generation largely occurs through the thermal decomposition of organic matter. The presence of oil-bearing fluid inclusions and pyrobitumen in Archean rocks suggests that similar processes operated as early as ca. 3.25 Ga. However, direct evidence of petroleum generation from potential source rocks is lacking, and an abiogenic origin has been proposed for some Archean carbonaceous residues. Pilbara craton ca. 3.2 Ga and ca. 2.63 Ga black shales were found to contain abundant kerogenous streaks and laminae, as well as bitumen nodules (comprising a radioactive mineral core surrounded by a carbonaceous rim) and pyrobitumen (formerly petroleum) globules, films, and aggregates. The bitumen nodules formed around detrital radioactive grains via polymerization of fluid hydrocarbons generated within the shale and represent diagnostic indicators of oil generation in ancient shales. The bitumen globules, films, and masses are preserved within authigenic pyrite and demonstrate that a separate hydrocarbon phase had developed in the shale matrix during burial, providing compelling evidence for in situ petroleum generation and expulsion. The abundance of bitumen nodules and residual pyrobitumen in black shales across the Pilbara craton suggests that hydrocarbon generation from kerogenous shales was a common phenomenon during the Middle to Late Archean. The petroleum was generated from organic matter that accumulated in marine environments, most probably comprising the remains of photosynthetic and chemosynthetic organisms, pointing to a sizeable biomass as early as 3.2 Ga.

Earth-Science Reviews, 2005

Recent advances in the field of geochronology have led to a greater understanding of the scale an... more Recent advances in the field of geochronology have led to a greater understanding of the scale and duration of geological processes. It is currently possible to date igneous and metamorphic rocks by a variety of radiometric methods to within a million years, but establishing the depositional age of sedimentary rocks has remained exceedingly difficult. The problem is most pronounced for Precambrian rocks, where the low diversity and abundance of organisms have prevented the establishment of any meaningful biostratigraphic framework for correlating strata. Also, most Precambrian successions have been metamorphosed, rendering original minerals and textures difficult to interpret, and resetting diagenetic minerals. Xenotime (YPO 4 ) is an isotopically robust chronometer, which is increasingly being recognized as a trace constituent in siliciclastic sedimentary rocks. It may start to grow during early diagenesis, typically forming syntaxial outgrowths on detrital zircon grains. Diagenetic xenotime occurs in a wide variety of rock types, including conglomerate, sandstone, siltstone, shale, phosphorite and volcaniclastic rocks, varying from early Archaean to Mesozoic in age. The formation of diagenetic xenotime is principally related to redox cycling of Fe-oxyhydroxides and microbial decomposition of organic matter, leading to elevated concentrations of dissolved phosphate and rare earth elements (REE) in sediment pore-waters. Xenotime has the properties of an ideal U -Pb chronometer, containing elevated levels of U (generally >1000 ppm) and very low concentrations of initial common Pb. In addition, it has an exceptional ability to remain closed to element mobility during later thermal events, and commonly yields concordant and precise dates. Because of the small size of diagenetic xenotime crystals and common textural complexities, an in situ isotopic technique with a spatial resolution of < 10 Am is required to successfully date xenotime; to date, this has only been achieved by ion microprobe. In metamorphosed sedimentary rocks, diagenetic xenotime retains its age information up to lower amphibolite facies in sandstone, and up to mid-upper greenschist facies in pelitic rocks. In many Precambrian basins (e.g., Witwatersrand Basin, South Africa), diagenetic xenotime is overgrown by chemically distinct and texturally younger xenotime related to burial diagenesis, contact metamorphism, hydrothermal alteration or regional metamorphism. With the aid of petrography, geochemical microanalysis and the use of isotopic techniques with fine spatial resolution, it may be possible to use xenotime to date early diagenesis, and potentially every major fluid and thermal event to have affected a depositional basin.

Contributions to Mineralogy and Petrology, 2005

Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimen... more Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimentary rocks worldwide, ranging in age from mid-Archaean to Palaeozoic. The outgrowths comprise minute (typically <3 lm) crystals that form an irregular fringe on detrital zircon grains, and in a few cases, on diagenetic xenotime outgrowths. Textural relationships indicate that while zircon growth postdates diagenetic xenotime precipitation, it precedes or is synchronous with metamorphic xenotime formation. Unlike xenotime, zircon outgrowths are absent in unmetamorphosed sedimentary rocks, and only appear in prehnite-pumpellyite facies rocks, suggesting that zircon growth commences at temperatures of $250°C. The greater abundance of zircon outgrowths in shales than in to other sedimentary rocks may relate to higher halogen concentrations, which have been linked to enhanced zirconium mobility in hydrothermal systems. The growth of zircon in metasedimentary rocks indicates that zirconium was transported in aqueous fluids, possibly as fluorine complexes, during very low-grade metamorphism.

Meso-to Neoproterozoic reworking in the Gascoyne Complex and what it means for mineral exploration

AAPG Bulletin, 1997

Minute rims of solid bitumen (∼40-50 µm thick) surround detrital radioactive grains in the Permia... more Minute rims of solid bitumen (∼40-50 µm thick) surround detrital radioactive grains in the Permian-Triassic sandstones and Arranoo Member of the Kockatea Shale from the northern Perth basin, Australia. The bitumen formed as Th-and U-bearing minerals (monazite, xenotime, zircon, thorite) irradiated and immobilized fluid hydrocarbons coming within range of alpha-particle emissions. Using transmitted light and scanning electron microscopy the rims appear compositionally homogeneous, but under blue/violet epifluorescent illumination the bitumen displays complex concentric and contorted banding. These fluorescent textures indicate that multiple influxes of hydrocarbons passed through the reservoir sandstones. Following radiation-induced immobilization of hydrocarbons from the first oil influx, the bitumen nodules grew through a process of swelling and expansion outward from the mineral core during subsequent oil influxes, producing graded fluorescent growth bands. Oil droplets and lamellae also were adsorbed onto the outer portion of the nodules. Such bitumen nodules are a new and potentially important source of data for understanding the movement of hydrocarbons in sedimentary basins, specifically for identifying hydrocarbon pathways, the number of discrete hydrocarbon pulses, and the relative timing of oil migration.

The origin of carbon in ancient sedimentary rocks is key to questions about the emergence and ant... more The origin of carbon in ancient sedimentary rocks is key to questions about the emergence and antiquity of life. Some of the oldest well-preserved carbon occurs in vertical black chert veins that transect Paleoarchean basalt-dominated successions in the Pilbara Craton, Australia, and the Kaapvaal Craton, South Africa. In the north Pilbara region west of Marble Bar, they famously host carbonaceous filaments in the Apex chert, which were interpreted to represent the fossilized remains of microbes resembling cyanobacteria (Schopf 1993). These findings were subsequently challenged by Brasier et al. (2002), who argued that the black chert is not sedimentary but occurs in veins that crosscut bedding and formed by precipitation from hydrothermal fluids. New in situ transmission electron microscopy and microanalysis of ~3.5 Ga black chert veins from the North Pole Dome area (Rasmussen & Muhling 2023) show that they contain abundant carbonaceous structures, including coalesced droplets, that resemble thermally altered relicts of migrated oil. Other textures, including characteristic black clots, resemble those preserved in bituminous chert from the 340 Ma Red Dog hydrothermal Zn-Pb-barite deposit, Alaska, suggesting that much of the carbon in the 3.5 Ga veins was originally an organic-rich liquid that co-precipitated with silica. These results suggest that carbonaceous matter in the 3.5 Ga black cherts is not exclusively indigenous cellular carbon (i.e. kerogen), but includes migrated carbon with implications for assessing the biogenicity of simple fossil-like microstructures. The lack of organic-rich source rocks in the volcanic-dominated succession points to a contribution from abiotic organic synthesis either during hydrothermal fluidrock reactions in the mafic-ultramafic lavas and/or polymerization of methane from mantle outgassing.

Geology, May 12, 2022

Xenotime and monazite were analysed for U-Pb dating with the Sensitive High-Resolution Ion MicroP... more Xenotime and monazite were analysed for U-Pb dating with the Sensitive High-Resolution Ion MicroProbe (SHRIMP-II) instrument at the John De Laeter Centre, Perth, Western Australia. Grains were identified in polished thin sections, and 3 mm diameter plugs were extracted from the thin sections with a hollow-core rotary drill and mounted in 25 mm diameter epoxy discs. The mounts were cleaned and gold coated before each analytical session. Xenotime and monazite standards were set into separate mounts and gold coated simultaneously with sample mounts. Standard and sample mounts were loaded together into the SHRIMP for concurrent analysis during each of the four analytical sessions. Instrument setup followed protocols for small-spot, in-situ analysis of xenotime and monazite developed in . A primary beam of O2 -ions was focused through a 50 µm Kohler aperture to produce an oval 10 µm wide spot on the sample surface with a current of 0.3 nA. The secondary ion system was focused through a 100 µm collector slit onto an electron multiplier to produce mass peaks with flat tops and a mass resolution of >5400 in all sessions. Background counts from scattered ions were reduced using a flight retardation lens, which is known to cause slight session-dependent instrumental mass fractionation (IMF) of Pb isotopes . IMF corrections were applied to all analyses. Data were collected in sets of 8 scans, with standard xenotime or monazite analysed every 4-6 sample analyses. Count times per scan for Pb isotopes 204, background position 204.045, 206, 207 and 208 were 10s, 10s, 10s, 30s and 10s, respectively. Monazite was analysed with a 13-peak run table as defined in , which includes mass stations for the estimation of La, Ce and Nd (REEPO2 + ), and Y (YCeO + ). Measurements on monazite standards FRENCH, Z2234 and Z2908 (see

Geology, Aug 12, 2021

Rhabdophane crystals were identified in polished thin sections using both optical microscopy and ... more Rhabdophane crystals were identified in polished thin sections using both optical microscopy and scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS). Rhabdophane crystals >15 µm in size were drilled from polished thin sections, cast into epoxy to make 25-mmdiameter mounts (Fig. ). Rhabdophane crystals in plugs of shale mounted in epoxy disc were analysed with an Oxford Instruments X-Max50 Energy Dispersive X-ray detector (EDS) mounted on a TESCAN VEGA3 scanning electron microscope (SEM). AZtec software from Oxford Instruments was used to collect and process the X-ray spectra. Fully quantitative analyses were collected at 15 kV accelerating voltage and 1-2 nA beam current. The system count rate was calibrated using pure copper, and standard spectra were collected on the VEGA3 from jadeite (Na), periclase (Mg), corundum (Al), wollastonite (Si and Ca), orthoclase (K), rutile (Ti), chromite (Cr), rhodonite (Mn), pyrite (Fe), Durango apatite for P, synthetic glasses (Si + Al + Ca + REY) for rare-earth-elements and Y, synthetic ThO2 and U metal. Analytical precision is ± 1-2% relative for major elements (>10 wt%) and ± 5-10% relative for minor elements (<10 wt%). Analysis totals are low (90-95%) whereas analyses of a monazite standard embedded in the same disc give totals 98-102%, confirming the hydrous nature of the rhabdophane. Rhabdophane and monazite analyses are presented in Tables and, respectively. Rhabdophane crystals in 25-mm-diameter mounts were analyzed using the Sensitive High-Resolution Ion MicroProbe (SHRIMP) housed in the John de Laeter Centre, Perth, Australia. Analytical procedures followed established methodologies for monazite (Fletcher et al., 2010), using O2 - primary ion beam focused onto an ellipsoidal spot 10-15 µm in dimension. The calibration of U-Th-Pb data collected from rhabdophane was carried out using a suite of monazite standards, a mineral that closely matches the chemical composition of rhabdophane. Monazite standards were set in separate mounts that were cleaned and gold-coated with the rhabdophane sample mounts. Rhabdophane was analyzed with a 13-peak run table as defined by Fletcher et al. (2010) for monazite, which includes mass stations for the estimation of La, Ce and Nd (REEPO2 + ), and Y (YCeO + ). Eight or seven scans of the mass spectrum were recorded for each analysis. Count times per scan for background position 204.045, and Pb isotopes 204, 206, 207 and 208 are, respectively, 10 seconds, 10 seconds, 10 seconds, 10 seconds and 30 seconds. A pre-collector retardation lens was activated to reduce arrivals of stray ions including isobars that interfere with the Pb isotopes.

Precambrian Research, Oct 1, 2019

The Capricorn Orogen records nearly one billion years of intraplate orogenesis within the West Au... more The Capricorn Orogen records nearly one billion years of intraplate orogenesis within the West Australian Craton, although the processes responsible for this protracted, punctuated reworking remain unclear. Of the major tectonic events that affected the region, the 1680-1620 Ma Mangaroon Orogeny is one of the least-well understood, mainly due to a lack of direct ages for metamorphism, an absence of pressure (P)-temperature (T) constraints, and uncertainty regarding the duration of granitic magmatism that is spatially and, possibly, temporally associated with deformation. In this study we define the P-T-time (t) conditions associated with the Mangaroon Orogeny based on in situ SHRIMP U-Pb monazite and xenotime geochronology and calculated P-T pseudosections. Data from a pelitic migmatite constrain the timing of low-P, high-T metamorphism to 1691 ± 7 Ma at conditions of 665-755 °C and 2.7-4.3

Precambrian Research, Jun 1, 2018

The tectonic history of the Proterozoic Capricorn Orogen, Western Australia, records complex intr... more The tectonic history of the Proterozoic Capricorn Orogen, Western Australia, records complex intraplate reworking lasting nearly one billion years. Although the Paleo-Mesoproterozoic reworking history is well defined in the crystalline basement of the Gascoyne Province, at the western end of the orogen, the younger reactivation history remains unclear. Four reworking events affected the orogen at 1820-1770 Ma, 1680-1620 Ma, 1320-1170 Ma, and 1030-900 Ma. These events were succeeded by a breakout in predominantly dextral strikeslip reactivation of major shear zones across the Gascoyne Province. Currently, the age of this reactivation is constrained by only one date of c. 570 Ma from a single shear zone, but field relationships imply that some of the shear zones must be older than a suite of c. 755 Ma dolerite dykes. In order to constrain the age of fault and shear zone reactivation we obtained new 40 Ar/ 39 Ar dates for mica and in situ SHRIMP U-Pb dates for xenotime within shear zones. Our results when combined with previously published data, show that reactivation occurred between 920 and 830 Ma. These dates overlap with the youngest reworking event, the 1030-900 Ma Edmundian Orogeny. Furthermore, Neoproterozoic U-Pb phosphate ages are known from the bounding cratons and faulting within the adjacent Mesoproterozoic sedimentary basins suggest this event is of regional significance. In contrast to previous suggestions that this Neoproterozoic reactivation was the result of a collision from the west, we propose that it reflects north-south compression that caused dextral strike-slip fault reactivation in the north and exhumation of the southern part of the orogen.

Geophysical Research Letters, Apr 2, 2018

 Neoarchean nanoparticle silicate inclusions appear to be the earliest iron mineral preserved in... more  Neoarchean nanoparticle silicate inclusions appear to be the earliest iron mineral preserved in cherts from Australia and South Africa  Our multiscale analyses indicate the particles are greenalite that are dominantly Fe(II) but also have low and variable Fe(III) content  We present four (bio)geochemical hypotheses that could produce low-Fe(III) greenalite

Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Loca... more Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned in the text are referenced using Map Grid Australia (MGA) coordinates, Zone 50. All locations are quoted to at least the nearest 100 m. U-Pb measurements were conducted using the SHRIMP II ion microprobes at the John de Laeter Centre of Mass Spectrometry at

Contributions to Mineralogy and Petrology, Nov 6, 2008

In situ SHRIMP U-Pb dating of magmatic zirconolite (CaZrTi 2 O 7 ) in the Golden Mile Dolerite fr... more In situ SHRIMP U-Pb dating of magmatic zirconolite (CaZrTi 2 O 7 ) in the Golden Mile Dolerite from the Mt Charlotte gold deposit (Yilgarn Craton, Australia) has yielded the first robust emplacement age (2,680 ± 9 Ma) for the principle host-rock of gold mineralization in the Kalgoorlie district. In contrast, comagmatic zircon gave ages from *2.68 Ga to *2.17 Ga, reflecting isotopic resetting of high-U and -Th crystals. In situ SHRIMP analysis of hydrothermal xenotime (YPO 4 ), which co-exists with gold in alteration pyrite, provided a Pb/Pb isochron age of 2,655 ± 13 Ma. This date indicates that the youngest deposit in the Kalgoorlie district (Mt Charlotte) formed at *2.65 Ga, and provides a new minimum age for the structurally older Golden Mile deposit. Our results indicate that gold mineralization at Mt Charlotte is *50 million years older than indicated by recent 40 Ar/ 39 Ar dating and places new constraints on the timing of late-stage regional faulting (D 4 ) in the province.

Exceptional preservation of organic matter and iron-organic colloidal mineralization in hydrothermal black smoker-type sulfide mineralization from the Paleoarchean seafloor

Chemical Geology, Feb 1, 2023

Geology, 2005

Petroleum generation largely occurs through the thermal decomposition of organic matter. The pres... more Petroleum generation largely occurs through the thermal decomposition of organic matter. The presence of oil-bearing fluid inclusions and pyrobitumen in Archean rocks suggests that similar processes operated as early as ca. 3.25 Ga. However, direct evidence of petroleum generation from potential source rocks is lacking, and an abiogenic origin has been proposed for some Archean carbonaceous residues. Pilbara craton ca. 3.2 Ga and ca. 2.63 Ga black shales were found to contain abundant kerogenous streaks and laminae, as well as bitumen nodules (comprising a radioactive mineral core surrounded by a carbonaceous rim) and pyrobitumen (formerly petroleum) globules, films, and aggregates. The bitumen nodules formed around detrital radioactive grains via polymerization of fluid hydrocarbons generated within the shale and represent diagnostic indicators of oil generation in ancient shales. The bitumen globules, films, and masses are preserved within authigenic pyrite and demonstrate that a separate hydrocarbon phase had developed in the shale matrix during burial, providing compelling evidence for in situ petroleum generation and expulsion. The abundance of bitumen nodules and residual pyrobitumen in black shales across the Pilbara craton suggests that hydrocarbon generation from kerogenous shales was a common phenomenon during the Middle to Late Archean. The petroleum was generated from organic matter that accumulated in marine environments, most probably comprising the remains of photosynthetic and chemosynthetic organisms, pointing to a sizeable biomass as early as 3.2 Ga.

Contributions to Mineralogy and Petrology, Jul 29, 2005

Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimen... more Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimentary rocks worldwide, ranging in age from mid-Archaean to Palaeozoic. The outgrowths comprise minute (typically <3 lm) crystals that form an irregular fringe on detrital zircon grains, and in a few cases, on diagenetic xenotime outgrowths. Textural relationships indicate that while zircon growth postdates diagenetic xenotime precipitation, it precedes or is synchronous with metamorphic xenotime formation. Unlike xenotime, zircon outgrowths are absent in unmetamorphosed sedimentary rocks, and only appear in prehnite-pumpellyite facies rocks, suggesting that zircon growth commences at temperatures of $250°C. The greater abundance of zircon outgrowths in shales than in to other sedimentary rocks may relate to higher halogen concentrations, which have been linked to enhanced zirconium mobility in hydrothermal systems. The growth of zircon in metasedimentary rocks indicates that zirconium was transported in aqueous fluids, possibly as fluorine complexes, during very low-grade metamorphism.

In Situ SHRIMP U-Pb Geochronology and Petrography of Mafic Dykes in the Yilgarn Craton, Western Australia, and Vestfold Hills and Bunger Hills, East Antarctica

AGUFM, Dec 1, 2015

THE IMPACT OF EARLY-DIAGENETlC ALUMINOPHOSPHATE PRECIPITATION ON THE OCEANIC PHOSPHORUS BUDGET

Marine Authigenesis: From Global to Microbial, 2000

Petrology and Stratigraphy of Subsurface Sandstones Near the Permo-Triassic Boundary, Northern Perth Basin, Western Australia

Diagenesis of Reservoir Sandstones from Selected Western Australian Sedimentary Basins, with Emphasis on Bitumen-rimmed Radioactive Heavy Minerals

Geology, 2005

Petroleum generation largely occurs through the thermal decomposition of organic matter. The pres... more Petroleum generation largely occurs through the thermal decomposition of organic matter. The presence of oil-bearing fluid inclusions and pyrobitumen in Archean rocks suggests that similar processes operated as early as ca. 3.25 Ga. However, direct evidence of petroleum generation from potential source rocks is lacking, and an abiogenic origin has been proposed for some Archean carbonaceous residues. Pilbara craton ca. 3.2 Ga and ca. 2.63 Ga black shales were found to contain abundant kerogenous streaks and laminae, as well as bitumen nodules (comprising a radioactive mineral core surrounded by a carbonaceous rim) and pyrobitumen (formerly petroleum) globules, films, and aggregates. The bitumen nodules formed around detrital radioactive grains via polymerization of fluid hydrocarbons generated within the shale and represent diagnostic indicators of oil generation in ancient shales. The bitumen globules, films, and masses are preserved within authigenic pyrite and demonstrate that a separate hydrocarbon phase had developed in the shale matrix during burial, providing compelling evidence for in situ petroleum generation and expulsion. The abundance of bitumen nodules and residual pyrobitumen in black shales across the Pilbara craton suggests that hydrocarbon generation from kerogenous shales was a common phenomenon during the Middle to Late Archean. The petroleum was generated from organic matter that accumulated in marine environments, most probably comprising the remains of photosynthetic and chemosynthetic organisms, pointing to a sizeable biomass as early as 3.2 Ga.

Earth-Science Reviews, 2005

Recent advances in the field of geochronology have led to a greater understanding of the scale an... more Recent advances in the field of geochronology have led to a greater understanding of the scale and duration of geological processes. It is currently possible to date igneous and metamorphic rocks by a variety of radiometric methods to within a million years, but establishing the depositional age of sedimentary rocks has remained exceedingly difficult. The problem is most pronounced for Precambrian rocks, where the low diversity and abundance of organisms have prevented the establishment of any meaningful biostratigraphic framework for correlating strata. Also, most Precambrian successions have been metamorphosed, rendering original minerals and textures difficult to interpret, and resetting diagenetic minerals. Xenotime (YPO 4 ) is an isotopically robust chronometer, which is increasingly being recognized as a trace constituent in siliciclastic sedimentary rocks. It may start to grow during early diagenesis, typically forming syntaxial outgrowths on detrital zircon grains. Diagenetic xenotime occurs in a wide variety of rock types, including conglomerate, sandstone, siltstone, shale, phosphorite and volcaniclastic rocks, varying from early Archaean to Mesozoic in age. The formation of diagenetic xenotime is principally related to redox cycling of Fe-oxyhydroxides and microbial decomposition of organic matter, leading to elevated concentrations of dissolved phosphate and rare earth elements (REE) in sediment pore-waters. Xenotime has the properties of an ideal U -Pb chronometer, containing elevated levels of U (generally >1000 ppm) and very low concentrations of initial common Pb. In addition, it has an exceptional ability to remain closed to element mobility during later thermal events, and commonly yields concordant and precise dates. Because of the small size of diagenetic xenotime crystals and common textural complexities, an in situ isotopic technique with a spatial resolution of < 10 Am is required to successfully date xenotime; to date, this has only been achieved by ion microprobe. In metamorphosed sedimentary rocks, diagenetic xenotime retains its age information up to lower amphibolite facies in sandstone, and up to mid-upper greenschist facies in pelitic rocks. In many Precambrian basins (e.g., Witwatersrand Basin, South Africa), diagenetic xenotime is overgrown by chemically distinct and texturally younger xenotime related to burial diagenesis, contact metamorphism, hydrothermal alteration or regional metamorphism. With the aid of petrography, geochemical microanalysis and the use of isotopic techniques with fine spatial resolution, it may be possible to use xenotime to date early diagenesis, and potentially every major fluid and thermal event to have affected a depositional basin.

Contributions to Mineralogy and Petrology, 2005

Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimen... more Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimentary rocks worldwide, ranging in age from mid-Archaean to Palaeozoic. The outgrowths comprise minute (typically <3 lm) crystals that form an irregular fringe on detrital zircon grains, and in a few cases, on diagenetic xenotime outgrowths. Textural relationships indicate that while zircon growth postdates diagenetic xenotime precipitation, it precedes or is synchronous with metamorphic xenotime formation. Unlike xenotime, zircon outgrowths are absent in unmetamorphosed sedimentary rocks, and only appear in prehnite-pumpellyite facies rocks, suggesting that zircon growth commences at temperatures of $250°C. The greater abundance of zircon outgrowths in shales than in to other sedimentary rocks may relate to higher halogen concentrations, which have been linked to enhanced zirconium mobility in hydrothermal systems. The growth of zircon in metasedimentary rocks indicates that zirconium was transported in aqueous fluids, possibly as fluorine complexes, during very low-grade metamorphism.

Meso-to Neoproterozoic reworking in the Gascoyne Complex and what it means for mineral exploration

AAPG Bulletin, 1997

Minute rims of solid bitumen (∼40-50 µm thick) surround detrital radioactive grains in the Permia... more Minute rims of solid bitumen (∼40-50 µm thick) surround detrital radioactive grains in the Permian-Triassic sandstones and Arranoo Member of the Kockatea Shale from the northern Perth basin, Australia. The bitumen formed as Th-and U-bearing minerals (monazite, xenotime, zircon, thorite) irradiated and immobilized fluid hydrocarbons coming within range of alpha-particle emissions. Using transmitted light and scanning electron microscopy the rims appear compositionally homogeneous, but under blue/violet epifluorescent illumination the bitumen displays complex concentric and contorted banding. These fluorescent textures indicate that multiple influxes of hydrocarbons passed through the reservoir sandstones. Following radiation-induced immobilization of hydrocarbons from the first oil influx, the bitumen nodules grew through a process of swelling and expansion outward from the mineral core during subsequent oil influxes, producing graded fluorescent growth bands. Oil droplets and lamellae also were adsorbed onto the outer portion of the nodules. Such bitumen nodules are a new and potentially important source of data for understanding the movement of hydrocarbons in sedimentary basins, specifically for identifying hydrocarbon pathways, the number of discrete hydrocarbon pulses, and the relative timing of oil migration.

The origin of carbon in ancient sedimentary rocks is key to questions about the emergence and ant... more The origin of carbon in ancient sedimentary rocks is key to questions about the emergence and antiquity of life. Some of the oldest well-preserved carbon occurs in vertical black chert veins that transect Paleoarchean basalt-dominated successions in the Pilbara Craton, Australia, and the Kaapvaal Craton, South Africa. In the north Pilbara region west of Marble Bar, they famously host carbonaceous filaments in the Apex chert, which were interpreted to represent the fossilized remains of microbes resembling cyanobacteria (Schopf 1993). These findings were subsequently challenged by Brasier et al. (2002), who argued that the black chert is not sedimentary but occurs in veins that crosscut bedding and formed by precipitation from hydrothermal fluids. New in situ transmission electron microscopy and microanalysis of ~3.5 Ga black chert veins from the North Pole Dome area (Rasmussen & Muhling 2023) show that they contain abundant carbonaceous structures, including coalesced droplets, that resemble thermally altered relicts of migrated oil. Other textures, including characteristic black clots, resemble those preserved in bituminous chert from the 340 Ma Red Dog hydrothermal Zn-Pb-barite deposit, Alaska, suggesting that much of the carbon in the 3.5 Ga veins was originally an organic-rich liquid that co-precipitated with silica. These results suggest that carbonaceous matter in the 3.5 Ga black cherts is not exclusively indigenous cellular carbon (i.e. kerogen), but includes migrated carbon with implications for assessing the biogenicity of simple fossil-like microstructures. The lack of organic-rich source rocks in the volcanic-dominated succession points to a contribution from abiotic organic synthesis either during hydrothermal fluidrock reactions in the mafic-ultramafic lavas and/or polymerization of methane from mantle outgassing.

Geology, May 12, 2022

Xenotime and monazite were analysed for U-Pb dating with the Sensitive High-Resolution Ion MicroP... more Xenotime and monazite were analysed for U-Pb dating with the Sensitive High-Resolution Ion MicroProbe (SHRIMP-II) instrument at the John De Laeter Centre, Perth, Western Australia. Grains were identified in polished thin sections, and 3 mm diameter plugs were extracted from the thin sections with a hollow-core rotary drill and mounted in 25 mm diameter epoxy discs. The mounts were cleaned and gold coated before each analytical session. Xenotime and monazite standards were set into separate mounts and gold coated simultaneously with sample mounts. Standard and sample mounts were loaded together into the SHRIMP for concurrent analysis during each of the four analytical sessions. Instrument setup followed protocols for small-spot, in-situ analysis of xenotime and monazite developed in . A primary beam of O2 -ions was focused through a 50 µm Kohler aperture to produce an oval 10 µm wide spot on the sample surface with a current of 0.3 nA. The secondary ion system was focused through a 100 µm collector slit onto an electron multiplier to produce mass peaks with flat tops and a mass resolution of >5400 in all sessions. Background counts from scattered ions were reduced using a flight retardation lens, which is known to cause slight session-dependent instrumental mass fractionation (IMF) of Pb isotopes . IMF corrections were applied to all analyses. Data were collected in sets of 8 scans, with standard xenotime or monazite analysed every 4-6 sample analyses. Count times per scan for Pb isotopes 204, background position 204.045, 206, 207 and 208 were 10s, 10s, 10s, 30s and 10s, respectively. Monazite was analysed with a 13-peak run table as defined in , which includes mass stations for the estimation of La, Ce and Nd (REEPO2 + ), and Y (YCeO + ). Measurements on monazite standards FRENCH, Z2234 and Z2908 (see

Geology, Aug 12, 2021

Rhabdophane crystals were identified in polished thin sections using both optical microscopy and ... more Rhabdophane crystals were identified in polished thin sections using both optical microscopy and scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS). Rhabdophane crystals >15 µm in size were drilled from polished thin sections, cast into epoxy to make 25-mmdiameter mounts (Fig. ). Rhabdophane crystals in plugs of shale mounted in epoxy disc were analysed with an Oxford Instruments X-Max50 Energy Dispersive X-ray detector (EDS) mounted on a TESCAN VEGA3 scanning electron microscope (SEM). AZtec software from Oxford Instruments was used to collect and process the X-ray spectra. Fully quantitative analyses were collected at 15 kV accelerating voltage and 1-2 nA beam current. The system count rate was calibrated using pure copper, and standard spectra were collected on the VEGA3 from jadeite (Na), periclase (Mg), corundum (Al), wollastonite (Si and Ca), orthoclase (K), rutile (Ti), chromite (Cr), rhodonite (Mn), pyrite (Fe), Durango apatite for P, synthetic glasses (Si + Al + Ca + REY) for rare-earth-elements and Y, synthetic ThO2 and U metal. Analytical precision is ± 1-2% relative for major elements (>10 wt%) and ± 5-10% relative for minor elements (<10 wt%). Analysis totals are low (90-95%) whereas analyses of a monazite standard embedded in the same disc give totals 98-102%, confirming the hydrous nature of the rhabdophane. Rhabdophane and monazite analyses are presented in Tables and, respectively. Rhabdophane crystals in 25-mm-diameter mounts were analyzed using the Sensitive High-Resolution Ion MicroProbe (SHRIMP) housed in the John de Laeter Centre, Perth, Australia. Analytical procedures followed established methodologies for monazite (Fletcher et al., 2010), using O2 - primary ion beam focused onto an ellipsoidal spot 10-15 µm in dimension. The calibration of U-Th-Pb data collected from rhabdophane was carried out using a suite of monazite standards, a mineral that closely matches the chemical composition of rhabdophane. Monazite standards were set in separate mounts that were cleaned and gold-coated with the rhabdophane sample mounts. Rhabdophane was analyzed with a 13-peak run table as defined by Fletcher et al. (2010) for monazite, which includes mass stations for the estimation of La, Ce and Nd (REEPO2 + ), and Y (YCeO + ). Eight or seven scans of the mass spectrum were recorded for each analysis. Count times per scan for background position 204.045, and Pb isotopes 204, 206, 207 and 208 are, respectively, 10 seconds, 10 seconds, 10 seconds, 10 seconds and 30 seconds. A pre-collector retardation lens was activated to reduce arrivals of stray ions including isobars that interfere with the Pb isotopes.

Precambrian Research, Oct 1, 2019

The Capricorn Orogen records nearly one billion years of intraplate orogenesis within the West Au... more The Capricorn Orogen records nearly one billion years of intraplate orogenesis within the West Australian Craton, although the processes responsible for this protracted, punctuated reworking remain unclear. Of the major tectonic events that affected the region, the 1680-1620 Ma Mangaroon Orogeny is one of the least-well understood, mainly due to a lack of direct ages for metamorphism, an absence of pressure (P)-temperature (T) constraints, and uncertainty regarding the duration of granitic magmatism that is spatially and, possibly, temporally associated with deformation. In this study we define the P-T-time (t) conditions associated with the Mangaroon Orogeny based on in situ SHRIMP U-Pb monazite and xenotime geochronology and calculated P-T pseudosections. Data from a pelitic migmatite constrain the timing of low-P, high-T metamorphism to 1691 ± 7 Ma at conditions of 665-755 °C and 2.7-4.3

Precambrian Research, Jun 1, 2018

The tectonic history of the Proterozoic Capricorn Orogen, Western Australia, records complex intr... more The tectonic history of the Proterozoic Capricorn Orogen, Western Australia, records complex intraplate reworking lasting nearly one billion years. Although the Paleo-Mesoproterozoic reworking history is well defined in the crystalline basement of the Gascoyne Province, at the western end of the orogen, the younger reactivation history remains unclear. Four reworking events affected the orogen at 1820-1770 Ma, 1680-1620 Ma, 1320-1170 Ma, and 1030-900 Ma. These events were succeeded by a breakout in predominantly dextral strikeslip reactivation of major shear zones across the Gascoyne Province. Currently, the age of this reactivation is constrained by only one date of c. 570 Ma from a single shear zone, but field relationships imply that some of the shear zones must be older than a suite of c. 755 Ma dolerite dykes. In order to constrain the age of fault and shear zone reactivation we obtained new 40 Ar/ 39 Ar dates for mica and in situ SHRIMP U-Pb dates for xenotime within shear zones. Our results when combined with previously published data, show that reactivation occurred between 920 and 830 Ma. These dates overlap with the youngest reworking event, the 1030-900 Ma Edmundian Orogeny. Furthermore, Neoproterozoic U-Pb phosphate ages are known from the bounding cratons and faulting within the adjacent Mesoproterozoic sedimentary basins suggest this event is of regional significance. In contrast to previous suggestions that this Neoproterozoic reactivation was the result of a collision from the west, we propose that it reflects north-south compression that caused dextral strike-slip fault reactivation in the north and exhumation of the southern part of the orogen.

Geophysical Research Letters, Apr 2, 2018

 Neoarchean nanoparticle silicate inclusions appear to be the earliest iron mineral preserved in... more  Neoarchean nanoparticle silicate inclusions appear to be the earliest iron mineral preserved in cherts from Australia and South Africa  Our multiscale analyses indicate the particles are greenalite that are dominantly Fe(II) but also have low and variable Fe(III) content  We present four (bio)geochemical hypotheses that could produce low-Fe(III) greenalite

Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Loca... more Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned in the text are referenced using Map Grid Australia (MGA) coordinates, Zone 50. All locations are quoted to at least the nearest 100 m. U-Pb measurements were conducted using the SHRIMP II ion microprobes at the John de Laeter Centre of Mass Spectrometry at

Contributions to Mineralogy and Petrology, Nov 6, 2008

In situ SHRIMP U-Pb dating of magmatic zirconolite (CaZrTi 2 O 7 ) in the Golden Mile Dolerite fr... more In situ SHRIMP U-Pb dating of magmatic zirconolite (CaZrTi 2 O 7 ) in the Golden Mile Dolerite from the Mt Charlotte gold deposit (Yilgarn Craton, Australia) has yielded the first robust emplacement age (2,680 ± 9 Ma) for the principle host-rock of gold mineralization in the Kalgoorlie district. In contrast, comagmatic zircon gave ages from *2.68 Ga to *2.17 Ga, reflecting isotopic resetting of high-U and -Th crystals. In situ SHRIMP analysis of hydrothermal xenotime (YPO 4 ), which co-exists with gold in alteration pyrite, provided a Pb/Pb isochron age of 2,655 ± 13 Ma. This date indicates that the youngest deposit in the Kalgoorlie district (Mt Charlotte) formed at *2.65 Ga, and provides a new minimum age for the structurally older Golden Mile deposit. Our results indicate that gold mineralization at Mt Charlotte is *50 million years older than indicated by recent 40 Ar/ 39 Ar dating and places new constraints on the timing of late-stage regional faulting (D 4 ) in the province.

Exceptional preservation of organic matter and iron-organic colloidal mineralization in hydrothermal black smoker-type sulfide mineralization from the Paleoarchean seafloor

Chemical Geology, Feb 1, 2023