Proceedings of the National Academy of Sciences of the United States of America, Jan 6, 2015
Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical informat... more Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U-Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5-30 nm nanospheres of metallic Pb. They are randomly dist...
ABSTRACT The Proterozoic Capricorn Orogen is a major tectonic zone that records the assembly and ... more ABSTRACT The Proterozoic Capricorn Orogen is a major tectonic zone that records the assembly and subsequent reworking of the West Australian Craton. Recent seismic transects across the orogen have identified major crustal structures, some of which are spatially associated with hydrothermal mineral deposits. The sedimentary-rock-hosted Abra deposit, which is the largest base-metal accumulation in the Capricorn Orogen, is localized within the crust-cutting Lyons River–Quartzite Well fault zone. Robust radiometric dates for the timing of sediment deposition and hydrothermal mineralization are essential for understanding the geological history of this long-lived orogen and the processes that formed the ore deposits. In situ U–Pb SHRIMP geochronology of xenotime intergrown with magnetite–hematite–galena from the Abra ore zone yields a weighted mean 207Pb/206Pb age of 1594 ± 10 Ma (n = 14, MSWD = 2.6) which is interpreted to represent a period of xenotime growth during the hydrothermal activity responsible for the mineralization. An older coherent cluster within this group gives a weighted mean age of 1610 ± 16 Ma (n = 5, MSWD = 1.5), which constrains the depositional age of the lower Edmund Group sediments to between c. 1680 Ma (maximum age of the basal Mt Augustus Sandstone) and c. 1610 Ma. Authigenic monazite from the ore zone gives 207Pb/206Pb ages of 1375 ± 14 (n = 16, MSWD = 0.99) Ma, interpreted to represent a hydrothermal event postdating the main phase of mineralization. Monazites in samples distal to mineralization yield weighted mean 207Pb/206Pb ages of 1221 ± 14 Ma (n = 5, MSWD = 1.04) and 995 ± 18 Ma (n = 6, MSWD = 1.3), interpreted as records of discrete episodes of hydrothermal fluid flow. Our results suggest that the Lyons River–Quartzite Well Fault, which is one of the principal structures in the Capricorn Orogen, has a long history of tectonic reactivation, spanning more than 600 million years and involving crustal extension and sediment deposition, hydrothermal mineralization and multiple episodes of fluid flow. Xenotime and monazite represent ideal chronometers for investigating the complex histories of hydrothermal mineralization and fluid flow in major crustal structures, and helping to unravel the geological evolution of intracratonic orogens.
Trends in whole-rock and mineral chemistry are seen along a 95 km traverse of lower Archean grani... more Trends in whole-rock and mineral chemistry are seen along a 95 km traverse of lower Archean granitic orthogneissic crust, in the Eastern Dharwar Craton, Tamil Nadu, south India (Hansen and Harlov 2007 J Petrol 48, 1641). Going from north (amphibolite ...
A recent claim to have found traces of Earth's earliest life (> 3.95 Ga) utilising isotopically l... more A recent claim to have found traces of Earth's earliest life (> 3.95 Ga) utilising isotopically light carbon in graphite-bearing metapelites from the Saglek Block of northern Labrador, Canada, is re-evaluated applying rigorous geological and geochronological criteria. The establishment of these criteria in previous evaluations of early life claims from southern West Greenland and northern Canada is reviewed in order to provide a backdrop to discussion of the Saglek claim. In particular, we emphasise the importance of the scale of lithological continuity in determining the veracity of such claims, which are considerably easier to demonstrate from large, relatively less tectonised supracrustal remnants like the Isua Greenstone Belt than they are from smaller, isolated enclaves of the kind found on Akilia or the highly tectonised and imbricated unit that is found in the Saglek Block. Unambiguous field relationships between ca. 3.9 Ga tonalitic gneiss and the graphite-bearing metasediments have not been demonstrated in the literature that the Saglek claim relies upon, and earlier U-Pb-Hf isotopic studies on zircon from metasediments at one of the localities used in the claim indicate a Mesoarchean to Neoarchean time of deposition. We conclude that, irrespective of the validity of the carbon isotopic evidence, field relationships and geochronological evidence fail to demonstrate an age of>3.95 Ga for the potential traces of life.
The Acasta Gneiss Complex of northwestern Canada contains the oldest known crustal rocks on Earth... more The Acasta Gneiss Complex of northwestern Canada contains the oldest known crustal rocks on Earth. Here we present a detailed geological map of the main area of the complex (around the sample locality of the oldest known rocks) and detailed sketch maps of critical geological outcrops. The geological map shows that the complex is divided, by a northeast-trending fault, into eastern and western domains. The eastern area is comprised from quartz dioritic-gabbroic gneisses and multi-phase tonalitic-granitic gneisses. The western area is comprised of layered quartz dioritic-dioritic and tonalitic-granitic gneisses and younger foliated granitic intrusions. The detail field observations reveal at least five tectonothermal events in the eastern area: (1 and 2) emplacement of mafic-intermediate magma (protolith of the quartz dioritic gneiss) and emplacement of felsic magma (protolith of the older felsic gneiss); (3) metamorphism to produce the gneissic structures of the felsic gneiss and quartz dioritic gneiss; (4) intrusion of felsic magma (protolith of the younger felsic gneiss), causing anatexis in some parts; (5) metamorphism and deformation to produce the gneissic structure of the younger felsic gneiss. In contrast, at least four tectonothermal events have been recognized in the western area: (1 and 2) emplacement of the protolith to the mafic-intermediate and felsic gneiss; (3) metamorphism and deformation to form the gneissic and layered structures; (4) intrusion of the granite sheet (the protolith of the foliated granite); (5) metamorphism and deformation of all lithologies.
Proceedings of the National Academy of Sciences of the United States of America, Jan 6, 2015
Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical informat... more Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U-Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5-30 nm nanospheres of metallic Pb. They are randomly dist...
Proceedings of the National Academy of Sciences of the United States of America, Jan 6, 2015
Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical informat... more Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U-Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5-30 nm nanospheres of metallic Pb. They are randomly dist...
ABSTRACT The Proterozoic Capricorn Orogen is a major tectonic zone that records the assembly and ... more ABSTRACT The Proterozoic Capricorn Orogen is a major tectonic zone that records the assembly and subsequent reworking of the West Australian Craton. Recent seismic transects across the orogen have identified major crustal structures, some of which are spatially associated with hydrothermal mineral deposits. The sedimentary-rock-hosted Abra deposit, which is the largest base-metal accumulation in the Capricorn Orogen, is localized within the crust-cutting Lyons River–Quartzite Well fault zone. Robust radiometric dates for the timing of sediment deposition and hydrothermal mineralization are essential for understanding the geological history of this long-lived orogen and the processes that formed the ore deposits. In situ U–Pb SHRIMP geochronology of xenotime intergrown with magnetite–hematite–galena from the Abra ore zone yields a weighted mean 207Pb/206Pb age of 1594 ± 10 Ma (n = 14, MSWD = 2.6) which is interpreted to represent a period of xenotime growth during the hydrothermal activity responsible for the mineralization. An older coherent cluster within this group gives a weighted mean age of 1610 ± 16 Ma (n = 5, MSWD = 1.5), which constrains the depositional age of the lower Edmund Group sediments to between c. 1680 Ma (maximum age of the basal Mt Augustus Sandstone) and c. 1610 Ma. Authigenic monazite from the ore zone gives 207Pb/206Pb ages of 1375 ± 14 (n = 16, MSWD = 0.99) Ma, interpreted to represent a hydrothermal event postdating the main phase of mineralization. Monazites in samples distal to mineralization yield weighted mean 207Pb/206Pb ages of 1221 ± 14 Ma (n = 5, MSWD = 1.04) and 995 ± 18 Ma (n = 6, MSWD = 1.3), interpreted as records of discrete episodes of hydrothermal fluid flow. Our results suggest that the Lyons River–Quartzite Well Fault, which is one of the principal structures in the Capricorn Orogen, has a long history of tectonic reactivation, spanning more than 600 million years and involving crustal extension and sediment deposition, hydrothermal mineralization and multiple episodes of fluid flow. Xenotime and monazite represent ideal chronometers for investigating the complex histories of hydrothermal mineralization and fluid flow in major crustal structures, and helping to unravel the geological evolution of intracratonic orogens.
Trends in whole-rock and mineral chemistry are seen along a 95 km traverse of lower Archean grani... more Trends in whole-rock and mineral chemistry are seen along a 95 km traverse of lower Archean granitic orthogneissic crust, in the Eastern Dharwar Craton, Tamil Nadu, south India (Hansen and Harlov 2007 J Petrol 48, 1641). Going from north (amphibolite ...
A recent claim to have found traces of Earth's earliest life (> 3.95 Ga) utilising isotopically l... more A recent claim to have found traces of Earth's earliest life (> 3.95 Ga) utilising isotopically light carbon in graphite-bearing metapelites from the Saglek Block of northern Labrador, Canada, is re-evaluated applying rigorous geological and geochronological criteria. The establishment of these criteria in previous evaluations of early life claims from southern West Greenland and northern Canada is reviewed in order to provide a backdrop to discussion of the Saglek claim. In particular, we emphasise the importance of the scale of lithological continuity in determining the veracity of such claims, which are considerably easier to demonstrate from large, relatively less tectonised supracrustal remnants like the Isua Greenstone Belt than they are from smaller, isolated enclaves of the kind found on Akilia or the highly tectonised and imbricated unit that is found in the Saglek Block. Unambiguous field relationships between ca. 3.9 Ga tonalitic gneiss and the graphite-bearing metasediments have not been demonstrated in the literature that the Saglek claim relies upon, and earlier U-Pb-Hf isotopic studies on zircon from metasediments at one of the localities used in the claim indicate a Mesoarchean to Neoarchean time of deposition. We conclude that, irrespective of the validity of the carbon isotopic evidence, field relationships and geochronological evidence fail to demonstrate an age of>3.95 Ga for the potential traces of life.
The Acasta Gneiss Complex of northwestern Canada contains the oldest known crustal rocks on Earth... more The Acasta Gneiss Complex of northwestern Canada contains the oldest known crustal rocks on Earth. Here we present a detailed geological map of the main area of the complex (around the sample locality of the oldest known rocks) and detailed sketch maps of critical geological outcrops. The geological map shows that the complex is divided, by a northeast-trending fault, into eastern and western domains. The eastern area is comprised from quartz dioritic-gabbroic gneisses and multi-phase tonalitic-granitic gneisses. The western area is comprised of layered quartz dioritic-dioritic and tonalitic-granitic gneisses and younger foliated granitic intrusions. The detail field observations reveal at least five tectonothermal events in the eastern area: (1 and 2) emplacement of mafic-intermediate magma (protolith of the quartz dioritic gneiss) and emplacement of felsic magma (protolith of the older felsic gneiss); (3) metamorphism to produce the gneissic structures of the felsic gneiss and quartz dioritic gneiss; (4) intrusion of felsic magma (protolith of the younger felsic gneiss), causing anatexis in some parts; (5) metamorphism and deformation to produce the gneissic structure of the younger felsic gneiss. In contrast, at least four tectonothermal events have been recognized in the western area: (1 and 2) emplacement of the protolith to the mafic-intermediate and felsic gneiss; (3) metamorphism and deformation to form the gneissic and layered structures; (4) intrusion of the granite sheet (the protolith of the foliated granite); (5) metamorphism and deformation of all lithologies.
Proceedings of the National Academy of Sciences of the United States of America, Jan 6, 2015
Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical informat... more Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U-Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5-30 nm nanospheres of metallic Pb. They are randomly dist...
Uploads
Papers by Dan Dunkley