Detrital zircon U-Pb age dating has become a widely used tool for determining sediment provenance... more Detrital zircon U-Pb age dating has become a widely used tool for determining sediment provenance in basins and orogenic systems. While traditional LA-ICPMS zircon geochronology is powerful, it has limitations when source regions are characterized by monotonous or non-diagnostic crystallization ages or by major sediment recycling and homogenization, leading to minimal zircon age variability. In the central Alps of Switzerland and Italy, for example, similar Cadomian, Caledonian, and Variscan zircons dominate with only minor Alpine ages. Samples collected from Oligocene-Miocene strata deposited in both the northern (Swiss Molasse) and southern (Apenninic foredeep) Alpine foreland basins document shifts in the relative abundance of Cadomian, Caledonian, Variscan and Alpine aged detrital zircon, but the exact source region and genesis of the grains remains poorly constrained based on zircon U-Pb age data alone. Laser Ablation Split Stream (LASS)-ICPMS depth profiling of detrital zircon...
ABSTRACT The validity of using the 40Ar/39Ar system for thermochronology relies on the assumption... more ABSTRACT The validity of using the 40Ar/39Ar system for thermochronology relies on the assumption that the source mineral is surrounded by a grain boundary reservoir defined by an effective 40Ar concentration of zero. However, the presence of extraneous 40Ar (AreAre) in metamorphic rocks shows that this assumption is invalid for a significant number of cases. AreAre is common in micas that have equilibrated under (ultra-)high pressure ((U)HP(U)HP) conditions: metasediments from six Phanerozoic (U)HP(U)HP terranes yield apparent 40Ar/39Ar phengite ages ≲≲50% in excess of the age of peak (U)HP(U)HP conditions, whereas cogenetic mafic eclogites yield ages up to ∼∼700% older despite lower K2O concentrations. A model is developed that calculates AreAre age fractions as a function of variable mica–fluid KDKD, bulk K2O and porosity under closed system conditions. Measured AreAre concentrations in mafic eclogites are reproduced only when porosities are ≲10-4≲10-4 volume fraction, showing that mafic protoliths operate as closed systems to advective solute transport during subduction. Porosities in eclogite-facies metapelites are ≲10-2≲10-2, reflecting loss of significant volumes of lattice-bound H2O relative to mafic rocks during subduction. Retention of locally-generated 40Ar in mafic eclogites shows that the oceanic crust is an efficient vehicle for volatile transport to the mantle.
Abstract Eclogite facies metamorphic rocks provide critical information pertaining to the timing ... more Abstract Eclogite facies metamorphic rocks provide critical information pertaining to the timing of continental collision in zones of plate convergence. Despite being amongst Earth's best studied orogens, little is understood about the rates of Alpine metamorphism within the Eastern Alps. We present LA–MC–ICPMS and ID–TIMS U–Pb ages of metamorphic allanite from the Eclogite Zone, Tauern Window, which when coupled with rare earth element analysis and thermobarometric modelling, demonstrate that the European continental ...
Budgets of 4-He and 40-Ar provide constraints on the chemical evolution of the solid Earth and at... more Budgets of 4-He and 40-Ar provide constraints on the chemical evolution of the solid Earth and atmosphere. Although continental crust accounts for the majority of 4-He and 40-Ar degassed from the Earth, degassing mechanisms are subject to scholarly debate. Here we provide a constraint on crustal degassing by comparing the noble gases accumulated in the Bravo Dome natural CO 2 reservoir, New Mexico USA, with the radiogenic production in the underlying crust. A detailed geological model of the reservoir is used to provide absolute abundances and geostatistical uncertainty of 4-He, 40-Ar, 21-Ne, 20-Ne, 36-Ar, and 84-Kr. The present-day production rate of crustal radiogenic 4 He and 40 Ar, henceforth referred to as 4-He * and 40-Ar * , is estimated using the basement composition, surface and mantle heat flow, and seismic estimates of crustal density. After subtracting mantle and atmospheric contributions, the reservoir contains less than 0.02% of the radiogenic production in the underlying crust. This shows unequivocally that radiogenic noble gases are effectively retained in cratonic continental crust over millennial timescales. This also requires that approximately 1.5 Gt of mantle derived CO 2 migrated through the crust without mobilizing the crustally accumulated gases. This observation suggests transport along a localized fracture network. Therefore, the retention of noble gases in stable crystalline continental crust allows shallow accumulations of radiogenic gases to record tectonic history. At Bravo Dome, the crustal 4-He */40-Ar * ratio is one fifth of the expected crustal production ratio, recording the preferential release of 4-He during the Ancestral Rocky Mountain orogeny, 300 Ma.
Detrital zircon U-Pb age dating has become a widely used tool for determining sediment provenance... more Detrital zircon U-Pb age dating has become a widely used tool for determining sediment provenance in basins and orogenic systems. While traditional LA-ICPMS zircon geochronology is powerful, it has limitations when source regions are characterized by monotonous or non-diagnostic crystallization ages or by major sediment recycling and homogenization, leading to minimal zircon age variability. In the central Alps of Switzerland and Italy, for example, similar Cadomian, Caledonian, and Variscan zircons dominate with only minor Alpine ages. Samples collected from Oligocene-Miocene strata deposited in both the northern (Swiss Molasse) and southern (Apenninic foredeep) Alpine foreland basins document shifts in the relative abundance of Cadomian, Caledonian, Variscan and Alpine aged detrital zircon, but the exact source region and genesis of the grains remains poorly constrained based on zircon U-Pb age data alone. Laser Ablation Split Stream (LASS)-ICPMS depth profiling of detrital zircon...
ABSTRACT The validity of using the 40Ar/39Ar system for thermochronology relies on the assumption... more ABSTRACT The validity of using the 40Ar/39Ar system for thermochronology relies on the assumption that the source mineral is surrounded by a grain boundary reservoir defined by an effective 40Ar concentration of zero. However, the presence of extraneous 40Ar (AreAre) in metamorphic rocks shows that this assumption is invalid for a significant number of cases. AreAre is common in micas that have equilibrated under (ultra-)high pressure ((U)HP(U)HP) conditions: metasediments from six Phanerozoic (U)HP(U)HP terranes yield apparent 40Ar/39Ar phengite ages ≲≲50% in excess of the age of peak (U)HP(U)HP conditions, whereas cogenetic mafic eclogites yield ages up to ∼∼700% older despite lower K2O concentrations. A model is developed that calculates AreAre age fractions as a function of variable mica–fluid KDKD, bulk K2O and porosity under closed system conditions. Measured AreAre concentrations in mafic eclogites are reproduced only when porosities are ≲10-4≲10-4 volume fraction, showing that mafic protoliths operate as closed systems to advective solute transport during subduction. Porosities in eclogite-facies metapelites are ≲10-2≲10-2, reflecting loss of significant volumes of lattice-bound H2O relative to mafic rocks during subduction. Retention of locally-generated 40Ar in mafic eclogites shows that the oceanic crust is an efficient vehicle for volatile transport to the mantle.
Abstract Eclogite facies metamorphic rocks provide critical information pertaining to the timing ... more Abstract Eclogite facies metamorphic rocks provide critical information pertaining to the timing of continental collision in zones of plate convergence. Despite being amongst Earth's best studied orogens, little is understood about the rates of Alpine metamorphism within the Eastern Alps. We present LA–MC–ICPMS and ID–TIMS U–Pb ages of metamorphic allanite from the Eclogite Zone, Tauern Window, which when coupled with rare earth element analysis and thermobarometric modelling, demonstrate that the European continental ...
Budgets of 4-He and 40-Ar provide constraints on the chemical evolution of the solid Earth and at... more Budgets of 4-He and 40-Ar provide constraints on the chemical evolution of the solid Earth and atmosphere. Although continental crust accounts for the majority of 4-He and 40-Ar degassed from the Earth, degassing mechanisms are subject to scholarly debate. Here we provide a constraint on crustal degassing by comparing the noble gases accumulated in the Bravo Dome natural CO 2 reservoir, New Mexico USA, with the radiogenic production in the underlying crust. A detailed geological model of the reservoir is used to provide absolute abundances and geostatistical uncertainty of 4-He, 40-Ar, 21-Ne, 20-Ne, 36-Ar, and 84-Kr. The present-day production rate of crustal radiogenic 4 He and 40 Ar, henceforth referred to as 4-He * and 40-Ar * , is estimated using the basement composition, surface and mantle heat flow, and seismic estimates of crustal density. After subtracting mantle and atmospheric contributions, the reservoir contains less than 0.02% of the radiogenic production in the underlying crust. This shows unequivocally that radiogenic noble gases are effectively retained in cratonic continental crust over millennial timescales. This also requires that approximately 1.5 Gt of mantle derived CO 2 migrated through the crust without mobilizing the crustally accumulated gases. This observation suggests transport along a localized fracture network. Therefore, the retention of noble gases in stable crystalline continental crust allows shallow accumulations of radiogenic gases to record tectonic history. At Bravo Dome, the crustal 4-He */40-Ar * ratio is one fifth of the expected crustal production ratio, recording the preferential release of 4-He during the Ancestral Rocky Mountain orogeny, 300 Ma.
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Papers by Andrew Smye