Thorium distribution in the crust: Outcrop and grain-scale perspectives

Williams, Megan A.; Kelsey, David E.; Baggs, Thomas; Hand, Martin; Alessio, Kiara L.

doi:10.1016/j.lithos.2018.09.016

Cited by 11 publications

(23 citation statements)

References 52 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Thorium enrichment and U depletion in the migmatitic crust in the Ivrea zone and at Mt Stafford result in similar calculated heat production in lower and higher grade rocks in both locations . Based on the modelling in this study and the measured concentrations of the heat-producing elements Bea & Montero, 1999;Williams et al, 2018), the heat production capacity of metasedimentary rocks is not expected to be transferred from the deep to shallow crust during intracrustal differentiation by anatexis.…”

Section: Discussionmentioning

confidence: 87%

Divergent behaviour of Th and U during anatexis: Implications for the thermal evolution of orogenic crust

Yakymchuk

Brown

2019

Journal Metamorphic Geology

View full text Add to dashboard Cite

Mobilization and migration of the heat‐producing elements (HPE) during anatexis is a critical process in the development of orogenic systems, the evolution of continental crust and the stabilization of cratons. In many crustal rocks the accessory minerals are the dominant hosts of Th and U, and the behaviour of these minerals during partial melting controls the concentrations of these elements in draining melt and residue. We use phase equilibrium modelling to evaluate if loss of melt saturated in the essential structural constituents of the accessory minerals can explain the concentrations of Th and U in residual metasedimentary migmatites and granulites along two well‐characterized crustal transects in the Ivrea zone, Italy and at Mt Stafford, Australia. While an equilibrium model of accessory mineral breakdown and melt loss approximates the depletion of U in the residual crust along both transects, it does not explain the relative enrichment of Th. We propose that the high Th concentrations in residual crust may be explained by either inhibition of monazite dissolution by kinetic factors or near‐peak growth of new high Th grains and overgrowth rims on undissolved monazite due to migration of melt through the orogenic crust. Retention of the HPE in the middle and deep orogenic crust may allow metasedimentary granulites to overcome the enthalpy barrier of melting to achieve ultrahigh temperature conditions and may be partly responsible for the slow cooling of many granulite terranes. Lastly, although the mantle was warmer and crustal heat production was higher in the past, peak temperatures and apparent thermal gradients of high‐temperature (HT)–ultrahigh temperature (UHT) granulite terranes have not decreased significantly since the Neoarchean. However, the pressure of HP granulite facies metamorphism has increased gradually from the Archean to the Phanerozoic, which suggests that the lithosphere became stronger as secular cooling of the mantle enabled plate collisions to form thicker orogens. Thus, as the lithosphere became stronger, the proportion of HT–UHT metamorphism associated with thin lithosphere and mantle heat has decreased, whereas the proportion associated with the formation of thick crust and radiogenic heat has increased.

show abstract

Section: Discussionmentioning

confidence: 87%

Divergent behaviour of Th and U during anatexis: Implications for the thermal evolution of orogenic crust

Yakymchuk

Brown

2019

Journal Metamorphic Geology

View full text Add to dashboard Cite

show abstract

“…Qualitative mapping and quantitative analyses of monazite chemistry were performed at Adelaide Microscopy, the University of Adelaide, using a Cameca SXFive electron microprobe following the method described in Williams et al (2018). Pb was not analysed for any samples herein to reduce individual spot analysis time.…”

Section: Methodsmentioning

confidence: 99%

“…Previous studies have successfully linked chemical zoning of Ce or Y in metamorphic monazite to mineral reactions involving garnet, apatite, xenotime and melt (Corrie & Kohn, 2008; Dumond et al, 2015; Kohn & Malloy, 2004; Pyle & Spear, 1999, 2003; Rubatto et al, 2006; Smith & Barreiro, 1990; Wing et al, 2003). However, to date, studies that have focused on Th behaviour in particular are few (Bea & Montero, 1999; Bingen et al, 1996; Skrzypek et al, 2018; Watt, 1995; Williams et al, 2018) and studies reporting Th zoning in monazite have produced seemingly contradictory results (e.g., decreasing Th from core to rim; Kohn & Malloy, 2004; Th‐in‐monazite is constant or increases with metamorphic grade; Skrzypek et al, 2018; Williams et al, 2018). Recent work by Williams et al (2018) and Skrzypek et al (2018), both on low pressure terranes, identified monazite chemical zones in prograde metamorphic sequences.…”

Section: Introductionmentioning

confidence: 99%

“…However, to date, studies that have focused on Th behaviour in particular are few (Bea & Montero, 1999; Bingen et al, 1996; Skrzypek et al, 2018; Watt, 1995; Williams et al, 2018) and studies reporting Th zoning in monazite have produced seemingly contradictory results (e.g., decreasing Th from core to rim; Kohn & Malloy, 2004; Th‐in‐monazite is constant or increases with metamorphic grade; Skrzypek et al, 2018; Williams et al, 2018). Recent work by Williams et al (2018) and Skrzypek et al (2018), both on low pressure terranes, identified monazite chemical zones in prograde metamorphic sequences. These monazite zones showed a systematic progression of increasing Th from the lowest to highest grade rocks.…”

Section: Introductionmentioning

confidence: 99%

“…These monazite zones showed a systematic progression of increasing Th from the lowest to highest grade rocks. At Mt Stafford this retention of Th in monazite resulted in the retention of Th in granulite facies rocks despite significant partial melting and interpreted melt loss (Bartoli, 2017; Palya et al, 2011; Williams et al, 2018). In this study, we explore bulk Th and monazite chemistry in the higher‐pressure Ivrea–Verbano Zone to explore similarities and differences in the record of monazite formation in these terranes.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Thorium zoning in monazite: A case study from the Ivrea–Verbano zone, NW Italy

Williams

Kelsey

Rubatto

2022

Journal Metamorphic Geology

Self Cite

View full text Add to dashboard Cite

Metamorphism and partial melting of the lower crust is commonly assumed to cause depletion in heat producing elements (HPEs; K, U, Th). In the deep crust, volumetrically subordinate metasedimentary layers, which are source to crustal granites (sensu lato), host the majority of Th AE U, primarily within the REE + Th + U + Y phosphate mineral monazite. We examine the spatial and temporal distribution of Th within monazite grains in metasedimentary rocks from the lower crustal section of the Ivrea-Verbano Zone (Italy), using textural, compositional and geochronological data. We link this to outcrop and regional scale trends described by in-field gamma-ray spectrometry data (infield GRS) for the purpose of understanding how Th distribution is controlled by progressive metamorphism and partial melting. In-field GRS data shows that the whole rock budget of Th does not change between granulite facies rocks and their unmelted equivalents but is significantly lower in rocks that have undergone more significant melt loss at ultra-high temperature (UHT) conditions. Concurrently, the bulk Th budget of monazite increases with metamorphic grade to granulite facies conditions and is greatly reduced in UHT samples. Monazite geochronology returns dates mostly in the range 240-320 Ma with two main peaks at circa 290 and 270 Ma. Textural and chemical constraints indicate that these dates record the timing of pre-peak to peak metamorphic conditions. Amphibolite facies monazite compositional zones are absent from granulite facies monazite, in contrast to examples from lowerpressure terranes. This is consistent with the expanded stability of allanite relative to monazite with increasing pressure having an important role in determining the internal structure, composition and extent of inheritance of monazite in going from amphibolite facies to granulite facies rocks. We propose high-pressure granulites should preserve less monazite inherited from amphibolite facies conditions than low-pressure granulites. Monazite is preserved at all metamorphic grades and presents a mineralogical mechanism for retaining Th in residual deep crust during partial melting and after melt loss.

show abstract

On Granites

Yakymchuk

2019

Journal of the Geological Society of India

View full text Add to dashboard Cite

Granites are important components of the Earth's continental crust and represent the net effect of thermochemical processes that operate during partial melting, magma extraction, ascent, emplacement and crystallization. Compositional and isotopic variations in granites arise from source heterogeneities, mixing in the source, and peritectic mineral entrainment as well as crystal fractionation and assimilation. Fluid-absent hydrate-breakdown melting reactions that accompany high-temperature metamorphism are responsible for differentiating the continental crust into a granitic upper portion and a residual lower portion. In some cases, melting can proceed through the influx of a hydrous fluid, although the significance of this on the long-term compositional differentiation of the continental crust is debated. Accessory minerals in granites are important chronometers and play a primary role in transferring the radiogenic isotope signature of sources to granites. Magma ascent and emplacement are guided by deformation and pluton construction is expected to be incremental with pluton heterogeneity being related to the interaction and differentiation of individual magma batches, as well as the extent to which melt separates from solid residual source material. The nature of the source plays a crucial role in the concentration of water and volatiles that are essential for the generation of hydrothermal–magmatic mineral deposits. Future studies of granites using non-traditional stable isotope systems are expected to provide new insights into the evolution of Earth’s continental crust.

show abstract

Thorium distribution in the crust: Outcrop and grain-scale perspectives

Cited by 11 publications

References 52 publications

Divergent behaviour of Th and U during anatexis: Implications for the thermal evolution of orogenic crust

Divergent behaviour of Th and U during anatexis: Implications for the thermal evolution of orogenic crust

Thorium zoning in monazite: A case study from the Ivrea–Verbano zone, NW Italy

On Granites

Contact Info

Product

Resources

About