Experimental alteration of monazite in granitic melt: Variable U–Th–Pb and REE mobility during melt-mediated coupled dissolution-precipitation

Varga, Jan; Raimondo, Tom; Daczko, Nathan R.; Adam, John A.

doi:10.1016/j.chemgeo.2020.119602

Cited by 36 publications

(12 citation statements)

References 76 publications

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“…The mismatch between monazite trace element concentration and age has previously been used to demonstrate variable resetting via coupled dissolutionprecipitation (CDP), commonly manifest as a continuum of concordant ages over >10 Myr (Varga et al, 2020;Weinberg et al, 2019). This pattern is exemplified by samples 94-91 and Shaw7 (Figure 7a,e).…”

Section: Timing Of Metamorphism In the Egcmentioning

confidence: 91%

Pressure–temperature–time constraints on gneiss dome formation in an intracontinental orogen

Varga

Raimondo

Morrissey

et al. 2021

Journal Metamorphic Geology

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Section: Timing Of Metamorphism In the Egcmentioning

confidence: 91%

Pressure–temperature–time constraints on gneiss dome formation in an intracontinental orogen

Varga

Raimondo

Morrissey

et al. 2021

Journal Metamorphic Geology

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“…This is likely a product of incomplete fluid-mediated coupled dissolution-precipitation (e.g. Grand'Homme et al, 2016;Harlov et al, 2011;Varga et al, 2020;Williams et al, 2011). Monazite with similar dates in the Woodnamoka Phyllite samples is likely to reflect the same event.…”

Section: Ark 2016mentioning

confidence: 96%

“…Accordingly, this post-deformational pegmatite is considered to have been emplaced during a c. 460-400 Ma hydrothermal-magmatic event at c. 424 Ma, with the U-Pb systematics in the analysed monazites variably modified by fluid-mediated processes (e.g. Grand'Homme et al, 2016;Harlov et al, 2011;Seydoux-Guillaume et al, 2012;Seydoux-Guillaume, Paquette, et al, 2002;Varga et al, 2020;Williams et al, 2011).…”

Section: Origin Of Pegmatitesmentioning

confidence: 99%

Th–U powered metamorphism: Thermal consequences of a chemical hotspot

Leeuwen

Hand

Morrissey

et al. 2021

Journal Metamorphic Geology

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The Arkaroola region of the northern Flinders Ranges, South Australia, records high geothermal gradient mineral assemblages that are not spatially or temporally associated with intrusive magmatism. Cordierite-bearing schists from the base of a ~12 km thick Neoproterozoic sedimentary sequence known as the Adelaide Rift Complex directly overlie Mesoproterozoic metasedimentary and granitic rocks with regional heat production values of ~7.9 µW/m 3 at 580 Ma, two to three times greater than global average values for granitic rocks. We integrate in-situ U-Pb monazite geochronology, Y+HREE-in-monazite thermometry and mineral equilibria modelling to show that rocks at the base of the sedimentary succession record amphibolite facies metamorphism at c. 580 Ma while the overlying sediments were still accumulating. Metamorphism took place under average geothermal gradient conditions in excess of 180°C/kbar (>60°C/km) that propagated to depths of at least 12 km. These thermal gradients persisted for upwards of 150 Ma, maintained by a lack of crustal erosion, and are documented by long-lived crustal anatexis. This system may be the archetypal example of Th-U powered metamorphism, recording the interplay between chemically extreme basement and thermally insulating sedimentary cover.

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“…The microstructures of monazites in contact to granitic melts were studied in experiments by Varga et al (2020). In nature, such a case could be when granites are re-melted during overprint by granulite facies metamorphism or when a regional high-grade metamorphic event leads to partial anatexis and migmatization.…”

Section: Monazite In High-grade Metamorphic Rocksmentioning

confidence: 99%

“…In nature, such a case could be when granites are re-melted during overprint by granulite facies metamorphism or when a regional high-grade metamorphic event leads to partial anatexis and migmatization. In the experiments (Varga et al, 2020), several distinct zones T1 to T4 are developed when monazite gets into contact to a melt: Zone T1 is a relict precursor monazite preserved in core domains of grain fragments that is brighter in BSE response and visually lacking discernible reaction textures. Zone T2 is monazite with fine melt-filled porosity (1-2 µm) that is spatially restricted to the margins of T1 and shows similar BSE brightness.…”

Section: Monazite In High-grade Metamorphic Rocksmentioning

confidence: 99%

Monazite Microstructures and Their Interpretation in Petrochronology

Schulz

2021

Front. Earth Sci.

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The phosphate mineral monazite (LREE,Y,Th,Ca,Si)PO4 occurs as an accessory phase in peraluminous granites and Ca-poor meta-psammopelites. Due to negligible common Pb and very low Pb diffusion rates at high temperatures, monazite has received increasing attention in geochronology. As the monazite grain sizes are mostly below 100 μm in upper greenschist to amphibolite facies meta-psammopelites, and rarely exceed 250 μm in granulite facies gneisses and in migmatites, microstructural observation and mineral chemical analysis need the investigation by scanning electron microscope and electron probe microanalyzer, with related routines of automated mineralogy. Not only the microstructural positions, sizes and contours of the grains, but also their internal structures in backscattered electron imaging gray tones, mainly controlled by the Th contents, can be assessed by this approach. Monazite crystallizes mostly euhedral to anhedral with more or less rounded crystal corners. There are transitions from elliptical over amoeboid to strongly emarginated grain shapes. The internal structures of the grains range from single to complex concentric over systematic oszillatory zonations to turbulent and cloudy, all with low to high contrast in backscattered electron imaging gray tones. Fluid-mediated partial alteration and coupled dissolution-reprecipitation can lead to Th-poor and Th-rich rim zones with sharp concave boundaries extending to the interior. Of particular interest is the corona structure with monazite surrounded by apatite and allanite, which is interpreted to result from a replacement during retrogression. The satellite structure with an atoll-like arrangement of small monazites may indicate re-heating after retrogression. Cluster structures with numerous small monazite grains, various aggregation structures and coating suggest nucleation and growth along heating or/and enhanced fluid activity. Microstructures of monazite fluid-mediated alteration, decomposition and replacement are strongly sutured grain boundaries and sponge-like porosity and intergrowth with apatite. Garnet-bearing assemblages allow an independent reconstruction of the pressure-temperature evolution in monazite-bearing meta-psammopelites. This provides additional potential for evaluation of the monazite microstructures, mineral chemistry and Th-U-Pb ages in terms of clockwise and counterclockwise pressure-temperature-time-deformation paths of anatectic melting, metamorphism and polymetamorphism. That way, monazite microstructures serve as unique indicators of tectonic and geodynamic scenarios.

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Experimental alteration of monazite in granitic melt: Variable U–Th–Pb and REE mobility during melt-mediated coupled dissolution-precipitation

Cited by 36 publications

References 76 publications

Pressure–temperature–time constraints on gneiss dome formation in an intracontinental orogen

Pressure–temperature–time constraints on gneiss dome formation in an intracontinental orogen

Th–U powered metamorphism: Thermal consequences of a chemical hotspot

Monazite Microstructures and Their Interpretation in Petrochronology

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