Closed system behaviour of argon in osumilite records protracted high‐Tmetamorphism within the Rogaland–Vest Agder Sector, Norway

Abstract

Here, we present results of the first 40Ar/39Ar dating of osumilite, a high‐T mineral that occurs in some volcanic and high‐grade metamorphic rocks. The metamorphic osumilite studied here is from a metapelitic rock within the Rogaland–Vest Agder Sector, Norway, an area that experienced regional granulite facies metamorphism and subsequent contact metamorphism between 1,100 Ma and 850 Ma. The large grain size (~1 cm) of osumilite in the studied rock, which preserves a nominally anhydrous assemblage, increases the potential for large portions of individual grains to have remained essentially unaffected by the effects of diffusive argon loss, potentially preserving prograde ages. Step‐heating diffusion experiments yielded a maximum activation energy of ~461 kJ/mol and a pre‐exponential factor of ~8.34 × 108 cm2/s for Ar diffusion in osumilite. These parameters correspond to a relatively high closure temperature of ~620°C for a cooling rate of 10°C/Ma in an osumilite crystal with a 175 µm radius. Fragments of osumilite separated from the sample preserve a range of ages between c. 1,070 and 860 Ma. The oldest ages are inferred to date the growth of coarse‐grained osumilite during prograde granulite facies regional metamorphism, which pre‐date contact metamorphism that has historically been ascribed to the growth of osumilite in the region. The majority of fragments record ages between c. 920 and 860 Ma, inferred to reflect the growth of osumilite and/or diffusive argon loss during contact metamorphism. The retention of old 40Ar/39Ar dates was facilitated by the low diffusivity of Ar in osumilite (i.e. a closed system), large grain sizes, and anhydrous metamorphic conditions. The ability to date osumilite with the 40Ar/39Ar method provides a valuable new thermochronometer that may constrain the timing and duration of high‐T magmatic and metamorphic events.

Interpreting and reporting 40Ar/39Ar geochronologic data

“…Thus, as noted by Ware and Jourdan (2018), 40 Ar/ 39 Ar thermochronometry of pyroxenes may yield improved constraints on the temperature-time evolution of exhumed granulite-facies rocks. The same could be said for 40 Ar/ 39 Ar thermochronometry of the rare cyclosilicate mineral osumilite (Blereau et al, 2019), which can be found in some Mg-rich, granulite-facies metapelites, given that its closure temperature is similar to that of orthopyroxene. Detailed temperaturetime paths for eclogite-facies terranes may also be improved through 40 Ar/ 39 Ar thermochronometry of pyroxenes found in mafic eclogites.…”

Deformation recorded in polyhalite from evaporite detachments revealed by 40Ar ∕ 39Ar dating

“…7) that show steady Ar diffusion at a moderate slope. All three Arrhenius plots show no signs of low-temperature rapid release of a low percentage volume of Ar at shallow slopes that is often interpreted as fast-release argon from cracks and defects, which would become a separate diffusion domain (Blereau et al, 2019;Thern et al, 2020).…”

“…The two diffusion parameters, E a and D 0 , are extracted from the array defined by the data presented in the Arrhenius plots up until temperatures at which the crystals broke down and began to melt. We used a crystal radius of 90 ± 15 µm and a spherical geometry for the calculation as this geometry is appropriate for all grain shapes, other than platy minerals, with little effect on the diffusion results (Blereau et al, 2019). Errors on the y-axis intercept D 0 and slope E a were calculated using a robust regression (Isoplot v3.7;Ludwig, 2003) since the scatter on the regression line is much larger than the uncertainties on the individual measurements.…”

Probing the history of ultra‐high temperature metamorphism through rare earth element diffusion in zircon

“…This terrain experienced prolonged high‐ T metamorphism during the Meso‐ to Neoproterozoic that culminated in the emplacement of a c . 930 Ma composite anorthosite massif, the Rogaland Igneous Complex (RIC) and the geographically associated Bjerkreim–Sokndal layered intrusion (Bingen et al, 2008; Blereau et al, 2017, 2019; Coint et al, 2015; Drüppel et al, 2013; Laurent et al, 2016; Laurent, Bingen, et al, 2018; Laurent, Duchene, et al, 2018; Möller et al, 2003; Schärer et al, 1996; Slagstad et al, 2013, 2018; Tomkins et al, 2005). In the aureole of the RIC (Figure 1), it is possible to study samples that experienced a wide range of peak metamorphic temperatures, combining both regional and contact metamorphic effects, and to compare and contrast their time‐integrated geochemical responses.…”

“…Firstly, we investigate systematically the variations in the REE compositions of zircon grains and compare these with modelled diffusion profiles, assuming equilibration between zircon and garnet, as an independent measure of their T–t evolution. Secondly, we differentiate between end‐member models accounting for the P–T–t history of the RVA Sector by combining new in situ zircon and monazite geochronological data from samples collected over a range of distances from the RIC with existing petrological and P–T data from the same samples (Blereau et al, 2017, 2019) and similar samples from other studies (e.g. Drüppel et al, 2013; Laurent, Bingen, et al, 2018; Laurent, Duchene, et al, 2018; Slagstad et al, 2018).…”