THE ZONE OF INCIPIENT 40AR* LOSS: INTRODUCING A NEW CONCEPT OF AR DEGASSING BEHAVIOR IN A CONTACT METAMORPHIC SETTING

Partial retention zones are well-known in thermochronology and define a range in which radiogenic nuclides are partially retained and/or partially outgassed via volume diffusion. Constraints for Ar closure temperatures are so far only reported for regional metamorphic settings but do not exist for contact metamorphic environments. This study aims to set new temperature constraints for the ⁴⁰Ar degassing behavior in illite fine fractions (<0.2 µm and <2 µm) in a contact metamorphic environment.

The thermal evolution of the Magallanes Basin in Chile (51°S) is characterized by a complex pattern of fold-and-thrust belt formation and magmatic events. The thrusting and associated uplift is dated at around 60 Ma with temperatures between 240 °C and 270 °C [1]. The Miocene Torres del Paine intrusion was emplaced between 12.59 and 12.43 Ma into the folded Cretaceous formations [2,3].

The Torres del Paine intrusion provides the possibility of studying profiles related to temperature changes as a function of distance to the intrusive body and their respective K/Ar ages. The temperature conditions of the regional metamorphosed pelitic host rock and the contact metamorphic pelite is monitored by illite crystallinities (Kübler Index) and by temperatures obtained from Raman spectroscopy on carbonaceous matter.

The metapelites were sampled in three profiles starting from the biotite-in isograde of the contact aureole to the regional anchizonal host rock unaffected by the intrusion. K-Ar ages were obtained for nine samples in their <0.2 µm and <2 µm fractions.

The results constrain the zone of incipient ⁴⁰Ar* loss to be between 248 and 320 °C for the <0.2 µm fraction and between ca. 260 °C and 345 °C for the <2 µm fraction. The continuous degassing of radiogenic ⁴⁰Ar is monitored up to a temperature of 296 °C by partially reset K/Ar ages. The upper temperature limit of the “zone of incipient ⁴⁰Ar* loss” is defined by extrapolating the ⁴⁰Ar/K ratio to a value of zero (i.e. total ⁴⁰Ar* loss) in a ⁴⁰Ar/K vs. temperature plot. The proposed temperatures are significantly higher than closure temperatures reported in the literature due to the short heating time and the absence of deformation and recrystallization of illite.

[1] Süssenberger et al. (2016). In GSA Abstracts, Denver.

[2] Michel et al. (2008). Geology, 459-462.

[3] Leuthold et al. (2012). EPSL, 85-92.