The zone of incipient 40Ar* loss-monitoring 40Ar* degassing behavior in a contact metamorphic setting

Abstract

The ⁴⁰Ar* retentivity behavior was studied in the contact metamorphic aureole of the Torres del Paine intrusion (51°S, Chile) and provides constraints on the closure temperature in contact metamorphic settings. The temperature conditions of the regional anchizonal to epizonal metamorphosed pelitic host rock and the contact metamorphic pelite were recorded by illite crystallinities (Kubler Index) and by Raman spectroscopy on carbonaceous matter. K/Ar age determinations were carried out on illite fine fractions (<0.2 and <2 µm). Differing reaction progresses are observed between organic matter and illite. Organic matter equilibrated with the ambient temperatures, whereas illite is lacking behind the contact metamorphic conditions evident by the absence or limited recrystallization progress. We propose two models to explain the degassing of ⁴⁰Ar* in the absence of recrystallization considering surface loss and volume diffusion as two main pathways. Incipient ⁴⁰Ar* loss appears at temperatures of about 248°C in the <0.2 µm and at 260°C in the <2 µm fraction. The high temperature end of ⁴⁰Ar* retentivity is derived by extrapolation yielding temperatures of 330°C for the <0.2 µm fraction and 345°C for the <2 µm fraction. Thus, the Zone of incipient ⁴⁰Ar* loss, called ZIAL, is constrained to between 248°C and 330°C for the <0.2 µm fraction and to between 260 and 345°C for the <2 µm fraction. The extrapolated values for the upper ZIAL limit are in good agreement with calculated bulk closure temperature values of 348°C (<2 µm) and 286°C (<0.2 µm) assuming ca. 10 kyrs of heating during the emplacement of the Torres del Paine intrusion.