Cryptic excess argon in metamorphic biotite: Anomalously old 40Ar/39Ar plateau dates tested with Rb/Sr thermochronology and Ar diffusion modelling

In ⁴⁰Ar/³⁹Ar geochronology, excess Ar is ⁴⁰Ar that does not derive from the in situ radioactive decay of ⁴⁰K, or from the measurable input of atmospheric Ar, and results in increased daughter-parent ratios that correspond to anomalously old apparent dates without geological age significance. Excess ⁴⁰Ar is commonly identified by a saddle-shaped ⁴⁰Ar/³⁹Ar degassing spectrum. However, biotite from the east Albany-Fraser Orogen of Western Australia contains excess ⁴⁰Ar, but yields well-defined ⁴⁰Ar/³⁹Ar plateau dates, reproducible upon replicate analysis of biotite from the same sample. This “cryptic excess ⁴⁰Ar” is inferred where plateau dates (1) are older than existing time constraints on cooling, such as U/Pb zircon ages for amphibolite to granulite facies metamorphism, and (2) vary between multiple samples from the same outcrop that have experienced the same thermal history. Rb/Sr geochronology is used to test the geologic significance of ⁴⁰Ar/³⁹Ar plateau dates, as the closure temperatures of both chronometers are similar, and the two chronometers are expected to yield similar cooling dates in an undisturbed system. Six Rb/Sr biotite-whole rock isochron ages from three outcrops yield a weighted mean age of 1133 ± 3 Ma (MSWD = 1.13, P = 0.34), and are interpreted to record post-orogenic cooling. ⁴⁰Ar/³⁹Ar plateau dates from the same samples are 31–394 Ma older, and in six of nine samples cannot be explained by simple monotonic cooling, by low-temperature biotite recrystallisation, or by alteration; instead, biotite contains cryptic excess ⁴⁰Ar. Diffusion modelling of ⁴⁰Ar/³⁹Ar step-heating plateaus suggests that Ar, both excess and radiogenic, is homogeneously distributed within the crystal lattice, and was incorporated during biotite crystallisation. Biotite likely crystallised in a rock with a high partial pressure of Ar, possibly due to a high rate of radiogenic ⁴⁰Ar generation in a K-rich lithology, together with a poorly-connected intergranular fluid network that inhibited Ar loss from the rock volume. This is supported by a homogeneous distribution of Ar at the cm scale, with reproducible biotite ⁴⁰Ar/³⁹Ar dates from each sample. Patchy fluid circulation during metamorphism led to the pervasive but heterogeneous distribution of excess ⁴⁰Ar across outcrops. Due to the presence of the ⁴⁰Ar/³⁹Ar plateau, cryptic excess ⁴⁰Ar is difficult to identify a priori, without comparison to additional geochronological constraints. Where cryptic excess ⁴⁰Ar on a regional scale has been identified, such as in the eastern Biranup Zone of the Albany-Fraser Orogen, Rb/Sr geochronology may provide a more robust alternative to constrain the cooling path of rocks from high temperatures.