Recycling argon through metamorphic reactions: The record in symplectites

Abstract

The 40 Ar/ 39 Ar ages of metamorphic micas that crystallized at high temperatures are commonly interpreted as cooling ages, with grains considered to have lost 40 Ar via thermally-driven diffusion into the grain boundary network. Recently reported laser-ablation data suggest that the spatial distribution of Ar in metamorphic micas does not always conform to the patterns predicted by diffusion theory and that despite high metamorphic temperatures, argon was not removed efficiently from the local system during metamorphic evolution. In the Western Gneiss Region (WGR), Norway, felsic gneisses preserve microtextural evidence for the breakdown of phengite to biotite and plagioclase symplectites during near isothermal decompression from c. 20-25 to c. 8-12 kbar at~700°C. These samples provide an ideal natural laboratory to assess whether the complete replacement of one K-bearing mineral by another at high temperatures completely 'resets' the Ar clock, or whether there is some inheritance of 40 Ar in the neocrystallized phase. The timing of the high-temperature portion of the WGR metamorphic cycle has been well constrained in previous studies. However, the timing of cooling following the overprint is still much debated. In-situ laser ablation spot dating in phengite, biotite-plagioclase symplectites and coarser, texturally later biotite yielded 40 Ar/ 39Ar ages that span much of the metamorphic cycle. Together these data show that despite residence at temperatures of~700°C, Ar is not completely removed by diffusive loss or during metamorphic recrystallization. Instead, Ar released during phengite breakdown appears to be partially reincorporated into the newly crystallizing biotite and plagioclase (or is trapped in fluid inclusions in those phases) within a close system. Our data show that the microtextural and petrographic evolution of the sample being dated provides a critical framework in which local 40 Ar recycling can be tracked, thus potentially allowing 40 Ar/ 39 Ar dates to be linked more accurately to metamorphic history.

Two‐Stage Cooling and Exhumation of Deeply Subducted Continents

“…The multigrain and whole‐grain incremental step‐heating white mica 40 Ar/ 39 Ar dates from host WGC basement gneisses increase from c. 370 in the northwest to c. 400 Ma in the southeast and may represent a cooling trend (Figure ; Hacker, ; Root et al, ; Walsh et al, , ). An older, pre‐Caledonian age component is also preserved in some white mica and biotite grains, although these may reflect excess argon (e.g., McDonald et al, ; Warren et al, ). The bulk of the U‐Pb titanite dates are bracketed by geochronologic results (U‐Pb zircon, Sm‐Nd and Lu‐Hf garnet) that constrain the timing of prograde‐to‐peak conditions and by 40 Ar/ 39 Ar dates.…”

“…To test and refine exhumation models for the WGC, a vast effort has been made during the past decades by applying multiple thermochronologic methods across the region. Most studies focus on 40 Ar/ 39 Ar dating of mica (Hacker, ; McDonald et al, ; Root et al, ; Walsh et al, , ; Warren et al, ) and U‐Pb dating of titanite (e.g., Spencer et al, ). These thermochronological data have allowed insight into the retrograde history of the WGC at an unprecedented level of spatial resolution and have led to well‐established exhumation models that are applied in the study of ( U ) HP terranes worldwide.…”

“…Recent studies, however, have demonstrated inconsistencies in the mineral chronometers that have been used to constrain existing exhumation models (e.g., Kohn, ; McDonald et al, ; Warren et al, , ). For example, the distribution of U‐Pb titanite dates from the WGC basement gneisses do not exhibit any systematic variation and many grains preserve pre‐Caledonian age components (Figure ; Spencer et al, ), indicating that the mineral persisted well beyond the theoretical limits of its stability (1.5–2.5 GPa at 700–850 °C; Kohn, , and references therein).…”

“…For example, the distribution of U‐Pb titanite dates from the WGC basement gneisses do not exhibit any systematic variation and many grains preserve pre‐Caledonian age components (Figure ; Spencer et al, ), indicating that the mineral persisted well beyond the theoretical limits of its stability (1.5–2.5 GPa at 700–850 °C; Kohn, , and references therein). Likewise, recent in situ 40 Ar/ 39 Ar mica dating on WGC rocks show that previous interpretations of the dates as uniformly reflecting cooling and cessation of diffusive Ar loss may be flawed (McDonald et al, ; Warren et al, ). Instead, the 40 Ar/ 39 Ar thermochronology results may be controlled by grain boundary Ar‐concentration, which could cause substantial, arbitrary age bias.…”

“…Instead, the 40 Ar/ 39 Ar thermochronology results may be controlled by grain boundary Ar‐concentration, which could cause substantial, arbitrary age bias. The step heating experiments that were used in these previous studies would have homogenized any intragrain and intergrain age variability and, thus, the interpretation of these results as representing cooling may not be accurate (McDonald et al, ; Warren et al, ). Adding to the complexity is the fact that both titanite and micas are susceptible to variable degrees of recrystallization during deformation or in the presence of fluids (e.g., Spencer et al, ; Warren et al, ).…”