(U-Th-Sm)/He thermochronological age distribution in a slowly cooled plutonic complex (Ploumanac’h intrusion, France): insights into helium diffusion processes.

Abstract:

(U-Th-Sm)/He thermochronology is widely used to determine the thermal histories of mountain ranges and sedimentary basins. Apatite crystals retain helium at low temperatures, thus giving an insight into upper crustal evolution (e.g. exhumation, subsidence, erosion). Advanced models predict He production and diffusion rates in apatite crystals, thereby allowing determination of helium dates by integration over time/temperature paths (e.g. Gautheron et al., 2009). However, scattered dates and discordance between predicted and measured dates suggest that other parameters than time or temperature may also influence helium contents in apatite.

The present study determines the variables that affect He diffusion in apatite over long timescales. We report the (U-Th-Sm)/He date distribution within a slowly cooled intrusion, along with AFT data, as well as extensive petrological and chemical characterization. The Ploumanac’h site (Brittany, France) was chosen because it includes small-scale spatial variations in petrology and chemistry (<km). The Hercynian Ploumanac’h complex is composed of four different lithologies that underwent the same thermal history. A first cooling event occurred 250 Myrs ago, followed by a long stay in the He partial retention zone, and a final Late Cretaceous exhumation.

Results evidence scattered (U-Th-Sm)/He dates, ranging from 80±8 to 250±25 Myrs, whereas AFT ages range from 120 to 160 ±10 Myrs. The old and scattered (U-Th-Sm)/He ages cannot be explained with current models. We investigate the influence of monograin chemistry, crystal defect, and sample petrology on (U-Th-Sm)/He dates. Data confirm that He can be stored at defect sites, but also support a decrease in He retentivity for high equivalent damage fraction (>6-910⁶ tracks/cm²).

 GAUTHERON C., TASSAN-GOT L., BARBARAND J., PAGEL M., 2009. Effect of alpha-damage annealing on apatite (U-Th)/He thermochronology. Chem. Geol. 266, 166-179.