TY - JOUR
T1 - Influence of crystal size on apatite (U-Th)/He thermochronology
T2 - An example from the Bighorn Mountains, Wyoming
AU - Reiners, Peter W.
AU - Farley, Kenneth A.
N1 - Funding Information:
We gratefully acknowledge the collaboration of the 1999 Keck–Bighorn participants (especially Peter Crowley, Joanna Reuter, and Grant Kaye), and Cathy Manduca. Thanks also to Charles Knaack for valuable technical assistance at WSU. We appreciate helpful reviews by Terry Spell and Peter Zeitler. This work was supported in part by NSF Grant EAR 0073576 to P.W.R. [RV]
PY - 2001
Y1 - 2001
N2 - Near-surface tectonic and geomorphic processes involve cooling of rocks through low temperatures (50-200°C). Because rates of helium diffusion in apatite, titanite, and zircon are sensitive to temperature variations in this range, uranium-thorium/helium thermochronometry ((U-Th)/Hedating is well-suited to to establishing the timing and rates of these processes in the geologic record. However,because fractional loss of He is controlled by crystal size such that larger crystals retain a larger fraction of radiogenic HE, (U-Th)/He ages must vary not only with thermal history but also with crystal size. Here we present crystal size-correlated He ages from co-existing apatites from the Bighorn Mountains, Wyoming that range from 100 to 350 Ma. These correlations are a sensitive indicator of the rock's thermal history in a temperature range below the system's nominal closure temperature (Tc̃ 70°C for apatite), and are consistent with a thermal history involving residence in the upper 2-3 km of crust since the Precambrian, with maximum temperatures of 65-80°C just prior to Laramide orogenic echumation. The influence of crystal size on He ages will be most apparent in rocks where temperatures have been in the range of partial He retention for long periods of time (̃30-70°C for > 10 7 years). In such cases, accurate interpretation of (U-Th)/he ages must incorporate the effect of crystal size, and this method may provide insights tothermal histories of rocks in previously inaccessible low-temperature ranges.
AB - Near-surface tectonic and geomorphic processes involve cooling of rocks through low temperatures (50-200°C). Because rates of helium diffusion in apatite, titanite, and zircon are sensitive to temperature variations in this range, uranium-thorium/helium thermochronometry ((U-Th)/Hedating is well-suited to to establishing the timing and rates of these processes in the geologic record. However,because fractional loss of He is controlled by crystal size such that larger crystals retain a larger fraction of radiogenic HE, (U-Th)/He ages must vary not only with thermal history but also with crystal size. Here we present crystal size-correlated He ages from co-existing apatites from the Bighorn Mountains, Wyoming that range from 100 to 350 Ma. These correlations are a sensitive indicator of the rock's thermal history in a temperature range below the system's nominal closure temperature (Tc̃ 70°C for apatite), and are consistent with a thermal history involving residence in the upper 2-3 km of crust since the Precambrian, with maximum temperatures of 65-80°C just prior to Laramide orogenic echumation. The influence of crystal size on He ages will be most apparent in rocks where temperatures have been in the range of partial He retention for long periods of time (̃30-70°C for > 10 7 years). In such cases, accurate interpretation of (U-Th)/he ages must incorporate the effect of crystal size, and this method may provide insights tothermal histories of rocks in previously inaccessible low-temperature ranges.
KW - Exhumation
KW - Helium
KW - Laramide orogeny
KW - Th/U
KW - Thermochronology
KW - U/He
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U2 - 10.1016/S0012-821X(01)00341-7
DO - 10.1016/S0012-821X(01)00341-7
M3 - Article
AN - SCOPUS:0034963676
VL - 188
SP - 413
EP - 420
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
SN - 0012-821X
IS - 3-4
ER -