U-PB GEOCHRONOLOGIC AND THERMOCHRONOLOGIC TIME-TEMPERATURE CONSTRAINTS FOR THE 40Ar/39Ar HORNBLENDE STANDARD, HB3gr

A now classic K-Ar and Rb-Sr study of the ca 1.1 Ga Lone Grove batholith, Texas, U.S.A. (Zartman, 1964) led to the development of its hornblende (HB3gr) as a geochronological standard and neutron flux monitor for the K-Ar and ⁴⁰Ar/³⁹Ar techniques, including use in early lunar studies. Zartman (1964) suggested that the hornblende K-Ar date may actually record thermal relaxation and not the crystallization age of the intrusion despite an estimated shallow ( <10 km) intrusion depth. In the 1960’s-1980’s, typical uncertainties associated with Rb-Sr and K-Ar were ca. 1-2 % or 10-20 Ma, making a test of this hypothesis difficult. More recently, internal uncertainties have been greatly reduced, the decay constants for ⁴⁰K are better estimated and it has been widely recognized that Ar in hornblende obeys thermally-activated volume diffusion, allowing this mineral to be exploited as a thermochronometer.

The EARTHTIME initiative has encouraged the high precision inter-calibration of the U-Pb and ⁴⁰Ar/³⁹Ar systems for quickly cooled samples and thus we applied the U-Pb system to evaluate the accuracy and precision of HB3gr as a Hornblende⁴⁰Ar/³⁹Ar age standard. Zircon and titanite U-Pb geo/thermochronology have the potential to constrain the thermal evolution of the sample, which in turn can be used to determine the expected effects of volume diffusion of Ar in hornblende during protracted cooling. Zircon grains from 3gr treated using the Chemical Abrasion technique yield a weighted mean ²⁰⁶Pb/²³⁸U date of ~1090 Ma. This date is within uncertainty of the ⁴⁰Ar/³⁹Ar hornblende date of 1083 ± 21 Ma, (1σ, including decay constant uncertainties) recalculated using astronomically calibrated neutron flux monitors and a statistically determined ⁴⁰K decay constant . Titanite U-Pb analyses, however, yield an array of ²⁰⁶Pb/²³⁸U dates between ~1087 and 1082 Ma, which we interpret as cooling dates (ca. 650 °C). Since the closure temperature for Ar diffusion in hornblende is nominally ca. 550 °C, ⁴⁰Ar/³⁹Ar dates of HB3gr also record post-emplacement cooling, resulting in rounded internal ⁴⁰Ar diffusion profiles and a dependency of calculated dates on grain size. Future use of HB3gr as a neutron flux monitor must consider the variation in dates produced by slow cooling through the Ar-hornblende partial retention zone.