Characterization and calibration of 40Ar/39Ar dating standards

Because 40Ar/39Ar ages are based on independent age determinations for standard minerals, it is critical that these standards be well characterized. This includes constraints both on the age of the standards themselves and the ages of the standards relative to one another. In an effort to better characterize standards in common use, we have intercalibrated five of these: the interlaboratory standards MMhb-1 hornblende, 92-176 Fish Canyon Tuff sanidine, and LP-6 biotite, along with two Australian National University intralaboratory standards, 77-600 hornblende and GA-1550 biotite. The results indicate that all standards studied are homogeneous using subsamples of tens of milligrams, indicating that total fusion analyses of such large aliquots will yield reproducible results. However, only 92-176 sanidine and GA-1550 biotite meet the criteria for acceptable standards at the single crystal sampling level, whereas MMhb-1 hornblende, 77-600 hornblende, and LP-6 biotite exhibit inhomogeneity, which compromises their usefulness. Intercalibration R values, which allow direct comparison of standards, have been derived as follows: R92-176GA-1550=3.575±0.005, RGA-1550MMhb-1=5.996±0.010, RGA-155077–600=4.650±0.014, RGA-1550LP-6=1.305±0.003, RLP-6MMhb-1=4.550±0.019, R92-176MMhb-1=21.53±0.09, and R77–600MMhb-1=1.289±0.002. These values generally agree with those derived in previous studies, although variability between studies significantly exceeds the analytical precision of the 40Ar/39Ar method, emphasizing the need for a widely accepted set of primary standards of well-established age. Based on a compilation of potassium measurements by flame photometry from 1968 to 2001, an updated K/Ar age of 98.5±0.8 Ma is calculated for GA-1550 biotite, which is used as a primary standard for the intercalibration. Relative to GA-1550, the ages for other standards are derived via the intercalibration R values as follows: 418.3±1.3 Ma for 77-600 hornblende, 523.2±0.9 Ma for MMhb-1 hornblende, 127.5±0.3 Ma for LP-6 biotite, and 28.10±0.04 Ma for 92-176 sanidine.