Improving 40Ar/39Ar Methods
Obtaining accurate ages is of primary importance in any geochronologic studies. Systematic error in 40Ar/39Ar dating originates mainly from uncertainties in the 40K decay constants and K/Ar isotopic data of neutron fluence monitors (standards). The 40K decay constant currently using in the geological society is known to no better than 2% at the 2 sigma level when the background activity data and 40K/K ratio are considered. The interlaboratory discrepancy for the age of the Fish Canyon sanidine (tFCs), one of the most widely distributed standards, is more than 2%. To constrain 40K decay constant and tFCs more accurately, I have suggested comparing, at the highest possible level of precision, 40Ar/39Ar isotopic data with inherently more accurate U/Pb ages. High quality isotopic data were obtained from volcanic and meteoritic samples of various ages, and combined with statistical procedures. Five data pairs yielded preliminary estimates for decay constant of 40K (5.476 ”¾ 0.017 E-10/year) which is 1~2 % smaller than the traditional values (5.543 E-10/year), and tFCs (28.27 ”¾ 0.066 Ma) which is consistent with 40Ar/39Ar age calibrated from K/Ar primary standard. Accumulating data pairs is under progress even though it is difficult to find high quality data pairs for samples which have experienced desirable thermal histories.
One of the assumptions of the intercalibration (between 40Ar/39Ar and U/Pb systems) is rapid cooling of the target materials over a temperature range of ~600-300 ”ĘC, decoupling any systematic bias between the two isotopic systems. This assumption is believed to be true for many volcanic or meteoritic units, and indirectly confirmed by examining petrographic features of each sample. More promising way to test the assumption is obtaining low-temperature cooling histories using the (U-Th)/He dating method. We have applied the (U-Th)/He dating to single crystals from the Acapulco meteorite confirming extremely fast cooling of this body down to ~100 ”ĘC, thus concluded that the Acapulco samples satisfy the assumption for the isotopic intercalibration.
40Ar/39Ar geochronologic work on chondrites produce a range of ages with a mean of 4.48 ”¾ 0.03 Ga, ~1-2% younger than corresponding U/Pb ages. This discordance is commonly attributed to slow cooling or high-temperature metamorphism after the formation of the parent body. However, such age bias could result from large uncertainties associated with the decay constants and the ages of neutron fluence monitors used for the 40Ar/39Ar age calculations. My work will continue to improve the fundamental parameters of the 40Ar/39Ar method and to scrutinize reliability of extant ages for better understanding of natural processes.Related publications:
Min
K, Farley KA, Renne
PR and Marti K (2003) Single grain (U-Th)/He ages from phosphates in Acapulco
meteorite and implications for thermal history. Earth and Planetary Science
Letters, v. 209, p. 323-337.
Kwon J, Min K, Bickel PJ and Renne PR (2002) Statistical methods for jointly estimating decay constant of 40K and age of a dating standard. Mathematical Geology, v. 34, p. 457-474.
Begemann F, Ludwig KR, Lugmair GW, Min K, Nyquist LE, Patchett PJ, Renne PR, Shih C-Y, Villa IM and Walker RJ (2001) Call for an improved set of decay constants for geochronological use. Geochimica et Cosmochimica Acta, v. 65, p. 111-121.
Min K, Mundil R, Renne PR and Ludwig KR (2000) A test for systematic errors in 40Ar/39Ar geochronology through comparison with U-Pb analysis of a 1.1 Ga rhyolite. Geochimica et Cosmochimica Acta, v. 64, p. 73-98.