MECHANISMS AND KINETICS OF ATMOSPHERIC, RADIOGENIC, AND NUCLEOGENIC ARGON RELEASE FROM CRYPTOMELANE DURING AR-40 AR-39 ANALYSIS/

Resistance furnace (RF) Ar-40/Ar-39 analyses of two compositionally di stinct cryptomelane samples indicate that atmospheric Ar-40, Ar-38 and Ar-36, radiogenic Ar-40(), and nucleogenic Ar-39(double dagger) and Ar-38(double dagger) isotopes are released from the mineral structure at discrete temperature steps ranging from 300-900 degrees C. The atmo spheric components are released at temperatures ranging from 300-500 d egrees C and most of the radiogenic and nucleogenic isotopes are liber ated between 650-900 degrees C. Correlation between the resistance fur nace results, laser-heating release spectra, and in situ observation o f the thermal behavior of cryptomelane in the high voltage electron mi croscope (HVEM) suggest that each release temperature is associated wi th degassing of a specific site in the mineral. The lower degassing te mperature (peak at similar to 400 degrees C) is associated with atmosp heric Ar release from intergranular spaces. Only minor Ar-40() and Ar -39(double dagger) are released at this stage. Minor amounts of Ar rel eased at intermediate steps (T = 500 and 600 degrees C) are associated with the break down of poorly crystallized phases or Ar displacement by volume diffusion from the channel structures within cryptomelane. T he most significant release temperature (peak at T similar to 800 degr ees C) is associated with phase transformations of cryptomelane into h ausmannite and manganosite. Most (>94% in one sample and >60% in anoth er) of the Ar-40() and Ar-39(double dagger) are released at the 700-9 00 degrees C temperature range, which makes the gas fractions released in this interval the most significant for cryptomelane dating by the Ar-40/Ar-39 method. The excellent match between the degassing behavior s of Ar-40() and Ar-39(double dagger) in K-rich cryptomelane suggests that these isotopes occupy the same crystallographic sites and that A r-39(double dagger) is not dislodged from the tunnel site by recoil du ring neutron irradiation. A detectable discrepancy between Ar-40() an d Ar-39(double dagger) release patterns in Cu-rich cryptomelane sugges ts that a maximum of 8% of the Ar-39(double dagger) may have been disp laced by recoil. The degassing profiles also indicate that if Ar-39(do uble dagger) is displaced from the tunnel sites by recoil, it is not l ost from the minerals but remains trapped in intercrystalline sites.