Other
Scientific paper
May 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.2057m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 10, pp.2057-2070
Other
3
Scientific paper
Resistance furnace (RF) 40 Ar/ 39 Ar analyses of two compositionally distinct cryptomelane samples indicate that atmospheric 40 Ar, 38 Ar and 36 Ar, radiogenic 40 Ar * , and nucleogenic 39 Ar and 38 Ar isotopes are released from the mineral structure at discrete temperature steps ranging from 300-900°C. The atmospheric components are released at temperatures ranging from 300-500°C and most of the radiogenic and nucleogenic isotopes are liberated between 650-900°C. Correlation between the resistance furnace results, laser-heating release spectra, and in situ observation of the thermal behavior of cryptomelane in the high voltage electron microscope (HVEM) suggest that each release temperature is associated with degassing of a specific site in the mineral. The lower degassing temperature (peak at ~400°C) is associated with atmospheric Ar release from intergranular spaces. Only minor 40 Ar * and 39 Ar are released at this stage. Minor amounts of Ar released at intermediate steps ( T = 500 and 600°C) are associated with the break down of poorly crystallized phases or Ar displacement by volume diffusion from the channel structures within cryptomelane. The most significant release temperature (peak at T ~ 800°C) is associated with phase transformations of cryptomelane into hausmannite and manganosite. Most (>94% in one sample and >60% in another) of the 40 Ar * and 39 Ar are released at the 700-900°C temperature range, which makes the gas fractions released in this interval the most significant for cryptomelane dating by the 40 Ar/ 39 Ar method. The excellent match between the degassing behaviors of 40 Ar * and 39 Ar in K-rich cryptomelane suggests that these isotopes occupy the same crystallographic sites and that 39 Ar is not dislodged from the tunnel site by recoil during neutron irradiation. A detectable discrepancy between 40 Ar * and 39 Ar release patterns in Cu-rich cryptomelane suggests that a maximum of 8% of the 39 Ar may have been displaced by recoil. The degassing profiles also indicate that if 39 Ar is displaced from the tunnel sites by recoil, it is not lost from the minerals but remains trapped in intercrystalline sites.
Becker Tim A.
Marcos Vasconcelos Paulo
Renne Paul R.
Wenk Hans-Rudolf
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