The in vacuo release of Ar and rare gases from minerals: 3. The degassing of He, Ne, Ar, Kr and Xe from irradiated apatite

The mechanisms of in vacuo release of Ar are investigated by bringing them into context with nine other rare gas isotopes and by studying the mineralogical modifications that occur in their host mineral. A 200 mg shard of Durango fluorapatite was step-heated after neutron irradiation. It contained radiogenic 4He from natural U decay and artificially produced major rare gas isotopes: 20Ne from F, 37Ar from Ca, 38Ar from Cl, 80Kr and 82Kr from Br, 128Xe from I, 131Xe from Ba and 134Xe from U. The 4He release rate was compared with that from an unirradiated aliquot. Helium was expected, and observed, to degas at the lowest furnace temperatures, with nearly complete exhaustion by 1300 °C. Neon followed a bimodal degassing pattern, with a peak of the release rate at 1178 °C and a higher one at 1406 °C. Argon degassing showed a similar bimodality. Krypton and xenon were both mostly released in a single, concentrated burst between 1362 and 1460 °C. The two Xe isotopes 128 and 131, produced from I and Ba, respectively, followed exactly the same degassing pattern. The crystallographic site of the target element had no control on the movement of the irradiation-produced rare gas atom. The He release from the irradiated and unirradiated aliquots gave two overlapping alignments in the Arrhenius diagram at temperatures between 450 and 1300 °C. The two slopes are indistinguishable at the 1 sigma level and yield an average activation energy of 62 ± 5 kJ/mol. In the 450–1300 °C temperature interval, all five rare gases showed parallel trends with an activation energy around 62 kJ/mol and release rate constants decreasing from He to Xe by about 4 orders of magnitude. Arrhenian trajectories for the Kr and Xe degassing rate merge and sharply steepen in the 1362 °C step, with a degassing rate in the 1406 °C step about 400 times higher than at lower temperatures and an activation energy of 1.28 MJ/mol. The high-T modes of the bimodal Ar and Ne release also merge on the same steep Arrhenius line, with release rate constants indistinguishable from those of Kr and Xe. This break in Arrhenius slope and the merger of four trajectories that were widely separated below 1300 °C indicate a major, energetically very costly event between 1300 and 1362 °C. The possible structural reordering was investigated with four mineralogical techniques: Raman spectroscopy, X ray diffractometry, transmission electron microscopy, and microchemical analysis by laser induced breakdown spectroscopy. Total loss of F may have transiently modified the apatite structure. Complete outgassing of Ne, Ar, Kr and Xe was only achieved after the defluorination reaction and attending displacive structural reorganization. Like in hydrated minerals, in K-feldspar, and in leucite, degassing of heavy noble gases in apatite occurs by at least two different physical mechanisms. In all of these minerals downslope extrapolation of in vacuo Ar degassing in the laboratory to temperatures of geological interest does not appear warranted.