HE-4 AGE DISCORDANCE AND RELEASE BEHAVIOR OF A DOUBLE SHELL BOTRYOIDAL HEMATITE FROM THE SCHWARZWALD, GERMANY

Microprobe analyses of a double shell botryoidal hematite from the Fah lenbach vein system, (Schwarzwald, Germany) show an Fe3+ depletion of 3-4% with respect to Al3+ and silicon within a approximately 0.5 mm wi de zone of alteration which separates both shells, and probably is the result of a subsequent hydrothermal event. Induced U-235 fission trac ks of grain mounts from both shells reveal several grain populations w ithin a range of approximately 3 to approximately 22 ppm total uranium . Mass spectrometer- (U) and photometer analyses (Th) of various local ities probed within each shell vary significantly at about 5 and 12 pp m U, as well as at 11 and 19 ppm Th. The Th/U ratios vary from approxi mately 1.5 to approximately 2. He-4 concentrations correlate with [U (0.23) Th] heterogeneities which permits construction of two shell-in ternal isochrons. The He-4 ages are discordant (inner shell, although mineralogically older: 150 Ma; outer shell: 182 Ma). He-4 release by s tepwise heating of one sample from each shell yielded activation energ ies of 124 +/- 8 and 142 +/- 7 [kJ/mol]. The extrapolated He-4 diffusi on coefficients at 20-degrees-C (logD20/r2 values) of -19.7 +/- 0.7 an d -22.2 +/- 0.6 [sec-1] reveal a higher He-4 retentivity at earth's su rface temperatures for the outer shell. 4He closure temperatures (afte r DODSON, 1973) are lower for the inner shell hematite by approximatel y 50-degrees-C, probably also indicating earlier opening during heatin g. The age discordance could be the result of a younger hydrothermal e vent which probably also has caused the observed alteration zone. Such an event is potentially related to the emplacement of the nearby Rapp enloch hematite generation (II) 156 Ma ago. Complete rejuvenation of t he inner shell hematite had occurred, whereas the He-4 age of the oute r shell was only slightly lowered due to the higher He-4 retentivity.