Mathematics – Logic
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58..421w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 1, pp.421-429
Mathematics
Logic
6
Scientific paper
Microprobe analyses of a double shell botryoidal hematite from the Fahlenbach vein system, (Schwarzwald, Germany) show an Fe 3+ depletion of 3-4% with respect to Al 3+ and silicon within a ~0.5 mm wide zone of alteration which separates both shells, and probably is the result of a subsequent hydrothermal event. Induced 235 U fission tracks of grain mounts from both shells reveal several grain populations within a range of ~3 to ~22 ppm total uranium. Mass spectrometer- (U) and photometer analyses (Th) of various localities probed within each shell vary significantly at about 5 and 12 ppm U, as well as at 11 and 19 ppm Th. The Th / U ratios vary from ~ 1.5 to ~2. 4 He concentrations correlate with [U+(0.23)Th] heterogeneities which permits construction of two shell-internal isochrons. The 4 He ages are discordant (inner shell, although mineralogically older: 150 Ma; outer shell: 182 Ma). 4 He release by stepwise heating of one sample from each shell yielded activation energies of 124 ± 8 and 142 ± 7 [kJ/mol]. The extrapolated 4 He diffusion coefficients at 20°C ( logD 20 / r 2 values) of -19.7 ± 0.7 and -22.2 ± 0.6 [sec -1 ] reveal a higher 4 He retentivity at earth's surface temperatures for the outer shell. 4 He closure temperatures (after , 1973) are lower for the inner shell hematite by ~50°C, probably also indicating earlier opening during heating. The age discordance could be the result of a younger hydrothermal event which probably also has caused the observed alteration zone. Such an event is potentially related to the emplacement of the nearby Rappenloch hematite generation (II) 156 Ma ago. Complete rejuvenation of the inner shell hematite had occurred, whereas the 4 He age of the outer shell was only slightly lowered due to the higher 4 He retentivity.
Lippolt Hans J.
Wernicke Rolf S.
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