PARAGENETIC SPECULARITE AND ADULARIA (ELBA, ITALY) - CONCORDANT (U-HEAND K-AR AGES(TH))

In order to test the usefulness of the helium method for detecting you ng geological events in hematite, U, Th and He-4 concentrations were a nalyzed in five specularite samples from Rio Marina-type hematite depo sits (northeastern Elba, Italy). Four specularite samples define a (U + 0.235Th)/He-4 isochron age of 5.39 +/- 0.46 Ma (2 sigma). The isochr on intercepts the He-4 axis at 49 +/- 88 pl/g (2 sigma). Adularia, whi ch is paragenetic with specularite in one sample used for the isochron , gives a mean K/Ar age of 5.32 +/- 0.11 Ma. In addition, two specular ite samples, one from Rio Marina and one from Terra Nera (southeastern Elba) give helium model ages of 6.4 +/- 0.4 and 7.3 +/- 0.4 Ma respec tively. Depending on the model of genesis, the high ages reflect incom plete He-4 degassing and/or He-4(excess). The study demonstrates that geologically meaningful helium ages as young as Pliocene can be obtain ed from specularite in spite of expected interferences from initial he lium. If the He-4 axis intercept is a general measure of initial He-4 in hematite its effect on the calculated ages in Mesozoic hematites in previous studies is negligible. The age concordance implies that simi lar concordance should exist in samples with a known thermal history m uch older than Pliocene. Moreover, the helium method could also work o n specularite with a thermal history younger than Pliocene.