Paragenetic specularite and adularia (Elba, Italy): Concordant (U + Th)-He and K-Ar ages

Abstract In order to test the usefulness of the helium method for detecting young geological events in hematite, U, Th and 4 He concentrations were analyzed in five specularite samples from Rio Marina-type hematite deposits (northeastern Elba, Italy). Four specularite samples define a(U + 0.235Th)/ 4 He isochron age of 5.39 ± 0.46 Ma (2σ). The isochron intercepts the 4 He axis at 49 ± 88 pl/g (2σ). Adularia, which is paragenetic with specularite in one sample used for the isochron, gives a meanK/Ar age of 5.32 ± 0.11 Ma. In addition, two specularite 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 respectively. Depending on the model of genesis, the high ages reflect incomplete 4 He degassing and/or 4 He excess . The study demonstrates that geologically meaningful helium ages as young as Pliocene can be obtained from specularite in spite of expected interferences from initial helium. If the 4 He axis intercept is a general measure of initial 4 He in hematite its effect on the calculated ages in Mesozoic hematites in previous studies is negligible. The age concordance implies that similar concordance should exist in samples with a known thermal history much older than Pliocene. Moreover, the helium method could also work on specularite with a thermal history younger than Pliocene.