Mt Ngauruhoe in the Taupo Volcanic Zone of New Zealand erupted andesite lava flows in 1949 and 1954, and avalanche deposits in 1975. Rb-Sr, Sm-Nd and Pb-Pb radioisotopic analyses of samples of these andesites, as anticipated, did not yield any “age” information, although the Pb isotopic data are strongly linear. When compared with recent andesite flows from the related adjacent Ruapehu volcano, the Sr-Nd-Pb radioisotopic systems plotted on correlation diagrams provide information about the depleted mantle source for the parental basalt magmas and the source of the crustal contamination that produced the andesite lavas from them. The variations in both the depleted mantle Nd “model ages” and the Pb isotopes also suggest radioisotopic heterogeneity in the mantle wedge 80 km below the volcano where partial melting has occurred, contaminated by mixing with trench sediments scraped off the interface with the subducting slab. Thus the radioisotopic ratios in these recent Ngauruhoe andesite flows were inherited, and reflect the origin and history of the mantle and crustal sources from which the magma was generated. By implication, the radioisotopic ratios in ancient lavas throughout the geologic record are likely fundamental to their geochemistry, characteristic of their origin and history rather than necessarily providing valid conventional “ages”.


Andesite, 1949–1975 flows, Mt Ngauruhoe, New Zealand, Rb-Sr, Sm-Nd, Pb-Pb, radioisotopes, petrogenesis, depleted mantle, magma genesis, crustal contamination, subduction, mixing, inherited radioisotopic ratios, invalid conventional “ages”


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