资源环境科技发展态势分析平台

At ocean spreading ridges, circulation of seawater through rock at elevated temperatures alters the chemical and isotopic composition of oceanic crust. Samples obtained from drilling into ocean floor and from ophiolites have demonstrated that certain isotope systems, such as O-18/O-16 and Sr-87/Sr-86, are systematically modified in hydrothermally altered oceanic crust. Although K is known to be mobile during hydrothermal alteration, there have not yet been any K-isotope analyses of altered oceanic crustal materials. Moreover, the K-41/K-39 of seawater was recently found to be significantly higher than that of igneous rocks, so the addition of seawater K to oceanic crust would be expected to generate K-41/K-39 variations in affected rocks. Here, we report high-precision K-41/K-39 measurements for samples from the Bay of Islands ophiolite, and we document large variations in K-41/K-39, covarying with previous determinations of Sr-87/Sr-86. Our data indicate that analytically resolvable K-41/K-39 effects arise in oceanic crust as a result of hydrothermal alteration. This finding raises the possibility that K-41/K-39 can be used as an effective tracer of oceanic crust recycled into the mantle, as a diagnostic criterion by which to identify ancient fragments of oceanic crust, and as a constraint on the flux of K between oceanic crust and seawater.