Geochemical evidence for formation of the Bay of Islands ophiolite above a subduction zone

K isotopes as a tracer of seafloor hydrothermal alteration

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 ¹⁸O/¹⁶O and ⁸⁷Sr/⁸⁶Sr, 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 ⁴¹K/³⁹K 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 ⁴¹K/³⁹K variations in affected rocks. Here, we report high-precision ⁴¹K/³⁹K measurements for samples from the Bay of Islands ophiolite, and we document large variations in ⁴¹K/³⁹K, covarying with previous determinations of ⁸⁷Sr/⁸⁶Sr. Our data indicate that analytically resolvable ⁴¹K/³⁹K effects arise in oceanic crust as a result of hydrothermal alteration. This finding raises the possibility that ⁴¹K/³⁹K 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.

Platinum-group element abundance patterns in different mantle environments

Mantle-derived xenoliths from the Cameroon Line and northern Tanzania display differences in their platinum-group element (PGE) abundance patterns. The Cameroon Line lherzolites have uniform PGE patterns indicating a homogeneous upper mantle over several hundreds of kilometers, with approximately chondritic PGE ratios. The PGE patterns of the Tanzanian peridotites are similar to the PGE systematics of ultramafic rocks from ophiolites. The differences can be explained if the northern Tanzanian lithosphere developed in a fluid-rich suprasubduction zone environment, whereas the Cameroon Line lithosphere only experienced melt extraction from anhydrous peridotites.