Revisiting Pb isotope signatures of Ni–Fe alloy hosted by antigorite serpentinite from the Josephine Ophiolite, USA

Abstract

Awaruite (Ni_2–3Fe) is a natural occurring Ni–Fe alloy in serpentinite, which represents a better candidate to assess Pb isotope signatures in the mantle wedge since the concentration of Pb in awaruite is almost ten times higher than that in serpentine minerals. Revisiting so–called josephinite from the Josephine Ophiolite confirmed that josephinite is characterized by aggregates of awaruite with minor Ni–arsenide. The Raman spectrum obtained from the josephinite–hosting serpentinite shows diagnostic peaks of antigorite, suggesting josephinite might have formed under stability field of antigorite. Using a stepwise leaching and partial dissolution method, we obtained Pb isotope ratios of josephinite by TIMS. Since all ratios converged to a homogeneous value towards the later steps of the partial dissolution, this allowed to calculate weighted mean values that give precise Pb isotope ratios: ²⁰⁶Pb/²⁰⁴Pb = 18.3283 ± 0.0020 (MSWD = 0.49), ²⁰⁷Pb/²⁰⁴Pb = 15.5645 ± 0.0020 (MSWD = 0.36), and ²⁰⁸Pb/²⁰⁴Pb = 38.0723 ± 0.0061 (MSWD = 0.50); these values can be evaluated as one of the reference Pb isotope ratios in serpentinites from supra–subduction zone ophiolite. The newly obtained Pb isotope ratios of josephinite are consistent with the previous reported isotope ratios, which are characterized by enriched ²⁰⁷Pb/²⁰⁴Pb ratio with MORB–source like ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios. Although these Pb isotope features interpreted as a reflection of arc magmatism in the previous study, the presence of Ni–arsenide and enriched ²⁰⁷Pb/²⁰⁴Pb ratios may indicate an involvement of As–rich fluids derived from slab sediments.