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

Abstract

Awaruite (Ni 2-3 Fe) 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: 206 Pb/ 204 Pb = 18.3283 ± 0.0020 (MSWD = 0.49), 207 Pb/ 204 Pb = 15.5645 ± 0.0020 (MSWD = 0.36), and 208 Pb/ 204 Pb = 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 207 Pb/ 204 Pb ratio with MORB-source like 206 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios. Although these Pb isotope features interpreted as a reflection of arc magmatism in the previous study, the presence of Ni-arsenide and enriched 207 Pb/ 204 Pb ratios may indicate an involvement of As-rich fluids derived from slab sediments.