jade gouge from Emirau Island, Papua New Guinea (Early Lapita context, 3300 BP): a unique jadeitite

摘要

A small stone artifact fashioned as a wood-carving gouge, discovered during an archaeological excavation on Emirau Island, Bismarck Archipelago, Papua New Guinea, has been determined to consist of jadeitite - jadeite jade. It is very unusual with respect to both pyroxene composition and minor mineral constituents. Pyroxene compositions lie essentially along the jadeite-aegirine join: Jd_(94)Ae_6 to Jd_(63)Ae_(36), and without any coexisting omphacite. This contrasts with Jd-Di or Jd-Aug compositional trends commonly observed in jadeitites worldwide. Paragonite and albite occur in veins and cavities with minor titanite, epidote-allanite, and zircon, an assemblage seen in a few jadeitites. Surprisingly, some titanite contains up to 6 wt% Nb_2O_5 with only trace Ta and a single grain of a Y-Nb phase (interpreted as fergusonite) is present; these are unique for jadeitite. A possible source is along the Torare River in northeast Papua, Indonesia, from which samples of "chloromelanite" (actually jadeitite) were collected by C.E.A. Wichmann in 1903 near Humboldt Bay along with stone adzes fashioned from the same material.