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This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Evans, B. W. Articles by Chopelas, A.

Magnetite-free, yellow lizardite serpentinization of olivine websterite, Canyon Mountain complex, N.E. Oregon

¹ Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98195-1310, U.S.A.
² Institute of Geophysics and Planetary Physics, University of California Los Angeles, Box 951567, Los Angeles, California 90095-1567, U.S.A.

Correspondence: ^* E-mail: bwevans{at}u.washington.edu

We document an example of serpentinization of olivine and orthopyroxene that produced virtually no magnetite, but instead relatively Fe-rich yellow-colored lizardite (X_Fe = 0.08 to 0.17), and the native Fe-Ni-Co metals, awaruite and wairauite. Lizardite’s identity was confirmed by micro-Raman spectroscopy, although peaks are broad. Electron microprobe analyses of the lizardite yield a continuous compositional trend of formula contents suggestive of the progressive uptake of Fe³⁺ exclusively on M sites, where it is charge balanced by vacancies. Although these observations are unusual, this secondary mineral assemblage can be explained in terms of the likely intensive variables T, f_H2O, f_H2, and a_SiO2 attending the alteration. The absence of magnetite in serpentinization does not signify a lack of oxidation. By forming the hydrated phase-component ferri-lizardite instead of magnetite from the fayalite and ferrosilite components, the yield of hydrogen is reduced by two-thirds. The usual inverse correlation of rock density with magnetic susceptibility is unlikely to be the case in this kind of serpentinization.

Key Words: Serpentinite • ferrian lizardite • olivine-websterite • micro-Raman • hydrogen • magnetic susceptibility