Testing Orthopyroxene Diffusion Chronometry on Rocks From the Lanzo Massif (Italian Alps)

Abstract

AbstractRocks from the northern and central zones of the Lanzo peridotite massif contain large (>3 mm) orthopyroxene crystals, often within a fine‐grained matrix. The rocks have been decompressed and cooled from the spinel peridotite field, via plagioclase peridotite to near the ocean floor. Orthopyroxene preserves evidence of the early part of this cooling history in the form of Cr and Al (plus various trace elements) loss profiles. Independent geological constraints can be placed on the cooling rates by considerations of absolute age (slowest possible cooling rate) and thermal modeling (fastest possible cooling rate), allowing a test of cooling rates retrieved from orthopyroxene diffusion chronometry. Quantitative profiles (mostly along the crystallographic c axis) and qualitative maps were measured for a series of orthopyroxene crystals, and the cooling rate extracted by diffusion modeling. It is possible to successfully extract reliable cooling rates from frozen Cr diffusion profiles in orthopyroxene, suggesting that the available experimental data are applicable to natural orthopyroxene. However, there are caveats related to the diffusion mechanism, boundary conditions when passing through the spinel‐plagioclase peridotite transition, and the potential effect of ubiquitous clinopyroxene exsolution lamellae on diffusion pathways and element partitioning. The usual issues of extracting 2‐D or 1‐D data from a 3‐D system apply, in addition to issues relating to shearing of orthopyroxene grains leading to moving boundaries. Al diffusion in orthopyroxene may be a more useful tool than Cr for assessing cooling rates, but its diffusivity is currently unconstrained.