Zoning in olivine xenocryst in hydrous systems
Résumé
Chemical zoning in xenocrysts may be inherited, the result of magmatic growth, or diffusive ion exchange with the host magma. To quantify magma residence times of xenocrysts based on diffusion zoning patterns thus requires knowledge of the contribution of diffusion to the zoning patterns and knowledge of appropriate diffusion coefficients. Using olivine xenocrysts (Fo 75-79) from melting experiments run for ca. 1 day, at 1000-1150 °C, 200 MPa, fO2 near NNO, and 1-5 wt% H2O, we characterize length-scales of growth versus Fe-Mg diffusion zoning, and suggest that hydrous diffusion coefficients for olivine need to be constrained to reliably model xenocryst residence times in magmatic systems. In the experiments, initially angular, unzoned olivine xenocrysts develop subhedral to euhedral crystal outlines, and continuous Fe-Mg core-rim zoning with rim thicknesses of ≤40 μm. The euhedral crystal outlines and thick rim zones indicate that the zoning patterns partly result from growth. However, abundant healed micro-fractures, originally present in the xenocrysts, extend into the compositional rim zones, and define minimum length scales for zoning as the result of diffusion. The maximum length scale of the diffusion zoning profiles are 9.5 and 17.5 μm in 1000 and 1150 °C experiments, and we suggest that these profiles represent diffusion close to [001]. Estimating the diffusion coefficient D as ~l2/t (l = diffusion length scale; t = time) gives log(D) of ca. -14.8 to -14.2 m2/s-1 at 1000 and 1150 °C, apparently independent of the amount of H2O present. Using the same approximation to calculate D for olivine xenocrysts in an equivalent dry system (using data from [1]), indicate that diffusion in olivine xenocrysts in lowpressure hydrous systems is over ten times faster than in equivalent anhydrous systems. To reliably quantify magma residence times of xenocrysts based on diffusion zoning patterns thus requires (i) that hydrous diffusion coefficient are rigorously constrained and applied, unless it is evident that the xenocrysts were immersed in dry magmas; and (ii) that growth contributions to xenocryst zoning patterns are constrained (e.g., using markers as above, or comparing zoning patterns for multiple elements). [1] Costa & Dungan (2005) Geology 33, 837-840.