Chemical zonation in olivine-hosted melt inclusions

• Published in: Contributions to mineralogy and petrology 2014-07, Vol.168 (1), p.1, Article 1030
• Main Authors: Newcombe, M. E., Fabbrizio, A., Zhang, Youxue, Ma, C., Le Voyer, M., Guan, Y., Eiler, J. M., Saal, A. E., Stolper, E. M.
• Format: Article
• Language: English
• Subjects: Analysis; Boundary layers; Chemicals; Crystallization; Earth and Environmental Science; Earth Sciences; Geology; Iron oxides; Melting; Mineral Resources; Mineralogy; Original Paper; Petrography; Petrology; Sciences of the Universe; Titanium dioxide; Zonation; Zoning
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-014-1030-6

Significant zonation in major, minor, trace, and volatile elements has been documented in naturally glassy olivine-hosted melt inclusions from the Siqueiros Fracture Zone and the Galapagos Islands. Components with a higher concentration in the host olivine than in the melt (e.g., MgO, FeO, Cr 2 O 3 , and MnO) are depleted at the edges of the zoned melt inclusions relative to their centers, whereas except for CaO, H 2 O, and F, components with a lower concentration in the host olivine than in the melt (e.g., Al 2 O 3 , SiO 2 , Na 2 O, K 2 O, TiO 2 , S, and Cl) are enriched near the melt inclusion edges. This zonation is due to formation of an olivine-depleted boundary layer in the adjacent melt in response to cooling and crystallization of olivine on the walls of the melt inclusions, concurrent with diffusive propagation of the boundary layer toward the inclusion center. Concentration profiles of some components in the melt inclusions exhibit multicomponent diffusion effects such as uphill diffusion (CaO, FeO) or slowing of the diffusion of typically rapidly diffusing components (Na 2 O, K 2 O) by coupling to slow diffusing components such as SiO 2 and Al 2 O 3 . Concentrations of H 2 O and F decrease toward the edges of some of the Siqueiros melt inclusions, suggesting either that these components have been lost from the inclusions into the host olivine late in their cooling histories and/or that these components are exhibiting multicomponent diffusion effects. A model has been developed of the time-dependent evolution of MgO concentration profiles in melt inclusions due to simultaneous depletion of MgO at the inclusion walls due to olivine growth and diffusion of MgO in the melt inclusions in response to this depletion. Observed concentration profiles were fit to this model to constrain their thermal histories. Cooling rates determined by a single-stage linear cooling model are 150–13,000 °C h −1 from the liquidus down to ~1,000 °C, consistent with previously determined cooling rates for basaltic glasses; compositional trends with melt inclusion size observed in the Siqueiros melt inclusions are described well by this simple single-stage linear cooling model. Despite the overall success of the modeling of MgO concentration profiles using a single-stage cooling history, MgO concentration profiles in some melt inclusions are better fit by a two-stage cooling history with a slower-cooling first stage followed by a faster-cooling second stage; the inferred to