Chemistry of grain boundaries in mantle rocks

Abstract The compositions of olivine grain boundaries have been analyzed with scanning transmission electron microscopy (STEM) via energy dispersive X-ray (EDX) spectrum profiling in three specimens: a peridotite ultramylonite, olivine phenocrysts in a basaltic rock, and synthesized compacts of olivine + diopside. Composition profiles across grain boundaries in both natural and synthetic samples exhibit a characteristic width of 5 nm and a depletion of Mg and concomitant enrichments of Ca, Al, Ti, and Cr. Chemical segregation is known to affect grain boundary processes such as grain boundary diffusion, sliding, fracture, and migration, all of which influence the rheological properties of polycrystalline aggregates. Also, because grain boundaries are enriched in trace elements, the boundaries can be important storage sites for such elements in mantle rocks. Mantlederived melts with unusual compositions, such as those rich in Ca and/or Ti, might be explained by preferential melting of olivine grain boundaries enriched in these elements. The common chemical signatures at grain boundaries in all samples indicate that chemical segregation is an energetically favorable phenomenon and thus should occur elsewhere in Earth’s mantle. Segregation of trace elements to grain boundaries may play an important role in dynamical and geochemical processes in Earth’s mantle.

[1]  D. Kohlstedt,et al.  Structure and chemistry of grain boundaries in deformed, olivine + basalt and partially molten lherzolite aggregates: evidence of melt-free grain boundaries , 2002 .

[2]  I. Jackson,et al.  High-temperature viscoelasticity of fine-grained polycrystalline olivine , 2001 .

[3]  R. de Kloe,et al.  Evidence for stable grain boundary melt films in experimentally deformed olivine-orthopyroxene rocks , 2000 .

[4]  J. Cottin,et al.  Armalcolite-bearing, Ti-rich metasomatic assemblages in harzburgitic xenoliths from the Kerguelen Islands: implications for the oceanic mantle budget of high-field strength elements , 2000 .

[5]  R. Cooper,et al.  Low-frequency shear attenuation in polycrystalline olivine: Grain boundary diffusion and the physical significance of the Andrade model for viscoelastic rheology , 1998 .

[6]  S. Karato,et al.  Mechanisms of shear localization in the continental lithosphere: inference from the deformation microstructures of peridotites from the Ivrea zone, northwestern Italy , 1998 .

[7]  G. Duscher,et al.  Atomic structure of a Ca-doped [001] tilt grain boundary in MgO , 1998 .

[8]  R. Wirth.  Thin amorphous films (1–2 nm) at olivine grain boundaries in mantle xenoliths from San Carlos, Arizona , 1996 .

[9]  J. Gerald,et al.  Grain boundary melt films in an experimentally deformed olivine‐orthopyroxene rock: Implications for melt distribution in upper mantle rocks , 1996 .

[10]  S. Karato,et al.  Ultramafic pseudotachylite from the Balmuccia peridotite, Ivrea-Verbano zone, northern Italy , 1995 .

[11]  Y. Chiang,et al.  Space Charge Segregation at Grain Boundaries in Titanium Dioxide: II, Model Experiments , 1993 .

[12]  R. Cooper.  Differential stress‐induced melt migration: An experimental approach , 1990 .

[13]  A. T. Anderson,et al.  Synneusis of Kilauea Iki olivines , 1989 .

[14]  Kazuhiro Suzuki.  Grain-boundary enrichment of incompatible elements in some mantle peridotites , 1987 .

[15]  F. Watt,et al.  Direct determination of strontium enrichment on grain boundaries in a garnet lherzolite xenolith by proton microprobe analysis , 1984, Nature.

[16]  R. Thompson,et al.  An assessment of the relative roles of crust and mantle in magma genesis: an elemental approach , 1984, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[17]  A. T. Anderson,et al.  Concentrations, sources, and losses of H2O, CO2, and S in Kilauean basalt , 1983 .

[18]  J. Shervais.  Thermal Emplacement Model for the Alpine Lherzolite Massif at Balmuccia, Italy , 1979 .

[19]  W. Kingery.  Plausible Concepts Necessary and Sufficient for Interpretation of Ceramic Grain‐Boundary Phenomena: II, Solute Segregation, Grain‐Boundary Diffusion, and General Discussion* , 1974 .

[20]  T. Vasilos,et al.  Origin of Grain-Boundary Diffusion in MgO , 1966 .