HR: 11:00h
AN: T41F-03    [PDF]
TI: The Composition of Olivine Grain Boundaries
AU: Fitz Gerald, J
EM: john.fitzgerald@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: * Faul, U
EM: uli.faul@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Cmiral, M
EM: cmiral@iol.cz
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AB: As an integral part of the characterisation of microstructures of experimentally produced olivine-rich samples, we have systematically studied grain boundaries in a conventional 300KV TEM by imaging and EDS analysis. Grain boundaries examined have misorientations $> 10\deg$ and show none of the arrays of dislocations present in either subgrain walls or grain boundaries in metals. In the TEM each grain boundary is oriented exactly parallel to the electron beam to produce the sharpest image of the grain boundary, and then TEM EDS analysis is performed with a beam ca. 20nm wide and 200nm long parallel to the grain boundary. Each grain boundary EDS spectrum is then compared to a spectrum acquired under the same conditions in the adjoining olivine grain. A large range of samples have been examined, including olivine aggregates derived from natural rocks and fully synthetically produced (trace element free, sol-gel origin). The samples were quenched from temperatures between 1100 and 1400$\deg$C after being run either under hydrostatic conditions from 0.3 to 3 GPa or after deformation in a Paterson gas-medium apparatus. The apparent width of the grain boundary region in high resolution TEM images is not detectably different for any of these samples, but significant EDX spectral changes show differences in composition of the grain boundaries that depend on bulk composition and sample treatment. For example, olivine in fully synthetically produced samples is trace element free, and correspondingly no trace elements are detectable in grain boundaries by TEM EDS. When a basaltic melt is added, EPMA WDS analyses show that the olivine grain interiors equilibrate with the melt by incorporating a small amount of Ca, Ti and Al as trace elements. Using TEM EDS we observe that the grain boundaries in these samples are now enriched in these elements relative to olivine grain interiors. Levels of trace elements in grain boundaries also appear to be affected by strain history: grain boundaries in newly recrystallised regions (deformed aggregate of San Carlos olivine + basaltic melt) have very low levels of trace elements immediately after deformation, yet after a long hydrostatic anneal (1300$\deg$C, $>$ 500 hrs) have re-equilibrated and contain significantly higher levels of trace elements. We conclude that the trace element enrichment in olivine grain boundaries relative to grain interiors reflects equilibrium partitioning between grain interiors, grain boundaries and other phases such as melt. This leads us to expect that olivine grain boundaries in natural rocks will also be enriched in trace elements depending on bulk chemistry.
DE: 1212 Earth's interior--composition and state (8105)
DE: 3630 Experimental mineralogy and petrology
DE: 3947 Surfaces and interfaces
SC: Tectonophysics [T]
MN: 2003 Fall Meeting