HR: 0800h
AN: V21B-0600    [Abstracts]
TI: Can kimberlites result from melting of ``normal'' mantle?
AU: * Khazan, Y
EM: ykhazan@ucsd.edu
AF: Institute of Geophysics, 32 Palladina Ave., Kiev, CA 03680 Ukraine
AU: * Khazan, Y
EM: ykhazan@ucsd.edu
AF: IGPP/UCSD, IGPP-225 UCSD, La Jolla, CA 92093 United States
AU: Fialko, Y
EM: fialko@radar.ucsd.edu
AF: IGPP/UCSD, IGPP-225 UCSD, La Jolla, CA 92093 United States
AB: The extreme enrichments of kimberlitic magmas in incompatible elements (e.g., compared to a typical undepleted mantle) are usually attributed to low degrees of melting and/or metasomatized source compositions. We present a model explaining the observed enrichment of kimberlitic magmas with rare earth elements (REE) in terms of melt migration through source rocks having the composition of ``normal'', primitive mantle. The model considers chemical evolution of a batch of melt that interacts with the solid matrix along its propagation path. This chromatographic process leads to abundances of trace elements that approach $C_0/D$ asymptotically, where $C_0$ is the concentration of a trace element in the primitive mantle and $D$ is the partition coefficient for this element between primitive mantle and the kimberlitic melt. This final, ``fully saturated'' melt composition is independent of the initial composition of the melt input and of the melt fraction. The fully saturated REE spectra that would result from this process are in excellent agreement with the maximum observed REE enrichments of kimberlites from South Africa, Siberia and India. This result may reconcile relatively high degrees of melting (i.e., $\sim$1 volume %) thought to be required for efficient melt segregation from the matrix with the extreme REE enrichments of kimberlitic magmas. This chromatographic process can also explain the low La/Yb ratios typical of many kimberlitic magmas if the length of the chromatographic column is shorter than that required for a full saturation of the most incompatible elements. The observed low La/Yb ratios may be consistent with porosities in excess of $\sim$1% since lower melting degrees should result in full saturation and high La/Yb. An important feature of our model is that the resulting saturated REE spectrum is practically independent of a source mineral composition, which may explain the observed similarity of the REE spectra of kimberlites from different geographic localities.
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting