HR: 0800h
AN: V41C-1467    [Abstracts]
TI: Garnet-melt partitioning at 10 GPa in the CMAS-CO2 system: a link between CO2-rich melts and majoritic-garnets in diamonds from the mantle?
AU: * Keshav, S
EM: s.keshav@gl.ciw.edu
AF: Geophys Lab, CIW, Washington DC, 20015 United States
AU: Gudfinnsson, G H
EM: g.gudfinnsson@gl.ciw.edu
AF: Geophys Lab, CIW, Washington DC, 20015 United States
AU: Presnall, D C
EM: presnall@gl.ciw.edu
AF: Geophys Lab, CIW, Washington DC, 20015 United States
AU: Minarik, W G
EM: minarik@eps.mcgill.ca
AF: EPS, McGill Univ, Montreal, H3A 2A7 Canada
AU: Fei, Y
EM: fei@gl.ciw.edu
AF: Geophys Lab, CIW, Washington DC, 20015 United States
AB: On the basis of mantle xenoliths and silicate inclusions in diamonds brought up by kimberlites, the mantle origin of kimberlite is beyond doubt. The mantle xenoliths and inclusions in diamonds have vastly improved constraints on the petrogenetic processes operating in the silicate portion of the Earth. Especially important among this group of mantle xenoliths is the rare suite of majoritic garnets trapped as inclusions in diamonds that have been interpreted as deeper (greater than 200 km) than usual samples from the mantle. While opinions vary on the ultimate origin of these majoritic garnets, on the basis of trace elements (for example, negative Eu anomalies indicating crystallization of feldspar and thus originally a much shallower origin) and light-element (carbon) isotope geochemistry, a popular view links them to subducted oceanic crust. In this contribution, we present experimentally determined partition coefficients of trace elements for majoritic garnets equilibrated with a kimberlitic melt at 10 GPa and 1800 C, in the CMAS-CO2 system, and test if there is a link between these inclusions and CO2-rich melts in the mantle. The experiments were performed in a MA-6/8 module using 14/8 assemblies with stepped Cr-doped MgO cells, MgO inner parts, Re-furnace, Type-C TC, and ZrO2 insulator. Starting mix was spiked with a suite of trace elements as AAS standard solutions and was contained in a sealed Pt capsule. Concentrations of major and trace elements were determined using EPMA and LA-ICPMS techniques, respectively. On the basis of calculated partition coefficients (D), almost all the trace elements, barring Lu (D greater than 1), are highly-to-moderately incompatible (D moderately-to-greatly less than 1). This behavior is perhaps a response to garnets becoming majoritic with increasing pressure. The data obtained here have been used to invert the trace element composition of melts that may have been in equilibrium with the majoritic garnets found as inclusions in diamonds. The majoritic character of garnets in the experiments allows direct testing of the model mentioned above. The modeling results suggest that, with a few exceptions, all the calculated melts resemble the sampled carbonatites, kimberlites, and alkalic liquids, suggesting that majoritic garnets in diamonds were last in equilibrium with such melts, and also indicating that there is perhaps a direct link between majoritic garnets in diamonds and the calculated melts. These results indicate that on the basis of Eu anomaly alone, it may be difficult to link majoritic garnets found as inclusions in diamonds to former oceanic crust. Although we do not have partitioning data on Eu in the present set of experiments, an alternative explanation to a crustal origin for the Eu anomaly might relate to some intrinsic feature of majoritic garnet crystal chemistry. It is also suggested that melts calculated to be in equilibrium with majoritic garnets in diamonds, even though not sampled on the surface of the Earth, may also have been the source of diamonds.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1065 Major and trace element geochemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005