Insights into the petrogenesis of the West Kimberley lamproites from trace elements in olivine

Date & time

12.30–1.30pm 8 September 2017

Location

Ringwood Room, J4

Speakers

Lynton Jaques

Contacts

 Michael Anenburg
 0451 406 232

The minor and trace element abundances in olivine can provide important information on the origin and evolution of a range of mantle-derived magmas. Mantle olivine may record a residue of partial melting, and the impact of metasomatism over time. Magmatic olivines can record the crystallisation history of the magma.

Olivine occurs as a magmatic phase (phenocrysts and microphenocrysts) and mantle fragments (dunite xenoliths and xenocrysts with solid-state deformation) in the Miocene lamproites of the West Kimberley province of Western Australia. Olivine is present over a wide compositional range from the diamondiferous Ellendale olivine lamproites with up to 30 wt % MgO through to leucite-rich lamproites with ~7 wt % MgO.

Combined EPMA and LA-ICP-MS analysis is used to distinguish mantle from magmatic olivine and to define magmatic trends and the origin of the mantle olivines. The magmatic olivines crystallised over a wide temperature range (~1240−850oC) and define trends of decreasing Ni and Cr, and increasing Mn and Ca with decreasing Mg/(Mg+Fe).

High Al contents indicate that the mantle olivines are mostly derived from garnet peridotite at temperatures ~1270−900oC based on the Al-in-olivine thermometer, with the higher temperatures similar to those of inclusions in Ellendale diamonds. Projection of the temperatures onto the local geotherm indicates that the lamproites sampled olivine from the diamondiferous cratonic root of the lithosphere at ~190 km to ~130 km in the spinel peridotite field. Olivines from shallower lithosphere are more depleted (higher Mg, lower Al, Ca, Ti, Fe etc) and there is a systematic increase in Fe, Ca, Ti etc at depth. This style of enrichment is typical of metasomatised cratonic lithosphere.

The West Kimberley lamproite olivines have low Li contents, again typical of cratonic lithosphere, and unlike the high Li contents of orogenic lamproites. This feature needs to be reconciled with the high Ba, low K/Rb and  K/Ba, and high Th/U contents of the West Kimberley lamproites which have been inferred to involve an ancient subducted sedimentary component. High 87sr/86Sr, unradiogenic Nd, and high 207Pb/204Pb but low 206Pb/204Pb indicate the lamproites are derived by very small degrees of partial melting of ancient (> 2 Ga) formerly depleted mantle that underwent multistage enrichment.

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