HR: 16:00h
AN: V42H-01 INVITED     [PDF]
TI: The Morphology and Composition of Groundmass Spinel in Kimberlite
AU: * Roeder, P L
EM: roeder@geol.queensu.ca
AF: Dept. of Geological Sciences, Queen's University, Kingston, Ont K7L 3N6 Canada
AB: Chromite and chromian spinel are a common, but very minor ($<$1%), early phase found in the groundmass of both basalt and kimberlite. The spinel is often zoned from a chromite core to a magnetite rim depending on the original melt composition, cooling rate and the nucleation of surrounding minerals. The primary silicate minerals in many kimberlites are often destroyed by late-stage alteration leaving spinel as one of the few minerals that retains morphological and chemical evidence of the progression from an early magmatic stage to a late-stage subsolidus alteration. The composition of basaltic and kimberlitic spinel can be compared by plotting FE2\#(Fe2/(Fe2+Mg)) versus FE3\#(Fe3/(Fe3+Al+Cr)). The primary chromite in both basalt and kimberlite usually have FE3\# $<$0.15 and FE2\# = 0.2-0.6 whereas the late stage magnetite for both rock types is high in FE3\# and FE2\#. Most spinel in basalts show a consistent trend of increasing FE2\# with increasing FE3\# whereas kimberlitic spinel can show a variety of different trends of increasing FE3\# at a constant, or even decreasing, FE2\#. Evidence will be presented that suggests that trends of FE3\# vs. FE2\# of spinel in various kimberlites may reflect variations in cooling rate of the kimberlite. It is useful to consider the morphology and compositional variation of spinel in kimberlite in terms of four stages: 1) High Cr2O3, TiO2$<$1 wt.%, FE2\# $<$ 0.5, FE3\# $<$0.1. 2) High Cr2O3, TiO2 1-3 wt.%, FE2\# $<$0.5, FE3\# $<$0.1. 3) Cr2O3 1-50wt.%, TiO2 3-20 wt.%, FE2\# 0.4-0.9, FE3\# 0.1-0.9. 4) Cr2O3 $<$1 wt.%, TiO2 $<$2 wt.%, FE2\# $>$0.8, FE3\#$>$0.9 . The chromite of stage 1 reflects the bulk melt composition well before intrusion of the kimberlite whereas stage 2 is thought to reflect the relatively rapid and local change in melt composition during intrusion . Stage 3 reflects a very large change in composition due to very local crystallization of the groundmass minerals and diffusion-controlled crystallization that gives rise to atoll spinels. Stage 4 reflects late subsolidus crystallization of magnetite from alteration and oxidation of silicates. The compositional boundary between stages is arbitrary and rarely are all four stages found in the same kimberlite. The varied morphology of spinel and the zoning of spinel in kimberlites reflect not only changes in the bulk melt compostion but on the relative cooling rates of kimberlites. The complex growth forms of chromite found in some MORB glasses and atoll chromites found in some kimberlites reflects relatively rapid growth rates from a melt. Thus the composition and morphology of spinel may be useful in estimating relative cooling rates and may even help us to better understand the survivability of diamonds during the cooling of kimberlites.
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting