HR: 10:35h
AN: V11F-02    [PDF]
TI: A multidisciplinary approach to understanding the origin of peridotite cumulates
AU: * Worrell, L M
EM: lworrell@merebrook.co.uk
AF: Merebrook, Suite 2B, East Mill, Bridgefoot, Belper, DE56 2UA United Kingdom
AU: Cheadle, M J
EM: cheadle@uwyo.edu
AF: Dept. of Geology and Geophysics, University of Wyoming, Laramie, WY 82071-3006 United States
AU: Coogan, L A
EM: lac8@leicester.ac.uk
AF: Dept. of Geology, Leicester University, Leicester, LE1 7RH United Kingdom
AU: Prior, D J
EM: davep@liv.ac.uk
AF: Dept of Earth Sciences, Liverpool University, Liverpool, L69 3BX United Kingdom
AU: Toplis, M J
EM: mtoplis@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, 15 rue Notre Dame des Pauvres, Vandoeuvre-Les-Nancy, BP 20, 545 France
AU: Wheeler, J
EM: johnwh@liverpool.ac.uk
AF: Dept of Earth Sciences, Liverpool University, Liverpool, L69 3BX United Kingdom
AB: The key to understanding the origin of igneous cumulates is to determine the relative importance of the competing processes of cumulate formation. Crystal sedimentation and/or in-situ growth, compaction, porous media convection and textural equilibration may all contribute to the final textural and chemical configuration of the rock. Using an integrated approach encompassing quantitative textural analysis, geochemistry and relatively new electron backscatter diffraction (EBSD) techniques has allowed the nature and extent of postcumulus textural and chemical modifications to be quantified, and illustrates that the final rock texture still carries essential information concerning magma-chamber processes and the early evolution of the cumulate rock. Quantitative textural analysis has highlighted several distinct olivine morphologies within individual peridotite layers from the Rum Intrusion, Scotland, and has demonstrated that in-situ growth alone cannot account for the textural variation within the peridotites because transport of at least one crystal phase is required from elsewhere in the magma chamber. The combined use of quantitative textural analysis to determine shape preferred orientations (SPOs) and EBSD to determine crystallographic preferred orientations (CPOs) has shown that the SPO and CPO for each peridotite layer are essentially in agreement. This, together with the presence of mixed olivine morphologies within a single rock, indicates that significant post-cumulus recrystalization cannot have taken place and furthermore compaction by pressure solution is unlikely to have occurred to any significant extent. Whole-rock geochemical analysis of several of these peridotite layers has highlighted positive Sr- and Eu- anomalies but no textural evidence for cumulus plagioclase, suggesting that interstitial plagioclase grew when melt was able to move through the crystal framework. However, in-situ ion microprobe analyses have shown that interstitial clinopyroxene formed from the final trapped melt fraction. This final melt fraction was heterogeneously distributed throughout the rock in a manner ultimately governed by the packing arrangement of the crystals which is itself controlled by primary magma chamber processes such as crystal transport.
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting