HR: 15:00h
AN: V53D-06 INVITED    [Abstracts]
TI: The Petrogenesis of the Unit 7/8 and 11/12 Chrome-spinel Seams of the Rum Eastern Layered Intrusion (NW Scotland) Re-evaluated
AU: * O'Driscoll, B
EM: brian.odriscoll@ucd.ie
AF: School of Geological Sciences, University College Dublin, Belfield, Dublin, 4, Ireland
AU: Daly, J S
EM: stephen.daly@ucd.ie
AF: School of Geological Sciences, University College Dublin, Belfield, Dublin, 4, Ireland
AU: Emeleus, C H
EM: c.h.emeleus@durham.ac.uk
AF: Department of Earth Sciences, University of Durham, Science Laboratories, South Road, Durham, DHI 3LE, United Kingdom
AU: Donaldson, C H
EM: chd@st-andrews.ac.uk
AF: School of Geography and Geosciences, University of St. Andrews, St Andrews, Fife, St Andrews, KY16 9AL, United Kingdom
AB: Laterally extensive (~2 mm thick) chrome-spinel seams in the Rum Layered Suite, NW Scotland, occur at the junctions of several of the coupled peridotite-troctolite macro-rhythmic units that make up the bulk of the eastern part of the intrusion. A detailed petrographic study of the rocks immediately above and below two of these seams suggests that existing models for seam formation involving early crystallisation and gravitational settling of chrome-spinel crystals from a newly emplaced body of picritic magma may be flawed. Instead, the textural relationships between minerals suggest that olivine crystallisation in the peridotite above each of the seams occurred before that of most of the chrome-spinel. Reaction textures between olivine and chrome-spinel crystals are commonly observed, with plagioclase usually occurring as thin rims between both olivine and chrome-spinel where both are in close proximity. The textural evidence suggests a significant degree of olivine crystal-shape change; it seems that many of the olivine crystals immediately above the main seams may initially have had much more complex (harrisitic) crystal shapes before modification to simpler morphologies in a crystal mush. Plagioclase occurs in the peridotite as large oikocrysts up to several cm in size. Additionally, the chrome-spinel seams occur only in those units that display extensive evidence of syn-magmatic deformation of unconsolidated cumulate in the underlying troctolite, and the seams themselves often exhibit small-scale load structures. A model suggesting in-situ crystallisation of the chrome-spinel seams is proposed, whereby mixing of an evolved interstitial liquid with a primitive picritic melt occurred approximately at the crystal mush-magma interface. The former was released from the unconsolidated troctolite mush as a response to re-mobilization and chaotic slumping, possibly triggered by emplacement of some of the hot picrite into the crystal mush pile. Significant undercooling in the picrite due to emplacement-related cooling had already produced a crystal framework comprising complex skeletal olivine crystal morphologies with very fast growth rates. It is envisaged that the significantly modified olivine textures in the peridotite immediately above both seams can be attributed to upward- moving porosity waves of the same ‘mixed' interstitial melt that precipitated the chrome-spinel seams. In addition to formation of the seams at the main unit junctions, ‘necklace' or ‘chain-like' distributions of chrome-spinel crystals around olivine crystals in the peridotite, as well as the large plagioclase oikocrysts, argue for the presence of a mobile interstitial melt with a protracted cooling history.
DE: 1036 Magma chamber processes (3618)
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting