Matheson
Geordie Lake
An Origin for Harrisitic and Granular Olivine in the Rum Layered Suite, NW Scotland: a Crystal Size Distribution Study

Brian O'Driscoll, Colin H. Donaldson, Valentin R. Troll, Dougal A. Jerram, C. Henry Emeleus

DOI:http://dx.doi.org/10.1093/petrology/egl059

Source: OAI



ABSTRACT

Dendritic crystal morphologies occur in a number of igneous rocks and are thought to originate from the rapid growth of crystals, yet many examples of dendritic morphologies are found in plutonic igneous rocks where cooling rates should be low. Results from crystal size distribution (CSD) measurements on harrisitic olivines from Rum, Scotland, combined with estimated olivine growth rates, suggest that the characteristic skeletal hopper and branching olivines of harrisitic cumulates that are up to centimetres long, may have exceptionally short crystal growth times (several hours to several hundreds of days). This, together with very low calculated nucleation densities for harrisitic olivine, supports the interpretation of harrisite being a 🐽disequilibrium texture, developed in response to supersaturation of the magma in olivine. 🐽We propose that this supersaturation arose through undercooling of thin picrite sheets emplaced along the Rum magma chamber floor, beneath cooler resident magma. It is envisaged that the picrite sheets were largely free of suspended olivine crystals. Coupled with the olivine-enriched composition of the melt and the increasing cooling rate, this allowed homogeneous nucleation of olivine to set in at 🐙deeper undercooling and greater olivine supersaturation than if there had been plentiful suspended olivines to act as heterogeneous nuclei. The enhanced supersaturation caused rapid growth of olivine once nucleation began, with skeletal and dendritic shapes. It is suggested that the observed, 🐠interlayered sequences of harrisite and cumulus peridotite found throughout the Rum Layered Suite are a result of multiple episodes of harrisite crystallization resulting from picrite emplacement that alternated with periods of crystal growth and accumulation in the main body of magma at lesser degrees of undercooling.

http://m.gsabulletin.gsapubs.org/content/85/11/1721
Skeletal and dendritic olivine crystals in the Archean volcanic ultramafic spinifex rocks and in harrisitic ultramafic layers of the Rhum pluton are classified as (1) plate, (2) randomly oriented, (3) porphyritic, or (4) branching type. Volcanic and plutonic examples are remarkably similar. Each type records the degree of pre-nucleation supersaturation with olivine that was attained by the parent melt. The rapid induction of supersaturation necessary to form skeletons and dendrites in a plutonic environment is attributed to changing water content or adiabatic expansion of the magma. The olivine crystals in harrisitic and spinifex rocks are not quench crystals; they grew rapidly from olivine-rich melts as the result of extreme supersaturation induced by slow cooling and slight supercooling below a liquidus with shallow slope in temperature-composition space. Skeletal and dense

W horizon
Primary in apatite
Assemblage 1 Vapour liquid


Main zone
Basal  zone

    
   Melt inclusions
   Vapour inclusions
    



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