Igneous rock associations 10. Komatiites

Geoscience Canada, March, 2008 by Jaroslav Dostal

SUMMARY

Komatiites are ultramafic volcanic rocks that occur mainly in Archean and Paleoproterozoic greenstone belts. These olivine-rich rocks are assumed to have crystallized from magmas that have about 28-30 wt% MgO. They are characterized by spinifex texture--platy or skeletal crystals of olivine set in a glassy matrix. Chemically, komatiites resemble peridotites and have high MgO but low Si[O.sub.2], Ti[O.sub.2] (<1 wt%), [K.sub.2]0 (<0.5 wt%) and incompatible trace element contents. Most of their compositional variations can be accounted for by olivine fractionation. Komatiites are conventionally considered to be derived from high temperature melts that have eruption temperatures of about 1600[degrees]C and are produced by high degrees of anhydrous melting of mantle plumes. The abundance of Archean komatiites, their decrease through the Proterozoic and extreme rarity in the Phanerozoic have been taken as evidence for secular cooling of the mantle. However, the plume model has recently been challenged. In marked contrast, it has been proposed that komatiites originate via hydrous melting at a shallow depth in subduction environments at significantly lower melting temperatures than those invoked by the plume hypothesis. This new model thus challenges traditional views of the early evolution of the Earth. Nevertheless, it appears that many komatiites are plume-related. In addition to the information they provide about the tectonics and the thermal evolution of Archean Earth, komatiites are economically important because they host locally significant magmatic Ni-sulfide (Ni-Cu-PGE) mineralization.

SOMMAIRE

Les komatiites sont des roches volcaniques ultramafiques que l'on retrouve principalement dans des bandes de roches vertes archeennes et paleoproterozoiques. On suppose que ces roches a fort contenu en olivine ont cristallise a partir de magmas ayant des teneurs de 28 a 30% en poids de MgO. Typiquement, elles presentent une textute spinifex; c'est-a-dire en cristaux lamellaires ou squelettiques d'olivine dans une matrice vitreuse. Chimiquement, les komattites ressemblent aux peridotites et presentent des teneurs elevees en MgO mais basses en SIO2, Ti[O.sub.2] (<1% en poids), [K.sub.2]O (<0.5% en poids) ainsi qu'en elements traces incompatibles. La plupart de leurs variations compositionnelles peuvent s'expliquer par le fractionnement de l'olivine. Par convention, on considere que les komatiites proviennent de magmas de hautes temperatures qui ont des temperatures d'eruption d'environ 1600[degrees]C, et qui proviennent d0un haut degre De fusion de panaches manteliques anhydres. L'abondance des komatiites archeennes, leur moindre abondance au Paleozoique et leur extreme rarete au Phanerozoique ont ete interpretes comme etant l'effet d'un refroidissement seculaire du manteau. Par-centre, recemment, le modele des panaches manteliques a ete remis en question. On a propose qui au contraire les komatiites proviendraient de fusions hydratees de faibles profondeurs en milieux de subduction, et a des temperatures de fusion significativement plus basses que celles supposees par l'hypothese des panaches. Ce nouveau modele remet donc en question la perspective traditionnelle sur les premiers stades de l'evolution de la Terre. Ceci dit, ii semble que nombre de komatiites soit relie a des panaches. Au-dela des informations qu'elles fournissent sur la tectonique et l'evolution thermique de la Terre archeenne, les komatiites sont importantes economiquement, etant les roches hotes de mineralisations magmatiques de sulfures de nickel (Ni-Cu-EGP) en certains endroits.

INTRODUCTION

Komattites are rare ultramafic volcanic and subvolcanic rocks that occur, predominantly, in Archean and Paleoproterozoic greenstone belts. These rocks contain more than 18 wt% MgO (Le Bas 2000) and are distinguished from other magnesium-rich rocks, such as picrites and meimechites, by having spinifex texture (characterized by spectacular arrays of subparallel or randomly-oriented skeletal, platy and bladed olivine crystals set in a glassy groundmass; Fig. 1). Because all komatiites do not display spinifex texture, Arndt and Fowler (2004) defined komatiites as ultramafic rocks that either contain spinifex texture or are related to rocks that do (see Kerr and Arndt 2001). Komatiitic lavas are assumed to crystallize from magmas that have ultramafic compositions.

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Komatiites were first identified by Viljoen and Viljoen (1969) in the Archean greenstone belt in the Barberton Mountainland of South Africa as a distinctive and "new" class of magnesium-rich (20-30 wt% MgO) volcanic rocks. Up until then, genuine ultramafic lavas were not known. Viljoen and Viljoen (1969) recognized that many high-Mg rocks in the Barberton greenstone belt were lava flows of significant lateral extent and thickness, with chilled of brecciated tops, amygdules, pillows and the distinctive quench textures that were subsequently named "spinifex" (after an Australian spiky grass--Triodia spinifex). The Viljoen brothers inferred that the rocks had characteristics of erupted liquids with distinctive chemical composition and named them after the Komati River flowing through the type locality. These 3.5 Ga flows are among the oldest known ultramafic rocks.