Komatiite

Komatiites erupted billions of years ago as pulsating streams of white-hot lava. Their unusual chemical compositions and exceptionally high formation temperatures produced highly fluid lava that crystallized as spectacular layered flows. Investigation of the extreme conditions in which komatiites formed provides important evidence about the thermal and chemical evolution of the planet, and the nature of the Precambrian mantle. This monograph, written by three experts with long experience in the field, presents a complete account of the characteristics of komatiites including their volcanic structures, textures, mineralogy and chemical compositions. Models for their formation and eruption are evaluated (including the anhydrous vs. hydrous magmas controversy). A chapter is also devoted to the valuable nickel and copper ore deposits found in komatiites. Komatiite is a key reference for researchers and advanced students interested in petrology, Archaean geology, economic geology, and broader questions about the evolution of the Earth's crust and mantle. Cover......Page 1 Half-title......Page 3 Title......Page 5 Copyright......Page 6 Dedication......Page 7 Contents......Page 9 Preface......Page 13 Part I Background information - description of the field characteristics, mineralogy and geochemistry of komatiites......Page 17 1.1 The discovery and early investigations of komatiite......Page 19 1.2 More recent studies......Page 25 1.3 Nomenclature......Page 26 1.4 Komatiite lithofacies......Page 29 1.5 Conclusion......Page 31 2.2 Komatiites in the Barberton greenstone belt......Page 32 2.3 Komatiites in the Abitibi greenstone belt......Page 43 2.4 Ore-bearing komatiites at Kambalda......Page 49 2.5 Komatiitic basalts on Gilmour Island......Page 56 2.6 Karasjok-type Fe-Ti-rich komatiites and picrites in Fennoscandia......Page 58 2.7 Komatiites and picrites from Gorgona Island......Page 62 3.1 Introduction......Page 69 3.2 Types of komatiite flows......Page 71 3.3 Dimensions of komatiite flows......Page 73 3.4 Spinifex texture......Page 77 Chilled margins (A1)......Page 83 Spinifex (A2)......Page 85 The B1 layer......Page 88 Cumulate layer (B2-B4)......Page 91 Basal chill zones......Page 92 Other unusual spinifex-textured komatiites......Page 93 Harrisitic textures......Page 94 3.6 Komatiite flows without spinifex textures......Page 95 3.7 Flows with thin spinifex layers......Page 96 3.8 Thick dunitic units......Page 100 3.9 Pillowed komatiites......Page 103 3.10 Volcaniclastic komatiites......Page 104 Flow-top breccia of the Alexo komatiite flow......Page 105 Ultramafic pyroclastic komatiites at Sattasvaara, Finland......Page 107 3.11 Komatiitic basalts......Page 108 Textures......Page 109 3.12 Intrusive komatiites......Page 112 4.1 Introduction......Page 114 Morphological characteristics......Page 115 Chemical compositions......Page 119 Major elements and zonation......Page 120 Variations in olivine compositions within komatiite flows......Page 121 Ni contents......Page 123 Cr contents......Page 126 Ca contents......Page 127 Chromite......Page 128 Pyroxenes......Page 132 Megacrysts in spinifex lavas......Page 133 Interstitial grains......Page 137 Pyroxenes in gabbros and cumulate layers......Page 138 Other magmatic minerals......Page 139 Glass......Page 140 4.3 Secondary minerals......Page 143 4.4 Summary......Page 145 5.1 Introduction......Page 146 5.3 Mobile and immobile elements: the olivine control line criterion......Page 147 Immobile elements in Zvishavane komatiites......Page 151 Mobile elements in Zvishavane komatiites......Page 154 5.4 Mobility of Mg......Page 157 5.5 Other approaches used to demonstrate element mobility......Page 159 5.6 Other types of mobile element behaviour......Page 164 Major elements......Page 165 Al2O3/TiO2......Page 167 Incompatible elements......Page 170 Compatible trace elements......Page 172 5.8 Igneous chemistry of komatiitic basalts......Page 176 6.2 The Rb–Sr system......Page 183 6.3 The Sm–Nd system......Page 187 Early dating attempts......Page 188 Crustal contamination......Page 190 Post-eruption changes in Sm–Nd ratios......Page 193 Nd isotopic constraints on the composition of the mantle source......Page 194 Age dating......Page 198 Initial isotopic compositions......Page 200 Age determination......Page 201 Initial isotope compositions......Page 202 6.6 The Lu–Hf system......Page 204 6.7 Helium isotopes......Page 208 6.8 Stable isotopes......Page 210 6.9 Conclusions emerging from the isotopic compositions of komatiites......Page 216 7.2 Experiments at 1 atm pressure......Page 218 Mg–Fe partitioning between olivine and komatiitic liquid......Page 229 Ni partitioning between olivine and komatiitic liquids......Page 230 Other partitioning data......Page 231 7.4 Experiments at moderate pressures (1–4 GPa)......Page 233 7.5 High-pressure experiments on komatiites......Page 234 Densities of komatiite liquids at high pressures......Page 235 Phase relations at high pressures......Page 236 7.6 Trace-element partitioning at high pressures......Page 242 7.7 Dynamic cooling experiments......Page 243 7.8 Huppert and Sparks’ experiments......Page 245 7.9 Summary and implications for komatiite petrogenesis......Page 246 Part II Interpretation – the manner of emplacement, the origin and the tectonic setting of komatiites......Page 249 8.1 Introduction......Page 251 8.2 Temperature......Page 252 8.4 Viscosity......Page 254 8.5 Density......Page 257 8.6 Other thermal properties......Page 258 9.1 Introduction......Page 265 9.2 Nomenclature and terminology......Page 266 Komatiite lithofacies......Page 267 Komatiite flow facies......Page 269 Size and scale......Page 272 Emplacement of thick basalt flows: the inflation hypothesis......Page 274 Inflation in komatiite flows......Page 277 Alternatives to the inflation mechanism......Page 279 Did komatiites flow turbulently?......Page 281 Thermomechanical erosion beneath komatiite flows......Page 283 Timing of crust development......Page 286 Mechanism of growth of spinifex texture......Page 287 Crystallization and accumulation of polyhedral olivine......Page 290 Crystallization of adcumulates......Page 298 9.6 Models for the emplacement and solidification of komatiite flows......Page 299 Solidification of thin differentiated flow lobes......Page 300 Solidification of flow lobes without spinifex texture......Page 303 Origin of B1 zones......Page 304 Thick cumulate-rich flows......Page 306 9.7 What did komatiite volcanoes look like?......Page 310 Structure of komatiite flow fields......Page 311 The form of komatiite volcanoes......Page 314 9.8 Concluding comments......Page 316 10.1 Introduction......Page 317 10.2 General characteristics......Page 318 10.3 Type I deposits......Page 321 The host komatiite sequence......Page 322 The orebodies......Page 325 Origin of the ‘troughs’......Page 326 Geochemistry of the Kambalda komatiites......Page 327 Timing of ore formation and flow crystallization......Page 328 Example: Alexo (Ontario)......Page 329 Example: Black Swan (Western Australia)......Page 332 Example: Raglan (New Quebec)......Page 334 10.4 Type II deposits......Page 336 Example: Thompson (Manitoba)......Page 339 10.6 Compositions of komatiite-hosted sulfide ores......Page 342 Influence of the R factor......Page 343 Monosulfide solid solution fractionation......Page 344 The thermomechanical erosion model for Type I deposits......Page 345 Origin of Type II ores......Page 347 Province selection......Page 348 Unit selection......Page 349 11.1 Introduction......Page 350 The Jamestown ophiolite complex and intrusive komatiites: early papers by Brooks and Hart and by de Wit and coauthors......Page 351 Formulation of the hydrous komatiite modelpapers by Grove et al. and Parman et al. published between 1996 and 1999......Page 353 The Boston Creek komatiitic basalt......Page 357 Experimental studies advocating a hydrous deep mantle source of komatiite......Page 358 Responses to the hydrous komatiite model: papers by Nisbet et al., McDonough and Danushevsky, and Arndt et al.......Page 359 11.3 Extrusive Barberton komatiites: papers by J. Dann published in 2000 and 2001......Page 361 11.4 Elaboration of the subduction zone model: papers by Parman et al. and Grove and Parman published between 2001 and 2004......Page 362 11.5 Other papers discussing the hydrous komatiite model......Page 366 11.6 A critical evaluation of the hydrous komatiite model......Page 371 12.2 Methods used to estimate liquid compositions......Page 374 Chilled margins......Page 376 Combined use of olivine compositions and whole-rock analyses......Page 377 Alexo komatiite......Page 380 Three unusual high-Mg komatiites: Commondale, Weltevreden and Murphy Well......Page 382 Thick dunitic units......Page 383 Eruption temperatures......Page 384 13.1 Nature of the komatiite source......Page 385 13.2 What does it take to make an ultramafic magma?......Page 387 13.3 Conditions of melting......Page 390 13.4 The formation of various types of komatiite......Page 395 Barberton-type komatiite......Page 397 Munro-type komatiite......Page 399 Gorgona komatiites and picrites......Page 403 Commondale and Weltevreden komatiites......Page 404 Fe-rich Karasjok-type komatiites from Boston Creek and Lapland......Page 406 13.5 The formation of komatiitic and tholeiitic basalts......Page 408 13.6 Summary......Page 410 14.1 Introduction......Page 412 ‘Mafic plains’......Page 413 Arcs and convergent margins......Page 420 Eruption in shallow water on continental basement......Page 422 14.2 The geodynamic setting of Archean greenstone belts......Page 427 14.3 A mantle plume does not know what it will meet at the surface......Page 429 14.4 Passage through the lithosphere......Page 431 14.5 The setting of komatiitic volcanism......Page 433 14.6 Conclusions......Page 435 References......Page 437 General Index......Page 481 Localities......Page 488 This Monograph, Written By Three Experts With Long Experience In The Field, Presents A Complete Account Of The Characteristics Of Komatiites Including Their Volcanic Structures, Textures, Mineralogy And Chemical Compositions. Models For Their Formation And Eruption Are Evaluated, Including Discussion Of The Controversial Issue Of Whether Komatiites Originate From Anhydrous Or Hydrous Magmas. Stephen Barnes And Michael Lesher Have Contributed One Chapter On Volcanology And Another On The Valuable Nickel And Copper Ore Deposits Found In Some Komatiites. Komatiite Is A Key Reference For Researchers And Advanced Students Interested In Petrology, Archaean Geology, Economic Geology, And Broader Questions About The Evolution Of The Earth's Crust And Mantle.--jacket. A Brief History Of Komatiite Studies And A Discussion Of Komatiite Nomenclature -- Brief Descriptions Of Six Classic Komatiite Occurences -- Field Characteristics, Textures, And Structures -- Mineralogy -- Geochemistry -- Isotopic Compositions Of Komatiites -- Experimental Petrology -- Interpretation : The Manner Of Emplacement, The Origin And The Tectonic Setting Of Komatiites -- Physical Properties Of Komatiites -- Physical Volcanology / By S.j. Barnes And C.m. Lesher -- Komatiite-associated Ni-cu-pge Deposites / By C.m. Lesher And S.j. Barnes -- The Hydrous Komatiite Hypothesis -- Compositions And Eruption Temperatures Of Komatiitic Liquids -- Petrogenesis Of Komatiite -- Geodynamic Setting. Nicholas Arndt ; With Contributions From C. Michael Lesher And Stephen J. Barnes. Includes Bibliographical References (p. 415-458) And Index.