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Geochemistry of Amphibole from Archean and Early Proterozoic Ultramafic Rocks: Implications for the Secular Evolution of the Earth’S Mantle

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Geochemistry of Amphibole from Archean and Early Proterozoic Ultramafic Rocks: Implications for the Secular Evolution of the Earth’S Mantle UNIVERSITÀ DEGLI STUDI DI MILANO Dottorato di Ricerca in Scienze della Terra Ciclo XXXI Geochemistry of Amphibole from Archean and Early Proterozoic Ultramafic Rocks: Implications for the Secular Evolution of the Earth’s Mantle Ph.D. Thesis Gianluca Sessa Matricola R11208 Tutor Academic Year Coordinator Prof. Massimo Tiepolo 2017-2018 Prof. ssa Elisabetta Erba Abstract .................................................................................................................................................... i Introduction ............................................................................................................................................ 1 Chapter 1 ................................................................................................................................................ 3 1.1 Komatiites ...................................................................................................................................... 3 1.2 Fe-picrites....................................................................................................................................... 5 1.3 Tholeiitic basalts ............................................................................................................................ 7 1.4 Amphibole in the Archean and Early Proterozoic rocks ................................................................ 7 1.5 Aim of this work ............................................................................................................................ 9 Chapter 2 .............................................................................................................................................. 11 Geological setting and petrography of selected samples .................................................................. 11 2.1 Agnew-Wiluna greenstone belt (Western Australia) ................................................................... 12 2.1.1 Mount Clifford ................................................................................................................................................................ 16 2.1.2 Mount Keith..................................................................................................................................................................... 19 2.2 Abitibi greenstone belt (Ontario - Canada) .................................................................................. 24 2.2.1 Boston Creek ................................................................................................................................................................... 26 2.2.2 Theo’s Flow ..................................................................................................................................................................... 31 2.2.3 Ghost Range .................................................................................................................................................................... 36 2.3 Paleoproterozoic samples ............................................................................................................. 40 2.3.1 Pechenga Complex (Russia) ....................................................................................................................................... 40 2.4 Phanerozoic samples .................................................................................................................... 50 2.4.1 Ross Orogen (Husky Ridge), Antarctica ................................................................................................................ 50 2.4.2 Alpine Orogen (Adamello batholith and Valmasino-Bregaglia/Bergell pluton), Italy ............................ 52 2.4.3 Continental arc, Japan .................................................................................................................................................. 55 2.4.4 Himalayan Orogen (Gangdese batholith), China ................................................................................................. 59 2.4.5 Zagros Orogen (Sanandaj–Sirjan Zone; Aligoodarz pluton), Iran ................................................................. 61 2.4.6 Sunda arc (Batu Hijau district, Sumbawa), Indonesia ....................................................................................... 63 2.4.7 Alkali amphibole megacrysts, USA ......................................................................................................................... 65 Chapter 3 .............................................................................................................................................. 73 3.2 Major, trace elements and isotope compositions of Archean and Early Proterozoic samples..... 76 3.2.1 Agnew-Wiluna greenstone belt (Australia) ........................................................................................................... 76 3.2.2 Abitibi greenstone belt (Canada) .............................................................................................................................. 81 3.2.3 Pechenga Complex (Russia) ....................................................................................................................................... 95 3.3 Major, trace elements and isotope compositions of Phanerozoic samples ................................ 118 Chapter 4 ............................................................................................................................................ 139 Discussion ............................................................................................................................................ 139 4.1 Amphibole stability in igneous environment ............................................................................. 139 4.2 Amphibole geothermobarometry ............................................................................................... 142 4.3 Is amphibole a reaction product between melt and clinopyroxene precursor? .......................... 147 4.4 Amphibole-Clinopyroxene equilibrium ..................................................................................... 148 4.5 Modelling of melt evolution ...................................................................................................... 152 4.6 The role of external components ................................................................................................ 162 4.6.1 Constrains from transition metals .......................................................................................................................... 163 4.6.2 Constrains from fluid-mobile elements................................................................................................................ 165 4.6.3 Constraints from δ18O and δD ................................................................................................................................ 170 4.7 Water contents of primary melts ................................................................................................. 176 4.8 Chlorine and Fluorine contents of primary melts ....................................................................... 178 4.9 Implications on the secular evolution of the Earth’s mantle ...................................................... 181 4.9.1 Inferences from incompatible trace element composition ............................................................................. 181 4.9.2 Inferences from the Nb/Ta ratio and its bearing with the Nb-paradox ...................................................... 188 4.9.3 Inferences from variations in water, fluorine and chlorine ............................................................................ 192 4.9.4 Inferences from Sc/V and its bearing on mantle oxygen fugacity ............................................................... 196 5. SUMMARY AND CONCLUSIONS ............................................................................................. 199 References ................................................................................................................................................ I Acknowledgments ................................................................................................................................... I Appendix A Appendix B Appendix C i Abstract Since its formation at 4.5 Ga, the Earth underwent a complex evolution that progressively differentiated its original composition into the reservoirs that we presently know. Our knowledge on the composition and differentiation mechanisms active in the Early and Ancient Earth are still fragmentary for the paucity of suitable preserved rock records. The poor knowledge on the Archean mantle composition arises a series of problems spanning from the effective chondritic composition of the Earth to how volatile elements (hydrogen, oxygen, chlorine and fluorine) were added to the Earth. For the unavailability of mantle sectors preserving the Archean geochemical signature, valuable information on the Archean mantle composition can be exclusively extracted from Archean mantle-derived igneous rocks. In the Archean greenstone belts, different products of mantle melting are found as lavas and sills spanning in composition from tholeiites through Fe-picrites to komatiites. All these rocks are generally affected by extensive alteration which prevent the bulk rocks to be fully informative on the primary mantle melt composition and particularly on its volatile element contents.
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