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Spinifex Komatiites Analytical Study

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Spinifex Komatiites Analytical Study Sébastien Bouquain Thèse en co-tutelle dirigée par: Collaborateurs: Nick Arndt, LGCA Grenoble François Faure, CRPG Nancy Guy Libourel, CRPG Nancy Eric Hellebrand, DGG Hawaï Steve Foley, Mainz The talk (1) Introduction (2) Scientific goals (3) Presentation of the rocks : komatiites and lunar basalts (4) Methods (5) Discussion (6) Conclusions and future research Introduction Æ ultramafic volcanic rocks [MgO>18%] Æ spinifex-textured rocks Æ mainly Archean (also Proterozoic and Cretaceous) Æ mainly extrusive rocks (lava flows) Introduction >> well-preserved minerals in fresh rocks! Æ Rocks from the lunar crust (mare basalts) >> crystallization sequence and pyroxene morphology comparableÆ Tholeiitic in komatiitesbasalts, (pyroxenites,and lunar basalts dunites) >> anhydrous reference Lunar komatiite basalt Introduction Æ Texture characterized by large, skeletal or dendritic, platy, bladed or acicular grains of olivine or pyroxene olivine pyroxene olivine pyroxene Scientific goals To use pyroxene compositions to gain a better understanding of •the chemical and physical processes that occurred during komatiite genesis • the thermal and compositional state of the Earth’s mantle in the Archean Æ models of crystallization of komatiite flows Æ models of early Earth’s mantle dynamics Scientific goals Two modelsThe try originto explainof komatiitethe formation magma: of komatiites: (1) hot and dry sourceDry (<plume ??? )> Wet (2)relatively cold and wet source (subduction zone) Two models try to explain the formation of komatiites: >> origin of spinifex texture (1)hot and dry source (plume) >> augite composition in Barberton komatiites (South Africa) Presentation of the rocks : komatiites and lunar basalts Alexo, Abitibi Greenstone Belt, Canada Presentation of the rocks : komatiites and lunar basalts Komatiite flow and komatiitic Ax104 basalt flow from Ax101 Alexo Ax102 Ax103 M668 Ax105 Ax106 Presentation of the rocks : komatiites and lunar basalts Komatiite flow and komatiitic basalt flow from Alexo pyroxenes in volumes between platy olivines Presentation of the rocks : komatiites and lunar basalts pyroxene needles zoned crystals: pigeonite core and augite rim aug pig skeletal crystals Presentation of the rocks : komatiites and lunar basalts Lunar Mare Basalts : Apollo 12 pigeonite basalt Apollo 15 pigeonite basalt >>> To interpret pyroxene textures and compositions pigeonite core augite rim Methods 1) Analytical approach • Detailed study of the mineralogy and geochemistry of pyroxene in komatiites and in lunar mare basalts • Use of SIMS (ion microprobe), electron microprobe, SEM >> Mainz (Institute of Petrology & Max Planck Institute) >> Nancy (CRPG) 2) Experimental work • Attempts to reproduce the spinifex texture, using pyroxene field composition • Studies on simplified materials representing the parental magma of komatiites (Ca-Mg-Al-Si system) >> Nancy (CRPG) Analytical study augite pigeonite augite augite pigeonite augite Analytical study Alexo komatiite - Barberton komatiite Analytical study Barberton augites: Barberton komatiites have high • small remnants preserved in largely CaO/Al O altered grains 2 3 • all (but one) analyzed pyroxenes were in olivine cumulates • from poorly preserved metamorphic The unusualrockscompositions of Barberton augites is due to the high CaO/Al2O3 of Barberton komatiites. >>> water is not necessary to form Barberton pyroxenes Clinopyroxene crystallizes at higher Augite that crystallizes a high temperature temperatures in compositions with high from liquids with high CaO/Al2O3 has high CaO/Al2O3 than in compositions with low Mg#. CaO/Al2O3. Analytical study Komatiites Lunar basalts 6 0,9 0,8 Mg # 8 Mg # d 0,6 0,8 e ed Æ Rare Earth Elementsliz profiles z 4 i a # al # m 0,4 0,7 r m 4 reflect crystallization Mg o r Yb Mg o n n 2 Yb M Sm 0,2 Sm 0,6 P PM La La 0 0 0 0,5 0 100 200 300 3 0,9 0,8 Mg # 8 Mg # 0,8 0,6 2 Be x100 # B x10 m # 0,4 m 0,7 4 Mg Li pp Mg pp 1 0,2 0,6 Li Be x100 0 0 0 0,5 0 100 200B x10300 Æ Be and Li profiles reflect Æ Be and Li profiles indicate alteration in komatiites geochemical behavior like REE Analytical study Equilibrium experiments (Arndt 1976) Early crystallization of cpx in natural komatiites Late crystallization of cpx under equilibrium conditions spinifex komatiites Analytical study Phase diagram Analytical study Early crystallization of pigeonite Alteration ? in komatiites: what are the solutions? Pyroxene accumulation ? olivine (Campbell & Arndt 1982) Wrong phase relations ? Chemical (Ca diffusion) boundary layer ? (Faure & Si Schiano 2005) Ca Water ? (Parman et al. 1997) Analytical study The tips of the spinifex crystals grow in liquid with high Si/Mg which stabilizes pigeonite Analytical study Phase diagram cold hot Analytical study (1) Diffusion coefficients -4 -5 2 Latypov (2006): 10 -10 cm /s for H2O-saturated melts LaTourrette et al. (1996): 10-7 cm2/s for anhydrous liquids Æ Our estimate : 5.10-5 cm2/s (2) A small amount of water (0.2-0.5 wt%) is necessary Æ Water in melt inclusions (McDonough & Danyushevsky 1995; Shimizu et al. 2001) Æ Degassing kinetics (Moune et al. 2007) (3) Moderate thermal gradient is sufficient >>> Soret differentiation allows pigeonite to crystallize from a liquid that would have normally crystallized augite Experimental investigation Æ we used a vertical high-temperature, gas-mixing furnace Æ inert atmosphere is maintained by a flow of Ar Æ presence of a variable thermal gradient in the furnace, depending of the exact location of the graphite crucible °C 1100 1300 1500 15 25 cm 35 Hot point 45 Thermal gradient Experimental investigation We performed two series of experiments: • Isothermal crystallization experiments • Dynamic crystallization experiments pigeonite tridymite diopside Experimental investigation Liquidus boundaries / Pyroxene composition field Starting composition Experimental investigation Typical procedure of cooling Temperature Cooling in Tstart +∆T the thermal 1386°C gradient Tliquidus 3-200°C/h Tquench Time 2h 3h Experimental investigation Example of a typical run ∆ ∆ ∆ Experimental investigation forsterite Crystals of pyroxene are zoned: pigeonite core is surrounded by a diopside rim Zoned pyroxene Natural komatiite Pyroxene needles crystallized parallel to one another and downwards in the capsule pig di Experimental investigation High cooling rate experiments pigeonite diopside Pigeonite crystallizes in the thermal gradient even at high cooling rates (100°C/h) Experimental investigation (1) We were able to crystallize zoned crystals of pyroxene that perfectly mimic those in komatiites. (2) Pyroxene crystals growth is constrained by the thermal gradient. (3) Pigeonite formed at high cooling rates (100°C/h) >>> what about olivine ? Experimental investigation Forsterite crystallization: (1) various morphologies Hopper forsterite Plate forsterite Forsterite morphologies reflects: (1) the effect of the thermal gradient10°C/h (2) the cooling rate Experimental investigation Forsterite crystallization: (1) various morphologies (2) metastable Low superheating High superheating Forsterite No forsterite Experimental investigation Conclusions (1) Zoned pyroxene crystallizes in natural komatiites and in lunar basalts. (2) For a given composition of komatiitic liquid, the type of pyroxene and the crystallization sequence depends on the conditions of crystallization. (3) Constrained growth in a thermal gradient causes pigeonite to crystallize earlier than at equilibrium conditions. Soret differentiation is an efficient mechanism in pyroxene spinifex- textured lavas. (4) There is no evidence for the presence of a significant amount of water in komatiites. Most komatiites are dry and hot Future research (1) To better understand the cooling mechanisms of komatiite flows – to have better contraints on Soret effect (2) To better understand the crystallization of pigeonite: Æ Experiments with olivine at the liquidus and pigeonite as the second mineral Æ Introduction of tracers such as Cr or REE, in order to follow the crystallization of pigeonite Thank you Model of a komatiite flow Solidification of the crust and growth of skeletal olivine crystals SettlingSettling ofof olivineolivine phenocrystsphenocrysts toto formform cumulatecumulate layelayerr.
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