Experimental Investigation On The Evolution Of Picritic Magma At High Temperature And High Pressure

Picritic magma is a significant primary magma.The evolution process of such magma is of importance to the properties of the deep mantle,the origin of oceanic island basalt(OIB)and continental flood basalt(CFB)and the formation of magmatic deposits.However,there has been no systematically experimental investigation on the evolution of such magma.As a consequence,the evolution process of the primary magma in the deep mantle and its role on the origination of related basalts and magmatic deposits remain poorly understood.To address these important problems,we have performed systematic studies on picritic magma under high temperature and high pressure,mainly including two experimental investigations on the evolution of picritic magma and its evolved magma-the high-titanium basaltic magma.(1)To investigate the evolution process of picritic magma,experiments were conducted by using three series of synthetical picrites with Mg O contents of 26 wt%,19wt%and 17 wt%as starting materials at pressures of 0.5-5.0 GPa and temperatures of1100-1700?.Experimental results show that the picritic magmas with different compositions have the similar evolution behavior at high pressure.At pressures higher than 3.0 GPa,clinopyroxene and garnet are the dominating equilibrium minerals while olivine is rarely in amount when the Mg O contents of picritic magma decrease to 20-17wt%.In this evolution process,the Si O₂,Mg O,Ca O and Al₂O₃ contents decrease and Ti O₂,Fe O^T and alkali contents increase with decreasing temperature due to the crystallization of clinopyroxene and garnet.(2)Despite the classic hypothesis that primary basalts are generated by partial melting of peridotite,several compositional characteristics of OIB and CFB,such as higher Fe O^T and lower Al₂O₃ and Ca O contents,are not easily reconciled with such an origin.Therefore,the majority of researchers proposed that a diverse mantle component(e.g.pyroxenite or Fe-rich peridotite)and partial melting of peridotite at higher pressures are account for such discrepancy.This thesis shows that such compositional characteristics can be resulted by the crystallization of clinopyroxene and garnet at high pressure.There is no need to involve different source components or melting conditions.(3)The hypothesis that Hawaiian OIB originated from an olivine-free pyroxenite source is popular.However,based on the experimental and calculated results of this thesis,a high-pressure crystallization model is proposed for the origination of Hawaiian OIB.The primary magmas of Hawaiian OIB were generated by partial melting of peridotite at pressures of 3-4 GPa.Then,the primary magmas precipitated large amounts of clinopyroxene and garnet in the deep lithosphere.The evolved magmas re-equilibrated with 0-30%orthopyroxene of harzburgite when passing through the lithospheric mantle.Finally,the magmas fractionated olivine at lower depth and form Hawaiian OIB.This model matches the compositional characteristics of Hawaiian OIB,especially its lower Ca O and higher Si O₂ contents compared to experimentally-derived partial melts of peridotite.Consequently,there is no need to attribute such characteristics of Hawaiian OIB to the olivine-free source pyroxenite which formed by a complex process.The Hawaiian post-shield alkali basalts and shield tholeiites originate from the same source by assimilating different amounts of orthopyroxene,where the former maintains their silica-poor features because the less orthopyroxene assimilation.Calculated results show that a correlation between peridotite-derived melt and Hawaiian lavas in the Ni-Mg O diagram might be attributed to the decrease in the Mg O content of the primary magmas in the deep mantle instead of Ni-rich source materials.The Ni partition coefficients between olivine and melt increase with decreasing pressure.In high-pressure crystallization model,the Ni content in the first olivine to crystallize from the parental magmas increase as the F_o content decrease and totally covered most Ni-rich olivine phenocrysts in Hawaiian OIB.(4)During partial melting of peridotite,olivine is the dominating minerals and commonly exceed 50%in amount.The Mg O and Fe O^T contents of peridotite-derived melts mainly buffered by the amount of olivine,because olivine is the most Mg-and Fe-rich silica minerals in the upper mantle.However,the crystallization of clinopyroxene and garnet would form a thin layer and reject further equilibrium between melt and surrounding olivine.Therefore,during high-pressure crystallization process,the variation of Mg O of magmas less changed while Fe O^T contents greatly increase.Experimental results show that Fe O^T contents in evolved magmas during high-pressure crystallization is 1.4 times higher than that in very low degrees of partial melting of peridotite.The Ti O₂content of melt is positively correlated with the variation of melt fraction.At the same variation of Mg O content of melt,the melt has a large variation of melt fraction at higher pressure,which increase the Ti O₂ contents of melt.This is the key factor to promote the final Ti enrichment of magmas and form the high-Ti basalts in oceanic island and large igneous province by buffering Mg O of residual melt to higher levels.(5)To investigate the crystallization process of the Fe-and Ti-rich basaltic magma-a residual magma after the evolution of picritic magma and constrain the origin of Fe-Ti-V oxide ore deposits,a series of crystallization experiments had been carried out by using natural Emeishan Ti-rich hydrous basalts as starting materials at a pressure of 0.5 GPa and temperatures of 700-1000?.Experimental results indicates that the solvus crest of Fe₂O₃-Fe₂Ti O₄(magnetite-ulv(?)spinel)solid solution in natural basaltic magmas should lie between 800? and 900?.Ilmenite is the first oxides to crystallize and then magnetite.This study provides definitive evidence that sandwich-and trellis-type lamellae in titanomagnetite is formed by the reaction of earlier crystallized ilmenite and the evolved parental magma.Exsolved magnetite in ilmenite is formed by the oxidization of the original Fe-rich ulv(?)spinel that formed at lower temperature during crystallization.The composition of magmas gradually changes from basaltic to acidic with decreasing temperature.(6)To investigate the origination of massive ores of Fe-Ti-V oxide ore deposits,experiments related to the saturation of Ti O₂ contents in picritic magmas were conducted at a pressure of 1.5 GPa and temperatures of 1150-1350?.Experimental results show that the saturation of Ti O₂ contents in picritic magmas can be 20 wt%,which is much higher than that in acidic magmas(less than 1 wt%).Based on this experiment and the crystallization experiment of high-Ti basalts,the evolution trend line of magmas is built and a three-stage formation of Fe-Ti-V oxide ore deposits is proposed.In the first stage,mafic picritic or basaltic magma mixing with acidic magma that formed from partial melting of crust or the evolved magma.The Ti O₂ content is saturated after magma mixing,thus large amount of Fe-Ti oxides precipitate from magmas and form the massive ores in the lower part of the intrusions.In the second stage,with decreasing temperature,the net-textural ores and disseminated ores are formed by the crystallization of basaltic magmas.It is similar like the results of crystallization experiment of high-Ti basalts.In the last stage,the basaltic magmas gradually evolved to acidic magmas and form the syenite in the upper part of the intrusions.