The Oxygen Fugacity Of Mantle-derived Magmas

The Earth’s mantle represents the largest reservoir of volatile elements（e.g.,carbon,oxygen,hydrogen,and sulfur）in the planet.The migration and release of volatile elements in the mantle play important roles in the mantle melting and the formation of the atmosphere.As heterovalent elements,the speciation and mobility of volatile elements in the mantle are governed by oxygen fugacity.Therefore,determining the oxygen fugacity of the mantle is a key aspect in the research of mantle and deep volatiles cycling.Current estimates of the mantle redox state are mostly based on the measurements of the oxygen fugacity of mantle-derived basalts.However,the oxygen fugacity of magmas is significantly affected by fractional crystallization,degassing and crustal assimilation processes,which precludes the determination of mantle’s redox conditions.Olivine and clinopyroxene are commonly the liquidus mineral of mantle-derived melts and therefore has great potential to decipher the oxygen fugacity of primary melts.This thesis presents two newly calibrated oxybarometers based on the exchange of V/Sc between olivine and basaltic melts and the partitioning of V between clinopyroxene and basaltic melt,respectively.These oxybarometers calibrated the influence of melt composition and temperature on the result of oxybarometer.By applying our oxybarometers to MORB,Kilauea Iki lava lake basalts,North Atlantic craton aillikite,Siberian craton aillikite and South Qinling Belt alkali magmas,this thesis explores the redox evolution of magma during differentiation and constrains the oxygen fugacity of the mantle.Determining the oxygen fugacity of the primary magma is the prerequisite for probing mantle oxygen fugacity through mantle-derived melts.However,most basalts ejected to the surface are differentiated melts,and their oxygen fugacity is affected by fractional crystallization and magma degassing,which limits our understanding of mantle oxygen fugacity.The congenetic intrusion and volcanic rock exposed in the orogenic belt record the redox state of magma at different depths and different evolution stages,which provides a unique opportunity for tracing the oxygen fugacity evolution during magma differentiation.The Early Paleozoic alkali basalts and mafic dykes in the South Qinling Belt display similar major and trace element and Sr-Nd isotopic compositions,indicating those magmas are derived from an identical mantle source.Systematic thermobarometer and oxybarometer calculations suggest alkali basalts and mafic dykes crystallized at different depths（230–773MPa）.Under high pressure,the primitive magmas are characterized by low oxygen fugacity（FMQ-1）,which gradually increases to FMQ+2 during decompression.The oxidizing trend is also supported by the correlation between V/Y and Mg^#values in clinopyroxene.MELTS simulation results show that the fractional crystallization of clinopyroxene could not oxidize the magma.Hygrometer results suggest that the H₂O contents of South Qinling Belt alkali magmas decrease with the decrease of pressure.The correlation between H₂O contents and oxygen fugacity indicates that the H₂O dissociation or degassing oxidizes hydrous magma during ascending.Therefore,evaluating the variation trend of oxygen fugacity during magma differentiation and determining the oxygen fugacity of primary melt is prerequisite for estimating the redox state of the mantle source.Geochemical characteristics of South Qinling Belt alkali basalts,Siberian craton aillikites and North Atlantic craton aillikites indicate these magmas are derived from lithospheric mantle sources.Based on petrography and thermobarometer results,we suggest the mantle source of South Qinling Belt alkali basalts are shallow lithospheric mantle（P=1.5–2.6 GPa）.In contrast,the mantle sources of aillikites are located in the deep lithospheric mantle where stabilizes diamond（P=5.2–5.4 GPa）.Oxybarometer results manifest the source of South Qinling Belt alkali basalts is slightly oxidized with oxygen fugacity of FMQ+0.20–+0.84,but the source of aillikites is highly reduced（FMQ-3.82–-2.51）,indicating the oxygen fugacity of lithospheric mantle decrease with increasing depth.MORB and OIB represent partial melts of the convective asthenospheric mantle at different depths.The olivine-melt V/Sc exchange coefficient oxybarometer results suggest the oxygen fugacity of MORB is FMQ+0.56±0.24,near one order of magnitude more oxidized than previously estimated.The presence of S⁶⁺in MORB glasses indicates some Fe³⁺has been reduced to Fe²⁺via Fe-S isochemical electron exchange reaction during the cooling of silicate melts.Fractional melting model suggests the mantle source of MORB contains 6-8%Fe³⁺/ΣFe,which is more oxidized than the lithospheric mantle.Along an adiabat upon upwelling,large amounts of carbonatite melts can form from a CO₂-bearing source at depth of 200–250 km explaining,therefore,the electrical conductivity anomaly of the upper mantle.Compared to MORB,Kilauea Iki lava lake basalts are more oxidized with oxygen fugacity of FMQ+1.27±0.24.High oxygen fugacity and high amount of recycled material in Hawaiian basalt suggest the recycling of oxidized surface materials causes the heterogeneous mantle oxidation state.Olivine and clinopyroxene oxybarometer results suggest asthenospheric mantle derived melts are more oxidized than lithospheric mantle conterparts.This thesis argues that the high oxygen fugacity observed in MORB and OIB does not indicate the whole asthenospheric mantle is more oxidized than lithospheric mantle,but reflects the oxidized domains in the asthenospheric mantle with lower solidus could melt during adiabatic upwelling.