Formation And Early Evolution Of Iron Meteorite Parent Bodies Constrained By Chromium Isotopes

Iron meteorite is a type of highly evolved meteorite composed primarily of ironnickel alloys.It is the only sample we can obtain from planetary core.Prior to the formation of iron meteorites,their parent body have undergone melting,the separation of metal and silicate melt,and the crystallization of the metal core.The timing,the physical and chemical conditions under which these processes occured,as well as the affinity and the origin of iron meteorite parent bodies can provide important constraints for the formation and early evolution of the Solar System.Chromium(Cr)isotopic system could be an effective tool to study the formation and evolution of iron meteorites.The mass-dependent fractionation of Cr isotopes can be used to trace the physicochemical conditions of different processes.The 53Mn-53Cr chronology can date the Mn-Cr fractionation event occurring within 15 Ma after the Solar System formation.The 54Cr nucleosynthetic anomaly can be used to track the material origin and the affinity of different meteorites.In this thesis,I aimed at constraining the formation and evolution of iron meteorites using the Cr isotopic system.I investigated the behavior of Cr isotopes during the core-mantle differentiation of iron meteorite parent bodies by high P-T experimental simulation.Based on the measurements of mass-dependent and mass-independent Cr isotopic compositions of Cr-rich minerals(daubreelite and chromite)in iron meteorites,I constrained the timing of core-mantle differentiation of iron meteorite parent bodies,tracked the affinity of different iron meteorite parent bodies,and investigated the behavior of Cr isotopes during the core crystallization process.Firstly,I simulated the separation of metal melt and silicate melt at 1 GPa and 1873 K using a piston cylinder apparatus and determined the Cr isotopic fractionation factor between the metal and silicate phases.Experimental results consistently indicate that the metal phase is isotopically heavier than the coexisting silicate phase,with Δ53Crmetalsilicate value up to 0.3‰ at the investigated experimental conditions.Oxygen fugacity and silicate composition do not have significant effects on the Cr isotopic fractionation factor.By contrast,increasing Ni content in the metal increases the Δ53Crmetal-silicate value,implying that the Ni content of the core could influence planetary isotopic signatures.Heavier Cr isotopes enter the core preferentially during planetary core formation.Our results indicate thatcore formation could increase the δ53Cr values of the cores of the parent bodies of iron meteorites by up to～0.2‰ at 1873 K.Therefore,the significantly heavy Cr isotopic composition(up to 2.85‰)of iron meteorites cannot be explained by equilibrium isotopic fractionation between the core and the mantle of the parent bodies of iron meteorites.On the other hand,the δ53Cr value of the terrestrial mantle could be lowered by up to～0.02‰ by core formation compared to chondrites,although this is unresolvable with the current analytical uncertainty.For smaller bodies such as Moon,Mars,and Vesta,the lower core formation temperatures could potentially generate a resolvable core-mantle Cr isotopic fractionation.However,the Moon’s small core size would limit the change in the Cr isotopic composition of the lunar mantle compared to the chondritic value.For Vesta and Mars,core formation could lower the δ53Cr values of their mantles by-0.01-0.02‰,which is trivial relative to the analytical uncertainty.Due to the low Fe/Cr values of daubreelite and chromite,these two minerals could preserve the primitive radiogenic 53Cr isotopic composition of iron meteorites.As daubreelite and chromite in iron meteorites and the core of differentiated planetesimal have low Mn/Cr values,the radiogenic 53Cr isotopic compositions in daubreelite and chromite only reflect the radiogenic accumulation before core-mantle differentiation.Therefore,I also measured the radiogenic 53Cr isotopic compositions of daubreelite and chromite from iron meteorite and calculated the core-mantle differentiation ages of various groups of iron meteorite parent bodies.The 53Mn-53Cr core-mantle differentiation ages for most iron meteorites are within 4 Ma after Solar System formation,indicating the early accretion and melting of the differentiated planetesimals.The 53Mn-53Cr modal ages are in accord with the 182Hf-182 Wmodal ages,suggesting the robustness of using 53Mn-53Cr chronology to date early metal-silicate segregation processes.Interestingly,the daubreelites in three IVA iron meteorites record three resolvable 53Mn-53Cr modal ages,possibly suggesting a complex impact history of IVA parent body.Daubreelite and chromite also preserve the primitive nucleosynthetic 54Cr anomaly in iron meteorites.In this thesis,I also measured the mass-independent 54Cr anomaly of daubreelite and chromite from iron meteorites and constrained the temporal and spatial distribution of neutron-rich nuclides in the solar nebula.Under the framework of isotopic dichotomy in meteorites,the NC meteorites formed in the inner Solar System(within the Jupitar orbit)and the CC meteorites formed in the outer Solar System(beyond the Jupitar orbit).The ε54Cr values of NC iron meteorites are positively correlated with the core formation ages of their parent asteroids,indicating a temporal evolution of neutron-rich nuclides in the formation region of these iron meteorite parent bodies(presumably 2-3 AU from the sun).The temporal evolution between 2-3 AU from the sun was possibly resulted from the gradual adding of materials from the inner part of the inner Solar System disk and essentially reflects the gradual homogenization of the inner Solar System disk.The ε54Cr values of CC iron meteorites are correlated with the Mn/Cr ratio of their parental planetesimals,indicating a heliocentric heterogeneous distribution of pre-solar 54Cr-enriched grains in the outer Solar System.The overlapped range of 54Cr anomalies for CC irons and carbonaceous chondrites indicates limited radial homogenization in the outer Solar System disk.Meanwhile,the chondrules of later formed chondrites seem to have more homogeneous 54Cr anomalies than the chondrules of earlier formed chondrites,suggesting a gradual local homogenization in the outer Solar System with time.At last,I measured the stable Cr isotopic compositions of daubreelite and chromite from iron meteorites to understand the behavior of Cr isotopes during core crystallization.Chromite could directly crystallize from the metallic melt,while daubreelite could only exsolve from pre-existing pyrrhotite which could also directly crystallize from the metallic melt.The daubreelite and chromite in iron meteorites recorded lighter Cr isotopic compositions compared to the metal phase,indicating the crystallization of chromite and pyrrhotite could both increase the Cr isotopic composition of the residual melt.Therefore,the extremely heavy Cr isotopic compositions in iron meteorites could reflect the fractional crystallization of these minerals.Also,the fractional crystallization of chromite and pyrrhotite could also be the cause of the compatible behavior of elemental Cr during core crystallization.As the core melt of differentiated planetesimal should be enriched in S and the early crystallized iron meteorites have more frequent occurrence of daubreelite,it suggests the fractional crystallization of pyrrhotite as the dominant cause for the compatible behavior of Cr and the gradual enrichment of heavy Cr isotopes in the core melt.Asthe density of pyrrhotite is lower than that of the metallic melt,the up-float of pyrrhotite could also cause the depletion of sulfur in the planetesimal core.