The Origin Indication Of Exsolution Structures In Corundum,Titanomagnetite And Ilmenite Megacrysts In Basalts From Changle,Shandong And Wenchang,Hainan

The redox state of the mantle determines the valence state of the variable valence elements,thereby controlling the form of occurrence of the elements in the geologic system.Research into the mantle redox state has attracted a great deal of attention.To assess the redox state of different geologic systems,predecessors proposed using oxygen fugacity to describe it,and define buffers such as FMQ,NNO and MH,the difference between the buffers reflects the level of oxygen fugacity.The oxygen fugacity decreases with decreasing mantle depth.Oxygen fugacity is related with Fe³⁺/Fe²⁺equilibrium.Basalt,as the sample most often used for the study of oxygen fugacity in the mantle,has become the point of entry for the study of the two important topics:the gradient of mantle oxygen fugacity and the difference in mantle oxygen fugacity in different tectonic settings.Previous researches have analyzed the process of basalt evolution across the rock analysis of basalt lithology,but the oxygen fugacity characteristics of the mantle source area of the Cenozoic basalt and the internal relationship between the oxygen fugacity of the source area and the associated geologic processes have not been clearly resolved.From these problems the megacrysts in the basalt are collected as research objects,in order to obtain data on the oxygen fugacity of basalt represented by megacrysts at the early stage of crystallization.Megacrysts are the minerals of early stage of magma crystallization and may represent the redox state of the early-stage crystallization of basalt.The megacrysts of corundum,titanomagnetite,and ilmenite in Cenozoic basalt are all oxides,which contain elements of varying Fe content and exsolution structures.Making them ideal scaffolds for studying the mechanism of exsolution and redox stage of megacrysts.Not only do the features of the exsolution structure reflect the composition of the original solid solution,but are also closely linked to temperature,pressure,oxygen fugacity,and the cooling rate.Thus,by studying the structure of the exsolution of megacrysts,the conditions for the formation of megacrysts as well as the evolution of basalt may be explored.Due to the complex composition and geologic conditions,Fe-Ti oxide exsolutions exhibit a variety of interface characters.In the past,because of the micron-to-submicron size and symbiosis of exsolutions with megacrysts,in-situ tests were difficult to perform,which posed a great challenge for studying the exsolution-host interface structure.Most of the previous studies on Ti-Fe oxide focused on its composition,crystallographic orientation relationships,and morphological characters.However,the relationships between interface structures and crystallographic orientation relationships,as well as its exsolution mechanism seem still necessary to be studied,which contain applications of titanomagnetite and ilmenite as typomorphic minerals for probing the evolution of intrusions and the genesis of basalts containing Ti-Fe oxides.The corundum,titanomagnetite,and ilmenite megacryst samples used in this thesis are selected from Changle,Shandong Province,and Wenchang,Hainan Province,which is located on the Cenozoic basalt belt in eastern China.On the basis of field investigation,mineralogical observation,X-ray diffraction and electron probe analysis,hematite exsolutions in corundum megacrysts from Changle,Shandong Province,ilmenite exsolutions in titanomagnetite megacrysts from Wenchang,Hainan Province,and two stages of exsolutions in ilmenite megacrysts from Wenchang,Hainan Province has been studied systematically using electron backscatter diffraction and high-resolution transmission electron microscopy.Phases,exsolution structures,chemical composition,crystallographic orientation relationships,and interface structures were analyzed.The mechanism of the exsolution formation was investigated,and the physical and chemical significance indicative of the mineralisation process was discussed.The key results are:1.Both corundum and hematite belong to the"corundum"structure,and hematite exsolutions in corundum megacrysts are formed as a result of the decomposition of the corundum-hematite solid-solution system.There are three groups of exsolutions intersected with～60°.Two groups of the exsolutions have the same orientation with the host and the other group is twinned to those two groups.The crystallographic orientation relationships and the regular interface confirm that the exsolution was formed under depressurization based on the crystal chemistry theory.High-resolution transmission electron microscopy photographs show that there are periodic coherency units at（0001）interface between hematite exsolutions and corundum host from Changle,Shandong Province.The measured unit lengths are 6.71-6.72nm,which are in good agreement with every_（）[projection of corundum（011 2）spacing on the interface]or 16-_（）[projection of hematite（011 2）spacing on the interface].Based on the results,at the interface,the hematite-corundum phases tent to modulate to get the maximum coherency in geological process during exsolution.Compared with the interface character of the structure between titanomagnetite and ilmenite,which is not straight periodic or regular,forming a certain range of transition zone at the adjacent area.The regular and periodic interface structure characters between corundum and hematite are due to the high matching degree of crystallographic orientation,similar crystal structure and cell parameters between the two phases.2.Titanomagnetite megacryst from Wenchang,Hainan Province is a composite phase mineral composed of magnetite-ulvospinel host,and exsolutions of ilmenite and hematite,among which ilmenite is the main exsolution phase.The ilmenite megacryst from Wenchang,Hainan Province is a composite phase mineral composed of ilmenite-hematite host and exsolutions of magnetite and ulvospinel.In the second-stage exsolutions,ilmenite-hematite exsolve,forming complex composite phases.Magnetite-ulvospinel and ilmenite-hematite belong to"spinel"type and"corundum type",which have the specific orientation relationships:（111）_Ti-Mag//（0001）_Ilm,[110]_Ti-Mag//[101 0]_Ilm.The ilmenite exsolutions can be converted from Fe₂Ti O₄ in the magnetite-ulvospinel solid-solution in two ways:（1）Oxidation-exsolution process,in which Fe₂Ti O₄ in the solid-solution oxidizes due to the increase of oxygen fugacity,and exsolves from the solid-solution as ilmenite;（2）Exsolution-oxidation process:When the slow cooling due to the miscibility gap at～600℃,ulvospinel exsolved,and then was oxidized to ilmenite at a lower temperature.The study of the mechanism of the exsolution structure provides a theoretical basis for the reconstruction of the initial composition of the solid-solution and the estimation of the temperature and oxygen fugacity during crystallization by using the temperature and oxygen fugacity meter of Ti-Fe oxides.3.In the titanomagnetite megacrysts from Wenchang,Hainan,the exsolution of ilmenite occurs at 920℃and oxygen fugacity of FMQ+1.In the hematite megacrysts from Wenchang,Hainan Province,the exsolutions of two phases occur at 920℃,oxygen fugacity of FMQ+1,and 880℃,oxygen fugacity of FMQ+0.75,respectively.The titanomagnetite exsolutions occurred earlier than the second-stage ilmenite exsolutions.On basis of the variation of oxygen fugacity of titanomagnetite and ilmenite megacrysts,the range of oxygen fugacity of basalt in the same area is calculated.This result confirms that the oxygen fugacity of basalt in Wenchang,Hainan is higher than that in Changle,Shandong,and oxidation-exsolution is more likely to occur in titanomagnetite in Wenchang,Hainan,which provides an explanation for the possible reason for the lack of exsolution in titanomagnetite in Changle,Shandong.The methods can be applied to research other oxide minerals and their Ti-Fe oxide exsolutions.