Geochemical Significance Of Silicon-bearing Rutile In Eclogites From Dabie-sulu Ultrahigh Pressure Metamorphic Terrane And ⁵³Mn-⁵³Cr Systematics Of Chondrules In CO Carbonaceous Chondrites

Silicon is the second most abundant element in bulk earth,and Si-O tetrahedron constitutes the vast majority of silicate minerals in BSE?Bulk Silicate Earth?.Variation of content and structure of Si in metamorphic rocks is an effective way to trace the environment of high temperature and pressure.Continental subduction and exhumation is a natural laboratory of environment of deep earth with high temperature and pressure,which can offer the samples experienced ultrahigh pressure metamorphism?UHP?.Rutile-bearing eclogites are one of the major rocks of the Dabie-Sulu UHP terrane,and the Si is one of the trace elements in these rutiles.Therefore,the rutile might become a window to investigate the geochemical behavior of Si which is a trace element in rutile,and meantime,Si-rutile may be a potential geo-manometer.Part 1 of this thesis reports the geochemical characteristics of silicon?bearing rutiles in ultra-high pressure metamorphic?UHPM?eclogites sampled from Dabie-Sulu orogen.In details,the samples?rutile-bearing eclogites?are collected from Shuanghe in region of Dabie Mountain and from Donghai in Jiangsu Province.After making them into rock thin sections,I use a series of in-situ geochemical analysis?SEM,EMP,LA-ICP-MS and NanoSIMS?to survey the Si content,distribution in the rutiles occurred in the eclogites from different areas.The main results of this study are as follows:1.Si-bearing rutiles occur in UHP metamorphic rocks from Dabie-Sulu regions,and LA-ICP-MS shows that the silicon contents in both intergranular rutiles and rutile inclusions are mostly over 400 ppm,and the highest value is 2113 ppm.Therefore,Si-bearing rutiles may become another special mineral to trace the UHP metamorphism.2.Elemental mapping of NanoSIMS shows that silicon exists in rutile as homogenous pattern?the signal in margin is slightly higher than that in core?instead of some hotspots,so this can exclude that the interference of silicate or quartz micro?inclusions reported in literatures.It suggests that the homogenous distribution of silicon comes from substitution of Si for Ti,and the Si is 6-fold coordinated?same as Ti?.This may trace the form and metallicity of silicon in deep earth.3.Si content in rutile positively correlates with the pressure according to previous studies of experimental petrology,so Si-in-rutile might develop into a new geological barometer which can measure the depth of continental subduction.Meanwhile,stishovite,a high-pressure phase of quartz,is with the same structure as rutile.If the occurrence mode of Si in rutile is related to stishovite,the possible depth of continental subduction might be updated to 300 km,deeper than previous studies?80?200 km?,according to the formation condition of stishovite?1000?,10 Gpa?.Chondrules are millimeter to submillimeter-sized spheres with igneous textures,believed to form as free-floating molten droplets during transient heating?up to 2000K?and rapid cooling?100-1000 K/h?events in the solar protoplanetary disk,and they are significant components?up to 80 wt%?in chondrites that are the most primitive materials in the Solar System.Formation of chondrules marked one of the key steps in the Solar System accretion.However,chromium isotope cosmochemistry can be used as not only a "timer"(⁵³Mn-⁵³Cr short-lived decay systematics with a half-life of 3.7 Myr)but also a "tracer"(?⁵⁴Cr systematics)to study the formation time,origin and probably formation mechanism of chondrules.Part 2 of this thesis mainly focuses on the ⁵³Mn-⁵³Cr systematics and major element contents?Cr isotope compositions and element contents are measured by TIMS and ICP-MS respectively?of Ornans?C03.4?chondrules and their implications for cosmochemistry and the evolution of early Solar System.Several achievements of this part have been produced:1.The initial ⁵³Mn/⁵⁵Mn of CO Ornans Chondrules?external isochron?,?7.2 ± 1,6?x 10^-6,can be translated to absolute age:4568.1 ± 1.3 Ma or 4567.7 ± 1.3 Ma,anchored to LEW86010 and U-corrected D'Orbigny respectively,so these results strongly support that the CO chondrules are as old as Ca-AI-rich inclusions?CAls?.2.Ornans chondrules show a large variability of ?⁵⁴Cr value,ranging from +0.20 to +1.22 and correlating with their ?⁵³Cr values.This Cr isotope evidence may reflect a"volatility process" or a "mixing process",and the former may suggest chondrule precursors come from a single reservoir,same,while the latter assumes that materials from inner Solar System were transported to the accretion region of CO chondrites and mix with the matrix-like precursors when chondrules formed.3.According to the ⁵³Mn-⁵³Cr chronology and heterogeneous ?⁵⁴Cr of Ornans CO chondrules,"X-wind" and "impact jetting" model among all the astrophysical models about formation of chondrules are supported.