锆石-吴元宝.ppt

小锆石 大故事 吴元保 中国地质大学(武汉) Electrovalence and Ion radii Zr4+ = 0.84? U4+ = 1.05? Th4+ = 1.10? Pb2+ = 1.29? Zircon has high U content and high U/Pb ratios, and thus is ideally suited for high precision geochronology Basic of radiogenic isotope dating(3) Lu-Hf同位素示踪 Hf同位素研究中的有关公式 εHf(0) = ((176Hf/177Hf)S/(176Hf/177Hf)CHUR, 0-1)×10000 εHf(t) = ((176Hf/177Hf)S-(176Lu/177Hf)S×(eλt-1))/((176Hf/177Hf)CHUR,0-(176Lu/177Hf)CHUR×(eλt-1))-1)×10000 TDM1 = 1/λ×(1+((176Hf/177Hf)S-(176Hf/177Hf)DM)/((176Lu/177Hf)S-(176Lu/177Hf)DM)) TDM2 = TDM1 - (TDM1 - t)((fcc - fs)/(fcc - fDM)) fLu/Hf = (176Lu/177Hf)S/(176Lu/177Hf)CHUR-1 其中，(176Lu/177Hf)S和(176Hf/177Hf)S为样品测定值，(176Lu/177Hf)CHUR = 0.0332, (176Hf/177Hf)CHUR,0 = 0.282772; (176Lu/177Hf)DM = 0.0384, (176Hf/177Hf)DM = 0.28325. fcc, fs, fDM 分别为大陆地壳、样品和亏损地幔的fLu/Hf。 t为样品形成时间，λ = 1.867×10-11 year-1。 锆石Hf同位素示踪 It has abundant Hf (0.5-1.5%) and very low Lu/Hf ratios (usually 0.0005); Its age can be determined directly from U-Pb isotope system; It is resistant to Lu/Hf disturbance; Disturbances, if have occurred, can be monitored by U-Pb systematics; Some geological processes can only be traced by zircon 矿物之间氧同位素分馏 定义：氧同位素以不同的比值分配到两种物质或两相中 表达方式： 分馏系数： 分馏值和分馏系数的关系： 岩石与水之间的氧同位素分馏 水是主要的流体组成 高温条件下使岩石的δ18O值降低，低温条件下则使岩石的δ18O值升高 以石英为例： ΔQz-H2O = δ18OQz - δ18OH2O = 3.38×106/T2 – 3.89 地球早期信息 Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago (Wilde et al., 2001; Mojzsis et al., 2001) Early record of crustal evolution The Early evolution of the Earth 确定地壳的生长历史 Magmatic and Crustal Differentiation History of Granitic Rocks from Hf-O Isotopes in Zircon (Kemp et al., 2007) Tracing magma mixing in granite genesis (Yang et al., 2007) Schematic temperature (T)–time (Δt) path for a hypothetical high-T terrain formed in the deeper parts of a hot orogen (Harley et al., 2007). Hf and O isotope evidence for a transition in the dynamics of continental growth 3.2Gyr ago