Application Of Melt Inclusions And Li Isotope In Earth Sciences

The recent and meteoric advance of analytical technics have lead to a booming development in melt inclusions and Li isotope studies in the international earth science research field over the past decades. However, techniques for analysis of single melt inclusions by LA-ICP-MS and high-precision measurement of Li isotope using MC-ICP-MS are currently scarce in China, which have impeded the development of melt inclusions and Li isotope studies and their applications in earth science. This doctoral dissertation focuses on the imperative techniques for melt inclusions and Li isotope analysis, and the application of melt inclusions and Li isotope as geochemical tracers for solving geological problems. The present results may help the expansion of these two promising research fields in the geological society.The first research subject in this dissertation is to establish a method using LA-ICP-MS to analyze chemical compositions of single melt inclusions. With the application of this method we studied the melt inclusions in the Fangcheng basalt and xenoliths therein to investigate the magma evolution of the basalt and melt-peridotite interaction in the lithospheric mantle beneath the North China Craton.Primary silicate melt inclusions were trapped in clinopyroxene and orthopyroxene phenocrysts in the Fangcheng basalt. Three types of melt inclusions (silicate, carbonate, and sulfide) coexisting with fluid inclusions occur in clinopyroxene xenocrysts and clinopyroxene in clinopyroxenite xenoliths. In situ laser-ablation ICP-MS analyses of major and trace element compositions on individual melt inclusions suggest that the silicate melt inclusions in the clinopyroxene and orthopyroxene phenocrysts were trapped from the same basaltic magma. The decoupling of major and trace elements in the melt inclusions indicates that the magma evolution was controlled by melt crystallization and contamination from entrapped ultramafic xenoliths. Trace element patterns of melt inclusions are similar to those of the average crust of North China Craton and Yangtze Craton, suggesting a considerable crustal contribution to the magma source. Calculated parental melt of the Fangcheng basalt has features of low MgO (5.96wt%), high Al2O3(16.81wt%), Sr (1670ppm), Y (-35ppm), and high Sr/Y (>40), implying that subducted crustal material was involved in the genesis of the Fangcheng basalt. The coexisting fluid-and melt-inclusions in clinopyroxene xenocrysts and in the clinopyroxene of xenoliths record a rare melt-peridotite reaction, that is olivine+carbonatitic melt1(rich in Ca)=clinopyroxene+melt2±CO2. The produced melt2is enriched in LREE and CO2and may fertilize the mantle significantly, which we consider to be the cause for the rapid replacement of lithospheric mantle during the Mesozoic in the region.The second research subject is to develop an approach for high-precision measurement of Li isotope using MC-ICP-MS, and investigate the Li isotopic compositions of the Jingshan post-collisional leucogranite from Bengbu and the Permian-Triassic Meishan section from Zhejiang. The results provided unique information on the petrogenesis of igneous rocks in the Jingshan area and global continental weathering rate at Permian-Triassic boundary.Major and trace element concentrations in a profile across a diabase dike to leucogranite show systematic variations, with fluid immobile elements (e.g., Ti, V, Nb, Zr) remaining constant, but fluid mobile elements (e.g., Li, Rb, Cs, Ba, Pb) and δ7Li values (+4.1%o to+2.2%o) exhibiting decreasing trends in the leucogranites toward to the diabase dike. All elements and Li isotopes (+2.0%o to+2.5%o), however, vary only slightly within the diabase dike. Such an observation, together with the increasing of loss on ignition (LOI) in the leucogranites near the diabase dike, indicate that fluid induced alteration of leucogranite occurred before the diabase dike intrusion, which led to light Li isotopic signature in the altered leucogranite.Lithium isotopic compositions of the diabase dikes (-+2.2%o) are lower than mantle rocks, implying a light Li input in the melting source. Modeling result demonstrates that the diabase dikes may correspond to melting of a mantle source hybridized by recycled regional lower crust. Their high Li concentrations (67.2-108.6ppm), high Li/Y, Ba/Nb ratios and low Rb/Cs ratios indicate involvement of a Li rich fluid phase during partial melting. The Jingshan leucogranite samples have high87Li values (+4.O‰to+9.0%o) with low Li concentrations (4.7-11.3ppm) compared to the worldwide granites. However, the residuary enclaves have very high Li concentrations (up to118.2ppm) and low δ7Li values (down to+0.8%o). Trace element modelling (REE and Sr/Y) and Li isotope mixing calculations are conducted to understand the petrogenesis of Jingshan leucogranite. The Li isotopic and elemental signatures of Jingshan leucogranite can be best explained as a result of crustal incongruent partial melting:bt+qtz+plag=grt+melt, leaving the Li isotopic composition light, but Li-rich garnet as residue. This explains the origin of high87Li signature in the Jingshan leucogranite and provides a quantitative model for its melting source. Both Li concentrations and δ7Li show large variations across the Meishan P-T section. Samples from Beds22-24show strong positively correlated Li-Al2O3, Li-Fe, Li-B and Sr-Pb concentrations, which can simply interpreted to result from mixing between chert and carbonate. Samples from Beds26-30define mixing trends between clays and carbonate+chert in Li-Al2O3, Li-Fe, Li-B, Sr-Pb,δ7Li-Al2O3and1/Li-δ7Li diagrams. Bed25, with high Kaolin proportion (thus have high Al2O3, B and low Ti, Na, Fe, Co concentrations), is obvious different with other samples. Mixing calculations and seawater δ7Li estimations suggest that the end-Permian seawater exhibits very light isotopic compositions (mostly+10‰to+16‰), about15%o lower than the present day value. Steady state and dynamic box models are presented for constraining the cause of low seawater δ7Li, the modeling results indicate that the observed low seawater δ7Li requires an enhanced riverine Li flux accompanied with very light isotopic compositions, and thus indicate enhanced global weathering rate and weathering intensity at PTB. The modeling results, the enormous Siberian flood basalts exposed to the Earth’s surface at PTB, together with the acid rains and hothouse climate, indicating that the extremely enhanced chemical weathering rate at PTB is likely to be a result of hot acid rain weathering of fresh erupt basalts.