The Behavior Of Mg Isotopes During Low-temperature Water-rock Interactions Processes

Magnesium is a fluid-mobile, major element in both the mantle and the crust, and has three isotopes (24Mg,25Mg,26Mg) with relative mass difference of～8%between24Mg and26Mg, which can potentially lead to large mass-dependent Mg isotope fractionation. This makes Mg isotopes have several advantages to serve as promising geochemical tracers. As new geochemical tracers, Mg isotopes have attracted more and more attentions from international geologists. At present, the studies on Mg isotope geochemistry are focusing on estimating the Mg isotopic compositions of the major geochemical resvervoirs and exploring the behavior of Mg isotopes during the different geological processes. However, due to the lack of high-precision Mg isotope data and the systematic studies, the behavior of Mg isotopes during the low temperature water-rock interaction processes, and the degree of influence of these processes on Mg isotopic compositions of continental crust and oceanic crust are still unclear. In order to provide a theoretical basis for better understanding of Mg geochemical cycle and the application of Mg isotopes as geochemical tracers for the geological processes, this thesis mainly focuses on the behavior of Mg isotopes during the major low temperature gaochmical processes, such as continental weathering, oceanic crust alteration and loess deposit formation. Consequently, detailed case studies are presented from basalt weathering profile in Hainan Island, alterated oceanic crust in the ODP Site801, and loess samples from the major loess deposits in the world, considering mineralogical abundance, major and trace element concentration, and isotopic composition, and the controls on the behaviors of Mg isotopes. The main knowledges based on these studies are following:(1) Magnesium isotopic compositions of a set of clay-rich saprolites developed on the Neogene tholeiitic basalt from Hainan island in southern China have been measured in order to document the behavior of Mg isotopes during continental weathering. Compared with unaltered basalts (δ26Mg=-0.36%o), the overlying saprolites are strongly depleted in Mg (i.e., τTh,Mg=-99.1to-92.9), and display highly variable δ26Mg, ranging from-0.49%o to+0.40‰. Magnesium concentration and δ26Mg value of the saprolites display a general increasing trend upwards in the lower part of the profile, but a decreasing trend towards the surface in the upper part. The variations of Mg concentration and isotopic composition in this weathering profile can be explained through adsorption and desorption processes,(a) adsorption of Mg to kaolin minerals (kaolinite and halloysite), with preferential uptake of heavy Mg isotopes onto kaolin minerals; and (b) desorption of Mg through cation exchange of Mg with the relatively lower hydration energy cations in the upper profile. Evidence for adsorption is supported by the positive correlation between δ26Mg and the modal abundance of kaolin minerals in saprolite of the lower profile, while negative correlations between δ26Mg and concentrations of lower hydration energy cations (e.g., Sr and Cs) in the upper profile support the desorption process. Our results highlight that adsorption and desorption of Mg on clay minerals play an important role in the behavior of Mg isotopes during extreme weathering, which may help to explain the large variation in Mg isotopic composition of river waters. Compilation of Mg isotopic data of weathered products reveals that behavior of Mg isotopes during continental weathering can be considered as a two-stage process,(a) limited Mg isotope fractionation during primary mineral dissolution in the incipient stage of weathering and (b) large Mg isotope fractionation during secondary mineral formation in the advanced stage of weathering.(2) Loess deposits serve as important continental archives for studying Quaternary climatic variations and for estimating the average chemical composition of the upper continental crust. Here, we report high-precision Mg isotopic data for nineteen loess samples from China, Argentina and Europe, which were previously used to estimate the composition of the upper continental crust. The results show that global loess have heterogeneous Mg isotopic compositions, with δ26Mg ranging from-1.64%o to+0.25‰and a weighted average of-0.89%o, which is lighter than both crust and mantle silicates. MgO content and826Mg of loess positively correlate with CaO/Al2O3ratio, which can be explained by a two-component mixing between carbonates and secondary silicate minerals. The large variation in Mg isotopic composition of loess results from the variations in the relative proportions of carbonates to clays in loess. Three factors:(a) source heterogeneity,(b) eolian sorting during transport and (c) chemical weathering after deposition, play important roles in controlling the distribution of carbonates and clays in loess. Both processes of eolian sorting and chemical weathering during formation of loess deposit tend to enrich clays in loess and in turn drive the Mg isotopic composition of loess toward heavier values. Significant correlations between δ26Mg values of loess from the same loss deposit and climatic indices such as CIA and SiO2/TiO2molar ratio indicate that Mg isotopic composition of loess may provide insights into paleoclimatic changes. However, our results suggest that Mg isotopic composition of loess may not represent the average Mg isotopic composition of the upper continental crust due to mixing and/or sorting of isotopically distinct components and isotope fractionation during loess deposit formation.(3) The altered oceanic crust (AOC) records the alteration processes and reveals much information about the global geochemical cycles of many elements such as Mg, which has high abundance in the solid Earth and ocean. The AOC therefore plays a central role in the global Mg cycle; however, the Mg isotope geochemistry of the AOC remains poorly constrained. The Mg isotopic composition of the altered oceanic crust from the ODP Site801in the western Pacific have been measured in order to investigate the behaviors of Mg isotopes during low-temperature interaction of seawater with oceanic basalt, and to estimate the flux of Mg isotopes of the oceanic crust that is recycled at subduction zones for the first time. Our results show that the AOC are depleted in Mg compared to fresh mid-ocean ridge basalt (MORB), and the change in MgO content (AMgO) decreases with depth. In addition, these AOC samples display highly variable δ26Mg, ranging from-2.82%o to+0.19%o. The SUPER composite, which represents the bulk upper oceanic basement sections of the ODP Site801, has a heavier826Mg value (=-0.02%o) relative to the bulk silicate earth or fresh MORB (δ26Mg=-0.25±0.08%o) previously reported by Teng et al (2010a). Composite samples prepared from volcanoclastics (VCL,δ26Mg=-1.01to0.14‰) and contained more Mg-rich secondary minerals have larger variations in826Mg compared to composite samples prepared from basaltic flows (FLO,δ26Mg=-0.54to-0.02%o), and on average is heavier than the later one. All these findings suggest that Mg isotopes have significantly fractionated during during low-temperature interaction of seawater with oceanic crust. Moreover, δ26Mg values of the AOCs positively correlated with their MgO contents. This correlationship can be explained by a three-component mixing among the fresh MORB, secondary carbonates and secondary silicate minerals, reflecting that Mg isotopic composition of the AOC is controlled by the relative proportion of primary and secondary Mg-rich mineral. The distribution of these secondary minerals in the AOC mainly results from combination of the water-rock ratio, temperature of alteration, and seawater oxidation during alteration of seawater with oceanic crust. For examples, the degree of alteration increases with the wate-rock ratio, and in turn produces higher abundance of the secondary carbonates in the AOC and lighter826Mg values; additionally, the AOC contains more secondary silicate minerals such as saponite and pyrite, and in turn has heavier826Mg value when the reaction environment trends to deoxidation; furthermore, the temperature of alteration decreases will induces higher abundance of carbonate and lighter Mg isotopic composition of the AOC. Based on the date from this study, the flux of Mg input from low-temperature alteration of oceanic crust is estimated about2×1010mol/yr, suggesting that this process might be the main factor driving the Mg concentration and Mg/Ca raio of seawater increasing over the past100million years. More importantly, the process of the AOC with highly heterogenous Mg isotopic composition along with the marine sediment recycled into mantle through subduction may lead to Mg isotopic composition of the mantle with heterogeneity.