Wang-Ye Li

Limited magnesium isotope fractionation during metamorphic dehydration in metapelites from the Onawa contact aureole, Maine

Knowledge on the behavior of Mg isotopes during metamorphic dehydration is the prerequisite for applying Mg isotopes as tracers for crustal recycling. Here we report Mg isotopic compositions of metapelites from the Onawa contact aureole, Maine. Except one sample, all metapelites across the aureole, from the wall-rock regional metamorphic rocks to the partially melted rocks adjacent to the pluton, have similar Mg isotopic compositions (δ26Mg = −0.09 to +0.12‰). This observation indicates limited Mg isotope fractionation during metamorphic dehydration and fluid-rock interaction, due to the low Mg concentration in fluids relative to rocks. Our results suggest that Mg isotopic compositions of metapelites can record those of their protoliths and, hence, recycled clastic sedimentary materials may preserve their low-temperature Mg isotopic signatures through subduction zones. Therefore, Mg isotopes may serve as new tracers for crustal recycling, for example, tracing components experienced weathering cycles within granite sources.

Magnesium isotopic composition of the deep continental crust

Abstract To constrain the behavior of Mg isotopes during deep crustal processes and the Mg isotopic composition of the middle and lower continental crust, 30 composite samples from high-grade metamorphic terranee and 18 granulite xenoliths were investigated. The composites derive from eight different high-grade metamorphic terranee ill the two largest Archean cratons of China, including 13 TTG gneisses, 5 amphibolites, 4 felsic, 4 intermediate, and 4 mafic granulites. They have variable bulk compositions with SiO2 ranging from 45.7 to 72.5%, representative of the middle crust beneath eastern China. The δ26Mg values of these samples vary from −0.40 to +0.12‰, reflecting heterogeneity of their protoliths, which could involve upper crustal sediments. The granulite xenoliths from the Cenozoic Hannuoba basalts also have a diversity of compositions with MgO ranging from 2.95 to 20.2%. These xenoliths equilibrated under high temperatures of 800–950 °C, corresponding to depths of the lower continental crust (>30 km). They yield a large δ26Mg variation of −0.76 to −0.24‰. The light Mg isotopic compositions likely result from interactions with isotopically light metamorphic fluids, probably carbonate fluids. Together with previously reported data, the average δ26Mg values of the middle and lower continental crusts are estimated to be −0.21 ±0.07‰ and −0.26 ±0.06‰, respectively. The bulk continental crust is estimated to have an average δ26Mg of −0.24 ±0.07‰, which is similar to the average of the mantle. The large Mg isotopic variation in the continental crust reflects the combination of several processes, such as continental weathering, involvement of supracrustal materials in the deep crust, and fluid metasomatism.

Interlaboratory comparison of magnesium isotopic compositions of 12 felsic to ultramafic igneous rock standards analyzed by MC-ICPMS

To evaluate the interlaboratory mass bias for high-precision stable Mg isotopic analysis of natural materials, a suite of silicate standards ranging in composition from felsic to ultramafic were analyzed in five laboratories by using three types of multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Magnesium isotopic compositions from all labs are in agreement for most rocks within quoted uncertainties but are significantly (up to 0.3‰ in 26Mg/24Mg, >4 times of uncertainties) different for some mafic samples. The interlaboratory mass bias does not correlate with matrix element/Mg ratios, and the mechanism for producing it is uncertain but very likely arises from column chemistry. Our results suggest that standards with different matrices are needed to calibrate the efficiency of column chemistry and caution should be taken when dealing with samples with complicated matrices. Well-calibrated standards with matrix elements matching samples should be used to reduce the interlaboratory mass bias.

Fluid/melt in continental deep subduction zones: Compositions and related geochemical fractionations

Plate subduction is the most magnificent process in the Earth. Subduction zones are important sites for proceeding matter- and energy- transports between the Earth’s surface and the interior, continental crust growth, and crust-mantle interactions. Besides, a number of geological processes in subduction zones are closely related to human beings’ daily life, such as volcanic eruptions and earthquakes, formation of mineral deposits. Subduction process thus has long been the centric topic of Earth sciences. The finding in 1980s that continental crust could be subducted to mantle depths is a revolutionary progress in plate tectonic theory. Compared to oceanic crust, continental crust is colder, drier, lighter, and much more geochemically/isotopically heterogeneous Hence, continental subduction process would affect the structure, compositions and evolutions of the overlying mantle wedge even more. During continental subduction and subsequent exhumation, fluids and melts can be generated in the (de)hydration process and partial melting process, respectively. These melts/fluids play important roles in crust-mantle interactions, elemental migrations, isotopic fractionations, and mantle metasomatism. By summarizing recent research works on subduction zones in this paper, we present a review on the types, physicochemical conditions and compositions of fluids/melts, as well as the migration behaviors of fluid-related characteristic elements (Nb-Ta-V) and the fractionation behaviors of non-traditional stable isotopes (Li-Mg) in subduction zones. The aim of this paper is to provide the readers an update comprehensive overview of the melt/fluid activities in subduction zones and of Li-Mg isotope systematics in subduction-related rocks and minerals.