Homogeneous and heavy potassium isotopic composition of global oceans

The oceans are an important interface for chemical exchange between the continents,the seafloor,and the atmosphere.Chemical weathering of continental silicate rocks removes atmospheric CO2 and releases rock-forming cations to the oceans through surface runoff.Secular changes in seawater chemistry thus serve as archives of long-term climate variation.

Chromite-induced magnesium isotope fractionation during mafic magma differentiation

To better understand the mechanism of Mg isotopic variation in magma systems, here we report high precision Mg isotopic data of 17 bulk rock samples including dunite, clinopyroxenite, hornblendite and gabbro and 10 pairs of dunite-hosted olivine and chromite separates from the well-characterized Alaskan-type Xiadong intrusion in NW China, which formed by continuous and high degree of lithological differentiation from mafic magmas. Chromite separates have highly variable δ^(26)Mg values from -0.10‰ to 0.40‰, and are consistently heavier than coexisting olivine separates(-0.39‰ to -0.15 T‰). Both mineral δ^(26)Mg values and the degrees of inter-mineral fractionation are well correlated with geochemical indicators of magma differentiation, indicating that these inter-sample and inter-mineral Mg isotope fractionations are caused by magma evolution. The δ^(26)Mg values range from -0.20‰ to -0.02‰ in the dunite,-043‰ in the clinopyroxenite,-043‰ to -0.28‰ in the hornblendite, 0.18 T‰ in the chromite-bearing hornblendite, and -0.56 T‰ to -0.16‰ in the gabbro. The Mg isotopic variations in different types of rocks are closely related to fractional crystallization and accumulation of different proportions of oxides vs. silicates. Chromite crystallization and accumulation is the most important factor in controlling Mg isotope fractionation during the formation of the Xiadong intrusion. Compared to basaltic and granitic magmas, differentiation of the Alaskan-type intrusions occurs at a relatively high oxygen fugacity, which favors chromite crystallization and consequently significant Mg isotope fractionations at both mineral and whole-rock scales. Therefore, Mg isotope systematics can be used to trace the degree of magma differentiation and related-mineralization.

High-precision potassium isotope analysis using the Nu Sapphire collision cell(CC)-MC-ICP-MS

This study presents high-precision analyses of stable potassium(K)isotope ratio using the recently-developed,collision-cell multi-collector inductively coupled plasma mass spectrometry(CC-MC-ICP-MS,Nu Sapphire).The accuracy of our analyses is confirmed by measuring well-characterized geostandards(including rocks and seawater).Our results are consistent with literature values and a precision of 0.04‰(2SD)has been achieved based on multiple measurements of BCR-2 geostandard over a six-month period.We also evaluate factors that may lead to artificial isotope fractionations,including the mismatches in K concentration and acid molarity between samples and bracketing standards,as well as potential matrices.As the K adsorption capacity of AGW50-X8(200-400 mesh)is reduced with an increasing amount of matrix elements,less than 150µg K was loaded during the column chemistry.To evaluate the potential use of K isotopes as an archive of paleo seawater composition,δ^(41)K values of an international seawater standard(IAPSO),a Mn-nodule(NOD-P-1),and two iron formation standards(FeR-2 and FeR-4)are reported.The δ^(41)K value of IAPSO is consistent with other seawater samples reported previously,further substantiating a homogeneous K isotopic distribution in modern global oceans.The K isotopes in Mn-nodule(NOD-P-1:−0.121±0.013‰)and iron formation samples(FeR-2:−0.538±0.009‰;FeR-4:−0.401±0.008‰)seem to be an effective tracer of their formation genesis and compositional changes of ancient seawater.Our results suggest that high-precision measurements of stable K isotopes can be routinely obtained and open up a large variety of geological applications,such as continental weathering,hydrothermal circulation and alteration of oceanic crust.