The evolution of Earth's surficial Mg cycle over the past 2 billion years.

The surficial cycling of Mg is coupled with the global carbon cycle, a predominant control of Earth's climate. However, how Earth's surficial Mg cycle evolved with time has been elusive. Magnesium isotope signatures of seawater (δ26Mgsw) track the surficial Mg cycle, which could provide crucial information on the carbon cycle in Earth's history. Here, we present a reconstruction of δ26Mgsw evolution over the past 2 billion years using marine halite fluid inclusions and sedimentary dolostones. The data show that δ26Mgsw decreased, with fluctuations, by about 1.4‰ from the Paleoproterozoic to the present time. Mass balance calculations based on this δ26Mgsw record reveal a long-term decline in net dolostone burial (NDB) over the past 2 billion years, due to the decrease in dolomitization in the oceans and the increase in dolostone weathering on the continents. This underlines a previously underappreciated connection between the weathering-burial cycle of dolostone and the Earth's climate on geologic timescales.

Significantly Enhanced Robustness of K Isotope Analysis by Collision Cell MC-ICP-MS and Its Application to the Returned Lunar Samples by China's Chang'e-5 Project.

Stable K isotope ratios, an emerging research tool for a wide range of problems, can be measured precisely with high sensitivity by using collision cell multicollector ICP mass spectrometers (CC-MC-ICP-MS). However, it has been shown that the accuracy of K isotope analysis by CC-MC-ICP-MS could be compromised severely by trace-level Ca contaminants, although the cause of such an effect remains poorly understood. Here, we report that the influence of Ca on K isotope analysis by CC-MC-ICP-MS can be dramatically reduced if D2 rather than H2 (the default gas) is used as the reaction gas that goes into the collision cell. This indicates the generation of positively charged calcium-hydride molecules in the collision cell. Usage of D2 as reaction gas circumvents the Ca-induced inaccuracy issues during K isotope analysis because 40CaD+ does not interfere with 41K+ as 40CaH+ does; as such, the robustness of K isotope analysis by CC-MC-ICP-MS is significantly enhanced. This improved method is verified by K isotope analysis of seven geostandards, and applied to China's Chang'e-5 lunar return samples at submicrogram K consumption, revealing significant K isotope variability within a 17 mg lunar basalt fragment.

Precise measurement of 41 K/39 K ratios by high-resolution multicollector inductively coupled plasma mass spectrometry under a dry and hot plasma setting.

RATIONALE: Stable K isotope geochemistry is becoming an important tool for various applications. Developments in analytical methods for K isotopes based on multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) without collision cell will bring research capability of K isotopes to many existing MC-ICP-MS labs. METHODS: Stable K isotopes were analyzed without applications of "cold plasma" and collision cell on a Nu 1700 Sapphire high-resolution multicollector inductively coupled plasma mass spectrometer. A conventional dry and hot plasma setting is used for analysis to maintain high K sensitivity and signal stability, and high mass resolution was applied to provide interference-free shoulders of 39 K+ for isotopic measurement of 41 K/39 K ratios. 40 Ar+ ion beam generated in ICP was neutralized in the ion guide rail for the Daly detector. RESULTS: Under such operating conditions, an external reproducibility of <±0.1‰ (2 standard deviation) for 41 K/39 K is achieved for K solutions of 1 ppm or above. Tests were carried out to evaluate the influence of total K loading, K concentration and acid molarity mismatch, matrix effects, and 40 Ar+ and 40 Ar1 H+ tailing on K isotope analysis. We found that the accuracy of K isotope analysis can be compromised by concentration mismatch of sample and standard K, by 0.007‰ in δ41 K per 1% mismatch of K content. By contrast, mismatch of HNO3 molarity or existence of HCl in HNO3 exerts negligible influences on the analytical precision and accuracy of K isotope analysis. Furthermore, K isotope analytical results remain accurate when Na/K, Mg/K, Ca/K, Rb/K, V/K, and Cr/K ratios are below 3%. CONCLUSIONS: The high-precision K isotope analytical method reported here is robust for studies on K isotopic variations in geological, cosmochemical, and biological samples. The f41 K values of six international geostandards measured using our method are consistent with data measured using different analytical methods from other laboratories.