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Elemental exchange during seafloor alteration exerts substantial control on the chemical compositions of seawater and oceanic crust, as well as the global elemental cycling. However, detailed processes and mechanisms for element transport and isotopic exchange remain unclear. Here, we report Li and K isotopic compositions across an altered MORB-like pillow basalt from Mariana Trench. We find large and systematic isotopic variations on a centimeter scale, with δ7Li decreasing from +9.07‰ to +6.21‰ and δ41K increasing from −0.12‰ to +0.33‰ from the rim to the core. The heavier-than-mantle Li and K isotopic compositions in the core likely inherit from an enriched mantle source rather than post-magmatic alteration processes. The low K and high Li isotopic compositions at the outmost rims result from adsorption during seawater-rock interactions. The basin-shaped elemental and isotopic variations were subsequently formed by chemical diffusion and spontaneous seawater penetration from the weathered rind to the interior of pillow basalt sample. This study uses a small-scale sampling approach and provides the first evidence for multi-mode chemical exchange in oceanic basalts and improves our understanding of the seafloor alteration processes. [ABSTRACT FROM AUTHOR]

Abstract: To explore the possibility of tracing recycled carbonate by using Mg isotopes and to evaluate the effects of the western Pacific oceanic subduction on the upper mantle evolution of the North China craton, Mg isotopic compositions of the Mesozoic–Cenozoic basalts and basaltic andesites from the craton have been investigated. The samples studied here come from a broad area in the craton with variable ages of 125–6Ma, and can be divided into two groups based on geochemical features: the >120Ma Yixian basalts and basaltic andesites, and the <110Ma Fuxin and Taihang basalts. Our results indicate that these two groups have distinct Mg isotopic compositions. The >120Ma Yixian basalts and basaltic andesites, with low Ce/Pb, Nb/U ratios and lower-crust like Sr-Nd-Pb isotopic compositions, have a mantle-like Mg isotopic composition, with δ26Mg values ranging from −0.31‰ to −0.25‰ and an average of −0.27±0.05‰ (2SD, n =5). This suggests that continental crust contamination may strongly modify their Ce/Pb, Nb/U ratios and Sr-Nd-Pb isotopic compositions but do not influence their Mg isotopic compositions. By contrast, the <110Ma basalts from Fuxin and Taihang exhibit lower δ26Mg values of −0.60‰ to −0.42‰, with an average of −0.46±0.10‰ (2SD). Since these basalts still preserve mantle-like Sr-Nd-Pb isotopic compositions and Ce/Pb, Nb/U ratios and have high U/Pb and Th/Pb ratios similar to those of HIMU basalts, the light Mg isotopic composition is most likely derived from interaction of their mantle source with isotopically light recycled carbonate melt. Since the Tethys and Mongolia oceanic subductions from south and north toward the North China craton were terminated in the Triassic and light Mg isotopic signature in basalts did not appear before 120Ma, the subducted Pacific oceanic crust could be the major source of the recycled carbonate. Therefore, this study not only presents an example to trace recycled carbonate using Mg isotopes but also confirms the important role of the western Pacific oceanic subduction in generating the <110Ma basalts in the North China craton. [Copyright &y& Elsevier]

We report a new method for precise analysis of gallium (Ga) isotopic composition in geological samples. The purification of Ga is achieved by a three-step ion exchange chromatography to remove matrix and interfering elements. The 71 Ga/ 69 Ga ratios are analyzed by multi-collector inductively-coupled-plasma mass-spectrometer (MC-ICP-MS). The external reproducibility of the measurements (0.05‰, 2 s.d.) was assessed by replicate analyses of the USGS BCR-2 and BHVO-2 standards. This newly developed technique was then used to investigate the extent of Ga isotopic fractionation during igneous processes by analyzing well-characterized samples from the Kilauea Iki lava lake, USA. These samples were formed in a closed system and have MgO contents ranging from 26.9 to 2.4 wt.%. We found that igneous processes do not fractionate Ga isotopes within the analytical uncertainty and that the Ga isotopic composition of mafic-ultramafic lavas can be used to estimate the composition of their mantle source. Twelve ocean island basalts, two mid-ocean-ridge basalts, one continental flood basalt and one komatiite have homogeneous and nearly identical Ga isotopic compositions within analytical uncertainties averaging 0.00 ± 0.06‰ (2 s.d.). This value represents the best estimate for the Ga isotopic composition of the bulk silicate Earth [ABSTRACT FROM AUTHOR]