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Abstract The widespread Cenozoic alkaline magmatism within and around the Tibetan Plateau offers a prime opportunity to probe the nature of the mantle at the depths where basalt magmas originate. The close temporal and spatial relationship between volcanism and regional strike‐slip fault systems also helps better understand the geodynamics of outward growth of the plateau in response to the continued India‐Asia convergence. We present a comprehensive study of the deeply sourced alkaline basalts formed along the Kunlun strike‐slip fault with the aim of understanding their petrogenesis and the composition of mantle sources beneath the northeastern Tibetan Plateau. High Nb/U and Ce/Pb ratios and relatively depleted bulk‐rock Sr‐Nd‐Pb isotope compositions corroborate the mantle origin of these alkaline basalts. Homogeneous and low 87Sr/86Sr of clinopyroxene indicates negligible crustal contamination during magmatic evolution. Low δ26Mg in the alkaline basalts and positive correlations with Hf/Sm and Ti/Ti* indicate that the basalts were derived from mantle that was metasomatized by melts derived from sedimentary carbonates during the Paleo‐Tethyan seafloor subduction. Based on 40Ar/39Ar dating results, it appears that the alkaline basaltic magmatism in the northeastern Tibetan Plateau occurred simultaneously with Kunlun strike‐slip faulting. These observations suggest that the India‐Asia convergence must have reactivated ancient subduction plate boundaries and resulted in strike‐slip faulting along these suture zones within and around the Tibetan Plateau. The eruption of low‐volume and deeply rooted alkaline basalts may have been controlled by fractures associated with the strike‐slip fault systems.

Abstract Investigating the carbonate preservation efficiency (CPE) of continental crust is crucial to understand the global carbon cycle, which requires constraints on initial carbonate abundances (ICAs) of crustal rocks. To link Mg isotopes to ICAs, we present elemental and Mg isotopic data for Himalayan carbonate‐bearing and carbonate‐free metasedimentary rocks. Given no evident melt extraction or external‐fluid infiltration, ICAs of these samples can be independently estimated by elemental data. Despite different carbonate species in the protoliths, all the samples show congruent relationship between their δ26Mg and ICAs, owing to the elevated carbonate δ26Mg and Mg/Ca in protoliths of calcite‐rich samples resulting from diagenetic processes. When collated with literature data, we suggest the observed correlation here can be applied to most carbonate‐bearing (meta‐)sedimentary rocks. Based on a steady state box‐model, we constrained the modern net carbonate accretion flux (9.50−5.56+9.50 Tmol/year) and the average time‐integrated CPE (∼80−43+20%) for continental crust.

Abstract It is debated whether there was strong climate seasonality during the Eocene, which provides a close geological analogy for near‐future scenarios of greenhouse gas emissions. Lithological data suggest the existence of a broad arid zone centered around 30°N paleo‐latitude, while a humid climate was supported by palaeobotanic assemblages in East Asia. Here, we report the occurrence of massive primary lacustrine dolomite and magnesite in the central East Asia during the middle Eocene. We provide a novel perspective from magnesium isotopes to link the formation of Mg‐carbonates to seasonal dry‐wet cycles. Rapid magnesium input during the rainy season and intense evaporation in the dry season likely caused the formation of magnesium carbonates in an enclosed lake. These findings provide insights into hydroclimatic seasonality during the Eocene, contributing to our understanding of the hydrological cycle response to a greenhouse climate.

Abstract Magnesium isotopic compositions of evaporite deposits may record information concerning brine evolution during deposition. We report Mg isotopic values (δ26MgDSM3) measured from an evaporite deposit of langbeinite (K2Mg2(SO4)3) found in the Permian Salado Formation. We used these data to model Mg isotope fractionation between langbeinite and its parent brine. In addition, both measured and theoretical results are used to estimate precipitation temperature and interpret depositional environment. The Salado langbeinite δ26Mg values are relatively low and fall within a relatively narrow range (–4.12 ± 0.03‰ to −3.81 ± 0.07‰). Equilibrium fractionation factors between langbeinite and aqueous Mg2+ solutions were calculated using quantum chemical density functional theory. All computations were performed at the B3LYP/6‐31 + G(d,p) level. Solvation effects were addressed using a solvent model (“water‐droplet” approach) and mineral structures were investigated using volume variable cluster models (VVCM). The equilibrium Mg isotopic fractionation factors α between langbeinite and model brine solution we obtained are 1.0005, 1.0004, and 1.0003 (Δ26Mglangb‐water≈103lnα = 0.473‰, 0.390‰, and 0.322‰) at 10°C, 25°C, and 40°C, respectively. These relatively large equilibrium fractionation factors indicate significant Mg isotope fractionation between langbeinite and its parent brine during precipitation, as langbeinite preferentially incorporates the heavier 26Mg and 25Mg isotopes. Rayleigh distillation modeling of the Salado langbeinite's relatively light Mg isotopic composition requires δ26MgDSM3 values of −4‰ for the parent brine. Models favor a precipitation temperature as high as 40°C under equilibrium conditions. Potential disequilibrium precipitation conditions suggested by Mg isotopic data also imply rapid deposition in a hot, arid sedimentary environment prevailing in the southwestern U.S. during the Late Permian.