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We apply the stable potassium isotope system (41K/39K) to well-studied Mariana arc lavas, in which inter-island geochemical variability has been interpreted to reflect near constant addition of an aqueous fluid flux, that dominantly samples the subducting mafic oceanic crust and variable amounts of sediment melt addition to the sub-arc mantle wedge. The nature of the sediment component in the Mariana arc lavas remains enigmatic, with a mixture of melts from all subducted sediment lithologies and the altered-mafic oceanic crust seeming likely, but some interpretations point towards a component from melting of the volcaniclastic sediment alone. We present K isotopic data on a set of well-characterised Mariana arc lavas and sediment samples (from Ocean Drilling Program site 801). Our data show that the majority of Mariana arc lavas are isotopically heavy (ẟ 41K), by up to ∼0.2 ‰, relative to mid-ocean ridge basalt (MORB), but most of the subducting sediment samples are slightly isotopically lighter than, or indistinguishable from MORB. The volumetrically important volcaniclastic sedimentary unit however is significantly isotopically lighter than MORB, by ∼0.8 ‰, which reflects marine diagenetic processes. Thus, volcaniclastic material significantly influences the bulk sediment K isotope composition. We show that ẟ 41K compositions of the Mariana arc lavas can be reproduced by the addition of an aqueous fluid with isotopically heavy K (relative to MORB) and an additional fraction of an incompatible-element-enriched isotopically light melt component. Modelling of K isotopes together with K/La and radiogenic Nd indicate that the melt component is best explained by a mix of melts from bulk sediment and altered-mafic oceanic crust. Our results show that distinctive K isotopic variations in subduction zone inputs and K isotopic fractionation during dehydration reactions makes K a useful tracer of subduction zone processes. [ABSTRACT FROM AUTHOR]

The alkaline, H 2 -rich vent fluids of the Lost City Hydrothermal Field (LCHF) have generally been attributed to serpentinization of tectonically uplifted mantle peridotite. However, elevated concentrations of highly incompatible and fluid mobile trace alkali metals, Rb and Cs, indicate reaction with relatively unaltered mafic components (Seyfried et al., 2015). Here, we present high-precision stable-K isotope analyses of LCHF vent fluids, which provide new constraints on source-rock lithology and support more quantitative estimates of fluid:rock ratio, providing a new constraint on heat and mass transfer processes. We find that δ41K values of LCHF vent fluids (0.03–0.07 ‰) are distinct from local seawater (0.13 ± 0.02 ‰). In combination with near-seawater concentrations of K (10.4 ± 0.1 mmol/kg), these data are incompatible with hydrothermal alteration of peridotite alone, which is highly depleted in K. Instead, K-isotope and concentration data are consistent with hydrothermal alteration of mafic rocks (e.g., gabbro or diabase) at a fluid:rock ratio of 6–26, a range that agrees well with Rb, Cs, and 87Sr/86Sr-based estimates of fluid:rock ratio, updated here to reflect derivation from mafic source rocks. Geochemical modeling of hydrothermal fluid-rock reactions indicates that LCHF vent fluids can be effectively reproduced by a two-stage reaction in which seawater initially reacts at 200–300 °C with mafic rocks at in a fluid:rock ratio of ∼ 10 —as constrained by the above isotopic and trace-element systematics— and subsequently reacts with peridotite at a fluid:rock ratio of ∼ 30 at roughly the same temperature, pressure conditions. We also note that LCHF vent fluids are ∼ 2 % depleted in Cl compared to seawater, which may reflect admixture of a vapor component derived from ongoing phase separation at higher (> 450 °C) temperatures. We thus propose that the LCHF represents waning-stage hydrothermal activity initiated by a discrete off-axis mafic intrusion that has subsequently cooled to intermediate temperatures conducive to olivine hydrolysis and production of alkaline vent fluids. In this interpretation, alkaline H 2 -rich vent fields occupy a narrow thermal window in the expected evolution of hydrothermal activity associated with episodic off-axis mafic intrusions into tectonically exposed ultramafic host rocks and are thus likely to be rare on the modern seafloor. [ABSTRACT FROM AUTHOR]

Potassium (K) isotopic compositions for a suite of crustal rocks (plutonic unit, sheeted dike complex, and volcanic sequence) in the Troodos ophiolite, Cyprus, were determined to investigate fluid-transport processes during subduction inception. The gabbros and plagiogranites in the plutonic unit have restricted K isotopic compositions (0.01–0.20 ‰ and −0.17 to 0.02 ‰, respectively), whereas diabases in the sheeted dike complex have variable δ41K values from −0.24 to 1.13 ‰. The δ41K values of the volcanic sequence range from −0.26 to −0.07 ‰ in the lower pillow lavas, and from −0.71 to 0.34 ‰ in the upper pillow lavas. These δ41K values are clearly distinct from those of subducting sediments and altered oceanic crust, ruling out direct crustal input as the cause of the unusually heavy K isotopic compositions. Rather, the high K/Th, K/U, and Sr/Th ratios of the investigated samples, together with binary mixing modelling, reveal that the anomalously heavy K isotopic compositions in Troodos rocks are attributed to the influx of fluids during subduction inception. Notably, the δ41K values in the crustal rocks of the Troodos ophiolite are more variable and extend to higher values than those of arc-front and rear-arc lavas of the Izu arc and arc lavas of Martinique in the Lesser Antilles, indicating continuous K isotope fractionation during slab dehydration. Thus, the heavy K isotopic compositions of the Troodos ophiolite reflect K isotope fractionation during dehydration of a shallow slab within a newly formed subduction system, where heavy K isotopes preferentially enter into the aqueous fluids. Later, progressive breakdown of K-bearing phases at deeper levels results in isotopically lighter hydrous melts, which can be delivered to the arc. [ABSTRACT FROM AUTHOR]

Background: More than 700 million people worldwide suffer from diseases of the pancreas, such as diabetes, pancreatitis and pancreatic cancer. Often dysregulation of potassium (K+) channels, co-transporters and pumps can promote development and progression of many types of these diseases. The role of K+ transport system in pancreatic cell homeostasis and disease development remains largely unexplored. Potassium isotope analysis (δ41K), however, might have the potential to detect minute changes in metabolic processes relevant for pancreatic diseases. Methods: We assessed urinary K isotope composition in a case-control study by measuring K concentrations and d41K in spot urines collected from patients diagnosed with pancreatic cancer (n=18), other pancreas-related diseases (n=14) and compared those data to healthy controls (n=16). Results: Our results show that urinary K+ levels for patients with diseased pancreas (benign and pancreatic cancer) are significantly lower than the healthy controls. For δ41K, the values tend to be higher for individuals with pancreatic cancer (mean δ41K = -0.58 ± 0.33‰) than for healthy individuals (mean δ41K= -0.78 ± 0.19‰) but the difference is not significant (p=0.08). For diabetics, urinary K+ levels are significantly lower (p=0.03) and δ41K is significantly higher (p=0.009) than for the healthy controls. These results suggest that urinary K+ levels and K isotopes can help identify K disturbances related to diabetes, an associated factors of all-cause mortality for diabetics. Conclusion: Although the K isotope results should be considered exploratory and hypothesis-generating and future studies should focus on larger sample size and δ41K analysis of other K-disrupting diseases (e.g., chronic kidney disease), our data hold great promise for K isotopes as disease marker. [ABSTRACT FROM AUTHOR]

Hydrothermal alteration is crucial in the formation of many ore deposits, with potassium (K) mobilization and cycling being prevalent. Potassic metasomatism of wall rocks generally forms K-bearing minerals, such as hydrothermal feldspar and mica. However, determining the source and redistribution of K (and other elements transported by the same fluid) in hydrothermal systems is challenging. K isotopes offer a potential solution to this problem. This study presents new K isotope data from two K-rich alteration assemblages — K-feldspar and sericite-quartz-pyrite — in the Jiaodong gold province of China. The data covers a compositional range from unaltered granites to syn -magmatic potassic alteration (formation of K-feldspar) and post-magmatic syn -mineralization phyllic alteration (formation of sericite). Potassic alteration in granite correlates with significant K addition, whereas phyllic alteration of earlier phases of magmatic and hydrothermal K-feldspar resulted in K loss. K-feldspar altered granites display similar δ41K values (–0.55 to –0.42 ‰ for whole-rocks and –0.56 to –0.48 ‰ for K-feldspar separates) as unaltered granite (–0.52 to –0.47 ‰). The narrow δ41K range suggests that magmatic fluid exsolution and magmatic-hydrothermal alteration have a minor effect on δ41K of the altered rock. Phyllic alteration of K-feldspar altered precursor rock leads to K loss and elevated δ41K values ranging from –0.36 to –0.19 ‰ for whole-rocks and –0.34 to –0.17 ‰ for sericite mineral separates. As sericite preferentially incorporates 41K, sericite will have higher δ41K values than the precursor K-feldspar, whereas the fluids will have lower δ41K values. Our study demonstrates that hydrothermal alteration may affect the K isotope composition of altered rocks in several ways, contingent on the nature of the involved phases, making K isotopes a promising tool for studying hydrothermal alteration and associated mineralization. [ABSTRACT FROM AUTHOR]

The genesis of peralkaline granites has fundamental implications for understanding their source characters and tectonic settings, yet it remains highly controversial after decades of research. Two major challenges in studies of peralkaline granites are how the peralkaline magmas evolve and how to trace their magmatic evolution. Here we use K isotopes in granites from Zhoushan archipelago of southeast China to explore these two issues. The Zhoushan archipelago hosts two groups of Cretaceous high-silica granites, which are the peralkaline rocks (PAG) and the calc-alkaline rocks (CAG). The CAG rocks exhibit a small variation (δ41K = −0.54 ± 0.06 ‰ to −0.36 ± 0.06 ‰) in K isotopic compositions, in contrast to the larger variation for the PAG rocks (δ41K = −0.67 ± 0.07 ‰ to −0.27 ± 0.05 ‰). The PAG rocks show strong correlations between δ41K values and various whole-rock major elements (i.e., Al 2 O 3 , Na 2 O, and K 2 O contents), but these correlations are absent for the CAG rocks. We propose that the separation of Na-rich alkali-feldspar, rather than low-Na alkali-feldspar and/or plagioclase, controlled the K isotope variability in PAG. The alkali-feldspar in PAG is enriched in Na, among which the alkali-feldspar with Or (orthoclase; mol. %) <15 is characterized by high δ41K ranging from −0.34 ‰ to 0.36 ‰. Geochemical modeling demonstrates that 23–33 % fractional crystallization of such Na-rich alkali-feldspar is required to generate the geochemical trends observed in PAG. Those findings suggest that crystallization of Na-rich alkali-feldspars dominates the magma evolution for peralkaline magmatism, supporting strong fractional crystallization in high-silica peralkaline magma. K isotopes show promise to trace detailed magmatic processes and provide constraints on the generation of peralkaline granites. [ABSTRACT FROM AUTHOR]

Potassium isotopes are a novel tracer for continental weathering. Previous K isotope studies on chemical weathering generally targeted weathering profiles under a particular climate region, yet the effects of chemical weathering on K isotopes under different climatic backgrounds remain unclear. Moreover, little is known about the K isotope signatures of modern unconsolidated detrital sediments. Here, we report K isotopic data of surficial seafloor sediments from continental shelves along the east coast of China (ECC), as well as those around the tropical Hainan island in the northern South China Sea. The ECC sediments have a relatively narrow distribution of δ41K (with reference to NIST3141a) values, which range from (−0.40±0.01)‰ to (−0.57±0.04)‰, with an average of (−0.51±0.09)‰. By contrast, δ41K values of Hainan offshore sediments display a larger variation, ranging from (−0.28±0.07)‰ to (−0.67±0.02)‰. The δ41K values of Hainan offshore sediments exhibit negative correlations with the chemical index of alteration (CIA), Al/K, Ti/K, and total iron (FeT), which underlines the control of chemical weathering on K isotopic signatures of detritus inputs into oceans. We also measured Mg isotope compositions for the same samples; interestingly, the variability in δ26Mg of the samples is small (~0.24‰) for all ECC and Hainan offshore sediments, and δ26Mg values do not show clear correlations with indexes of chemical weathering. Our study demonstrates the link between K isotopic variability of detrital sediments and climatic conditions including rainfall intensity, which indicates that K isotopes of the detrital component of marine sediments could be applied to study Earth's climate in deep time. The δ41K values of the offshore detrital sediments are significantly less variable than those of pelagic marine sediments, highlighting the importance of distinguishing the effects of diagenesis and neoformation of clay minerals from continental weathering in attempts to study deep-time climate-weathering link by K isotopes in detrital sedimentary records. [ABSTRACT FROM AUTHOR]

BackgroundMore than 700 million people worldwide suffer from diseases of the pancreas, such as diabetes, pancreatitis and pancreatic cancer. Often dysregulation of potassium (K+) channels, co-transporters and pumps can promote development and progression of many types of these diseases. The role of K+ transport system in pancreatic cell homeostasis and disease development remains largely unexplored. Potassium isotope analysis (δ41K), however, might have the potential to detect minute changes in metabolic processes relevant for pancreatic diseases.MethodsWe assessed urinary K isotope composition in a case-control study by measuring K concentrations and δ41K in spot urines collected from patients diagnosed with pancreatic cancer (n=18), other pancreas-related diseases (n=14) and compared those data to healthy controls (n=16). ResultsOur results show that urinary K+ levels for patients with diseased pancreas (benign and pancreatic cancer) are significantly lower than the healthy controls. For δ41K, the values tend to be higher for individuals with pancreatic cancer (mean δ41K = -0.58 ± 0.33‰) than for healthy individuals (mean δ41K = -0.78 ± 0.19‰) but the difference is not significant (p=0.08). For diabetics, urinary K+ levels are significantly lower (p=0.03) and δ41K is significantly higher (p=0.009) than for the healthy controls. These results suggest that urinary K+ levels and K isotopes can help identify K disturbances related to diabetes, an associated factors of all-cause mortality for diabetics.ConclusionAlthough the K isotope results should be considered exploratory and hypothesis-generating and future studies should focus on larger sample size and δ41K analysis of other K-disrupting diseases (e.g., chronic kidney disease), our data hold great promise for K isotopes as disease marker.

We report high-precision K isotopes, apatite U–Pb ages, whole-rock elements and Sr–Nd isotopes for the Saima nephelite syenite in the North China Craton. Trace-element and Sr–Nd–Hf–O isotope data indicate the presence of subducting sediments in the source region, while K isotopic compositions show a narrow range between –0.54 ‰ and –0.28 ‰, with an average of –0.41 ± 0.06 ‰, identical to the value of the asthenosphere. The nearly identical K isotopic compositions are low probability events compared with the K isotopic compositions of island arc lavas reported previously (–1.55 ‰ to +0.2 ‰). Although crustal contamination is consistent with the Sr–Nd–K isotopic data, alternatively we propose that the isotopic data also reconcile with the interaction between cratonic roots and the underlying convective asthenosphere, if this interaction is over prolonged periods of time. Numerical simulations successfully reproduced the observed data, if the metasomatism of the lithospheric mantle root, the source of the Saima alkaline rocks, occurred 500 Ma ago. Our study reveals that the isotopic compositions of fast-diffusion components in a lithospheric mantle metasomatized by ancient subducting melts can be effectively homogenized by convective asthenosphere through diffusion over a long time interval. [ABSTRACT FROM AUTHOR]

To constrain how impacts influence the behavior of moderately volatile elements (MVEs), we report potassium (K) and zinc (Zn) contents and isotopic compositions of shock melt pockets (SMPs) and unmelted parts of three heavily shocked ordinary chondrites and bulk rocks of Chelyabinsk meteorite. All SMPs are enriched in K content and have lower isotopic values (δ41K = −1.99‰, −1.22‰ and −1.40‰) while the adjacent unmelted parts are enriched in heavy K isotopes (δ41K = −0.41‰, −0.01‰ and 0.04‰) compared to the bulk rocks of Chelyabinsk meteorite (δ41K = −0.77‰ and −0.73‰). By contrast, Zn is depleted in SMPs and the isotopic compositions are heavier (δ66Zn = −0.19‰, 2.42‰, 1.74‰) in SMPs than that in unmelted parts (δ66Zn = −0.65‰, 1.76‰, −0.97‰). Our results indicate a decoupling between the two MVEs that Zn is lost from shock melts while K is dramatically enriched in shock melts during impacts. The isotope fractionation of Zn is probably caused by evaporation of shock melts, while K isotope fractionation is most likely caused by solid-melt diffusion which is controlled by its incompatibility. The isotopic decoupling of K from Zn during major impacts further enhances our understanding of high temperature elemental and isotopic behavior of MVEs and may shed new light on the variously heterogeneous distribution of MVEs in solar system. [ABSTRACT FROM AUTHOR]

To constrain the behavior of K isotopes during granitic magmatic differentiation, we present high-precision K isotope data for bulk granitoids and their K-bearing minerals from Dabie and Himalayan orogens in China. Plagioclase displays extremely large K isotopic variation, with δ41K ranging from −0.56 to 1.91‰. Other minerals also have heterogeneous K isotopic compositions, with δ41K varying from −0.56 to −0.24‰ in hornblende, from −0.72 to −0.39‰ in biotite, from −0.79 to −0.47‰ in K-feldspar, and from −0.74 to −0.61‰ in muscovite. The general trend of 41K enrichment follows the order of plagioclase ≫ hornblende > biotite ≈ K-feldspar ≥ muscovite, which can be attributed to the difference in K O bond strengths. Both δ41K of plagioclase and K isotope fractionation factors between plagioclase and other minerals are negatively correlated with K concentration in plagioclase, suggesting a compositional control on isotope fractionations. Inter-mineral K isotope fractionations among other minerals are roughly temperature-dependent. Potassium isotopic compositions of whole rocks display measurable variations (δ41K = –0.69 to −0.28‰), which are correlated with proxies for plagioclase fractionation (e.g., Sr and Rb/Sr). The correlation indicates that fractional crystallization may account for the whole-rock K isotopic variations observed in high-silica igneous rocks, which is supported by modeling of fractional crystallization using the apparent isotope fractionation factors determined in this work. Our study demonstrates the existence of large K isotope fractionation during granitic magmatism, making K isotopes a potential tool for studying differentiation of felsic magmas. [ABSTRACT FROM AUTHOR]

Potassium (K)-containing fertilizers are widely applied in agriculture to improve plant growth, yet fertilizer overuse induce pollution of waters, calling for methods to trace the sources of K. K isotope compositions in ecosystems could potentially indicate the source of K, yet there is actually few isotope data on fertilizer K. Here we report stable K isotope data for various fertilizer types from different countries, and we compare these values with K isotope data for plants, soil, mantle, ocean, and rivers. K isotope compositions were determined by multi-collector inductively coupled plasma mass spectrometry. δ41K values of K2SO4 fertilizers ranged from − 0.16 to 0.51‰, those of KH2PO4 fertilizers from 0.04 to 0.19‰, those of NPK fertilizers from 0.04 to 0.22‰, and those of NK fertilizers from 0.16 to 0.23‰. There is a considerable variation in the K isotope compositions by fertilizer types but limited variation by countries, suggesting these data could be reference materials worldwide. Comparison with other K sources shows that, in some cases, K isotope ratios and concentrations could be used to identify the input of fertilizers in ecosystems. [ABSTRACT FROM AUTHOR]

Meteorites record processes that occurred before and during the formation of the Solar System in the form of nucleosynthetic anomalies: isotopic compositions that differ from the Solar System patterns. Nucleosynthetic anomalies are rarely seen in volatile elements such as potassium at bulk meteorite scale. We measured potassium isotope ratios in 32 meteorites and identified nucleosynthetic anomalies in the isotope potassium-40. The anomalies are larger and more variable in carbonaceous chondrite (CC) meteorites than in noncarbonaceous (NC) meteorites, indicating that CCs inherited more material produced in supernova nucleosynthesis. The potassium-40 anomaly of Earth is close to that of the NCs, implying that EarthÕs potassium was mostly delivered by NCs. [ABSTRACT FROM AUTHOR]

High-temperature hydrothermal alteration of oceanic crust is one of the two main sources of potassium (K) to the oceans, with modern flux estimates ranging roughly from ∼8 to 30 % of the K flux from rivers. Despite the role of high-temperature hydrothermal fluids in the global seawater K budget, little is known about its effect on the K isotopic composition of the oceans. Here we present stable K isotope measurements (δ41K SRM3141a) of globally distributed high-temperature hydrothermal fluids from three mid-ocean ridge systems: the East Pacific Rise (n = 21), the Juan de Fuca Ridge (n = 34), and the Mid-Atlantic Ridge (n = 18). We find a strong correlation between δ41K and Mg/K ratios, consistent with conservative mixing between a high-temperature hydrothermal fluid (i.e., Mg = 0) and seawater as fluids ascend to the seafloor and/or due to seawater entrainment during sampling. The δ41K of end-member hydrothermal fluids is found to range between −0.80 ‰ and 0.07 ‰, with an average value of −0.36 ± 0.30 ‰ (2σ, n = 38). Most (∼76 %) of the variability in end-member fluid δ41K compositions observed here can be explained by high-temperature fluid-rock K exchange, with little (∼0.2 ‰) to no K isotope fractionation between hydrothermal fluid and altered crust. Larger deviations from the average end-member hydrothermal fluid value are likely to result from processes other than high-temperature fluid-rock exchange, such as (1) low-temperature hydrothermal reactions during fluid recharge, (2) reaction of fluids with local sedimentary sources, and (3) phase separation. The K contents of end-member fluids vary considerably, from ∼1 to 38 mM, thus a K-weighted average of −0.37 ± 0.24 ‰ (2σ, n = 38) is estimated to represent the δ41K composition of the global hydrothermal K flux. Our results suggest that K sourced from axial hydrothermal alteration does not contribute to the elevated 41K/39K of seawater compared to bulk silicate Earth (BSE). In addition, subduction of oceanic crust altered under high-temperature conditions is unlikely to be a significant source of K isotopic heterogeneity to Earth's mantle. [ABSTRACT FROM AUTHOR]

• Arc lavas from the circum-Pacific subduction zones show large variations in δ41K. • Sedimentary input to the mantle source could produce arc lavas with lower δ41K. • AOC-derived fluids contribute to the formation of arc lavas with heavy k isotopes. • Fluids derived from deserpentinizing slab mantle involve the generation of adakite. • K isotopes have the potential to trace the origins of different types of arc lavas. Arc lavas display significant chemical and isotopic heterogeneity mainly due to recycled materials from subducting slabs. However, the extent to which different types of subducted sediments and oceanic crust contribute to the petrogenesis of arc magmas, as well as the roles of the mantle wedge and overlying crust, remain debated. Potassium (K) isotopes have the potential to provide new insights into the processes and sources of arc magmatism because sediments and altered oceanic crust are highly enriched in K and have distinct δ41K values compared with the mid-ocean ridge basalts and upper mantle (-0.42 ± 0.08‰, 2SD). Here we report K isotopic compositions of 32 well-characterized arc lavas from the circum-Pacific margins. We find low δ41K values (-0.86‰ ∼ -0.38‰) in the Setouchi arc samples, which we interpret as the result of incorporation of isotopically light sediments into the subarc mantle. The Kurile and Panama arc lavas have high δ41K values (-0.36‰ ∼ 0.02‰) and their δ41K values correlate positively with the Ba/Th ratios, indicating ∼0.5–2% fluid additions from dehydrated altered crust. Adakites have variable but overall heavy K isotope compositions (-0.44‰ ∼ -0.01‰) , which correlate with the diagnostic trace-element indicators of Sr/Y, La/Yb and K/U. These results indicate limited K isotope fractionation during metamorphic dehydration of hot slab subduction, and/or fluid metasomatism before the slab melting. Our study suggests that fluids and melts originating from subducting sediments and slabs exerted distinct influences on the origins of different types of arc magmas, demonstrating the significance of K isotopes in studying the petrogenesis of arc magmas. [ABSTRACT FROM AUTHOR]

[Display omitted] • The El-Shadli plagiogranites are characterized by a wide range of δ 41K values (–0.31‰ ± 0.06‰ to 0.36‰ ± 0.05‰; 2 SE) • The plagiogranites were produced by partial melting of hydrothermally-altered mid-to-lower oceanic crust. • K isotopes are powerful source tracers for granitoids. Constraining the processes associated with the formation of new (juvenile) continental crust from mantle-derived (basaltic) sources is key to understanding the origin and evolution of Earth's landmasses. Here we present high-precision measurements of stable isotopes of potassium (K) from Earth's most voluminous plagiogranites, exposed near El-Shadli in the Eastern Desert of Egypt. These plagiogranites exhibit a wide range of δ 41K values (–0.31‰ ± 0.06‰ to 0.36‰ ± 0.05‰; 2 SE, standard error) that are significantly higher (isotopically heavier) than mantle values (–0.42‰ ± 0.08‰). Isotopic (87Sr/86Sr and 143Nd/144Nd) and trace element data indicate that the large variation in δ 41K was inherited from the basaltic source rocks of the El-Shadli plagiogranites, consistent with an origin through partial melting of hydrothermally-altered mid-to-lower oceanic crust. These data demonstrate that K isotopes have the potential to better constrain the source of granitoid rocks and thus the secular evolution of the continental crust. [ABSTRACT FROM AUTHOR]

Characterizing the K-isotope compositions of marine sediments is a necessary step to using this novel isotope system to investigate K cycling both in the ocean system and in subduction zones. Here we report data for 70+ samples in order to characterize the K-isotope compositions of deep-sea sediments for two drill sites in the western Pacific: (1) IODP Site C0011 adjacent to the Nankai Trough and Southwest Japan arc and (2) Site 1149 adjacent to the Izu-Bonin trench and arc. Variation in δ 41 K at both sites is mostly subtle, reflecting a consistently strong contribution of K from sources that have δ 41 K similar to the upper continental crust (UCC). Yet, in detail, the mean δ 41 K at both sites is about 0.05–0.10 ‰ lower than a recently proposed best estimate for UCC ( δ 41 K = −0.44 ‰). We infer that the slightly low mean δ 41 K values relative to UCC are likely caused by secondary processes acting on the volcanogenic component of the sediments, though chemical weathering of (non-volcanogenic) terrigenous materials may also play a role. We measured a very low δ 41 K value (−1.33 ± 0.04 ‰) for one discrete ash layer at Site C0011, so volcanoclastic materials may lower the δ 41 K values of the bulk sediments, despite only accounting for a small fraction of the K in the sediments. Correlations between δ 41 K , Cr/Al, and K/Al in the upper sediments of Site 1149 were used to investigate mixing between volcanic ash and terrigenous materials. This approach indicates that mixing of isotopically light volcanic ash (with δ 41 K about −0.75 ‰) and terrigenous materials (about −0.46 ‰) can account for the observed variation in δ 41 K values in these sediments. In addition, we observe that at Site C0011 K isotopes exhibit a shift (by ~0.08 ‰) at ~250 m below the seafloor from UCC-like compositions in the upper sediments to slightly lighter compositions in the lower sediments. This shift in δ 41 K coincides with an abrupt change in certain physical properties (porosity and cementation) of the sediments that may relate to the extent of weathering/degradation of the dispersed volcaniclastics, with more extensively weathered materials present in the lower sediments. Finally, we find evidence for variable admixture of a high δ 41 K silicic volcanic ash in the lowermost sediments at Site C0011, supporting the view that heterogeneous volcaniclastics can give rise to significant δ 41 K variations in marine sediments. [ABSTRACT FROM AUTHOR]

Plants play a critical role in the surficial environment, influencing energy transfer, the global carbon cycle, and climate change. Potassium (K) controls many essential plant physiological features like photosynthesis, cell osmosis adjustment, and enzyme activity. However, few plant species of K isotopes have been investigated and knowledge regarding the mechanism of isotopic fractionations in biological processes is still limited. This study investigated foliar K isotopic compositions of C3 and C4 vegetation and soil in the typical karst region of Guizhou Province, Southwest China. The results showed that: (a) The δ41K of soil ranged from −0.04‰ to −0.08‰, with no K isotope ratios change between topsoil and subsoil. (b) Foliar δ41K of C3 vegetation ranged from −1.06 ± 0.06‰ to −0.15 ± 0.06‰ and foliar δ41K of C4 vegetation varied from 0.28 ± 0.05‰ to 1.15 ± 0.06‰. (c) Foliar δ41K was negatively related to foliar K concentration (R2 = 0.78; p < 0.01); The significant K isotope differences between C3 and C4 vegetation are likely attributed to their different mechanisms for drought stress or their different foliar K concentration. This study firstly documented the potential of K isotopes as a novel tool for biological processes to distinguish between C3 and C4 plants and also shed light on the potential future application of K isotope signatures in soil‐plant interactions. Plain Language Summary: Potassium (K) is an essential nutrient that controls many ecological processes like photosynthesis, cell osmosis adjustment, and enzyme activity. Isotope methods are beneficial for tracing sources, biological processes, and nutrient cycling. However, due to limited instrumental measurements, little is known about the K isotopes in plants. We identified significant K isotope differences occurring in leaves of C3 and C4 vegetation, and a good linear relationship between foliar K isotope and its concentration. The findings presented in this study broaden the knowledge of K isotopes in plants, suggesting a potential tracer for biological processes. Key Points: Significant potassium isotope differences exist between leaves of certain C3 and C4 plantsA good linear relationship emerges between foliar δ41K and foliar K concentration [ABSTRACT FROM AUTHOR]

Potassium isotopes may provide a novel approach for fingerprinting recycled sediments in the mantle due to the significant differences in K abundance and isotopic ratio between subducting sediment and the mantle. However, the behavior of K isotopes in sediments during subduction zone metamorphism is still unknown. Here we investigate K isotopic composition of a set of well-characterized high- to ultrahigh-pressure metasediments from the Schistes Lustrés nappe (western Alps), which represents marine sediments subducted down to ~90 km depth in a cold subduction zone, and their protoliths from the Lavagna nappe (Apennines, Italy). The metasediments display δ41KSRM 3141a values from –0.76‰ to –0.48‰, which are on average lower than the mantle value (–0.43‰) but similar to those of non-metamorphic equivalents (–0.79‰ to –0.49‰). No systemic variation of δ41K with metamorphic grade is observed, suggesting negligible K isotope fractionation in these sediments during prograde metamorphism. This is in accord with the limited loss of K during the entire metamorphic history as evidenced by the constancy of K/Rb and K/Cs ratios between metamorphic and non-metamorphic sediments and the absence of correlations of δ41K with K/Rb and K/Cs. The heterogeneous δ41K values of metasediments are most likely inherited from their protoliths, which experienced different degrees of chemical weathering depending on their provenances. Our results demonstrate that the variable and light K isotopic signatures in subducting sediments could be preserved to depths of at least 90 km along a cold geotherm gradient, indicating that the introduction of sediments into the mantle could produce K isotope heterogeneity in the source regions of mantle-derived lavas. [ABSTRACT FROM AUTHOR]

This work presents the systematic investigation of K contents and stable K isotopic compositions of surface waters, groundwater, wastewaters, suspended particles, bed sediments, and fertilizers in the Upper Mun Rive catchment, northeast Thailand. This area is of particular interest because its abundant potash deposits and intensive agricultural activities and urbanization, therefore this will improve our understanding whether the K isotope is robust enough to resist evaporite dissolution and anthropogenic disturbances on tracing silicate weathering in the highly weathered tropical region. The dissolved loads in surface waters and shallow groundwater display the large variation in δ41K diss values from −0.54‰ to +0.09‰ relative to suspended particles (−0.60‰ to −0.41‰), river bed sediments (−0.54‰ to −0.47‰), the upper continental crust (UCC, −0.44 ± 0.05‰), which agrees with the fact that 39K is preferentially retained in weathering products during silicate weathering. The wet and dry seasons difference ranging from −0.05‰ to +0.10‰ δ41K, which is slightly greater than our long-term analytical uncertainty of ±0.06‰. There is likely a limited K input from evaporites dissolution due to weak correlation between δ41K diss values and Cl concentrations or Cl/K ratios in the UMR and world rivers, while a major tributary Takhong River is clearly influenced by the contribution of domestic sewage with lower δ41K and higher δ15N values. Based on a mass balance of K budge, the dissolved K in the UMR is mostly (>90%) derived from silicate weathering in the unpolluted area, this is consistent with previous studies of large river basins, and the evaporite dissolution and other non-silicate sources may not strongly influence dissolved K and δ41K diss values in rivers, even in evaporite-rich catchment. Therefore, K isotopes cannot be used as lithological tracers in catchments, rather the K isotopic fractionation mainly occurs during silicate weathering, such as the formation of secondary minerals which favor light isotope. Hence, the mechanism of K isotopic fractionation linked to secondary minerals K uptake needs further consideration. Overall, these results clearly show that K isotopes could be an ideal indicator to constrain silicate weathering processes and anthropogenic inputs at catchment scale. [ABSTRACT FROM AUTHOR]

Potassium elemental and isotope systematics were investigated for a suite of central European tektites from three strewn sub-fields in Czech Republic and possible parent sedimentary materials from the vicinity of the Ries impact structure in SE Germany, supplemented by data for several other impact-related materials (bediasites, Ivory Coast tektites, Libyan Desert Glass). This is paralleled by computation of potential K loss and attendant isotope fractionation for physico–chemical conditions typical for formation of tektite precursor melts. These theoretical calculations indicate a <0.1% loss of K from tektite precursor melts up to 2,500 K and <0.002‰ change in the 41K/39K ratio even for a small sphere of 0.002 m at 2,500 K, precluding any significant K loss and isotope fractionation. Numerical modelling also indicates that differential velocities between surrounding gas and liquid are not sufficient to remove the gaseous boundary layer, such that the partial pressure of potassium developed around the molten moldavite beads impedes further evaporation and also contributes to back-condensation of the already evaporated potassium. Central European tektites (moldavites) are enriched in K compared to the assumed sedimentary sources from the wider Ries area whereby the latter materials do not exceed 2.9 wt.% K 2 O compared to 2.5–4.1 wt.% K 2 O in moldavites. The apparent K enrichment in moldavites may be explained by a yet unaccounted process during formation of tektite precursor melts and/or unidentified source, such as volcanoclastic deposits that were produced by large Mid-Miocene volcanic centers in the Pannonian Basin. The K isotope compositions of tektites are more variable than those of sediments from the wider Ries area but they largely overlap (δ41K from −0.78 ± 0.03‰ to −0.13 ± 0.03‰ versus −0.72 ± 0.03‰ to −0.28 ± 0.02‰, respectively). These ranges mimic 41K/39K variations reported for igneous and sedimentary portions of the upper continental crust (δ41K roughly between −0.7 and −0.1‰). They show a slight difference among the three investigated strewn sub-fields, depending on their respective distance from the impact. In detail, moldavites from the closest strewn sub-field in the Cheb Basin show predominantly heavy K isotope compositions and those from the farthest strewn sub-field in Western Moravia are uniformly isotopically light. The origin of this difference may reflect lithological heterogeneity of the target area. Potassium contents in bediasites and Ivory Coast tektites range between 1.3 and 1.8 wt.% K 2 O and their corresponding δ41K values vary from −0.57 ± 0.02‰ to −0.41 ± 0.03‰. Both ranges are significantly narrower than those observed for moldavites. When compared to data for possible sedimentary precursors in the Chesapeake Bay and Bosumtwi impact structure, respectively, it is apparent that these tektites were neither depleted nor enriched in potassium. The extent of their K isotope fractionation relative to plausible sources remains unconstrained. The Libyan Desert Glass displays invariant δ41K of ~ −0.57 ± 0.06‰ at ≤0.01 wt.% K 2 O. Given the silica-rich nature of LDG and the lack of possible parent materials, no further constraints can be placed at present to further resolve the source material or reveal details of LDG formation process. [ABSTRACT FROM AUTHOR]

Stable potassium (K) isotopes are emerging as tracers of terrestrial recycling and planetary processes. These applications require well‐defined K isotopic compositions for the mantle and the bulk silicate Earth (BSE). Both values are determined primarily by basalts formed via partial melting of the mantle. Basaltic melts experience igneous differentiation before reaching the surface, which may alter their isotopic compositions compared to their mantle sources. This study investigates K isotope fractionation during the differentiation and solidification of the Kilauea Iki lava lake, Hawaii, for which crystallization and thermal histories are well documented. High‐precision K isotopic ratios (δ41K) are measured in 13 Kilauea Iki samples that cover its complete differentiation history, ranging from olivine‐rich, high‐MgO cumulates to increasingly differentiated, MgO‐depleted samples. The limited δ41K range of −0.42 to −0.37‰ in all but one sample reveals no analytically resolvable fractionation across diverse bulk compositions, even though their bulk MgO contents varied from 26.9 to 2.37 wt.%. The lack of K isotopic fractionation is consistent with an absence of K‐rich minerals in the crystallizing assemblage, where only plagioclase can accommodate a small amount of K. A highly differentiated vein displays the lowest δ41K of −0.47‰ among the Kilauea Iki suite, which is consistent with our modeling calculations that suggest measurable K isotope fractionation at more advanced magmatic differentiation stages. Combining our new results with literature data, we propose an average δ41K of −0.42 ± 0.08‰ (2SD) for the pristine mantle and of −0.42 ± 0.07‰ (2SD) for the BSE. Key Points: Limited potassium (K) isotope fractionation during basaltic differentiationMeasurable K isotope fractionation occurs at advanced stages of differentiationThe K isotopic composition of the mantle can be constrained by oceanic basalts [ABSTRACT FROM AUTHOR]

Arc magmas derived from mantle melting often have trace element and isotopic signatures that indicate crustal contributions. The origin and extent of crustal contributions are critical constraints for quantifying crust-mantle recycling at subduction zones; however, it is difficult to distinguish between inputs from the downgoing oceanic slabs and that of the over-riding arc crust. Here we present a novel study using stable potassium (K) isotopes to fingerprint the long-debated crustal signatures in lavas from Martinique Island, Lesser Antilles arc. We find δ41K of Martinique lavas varies from −0.66 to 0.01‰ and correlates with chemical parameters and radiogenic isotope ratios. These correlations cannot be produced by assimilation of arc crust during magma ascent. Rather, they reflect mantle sources that have been modified by the input of subducted sediments. Most lavas display a strong negative correlation between δ41K and radiogenic isotopic compositions, from typical mantle values of −0.37‰ to much lower values of −0.66‰, suggesting the addition of <1% to 5% subducted sediments to their mantle sources. Notable divergence in this correlation occurs for three samples with variably high δ41K (−0.28 to 0.01‰), which is likely caused by the addition of isotopically heavy K-bearing fluids derived from slab dehydration. Our study indicates that a substantial fraction of large ion lithophile elements, including K in arc magmas and thus nascent crust, is inherited from subducted crust; K isotopes are useful tracers of this cycling process. [ABSTRACT FROM AUTHOR]

Evaporation rates of K2O, Na2O, and FeO from chondrule-like liquids and the associated potassium isotopic fractionation of the evaporation residues were measured to help understand the processes and conditions that affected the chemical and isotopic compositions of olivine-rich Type IA and Type IIA chondrules from Semarkona. Both types of chondrules show evidence of having been significantly or totally molten. However, these chondrules do not have large or systematic potassium isotopic fractionation of the sort found in the laboratory evaporation experiments. The experimental results reported here provide new data regarding the evaporation kinetics of sodium and potassium from a chondrule-like melt and the potassium isotopic fractionation of evaporation residues run under various conditions ranging from high vacuum to pressures of one bar of H2+CO2, or H2, or helium. The lack of systematic isotopic fractionation of potassium in the Type IIA and Type IA chondrules compared with what is found in the vacuum and one-bar evaporation residues is interpreted as indicating that they evolved in a partially closed system where the residence time of the surrounding gas was sufficiently long for it to have become saturated in the evaporating species and for isotopic equilibration between the gas and the melt. A diffusion couple experiment juxtaposing chondrule-like melts with different potassium concentrations showed that the diffusivity of potassium is sufficiently fast at liquidus temperatures (DK>2-10-4cm2/s at 1650-C) that diffusion-limited evaporation cannot explain why, despite their having been molten, the Type IIA and Type IA chondrules show no systematic potassium isotopic fractionation.

The Moon is depleted in water and other volatiles compared to Earth and the bulk solar composition. Such depletion of volatile elements and the stable isotope fractionations of these elements can be used to better understand the origin and early differentiation history of the Moon. In this study, we focus on the moderately volatile element, potassium, and we report the K elemental abundances and isotopic compositions (δ41K relative to NIST SRM 3141a) for nineteen Apollo lunar rocks and lunar meteorites (twenty-two subsamples), spanning all major geochemical and petrologic types of lunar materials. The K isotopic compositions of low-Ti and high-Ti basalts are indistinguishable, providing a lunar basalt average δ41K of –0.07 ± 0.09‰ (2SD), which we also consider to be the best estimate of the lunar mantle and the bulk silicate Moon. The significant enrichment of K in its heavier isotopes in the bulk silicate Moon, compared with the bulk silicate Earth (δ41K = –0.48 ± 0.03‰), is consistent with previous analyses of K isotopes and other moderately volatile elements (e.g. , Cl, Cu, Zn, Ga, and Rb). We also report analyses of K isotopes for lunar nonmare samples, which show large variations of K isotopic ratios compared to lunar basalts. We interpret this large K isotopic fractionation as the result of late-stage magma ocean degassing during urKREEP formation, which is also coupled with Cl isotope fractionation. Degassing of urKREEP likely triggered redistribution of K isotopes in the Moon, enriching the urKREEP reservoir in heavy K isotopes while implanting the light K isotopic signatures onto the lunar surface. This scenario suggests a heterogeneous distribution of K isotopes in the Moon as a consequence of its magmatic evolution. [ABSTRACT FROM AUTHOR]

Assessing the K isotopic composition of the upper continental crust is important for understanding the processes by which the crust is generated and modified, and constraining the K isotopic budget of the silicate Earth. High-precision K isotopic data are reported for 70 well-characterized individual and composite samples from the upper continental crust, including diorite, granodiorite, granite, loess, shale, graywacke, pelite, and tillite, to constrain its K isotopic composition. δ41K varies significantly in eight I-type and two S-type granites from Australia (−0.57 to −0.40‰), nine A-type granites (−0.53 to −0.38‰), and three granitoid composites including diorite, granodiorite, and granite (−0.50 to −0.37‰) from China, mainly reflecting source heterogeneity. The 22 post-Archean Australian shales (PAAS) (δ41K = −0.68 to −0.12‰) and the 12 sedimentary composites including graywacke, pelite, and tillite from China (δ41K = −0.57 to −0.23‰) have heterogeneous K isotopic compositions while the 12 loess samples from around the world display more limited K isotopic variation (δ41K = −0.47 to −0.35‰). δ41K values display a smaller dispersion in loess compared to shales, which have comparatively more intense weathering and higher chemical index of alteration (CIA). δ41K correlates with Fe 2 O 3 /Al 2 O 3 and Fe 2 O 3 /K 2 O in shales and Al 2 O 3 /SiO 2 , K 2 O/Al 2 O 3 , and δ7Li in loess, suggesting that K isotopes are fractionated during chemical weathering. Overall, the upper continental crust has a heterogeneous K isotopic composition, ranging from −0.68 to −0.12‰ with an average δ41K of −0.44 ± 0.05‰ (2SD, n = 88), which is indistinguishable from the mantle. [ABSTRACT FROM AUTHOR]

Substantial quantities of oceanic slabs are known to be recycled into the mantle through subduction. However, tracing their fate in the mantle through studies of mantle-derived samples is not always straightforward. As K represents an omnipresent element in subducting crustal materials, its two stable isotopes (39K and 41K) display outstanding potential in tracing recycled crustal materials in the mantle. Here we report the first set of K isotopic data for a suite of well-characterized continental basaltic lavas from Northeast China. The δ41K of these lavas exhibit large variation from −0.81‰ to −0.15‰, compared with the narrow range of unaltered oceanic basalts (−0.43 ± 0.03‰). The isotopic variations, together with geochemical data obtained in earlier studies, cannot be ascribed to chemical weathering, crystal fractionation, crustal contamination, or partial melting, but are consistent with the presence of diverse crustal materials in magma sources. Recycled oceanic crust and sediments both have contributed to the contrasting K isotopic signature observed in these lavas. Our study here highlights the potential of using K isotopes to trace different types of recycled crustal materials in the mantle and places new constraints on the generation of enriched mantle reservoirs. [ABSTRACT FROM AUTHOR]

The local concentration of potassium ion in a single HeLa cell was observed with an ion-selective nano-pipette probe. Ion selectivity was achieved by using a polyvinyl chloride film with selected ionophores placed within the nano-pipette. Both alternating and constant bias voltages were applied to the counter electrode for the observation of local ion concentrations with a response time of less than 0.1 s. These measurements were enabled by a low-current detection system prepared specifically for this study. The difference in local potassium concentrations between inside a living HeLa cell and the surrounding solution was approximately 100 mM, while no difference in potassium ion concentration was observed between the interior of dead cells and the surrounding solution. [ABSTRACT FROM AUTHOR]

• K isotopes of groundwater with different ages are reported for the first time. • Extremely high δ41K (0.69‰) indicates intensive clay formation in modern groundwater. • Clay formation followed by long-time adsorption causes low K/Na in fossil groundwater. • K isotopes are a robust indicator of the degree of subsurface chemical weathering. Chemical weathering of rocks largely determines sources of groundwater hydrochemical components, but the superposition of processes removing cations leads to a wide variability of elemental ratios. In this study, potassium (K) isotopes are used for the first time to quantify the roles of silicate dissolution, clay incorporation and clay adsorption in subsurface K cycle. We investigate the behavior of K isotopes in groundwater with contrasting ages from a sandstone aquifer in the Ordos Basin, NW China. For modern groundwater (< 50 yrs) in the recharge area with abundant CO 2 , the silicate-derived K has been largely removed by 10-year-scale clay incorporation, generating low [K]/[Na] ranging between 0.026 and 0.048 and extremely high δ41K varying between 0.57‰ and 0.69‰; our calculations reveal that 92% of total K released from silicate dissolution has been removed by clay incorporation, suggesting that using measured [K] alone would underestimate the degree of silicate dissolution. For fossil groundwater (∼104 yrs) in the discharge area, clay adsorption during long-time circulation leads to extremely low [K]/[Na] ranging from 0.001 to 0.011 and much lower δ41K varying from -0.32‰ to 0.37‰; as a result of the combined effect of clay incorporation and clay adsorption, up to 99.2% of silicate-derived K has been removed from the dissolved load. This study provides novel insights into the complex processes controlling the K cycle in groundwater and highlights the potential of K isotopes in understanding subsurface weathering and associated elemental cycles. [Display omitted] [ABSTRACT FROM AUTHOR]

Rainwater dissolves urban air contaminants that threaten sustainable urbanization and human health. Tracing rainwater contamination supports urban management and environmental protection. Potassium isotopes (δ41K) have become new tracers in recent years, but K isotopes in urban rainwater and their application in tracing anthropogenic contamination have not yet been reported. Therefore, novel data on rainwater from Beijing, the capital of China, were presented in this study (δ41K: −1.16‰ to −0.17‰). Rainwater δ41K values showed a good nonlinear correlation with NO 3 -/K+ (R = 0.71; p < 0.05). Nitrogen (N) and K isotopes data were applied in three-end-member mixing model, and the three anthropogenic sources are biomass burning (δ15N: −1.3 ± 4.3‰, δ41K: −1.16 ± 0.06‰), fertilizer (δ15N: 1.6 ± 1.9‰, δ41K: 0.14 ± 0.09‰), and traffic emission (δ15N: −8.0 ± 4.5‰, δ41K: −0.17 ± 0.06‰). The MixSIAR model and backward trajectory with fire maps indicated that biomass burning and traffic emission were the primary contributors, with most biomass burning coming from southwest and southeast Beijing. This study expands rainwater K isotope composition data and first shows its potential to trace anthropogenic contamination. With K isotope ratios effectively tracing pollution sources in Beijing rainwater, it will be widely applied in environmental research. [Display omitted] [ABSTRACT FROM AUTHOR]

To constrain the mechanism and magnitude of inter-mineral K isotope fractionation during high-temperature magmatic processes, we conducted high-precision K isotope measurements on ultrapotassic and granitic rocks and mineral separates from the Qinling Orogenic Belt, China, as well as first-principles investigations of the corresponding K-bearing minerals. Our results show that the ultrapotassic rocks (−0.33 to −0.14‰) have considerably higher δ41K than the primitive mantle (−0.42 ± 0.08‰), confirming the previous conclusion that slab-derived fluids have contributed heavy K isotopic signatures to their mantle sources. Specifically, the K isotopic fractionation between biotite and microcline (Δ41K biotite-microcline = −0.11 to 0.05‰, with biotite and microcline ranging from −0.49 to −0.14‰ and − 0.48 to −0.12‰, respectively) agrees well with the calculated equilibrium isotopic fractionation scale. This evidence, together with the homogeneous mineral chemical compositions, suggests equilibrium K isotopic fractionation between biotite and microcline. Notably, amphiboles (δ41K ranging from −0.41 to 0.48‰) have heterogeneous K isotopic compositions and are generally heavier than coexisting microcline and biotite. In addition, the determined K isotopic fractionation between amphibole/plagioclase and microcline is inconsistent with the calculated results and is likely caused by the kinetic fractionation processes. To reveal the potential controls on the K isotopic compositions of arc magma, we utilized theoretical and measured K isotopic fractionation factors to model K isotopic fractionation behavior during amphibole crystallization. The results show that when Δ41K amphibole-melt = 0.05‰, the fractional crystallization of amphibole shows negligible effect on the K isotopic composition of the residual melts. Even in the case of Δ41K amphibole-melt = 0.24‰, fractional crystallization of amphibole exerts an insignificant control on the K isotopic composition of arc magma. Therefore, although amphibole may have heavier K isotopic compositions than the melts, its fractional crystallization may not distinctly decrease the K isotopic composition of residual melts. • The equilibrium K isotope fractionation between Bt and Mic is negligible • Amp and Pl show considerably higher δ41K than coexisting Bt and Mic • The fractionation of Amp would not obviously drive the melt toward lighter K isotope composition [ABSTRACT FROM AUTHOR]

A method for preparation of Ti-doped NaAlH4 with high hydrogen capacity was developed, in which the NaH/Al mixture was mechanically milled with a catalytic amount of KH together with metallic Ti. The addition of KH was found to result in a pronounced improvement in the dehydriding performance of the Na3AlH6/NaH+Al step. As a result, the practical cycling hydrogen capacity has been markedly enhanced from 3.3 wt % for the Ti-doped hydride to 4.7 wt % for KH+Ti co-doped material. Moreover, the pronounced enhancement on hydrogen capacity arising upon adding KH was observed to persist in the following dehydrogenation/hydrogenation cycles. Structural investigation shows that the addition of KH has led to a lattice expansion. Moreover, it was found that the enthalpy change of the Na3AlH6/NaH+Al decomposition step underwent a considerable decrease upon adding KH. Therefore, the observed property improvement may be ascribed to a favorable thermodynamic adjustment arising upon the addition of KH. [ABSTRACT FROM AUTHOR]

High-temperature equilibrium and kinetic stable isotope fractionation during partial melting, fractional crystallization, and other igneous differentiation processes has been observed in many isotope systems, but due to the relative nascence of high-precision analytical capabilities for K, it is still unclear whether igneous processes induce systematic and resolvable K isotope fractionation. In this study, we look to the natural laboratory of Hekla volcano in Iceland to investigate the behavior of K isotopes during magmatic differentiation of basalt to rhyolite. Using a novel MC-ICP-MS method, we analyzed 24 geochemically diverse samples from Hekla, including 7 basalts, 8 basaltic andesites, 3 andesites, 4 dacites, and 2 rhyolites, along with 2 additional samples from Burfell, Iceland, for comparison (1 basalt and 1 trachyte). We observed extremely limited variation of 41K/39K ratios throughout our suite of samples, which is not resolvable within the best current analytical uncertainty. The average value of all samples is δ41K NIST SRM3141a = −0.46 ± 0.07‰ (2sd). This value agrees with the Bulk Silicate Earth value previously defined by average global oceanic basalts in literature. The lack of variation throughout this suite of samples from a single volcano system indicates that K does not fractionate during magmatic differentiation (of basalt to rhyolite) through processes such as partial melting and fractional crystallization. This conclusion is important to the estimation of the Bulk Silicate Earth K isotope composition, to placing a more robust estimate on the composition bulk continental crust, and to fostering a better understanding of the behavior of K isotopes during differentiation of the terrestrial planets. • We measured K isotopes of a series of cogenetic igneous rocks from Hekla, Iceland. • No resolvable K isotopic variations among volcanic rocks from basalt to rhyolite. • K isotopes are likely not fractionated during igneous differentiation processes. • Data agrees with prior estimates for the bulk K isotope composition of the mantle. [ABSTRACT FROM AUTHOR]

Trinitite materials are the post-detonation glassy residues formed from melting and evaporation of arkosic sands during the first nuclear detonation at the Trinity test site, New Mexico on 16th July, 1945. These trinitites provide useful materials for studying elemental and isotopic behaviors associated with high temperature melting and evaporation that is otherwise difficult to achieve under laboratory conditions. Using a high-precision method, we measured the potassium (K) isotopic compositions of six bulk trinitite samples taken at different distances from the epicenter of detonation of the Gadget (ground zero). 15 leachates and etchates of trinitite samples were also analyzed to examine the distribution of K within the samples. All trinitites but IF_m (taken within 10 m from the epicenter) show no resolvable K loss and span a narrow range in K isotopic compositions (δ41K: −0.42 ± 0.05‰ to −0.48 ± 0.05‰), revealing no discernible K isotopic fractionation from the Bulk Silicate Earth (BSE) value (−0.48 ± 0.03‰). Residues and etchates of the trinitite material are identical in composition to the bulk samples implying that K isotopes were homogeneous within the arkosic sand at the Trinity test site prior to the nuclear detonation. The most strongly melted green trinitite IF_m, is the only trinitite that shows loss of K (~7%) coupled with a resolvable heavier K isotope composition (0.2‰ higher in δ41K than the BSE value). This coupled K loss and isotopic fractionation corresponds to a fractionation factor (α vapor-melt) between 0.995 and 0.998 during the Trinity nuclear detonation. These results confirm that K isotopic fractionation occurs through evaporation processes at high temperatures. We also show that, compared with Zn isotopes measured in the same samples, the isotopes of K were significantly less fractionated during evaporation, indicating that K is less volatile during processes such as magma ocean degassing, volcanic outgassing, and impact volatile loss. Our findings support the concept that the heavy K isotopic composition observed in lunar mare basalts reflects the primary signature imprinted by the Moon-forming giant impact event. • Improved mass spectrometry technique for potassium isotope analysis. • Potassium isotopic compositions were measured in trinitites that were produced by the high heat of nuclear detonation. • Potassium isotopic fractionation occurs through evaporation processes at high temperatures. • Absence of radiogenic 41K influence on δ41K in trinitites. • Enrichment of heavy K isotopes in lunar rocks is most likely the result of an initial Giant Impact event. [ABSTRACT FROM AUTHOR]

High-temperature isotope fractionation during partial melting and other igneous differentiation processes has been observed in many non-traditional isotope systems. The potassium (K) isotope system has not been extensively investigated historically due to the lack of high-precision analysis methods; however, the recent development of the Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) now allows for high-precision potassium isotope analysis. In this study, we utilized this new method to analyze 51 geologically, geographically, and geochemically diverse oceanic basalt samples including 32 mid-ocean ridge basalts (MORB), 3 back-arc basin basalts (BABB), and 16 oceanic island basalts (OIB). We observed a limited variation of 41K/39K ratios across our spread of samples. This variation in mantle-derived rocks is restricted compared to the large K isotopic fractionation observed in low-temperature systems. The averages of MORBs, BABBs, and OIBs are −0.44 ± 0.17‰ (2sd), −0.44 ± 0.08‰, and −0.41 ± 0.16‰, respectively, and there is no geographical variation (e.g., Indian vs. Pacific MORBs) in terms of K isotopes. Among all samples, there are two outliers, in which we have observed evidence of secondary mineral formation (i.e., palagonite) due to interaction with seawater. These two outliers have a K isotopic composition significantly heavier than other unaltered samples, close to the K isotopic composition of seawater. The grand average of all pristine samples is −0.43 ± 0.17‰ (2sd) which agrees well with the Bulk Silicate Earth (BSE) value previously defined. This new study indicates the homogeneity of K isotopes in the mantle and suggests that, since K will not resolvably fractionate during partial melting, any observable fractionation of K isotopes in primitive basalts is likely due to low-temperature, post-eruptive alteration processes. This conclusion is critical for understanding the initial bulk composition of the Earth and it is essential for any interplanetary comparison. [ABSTRACT FROM AUTHOR]

There is a renewed interest in K isotope geochemistry driven by the advances in analytical precision and it is emerging as a useful tracer in a variety of disciplines ranging from Earth sciences to biology. However, high-quality K isotopic data for reference materials are still limited but highly needed. Here, we report high-precision stable K isotopic compositions (δ41K) for 23 commercially available international reference materials, including igneous, sedimentary, and metamorphic rocks, as well as an in-house seawater standard. Potassium in digested samples was separated by cation-exchange chromatography with Bio-Rad AG50W-X8 (200–400 mesh) resin in 0.5 N HNO 3 media. Potassium isotopes were measured on a Nu Plasma II high-resolution MC-ICPMS. The reproducibility of K isotopic analysis, based on over one year of measurements of pure K solutions and rock standards, was ≤0.06‰ (95% confidence interval). Synthetic solutions made by mixing single element standards to represent various rock matrices confirmed the accuracy of our methods. The 23 reference materials have δ41K values ranging from −0.562‰ to −0.253‰ and the seawater standard has a much higher δ41K value of 0.143‰. The comprehensive dataset presented here provides a reference for quality control and inter-laboratory comparison of high-precision K isotopic analyses for future studies. [ABSTRACT FROM AUTHOR]

The causal effects among uplift, climate, and continental weathering cannot be fully addressed using presently available geochemical proxies. However, stable potassium (K) isotopes can potentially overcome the limitations of existing isotopic proxies. Here we report on a systematic investigation of K isotopes in dissolved load and sediments from major rivers and their tributaries in China, which have drainage basins with varied climate, lithology, and topography. Our results show that during silicate weathering, heavy K isotopes are preferentially partitioned into aqueous solutions. Moreover, d41K values of riverine dissolved load vary remarkably and correlate negatively with the chemical weathering intensity of the drainage basin. This correlation allows an estimate of the average K isotope composition of global riverine runoff (d41K = -0.22?), as well as modeling of the global K cycle based on mass balance calculations. Modeling incorporating K isotope mass balance better constrains estimated K fluxes for modern global K cycling, and the results show that the d41K value of seawater is sensitive to continental weathering intensity changes. Thus, it is possible to use the d41K record of paleo-seawater to infer continental weathering intensity through Earth's history. [ABSTRACT FROM AUTHOR]

In the last a few years, several groups proposed new methods of high-precision K isotope analysis using Multiple Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) either through “collision gas” or “cold plasma” methods. Here we report a detailed method analyzing K isotopes in high-precision using Neptune Plus MC-ICP-MS in cold plasma and conduct an inter-laboratory comparison. The precision of measurements of 41K/39K ratios of a single run in this study can routinely reach ∼0.05 per mil (95% confidence interval; n = 10). The long-term (20 months) robustness and reproducibility of this method have also been evaluated (0.11 per mil; 2 standard deviation; n = 890). We also report K isotopic compositions of 20 geological reference materials and compare this method with recent methods from other laboratories. These independent measurements of the same reference materials agree well with each other within reported analytical uncertainties. Because of the newly improved methods and observed K isotopic fractionation among reference materials, the International Union of Pure and Applied Chemistry (IUPAC) recommended values for the weight of potassium and the atomic fractions of K isotopes need to be revised. [ABSTRACT FROM AUTHOR]

Abstract The analysis of 41Ca can be quantified by measuring the ratio 41Ca/40Ca. At present, ratios of 41Ca/40Ca ≤ 10−11 can only be measured by accelerator mass spectrometry (AMS). The sensitivity is mainly limited by the interference of the isobar 41K. With compact AMS (≤1 MV), abundance sensitivities of 10−11 to 10−12 have been demonstrated, but this may necessitate the measurement of 39K to correct for the 41K background. With a 3 MV AMS, the dE/dx method can still be used to partially separate 41Ca from 41K. In this work, a new capability for 41Ca analysis has been tested at the Xi'an-AMS. Ca4+ was chosen to achieve a balanced performance between 41Ca/41K separation and transmission efficiency. A 41Ca/40Ca background level of near 10−13 was achieved without the need for 39K correction. This new analytical capability at our facility was then applied to 41Ca tracer samples from our studies on the "White Jade" land snail shells, in which 41Ca was used as a tracer for studying the absorption rate of calcium. [ABSTRACT FROM AUTHOR]

Abstract Dealing with the 41K interference during 41Ca measurement with low energy AMS systems is challenging. The detection of 39K to correct the measured 41Ca/40Ca ratios is a powerful tool that improves precision and accuracy. However, the study of the sources of this interference can help to reduce it to its minimum. In this work, it is shown that iron is a factor enhancing the production of the 41K interference, since the (41K57Fe)− ion has the same mass as (41K19F 3)−. To validate this observation, we measured 57Fe2+ together with 41M2+ (all ions with mass 41) and 39K2+ in a blank sample and an aluminum target at the 600 kV AMS system at ETH Zurich. We also show the temporal evolution of 41M/40Ca and 39K/40Ca ratios in two blank samples during a measurement on the 1 MV AMS system at CNA Seville. Even when unstable behavior for these ratios is observed for one of the blanks, the relationship between both, 41M/39K, is relatively stable over time. This supports the accuracy of the K–correction method, providing that it is applied sequence by sequence. Two programs, one for each of these compact AMS systems, were written in FORTRAN code to deal with the complexity of the data analysis due to the K-correction. [ABSTRACT FROM AUTHOR]

Quasibound levels and shape resonances in the (B 1Πu -X 1Σ+g) band system of 39K2 have been recorded by crossed laser-molecular beam techniques. Using optical–optical double resonance, individual rovibrational levels (v‘=15–18, J‘=3–25) of the K2 state are prepared by Franck–Condon pumping (FCP) in a supersonic nozzle beam. Excitation into quasibound levels below and above the (B 1Πu) state barrier is detected as molecular and atomic (K4 2P3/2→4 2S1/2 only) fluorescence. The resonance transition frequencies and shapes are measured and the results are used (a) to determine the scattering resonance energies, widths, and lifetimes; (b) to compare them with values obtained by a ‘‘maximum internal amplitude’’ approach [R. J. LeRoy and R. B. Bernstein, J. Chem. Phys. 54, 5114 (1971)]; and (c) to check the agreement with exact calculations of the B state potential using the ‘‘inverted perturbation approach (IPA).’’ The bound and quasibound part of the B 1Πu state including the locus (R=8.08±0.05 Å) of the barrier maximum (298±8 cm-1 above the adiabatic dissociation limit) is found in excellent agreement with previous results. The shape resonances are not highly sensitive to the long-range interatomic forces, here the repulsive dipole–dipole resonance interaction. [ABSTRACT FROM AUTHOR]

Mountain uplift produces steep slopes and rapid erosion rates, and hence exposes more silicate rocks to weathering and accelerates the drawdown of atmospheric CO 2 , driving global cooling over the Cenozoic. Riverine potassium (K) and lithium (Li) isotopes are two promising proxies for silicate weathering because the isotopes fractionate during secondary mineral formation associated with weathering and these two elements are highly enriched in silicate rocks. This study explores similarity and difference between K and Li isotope behaviors during weathering across the Lancang River in the southeastern Tibetan Plateau, in order to evaluate the role of these two isotopes in tracing silicate weathering in tectonically active mountains. Dissolved K and Li in the Lancang River mainstream are mainly derived from the dissolution of silicates. River waters are enriched in the heavy isotope of K (−0.35‰ to +0.05‰) and Li (+7.1‰ to +20.4‰) relative to suspended sediments (−0.56‰ to −0.42‰ and −2.3‰ to 0‰, respectively), and the element partitioning between dissolved and suspended loads controls dissolved δ 7 Li and δ 41 K values. Interesting, dissolved δ 7 Li and δ 41 K show the opposite evolution trends toward downstream. As the weathering intensity increases downstream (higher Si/(Na + K)⁎ ratios), the dissolved δ 7 Li values increase from 7.4‰ to 13.6‰, whereas dissolved δ 41 K values decrease from −0.04‰ to −0.26‰. Compared to its headwater with limited weathering, more secondary minerals (e.g., clays) are formed in the lower reaches (e.g., floodplains) with low physical erosion rates and high annual precipitation, driving more 6Li incorporated into clays, thus high dissolved δ 7 Li values downstream. While dissolved δ 41 K values and the fraction of 39K incorporated into secondary minerals are low in floodplains, this is consistent with observations that illite is a main K-bearing phase among clay minerals, and its content in the clay fraction in river sediment decreases as weathering intensity increases. Our study further supported that the increase in Cenozoic seawater δ 7 Li is related to weathering in floodplains rather than weathering of mountains, whereas K isotope fractionation mainly occurs in mountain regions. If Cenozoic seawater δ 41 K rise, as with δ 7 Li change, weathering at higher elevation may exerts more effect on high riverine and seawater δ 41 K values. This study improves our knowledge of K-Li isotope behaviors during present-day silicate weathering, and the response of K-Li isotopes in sedimentary archives to silicate weathering over geological time. • First comparative study of riverine K and Li isotope signatures in Tibetan Plateau. • River water dissolved δ 7 Li values increase while δ 41 K values decrease downstream. • More Li incorporated into clays results in high δ 7 Li values at lower elevation. • K isotope fractionation mainly occurs in mountain regions where more illite formation. • Weathering in high mountains may exerts more effect on riverine and seawater δ 41 K values. [ABSTRACT FROM AUTHOR]

Subduction recycles water, other volatiles and incompatible elements into Earth's mantle and plays a major role in exchanges between its surface and interior. Potassium (K), a highly fluid-mobile and incompatible alkali element, is strongly affected by these processes. The emerging K stable isotope system has been proposed as a new tracer of subduction events, e.g. to distinguish the inputs to arc magmas from sediments, altered basalts and other near-surface materials. Yet, it remains unknown if and how the K isotopic signatures of these components are affected by extraction of K-bearing media from dehydrating slabs and their transfer to the mantle wedge. Fragments (xenoliths) of sub-arc mantle carried to the surface by arc magmas may retain these slab-derived components. We report high-precision K isotope data on mantle harzburgite xenoliths from Avacha, an active arc volcano in Kamchatka (Russia) to further investigate the nature of slab-derived fluids and their impact on arc mantle. We find a narrow δ 41 K range for amphibole-rich cumulates from host andesite magmas (−0.53 ± 0.09‰ to −0.34 ± 0.09‰) that overlaps those of the bulk silicate Earth (BSE) and the upper continental crust (UCC). In contrast, the Avacha peridotites derived from the uppermost lithospheric mantle (refractory melting residues metasomatized by subduction zone fluids) show an extremely broad K isotopic range (δ 41 K ∼ 3‰) and are commonly enriched in heavy K isotopes (up to +1.16 ± 0.08‰) relative to the BSE and UCC; some of the peridotites show the highest δ 41 K values for any reported mantle rocks. The new data on the Avacha xenoliths, along with available K isotope data on arc lavas and eclogites, suggest that slab-derived fluids may develop heavy K isotopic signatures, distinct from those in the original subducting materials (e.g., marine sediments and altered oceanic crust). These results provide direct evidence that (a) subduction dehydration and fluid percolation in the mantle lithosphere can significantly fractionate K isotopes, and (b) slab-derived fluids infiltrated the sub-arc mantle wedge. Due to the large K concentration difference between the depleted mantle and slab-derived fluids, K isotopes may be a sensitive tracer of fluid activity in sub-arc mantle. • K isotopic composition of lithospheric mantle is heterogeneous deviating from BSE. • Peridotites have lighter K isotopes due to diffusion-driven kinetic fractionation. • Subduction zone peridotites exhibit heavy K isotopes due to dehydration fluids. • Study reveals slab-derived fluid infiltration in arc mantle using K isotope tracer. [ABSTRACT FROM AUTHOR]