On September 24, 2023, NASA's OSIRIS‐REx mission dropped a capsule to Earth containing ~120 g of pristine carbonaceous regolith from Bennu. We describe the delivery and initial allocation of this asteroid sample and introduce its bulk physical, chemical, and mineralogical properties from early analyses. The regolith is very dark overall, with higher‐reflectance inclusions and particles interspersed. Particle sizes range from submicron dust to a stone ~3.5 cm long. Millimeter‐scale and larger stones typically have hummocky or angular morphologies. Some stones appear mottled by brighter material that occurs as veins and crusts. Hummocky stones have the lowest densities and mottled stones have the highest. Remote sensing of Bennu's surface detected hydrated phyllosilicates, magnetite, organic compounds, carbonates, and scarce anhydrous silicates, all of which the sample confirms. We also find sulfides, presolar grains, and, less expectedly, Mg,Na‐rich phosphates, as well as other trace phases. The sample's composition and mineralogy indicate substantial aqueous alteration and resemble those of Ryugu and the most chemically primitive, low‐petrologic‐type carbonaceous chondrites. Nevertheless, we find distinct hydrogen, nitrogen, and oxygen isotopic compositions, and some of the material we analyzed is enriched in fluid‐mobile elements. Our findings underscore the value of sample return—especially for low‐density material that may not readily survive atmospheric entry—and lay the groundwork for more comprehensive analyses.

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.

At ocean spreading ridges, circulation of seawater through rock at elevated temperatures alters the chemical and isotopic composition of oceanic crust. Samples obtained from drilling into ocean floor and from ophiolites have demonstrated that certain isotope systems, such as ¹⁸O/¹⁶O and ⁸⁷Sr/⁸⁶Sr, are systematically modified in hydrothermally altered oceanic crust. Although K is known to be mobile during hydrothermal alteration, there have not yet been any K-isotope analyses of altered oceanic crustal materials. Moreover, the ⁴¹K/³⁹K of seawater was recently found to be significantly higher than that of igneous rocks, so the addition of seawater K to oceanic crust would be expected to generate ⁴¹K/³⁹K variations in affected rocks. Here, we report high-precision ⁴¹K/³⁹K measurements for samples from the Bay of Islands ophiolite, and we document large variations in ⁴¹K/³⁹K, covarying with previous determinations of ⁸⁷Sr/⁸⁶Sr. Our data indicate that analytically resolvable ⁴¹K/³⁹K effects arise in oceanic crust as a result of hydrothermal alteration. This finding raises the possibility that ⁴¹K/³⁹K can be used as an effective tracer of oceanic crust recycled into the mantle, as a diagnostic criterion by which to identify ancient fragments of oceanic crust, and as a constraint on the flux of K between oceanic crust and seawater.