Ion microprobe elemental and isotopic determinations can be precise but difficult to quantify. Error is introduced when the reference material and the sample to be analysed have different compositions. Mitigation of such matrix effects' is possible using ion implants. If a compositionally homogeneous reference material is available which is matrix-appropriate' (i.e., close in major element composition to the sample to be analysed, but having an unknown concentration of the element, E, to be determined) then ion implantation can be used to introduce a known amount of an E isotope, calibrating the E concentration and producing a matrix-appropriate calibrator. Nominal implant fluences (ions cm(-2)) are inaccurate by amounts up to approximately 30%. However, ion implantation gives uniform fluences over large areas; thus, it is possible to co-implant' an additional reference material of any bulk composition having known amounts of E, independently calibrating the implant fluence. Isotope ratio measurement standards can be produced by implanting two different isotopes, but permil level precision requires postimplant calibration of the implant isotopic ratio. Examples discussed include (a) standardising Li in melilite; (b) calibrating a Mg-25 implant fluence using NIST SRM 617 glass and (c) using Si co-implanted with Mg-25 alongside NIST SRM 617 to produce a calibrated measurement of Mg in Si.

We performed an in-depth exploration of the Al-Mg system for presolar graphite, SiC, and Si3N4 grains found to contain large excesses of Mg-26, indicative of the initial presence of live Al-26. Ninety of the more than 450 presolar grains processed in this study contain well-correlated delta Mg-26/Mg-24 and Al-27/Mg-24 ratios, derived from Nano-scale Secondary Ion Mass Spectrometer depth profiles, whose isochron-like regression lines yield inferred initial Al-26 Al-27 ratios that, on average, are similar to 1.5-2 times larger than the ratios previously reported for the grains. The majority of presolar graphite and SiC grains are heavily affected by Al contamination, resulting in large negative delta Mg-26/Mg-24 intercepts of the isochron lines. Al contamination is potentially due to etching of the grains' surfaces and subsequent capture of dissolved Al during the acid dissolution of their meteorite host rocks. From the isochron fits, the magnitude of Al contamination was quantified for each grain. The amount of Al contamination on each grain was found to be random and independent of grain size, following a uniform distribution with an upper bound at 59% contamination. The Al contamination causes conventional whole-grain estimates to underpredict the initial Al-26/Al-27 ratios. The presolar grains with the highest Al-26/Al-27 ratios are from Type II supernovae whose isochronderived initial Al-26/Al-27 ratios greatly exceed those predicted in the He/C and He/N zones of SN models.

Presolar stardust is present at low levels in meteorites and cometary dust and identified as ancient stellar matter by unusual isotopic compositions reflecting nuclear processes in stellar interiors and galactic chemical evolution. Most grains originated in winds from asymptotic giant branch (AGB) stars and supernova and their isotopic compositions provide important constraints on models of evolution and nucleosynthesis in these environments. The presolar grains from AGB stars appear to have formed in a lower-mass population of stars than predicted by GCE models. A merger of the Milky Way with a dwarf galaxy some 1Gyr before the birth of the Solar System may explain this and other grain observations and the data thus can provide a unique window into the presolar history of the solar neighborhood.

Extreme excesses of C-13 (C-12/C-13 < 10) and N-15 (N-14/N-15 < 20) in rare presolar SiC grains have been considered diagnostic of an origin in classical novae, though an origin in core collapse supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data for 14 submicron-to micron-sized C-13-and N-15-enriched presolar SiC grains (C-12/C-13 < 16 and N-14/N-15 < similar to 100) from Murchison, and their correlated Mg-Al, S, and Ca-Ti isotope data when available. These grains are enriched in C-13 and N-15, but with quite diverse Si isotopic signatures. Four grains with Si-29,Si-30 excesses similar to those of type C SiC grains likely came from CCSNe, which experienced explosive H burning occurred during explosions. The independent coexistence of proton-and neutron-capture isotopic signatures in these grains strongly supports heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven putative nova grains with Si-30 excesses and Si-29 depletions show lower-than-solar S-34/S-32 ratios that cannot be explained by classical nova nucleosynthetic models. We discuss these signatures within the CCSN scenario. For the remaining five putative nova grains, both nova and supernova origins are viable because explosive H burning in the two stellar sites could result in quite similar proton-capture isotopic signatures. Three of the grains are sub-type AB grains that are also 13C enriched, but have a range of higher 14N/15N. We found that N-15-enriched AB grains (similar to 50 < 14N/15N < similar to 100) have distinctive isotopic signatures compared to putative nova grains, such as higher 14N/15N, lower Al-26/Al-27, and lack of 30Si excess, indicating weaker proton-capture nucleosynthetic environments.

The X-Ray Spectrometer (XRS) that flew on the MESSENGER spacecraft measured X-rays from the surface of Mercury in the energy range similar to 1-10 keV. Detection of characteristic K-alpha-line emissions from Mg, Al, Si, S, Ca, Ti, and Fe yielded the surface abundances of these geologically important elements. Spatial resolution as fine as similar to 40 km (across track) was possible at periapsis for those elements for which counting statistics were not a limiting factor. Four years of orbital observations have made it possible to generate from XRS spectra detailed elemental composition maps that cover a majority of Mercury's surface. Converting measurements to compositions requires a thorough understanding of the XRS instrument capabilities. The ground and flight calibration measurements presented here are necessary for the reduction and analysis of the X-ray data from the MESSENGER mission. (c) 2016 Elsevier Ltd. All rights reserved.

Many cosmic dust particles have escaped the aqueous and thermal processing, the gravitational compaction, and the impact shocks that often overprint the record, in most larger samples, of how Solar System materials formed. The least-altered types of cosmic dust can, therefore, act as probes into the conditions of the solar protoplanetary disk when the first solids formed. Analyses of these "primitive" particles indicate that the protoplanetary disk was well mixed, that it contained submicron grains formed in a diversity of environments, that these grains were aerodynamically transported prior to aggregation, which was likely aided by organic grain coatings, and that some minerals that condensed directly from the disk are not found in other materials. These protoplanetary aggregates are not represented in any type of meteorite or terrestrial rock. They can only be studied from cosmic dust.

Targeted MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) X-Ray Spectrometer measurements of Mercury's largest identified pyroclastic deposit are combined with neutron and reflectance spectroscopy data to constrain the composition of volatiles involved in the eruption that emplaced the pyroclastic material. The deposit, northeast of the Rachmaninoff basin, is depleted in S (relative to Ca and Si) and C, compared with the rest of Mercury's surface. Spectral reflectance measurements of the deposit indicate relatively high overall reflectance and an oxygen-metal charge transfer (OMCT) absorption band at ultraviolet wavelengths. These results are consistent with oxidation of graphite and sulfides during magma ascent, via reaction with oxides in the magma or assimilated country rock, and the formation of S-and C-bearing volatile species. Consumption of graphite during oxidation could account for the elevated reflectance of the pyroclastic material, and the strength of the OMCT band is consistent with similar to 0.03-0.1 wt% FeO in the deposit.

Mercury's global surface is markedly darker than predicted from its measured elemental composition. The darkening agent, which has not been previously identified, is most concentrated within Mercury's lowest-reflectance spectral unit, the low-reflectance material(1). This low-reflectance material is generally found in large impact craters and their ejecta(2,3), which suggests a mid-to-lower crustal origin. Here we present neutron spectroscopy measurements of Mercury's surface from the MESSENGER spacecraft that reveal increases in thermal-neutron count rates that correlate spatially with deposits of low-reflectance material. The only element consistent with both the neutron measurements and visible to near-infrared spectra(4) of low-reflectance material is carbon, at an abundance that is 1-3 wt% greater than surrounding, higher-reflectance material. We infer that carbon is the primary darkening agent on Mercury and that the low-reflectance material samples carbon-bearing deposits within the planet's crust. Our findings are consistent with the formation of a graphite flotation crust from an early magma ocean(5), and we propose that the heavily disrupted remnants of this ancient layer persist beneath the present upper crust. Under this scenario, Mercury's globally low reflectance results from mixing of the ancient graphite-rich crust with overlying volcanic materials via impact processes or assimilation of carbon into rising magmas during secondary crustal formation.