Corliss K. Sio

Melt inclusions are used to study the origin and evolution of magmas. The extent to which they represent equilibrium melt compositions, however, critically hinges on the ratio of crystal growth rate to diffusion rate in melts. If the rate of crystal growth is limited by the supply of nutrients and the dissipation of unwanted components, the trapped melt will be depleted in compatible elements and enriched in incompatible elements. Despite widespread recognition of the potential for melt inclusions to trap disequilibrium compositions, their identification remains a challenge. Here, we demonstrate how stable isotopes provide a solution to this problem. Melt inclusions in basaltic tephra from Kīlauea volcano, Hawai‘i, display Mg isotope fractionation up to 0.49‰ (26Mg/24Mg) relative to bulk rock. These observations indicate that kinetic processes, such as chemical diffusion, are at play, as equilibrium mineral-melt isotope fractionation at high temperatures is an order of magnitude smaller. We propose that the heavy Mg isotopic compositions of the melt inclusions were generated by the faster diffusion and thus preferential incorporation of 24Mg by the growing olivine, leaving the slower diffusant (26Mg) behind. The incompatible elements (e.g., Ca, Ti, and K) are predicted to display similarly large fractionations when boundary layer effects are significant. These findings show that stable isotopes can be a useful tool in identifying disequilibrium compositions in melt inclusions.

Rationale: Lithium isotope geochemistry is an important tool in the studies of Earth and planetary materials. In situ Li isotope analyses are typically performed using secondary ion mass spectrometry (SIMS) or laser ablation multicollector inductively coupled plasma mass spectrometry (LA‐MC‐ICPMS), but these instruments are not widely accessible. Here, the capability of laser ablation quadrupole ICPMS for conducting Li isotopic analyses is evaluated. Methods: An array of MPI‐DING and USGS silicate glass reference materials was analyzed repeatedly over the course of 6 months. These materials range from komatiite to rhyolite in terms of silica content (45.5–75.6 wt%) with 9–45 ppm Li. Their Li isotope compositions have been previously characterized so that matrix effects could be tested with these reference materials. Analyses were conducted using an NWR193 laser ablation system coupled to an Agilent 7900 ICPMS system. Results: Analytical precision is primarily limited by Li concentration in the samples. For samples with ~9 ppm Li, the internal precision is 6‰ (2 SD, 150 μm spot diameter), whereas that for a sample with ~45 ppm Li is 4‰ (2 SD, 120 μm spot diameter). The technique is somewhat sensitive to sample matrix: samples with SiO ₂ content that deviates from the bracketing standard display fractionated δ ⁷ Li, necessitating correction using a session‐specific matrix correction curve. Conclusion: Lithium isotope analysis by ns‐LA‐QICPMS is worthwhile for samples with high Li concentrations and when a matrix‐matched standard can be obtained. Although the precision of this method is not as high as those achievable with SIMS and LA‐MC‐ICPMS, it remains adequate for resolving large isotope fractionations found in natural and laboratory settings.

Rationale: The microanalytical community has an outstanding need for platinum group element (PGE) reference materials, particularly for trace element analysis by laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS). National Institute of Standards and Technology (NIST) glasses contain Rh, Pd, and Pt, but lack Ru, Os, and Ir. Synthesis of silicate PGE standards has proven difficult due the tendency of PGEs to form metallic nuggets. Methods: Additive manufacturing methods were used to produce PGE standards with a silica matrix. Monodispersed submicron PGE‐doped Stöber particles were used as feedstock materials for electrophoretic deposition (EPD). Two‐cm‐sized samples produced by EPD were subsequently densified by thermal processing. The homogeneity of PGEs was tested using LA‐ICPMS and concentrations were measured by laser ablation and solution ICPMS. Results: The PGE concentrations ranged from 0.5 to 3 μg/g. The inhomogeneity was at the 3% RSD level for Ru, Rh, Ir, and Os throughout and 5% for Pt and Pd in the interior of the samples. Based on LA‐ICPMS analyses, the interiors of the two samples have near identical concentrations in PGEs. Conclusions: The samples fabricated in this study represent the most complete and homogeneous PGE standards produced with a silicate matrix. The ability to produce multiple samples with the same composition provides opportunities for validating methods, monitoring long‐term reproducibility, and facilitating interlaboratory comparisons.