Marek Locmelis

Extremely Ni-rich Fe–Ni sulfide assemblages in komatiitic dunite at Betheno, Western Australia: results from synchrotron X-ray fluorescence mapping

Fresh unserpentinised komatiitic dunite at Betheno (Western Australia) contains a distinctive sulfide assemblage of pentlandite, pyrite and millerite. The Ni tenor (i.e. Ni concentration in the original sulfide liquid) of this assemblage is in excess of 30 wt%, and the bulk sulfide composition falls within the compositional range of monosulfide solution (MSS) above 800°C. Such assemblages have conventionally been interpreted as the result of hydrothermal upgrading of normal lower Ni-rich magmatic assemblages, but this explanation is not applicable at Betheno. Subtle zonation of Ni concentration in the host olivine, revealed by high-resolution X-ray fluorescence mapping using the Maia detector on the Australian Synchrotron, suggests that coupled subsolidus re-equilibration of Ni and Fe between olivine and sulfide is not a plausible explanation, and the olivine appears to have gained Ni from sulfides rather than the other way around. This leaves a primary magmatic origin as the favoured interpretation, and supports the existence of a stable pyrite–millerite tie-line in the Fe–Ni–S system at low temperatures. Further evidence for this comes from the existence of similar assemblages in fresh dunites from the nearby Perseverance nickel deposit. Hydrothermal alteration is evidently not necessary to form unusually Ni-rich sulfide assemblages. The exceptionally high Ni tenors are attributed to open-system equilibration of sulfide liquid with typical Ni-undepleted olivine, under conditions where sulfide compositions are essentially buffered by the olivine composition, and to the known positive correlation between the Fe/Ni distribution coefficient between olivine and sulfide and the Ni tenor. Other Ni-rich, millerite-bearing assemblages, such as those from the Black Swan nickel deposit, may also have primary origins.

The capacity of hydrous fluids to transport and fractionate incompatible elements and metals within the Earth's mantle

Both silicate melts and aqueous fluids are thought to play critical roles in the chemical differentiation of the Earths crust and mantle. Yet their relative effects are poorly constrained. We have addressed this issue by measuring partition coefficients for 50 trace and minor elements in experimentally produced aqueous fluids, coexisting basanite melts, and peridotite minerals. The experiments were conducted at 1.0–4.0 GPa and 950–1200°C in single capsules containing (either 40 or 50 wt %) H2O and trace element-enriched basanite glass. This allowed run products to be easily identified and analyzed by a combination of electron microprobe and LAM-ICP-MS. Fluid and melt compositions were reconstructed from mass balances and published solubility data for H2O in silicate melts. Relative to the basanite melt, the solutes from H2O-fluids are enriched in SiO2, alkalis, Ba, and Pb, but depleted in FeO, MgO, CaO, and REE. With increasing pressure, the mutual solubility of fluids and melts increases rapidly with complete miscibility between H2O and basanitic melts occurring between 3.0 and 4.0 GPa at 1100°C. Although LREE are favored over HREE in the fluid phase, they are less soluble than the HFSE (Nb, Ta, Zr, Hf, and Ti). Thus, the relative depletions of HFSE that are characteristic of arc magmas must be due to a residual phase that concentrates HFSE (e.g., rutile). Otherwise, H2O-fluids have the capacity to impart many of the geochemical characteristics that distinguish some rocks and melts from the deep mantle lithosphere (e.g., MARID and lamproites).

Improved Precision and Accuracy of Quantification of Rare Earth Element Abundances via Medium-Resolution LA-ICP-MS

AbstractLaser ablation ICP-MS enables streamlined, high-sensitivity measurements of rare earth element (REE) abundances in geological materials. However, many REE isotope mass stations are plagued by isobaric interferences, particularly from diatomic oxides and argides. In this study, we compare REE abundances quantitated from mass spectra collected with low-resolution (m/Δm = 300 at 5% peak height) and medium-resolution (m/Δm = 2500) mass discrimination. A wide array of geological samples was analyzed, including USGS and NIST glasses ranging from mafic to felsic in composition, with NIST 610 employed as the bracketing calibrating reference material. The medium-resolution REE analyses are shown to be significantly more accurate and precise (at the 95% confidence level) than low-resolution analyses, particularly in samples characterized by low (<μg/g levels) REE abundances. A list of preferred mass stations that are least susceptible to isobaric interferences is reported. These findings impact the reliability of REE abundances derived from LA-ICP-MS methods, particularly those relying on mass analyzers that do not offer tuneable mass-resolution and/or collision cell technologies that can reduce oxide and/or argide formation. Graphical Abstractᅟ