Extreme enriched and heterogeneous ⁸⁷Sr/⁸⁶Sr ratios recorded in magmatic plagioclase from the Samoan hotspot

We report the major-element, trace-element, and 87Sr/86Sr compositions of six plagioclase crystals from two Samoan lavas with extreme EM2 isotopic compositions (ALIA-115-18 with whole-rock 87Sr/86Sr of 0.718592, and ALIA-115-21 with whole-rock 87Sr/86Sr of 0.720469). We employed laser-ablation split-stream mass spectrometry (LASS) to simultaneously measure 87Sr/86Sr ratios, major-element concentrations, and trace-element concentrations in the same plagioclase crystal volume. We find that two plagioclase crystals have extreme 87Sr/86Sr heterogeneity in excess of 5000 ppm (where ppm of 87Sr/Sr variability86=106⋅[Sr/8687Srmax−87Sr/86Srmin]/87Sr/86Sravg). In two of the plagioclase crystals, we identify the highest 87Sr/86Sr ratios (0.7224) ever measured in any fresh, mantle-derived ocean island basalt (OIB) or OIB-hosted mineral phase.We find that in 87Sr/86Sr-versus-Sr concentration space, the six plagioclase crystals overlap in a “common component” region with higher 87Sr/86Sr than has been previously identified in whole-rock Samoan lavas or mineral separates. We use the occurrence of olivine mineral inclusions (Fo=74.5±0.8, 2 SD) in the high-87Sr/86Sr zone of one plagioclase crystal to infer the bulk composition (Mg#=46.8±0.8, 2 SD) of the extreme EM2 magma from which the olivine and high-87Sr/86Sr plagioclase crystallized. We argue that a relatively evolved EM2 endmember magma mixed with at least one lower-87Sr/86Sr melt to generate the observed intra-crystal plagioclase isotopic heterogeneity.By inferring that subducted terrigenous sediment gives rise to EM2 signatures in Samoan lavas, we estimate that the quantity of sediment necessary to generate the most-elevated 87Sr/86Sr ratios observed in the Samoan plagioclase is ∼7% of the mantle source. We also estimate that sediment subduction into the mantle over geologic time has generated a sediment domain that constitutes 0.02% of the mass of the mantle, a much lower proportion than required in the EM2 mantle source. Even if subducted sediment is concentrated in large low-shear-velocity provinces (LLSVPs) at the base of the mantle (which constitute up to 7.7% of the mantle's mass), then only 0.25% of the LLSVPs are composed of sediment. This requires that the distribution of subducted sediment in the mantle is heterogeneous, and the high relative abundance of sediment in the Samoan EM2 mantle is an anomalous relic of ancient subduction that has survived convective attenuation

Non-destructive identification of surface enrichment and trace element fractionation in ancient silver coins

A common issue in non-destructive surface analysis of historical silver coins is depletion of Cu from the near-surface areas, which in turn results in higher Ag content at a coin's surface. This paper reports a non-destructive analytical strategy using µXRF for identification of Ag and Cu surface enrichments and depletions by comparing peak intensity ratios of Ag Kα/Ag Lα, Cu Kα/Ag Kα and Cu Lα/Ag Lα for coins and Ag-Cu standards of similar composition. Our characterization of coins from different contexts and chronologies shows that a multi-standard approach provides the most reliable identification of surface enrichment of Ag and depletion of Cu. Coins possessing Ag surface enrichment were further analysed with LA-ICP-MS to determine any differences in trace element composition between the cores and surface of the coins. We show that the near-surface regions of these coins are enriched in Au and depleted in Co, Ni, As, and Pt relative to their cores. These systematics allow for a more robust assessment of the degree of silver coin surface alteration critically important in measuring the original composition of historical silver coins

The trophodynamics of Southern Ocean Electrona (Myctophidae) in the Scotia Sea

The Scotia Sea is one of the most productive regions of the Southern Ocean, but its surface waters are experiencing a rapid increase in temperature, which may be changing the behaviour and distribution of many myctophids and their prey species. Electrona antarctica and Electrona carlsbergi are two of the most abundant myctophids in the region, but their ecology is poorly understood and their response to ongoing environmental change is difficult to determine. This study investigated spatial and temporal patterns in their abundance, population structure and diets using mid-water trawl nets deployed across the Scotia Sea during spring, summer and autumn. E. antarctica was the most numerically abundant species (0.09–0.21 ind. 1,000 m−3), with greatest concentrations occurring in the sea-ice sectors. E. carlsbergi occurred in more northern regions, comprising densities of 0.02–0.11 ind. 1,000 m−3. There was evidence of seasonal variation in depth distribution, size-related sexual dimorphism and size-specific vertical stratification for both species. Latitudinal trends in sex ratio and female body size were apparent for E. antarctica. Its diet varied between regions, seasons and size classes, but overall, Euphausia superba, Metridia spp. and Themisto gaudichaudii were the dominant prey items. E. carlsbergi appeared not to recruit in the Scotia Sea. Its diet was dominated by copepods, particularly Rhincalanus gigas and Metridia spp., but regional, seasonal and ontogenetic variations were evident. This study contributes to our understanding of how mid-water food webs are structured in the Southern Ocean and their sensitivity to ongoing environmental change