The composition of amphibole phenocrysts in Neogene mafic volcanic rocks from the Puna plateau: Insights on the evolution of hydrous back-arc magmas
Authors: Maro, Guadalupe; Trumbull, Robert B.; Caffe, Pablo J.; Jofre, Cynthia B.; Filipovich, Ruben E.; Frick, Daniel A.
In typical Andean arc magmas, amphibole appears as a phenocryst phase only after considerable differentiation. However, some near-primitive volcanic rocks (high-Mg andesites and basalts) from monogenetic centers in the Puna plateau of Argentina also contain amphibole phenocrysts, implying special conditions of hydrous magma generation in this back-arc setting. This study documents typical examples from Southern and Northern Puna regions and uses the major and trace-element compositions of amphibole to constrain a petrogenetic model for the hydrous magmas. There are significant differences in the nature of amphiboles and their host lavas depending on location of the volcanic centers in the Southern and the Northern Puna regions. In the Southern Puna, basaltic andesitic lavas have Sr/Y values>40 and amphiboles showskeletal forms and occur in an assemblagewith olivine and pyroxene. The amphibole compositions are relatively Al- and Ti-poor compared to the Northern Puna. Thermobarometry indicates amphibole crystallization temperatures of 960-1000 degrees C at moderate pressure (< 5 kbar). In contrast, the mafic lavas from centers in the Northern Puna show Sr/Y ratios lower than 20 and amphiboles in these rocks coexist with a plagioclase-orthopyroxene assemblage. The Northern Puna amphiboles have higher Ti and Al contents than those in the southern region and the thermobarometry estimates imply generally higher crystallization temperatures (>1000 degrees C) and pressures (6-8 kbar). Furthermore, the chemical composition of amphibole phenocrysts in the Northern Puna Campo Negro center suggests an alkaline affinity of the parentalmagmas which, together with radiogenic isotope data from earlier studies, indicates a significant contribution of the enriched lithosphere in the magma source. The new data collectively suggest high pressure evolution of hydrousmagmas in the Southern Puna, whereas the Northern Puna magmas underwent more differentiation at higher levels in the crust. This contrast in the evolution history ofmagmas belowboth regions can be connectedwith their position relative to partialmelting zones in the mid-upper crust, which are larger and longer-lived in the north than in the south, thus favoring a slower ascent of magmas in that region. (C) 2020 Elsevier B.V. All rights reserved.
Spatial distribution of microplastics around an inhabited coral island in the Maldives, Indian Ocean
SCIENCE OF THE TOTAL ENVIRONMENT
Authors: Patti, Toby B.; Fobert, Emily K.; Reeves, Simon E.; da Silva, Karen Burke
Small plastic particles are considered environmental pollutants and are highly concentrated in marine sediments. However, knowledge about plastic abundance within coral reef habitat and beach sediments surrounding remote inhabited coral islands is scarce. In this study, microplastic accumulation was investigated on a small inhabited coral island located in the Maldives. Sediments from 22 sampling sites across fore reef, reef flat, and beach environments were analysed for plastic particles <5 mm. Density separation and microscope enumeration revealed a total of 1244 individual microplastic pieces, in filamentous (49%) and fragmented (51%) forms, found across all sampling sites. High concentrations were recorded at all sites, however, there was no significant relationship between microplastic concentration or size across regions (inner atoll and outer atoll) or environments (fore reef, reef flat, and beach). Furthermore, concentrations of microplastic fragment and filament forms, total concentration, and the microplastic community, showed little correlation with sediment particle size. Our findings show microplastics arc ubiquitous in marine sediments around a remote coral island, at sizes ingestible by marine organisms, raising concerns about potential effects of microplastic ingestion by coral reef species. (C) 2020 Elsevier B.V. All rights reserved.