The genetic relationship between andesites and dacites at Tungurahua volcano, Ecuador

Volcanic eruptions of intermediary and silica-rich magmas (andesites, dacites and rhyolites) in convergent arc settings generate voluminous and explosive eruptions that can strongly affect human activity and have significant environmental impacts. It is therefore crucial to understand how these magmas are generated in order to anticipate their potential impact. At convergent margins, primitive magmas (primitive basalts and/or andesites) are derived from the mantle wedge and they are progressively modified by physical and chemical processes operating between the melting zone and the surface to produce silica-rich magmas. In order to elucidate the relationship between andesites and dacites, we focus on Tungurahua volcano, located in the Ecuadorian Andes. We collected a set of samples comprising such lithologies that were erupted during the last 3000 year BP. This relatively short period of time allows us to assume that the geodynamic parameters remain constant. Petrology and major-trace element compositions of these lavas have already been examined, and so we performed a complementary Pb-Sr isotope study in order to determine the nature and origin of the components involved in andesite and dacite genesis. Sr isotopes range from 0.70417 to 0.70431, and Pb isotope compositions range from 18.889 to 19.154 for Pb-206/Pb-204, from 15.658 to 15.696 for Pb-207/Pb-204, and from 38.752 to 38.918 for Pb-208/Pb-204. Dacites display a remarkably homogeneous Pb isotopic composition, with higher Pb-206/Pb-204 values for a given Pb207-208/Pb-204 compared to andesites. Andesites show notable Pb-207/Pb-206 variations for a given SiO2 content, whereas dacites have lower and homogenous Pb-207/Pb-206 values. Andesite and dacite altogether plot in a roughly triangular distribution, with dacitic magmas systematically plotting at the high SiO2 and Sr-87/Sr-86 and low Pb-207/Pb-206 fields. Based on our new dataset, we show that at least 3 different components are required to explain the Tungurahua compositional and isotope variation: one corresponds to the mantle, the second has a deep origin (slab component or lower crust), and a mixture between these two components explains andesite heterogeneity. The third component is derived from the underlying upper continental crust. While andesites are derived from deep components, dacites are derived from the andesitic magmas that underwent an assimilation-fractional crystallization (AFC) process with incorporation of the local metamorphic basement. Finally, we used the geochemical and isotopic data to produce a model of the magmatic plumbing system beneath Tungurahua, consistent with geophysical and experimental petrology constraints. We conclude that melt migration and storage in the upper crust appears to be a key parameter for controlling volcanic behavior though time

Rare earth element partitioning between sulphides and melt : evidence for Yb2+ and Sm2+ in EH chondrites

We present the first complete dataset of partition coefficients of Rare Earth Elements (REE) between oldhamites or molten FeS and silicate melts. Values have been determined at 1300 and 1400 °C from experiments on mixtures of a natural enstatite chondrite and sulphides powders (FeS or CaS) performed in evacuated silica tubes for different fO2 conditions (from IW-6.9 to IW-4.1). Obtained REE partitioning values are between 0.5 and 5 for oldhamites and between 0.001 and 1 for FeS. In both sulphides, Eu and Yb are preferentially incorporated compared to neighbouring REE. X-ray Absorption Near Edge Structure measurements on Yb and Sm demonstrate the partial reduction to 2+ valence state for both elements, Yb reduction being more pronounced. Therefore, the Yb anomaly in the sulphides is interpreted to be an effect of the presence of Yb2+ in the system and the amplitude of the anomaly increases with decreasing oxygen fugacity. The obtained oldhamite/silicate melt partition coefficients patterns are unlike any of the observed data in natural oldhamites from enstatite chondrites and achondrites. In particular, the low values do not explain the observed enrichments in oldhamite crystals. However, positive Eu and Yb anomalies are observed in some oldhamites from EH chondrites and aubrites. We attribute these anomalies found in meteorites to the sole oldhamite control on REE budget. We conclude that the presence of positive Eu and Yb anomalies in oldhamites is a good indicator of their primordial character and that these oldhamites carry a condensation signature from a highly reduced nebular gas

182W evidence for core-mantle interaction in the source of mantle plumes

Tungsten isotopes are the ideal tracers of core-mantle chemical interaction. Given that W is moderately siderophile, it preferentially partitioned into the Earth's core during its segregation, leaving the mantle depleted in this element. In contrast, Hf is lithophile, and its short-lived radioactive isotope 182Hf decayed entirely to 182W in the mantle after metal-silicate segregation. Therefore, the 182W isotopic composition of the Earth's mantle and its core are expected to differ by about 200 ppm. Here, we report new high precision W isotope data for mantle-derived rock samples from the Paleoarchean Pilbara Craton, and the Réunion Island and the Kerguelen Archipelago hotspots. Together with other available data, they reveal a temporal shift in the 182W isotopic composition of the mantle that is best explained by core-mantle chemical interaction. Core-mantle exchange might be facilitated by diffusive isotope exchange at the core-mantle boundary, or the exsolution of W-rich, Si-Mg-Fe oxides from the core into the mantle. Tung-sten-182 isotope compositions of mantle-derived magmas are similar from 4.3 to 2.7 Ga and decrease afterwards. This change could be related to the onset of the crystallisation of the inner core or to the initiation of post-Archean deep slab subduction that more efficiently mixed the mantle

A Precambrian microcontinent in the Indian Ocean

The Laccadive–Chagos Ridge and Southern Mascarene Plateau in the north-central and western Indian Ocean, respectively, are thought to be volcanic chains formed above the Réunion mantle plume1 over the past 65.5 million years2,3. Here we use U–Pb dating to analyse the ages of zircon xenocrysts found within young lavas on the island of Mauritius, part of the Southern Mascarene Plateau. We find that the zircons are either Palaeoproterozoic (more than 1,971 million years old) or Neoproterozoic (between 660 and 840 million years old). We propose that the zircons were assimilated from ancient fragments of continental lithosphere beneath Mauritius, and were brought to the surface by plume-related lavas. We use gravity data inversion to map crustal thickness and find that Mauritius forms part of a contiguous block of anomalously thick crust that extends in an arc northwards to the Seychelles. Using plate tectonic reconstructions, we show that Mauritius and the adjacent Mascarene Plateau may overlie a Precambrian microcontinent that we call Mauritia. On the basis of reinterpretation of marine geophysical data4, we propose that Mauritia was separated from Madagascar and fragmented into a ribbon-like configuration by a series of mid-ocean ridge jumps during the opening of the Mascarene ocean basin between 83.5 and 61 million years ago. We suggest that the plume-related magmatic deposits have since covered Mauritia and potentially other continental fragments