The trace element distribution in peat soils affected by natural burning events: A proxy of the original composition and metals mobility assessment.

This work evaluates for the first time the effects on the trace element composition of peat soils affected by natural burning events, a recurrent phenomenon in the reclaimed wetland of the Mezzano Lowland (Padanian plain, NE Italy). The trace element distribution of two neighboring soil profiles, one pristine and one deeply affected by burning events, were compared to identify the original geochemical fingerprint of saltmarsh peat environment. The pre-combustion composition of the fired profile was reconstructed to infer the physico-chemical changes occurred as a consequence of the burning event, with a special attention to the mobility of elements of environmental concern, such as potentially toxic trace metals. The increase in concentration of potentially toxic elements (PTE) was particularly evident in two layers of the fired profile. V, Cr, Cu, Zn, Pb, and As contents progressively increase toward intermediate depths (30-75 cm) together with Th, Sr, Ba, U. On the contrary, Tl, Bi and Cd show a concentration peak in a thin, shallower (14-17 cm depth) horizon. The trace element composition of the unfired profile allowed the identification of specific ratios between immobile elements that can be used as geochemical fingerprint of the soils horizons with different soil organic matter (SOM) content. On the basis of Sr/Rb, Th/U and Ba/Sr it was possible to classify three types of sedimentary deposits characterizing both the unfired and fired profile, as well as to delineate the fire severity trends occurred in the different soil horizons of the fired profile. The distribution of immobile trace element, representative of the organic (U) and mineral (silicate, Th, Ba, REE and non-silicate, Sr) soil fractions with organic matter and bulk density in the non-fired profile, allowed the reconstruction of the original physico-chemical composition of the fired/burned profile and the accurate determination of the relative CO2 lost during the burning event. Moreover, the distribution of PTE with respect to immobile trace elements, used to estimate the element redistribution and mobility after burning in the fired profile, suggested that elements such as Cr, Ni, Zn, V were mainly immobile, whereas Pb, Mo and in particular Tl and Bi suffered a significant redistribution along the burned profile. Nonetheless, results of the gain/loss calculation for the whole soil profile suggested that no significant entry or leak of these elements occurred, limiting their redistribution inside the investigated soil system.

Quartz-bearing rhyolitic melts in the Earth's mantle.

The occurrence of rhyolite melts in the mantle has been predicted by high pressure-high temperature experiments but never observed in nature. Here we report natural quartz-bearing rhyolitic melt inclusions and interstitial glass within peridotite xenoliths. The oxygen isotope composition of quartz crystals shows the unequivocal continental crustal derivation of these melts, which approximate the minimum composition in the quartz-albite-orthoclase system. Thermodynamic modelling suggests rhyolite was originated from partial melting of near-anhydrous garnet-bearing metapelites at temperatures ~1000 °C and interacted with peridotite at pressure ~1 GPa. Reaction of rhyolite with olivine converted lherzolite rocks into orthopyroxene-domains and orthopyroxene + plagioclase veins. The recognition of rhyolitic melts in the mantle provides direct evidence for element cycling through earth's reservoirs, accommodated by dehydration and melting of crustal material, brought into the mantle by subduction, chemically modifying the mantle source, and ultimately returning to surface by arc magmatism.

Heavy oxygen recycled into the lithospheric mantle.

Magmas in volcanic arcs have geochemical and isotopic signatures that can be related to mantle metasomatism due to fluids and melts released by the down-going oceanic crust and overlying sediments, which modify the chemistry and mineralogy of the mantle wedge. However, the effectiveness of subduction-related metasomatic processes is difficult to evaluate because the composition of arc magmas is often overprinted by interactions with crustal lithologies occurring during magma ascent and emplacement. Here, we show unequivocal evidence for recycling of continental crust components into the mantle. Veined peridotite xenoliths sampled from Tallante monogenetic volcanoes in the Betic Cordillera (southern Spain) provide insights for mantle domains that reacted with Si-rich melts derived by partial melting of subducted crustal material. Felsic veins crosscutting peridotite and the surrounding orthopyroxene-rich metasomatic aureoles show the highest 18O/16O ratios measured to date in upper mantle assemblages worldwide. The anomalously high oxygen isotope compositions, coupled with very high 87Sr/86Sr values, imply the continental crust origin of the injected melts. Isotopic anomalies are progressively attenuated in peridotite away from the veins, showing 18O isotope variations well correlated with the amount of newly formed orthopyroxene. Diffusion may also affect the isotope ratios of mantle rocks undergoing crustal metasomatism due to the relaxation of 18O isotope anomalies to normal mantle values through time. Overall, the data define an O isotope "benchmark" allowing discrimination between mantle sources that attained re-equilibration after metasomatism (>5 Myr) and those affected by more recent subduction-derived enrichment processes.

High-Precision In Situ 87Sr/86Sr Analyses through Microsampling on Solid Samples: Applications to Earth and Life Sciences.

An analytical protocol for high-precision, in situ microscale isotopic investigations is presented here, which combines the use of a high-performing mechanical microsampling device and high-precision TIMS measurements on micro-Sr samples, allowing for excellent results both in accuracy and precision. The present paper is a detailed methodological description of the whole analytical procedure from sampling to elemental purification and Sr-isotope measurements. The method offers the potential to attain isotope data at the microscale on a wide range of solid materials with the use of minimally invasive sampling. In addition, we present three significant case studies for geological and life sciences, as examples of the various applications of microscale 87Sr/86Sr isotope ratios, concerning (i) the pre-eruptive mechanisms triggering recent eruptions at Nisyros volcano (Greece), (ii) the dynamics involved with the initial magma ascent during Eyjafjallajökull volcano's (Iceland) 2010 eruption, which are usually related to the precursory signals of the eruption, and (iii) the environmental context of a MIS 3 cave bear, Ursus spelaeus. The studied cases show the robustness of the methods, which can be also be applied in other areas, such as cultural heritage, archaeology, petrology, and forensic sciences.