Isotopic evolution of prehistoric magma sources of Mt. Etna, Sicily: Insights from the Valle Del Bove

Mount Etna in NE Sicily occupies an unusual tectonic position in the convergence zone between the African and Eurasian plates, near the Quaternary subduction-related Aeolian arc and above the down-going Ionian oceanic slab. Magmatic evolution broadly involves a transition from an early tholeiitic phase (~ 500 ka) to the current alkaline phase. Most geochemical investigations have focussed on either historic (> 130-years old) or recent (< 130-years old) eruptions of Mt. Etna or on the ancient basal lavas (ca. 500 ka). In this study, we have analysed and modelled the petrogenesis of alkalic lavas from the southern wall of the Valle del Bove, which represent a time span of Mt. Etna’s prehistoric magmatic activity from ~ 85 to ~ 4 ka. They exhibit geochemical variations that distinguish them as six separate lithostratigraphic and volcanic units. Isotopic data (143Nd/144Nd = 0.51283–0.51291; 87Sr/86Sr = 0.70332–0.70363; 176Hf/177Hf = 0.28288–0.28298; 206Pb/204Pb = 19.76–20.03) indicate changes in the magma source during the ~ 80 kyr of activity that do not follow the previously observed temporal trend. The oldest analysed Valle del Bove unit (Salifizio-1) erupted basaltic trachyandesites with variations in 143Nd/144Nd and 87Sr/86Sr ratios indicating a magma source remarkably similar to that of recent Etna eruptions, while four of the five subsequent units have isotopic compositions resembling those of historic Etna magmas. All five magma batches are considered to be derived from melting of a mixture of spinel lherzolite and pyroxenite (± garnet). In contrast, the sixth unit, the main Piano Provenzana formation (~ 42–30 ka), includes the most evolved trachyandesitic lavas (58–62 wt% SiO2) and exhibits notably lower 176Hf/177Hf, 143Nd/144Nd, and 206Pb/204Pb ratios than the other prehistoric Valle del Bove units. This isotopic signature has not yet been observed in any other samples from Mt. Etna and we suggest that the parental melts of the trachyandesites were derived predominantly from ancient pyroxenite in the mantle source of Etna

Triple oxygen isotopic composition of the high-³He/⁴He mantle

Measurements of Xe isotope ratios in ocean island basalts (OIB) suggest that Earth’s mantle accreted heterogeneously, and that compositional remnants of accretion are sampled by modern, high-3He/4He OIB associated with the Icelandic and Samoan plumes. If so, the high-3He/4He source may also have a distinct oxygen isotopic composition from the rest of the mantle. Here, we test if the major elements of the high-3He/4He source preserve any evidence of heterogeneous accretion using measurements of three oxygen isotopes on olivine from a variety of high-3He/4He OIB locations. To high precision, the Δ17O value of high-3He/4He olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (Δ17OBulk Mantle − Δ17OHigh 3He/4He olivine = −0.002 ± 0.004 (2 × SEM)‰). Thus, there is no resolvable oxygen isotope evidence for heterogeneous accretion in the high-3He/4He source. Modelling of mixing processes indicates that if an early-forming, oxygen-isotope distinct mantle did exist, either the anomaly was extremely small, or the anomaly was homogenised away by later mantle convection. The δ18O values of olivine with the highest 3He/4He ratios from a variety of OIB locations have a relatively uniform composition (∼5‰). This composition is intermediate to values associated with the depleted MORB mantle and the average mantle. Similarly, δ18O values of olivine from high-3He/4He OIB correlate with radiogenic isotope ratios of He, Sr, and Nd. Combined, this suggests that magmatic oxygen is sourced from the same mantle as other, more incompatible elements and that the intermediate δ18O value is a feature of the high-3He/4He mantle source. The processes responsible for the δ18O signature of high-3He/4He mantle are not certain, but δ18O–87Sr/86Sr correlations indicate that it may be connected to a predominance of a HIMU-like (high U/Pb) component or other moderate δ18O components recycled into the high-3He/4He source

An Integrated Site-Wide Assessment of Nuclear Wastes to Remain at the Hanford Site, Washington

Since its creation in 1943 until 1988, the Hanford Site, a facility in the U.S. Department of Energy (DOE) nuclear weapons complex was dedicated to the production of weapons grade plutonium and other special nuclear materials. The Hanford Site is located in eastern Washington State and is bordered on the north and east by the Columbia River. Decades of creating fuel, irradiating it in reactors, and processing it to recover nuclear material left numerous waste sites that involved the discharge of contaminated liquids and the disposal of contaminated solid waste. Today, the primary mission of the Hanford Site is to safely cleanup and manage the site's legacy waste. A site-wide risk assessment methodology has been developed to assist the DOE, as well as state and federal regulatory agencies, in making decisions regarding needed remedial actions at past waste sites, and safe disposal of future wastes. The methodology, referred to as the System Assessment Capability (SAC), utilizes an integrated set of models that track potential contaminants from inventory through vadose zone, groundwater, Columbia River and air pathways to human and ecological receptors

Mantle dynamics and secular variations beneath the East African Rift: Insights from peridotite xenoliths (Mega, Ethiopia)

Segments of the Main Ethiopian Rift (MER), a key sector of the East African Rift (EAR) linking the Afar and Turkana depressions, record different stages of lithospheric evolution from initiation to continental break-up and incipient oceanic spreading. Although EAR tectonic and magmatic activity is generally ascribed to a manifestation of sublithospheric mantle processes, the number, depth of provenance and triggering mechanisms of the related mantle upwellings remain topics of debate.Wepresent newHf and Pb isotope data for EAR mantle xenolith clinopyroxenes from Southern Ethiopia (Mega, the Sidamo region) that compellingly testify to multiple episodes of mantle depletion and metasomatic enrichment. Radiogenic values (εHf up to +1076) suggest that present-day MER lithospheric mantle domains underwent melting, possibly in the presence of residual garnet, billions of years ago, thereby fractionating (increasing) the Lu/Hf ratio and ultimately leading to extremely high 176Hf/177Hf. Although the precise dating of these depletion episodes is hampered by possible metasomatic overprinting, positively correlated Lu/Hf and 176Hf/177Hf indicates apparent ingrowth at ca. 1.9 Ga, providing a minimum age for the delineated Proterozoic melting events. Our findings of Early Proterozoic melting episodes are complementary to previously determined Os model ages on xenoliths fromthe same site indicating melt extraction between 2.4 and 2.8 Ga (Reisberg et al., 2004; Chem. Geol. 208, 119–140). Taken together, the geochemical and isotopic characteristics of the Mega xenoliths therefore record melt extraction events and preserve memory of ancient mantle dynamics beneath this site, where important lithospheric discontinuities exist between the Archean/Early Proterozoic Tanzanian craton and the Late Proterozoic Panafrican mobile belt. Subsequent metasomatic reactions variably affected the Nd and Hf isotopic compositions of some samples and completely overprinted the Pb isotopic composition of the whole xenolith suite. The persistence of these geochemical heterogeneities within the investigated suites of xenoliths precludes pervasive melt refertilization, which would have homogenized existing compositions and obliterated the record of previous petrologic processes. This suggests that the MER segment considered here developed on a lithospheric section isolated by preexisting tectonic structures, far from the influence of plumes originating deep in the convecting mantle

Magma dynamics of ancient Mt. Etna inferred from clinopyroxene isotopic and trace element systematics

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Elemental and isotopic perspectives on the impact of arbuscular mycorrhizal and ectomycorrhizal fungi on mineral weathering across imposed geologic gradients

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