No abstract

No abstract

No abstract

The volcanic hazard potential of Mount Etna volcano is currently nourished by longlasting , powerful eruptions of basaltic magmas coupled with increased seismicity and ground deformation, and the world's largest discharge of volcanic gases. The current evolutionary cycle of Mount Etna activity is consistent with subduction-related chemical modifi cations of the mantle source. Arrival of a new mantle-derived magma batch beneath the volcano has been hypothesized, but is still elusive among the erupted products. Here we demonstrate petrological and geochemical affi nities between the magmas supplying modern eruptions and high-Mg, fallstratifi ed (FS) basalts ejected violently ~4 k.y. ago. The FS primitive magmas (~13 wt% MgO) are characteristically volatile enriched (at least 3.8 wt% H 2 O and 3300 ppm CO 2 ), and bear a trace element signature of a garnet-bearing, metasomatized source (high Gd/Yb, K/La, U/Nb, Pb/Ce, Ca/Al). They started crystallizing olivine (Fo 91 ), clinopyroxene (Mg# 92.5), and Cr spinel deep in the plumbing system (>5 kbar), contributing to the cumulate piles at depth and to differentiated alkaline basalt and trachybasalt magmas in the shallow conduit. Continuous infl ux of mantle-derived, volatile-rich magmas, such as those that supplied the FS fallout, provides a good explanation for major compositional and eruptive features of Mount Etna.

The origin of iron oxide-apatite deposits is controversial. Silicate liquid immiscibility and separation of an iron-rich melt has been invoked, but Fe–Ca–P-rich and Si-poor melts similar in composition to the ore have never been observed in natural or synthetic magmatic systems. Here we report experiments on intermediate magmas that develop liquid immiscibility at 100 MPa, 1000–1040 °C, and oxygen fugacity conditions (fO2) of ∆FMQ = 0.5–3.3 (FMQ = fayalite-magnetite-quartz equilibrium). Some of the immiscible melts are highly enriched in iron and phosphorous ± calcium, and strongly depleted in silicon (<5 wt.% SiO2). These Si-poor melts are in equilibrium with a rhyolitic conjugate and are produced under oxidized conditions (~FMQ + 3.3), high water activity (aH2O ≥ 0.7), and in fluorine-bearing systems (1 wt.%). Our results show that increasing aH2O and fO2 enlarges the two-liquid field thus allowing the Fe–Ca–P melt to separate easily from host silicic magma and produce iron oxide-apatite ores.

Nyerereite and nahcolite have been identified as micro- and nano-inclusions in diamond from the Juina area, Brazil. Alongside them are Sr- and Ba-bearing calcite minerals from the periclase-wiistite series, wollastonite II (high), Ca-rich garnet, spinels, olivine, phlogopite and apatite. Minerals of the periclase- wustite series belong to two separate groups: wustite and Mg-wustite with Mg# = 1.9—15.3, and Fe- periclase and periclase with Mg# = 84.9—92.1. Wollastonite-II (high, with Ca:Si = 0.992) has a triclinic structure. Two types of spinel were distinguished among mineral inclusions in diamond: zoned magnesioferrite (with Mg# varying from 13.5—90.8, core to rim) and Fe spinel (magnetite). Olivine (Mg# = 93.6), intergrown with nyerereite, forms an elongate, lath-shaped crystal and most likely represents a retrograde transformation of ringwoodite or wadsleyite. All inclusions are composed of poly-mineralic solid mineral phases. Together with previously found halides, sulphates and other mineral inclusions in diamond from Juina, they form a carbonatitic-type mineral paragenesis in diamond which may have originated in the lower mantle and/or transition zone. Wustite inclusions with Mg# = 1.9—3.4, according to experimental data, may have formed in the lowermost mantle. The source for the observed carbonatitic-type mineral association in diamond is lower-mantle natrocarbonatitic magma. This magma may represent a juvenile mantle melt, or be the result of low-degree partial melting of deeply-subducted carbonated oceanic crust. This magma was rich in volatiles, such as Cl, F and H, which played an important role in the formation of diamond.

No abstract

No abstract