Asthenosphere dynamics based on the H2O dependence of element diffusivity in olivine

The oceanic asthenosphere shows two enigmatic features: low viscosity and high electrical conductivity. Their origins gather wide attention, but remain unsolved. Recent self-diffusivity measurements as a function of H2O content in olivine demonstrated that the H2O-incorporation in olivine cannot soften the asthenosphere, but it enhances the ionic conductivity, and causes the high-conductivity anomaly.

Zircon survival in shallow asthenosphere and deep lithosphere

Zircon (ZrSiO4) is the most frequently used geochronometer of terrestrial and extraterrestrial processes. To shed light on question of zircon survival in the Earth's shallow asthenosphere, high-temperature experiments of zircon dissolution in natural mid-ocean ridge basaltic (MORB) and synthetic haplobasaltic melts have been performed at temperatures of 1250–1300 °C and pressures from 0.1 MPa to 0.7 GPa. Zirconium measurements were made in situ by electron probe microanalyses (EPMA) at high current. Taking into account secondary fluorescence effects in zircon-glass pairs during EPMA, a zirconium diffusion coefficient of 2.87E-08 cm2/s was determined at 1300 °C and 0.5 GPa. When applied to the question of zircon survival in asthenospheric melts of tholeiitic basalt composition, the data are used to infer that typical 100 mm zircon crystals dissolve rapidly (~10 h) and congruently upon reaction with basaltic melt at pressures of 0.2–0.7 GPa. We observed incongruent (to crystal ZrO2 and SiO2 in melt) dissolution of zircon in natural mid-ocean ridge the basaltic melt at low pressures <0.2 GPa and in the haplobasaltic melt at 0.7 GPa pressure. Our experimental data raise questions about the origin of zircon crystals in mafic and ultramafic rocks, in particular, in shallow oceanic asthenosphere and deep lithosphere, as well as the meaning of the zircon-based ages estimated from these minerals. The origin of zircon in shallow (ultra-) mafic chambers is likely related to the crystallization of intercumulus liquid. Large zircon megacrysts in kimberlites, peridotites, alkali basalts, and carbonatite magmas suggest fast transport and short interaction durations between zircon and melt. The origin of zircon megacrysts is likely related to metasomatic addition of Zr into the mantle as an episode of mantle melting should eliminate them on geologically short timescales.

Ancient Melt Depletion and Metasomatic History of the Subduction Zone Mantle: Osmium Isotope Evidence of Peridotites from the Yap Trench, Western Pacific

Highly depleted peridotites from the Yap Trench in the western Pacific Ocean have been studied for Re-Os elements and Re-Os isotopes. These peridotites have a low Re-Os content and variable 187Os/188Os ratios (0.12043–0.14867). The highest 187Os/188Os ratio is far higher than that of the primitive upper mantle and the lowest 187Os/188Os ratio is comparable to the most unradiogenic 187Os/188Os ratio (0.11933) discovered in subduction zone peridotites. The suprachondritic 187Os/188Os ratios of the Yap Trench peridotites results from modification of the mantle wedge by slab-derived fluid and melt. This is consistent with the observation that high 187Os/188Os ratios generally occur in oceanic peridotites with low Os content (<2 ppb) since Os may be reduced during late processes such as fluid alteration and melt refertilization. The sub-chondritic 187Os/188Os ratios of the Yap Trench peridotites correspond to a Re depletion age of 0.24–1.16 billion years, which means that these peridotites represent old mantle residue of ancient melting events. This ancient melting, combined with probable back-arc melting and forearc melting during subduction initiation, indicates that the Yap Trench mantle has a complex evolutionary history. The amount of old mantle residue in the oceanic asthenosphere was underestimated because the 187Os/188Os ratio in mantle peridotites is elevated during late processes. Therefore, old depleted mantle fragments may contribute substantially to the chemical heterogeneity of the oceanic mantle.

"Diffusive" border of isotopic reservoirs of indian and pacific morb types beneath Kamchatka

The convergence zones of lithospheric plates in the Northwest Pacific are the boundaries of the two main isotopic domains of the Earth - the Indian and Pacific MORB types, separated be cold oceanic lithosphere. This configuration limits of their interaction by special geodynamic environments - the influence of deep plume sources or the distraction of the subducted slab and intrusion of the oceanic asthenosphere into the subcontinental mantle wedge. The latter mechanism is reconstructed in the Central Kamchatka Depression on the basis of geological, geochemical, and high-precision (double-spike) lead isotopic data. The role of the oceanic asthenosphere in magma generation in the zones of convergence of oceanic and continental lithospheres is a poorly studied but not unique phenomenon that must be considered under geodynamic reconstructions and the creation of new, more realistic models of the juvenile continental crust formation.

Evolution of the isotopic-geochemical composition of rocks of Uksichan volcano, Sredinnyi range, Kamchatka, and its relations to the tectonic restyling of Kamchatka in the neogene

The paper presents newly acquired data on concentrations of major and trace elements and on Sr, Nd, and Pb isotope composition in Pliocene and Late Pleistocene–Holocene mafic volcanic rocks of the Uksichan volcanic center, one of the largest in the Sredinnyi Range of Kamchatka. Based on these data, the mafic Pliocene volcanics are thought to be produced by the melting of heterogenized mantle material, which had been hybridized by subduction and asthenospheric processes. The behavior of HFSE and Pb isotopic systematics provide evidence of the melting of subducted sediment and origin of pyroxenite segregations in the peridotite matrix. The low ∆8/4Pb values of the Pliocene lavas of Uksichan shield volcano and in modern large volcanic centers in the Central Kamchatka Depression are correlated with the magmatic productivity, which indicates, when considered together with HFSE and HREE behavior, that the Pacific asthenosphere was involved in the magma-generating processes. The Late Pleistocene–Holocene basalt volcanism, which was spatially constrained to the peripheries of the Pliocene shield edifice, developed in an extensional environment as a result of the melting of an enriched mantle source. The attenuation and then complete termination of volcanic activity in the Sredinnyi Range in the Late Pleistocene–Holocene was associated with an increase in the ∆8/4Pb of the mafic lavas, which indicates that the center of the activity related to the oceanic asthenosphere shifted eastward toward the Central Kamchatka Depression. The influence of the oceanic asthenosphere on subduction-related magmatism is not unique to convergence zones alone and should be taken into consideration when models are constructed for the origin of juvenile continental crust.