Pyrrhotite–silicate melt partitioning of rhenium and the deep rhenium cycle in subduction zone

The Re-Os isotopic system serves as an important tracer of recycled crust in Earth’s deep mantle because of the large Re/Os ratios and time-integrated enrichment of radiogenic Os in Earth’s crust. However, the Re distribution in Earth’s known reservoirs is mass imbalanced, and the behavior of Re during subduction remains little understood. We performed laboratory experiments to determine the partition coefficients of Re between pyrrhotite and silicate melt (DRepo/sm) at 950–1080 °C, 1–3 GPa, and oxygen fugacities (in log units relative to the fayalite-magnetite-quartz [FMQ] buffer) of FMQ –1.3 to FMQ +2. The obtained DRepo/sm values are 200–25,000, which increase with decreasing oxygen fugacity and the total iron content (FeOtot) of silicate melt but decrease with increasing temperature or decreasing pressure. Applying DRepo/sm to constrain the behavior of Re during slab melting demonstrates that slab melts contribute minimal Re to the sub-arc mantle, with most Re dissolved in sulfides subducted into Earth’s deep mantle. Deep storage of recycled oceanic basalts and sediments can explain the mass imbalance of Re in Earth’s primitive mantle, depleted mantle, and crust.

Petrological and geochemical characteristic of the rocks of the Voznesensky intrusive massif (Southern Urals): Оn the question of the composition and sources of magma producing gold and copper porphyry mineralization

Research subject. The petrological and geochemical features of the rocks of the Voznesensky intrusive massif and its dyke series were studied in order to clarify the composition, possible sources and geodynamic settings of magma generation that produced Au- and Cu-porphyry mineralization.Methods. The content of petrogenic oxides was determined by the chemical method, trace elements – by ICP-MS analysis.Results. Among the rocks of the Voznesensky massif, which have the geochemical characteristics of suprasubduction formations, varieties with calc-alkaline and adakite-like properties were established. The main phase of the massif is represented by gabbro-diorites and diorites belonging to the calc-alkaline series. Ore-bearing dykes of gabbro-diorites, diorites and granodiorites of the Au-porphyry Bolshekaransky deposit are of calc-alkaline composition, while the post-ore dykes of granodiorites and plagiogranites of this deposit exhibit adakite-like characteristics.Conclusions. The ore-bearing dyke series of the Voznesensky deposit is represented by calc-alkaline diorites and adakite-like granodiorites and plagiogranites. The metallogenic specialization of the dykes was influenced by the silicic acidity and the redox state of the ore-generating melts. Granitoids with Cu-porphyry mineralization, compared to their gold-bearing varieties, crystallized from more acidic melts with a higher degree of oxidation. It is assumed that the main mantle component of magmas for the Voznesensky rocks were relatively weakly depleted spinel peridotites of the suprasubduction lithospheric mantle. Calc-alkaline magmas were melted from a mantle substrate previously metasomatized by aqueous fluids, and magmas with adakite-like properties – metamorphosed by melts of basalts and sedimentary rocks of slab. Melting of slab rocks may have been associated with additional heating due to friction caused by changes in direction and/or velocity of oblique subduction.

The Cretaceous igneous rocks in southeastern Guangxi and their implication for tectonic environment in southwestern South China Block

Southeastern Guangxi is located in the southwestern South China Block and to the northwest of the South China Sea (SCS), with abundant records of the Cretaceous magmatism. A detailed study of igneous rocks will contribute to a better understanding of the late Mesozoic tectonic environment. Zircon U–Pb dating yields ages of 93.37 ± 0.43 Ma for Yulin andesites and 107.6 ± 1.2 Ma for Luchuan granites. Yulin andesites are hornblende andesites, of which w(MgO) is between 7.72% and 8.42%, and Mg# is between 66.7 and 68.0, belonging to high magnesian andesites (HMAs) from peridotite sources. Luchuan granites are medium- to fine-grained monzogranites. Monzogranites and clastoporphyritic lava are high-K calc-alkaline series and metaluminum to weakly peraluminous series, which belong to the I-type granites. Those are enriched in Rb, Th, K and LREEs and depleted in Nb, Ta, P and Eu, showing the geochemical characteristics related to subduction. Unlike the contemporary “bimodal igneous rock assemblages” in Zhejiang and Fujian, the intermediate-acid magmatites in the southeastern Guangxi imply the compressive tectonic environment. The assemblage of HMAs and adakitic rocks indicates that the southwestern South China Block was under the Neo-Tethyan subduction during Cretaceous, and slab melting contributed to the magma in this area.

Seismic Imaging, Arc Magamtism and Megathrust Earthquake under the Western Pacific Subduction Zone

<p>Arc magmatism and megathrust earthquake occurrence in a subduction zone are deemed to attribute to many factors, including structural heterogeneities, fluid contents, temperature, depth of subducting oceanic crust, and etc. However, how these factors affect them is unclear. The extensive arc magmatism observed on the island arcs is considered to be an indicator on chemical exchange between the wedge mantle and the surface in a subduction zone. Megathrust earthquake frequently occurrence is also considered to be attributed to the slab melting and associated interplate coupling of the subducting plate. The Western Pacific subduction zone is regarded as one of the best Laboratory for seismologists to examine these processes due to the densest seismic networks recording numerous earthquakes. Some of the previous studies suggest that the island-arc magmatism is mainly contributed to the melting of peridotite in the mantle wedge due to fluids intrusion from the dehydration process associated with the subducting oceanic crust. They further argued that the oceanic plate could only provide water to the overlying mantle wedge for arc magmatism, but not melt itself due to that it is too cold to melt at a depth between 100 and 200km. However, some of other studies revealed that the hydrated basalt derived from the mid-ocean ridge will be melted with high T and water saturated on the upper interface of the sbuducting plate in the mantle wedge. We consider that the three-dimensional (3-D) P- and S- wave velocity (Vp, Vs) and Poisson’s ratio (σ) structures of the subduction zone, synthesized from a large-quantity of high-quality direct P-, and S-wave arrival times of source-recive pairs from the well located suboceanic events with sP depth phase data could provide a compelling evidence for structural heterogeneity, highly hydrated and serpentinized forearc mantle and dehydrated fluids in the subduction zones. In this study, we combined seismic velocities and Poisson’s ratio images under the entire-arc region of the Western Pacific subduction zone to reveal their effects on megathrust earthquake generation and arc magmatism. We find that a ~10 km-thick low-velocity layer with high-V and high-Poisson’s ratio anomalies is clearly imaged along the upper interface of the subducting Pacific slab. This distinct layer implies partial melting of the oceanic crust due to the deep-seated metamorophic reactions depending on the source of fluids and temperature regime. Such a process could refertilize the overlying mantle wedge and enrich the peridotite sources of basalts under the island arc. Significant low-V and high-Poisson’s ratio anomalies were observed in the mantle wedge along the volcanic front, indicating melting or partial melting of peridotite-rich mantle and then yield tholeiitic magma there. The present study demonstrates that the combined factors of fluid content, mineral composition and thermal regime play a crucial role in both slab melting and arc-magmatism under the Western Pacific subduction zone.</p>