Petrology最新文献

The origin of native Fe in the Earth’s crust was experimentally modeled by reproducing interaction between basalt melts and fluid (H₂ and/or H₂ + CH₄) at a temperature of 1100–1250°C, fluid pressure of 1–100 MPa, and strongly reducing conditions with fO₂ = 10⁻¹² to 10⁻¹⁴ bar. The experiments were carried out in an IHPV equipped with a unit of original design for conducting long lasting experiments under a high pressure of reduced fluid. The experiments were done using samples of natural magmatic rock: magnesian basalt from Tolbachik volcano, Kamchatka, and this magnesian basalt enriched in Ni and Co oxides. The experimental results highlight the following features of interaction between reducing fluid and basalt melt: (1) In spite of the high reducing potential of the system of H₂ or (H₂ + CH₄) fluid with magmatic melt, hydrogen oxidation and reduction reactions of metals of variable valence are not completed in the system. Redox reactions in the basalt melt are terminated because H₂O forms in the system and buffers the redox potential of H₂ of the (H₂ + CH₄) mixture. (2) The initially homogeneous magmatic melt becomes heterogeneous: the newly formed H₂O dissolves in the melt and partially in the fluid phase, and this results in more silicic melt and small metal segregations morphologically resembling liquid immiscibility textures. (3) The onset of metal–silicate liquid immiscibility in magmatic melts interacting with reducing fluid can occur at geologically realistic temperatures (≤1250°C), which are much lower than the melting temperatures of iron and its alloys with nickel and cobalt. (4) Carbon, which is formed in the experiments by methane pyrolysis, is dissolved in the metal phase. Our experimental results illustrate a mechanism explaining the occurrence of carbon in natural native iron. (5) The texture and size of the experimentally reproduced metal segregations are consistent with data on naturally occurring native metals, primarily, iron and its alloys with nickel and cobalt, in magmatic rocks of various composition and genesis.

Large single crystals of natural zircon were shock-loaded at 13.6 and 51.3 GPa in planar geometry. No structural changes were observed in zircon after loading at 13.6 GPa. Loading to 51.3 GPa resulted in zircon transformation to a denser scheelite-structured phase, reidite. The investigation of reidite samples by X-ray diffraction, Raman, photo- and cathodoluminescence spectroscopies revealed segregation of some trace cations (e.g., REE) on planar defects during the transformation. Importantly, the segregation occurred in a laboratory experiment without long-term annealing after shock loading. A possible mechanism of segregation of trivalent trace cations in zircon includes local violation of charge balance during the zircon–reidite reconstructive transformation, which is accompanied by considerable changes in the topology of polyhedra and second coordination spheres (Si–Zr). This results in expulsion of a fraction of the trace element into energetically expensive interstitial positions with high diffusivity even at relatively low temperatures.

To explore the tectonic evolution of the Bairiqili area in the East Kunlun Orogen, we conducted petrological, geochemical, and geochronological analyses of fine-grained quartz diorite, coarse-grained quartz diorite, and granite porphyry from the region. The results show that the weighted average ages of these rocks are 251.6 ± 1.2 Ma for fine-grained quartz diorite, 250.2 ± 0.6 Ma for coarse-grained quartz diorite, and 245.8 ± 0.9 Ma for granite porphyry, indicating formation during the Early and Middle Triassic periods. The coarse-grained quartz diorite is peraluminous, low in silicon, and high in iron. It belongs to the calc-alkaline series of magmatic rocks, enriched in large ion lithophile elements (Rb, K, Ba), and relatively depleted in high-field strength elements (P, Nb, Ti). The zircon ε_Hf(T) values for fine-grained quartz diorite range from –8.68 to –3.39, with two-stage model ages (T_DM2) between 1493 and 1828 Ma. The zircon ε_Hf(T) values for coarse-grained quartz diorite range from –9.46 to –2.88, with T_DM2 ages from 1459 to 1875 Ma, and for granite porphyry, the zircon ε_Hf(T) values range from –3.37 to –1.56, with T_DM2 ages from 1374 to 1488 Ma. These geochemical data suggest that the three plutons primarily derive from crust-mantle mixed sources. All plutons formed in a volcanic arc environment, related to arc magmatism driven by the northward subduction of the Paleo-Tethys oceanic crust. Regional data indicate that the Paleo-Tethys Ocean closed likely in the Middle Triassic.

The Elimala and Markanje Plutons in the Mangalore-Gundlupet Crustal Corridor of the southern part of the Western Dharwar Craton were investigated to elucidate their geochemistry, geochronology, and petrogenesis. Zircon geochemistry revealed high Hf concentrations (9019–10011 ppm), steep HREE enrichment, positive Ce anomalies, and negative Eu anomalies, typical of magmatic zircons derived from highly evolved melts. U–Pb dating of magmatic zircons from the Elimala Pluton yielded a Concordia age of 832 ± 4 Ma, indicating crystallization during the Neoproterozoic. Bulk-rock geochemical analyses classified the granitoids as calc-alkaline, metaluminous to weakly peraluminous, I-type granitoids. Rare earth element patterns and trace element compositions, such as high Sr/Y and low (Dy/Yb)_N ratios, indicate derivation from the partial melting of thickened lower crustal rocks within the garnet and/or amphibole stability field. The studied rocks exhibit characteristics of adakite-like granitoids, displaying affinities to high-silica adakites but with distinct compositional features that differentiate them from slab-derived melts. Their petrogenesis is attributed to low-degree partial melting of thickened lower continental crust rather than fractional crystallization or slab melting. Zircon trace element signatures and bulk-rock chemistry collectively suggest a crustal origin with minimal post-magmatic alteration, supported by low loss-on-ignition values and immobile element behavior. These findings provide new insights into the diverse lithology and crustal evolution of the southern part of the Western Dharwar Craton during the Neoproterozoic and the processes that led to the formation of adakite-like rocks.

Relatively large crystals of baddeleyite (up to 100 μm) and zircon (up to 400 μm) were found in Cenozoic subalkaline andesites in the northern Sikhote-Alin. The reasons for such a rare association of Zr minerals in volcanics are discussed based on the petrological characteristics of the andesites, their U−Pb isotope dating, and contents of trace elements in the baddeleyite and zircon. The andesites contain rare clinopyroxene phenocrysts, which crystallized in the temperature range of 1090–1150°C. The fine-grained matrix is composed of pyroxene, plagioclase, hornblende, ilmenite, and felsic domains, which consist of K−Na feldspar, quartz and silicic glass and were formed at 926°C. The accessory baddeleyite and zircon are confined to the felsic domains. The obtained U−Pb age of the baddeleyite 52.7 ± 1.1 Ma (MSWD = 2.6) can be used as an estimation of the crystallization age of the andesite melts. The zircon ²⁰⁶Pb/²³⁸U age broadly varies within the range of 46 to 56 Ma. The youngest values (46–49 Ma) probably reflect the partial disturbance of the isotope system caused by the high contents of U (up to 1.3 wt %) and Th (up to 3.8 wt %). The baddeleyite shows a simultaneous decrease in concentrations of Hf (from 7742 to 2869 ppm), Y, and heavy REE, which may be explained by its competitive crystallization with amphibole. Well-pronounced negative Eu anomalies in the baddeleyite and zircon suggest their growth simultaneously with feldspars. High concentrations of HREE, U, and Th in the zircon indicate its crystallization from enriched residual melts. Zircon crystallization temperatures estimated using the Ti-in-zircon geothermometer (from 800 to 990°C) are comparable with temperature estimations for the felsic domains. The baddeleyite and zircon compositions imply that the minerals crystallized during the late stages of the melt evolution, perhaps in an intermediate magma chamber. In the magmatic history of the Sikhote-Alin, the Early Eocene andesites formed between the older Paleocene−Early Eocene A-type rhyolites (61–53 Ma) and the younger Eocene–Miocene basalts (40–20 Ma). This time span is thought to have been associated with lithospheric extension due to the break-up of the downgoing oceanic slab and the opening of a mantle window, with the Early Eocene andesites likely marking this tectonic event.