The balance of CO 2 during abundant basaltic magma production is an important factor of volcanic hazards and climate. In particular, this can be explored based on CO 2 -rich mantle-derived magmas or carbonate assimilation by basaltic melts. To reconstruct the origin of Fe-rich carbonates hosted by Cenozoic basalts from Wangtian’e volcano (northeast China), we studied elemental compositions of melt, crystalline and fluid inclusions in magmatic minerals as well as the oxygen and carbon isotope compositions of the plagioclase and carbonates from basalts. The crystallization of basaltic magmas occurred in shallow chamber (∼4 km) at temperatures of 1,180°C–1,200°C and a pressure of 0.1 ± 0.01 GPa. Stable Fe-rich carbonates occur in the Wangtian’e tholeiite basalts as groundmass minerals, crystalline inclusions in plagioclase and globules in melt inclusions, which suggests that they crystallized from a ferrocarbonate melt. The values of δ 18 О and δ 13 С in the minerals analyzed by laser fluorination method are in line with the sedimentary source of Fe-rich carbonates, indicating assimilation and partial decomposition of carbonate phases. The parent ferrocarbonate melt could be produced during interactions between the basaltic magma and the crustal marbles. The phase diagram and thermodynamic calculations show that the ferrocarbonate melt is stable at a temperature of 1,200°C and a pressure of 0.1 GPa. Our thermodynamic calculations show that carbonate melt containing 73 wt% FeCO 3 , 24 wt% MgCO 3 and 3 wt% CaCO 3 is in thermodynamic equilibrium with silicate melt in agreement with our natural observations. The proposed mechanism is crustal carbonate sediment assimilation by the intraplate basaltic magma resulting in the melt immiscibility, production of the ferrocarbonate melt and the following Fe-rich carbonate mineral crystallization during magma residence and cooling.
Abstract We have determined the Sr, Nd, Pb and Hf isotopic compositions of clinopyroxene separated from mantle-derived ultramafic xenoliths (six spinel peridotites, two composite Cr-diopside pyroxenites, and one discrete Al-augite pyroxenite) hosted by Cenozoic alkali basalts at Hannuoba, North China, in order to understand the nature of the mantle source for this intraplate volcanism, and the petrologic history of the mantle lithosphere beneath North China Block, a crustal segment of the Sino-Korean Craton. Measured Sr, Nd, Pb and Hf isotopic compositions in the clinopyroxene grains separated from spinel peridotite and Cr-diopside pyroxenite (87Sr/86Sr = 0.70265 to 0.70485; 206Pb/204Pb = 17.75 to 19.15; ɛNd = 0 to + 11; ɛHf = + 10 to + 38) display mixing hyperbolas between mantle compositional end members DMM and EMII on the Sr–Pb and Nd–Pb isotope correlation diagrams. This is distinctly different from the host basalt data which show a mixture of DMM and EMI components on the diagrams. We interpret this to reflect infiltration by metasomatic agents, possibly silicate melts, having an EMII-like isotopic signature, which enriched a precursor time-integrated depleted mantle. An Al-augite pyroxenite, also hosted by these basalts, is characterized by highly enriched Sr, Nd, and Hf isotopic compositions (87Sr/86Sr = 0.70733; ɛNd = − 16; ɛHf = − 18) with only moderately radiogenic Pb that has a 206Pb/204Pb value of 18.23. All of these data plot outside (1) the fields for oceanic basalts, and (2) the mixing arrays defined on the isotopic correlation diagrams by peridotites/Cr-diopside pyroxenite with their metasomatic agents, and by the host basalt. These observations suggest that (1) Al-augite pyroxenite is not cogenetic with the Cr-diopside pyroxenite, (2) parental melts of the pyroxenites are not likely to be the source for the metasomatism, and (3) the thermo/mechanically reactivated pyroxenite and/or spinel peridotite, is not likely to be the source for host basalt magmatism. The Cenozoic intraplate volcanism, therefore, must have originated in the asthenosphere. We observe that the relatively little-metasomatized Hannuoba peridotites define a Lu–Hf isochron of 2587 ± 86 Ma (2σ). This value is, within error, indistinguishable from the Sm–Nd isochron age of the overlying granulite terrain. We suggest, therefore, that the Lu–Hf system can be used to constrain the timing of lithospheric mantle differentiation. Preservation of the Neoarchean mantle lithosphere beneath Hannuoba, despite the protracted tectono-magmatic reactivation during the Mesozoic and Cenozoic in this area, suggests that complete removal of the lithospheric mantle beneath East Asia by wholesale delamination is unlikely.
Abstract Despite extensive modeling efforts, the dynamics of subduction initiation (SI), including the role of elasticity, are not fully understood. Using two‐dimensional thermomechanical models with visco‐plastic (VP) and visco‐elasto‐plastic (VEP) rheologies, we systematically investigate the role of elasticity in intraoceanic SI using two model setups: spontaneous initiation without imposed convergence and induced initiation with imposed convergence. In spontaneous models, the overriding plate age of <20 Ma and the subducting plate age of >50 Ma generally lead to vertically driven SI with either rheology, but for a given age contrast, SI is easier to occur with the VEP rheology. In induced model with either rheology, when the two plates are young and have a small age contrast, the resulting SI is horizontally driven, and elasticity does not affect SI significantly, regardless of the convergence rate. However, when the thermal age contrast is large and a convergence rate is relatively low, the SI in induced models is vertically driven and similar to that in the spontaneous models, and the VEP rheology leads to faster SI than the VP rheology. This effect of elasticity becomes smaller with increasing initial horizontal compressional stress but does not become fully negated by the initial stress of <∼50 MPa. Therefore, inclusion of elasticity with reasonable shear modulus and initial stress values results in a weaker slab, making it easier for vertically driven SI to occur when the age contrast is relatively large.
The Fe-V-Ti-PGE-bearing Hongge layered intrusion in the Pan-Xi area of Sichuan Province in southwestern China consists, from the base upward, of zones of olivine clinopyroxenite (Cycle I), clinopyroxenite (Cycles II, III) and gabbro (Cycle IV). Abrupt reversals of major- and trace-element values at the boundaries of individual units suggest that new, more primitive magma was injected into the resident liquids in each cyclic unit. The new magmas are interpreted to have originated from a mantle plume, and were subjected to different degrees of contamination by continental lithospheric mantle and the upper crust. The homogeneous, decoupled Sr-Nd isotopes and cyclic variations of major and trace elements imply that each cyclic unit crystallized from a magma that was thoroughly mixed before crystallization. Assimilation of wallrocks accounted for the PGE mineralization in Cycle I. Mixing between a primitive and an evolved magma resulted in the formation of the PGE-enriched layer in Cycle II and the magnetite layers in Cycles II and III.
Abstract Tholeiitic basalts and trachytes of the bimodal association of Wangtian’e volcano are studied. It is shown that trachytes formed with the leading role of crystal fractionation of the parental tholeiitic magma. It has been identified from the melt inclusion study that hedenbergite and plagioclase phenocrysts of trachytes from Wangtian'e volcanic neck crystallized in the temperature range of 1080–1100 and 1050–1060°C, respectively. After heating experiments with melt inclusions in plagioclase of trachytes, in some cases relics of carbonates and CO 2 were observed in the inclusions. The trachyte phenocrysts host water-bearing Fe-rich globules and CO 2 inclusions with carbonate phases. The Fe-rich silicate globules are often intergrown with titanomagnetite and are covered by amorphous carbon films. A model is suggested explaining the formation of water-bearing Fe-rich globules and CO 2 inclusions with carbonate relics as a result of silicate–silicate and silicate–carbonate liquid immiscibility, that was caused by the evolution of the parental basaltic melt. Upon the trachytic melt ascending to the surface, the ferrocarbonate liquid was decomposed on magnetite, carbon, and carbon dioxide.
Numerical modeling results and input files of the manuscript "Effects of elasticity on subduction initiation: Insight from 2-D thermomechanical models".
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