Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy最新文献

Partial melting of granodioritic gneisses in the contact aureole of the Bergell Pluton (Central Alps) occurred during regional deformation. Melting occurred in the presence of water, which was released from the pluton. The presence of melt is evidenced by local segregations of granite in shear zones, veins and dykes, and by grain-scale interstitial films of K-feldspar and quartz that do not occur in the unmelted protolith. These films are oriented parallel, as well as perpendicular to the foliation plane. In contrast, on the outcrop scale, cm-wide leucosome veins are oriented almost exclusively parallel to the foliation plane, indicating foliation-parallel flow. The partially molten granitic rocks contain dm-long clasts of restitic, well-foliated gneiss, showing a higher competence than their granitic matrix. K-feldspar is lacking in these clasts, the microstructure of which is characterized by elongate aggregates of quartz and feldspar, both dynamically recrystallized. In contrast, the granitic matrix is characterised by a random distribution of minerals, whith a shape preferred orientation defining a weak foliation. These microstructures are indicative of granular flow, whereas the microstructures of the clasts indicate dislocation creep involving dynamic recrystallization. The presence of K-feldspar controls the onset of melting and thus the transition from dislocation creep to granular flow. The weakening resulting from this transition is indicated by the formation of strong clasts in a weaker matrix.

Structural analysis of migmatites based on the distribution and proportion of the granitic fraction at the outcrop scale is taken as a guide to decipher the behavior of partially molten rocks during orogenesis. This approach evaluates the various mechanisms that control melt segregation and magma mobility from pores to orogens. During partial melting, the first liquid appears at grain interfaces but such textures are rarely preserved in igneous rocks. Mechanisms of melt movement are scale-dependent and it is important to distinguish melt segregation at grain-scale from melt migration which occurs over larger distances. Melt segregation is controlled by melt connectivity and ubiquitous localization of granites in structurally-controlled dilatant sites provides evidence of the efficiency of melt segregation at the outcrop scale, probably achieved by porous flow at the grain scale. Magma mobility is controlled by the continuity of the solid framework which controls the rheologic threshold at the transition from solid-dominated metatexites to liquid-dominated diatexites. The presence of laccoliths of homogeneous leucogranite, emplaced at higher structural levels far from their source, indicates the efficiency of melt migration beyond the grain scale. The transition zone between diatexites (melt source) and granitic laccoliths (melt sink) is characterized by a network of granitic veins centimeter- to meter-thick. The geometric characteristics of this network suggest that, depending on structural level and the competency contrast between liquid and solid, veins propagate by either channeled porous flow, ductile deformation or fracturing. The main driving forces for upward melt migration appear to be buoyancy and dilatancy; the characteristics of local and regional deformation control the patterns of the granitic vein networks. Partial melting and redistribution of melt and magma from segregation by percolation at the grain scale relayed by pervasive migration through vein networks, is associated with chemical differentiation and generation of new rheological layering of the orogenic crust.

Chemical and carbon isotope compositions of fluid inclusions were measured for peridotite xenoliths enclosed in Cenozoic basalts and Triassic ultrahigh pressure eclogites from the Dabie terrane in eastern China to provide insight into the nature of their related fluids. The results show that the inclusions contain different amounts of gaseous CO₂, N₂, H₂S, CO, CH₄ and H₂, some of which have significant amounts of H₂. This may reflect chemical heterogeneity in mantle fluids of eastern China and possible metasomatism to the mantle-derived rocks during their eruption and exhumation to surface. There is a variation in δ ¹³C from −12.1 to 0.7‰ for the fluid inclusions in the peridotite xenoliths. Although the higher δ ¹³C values may be either derived from decarbonation of sedimentary carbonates or responsible for the primary mantle carbon remained in the mantle-derived rocks, the lower δ ¹³C values below −15‰ suggest the incorporation of organic carbon by plate subduction into the continental lithospheric mantle in eastern China.

The fluid inclusions in the eclogites from the Dabie terrane also exhibit a large δ ¹³C variation from −18.5 to 4.6‰, similar to previously reported δ ¹³C features measured for bulk carbon in eclogites from the Dabie-Sulu terranes. While the low δ¹³C values result from hydrothermal alteration to the eclogite protoliths by ¹³C-depleted surface fluid before plate subduction, the high δ¹³C values indicate overprinting of ¹³C-rich CO₂-bearing fluid derived from leaching marble lithologies subsequent to the ultrahigh pressure metamorphism. A geodynamic model is proposed to account for the geochemical recycling of carbon in the processes of plate subduction and prograde metamorphism as well as plate break-off and exhumation. It appears that the ¹³C-depleted slab may be broken off at mantle depths and undergone partial melting to generate basaltic magma and peridotite melt. This provides a genetic links between the ¹³C-depleted peridotites and eclogites.

The consequences of large reservoir compaction on the surface subsidence and the stress path of the overburden formations were investigated with physical modeling experiments where the compacting reservoir was modeled with a circular retracting trap door (TD) under a deformable overburden of sand. Tests with various overburden heights showed the formation of shear bands starting almost vertically at the TD edges and converging successively to the symmetry axis. A shallow and a deep formation mechanism were identified. In the shallow mechanism, the shear bands reach immediately the upper surface and thus the TD displacement is felt immediately at the surface as subsidence. In the deep mechanism, at low TD displacements the shear bands meet initially each other forming an arch and thus only part of the TD displacement is felt at the surface. At higher TD displacements, additional shear bands form above the initial shear band and eventually reach the surface. After that, the TD displacement is directly felt at the surface as subsidence. The TD stress drops rapidly to a minimum value, which appears to be independent of the overburden height.

Mechanically high porosity chalks behave as frictional materials, but with an end-cap reflecting pore collapse failure. Shear failure between the grains seems to be the basic failure mechanism, not only in compression tests at low confining pressures where shear bands are formed, but also in pore collapse where distributed shear failure will occur in the material. Also tensile failure seems to be initiated by a shear failure mechanism, and tensile strength can be predicted from the compressive yield criterion (Mohr-Coulomb). The mechanical properties of chalk are strongly dependent on the type of fluid in the pores. Water saturated chalk is considerably weaker than dry or oil saturated chalk. Chalk and fluids may interact through capillary forces and through surface physical/chemical reactions. Capillary forces will always be present when two immiscible fluids are present in the pore space. But such effects can only partly explain the experimental observations. There are however indications that the water weakening effect might be caused by repulsive forces generated by dipole-dipole interactions in the very narrow grain contact areas.