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Oscillatory Magma Crystallization by Feedback between the Concentrations of the Reactant Species and Mineral Growth Rates 总被引：1，自引：0，他引：1
During crystallization of silicate magmas, silica polymers advectinto the reaction zone carrying other chemical species withthem. If some species forming minerals are more abundant relativeto Si in the liquid than in the minerals, then the species becomemore concentrated in the reaction zone as crystallization progresses.This enrichment may cause mineral growth rates to increase,thereby accelerating the advection of silica and further increasingthe concentrations. This positive feedback may be slowed downby the depletion of other reactant species that have lower concentrationratios to Si in the liquid than in the minerals. A quantitativetransport-reaction model that incorporates these competing effects,mass-action mineral-growth rate laws, and diffusion plus advection,shows that the feedback can cause oscillatory crystallization.The model represents a plausible mechanism for the origin ofrepetitive igneous layering. It predicts the repeated layeringitself, its occasional occurrence in general, its common occurrencein alkaline rocks, and paired layering. Also, it accounts directlyfor a significant property of microrhythmic layering: that theoscillatory modes of two or more minerals are spatially staggered. 相似文献
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During the Late Palaeozoic Variscan Orogeny, Cambro‐Ordovician and/or Neoproterozoic metasedimentary rocks of the Albera Massif (Eastern Pyrenees) were subject to low‐pressure/high‐temperature (LPHT) regional metamorphism, with the development of a sequence of prograde metamorphic zones (chlorite‐muscovite, biotite, andalusite‐cordierite, sillimanite and migmatite). LPHT metamorphism and magmatism occurred in a broadly compressional tectonic regime, which started with a phase of southward thrusting (D1) and ended with a wrench‐dominated dextral transpressional event (D2). D1 occurred under prograde metamorphic conditions. D2 started before the P–T metamorphic climax and continued during and after the metamorphic peak, and was associated with igneous activity. P–T estimates show that rocks from the biotite‐in isograd reached peak‐metamorphic conditions of 2.5 kbar, 400 °C; rocks in the low‐grade part of the andalusite‐cordierite zone reached peak metamorphic conditions of 2.8 kbar, 535 °C; rocks located at the transition between andalusite‐cordierite zone and the sillimanite zone reached peak metamorphic conditions of 3.3 kbar, 625 °C; rocks located at the beginning of the anatectic domain reached peak metamorphic conditions of 3.5 kbar, 655 °C; and rocks located at the bottom of the metamorphic series of the massif reached peak metamorphic conditions of 4.5 kbar, 730 °C. A clockwise P–T trajectory is inferred using a combination of reaction microstructures with appropriate P–T pseudosections. It is proposed that heat from asthenospheric material that rose to shallow mantle levels provided the ultimate heat source for the LPHT metamorphism and extensive lower crustal melting, generating various types of granitoid magmas. This thermal pulse occurred during an episode of transpression, and is interpreted to reflect breakoff of the underlying, downwarped mantle lithosphere during the final stages of oblique continental collision. 相似文献
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Few studies have examined the hydrodynamic behaviour of carbonate sediments. The data presented here are the result of preliminary research on entrainment in well- and poorly sorted carbonate sands. Experiments were performed using naturally occurring sediments in a tilting, recirculating freshwater flume. Results indicate that when of similar size, shape and density, the transport threshold of carbonate sands is similar to that of quartz. However, owing to their lower density and often platy or irregular shape, skeletal sands require a lower shear stress to initiate transport. Because the density of carbonate particles may increasingly vary with grain size, the threshold of motion in coarse carbonate grains may differ more markedly from that of quartz. In poorly sorted samples, results show that the coarse-grained constituents move before the finer-grained components. Grain properties and boundary-layer dynamics are believed to explain this phenomenon. Rollability of the larger grains combined with physical trapping and immersion within a low velocity sublayer are believed to prevent finer particles from moving. Given the appropriate sediments and flow conditions, it may therefore be possible to deposit and preserve fine-grained sediments in a flow regime typically thought to transport such materials. 相似文献
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