Metamorphic buffer

It is obvious that in the case of pressure gradients determined by the maximum controlled by buffer reactions in the pile of rocks, and by the minimum in fractures (in the case of open circulation Pf(n,in) - (hydr) of the column of fluid), mechanical movement of the fluid was in one direction — from rock to fracture. For movement in the opposite direction—from fracture to rock—it was necessary to create a corresponding pressure gradient. Such phenomena, in addition to diffusion along the concentration gradient, presumably have occurred in hydrothermal metamorphism with typical reactions of hydration and carbonation. However, for normal progressive metamorphism it is hard to imagine a mechanical model in which a fluid with a strictly constant value of / h,o " h,o introduced from... 

It is assumed that in such rocks oxides—hematite and magnetite—were essential minerals as well as siderite. The association of siderite with magnetite and fayalite in equilibrium with graphite in reducing conditions has already been examined now it remains to analyze in detail the particulars of metamorphism in oxidizing conditions in equilibrium with a hematite-magnetite buffer. 

Fig. 81. Diagrams of mineral equilibria in oxide-carbonate iron-formations (isothermal sections in coordinates of log /co l S /hjO magnetite-hematite buffer) 1 = isobars of total fluid pressure (Ff = - — vertical thin straight lines—isobars of log/y 3 = isobars of log/cc Fields of actual pressures in metamorphism are hatched.

A second interpretation of carbonaceous chondrites is as primary condensates of the solar nebula. By this view, their hydrolytic alteration is due to melting in cometary nuclei during close passes with the Sun, or due to transient heating events by shock waves or collisions (McSween, 1999). Other carbonaceous chondrites show metamorphic alteration with minerals similar to those in Earth formed during deep burial under elevated temperatures and pressures (Brearley, 1999). Like soils and paleosols on Earth and Mars, carbonaceous chondrites demonstrate the great antiquity of hydrolytic weathering in dilute acidic solutions, presumably of carbonic acid derived from water vapor and CO2. These remain the principal gases released from volcanoes, and soils remain important buffers for this environmental acid. 

It is well known that the crustal Urey cycle of silicate weathering and metamorphism can function as a dynamic buffer for atmospheric... 

Rice J.M., and Ferry, J.M., 1982. Buffering infiltration, and the control of intense variables during metamorphism, in J.M. Ferry, ed. Characterization of Metamorphism Through Mineral Equilibria, Reviews in Mineralogy, vol. 10, Washington D.C., Mineralogical Society of America, pp. 263-326. 

Jamtveit B, Bncher-Nnrminen K, Stijfhoom DE (1992a) Contact metamorphism of layered shale carbonate seqnences in the Oslo Rift 1. Buffering infiltration and mechanisms of mass transport. J Petrol 33 ... 

Carbon is highly important for our biosphere, not just because it forms organic compounds it also creates atmospheric greenhouse gases, pH buffers in seawater, and redox buffers virtually everywhere. Carbon species can stabilize metamorphic minerals and they can affect plutonism and volcanism. These various C constituents all interact via the biogeochemical C cycle, an array of C reservoirs linked by a network of physical, chemical and biological processes. The overall C cycle actually consists of multiple nested cyclic pathways that differ with respect to some of their reservoirs and processes (Fig. 1). However, all pathways ultimately pass through the hydrosphere and atmosphere, and it is this common course that unites the entire carbon cycle and allows even its most remote constituents to influence our environment and biosphere. 

Such buffered assemblages are quite common in nature and make contact aureoles of impure carbonate rocks one of the best areas to study the evolution of metamorphic fluids, because fluid compositions are essentially controlled by the mineral reactions. Progressive metamorphism initially leads to a rapid enrichment in CO2/ because the talc and tremolite reactions consume H2O and release CO2. At higher temperatures, water enrichment can take place by the decomposition of tremolite to diopside and/or forsterite. Changes in the compositions of the solids will affect the fluid ratios, and in fig. 6 the compositional changes of coexisting calcite and dolomite as a function of temperature have been taken into account. 

Aluminosilicates abound in most rocks considered suitable for the disposal of radioactive wastes (acidic-basic igneous and metamorphic rock types, and clays), so that the abundance of these minerals as potential buffers of pH should not be a limiting factor. Calcite may be less abundant in fractured hard rock systems than sedimentary rocks such as clays. 

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