Metamorphism oxide-carbonate

Oxide-carbonate iron-formations. Hematite-siderite rocks are relatively rarely encountered, which apparently is explained not by the exceptional nature of this association in sediments, but by metamorphic transformation into magnetite rocks at relatively low temperatures ... 

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.

If there is no constant influx of fluid of a certain composition, decomposition of magnetite ceases. The limiting case is a dry system closed to CO2. By analogy with systems closed to water, in such a system with constant pressure P — Pf = const) the fluid phase disappears entirely, and the Mgt + Sid + Hem association (system Fe-C-O) becomes bivariant and can exist stably below the P-T curve (see Fig. 77) in the stability field of the Sid -1- Hem (+ fluid) association. From these considerations the Mgt -I- Sid + Hem association cannot be used to judge the low-temperature limit of mineral formation the upper limit is fixed quite definitely inasmuch as removal of CO2 begins at P P and the reaction proceeds irreversibly to the right. The extensive occurrence of magnetite in oxide-carbonate iron-formations of low-rank metamorphism apparently indicates the absence of equilibrium or even a deficiency of COj and special dry conditions. 

The aqueous fluids formed by melting of ices in asteroids reacted with minerals to produce a host of secondary phases. Laboratory studies provide information on the identities of these phases. They include hydrated minerals such as serpentines and clays, as well as a variety of carbonates, sulfates, oxides, sulfides, halides, and oxy-hydroxides, some of which are pictured in Figure 12.15. The alteration minerals in carbonaceous chondrites have been discussed extensively in the literature (Zolensky and McSween, 1988 Buseck and Hua, 1993 Brearley, 2004) and were most recently reviewed by Brearley (2006). In the case of Cl chondrites, the alteration is pervasive and almost no unaltered minerals remain. CM chondrites contain mixtures of heavily altered and partially altered materials. In CR2 and CV3oxb chondrites, matrix minerals have been moderately altered and chondrules show some effects of aqueous alteration. For other chondrite groups such as CO and LL3.0-3.1, the alteration is subtle and secondary minerals are uncommon. In some CV chondrites, a later thermal metamorphic overprint has dehydrated serpentine to form olivine. 

It is difficult to believe that oxygen per se was available during metamorphism. However, it is known that at 500°-600°C. steam can react with carbons to give a surface oxide and hydrogen water would be available from clay minerals and bed moisture in the Antarctic strata, and this reaction could well have occurred at the high pressures prevailing in the seams. On the other hand,... 

It cannot be assumed a priori that conditions become acid when organic matter is oxidized pH will be controlled by the extent to which hydroxide- and hydrogen ion-producing reactions balance each other. Possibly the final pH during late diagenesis and metamorphism is not controlled by carbonates which are unstable at low e values, owing to the reaction ... 

The rock types distinguished hardly ever occur in pure form, for the oxide facies always contains a certain amount of carbonate in addition to hematite and magnetite, and the iron silicate rocks usually contain carbonate, sometimes magnetite. Sustained mineralogical composition is observed only in thin bands on the whole, in conditions of low and medium degrees of metamorphism the pile of BIF looks like a layer cake . The composition of highly metamorphosed rocks is more uniform, which can be explained both by the disappearance of carbonate rocks due to thermal dissociation and by more intensive diffusion. 

Thus metamorphic transformation of hematite into magnetite is possible only in the presence of a reducing agent (free carbon, gaseous CO, H2, or CH4), but it is expedient to consider the oxide-reducing reactions that take place when we analyze mineral equilibria in silicate and carbonate rocks. 

Therefore the formation of magnetite in that way could hardly be of essential importance in the metamorphism of iron-formations, and martitiza-tion is still less hkely. However, in deposits of other genetic types, for instance skam deposits, oxidation of iron silicates to magnetite at the contact with large masses of carbonate rocks (dolomite, magnesite) can be considered an ore-forming process. The last conclusion is still feasible because the carbon dioxide released in the dissociation of carbonates probably had an undisturbed CO O2 ratio. 

In the case of a high iron content in the carbonate, olivine is formed instead of orthopyroxene. A 10% increase in magnesium content leads to a 5-8° shift of the P-T curve into the higher temperature region. The shift is relatively small, but it plays a definite role in the metamorphic redistribution of iron among the minerals, particularly in reactions in which iron oxides are formed in the absence of water ... 

The oxidation of sulfides preferentially dissolves carbonates, rather than silicates, because the rate of carbonate dissolution is orders of magnitude faster. For example, Haut Glacier d Arolla has a bedrock which is composed of metamorphic silicate rocks. Carbonates and sulfides are present in trace quantities in bedrock samples (0.00-0.58% and <0.005-0.71%, respectively). There are also occasional carbonate veins present in the schistose granite. Despite the bedrock being... 

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