Oceanic crust carbonates

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

The geological process of the formation of serpentine from peridotite probably involves the synthesis of carbon compounds under FTT conditions (see Sect. 7.2.3). The hydrogen set free in the serpentinisation process can react with CO2 or CO in various ways. The process must be quite complex, as CO2 and CO flow through the system of clefts and chasms in the oceanic crust and must thus pass by various mineral surfaces, at which catalytic processes as well as adsorption and desorption could occur. 

The latter two assumptions are simplistic, considering the number of factors that affect pH and oxidation state in the oceans (e.g., Sillen, 1967 Holland, 1978 McDuff and Morel, 1980). Consumption and production of CO2 and O2 by plant and animal life, reactions among silicate minerals, dissolution and precipitation of carbonate minerals, solute fluxes from rivers, and reaction between convecting seawater and oceanic crust all affect these variables. Nonetheless, it will be interesting to compare the results of this simple calculation to observation. 

You CF, Chan LH, Gieskes JM, Klinkhammer GP (2004) Seawater intrusion through the oceanic crust and carbonate sediment in the Equatorial Pacific Lithium abundance and isotopic evidence. Geophys Res Lett 30 (in press)... 

Moecher DP, Valley JW, Essene EJ, (1994) Exhaction and carbon isotope analysis of COj from scapolite in deep crustal granulites and xenoliths. Geochim Cosmochim Acta 58 959-967 Mojzsis SJ, Harrison TM, Pidgeon RT (2001) Oxygen-isotope evidence from ancient zircons for liquid water at the Earth s surface 4,300 Myr ago. Nature 409 178-181 Muehlenbachs K, Clayton RN (1976) Oxygen isotope composition of the oceanic crust and its bearing on seawater. J Geophys Res 81 4365-4369... 

During this zone refining, the primary (igneous) rocks are transformed into secondary minerals. These include (1) clay minerals, such as phillipsite, chlorite, montmo-rillonite (smectite), saponite, celadonite, and zeolite (2) iron oxyhydroxides (3) pyrite (4) various carbonates and (5) quartz. These minerals form rapidly, within 0.015 and 0.12 million years after creation of the oceanic crust at the MOR. During these alteration... 

Spero HJ, Bijma J, Lea DW, Bemis BE (1997) Effect of seawater carbonate concentration on foraminiferal carbon and oxygen isotopes. Nature 390 497-500 Spivack AJ, Edmond JM (1986) Determination of boron isotope ratios by thermal ionization mass spectrometry of the dicesium metaborate cation. Anal Chem 58 31-35 Spivack AJ, Edmond JM (1987) Boron isotope exchange between seawater and the oceanic crust. Geochim Cosmochim Acta 51 1033-1043... 

Recently, the first fossil fungi have been found in the submarine, deep basaltic earth crust (Schumann et al., 2004). The fossils were detected in thin sections obtained from drilled basaltic cores collected during Ocean Drilling Program, Leg 200, in the North Pacific Ocean (Stephen et al., 2003). Unique filamentous fossilized fungi were observed in the carbonate-filled vesicles of a massive tholeiitic lava flow from the upper oceanic crust at a depth of 51 m below sea floor, beneath sediment and magmatic rocks, and under an overlying water column of about 5000 m (Fig. 16.1). These... 

The total uptake of the crust is calculated here as 0.355 g/100 g, which is slightly above the estimate made by Staudigel et al. (1989), and also similar to the total carbon uptake of the oceanic crust inferred by Alt and Teagle (1999) and for the Troodos ophiolite (Bednarz and Schmincke, 1989). The above extrapolation of data for intermediate depths (lower extrusive crust, sheeted dikes) does not contribute major uncertainties to the total flux estimate, because most of the carbon inventory is located in the upper 600 m of the crust, and, therefore, most of the uncertainties lie in this depth interval. 

Alt J. C. and Teagle D. A. H. (1999) The uptake of carbon during alteration of ocean crust. Geochim. Cosmochim. Acta 63, 1527-1535. 

Staudigel H., Hart S. R., Schmincke H. U., and Smith B. M. (1989) Cretaceous ocean crust at DSDP sites 417 and 418 Carbon uptake from weathering versus loss by magmatic outgassing. Geochim. Cosmochim. Acta 53, 3091-3094. 

A- -Vi = B- -V2 (where A, B are volatile free phases and Vi, V2 are hydrous phases or carbonates), involve hydrates and/or carbonates and change the mineralogy of a rock volume according to the stability fields of the minerals, but do not liberate a fluid. Prograde subduction zone metamorphism (as is true for any type of prograde metamorphism) generally reduces the amount of H2O that can be stored in hydrous minerals with depth. Thus, almost any part of the oceanic crust sooner or later becomes fluid saturated. In an equilibrium situation, the volatile content bound in hydrous phases and carbonates remains constant until fluid saturation occurs. Either continuous or discontinuous reactions may lead to fluid saturation in a rock. The point at which this occurs depends on initial water content, and pressure and temperature, and somewhat counter-intuitively, initial low water contents do not cause early complete dehydration, but delay the onset of fluid production to high pressures. 

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