Oceans oceanic crust

Encompassing the mantle lies the outermost—and thinnest—layer called the crust. It is only about 8 km beneath the oceans (oceanic crust) and an average of about 40 km under the continents (continental crust). Its mass is estimated at 2.4 x 1022 kg, and is constituted primarily of (oxygenated) compounds of Si (approximately 60%) and A1 (approximately 15%) silica, quartz, silicates, silico-aluminates, and others (e.g., metal oxides). As can... 

A simple model can be used to describe this control of the concentration. In this model the input is from rivers and the output is uptake by reactions in the ocean crust under hydrothermal systems. (An application of this model is given in Section 13.5). Thus... 

The present-day best estimates are that l riv/l hydro is about 300. As l hydro increases, e.g. faster spreading of ocean crust at ridges, Csw responds. The dominant control is tectonics. 

Alt, J.C. and Honnorez, J. (1984) Alteration of the upper oceanic crust, DSDP site 417 mineralogy and chemistry. Contr. Mineral. Petrol, 87, 149-169. 

Wolery, T.J. (1978) Some chemical aspects of hydrothermal processes at midoceanic ridges — A theoretical study I, Basalt-seawater reaction and chemical cycling between the oceanic crust and the oceans. II, Calculation of chemical equilibrium between aqueous solutions and minerals. Ph.D. Thesis, Northwestern U. 

Using 2.5 X lO g/m.y. as oceanic production rate and 5-20 as seawater/rock ratio and assuming that 30% of oceanic crust interacts with circulating seawater, and the crustal production rate is (0.8-1.1) x 10 kg/m.y., then the rate of seawater cycling through back-arc basin is estimated to be (4-22) x lO kg/m.y. Using this value and the CO2 concentration of hydrothermal solution ((0.05-0.3) mol/kg -H20) (Table 3.2), hydrothermal CO2 flux into the ocean is estimated to be (0.2-6) x lO kg/m.y. 

Sulfur in the sediments and oceanic crust which is derived from seawater subducts to deeper parts. This subduction flux is estimated to be ca. 4 x lO mol/m.y. (Shikazono, 1997). Therefore, degassing S flux from back-arc and island arc ((2.3-8.2) x lO mol/m.y.) seems to be not different from the subduction flux, although uncertainty of estimated degassing and subduction flux is large. 

It seems unlikely that all of the oceanic crust produced interacts with seawater (Holland, 1978). Accepting that 30% of oceanic crust interacts with circulating seawater, hydrothermal As flux is estimated to be (3.8-0.1) x lO g As/year. This flux, although... 

Hydrothermal flux from back-arc basins estimated based on the H2O flux which was estimated from oceanic crust production rate and seawater/rock ratio at back-arc... 

Kaiho, K. and Saito, S. (1994) Oceanic crust production and climate during the last 100 Myr. Terra Nova, 6, 376-384. 

Spivack AJ, Edmond JM (1987) Boron isotope exchange between seawater and the oceanic crust. Geochim Cosmochim Acta 51 1033-1043... 

The upper portions of the subducting plate are comprised of hydrothermally altered oceanic crust and an overlying layer of sediments, sometime jointly referred to as the... 

An important aspect in the preceding discussion is the need to separate the fluid and sediment components spatially and (as we will see) also temporally. Quantitative mass balance estimates (e g., McCulloch and Gamble 1991 Stolper and Newman 1994 Ayers 1998) often conclude that there is as much, or even more, Th and U in the bulk slab componenf (i.e., sediment plus fluid from the altered oceanic crust). However, if the sediment component added is in U-Th isotope equilibrium (or returns to this state prior to fluid addition see Section 5.3), then addition of only 0.02 ppm U in the fluid will result in significant U-excesss in the composite source (e.g., Condomines and Sigmarsson 1993 Turner et al. 1997). 

Altered oceanic crust composition based on Staudigel et al. (1996)... 

Assuming U-series equilibrium in altered oceanic crust... 

Bebout GE et al. (eds) AGU Geophys Monogr Ser 96 119-133 Peacock SM, Rushmer T, Thompson AB (1994) Partial melting of subducting oceanic crust. Earth Planet Sci Lett 121 227-244... 

The initial U activity in the mantle wedge (Uw) is set to an arbitrary value of 1 and all the other nuclides are scaled relative to Uw The initial U activity in the oceanic crust is twice the activity in the mantle wedge. The Th/U ratios of the mantle wedge and the slab are both equal to 2.5. This value is relevant for modeling the higher ( Th/ Th) observed in some arc lavas. Fluid is added to a portion of mantle wedge, and the mass fraction of fluid (f) and the composition of the mixture at time step i is given by (same equation for all the nuclides) ... 

Staudigel H, Plank T, White B, Schmincke H-U (1996) Geochemical fluxes during seafloor alteration of the basaltic upper oceanic crust DSDP Sites 417 and 418. In Subduction Top to Bottom. Bebout GE, Scholl DW, Kirby SH, Platt JP(eds), AGU, Washington DC, p 19-37 Suman DO, Bacon MP (1989) Variations in Holocene sedimentation in the North American Basin determined by °Th measurements. Deep Sea Res 36 869-787... 

As the Earth s tectonic plates drift apart, a new oceanic crust is formed from the basalt rising from the depths cold sea water acts as a cooling agent. Geologists distinguish between two types of hydrothermal systems (Holm, 1992) ... 

Off-axis systems, on the flanks of spreading centres, driven by free convection due to cooling of the ocean crust, where the mean water temperature is around 420 K. 

The two mineral systems in the ocean crust are compatible, as has been shown by calculations for the following reactions (Helgeson et al., 1978) ... 

At lower temperatures, reducing conditions are present (CH4 is stable) this is typical for the oceanic crust. Most of the hydrothermal water circulates in the oceanic crust at a temperature of around 420 K, and the reducing conditions present there are mainly controlled by the PPM mineral mixture (Alt et al., 1989). 

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 yields of methane are directly proportional to the amount of alloy added. Abiotic methane formation is probably more general than had previously been thought, in particular since nickel-iron alloys have been found in the oceanic crust. 

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. 

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