Ocean crust spreading rate

As discussed in sections 1.5.3, 2.3 and 2.4.1, the hydrothermal solutions both from back-arc basins and midoceanic ridges are dominantly of seawater origin. Therefore, the fluxes of hydrothermal solution are estimated from seawater cycling rate. This rate is considered to be equal to oceanic production rate times seawater/rock ratio. Kaiho and Saito (1994) estimated the crustal production rate at back-arc basins (Okinawa, Mariana, Andaman, Manus, Woodlark, North Fiji, Lau-Havre, East Scotia and Cayman) based on the spreading rate, thickness of crust and length of ridge axis. Their estimated oceanic crustal production rate is 8.5 x 10 km /m.y. which is roughly equal to 2.5 x lO g/m.y. 

Karson J. A. (2002) Geologic structure of the uppermost oceanic crust created at fast- to intermediate-rate spreading centers. In Annual Reviews Earth Planet of Sciences, Annual Reviews (eds. R. Jeanloz, A. L. Albee, and K. C. Burke). Annual Reviews, Palo Alto, CA, vol. 30, pp. 347-384. 

Present-day spreading rates allow us to estimate the volume of oceanic crust, which is subducted annually. Estimates are between 18 and 20 km3/ yr, the differences being in part due to different assumptions about the average thickness of the oceanic crust. If this rate of subduction has been constant over geological time, then the Earth s mantle should contain 5 % recycled oceanic crust (Helffrich Wood, 2001), although if this were concentrated in the upper part of the mantle then, of course, the proportion would be considerably greater. 

Assuming that thickness of weathered basalt is 200-800 m and spreading rate of oceanic crust is 2.94 km my (Chase 1972), the amount of weathered basalt is estimated to be (1.8-7.0)... 

We start with another set of isotope signatures. The rate of erosion in the distant past can be estimated by measuring the ratio of strontium isotopes in marine carbonates. Two stable isotopes of strontium — strontium-86 and strontium-87 — differ in their distribution between the Earth s crust and the mantle underneath it. The mantle is rich in strontium-86, whereas the crust is more richly endowed with strontium-87. The major source of strontium-86 in the oceans is the igneous rock basalt. This rock is extruded continuously from the mantle at the mid-ocean ridges, from where it spreads slowly across the ocean floor before diving back into the mantle beneath the ocean trenches. A little strontium dissolves from the basalt into seawater. The speed of dissolution is more or less constant. The gradual build-up of dissolved strontium-86 in the oceans is balanced by a steady uptake of strontium by marine carbonates, such as limestone (calcium carbonate). This is because strontium can displace its sister element, calcium, in the crystalline structure of limestone. As each of these processes takes place at a steady rate, we would not expect the relative amount of strontium-86 in limestone to fluctuate a great deal. In fact it varies quite a lot. Strontium-87 is to blame. 

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