Ridge axis

Fiji Transform Fault Extensional Relay Zone A (16°10 S, 177°25 E) 1869-2335 Short spreading ridge axis which displaces Fiji transform fault as interpreted by Jarvis et al. (1994). Hydrothermal sulfide impregnation in MORB-like ba.salt dredged from axial valley. M etite, pyrrhotite, chalcopyrite and opal on fracture surface. 

Kaiho and Saito (1994) estimated 20 x 10 km /m.y. and 2x 10 km /m.y. for present-day midoceanic ridge crustal production rate and back-arc basin crustal production rate, respectively. If their estimates are correct. Mg removal to midoceanic ridge basalt during early-middle Miocene age is estimated to be 2.6 1 x 10 g/year. Although estimates of annual Mg removal by interaction of circulating seawater with midoceanic ridge basalt are uncertain, it seems likely that Mg removal by seawater-volcanic rock interaction at back-arc basins corresponds to that of Mg removal at midoceanic ridge axis. 

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. 

Figure 20. Calculated Fe isotope composition of seawater from different ocean basins based on a simple two-component mixing between Fe from aerosol particles and Fe from mid-oceanic-ridge (MOR) hydrothermal solutions. Atmospheric Fe fluxes (Jatm) for different ocean basins from Duce and Tindale (1991) MOR hydrothermal Fe flux ( mor) to different ocean basins were proportioned relative to ridge-axis length. Modified from Beard et al. (2003a).

Some ridge sections are underlain by mantle melt anomalies, or hot spots, such as at the Azores and Galapagos Islands. These are marked by the stars in Figure 19.1 and data points 12 and 13 in Figure 19.5. Mantle upwelhng beneath both these ridge sections has abnormally thickened the oceanic crust to at least about 10 km. Most of the 47 known hot spots lie more than 500 km from a ridge axis. The Hawaiian islands are a notable example. 

Lang, S. Q., Butterfield, D. A., Lilley, M. D., Johnson, H. D., and Hedges, J. I. (2006). Dissolved organic carbon in ridge-axis and ridge-flank hydrothermal systems. Geochim. Cosmochim. Acta 70, 3830-3842. 

Relatively young (<250 Ma) oceanic upper mantle is formed at the lowest average pressures and temperatures by melt extraction at mid-ocean ridges. Melt extraction is expected to be a polybaric process, and likely involves a combination of near-fractional and batch melt extraction. Assuming an average pressure of melt extraction of 1 GPa and 15-20% melt extraction at the ridge axis, an average temperature of 1,315 50 °C is estimated. 

Macdonald K. C., Fox P., Perram L., Eisen M., Haymon R., Miller S., Carbotte S., Cormier M., and Shor A. (1988) A new view of the mid-ocean ridge from the behavior of ridge axis discontinuities. Nature 335, 217-225. 

Sun S.-S., Tatsumoto M., and Schilling J.-G. (1975) Mantle plume mixing along the Reykjanes ridge axis lead isotope evidence. Science 190, 143-147. 

Vogt P. R. (1986) Portrait of a plate boundary the Mid-Atlantic Ridge axis from the equator to Siberia. In The Western North Atlantic Region, The Geology of North America, vol. M., plate 8A (eds. P. R. Vogt and B. E. Tucholke). Geological Society of America, Boulder, CO. 

To put these volume fluxes in context, the maximum flux of cool (<20 °C) hydrothermal fluids, calculated above is almost identical to the global riverine flux estimate of 3.7-4.2 X 10 g yr (Palmer and Edmond, 1989). The flux of high-temperature fluids close to the ridge axis, by contrast, is 1,000-fold lower. Nevertheless, for an ocean volume of 1.4XlO " g, this still yields a (geologically short) oceanic residence time, with respect to high-temperature circulation, of —20-30 Ma— and the hydrothermal fluxes will be important for those elements which exhibit high-temperature... 

Major differences in substrate are, therefore, reflected in the compositions of vent fluids. Insufficient trace-metal data for vent fluids exist, however, to discern more subtle substrate differences, e.g., between EMORB and NMORB on non-hotspot influenced ridges. Where the ridge axis is influenced by hot-spot volcanism, some differences may be seen in the fluid compositions, as, for example, the high barium in the Lucky Strike vent fluids, but in this case most of the fluid characteristics (e.g., potassium concentrations) do... 

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