Seawater subduction

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. 

Holland, G. and C. J. Ballentine (2006) Seawater subduction controls the heavy noble gas composition of the mantle. Nature 441, 186-191 Holland, G., M. Cassidy and C. I Ballentine (2009) Meteorite Kr in earth s mantle suggests a late accretionary source for the atmosphere. Science 326, 1522-1525 Holland, H. D. (1984) The chemical evolution of the atmosphere and oceans. Princeton Univ. Press, New Jersey (USA), 582 pp. 

First we compare the results for clinopyroxene which was the only phase common to both of these studies. The results from the pure H2O experiments are somewhat inconsistent. Keppler (1996) found that Th was more mobile than U, contrary to all observations, whereas Brenan et al. (1995) found the reverse. However, subduction zone fluids are almost certain to contain solutes like Na and Cl derived from seawater and Keppler and Wyllie (1990) showed that the solubility of U, but not Th, is enhanced by the presence of Cl, although the salinities used by Keppler (1996) were very high. Both Brenan et al. (1995) and Keppler (1996) found that U was an order of magnitude more fluid mobile than Th when NaCl was present, although, in the Brenan et al. (1995) experiments, the absolute D "opyroxene/flu.d lower in the presence of NaCl than... 

One of the most notable features of seawater is its high degree of saltiness. In previous chapters, we have discussed various sources of this salt, these being rivers, volcanic gases, and hydrothermal fluids. These elements have ended up in one of four places (1) as dissolved ions in seawater, (2) as sedimentary minerals, (3) as hydrothermal minerals, and (4) as volatiles that reside in the atmosphere. The minerals are recycled via geologic uplift and subduction. Upon return to Earth s surface, these minerals are chemically weathered via acid attack by the atmospheric volatiles remobilizing the salts for return to the ocean in river runoff. 

What has happened to the bicarbonate and calcium delivered to the ocean by river runoff As described later, these two ions are removed from seawater by calcareous plankton because a significant fraction of their hard parts are buried in the sediment. In contrast, the only sedimentary way out of the ocean for chloride is as burial in pore waters or precipitation of evaporites. The story with sodium is more complicated— removal also occurs via hydrothermal uptake and cation exchange. Because the major ions are removed from seawater by different pathways, they experience different degrees of retention in seawater and uptake into the sediments. Another level of fractionation occurs when the oceanic crust and its overlying sediments move through the rock cycle as some of the subducted material is remelted in the mantle and some is uplifted onto the continents. 

The oceanic crust is also a sink for seawater as its porosity increases with age because cooling and chemical weathering lead to the continuing development of fractures and fissures. Most of this trapped water is degassed back into ocean when the crust and its overlying sediment undergo subduction. 

A number of eclogites have both radiogenic neodymium isotopes and radiogenic strontium relative to bulk Earth. For these samples, some of the Sr/ Sr values exceed modem and Archean seawater compositions (see Chapter 6.17). Hence, in a subduction-type model, the radiogenic strontium isotopic composition of the eclogite protohth is not solely inherited from oceanic cmst but is likely to be a time-integrated response to Rb/Sr enrichment during hydrothermal alteration... 

Scambelluri M., Piccardo G. B., Philippot P., Robbiano A., and Negretti L. (1997) High salinity fluid inclusions formed from recycled seawater in deeply subducted alpine serpentinite. Earth Planet. Sci. Lett. 148, 485 -499. 

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