Lithosphere, characterization

As a specific example, consider oceanic sulfate as the reservoir. Its main source is river runoff (pre-industrial value 100 Tg S/yr) and the sink is probably incorporation into the lithosphere by hydrogeothermal circulation in mid-ocean ridges (100 Tg S/yr, McDuff and Morel, 1980). This is discussed more fully in Chapter 13. The content of sulfate in the oceans is about 1.3 X lO TgS. If we make the (im-realistic) assumption that the present runoff, which due to man-made activities has increased to 200 Tg S/yr, would continue indefinitely, how fast would the sulfate concentration in the ocean adjust to a new equilibrium value The time scale characterizing the adjustment would be To 1.3 X 10 Tg/(10 Tg/yr) 10 years and the new equilibrium concentration eventually approached would be twice the original value. A more detailed treatment of a similar problem can be found in Southam and Hay (1976). 

The lithosphere is both literally and figuratively the bedrock, the solid foundation, on which understanding of geology rests. To characterize the lithosphere is to characterize not only what is, but, because of the chemical reactivity of rocks and their surface activity, it is also to characterize what was and what may become. 

The mantle up to several hundred kilometers beneath the continents, particularly under the older cratons, forms deep keels characterized by fast seismic velocities, is Mg-rich, and depleted in Fe. It is less dense than the surrounding mantle, a factor that imparts stability to the keels, which thus float under the continents. Although the low Fe/Mg ratio and low Ca and A1 contents are generally attributed to the extraction of a partial melt, this subcontinental lithospheric mantle (SCLM) is apparently enriched in incompatible elements such as Ba, Th, U, Ta, Nb, La, Ce, and Nd and depleted in HREE, Ti, Sc, V, Al, and Ca relative to average abundances in the mantle. This element pattern indicative of both enrichment and depletion indicates that multistage processes must have occurred, with an initial extraction of a partial melt, followed by at least one stage of a secondary enrichment, often referred to a metasomatic event. Curiously, this event has apparently not affected the Fe/Mg ratios. 

Characterization of chemical processes in the atmosphere, biosphere, and lithosphere... 

In general, in order to characterize the noble gas state of the continental lithosphere, it is necessary to measure or clearly deduce the pre-entrainment noble gas isotope inventories of xenolith samples. This first requires subtracting the effects of post-eruption... 

Distinctive mantle source melt source regions. While many OIB have He characterized by He/ He ratios that are higher than those in MORB, some have been found to have lower ratios (see Graham 2002). As discussed in the Regional studies section, the presence of such a component has implications for the causes of host magma volcanism as well as the sources of lithosphere He. 

MORB mantle. The convecting upper mantle sampled by MORB has He/" He = (8 1) Ra away from hotspots (see Graham 2002, this volume). In some areas, such as southeast Australia, xenolith He appears to have MORB He isotope compositions. This is not surprising, considering that much of the mantle underlying the continents must have this composition. It is also not incompatible with the involvement of diapiric mantle hotspots in the local volcanism. The He isotope composition of many hotspots have not been clearly characterized, and while hotspots are often assumed to have very high He/" He ratios as seen in Iceland and Hawaii, it has not been established that other hotspots, with very different trace element characteristics, all have such ratios. Further, the presence of such material does not preclude the involvement of MORB mantle as well within a particular lithospheric region. 

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