Basalts reservoirs

Stegman D. R., Richards M. A., and Baumgardner J. R. (2002) Effects of depth-dependent viscosity and plate motions on maintaining a relatively uniform mid-ocean ridge basalt reservoir in whole mantle flow. J. Geophys. Res. 107, 10.1029/2001JB000192. 

It is clear that the Earth s mantle has at least two Os-isotopic reservoirs - a plume-related isotopically enriched reservoir and a chondritic upper mantle reservoir. Both have long histories (Fig. 3.32). The variations in composition within the upper mantle reservoir reflect Re-depletion and enrichment related to melt extraction. The isotopically enriched plume reservoir represents chemically isolated, rhenium-enriched, recycled oceanic lithosphere. There is some evidence to suggest that this enriched reservoir may have been in existence since the early Archaean (Walker Nisbet, 2002) and was the source of some Archaean komatiites and the 3.81 Ga Itsaq Gneiss chromitites. If this is true, then basaltic crust was being created and recycled even before 4.0 Ga. Estimates of the present size of this high Re/Os basaltic reservoir vary from 5% to >10% of the whole mantle (Bennett et al., 2002 Walker et al., 2002). 

Variations in ( Ra/ Th) ratios are also apparent in the Ardoukoba (Asal Rift) 1978 basaltic eruption studied by Vigier et al. (1999). ( Ra/ jj ratios decrease with increasing Th content, as do the ( Ra)/Ba ratios. These variations cannot be entirely explained by plagioclase fractionation, which led the authors to propose a model of a laterally zoned fissural reservoir, with several injections of basaltic magmas. In this case, the oldest magma would have stayed in the reservoir for about 1.9 ka. However small Sr... 

Model of Pb, Bi, and Po degassing. For a purpose of clarity, it is considered here that the degassing reservoir has reached a chemical steady-state (i.e., radionuclide activities in the degassing reservoir are constant, that is d(Ik)iydt = 0 in Eqn. 4). This assumption usually is valid for very active basaltic systems like Stromboli, where erupted products display an almost constant chemical composition as shown above, and where the degassing reservoir is quickly and continuously replenished with deep undegassed magma. 

Mantle reservoirs. The only quasi-systematic studies of igneous materials have centered on the mantle in particular mid-ocean ridge basalts (MORE), ocean island basalts, and mantle peridotites. After reporting one MORE analysis in Chan and Edmond (1988), the first full study of MORE (Chan et al. 1992) reported three apparently imaltered Atlantic basalts and one from the East Pacific Rise, with a range in 8 Li of +3.4 to +4.7 (Fig. 5). Subsequent studies have increased the global range of samples, the diversity of bulk compositions analyzed. 

Figure 9. Plots of Li and radiogenic isotopes for mantle rocks, (a) 5 Li vs. Sr/ Sr (b) 5 Li vs. Nd/ Nd (c) "Sr/ Sr vs. Pb/ Pb (d) 5"Li vs. Pb/ Pb (Nishio et al. 2003, 2004). Symbols + = south Pacific island basalts (six islands) O = Iherzolite xenolith, Bullenmerri, Australia = Iherzolite xenolith, Sikhote-Alin, Russia (three localities) A = dunite-peridotite-pyroxenite xenolith, Kyushu, Japan (two localities) V = Iherzolite xenolith, Ichinomegata, Japan. The ocean island data are from bulk rocks, the xenolith data are clinopyroxene separates. For explanations of the derivation of radiogenic isotope fields (DM, EMI, EM2, HIMU), see Zindler and Hart (1986). The estimate for Li isotopes in DM is based on MORE. The Li isotopic ranges for the other mantle reservoirs are based on Nishio et al. (2004) and Nishio et al. (2003), but these will require further examination (hence the use of question marks).

Support for this conclusion comes from laser ablation analyses of mantle olivines recently reported by Norman et al. (2004). The loess and continental basalt samples suggest that evolved crustal materials may be on average approximately 0.4-0.6%o lower in 5 Mg than the primitive Cl/mantle reservoir (Fig. 1). 

Figure 5. A plot of A Mg vs. 5 Mg for terrestrial Mg materials. Within best estimates of uncertainties (cross) all of the data lie in the region bounded by equilibrium and kinetic mass fractionation laws. Waters, carbonates, and organic Mg (chlorophyll) have higher A Mg values than mantle and crustal Mg reservoirs represented by mantle pyroxene, loess, and continental basalts. The difference in A Mg values is attributable to episodes of kinetic mass fractionation.

Grustal reservoirs are also variable in Gl-isotope compositions (Figs. 1-6) due to fractionation of the Gl-isotope compositions inherited from their mantle source through fluid-mineral reactions, incorporation of G1 derived from the oceans and fractionation within fluid reservoirs by diffusion (see below). For example, the oceanic crust is enriched in Gl (and pore fluids depleted in Gl) through reaction of seawater with basaltic crust derived from the depleted mantle (Fig. 1 Magenheim et al. 1995). Undoubtedly, future investigations of Gl-isotopes in whole rocks and mineral separates will address the Gl-isotope compositions of these reservoirs and their evolution. 

T1 isotope ratios might be also used as a tracer in mantle geochemistry (Nielsen et al. 2006 2007). Since most geochemical reservoirs except Fe-Mn marine sediments and low temperature seawater altered basalts are more or less invariant in T1 isotope composition, admixing af small amounts of either of these two components into the mantle should induce small T1 isotope fractionations in mantle derived rocks. And indeed, evidence for the presence of Fe-Mn sediments in the mantle underneath Hawaii was presented by Nielsen et al. (2006). 

Richter FM, Davis AM, DePaolo D, Watson BE (2003) Isotope fractionation by chemical diffusion between molten basalt and rhyolite. Geochim Cosmochim Acta 67 3905-3923 Riciputi LR, Cole DR, Machel HG (1996) Sulfide formation in reservoir carbonates of the Devonian Nishu Formation, Alberta, Canada an ion microprobe study. Geochim Cosmochim Acta 60 325-336... 

To begin the discussion, we will present briefly a view of the modern carbon cycle, with emphasis on processes, fluxes, reservoirs, and the "CO2 problem". In Chapter 4 we introduced this "problem" here it is developed further. We will then investigate the rock cycle and the sedimentary cycles of those elements most intimately involved with carbon. Weathering processes and source minerals, basalt-seawater reactions, and present-day sinks and oceanic balances of Ca, Mg, and C will be emphasized. The modern cycles of organic carbon, phosphorus, nitrogen, sulfur, and strontium are presented, and in Chapter 10 linked to those of Ca, Mg, and inorganic C. In conclusion in Chapter 10, aspects of the historical geochemistry of the carbon cycle are discussed, and tied to the evolution of Earth s surface environment. 

The Cl/Br ratio of seawater is 290, but that of evaporites is considerably higher (>3,000), such that the exospheric ratio is —400 50. The ratio in MORE and other basalts is the same (Jambon et al, 1995). Iodine in the Earth is concentrated in the organic matter of marine sediments this reservoir contains 1.2 X 10 kg I (O Neill and Palme, 1998), corresponding to 6 ppb if this iodine comes from 50% of the mantle. MORBs have —8 ppb I (Deruelle et al, 1992) implying —1 ppb in the depleted (degassed) mantle, for a PM abundance of 7 ppb. 

Anderson D. L. (1994) The sublithospheric mantle as the source of continental flood basalts the case against the continental lithosphere and plume head reservoirs. Earth Planet. Set Lett. 123, 269-280. 

Two extreme notions about the meaning of these components or end members (sometimes also called flavors ) can be found in the hterature. One holds that the extreme isotopic end members of these exist as identifiable species, which may occupy separate volumes or reservoirs in the mantle. In this view, the intermediate compositions found in most oceanic basalts are generated by instantaneous mixing of these species during melting and emplacement of OIBs. The other notion considers them to be merely extremes of a... 

McKenzie D. and O Nions R. K. (1983) Mantle reservoirs and ocean island basalts. Nature 301, 229-231. 

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