Silicate Budgets

The Si balance can also be represented by equation (5.2). The relevant fluxes are given in Table 5.3 and Fig. 5.6. Unlike P and N, far less Si is transported out of the SCS by the intermediate water than is brought in through deepwater influx. This is because siliceous particles do not dissolve as readily as 

Positive and negative numbers represent inflow and outflow, respectively 

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The offshore advective flux for Si shown in Fig. 17.3 (30 X 10 mol d l) was calculated by difference, based on the total flux of dissolved Si supplied to the shelf system (32 X 10 mol Si d-1), the estimated deltaic burial rate (1-3 x 10 mol Si d ), and the nearshore particulate flux (0.1-0.7 x 10 mol Si d ). This advective flux is in good agreement with the results of Daley (1997), who estimated that 30 x 10 mol d of Si leave the shelf, based on seasonal field data and a multibox model for the shelf. Most of the silicate (94%) supplied to the shelf by external sources appears to be transported to the open ocean in either dissolved or particulate form. Approximately 36% of the Si leaving the outer shelf is in particulate form according to these calculations. Biogenic silica export may have contributed to the lack of closure in the Edmond et al. (1981) silicate budget for the shelf, although deltaic burial also remains as a potentially important sink. 

Codispoti, L.A. and Owens, T.G. (1975) Nutrient transport through Lancaster Sound in relation to the Arctic Ocean s reactive silicate budget and the outflow of Bering Strait Waters. Limnology and Oceanography, 2d, 115-119. 

Other accessories that may play an important role in the fractionation of some U-series elements, include, monazite, apatite, allanite, titanite, thorite and chevkinite. Hermann (2002) has recently determined experimentally the partitioning of U, Th and lanthanides between allanite and granitic melt at 2.0 GPa and 900°C. He finds D ] = 20 and Z)tii = 60, confirming that allanite can play an important role in controlling U-Th budgets in silicic melts. The very high Z La in the same experiment (-200), indicates that allanite will also be an important host for Bi and Ac. 

The metal budget of any individual shale horizon reflects a variable admixture of materials with a number of end member compositions. Even where the sulfide component of a sample is >10%, conventional discrimination plots used to identify and quantify hydrothermal input such as Co/Ni ratio (e.g., Meyer et al., 1990) or rare earth element plots (e.g., Johannesson et al., 2006) are hindered in their application due to dilution by the nonsulfide silicate detrital minerals. 

Most of the dissolved calcium in groundwater in northern Wisconsin is the result of silicate hydrolysis of the aquifer materials. The assumption of conservancy is accurate only because of the relatively slow rates of silicate dissolution. The presence of more soluble calcium-containing minerals, such as calcite or gypsum, would invalidate assumptions of conservancy and would lead to significant errors in solute budgets. 

The budget can be balanced by consideration of a variety of silicate reactions producing different ratios of cations HCC>3 SiC>2. The simplest approach was that of Garrels and Mackenzie (1971a), in which they considered the reactions of feldspars with C02-charged water to produce kaolinite and a montmorillonite-type mineral ... 

The silicate minerals account for aU the HREE budget and 50-90% of the LREE, strontium, and Zr-Hf in apatite-free peridotites. 

In contrast, peridotites metasomatized by small melt fractions show enrichment in platinum and palladium and elevated (Pd/Ir) . Bulk mineral separate PGE-Re analyses of two fertile xenoliths from southeastern Australia indicate less than 6% of the whole-rock PGE budget resides in either silicate or oxide phases and further implicates sulfides and alloys as the main controls of PGE-Re abundance. Comparison of sulfide versus whole-rock budgets by Lorand and Alard (2001) demonstrates the dominance of sulfide as the main PGE host in relatively fertile peridotites. This confirms the results of earlier studies of xenolith PGE mass balance (Hart and Ravizza, 1996 Mitchell and Keays, 1981) plus xenolith-derived and diamond inclusion sulfide studies (Jagoutz et al, 1979 Pearson et al, 1998b). As with cratonic xenoliths, sulfur-PGE and major-element-PGE correlations in more depleted noncratonic peridotites indicate that I-PGEs are probably not hosted entirely by sulfide (Lee, 2002). 

In summary, amphibole, along with mica, is the dominant silicate host for niobium in peridotitic xenoliths. In mica-absent assemblages amphibole also dominates the barium and tantalum budgets (Ionov et al, 1997 Eggins et al, 1998) and its presence strongly alfects the bulk rock Zr/Nb ratio. 

There is —90 ppm of phosphorus in the silicate Earth (McDonough et al., 1985), and the bulk Earth is estimated to have — 0.1 wt.% phosphorus. Using the relationships in Figure 6 the core is thus estimated to have —0.20 wt.% phosphorus (Table 4). Thus, 90% of the planet s inventory of phosphorus is in the core (Table 6) and the core s metal/silicate phosphorus enrichment factor is —22. Similarly, the core hosts —90% of the planet s carbon budget, and has a metal/silicate enrichment factor only slightly lower at —17. 

It has been estimated that the silicate portion of Mars contains 32 ppb I and 16 ppb of U at present, equivalent to 32.3 ppb initially (Wanke and Dreibus, 1988), so that I/U = 0.54 (molar ratio) at 4.57 Ga. If the atmosphere contains plutonium-derived xenon comprising 5% of the atmospheric Xe, so that ( Xe/ Xe) = 83, then using Equation (2), a closure age of 46 Ma is obtained, which is a factor of 2 lower than that of the Earth. In this case, —2% of the mantle has degassed radiogenic xenon, which is compatible with the " Ar budget (see Chapter 4.11), and so... 

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