Compatible elements

This equation shows that, at constant growth rate, the more incompatible the elements, the longerrit takes for steady-state to establish. We therefore can expect kinetic disequilibrium beween mineral and liquid to be more conspicuous for incompatible than for compatible elements. 

Figure 9.6 Comparison of the equilibrium [equation (9.2.2)] and fractional melting [equation (9.3.15)] models for a bulk solid-liquid partition coefficient Dt of 0.1 (top) and 2 (bottom). Although the concentrations predicted by the two models diverge rapidly for incompatible elements in instantaneous melts, they remain virtually identical for compatible elements.

The residence time of a trace element is xJaLi. compatible elements can be thought of as reactive and have shorter residence times than inert incompatible elements. As shown in Chapter 7, equation (9.4.4) can be integrated from 0 to t into equation (7.2.12)... 

A comparison of the two models described by equations (9.4.7) and (9.4.8) with fractional crystallization for >j = 0.1 and Dt = 5 and assuming an erupted fraction Y of 50 percent is shown in Figure 9.5 (p. 493). Use of either equation (9.4.7) or equation (9.4.8) leads to quite different patterns of incompatible and compatible elements (Albarede, 1985 Caroff et al., 1993) which makes it possible to discuss the timing of replenishment and eruption events. 

Incompatible elements can achieve very large enrichment in the liquid. Steady-state is achieved over a characteristic length in proportion with (ktL/ktR)L. For small porosities, this length is in the order of (4>/)L for incompatible elements, in the order of L for compatible elements (Figure 9.12). A very small fraction

limit concentration ( C0 / >) and the characteristic length of incompatible elements. 

This relationship has been displayed in Figure 9.13. For small values of d> and zone-refining. Incompatible elements are such that ktR/ktLat

efficient scavenging by ascending molten zones. Again, residual porosity is a critical factor for incompatible-element distributions. 

Incompatible elements keep pace with the liquid, compatible elements lag significantly behind. 

For small extents of crystallization, the maximum change, and thereby the most valuable information on F, will be obtained from elements with high Dt (compatible elements) such as Ni in basaltic olivine. Elements with ), 1 (incompatible elements), such as Th, Ba or rare-earth elements in basaltic systems, will provide basically no clue to F variations. In addition, information carried by incompatible elements, which do not fractionate with respect to each other, is entirely redundant. This is better shown by taking the relative change in the ratio of two elements il and i2 per increment of crystallization... 

We will assume small degrees of melting, i.e., F 1. Two extreme cases will be considered. For compatible elements (D, F)... 

The concentration profiles for >,-=0.1 and Dt=5 have been depicted in Figure 9.16 as a function of the dimensionless distance vx/. Accumulation of incompatible elements and depletion of compatible elements in the vicinity of the interface are the remarkable features of this model. Concentrations at the interface are given by... 

Chromium is a compatible element in the Earth s mantle, and tends be present in much greater concentrations in mafic igneous rocks than in felsic ones (Faure 1991). Ultramafic rocks often contain over 1,000 ppm Cr and can generate environmental problems when they weather (Robertson 1975 Robles and Armienta 2000). Granites may contain less than 20 ppm Cr, whereas shales contain roughly 90 ppm. 

Given that olivine-compatible elements such as Mg or Co are likely to be most concentrated in the first olivine crystals to form an olivine cumulate, assays through two cumulate peridotite units of the BLUC reveal trends that are consistent with a downward younging direction (Fig. 5). 

In any application the fluorescence detection systems must have four compatible elements ... 

These examples involve partitioning of elements as liquids cooled and crystallized. Partial melting of a solid rock also results in partitioning of incompatible elements into the liquid phase, which contains no rigid crystalline sites. Separation of the melt then fractionates incompatible elements from the compatible elements left behind in the solid residue. 

Common igneous processes (partial melting and fractional crystallization) lead to element fractionations. Incompatible elements tend to be concentrated in melts and compatible elements in solids. Separation of partial melts from residual crystals as the melts ascend to higher levels, or accumulation of early-formed crystals from melts, ultimately produces rocks with compositions different from the starting materials. These processes account for the fractionations seen in differentiated meteorites and planetary samples. 

Copper. The abundance of copper in the depleted mantle raises a particular problem. Unlike other moderately compatible elements, there is a difference in the copper abundances of massive peridotites compared to many, but not all, of the xenolith suites from alkali basalts. The copper versus MgO correlations in massive peridotites consistently extrapolate to values of 30 ppm at 36% MgO, whereas those for the xenoliths usually extrapolate to <20 ppm, albeit with much scatter. A value of 30 ppm is a relatively high value when chondrite normalized ((Cu/Mg)N = 0.11), and would imply Cu/Ni and Cu/Co ratios greater than chondritic, difficult to explain, if true. However, the copper abundances in massive peridotites are correlated with sulfur, and may have been affected by the sulfur mobility postulated by Lorand (1991). Copper in xenoliths is not correlated with sulfur, and its abundance in the xenoliths and also inferred from correlations in basalts and komatiites points to a substantially lower abundance of 20 ppm (O Neill, 1991). We have adopted this latter value. 

Figure 8 Abundances of RLEs in fertile spinel-UierzoUte xenoliths from various occurrences. Compatible elements bave constant enrichment factors. Abundances decrease with increasing degree of incom-patibiUty, reflecting removal of very small degrees of partial melts (after Jochum et al., 1989).

Osmium is of great interest to mantle geochemists because, in contrast with the geochemical properties of strontium, neodymium, hafnium, and lead, all of which are incompatible elements, osmium is a compatible element in most mantle melting processes, so that it generally remains in the mantle, whereas the much more incompatible rhenium is extracted and enriched in the melt and ultimately in the crust. This system therefore provides information that is different from, and complementary to, what we can learn from... 

Rehnements of the Taylor and McLennan (1985) model are provided by McLennan and Taylor (1996) and McLennan (2001b). The latter is a modihcation of several trace-element abundances in the upper crust and as such, should not affect their compositional model for the bulk crust, which does not rely on their upper crustal composition. Nevertheless, McLennan (2001b) does provide modihed bulk-crust estimates for niobium, rubidium, caesium, and tantalum (and these are dealt with in the footnotes of Table 9). McLennan and Taylor (1996) revisited the heat-flow constraints on the proportions of mahc and felsic rocks in the Archean crust and revised the proportion of Archean-aged crust to propose a more evolved bulk crust composition. This revised composition is derived from a mixture of 60% Archean cmst (which is a 50 50 mixture of mahc and felsic end-member lithologies), and 40% average-andesite cmst of Taylor (1977). McLennan and Taylor (1996) focused on potassium, thorium, and uranium, and did not provide amended values for other elements, although other incompatible elements will be higher (e.g., rubidium, barium, LREEs) and compatible elements lower in a cmst composition so revised. 

Figure 22 Schematic plot of concentration versus distance for compatible versus incompatible elements in a mineral that is resorbed and regrown. Solid line shows final profile. Dashed line shows original profile for compatible element, prior to dissolution. If trace element zoning is radially distributed, this process would lead to an annulus and moat in compatible and incompatible elements, respectively (source Yang and Rivers, 2002).

Uncertainties in phenocryst abundance estimates are a significant source of error, particularly for major and highly compatible elements in the main phenocryst phases. In most cases related to ocean drilling samples, phenocryst abundances in a particular sample are rarely known to better than about 5 vol.%. The most abundant phenocrysts are olivine and plagioclase, which contain the elements magnesium, silicon, calcium, aluminum, and nickel. 

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