Incompatible element ratios

Incompatible-element ratios (e.g., Th/La, Nb/Zr, Ce/Yb in basalts) are therefore expected to be very insensitive to mineral separation from the melt and, for differentiated lavas, can be used as a parameter characteristic of their parent liquid see below). 

The corresponding expression for isotopic (or incompatible-element) ratios is given by DePaolo (1981). Let us label i 1 and /2 the two isotopes of the same element. We further assume that their partition coefficient is identical, as are their r and z, values. Dividing equation (9.4.17) for isotope i2 by the corresponding equation for isotope il, we get... 

In Figure 9.15, the relationship between the fractional change in the elemental ratio and the extent of crystallization F is plotted for different values of AD=Di2—Dn for partition coefficients less than 0.1, several tens of percent fractionation are needed before a change of a few percent in the ratio becomes visible. Crystal fractionation does not change incompatible-element ratios such as La/Yb, Zr/Nb,. .. except in extremely residual melts. 

However, variations of Pb isotopes and incompatible element ratios for both orogenic and anorogenic magmas (Figs. 1.6, 1.7b), reveal a more complex picture, which does not fit simple two-end-member mixing between mantle and crust. In general, these data suggest interaction between distinct types of mantle sources, as well as multiple events of mantle contamination by various types of crustal rocks (D Antonio et al. 1995 Pec-cerillo 1999). These issues will be discussed in detail in later chapters. 

Variation diagrams of major and trace elements vs. MgO at Colli Albani (Fig. 4.19) show a positive correlation for CaO, TiC>2, FeOtotai and ferro-magnesian trace elements (Cr, Ni, Co, etc.), negative correlations for Na20, K2O, AI2O3 and incompatible elements (Th, La, Ta, etc.), and a bell shaped trend for P2O5. Incompatible elements show smooth inter-element positive trends (Fig. 4.19g). The pre-caldera lavas seem to define different trends on some major and trace element variation diagrams, especially on plots of incompatible element vs. incompatible element ratios (Fig. 4.19h). REE and incompatible element patterns have shapes that are similar to those for other ultrapotassic rocks from the Roman Province (Fig. 4.20). 

The Emici and Roccamonfina magmas exhibit peculiar compositional characteristics in terms of their very variable geochemical and isotopic signatures for potassic to ultrapotassic mafic rocks. These are not encountered in other potassic volcanoes of central Italy (i.e. Roman and Campanian provinces, with the possible exception of Vico volcano), where coexisting KS and HKS rocks have different potassium and incompatible trace element abundances but exhibit similar incompatible element ratios and radiogenic isotope signatures (Peccerillo 1999, 2002). 

Fig. 5.8. Plots of incompatible element ratios for the mafic rocks (MgO > 3%) of Emici and Roccamonfina volcanoes, compared with the mafic rocks from the Campania and Roman provinces. Symbols as in Fig. 5.4.

Fig. 6.25. Variation of incompatible element ratios for the Vulture, Campania and Pontine mafic volcanic rocks (MgO > 3 wt%). Compositions for the intraplate Eocene Pietre Nere rocks (Fig. 6.1) and for other Italian magmatic provinces are shown for comparison.

Radiogenic isotopic variations for Sardinia Plio-Quaternary volcanics are accompanied by systematic modifications of trace element ratios (Lustrino et al. 2004a). For instance, Ba/Nb is higher in the northern than in the southern outcrops, whereas Ce/Pb is lower (Fig. 9.9). In general terms, the rocks from the southern occurrences have incompatible element ratios close to typical anorogenic magmas such as those of the Sicily Province,... 

Angrites are believed to have formed as partial melts of primitive source materials under relatively oxidizing conditions (Longhi, 1999 Mittlefehldt and Lindstrom, 1990 Mittlefehldt et al., 2002). Angra dos Reis is distinct from the others—its major-element composition indicates a different fractionation path for its parent melt (Longhi, 1999). Key incompatible element ratios and... 

Condie K. C. (2003) Incompatible element ratios in oceanic basalts and komatiites tracking deep mantle sources and... 

The incompatible element ratios of oceanic basalts provide a convenient template against... 

Figure 7 Illustration of the effects of equilibrium (batch) crystallization or melting on trace element abundances, (a) Variation in liquid concentration (Cl) (normalized to unit source concentration Cq= 1) as a function of melt fraction (F) for six elements with different bulk distribution coefficients (D). (b) Change in the ratios of incompatible elements with different Ds as a function of F. Each curve is for a different pair of elements that have the Z)s indicated. Note that when D < 0.1, incompatible element ratios can be changed only at very low extents of melting (or high extents of crystallization) (Langmuir et al., 1992) (reproduced by permission of American Geophysical...

A second finding in the study of dikes exposed at Hess Deep concerns the spatial variation in their compositions (Stewart et al., 2002). Dikes were sampled over an area encompassing 25 km of an east-west flowhne, representing —3.7 X 10 yr of crustal accretion at the EPR. Indices of fractionation (MgO), and incompatible element ratios (La/Sm, Nb/Ti) show no systematic trends along flowline. Rather, over short (<4m) and long (—25 km) distances. 

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