Variation diagrams

An extension of the tree of causes, called variation diagrams (Leplat and Rasmussen, 1984) was developed to answer some of these criticisms. In this method, the Rasmussen stepladder model of human error (see Chapter 2) is applied to analyze causal factors at each node of the tree. A detailed example of the use of this technique is provided in Chapter 7 (Case Study 1). 

Fig. 2. Thorium variation diagram (ppm) versus seleoted traoe-elements (ppm).

The two-mica granite and the muscovite Ieucogranite phases of the NPSG fall in the field of specialized granites on a Rb-Sr-Ba ternary variation diagram for granitic intrusions (Fig. 3). Based on the elevated uranium contents in these two phases, it is likely that the uranium at Long Lake was derived predominantly from them. 

Shown in Figs. 3.2 and 3.3 are M variation diagrams for chain and sheet silicates plotted by Povarennykh (1963). The remaining procedure in hardness calculation is exactly the same as for isodesmic minerals, and to the result we add a correction determined on the structural relations for the diagrams. 

Fig. 1.6. Variation diagrams of incompatible trace element ratios and 87Sr/86Sr for mafic (MgO > 4 wt %) Plio-Quatemary volcanic rocks from Italy.

Fig. 2.3. Variation diagrams of MgO vs. selected major and trace elements and 87Sr/86Sr for magmatic rocks of the Tuscany Province. Symbols as in Fig. 2.2.

Fig. 4.6. Variation diagrams of selected major, trace elements and 87Sr/86Sr ratios vs. MgO for the Vulsini rocks. Symbols as in Fig. 4.4. Crosses indicate samples of uncertain location.

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). 

Fig. 4.19. Variation diagrams for the Colli Albani volcanic rocks.

Fig. 6.5. Variation diagrams of selected major and trace elements vs. SiC>2 for Somma-Vesuvio rocks. For symbols see Fig. 6.4.

Fig. 6.11. Variation diagrams for Campi Flegrei rocks and for Pliocene buried vol-canics from Parete-2 well. Symbols as in Fig. 6.10.

Major and trace elements of the Ischia rocks define continuous trends on variation diagrams, although with scattering (Fig. 6.16). Ti02, MgO, FeOto-tai, CaO, P2O5, ferromagnesian trace elements, Sr and Ba decrease with in-... 

Fig. 6.23. Variation diagram for selected major and trace elements of Mount Vulture rocks.

Fig. 7.4. Variation diagrams for the islands of Alicudi, Filicudi and Salina.

Variation diagrams of major and trace elements vs. SiC>2 (Fig. 8.13) show a bimodal distribution between basalts and rhyolites, with a gap for intermediate compositions (Daly gap). Such a distribution of rock types is typical of volcanoes from continental rift settings (see Peccerillo et al. 2003 and references therein). A few intermediate samples have been found as xenoliths, and these have been suggested to represent hybrids between mafic and felsic magmas (e.g. Mahood and Baker 1986). 

Fig. 8.14. Inter-element variation diagrams for Pantelleria volcanic rocks.

Variation diagrams of major and trace elements show a decrease in Ti02, FeOtotai, MgO, CaO, Cr, Ni and Sc, and an increase in K20 and incompatible elements with increasing silica. P2O5 shows a wide scattering in the mafic rocks (Fig. 8.20). REE patterns exhibit variable degrees of fractionation, sometimes with a small positive anomaly of Eu (Fig. 8.21a). Mantle-normalised incompatible element plots of mafic rocks show an upward convex shape and negative anomalies of K, Flf and Ti (Fig. 8.21b). 

Fig. 9.5. Major element variation diagram for Plio-Quatemary volcanic rocks from Sardinia. Symbols as in Fig. 9.3.

Fig. 9.9. Ba/Nb vs. Ce/Pb variation diagram for Sardinian Plio-Quatemary volcanic rocks (MgO >4 wt %). The composition of the Sardinian Oligo-Miocene orogenic volcanism and of the anorogenic rocks from Sicily are shown for comparison. Symbols as in Fig. 9.3.

MORB, OIB and island-arc geochemical signatures, respectively. The distribution of various rock types are schematically shown in Fig. 9.1. Compositional characteristics and ages are given in Tables 9.2. Compositions for selected rocks are reported in Table 9.4. The TAS diagram based on oxide concentrations recalculated on a water-free basis is shown in Fig. 9.10. Variation diagrams of some major and trace elements and mantle-normalised incompatible element patterns for selected samples are shown in Figs. 9.11 and 9.12. 

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