Spidergrams

Fig. 3. Total alkalis versus silica (TAS) and (b) AFM plot of Irvine and Baragar (1971), (c) modified Zr/Ti02-Nb/Y plot (Pearce, 1996) of Winchester and Floyd (1977), (d) Rock/chondrite-normalized REE diagram for rocks of amli-llica pluton and (e) rock/MORB-normalized spidergrams, (f) Th/Yb vsTa/Yb diagram.

Trace-element abundance patterns, often called spidergrams, of MORE are shown in Figure 11 ( spidergram is a somewhat inappropriate but a convenient term coined by R. N. Thompson (Thompson et al., 1984), presumably because of a perceived resemblance of these patterns to spider webs, although the resemblance is tenuous at best). The data chosen for this plot are taken from le Roux et al. (2002) for MORE glasses from the MAR (40-55° S), which encompasses... 

Figure 11 Trace element abundances of 250 MORE between 40° S and 55° S along the Mid-Atlantic Ridge. Each sample is represented by one Une. The data are normalized to primitive-mantle abundances of (McDonough and Sun, 1995) and shown in the order of mantle compatibility. This t)fpe of diagram is popularly known as spidergram. The data have been filtered to remove the most highly fractionated samples containing less than 5% MgO (source...

Trace-element ratios of similarly incompatible pairs, such as Th/U, Nb/U, Nb/La, Ba/Th, Sr/Nd, or Pb/Nd, tend to be more useful in identifying source differences, because they are fractionated relatively little during partial melting. Elements that appear to be diagnostic of distinctive source types in the mantle are niobium, tantalum, lead, and to a lesser extent strontium, barium, potassium, and rubidium. These will be discussed in connection with the presentation of specific spidergrams in Section 2.03.5.2.2. 

Figure 20 shows examples of spidergrams for representative samples of HIMU, EM-1, EM-2, and Hawaiian basalts, in addition to average MORB and average normal MORB, average... 

Spidergrams for mafic garnet granulites from the Tonsina area show no evidence for the presence of... 

Figure 18 Extended trace-element diagrams (hereafter, spidergrams) for volcanics and felsic plutonic rocks from the Talkeetna arc section, south central Alaska. Concentrations are normalized to N-MORB (Hofmann, 1988). Bold red lines are average values from Table 3. Talkeetna lavas, and plutonic rocks interpreted as hquid compositions, are only shghtly enriched in light REE compared to MORB, but show depletion of Nb and Ta, and enrichment of Pb and Sr, t)fpical for arc lavas worldwide. Their trace-element contents are similar to, for example, lavas from the modem Tonga arc. Data from Greene et al. (2003) and our unpublished research.

Figure 19 MORE-normalized spidergrams for lower crustal gabbronorites in the Talkeetna arc section, south-central Alaska. Bold red line is average from Table 3. In these cumulate gabbros, some of the Nb and Ta depletion, and Pb and Sr enrichment, is inherited from parental, arc magmas, but the pattern is modified by high Ti (and probably Nb and Ta) in cumulate magnetite and high Ph and Sr in cumulate plagioclase. Data from Greene et al. (2003) and...

Figure 21 MORE normalized spidergrams for mafic garnet granuUtes from the Tonsina area, at the base of the Talkeetna arc section at 1 GPa, 1,000 °C (DeBari and Coleman, 1989 Kelemen et al., 2003a). Bold red line is average from Table 3. These orthogneisses formed via metamorphic recrystaUization of protoliths with major and trace-element contents identical to gabbronorites from higher in the Talkeetna section. Garnet in these rocks is metamorphic, as previously proposed (DeBari and Coleman, 1989). Data from our unpublished research.

MORB- MORB-normalized spidergrams are most appropriate for evolved basalts, andesites normalized and crustal rocks — rocks to which MORB rather than primitive mantle could be spider diagrams parental. This form of spider diagram was proposed by Pearce (1983) and is based upon two parameters (Figure 4.25). Firstly, ionic potential (ionic charge for the element in its normal oxidation state, divided by ionic radius) is used as a measure of the mobility of an element in aqueous fluids. Elements with low (<3) and high (>12) ionic potentials are mobile and those with intermediate values are generally... 

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