Spidergrams mantle

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. 

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... 

---