VARIABILITY - A NEW PARAMETER WHICH EMPHASIZES THE LIMITS OF EXTENDEDRARE-EARTH DIAGRAMS

Spidergrams or extended Coryell-Masuda diagrams have been widely used over the past several years to apply the result of trace element analy ses to the understanding of numerous geochemical processes including t he behaviour of the hygromagmaphile elements in mid-ocean ridge basalt s genesis, the behaviour of elements during the continental crust extr action, and for tracing the genesis of oceanic island basalts. However , the use of these diagrams is problematical due to ambiguities linked to the assumptions made for classifying the elements along the X axis and to the lack of information concerning the real conditions of part ial melting. Selected examples are used to illustrate the assumptions, aims, and limitations of spidergrams commonly used in the field of oc eanic basalts genesis and magmatic processes. A parameter, the variabi lity (V), is defined to better constrain the order of elements on the X axis of spidergrams. The variability of an incompatible element in a basalt population has been demonstrated to be related to its bulk par tition coefficient D [Schiano, 1991 ; Dupre et al., 1992]. It follows that for any population, elements can then be arranged following their D. When applied to different basalt populations, different D orders c an be obtained, which illustrates the ambiguities of using an assumed fixed order as in classical spidergrams. The utility of this approach is discussed by comparing variabilities of basalt populations with dif ferent origins. For example, the so-called ''St Helen'' geochemical si gnature cannot be due to residual material from a dehydrated subducted plate. In addition, when comparing variabilities of incompatible elem ents in arc volcanism basalts and mid-ocean ridge basalts, it is found that Ta anomaly in arc related basalts is more likely linked to the i mpossibility for Ta to migrate with the fluids extracted from the dehy drating subducting plate than to its capacity of being trapped in a re sidual phase.