GEOLOGY 330 - SPIDERGRAMS

Prior knowledge: plate tectonic environments; names of rare earth elements; meaning of chondrite'.
Spidergrams are histogram like plots of the abundances of a set of elements in an analyzed sample
relative to their abundance in some standard such as chondrite, primitive Earth, or ocean floor basalt. What
makes a spidergram unique is the order in which the elements are arranged along the abscissa. As in the case
of conventional REE plots, what is important is the pattern of the relative abundances rather than the abundance
values themselves.
One of the earliest spidergrams (Jahn, B-M, Chi-Yu, S., and Rama Murthy, V., 1974. Trace element
geochemistry of Archean volcanic rocks. Geochim. Cosmochim Acta, 38, 611-627) attempted to classify
Archean rocks in terms of their relative abundance of the four alkali elements Sr, K, Rb, Ba. Unfortunately,
these elements, and also U and Th, are highly susceptible to alteration, and most spidergrams now include
the REE, Nb (Ta), Zr, Ti, P, and Y (which nevertheless behaves in a similar manner to the REE Ytterbium).
REE possess a number of physical and chemical properties that make them especially useful in
geochemical studies of igneous and sedimentary rocks, ocean water/rock, and continental rock weathering
systems. All the REE are refractory with oxide condensation temperatures similar to Sr, U and Th. They were
not fractionated during the formation of the Earth, and REE patterns are similar to those of chondrites and the
solar photosphere. The abundances quoted above, including that for chondrite, are about 1.5 times those
of chondrite "C1" to take into account volatile loss in the formation of the Earth. The REE content
of primitive mantle (present mantle plus crust) is further enriched 1.5 times over that of the chondritic bulk earth.

RETURN TO:

Click here to return to beginning.

Click here to return to course outline.

Click here to return to W.R. Church's page.

Click here to return to course list.