Click here to go to 'Figures/Overheads' section.

Click here to return to course outline.

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

Click here to return to Earth Science course list.

 
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.

FIGURES

Structural Provinces of North America.

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.