KIsH GEOCHEMICAL STUDIES OF PUMICE 223

 

3.0

N
on

N
oO

Metaluminous

AlzO3/(Na2O + K2O0)
on

=
>)

Peraikaline

0.5
0.5 1.0

 

Peraluminous

1.5 2.0

AlzQO3/(CaO + Na.O + K20)

Figure 8. Shand’s alumina saturation classification of south Floirda pumice samples that have a rhyolitic compsition. A single
sample from the Wynnhaven Beach site in northwestern Florida is shown as an open circle.

limited range in major element composition (Hildreth 1981).

The principle trace element compositions for the Florida
pumice samples are reported in Table 7. Barium (Ba) - a trace
element that is selectively removed by the fractional crystalliza-
tion of potassium feldspar, is uniformly high (850-1100 ppm)
in all felsic pumice samples. The elevated concentration of Ba
indicates that potassium feldspar was not a residual phase in the
source material for the pumice and was not a crystallizing phase
during magma evolution. Strontium (Sr) exhibits a moderate,
negative correlation with silica concentration and a positive
correlation with the sodium content of felsic samples. High
silica, high-K pumice has Sr concentrations that range from
approximately 100-200 ppm. Low silica samples and medium-
K samples have Sr concentrations in the range 250-380 ppm.
Zirconium (Zr) concentration exhibits significant variation for
different samples (55-245 ppm), however, the variation of Zr
concentration does not appear to have any significant correla-
tion with the major element composition and may be a product
of inhomogeneous distribution of the trace mineral zircon
between different samples.

Rare Earth Elements

The 14 rare earth elements (REE) are often studied sepa-
rately from other trace elements due to their high, internal
chemical coherence and resistance to mobility or modification
associated with hydrothermal alteration or surface weathering.
The interrelationship between the different REE is normally
represented by a graphical plot of the rare earth elements
arranged in increasing atomic number on the x axis, versus
REE concentration logarithmically normalized to chondritic
meteorite reference values presented on the y axis (Rollinson
1993). The REE concentrations for Florida pumice samples are
reported in Table 8. The average chondritic values reported by
Boynton (1984) were used to calculate the normalized values
shown in Figures 9 and 10. There is a strong correlation
between the major element chemistry and observed REE
distribution pattern (Figures 9 and 10). High-K, high-silica
pumice (12 samples) has “lazy L” shaped REE patterns (Figure
9) that are associated with strong enrichment in the light REE,
with Lanthanum (La) concentrations approximately 70-90 times
chondritic abundances, and La,,/Yb,, ratios ranging from