KISH , GEOCHEMICAL STUDIES OF PUMICE 229

 

discrimination diagrams that utilize trace elements patterns have
proven to be especially useful in identifying igneous materials
from similar sources. Pearce et al. (1984) developed several
trace element diagrams useful for the study of high SiO, igneous
rocks. While these diagrams were initially intended for the
study of plutonic rocks, they also can be used in the study of
felsic volcanics. The Rb versus (Yb + Nb) discrimination
diagram (Figure 11) allows discrimination between ocean-ridge
granites (ORG), within-plate granites (WPG), volcanic-arc
granites (VAG) and syn-collisional granites (syn-COLG). All
the felsic pumice samples in this study that have a rhyolitic
composition (16 samples) that form a cluster within the VAG
field (Figure 11). Some of the samples also occupy distinct
subfields that correlate with major element composition. The
implied igneous association, volcanic arc granite or a normal
“calc-alkaline” association, is compatible with the overall
chemical and mineralogical nature of the pumice.

Summary of Petrographic and Geochemical Studies

The combined petrographic and geochemical characteristics
of the pumice artifacts analyzed in this study indicate the most
of the samples have a high-silica, rhyolite composition. The
observed trace element composition of rhyolitic to andesitic
pumice indicates these rocks are associated with a calc-alkaline
suite of volcanic rocks; however, the two distinct potassium
concentrations for the rhyolitic pumice (high- K and medium-
K) and the medium-K composition of andesitic samples
(Figure 6) suggests the felsic pumice was derived from at least
two separate sources. The variation in potassium content is
probably not associated with two separate eruptions from the
same source, but more likely, the two compositional groups
were derived from separate volcanic sources — the sources may
have been widely separated. The relatively high-K content for
most of the high-silica pumice is commonly associated with a
calc-alkaline volcanism in a continental setting, while the
medium-K content samples are more characteristic of calc-
alkaline volcanism in an oceanic or continental margin setting.

The two mafic pumice-scoria artifacts, one from southern
Florida and the other from the Panhandle of the Gulf Coast,
have a distinct alkaline signature. It is unlikely that these mafic
samples were derived from the same volcanic sources that
produced the andesitic to rhyolitic pumice. Mafic alkaline
volcanic rocks can be found in a wide range of tectonic setting
including “within-plate” volcanic oceanic islands, some “back-
arc” portions of subduction zones, continental rift zones and
localized “hot spot” regions such as the Yellowstone region of
Montana and Wyoming.

| Potential Provenance of the Pumice Artifacts

The extensive distribution of high-silica, high-K pumice,
which has a limited range in trace element composition, may
indicate that this material was derived from a single eruptive
source, Other types of pumice, including medium-K rhyolite
pumice, andesitic-dacitic pumice and basaltic scoria-pumice
were probably derived from separate sources. Limited numbers
of modern pumice fragments, washed ashore in the West Palm
Beach area in southeastern Florida (Bryan 1972), have a

cummingtonite-dacite composition, which is not similar to the
high-silica, rhyolitic pumice artifacts. Bryan (1972) suggested
the modern pumice originated in the Lesser Antilles, but he did
not cite specific evidence for this conclusion. Examples of
modern drift pumice that probably originated in the Lesser
Antilles and then traveled several hundred kilometers to the
north, include a single piece of dacite pumice washed ashore
in Puerto Rico (Donovan 1999).

Wheeler (2006) reviewed the different hypotheses for the
provenance of Florida pumice artifacts. He found that earlier
workers had proposed a wide range of volcanic sources
including the Lesser Antilles, Atlantic Islands such as the
Azores and more exotic sources such as the volcanoes of the
Peruvian Andes. Modern studies of pumice have demonstrated
that moderately large samples (5-10 cm) can stay afloat for a
period of many months (Manville et al. 1998). Other examples
of the durability of drift pumice includes medium to large
blocks of pumice that formed during a shallow, submarine
eruption at Protector Shoals in the South Sandwich Islands.
Blocks of this pumice nearly circumnavigated the oceans of the
southern hemisphere, traveling over 20,000 km around the
continent of Antarctica (Risso et al. 2002). The ability of
pumice to be such a far-traveled material makes it necessary to
evaluate the entire Atlantic (northern hemisphere) and Gulf of
Mexico region for the potential sources of Florida drift pumice.

Potential sources for the pumice are considered to be
volcanoes that were active at the time the pumice drifted to
Florida (direct eruptive sources) or volcanic sources that are
relatively young (<11,000 years old) and could have provided
unconsolidated pumaceous material that was transported to the
sea by mass wasting and erosion. A database of Holocene
volcanic activity compiled by Simkin and Siebert (1994) was
used to search for potential provenance localities. Four regions
of volcanic activity were considered as potential sites (Figures
12 and 13). These include: (1) the tholeiitic to alkaline
volcanoes associated with oceanic ridge and within-plate
(plume) igneous activity along the North Atlantic ridge system,
including the islands of Iceland and Jan Mayan (Macdonald et
al. 1990; Lacasse and Garbe-Schénberg 2001); (2) Mafic to
felsic, alkaline volcanic activity associated with the volcanoes
in the eastern Atlantic such as the Canary Islands (Edgar et al.
2002); the Azores (Guest et al. 1999) and the Cape Verde
Islands (Plesner et al. 2003); (3) Volcanic activity in the
western Atlantic restricted to calc-alkaline dominated volcanism
of the Lesser Antilles island arc (Macdonald et al. 2000); and
(4) calc-alkaline volcanism of the easternmost portion of the
Trans Mexico Volcanic Belt located 100 km west of the Gulf
of Mexico (Héskuldsson and Robin 1993) and mafic, alkaline
volcanic rocks that are present in the Tuxtla volcanic field
situated on the Gulf of Mexico near Veracruz, Mexico (Thorpe
1977; Nelson et al. 1995).

Dispersal of Pumice

Dispersal of pumice by ocean currents may be associated
with several possible mechanisms. These include direct
dispersal of pumice into the sea or rivers flowing into the sea by
large airborne eruptions of pyroclastic material. Plinian
eruptions and fallout deposits produce great volumes of pumice