Crossing the Borders: New Methods and Techniques in the Study of Archaeological Materials from the Caribbean

Geochemical Techniques in Discriminating Chert Materials / 55
reprecipitation in the pore space of the chert during later weathering (Bush and
Sieveking 1986; Cowell 1981; Kars et al. 1990; Luedtke 1992). The following is proposed
concerning distribution of trace metals in chert:
(1) Li and Cr, but also to a minor extent K, Al, and Na, can occur as impurities in
the quartz crystal lattice itself, however generally in very low concentrations.
(2) Ca, Mg, and Sr may occur in fairly high concentrations, in particular if chert
is formed in limestone and if less soluble and often coarse crystalline carbonate
relics of the original host- rock have been incorporated and are still
present.
(3) Al, K, Ti, and Cr, and to a lesser extent Fe, Ca, Mg, Sr, and Na may occur in
relatively high concentrations if rather insoluble siliciclastic compounds have
been included, such as clay, volcanic tuff, and detrital rock fragments.
(4) Al/ K ratio is higher in chert with land derived siliciclastic inclusions as compared
to chert with marine (authigenic) clay inclusions that have incorporated
potassium from seawater.
(5) Fe, Mn, and S stem from sulfides, such as pyrite, that form authigenically in
marine sediments, moreover as a consequence of bacterial metabolism.
(6) S is also associated with organic matter.
(7) Na is introduced by seawater and is concentrated in remaining interstitial
water.
(8) Ca, Mg, and Sr concentrations may decrease as a consequence of weathering
and the dissolution of carbonates, in particular at the margins of chert concretions.
(9) S and Fe concentrations decrease as a consequence of pyrite oxidation during
weathering. Sulfate diffuses out off the concretion, while Fe2+ oxidizes and
precipitates as rust colored Fe3+, in particular toward the margin of the chert
concretion.
Discussion of Trace Element Variability in Caribbean Cherts
More than 150 chert samples from the 15 different sources were analyzed using
ICPAES. Thorough study of the chemical data from samples belonging to all these
sources has come up with some clear results. First, variation in chert trace- element
composition is much dependent on the type of host- rock in which it was formed
and reflects the characteristics of the different geological formations. As a result,
variation is seen between materials from different geological formations (Figure
4.5). Second, the trace element composition reflects a super imposed chemical
weathering effect. Third, the trace element composition of chert derived from different
localities in the same geological formation is comparable, but may vary in
certain cases.
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