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Basalts cored on DSDP Leg 16 fall into two groups. Those from the west flank of the East Pacific Rise (DSDP 159-163) are geochemically homogeneous tholeiites. In contrast, the Panama Basin samples (DSDP 155-158) range from alkali basalts to tholeiites and may have formed over a mantle "hot spot" rather than at a mid-ocean ridge. Based on core relationships, all the Leg 16 basalts are extrusive.DSDP 163 penetrated 18 meters of basalt made up of seven cooling units with chilled glassy margins. Even in this Campanian (Cretaceous) sequence, severe alteration affects only the uppermost few centimeters. Sideromelane appears particularly resistant to low-temperature alteration. Microprobe analyses of spheroidal "globules" (probably spherulites) at the inner margins of sideromelane masses show bimodal chemical compositions: reddish spherulites are enriched in Na2θ, A1 2 O 3 and SiC>2, whereas opaque ones are enriched in MgO and FeO relative to glass. The bulk composition of the "globular" zone is the same as that of the bulk rock, pointing to partitioning in situ.

After immersion for 30 seconds in stannous fluoride, tin and fluorine were detected in enamel. When the enamel was immersed for a longer time, it was etched by stannous fluoride treatment. A second tin-rich zone formed on the enamel surface after prolonged exposure and possibly consisted of Sn3F3PO4 and a hydrated tin compound.

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