Ten basalt samples recovered from the FAMOUS area were selected so as to obtain representatives of a wide geographical and compositional range. The samples were analyzed for 87Sr/86Sr, K, Rb, Cs, Sr, Ba, and rare-earths. Sr-isotope ratios fall in the narrow range of 0.70288 to 0.70307, which implies that these samples were derived from an isotopically homogeneous source. The FAMOUS area lies in a geochemical transition zone between the Azores Plateau and “normal” ridge areas south of lat 33°N. The LIL (large-ion-lithophile) and Sr-isotope geochemistry of FAMOUS basalts is thus influenced by the Azores mantle plume; this results in higher Sr-isotope and LIL concentrations in these basalts than is typical of Mid-Atlantic Ridge basalts. Trace-element distributions in FAMOUS area basalts cannot be entirely accounted for by fractional crystallization models that are based on major-element chemistry. The LIL distribution in FAMOUS basalts could be due to variable extents of partial melting. Zonation within the magma chamber may result from incomplete mixing of successive batches of magma entering the chamber and could be further enhanced by fractional crystallization. The variation in partial melting would require significant increases in mantle temperature over a relatively short period of time. According to this model, the Mount Pluto magma represents the highest degree of partial melting and may mark the initiation of a new cycle of eruptive activity in the median valley.

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