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Leg 144 of the Ocean Drilling Program (ODP) recovered lavas and volcaniclastics from the volcanic basement of five northwest Pacific guyots (Limalok, Lo-En, and Wodejebato guyots in the Marshall Islands group; MIT Guyot, an isolated edifice midway between the Marshall Islands and Japan; and Takuyo-Daisan Guyot in the Japanese seamount group). Most of the lavas have undergone extensive low-temperature alteration, but their petrography, mineral chemistry, and, in some cases, whole-rock chemistry clearly demonstrate that almost all are of alkalic affinity, ranging from highly magnesian basanites to hawaiites. The sole exception is the single lava recovered from Takuyo-Daisan, which is tholeiitic or transitional in character. Tectonic reconstructions suggest that all the seamounts investigated during Leg 144 originated, in Cretaceous time, as intraplate volcanoes in what is now known as the SOPITA (South Pacific isotopic and thermal anomaly) region. One of the principal objectives of basement sampling was to determine whether the present-day manifestations of this region have persisted since the Cretaceous or whether they have evolved through time. Ratios and relative abundance patterns of those incompatible trace elements that are not readily affected by alteration processes are remarkably uniform in the Leg 144 alkalic lavas, falling well within the overall field of modern SOPITA lavas and strongly resembling those of Tahiti in particular. In this uniformity, they differ from the pronounced diversity of modern SOPITA lavas, although the sample is rather small. Thus, the Cretaceous SOPITA mantle source was little different in trace element abundances from today, although the Leg 144 data do not preclude an increase in diversity through time.

Petrologic Studies of Basement Lavas from Northwest Pacific Guyots