The South Pacific Isotopic and Thermal Anomaly (SOPITA) is isotopically the most diverse intraplate volcanic province of the ocean basins, and shows evidence for volcanism since at least the Cretaceous. Studies of the SOPITA over this 120 m.y. timespan have great potential for expanding our understanding of chemical geodynamics: length scales of isotopic diversity, interaction of lithosphere and plume reservoirs, and long-term evolution of mantle plumes.
The SOPITA is the source region for widespread volcanism of the French Polynesian Islands, and the Cretaceous West Pacific Seamount Province (located north of the Ontong Java Plateau). The Cretaceous seamounts were formed between 130-65 Ma, with the highest peak in volcanic activity near 85-80 MA and a lower peak near 125-120 Ma. Vlinder Guyot (16°55'N/154°20'E; 92-95Ma), an unnamed seamount "SM1" (15°29'N/155°01'E; 90Ma), Ita-Mai-Tai Guyot (12°58'N/156°45'E; 120Ma), and Ioah Guyots (14°11'N/155°59'E) are part of the NW-SE trending Magellan Seamount Trail (MST) that is located to the east of the Mariana trench. The MST is terminated at the E-Mariana subduction zone, and at the young end the seamount trail ends at Ita-Mai-Tai Guyot.
The MST guyots are characterized by complex volcanic histories involving large volumes of magma. Their morphologies are dominated by the presence of multiple rift zones, and both eroded and post-erosional satellite cones. Cross-cutting relations indicate that Vlinder Guyot is attached to an older, independent volcano (NW ridges) and that a younger, post erosional cone is located on top of its summit platform (17°00'N/ 154°16'E). An older, reef-bearing satellite ("Oma Vlinder" Guyot) is located in a southern rift zone. These morphological features are indicative of a prolonged duration of volcanism (up to 30 m.y.) at Vlinder Guyot, in which the 40Ar/39Ar dates represent crystallization ages of late shield volcanism.
Ita-Mai-Tai Guyot is located at the young end of the MST, yet it is significantly older than Vlinder Guyot and SM1 seamount. Since 120 m.y. ago Pacific plate movement was westward, the formation of Ita-Mai-Tai Guyot cannot totally be explained by NW-SE trending MST hot spot volcanism. More likely, an older part of it was produced by a precursory volcanic event. Extrapolation of Pacific plate movement prior to 80 Ma shows that MS hot spot volcanism could only have existed for at least 40-50 m.y. Because the time span of the volcanic history for single guyotsis in the same range as the length of the MST, and the possible evidence for a precursory volcanic event at 120 Ma, it is very difficult to establish an age progression for the MST with the limited dredge samples available.
Despite this complex volcanic history, basaltic samples of the MST guyots have similar strong affinities, and strongly enriched trace element and isotopic signatures (87Sr/86Sr = 0.70455-70543; 143Nd/144Nd = 0.51252-0.51274; 206Pb/204Pb = 17.96-19.12; 207Pb/204Pb = 15.60-15.69; 208Pb/204Pb = 38.53-39.40). This uniform geochemical signature suggests small admixture of the EM1 mantle component in the MST mantle source region similar to that of the Rarotonga hot spot in the French Polynesian area. Moreover, this preliminary data possibly indicates the maintained presence of a single mantle reservoir (dominated by EM1) to produce the MST between 120-180 Ma.