Lavas erupted from near-ridge seamounts, such as the southeast-northwest trending, 50 km long Lamont seamount chain, yield important information regarding the petrogenesis of mid-ocean ridge basalts (MORBs), and provide constraints on tectonic processes which lead to the development of ridge-flank central volcanoes (seamounts) and seamount chains. The Lamont seamount chain consists of five 800–1400 m high volcanoes surrounded by abundant 50–l200 m high lava cones, erupted west of the East Pacific Rise (EPR) axis just south of the Clipperton Transform. Lavas erupted from the chain are MgO-rich (MgO averages 8.2%, whereas lavas from the adjacent EPR average 7.6% MgO), and include some of the most primitive lavas yet found in the eastern Pacific (MgO to 9.7%, mg-number 0.72, Cr to 460 ppm, Ni to 240 ppm). They also contain low abundances of incompatible elements, classifying as LREE-depleted N-type MORBs, with the following variations in concentration: Na2O, 21.3–3% TiO2,0.81–1.43% Hf, 1.19–2.69 ppm; and La, 0.88–3.27 ppm. (La/Sm)N varies from 0.25 to 0.57, and K2O is always less than 0.13%. In contrast, the lavas from the adjacent EPR are richer on average in the LREE and other incompatible elements, with La ranging from 2.67 to 3.77 and (La/Sm)N varying from 0.53 to 0.57. The seamount lavas are also characterized by a relatively simple mineralogy, containing predominantly unzoned olivine (to Fo91), plagioclase (to An91), and Cr-rich spinel. Clinopyroxene is typically lacking. The simple mineralogy and primitive nature of the basalts, combined with the absence of active or recent hydrothermal activity, suggest that the Lamont seamount magmas had short crustal residence times, and did not evolve in long-lived shallow magma chambers. Consequently, these lavas serve as excellent probes of mantle source compositions and processes.

Our results indicate that the seamounts were derived from heterogeneous mantle sources variably depleted in the LREE and other incompatible elements relative to the adjacent EPR MORB sources, but that otherwise the seamount sources are similar to the EPR sources. Magmas erupted from individual centers are required to have been derived from a variety of mantle sources, indicating major and trace element heterogeneity over lateral distances of 5 km or less. The point source nature of the volcanism and apparent rapidity with which the lavas reached the surface indicates that magmas feeding near-ridge seamounts are transported primarily by propagation of veins and dikes rather than by porous media flow. The normative compositions of the most primitive Lamont seamount lavas suggest that they last equilibrated with mantle mineral assemblages at around 9–10 kb and 1200–1240°C, though slight depletions in the HREE relative to Sm imply that the seamount magmas may contain a magmatic component derived from a garnet-bearing source. The seamounts demonstrate the presence of partial melt underneath 0.7–0.1 m.y. old oceanic crust away from the EPR axis at 10°N, and the pressure estimates derived from the compositions of the seamount lavas indicate that porous media flow of melt at distances of 8–50 km west of the EPR at this latitude can only occur at pressures greater than 9 kb.

Author notes

* Present address: Ocean Drilling Program, Texas A & M University, College Station, TX 77843.