Geochemical and structural studies of the Lamont seamounts: seamounts as indicators of mantle processes

Abstract

A chain of seamounts located just west of the East Pacific Rise (EPR) axis at 09°55′N (herein called the Lamont seamounts) have been investigated using Sea MARC I and Sea Beam sonar surveys, “Alvin” submersible dives, deep-sea camera tows and dredging. The most recently active volcanic vents on the summits of these volcanoes are located at the periphery of each caldera or crater rim or at the margins of caldera floors. The distribution of recent volcanic vents and the morphology of the summit plateaus suggest the possible existence of ring fractures on these seamounts. Ring fractures could be important in focusing post-caldera collapse eruptions around summit craters and building summit plateaus. Structural and morphological data and the petrology of recovered lavas suggest that calderas in the Lamont seamounts probably formed by collapse associated with magma withdrawal into large, primary vertical magma conduit systems, rather than by repeated inflation/deflation cycles associated with steady-state shallow-level magma chambers. Calderas in the Lamont seamounts have evolved through coalescence of adjoining craters and the plan-view shape and morphology of each caldera suggest that primary magma conduits have migrated beneath the summit of each seamount through time. Compared to lavas from the adjacent EPR, the majority of Lamont seamount and cone lavas are more primitive, depleted in incompatible elements and compositionally heterogeneous. Variations in light rare earth elements relative to major elements (e.g. TiO 2, Na 2O/(Na 2O + CaO) ) and radiogenic isotopes cannot be ascribed to differences in the amount of partial melting of a single N-type MORB source. Rather it appears that Lamont seamount lavas were derived from heterogeneous mantle sources variably depleted in highly incompatible elements. Comparison with lavas from the adjacent ridge crest suggests that magma plumbing and ascent mechanisms beneath the Lamont seamounts are separate from those along the EPR. In particular seamount magmas appear to reflect near-primary melting events in the mantle whereas EPR lavas are the products of shallow-level crystallization and mixing in crustal magma chambers. Absence of long-lived magma chambers within the Lamont seamounts may account for the low level of hydrothermal activity observed throughout the chain. Furthermore, because of the tight control on sample location and the recovery of lavas having distinct parents from individual volcanoes as well as the Lamont chain as a whole, we propose that the scale of mantle chemical heterogeneities is less than 5 km.