Variations in the geochemistry of magmatism on the East Pacific Rise at 10°30′N since 800 ka

Samples of volcanic rock, collected from the flanks of the East Pacific Rise at 10°30′N, were used to investigate changes in the geochemistry of magmatism at the ridge axis, over the past 800 ka at this location. We show that there have been large variations in the major element chemistry of the lavas erupted at the spreading axis on this ridge segment over this period. For example, the average MgO content of lavas erupted at the ridge axis increased from about 3.0% at 600 ka, to about 7.0% at 300 ka. Since 300 ka the average MgO content has systematically decreased, and the average MgO content of lavas collected from within the neovolcanic zone at 10°30′N is 6.0%. These temporal changes in major element chemistry are not accompanied by systematic changes in isotope composition or incompatible trace element ratios, and are interpreted to reflect changes in the average rate of supply of melt to the ridge axis during this period. The data support previous arguments that changes in melt supply rate over periods of 100–1000 ka have an important influence on the major element chemistry of the lavas erupted at fast spreading ridges. At 10°30′N, the melt supply rate appears to have been relatively low for much of the past 800 ka. Samples younger than 50 ka, collected from within 3 km of the ridge axis at 10°30′N (inside the neovolcanic zone), have a smaller range in major element chemistry compared to the samples dredged from the ridge flanks. Variations in the chemistry of lavas erupted over periods of less than about 100 ka may be controlled by the geometry of the magma plumbing system beneath the ridge axis.