Depleted and enriched upper mantle sources for basaltic rocks from diverse tectonic environments in the northeast Pacific ocean: The generation of oceanic alkaline vs. tholeiitic basalts
Oceanic basaltic rocks erupted in different tectonic environments in the northeast Pacific define a near-continuous spectrum of major and trace element compositions, ranging from highly depleted tholeiitic basalts to alkali basalt, hawaiite, and mugearite. The degree of incompatible element enrichment correlates with radiogenic isotope ratios: basalts with higher La/Sm have higher 87Sr/86Sr, 206Pb/204Pb, and lower 143Nd/144Nd, although all the basalts fall within the range of East Pacific Rise MORB in a Sr-Nd isotope plot. The geochemical differences between the tholeiitic and alkaline basalts require two mantle sources, but the geographic proximity of depleted and enriched basalts along the Explorer and northern Juan de Fuca Ridges implies that the two sources are intimately mixed. The first source, DM, is the dominant source of MORB. The second source, enriched mantle, has higher H2O, K/Na, K/La, K/Ti but similar Sm/Yb and HREE abundances, as well as lower Mg/(Mg+Fe) and CaO/Al2O3, and is the dominant source for alkaline intraplate rocks. Enriched mantle has a HIMU-type isotopic composition. The high volatile contents and the geochemistry of the alkaline lavas are consistent with an amphibole-bearing peridotite source, perhaps enriched in clinopyroxene. Veins or plums of enriched mantle are dispersed throughout the northeast Pacific upper mantle, and mixing of melts from enriched and depleted mantle sources produces the spectrum of MORB compositions (West Valley La/Smcn = 0.2-1.8, 206Pb/204Pb=18.2-19.2). The subtle isotopic distinction between depleted and enriched basalts limits the age of the enriched source to between 0.5 and 2.0 Ga. The source of the enriched mantle may be hydrated, subducted oceanic crust, stored in the transition zone, that either is transported into the upper mantle via mantle plumes or rises into the upper mantle as partial melts.
|Series||Geophysical Monograph Series|
Cousens, B. (1995). Depleted and enriched upper mantle sources for basaltic rocks from diverse tectonic environments in the northeast Pacific ocean: The generation of oceanic alkaline vs. tholeiitic basalts. In Geophysical Monograph Series. doi:10.1029/GM095p0207