Ancient seafloor signals in Pitcairn Island lavas and evidence for large amplitude, small length-scale mantle heterogeneities

Details Ancient seafloor signals in Pitcairn Island lavas and evidence for large amplitude, small length-scale mantle heterogeneities Ancient seafloor signals in Pitcairn Island lavas and evidence for large amplitude, small length-scale mantle heterogeneities

Sep 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989e%26psl..94..257w&link_type=abstract

Earth and Planetary Science Letters, Volume 94, Issue 3-4, p. 257-273.

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Scientific paper

Isotopic data for Pitcairn island volcanic rocks reveal the presence of two markedly different sources which appear to be closely related both spatially and temporally. The isotopic variability (or amplitude ratio) of this suite relative to all other oceanic basalts (mid-ocean ridges and oceanic islands) is ~ 30% for Pb, ~ 40% for Sr and 50% for Nd. This confirms the findings of several recent studies indicating that, contrary to earlier conclusions, large amplitude variations can exist over limited (~ 10 km) length scales. The unusual, unradiogenic 206Pb/204Pb and 143Nd/144Nd and radiogenic 87Sr/86Sr isotopic compositions, lack of strong covariation between isotopic and other geochemical parameters, and variable Nb/U ratios place strong constraints on any petrogenetic model. It is suggested that the two magma types may reflect the presence of variable quantities of ancient subducted oceanic crust and ancient subducted sedimentary material in the mantle source beneath Pitcairn. The critical role of subduction zone ``processing'' in modifying the chemical composition of such sources is stressed; thus, for example, the trace element composition of modern pelagic sediment cannot be used as a direct analogue of the subducted sediment residuum which may be important in the source of Pitcairn and some other ocean islands. The complementary trace element abundance patterns often noted between ocean island basalts (OIB) and island arc tholeiites (IAT) are seen as a direct result of this process.

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