ausblenden:
Schlagwörter:
Pitcairn Island; hot spots; mantle; stable isotopes; osmium; hafnium; lead
Zusammenfassung:
We present comprehensive radiogenic isotope (Os, Pb, Hf, Nd, Sr) and trace element data on basaltic lavas from Pitcairn Island and the Pitcairn seamounts and examine the origin of the enriched mantle isotopic signature (EM-1) found in these lavas. The Os-187/Os-188 ratios of the lavas range from 0.131 to 0.254, while those of the high-Os concentration samples (> 50 pg/g) lie between 0.131 and 0.148. All Os-187/Os-188 ratios are higher than the bulk silicate Earth reference value of 0.127. Since ancient subcontinental lithospheric mantle (SCLM) is expected to have a Os-187/Os-188 ratio less than 0.127, it appears that recycled SCLM plays no role in the Pitcairn source. Variations in Os-187/Os-188 ratios appear to be unconnected with those of Pb-206/Pb-204 ratios in Pitcairn lavas, suggesting that Pb and Os isotopic variations are controlled by different factors. Modeling shows that variations in Pb isotopic compositions may mainly reflect the proportion of recycled sediment in the source, while those of Os-187/Os- 188 ratios may reflect the proportion of peridotite mantle versus recycled oceanic crust. The occurrence of negative Nb anomalies in some of the lavas, a correlation between Nb anomaly and Sr-87/Sr-86 ratios (0.7036-0.7051), and extremely unradiogenic and strongly correlated Nd and Hf isotopic compositions (epsilon(Nd) of -5.9 to +1.1 and epsilon(Hf) of - 5.3 to +2.2) together suggest that the Pitcairn mantle source contains a recycled continental crustal component. The slope of the epsilon(Hf) vs. epsilon(Nd) correlation is shallower for Pitcairn Island than for the Pitcairn seamounts or the global OIB array, and may be due to a variable ratio of recycled mud to sand in the Pitcairn source. A trace element mixing model also indicates the presence of small amounts of recycled pelagic and terrigenous sediment and permits variable amounts of depleted components such as recycled MORB, gabbro and depleted mantle. The Pb-206/Pb-204 ratios of the Pitcairn lavas vary between 17.47 and 18.10 and are very unradiogenic compared to those of other ocean island basalts. By contrast, Pb-206/Pb- 204 ratios are high and relatively homogeneous at values of similar to39.0. This observation along with the measured Th/U ratios of the lavas, which range up to 14.1, indicate a long- term history of U loss in the Pitcairn source. In Pb-207/Pb- 204-Pb-206/Pb-204 space, the data form a linear array that can be interpreted in terms of mixing between a minor recycled sediment end member and more depleted material. Lead isotopic compositions suitable for the recycled end member here investigated using a three-stage evolution model by Monte Carlo methods and suggest ages between 0.7 and 1.9 Ga for the recycled sediment. The relationships between measured Th/U and radiogenic Pb-208*/Pb-206* ratios suggest that the isotopic arrays displayed by the lavas were produced by mixing, probably occurring during magma genesis. (C) 2002 Elsevier Science B.V. All rights reserved.
