Generation of Hawaiian post-erosional lavas by melting of a mixed lherzolite/pyroxenite source

Melting of mafic veins in a marble-cake mantle may play an important role i n generating isotopic and chemical heterogeneities in mid-ocean ridge and o cean island basalts. Mafic veins have lower solidi than mantle peridotite a nd will be preferentially sampled during partial melting, particularly at l ow melt fractions. However, the abundance of mafic components in the mantle or their role during melt generation has been difficult to quantify becaus e most isotopic systems (e.g. Rb-Sr, Sm-Nd, U-Th-Pb) are not diagnostic of the presence or absence of mafic components. The compatible behavior of Os during mantle melting combined with the incompatible behavior of Re makes t he Re-Os isotopic system uniquely well suited for distinguishing mafic and ultramafic contributions to melt generation. Almost all peridotites have lo w Os-187/Os-188 (e.g. chondritic to subchondritic). In contrast, mafic rock s have much higher Re/Os than peridotites, which results in the rapid ingro wth of Os-187 and the development of large isotopic contrasts between mafic and ultramafic components within the mantle. In this paper, we show that O s-isotopes in Hawaiian post-erosional lavas extend to more radiogenic value s than are found in Hawaiian lherzolites, abyssal peridotites or most other ultramafic samples. Os-isotopes are not correlated with other isotopic tra cers, in contrast with plume-derived Hawaiian shield-stage lavas. The lack of correlation between Os-isotopes and Sr-, Nd- or Pb-isotopes and the more 'depleted' or MORB-like Sr-Nd isotopic signature of the post-erosional lav as relative to other Hawaiian lavas precludes significant melt input from t he Hawaiian plume. However, Os-isotopes are correlated with major and trace elements. Lavas with more radiogenic Os-isotope compositions have higher s ilica and alumina and lower calcium and incompatible trace element abundanc es than lavas with less radiogenic Os-isotopes. These correlations result f rom mixing of pyroxenite- and peridotite-derived melts, both likely derived from the similar to 100 Ma Pacific lithospheric mantle. (C) 2000 Elsevier Science B,V. All rights reserved.