Melt production beneath oceanic islands

Details Melt production beneath oceanic islands Melt production beneath oceanic islands

Apr 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998pepi..107..143m&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 107, Issue 1-3, p. 143-182.

Physics

8

Scientific paper

Ocean island basalts are well-known to have distinctive isotopic signatures whose origin is not yet understood. It is, however, clear that these signatures impose important constraints on models of mantle circulation and melt generation. Large numbers of high-quality trace element analyses are now available from such islands, and are used to model the process of melt generation by inversion of the rare-earth element concentrations. This approach shows that about one-third of the islands contain incompatible element concentrations that are too large to have been generated by melting of either the MORB source or Primitive Mantle. However, they can be produced from a source that has previously been enriched by the addition of a few % of metasomatic melt. Those islands that do not require such source enrichment can be also be produced from the same enriched sources if the extent of melting is sufficiently large. A surprising feature of all models is that the melt is generated in the depth range where both garnet and spinel are stable. The decay of 147Sm in the enriched sources can generate the observed isotopic anomalies in 143Nd/144Nd in ~0.5 Ga. Though the melting models successfully reproduce most of the observed concentrations of minor and trace elements, the partition coefficients for Na, Sr and Pb used in the calculations appear to be too small. The similarity between the melting processes responsible for generating most oceanic islands is most clearly demonstrated using principal component analysis, which is a simple method of representing large numbers of analytic results. In the eight-dimensional space consisting of La, Ce, Sm, Eu, Yb, Lu, Ti and Nb, the islands lie on a single straight line passing through the origin. This is the behaviour expected if a source of constant composition undergoes variable amounts of melting. The trace element modelling shows that the sources of the basalts have the composition required to generate the HIMU reservoir required by the Pb isotopes, but cannot account for the other components required to account for the interisland variability of the Pb isotopic ratios.

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