Magma sources of the Cape Verdes archipelago: Isotopic and trace element constraints

Details Magma sources of the Cape Verdes archipelago: Isotopic and trace element constraints Magma sources of the Cape Verdes archipelago: Isotopic and trace element constraints

Dec 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52.2979g&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 52, Issue 12, pp.2979-2992

Physics

26

Scientific paper

The Sr-, Nd-, and Pb-isotopic compositions of Late Tertiary to Recent mafic alkaline rocks of the Cape Verdes archipelago vary from 143 Nd / 144 Nd = 0.512606-0.513045 ( Nd = -0.5 to +8.1), 87 Sr / 86 Sr = 0.702922-0.703934, and 206 Pb / 204 Pb = 18.743-19.881. The variation of 207 Pb / 204 Pb with 206 Pb / 204 Pb in Cape Verdes lavas coincides with data for MORB, Hawaii, and Iceland, while 208 Pb / 204 Pb vs . 206 Pb / 204 Pb is an oblique, positive trend, i.e. , 208 Pb is higher in samples from the southern Cape Verdes islands which have lower 206 Pb / 204 Pb . At least three isotopically distinct components, including depleted upper mantle, are required in Cape Verdes magma sources. Samples from the southern islands have less radiogenic Nd and Pb and more radiogenic Sr relative to northern islands. Isotopic and trace element characteristics of an enriched source component (EM) present only in the southern Cape Verdes suggest an origin as recycled subcontinental mantle or lithosphere, except that this component in the Cape Verdes had a significant long-term depletion of U relative to Th. The isotopic and trace element characteristics of a component predominant in northern Cape Verdes magma sources with relatively radiogenic Pb and Nd (HIMU) suggest an origin for this component as ancient recycled oceanic crust. The trace element and isotopic variations, the large scale isotopic heterogeneity, the intra-island stratigraphic variations, the apparent geographic age progression of magmatism, and the geophysical constraints are best explained by variable partial melting of a heterogeneous plume (HIMU + depleted mantle) in the northern Cape Verdes, and mixing of plume-derived melts with lithospheric (EM) melts in the southern Cape Verdes. Alternatively, an EM plume produces magma supplied to the southern Cape Verdes, and the northern Cape Verdes, located off the plume track, are supplied by relatively lower degrees of melting at the plume margin which has entrained pre-existing HIMU components from a heterogeneous (DM + HIMU) upper mantle.

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