Abstract
Isotopic compositions of the Cape Verde (Central) hotspot magmas indicate a predominant influence from young HIMU and EM-1 type sources. Detailed modelling based on high precision Sr, Nd and Pb (DS) isotope data suggests that seven local mantle end-members explain the isotopic variation within five of the major islands in the archipelago. Four source end-members are dominant for three of the islands - young HIMU at Santo Antão (SA) with 206Pb/204Pb = 20.0, 208Pb/204Pb = 39.8, 143Nd/144Nd = 0.51288, 87Sr/86Sr= 0.70335, EM-1 at Fogo with 206Pb/204Pb = 18.8, 208Pb/204Pb = 38.7, 143Nd/144Nd = 0.51274 , 87Sr/86Sr= 0.7038) A moderately depleted end-member at São Nicolau (SN) with 206Pb/204Pb = 19.2, 208Pb/204Pb = 38.8, 143Nd/144Nd = 0.51302, 87Sr/86Sr= 0.7029. A fourth end-member represented on all these islands has 206Pb/204Pb = 20.1, 208Pb/204Pb = 39.4, 143Nd/144Nd = 0.51295 , 87Sr/86Sr= 0.7031. Incompatible trace element ratios such as Ba/Nb, Ba/La, Th/Rb, La/Nb, La/Th of Fogo-EM1 are comparable to other EM1-type OIB, whereas SA young HIMU-rocks overlap with classical HIMU. The SN end-member has Ba/Nb=6-9, Ba/La=8-14, and La/Th=9-11 in the range of EM1 and HIMU, but has relatively lower Rb than even HIMU. Carbonatites are widespread throughout Cape Verde Islands but volumetrically minor and are low in Ti, K, and Rb. In several silicate rocks from all three islands low Ti/Eu is evidence for a carbonatite component and is accompanied by LREE enrichment, and relatively low K and Rb. Other rocks have a relative depletion in LREEs and high Ti/Eu and seem to be complementary to carbonatites; they may be derived from sources that lost carbonatite or from immiscibility of carbonatite and silicate melts. Fogo rocks show a distinct trend in isotopic compositons towards carbonatite from this island, but other carbonatites range over much of the isotopic span of the silicate rocks, and no carbonatite trend is present isotopically for SA and SN. Nb/U increases from oceanic values at SA and SN towards 80 in EM-1 at Fogo. Nb/U shows a broad negative correlation with 206Pb/204Pb and a poisitve with 143Nd/144Nd. Nb/U range to >110 in some SN-rocks without isotopic correlation, and a relatively young process is indicated that may include melt-crystal equilibration for an exotic phase during melting and transport. The variation of Rb and K is very large compared to other incompatible elements and show island-related characteristics with very low Rb/Sr = 0.06-0.15 exclusively for many SN rocks, whereas Fogo generally has high Rb/Sr typically 0.5-0.7, and the majority of SA is intermediate. The low Rb and K rocks from SN are rather pristine typically with 1-2 wt.% loi.. The Rb/La ratio correlates broadly with 143Nd/144Nd and the rocks have mantle Ti/Eu. There is a strong correlation of Rb/La and Rb/Sr demonstrating that fluids are unlikely agents to explan this variation. Phlogopite control is also excluded due to decoupling of Rb and K from Ba. A possible explanation for the low K and Rb is dehydration of the mantle source in the presence of carbonate that could accommodate released Ba and Sr.
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| Original language | English |
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| Publication date | Dec 2010 |
| Number of pages | 1 |
| Publication status | Published - Dec 2010 |
| Event | American Geophysical Union, Fall Meeting 2010 - San Francisco, United States Duration: 13 Dec 2010 → … |
Conference
| Conference | American Geophysical Union, Fall Meeting 2010 |
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| Country/Territory | United States |
| City | San Francisco |
| Period | 13/12/2010 → … |