A combined chemical and Pb-Sr-Nd isotope study of the Azores and Cape Verde hot-spots: The geodynamic implications

G. R. Davies, R. A. Cliff, M. J. Norry, D. C. Gerlach

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

The mafic lavas of the Azores and Cape Verde Islands are of a highly varied composition, reflecting a complex history of magma genesis and a variety of source compositions. The lavas of the Cape Verde Islands are characteristically highly silica undersaturated, with alkali-rich ankaramites, larnite-normative melilitites and carbonatites. In contrast, the lavas of the Azores vary from strongly nepheline- to hypersthene-normative types. Isotopic ratios and trace elements also show considerable variation, consistent with derivation from multicomponent mantle sources. Three distinct groupings of lava compositions can be seen in Pb-Sr-Nd isotope space, and each is characterized by its own trace-element enrichment. (i) The majority of the lavas from the Azores and northern Cape Verdes Islands have isotope and trace-element systematics similar to basalts recovered from elevated segments of the mid-Atlantic Ridge (eg during Deep Sea Drilling Project Leg 82) (Ba/La <12, 206Pb/ 204Pb=19.3-20.0). (ii) The islands of Sao Miguel and Faial in the Azores are characterized by radiogenic strontium and lead isotope ratios and marked fractionation of the most incompatible trace elements (eg 87Sr/ 86Sr=0.70522, 206Pb/ 204Pb=19.88, 207Pb/ 204Pb=15.75, Ba/Nb=12, La/Nb=0.84). (iii) In contrast, the southern Cape Verde Islands have relatively unradiogenic Pb-Sr-Nd isotope ratios (eg 87Sr/ 86Sr=0.70393, 206Pb/ 204Pb=18.74, 207Pb/ 204Pb=15.53, Ba/La=22, Ba/Nb=15, La/Nb=0.67).Mixing relationships between isotope and trace-element ratios indicate the involvement of at least four, and possibly five, chemically distinct source regions in the petrogenesis of mafic lavas. It is shown that these different source regions represent combinations of the depleted upper mantle (mid-ocean ridge basalt source), recycled oceanic lithosphere and two components of the subcontinental lithospheric mantle. No direct unequivocal evidence is found for a fifth component, namely undepleted primitive mantle reservoir, although helium isotope data suggest influxes of material and heat from the lower mantle. Similarly, it is suggested that continental crust has no direct contribution to the petrogenesis of these ocean island basalts and its role in the mantle is limited to the production/modification of the subcontinental lithosphere above subduction zones.

Original languageEnglish
Pages (from-to)231-255
Number of pages25
JournalGeological Society Special Publication
Volume42
DOIs
Publication statusPublished - 1 Dec 1989

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Geodynamics
geodynamics
Isotopes
Trace elements
trace element
isotope
Basalt
petrogenesis
mantle
helium isotope
Chemical analysis
nepheline
strontium isotope
ocean island basalt
lead isotope
oceanic lithosphere
lower mantle
Deep Sea Drilling Project
isotopic ratio
mid-ocean ridge basalt

Cite this

@article{7441640cc07e4602992827aa225d4974,
title = "A combined chemical and Pb-Sr-Nd isotope study of the Azores and Cape Verde hot-spots: The geodynamic implications",
abstract = "The mafic lavas of the Azores and Cape Verde Islands are of a highly varied composition, reflecting a complex history of magma genesis and a variety of source compositions. The lavas of the Cape Verde Islands are characteristically highly silica undersaturated, with alkali-rich ankaramites, larnite-normative melilitites and carbonatites. In contrast, the lavas of the Azores vary from strongly nepheline- to hypersthene-normative types. Isotopic ratios and trace elements also show considerable variation, consistent with derivation from multicomponent mantle sources. Three distinct groupings of lava compositions can be seen in Pb-Sr-Nd isotope space, and each is characterized by its own trace-element enrichment. (i) The majority of the lavas from the Azores and northern Cape Verdes Islands have isotope and trace-element systematics similar to basalts recovered from elevated segments of the mid-Atlantic Ridge (eg during Deep Sea Drilling Project Leg 82) (Ba/La <12, 206Pb/ 204Pb=19.3-20.0). (ii) The islands of Sao Miguel and Faial in the Azores are characterized by radiogenic strontium and lead isotope ratios and marked fractionation of the most incompatible trace elements (eg 87Sr/ 86Sr=0.70522, 206Pb/ 204Pb=19.88, 207Pb/ 204Pb=15.75, Ba/Nb=12, La/Nb=0.84). (iii) In contrast, the southern Cape Verde Islands have relatively unradiogenic Pb-Sr-Nd isotope ratios (eg 87Sr/ 86Sr=0.70393, 206Pb/ 204Pb=18.74, 207Pb/ 204Pb=15.53, Ba/La=22, Ba/Nb=15, La/Nb=0.67).Mixing relationships between isotope and trace-element ratios indicate the involvement of at least four, and possibly five, chemically distinct source regions in the petrogenesis of mafic lavas. It is shown that these different source regions represent combinations of the depleted upper mantle (mid-ocean ridge basalt source), recycled oceanic lithosphere and two components of the subcontinental lithospheric mantle. No direct unequivocal evidence is found for a fifth component, namely undepleted primitive mantle reservoir, although helium isotope data suggest influxes of material and heat from the lower mantle. Similarly, it is suggested that continental crust has no direct contribution to the petrogenesis of these ocean island basalts and its role in the mantle is limited to the production/modification of the subcontinental lithosphere above subduction zones.",
author = "Davies, {G. R.} and Cliff, {R. A.} and Norry, {M. J.} and Gerlach, {D. C.}",
year = "1989",
month = "12",
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doi = "10.1144/GSL.SP.1989.042.01.15",
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A combined chemical and Pb-Sr-Nd isotope study of the Azores and Cape Verde hot-spots : The geodynamic implications. / Davies, G. R.; Cliff, R. A.; Norry, M. J.; Gerlach, D. C.

In: Geological Society Special Publication, Vol. 42, 01.12.1989, p. 231-255.

Research output: Contribution to JournalArticleAcademicpeer-review

TY - JOUR

T1 - A combined chemical and Pb-Sr-Nd isotope study of the Azores and Cape Verde hot-spots

T2 - The geodynamic implications

AU - Davies, G. R.

AU - Cliff, R. A.

AU - Norry, M. J.

AU - Gerlach, D. C.

PY - 1989/12/1

Y1 - 1989/12/1

N2 - The mafic lavas of the Azores and Cape Verde Islands are of a highly varied composition, reflecting a complex history of magma genesis and a variety of source compositions. The lavas of the Cape Verde Islands are characteristically highly silica undersaturated, with alkali-rich ankaramites, larnite-normative melilitites and carbonatites. In contrast, the lavas of the Azores vary from strongly nepheline- to hypersthene-normative types. Isotopic ratios and trace elements also show considerable variation, consistent with derivation from multicomponent mantle sources. Three distinct groupings of lava compositions can be seen in Pb-Sr-Nd isotope space, and each is characterized by its own trace-element enrichment. (i) The majority of the lavas from the Azores and northern Cape Verdes Islands have isotope and trace-element systematics similar to basalts recovered from elevated segments of the mid-Atlantic Ridge (eg during Deep Sea Drilling Project Leg 82) (Ba/La <12, 206Pb/ 204Pb=19.3-20.0). (ii) The islands of Sao Miguel and Faial in the Azores are characterized by radiogenic strontium and lead isotope ratios and marked fractionation of the most incompatible trace elements (eg 87Sr/ 86Sr=0.70522, 206Pb/ 204Pb=19.88, 207Pb/ 204Pb=15.75, Ba/Nb=12, La/Nb=0.84). (iii) In contrast, the southern Cape Verde Islands have relatively unradiogenic Pb-Sr-Nd isotope ratios (eg 87Sr/ 86Sr=0.70393, 206Pb/ 204Pb=18.74, 207Pb/ 204Pb=15.53, Ba/La=22, Ba/Nb=15, La/Nb=0.67).Mixing relationships between isotope and trace-element ratios indicate the involvement of at least four, and possibly five, chemically distinct source regions in the petrogenesis of mafic lavas. It is shown that these different source regions represent combinations of the depleted upper mantle (mid-ocean ridge basalt source), recycled oceanic lithosphere and two components of the subcontinental lithospheric mantle. No direct unequivocal evidence is found for a fifth component, namely undepleted primitive mantle reservoir, although helium isotope data suggest influxes of material and heat from the lower mantle. Similarly, it is suggested that continental crust has no direct contribution to the petrogenesis of these ocean island basalts and its role in the mantle is limited to the production/modification of the subcontinental lithosphere above subduction zones.

AB - The mafic lavas of the Azores and Cape Verde Islands are of a highly varied composition, reflecting a complex history of magma genesis and a variety of source compositions. The lavas of the Cape Verde Islands are characteristically highly silica undersaturated, with alkali-rich ankaramites, larnite-normative melilitites and carbonatites. In contrast, the lavas of the Azores vary from strongly nepheline- to hypersthene-normative types. Isotopic ratios and trace elements also show considerable variation, consistent with derivation from multicomponent mantle sources. Three distinct groupings of lava compositions can be seen in Pb-Sr-Nd isotope space, and each is characterized by its own trace-element enrichment. (i) The majority of the lavas from the Azores and northern Cape Verdes Islands have isotope and trace-element systematics similar to basalts recovered from elevated segments of the mid-Atlantic Ridge (eg during Deep Sea Drilling Project Leg 82) (Ba/La <12, 206Pb/ 204Pb=19.3-20.0). (ii) The islands of Sao Miguel and Faial in the Azores are characterized by radiogenic strontium and lead isotope ratios and marked fractionation of the most incompatible trace elements (eg 87Sr/ 86Sr=0.70522, 206Pb/ 204Pb=19.88, 207Pb/ 204Pb=15.75, Ba/Nb=12, La/Nb=0.84). (iii) In contrast, the southern Cape Verde Islands have relatively unradiogenic Pb-Sr-Nd isotope ratios (eg 87Sr/ 86Sr=0.70393, 206Pb/ 204Pb=18.74, 207Pb/ 204Pb=15.53, Ba/La=22, Ba/Nb=15, La/Nb=0.67).Mixing relationships between isotope and trace-element ratios indicate the involvement of at least four, and possibly five, chemically distinct source regions in the petrogenesis of mafic lavas. It is shown that these different source regions represent combinations of the depleted upper mantle (mid-ocean ridge basalt source), recycled oceanic lithosphere and two components of the subcontinental lithospheric mantle. No direct unequivocal evidence is found for a fifth component, namely undepleted primitive mantle reservoir, although helium isotope data suggest influxes of material and heat from the lower mantle. Similarly, it is suggested that continental crust has no direct contribution to the petrogenesis of these ocean island basalts and its role in the mantle is limited to the production/modification of the subcontinental lithosphere above subduction zones.

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