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Titolo:
INCOMPATIBLE TRACE-ELEMENTS IN OIB AND MORB AND SOURCE ENRICHMENT IN THE SUB-OCEANIC MANTLE
Autore:
HALLIDAY AN; LEE DC; TOMMASINI S; DAVIES GR; PASLICK CR; FITTON JG; JAMES DE;
Indirizzi:
UNIV MICHIGAN,DEPT GEOL SCI,1006 CC LITTLE BLDG ANN ARBOR MI 48109 UNIV EDINBURGH,DEPT GEOL & GEOPHYS EDINBURGH EH9 3JW MIDLOTHIAN SCOTLAND
Titolo Testata:
Earth and planetary science letters
fascicolo: 3-4, volume: 133, anno: 1995,
pagine: 379 - 395
SICI:
0012-821X(1995)133:3-4<379:ITIOAM>2.0.ZU;2-R
Fonte:
ISI
Lingua:
ENG
Soggetto:
ISOTOPE GEOCHEMISTRY; CAMEROON LINE; EARLY DIFFERENTIATION; EARTHS MANTLE; RIDGE BASALT; CONSTRAINTS; EVOLUTION; PB; XENOLITHS; AFRICA;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Science Citation Index Expanded
Citazioni:
72
Recensione:
Indirizzi per estratti:
Citazione:
A.N. Halliday et al., "INCOMPATIBLE TRACE-ELEMENTS IN OIB AND MORB AND SOURCE ENRICHMENT IN THE SUB-OCEANIC MANTLE", Earth and planetary science letters, 133(3-4), 1995, pp. 379-395

Abstract

The concentrations of incompatible trace elements in ocean island basalts (OIB) from the central Atlantic extend to relatively enriched andfractionated compositions in regions of older oceanic lithosphere. Certain trace element ratios normally considered to be uniform in the mantle, such as Ce/Pb, are particularly variable. However, other trace element ratios that are expected to be variable because of differences in bulk distribution coefficient, such as Ce/U, are relatively uniform. The Ce/Pb ratios in enriched OIB are correlated with unusually high U/Pb and low K/U. These U/Pb ratios would have generated excessively radiogenic Pb if they were long-term (10(9) yr) features of the source such as might result from core formation or recycling of hydrothermally altered ocean floor basalts. However, volcanic centers with high U/Pb do have high Pb-206/Pb-204 for their Pb-207/Pb-204, a feature that is most easily modelled by enrichment in U relative to Pb about 10(8) yr prior to melting, a time similar to the age of the lithosphere. We propose that the source regions of these magmas are enriched by the introduction of small degree partial melts soon after the formation of the oceanic lithosphere. Metasomatism of the uppermost mantle by small degree partial melts produced in equilibrium with a combination of residual upper mantle major silicate phases, together with minor amphibole(less than or equal to 2%), sulfide (less than or equal to 0.2%) and phlogopite (less than or equal to 0.2%) at about the time of formationof the lithosphere, would generate a 'near-surface fractionated' (NSF) source with low K/U and high U/Pb, Delta(206)Pb/Pb-204 and Ce/Pb, while maintaining Ce/U, Nb/U, Ba/Ce and Ba/Nb that are only slightly fractionated relative to other OIB. An important feature of the modellingof NSF mantle is that U is more incompatible than Ba or Rb. This is confirmed by the variability in incompatible trace element ratios with U concentration for enriched OIB. However, this contrasts with the relative incompatibility deduced from U-Th-Ra disequilibrium data for MORB and OIB, endorsing the view that the variability in highly incompatible trace element ratios in enriched OIB is dominated by source enrichment effects that are distinct from the fractionation that takes placeduring the production of the erupted magmas. The Ce/U, Ba/Ce and U/Pbratios of all OIB, including enriched OIB from regions of old lithosphere, are uniform relative to data for MORB. This appears inconsistentwith the degree of isotopic variability in OIB relative to MORB and is difficult to explain unless the variations in incompatible trace element ratios in MORB are dominated by effects other than melting. Ratio-element plots provide evidence that the incompatible element ratios of MORB are affected by OIB-component contamination in the source or intransit to the surface and this is consistent with covariation between trace element ratios and some isotopic compositions in MORB. The ratios and concentrations of highly incompatible trace elements in MORB vary as a consequence of this contamination, as well as degree of partial melting. The relative uniformity and near chondritic proportions ofCe/U and, to a lesser extent, Ba/Ce in OIB compared with MORB are difficult to reconcile with recycling models that advocate material resembling present-day MORB or hydrothermally altered MORB as the dominant component of OIB sources but are consistent with NSF mantle recycling. Similarly, the Ba/U/Ce ratios are inconsistent with models in which the OIB source was affected by Ca perovskite fractionation in a magma ocean on the early Earth.

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Documento generato il 24/11/20 alle ore 07:45:08