HR: 0800h
AN: V51B-0564    [Abstracts]
TI: Tristan-Gough Plume: Negative Ce-Anomalies as Evidence of a Recycled Sediment Component in the Deep Mantle
AU: * Class, C
EM: class@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, POB 1000 RT 9W, Palisades, NY 10964 United States
AU: le Roex, A
EM: aleroex@geology.uct.ac.za
AF: University of Cape Town, Private Bag, Rondebosch, 7701 South Africa
AB: Recent volcanism on Tristan and Gough Islands, the associated age-progressive volcanism forming the Rio Grande Rise and Walvis Ridge symmetric to the Mid-Atlantic Ridge and extending towards the large Paran\'{a} (S America) and Etendeka (Africa) flood basalt province, as well as the near uniform composition of the related mantle source since $\sim$130 Ma cannot be reconciled with models other than the plume model. We thus consider Tristan-Gough basalts as probes for deep mantle sources. New high precision trace element data (acquired by ICPMS) on mafic volcanic rocks from Gough Island reveal variable depletion in Ce relative to adjacent rare earth elements when normalized to chondrites, i.e. so-called negative Ce anomalies (Ce/Ce*$<$1). Ce/Ce* values in Gough lavas extend down to values of $\sim$0.93. The magnitude of the anomaly varies with other chemical parameters (e.g. with $^{87}$Sr/$^{86}$Sr ratios) and is therefore not an analytical artifact. Correlations of Ce anomalies with $^{87}$Sr/$^{86}$Sr ratios has been taken as evidence for alteration by sea-spray, but in Gough basalts the accompanying alteration-induced rare earth element enrichment is missing. Thus Ce anomalies in Gough samples are not a weathering phenomenon. Ce anomalies are only formed in the oceans, being characteristic of certain deep sea sediments. Positive Pb and negative Nb anomalies, typical for such sediments, are absent in Gough Island lavas and thus preclude a contribution from sediment within the volcanic pile. Whereas the negative Ce anomaly combined with anomalous isotope ratios and higher LIL/REE strongly indicates a sediment component, the absence of a significant Nb depletion or Pb enrichment shows that the sediment component has been modified during subduction, before it became part of the source of some Gough Island lavas. The presence of a negative Ce anomaly is the strongest indication yet for recycled sediment in the Tristan-Gough plume source. The data confirm that the processes occurring on the Earth's surface affect the evolution of the deep earth.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 3640 Igneous petrology
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting