Ce anomalies in Gough Island lavas - Trace element characteristics of a recycled sediment component

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Earth and Planetary Science Letters
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Jan 30
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High precision trace element data obtained by inductively coupled plasma mass spectrometry and Sr-Nd isotope analyses are presented for mafic volcanic rocks from Gough Island, South Atlantic. The new data reveal negative Ce anomalies, with Ce/Ce* values in Gough lavas extending down to similar to 0.92. Ce is only fractionated from other rare earth elements (REE) due to formation of Ce4+ under oxidizing conditions of near-surface environments while other REE remain trivalent. Ce anomalies in convergent margin magmas have been shown to indicate a contribution of a subducted sediment component. In contrast, Ce anomalies in intraplate basalts have been attributed to weathering processes, but can be excluded here based on element-element systematics indicating magmatic trends rather than weathering-induced element mobility. Shallow-level contamination by local marine sediments with negative Ce anomaly inherited from seawater can be excluded because Gough lavas with increasingly negative Ce anomalies do not trend towards low Ce/Pb ratios characterizing such sediments. Rather, it is argued that the negative Ce anomalies in Gough Island lavas are consistent with variable amounts of a sediment component in the mantle plume source. Mixtures between estimates of subducting sediment columns with negative Ce anomaly and mantle capable of giving rise to Gough Island magmas without Ce anomalies reproduce the Gough compositional array with the exception of highly fluid-mobile elements. The calculated trace element composition of the deeply recycled sediment in the Gough plume source is depleted in fluid-mobile elements such as Ba and Pb relative to the composition of some present-day subducting sediments. This loss is attributed to the dehydration or flushing of sediment in the subduction factory, consistent with constraints from arc magmas. (C) 2007 Elsevier B.V. All rights reserved.


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DOI 10.1016/j.epsl.2007.10.030