East Scotia Ridge unsecured

Mcdonough sun 1995 similar to the source of bouvet

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Unformatted text preview: e correlation of Th/Nb with H2O suggests that Th is transported into the mantle source either in an aqueous fluid, or in a hydrous sediment melt. Several workers have argued that Th is transported from the slab in silicate melts (e.g. Elliott et al., 1997; Turner & Hawkesworth, 1997; Class et al., 2000). This is consistent with experimental data indicating that Th is preferentially partitioned into silicate melts but not into aqueous fluids (Brenan et al., 1995; Johnson & Plank, 1456 FRETZDORFF et al. PETROGENESIS OF EAST SCOTIA RIDGE Fig. 10. Estimation of the addition of subduction-related components (Ba, Rb, K, Pb, Th, U, Sr), termed ‘enrichment factors’, to the East Scotia Ridge magma source relative to a MORB-like ‘baseline’ sample (WX48) from segment E5. Enrichment factors were calculated by dividing measured element/Yb ratios by the element/Yb ratio of sample WX48 normalized to the Nb/Yb ratio of the sample. This method removes the effects of fractional crystallization, different degrees of partial melting for elements of similar incompatibility to Nb and the effects of different mantle compositions. It should be noted that the scale of the plot showing segment E4 extends to higher values than the other diagrams. (See text for details.) 1999), suggesting that Th requires sediment melting to be efficiently transferred to the arc. However, the partition behaviour of Th in aqueous fluids remains uncertain (Pearce & Peate, 1995) and some workers have suggested that, at high temperature, Th could be transported in an aqueous fluid (Stolper & Newman, 1994). We have already shown, using Nb/Yb ratios, that there are variations in the composition of the back-arc mantle source independent of subduction components (Fig. 7). In Fig. 13 we investigate the compositional effects of the slab-derived components and plume vs MORBsource mantles on the back-arc lavas. Trends A and B in Fig. 13 diverge strongly from N-MORB. Trend A is characterized by positive correlation of Nb/Yb with Th/ Yb, increasing Ta/Nd at constant Th/Nb, and slightly increasing Th/Nd with decreasing 143Nd/144Nd. This 1457 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 8 AUGUST 2002 Fig. 11. Variation of Ba/Th vs Th/Nb ratios for lavas from the East Scotia Ridge, South Sandwich island arc (Pearce et al., 1995), Bouvet Island (Le Roex & Erlank, 1982; Weaver et al., 1987), and the composition of N-MORB (Hofmann, 1988). Ba/Th and Th/Nb ratios for samples DR.157 and DR.158 are from D. Harrison (unpublished data, 2000; location sites shown in Fig. 2). The continuous lines with arrows are putative enrichments interpreted to have been caused by aqueous fluid (increasing Ba/Th for constant low Th/Nb ratios) and sediment addition (increasing Th/Nb for constant low Ba/Th ratios) from the subducting slab (see text for details). trend could be the result of MORB–plume source mixing and/or variable degrees of mantle partial melting. Trend B displays increases in Th/Yb and Th/Nb with moderate increases in Nb/Yb and Ta/Nd, and stron...
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This document was uploaded on 02/01/2014.

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