Consequences of off-axis melt delivery at the Moho: Sr and Nd isotopic results from the Oman ophiolite
Abstract
Recent tomographic images beneath the East Pacific Ridge (Toomey et al., 2007) indicate that at the Moho level, much of the melt is delivered off-axis (sometimes more than 10 km away from the axis) instead of beneath the ridge axis. In off-axis melt delivery sites, rising melt may enter directly into contact with cold and hydrothermally altered lithosphere, and thus might be subject to contamination by assimilation of this material. Structural mapping of the Oman ophiolite shows the presence of on and off-axis mantle diapirs, which offer the possibility to investigate the consequences of off-axis melt delivery. Off-axis diapirs display several particularities; for example, they are capped by a clinopyroxene-rich Moho Transition Zone (MTZ) whereas on-axis MTZ contain troctolite and gabbro lenses. To explore whether the differences between the on and off-axis MTZ arise from variations in the melt source or instead from interaction with melt-hydrated lithosphere, we are analyzing the strontium and neodymium isotopic compositions of ~20 samples, including pyroxenites, gabbros, diorites, and dunites, from one on-axis and one off-axis diapir in the Oman ophiolite. Our first results, obtained from whole rock powders, show some overlap between the initial (95 Ma) Nd isotopic compositions of the two diapirs (ɛNd = 8.5-9.2 on-axis, 3 samples; ɛNd = 5.6-8.8 off-axis, 8 samples, excluding one sample with a value of -0.3). Though some variations may be suggested by these data, they indicate that both diapirs were derived from roughly comparable mantle sources. This similarity is not surprising, since incorporation of hydrothermally altered lithosphere is not expected to strongly influence Nd isotopic ratios. In contrast, our first Sr isotopic results show a marked difference in composition between on and off- axis localities (87Sr/86Sr = 0.703010 - 0.703438 on-axis, 5 samples; 87Sr/86Sr =0.703714 - 0.706192 off-axis, 13 samples). The on-axis samples display a limited range of enriched MORB-like compositions whereas those from the off-axis diapir have more radiogenic and highly heterogeneous compositions. We tentatively interpret this difference to result from contamination by magmatic assimilation of the overlying hydrated lithosphere. The lack of correlation between Sr content and isotopic composition suggests that the radiogenic compositions are not late-stage features acquired during alteration related to ophiolite emplacement. Analyses of separated clinopyroxenes are in progress to verify this interpretation. If confirmed, our results may indicate that off-axis melt delivery at mid-ocean ridges can produce hydrated melts with isotopic compositions that mimic those of magmas from supra-subduction zone settings. Toomey, D.R., Jousselin, D., Dunn, R.A., Wilcock, W.S.D. and Detrick, R.S. Skew of mantle upwelling beneath the East Pacific Rise governs segmentation. Nature, 446: 409-414, 2007.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMOS21C1505N
- Keywords:
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- 1021 GEOCHEMISTRY / Composition of the oceanic crust;
- 1032 GEOCHEMISTRY / Mid-oceanic ridge processes;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 8140 TECTONOPHYSICS / Ophiolites