High H2O in MORB From Gakkel Ridge
Abstract
Enriched and depleted mid-ocean ridge basalts (MORB) from western Gakkel Ridge (GR) in the Arctic Ocean have distinctive trace element and volatile characteristics compared to MORB globally. They are enriched in H2O and large ion lithophile (LIL) relative to rare earth elements (REE), so have high H2O/Ce and Ba/La. To a lesser degree, they are enriched in REE relative to HFS (e.g., high La/Nb). The order of enrichment is H2O >Ba>Rb>K>La and is similar to the subduction zone signature displayed by some back-arc basin basalts. There has been about 200 km of shortening in this region as Greenland converged with the Arctic Basin during the Cenozoic (Brozena et al, 2003) but true subduction is less likely. The enrichment pattern also has some similarity with continental lithosphere, which may be present in the source region since continental rifting was relatively recent in this region. Continental involvement was suggested for Knipovich R. (Schilling et al., 1999). The geographic extent of the distinctive enrichment should help constrain its origin, since an anomaly associated with Greenland convergence should be localized to western GR. H2O/Ce and Ba/La both diminish toward the eastern GR, where basalts resemble Pacific MORB.However, H2O/Ce remains high toward the North Atlantic along Knipovich, Mohns, Kolbeinsey and Reykjanes Ridges, suggesting a much larger region, perhaps related to a larger and more ancient subduction event. The extent of the high Ba/La in the northernmost Atlantic is not clear from current data. Isotopic data from Gakkel ridge MORB should provide additional constraints on the nature of the high- H2O component on GR. When MORB are considered globally, there are no consistent associations between H2O/Ce and other trace element and isotopic characteristics. Some MORB with high Ba/Nb and high 87Sr/86Sr relative to 206Pb/204Pb have high H2O (SEIR) whereas others (southern MAR) have distinctive low H2O (Dixon et al; 2002; Michael and Kamenetsky, 2002). This might reflect very different high-Ba components, or it could reflect different behavior of H2O within the high Ba component compared to the normal peridotite. Brozena et al. (2003) Geology Dixon et al., (2002) Nature 420, 385-389 Schilling et al., (1999) JGR 104, 10543-10569
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.V22F..07M
- Keywords:
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- 1025 Composition of the mantle;
- 3035 Midocean ridge processes;
- 3670 Minor and trace element composition