Variability of Water in the Convecting Mantle

Abstract:

Estimation of the abundance of water in mantle sources first requires careful consideration of the shallow-level effects of degassing and contamination by seawater-derived components. Use of submarine glasses erupted at >500m water depth, and critically-evaluated use of melt inclusion volatile contents, can be used to identify and eliminate degassing as an important mitigating factor; widespread evidence for seawater-derived components are evident in halogen contents, but these effects do not typically correlate with water though there may be subtle effects in long-lived magmatic systems at mid-ocean ridges. H₂O/Ce ratios show large differences between mid-ocean ridges, hotspots, back-arc basins and arc-front volcanoes that testify to the large input of water at subduction zones; however, at arcs most of the Ce (like most of the water) is derived from the subducting slab, and at hotspots isotopic tracers of recycled components indicate the presence of materials that can, in sufficient quantity, dominate the Ce budget of mantle plumes. Thus H₂O/Ce ratios, while useful, are problematic when the goal is to determine the absolute abundances of water in mantle sources because the abundance of Ce cannot normally be assumed with confidence, except perhaps at mid-ocean ridges.

A more complete understanding of the abundance of water in mantle sources can be obtained when data for radiogenic isotopes are used as tracers of mantle composition, and when major and trace element data illuminate the process of mantle melting. In areas far from hotspots, normal mid-ocean ridges reveal a remarkably narrow range of H₂O/Ce ratios yet display large-scale regional differences between ocean basins [1]. Isotopically enriched signatures at hotspots suggest low absolute H₂O abundances [2], yet there is so much water delivered to the sources of arc volcanoes that even >95% dehydration of slabs results in delivery of water to the deep mantle in excess of that observed in MORB sources. The modern net flux of water from the Earth’s surface to the interior may thus point toward mantle enrichment, and may have been so throughout Earth’s history.

[1] Michael, P. (1995), EPSL 131, 301-320. [2] Dixon, J.E., Leist L., Langmuir, C., Schilling, J.-G. (2002) Nature 420, 385-389.