Water in the oceanic upper mantle: implications for rheology, melt extraction and the evolution of the lithosphere

Patterns in Galápagos Magmatism Arising from the Upper Mantle Dynamics of Plume‐Ridge Interaction

“…The more refractory DC matrix is treated as peridotite with 100 ppm water (i.e., C sw0 = 100 ppm) and the more fusible EC veins are treated as hydrous peridotite with 500 ppm water (i.e., C sw0 = 500 ppm). The bulk water content is thus 140 ppm, which is intermediate between estimated contents of the source of normal mid-ocean ridge basalt (NMORB) and the mantle component FOZO [Hirth and Kohlstedt, 1996;Workman et al, 2006]. One effect of water is on viscosity in Model 2.…”

“…The above result is puzzling because the dependence of mantle viscosity on water content is reasonably wellcharacterized [Hirth and Kohlstedt, 1996;Hirth and Kohlstaedt, 2003] and has been shown to be important in explaining analogous observations at the Iceland hotspot-ridge system [Ito et al, 1999;Bianco et al, 2013]. A possible explanation for this apparent contradiction comes from considering the strain-rate dependence of mantle rheology.…”

“…The temperature effect is important in creating the thermal lithosphere, which generally thickens with distance from the ridge segments and evolves self-consistently with the solution of the energy equation due to changes in ridge segmentation and spreading rate. In Model 2, r = −1, so viscosity also depends on water content [Hirth and Kohlstedt, 1996]. Water concentration in the solid residue (C sw ) decreases from its starting concentration (C w0 ) with melt depletion F according to the fractional melting equation (with bulk partition coefficient of 0.01 [Cushman et al, 2004]).…”

“…Water concentration in the solid residue (C sw ) decreases from its starting concentration (C w0 ) with melt depletion F according to the fractional melting equation (with bulk partition coefficient of 0.01 [Cushman et al, 2004]). Thus, in Model 2, the dehydration of the mantle causes viscosity to increases from its value just below the melting zone to a maximum of 100 times (i.e., 1 ≤ (C sw /C w0 ) r ≤ 100) after a couple of percent of melting [Hirth and Kohlstedt, 1996]. This creates a compositional lithosphere that encompasses most of partial melting zone in the sense that the buoyancy-driven flow of the plume is largely deflected beneath the dehydrated mantle layer, and within the layer, spatial variations in mantle flow and decompression melting are primarily driven passively by seafloor spreading [Ito et al, 1999;Bianco et al, 2013].…”

Three-dimensional numerical models of the evolution of pull-apart basins

“…(1) Gleason and Tullis (1995) (2) Rybacki and Dresen (2000) (3) Hirth and Kohlstedt (1996) Source for diffusion and Peierls' creep laws in mantle: Kameyama et al (1999).…”