Once melting commences within the Earth's mantle, melt movement commences by porous flow. Whether or not the entire path to the surface occurs via porous flow or whether channeled flow takes over is not well understood. Since channeled flow will be much faster than porous flow, time scale information provides one of the most powerful means of distinguishing the mechanism involved. U-series isotopes are the principle source of this time scale information. Studies of 238U-230Th and 235U-231Pa disequilibria in mid-ocean ridge, ocean island and island arc basalts all place minimum constraints on ascent rates of 1-20 m/yr. 226Ra-230Th disequilibria in island arc basalts (IAB) arguably provide the most stringent constraint suggesting ascent rates of 70 m/yr. This requires channeled melt transport for the majority of source to surface path. It remains possible that 210Pb disequilbria are a melting signature and if so this would require melt ascent rates of km/yr. Such estimates are consistent with available seismic and experimental constraints that melt transport is in general fast.
|Title of host publication||Timescales of Magmatic Processes|
|Subtitle of host publication||From Core to Atmosphere|
|Editors||Anthony Dosseto, Simon P. Turner, James A. Van Orman|
|Place of Publication||Chichester, West Sussex, UK|
|Publisher||John Wiley & Sons|
|Number of pages||14|
|Publication status||Published - 30 Nov 2010|